Patent Application
20060060781
Disclosure gleaned from the first application is as follows. More particularly, the present invention relates to a charged-particle beam apparatus for automatically adjusting astigmatism or the like in a charged-particle optical system for carrying out inspection, measurement, fabrication and the like with a high degree of precision by using a charged-particle beam, and the invention also relates to a method of automatically adjusting the astigmatism in such a charged-particle beam apparatus.
For example, an electron-beam microscope is used as an automatic inspection system for inspecting and/or measuring a microcircuit pattern created on a semiconductor wafer or the like. In the case of defect inspection, a detected image, which is an electronic beam image detected by a scanning electron-beam microscope, is compared with a reference picture used as a reference. In addition, in the case of measurement of a line width, a hole diameter and other quantities of a microcircuit pattern, the measurement is carried out in image processing by using an electron-beam image detected by a scanning electron-beam microscope. The measurement of such quantities of a microcircuit pattern is carried out in setting and monitoring conditions of a process used in the manufacture of a semiconductor device.
In comparative inspection for detecting a defect in a pattern by comparing electronic images of patterns and in measurement of line width or another quantity of a pattern by processing an electronic image, as described above, the quality of the electronic image has a big effect on reliability of a result of the inspection. The quality of an electronic image deteriorates due to deterioration in resolution or the like caused by aberration and defocus of an electron-beam optical system. The deterioration in image quality deteriorates the inspection sensitivity and the measurement performance. In addition, the width of an image on a picture changes and a stable result of detection of an edge cannot be obtained. Thus, the sensitivity of detection of a defect and a result of measurement of the line width of a pattern, as well as a result of measurement of a hole diameter, also become unstable.
Traditionally, the focus and astigmatism of an electron-beam optical system are adjusted by adjusting the control current of an objective lens and control currents of two sets of astigmatism correction coils while visually observing an electronic image. To be more specific, the focus is adjusted by changing the current flowing to the objective lens in order to change the convergence height of a beam.
It takes time to adjust the focus and astigmatism of an electron-beam optical system by adjusting the control current of an objective lens and control currents of two sets of astigmatism correction coils, while visually observing an electronic image, as described above. In addition, if the surface of a sample is scanned by using an electron beam a number of times, it is quite within the bounds of possibility that a problem of damage inflicted on the sample is raised. Furthermore, by carrying out the adjustment manually, the result of adjustment may inevitably vary from operator to operator. Moreover, the astigmatism and the focal position normally vary with the lapse of time. Thus, in automatic inspection and measurement, it is necessary to adjust the astigmatism and the focal position periodically, presenting a hindrance to automation.
In order to solve the problems described above, a variety of conventional automatic astigmatism correction methods have been proposed. In Japanese Patent Laid-open No. Hei 7-153407, for example, there has been disclosed an apparatus (referred to as Example 1) wherein a 2-dimensional scanning operation is carried out on a sample by using a charged-particle beam to produce a secondary-electron signal from the sample; the secondary-electron signal is then differentiated and digital data with a large change is extracted; then, a position on the sample, at which the large change of the extracted data occurs, is found; subsequently, a charged-particle beam is used for scanning in the X direction only and in the Y direction only while excitement flowing to an objective lens is being changed with the found position taken as a center; a maximum value of digital data of a secondary-electron signal generated by these scanning operations is then used for detecting focal information in the X direction and focal information in the Y direction; from the focal information in the X direction and the focal information in the Y direction, a current to flow to the objective lens is then determined and output to the objective lens; afterward, a current flowing to an astigmatism correction coil is changed and a charged-particle beam is then used for carrying out a scanning operation in the X or Y direction to produce a secondary-electron signal; and a maximum value of digital data of the secondary-electron signal is used for determining the magnitude of a current to flow to the astigmatism correction coil in order to adjust the astigmatism and the focus of the charged-particle beam.
In addition, in Japanese Patent Laid-open No. Hei 9-161706, there has been disclosed a method (referred to as Example 2) whereby the focus is changed back and forth by carrying out a scanning operation using an electron beam in a variety of directions in order to recognize the direction of astigmatism; then, two different astigmatism correction quantities are changed, while the relation between these astigmatism correction quantities is being maintained, so that the astigmatism changes only in this direction; and finally, a condition for the image to become bright is searched for. Thus, the adjustment can be carried out by limiting conditions of an astigmatism correction quantity with two degrees of freedom compared to a condition of an astigmatism correction quantity with one degree of freedom.
Furthermore, in Japanese Patent Laid-open No. Hei 10-106469, there has been disclosed a method (referred to as Example 3) whereby, first of all, the focus is adjusted automatically to a position slightly shifted from an in-focus state; then, the direction of astigmatism is found by adoption of FFT of a 2-dimensional picture; subsequently, two different astigmatism correction quantities are changed while the relation between these astigmatism correction quantities is being maintained, so that the astigmatism changes only in this direction; and finally, a condition for the image to become bright is searched for.
Moreover, in Japanese Patent Laid-open No. Hei 9-82257, there has been disclosed a method (referred to as Example 4) whereby, by adopting Fourier transformation of a 2-dimensional SEM image, a point at which a change of the magnitude of the Fourier transformation is inverted is first of all found, while the focus is being changed in order to determine an in-focus position; then, a 2-dimensional particle image at a focal point before the in-focus position and a 2-dimensional particle image at a focal point after the in-focus position are found; subsequently, the direction of astigmatism is found from a distribution of magnitudes of the Fourier transform; and finally, the astigmatism is corrected so that the astigmatism changes in this direction.
In addition, in U.S. Pat. No. 6,025,600, there has been disclosed a method (referred to as Example 5) whereby, 4-direction sharpness values of an acquired SEM picture are found by increasing the focal position; then, the focal position is increased until maximums of these values are obtained; and, finally, a correction quantity of astigmatism is found from the maximums of the sharpness values in the 4-direction.
Furthermore, in Japanese Patent Laid-open No. Sho 59-18555 and U.S. Pat. No. 4,554,452, which is a U.S. patent corresponding to Japanese Patent Laid-open No. Sho 59-18555, there has been disclosed a method (referred to as Example 6) whereby, an SEM picture is scanned in a variety of directions by increasing a focal position in order to find the sharpness in each of the directions; and the correction quantity of astigmatism is found from a maximum value of the sharpness found in each of the directions.
Example 1 adopts a method whereby, while three kinds of control quantity, namely, two kinds of astigmatism correction quantity and a focal correction quantity, are each being changed one by one, a point providing a maximum sharpness value of a secondary particle image is found by a trial-and-error technique. Thus, it takes too long a time to complete the correction of astigmatism. As a result, since the sample is exposed to a charged-particle beam for a long time, the sample may also be damaged by charge-up, contamination and the like. In addition, if an astigmatism is adjusted automatically or visually by taking sharpness as a reference, a state in which the astigmatism is not correctly eliminated easily results in dependence on the sample pattern.
Also in the case of Example 2, after examining the direction of astigmatism by changing the focal point back and forth, it is necessary to carry out a 1-dimensional scanning operation by changing the focal point back and forth while changing the astigmatism adjustment quantity in order to repeatedly carry out an operation to search for a condition in which in-focus positions in two directions coincide with each other, so that Example 2 has a problem in that it takes too much time. In addition, there is also a problem in that a post-radiation mark is left on the sample due to the fact that the scanning operation using an electron beam is a one-dimensional operation. Moreover, there is also a problem in that stable astigmatism correction cannot be carried out since a sufficient signal cannot be obtained in dependence on the location of the one-dimensional scanning operation, if the sample does not have a uniform texture thereon.
Also in the case of Example 3, since the adjustment comprises two steps, namely, the step of changing the focus back and forth and the step of changing the astigmatism correction quantity up and down, there are problems in that it takes time to carry out the adjustment, and, in addition, the damage inflicted on the sample is great. Furthermore, in order to find the direction of the astigmatism by adoption of the FFT, the method requires a precondition that the spectrum of an image in which no astigmatism is generated is uniform. Thus, there is a problem in that the number of usable samples is inevitably limited.
As described above, Examples 1, 2 and 3 include neither a method of finding the direction and the magnitude of astigmatism in a stable manner from a particle image, nor the computation of a correction quantity to be supplied to an astigmatism adjustment means from the direction and the magnitude of the astigmatism. Thus, the astigmatism correction quantity must be changed and the result must be checked repeatedly on a trial-and-error basis, so that it takes time to carry out the adjustment; and, at the same time, the sample is contaminated, whereas damage caused by charge-up is inflicted upon the sample. In addition, in the case of a one-dimensional beam scanning operation, there is a problem of precision deterioration for scanning of a location with a coarse pattern on the sample.
Moreover, in the case of Example 4, the direction and the strength of an astigmatism are found from Fourier transformation of a 2-dimensional image with the focus being changed back and forth. However, Example 4 does not include a specific method of computing a correction quantity to be supplied to an astigmatism adjustment means from the direction and the strength of the astigmatism. Furthermore, the meaning of the strength seen from the physics point of view is not defined clearly. Thus, there is a problem in that the correction quantity to be supplied to the astigmatism adjustment means cannot be found with a sufficient degree of accuracy.
In addition, in the case of Example 5, an astigmatism correction quantity can be found from an SEM image with the sequence of focal points being shifted, and the amount of damage inflicted on the sample can be reduced. However, this method does not consider the case of a sharpness curve becoming unsymmetrical or having two peaks for a large astigmatism. Furthermore, when degrees of directional sharpness are to be found from a picture, the sharpness in the vertical direction and the sharpness in the horizontal direction include many noises in comparison with the sharpness in the slanting direction, due to the beam noises and response characteristics of the detector. As a result, there is a problem of unstable operation for a dark sample.
In the case of Example 6, the scanning axis is rotated in more than three directions to obtain a signal, and the sharpness in each of the directions is found from this cross-sectional signal, so that it takes time to carry out the scanning operation. More specifically, there is a problem in that the determined sharpness is susceptible to an error, because of an effect of the edges in other directions, due to the fact that the processing is a one-dimensional differentiation process.
As a problem common to Examples 5 and 6, the astigmatism correction quantity cannot be found with a high degree of accuracy, or it takes time to converge the astigmatism correction if the edge of a sample pattern is one-sided in a certain direction, so that the sharpness in this certain direction is affected by an edge in another direction and inevitably increases, This phenomenon is caused by the fact that the astigmatism correction quantity is found by adopting a linear junction of maximum values of the sharpness.
Background disclosure gleaned from the second application is as follows. More particularly, The present invention relates to a convergent charged particle beam apparatus using a charged particle beam such as an electron beam or ion beam for microstructure fabrication or observation and an inspection method using the same, and more particularly to an automatic focusing system and arrangement in the convergent charged particle beam apparatus.
As an example of an apparatus using a charged particle beam, there is an automatic inspection system intended for inspecting and measuring a microcircuit pattern formed on a substrate such as a semiconductor wafer. In defect inspection of a microcircuit pattern formed on a semiconductor wafer or the like, the microcircuit pattern under test is compared with a verified non-defective pattern or any corresponding pattern on the wafer under inspection. A variety of optical micrograph imaging instruments have been put to practical use for this purpose, and also electron micrograph imaging has found progressive applications to defect inspection by pattern image comparison. In a scanning electron microscope instrument which is specifically designed for critical-dimension measurement of line widths and hole diameters on microcircuit patterns used for setting and monitoring process conditions of semiconductor device fabrication equipment, automatic critical-dimension measurement is implemented through use of image processing.
In comparison inspection where electron beam images of corresponding microcircuit patterns are compared for detecting a possible defect or in critical-dimension measurement where electron beam images are processed for measuring such dimensions as pattern line widths, reliability of results of inspection or measurement largely depends on the quality of electron beam images.
Deterioration in electron beam image quality occurs due to image distortion caused by deflection or aberration in electron optics, decreased resolution caused by defocusing, etc., resulting in degradation of performance in comparison inspection or critical-dimension measurement.
In a situation where a specimen surface is not uniform in height, if inspection is conducted on the entire surface area under the same condition, an electron beam image varies with each region inspected as exemplified in FIGS. 21(a)-21(d), wherein
In Japanese Non-examined Patent Publication No. 258703/1993, there is disclosed a method intended for circumventing the abovementioned disadvantages, wherein an optimum control current to an objective lens for each surface height of a specimen is pre-measured at some points on the specimen and then, at the time of inspection, focus adjustment at each point is made by interpolation of pre-measured data. However, this method is also disadvantageous in that a considerable amount of time is required for measuring an optimum objective lens control current before inspection and each specimen surface height may vary during inspection depending on wafer holding conditions.
A focus adjustment method for a scanning electron microscope using an optical height detecting arrangement is found in Japanese Non-examined Patent Publication No. 254649/1988. However, since an optical element for height detection is disposed in a vacuum system, it is rather difficult to perform optical axis alignment.
In microstructure fabricating equipment using a convergent charged particle beam, focus adjustment of the charged particle beam has a significant effect on fabrication accuracy, i.e., focus adjustment is of extreme importance as in instruments designed for observation. Examples of microstructure fabricating equipment include an electron beam exposure system for forming semiconductor circuit patterns, a focused ion beam (FIB) system for repairing circuit patterns, etc.
In a scanning electron microscope, a method of measuring an optimum control current to an objective lens thereof through electron beam imaging necessitates attaining a plurality of electron beam images for detecting a focal point, thus requiring a considerable amount of time for focus adjustment. That is, such a method is not suitable for focusing in a short time. Further, in an application of automatic inspection or critical-dimension measurement over a wide range, focus adjustment at every point using the abovementioned method is not practicable, and it is therefore required to perform pre-measurement at some points before inspection and then estimate a height at each point through interpolation, for instance.
A semiconductor wafer to be inspected in a fabrication process may deform due to heat treatment or other processing, and a degree of deformation will be on the order of some hundreds of micrometers in the worst case. However, it is extremely difficult to hold the specimen wafer stably without causing interference with electron optics in a vacuum specimen chamber, and also it is impossible to adjust specimen leveling as in an optical inspection system using vacuum chucking.
Further, since a substantial amount of time is required for inspection, a specimen holding state may vary due to acceleration/deceleration in reciprocating stage movement, thereby resulting in a specimen surface height being different from a pre-measured level.
For the reasons mentioned above, there is a rather high degree of possibility that a surface height of a specimen under inspection will vary unstably exceeding a focal depth of the electron optical system (a depth of focus is generally on the order of micrometers at a magnification of 100×, but that necessary for semiconductor device inspection depends on inspection performance requirements concerned). For focus adjustment using electron beam images, a plurality of electron images must be attained at each point of interest with each stage being stopped. It is impossible to conduct focus adjustment continuously while detecting a height at each point simultaneously with stage movement for the specimen under inspection.
In an approach that focus adjustment using electron beam images is performed at some points on a specimen surface before the start of inspection, an amount of time is required for calibration before inspection. This causes a significant decrease in throughput as a size of wafer becomes larger. Since there is a technological trend toward larger-diameter wafers, a degree of wafer deformation such as bowing or warping will tend to be larger, resulting in more stringent requirements being imposed on automatic focusing functionality. Depending on the material of a specimen, exposure with an electron beam may alter an electric charge state on specimen surface to cause an adverse effect on electron beam images used for inspection.
In consideration of the above, it is difficult to ensure satisfactory performance in long-period inspection on a scanning electron microscope instrument using the conventional methods. Where stable holding of a specimen is rather difficult, it is desirable to carry out specimen surface height detection in a range of electron optical observation immediately before images are attained during inspection. Further, where inspection is conducted while each stage is moved continuously, specimen surface height detection must also be carried out continuously at high speed without interrupting a flow of inspection operation. For realizing continuous surface height detection simultaneously with inspection, it is required to detect a height of each inspection position or its vicinity at high speed.
However, if any element which affects an electric or magnetic field, e.g., an insulating or magnetic element, is disposed in the vicinity of an observation region, electron beam scanning is affected adversely. It is therefore impracticable to mount a sensor in the vicinity of electron optics. Further, since the observation region is located in the vacuum specimen chamber, measurement must be enabled in a vacuum. For use in the vacuum specimen chamber, it is also desirable to make easy adjustment and maintenance available. While there have been described conditions as to an Example of an electron-beam inspection system, these conditions are also the same in a microstructure observation/fabrication system using an ion beam or any other convergent charged particle beam. Further, since there are the same conditions in such systems that images of an aperture, mask, etc. are formed or projected as well as in a system where a charged particle beam is converged into a single point, it is apparent that the present invention is applicable to charged particle beam systems comprising any charged particle beam optics for image formation/projection.
