Patent Application
20090236540
1. Field of the Invention
The present invention relates to an electron microscope and a stage mechanism for an electron microscope, and more particularly, to an electron microscope that are suitable for measuring dimensions of fine patterns of a semiconductor device or observing the fine patterns and a stage mechanism for the electron microscope.
2. Background Art
A scanning electron microscope (SEM) is used in various fields of research and development, and has been applied in a manufacturing field in recent years. In particular, the measurement of dimensions of fine structure or observation of the fine structure, which is performed by a scanning electron microscope, becomes necessary in a process for manufacturing a semiconductor.
The design rule of a semiconductor integrated circuit becomes finer every year, then the width of a pattern reaches 100 nm or less. Accordingly, a length measuring SEM or a review SEM is used to measure dimensions of fine patterns, to observe the shape of the fine patterns, or to observe the defects of foreign materials. For example, the length measuring SEM among them, which is an apparatus used for measuring the width of a circuit pattern, includes an electron optical system that converges an electron beam and scans, a sample chamber and a stage for positioning a wafer, which is a sample, in a vacuum.
Conventionally, a structure using a sliding screw as driving means has been generally employed in the sample stage of the electron microscope for inspecting and measuring a semiconductor. However, in recent years, in order to improve positioning speed and accuracy, and to avoid chemical pollution of a wafer caused by a lubricant oil, there has been proposed a stage using an ultrasonic motor, which is a linear actuator using a piezoceramic actuator (piezo actuator), as a drive source (see Japanese Patent Application Laid-Open (JP-A) No. 3-129653 (corresponding to U.S. Pat. No. 5,149,967) and Japanese Patent No. 3834486).
There are some types as a specific structure of the ultrasonic motor. However, the types of the ultrasonic motor may be broadly classified into (1) a type that generates a surface elastic wave on a surface contacting an object to be slid, and (2) a type that displaces or vibrates a driving part contacting an object to be slid by deformation of an actuator. In general, the type (2) using the deformation of the actuator is suitable for the electron microscope for a semiconductor requiring a large slide stage since being capable of sliding a heavy object at high slide speed.
Further, the type (2) may be classified into a type that uses the resonance of a motor structure and a non-resonant type that does not resonate. An example disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-184382 (corresponding to U.S. Pat. No. 6,064,140) is a type that uses resonance in an ultrasonic motor using the deformation of an actuator, and vibrates a ceramic spacer (driving part) by the combination of the resonance in an expansion and contraction mode of a piezoelectric plate in a longitudinal direction and the resonance in a bend mode, thereby driving an object. In this case, there has been disclosed that a slide direction can be changed by switching a phase relationship between the two kinds of resonance into a positive or negative relationship.
Further, an example of a non-resonant ultrasonic motor structure has been disclosed in Japanese Patent No. 3834486. In this example, two actuators, which can expand and contract and be displaced at an end in a transverse direction, are employed in parallel. Drive voltages are applied so that a phase difference between the respective expansions and contractions and transverse displacements becomes 90°. Accordingly, the end (driving part) is moved in elliptical form, so that the object is driven.
In the electron microscope for inspecting and measuring a semiconductor, an observation portion of a sample is moved to a field of observation view at high speed by a stage, and an electron beam is irradiated onto the observation portion in a vacuum chamber for observation and measurement. Meanwhile, if drift occurs on a sample stage, that is, if the stop position of the sample is minutely deviated with time after the sample is completely positioned while the sample is observed with high magnification, there is a problem in that measurement accuracy deteriorates during the measurement of the dimensions of fine patterns.
In recent years, in the electron microscope for inspecting and measuring a semiconductor, the resolution of a beam reaches about 1.5 nm, and the measurement reproducibility of 0.2 nm has been required for measuring the width of the fine pattern. For this reason, drift needs to be suppressed to 0.5 nm/sec or less during a period of 1 to 2 seconds after the stage is stopped, until the stage starts to move toward the next portion to be measured.
Meanwhile, since an electron beam can be electrically deviated, the required positioning accuracy of the stage is about 1 μm. Even if the positioning error of such degree is generated, the object pattern on the wafer can be positioned at the center of an image by the deviation of the beam prior to the observation and measurement. However, in order to compensate the drift by the deviation of a beam, compensation should be performed in real-time during the observation and measurement. Accordingly, the compensation needs to be performed with high speed and accuracy. As a result, there are problems in that the circuit is complicated and that the electrical resistance against external noise deteriorates.
