Abstract:
An improved method and apparatus for coating semiconductor substrates with organic photoresist polymers by extruding a ribbon of photoresist in a spiral pattern which covers the entire top surface of the wafer. The invention provides a more uniform photoresist layer and is much more efficient than are current methods in the use of expensive photoresist solutions. A wafer is mounted on a chuck, aligned horizontally and oriented upward. An extrusion head is positioned adjacent to the outer edge of the wafer and above the top surface of the wafer with an extrusion slot aligned radially with respect to the wafer. The wafer is rotated and the extrusion head moved radially toward the center of the wafer while photoresist is extruded out the extrusion slot. The rotation rate of the wafer and the radial speed of the extrusion head are controlled so that the tangential velocity of the extrusion head with respect to the rotating wafer is a constant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from our copending provisional U.S. patent application Ser. No. 60/050,017, filed Jun. 16, 1997; Ser. No. 60/050,159, filed Jun. 19, 1997; and Ser. No. 60/055,789, filed Aug. 14, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an improved method and apparatus for coating semiconductor substrates with organic photoresist polymers. In particular, this invention relates to an improved method and apparatus for coating semiconductor substrates which provides a more uniform photoresist layer and is much more efficient than are current methods in the use of expensive photoresist solutions. 
     BACKGROUND OF THE INVENTION 
     The manufacture of integrated circuits involves the transfer of geometric shapes on a mask to the surface of a semiconductor wafer. Thereafter, the semiconductor wafer corresponding to the geometric shapes, or corresponding to the areas between the geometric shapes, is etched away. The transfer of the shapes from the mask to the semiconductor wafer typically involves a lithographic process. This includes applying a solution of a pre-polymer solution to the semiconductor wafer, the pre-polymer being selected to form a radiation-sensitive polymer which reacts when exposed to ultraviolet light, electron beams, x-rays, or ion beams, for example. The solvent in the pre-polymer solution is removed by evaporation, and the resulting polymer film is then baked. The film is exposed to radiation, for example, ultraviolet light, through a photomask supporting the desired geometric patterns. The images in the photosensitive material are then developed by soaking the wafer in a developing solution. The exposed or unexposed areas are removed in the developing process, depending on the nature of the radiation-sensitive material. Thereafter, the wafer is placed in an etching environment which etches away the areas not protected by the radiation-sensitive material. Due to their resistance to the etching process, the radiation sensitive-materials are also known as photoresists, and the term photoresist is used hereinafter to denote the radiation-sensitive polymers and their pre-polymers. 
     The photoresist film thickness required depends on the desired resolution, defect protection, and step coverage. Thicker films provide better adhesion, greater protection for reactive ion erosion, and improved defect protection. However, thicker films also result in lower resolution because they take longer to expose and develop. Photoresist film thicknesses used in current semiconductor manufacturing may be typically 0.5 to 4 μm thick. 
     Thickness uniformity of the photoresist layer is an important criterion in the manufacture of integrated circuits. When the radiation is focused through the mask onto the coating, variations in thickness of the coating prevent the precise focus over the entire surface of the wafer which is required to obtain the sharpness necessary to ensure satisfactory reproduction of the geometric patterns on the semiconductor wafer for advanced circuits with line width dimensions approaching 0.25 μm line widths and smaller over a surface. Photoresist film thickness uniformity is required to maintain good transfer of the mask pattern to the photoresist. Uniformity is important to maintain a constant exposure level across the surface of the wafer. Nonuniformities cause position overlay errors when optical steppers attempt to sense alignment marks beneath the photoresist film. Nonuniformities also change the reflectivity of a photoresist deposited over an oxide. 
     The small critical dimensions of microelectronic devices require photoresist coating thickness typically to be uniform to within 10 Å (3σ). As the critical dimension decreases further, even better uniformities will be required. 
     The high cost of the photoresist pre-polymer solutions makes it desirable to devise methods of improving the efficiency of the coating process so as to minimize the amount of the polymer solution required to coat a substrate. 