The present invention relates to an electron beam exposure or system inspection or measurement or processing apparatus having an observation function using charged particle beams such as electron beams or ion beams and its method and an optical height detection apparatus.
Heretofore, a focus of an electron microscope has been adjusted by adjusting a control current of an objective lens while an electron beam image is observed. This process requires a lot of time, and also, a sample surface is scanned by electron beams many times. Accordingly, there is the possibility that a sample will be damaged.
In order to solve the above-mentioned problem, in a prior-art technique (Japanese laid-open patent application No. 5-258703), there is known a method in which a control current of an optimum objective lens relative to a height of a sample surface in several samples are measured in advance before the inspection is started and focuses of respective points are adjusted by interpolating these data when samples are inspected.
In this method, SEM images obtained by changing an objective lens control current at every measurement point are processed, and an objective lens control current by which an image of a highest sharpness is recorded. It takes a lot of time to measure an optimum control current before inspection. Moreover, there is the risk that a sample will be damaged due to the irradiation of electron beams for a long time. Further, there is the problem that a height of a sample surface will be changed depending upon a method of holding a wafer during the inspection.
Moreover, as the prior-art technique of the apparatus for inspecting a height of a sample, there are known Japanese laid-open patent application No. 58-168906 and Japanese laid-open patent application No. 61-74338.
According to the above-mentioned prior art, in the electron beam apparatus, the point in which a clear SEM image without image distortion is detected and a defect of a very small pattern formed on the inspected object like a semiconductor wafer such as ULSI or VLSI is inspected and a dimension of a very small pattern is measured with high accuracy and with high reliability has not been considered sufficiently.
FIGS. 4(a) and 4(b) are diagrams of patterns for focus and astigmatism correction according to the invention;
FIGS. 7(a) and 7(b) are diagrams each showing typical picture processing to find directional sharpness;
FIGS. 8(a) and 8(b) are diagrams each showing an Example of the shape of a sample serving as a calibration target for fast focus and astigmatism correction;
FIGS. 12(b) and 12(c) are graphs each representing a relation between the focus value and the sharpness and serving as a means for explaining a case of a double-peak curve of directional sharpness;
FIGS. 15(a) and 15(b) are graphs representing a relation between the focus value and the sharpness and serving as a means for explaining a method of finding a central position of a directional-sharp curve by adopting a symmetry-matching technique;
FIGS. 21(a)-21(d) show inspection of a wafer at different regions and electron beam images of the different regions;
FIGS. 34(a) and (b) are schematic sectional views showing a change in optical path size on a flat-plate electrode according to incidence angle;
FIGS. 47(a) and (b) show views of a specimen surface for explaining a method of presuming an observation region height using height data detected continuously;
FIGS. 48(a)-(c) show views of a specimen surface for explaining a method of presuming an observation region height using height data detected continuously;
FIGS. 49(a) and (b) show views of a specimen surface for explaining a method of presuming an observation region height using height data detected continuously in a different manner;
FIGS. 53(a)-(c) show diagrams for explaining height detection using a plurality of fine slit light beams;
FIGS. 54(a)-54(d) (similar to FIGS. 21(a)-21(d)) show a semiconductor wafer and image obtained at different areas thereof so as to explain that electron beams need be focused on an inspected object such as a semiconductor wafer in an electron beam inspection according to the present invention;
FIGS. 58(a)-(c) show a semiconductor wafer in which a semiconductor memory is formed according to the present invention and enlarged portions thereof;
FIGS. 59(a) and (b) show a detection image f1(x, y) and a comparison image g1(x, y) which are compared and inspected in the electron beam inspection apparatus (SEM inspection apparatus) according to the present invention;
FIGS. 64(a) and (b) are used to explain a principle in which a detection error is reduced by a multi-slit;
FIGS. 72(a) and (b) are diagrams used to explain the manner in which a height is detected by the selection of the slit under the condition that a detection position is not displaced by a height of a sample surface in the height detection apparatus according to the present invention;
FIGS. 82(a) and (b) are diagrams showing a detection position displacement and the manner in which a detection position displacement is decreased in a height detection optical apparatus according to the present invention;
FIGS. 86(a) and (b) show an embodiment in which a position of a multi-slit image is measured by a vibrating mask in a height detection apparatus according to the present invention;
FIGS. 89(a)-(c) are graphs used to explain a correction curve obtained by a standard pattern for correction in an electron beam apparatus according to the present invention;
FIGS. 90(a) and (b) show in perspective view standard patterns for correction according to other embodiments of the present invention;
FIGS. 94(a) and (b) show multi-slit patterns in which the center spacing between the multi-slit patterns is increased and in which the center slit is made wider, respectively.
More particularly, a description will be made of a charged-particle beam apparatus, an automatic astigmatism correction method and a sample used in adjustment of astigmatism of a charged-particle beam according to preferred embodiments of the present invention with reference to the drawings. Mathematical formula within the disclosure gleaned from the first application will be referenced as “equations” (Eq.).
As shown in
The charged-particle optical system 10 comprises a charged-particle beam source 14, an astigmatism corrector 60, a beam deflector 15, an objective lens 18, a sample base 21, an XY stage 46, a grid electrode 19, a retarding electrode (not shown in the figure), an optical-height detection sensor 13 and the particle detector 16. The charged-particle beam source 14 emits a charged-particle beam, such as an electron beam or an ion beam. By application of an electric field, the astigmatism corrector 60 corrects astigmatism of the charged-particle beam emitted by the charged-particle beam source 14. The beam deflector 15 carries out a scanning operation by deflecting the charged-particle beam emitted by the charged-particle beam source 14. By using a magnetic field, the objective lens 18 converges the charged-particle beam deflected by the beam deflector 15. On the sample base 21, a sample 20 is mounted. A target 62 for calibration use is fixed at a location on the sample base 21 beside the sample 20. The n stage 46 moves the sample base 21. The grid electrode 19 has an electric potential close to ground potential. Provided on the sample base 21, the retarding electrode has a negative electric potential if the charged-particle beam radiated to the sample 20 and the calibration target 62, which are provided on the sample base 21, is an electron beam, but has a positive electric potential if the charged-particle beam is an ion beam. The optical height detection sensor 13 measures the height of the sample 20 or the like by adopting a typical optical technique. The particle detector 16 detects secondary particles emitted from the surface of the sample 20 as a result of radiation of the charged-particle beam to the sample 20. The particle detector 16 may also detect particles reflected by a typical reflecting plate. It should be noted that the astigmatism corrector 60 can be an astigmatism correction coil based on use of a magnetic field or an astigmatism correction electrode based on use of an electric field. In addition, the objective lens 18 can be an objective coil based on use of a magnetic field or an electrostatic objective lens based on use of an electric field. Furthermore, the objective lens 18 may be provided with a coil 18a for focus correction. In this way, the astigmatism corrector 60, an astigmatism correction circuit 61 and other components constitute an astigmatism adjustment means.
A stage control unit 50 controllably drives the movement (the travel) of the XY stage 46 while detecting the position (or the displacement) of the XY stage 46 in accordance with a control command issued by an overall control unit 26. It should be noted that the XY stage 46 has a position-monitoring meter for monitoring the position (or the displacement) of the XY stage 46. The monitored position (or the displacement) of the XY stage 46 can be supplied to the overall control unit 26 by way of the stage control unit 50.
A focal-position control unit 22 controllably drives the objective lens 18 in accordance with a command issued by the overall control unit 26 and on the basis of the sample surface's height measured by the optical height detection sensor 13, so as to adjust the focus of the charged-particle beam to a position on the sample 20. It should be noted that by adding a Z-axis component to the XY stage 46, the focus can be adjusted by controllably driving the Z-axis component instead of the objective lens 18. In this way, a focus control means can be configured to include the objective lens 18 or the Z-axis component and the focal-position control unit 22.
A deflection control unit 47 supplies a deflection signal to the beam deflector 15 in accordance with a control command issued by the overall control unit 26. In this case, the deflection signal may be properly corrected so as to compensate for variations in magnification, which accompany variations in surface height of the sample 20, and a picture rotation accompanying control of the objective lens 18.
In accordance with an electric-potential adjustment command issued by the overall control unit 26, a grid-electric-potential adjustment unit 48 adjusts an electric potential given to the grid electrode 19 provided at a position above and close to the sample 20. On the other hand, in accordance with an electric-potential adjustment command issued by the overall control unit 26, a sample-base-electric-potential adjustment unit 49 adjusts an electric potential given to the retarding electrode provided at a position above the sample base 21. In this way, the grid electrode 19 and the retarding electrode can be used for giving a negative or positive electric potential to the sample 20 in order to reduce the velocity of an electron beam or an ion beam traveling between the objective lens 18 and the sample 20. Thus, the resolution in a low-acceleration-voltage area can be improved.
In accordance with a command issued by the overall control unit 26, a beam-source-electric-potential adjustment unit 51 adjusts the electric potential applied to the charged-particle beam source 14 in order to adjust the acceleration voltage of the charged-particle beam emitted by the charged-particle beam source 14 and/or adjust the beam current.
The beam-source-electric-potential adjustment unit 51, the grid-electric-potential adjustment unit 48 and the sample-base-electric-potential adjustment unit 49 are controlled by the overall control unit 26 so that a particle image with a desired quality can be detected by the particle detector 16.
In the correction of astigmatism and focus, an astigmatism adjustment unit 64 provided in accordance with the present invention issues a control command for changing the focal position (a focus f) to the focal-position control unit 22 so that the focal-position control unit 22 controllably drives the objective lens 18. As a result, while the charged-particle beam is being radiated to an area on the sample 20 or the calibration target 62, the focus is changed. In the area, a pattern including edge elements of the same degree in all directions, like one shown in
The overall control unit 26 computes astigmatism correction quantities (Δstx, Δsty) for the astigmatic differences found as described above and stored in the storage unit 57 from a relation between the astigmatic difference and the astigmatism correction quantity. The relation between the astigmatic difference and the astigmatism correction quantity is found in advance as a characteristic of the astigmatism corrector 60. The overall control unit 26 also computes a focus correction quantity for the focal offset z found as described above and stored in the storage unit 57 from a relation between the focal offset z and the focus correction quantity. The relation between the focal offset z and the focus correction quantity is found in advance as a characteristic of the objective lens 18. The astigmatism correction quantities (Δstx, Δsty) and the focus correction quantity, which are found by the overall control unit 26, are supplied to the astigmatism adjustment unit 64.
The astigmatism adjustment unit 64 provides the astigmatism correction quantities (Δstx, Δsty) received from the overall control unit 26 to an astigmatism correction circuit 61 so that the astigmatism corrector 60 is capable of correcting the astigmatism of the charged-particle beam. The astigmatism corrector 60 comprises an astigmatism correction coil based on a magnetic field or an astigmatism correction electrode based on an electric field. The astigmatism adjustment unit 64 supplies the focus correction quantity to the focal-position control unit 22 so as to control a coil current flowing to the objective lens 18 or a coil current flowing to a focus correction coil 18a (not shown in the figure). As a result, the focus is corrected.
As another method, a Z-axis component is provided as a portion of the XY stage 46. In this case, the astigmatism adjustment unit 64 issues a control command for moving the focus back and forth or changing the height of the sample 20 to a stage control unit 50 by way of the overall control unit 26 or directly. In accordance with this control command, the stage control unit 50 drives the Z-axis component in the direction of the Z axis in order to move the focus back and forth, so that a particle picture with a varying focus is obtained from the particle detector 16. Then, the astigmatism & focus-correction-quantity-computation image-processing unit 53 determines the astigmatism correction quantities and a focus correction quantity. The focus correction quantity is fed back to the Z-axis component of the XY stage 46, while the astigmatism correction quantities are fed back to the astigmatism corrector 60. The fed-back quantities are used for correction. Of course, the component for acquiring an image by moving the focus back and forth is different from the component for carrying out final focus correction. That is to say, one of the components may be the focal-position control unit 22, while the other component may be the Z-axis component of the XY stage 46. As an alternative, it is nice to control both components at the same time as a combination so as to adjust the position of the sample 20 or the calibration target 62 relative to the focal position to a desired distance. It should be noted that, by controlling the objective lens 18 rather than the Z-axis component, excellent responsiveness can be obtained.
As described above, the correction of the astigmatism and the focus is based on control executed by the astigmatism adjustment unit 64 in accordance with a command issued by the overall control unit 26. The overall control unit 26 receives a particle image with corrected astigmatism and a corrected focus, which are values stored in the image memory 52, directly or by way of the astigmatism & focus-correction-quantity-computation image-processing unit 53, and displays the image on a display means 58. As a result, the overall control unit 26 is capable of allowing the operator to visually examine corrected data, such as the astigmatism, and indicate acceptance or denial of the corrected data.
In addition, during an inspection and/or a measurement, for example, the XY stage 46 is controlled to bring a predetermined position on the sample 20 to the visual field of the charged-particle optical system. Then, the particle detector 16 acquires a particle-image signal, which is converted by the A/D converter 24 into a particle digital image signal to be stored in an image memory 55.
Subsequently, on the basis of the detection particle digital image signal stored in the image memory 55, an inspection & measurement image-processing circuit 56 measures the dimensions of a fine pattern created on the sample 20 and/or inspects a fine pattern generated on the sample 20 for a defect inherent in the pattern and/or for a defect caused by a foreign material. Results of the measurement and the inspection are supplied to the overall control unit 26. By correcting the astigmatism and the focus in accordance with the present invention at least periodically in this way, it is possible to implement inspection or measurement based on a particle image in which the aberration thereof is always corrected.
It should be noted that, in the case of particle-image-based inspection of a defect or the like, the inspection & measurement image-processing unit 56 repeatedly delays a detected detection particle digital image signal by a period of time corresponding to a pattern in order to create a reference particle digital image signal. The inspection & measurement image-processing unit 56 then compares the detection particle digital image signal with the reference particle digital image signal by making the position of the former coincide with the position of the latter in order to detect a discrepancy or a difference image as a defect candidate. Then, the inspection & measurement image-processing unit 56 carries out processing wherein a characteristic quantity of the defect candidate is extracted and false information to be eliminated from the characteristic quantity is identified. As a result, the sample 20 can be inspected for a true defect.
Since the effects of charge-up, dirt, damage and the like on the sample 20 are small, the optical height detection sensor 13 is capable of detecting variations in surface height of the sample 20 at the time of inspection or measurement of positions. The detected variations are fed back to the focal-position control unit 22 so that an in-focus state can always be maintained. If the optical height detection sensor 13 is used in this way, by carrying out automatic adjustment of astigmatism and focus at another position on the sample 20, or at the calibration target 62 placed on the sample base 21, either in advance or periodically during an inspection or a measurement, the radiation of a converged charged-particle beam used for the automatic adjustment of astigmatism and focus can be removed from the actual sample 20, or reduced substantially. As a result, the effects of charge-up, dirt, damage and the like on the sample 20 can be eliminated.
The following description is directed to the automatic adjustment of astigmatism and focus in the converged charged-particle optical system provided by the present invention. In accordance with the present invention, astigmatism values and focal offsets are collected from a small number of 2-dimensional particle images, and are converted into astigmatism and focus correction quantities, which are used in one correction.
Next, the state of astigmatism will be explained with reference to
In the middle of
On the right side of
Thus, by causing currents to flow to both of the sets stx and sty, astigmatism of any arbitrary orientation can be deliberately generated in any arbitrary direction. As a result, pre-adjustment astigmatism of the charged-particle optical system can be canceled by the deliberately generated astigmatism to result in a corrected astigmatism.
That is to say, in a state in which an astigmatism is being generated, the charged-particle beam blurs into an elliptical shape for a shift from an in-focus condition, as shown in
Next, correction of the astigmatism and the focus will be explained with reference to FIGS. 4(a) to 7(b). FIGS. 4(a) and 4(b) are diagrams each showing an example of a pattern created on the sample 20 or the calibration target 62 to be used for correction of focus and astigmatism. As a pattern for correcting astigmatism and focus, it is nice to use a pattern including edge elements generated by the astigmatism in three or more directions to the same degree.
