Abstract:
A system for aligning a mask and a product with high precision, featuring a means for supporting a product in a fixed position and a mask adjacent to the product so that the mask is movable relative to the product. A plurality of computer controlled plungers push the surrounding mask frame into alignment. A computer is used to determine the relative position of the mask and the product prior to alignment; and that, to achieve proper alignment of the mask and the product, and determines the distance that the mask must be moved.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of Ser. No. 09/399,087 filed on Sep. 20, 1999 now U.S. Pat. No. 6,327,034. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention relates to an improved apparatus for automatically and precisely aligning two superimposed objects (i.e., aligning a semiconductor wafer or other substrate coated with photoresist material with a mask), and to a method of aligning objects using the apparatus. 
     BACKGROUND ART 
     In the present day manufacture of integrated circuits, complex circuit patterns are formed on a silicon wafer by photoresist techniques employing a series of contact printings on the wafer. These contacts printings are made from several transparent masks used in succession and in a preselected order. Each successive mask must be accurately aligned with the previous print or prints made on the wafer from the prior masks so that the completed pattern is accurate within a few microns. 
     The alignment of each mask with the wafer may be accomplished manually by manipulation of the mask over the wafer while the operator observes the mask and wafer through a high power microscope. Alignment may be aided by the use of a pair of spaced-apart detection marks or alignment patterns, for example, crosses, bull&#39;s-eyes, or pinpoint alignment holes, formed on each of the wafer and mask, the pair of alignment patterns on the mask being arranged so that they are superimposed over and aligned with the pair of alignment patterns on the wafer when the wafer and mask are properly aligned. 
     Apparatus has been proposed for producing alignment of the mask and wafer automatically, thus relieving the operator of this tedious task. One form of such automatic apparatus is described in U.S. Pat. No. 3,497,705 issued Feb. 24, 1970, to A. J. Adler and entitled “Mask Alignment System Using Radial Patterns And Flying Spot Scanning”. In that system a pair of spaced-apart radial alignment patterns on the wafer is adapted for alignment with a superimposed pair of spaced-apart radial alignment patterns on the transparent mask. The radials of the alignment patterns on the mask are uniformly angularly displaced relative to the radials of the alignment patterns on the wafer when the mask and wafer are properly aligned. A scanning system employing two flying spot scanners scans each of the two pairs of alignment patterns in a circular manner about a center point, measuring the angular distance between the successive radials encountered by the scanning beam. Error signals derived from misalignment of the radials of the alignment patterns on the mask relative to the radials of the alignment patterns on the wafer are utilized to produce relative movement in X, Y, and rotational directions to bring the two pairs of alignment into proper alignment. 
     Karlson et al., in U.S. Pat. No. 4,052,603, published Oct. 4, 1977, discloses a system for aligning a pattern mask and a photoresist-coated substrate which are separated by an optical element which involves a system for correcting misalignment in the x-or y-directions between the mask and the substrate by using stepper motors to move the substrate in the x-direction or in the y-direction relative to the mask. Rotational misalignment is corrected using a separate mechanism involving gears and levers to rotate the mask relative to the substrate. This apparatus involves use of two separate systems simultaneously to achieve successful alignment, and is rather complicated. It would be desirable to have a system which moves the mask relative to a fixed substrate, rather than moving both the mask and the substrate relative to each other. It would also be desirable to achieve rotational motion and motion in the x-direction or in the y-direction relative to the substrate with a single mechanism, rather than with two separate mechanisms. 
     SUMMARY OF THE INVENTION 
     The objects of the invention are to provide an improved apparatus for bringing two products into alignment with high precision, and to provide a method for using the apparatus to successfully align two objects. 
     The invention provides a system for aligning a product, such as a semiconductor wafer or other substrate coated with a photoresist material, and a mask with high precision. The mask has a plurality of detection marks, and the product has a plurality of corresponding detection marks. When the mask and the product are properly aligned, each of the mask detection marks is precisely aligned with the appropriate product detection mark. The system features a product securing surface, where the product securing surface may be made from a transparent having porous sections therethrough adapted to support the product, a means for securing the product to the plate by applying vacuum to the product through the porous sections, and at least two registration pins adapted to fit through registration holes in the product. The invention also features a surface for positioning a mask. Normally, the mask is surrounded by a rectangular mask frame having at least two holes in it. The mask frame is positioned on the mask positioning surface. The mask positioning surface features a holding plate adapted to support the mask frame and at least two alignment pins adapted to fit through alignment holes in the mask frame. In the embodiment described herein, the transparent plate and its porous sections are a recessed portion of the holding plate supporting the mask frame. 