     Methods which have been used or proposed for coating wafers include dip coating, meniscus coating, spray coating, patch coating, bubble coating, chemical vapor deposition, and spin coating. Only a few of these methods produce photoresist films with the thicknesses and uniformities required for semiconductor production. Of these methods, only spin coating has a production rate fast enough to meet the demands of chip manufacturers. One major shortcoming of spin coating, however, is that it can waste as much as 90%, or more, of the photoresist applied to the wafer surface. 
     About one million gallons of photoresist are consumed each year at a cost of several hundred million dollars. As the critical dimension of semiconductor devices becomes smaller, new deep UV photoresists will be used. These new photoresists can cost five or more times the cost of the i-line photoresists used currently. Therefore, a new coating method is needed which wastes less photoresist while producing uniform, defect-free coatings at a rate comparable to that of spin coating. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     One object of this invention is to provide an improved wafer coating process and apparatus which provide greater coating uniformity across the entire surface of the wafer. 
     Another object of the invention is to provide an improved wafer coating process and apparatus which provide coating uniformity with less waste and more efficient use of the photoresist. 
     In a first aspect the invention provides a method of applying a coating of photoresist to a circular semiconductor wafer, the wafer having a top surface, a center, and an outer edge, the method comprising extruding a ribbon of photoresist, the ribbon having a width bounded by outer and inner sides, the ribbon extruded in a spiral pattern which covers the entire top surface of the wafer. 
     In a second aspect, the invention provides a method of applying a coating of photoresist to a circular semiconductor wafer, the wafer having a top surface, a center, a diameter, and an outer edge, the method comprising the steps of mounting the wafer on a chuck, the top surface of the wafer aligned horizontally and oriented upward; positioning an extrusion head adjacent to the outer edge of the wafer and above the top surface of the wafer, the extrusion head configured to extrude photoresist out an extrusion slot, the extrusion slot having a length bounded by a first end and a second end, the extrusion head positioned with the extrusion slot aligned radially with respect to the wafer, the first end of the extrusion slot located adjacent to the outer edge of the wafer, and the second end of the extrusion slot outside the outer edge of the wafer; rotating the wafer about its center; extruding a ribbon of photoresist from the extrusion slot, the ribbon having a width bounded by outer and inner sides, the width of the ribbon substantially equal to the length of the slot; and, while extruding photoresist from the extrusion slot, and maintaining the extrusion slot aligned radially with respect to the wafer, moving the extrusion head radially inward from the outer edge of the wafer toward the center of the wafer until the photoresist covers the entire top surface of the wafer. 
     In a third aspect, the invention provides a method of applying a coating of photoresist to a circular semiconductor wafer, the wafer having a top surface, a center, a diameter, and an outer edge, the method comprising the steps of mounting the wafer on a chuck; positioning an extrusion head at the center of the wafer and above the top surface of the wafer, the extrusion head configured to extrude photoresist out an extrusion slot, the extrusion slot having a length bounded by a first end and a second end, the extrusion head positioned with the extrusion slot aligned radially with respect to the wafer, the second end of the extrusion slot located at the center of the wafer and the first end of the extrusion slot located between the center of the wafer and the outer edge of the wafer; rotating the wafer about its center; extruding a ribbon of photoresist from the extrusion slot, the ribbon having a width substantially equal to the length of the slot; and, while extruding photoresist from the extrusion slot, and maintaining the extrusion slot aligned radially with respect to the wafer, moving the extrusion head radially outward toward the outer edge of the wafer until the second end of the extrusion slot reaches the outer edge of the wafer. 
     In a fourth aspect, the invention provides an apparatus for applying a coating of photoresist to a circular semiconductor wafer, the wafer having a top surface, a center, a diameter, and an outer edge, the apparatus comprising means for mounting a wafer with the top surface of the wafer aligned horizontally and oriented upward; an extrusion head positioned adjacent to the outer edge of the wafer and above the top surface of the wafer, the extrusion head configured to extrude photoresist out an extrusion slot, the extrusion slot having a length bounded by a first end and a second end, the extrusion head positioned with the extrusion slot aligned radially with respect to the wafer, the first end of the extrusion slot located adjacent to the outer edge of the wafer, and the second end of the extrusion slot outside the outer edge of the wafer; means for rotating the wafer about its center; means for extruding a ribbon of photoresist from the extrusion slot, the ribbon having a width substantially equal to the length of the slot; and means for, while extruding photoresist from the extrusion slot, and maintaining the extrusion slot aligned radially with respect to the wafer, moving the extrusion head radially inward toward the center of the wafer until the photoresist covers the entire top surface of the wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a static dispense method employed to dispense photoresist on a wafer surface in a spin coating process. 
