Abstract:
A projection eyepiece and method for aligning pattern areas on a substrate surface having a micro-optical device on an opposite surface side of the substrate is disclosed. The projection eyepiece enables projection of a reticle image onto a first surface of a substrate, enabling receipt of a reflection of that reticle image from a micro-optical device located on a second and opposing surface of the substrate, and enabling comparison of the projected and received image to determine alignment of the point of incidence on the first surface with the micro-optical device of the second surface. The projection eyepiece therefore determines alignment of pattern areas on opposing substrate surfaces by comparing a projected reticle image to a reflection of that projected reticle image.

Description:
PRIORITY INFORMATION 
     This application is a continuing application and claims priority under 35 U.S.C. §120 based on the U.S. patent application Ser. No. 09/196,784 filed Nov. 20, 1998 by the same inventor, now U.S. Pat. No. 6,222,198, which application was entitled “A System and Method for Aligning Pattern Areas on Opposing Substrate Surfaces”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a projection eyepiece and method for aligning pattern areas on a substrate surface having a micro-optical device on an opposite surface side of the substrate. The projection eyepiece enables projection of a reticle image onto a first surface of a substrate, enabling receipt of a reflection of that reticle image from a micro-optical device located on a second and opposing surface of the substrate, and enabling comparison of the projected and received image to determine alignment of the point of incidence on the first surface with the micro-optical device on the second surface. 
     2. Description of the Related Art 
     Optical devices fabricated using photolithographic technology often require precise alignment of devices on both sides of a single substrate. For instance, it is sometimes necessary to etch optical lenslets, alignment marks, detectors or other devices into both sides of a thick (several millimeter) substrate, and to obtain a precise lateral arrangement of devices positioned on one side of the substrate with corresponding devices positioned on the opposite side of the substrate. Such precise alignment is difficult to achieve, particularly when the substrate is too thick for the mask aligner microscope or the substrate is opaque to visible light. 
     FIGS. 1A-1B illustrate how a conventional mask aligner (either visible or infrared) is used to align devices on opposite sides of a substrate, FIG. 1A showing the mask aligner focused on the distal (lower) substrate surface and FIG. 1B showing focus on the proximate (upper) substrate surface. More specifically, the microscope objective  11  of the mask aligner is positioned above the mask  12  and substrate  13 . The mask pattern  15  is positioned on the lower surface of the mask and in contact with the photoresist coated on top of substrate  13 . An alignment mark  14  has been previously etched into the lower surface of a substrate. 
     The mask aligner is designed to align an alignment mark  15  of mask  12  with the alignment mark  14  positioned on the lower opposing surface of substrate  13 , so that the mask pattern can be transferred into the photoresist on the top surface of substrate  13 . To achieve alignment, the microscope objective  11  of the mask aligner is alternatingly focused on the top and bottom alignment marks  14  and  15  by translating the microscope objective  11  perpendicular to the surface of substrate  13 . 
     The distance that the microscope objective must be translated is equivalent to the thickness W 1  of the substrate  13  divided by the index of refraction n of the substrate  13  (e.g., n=1.5). For instance, the microscope is first centered on the lower alignment mark  14 , often with the aid of a reticle or cross hair in the eyepiece of the microscope. The microscope is then vertically translated to focus on the top or photoresist surface of the substrate, where the mask is moved laterally to center its alignment mark in the field of view of the microscope. After exposing and developing the photoresist, the substrate is etched to transfer the pattern from the photoresist into the surface of the substrate. 
     To achieve accurate top-to-bottom alignment using a conventional mask aligner, as described, the microscope must be precisely translated in a direction perpendicular to the surfaces of the substrate. If the microscope is not translated perpendicular to the surfaces, a lateral change in position of the microscope will result, causing the two patterns on the opposite surfaces to be misaligned. 
     Conventional mask aligners are not generally designed for precise perpendicular translation of the microscope body. Rather, the normal wobble and straightness of travel tolerances in mask aligner microscope translation stages is large enough to introduce several microns of lateral error in the alignment. In fact, recent experiments using a state-of-the-art conventional mask aligner showed more than twenty (20) microns of lateral alignment error between the patterns placed on opposite surfaces of a typical substrate. Consequently, conventional mask aligners of this type are susceptible to error. 
