Patent Application
20130248731
The present application claims priority from Japanese patent application JP 2012-063816 filed on Mar. 21, 2012, the content of which is hereby incorporated by reference into this application.
The present invention relates to an electron beam application technology, and in particular, to an electron beam apparatus such as an inspection apparatus, a microscope, and so forth, used in a semiconductor process, and a lens array incorporated therein.
In a semiconductor process, use is made of an electron microscope for irradiating an electron beam called a primary beam onto a specimen to thereby make an observation of a pattern, and a structure, formed on the specimen such as a wafer, and so forth from a signal of a secondary electron, a reflection electron, and so forth (hereinafter called a secondary beam), that is, to carry out observation, measurement, inspection, and suchlike. The electron beam apparatus includes, for example, an electron beam measuring apparatus for measuring a shape and a size, an electron beam inspection apparatus for use in the inspection of a pattern formed on a wafer, and so forth.
In these electron microscopes, enhancement in an inspection speed and a measurement speed is an important problem, and various schemes have been proposed in order to solve the problem. For example, in a multi-beam electron inspection apparatus proposed in Japanese Unexamined Patent Application Publication No. 2001-267221, a scheme has been proposed whereby multiple beams formed by splitting a beam with the use of a plate having plural openings are caused to individually focus using lenses arranged in array to thereby form multiple intermediate images, whereupon the plural intermediate images are projected on a specimen using an objective lens, and a deflector, provided in the downstream, to be then scanned.
With the multi-beam electron inspection apparatus described as above, uniformity in beam diameter is one of factors deciding a measurement precision, and an inspection precision. For this reason, there is the need for correcting a curvature of field aberration of an objective lens for use in projecting the plural intermediate images on the specimen. The curvature of field represents a phenomenon in which an image surface projected by a lens is not flat, meaning that if a beam passing through a track close to the center axis is brought to a focus, a beam passing through a track away from the center axis will be out of focus in the optical system of the multi-beam electron inspection apparatus.
In contrast, for example, with an electron beam exposure apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-123599, there has been shown another scheme for correcting the curvature of field aberration. More specifically, the curvature of field of a lens provided in the downstream is found beforehand, and openings provided in at least one plate of electrode among three plates of electrodes composing a lens array is set to have diameters not less than two types so as to correct the curvature of field. By so doing, the image surface of the lens array has a preset curvature, thereby offsetting the curvature of field of the objective lens.
In the case of correcting the curvature of field aberration of an objective lens using, for example, the scheme disclosed in Japanese Unexamined Patent Application Publication No. 2007-123599, a likely problem will be that the diameter of the opening of the lens array cannot be easily changed, and therefore, an optical condition under which the curvature of field can be corrected will be restricted. More specifically, if a magnification of the objective lens is changed after once the lens array is installed in the apparatus, it will be difficult to control the curvature of field of the objective lens so as to match a change in the curvature of field aberration of the objective lens, accompanying a change in the magnification.
As a method for avoiding this problem, it is conceivable to use a lens array where individual voltages can be set to respective electron beams, such as a lens array shown in, for example, Japanese Unexamined Patent Application Publication No. 2001-267221. More specifically, the curvature of a lens array image surface can theoretically be controlled by, for example, individually controlling a voltage for every electron beam in such a way as to match a change in the curvature of field aberration although this is not described in the relevant literature. However, in reality, many technical problems are involved in preparing the lens array described in Japanese Unexamined Patent Application Publication No. 2001-267221. Further, in consideration of many power supplies, and circuits, necessary in order to control voltages applied to the respective electron beams, it can be said that this method has a problem from a cost point of view, as well.
Meanwhile, as a method for avoiding this problem without setting individual voltages to the respective electron beams, adjustment of a voltage applied to the lens array can be cited. This is because an applied voltage can be controlled from outside even after the lens array is installed in the apparatus. However, even if the voltage applied to the lens array disclosed in Japanese Unexamined Patent Application Publication No. 2007-123599 is adjusted, an image forming position of the beam passing through the track close to the center axis, intrinsically posing no problem with the curvature of field, undergoes a change concurrently with a change in the curvature of the lens array image surface. As a result, not only an optical condition on the downstream side of the lens array should be changed again, but also the magnification of an image projected on the specimen, as well, is changed.
