Abstract:
A system for of aligning a mask to a substrate comprising: a fixture for holding the mask and the substrate in fixed positions relative to each other; means for holding the substrate, the means for holding the substrate protruding through openings in a table and the fixture, the means for holding fixedly mounted on a stage, the stage moveable in first and second directions and rotatable about an axis relative to the table; means for affixing the fixture containing the mask and the substrate to the table; means for controlling the means for temporarily affixing so as to generate a uniform force around a perimeter of the fixture to effectuate the temporarily affixing; means for aligning the mask to the substrate, the means for aligning controlling movement of the stage in the first and second directions and rotation about the axis; and means for fastening the fixture together.

Description:
This application is a continuation of U.S. patent application Ser. No. 10/604,142 filed on Jun. 27, 2003. now U.S. Pat. No. 7,410,919issued Aug. 12, 2008. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of semiconductor processing; more specifically, it relates to an apparatus and method for aligning a solder bump mask to a substrate. 
   BACKGROUND OF THE INVENTION 
   The formation of solder bumps or controlled collapse chip connection (C 4 ) interconnects on semiconductor substrates requires assembly of an alignment fixture holding the substrate and a metal mask having holes through which the solder bump processes of sputter clean, pad limiting metallurgy evaporation and solder bump evaporation are performed. Prior to these process steps, the mask must be aligned to the substrate. Traditionally, alignment of mask to substrate has been done manually, however as solder bump sizes and spacing between solder bumps has decreased; manual alignment has been shown to be unable to provide the alignment accuracy needed. 
   SUMMARY OF THE INVENTION 
   A first aspect of the present invention is a system for of aligning a mask to a substrate comprising: an alignment fixture for temporarily holding the mask and the substrate in fixed positions relative to each other; means for holding the substrate by a bottom surface, the means for holding the substrate protruding through an opening in a table and an opening in the alignment fixture, the means for holding fixedly mounted on a stage assembly, the stage assembly moveable in first and second directions and rotatable about an axis relative to the table; means for temporarily affixing the alignment fixture containing the mask and the substrate to the table; means for controlling the means for temporarily affixing so as to generate a uniform force around a perimeter of the alignment fixture to effectuate the temporarily affixing; means for aligning the mask to the substrate, the means for aligning controlling movement of the stage assembly in the first and second directions and rotation about the axis; and means for temporarily fastening the alignment fixture together. 
   A second aspect of the present invention is a method for of aligning a mask to a substrate comprising in the order recited: (a) placing a bottom ring of an alignment fixture on an alignment tool; (b) loading a substrate onto a chuck; (c) securing the substrate on the chuck; (d) locating alignment targets on the substrate relative to fixed positions of a first X-Y stage and a rotational stage mounted on the first X-Y stage; (e) placing the mask on the bottom ring and placing a top ring of the alignment fixture on the mask, the top ring aligned to the bottom ring; (f) applying a clamping force of a first predetermined amount of force to the alignment fixture sufficient to prevent the mask from moving relative to the top and bottom rings; (g) locating alignment marks on the mask relative to a fixed position of a second X-Y-stage, the first X-Y stage and the table mounted to the second X-Y stage, X and Y orthogonal displacement directions associated with each of the first and second X-Y stages being co-aligned; (h) calculating an X distance in the X direction and a Y distance in the Y direction to move the first X-Y stage and an angle to rotate the rotational stage through in order to align the alignment marks to the alignment targets; (i) increasing the applied clamping force to a second predetermined amount of force, releasing the substrate from the chuck, and increasing the applied clamping force to a third predetermined amount of force; (j) temporarily fastening the alignment fixture containing the mask and the substrate together; and (k) releasing the applied clamping force. 