     Another conventional system used to achieve front-to-back alignment involves two video cameras used to focus upon the alignment marks positioned on opposite sides of the substrate, the two images from the cameras being superimposed electronically to show lateral alignment of the two marks. However, use of this system to align substrates of different thicknesses is limited, since the system must be calibrated for a fixed substrate thickness using a calibration plate which has alignment marks precisely placed on both sides of the plate by the manufacturer of the mask aligner. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method that substantially obviates one or more of the problems experienced due to the above and other limitations and disadvantages of the conventional art. 
     An object of the present invention is to provide a projection eyepiece and method for aligning pattern areas on opposing substrate surfaces with improved accuracy. 
     Other and further objects, features, and advantages of the present invention will be set forth in the description that follows, and in part will become apparent from the detailed description, or may be learned from the practice of this invention. 
     To achieve these and other objects, features, and advantages in accordance with the purpose of the present invention as embodied and broadly described, the present invention includes a protection eyepiece device that detects alignment between positions on opposing surfaces of a substrate having a reflective surface on at least a portion of one surface side, the eyepiece including a reticle source structured and arranged to project a reticle image toward the substrate, and a detection device structured and arranged to receive a reflection of the reticle image from the substrate and to determine alignment of the positions on opposing surfaces of the substrate based on the received reflection. The reticle source is structured and arranged to project the reticle image with a focal point on a first side of the substrate, the reflection of the reticle image generally being received by the detection device from a second and opposing surface of the substrate. The reticle source generally includes a source capable of projecting light and a reticle position to receive the projected light from the source, the reticle being structured and arranged such that light therefrom forms a reticle image. The detection device generally includes a plane upon which the received reflection is compared with at least one of the projected reticle image and a representation of the projected reticle image. A second reticle is sometimes used to generate the representation of the projected reticle image on the plane with which the received reflection is compared to determine alignment. The reticle source may further include a beam splitter on which the reticle image is instant, the beam splitter being structured and arranged to split the instant reticle image such that the reticle image is projected toward the substrate and toward the detection device. The reticle device may alternatively include a polarization sensitive beam splitter upon which the reticle image is incident, the beam splitter being structured and arranged to reflect instant light of a first predetermined polarity and pass instant light of a second predetermined polarity, and a linear polarizer positioned to receive light from the source, the linear polarizer being structured and arranged to pass light of the first predetermined polarity such that the light passing through the linear polarizer is reflected by the polarization sensitive beam splitter. The beam splitter is structured and arranged to reflect light of the first predetermined polarity toward the detection device. The detection device generally includes an image perceiving device structured and arranged to receive at least the reflection of the reticle image from the substrate and a second linear polarizer positioned between the reticle and the image perceiving device, the linear polarizer being structured and arranged to block light from the source that passes through the reticle. In the projection eyepiece, the reticle is a reflective reticle that generates the reticle image by reflecting light incident from the beam splitter, the reticle being positioned between the image perceiving device and the beam splitter. The reticle source generally includes a quarter wave plate positioned between the beam splitter and the reflective reticle, and the detection device includes a quarter wave plate positioned between the reflective reticle and the second linear polarizer. 
     When the source is structured and arranged to project polarized light, the reticle source generally includes a non-polarizing beam splitter upon which the polarized light is incident, the beam splitter being structured and arranged to reflect the incident polarized light and to pass light that is orthogonal to the incident polarized light. The detection device generally includes an image perceiving device structured and arranged to receive at least the reflection of the reticle image from the substrate, and a linear polarizer positioned between the reticle and the image perceiving device, the linear polarizer being structured and arranged to block light passing through the reticle from the source. The detection device may also include a quarter wave plate positioned between the beam splitter and the substrate. 