The invention has been developed under circumstances described as above, and it is one of objects of the invention to provide both an electron beam apparatus and a lens array, capable of correcting a curvature of field aberration under various optical conditions. The above and other objects, novel features of the present invention will be apparent from the following description and the accompanying drawings.
The gist of a representative means for solving the problem disclosed under the present application is described as follows.
The lens array according to one aspect of the invention is capable of causing multiple electron beams to be individually converged on individual axes, respectively, thereby forming an image-forming surface of the plural electron beams, having a unit for adjusting a shape of the image-forming surface in response to a change in various parameters for setting an optical condition. The relevant unit independently controls an image forming position of one length of electron beam among the plural electron beams, serving as a reference, and a curvature of the image-forming surface.
According to the one aspect of the invention, a curvature of field aberration can be corrected under a variety of optical conditions.
In any of embodiments described hereinafter, the embodiment is divided into plural sections or plural embodiments as necessary for convenience's sake, however, it is to be understood that these sections or these embodiments are not unrelated to each other unless otherwise specified, and one part represents a variation, detail, and supplementary remarks of a part or the whole of the other. Further, with the embodiments described hereinafter, it is to be understood that if the number of elements, and so forth (including the number of pieces, a numerical value, a quantity, a scope, and so forth) are referred to, the number, and so forth be not limited to a specific number, and may be not less than the specific number, or less than the specific number unless otherwise specified, and obviously theoretically limited to the specific number.
Still further, in the embodiments described hereinafter, it is needless to say that constituent elements thereof (including an element step, and so forth) are not necessarily essential unless otherwise specified, and obviously theoretically considered as essential. Similarly, with the embodiments described hereinafter, it is to be understood that if a shape of the constituent element, and so forth, and a positional relationship are referred to, a constituent element that is effectively approximated thereto, or is analogues thereto is included unless otherwise specified, and obviously theoretically considered otherwise. The same can be said of the value and the scope.
Embodiments of the invention are described in detail hereinafter with reference to the drawings. Further, in all the figures for describing the embodiments of the invention, members identical to each other are denoted by like reference numerals, omitting repeated description thereof.
In a microscope for application to a semiconductor process, such as, for example, an electron beam inspection apparatus, an electron beam measuring apparatus, and so forth, the variety of controls as to an optical condition, according to a specimen, are required. Under such circumstances, a lens array according to the related art is unable to independently control an image forming position using a lens close to a center axis, and a curvature of a lens array image surface (a lens array image-forming surface or a crossover image surface), so that it has become difficult to have a desirable optical condition compatible with the correction of the curvature of field aberration. In the first embodiment of the invention, with an eye on this point, it is intended to implement an electron beam apparatus capable of independently controlling the image forming position by the lens close to the center axis, and the curvature of the lens array image surface. As one of specific unit (will be described in detail later on) for implementing the above, there is adopted a configuration where at least four plates of electrodes for forming a lens array are prepared, and an individual voltage can be applied to at least the two plates of the electrodes, respectively. Openings provided in the two plates of the electrodes to which the individual voltage can be applied, respectively, differ in size from each other. The diameter of the opening in at least one plate of the electrode of the two plates of the electrodes is set so as to vary according to a distance from the center axis.
The split primary beams are individually converged by a lens array 110, and 25 pieces of crossover images are formed on a lens array image surface (a lens array image-forming surface, or a crossover image surface) 112. The lens array image surface 112 is a curved surface symmetrical around the center axis as described later on. Reference numerals 111a, 111b, 111c each are the crossover image with respect to each of the 3 lengths of the beams shown in the figure. The 25 lengths of the beams are subjected to a convergence action of the lens array, subsequently forming images on a transfer lens image-forming surface 115 by the respective convergence actions of transfer lenses 113a and 113b.