For the reasons described above, first of all, the XY stage 46 is controllably driven on the basis of positional information of a pattern for correction of astigmatism and focus to position the pattern at a location in close proximity to the optical axis of the charged-particle optical system. The positional information is supplied by the overall control unit 26 to the stage control unit 50. Then, while the charged-particle beam is being radiated to the pattern for correction of astigmatism and focus in a scanning operation in response to a command issued by the overall control unit 26 to the deflection control unit 47, the astigmatism adjustment unit 64 issues commands to the focal-position control unit 22 to have the following operations take place:
(1) At a step S51 in the flowchart shown in
(2) Subsequently, at the next step S52, the astigmatism & focus-correction-quantity-computation image-processing unit 53 is driven to find center positions p0, p45. p90 and p135 of curves representing the degrees of directional sharpness at the angles of 0, 45, 90 and 135 degrees, namely, d0(f), d45(f), d90(f) and d135(f), respectively, each as a function of the focus f as shown in the upper part of
(3) Then, at the following step S53, the astigmatism & focus-correction-quantity image-processing unit 53 is driven to find a focal-position shift (astigmatic difference) direction α and magnitude δ, as well as a focal offset z, in a direction caused by the astigmatic difference from a sinusoidal relation shown in the lower part of
It should be noted that a storage unit 54 is used for storing, among others, a program for finding the degrees of directional sharpness d0(f), d45(f), d90(f) and d135(f), a program for finding the center positions p0, p45, p90 and p135 from the degrees of directional sharpness d0(f), d45(f), d90(f) and d135(f) and a program for finding the astigmatic difference and the offset value. The astigmatism & focus-correction-quantity-computation image-processing unit 53 is capable of executing these programs. The storage unit 54 can be a ROM or the like.
(4) There has been found in advance a relation between variations in astigmatism control values (stx, sty), which are characteristics of the astigmatism corrector 60, and variations in astigmatic difference direction α and magnitude δ or variations in the astigmatic-difference vector (dx, dy). The variations in the astigmatic difference direction α and magnitude δ or variations in astigmatic-difference vector (dx, dy) are known as sensitivity. Thus, at the next step S54, the overall control unit 26 is capable of converting and splitting the astigmatic difference direction α and magnitude δ or the vector (dx, dy) into required astigmatism correction quantities (1, 2) (Δstx, Δsty) on the basis of this relation. Then, at the next step S55, the overall control unit 26 is capable of setting the astigmatism correction quantities (1, 2) (Δstir, Δsty) as well as a focal offset z and supplying them to the astigmatism adjustment unit 64. It should be noted that the astigmatism correction quantities (1, 2) (Δstx, Δsty) and the focal offset z can also be computed by the astigmatism & focus-correction-quantity-computation image-processing unit 53, instead of the overall control unit 26. In this case, the astigmatism & focus-correction-quantity-computation image-processing unit 53 receives characteristics of the astigmatism corrector 60 and the objective lens 18 from the overall control unit 26.
(5) The astigmatism adjustment unit 64 transmits the focal offset z received from the overall control unit 26 to the focal-position control unit 22, which uses the focal offset z to correct an objective-coil current flowing through the objective lens 18, or a focus correction coil current flowing through the focus correction coil 18a. The astigmatism adjustment unit 64 transmits the astigmatism correction quantities (Δstx, Δsty) received from the overall control unit 26 to an astigmatism correction circuit 61, which uses the astigmatism correction quantities (Δstx, Δsty) to correct an astigmatism correction coil current or an astigmatism correction static voltage. In this way, the correction and the adjustment of the astigmatism can be carried out at the same time.
(6) For a small astigmatism, an auto-stigma operation is completed in one processing as described above. For a large astigmatism, however the correction cannot be completed in one processing due to causes of the aberration other than astigmatism. Examples of such causes are high-order astigmatism and picture distortion. In this case, the processing goes back to step (1) to apply an auto stigma and repeat the loop until the astigmatism correction quantities (Δstx, Δsty) and the focal offset z are reduced to small values.
In accordance with the method described above, it is possible to implement simultaneous adjustment of astigmatism and focus in a short period of time with little damage inflicted upon the sample 20 and the calibration target 62. In addition, by comparing the directional sharpness of images of the same sample 20 or the same calibration target 62, while varying the focal distance, an astigmatic difference can be found. Thus, the simultaneous adjustment of astigmatism and focus can be implemented independently of a pattern on the sample 20 or the calibration target 62, that is, a pattern for astigmatism and focus correction. The only condition imposed on the pattern on the sample 20 or the calibration target 62 is that the pattern shall include edge elements to the same degree in all directions.
In the embodiment described above, four types of directional sharpness at θ=0, 45, 90 and 135 degrees are used. It should be noted, however, that if the astigmatic difference direction α and magnitude δ are known, not all the four directions at θ=0, 45, 90 and 135 degrees need be used. That is to say, only degrees of directional sharpness dθ(f) for at least 3 angles θ corresponding to three directions are required. In this case, for each value of θ, a center position pθ of the curve dθ(f) is found. Then, a sinusoidal waveform or a waveform close to the sinusoidal waveform is applied to pθ. The astigmatic difference direction α and magnitude δ can be found as the phase and the amplitude of the sinusoidal waveform, respectively.
The following description is directed to a specific embodiment implementing processing carried out by the astigmatism & focus-correction-quantity-computation image-processing unit 53 to find the directional sharpness of a particle image.
As a first embodiment, a particle image is detected and observed by the particle detector 16. The particle image is detected by radiating a charged-particle beam to a sample (target) 62 in a scanning operation. The target 62 is used specially for automatic correction of astigmatism. The sample 62 has a striped pattern with a stripe direction varying from area, to area as shown in
V=Σxy(s(x,y)−s mean)2/N.
As an alternative, the amplitude can also be found by computing a sum of absolute values Σxy|t(x, y)| or a sum of squares Σxy(t(x, y))2, where notation t (x, y) denotes a differential obtained as a result of 2-dimensional differentiation, such as Laplacian differentiation, of s(x,y), notation |t(x,y)| denotes the absolute value of the differential t(xy) and notation (t(x,y))2 denotes the square of the differential t(x,y). In this case, the result defines the directional sharpness dθ. The angular direction θ can be defined in any way. In the figure, an angular direction of 0 degrees is defined for a normal direction of the pattern coinciding with the horizontal direction. The angular direction θ is then defined in a clockwise manner with the angular direction of 0 degrees taken as a reference. Directions of the pattern are not limited to the four directions shown in the figure. That is to say, the directions of the pattern may be a combination of arbitrary angles that divide a 180-degree-area into about n equal parts, where n is any arbitrary integer equal to or greater than 3.
A second embodiment is provided for a pattern created on the sample 20 or the target 62, as shown in
Moreover, in order to find the directional sharpness with a high degree of accuracy, the following technique can be adopted. As shown in
For the reasons described above, the scanning direction is changed from the first focus sweep to the second focus sweep by about −45 degrees, as shown in
Thus, astigmatism can be measured with a high degree of accuracy and without being affected by noise even for a dim pattern. In addition, astigmatism can be measured and corrected even for a pattern that is darkened due to contamination of the sample or the like.
(1) In a loop L51, while a charged-particle beam is being radiated to a pattern for correction of astigmatism and focus in a scanning operation according to a command issued by the overall control unit 26 to the deflection control unit 47, the astigmatism adjustment unit 64 issues a command to the focal-position control unit 22 to make the following happen. While the focus f is being changed, the particle detector 16 acquires a plurality of images and stores them in the image memory 52. The astigmatism & focus-correction-quantity-computation image-processing unit 53 computes degrees of directional sharpness at angles of 45 and 135 degrees for the images, that is, the degrees of directional sharpness d45(f) and d135(f), which are shown in
(2) Then, in the next loop L51′, while the charged-particle beam is being radiated to the pattern for correction of astigmatism and focus in a scanning operation, with the angle rotated from that of the loop 51 by −45 degrees in accordance with a command issued by the overall control unit 26 to the deflection control unit 47, the astigmatism adjustment unit 64 issues a command to the focal-position control unit 22 to make the following happen. While the focus f is being changed, the particle detector 16 acquires a plurality of images and stores them in the image memory 52. The astigmatism & focus-correction-quantity-computation image-processing unit 53 computes degrees of directional sharpness at angles of 45 and 135 degrees for the images, that is, the degrees of directional sharpness d0(f) and d90(f), which are shown in
(3) Subsequently, at the next step S52, the astigmatism & focus-correction-quantity-computation image-processing unit 53 is driven to find center positions p0, p45, p90 and p135 of curves representing the degrees of directional sharpness at the angles of 0, 45, 90 and 135 degrees, namely, d0(f), d45(f), d90(f) and d135(f) respectively, each as a function of focus f, as shown in the upper portion of
(4) Then, at the following step S53, the astigmatism & focus-correction-quantity-computation image-processing unit 53 is driven to find a focal-position shift (astigmatic difference) direction α and magnitude δ, as well as an focal offset z, in a direction caused by the astigmatic difference from a sinusoidal relation, as shown in the lower portion of
(5) There has been found in advance a relation between variations in astigmatism control values (stx, sty), which are characteristics of the astigmatism corrector 60, and variations in astigmatic difference direction α and magnitude δ, or variations in astigmatic-difference vector (dx, dy). The variations in astigmatic difference direction α and magnitude δ, or variations in the astigmatic-difference vector (dx, dy), are known as sensitivity. Thus, at step S54, the overall control unit 26 is capable of converting and splitting the astigmatic difference direction α and magnitude δ or vector (dx, dy), into required astigmatism correction quantities (1, 2) (Δstx, Δdty) on the basis of this relation. At step S55, the overall control unit 26 is capable of setting the astigmatism correction quantities (1, 2) (Δstx, Δsty) and a focal offset z and supplying them to the astigmatism adjustment unit 64.
(6) The astigmatism adjustment unit 64 transmits the focal offset z received from the overall control unit 26 to the focal-position control unit 22, which uses the focal offset z to correct an objective coil current flowing through the objective lens 18, or a focus correction coil current flowing through the focus correction coil 18a. The astigmatism adjustment unit 64 transmits the astigmatism correction quantities (Δstx, Δsty) received from the overall control unit 26 to the astigmatism correction circuit 61, which uses the astigmatism correction quantities (Δstx, Δsty) to correct an astigmatism correction coil current or an astigmatism correction static voltage. In this way, the correction and the adjustment of the astigmatism can be carried out at the same time.
(7) For a small astigmatism, an auto-stigma operation is completed in one processing, as described above. For a large astigmatism, however, the correction cannot be completed in one processing due to causes of aberration other than astigmatism. Examples of such causes are high-order astigmatism and picture distortion. In this case, the processing goes back to step (1) to apply an auto stigma and repeat the loop until the astigmatism correction quantities (Δstx, Δsty) and the focal offset z are reduced to small values.
The following description is directed to a method based on another principle. The method is adopted to solve a phenomenon of differences in property among sharpness curves at 0, 90, 45 and 135 degrees, as shown in
The following description is directed to an embodiment of a method adopted by the astigmatism & focus-correction-quantity-computation image-processing unit 53 to find the center position pθ of a directional-sharpness curve dθ(f), which is a function of focal position f. In accordance with a method to find the center position pθ of a directional-sharpness curve dθ(f), a quadratic function, a Gaussian function or the like is applied to values in close proximity to a focal position f corresponding to the peak of the directional-sharpness curve dθ(f). Thus, the center position pθ is found as the center position of the function. In accordance with a method used to find the center position pθ of a directional-sharpness curve dθ(f), the center position pθ is found as the center of gravity of points representing values greater than a predetermined threshold. A proper method can be selected.
With this simple technique to find a position corresponding to a peak or the interpolation technique to find such a position, however, an error is generated, particularly in the case of a large astigmatism. This problem will be explained with reference to FIGS. 12(a) to 12(c). Consider sharpness in the 0-degree direction for a case in which an astigmatism is generated in about ±45 degree directions, as shown in
In the example shown in FIGS. 12(a) to 12(c), if the simple maximum-value method is adopted, point p0 corresponding to the peak of the d0(f) curve will be close to point p45 corresponding to the peak of the d45(f) curve, while point p90 corresponding to the peak of the d90(f) curve will be close to point p135 corresponding to the peak of the d135(f) curve. In this case, components p45-p135 of the astigmatic difference in the ±45 degree directions have magnitudes at least twice the magnitudes which are supposed to occur. Thus, if those components are used for correction, the astigmatism in these directions will be inevitably over corrected, causing an instability.
On the other hand, the method used to search for a peak may determine a point C, as shown in
In order to find such a middle point, in accordance with the present invention, the sizes of peaks B and C are taken into consideration, so that the middle point between points B and C truly represents the center of the directional sharpness. There are a variety of conceivable methods implemented by embodiments described below to find such a middle point. However, the methods to find such a middle point are not limited to the embodiments described below. In the case of a double-peak sharpness curve, any method provided by the present invention can be adopted to find such a middle point by taking the sizes of the peaks into consideration.
pθ=Σf*(dθ(f)−αMax Value)/pθ=Σd(dθ(f)−αMax Value)
pθ=(dθ(C)*B+dθ(B)*C)/(dθ(C)dθ(B))
FIGS. 15(a) and 15(b) are graphs representing a relation between the focus value and the sharpness and serving as a means for explaining a method of finding a central position of a directional-sharp curve by adopting a symmetry-matching technique. In the figures, a curve dθ(f) represents variations in directional sharpness with variations in focal position. Consider a vertical line f=a passing through a position a as a symmetrical axis. The position a is selected so that the portion of a curve dθ(a−f) on the left side of the symmetrical axis becomes the most matching image of the portion of the curve dθ(f) on the right side of the symmetrical axis serving as an error. On the other hand, the portion of the curve dθ(a−f) on the right side of the symmetrical axis becomes the most matching image of the portion of the curve dθ(f) on the left side of the symmetrical axis. The curves on the lower side each represent variations in degree of matching with variations in position a. The position a at which the degree of matching reaches a maximum is taken as the in-focus position pθ. The degree of matching can be computed as a correlation quantity between the curves. In this case, at the in-focus position pθ, the correlation quantity reaches a maximum. The degree of matching can also be computed as a sum of squared differences between the curves. In this case, at the in-focus position pθ, the correlation quantity reaches a minimum. It is needless to say that the degree of matching can also be computed as any quantity that is generally used as an indicator of matching.
The following description is directed to an embodiment implementing a technique adopted by the overall control unit 26 to compute an astigmatism correction quantity from an astigmatic difference received from the astigmatism & focus-correction-quantity-computation image-processing unit 53. When the four directions of the in-focus positions p0, p45, p90 and p135 at 0, 45, 90 and 135 degrees are used, first of all, the astigmatism & focus-correction-quantity-computation image-processing unit 53 computes an astigmatic-difference vector (dx, dy)=(p0−p90, p45−p135) and supplies the vector to the overall control unit 26. Then, the overall control unit 26 splits astigmatism correction quantities (Δstx, Δsty) on the basis of Eq. (4) given as follows:
Δstx=mxx*dx+mxy*dy
Δsty=myx*dx+myy*dy (4)
where notations mxx, mxy, myx and myy each denote a parameter of astigmatism correction quantity splitting, which are computed on the basis of characteristics of the astigmatism corrector 60. Typically, the parameters are stored in the storage unit 57. Thus, the astigmatism adjustment unit 64 supplies the astigmatism correction quantities obtained from the overall control unit 26 to the astigmatism correction circuit 61 so that the astigmatism correction circuit 61 changes the quantities by (βΔstx, βΔsty) where notation β denotes a correction quantity reduction coefficient. In turn, the astigmatism correction circuit 61 drives the astigmatism corrector 60 to change the astigmatism correction quantities by (βΔstx, βΔsty).
In addition, since the focal offset z obtained from the image-processing circuit 53 is an average value of focal positions in different directions, the overall control unit 26 sets the focus correction quantity at (p0+p45+p90+p135)/4. Thus, the astigmatism adjustment unit 64 supplies the focus correction quantity obtained from the overall control unit 26 typically to the focal-position control unit 22, which then corrects the objective lens 18 by the focus correction quantity.