     The source generally includes a light source capable of generating light, a collimator capable of collimating the generated light, and a linear polarizer capable of polarizing the collimated light. The source may alternative include a light source capable of generating light, a focusing lens and diffuser grating capable of focusing the generated light onto reflected positions of the reticle and a linear polarizer capable of polarizing the collimated light. 
     The present invention also includes a method for detecting alignment between positions on opposing surfaces of a substrate having a reflected surface on at least a portion of one surface side. 
     Both the foregoing general description and following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Thus, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention are given by way of example only, since various changes and modifications that are within the spirit and scope of the invention will become apparent to those of ordinary skill in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description below along with the drawings, which are given by way of illustration and thus do not limit the actual implementation of the present invention, wherein: 
     FIGS. 1A-1B illustrate the operation of a conventional mask aligner; 
     FIGS. 2A-2C illustrate a mask aligner according to a first embodiment of the present invention, and show the relationship of that mask aligner to a mask and a substrate having opposing surfaces on which alignment marks will be aligned; 
     FIGS. 3A-3C illustrate projection eyepieces according to second, third and fourth preferred embodiments of this invention; 
     FIGS. 4A-4C illustrate steps of an exemplary process used to align a substrate and mask according to the present invention; 
     FIGS. 5A-5B illustrate the lateral displacement between the two images of the reticle due to misalignment between the patterns on opposite sides of the substrate; and 
     FIG. 6 illustrates the inconsequential effects of a substrate whose surfaces are inclined with respect to the microscope of the mask aligner using this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. In the drawings, redundant description of like elements and processes, which generally are designated with like reference numerals, is omitted for brevity. 
     The following is a detailed description of several preferred projection eyepieces according to the present invention. The projection eyepieces are described as being incorporated into a mask aligner. As such, a description is also provided with respect to a substrate and mask and a process used to align the substrate and mask using a mask aligner incorporating one of the preferred projection eyepieces. 
     The Mask Aligner 
     FIGS. 2A-2B illustrate an exemplary mask aligner in accordance with a first embodiment of the present invention. The mask aligner of FIGS. 2A-2B includes a projection eyepiece  23  and a microscope body  24 . The projection eyepiece  23  shown in FIGS. 2A-2B is similar to that typically used in autocollimation telescopes. 
     The projection eyepiece  23  contains a visible or infrared source  231  which illuminates a source reticle  232 . An image of the source reticle  232  is projected down the microscope body  24  by beam splitter  233  and relay lens  241 . Detection device  234  is typically either a visible or infrared camera focused on reticle  235 . However, detection device  234  may be a conventional microscope eyepiece for human visual operation. 
     In the projection eyepiece  23 , reticles  232  and  235  are aligned with respect to each other such that their positions are mirror images of each other about the reflecting plane of beam splitter  233 . In other words, if an observer were to look back into the eyepiece from the right (e.g. from position  241 ), the observer would observe the two reticles  232  and  235  superimposed on top of each other. Image plane  25  is therefore simultaneously a conjugate image plane of both reticles  232  and  235 . 
     Microscope body  24  includes an arrangement of lenses, including a microscope objective  242  arranged to produce an image of the mask pattern (located at the interface between mask  22  and substrate  21 ) onto the image plane  25 . Together with the projection eyepiece  23 , the microscope body  24  also focuses light from source reticle  232  and forms an image of the source reticle  232  onto the photoresist-coated surface of substrate  21 , which is also located at the interface of mask  22  and substrate  21 . The four conjugate image-object planes of the system include; reticles  232  and  235 , image plane  25 , and the interface between mask  22  and substrate  21 . 
     Generally, image plane  25  of microscope objective  242  is located inside the microscope body  24 . For proper operation, the reticles  232  and  235  must be coincident with image plane  25 , but the beam splitter  233  may prevent physical location of the reticles at image plane  25 . In such a case, a relay lens  241  is preferably included as an attachment to the microscope body  24  (as shown) to reimage the reticles onto image plane  25  of microscope objective  242  with the proper magnification. The light returned from the substrate  21  produces an image at image plane  25  before being reimaged by relay lens  241  onto reticle  235 . The displacement between the image formed at reticle  235  and the actual reticle  235  is used to determine alignment between the mask  22  and substrate  21 . 