A Wien filter 114 is provided in the vicinity of the transfer lens image-forming surface 115. The Wien filter 114 causes a magnetic field and an electric field orthogonal to each other to be generated in a plane substantially perpendicular to the center axis to thereby impart a deflection angle corresponding to the energy of a passing electron to the passing electron. With the present embodiment, the intensity of the magnetic field, and the intensity of the electric field are set such that the primary beams travel in a straight line.
Reference numerals 116a, 116b each are an objective lens, being two electromagnetic lenses in pairs. A negative voltage is applied to a specimen 120, and an electric field for causing the primary beams to decelerate is formed between the specimen 120 and a ground electrode 118 connected to a ground voltage. Meanwhile, a surface electric field control electrode 119 is an electrode for adjustment of the intensity of an electric field in the vicinity of the surface of the specimen 120. An electric field generated by the ground electrode 118, the surface electric field control electrode 119, and the specimen 120 acts as an electrostatic lens against the primary beams.
The 25 lengths of the primary beams are subjected to a convergence action of the electrostatic lens, and the respective convergence actions of the objective lenses 116, 116b, whereupon the 25 pieces of the crossover images are finally formed on the specimen 120.
A deflector 117 of an electrostatic octupole type is installed inside the objective lenses. Upon a scan-signal generated from a scan-signal generation circuit 135 being inputted to the deflector 117, substantially uniform deflecting electric fields are formed in the deflector, and the 25 lengths of the primary beams passing through the deflector are subjected to deflection actions in directions substantially identical to each other, and at angles substantially identical to each other, respectively, to scan over the specimen 120. Because the specimen 120 is mounted on a stage 121 movable by a control of a control device 136, desired locations on the specimen are scanned by the 25 lengths of the primary beams, respectively.
The primary beams having reached the surface of the specimen 120 come into mutual actions with a constituent substance of the surface of the specimen. Respective flows of secondary electrons, such as a reflection electron, a secondary electron, an Auger electron, and so forth, generated from the specimen 120, as a result of the mutual actions, are referred to as a secondary beam hereinafter. With the present embodiment, since the 25 lengths of the primary beams reach the surface of the specimen, 25 lengths of the secondary beams are generated, however,
Because the negative voltage has been applied to the specimen, the secondary beams generated from the specimen 120 are accelerated toward the objective lenses 116a, 116b. Thereafter, the secondary beams are subjected to the respective convergence actions of the objective lenses 116, 116b, and are further subjected to a reflection action of the Wien filter 114. By so doing, the tracks of the secondary beams are separated from the tracks of the primary beams, respectively. The secondary beams in the respective tracks separated from the respective tracks of the primary beams are subjected to a convergence action of an electromagnetic lens 123 acting only on the secondary beams. A swing-over deflector 124 is a deflector for causing the secondary beams to always fall on respective detectors corresponding thereto, and a scan-signal in sync with the scan-signal inputted to the deflector 117 is inputted to the swing-over deflector 124 by the scan-signal generation circuit 135. More specifically, the secondary beams (the 3 lengths of the secondary beams shown in
Signals detected by the detectors 125a, 125b, 125c, respectively, are amplified by amplifiers 126a, 126b, and 126c, respectively, to be digitized by an A/D converter 127.
Digitized signals in the form of image data are once stored in a storage 129 inside a system control unit 128. Thereafter, an operation part 130 executes computation of various statistics of an image. Computed statistics are displayed on an image display unit 131. Processes from the detection of the secondary beams up to the computation of the statistics are executed in parallel with each other on a detector-by-detector basis. Further, reference numeral 133 denotes an input unit including a keyboard, and a mouse, serving as the user-interface of the system control unit 128. Further, the condenser lens 107, and the collimator lens 108 are primarily responsible for shaping up an electron beam from the electron gun 101, therefore being called an irradiation optical system, while the transfer lenses 113a, 113b, and the objective lenses 116a, 116b are primarily responsible for projecting the electron beam obtained via the irradiation optical system on the specimen 120, therefore being called a projection optical system.