     FIG. 2 illustrates a forward radial dynamic dispense method employed to dispense photoresist on a wafer surface in a spin coating process. 
     FIG. 3 illustrates a reverse radial dynamic dispense method employed to dispense photoresist on a wafer surface in a spin coating process. 
     FIG. 4 is an assembly drawing of a side view of an extrusion head of the invention. 
     FIG. 5 is a front view of a front plate of an extrusion head of the invention. 
     FIG. 6 is a front view of a rear plate of an extrusion head of the invention. 
     FIG. 7 is a front view of a shim of an extrusion head of the invention. 
     FIG. 8 is a front view of a shim against a back plate. 
     FIG. 9 is a cross sectional view of an assembled extrusion head of the invention. 
     FIG. 10 is a perspective view of an assembled extrusion head of the invention. 
     FIG. 11 is a cross sectional view of the lips of an extrusion head with a substrate moving beneath the lips of the extrusion head. 
     FIGS. 12,  13  and  14  are a front view, top view and rear view, respectively of an extrusion spin coating assembly of the invention. 
     FIG. 15 is a block diagram of an embodiment of a control system in the extrusion spin coating assembly of the invention. 
     FIGS. 16,  17 ,  18  and  19  illustrate the configuration of an extrusion spin coating assembly during several steps of the extrusion spin coating process of the invention. 
     FIG. 20 is a diagram which illustrates certain parameters of extrusion spin coating motion according to the invention. 
     FIG. 21 illustrates an extrusion spin coating spiral pattern according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1,  2  and  3  shows three primary methods currently employed to dispense photoresist on a wafer surface in a spin coating process. The method depicted in FIG. 1 is referred to as “static dispense.” In static dispense, the photoresist is dispensed directly from dispense nozzle  20  into the center of a stationary wafer  10 , producing a circular pool of photoresist  12 . Alternatively, the entire surface of the wafer  10  may be flooded with photoresist. Often, the wafer  10  is rotated slowly after a static dispense to begin spreading the photoresist  12  over the wafer  10  surface. 
     The methods illustrated in FIGS. 2 and 3 are referred to as “dynamic dispenses,” because the wafer  10  is rotating slowly while the photoresist  14 ,  16  is dispensed. During forward radial dispense, illustrated in FIG. 2, the dispense nozzle  20  is initially located at the center of the wafer  10  and moves radially outward as the photoresist  14  is deposited. For reverse radial dispense, illustrated in FIG. 3, the dispense nozzle begins at the outer edge of the wafer and moves radially inward. In both FIGS. 2 and 3 the dispense nozzle  20  is illustrated at the end of travel after having deposited photoresist on the slowly spinning wafer  10 . Both forward and reverse radial dispense produce a spiral pattern  14 ,  16  of photoresist. The geometry of the spiral  14 ,  16 , i.e. number of turns of the spiral and volume of photoresist per unit length along the spiral, is determined by the angular rotation of the wafer  10 , the radial velocity of the nozzle  20  with respect to the wafer  10 , and the volumetric flow of the photoresist during the dispense. Dynamic dispenses use less photoresist, but static dispenses produce a more uniform film. 
     After the photoresist is deposited on the wafer, the wafer is accelerated to create a centrifugal force which spreads the photoresist toward the edge of the wafer. The wafer may be spun at an intermediate speed for a few seconds before being accelerated to the final high-speed spin. When the bulk of the photoresist reaches the edge of the wafer, most of the photoresist is flung off in many tiny droplets. It has been shown that while the acceleration rate does not affect the final film thickness, higher acceleration rates do tend to produce more uniform films. 
     Once the wafer is spun up to the final high speed, the wafer continues to spin to cause the photoresist to reach the desired thickness. Photoresist continues to flow outward and off the wafer in concentric waves. Simultaneously, the solvent in the photoresist evaporates quickly because of high convection over the wafer surface. As the solvent fraction in the photoresist decreases, the viscosity of the photoresist gradually increases, causing the outward flow of photoresist to diminish until it almost ceases. Subsequent thinning of the photoresist comes almost entirely from solvent evaporation. When the solvent is mostly evaporated, typically after about 30 seconds, spinning is stopped, and the wafer is soft baked at a high temperature to evaporate the remaining solvent from the photoresist. 