   A third aspect of the present invention is a method for of aligning a mask to a substrate comprising in the order recited: (a) providing an alignment fixture for temporarily holding the mask and the substrate in fixed positions relative to each other; (b) providing an alignment tool including a stage assembly and a table; (c) placing a bottom ring of the alignment fixture on the table; (d) securing the substrate on the chuck and locating alignment targets on the substrate relative to a fixed position of the stage assembly; (e) placing the mask on the bottom ring and placing a top ring of the alignment fixture on the mask, the top ring aligned to the bottom ring; (f) applying a affixing force of a first predetermined amount of force to the alignment fixture sufficient to prevent the mask from moving; (g) locating alignment marks on the mask relative to a fixed position of the stage assembly; (h) moving the substrate relative to the mask in order to align the alignment marks to the alignment targets; (i) increasing the applied affixing force to a second predetermined amount of force, releasing the substrate from the chuck, increasing the applied affixing force to a third predetermined amount of force; (j) temporarily fastening the alignment fixture containing the mask and the substrate together; and (k) releasing the applied affixing force. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1A  is a top view of a bottom ring of a substrate to mask alignment fixture for forming interconnects according to the present invention; 
       FIG. 1B  is a cross-section view through line  1 B- 1 B of  FIG. 1A ; 
       FIG. 2  is a top view of an evaporative mask for forming interconnects according to the present invention; 
       FIG. 3A  is a top view of a top ring of the substrate to mask alignment fixture for forming interconnects according to the present invention; 
       FIG. 3B  is a cross-section view through line  3 B- 3 B of  FIG. 3A ; 
       FIG. 4A  is a partial cross-section view through the assembled substrate to mask alignment fixture for forming interconnects according to the present invention; 
       FIG. 4B  is a top view and  FIG. 4C  is a side view of a spring clip illustrated in  FIG. 4A ; 
       FIG. 5  is a top view of an alignment tool according to the present invention; 
       FIG. 6  is a cross-section view through line  6 - 6  of  FIG. 5 ; 
       FIG. 7A  is a cross-section view through an alignment pin mechanism according to the present invention; 
       FIG. 7B  is a diagram illustrating the tolerances between the alignment pin of  FIG. 7A  and a mask; 
       FIG. 8  is a side view of a clamping mechanism according to the present invention; 
       FIG. 9  is a side view of a clipping mechanism, according to the present invention; 
       FIG. 10A  is a top view of a substrate having alignment marks according to the present invention; 
       FIG. 10B  is a top view of a mask having alignment marks according to the present invention; 
       FIG. 10C  is a top view of the substrate and mask alignments are they would appear in perfect alignment; 
       FIG. 11A  is a diagram of initial wafer alignment fiducials prior to mask to wafer alignment according to the present invention; 
       FIG. 11B  is a diagram of initial mask alignment fiducials coordinates prior to mask to wafer alignment according to the present invention; and 
       FIG. 12  is a flowchart of the method for aligning a substrate to a mask according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For the purposes of the present invention the term interconnect is defined as a solder bump or C 4  interconnection that is formed by evaporation onto a substrate through holes formed in a mask. The term substrate is defined to include but is not limited to semiconductor substrates (or wafers) including bulk silicon substrates and silicon-on-insulator (SOI) substrates. The term mask includes but is not limited to metal masks fabricated from molybdenum or other metals. Movement in any of the mutually orthogonal X, Y and Z directions and the rotational Θ direction (as described infra) includes movement in both positive and negative directions. 
     FIG. 1A  is a top view of a bottom ring  100  of a substrate to mask alignment fixture for forming interconnects according to the present invention and  FIG. 1B  is a cross-section view through line  1 B- 1 B of  FIG. 1A . In  FIGS. 1A and 1B , bottom ring  100  includes an outer lip  105  and an inner lip  110  joined by an integral plate  115 . Inner lip  110  defines the extent of an opening  120  centered in bottom ring  100 . Plate  115  includes a multiplicity of openings  125 . Opening  120  provides access for a substrate positioning chuck (see  FIG. 6 ) and openings  125  are for thermal expansion and heat retention control during evaporative processes and to make bottom ring  100  lighter. The difference in height between outer lip  105  and inner lip  110  is H 1  In one example, for a standard 200 mm diameter wafer about 650 microns thick, H 1  is about 0.007 inches. Bottom ring  100  also includes an alignment pin hole  130 A and a diametrically opposed alignment pin slot  130 B, each positioned adjacent to an outer perimeter  135  of the bottom ring. Bottom ring  100  further includes a multiplicity (in the present example 6) of retainer post holes  140  evenly spaced about and adjacent to outer perimeter  135  of the bottom ring. 