Next, controls of respective optical elements are described. An optical system control circuit 134 controls the respective optical elements in a unified manner according to a measuring-condition setting program 132 installed in the system control unit 128. More specifically, the optical system control circuit 134 controls a voltage applied to an extraction electrode (not shown) mounted in the electron gun 101, an acceleration voltage of the electron gun (a voltage applied between the cathode 102 and the anode 105), and a current to be applied to the electromagnetic lens 104 for superimposing the magnetic field inside the electron gun. Further, the optical system control circuit 134 controls respective currents applied to the condenser lens 107, and the collimator lens 108, and a voltage applied to the lens array 110. Still further, the optical system control circuit 134 controls respective currents applied to the transfer lenses 113a, 113b, and the objective lenses 116a, 116b. Yet further, the optical system control circuit 134 controls respective voltages applied to the ground electrode 118, and the surface electric field control electrode 119. Further, the optical system control circuit 134 controls a voltage as well as a current applied to the Wien filter 114. Furthermore, the optical system control circuit 134 controls a current applied to the electromagnetic lens 123.
Now, the gist of the correction of a curvature of field aberration is described with reference to
In contrast,
Now, referring to
Further, even in the case of using the scheme for correcting the curvature of field aberration as shown in Japanese Unexamined Patent Application Publication No. 2007-123599, the curvature of field aberration is found by assuming a specific magnification, and on the basis of the curvature of field aberration, the respective diameters of openings in the lens array 110 are adjusted, so that if a magnification differs from the assumed magnification, it will be difficult to carry out an optimum correction. For example, an excessive correction occurs as shown in
In contrast, with the lens array 110 according to the first embodiment, the curvature of the image surface of the lens array 110 is optimally controlled such that even if the magnification of a zoom lens is changed, the curvature of field on the specimen is minimized. More specifically, by installing the lens array 110 capable of adjusting as appropriate the curvature of the lens array image surface (the lens array image-forming surface, or the crossover image surface) so as to match a change in the curvature of field aberration, accompanying a change in strength, and so forth of the lenses 202, 203, respectively, as shown in
An example of the configuration of the lens array according to the first embodiment is described with reference to
The lens array shown in
The respective diameters of the openings in the second electrode are all equal, as shown in
Next, referring to
In order to execute such a control as described, it need only be sufficient to control respective strengths of the lenses 401, 402, as follows. In
Meanwhile,
Thus, the respective diameters of the openings in the second electrode (302, 401) of the lens array comprised of the four plates of the electrodes is varied in distribution from the respective diameters of the openings in the third electrode (303,402), and the voltage V1 to be applied to the second electrode 302, and the voltage V2 to be applied to the third electrode 303 are controlled as appropriate, whereupon the curvature of the lens array image surface, and the image forming position of the lens close to the center axis can be independently controlled. More specifically, in this example, the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface) is controlled by the third electrode, and V2, and the image forming position of the lens close to the center axis is controlled by the second electrode, and V1. By so doing, even in the case of changing a variety of the optical conditions (a focal distance, and so forth, with respect to other lenses of the electron beam apparatus), a lens array image surface corresponding thereto can be set as appropriate, so that it is possible to constantly minimize the curvature of field aberration on a specimen.
In the first embodiment, at the time of adjustment of the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface), the voltages V1, and V2 are controlled in such a way as to keep the image forming position of the center beam (c) to remain constant. However, since the essence of the first embodiment lies in that the two parameters, that is, the image forming position of the lens close to the center axis, and the curvature of the lens array image surface are controlled by adjusting the two voltages (V1, V2), the image forming position of the center beam (c) need not necessarily be constant, and can be adjusted to a desired value as necessary.