It should be noted that, as another embodiment, the astigmatism & focus-correction-quantity-computation image-processing unit 53 first computes the astigmatic difference magnitude δ−|dx,dy)| and direction α=½ arctan (dy/dx), supplying the magnitude and the direction to the overall control unit 26. The overall control unit 26 may then convert the astigmatic difference magnitude δ and direction α into the astigmatism correction quantities (Δstx, Δsty).
In addition, when directional sharpness pθ in n directions is used, where n is an integer of at least 3, the astigmatism & focus-correction-quantity-computation image-processing unit 53 needs to apply a sinusoidal waveform to these pieces of data and then find the astigmatic difference magnitude δ and direction α, as well as the focal offset z, from the phase, the amplitude and the offset of the waveform.
Furthermore, if the astigmatism correction quantity is changed, the focal position may be affected by the change, being slightly shifted in some cases. Thus, in this case, the overall control unit 26 typically multiplies each of the astigmatism correction quantities (Δstx, Δsty) by a proper coefficient and adds the products to variations of the astigmatism correction quantities (Δstx, Δsty) to produce new astigmatism correction quantities.
The following description is directed to a method to compute the astigmatism correction quantities more accurately, in a shorter period of time and with a higher degree of precision, in comparison with the embodiment described above. With the method described above, there occurs a phenomenon wherein the position of the gravitational center of the sharpness is dragged by the sharpness in the adjacent direction. Consider a sharpness curve d45 in a 45-degree direction relative to, for example, a pattern like the one shown in
Thus, a corrected astigmatic-difference vector is used to find the astigmatism correction quantities (Δstx, Δsty). As shown in
Eqs. (5) and (6) are used for splitting the astigmatism correction quantities. Notations mxx, mxy, myx and myy each denote a parameter for splitting the astigmatism correction quantities. In the above equations, notation p denotes a parameter for correcting a phenomenon in which the position of the sharpness center of gravity is dragged by sharpness in the adjacent direction. The parameter p has a value in the range 0<p<1.
With the second correction equation, on the other hand, the astigmatic-difference vector (dx, dy) is corrected in accordance with the heights of the peaks of the directional-sharpness curves in addition to the relation between the magnitudes of the components dx and dy of the astigmatic-difference vector. Assume that the values pd0, pd45, pd90 and pd135 are used as the heights of the peaks of the sharpness curves d0, d45, d90 and d135, respectively, and assume that px=pd0+pd90, whereas py=pd45+pd135. In this case, the following equations hold true:
Eqs. (7) and (8) are used for splitting the astigmatism correction quantities. Notations a, bp, bd, cp and cd each denote a correction parameter. The a parameter has a value in the range of 1 to 2. A typical value of the parameter a is 1.8. The parameters bp and bd each have a value of 5, whereas the parameters cp and cd each have a value of about 0.5. That is to say, for px<py and dx>dy, the component dx is corrected by a factor not exceeding a times. For px>py and dx<dy, on the other hand, the component dy is corrected by a magnification factor not exceeding a times.
Eqs. (9) and (10) are used for splitting the astigmatism correction quantities. Notations a, bp, bd, cp and cd each denote a correction parameter. The a parameter has a value in the range of 1 to 2. A typical value of the parameter a is 1.8. The parameters bp and bd each have a value of about 2, whereas the parameters cp and cd each have a value of about 4. That is to say, for px<py and dx>dy, the component dx is corrected by a factor not exceeding a times. For px>py and dx<dy, on the other hand, the component dy is corrected by a magnification factor not exceeding a times.
By using these equations, even if a sample pattern exhibits a one-sided property in the direction thereof, the one-sided property can be corrected so that the astigmatism correction quantities can be computed with a high degree of precision. As a result, the astigmatism can be corrected in a short period of time and with a high degree of precision.
Referring to
That is to say, since the calibration target 62a shown in
In addition, even if a calibration target 62 with a horizontal planar shape, or the actual sample 20, is used, the same effects as the embodiment described above can be obtained. That is to say, if a particle image is taken by varying the focal position at a high speed, an image with a focus varying from area to area can be obtained in the same way as in the embodiment described above. As a result, by using only an image, the astigmatism and the focus can be adjusted in a short period of time.
The following description is directed to a relation between inspection or measurement of an object substrate and correction of astigmatism, as well as correction of focus. First of all, the object substrate (or the actual sample) 20 is mounted on the sample base 21. Then, the overall control unit 26 inputs and stores information concerning positions on the object substrate 20 to be scanned or measured. The information is acquired from an input means 59, which typically comprises a recording medium or a network. Thus, in an operation to scan or measure the object substrate 20, the overall control unit 26 issues a command to the XY stage 46 to control the XY stage 46 in order to take a predetermined position on the sample 20 to the visual field of the charged-particle optical system. Subsequently, a charged-particle beam is radiated to the predetermined position in a scanning operation, and a particle image generated as a result of the scanning operation is detected by the particle detector 16. A signal representing the particle image is then subjected to an A/D conversion to generate digital data to be stored in the image memory 55. Then, the inspection & measurement image-processing unit 56 carries out image processing on the digital data stored in the image memory 55 in an inspection or measurement operation. In the inspection or measurement operation, the astigmatism and the focus are corrected at each inspection or measurement position in accordance with the present invention so as to allow implementation of the inspection or the measurement based on a particle image with the aberration always being corrected.
Assume that the height detection sensor 13 employed in the inspection & measurement apparatus is an optical height detection sensor, which has small bad effects, such as charge-up, dirt and damage on the object substrate 20. With such, sensor characteristics, a sample height detected by the optical height detection sensor 13 at each inspection or measurement position is fed back to the focal-position control unit 22 so that only a converged charged-particle beam for inspection or measurement is radiated to the object substrate (sample) 20 in a scanning operation without radiating a converged charged-particle beam for correcting astigmatism and focus to the object substrate (sample) 20 in a scanning operation. As a result, bad effects such as charge-up, dirt and damage on the object substrate can be reduced to a minimum. In this case, automatic adjustment of astigmatism and focus is carried out at another position on the sample 20, or at the calibration target 62 placed on the sample base 21, either in advance or periodically during an inspection or a measurement.
By the way, it is possible to use a sample having an inclined or staircase-shaped surface as shown in FIGS. 8(a) and 8(b), or a sample having a planar top surface as shown n
By carrying out automatic adjustment of astigmatism and focus in accordance with the present invention, as described above, it is possible to correct shifts in focal position and astigmatism, which normally occur with the lapse of time. In order to carry out the automatic adjustment of astigmatism and focus in accordance with the present invention, however, it is necessary to adjust the detection offset of the optical height detection sensor 13 in advance. Differences (or variations) in height between inspection or measurement positions on the actual sample (object substrate) 20 are detected for use in correction of an in-focus state. Thus, a converged charged-particle beam with no astigmatism is radiated to the actual sample 20 in a scanning operation in an in-focus state only during an inspection or a measurement. Therefore, a particle image can be detected with the effects, such as charge-up, dirt and damage, on the object substrate reduced to a minimum. As a result, the object substrate 20 can be inspected or measured with a high degree of precision.
In addition, when it is desired to calibrate not only an offset between the optical height detection sensor 13 and the focal-position control unit 22, but also the gain, a plurality of calibration targets 62, each having a known height, are provided in advance. Such calibration targets 62 are used for carrying out both automatic correction of focus and detection using the optical height detection sensor 13, so that the gain and, furthermore, the linearity can also be calibrated as well. In addition, by carrying out both automatic correction of focus and detection using the optical height detection sensor 13, while changing the height of the calibration target 62 or the sample 20 by using the Z-axis component of the XY stage 46, the gain and, furthermore, the linearity can also be calibrated.
In addition, an inspection or a measurement can be carried out at a high speed by driving the beam deflector 15 to move a converged charged-particle beam in a scanning operation in a direction crossing (or, particularly, perpendicular to) the movement of the XY stage 46, while continuously moving the XY stage 46 in the horizontal direction, as shown in
The height detected by the optical height detection sensor 13 is always fed back to the focal-position control unit 22 and the deflection control unit 47. In addition, while the focal shift and deflection rotation are being corrected, a particle image is being detected continuously. As a result, the entire surface of the actual sample 20 can be inspected or measured with a high degree of precision and a high degree of sensitivity. It should be noted that, in order to correct the focus, it is of course also possible to drive the Z-axis component of the XY stage 46 instead of driving the focal-position control unit 22 to provide the same effects as well. In the mean time, the radiation of the charged-particle beam is moved to the calibration target 62 periodically, as shown in
The embodiments described above are applied to cases in which the charged-particle beam apparatus is applied to an inspection & measurement apparatus. It should be noted, however, that the present invention can also be applied to fabrication equipment and the like.
The present invention exhibits an effect such that astigmatism and focus can be automatically adjusted at a high speed and with a high degree of precision without inflicting damage upon a sample by using only a small number of particle images obtained by detection of a converged charged-particle beam radiated to the sample in a scanning operation.
In addition, the present invention also exhibits another effect in that inspection or measurement can be carried out automatically with a high degree of stability and a high degree of precision, while the quality of a particle image detected over a long period of time is being maintained in operations to inspect defects, such as impurities in a pattern, or to measure the dimensions of the pattern on the basis of a particle image detected by radiation of a converged charged-particle beam to an object substrate, including the pattern in a scanning operation, wherein the converged charged-particle beam has been subjected to high-speed and high-precision automatic adjustment of astigmatism and focus without inflicting damage on the sample.
More particularly, shown in
The specimen wafer under inspection can be moved by an X-Y stage 7 and a Z stage 8. By moving each stage, an arbitrary point on the surface of the specimen wafer is observable through the electron optical system. Electron beam irradiation and image input can be performed in synchronization with stage movement, which is controlled under direction of a control computer 2010. A height detector 2011 is of an optical non-contact type which does not cause interference with the electron optical system, and it can speedily detect a height of the specimen surface at or around an observation position in the electron optical system by a height calculator 2011a. Resultant data of height detection is input to the control computer 2010.
According to the height of the specimen surface, the control computer 2010 adjusts a focal point of the electron optical system, i.e., a position of the Z stage, and it receives input of the image signal. Using the image signal input in a focused state and inspection position data detected by a position monitoring measurement device, defect judgment is carried out through comparison with a pattern pre-stored by an image processing circuit 9, a corresponding pattern at a location on the specimen wafer under inspection, or a corresponding pattern on a different wafer with a defect being detected by defect detector 100. While the automatic semiconductor device inspection system using secondary electron images is exemplified in
In the example shown in
The height detector described above may also be applicable to different types of microstructure observation/fabrication systems using other convergent charged particle beams as in the inspection system exemplified in
In the light-reflected position detecting method mentioned above, since a height detection optical element is not located directly above a detection position, a height in an observation region in a charged particle beam optical system can be detected simultaneously with observation by the charged particle beam optical system in a fashion that virtually no interference takes place. By making a height point detected by the height detector meet an observation region in the charged particle beam optical system, a surface height of an object item can be known at the time of observation. In this arrangement, through feedback of height data thus attained, observation can be conducted using a charged particle beam which is always in focus.
It is not necessarily required to provide such a condition that a desired observation region in the charged particle beam optical system meets a corresponding height point detected by the height detector, but rather it is just required that a surface height of the object is recognizable at the time of observation using vicinal height data attained successively. In use of the light-reflected position detecting method, optical parts may be arranged flexibly to some extent in optical system design, and it is therefore possible to dispose the optical parts to prevent interference with the charged particle beam optical system.
Disposition of the height detector in the light-reflected position detecting method is substantially limited by an angle of incidence on the object surface. In the light-reflected position detecting method, since a degree of incidence angle has an effect on height detection performance, an incidence angle cannot be determined only by part disposition in the system.
Referring to
Exemplary preferred embodiments of disposition of optical parts in a height detection optical system are described in the following description wherein in general, if an insulator is located in the vicinity of a charged particle beam optical system, a possible charge build-up in the insulator affects an electric field around it to cause an adverse effect on charged particle beam deflection, resulting in degradation in image quality. Since such a charging effect varies with time as a charged condition changes, compensation for it is difficult practically.
For attaining a stable charged particle beam image, disposition of an insulator such as a lens at a position encountered with the charged particle beam must be avoided. If the insulator is coated with a conductive film and disposed at a position sufficiently apart from the charged particle beam optical system, an adverse effect may be reduced. A degree of requirement for preventing an adverse effect of the insulator (lens) on the charged particle beam optical system depends on specifications of the charged particle beam optical system such as visual field condition, accuracy, resolution, etc. According to the specifications of the charged particle beam optical system, a range influential on the charged particle beam optical system may be determined, and an optical path may be designed so that the insulator is not disposed in the influential range, thus preventing an adverse effect on the charged particle beam optical system.
When a lens for the height detector is disposed in the periphery of the charged particle beam optical system, an effect on the charged particle beam can be presumed experimentally through computer simulation. The height detection optical system may be designed after determining a suitable mounting position of each lens as illustrated in
In the preferred embodiment mentioned above, each lens is disposed at a position which does not cause an adverse effect on the charged particle beam optical system. Further, as shown in
As in the preferred embodiment exemplified above, some or all of the optical parts of the height detection optical system may be arranged outside the vacuum specimen chamber. As illustrated in
Referring to
As shown in
Accordingly, as shown in
The following description describes exemplary preferred embodiments for reducing an effect of chromatic aberration due to variance in refractive index of glass material used for a window for light passage. When a light beam for height detection passes though the window made of glass, its optical path is made to shift. As shown in
Further, the degree of optical path shift is dependent on an angle of incidence and proportional to a thickness of glass plate. If the incidence angle to the glass plate of the window is decreased as in the foregoing preferred embodiment, the degree of optical path shift can be reduced. However, if the incidence angle is rather large, there arises a particular problem. (For example, in case that the incidence angle is 70 deg., glass BK7 is used and the thickness of glass plate is 2 mm, there occurs a difference of 9 μm in optical path shift between wavelengths of 656.28 nm and 404.66 nm.).
Where white light is used, an effect of chromatic aberration varies with color of an object under inspection and therefore its correction is rather difficult. For reduction in effect of chromatic aberration, there may be provided such arrangements that the window glass plate is made thinner and a glass plate for correcting chromatic aberration is inserted on the optical path. Since the degree of optical path shift is proportional to the thickness of window glass plate, it is preferred to use a glass plate having a thickness which will not cause significant chromatic aberration, in consideration of applicable wavelength coverage and desired accuracy of height detection.
It is not necessarily required to use glass material if a required strength can be satisfied, and therefore an optically transparent part made of pellicle material, for example, may be employed. However, in case of the window on the vacuum specimen chamber, considerable strength is required and it is not permitted to make the glass plate sufficiently thinner. Therefore, in such a case, the glass plate for correcting chromatic aberration may be inserted on the optical path.
Referring to
Further, in
For the purpose of decreasing an accelerating voltage for the charged particle beam to be applied onto a specimen, a flat-plate electrode may be arranged at a position over a surface of the specimen in parallel thereto. In this arrangement, it is required to provide an opening or window on the flat-plate electrode to allow passage of light on an optical path for the height detector. Since a shape of the flat-plate electrode has an effect on electric field distribution in the vicinity of the specimen, it may affect the quality of charged particle beam images adversely. Exemplary embodiments for reducing an adverse effect on the charged particle beam images are described in the following description. A degree of adverse effect on the charged particle beam optical system varies depending on the size or position of the opening to be provided on the flat-plate electrode. An permissible level of adverse effect by the opening depends on performance required for the charged particle beam optical system. When the size of the opening is considerably small, its adverse effect may be negligible. Therefore, a method for reducing the opening size is explained below.
As shown in FIGS. 34(a) and 34(b), when an incidence angle to a surface of an object with respect to the vertical is increased from the small incidence angle of
Reduction of the size of the opening without decreasing the entire quantity of light for detection is described below. Commonly, an optical lens aperture having a circular shape whose center coincides with the optical axis is employed. According to one aspect of the present invention, there is provided an elliptic or rectangular optical lens aperture having its major axis which is in the axial direction across the optical axis and parallel to the object surface and having its minor axis which is in the axial direction across the major axis and the optical axis. In this arrangement, the entire quantity of light necessary for height detection can be ensured by providing an elliptic or rectangular area which is equal to that of a circular lens aperture.