     After the mask frame is placed on the positioning surface, a number of plungers are activated to resiliently press against two perpendicular edges of the mask frame. These plungers serve to prevent inadvertent or accidental movement of the mask frame and, in the embodiment described herein, are three in number. Computer controlled push plates, also three in number in the present contemplated embodiment, provide the necessary movements to align the corresponding detection marks on the mask frame and the product. In the contemplated embodiment one push plate is set opposite one of the plungers and the remaining pair of push plates are disposed opposite the remaining plungers. Given an arbitrary Cartesian coordinate system imposed on the mask frame and product, sat the single push plate and the corresponding plunger represent movement along the x-axis. Thus, it should be clear that movement along the y-axis is accomplished by synchronous movement of both the other push plates in the desired direction. If rotation is required for alignment, all three push plates may be moved simultaneously using mathematic additions of each individual calculated movement distance. 
     The system additionally features an imaging means comprising a video camera and an optical microscope adapted to focus an image of the mask and/or the product onto the imaging plane of the video camera. The video camera records an image of each of the detection marks on the product and an image of the detection marks on the mask, and transmits these images to a computer. 
     The computer then records the position of each of the detection marks on the product; and also records the position of the detection marks on the mask. The computer then compares the positions of the respective detection marks to each other and determines that, to bring the detection marks into alignment with each other the mask needs to be moved in a first direction by a first distance and moved in a second direction by a second distance, and rotated by a defined amount. This can be accomplished be either moving all three simultaneously or by moving them one at a time. It should be noted that the distances involved would be measured in micrometers due to pre-positioning allowed for by the initial positioning pins, as will be discussed further below. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an alignment tool for use in the inventive system with neither the product or the mask in place 
     FIG. 2 shows an alignment tool for use in the inventive system with a product positioned thereon. 
     FIG. 3 shows an alignment tool for use in the inventive system with a mask positioned thereon. 
     FIG. 4 is a side view taken along line  4 — 4  in FIG.  3 . 
     FIG. 5 shows an exploded view of a stage suitable for supporting the tool of FIG.  1 . 
     FIGS. 6 and 7 show two microscopes that may be used in conjunction with the tool of FIG.  1 . 
     FIG. 8 shows a mechanism for adjusting the focus of the microscope. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning first to FIG. 1, the mask alignment portion of the invention is indicated generally at  10 . The stage top plate is indicated at  12 . The product securing surface is indicated at  14  and is disposed between 0.01 and 0.1 inch below the rest of the top plate  12  to prevent the product and the mask from touching one another as they are brought into alignment. The central part of the product securing surface as denoted at  16 , is transparent and, in part at least, is porous, as will be discussed further below. At the outer margins of this porous surface are retractable registration pins  18 . The flat product, such as a green sheet, a semiconductor wafer, or other flat type of product is positioned on the surface  16  by placing the retractable pins  18  through corresponding holes (indicated at  20  in FIG. 2) in the product. The product  22  is seen placed on the surface  16  in FIG.  2 . After the product  22  is placed on the surface  16 , a vacuum is applied from a vacuum chamber  24  through a tube  26 . The product securing surface, as mentioned above, is porous, and in the contemplated embodiment is made from a material such as Porex, a ceramic material that allows the vacuum to be applied while maintaining rigidity. As mentioned, a portion of the surfaces  14  and  16  would be light transmitting that would allow for a ring illuminator, or similar light source to be placed underneath plate  12  and enables the user of the apparatus  10  to see the various detection marks (as will be discussed further below) on the product and the mask. The light could be supplied through a fiber optic conduit such as indicated at  28  in FIGS. 1,  2 , and  3  and will be discussed further below. As mentioned above, the product  22  is seen placed on the securing surface  16  in FIG.  2 . Outside this securing surface  16  is the mask frame support surface  30 . This allows for the placement of the mask  32 , seen placed on the apparatus  10  in FIG.  3 . The mask has a central portion  34  that substantially covers the product  22  and further includes mask frame holes  35 . Note in FIGS. 1 and 2 the dual sets of alignment pins  36  and  38 . Retractable rough alignment pins  36  are seen on opposing corners of the mask frame support surface  30 . On the other set of corners are the fixed fine alignment pins, which are threaded and allow the user to secure the mask  32  by screwing down securing nuts, or the like, indicated at  40  in FIG.  3 . It should be noted that in the Figs. the size of the holes  20 ,  35  on the product and the mask along with the size of the various pins is exaggerated for clarity. Thus, at this point in the process, the product  22  and the mask  32  are generally in alignment with one another and now the process of fine, exact alignment can begin. 