     In each of the dispense methods depicted in FIGS. 1,  2  and  3 , the photoresist is dispensed onto the wafer in a thick puddle or ribbon, and must be spread by some means, e.g. slow spin, to spread the photoresist to cover the wafer and to reduce the photoresist to a thin layer. In the method of the invention, the photoresist is applied in a thin uniform layer over the entire surface of the wafer. This eliminates the need for the slow spin step, and requires less photoresist to be deposited on the wafer to achieve the desired final thickness and uniformity. 
     The method of the invention employs extrusion slot coating to dispense a thin ribbon of photoresist over the entire surface of the wafer. Extrusion slot coating is a member of the class of pre-metered coating methods. With extrusion slot coating, the coating thickness can be controlled by the photoresist dispense rate, the efficiency can be near 100%, and the thickness uniformity is very good. 
     In extrusion slot coating, the photoresist is extruded onto the wafer through a narrow slot. FIGS. 4-11 illustrate an embodiment of an extrusion head  30  which may be used in the invention. The extrusion head  30  may also be referred to as an extrusion die. FIG. 4 shows a side assembly view of the extrusion head  30  which is constructed of a stainless steel U-shaped shim  31  sandwiched between a stainless steel front plate  32  and a stainless steel back plate  33 . FIGS. 5,  6  and  7  show a front view of the front plate  32 , back plate  33 , and shim  31 , respectively. FIG. 8 shows a front view of the shim  31  against the back plate  33 . Referring to FIG. 4, the front plate  32  and back plate  33  are grounded and polished on their inner edges, facing the shim  31 , to provide good seal with the shim  31  and a smooth surface for extrusion. Photoresist enters the extrusion head  30  through a port  34  in the top of the back plate  33 . The port  34  directs the photoresist through a tube  35  to a flow channel  36  (FIGS. 4,  6 ). The flow channel  36  is as wide as the opening of the “U” of the shim  31  (FIGS. 7,  8 ). 
     FIG. 9 is a sectional view of the extrusion head  30  illustrated in FIG.  4 . The void created by the u-shape of the shim  31  leaves a narrow gap  38  between the front plate  32  and back plate  33  through which photoresist can flow. At the base of the extrusion head  30 , the gap  38  continues downward between two narrow “lips”  41 ,  42  which extend the inner surface of the front plate  32  and back plate  33 . 
     FIG. 10 is a perspective view of the extrusion head illustrated in FIG.  4 . The gap  38  extends across the opening of the “U”  37  (FIGS. 7,  8 ) of the shim  31  to form an extrusion slot  39  in the extrusion head  30 . 
     FIG. 11 is a cross sectional view of the lips  41 ,  42  of an extrusion head  30  with a substrate  50  moving beneath the extrusion lips  41 ,  42 . Photoresist is extruded out the slot  39  at the base of the lips  41 ,  42  onto the top surface  51  of the substrate  50 . The width of the gap  38  between the front plate  32  and rear plate  33 , indicated as d, is equal to the thickness of the shim  31  (FIGS. 4,  9 ). The coating gap between the lips  41 ,  42  and the substrate  50  is filled with a bead  46  of coating fluid coming from the slot  39 . When the substrate  50  is moved perpendicular to the slot  39 , keeping the coating gap constant, fluid is drawn out of the bead  46  and remains as a thin film on the substrate  50 . The width of the extruded film, w (FIGS. 19,  20 ) is approximately equal to the length of the extrusion slot  39 , i.e. the opening of the “U” of the shim  31  (FIGS. 7,  8 ). The average thickness of the extruded film, h, is        h   =     Q   wv                            
     where v is the coating speed, and Q is the fluid dispense rate. The menisci  44 ,  45  at the leading and trailing edges of the coating bead  46  are pinned to the corners of the extrusion head lips  41 ,  42 . The corners of the extrusion head lips  41 ,  42  should have a radius of curvature less than approximately 50 μm to keep the menisci  44 ,  45  pinned. The capillary, viscous, and inlet pressures in the coating bead  46  must balance the external pressure to maintain stability in the coating bead  46 . A slight vacuum at the leading edge of the coating bead  46  can be used to stabilize the coating bead  46  when coating thinner films or at higher coating speeds. The extrusion head lips  41 ,  42  are normally of equal length (G 1 =G 2 ) and the extrusion head  30  is perpendicular to the substrate  50 . For very thin coatings, however, it is sometimes beneficial to have one of the lips extend beyond the other (G 1 ≠G 2 ) or to have the extrusion head  30  slightly tilted from perpendicular to the substrate  50 , thereby tilting the coating slot  39  with respect to the substrate  50 . 