In the first embodiment, the respective diameters of the openings corresponding to all the beams are set identical to each other with respect to the second electrode of the lens array comprised of the four plates of the electrodes. However, the principle behind the lens array according to the first embodiment lies in the control of the lens strength distribution through independent control of respective voltages applied to the two plates of the electrodes differing from each other in terms of a distribution of the respective diameters of the openings, and therefore, only if the second electrode differs in the diameter of the opening from the third electrode, the same effect can be obtained.
Further, with the first embodiment, in order to cause the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface) to be inverted from the curvature of field of the lens provided in the downstream, that is, to be convex upward, the opening in the third electrode is formed such that the further the opening is away from the center, the larger the diameter of the opening is. However, for example, in the case where the curvature of the lens array image surface is directed so as to be convex downward, use may be made of an electrode having openings, the diameter of each of the openings decreasing in size as the opening is further away from the center. Further, if the diameter of each of the openings in, for example, the second electrode increases in size as the opening is further away from the center, and conversely, if the diameter of each of the openings in the third electrode decreases in size, contrary to the case of the second electrode, as the opening is further away from the center, this will enable the curvature of the lens array image surface to be controlled with greater accuracy.
Still further, with the first embodiment, because the lenses in the downstream are the electromagnetic lenses that are rotationally symmetrical, the curvature of field aberration as well is rotationally symmetrical. However, there can be the case where the respective curvatures of field aberration of the lenses in the downstream are not rotationally symmetrical, including the case of using a lens that is non-rotationally symmetrical, such as a quadrupole lens, an octupole lens, and so forth. In such a case, the same effect can be obtained by varying the distribution of the respective diameters of the openings, in the lens array, according to respective azimuths instead of varying the same according to only the distance from the center axis. Further, with the first embodiment, adjustment of the respective magnifications of the objective lenses 116a, 116b is intended, and a scheme for correcting a change in the curvature of field aberration, accompanying the adjustment, is described. However, even in the case of intending to change energy of the primary beam falling on a specimen, and in the case of intending to change the intensity of an electric field in the vicinity of a specimen surface, the first embodiment is effective as a unit for correcting a change in the curvature of field aberration, accompanying those actions.
Furthermore, the scheme for controlling the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface) according to the first embodiment is effective as a unit for correcting a spherical aberration in the lenses in the upstream of the lens array. This is described with reference to
The spherical aberration is a phenomenon in which a beam on a track departing from a point on an optical axis does not form an image at one point on an image surface.
Further, in
Next, a procedure for setting an optical condition of the electron beam apparatus according to the first embodiment is described with reference to the example of the schematic configuration shown in
In step S602, the measuring-condition setting program 132 installed in the system control unit 128 decides parameters of the respective optical elements on the basis of the measurement condition set in the step S601. The parameters include, for example, the magnification of the condenser lens 107, the focal distance of the collimator lens 108, the respective magnifications of the transfer lenses 113a, 113b, the respective magnifications of the objective lenses 116a, 116b, the voltage applied to the surface electric field control electrode 119, the focal distance of the electromagnetic lens 123, and so forth. Further, the parameters include the acceleration voltage of the electron gun, both a current and a voltage that are applied to the Wien filter 114, and so forth.
In step S603, the optical system control circuit 134 sets voltage•current to be applied to the respective optical elements on the basis of the parameters set in the step S602, under control of the measuring-condition setting program 132.
In step S604, the measuring-condition setting program 132 refers to a relationship between pre-inputted magnifications of the respective lenses, and curvatures of field thereof, thereby calculating a curvature of field on the specimen 120, predicated on the precondition of the parameters set in the step S602.
In step S605, the measuring-condition setting program 132 calculates an optimum curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface). More specifically, the measuring-condition setting program 132 converts the curvature of field on the specimen 120, found in the step S604, into a curvature of field of the lens array 110 on the basis of respective longitudinal magnifications of the transfer lenses 113a, 113b, and respective longitudinal magnifications of the objective lenses 116a, 116b.