     Note that in FIGS. 1,  2 , and  3  there are seen resilient plungers  42  and push plate  50 . These will now be discussed in more detail. Resilient plungers  42  are preferably run on compressed air supplied by the user but any number of other methods of urging them against mask  32 , as seen in FIG. 3, may be used. They could, for instance, be spring biased. The plungers  42  are held against the mask  32  with enough to force to prevent any inadvertent movement. Note that in the embodiment seen herein, the plungers  42  are arranged with a single plunger on the side indicated at  46  of the mask  32 . Perpendicular and next to side  46  is side  48 . On side  48  there are disposed a pair of the plungers  42 . Opposite the single plunger  42  is a single push plate  50  located on the side of the mask indicated at  52 . In a similar manner, opposite side  48  with its accompanying pair of plungers  42  is side  54 , perpendicular to side  52  and having a pair of push plates, both indicated at  50 . With this arrangement of the plungers  42  and the push plates  50 , the mask  32  and the product  22  may now be brought into precise alignment. Located at the end of each of the push plates  50  are limit switches  56 . These are actuated on contact with the surface of the mask  32  and may be photoelectric switches, differential pressure switches, or spring-actuated switches. In any case, when contact with the edge of the mask  32  is made, power is cut off to the stepper motors  58  that drive the push plates. The stepper motors are preferably electronically driven 0.45 degree motor. This allows a resolution of approximately 0.5 micrometers per step. Please note that the movement of the mask may be accomplished by either moving the push plates forward or by moving them backwards, which allows the resilient plungers to move the mask in the desired direction. 
     A computer, seen in FIGS. 6 and 7 records the position of the push plates when they contact the mask and this allows for calculations that will. be discussed further hereinbelow in regard to the fine alignment process. 
     The discussion now turns to the stage assembly of the apparatus. This is seen in FIGS. 4 and 5 in the drawings. The alignment tool  10  is positioned on the top stage plate  12 . Bottom plate  60  is attached to a tabletop or the like (not shown) and a second plate  62  is then mounted on the bottom plate using linear bearings or the like indicated at  64 . The top plate  12  is then in turn located on the second plate  62  using another set of bearings indicated at  66 . These bearings  64 ,  66  allow the top plate  12  to move in an X-Y direction controlled by the computer to bring the workpiece into the desired position. It is contemplated that stepper motors driving horizontally threaded screws  68 ,  70  would accomplish this. 
     Turning now to FIGS. 6,  7 , and  8 , the optical assembly of the present invention will be discussed. In the Figs. the microscope, indicated at  72 , can be used with first, as seen in FIG. 6, a light transmissive system, that includes the light source  74 . The light would travel in the line as indicated at A and through the transparent and partially porous surface  16 . The light passes through the objective lens  76  and then the image is focused through the lens system indicated at  78  onto the imaging plane of the video camera  80 . This image is then transmitted to the computer as indicated in the Figures. Turning to FIG. 7, in the case that mask and product could be opaque, they could be illuminated by reflected light directed through the side of the microscope  72  as indicated by line B from a light source seen at  82 . The light beam reflects off a splitter  84 , passes in turn through the objective lens  76 , off of mask and product  32 ,  22 , and then is focused onto the image plane of the camera  80  and transmitted to the computer. Travel to provide focus for the microscope  72  is seen in FIG.  8 . Stepper motor  58  drives vertical threaded screw  86  carrying the microscope  72  up and down as desired (preferably through computer control) by means of the bracket  88 . The distance that the microscope  72  is from product securing surface  14  may be saved in memory so that the computer can return the microscope  72  to the same position in the case of a similar product and mask. 
     The operation of the alignment system will now be described. Initially, the retractable registration pins  18  are in position and the product  22  that includes a plurality of detection marks thereon is positioned on the porous surface on these pins. A vacuum is then applied through the porous surface  16  and the pins  18  are then retracted. By moving the stage top plate  12  by means of the stepper motors and the associated threaded screws  68 ,  70 , one of the aforementioned detection marks is focused on and transmitted to the computer which records the location of this mark in relation to one of the edges of the porous plate  16  or a reference grid imprinted on the porous plate  16 . The various detection marks are brought into the field of view of the microscope, transmitted to the computer, and subsequently recorded. The same process, with the detection marks on the mask, is also performed. The mask  32  is first placed on the retractable rough alignment pins  36  and the fixed fine alignment pins  38 . Pins  36  are then retracted. After the computer has recorded the position of the respective alignment marks on the product  22  and the mask  32 , the positions are compared and, using analytic geometry, a calculation is made to determine what movements are necessary to achieve the desired alignment between the mask  32  and the product  22 . By using the stepper motors  58  and their associated push plates  50 , which are secured against the edges of the mask  32 , along with the plungers  42 , the mask  32  may be moved in a X or Y direction (arbitrarily arrived at by the imposition of a Cartesian coordinate plane onto the securing surface  14 ) or the mask  32  may be rotated by moving two or three of the push plates  50  simultaneously. As explained above, the resilient plungers will act as prime movers if the stepper motors  58  are used to move the push plates  50  away from the edges of the mask  32 . Finally, the nuts  40  are tightened, securing the mask  32  alignment with the product  22 . 
     Thus, the instant invention is useful in various capacities in materials science such as semiconductor technology.