If the charged particle beam optical system is affected by the size of the opening so that performance required for it cannot be attained, it is necessary to provide a further measure. For example, instead of merely a hollow opening formed on the flat-plate electrode, there may be provided such an arrangement that a window made of glass coated with a conductive film or other material is formed on the flat-plate electrode to allow passage of light on an optical path. In this arrangement, an adverse effect due to electric field to be given to an object or its periphery can be reduced. As exemplified in
The opening or window formed on the flat-plate electrode in the foregoing examples has a considerable effect on electric potential distribution in the vicinity of the object. The following describes an opening/window disposition method for reducing this effect. Since the window and opening can be disposed in the same manner, the window is taken in the description given below.
In a microstructure observation/fabrication system to which the present invention is directed, two-dimensional observation or fabrication is mostly carried out through two-dimensional scanning by deflecting a convergent charged particle beam or through stage scanning by combination of one-dimensional scanning based on charged particle beam deflection and stage movement in the direction orthogonal to the one-dimensional scanning. According to the present invention, the window is disposed in consideration of charged particle beam deflection and stage movement direction in charged particle beam scanning. Thus, an effect of variation in electric field due to the window can be reduced as proposed below.
Referring to
In case of stage scanning, electric field distribution in a deflection range can be made uniform by disposing windows 2029 in parallel to the deflection direction as shown in
The following describes exemplary embodiments for charged particle beam focus adjustment using height detection result data attained by the height detector. A focal point of the charged particle beam is adjusted by an objective lens control current. Using input data of an object surface height detected by the height detector in an observation region of the charged particle beam optical system, the objective lens control current is regulated to enable observation of a charged particle beam image which is always in focus. For this purpose, in the charged particle beam optical system, a level of objective lens control current is to be calibrated beforehand with respect to variation in object surface height. Further, an offset and gain in relation between the height detector and the charged particle beam optical system are to be calibrated beforehand.
Calibration methods for offset and gain will be described in the following exemplary embodiments. When the charged particle beam optical system is not structured in a telecentric optical arrangement, variation in object surface height will cause a magnification error in addition to a defocused condition. As to the magnification error, correction can be made through feedback control of a deflection circuit using height variation data, thus making it possible to always attain a charged particle beam image at the same magnification. Further, if the microstructure observation/fabrication system using the convergent charged particle beam is provided with a mechanism capable of moving an object in the Z-axis direction with high accuracy and at response speed sufficient for focal point control, resultant data of height detection may be used for object stage height feedback control instead of feedback control of the charged particle beam optical system.
Where stage height feedback control is carried out, a surface of the object can always be maintained at a constant height with respect to the height detector and the charged particle beam optical system. Therefore, no problem will arise even if a guaranteed detection accuracy range of the height detector is narrow. As a drive mechanism for an object stage, there may be provided a piezoelectric mechanism enabling fine movement at high speed under vacuum, for example. When such a piezoelectric mechanism is used, a magnification error does not occur since a height of the object surface is always maintained at a constant level with respect to the charged particle beam optical system.
Calibration of objective lens control current and focal point in the charged particle beam optical system may be carried out in the following manner. In an instance where there is a nonlinear relationship between objective lens control current and focal point, it is required to make correction for nonlinearity. Linearity evaluation and correction value determination may be effected as described below.
Referring to
When the object holding stage is movable on a plane as in the inspection system shown in
The standard pattern 31a shown in
Where the object holding stage is not provided with a movement mechanism, the charged particle beam optical system can be calibrated by disposing the standard pattern so that it will always be located in a visual field of the charged particle beam optical system. Further, the standard pattern may be formed so that it can be attached to an object holding jig. Thus, even when the object holding stage is not provided with a movement mechanism, it is possible to perform calibration by setting the standard pattern on the stage and thereafter exchange the standard pattern with the object for observation.
In case that the charged particle beam system is provided with a mechanism for moving an object in the height direction as shown in
Calibration of the charged particle beam optical system using the standard pattern shown in
The following describes exemplary embodiments concerning focal point correction and relationship between height measurement position under inspection and observation position in the charged particle beam optical system. If the observation position of the charged particle beam optical system completely meets the height detection position of the height detector, focus adjustment may be made according to height data detected by the height detector. However, in the light-reflected position detecting method, a deviation of detection position occurs due to variation in object surface height as illustrated in
In the inspection system according to the present invention, since continuous inspection is performed by moving the stage, height data at each point can be attained continuously. Using resultant data of height detection, a height of object surface in an observation region in the charged particle beam optical system may be presumed or predicted to enable focus adjustment. Focus adjustment when there is a positional deviation between the height detection position and the observation region in the charged particle beam optical system may be effected in the following manner. In the following description, it is assumed that stage scanning is performed by deflecting the beam of the charged particle beam optical system in the Y-axis direction and moving the stage in the X-axis direction to produce a two-dimensional image.
Where each of X-axis and Y-axis stage scanning movements is always limited to one direction at the time of inspection, if each of the X-axis and Y-axis scanning movements is always made in one direction only as shown in
As shown in
Another exemplary embodiment is considered in that either one of X-axis and Y-axis stage scanning movements is always limited to one direction and also the axis movable only in one direction coincides with the projection direction of the height detection optical system. As shown in
Further, as shown in
In an arrangement that the optical axis of the charged particle beam optical system is made to coincide with a reference position of height detection, it is possible to presume a height of the inspection position using height detection data attained. However, since a height of the inspection position cannot always be determined through interpolation, its reliability is not ensured. For reliable height detection, there may be provided such an arrangement that the height detection optical system is equipped with a movable mechanism and the entire optical system is shifted in parallel as shown in
Exemplary embodiments for optical height detection in which a height of a specimen surface can be detected reliably without being affected by a state of the specimen surface are now considered. In case that a specimen surface height is detected by the light-reflected position detecting method as shown in
According to the present invention, it is possible to detect a height of an observation position in the electron beam optical system using the optical height detector and attain an in-focus electron beam image while conducting inspection. In an electron beam inspection system, inspection performance and reliability thereof can be improved by carrying out inspection using an electron beam image which is always focused in a consistent state. Furthermore, since height detection can be made simultaneously with inspection, continuous stage movement is applicable to inspection to reduce a required inspection time substantially. This feature is particularly advantageous in inspection of semiconductor wafers which will become still larger in diameter in the future. Similarly, the same advantageous effects can be attained in a microstructure observation/fabrication system using a convergent charged particle beam. Further, by disposing the height detection optical system outside the vacuum specimen chamber, adjustment and maintenance can be carried out with ease.
Mathematical formula within the disclosure gleaned from the first application will be referenced as “expressions.”
An embodiment of an automatic inspection system for inspecting/measuring a micro-circuit pattern formed on a semiconductor wafer which is an inspected object according to the present invention will be described. A defect inspection of the micro-circuit pattern formed on the semiconductor wafer or the like is executed by comparing inspected patterns and good pattern and patterns of the same kind on the inspected wafer. Also in the case of an appearance inspection using an electron microscope image (SEM image), a defect inspection is executed by comparing pattern images. Furthermore, also in the case of the length measurement (SEM length measurement) executed by a scanning-type electron microscope which measures a line width or a hole diameter of a micro-circuit pattern used to set or monitor a manufacturing process condition of semiconductor devices, the length measurement can be automatically executed by the image processing.
In the comparison inspection for detecting a defect by comparing electron beam images of a similar pattern or when a line width of a pattern is measured by processing an electron beam image, a quality of an obtained electron beam image exerts a serious influence upon the reliability of the inspected results. The quality of electron beam image is deteriorated by an image distortion caused by deflection and aberration of an electron optical system and is also deteriorated as resolution is lowered by a de-focusing. The deterioration of the image quality lowers a comparison and inspection efficiency and a length measurement efficiency.
Referring now to the drawings, a height of a surface of an inspected object is not even and an inspection is executed over the whole range of heights under the same condition for a wafer as shown in
An electron beam apparatus according to an embodiment of the present invention will be described with reference to
In accordance with the present invention, an electron beam image is prevented from being deteriorated in the above-mentioned electron beam apparatus (observation SEM apparatus, length measuring SEM apparatus).
The quality of the electron beam image is deteriorated due to image distortion caused by deflection and aberration of the electron optical system, and a resolution is lowered by de-focusing. For preventing the image quality from being deteriorated, the present invention provides, as shown in
The height detection apparatus 200 composed of the height detection optical apparatus 200a and the height calculating unit 200b is arranged substantially similarly to a second embodiment which will be described later, and is installed about an optical axis 110 of an electron beam symmetrically with respect to the sample 106. An illumination optical system of each height detection optical apparatus 200a comprises a light source 201, a condenser lens 202, a mask 203 with a multi-slit pattern, a half mirror 205, and a projection/detection lens 220. A detection optical system of each height detection optical apparatus 200a comprises a projection/detection lens 220, a magnifying lens 264 for focusing an intermediate multi-slit image focused by the projection/detection lens 220 on a line image sensor 214 in an enlarged scale, a mirror 206, a cylindrical lens (cylindrical lens) 213, and a line image sensor 214.
By the illumination optical system of the respective height detection optical apparatus which is installed symmetrically, a multi-slit shaped pattern is projected at the measurement position on the sample 106 for detecting an SEM image with the above-mentioned irradiation of electron beams. This regularly-reflected image is focused by the detection optical system of each height detection optical apparatus 200a and thereby detected as a multi-slit image. Specifically, since the height detection optical apparatus 200a projects and detects patterns of multi-slit shape from the left and right symmetrical directions and the height calculating unit 200b constantly obtains a height of a constant point 110 by averaging both detected values, it is necessary to locate a pair of height detection optical apparatus 200a in the left and right directions. Initially, a light beam emitted from the light source 201 is converged by the condenser lens 202 in such a manner that a light source image is focused at the pupil of the projection/detection lens. This light beam further illuminates the mask 203 on which the multi-slit shaped pattern is formed. Of the light beams, the light beam that was reflected on the half mirror 205 is projected by the projection/detection lens 220 onto the sample 106. The multi-slit pattern that was projected onto the sample is regularly reflected and passed through the projection/detection lens 220 of the opposite side. Then, the light beam passed through the half mirror 205 is focused in front of the magnifying lens 264. This intermediate image is focused on the line image sensor 214 by the magnifying lens 264. At that time, of the luminous flux, the portion that was passed through the half mirror 205 is focused on the line image sensor 214. In this embodiment, the cylindrical lens 213 is disposed ahead of the line image sensor 214 to compress the longitudinal direction of the slit and thereby the light beam is converged on the line image sensor 214. Assuming that m is a magnification of the detection optical system, then when the height of the sample is changed by z, a multi-slit image is shifted by 2mz·sin θ on the whole. By utilizing this fact, the height calculating unit 200b calculates a shift amount of the multi-slit image from a signal of a multi-slit image detected from the detection optical system of each height detection optical apparatus 200a, calculates a height of a sample from the calculated shift amount of the multi-slit image, and obtains a height on the electron beam optical axis 110 on the sample by averaging these calculated heights of the sample. Specifically, the height calculating means 200b calculates the height of the sample 106 from the shift amounts of the right and left multi-slit images. Here, an average value therebetween is calculated by using the height detected values obtained from the right and left detection system 200a, and is set to a height detection value at the final point 110. The position 110 at which the height is to be detected becomes an optical axis of the upper observation system.
Incidentally, while the height detection optical apparatus 200a is arranged substantially similarly to a second embodiment as shown in
The focus control apparatus 109 drives and controls an electromagnetic lens or an electrostatic lens on the basis of height data 190 obtained from the height calculating unit 200b to thereby focus an electron beam on the surface of the sample 106.
A deflection signal generating apparatus 108 generates the deflection signal 141 to the deflection element 102. At that time, the deflection signal generating apparatus 108 corrects the deflection signal 141 on the basis of the height data obtained from the height calculating unit 200b in such a manner as to compensate for an image magnification fluctuation caused by the fluctuation of the height of the surface of the sample 106 and an image rotation caused by the control of the electromagnetic lens 103. Incidentally, if an electrostatic lens is used as the objective lens 103 instead of the electromagnetic lens, then the image rotation caused when the focus is controlled does not occur so that the image rotation need not be corrected by the height data 190. Further, if lens 103 is comprised of a combination of an electromagnetic lens and an electrostatic lens, the electromagnetic lens has a main converging action and the electrostatic lens adjusts the focus position, then the image rotation, of course, need not be corrected by the height data 190.
Further, instead of directly controlling the focus position of the electromagnetic lens or the electrostatic lens 103 by the focus control apparatus 109 under the condition that the stage 105 is used as an XYZ stage, the height of the stage 105 may be controlled.
The entirety control apparatus 120 controls the whole of the electron beam apparatus (SEM apparatus), displays a processed result processed by the image processing apparatus 124 on a display 143 or stores the same in a memory 142 together with coordinate data for the sample. Also, the entirety control apparatus 120 controls the height calculating unit 200b, the focus control apparatus 109 and the deflection signal generating apparatus 108 thereby to realize a high-speed auto focus control in the electron beam apparatus and an image magnification correction and an image rotation correction caused by this focus control. Furthermore, the entirety control apparatus 120 executes a correction of a height detected value, which will be described later.
When the appearance of the inspected object is inspected based on the SEM image, a two-dimensional SEM image should be obtained over a certain wide area. As a result, driving the beam deflector 102 to scan electron beams in the direction substantially perpendicular to the movement direction of the stage 105 while the stage 105 is being continuously moved, it is necessary to detect a two-dimensional secondary electron image signal by the secondary electron detector 104. Specifically, while the stage 105 is being continuously moved in the X direction, for example, the beam deflector 102 is moved to scan electron beams in the Y direction substantially perpendicular to the movement direction of the stage 105, and then the stage 105 is moved in a stepwise fashion in the Y direction. Thereafter, while the stage 105 is being continuously moved in the X direction, the beam deflector 102 is driven to scan electron beams in the Y direction substantially perpendicular to the movement direction of the stage 105, and a two-dimensional secondary electron image signal has to be detected by the secondary electron detector 104. The processes of (1) continuous movement of the stage, (2) beam scanning, (3) optical height detection, (4) focus control and/or deflection direction and width correction, and (5) secondary electron image acquisition should be executed simultaneously. In this way, the acquired SEM image is kept focused and distortion-corrected while the image is being acquired continuously and speedily. By this control, fast and high-sensitivity defect detection can be achieved. Then, the image processing circuit 124 compares corresponding images or repetitive patterns by comparing an electron beam image delayed by the image memory and an image directly inputted from the A/D converter 124, thereby resulting in the comparison inspection being realized. The entirety control unit 120 receives the inspected result at the same time it controls the image processing circuit 124, and then displays the inspected result on the display 143 or stores the same in the memory 142. Incidentally, in the embodiment shown in
Further, the appearance inspection apparatus using an SEM image will be described with reference to FIGS. 57 to 62.
As the inspected object 106, there is the semiconductor wafer 303 as shown in FIGS. 58(a)-58(c), for example. On this semiconductor wafer 3, there are arrayed a number of chips 3a which form the same product finally as shown in
If the repetitive patterns (chips or cells of the semiconductor wafer, by way of example) of the inspected object 106 are equal to each other strictly and if equal detected images are obtained, then only defects cannot agree with each other when images are compared with each other. Thus, it is possible to recognize a defect.
However, in actual practice, a disagreement between images exists in the normal portion. As a disagreement at the normal portion, there are a disagreement caused by the inspected object, and a disagreement caused by the image detection system. The disagreement caused by the inspected object is based on a subtle difference caused between the repetitive patterns by a wafer manufacturing process such as exposure, development or etching. This disagreement appears as a subtle difference of pattern shape and a difference of gradation value. The disagreement caused by the image detection system is based on a fluctuation of a quantity of illumination light, a vibration of stage, various electrical noises, and a disagreement between detection positions of two images or the like. These disagreements appear as a difference of gradation value of a partial image, a distortion of pattern, and a positional displacement of an image on the detected image.
In the embodiment according to the present invention, a detection image (first two-dimensional image) in which gradation values of coordinates (x, y) aligned at the pixel unit are f1(x, y) and a compared image (second two-dimensional image) in which gradation values of coordinates (x, y) are g1(x, y) are compared with each other, a threshold value (allowance value) used when a defect is determined is set at every pixel considering the positional displacement of pattern and a difference between the gradation values, and a defect is determined on the basis of a threshold value (allowance value set at every pixel.