In step S606, the measuring-condition setting program 132 decides the voltage V1 to be applied to the second electrode of the lens array 110, and the voltage V2 to be applied to the third electrode of the lens array 110, as shown in
In step S607, the measuring-condition setting program 132 determines whether or not setting of the voltage of the lens array 110 can be implemented from the viewpoint of resistance to a voltage difference. The lens array is comprised of the four plates of the electrodes, as previously described, and various voltages are applied to the respective electrodes, thereby causing occurrence of a lens action. An insulating member is sandwiched between the adjacent electrodes in the four plates of the electrodes, and if a voltage difference exceeds a predetermined value, this will raise the risk that an electrical discharge occurs to thereby impair the function of the lens, and break down the lens array, or a power supply. Accordingly, it is necessary to impose a limitation to the respective absolute values of the voltages V1, V2 and a voltage difference between the voltages V1 and V2.
For example, a diagonally shaded region in
In step the S608, the optical system control circuit 134 sets the voltage V1, and the voltage V2, decided in the step 606, to the second electrode of the lens array 110, and the third electrode of the lens array 110, respectively, under control of the measuring-condition setting program 132.
In step S609, the electron beam apparatus measures the image forming position with respect to the respective beams under control of the measuring-condition setting program 132, thereby measuring the curvature of field on the specimen 120. A calibration mark (not shown in
In step S610, the measuring-condition setting program 132 determines whether or not the curvature of field measured in the step S609 is within tolerance. If it is determined that the curvature of field is outside the tolerance, the processing reverts to the step 605, thereby re-calculating the optimum curvature of the lens array image surface (the lens array image-forming surface is or the crossover image surface). If it is determined that the curvature of field is within the tolerance, this indicates completion of the setting of the optical condition, whereupon measurement of the specimen 120 is started in step S611.
Adoption of the flow chart described as above enables the correction of the curvature of field aberration to be executed so as to correspond to various optical conditions. Furthermore, in this case, protection of the lens array can be achieved by taking the resistance to the voltage difference with respect to the lens array 110 into consideration, and the correction of the curvature of field aberration can be implemented with higher precision by verifying whether or not the respective voltages V1, V2 of the lens array are appropriate on the basis of actual measurement of the curvature of field on the specimen 120. In this case, the measurement of the image forming position, in the step 609, is executed using the calibration mark provided on the stage 121; however, a beam detection unit may be installed at another position in the case where measurement with higher sensitivity is required, and so forth. For example, if an aperture having a sharp end face is provided in the vicinity of the lens array image surface, and the beam having scanned over the aperture is detected by a detector such as a photodiode, a Faraday cup, and so forth, a beam shape on the aperture can be measured by a knife-edge method.
According to the present embodiment, various explanations are given hereinabove on the assumption that the electron beam measuring apparatus is one example of the electron beam apparatus, however, the present invention can be similarly applied to all the apparatuses having an electron optical system using a lens array capable of causing multiple beams to be individually converged, thereby obtaining the same advantageous effects. More specifically, the invention can be applied to, for example, an inspection apparatus for examining the presence or absence of a defect in a pattern formed on a specimen, an electron microscope such as a review SEM for observing a defect in a pattern formed on a specimen, and so forth. Furthermore, the invention can be applied to, for example, an electron bean imaging apparatus with an electron microscope applied thereto.
The second lens array 805 as well is comprised of 3 plates of electrodes, including a first electrode 806, a second electrode 807, and a third is electrode 808, provided in this order from the upstream side (the side of the lens array, adjacent to the electron gun). The respective electrodes have 25 pieces of openings formed therein. The respective openings are circular in shape, and the respective openings in each of the electrodes are disposed such that a beam axis of each of the 25 lengths of the beams, indicated by a solid line in the figure, penetrates through the center of the opening. A common voltage (in this case, the ground voltage) is connected to the first electrode 806, and the third electrode 808, respectively, and the voltage V1 from the power supply is supplied to the second electrode 807.