     FIG. 2  is a top view of an evaporative mask for forming interconnects according to the present invention. In  FIG. 2 , a circular mask  145  includes a multiplicity of openings  150  arranged in groups  155 . Each group  155  corresponds to a chip on a substrate (not illustrated) that will be placed under mask  145  as illustrated in  FIG. 4  and described infra. Mask  145  also includes an alignment pin hole  160 A and a diametrically opposed alignment pin slot  160 B, each positioned adjacent to an outer perimeter  160  of the mask. Mask  145  further includes a multiplicity (in the present example 6) of retainer post holes  165  evenly spaced about and adjacent to outer perimeter  160  of the mask. 
     FIG. 3A  is a top view of a top ring  175  of the substrate to mask alignment fixture for forming interconnects according to the present invention and  FIG. 3B  is a cross-section view through line  3 B- 3 B of  FIG. 3A . In  FIGS. 3A and 3B , top ring  175  has a lower lip  180  protruding from a flange  185 . Lower lip  180  protrudes a distance H 2 . In one example, for a standard 200 mm diameter wafer having a thickness of about 725 microns, H 2  is about 0.002 inches. Top ring  175  includes an opening  190  centered within top ring  175 . Top ring  175  also includes an alignment pin hole  195 A and a diametrically opposed alignment pin slot  195 B, each positioned adjacent to an outer perimeter  200  of the mask. Top ring  175  further includes a multiplicity (in the present example 6) of retainer posts  205  evenly spaced about and adjacent to outer perimeter  200  of the mask. 
     FIG. 4A  is a partial cross-section view through an assembled substrate to mask alignment fixture  210  for forming interconnects according to the present invention. In  FIG. 4A , only a portion of assembled fixture  210  is illustrated. Substrate  215  and mask  145  are illustrated contained in alignment fixture  210 . Retainer posts  205  protrude through retainer post holes  140  in bottom ring  100  and pass through retainer post holes  165  in mask  145 . When spring clips  220  are slid onto retainer posts  205  perimeter  160  of mask  145  is held in a slightly pressed down position by lower lip  180  of top ring  175  against outer lip  105  of spring clips  220  thus holding substrate  215 , mask  145 , top ring  175  and bottom ring  100  together. Spring clips  220  are not put in place until mask  145  is aligned to substrate  215 . 
   The combination of the difference in heights between outer and inner lips  105  and  110  of bottom ring  100  and lower lip  180  of top ring  175  deflects (or bows) substrate  215  and mask  140  into very shallow but semi-spherical shape by pressing perimeter  160  of mask  145  and perimeter  225  of substrate  215  towards bottom ring  100 . 
   Since alignment fixture  210  is mounted in a dome of a metal evaporator, the bow imparted to substrate  215  prevents or reduces such problems associated with evaporation through an knife edge opening in a mask such as sputter haze, PLM flaring and solder pad haloing. 
     FIG. 4B  is a top view and  FIG. 4C  is a side view of spring clip  220  illustrated in  FIG. 4A . Spring clip  220  includes a notch  230  that engages a lower end  235  of retainer post  205  as illustrated in  FIG. 4A . Spring clip  220  also includes a retraction hole  240  to enable removal of spring clips  205  and thus disassembly of alignment fixture  210  (see  FIG. 4A ).  FIG. 4C  illustrates spring clip  220  before engagement with retaining post  205 . 