A pattern inspection system according to the present invention comprises, as shown in
Initially, the inspection unit 115 will be described with reference to
Of the electron detection unit 117, the electron detector 335 (104) for detecting electrons such as secondary electrons or reflection electrons is installed above the objective lens 333 (103), for example, within the inspection chamber 2100. An output signal from the electron detector 335 is amplified by an amplifier 336 installed outside the inspection chamber 2100.
The sample chamber 119 comprises a sample holder 330, an X stage 331 and a Y stage 332 previously referred to as stage 105, a position monitoring length measuring device 107 and a height measuring apparatus 200 such as an inspected based plate height measuring device. Incidentally, there may be provided a rotary stage on the stage.
The position monitoring length measuring device 107 monitors a position such as the stages 331, 332 (stage 105), and transfers a monitored result to the entirety control unit 120. The driving systems of the stages 331, 332 also are controlled by the entirety control unit 120. As a result, the entirety control unit 120 is able to precisely understand the area and the position irradiated with electron beams 112 on the basis of such data.
The inspected base plate height measuring device is adapted to measure the height of the inspected object 106 resting on the stages 331, 332. Then, a focal length of the objective lens 333 (103) for converging the electron beam 112 is dynamically corrected on the basis of measured data measured by the inspected base plate height measuring device 200 so that electron beams can be irradiated under the condition that electron beams are constantly properly-focused on the inspected area. Incidentally, in
The optical microscope unit 118 is located at the position near the electron optical system 116 within the room of the inspection chamber 2100 and which position is distant to the extent that the optical microscope unit and the electron optical system cannot affect each other. A distance between the electron optical system 116 and the optical microscope unit 118 should naturally be a known value. Then, the X stage 331 or the Y stage 332 is reciprocally moved between the electron optical system 116 and the optical microscope unit 118. The optical microscope unit 118 comprises a light source 361, an optical lens 362, and a CCD-camera 363. The optical microscope unit 118 detects the inspected object 106, e.g. an optical image of a circuit pattern formed on the semiconductor wafer 303, calculates a rotation displacement amount of circuit patterns based on the optical image thus detected, and transmits the rotation displacement amount thus calculated to the entirety control unit 120. Then, the entirety control unit 120 becomes able to correct this rotation displacement amount by rotating a rotating stage forming a part of stage 302 (105) which includes stages 331 and 332, for example. Also, the entirety control unit 120 sends this rotation displacement amount to a correction control circuit 120′, and the correction control circuit 120′ becomes able to correct the rotation displacement by correcting the scanning deflection position of electron beams caused by the scanning deflector 334, for example, on the basis of this rotation displacement amount. Moreover, the optical microscope unit 118 detects the inspected object 106, e.g. the optical image of the circuit pattern formed on the semiconductor wafer 303, observes this optical image, for example, displayed on the monitor 350, and sets the inspection area on the entirety control unit 120 by entering the coordinates of the inspection area into the entirety control unit 120 by using an input based on the optical image thus observed. Furthermore, the pitch between the chips on the circuit pattern formed on the semiconductor wafer 303, for example, or the repetitive pitch of the repetitive pattern such as the memory cell can be measured in advance and can be inputted to the entirety control unit 120. Incidentally, while the optical microscope unit 118 is located within the inspection chamber 2100 in
As shown in
Next, the image output unit 140 will be described with reference to
The pre-processing circuit (image correcting circuit) 340 comprises, as shown in
As described above, after the dark level correction (dark level is corrected on the basis of the detection signal 71 during the beam blanking period), the electron beam current fluctuation correction (beam current intensity is monitored and a signal is normalized by a beam current) and the shading correction (fluctuation of quantity of light at the beam scanning position is corrected) are effected on the digital image data (gradation image data) 71 obtained from the electric converter (light-receiving element) 325, the filtering processing is effected on the corrected digital image data (gradation image data) 80 by a Gaussian filter, a mean value filter or an edge-emphasizing filter in the filtering processing circuit 81, thereby resulting a digital image signal 82 with an image quality being improved. If necessary, a distortion of an image is corrected. These pre-processings are executed in order to convert a detected image so as to become advantageous in the later defect judgment processing.
Although the delay circuit 341 formed of a shift register or the like delays the digital image signal 82 (gradation image signal) with an improved image quality from the pre-processing circuit 340 by a constant time, if a delay time is obtained from the entirety control unit 120 and set to a time during which the stage 302 is moved by a chip pitch amount (d1 in
Moreover, an electron beam image latched within the pre-processing circuit 340 and the second image memory unit 347 or the like or the optical image detected by the optical microscope unit 118 may be displayed on the monitor and can be observed.
The image processing unit 124 will be described with reference to
The pixel unit position alignment unit 342 of image processing unit 124 displaces the position of comparison image, for example, in such a manner that the position displacement amount of the comparison image g0(x, y) relative to the above-mentioned detection image f0(x, y) falls in a range of from 0 to 1 pixel, in other words, the position at which a “matching degree” between f0(x, y) and g0(x, y) becomes maximum falls within a range of from 0 to 1 pixel. As a consequence, as shown in FIGS. 59(a) and 59(b), for example, the detection image f0(x, y) and the comparison image g0(x, y) are aligned with an alignment accuracy of less than one pixel. A square portion shown by dotted lines in
max|f0−g0|, ΣΣ|f0−g0|, ΣΣ(f0−g0)2 (expression 1)
max |f0−g0| shows a maximum value of an absolute value of a difference between the detection image f0(x, y) and the comparison image g0(x, y). ΣΣ|f0−g0| shows a total of absolute value of a difference between the detection image f0(x, y) and the comparison image g0(x, y) within the image. ΣΣ|(f0−g0)| shows a value which results from squaring a difference between the detection image f0(x, y) and the comparison image g0(x, y) and integrating the squared result in the x direction and the y direction.
Although the processed content is changed depending upon the adoption of any one of the above-mentioned (expression 1), the case that ΣΣ|f0−g0| is adopted will be described below.
Mx assumes the displacement amount of the comparison image g0(x, y) in the x direction, and my assumes the displacement in the y direction (mx, my are integers). Then, e1(mx, my) and s1(mx, my) are defined by equations of (expression 2) and (expression 3), respectively:
e1(mx,my)=ΣΣ|f0(x,y)−g0(x+mx,y+my) (expression 2)
s1(mx,my)=e1(mx,my)+e1(mx+1,my)+e1(mx,my+1)+e1(mx+1,my+1) (expression 3)
In the expression 2, ΣΣ shows a total within the image. Since what is required to calculate is a value obtained when mx assumes the displacement amount of the x direction in which s1(mx, my) becomes minimum and a value obtained when my assumes the displacement amount of the y direction, by changing mx and my as ±0, 1, 2, 3, 4, . . . n, in other words, by changing the comparison image g0(x, y) with a pixel pitch, there is calculated s1(mx, my) of each time. Then, a value mx0 of mx in which the calculated value becomes minimum and a value my0 of my in which the calculated value becomes minimum are calculated. Incidentally, the maximum displacement amount n of the comparison image should be increased as the positional accuracy is lowered in response to the positional accuracy of the detection unit 115. The pixel unit position alignment unit 342 outputs the detection image f0(x, y) at it is, and outputs the comparison image g0(x, y) with a displacement of (mx0, my0). That is, f1(x, y)=f0(x, y), g1(x, y)=g0(x+mx0, y+my0).
A positional displacement detection unit (not shown) for detecting a positional displacement of less than a pixel divides the images f1(x, y), g(x, y) aligned at the pixel unit into small areas (e.g. partial images composed of 128*256 pixels), and calculates positional displacement amounts (positional displacement amounts become real number of 0 to 1) of less than the pixel at every divided area (partial image). The reason that the images are divided into small areas is in order to cope with a distortion of an image, and hence should be set to a small area to the extent that a distortion can be neglected. As a measure for measuring a matching degree, there are the selection branches shown in the expression 1. An example is shown in which the third “sum of squares of difference” (ΣΣ(f0−g0)2) is adopted.
Let it be assumed that an intermediate position between f1(x, y) and g1(x, y) is held at the positional displacement amount 0 and that, as shown in
f2(x,y)=f1(x+δx,y+δy)=f1(x,y)+δx(f1(x+1,y)−f1(x,y))+δy(f1(x,y+1)−f1(x,y)) (expression 4)
g2(x,y)=g1(x−δx,y−δy)=g1(x,y)+δx(g1(x−1,y)−g1(x,y))+δy(g1(x,y−1)−g1(x,y)) (expression 5)
The expression 4 and the expression 5 are what might be called linear interpolations. A matching degree e2(δx, δy) of f2 and g2 is represented by the following equation of (expression 6) if “sum of squares of difference” is adopted.
e2(δx, δy)=ΣΣ(f2(x,y)−g2(x,y))2 (expression 6)
ΣΣ denotes a total within small areas (partial images). The object of the positional displacement detection unit (not shown) for detecting a positional displacement of less than the pixel unit is to obtain a value δx0 of δx and a value δy0 of δy in which e2(δx, δy) takes the minimum value. To this end, an equation which results from partially differentiating the above-mentioned expression 6 by δx, δy is set to 0 and may be solved. The results are obtained as shown by the following equations of (expression 7) and (expression 8):
δx={(ΣΣC0*Cy)*(ΣΣCx*Cy)ΣΣC0*Cx)*(ΣΣCy*Cy)}/{(ΣΣCx*Cx)*(ΣΣCy*Cy)−(ΣΣCx*Cy)*(ΣΣCx*Cy)} (expression 7)
δx={(ΣΣC0*Cx)*(ΣΣCx*Cy)ΣΣC0*Cy)*(ΣΣCx*Cx)}/{(ΣΣCx*Cx)*(ΣΣCy*Cy)−(ΣΣCx*Cy)*(ΣΣCx*Cy)} (expression 8)
However, respective ones of C0, Cx, Cy establish relationships shown by the following equations of (expression 9), (expression 10) and (expression 11):
C0=f1(x,y)−g1(x,y) (expression 9)
Cx={f1(x+1,y)−f1(x,y)}−{g1(x−1,y)−g1(x,y)} (expression 10)
Cy={f1(x,y+1)−f1(x,y)}−{g1(x,y−1)−g1(x,y)} (expression 11)
In order to obtain δx0, δy0, respectively, as shown by the (expression 7) and the (expression 8), it is necessary to obtain a variety of statistic amounts ΣΣCk*Ck (Ck=C0, Cx, Cy). The statistic amount calculating unit 344 calculates a variety of statistic amount ΣΣCk*Ck on the basis of the detection image f1(x, y) composed of the gradation value (light and shade value) aligned at the pixel unit obtained from the pixel unit position alignment unit 342 and the comparison image g1(x, y).
The sub-CPU 345 obtains δx0, δy0 by calculating the (expression 7) and the (expression 8) by using the ΣΣCk*Ck which was calculated in the statistic amount calculating unit 344.
The delay circuits 346, 347 formed of the shift register or the like are adapted to delay the image signals f1 and g1 by the time which is required by the less than pixel positional displacement unit (not shown) to calculate δx0, δy0.
The difference image extracting circuit (difference extracting circuit:distance extracting unit) 349 is adapted to obtain a difference image (distance image) sub(x, y) between f1 and g1 having positional displacements 2*δx0, 2*δy0 from a calculation standpoint. This difference image (distance image) sub(x, y) is expressed by the equation of (expression 12) as follows:
sub(x,y)=g1(x,y)−f1(x,y) (expression 12)
The threshold value computing circuit (allowance range computing unit) 348 is adapted to calculate by using the image signals f1, g1 from the delay circuits 346, 347 and the positional displacement amounts δx0, δy0 of less than the pixel obtained from the less than pixel positional displacement detection unit (not shown) two threshold values (allowance values indicative of allowance ranges) thH(x, y) and thL(x, y) which are used by the defect deciding circuit (defect judgment unit) 350 to determine in response to the value of the difference image (distance image) sub(x, y) obtained from the difference image extracting circuit (difference extracting circuit:distance extracting unit) 349 whether or not the inspected object is the nominated defect. ThH(x, y) is the threshold value (allowance value indicative of allowance range) which determines the upper limit of the difference image (distance image) sub(x, y), and thL(x, y) is the threshold value (allowance value indicative of allowance range) which determines the lower limit of the difference image (distance image) sub(x, y). Contents of the computation in the threshold value computing circuit 348 are expressed by the equations of (expression 13) and (expression 14) as follows:
thH(x,y)=A(x,y)+B(x,y)+C(x,y) (expression 13)
thL(x,y)=A(x,y)−B(x,y)−C(x,y) (expression 14)
However, A(x, y) is a term expressed by a relationship of the following equation of (expression 16) and which is used to correct the threshold values by using the less than pixel positional displacement amounts δx0, δy0 in response to the value of the difference image (distance image) sub(x, y) substantially.
Also, B(x, y) is a term expressed by a relationship of the equation of the (expression 16) and which is used to allow a very small positional displacement of a pattern edge (very small difference of pattern shape, pattern distortion also returns to a very small positional displacement of pattern edge from a local standpoint) between the detection image f1 and the comparison image g1.
Also, C(x, y) is a term expressed by a relationship of the equation of (expression 17) and which is used to allow a very small difference of gradation value (light and shade value) between the detection image f1 and the comparison image g1).
where α, β are the real numbers ranging from 0 to 0.5, γ is the real number greater than 0, and ε is the integer greater than 0.
dx1(x, y) is expressed by a relationship of the equation of (expression 18), and indicates a changed amount of a gradation value (light and shade value) with respect to the x direction+1 adjacent image in the detection image f1(x, y).
dy2(x, y) is expressed by a relationship of the equation of (expression 19), and indicates a changed amount of a gradation value (light and shade value) with respect to the x direction−1 adjacent image in the comparison image g1(x, y).
dy1(x, y) is expressed by a relationship of the equation of (expression 20), and indicates a changed amount of a gradation value (light and shade value) with respect to the y direction+1 adjacent image in the detection image f1(x, y).
dy2(x, y) is expressed by a relationship of the equation of (expression 21), and indicates a changed amount of a gradation value (light and shade value) with respect to the y direction−1 adjacent image in the comparison image g1(x, y).
dx1(x,y)=f1(x+1,y)−f1(x,y) (expression 18)
dx2(x,y)=g1(x,y)−g1(x−1,y) (expression 19
dy1(x,y)=f1(x,y+1)−f1(x,y) (expression 20)
dy2(x,y)=g1(x,y)−g1(x,y−1) (expression 21)
max1 is expressed by a relationship of the equation of (expression 22), and indicates maximum gradation values (light and shade values) of x direction+1 adjacent image and y direction+1 adjacent image including itself in the detection image f1(x, y).
max2 is expressed by a relationship of the equation of (expression 23), and indicates maximum gradation values (light and shade values) of x direction−1 adjacent image and y direction−adjacent image including itself in the comparison image g1(x, y).
max1=max{f1(x,y),f1(x+1,y),f1(x,y+1),f(x+1,y+1)} (expression 22)
max2=max{g1(x,y),g1(x−1,y),g1(x,y−1),g(x−1,y−1)} (expression 23)
First, the first term A(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y), thL(x, y) will be described. Specifically, the first term A(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y) and thL(x, y) is the term used to correct the threshold values in response to the less than pixel positional displacement amounts δx0, δy0 which were calculated by the positional displacement detection unit 343. Since dx1 expressed by (expression 18), for example, is a local changing rate of a gradation value of f1 in the x direction, dx1(x, y)*δx0 expressed by (expression 15) can be regarded as a predicted value of the change of the gradation value (light and shade value) of f1 obtained when the position is shifted by δx0. Therefore, the first term {dx1(x, y)*δx0−dx2(x, y)*(−δx0)} can be regarded as a value which predict at every pixel a changing rate of a gradation value (light and shade value) of the difference image (distance image) of f1 and g1 obtained when the position of f1 is displaced by δx0 in the x direction and the position of g1 is displaced by −δx0 in the x direction. Similarly, the second term can be regarded as the value which predicts a changing rate with respect to the y direction. Specifically, {dx1(x, y)+dx2(x, y)}*δx0 is a value which can predict a changing rate of a gradation value (light and shade value of difference image (distance image) of f1 and g1 in the x direction by multiplying a local changing rate {dx1(x, y)+dx2(x, y)} of the difference image (distance image) between the detection image f1 and the comparison image g1 in the x direction with the positional displacement δx0. Also, {dy1(x, y)+dy2(x, y)}*δy0 is a value which predicts at every pixel a changing rate of a gradation value (light and shade value) of the difference image (distance image) of f1 and g1 by multiplying a local changing rate {dy1(x, y)+dy2(x, y) of the difference image (distance image) between the detection image f1 and the comparison image g1 in the y direction with the positional displacement δy0.