As described above, a permissible value of drift in positioning accuracy is stricter by 1000 times or more in the electron microscope for inspecting and measuring a semiconductor while such strict performance is not needed in other fields.
In the ultrasonic motor and the stage using the ultrasonic motor, which are disclosed in JP-A Nos. 3-129653 (corresponding to U.S. Pat. No. 5,149,967) and 7-184382 (corresponding to U.S. Pat. No. 6,064,140) and Japanese Patent No. 3834486, positioning can be performed with high speed and accuracy, but the drift has been not considered. Accordingly, if the ultrasonic motor and the stage using the ultrasonic motor are used in the electron microscope for inspecting and measuring a semiconductor, there is a problem in that it is difficult to reduce drift.
An ultrasonic motor using a piezo electric actuator has residual deformation of the piezo electric actuator as a peculiar problem. The residual deformation is a phenomenon where the piezo electric actuator continues to be minutely deformed even though the drive voltage is kept constant after the piezo electric actuator is deformed in quick response to the change of a drive voltage. The residual deformation causes drift. Meanwhile, the magnitude of the residual deformation is generally about 1 μm or less and does not cause a problem in an application that does not strictly require low drift of the stage. However, drift causes a serious problem in the electron microscope for inspecting and measuring a semiconductor where a permissible value of drift is strict as described above.
The object of the present invention is to provide a stage mechanism that has small drift when stopped and an electron microscope apparatus using the stage mechanism.
According to an embodiment of the invention, there is provided a sample stage. The sample stage includes two or more driving actuators, which can expand and contract or slide, and moves a stage by the cooperation of the two driving actuators. With a coordination of the two driving actuators, various controls are available by combining the operations of the two driving actuators. Accordingly, a stage mechanism capable of reducing a stop drift as well as moving a stage can be provided.
According to the embodiment of the invention, it may be possible to achieve a sample stage that has small drift when stopped, and an electron microscope using the sample stage.
In a sample stage according to an embodiment of the invention that is driven by an ultrasonic motor including piezo electric actuators disposed at angles symmetric about a surface to be driven while stopping in positioning, after positioning conditions are satisfied, a phase difference in the variation of a drive voltages applied to the piezo electric actuators is kept for a predetermined time O.
Alternatively, in the same sample stage while stopping in positioning, after positioning conditions are satisfied, a drive voltage is fixed at a time when a phase of the variation of a drive voltage applied to each of the piezo electric actuators becomes a predetermined phase. Further, this cutoff phase is determined according to the positioning stop coordinates and moving direction of the stage immediately before the stop in positioning.
Further, a sample stage includes a piezo electric actuator that presses a drive tip against the surface to be driven, and a piezo electric actuator that moves the drive tip in a driving direction. After positioning conditions are satisfied while stopping in positioning, an applied voltage is fixed at a time when a phase of a voltage applied to the latter piezo electric actuator becomes a predetermined phase. Further, the cutoff phase is a phase corresponding to a point where a deformation response to the applied voltage by the piezo electric actuator intersects with a deformation convergence line of the piezo electric actuator.
In addition, an electron microscope includes these sample stages.
Further, in order to control the sample stage that is driven by the ultrasonic motor, the electron microscope includes a phase difference oscillating circuit that controls a phase difference of a variations of a voltages respectively applied to a pair of the piezo electric actuators of the ultrasonic motor, and a delay circuit and a hold circuit for maintaining the phase difference of the variations of the applied voltages for a predetermined time after positioning conditions are satisfied and then fixing the phase difference. Further, the electron microscope further includes a memory circuit that stores a phase difference value to be added to an input of the phase difference oscillating circuit in association with a moving direction of the stage and a positioning coordinate.
Alternatively, electron microscope includes a similar phase difference oscillating circuit, and a synchronization circuit and a hold circuit for fixing the applied voltages at a time when the applied voltages reach predetermined phases after positioning conditions are satisfied. Further, the electron microscope further includes a memory circuit that stores a phase difference value to be added to an input of the phase difference oscillating circuit and a cutoff phase to be input to the fixing circuit in association with positioning coordinates and a moving direction of the stage.