     The description of the extrusion spin coating assembly  100  will be with reference to FIGS. 12,  13  and  14 , which illustrate front, top and rear views, respectively, of an extrusion spin coating assembly  100  according to the invention. Components of the extrusion spin coating assembly  100  illustrated in FIGS. 12,  13  and  14  include a coating module  110  and a positioning system  130 . Not illustrated in FIGS. 12,  13  and  14 , but described with reference to FIG. 15, is a control system  210  which includes a positioning controller  220  and a spinner controller  280 . 
     The coating module  110  includes a spinner assembly  111  which includes a spinner servomotor (not illustrated, reference numeral  113  in FIG. 15) connected to a vertical shaft  112 . The vertical shaft  112  supports a Teflon vacuum chuck  114 . The spinner assembly  111  can be moved vertically using a chuck elevator servomotor (not illustrated, reference numeral  115  in FIG.  15 ). The chuck elevator servomotor is equipped with an elevator motor brake (not illustrated, reference numeral  135  in FIG.  15 ). With the spinner assembly  111  at its lowest position, the chuck  114  is surrounded by a catch cup  116  (sectional view illustrated). The catch cup  116  is a circular cup having an open top  117 . The upper portion  120  of the cup wall  118  tilts inward to facilitate retaining waste photoresist within the catch cup  116 . The catch cup  116  serves three purposes. The catch cup  116  catches and drains waste photoresist out a liquid waste drain  122 . The catch cup has an exhaust vent  118  through which evaporated solvent is removed. The catch cup  116  directs the flow of air over a spinning wafer to avoid turbulence. Both the exhaust vent  118  and waste drain  122  exit the bottom  124  of the catch cup  116 . Means for removing waste photoresist and exhausted vapors are well known to those skilled in the art and are therefore not illustrated. 
     The spinner assembly  111  has a centering device including eight Teflon pins  138  for centering wafers on the chuck  114 , and three vertical pins (not illustrated) for supporting loose wafers before and after processing. The centering pins  138  are controlled by a centering solenoid (not illustrated, reference numeral  119  in FIG.  15 ). 
     Sensors on the coater module  110  indicate chuck  114  vertical home position (not illustrated, reference numeral  121  in FIG.  15 ), vacuum state (on/off) (not illustrated, reference numeral  123  in FIG.  15 ), and centering pin position (not illustrated, reference numeral  125  in FIG.  15 ). These features of the coating module  110  are well known to those skilled in the art and are therefore not illustrated. 
     A coater module  110  suitable for use with the invention is a 90SE coater module which is commercially available from Silicon Valley Group, Inc. The 90SE coater module is one component of a 90SE Wafer Processing track also commercially available from Silicon Valley Group, Inc. 
     The positioning system  130  is supported by an aluminum baseplate  132  which is mounted above the coater module  110 . The baseplate  132  has a center cut-out  134  positioned over the coater module  110 . First and second vertical support plates  134 ,  136  mounted above the baseplate support a cross-support  137  on which a two-axis positioning system  150  is mounted. The positioning system  150  includes an x-axis positioning table  152  and a z-axis positioning table  162 . The x-axis positioning table  152  includes an x-axis table motor  154  and x-axis table base  156 . Likewise, the z-axis positioning table  162  includes a z-axis table motor  164  and z-axis table base  166 . The z-axis positioning table  162  also includes a z-axis brake  160 . The z-axis positioning table  162  is mounted on the carriage  158  of the x-axis positioning table  152 . The x-axis positioning table  152  moves in a horizontal plane, parallel to the surface  51  of a wafer  50  mounted on the chuck  114 , and the z-axis positioning table  162  moves in a vertical direction perpendicular to the plane of the surface  51  of a wafer  50  mounted on the chuck  114 . A positioning system suitable for use in the x-axis and z-axis positioning tables  152 ,  162  of the invention is the Parker Daedal Motion Table driven by 5-pitch ball screws. 