This configuration example can be regarded as a configuration of a lens array, made up by splitting the lens array shown in
The merit of the lens array being split into the two units lies in that an aligner (not shown) can be installed between the two lens array units. More specifically, the track of the beam can be corrected even in the case where misalignment occurs at the time of assembling the two lens array units with each other, so that the plurality of the beams can be excellently converged.
The respective diameters of 25 pieces of the openings are all equal with respect to the first, third, and fifth electrodes (901, 903, 905, respectively, as is the case with the configuration example shown in
Next, referring to
Since the openings in the second electrode (902) are formed such that the further the opening is away from the center of the array, the larger the diameter of the opening is, as described with reference to
With the third embodiment of the invention, as well, the two parameters, that is, the image forming position of the lens close to the center axis, and the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface) can be independently controlled by adjusting the two voltages (V1, V2), as is the case with the first embodiment. With the third embodiment of the invention, in addition to this, a lens principal plane can be independently controlled. Herein, the lens principal plane represents the center of gravity of lens strength on a track passed by one length of a beam. Variation of the lens principal plane will cause variation in a beam spread angle on the image surface, whereupon defocusing (aberration) of a beam undergoes variation, thereby raising the risk of causing variation in the diameter of the beam, on the specimen. Meanwhile, with the third embodiment of the invention, because the lens array is made up so as to be vertically symmetrical about the third electrode 903, as shown in
Further, in
Further, the principle behind the present embodiment lies in that respective voltages applied to the two plates of the electrodes differing from each other in terms of distribution of the respective diameters of the openings are independently controlled in the lens array having a vertically symmetrical structure, thereby controlling the lens strength distribution, and therefore, only if the second electrode is identical in the diameter of the opening to the fourth electrode, and the third electrode differs in the diameter of the opening from both the second and fourth electrodes, the same effect can be obtained.
Further, the essence of the present embodiment lies in that the lens principal plane is always kept constant, so that even if the second electrode differs in electrode diameter from the fourth electrode, the same effect can be obtained provided that the lens strength distribution shown in
With the present embodiment, the lens array is made up in order to cause the lens principal plane to be kept constant against all the beams. However, if the voltages V1, V2, V3 to be applied to the second electrode 902, the third electrode 903, and the fourth electrode 904, respectively, are individually controlled, as shown in
{Gist of a Scheme for Correction of the Curvature of Field Aberration (Application Example [1])}
In the respective cases of the first, and second embodiments described hereinabove, the curvature of field aberration as the target of correction is static, that is, is constant time-wise, and accordingly, the voltage applied to the lens array is a DC voltage which is constant time-wise too. With a fourth embodiment of the invention, dynamic correction of a change in the curvature of field aberration, accompanying scanning over a specimen, with the beam. In this case, explanation is given by taking the electron measuring apparatus as an example of the electron beam apparatus, as is the case with the first embodiment, however, it is to be pointed out that the invention is particularly effective in both the electron beam inspection apparatus, and the electron beam exposure apparatus, having a wide beam scanning scope on a specimen.
In this case, scanning over a specimen 120, with a beam, is executed by the deflector 117 installed inside the objective lens 116a, 116b, as described in the first embodiment, with reference to
Upon a scan-signal generated from the scan-signal generation circuit 135 being inputted to the deflector 117, substantially uniform deflection electric fields are formed in the deflector, and the primary beams passing through the deflector are deflected. At this point in time, a deflection curvature of field aberration, and a hybrid curvature of field aberration are included in an aberration occurring as a result of deflection. Because the deflection curvature of field aberration, among those aberrations, occurs in common with all the beams, there is provided a dynamic focus lens (not shown) that acts in common with all the beams of the projection optical system, and a voltage or a current is supplied thereto in sync with the deflection, whereupon the deflection curvature of field aberration can be corrected. On the other hand, the hybrid curvature of field aberration is decided by both a position vector, and a deflection vector, so that the hybrid curvature of field aberration cannot be corrected by the dynamic focus lens. Accordingly, with the fourth embodiment of the invention, a voltage in sync with the deflection is supplied to the lens array, thereby executing the dynamic correction of the curvature of field aberration.