     The relay lens  241  may alternatively be included in the projection eyepiece  23  (not shown). Also, relay lens  241  can serve to remove aberrations produced by focusing light through beam splitter  233  if beam splitter  233  is a cube beam splitter. 
     In the preferred embodiment, the distance between lenses  241  and  242 , as well as the specific optical power of relay lens  241 , is defined by the specific optical and mechanical configuration of the mask aligner and the required magnification at camera  234 . The specific optical prescription may therefore be determined by routine lens design or experimentation. Image plane  25  is generally located at a position corresponding to a reticle or cross hair in the conventional eyepiece of a microscope or mask aligner. Furthermore, the substrate  21  and mask  22  may be positioned precisely with respect to the microscope lens using well-established conventional means, such as those customarily used on conventional mask aligners. 
     FIGS. 3A and 3B illustrate projection eyepieces in accordance with a second and third embodiment of the present invention, respectively. The eyepieces shown in FIGS. 3A and 3B can be substituted for the projection eyepiece shown at reference numeral  23  of FIG.  2 A. Unlike the projection eyepiece  23  of FIG. 2A which has two distinct reticles  232  and  235  that must be precisely aligned relative to beam splitter  233 , the projection eyepieces shown in FIGS. 3A and 3B require only one reticle. 
     Specifically, the projection eyepiece  63  of FIG. 3A consists of the following components: a visible or infrared source  631 A, source collimating lens  631 B, linear polarizer  632 , polarization sensitive cube beam splitter  633 , quarter wave plate  638 , reflective reticle  637 , quarter wave plate  636 , linear polarizer  635 , and camera or eyepiece lens  634 . As in FIG. 2A, the projection eyepiece  63  may include a relay lens  241  for the same reasons as discussed earlier. The polarization beam splitter  633  causes s-polarized light to be reflected at the reflecting interface and p-polarized light to be transmitted through it. The linear polarizer  632  is oriented to produce s-polarized light incident on the polarization cube beam splitter  633 . The source light from source  631 A, which passes through source collimating lens  631 B and linear polarizer  632 , is reflected off the interface of beam splitter  633  toward quarter wave plate  638 . Quarter wave plate  638  is oriented 45 degrees with respect to the polarization direction so that the light becomes circularly polarized upon transmission therethrough. Reticle  637  consists of a reflective, metallic cross hair or other reflective reticle pattern with clear surrounding regions, e.g., produced by photolithography and chemical etching. Some of the source light reflects off the metallic regions of the reticle and makes a second pass through quarter wave plate  638 , whereupon the reflected light becomes p-polarized light with respect to the beam splitter  633 . This p-polarized light, which appears to be emanating from reticle  637 , passes through the beam splitter and is imaged by relay lens  241  onto image plane  25 . The circularly polarized source light, which is not reflected at reticle  637 , passes through quarter wave plate  636  where it is converted into p-polarized light. Linear polarizer  635  is oriented to block p-polarized light, preventing this extraneous light from reaching the camera or eyepiece lens  634 . 
     The p-polarized light projected down the microscope body by relay lens  241  is imaged onto the substrate as described previously. When this light is reflected from the substrate, it remains p-polarized. The p-polarized light reflected from the micro-mirrors (not shown in this figure, but shown as  213  in FIG. 2C) again passes through the cube beam splitter  633 , through quarter wave plate  638 , where it is converted to circularly polarized light. The light then passes through the second quarter wave plate  636 , where it is converted into s-polarized light which passes through linear polarizer  635 . Thus the light from the micro-mirrors passes through to the camera or eyepiece lens  634 , but the extraneous light from source  631  is blocked, providing a high signal-to-noise ratio. 