As described above, the first term A)x, y) in the threshold values thH(x, y) and thL(x, y) is the term used to cancel the known positional displacements δx0, δy0.
The second term B(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y) and thL(x, y) will be described. Specifically, the second term B(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y) and thL(x, y) is the term used to allow a very small positional displacement of pattern edge (very small difference of pattern shape and pattern distortion also are returned to very small positional displacements of pattern edge from a local standpoint). As will be clear from the comparison of the (expression 15) for calculating A(x, y) and the (expression 16) for calculating B(x, y), B(x, y) is an absolute value of a change prediction of a gradation value (light and shade value) of the difference image (distance image) brought about by the positional displacements α, β. If the positional displacement is canceled by A(x, y), then the addition of B(x, y) to A(x, y) means that the position aligned state is further displaced by α in the x direction and by β in the y direction considering a very small positional displacement of pattern edge caused by a very small difference based on the pattern shape and the pattern distortion. That is, +B(x, y) expressed by the equation of (expression 13) is to allow the positional displacement of +α in the x direction and the positional displacement of +β in the y direction as the very small positional displacements of the pattern edge caused by the very small differences based on the pattern shape and the pattern distortion. Further, the subtraction of B(x, y) from A(x, y) in the equation of (expression 14) means that the positional aligned state is positionally displaced by −α in the x direction and by −β in the y direction. −B(x, y) expressed by the equation of (expression 14) is adapted to allow the positional displacement of −α in the x direction and −β in the y direction. As shown by the equations of (expression 13) and (expression 14), if the threshold value includes the upper limit thH(x, y) and the lower limit thL(x, y), then it is possible to allow the positional displacements of ±α, ±β. Then, if the threshold value computing circuit 348 sets the values of the inputted parameters α, β to proper values, then it becomes possible to freely control the allowable positional displacement amounts (very small positional displacement amounts of pattern edge) caused by the very small difference based on the pattern shape and the pattern distortion.
Next, the third term C(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y) and thL(x, y) will be described. The third term C(x, y) in the equations of (expression 13) and (expression 14) for calculating the threshold values thH(x, y) and thL(x, y) is a term used to allow a very small difference of a gradation value (light and shade value) between the detection image f1 and the comparison image g1. As shown by the equation of (expression 13), the addition of C(x, y) means that the gradation value (light and shade value) of the comparison image g1 is larger than the gradation value (light and shade value) of the detection image f1 by C(x, y). As shown by the equation of (expression 14), the subtraction of C(x, y) means that the gradation value (light and shade value) of the comparison value g1 is smaller than the gradation value (light and shade value) of the detection image by C(x, y). While C(x, y) is a sum of a value which results from multiplying a representing value (max value) of a gradation value at the local area with the proportional constant γ and the constant ε as shown by the equation of (expression 17), the present invention is not limited to the above-mentioned function. If the manner in which the gradation value is fluctuated is already known, then it is possible to use a function which can cope with such manner. For example, if it is clear that a fluctuation width is proportional to a square root of a gradation value, then the equation of (expression 17) should be replaced with C(x, y)=(square root of (max1+max2))*γ+ε. Thus, the threshold value computing circuit 348 becomes able to freely control a difference of allowable gradation value (light and shade value) by the inputted parameters γ, ε similarly to B(x, y).
Specifically, the threshold value computing circuit (allowable range computing unit) 348 includes a computing circuit for computing {dx1(x, y)+dx2(x, y)} by the equations of (expression 18) and (expression 19) based on the detection image f1(x, y) composed of a gradation value (light and shade value) inputted from the delay circuit 346 and the comparison image g1(x, y) composed of a gradation value (light and shade value) inputted from the delay circuit 347, a computing circuit for computing {dy1(x, y)+dy2(x, y)} by the equations of (expression 20) and (expression 21) and a computing circuit for computing (max1+max2) by the equations of (expression 22) and (expression 23). Further, the threshold value computing circuit 348 includes a computing circuit for computing ({dx1(x, y)+dx2(x, y)}*δx0±|{dx1(x, y)+dx2(x, y)}|*α) which is a part of (expression 15) and a part of (expression 16) on the basis of {dx1(x, y)+dx2(x, y)} obtained from the computing circuit, δx0 obtained from the less than pixel displacement detection unit 343 and the inputted a parameter, a computing circuit for computing (dy1(x, y)+dy2(x, y))*δy0±|{dy1(x, y)+dy2(x, y)}|*β) which is a part of (expression 15) and a part of (expression 16) on the basis of {dy1(x, y)+dy2(x, y)} obtained from the computing circuit, δy0 obtained from the less than pixel displacement detection unit 343 and the inputted β parameter and a computing circuit for computing ((max1+max2)/2)*γ+ε) in accordance with the equation of (expression 17), for example, on the basis of (max1+max2) obtained from the computing circuit and the inputted γ, ε parameters. Further, the threshold value computing circuit 348 includes an adding circuit for positively adding ({dx1(x, y)+dx2(x, y)}*δx0+|{dx1(x, y)+dx2(x, y)}|*α), ({dy1(x, y)+dy2(x, y)}*δy0+|{dy1(x, y)+dy2(x, y)}|*β) obtained from the computing circuit and ((max1+max2)/2)*γ+ε) obtained from the computing circuit to output the threshold value thH(x, y) of the upper limit, a subtracting circuit for negatively computing (((max1+max2)/2)*γ+ε) obtained from the computing circuit and an adding circuit for positively computing ({dx1(x, y)+dx2(x, y)}*δx0−|{dx1(x, y)+dx2(x, y)|*α} obtained from the computing circuit, ({dy1(x, y)+dy2(x, y)}*δy0−|{dy1(x, y)+dy2(x, y)}|*β) obtained from the computing circuit and −((max1+max2)/2*γ+ε) obtained from the subtracting circuit to output the threshold value thL(x, y) of the lower limit.
Incidentally, the threshold value computing circuit 348 may be realized by a CPU by software processing. Further, the parameters α, β, γ, ε inputted to the threshold value computing circuit 348 may be entered by an input means (e.g. keyboard, recording medium, network or the like) disposed in the entirety control unit 120.
The defect deciding circuit (defect judgment unit) 350 decides by using the difference image (distance image) sub(x, y) obtained from the difference image extracting circuit (difference extracting circuit) 349, the threshold value of the lower limit (allowable value indicating the allowable range of lower limit) thL(x, y) obtained from the threshold value computing circuit 348 and the threshold value of the upper limit (allowable value indicating the allowable range of upper limit) thH(x, y) that the pixel at the position (x, y) is a non-defect nominated pixel of the following equation of (expression 24) is satisfied and that the pixel at the position (x, y) is a defect nominated pixel if it is not satisfied. The defect deciding circuit 350 outputs def(x, y) which takes a value of 0, for example, with respect to the non-defect nominated pixel and which takes a value greater than 1, for example, the defect-nominated pixel indicating a disagreement amount.
thL(x,y)≦sub(x,y)≦thH(x,y) (expression 24)
The feature extracting circuit 350a executes a noise elimination processing (e.g. contracts/expands def(x, y). When all of 3×3 pixels are not simultaneously the defect-nominated pixels, the center pixel is set to 0 (non-defect nominated pixel), for example, and eliminated by a contraction processing, and is returned to the original one by an expansion processing. After a noise-like output (e.g. all 3×3 pixels are not simultaneously the defect-nominated pixels) is deleted, there is executed a defect-nominated pixel merge processing in which nearby defect-nominated pixels are collected into one. Thereafter, barycentric coordinates and XY projection lengths (maximum lengths in the x direction and the y direction) are demonstrated at the above-mentioned unit. Incidentally, the feature extracting circuit 350a calculates a feature amount 88 such as a square root of (square of X projection length+square of Y projection length) or an area, and outputs the calculated result.
As described above, the image processing unit 124 controlled by the entirety control unit 120 outputs the feature amount (e.g. barycentric coordinates, XY projection lengths, area, etc.) of the defect-nominated portion in response to coordinates on the inspected object (sample) 106 which is detected with the irradiation of electron beams by the electron detector 335 (104).
The entirety control unit 120 converts position coordinates of the defect-nominated portion on the detected image into the coordinate system on the inspected object (sample) 106, deletes a pseudo-defect, and finally forms defect data composed of the position on the inspected object (sample) 106 and the feature amount calculated from the feature extracting circuit 350a of the image processing unit 124.
According to the embodiment of the present invention, since the whole positional displacement of the small areas (partial images), the very small positional displacements of individual pattern edges and the very small differences of gradation values (light and shade values) are allowed, the normal portion can be prevented from being inadvertently recognized as the defect. Moreover, by setting the parameters α, β, γ, ε to proper values, it becomes possible to easily control the positional displacement and the allowance amount of the fluctuation of the gradation values.
Further, according to the embodiment of the present invention, since an image which is position-aligned by the interpolation in a pseudo-fashion, an image can be prevented from being affected by a smoothing effect which is unavoidable in the interpolation. There is then the advantage that the present invention is advantageous in detecting a very small defect portion. In actual practice, according to the experiments done by the inventors of the present invention, having compared the result in which the defect is decided by calculating the threshold value allowing the positional displacement and the fluctuation of the gradation value similarly to this embodiment after an image which is position-aligned by the interpolation in a pseudo-fashion by using the result of the positional displacement detection of less than pixel and the result obtained by the defect judgment according to this embodiment, the defect detection efficiency can be improved by greater than 5% according to the embodiment of the present invention.
The arrangement for preventing the electron beam image in the aforementioned electron beam apparatus (observation SEM apparatus, length-measuring SEM apparatus) from being deteriorated will be described further. Specifically, the quality of the electron beam image is deteriorated by the image distortion caused by the deflection and the aberration of the electron optical system and by the resolution lowered by the de-focusing. The arrangement for preventing the image quality from being deteriorated is comprised of the height detection apparatus 200 composed of the height detection optical apparatus 200a and the height calculating unit 200b, the focus control apparatus 109, the deflection signal generating apparatus 108, and the entirety control apparatus 120.
Light emitted from the light source 201 irradiates the mask 203 on which there is drawn the multi-slit shaped pattern which results from repeating the rectangular-shaped pattern, for example. As a result, the multi-slit-shaped pattern is projected by the projection lens 210 onto the height measuring position 217 on the sample surface 106. The multi-slit-shaped pattern drawn on the mask 203 is not limited to the slit-shaped pattern, and may be shaped as any shape such as an ellipse or a square so long as it is formed by the repetition of the same pattern. Generally, it can be a pattern that comprises a row of patterns with different shapes. Moreover, the spacing between the neighboring patterns can be different from each other. What is essential, as will be described later in detail using
Reference numeral 110 denotes the optical axis of the upper observation system, i.e. the height detection position. Specifically, when the above-mentioned height detection apparatus is used as an auto focus height sensor, reference numeral 110 becomes the optical axis of the upper observation system. Incidentally, assuming that p is the pitch of the multi-slit-shaped pattern of the projected image of the projection lens 210, then the pitch of the pattern projected onto the sample surface 106 becomes p/cos θ, and the pitch of the pattern on the image sensor 214 becomes pm. Also, assuming that m′ is the magnification of the illumination projection system, then the pitch of the pattern on the mask 203 becomes p/m′. That is, the pitch of the multi-slit-shaped pattern formed on the mask 203 becomes p/m′.
As shown in FIGS. 64(a), 64(b), when a height is detected on the sample 106 at its boundaries having different reflectances, an intensity distribution of a signal detected on the line image sensor 214 is affected by a reflectance distribution of a sample. However, if the multi-slit-shaped pattern is as thin as possible so long as a clear image can be maintained within a height detection range, then it is possible to suppress a detection error caused by a reflectance distribution on the surface of the object. Because, the detection error is caused as a center of gravity of a slit image is deviated due to a reflectance distribution of a sample, and an absolute value of this deviation increases in proportion to the width of the slit. In the embodiment as shown in
Although the detection error decreases as the slit width is reduced, this has a limitation. Thus, even when the slit width is reduced over a certain limit, no slit is clearly focused on the image sensor 214, and a contrast is lowered. This has the following relationship.
Specifically, assuming that ±zmax is a target height detection range, then at that time, the multi-slit image on the image sensor 214 is de-focused by ±2zmax·cos θ. On the other hand, assuming that p is the cycle of the multi-slit-shaped pattern on the projection side and that NA is an NA (Numerical Aperture) of the detection lens 215, then this focal depth becomes ±a·0.61p/NA. That is, the condition that the slit cycle p satisfies (2zmax·cos θ)<(a·0.61p/NA) is the condition under which the multi-slit image can be constantly detected clearly. In the above, a is the constant determined by defining the focus depth such that its amplitude is lowered. When the focus depth is defined under the condition that the amplitude is lowered to ½, a is about 0.6.
In the embodiment shown in
Also, in the embodiment shown in
As shown in
In the embodiment shown in
A height detection algorithm of the sample surface 106 according to an embodiment will be described next with reference to
1. Scan y(x) and calculate a position xmax of maximum value.
2. Calculate the substantial position of the peak i by searching left and right directions from xmax by each pitch p.
3. Assuming that xo represents the peak position of the left end, then the substantial position of the peak i becomes xo+p*i. The positions of the left and right troughs xo+p*i−p/2, xo+p*i+p/2.
4. Set ymin=max(y(xo+p*i−p/2), y(xo+p*i+p/2). That is, a larger one of left and right troughs is set to ymin.
5. Set k to a constant of about 0.3, and set yth=ymin+k*(y(xo+p*i)−ymin). That is, set amplitude (y(xo+p*i)−ymin)*k to a range value (threshold value) yth.
6. Calculate a center of gravity of y(x)−yth relative to a point at which y(x)>yth is satisfied between xo+p*i−p/2 and xo+p*i+p/2, and set the value thus calculated to yg(i).
7. Calculate weighted mean of yg(i)−ygo(i), and set the calculated weighted mean to image shift.
8. Calculate the height z by adding an offset to a value which results from multiplying the image shift with a detection gain (1/(2m·sin θ)).
In this manner, there is realized the height detection which is difficult to be affected by the surface state of the sample 106. Incidentally, in this embodiment, the peak of the slit image is used but instead a trough between the slit images may be used. Specifically, a center of gravity of yth−y(x) is calculated with respect to a point of y(x)<yth and set to a center of gravity of each trough. Then, the shifted amount of the whole image is obtained by averaging the movement amount of these trough images. Thus, there can be achieved the following effects. Since the detection waveform is determined based on the product of the projection waveform and the reflectance of the sample surface, the bright portion of the slit image is largely affected by the fluctuation of the reflectance, and the shape of the detection waveform tends to change. On the other hand, the trough portion of the waveform is difficult to be affected by the reflectance of the sample surface. Therefore, by the height detection algorithm based on the measurement of the movement amount of the trough between the slit images, it is possible to reduce the detection error caused by the surface state of the object much more.
The height detection optical apparatus 200a according to a second embodiment according to the present invention will be described next with reference to
The second embodiment shown in
The height detection optical apparatus 200a according to a third embodiment of the present invention will be described next with reference to
Further, the manner in which an error caused by another cause can be canceled out by using the arrangement of the second or third embodiment shown in
Next, the manner in which the height of the sample surface 106 can be obtained accurately by the height calculating unit 200b even when the height z of the sample surface 106 is changed will be described with reference to FIGS. 72(a)-72(b). Although the optical system shown in
Therefore, the height calculating unit 200b can select a plurality of slits containing this slit at the center, average height detection values from these slits, determine the value thus averaged as a final height detection value, and can accurately obtain the height at the visual field center 110 of the upper observation system. In order for the height calculating unit 200b to calculate z·sin θ/p, it is necessary to know the height z. Since the z required may be an approximate value for selecting the slit, the height that was calculated previously or the detection height obtained before the detection position displacement is corrected may be used as the height z. Incidentally, the position equivalent to the visual field center 110 is shifted on the image sensor by zm·sin θ as the height of the sample 106 is changed by z.