     FIG. 5  is a top view of an alignment tool  250  according to the present invention. In  FIG. 5 , alignment tool  250  includes a top plate  255  a chuck  260 , alignment pin mechanisms (illustrated in  FIG. 8A  and described more fully infra), a multiplicity of clamping mechanisms  270  (illustrated in  FIG. 9  and described more fully infra) and a multiplicity of clipping mechanisms  275  (illustrated in  FIG. 10  and described more fully infra). Chuck  260  extends through an opening  280  in top plate  255  and includes a multiplicity of O-rings  285 . Each O-ring surrounds a vacuum port  290  centered within the ring. O-rings  285  are arranged in a ring and located adjacent to a perimeter  295  of chuck  260 . Clamping mechanisms  270  are evenly spaced around a locator ring  300  centered on chuck  260  that roughly defines the position occupied by alignment fixture  210 . Clipping mechanisms  275  are evenly spaced around locator ring  300 . Alignment pin mechanisms (containing alignment pins  305 ) are positioned diametrically opposed adjacent to and interior of locator ring  300 . Attached to an underside of top plate  255  is a fixed inner actuator ring  310  having outwardly protruding spokes  315 . Spokes  315  extend through eccentric slots  350  (not shown in  FIG. 5 , see  FIG. 6 ) in a rotatable outer actuator ring  320 . Outer actuator ring  320  can move up and down in the Z direction (see  FIG. 6 ) as well as rotate in the θ direction (see  FIG. 6 ) about an axis defined by the Z direction. 
   While six clamping mechanisms  270  and six clipping mechanisms  275  are illustrated in  FIG. 5 , any number greater than or equal to three mechanisms of each type may be used. While two alignment pins mechanisms are illustrated in  FIG. 5 , a greater number of alignment pin mechanisms may be employed in which case the number and arrangement of alignment pin holes  130 A and  195 A and alignment pin slots  130 B and  195 B (see  FIGS. 1A and 3A  respectively) will change. 
   While eight O-rings  285  are illustrated in  FIG. 5 , chuck  260  may include a lesser or greater number of O-rings, the minimum number being three. The inventors have discovered that chucks using a conventional single O-ring design can induce errors up to 10 times greater then the accuracy of the encoders because of O-ring distortion during the alignment process. This distortion is caused by the fact that mask  145  (see  FIG. 2 ) and substrate  215  (see  FIG. 4A ) are in slight frictional contact that can cause a single O-ring to distort or creep. The radially orientated multi O-ring design of chuck  260  all but eliminates translation errors caused by O-ring creep. Note, a single O-ring design using a hard O-ring, presents other problems such as substrate breakage, since being less flexible there is insufficient compressibility in the O-ring to absorb acceleration induced shock. 
     FIG. 6  is a cross-section view through line  6 - 6  of  FIG. 5 . In  FIG. 6 , chuck  260  is mounted on an rotational stage  325  for θ adjustment that in turn is mounted on an upper X-Y stage  330  for X direction and Y direction (the Y direction is into plane of the paper) adjustment of the position of substrate  215 . Upper X-Y stage  330  is in turn mounted on a lower X-Y stage  340 . It is preferred, but not necessary that the X movement of upper X-Y stage  330  is perpendicular to the Y movement of lower X-Y stage  340 , the Y movement of upper X-Y stage  330  is perpendicular to the X movement of lower X-Y stage  340  and the top surfaces  325 A,  330 A and  340 A respectively of rotational stage  325 , upper X-Y stage  330  and lower X-Y stage  340  are parallel. Top plate  255  is also mounted on lower X-Y stage  340  via brackets  345 . Since alignment pin mechanism  265  is fixed to top plate  255 , a measurable and repeatable relationship exists between alignment pins  305  and chuck  260  as long as the relative positions of upper X-Y stage  325  and lower X-Y stage  340  are known. 
   As outer actuator ring  320  rotates spokes  315  fixed to inner actuator ring  310  and passing through slanted slots  350  in the outer actuator ring cause the outer actuator ring to translate in the Z direction. Note, the X, Y and Z directions are orthogonal to each other. A lip  355  attached to outer actuator ring  320  thus also moves in the Z direction. A push rod of clamping mechanism  270  rides on lip  355  (see  FIG. 8 ) so clamping is controlled by the rotation of outer actuator ring  315  and clamping force is uniformly applied by all clamping mechanisms  275  (see  FIG. 5 ). Alignment tool  250  also includes an optical system  360  (generally a lens and a camera) linked to a computer  365  containing pattern recognition software as well a software for controlling movement rotational stage  325 , upper X-Y stage  330  and lower X-Y stage  340 . The pattern recognition software detects the position of alignment targets on substrate  215  and alignment marks on mask  145 . Computer  365  is linked to stepping motors on rotational stage  325 , upper X-Y stage  330  and lower X-Y stage  340  for controlling movement of substrate  215 . Computer  365  calculates the amount of upper X-Y stage  330 , lower X-Y stage  340  and rotational stage  325  movement required to align mask  145  with the substrate  215 . 
