1.  Field of the Invention
The present invention relates to a system and method for achieving top-to-bottom mask alignment.  More particularly, the present invention relates to a system and method for aligning pattern areas on a photoresist-coated substrate surface having pattern areas on the opposite surface of the substrate.  The alignment performed by the present invention is preferably based on a reticle image that is formed on the photoresist and reflected from the opposite surface by a micro-optical device installed on that opposite surface. 
2.  Description of the Related Art
Photolithographic technology involves the process of transferring a pattern on a mask into a photosensitive photoresist coated onto a substrate.  The mask is usually a quartz or glass plate with one side coated with a thin opaque chrome layer.  To form a desired pattern on the mask, portions of the chrome layer are precisely removed to form a complex pattern of transparent and opaque areas.  In the microelectronics industry, this pattern represents a microcircuit.  In the micro-optics industry, this pattern represents an optical device such as a lens or diffraction grating.  The goal of photolithography is to precisely transfer the mask pattern into the photoresist which is coated onto the substrate.  The two categories of photolithography generally used in conventional practice are projection lithography and contact lithography.  Projection lithography involves the use of a lens to image the mask pattern onto the surface of the photoresist.  Devices used to achieve projection lithography are called steppers.  Contact lithography involves direct contact between the mask and the photoresist.  Devices used to achieve contact lithography are called mask aligners.  This invention is related to the latter category of contact lithography. 
Once the photoresist on the substrate has been properly exposed to UV light transmitted through the mask pattern, the substrate is removed from the stepper or mask aligner and the photoresist is chemically developed.  Areas of photoresist not exposed to UV light remain on the substrate.  By contrast, areas of photoresist exposed to UV light are removed by the chemical developer, leaving the substrate susceptible to subsequent ion etching in those areas.  After developing the photoresist, the substrate is placed in a reactive ion etcher which bombards the surface with reactive gas ions.  The ions etch the unprotected areas of the substrate and thus transfer the photoresist pattern into the substrate.  After etching to the correct etch depth (usually less than a few microns), the substrate is removed from the etcher and all remaining photoresist is chemically removed. 
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. 
To enable alignment of devices on opposing sides of a substrate which only transmits invisible infrared light (e.g. silicon or germanium substrates), a conventional mask aligner may be equipped with an infrared source and camera.  The infrared source and camera enable the user to "see" through the substrate and alternatingly focus on the alignment marks positioned on both sides of the substrate by translating the microscope perpendicular to the substrate surface.  However, to focus the microscope on the distal surface of the substrate, the working distance of the microscope objective must be greater than the thickness of the substrate and mask plate combined. 
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 e(Y. 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 W1 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. 
However, 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.