Further, when the appearance is inspected on the basis of the SEM image shown in
Also in this embodiment, the height detection apparatus 200 should constantly detect the height of the surface of the inspected object 106 from which the secondary electron image signal is detected and obtain the correct inspected result by executing the automatic focus control.
However, due to an image accumulation time of the image sensor 214 in the height detection optical apparatus 200a, a calculation time in the height calculating unit 200b, the responsiveness of the focus position control apparatus 109 or the like, it is frequently observed that a focus control is delayed. Therefore, even when the focus control is delayed, light should be accurately focused on the surface of the inspected object 106 from which the secondary electron image signal is detected. In
In this embodiment, the detection position displacement caused by the change of the height of the sample surface 106 shown in
In the embodiment shown in
Zc=Z0+((Z0)−(Z−1))×((T1/2)+T2)/T1 (expression 25)
Extrapolation straight lines may of course be applied to more than three points Z−m, Z−(m−1), . . . Z−2, Z−1, Z0 so as to reduce an error or a quadratic function, a cubic function or the like may be applied to these points. These extrapolation methods are mathematically well known, and when in use, the most suitable one may be selected in accordance with the magnitude of the change of the height detection value and the magnitude of the fluctuations.
As another embodiment, the manner in which the height detection value is corrected and outputted will be described. When the height detection value changes stepwise at the interval T1, if the feedback is applied to electron beams by using such stepwise height detection values, then it is not preferable that the quality of electron beam image is changed rapidly at the interval T1. In this case, in addition to the extrapolation height detection value Zc, an extrapolation height detection value Zc′ which is delayed by a time T1 from a time a is calculated similarly. In the embodiment shown in
Zc=(Z−1)+(((Z−1)−(Z−3))/(2T1))×2.5T1
Zc′=(Z0)+(((Z0)−(Z−2))/(2T1))×2.5T1 (expression 26)
On the basis of these Zc and Zc′, the height Z1 which is delayed by t from the time a can be calculated by interpolation as in the following equation of (expression 27):
Z1=Zc+(Zc′−Zc)t/T1 (expression 27)
As described above, the detection time delay caused by the CCD storage time and the height calculation time can be corrected. Thus, even when height of the inspected object 106 is change every moment, a height detection value with a small error can be obtained, and a feedback can be stably applied to the electron optical system which controls electron beams.
Further, in the electron optical system shown in
Furthermore,
As another embodiment, instead of an arrangement for controlling an angle of the mirror 206 electrically, if the mirror 206 is controlled in such a manner that the position at which the slit-shaped pattern image is focused on the image sensor 214 always becomes constant, then the irradiated position 217 of detection light on the sample can be maintained constant regardless of the height z of the sample 106. When the mirror is controlled as described above, the rotation angle of the mirror 206 and the height z are in proportion to each other so that the height z of the sample can be detected by detecting the rotation angle of the mirror 206.
A method of further decreasing a detection error by properly selecting the slit direction will be described next with reference to
While the method of correcting the detection position of the projection direction by the multi-slit and the method of canceling out the positional displacement by the two-side projection have been described so far with respect to the phenomenon in which the detection position is displaced by the height z of the sample surface 106, a method of reducing a displacement of a detection position in the longitudinal direction of the slit, i.e. in the direction perpendicular to the projection direction will be described. When the longitudinal direction of the slit is projected across areas having different reflectances on the sample as shown in
An embodiment in which a two-dimensional distribution of the height of the sample 106 is obtained will be described next with reference to
In the embodiment shown in
Moreover, as in the embodiment shown in
With respect to a multi-slit shift amount detection algorithm executed by the height calculating unit 200b, an embodiment different from
Furthermore, while the above-mentioned algorithm constructs the filter matched with the pitch p of the well-known multi-slit shaped pattern and uses this filter to detect the phase, the present invention is not limited thereto, and an FFT (Fast Fourier Transform) is effected on y(x) and a phase corresponding to a peak of a spectrum is obtained, thereby making it possible to detect the phase of the waveform y(x).
An embodiment of another slit shift amount measuring algorithm will be described next with reference to
Moreover, the peak position of the slit can be determined as the position of the slit. The interpolation is executed in order to calculate the peak position with an accuracy below pixel. There are various interpolation methods. When a quadratic function interpolation, for example, is carried out, if three points before and after the maximal value are set to (x1−Δx, y0), (x1, y1) and (x1+Δx, y2), then the peak position is expressed by x1+Δx (y2−y0)/{2(2·y2−y2−y0)}.
While the above-mentioned methods have been described so far on the assumption that the position of the slit is calculated, the present invention is not limited thereto, and the position of the trough of the detection waveform is calculated and the shift of this position is detected, thereby making it possible to obtain the height of the sample. If so, the following effects can be achieved. The amount in which the waveform of the detection multi-slit pattern is fluctuated by the reflectance distribution on the surface of the sample increases much more when the reflectance boundary coincides with the peak portion of the multi-slit image as compared with the case in which the reflectance boundary coincides with the trough portion. The reason for this is that the detected light quantity distribution is determined based on a product of the light quantity distribution obtained when the reflectance of the sample is constant and the reflectance of the sample. Consequently, the bright portion tends to cause the change of the detected light quantity relative to the change of the same reflectance. Accordingly, if the position of the trough portion having the small fluctuation of the waveform is calculated, the position of the slit image can be detected and the height of the sample can be detected with a small error independently of the state of the reflectance of the sample. As the method of detecting the position of the trough portion, there may be used the algorithm for calculating a center of gravity relative to a code-inverted waveform −y(x) shown in
A method of detecting the position of the multi-slit image without the linear image sensor will be described next with reference to FIGS. 86(a)-86(b). As shown in
An embodiment concerning a method of correcting a focus control current or a focus control voltage and a focus position of charged particle optical system (objective lens 103) in the observation SEM apparatus and the length measuring SEM apparatus including the appearance inspection SEM apparatus shown in
Upon correction, on the basis of the command from the entirety control unit 120, the stage control apparatus 126 is controlled in such a manner that this correction standard pattern 130 is moved about the upper observation system optical axis 110 in the observation area. The entirety control unit 120 uses this standard pattern 130 to obtain from the focus control apparatus 109 the focus control current or the focus control voltage under which the secondary electron image signal (SEM image signal) which is the charged particle beam image detected by the secondary electron detector 104 which is the charged particle detector becomes clearest at each point, and measures the same. At that time, the visibility of the secondary electron image (SEM image) which is the charged particle beam image is detected by the secondary electron detector 104. A digital SEM image signal converted by the A/D converter 339 (122) or the digital SEM image signal pre-processed by the pre-processing circuit 340 is inputted to the entirety control unit 120 and thereby displayed on the display 143 or stored in the image memory 347 and displayed on the display 350, thereby being visually confirmed or determined by the image processing for calculating a changing rate of an image at the edge portion of the SEM image inputted to the entirety control unit 120. Since the real height of the correction sample surface (correction standard pattern 130) is already known, if this height information is inputted by using an input (not shown), then the entirety control unit 120 is able to obtain a relationship between the real height of the sample surface and the optimum focus control current or focus control voltage by the above-mentioned measurement as shown in
Specifically, as shown in
In the next step S39, the entirety control unit 120 issues a command to the image processing unit 124 in such a manner that the image processing unit obtains an image distortion parameter composed of an image magnification correction, an image rotation correction or the like in each height Zn in the correction sample piece 130 and stores the image distortion correction parameter thus obtained in the memory 142. In the next step S40, the position n on the sample piece 130 is set to n=n+1. Then, until n≦Nn is satisfied in a step S41, the steps S31 to S39 are repeated to thereby obtain the focus control signal In under which the visibility of the image in the height Zdn of each sample piece becomes maximum and the image distortion correction parameter composed of the image magnification correction, the image rotation correction or the like. If a NO is outputted in the inequality of n≦Nn at the step S41, then in a step S42, the entirety control unit 120 obtains a correction curve shown in
Incidentally, the correction standard pattern 130 shown in
Furthermore, when the charged particle beam apparatus such as the SEM apparatus has the Z stage, the Z stage is moved and detected in height not by the standard pattern shown in
The manner in which the correction is executed by using the correction parameter thus obtained and an appearance is inspected on the basis of the SEM image in the SEM apparatus shown in
In the next step S72, the entirety control unit 120 issues a command to the stage control apparatus 126 so that the stage control apparatus moves the stage to a stage scanning start position. Then, steps S73, S74, S75, S76 are executed in parallel to each other. In the step S73, the entirety control unit 120 issues a command to the stage control apparatus 126 so that the stage control apparatus 126 drives the stage 302 with the inspected object 106 resting thereon at a constant speed. Simultaneously, in the step S74, the entirety control unit 120 issues a command to the height calculating unit 200b such that the height calculating unit 200b outputs correction detection height information 190 based on real time height detection and height detection apparatus correction parameters obtained from the height detection optical apparatus 200a to the focus control apparatus 109 and the deflection control apparatus 108. Further, at the same time, in the step S75, the entirety control apparatus 120 issues commands to the focus control apparatus 108 and the deflection control apparatus 109 such that the focus control apparatus 108 and the deflection control apparatus 109 continuously execute the focus control by using height-focus control signal correction parameters based on the scanning of electron beams and the corrected detection height and the deflection distortion correction by using the image distortion correction parameters such as image magnification correction based on the corrected detection height. Furthermore, at the same time, in the step S76, the entirety control unit 120 issues a command to the image processing unit 124 such that the appearance inspection is executed by obtaining SEM images continuously obtained from the image processing unit 124.
In the next step S77, at the stage scanning end position, the entirety control unit 120 displays the inspected result received from the image processing unit 124 on the display 143 or stores the above inspected result in the memory 142. If it is determined at the next step S78 that the inspection is not ended, then a control goes back to the step S72. If it is determined at the step S78 that the inspection is ended, the processing is ended (step S79).
While the SEM apparatus (electron beam apparatus) has been described so far in the above-mentioned embodiments, the present invention may be applied to other converging charged beam apparatus such a converging ion beam apparatus. In that case, the electron gun 101 may be replaced with an ion source. Then, in this case, while the secondary electron detector 104 is not always required, in order to monitor the state manufactured by the ion beams, a secondary electron detector or secondary ion detector may be disposed at the position of the secondary electron detector 104. Further, the present invention may also be applied to manufacturing apparatus of a wide sense which includes a pattern writing apparatus using electron beams. In this case, while the secondary electron detector 104 is not always required, because the main purpose is to utilize the electron beam for writing patterns on the sample 106, the secondary electron detector should preferably be used similarly in order to monitor the processing state or to align the position of the sample.
It is apparent that optical apparatus such as ordinary optical microscope, optical appearance inspection apparatus and optical exposure apparatus may similarly construct an automatic focus mechanism by using the present height detection apparatus if they have a mechanism for controlling a focus position. In the case of apparatus in which a sample is not elevated and lowered in order to achieve the properly-focused state but a focus position of an optical system is changed, such apparatus can receive particularly remarkable effects of characteristics of highly-accurate height detection of wide range achieved by the present height detection apparatus.
According to the present invention, the image distortion caused by the deflection and the aberration of the electron optical system can be reduced, and the decrease of the resolution due to the de-focusing can be reduced so that the quality of the electron beam image (SEM image) can be improved. As a result, the inspection and the measurement of length based on the electron beam image (SEM image) can be executed with high accuracy and with high reliability.
Additionally, according to the present invention, if the height information of the surface of the inspected object detected by the optical height detection apparatus and the correction parameters between the focus control current or the focus control voltage of the electron optical system and the image distortion such as the image magnification error are obtained in advance, then the most clear electron beam image (SEM image) can be obtained from the inspected object without image distortion, and the inspection and the measurement of length based on the electron beam image (SEM image) can be executed with high accuracy and with high reliability.
Further, according to the present invention, in the electron beam system inspection apparatus, since the height of the surface of the inspected object can be detected real time and the electron optical system can be controlled real time, an electron beam image (SEM image) of high resolution without image distortion can be obtained by the continuous movement of the stage, and the inspection can be executed. Hence, an inspection efficiency and its stability can be improved. In addition, an inspection time can be reduced. In particular, the reduction of the inspection time is effective in increasing a diameter when the inspected object is the semiconductor wafer.
Furthermore, according to the present invention, similar effects can be achieved also in observation manufacturing apparatus using converging charged particle beams.
At least a portion (if not all) of the present invention may be practiced as a software invention, implemented in the form of one or more machine-readable medium having stored thereon at least one sequence of instructions that, when executed, causes a machine to effect operations with respect to the invention. With respect to the term “machine”, such term should be construed broadly as encompassing all types of machines, e.g., a non-exhaustive listing including: computing machines, non-computing machines, communication machines, etc. With regard to the term “one or more machine-readable medium”, the sequence of instructions may be embodied on and provided from a single medium, or alternatively, differing ones or portions of the instructions may be embodied on and provided from differing and/or distributed mediums. A “machine-readable medium” includes any mechanism that provides (i.e., stores and/or transmits) information in a form readable by a machine (e.g., a processor, computer, electronic device). Such “machine-readable medium” term should be broadly interpreted as encompassing a broad spectrum of mediums, e.g., a non-exhaustive listing including: electronic medium (read-only memories (ROM), random access memories (RAM), flash cards); magnetic medium (floppy disks, hard disks, magnetic tape, etc.); optical medium (CD-ROMs, DVD-ROMs, etc); electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals); etc.
Method embodiments may be emulated as apparatus embodiments (e.g., as a physical apparatus constructed in a manner effecting the method); apparatus embodiments may be emulated as method embodiments. Still further, embodiments within a scope of the present invention include simplistic level embodiments through system levels embodiments.
In concluding, reference in the specification to “one embodiment”, “an embodiment”, “example embodiment”, etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment or component, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments and/or components. Furthermore, for ease of understanding, certain method procedures may have been delineated as separate procedures; however, these separately delineated procedures should not be construed as necessarily order dependent in their performance, i.e., some procedures may be able to be performed in an alternative ordering, simultaneously, etc.
This concludes the description of the example embodiments. Although the present invention has been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001-202904 | Jul 2001 | JP | national |
| 10-046725 | Feb 1998 | JP | national |
| 9-216604 | Aug 1997 | JP | national |
The present application is a continuation-in-part (CIP) of at least three prior applications, i.e., a first application being Ser. No. 10/114,938 filed 4 Apr. 2002, pending; a second application being Ser. No. 10/851,322 filed 24 May 2004 and issued as U.S. Pat. No. 6,885,012; and a third application being Ser. No. 10/853,225 filed 26 May 2004, pending. The above-noted second application is a continuation application of U.S. application Ser. No. 10/426,702, filed May 1, 2003, which is a continuation of U.S. application Ser. No. 10/012,400, filed Dec. 12, 2001, now U.S. Pat. No. 6,559,459, which is a continuation of U.S. application Ser. No. 09/258,461, filed Feb. 26, 1999, now U.S. Pat. No. 6,335,532, which is a continuation-in-part application of U.S. application Ser. No. 09/132,220, filed Aug. 11, 1998, by some of the inventors herein, now U.S. Pat. No. 6,107,637. The above-noted third application is a continuation of U.S. application Ser. No. 10/012,454, filed Dec. 12, 2001, which is a continuation of U.S. application Ser. No. 09/642,014, filed Aug. 21, 2000, now U.S. Pat. No. 6,333,510, which is a continuation of U.S. application Ser. No. 09/132,220, filed Aug. 11, 1998, now U.S. Pat. No. 6,107,637. The teachings and subject matter of every one of the above-mentioned disclosures is incorporated by reference in its entirety into the present application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10114938 | Apr 2002 | US |
| Child | 11114203 | Apr 2005 | US |
| Parent | 10851322 | May 2004 | US |
| Child | 11114203 | Apr 2005 | US |
| Parent | 10853225 | May 2004 | US |
| Child | 11114203 | Apr 2005 | US |