   During the alignment process, it is important that movement of substrate  215  is precise and accurate. In one example, encoders within rotational stage  325  are accurate to 0.001 degree encoders within upper X-Y stage  330  and lower X-Y stage  340  are accurate to 1 micron. 
     FIG. 7A  is a cross-section view through alignment pin mechanism  265  according to the present invention. In  FIG. 7 , alignment pin mechanism  265  includes a body  370  having a lower chamber  375  open to an upper chamber  380 . Alignment pin  305  includes an upper narrow portion  385  and a wide lower portion  390 . Alignment pin  305  extends through upper chamber  380  into lower chamber  375 . Alignment pin  305  is restricted in movement in the X and Y directions by sleeve bearing  395  and is moveable in the Z direction due to spring  400  contained within lower chamber  375 . Lower portion  390  of pin  305  passes through alignment pin hole  130 A (or alignment pin slot  130 B) in bottom ring  100  as well as an opening  405  in top plate  255 . Upper portion  385  of pin  305  passes through alignment pin hole  160 A (or alignment pin slot  160 B) in mask  145 . Upper portion  385  of pin  305  also passes through alignment pin hole  195 A (or alignment pin slot  195 B) in top ring  175 . 
     FIG. 7B  is a diagram illustrating the tolerances between alignment pin  305  of  FIG. 7A  and mask  145 . In  FIG. 7B , upper portion  385  of alignment pin  305  has a diameter of D 1 . Lower portion of alignment pin  305  has diameters D 2 . Alignment pin hole  160 A of mask  145  has a diameter of D 3 . Note alignment pin slot  160 B (see  FIG. 2 ) has a width of D 3  and is about 2D 3  long. D 2  is greater than D 1 . In one example D 2 −D 1 =0.010 inch and D 3 −D 1 =0.002 inch. 
   Returning to  FIG. 7A , during alignment of mask  145  to a substrate  215 , it is important that the mask does not move. Mask  145  experiences forces in the X, Y and θ directions. Alignment pins  305  restrict this movement. It is also important that alignment pins  305  move freely in the Z direction. Spring  400  ensures that there is always a net upward force (positive Z direction) on alignment pin  305  to resist downward force (negative Z direction) imparted to mask  145  by clamping mechanisms  270  (see  FIG. 5 ) during the alignment process to keep constant contact between the mask and bottom ring  100 . 
     FIG. 8  is a side view of clamping mechanism  270  according to the present invention. In  FIG. 8 , clamping mechanism  270  includes a mounting bracket  410  mounted to top plate  255 , a body  415 , a moveable clamp finger  420  slidably mounted in body  415  and a push rod  430  that operably engages lip  355  of outer actuator ring  320 . Clamp finger  420  can be slid over or retracted from top ring  175  by a mechanism not illustrated. As outer actuator ring rotates  320 , because spokes  315  are fixed to inner actuator ring  310  and extend through eccentric slots  350 , lip  355  moves up or down depending on the direction of rotation of outer actuator ring  320 . Push rod  430 , engaged on lip  355 , moves up and down with lip  355 , causing clamp finger  420  to apply pressure to the assembled alignment fixture  210  comprising bottom ring  100 , substrate  215 , mask  145  and top ring  175 . Clamp finger  420  is spring loaded so movement of lip  355  toward top plate  255  works against the spring and moves clamp finger  420  away from alignment fixture  210 . As lip  355  lowers, increasing pressure is applied to alignment fixture  210 . 