     FIG. 3B shows yet another embodiment of the projection eyepiece of the present invention, which embodiment uses a non-polarizing beam splitter  633 . In FIG. 3B, quarter-wave plates  638  and  636  are replaced with a single quarter wave plate  639  located between the beam splitter  633  and relay lens  241 . Polarized light from the source  631  is therefore again blocked by polarizer  635  which is oriented to pass light only in the orthogonal direction. However, after making two passes through the quarter wave plate, once down the microscope body and again on its return trip, the reflected light becomes polarized in the orthogonal direction, causing it to pass through polarizer  635  to reach the camera or eyepiece lens  634 . 
     FIG. 3C shows an alternative embodiment of the projection eyepiece of FIG. 3A, which embodiment moves the quarter wave plate  638  to a position between the polarization beam splitter  633  and image plane  25 . Reticle  637  is also moved to be coincident with image plane  25 . Reticle  637  is reimaged to position  639  by relay lens  241 . Beam splitter  633  is oriented to reflect s-polarized light from source  631  toward reticle  637 . After two passes through quarter wave plate  638 , the s-polarized light becomes p-polarized light and passes through beam splitter to eyepiece or camera  634 . 
     Yet another alternative embodiment (not shown) replaces collimating lens  631 B of FIGS. 3A-3C with a focusing lens and diffuser grating to focus the majority of the light from source  631 A onto the reflective portions of reticle  637 . 
     From the operator&#39;s perspective, the projection eyepieces of FIGS. 3A and 3B are identical in operation to that of FIG.  2 A. However, as mentioned previously, the projection eyepieces shown in FIGS. 3A and 3B do not require the optical alignment of two distinct reticles to be maintained. 
     Specifically, the projection eyepiece of FIG. 2A requires reticle  232  to be aligned with reticle  235 . If the two reticles are misaligned, their images will not be optically superimposed at the projection eyepiece, nor will their images overlap at image plane  25  when true alignment is realized, causing a systematic mask alignment error. However, it is difficult to attain this alignment, and even more difficult to maintain this alignment with normal handling and wear. Since one physical reticle is used to produce both the source and image comparison reticles, alignment of multiple reticles is not necessary to achieve optical overlap. Thus, the projection eyepieces of FIGS. 3A and 3B each alleviate the need for precise alignment by using the same reticle for both the source and image comparison reticles. 
     Note that in each of the preferred embodiments, the existing eyepiece or camera of a conventional device mask aligner may be replaced with the projection eyepieces of the present invention, such as those shown in FIGS. 2A,  3 A and  3 B. Thus, by adopting the unique design of the above-described projection eyepieces and by including specific optical devices on the substrate, existing commercial mask aligners can be modified to implement this invention. 
     The Substrate 
     The substrate is identified in FIGS. 2B and 2C by reference numeral  21 . Substrate  21  is fabricated from a material that is transparent to the light used to project the reticle image onto the substrate. If the substrate is transmissive to infrared light but not visible light, such as silicon or germanium, then an infrared source and camera are used in the projection eyepiece. By contrast, for substrates which transmit visible light, such as fused silica, gallium phosphide or zinc selenide, a visible source and camera (or microscope eyepiece) are used in the projection eyepiece. Various other materials may also be used for the substrate, so long as they are optically transparent to either visible light or infrared light. 
     Substrate  21  has a first substrate surface  211  and an opposing second substrate surface  212 . First substrate surface  211  is coated with photoresist  23  which contacts mask  22 . The mask pattern lies on the surface of mask  22  that contacts the photoresist  23 . The photoresist layer  23  between the mask pattern of mask  22  and first substrate surface of  211  ranges in thickness from less than one micron to several microns, depending on the application or device to be transferred into the substrate. At least one alignment mark and at least one reflective micro-optical device  213  (hereinafter a “micro-mirror”) are fabricated onto the second substrate surface  212 . Generally, more than one alignment mark and one micro-mirror are used in order to remove both rotational and translational alignment errors. Alignment marks may be positioned anywhere on the second substrate surface  212 , but the position of the alignment marks must be precisely known relative to the position of the micro-mirrors  213  in order for the alignment marks to be used for subsequently fabricating micro-devices on the second substrate surface  212 . Alternatively, if the micro-devices and the micro-mirrors  213  are simultaneously fabricated on surface  212  using the same mask so that no further processing on surface  212  is required, then additional alignment marks other than the micro-mirrors  213  would not be necessary on surface  212 . 