Further, an electron microscope includes a sample stage that is driven by a linear drive source, such as an ultrasonic motor or a linear motor. The electron microscope has a mechanism, in which steps for (1) positioning the sample stage, and (2) evaluating a stage drag while stopping in positioning are performed for the positioning from both directions of a moving axis at points of a plurality of coordinates that are disposed within a movement stroke of the sample stage, so as to measure the distribution of a stop drag of the stage within the movement stroke and then based on the measurement result, drift during the positioning of the stage is reduced.
Alternatively, an similar electron microscope has a mechanism, in which steps for (1) positioning the sample stage, (2) evaluating drift after stop, and (3) compensating a control parameter of the stage are repeated at points of a plurality of coordinates that are disposed within a movement stroke of the sample stage, so as to reduce drift of the stage within the movement stroke.
In addition, the mechanism for reducing the stop drift may be automatically performed by issuing an operation command.
An embodiment of the invention will be described below with reference to drawings. Meanwhile, a length measuring SEM is exemplified as an aspect of an SEM in the following description, but the invention is not limited thereto. For example, the invention may be applied to a review SEM that has been described above, particularly, a general charged particle beam apparatus that performs fine measurement, inspection, processing, observation, and the like.
The sample chamber 2 is kept in a vacuum state, which corresponds to a vacuum pressure of about 10−4 Pa, by a vacuum pump (not shown) or the like. The sample stage disposed in the sample chamber 2 is a mechanism that moves and positions an arbitrary portion of the wafer to a length measurement position, onto which electron beams are irradiated, at high speed.
The structure of the ultrasonic motor 18 is shown in
Meanwhile, it is preferable that a driving frequency be 20 kHz or more. Like in the case of a known resonant ultrasonic motor, when the drive tip recedes, the contact between the drive tip and the object surface to be fed is not maintained during the vibration in this band. As a result, the drive tip drives the object surface by the transverse displacement speed in an area close to a protruding end of the Lissajous waveform.
According to the above-mentioned disposition, it may be possible to press the drive tip 24 against the drive face or to separate the drive tip from the drive face by the cooperation of the piezo electric actuators 23A and 23B.
The peculiar drift generated by the piezo electric actuator will be described below.
Since the polycrystal orientation of the actuator is rotated due to an electric field that is generated inside the piezo electric actuator by the application of a drive voltage, the deformation of the piezo electric actuator is generated. However, since internal friction is applied to the rotation, hysteresis occurs in an applied voltage-deformation graph. This relationship is shown in
If an applied voltage is fixed at a point C on the loop in
The piezo electric actuators of the ultrasonic motor 18 are provided with an angle therebetween not to be parallel to a pressing direction where the ultrasonic motor is pressed against the surface to be driven. Accordingly, if the piezo electric actuator has the residual deformation after the positioning, displacement is generated in a direction where the drive tip 24 fixing the table moves the table, which causes drift.
However, the piezo electric actuators 23A and 23B are symmetrically disposed. Accordingly, when the same residual deformation is generated, the drive tip 24 generates the displacement only in the pressing direction, so that drift is not generated. If the table reaches near a positioning point in a normal positioning operation, the table is stopped at the positioning point by the position servo. However, since the motor generates a thrust due to a residual friction force in this case, the voltages applied to the piezo electric actuators are not same as in normal case. In this example, after the table reaches near the positioning point, the phase difference between the applied voltages Vp1 and Vp2 is kept at 0° for a short time δ. Then, after the deformation hysteresis of the piezo electric actuators is kept evenly, the drive is cut off and the applied voltages are fixed. Since the voltages applied to the piezo electric actuators 23A and 23B are equal to each other for the time δ, residual deformation becomes equal after drive cutoff. If about several cycles of the driving frequency or 1/100 or less of a normal positioning time is delayed, δ is sufficiently effective. Meanwhile, if driving is performed immediately before the positioning while the phase difference is kept at 0° without employing the position servo unlike the invention, a deviation of about 1 μm or less occurs in the position of the table due to the residual friction force and the like. However, as described above, as for the stage mechanism of the electron microscope that is used to inspect and measure a semiconductor, a permissible value of drift in positioning accuracy is stricter by 1000 times or more. Accordingly, the deterioration of this positioning accuracy does not affect the performance of the apparatus, and it may be possible to achieve the inspection and measurement accuracy caused by the reduction of the drift.