   Once the alignment process is completed, it is important that the clamping process be uniform across alignment fixture  210 , smooth and reproducible time to time. Any non-uniformity will result sideway slippage of mask  145  and/or substrate  215  and thus misalignment after the alignment process. Any non-smoothness in clamping can result in a shock that can cause mask  145  and/or substrate  215  movement, again resulting in misalignment after the alignment process. Non-reproducibility in clamping pressure can result in clipping (see  FIG. 9  and discussion infra) problems. Since all clamping mechanisms  270  are driven by outer actuator ring  320  clamping is uniform and smooth. A precision drive mechanism (not shown) driving outer actuator ring  320  ensures reproducible and precision controlled clamping pressure. 
     FIG. 9  is a side view of clipping mechanism  275  according to the present invention. In  FIG. 9 , clipping mechanism  275  includes a base  435  mounted to top plate  255 , a slide  440  having a slot (not shown) to hold a clip  220 , and a roller  445  mounted to a support  450  attached to base  435 . In operation, a clip  220  placed on slide  440 . As slide  440  is pushed forward toward retaining post  235 , clip  220  is compressed by roller  445 . As slide  440  continues forward, clip  220  engages retaining post  235  and becomes released from roller  445  allowing clip  220  to “spring” open. When slide  440  is retracted, clip  220  remains in place due to friction forces between the clip and bottom ring  100 . 
   It is important that insertion of clips  220  do move top ring  175  to which retainer posts  235  are fixedly attached. Movement of top ring  175  will cause mask  145  to move, thus changing the alignment of mask  145  to substrate  215 . Clipping mechanism  275  “preloads” clips  220  so that forces in the Z direction are eliminated during insertion, resulting in minimal X direction and Y direction forces being applied to retaining post  235  and top ring  175  during insertion. Each slide  440  moved to engage retaining posts  235  simultaneously by a mechanical mechanism. 
     FIG. 10A  is a top view of a substrate having alignment marks according to the present invention. In  FIG. 10A , substrate  100  includes diametrically opposed (or nearly diametrically opposed) left and right course alignment targets  455 A and  455 B respectively and diametrically (or nearly diametrically opposed) left and right fine alignment target sets  460 A and  460 B respectively. Left and right course alignment targets  455 A and  455 B are used by pattern recognition software residing on computer  365  (see  FIG. 6 ) during alignment operations. Left and right fine alignment targets  460 A and  460 B are used by an operator to check the quality of alignment operations. 
     FIG. 10B  is a top view of a mask having alignment marks according to the present invention. In  FIG. 10B , mask  145  includes diametrically opposed (or nearly diametrically opposed) left and right course alignment marks  465 A and  465 B respectively and diametrically (or nearly diametrically opposed) left and right fine alignment mark sets  470 A and  470 B respectively. Left and right course alignment targets  465 A and  465 B are used by pattern recognition software residing on computer  365  (see  FIG. 6 ) during alignment operations. Left and right fine alignment targets  470 A and  470 B are used by an operator to check the quality of alignment operations. Also illustrated in  FIG. 10B  are alignment pin hole  160 A and alignment pin slot  160 B. 
     FIG. 10C  is a top view of the substrate and mask alignments are they would appear in perfect alignment. In  FIG. 10C , course substrate alignment target  455 A ( 455 B) is centered on the middle two course mask alignment marks  465 A ( 465 B). All mask fine alignment marks  470 A ( 470 B) are centered in substrate fine alignment targets  460 A ( 460 B). 
     FIG. 11A  is a diagram of initial wafer alignment fiducial coordinates prior to mask to wafer alignment according to the present invention. In  FIG. 11A , rotational stage in  325  can move the Θ direction and upper X-Y stage  330  can move in the X and Z direction. A reference location  475  has upper X-Y stage coordinates X 0  and Y 0  and rotational stage  325  coordinate Θ 0 . Rotational stage  325  and upper X-Y stage  330  are initially moved to coordinates X 0 , Y 0  and Θ 0  respectively (as described infra in reference to  FIG. 12 ) and all subsequent motions of rotational stage  325  and upper X-Y stage  330  stage motions are referenced to coordinates X 0 , Y 0  and Θ 0  respectively. Wafer  215  includes a left fiducial mark  480  containing left course alignment target  455 A (see  FIG. 10A ) and fine alignment targets  460 A (see  FIG. 10A ) and a right fiducial mark  485  containing right course alignment target  455 B (see  FIG. 10A ) and fine alignment targets  460 B (see  FIG. 10A ). When rotational stage  325  and upper X-Y stage  330  are moved to coordinates X 0 , Y 0  and Θ 0  respectively, left fiducial  480  is at location X LW0 , Y LW0  and Θ LW0  and right fiducial  485  is at location X RW0 , Y RW0  and Θ RW0 . 