     Micro-mirrors  213  are used to establish reference positions on the first substrate surface  211  that are precisely positioned with respect to reference positions on the second substrate surface  212 . In the simplest embodiment, each micro-mirror  213  is a concave mirror with a radius of curvature equal to the thickness of the substrate  21  and with an optical axis  214  which is perpendicular to the surface of the substrate  21 , as illustrated in FIG.  2 C. In this embodiment, point P, which lies on the optical axis  214 , is simultaneously a conjugate object and image point, and surface  211  is simultaneously a conjugate object and image plane. Therefore, in this embodiment, a point of light focused to the left of P on surface  211  will be imaged to the right of P on surface  211 , and vice versa. Alternate embodiments include micro-mirrors  213  formed of reflective diffractive optical elements, diffraction gratings, or a complex holographic optical elements. However, in each of these embodiments, the micro-mirror  213  collects light from an image formed on the first substrate surface  211  near its optical axis  214 , and focuses the light to form a second image on the first substrate surface  211 . Thus, the micro-mirrors  213  are sensitive to the lateral position of the first image formed on the first substrate surface  211 , such that a first image that is centered on the optical axis of the micro-mirror  213  will coincide with its reflected second image from the micro-mirror  213 . Otherwise, when a first image is not centered on the optical axis of micro-mirror  213 , a detectable lateral displacement will be observed between the first image and its reflected second image. 
     There are a number of different methods for fabricating the micro-mirrors and alignment marks on the second substrate surface  212 , most all of which use some form of photolithography. The micro-mirrors  213  may be etched into the surface of the substrate  21 , or they may be fabricated in a thin film or coating on the second substrate surface  212 . Some of the methods for fabricating micro-optical devices are discussed in  Micro-optics and Lithography,  Maria and Stefan Kufner, VUB University Press, Brussels, Belgium, 1997. 
     The Mask 
     The mask is identified in FIGS. 2B and 2C by reference numeral  22 . Mask  22  is preferably a gray scale mask which is capable of achieving one-step fabrication of the micro-optical device. However, other conventional masks such as chrome masks may be used. Mask  22  is placed in contact with the photoresist-coated substrate  21 . The side of mask  22  in contact with the photoresist  23  contains the mask pattern to be transferred into the photoresist  23 . The mask pattern contains alignment marks which are to be centered on the optical axes of the micro-mirrors  213 . The mask aligner has devices, such as a micrometer adjustment, to precisely translate the mask  22  laterally with respect to the substrate  21 . In addition to the alignment marks for centering the optical axes of the micro-mirrors  213 , the mask  22  may contain patterns for other micro-devices, including additional alignment marks for subsequent fabrication processes. 
     Process for Aligning the Substrate and the Mask 
     The flowcharts of FIGS. 4A-4C show steps in an exemplary process used to align a substrate and mask using a mask aligner having the above-described mask, substrate, and projection eyepiece. In step  31  of FIG. 4A, the cross hair or reticle of the mask aligner microscope is aligned with the optical axis of one of the micro-mirror devices positioned on the distal substrate surface. In this step, the relative positions of the microscope and substrate are changed until coincidence is achieved between the two images of the reticle at image plane  25 , indicating alignment of the microscope with the optical axis of the target micro-mirror device on the substrate. Once the microscope is aligned with the optical axis of the micro-mirror device, an alignment mark on the mask is brought into coincidence with the microscope reticle in step  32 , thereby aligning the mask with the substrate. 