Meanwhile, it may be possible to prevent the piezo electric actuator from being deformed in the slide direction by the above-mentioned method, but residual deformation is generated in the pressing direction perpendicular to the slide direction. However, the support stiffness of the table is high in the direction perpendicular to the slide direction. In this example, the ultrasonic motors 18 are provided on both sides of the table so that the table is provided between the ultrasonic motors. Accordingly, the residual deformation is cut off in the pressing direction, and the movement of the table is not caused.
Another method of reducing the drift of the table in the same apparatus as Example 1 will be described with reference to
In this example, this problem is solved by limiting the timing of the control cutoff. A point A and a point B are shown near a contact point in
In this example, phase values φt1 and φt2 satisfying the above-mentioned condition are previously calculated, and the drift is reduced by performing the drive cutoff of the ultrasonic motor 18 under this condition.
Since the residual friction force is determined depending on the characteristics of a distortion or sliding mechanism of the rail, the residual friction force may have coordinates dependence and directional dependence. For this reason, if the residual friction force varies according to the positioning coordinates or direction when the positioning is performed by the above-mentioned method, drift may not be sufficiently reduced by using the fixed additional phase value φr or cutoff phase φt. Accordingly, it is preferable that the cutoff phase is stored in association with the moving direction and the coordinates to be used during the positioning.
An example of a circuit, which performs this control, is shown in
Meanwhile, the stage coordinate value p may be input momentarily. However, if the stage coordinate value is fixed at a target position at the beginning of the positioning operation, it may be possible to perform a stable control.
This example provides a method of reducing drift when a serially disposed ultrasonic motor is used unlike in Examples 1 and 2.
The same stage as the stage of Example 1 may be used as a stage on which the ultrasonic motor 18 of this example is mounted. Further, the circuit shown in
In this example, the piezo electric actuator 23A, which expands or contracts only in the pressing direction, does not affect the drift in the slide direction. Only the piezo electric actuator 23B, which is sheared in the slide direction, affects the drift in the slide direction. Accordingly, there is a demand for the control that makes the residual deformation of the piezo electric actuator 23B be 0. In
There has been already described in Example 2 or 3 that the additional phase value φr and the cutoff phase φt need to be previously stored in the memory circuit in order to accurately reduce drift by the circuit of
After the procedure corresponding to the positive direction is completed, positioning is performed in a negative direction and storing is performed. The procedures corresponding to both directions are performed for all measurement points, so that storage scanning on the memory circuit is completed.
Meanwhile, the measurement or compensation needs to be performed for each of the X and Y axes in a stage mechanism including two (X and Y) axes. However, it may be possible to achieve the accurate reduction of drift by setting measurement points, which are disposed in the form of grid points, on the X-Y plane, and performing measurement or compensation on each of the points.
It is considered that the stop drag caused by the residual friction force or the like is changed due to the temporal change caused by the abrasion of parts of a sample stage mechanism, or the service such as maintenance. Accordingly, it is preferable that the contents of the memory circuit of
Further, in Examples 1 to 3, there has been described a method of reducing drift in consideration of the residual deformation of the piezo electric actuator of the sample stage using the ultrasonic motors. However, even in the case of other linear drive sources such as a linear motor, drift is generated due to a stop drag that is caused by the residual friction force after the positioning. Accordingly, the reduction of the drift, which is caused by the measurement of the stop drag or the compensation of the control parameter in this example, is available in the case of a sample stage using other linear drive sources. In this case, for example, in the case of the drive of the linear motor, a value of holding current flowing through a field coil may be employed as a control parameter in order to maintain a constant thrust after the positioning.
Meanwhile, the invention has been described herein by an example where the invention is applied to a scanning electron microscope (SEM) for inspecting measuring a wafer (sample). The stage apparatus according to the embodiment of the invention is not limited to an SEM. The invention may also be applied to general charged particle beam apparatuses, such as an electron beam drawing apparatus and an FIB, that include stage devices for picking up a sample and moving the sample in two (X and Y) directions. In addition, the invention is not limited to the charged particle beam apparatus, and may also be applied to an optical inspection apparatus that inspects foreign materials or defects by light scattering. Further, the sample to be held is not limited to a wafer, and may be applied to inspect and measure a sample having fine patterns, such as a reticle for lithography and a mask.
The invention may be suitable for a charged particle beam apparatus such as an electron microscope for inspection and measurement in a field of manufacture of a semiconductor device, and a sample stage mechanism used for the charged particle beam apparatus.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-068660 | Mar 2008 | JP | national |