     FIG. 11B  is a diagram of initial mask alignment fiducial coordinates prior to mask to wafer alignment according to the present invention. In  FIG. 11B , lower X-Y stage  340  can move in the X and Z direction. Lower stage X-Y  340  is referenced to reference position  475 . All subsequent motions of lower X-Y stage  340  are referenced to coordinates X 0  and Y 0 . As long as the X movement of upper X-Y stage  330  is perpendicular to the Y movement of lower X-Y stage  340 , the Y movement of upper X-Y stage  330  is perpendicular to the X movement of lower X-Y stage  340  and the top surfaces of rotational stage  325 , upper X-Y stage  330  and lower X-Y stage  340  are parallel. Mask  145  includes a left fiducial mark  490  containing left course alignment mark  465 A (see  FIG. 10B ) and fine alignment marks  470 A (see  FIG. 10B ) and a right fiducial mark  495  containing right course alignment mark  465 B (see  FIG. 10A ) and fine alignment marks  470 B (see  FIG. 10B ). When lower X-Y stage  330  is referenced to coordinates X 0  and Y 0 , left fiducial  490  is at location X LM0  and Y LM0  and right fiducial  495  is at location X RM0  and Y RM0 . 
   Reference to  FIGS. 11A and 11B  will be useful in understanding the process illustrated in  FIG. 12  and described infra. In  FIG. 12  it is assumed that the X movement of upper X-Y stage  330  is perpendicular to the Y movement of lower X-Y stage  340 , the Y movement of upper X-Y stage  330  is perpendicular to the X movement of lower X-Y stage  340  and the top surfaces of rotational stage  325 , upper X-Y stage  330  and lower X-Y stage  340  are parallel. Any deviations from these conditions, requires more complex calculations then described infra in reference to  FIG. 12 . 
     FIG. 12  is a flowchart of the method for aligning a substrate to a mask according to the present invention In step  500 , a bottom ring of the alignment fixture is loaded onto the alignment tool and in step  505  a substrate is placed on the chuck of the alignment tool and vacuum applied to the chuck, holding the substrate fast to the chuck. 
   In step  510 , the course substrate alignment targets are located. First, the upper X-Y and rotational stages are moved to a predetermined X 0 , Y 0  and θ 0  positions. For the substrate left course alignment target the pattern recognition software locates the center of the left course alignment target and moves the upper X-Y stage and the rotational stage so the center of the left course alignment target is aligned with the center of the optical system. The pattern recognition software again locates the center of the substrate left course alignment target and moves the upper X-Y stage and the rotational stage so the center of the left course alignment target is aligned with the center of the optical system. The double locating brings the substrate left course alignment target to the area of least distortion within the optical system. This fixes the substrate left course target starting position on the upper X-Y stage and the rotational stage as X LW0 , Y LW0  and θ LW0 . 
   For the substrate right course alignment target the pattern recognition software locates the center of the right course alignment target and moves the upper X-Y stage and the rotational stage so the center of the right course alignment target is aligned with the center of the optical system. The pattern recognition software again locates the center of the substrate right course alignment target and moves on the upper X-Y stage and the rotational stage so the center of the right course alignment target is aligned with the center of the optical system. The double locating brings the substrate right course alignment target to the area of least distortion within the optical system. This fixes the substrate right course target starting position on the upper X-Y stage and the rotational stage as X RW0 , Y RW0  and θ RW0 . 
   Next, in step  515 , the upper X-Y stage and the rotational stage are moved fixed distances D X , D Y  and angle D θ . This motion compensates for the slack that is inherent in the stage mechanics. 