Herein, the principle behind the correction of the hybrid curvature of field aberration is described hereinafter. The hybrid curvature of field aberration can be represented by a formula A×M×R×cos(α−θ+φ) where R=a distance between the primary beam and the center beam, θ=an azimuth, M=a deflection distance, φ=an azimuth, A=the absolute value of a hybrid curvature of field aberration, and α=an azimuth. In this case, the hybrid curvature of field aberration will be at the maximum if α−θ+φ=0, will be zero if α−θ+φ=90°, and will be at the minimum if α−θ+φ=180°. If the case of the azimuth α=zero is assumed for brevity, the curvature of field aberration will be at the maximum when the position vector of a beam, and the deflection vector thereof are oriented in the same direction, while the curvature of field aberration will be at the minimum when the position vector of the beam, and the deflection vector thereof are oriented in directions opposite from each other. If a lens array image surface (a lens array image-forming surface or a crossover image surface) is tilted as shown in
In
The respective diameters of 25 pieces of the openings are all equal with respect to the first, and fourth electrodes (1101, 1104), respectively, as is the case with
With the fourth embodiment of the invention, there has been described only the correction of the curvature of field aberration, accompanying the scanning over a specimen, with the beam, however, in reality, if the correction of the static curvature of field aberration described in the first to the third embodiments, respectively, or the correction of the deflection curvature of field aberration, using dynamic focus lens is combined with the former, this will enable a curvature of field aberration to be more suitably corrected. More specifically, for example, the lens array shown in
With a fifth embodiment of the invention, there is described an example in which the scheme for controlling the curvature of the lens array image surface (the lens array image-forming surface or the crossover image surface), as described in the foregoing, is applied to a reflection electron-beam imaging apparatus. The reflection electron-beam imaging apparatus is an imaging apparatus where electron beams in a shape corresponding to a pattern to be rendered are reflected using a reflecting mirror capable of controlling reflection/absorption on a pixel-by-pixel basis, and the electron beams each are focused in reduced size, thereby rendering a desired pattern on a wafer. The reflecting mirror is provided with an array of micro-electrodes, thereby controlling reflection/absorption on the pixel-by-pixel basis by controlling voltages applied to the respective micro-electrodes.
More specifically, the reflecting mirror is comprised of a lens array, and respective units of a pattern generator 1205, as shown in
A positive voltage or a negative voltage, according to the pattern to be rendered, is applied to the respective micro-electrodes. If a negative voltage greater in energy than the incident beam is applied, the incident beam is reflected. Conversely, if a positive voltage is applied, the incident beam is absorbed by the micro-electrode. Voltages applied to the respective micro-electrodes are controlled by a pattern-generator control circuit 1207. Reflected beams reach onto a wafer via a contraction optical system (not shown) is provided on an upper side in plane of the figure.
Even with the electron beam apparatus of such a reflection type described as above, the curvature of field aberration can pose a problem. More specifically, the beam reflected by the reflecting mirror has an areal spread, the image forming position of the beam passing through the track close to the center axis, on the wafer, ends up differing from that of the beam passing through the track away from the center axis, due to the curvature of field aberration of the contraction optical system, at the time when the electron beams each are focused in reduced size on the wafer. With the fifth embodiment of the invention, the respective diameters of the openings in any one of the lens electrodes 1202, 1203, 1204 are set so as to vary according to a distance from the center axis of the contraction optical system in order to prevent occurrence of such a situation described as above. Further, the image forming position of the beam close to the center axis, and the curvature of the lens array image surface are independently controlled by controlling respective voltages applied to the lens electrodes 1201 to 1204.
Having specifically described the invention developed by the inventor, et al. with reference to the embodiments, as above, it is our intention that the invention be not limited thereto, and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-063816 | Mar 2012 | JP | national |