     More specifically, FIG. 4B shows steps in an exemplary process for aligning the microscope with the optical axis of a substrate micro-mirror according to step  31  of FIG. 4A, and FIG. 4B shows steps in an exemplary process for aligning the mask with the substrate according to step  32  of FIG.  4 A. The mask holder and substrate chuck on most all-commercial mask aligners are generally able to roughly align the mask with the substrate within a tolerance of better than 1 mm. Therefore, the optical axis of the micro-mirrors will be within the vicinity (&lt;1 mm) of the corresponding alignment marks on the mask, which are easily found under low power magnification of the microscope. If the substrate is thin enough, the user will also see a blurred outline of the micro-mirror which can be used to help align the microscope on the optical axis of the micro-mirror device. The rough alignment achieved by this method is generally sufficient to detect reflected light from the micro-mirror. 
     In step  311  of FIG. 4B, the microscope of the mask aligner is roughly aligned with the micro-mirror of the substrate. In step  312 , the reticle image projected from the mask aligner is compared with the reticle image reflected from the micro-mirror on image plane  25 . If the images coincide, the microscope is deemed to be aligned with the optical axis of the substrate micro-mirror, and the process proceeds to step  32  for alignment of the mask with the substrate. By contrast, if the images do not coincide, the relative positions of the microscope and substrate are changed in step  313 , and the process is returned to step  312  for an updated comparison of the images. Micrometer or micropositioner devices are generally used to change the relative positions of the microscope or the substrate chuck. 
     In step  321  of FIG. 4C, the mask is coarsely aligned with the substrate. The image of an alignment mark on the mask is then compared with the coincident images produced by the micro-mirror of the substrate in step  322 . If the alignment marks and coincident reticle images are determined to coincide in step  322 , the mask and substrate are deemed aligned in step  324 . However, if the alignment marks are not determined to coincide in step  322 , the relative positions of the mask and microscope/substrate are changed in step  323 , and the process is returned to step  322  to perform an updated comparison of the alignment marks. 
     The processes described with respect to FIGS. 4A-4C can be repeated for at least two widely separated micro-mirrors on the substrate in order to remove both translational and rotational errors between the mask and substrate. 
     FIGS. 5A-5B illustrate the lateral displacement (d) of the two images at image plane  25  when microscope objective  242  is not aligned with the optical axis of micro-mirror  213 . The magnitude of the displacement is equal to twice the alignment error times the magnification of the objective. Thus, using a 40×microscope objective, a 5-micron alignment error will result in a 400-micron displacement between the two reticle images. 
     Unlike the conventional top-to-bottom alignment method of vertically translating the microscope to alternately focus on opposite surfaces of the substrate, the above-described process is relatively stable, even when tilt is experienced between the substrate and microscope. FIG. 6 illustrates an example of a substrate and mask tilted with respect to the microscope, where the degree of the tilt is exaggerated for clarity. 
     In FIG. 6, microscope objective  242  is focused on the optical axis  214  of the micro-mirror  213  at the conjugate object and image planes of the micro-mirror (located at the interface between mask  22  and substrate  21 ). As shown, no displacement occurs between the two images at image plane  25  and no misalignment is detected as a result of the tilt. From this example, one of ordinary skill will appreciate that the system&#39;s capability to perform the top-to-bottom alignment is not affected by the perpendicularity of the microscope with respect to the substrate surface. Rather, alignment can be successfully achieved by the present invention so long as the microscope objective is focused on the optical axis of the micro-mirror at the conjugate object and image planes of the micro-mirror. 
     While this description illustrates what are at present considered to be preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. For instance, although ideal thin paraxial lenses are shown in the figures, it would be readily apparent to those of skill that this invention is not limited to any particular lens prescription. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the present invention without departing from the central scope thereof. Therefore, the present invention should not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention. Rather, the present invention is intended to include all embodiments falling within the scope of the appended claims. 
     Furthermore, the foregoing description and the drawings are regarded by the applicant as including a variety of individually inventive concepts, some of which may lie partially or wholly outside the scope of some or all of the following claims. The fact that the applicant has chosen at the time of filing of the present application to restrict the claimed scope of protection in accordance with the following claims is not to be taken as a disclaimer of alternative inventive concepts that are included in the contents of the application and could be defined by claims differing in scope from the following claims, which different claims may be adopted subsequently during prosecution, for example, for the purposes of a continuation or divisional application.