   In step  520 , the mask is positioned (using the alignment pins) over the substrate and in step  525  the top ring of the alignment fixture is positioned (using the alignment pins). 
   In step  530 , a light clamping pressure is applied by the clamping mechanisms in order to prevent motion of the mask during subsequent upper X-Y stage, rotational stage and lower X-Y stage movements. The pressure applied is just sufficient to bring the mask and top ring of the alignment fixture into contact. 
   In step  535  the upper X-Y stage and the rotational stage are moved a distance −D X /2, −D Y /2 and angle −D θ /2 (i.e. halfway back to X 0 , Y 0  and θ 0 ). This motion compensates for the slack that is inherent between the mask and the alignment pins and places the mask in a stable position. 
   In step  540 , the pattern recognition software locates the center of the mask left course alignment mark and moves the lower X-Y stage so the center of the left course alignment mark is aligned with the center of the optical system. The pattern recognition software again locates the center of the mask left course alignment mark and moves the lower X-Y stage so the center of the left course alignment mark is aligned with the center of the optical system. The double locating brings the mask left course alignment mark to the area of least distortion within the optical system. This fixes the left course mark starting position on the lower X-Y stage as X LM0  and Y LM0 . 
   For the mask right course alignment mark the pattern recognition software locates the center of the right course alignment mark and moves the lower X-Y stage so the center of the right course alignment mark is aligned with the center of the optical system (generally a lens and a camera). The pattern recognition software again locates the center of the mask right course alignment mark and moves the lower X-Y stage so the center of the right course alignment mark is aligned with the center of the optical system. The double locating brings the mask right course alignment mark to the area of least distortion within the optical system. This fixes the right course mark starting position on lower X-Y stage as X RM0  and Y RM0 . 
   In step  545 , first the rotational displacement of the substrate θ W =tan −1  ((Y LW0 −Y RW0 )/(X LW0 −X RW0 )) and the rotational displacement of the mask θ M =tan −1  ((Y LM0 −Y RM0 )/(X LM0 −X RM0 )) are calculated. Second, the relative rotational displacement (and correcting theta displacement) between the mask and substrate Δθ MW =θ M −θ W  is calculated. In order to align the substrate to the mask in the X and Y directions compensation for the applied rotation is necessary. Third, the X-translation, X′=X WL0  cos Δθ MW −Y WL0  sin Δθ MW  (or X′=X WR0  cos Δθ MW −Y WR0  sin Δθ MW ) and the Y-translation Y′=X WL0  sin Δθ MW +Y WL0  cos Δθ MW  (or Y R ′=X WR0  sin θ MW +Y WR0  COS θ MW ) are calculated. Fourth, the correcting displacements ΔY=Y ML0 −Y′ or (ΔY=Y MR0 −Y′) and ΔX=X ML0 −X′ or (ΔX=X MR0 −X′) are calculated. Fifth, the Y ALIGN =ΔY+(−D Y /2), X ALIGN =ΔX+(−D X /2) and θ ALIGN =Δθ MW +(−D θ /2) movements of the upper X-Y and rotational stages are calculated. 
   In step  550 , the mask and substrate are aligned by moving the upper X-Y stage distances X ALIGN  and Y ALIGN  and moving the rotational stage angle θ ALIGN . 
   In step  555 , the clamping mechanism fully compresses the top ring, mask and substrate against the bottom ring. The chuck vacuum is released at about 75% full compression in order to avoid breaking the substrate, which is now slightly bowed by the clamping and could be further shocked by air entering the chuck when the substrate releases from the chuck under the condition of the clamping pressure being greater than atmospheric pressure. 
   In step  560 , clipping mechanisms install the clips on the retaining posts which keep the bottom ring, substrate, mask and top ring stack under compression and in alignment. 
   In step  565 , the fine alignment targets/marks are inspected to the alignment of mask to substrate is within specification. 
   Using the alignment fixture, alignment tool and alignment/clamping/clipping procedure described supra, the inventors have been able to achieve alignments between mask and substrate on 200 mm wafers of better than 20 microns. 
   The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.