     An extrusion head  30  is mounted at the bottom of an aluminum extrusion head support  172  which, in turn, is mounted on the z-axis positioning table  162 . The z-axis positioning table  162  has sufficient range of motion to move the extrusion head  30  from a position above the base plate  132 , down, through the center cut-out  134  in the baseplate  132 , to the proximity of a wafer  50  on the chuck  114 . 
     An optical sensor  174  is mounted on the extrusion head support  172 . The optical sensor  174  is used to measure the gap between the extrusion head  30  and a wafer  50  mounted on the chuck  114 . A sensor suitable for use in an embodiment of the invention is a Philtec RC140L reflectance compensated optical displacement sensor. The optical sensor  174  shines a light on the surface of the wafer  50 , measures the reflected light, and generates a voltage proportional to the intensity of the measured light. The spot size of the Philtec sensor is 6 mm and has a bandwidth from DC to 100 Hz. The voltage-distance curve of the Philtec sensor is generally non-linear, but has a linear region when the sensor-wafer distance is between, for example, 5.51 and 6.17 mm (0.217 and 0.243 inch). The optical sensor  174  is positioned on the extrusion head support  172  so that all measurements fall within the linear range of the optical sensor  174 . 
     Means for controlling flow of the photoresist includes a photoresist pump (not illustrated) and a photoresist shutoff valve  129 . Such arrangements are well known to those skilled in the art, and therefore is not fully illustrated in FIGS. 12,  13  or  14 . However, the following description of the control system  210  of the extrusion spin coating assembly  100  includes reference to the photoresist pump (not illustrated, reference numeral  127  in FIG. 15) and the photoresist shutoff valve  129 . 
     FIG. 15 is a block diagram which illustrates an embodiment of a control system  210  suitable for controlling the extrusion spin coating assembly  100  of the invention. The control system  210  includes a computer  212 , a positioning controller  220  and a spinner controller  280 . The computer  212  downloads programs to the positioning controller  220 , the spinner controller  280  and the photoresist dispense pump  127  via serial interfaces  213 ,  214 ,  215 . The positioning controller  220  sends commands to the photoresist dispense pump  127  to start and stop photoresist flow and to control the photoresist shutoff valve  129 . The positioning controller  220  also controls the position of the x-axis positioning table  152  via the x-axis motor  154  and z-axis positioning table  162  via the z-axis motor  164 , and the chuck elevator servomotor  115 . The positioning controller  220  receives the output of the optical sensor  174 , calculates the distance between the extrusion head  30  and the wafer  50 , and uses the results to control the z-axis positioning table  162  via the z-axis motor  164 . 
     A computer suitable for use in the control system  210  is an IBM-compatible PC. Suitable for use as the positioning controller  220  is the Parker Compumotor AT6450 Servo Controller, including the optional ANI analog input PC card and the AUX board. Suitable for use as the spinner controller  280  is The Pacific Scientific SC 755. Although the computer  212 , positioning controller  220  and spinner controller  280  are shown separately in the block diagram of FIG. 15, in an embodiment which includes the Parker Compumotor AT6450 and Pacific Scientific SC755 controllers, the Compumotor AT6450 plugs into the motherboard of the PC. The invention also contemplates an embodiment in which both the positioning controller  220  and spinner controller  280  functions are provided by a single, combined controller. 
     The positioning controller  220  includes a positioning controller processor and several inputs and outputs. The inputs and outputs include a 14-bit analog to digital (A/D) converter, several discrete digital inputs and outputs, and servomotor outputs (the processor and inputs and outputs are well known to those skilled in the art and are not individually illustrated). The output of the optical sensor  174  is coupled to the A/D converter input  224 . The positioning controller  220  discrete digital inputs are optically isolated interfaces, and include a chuck position home indicator input  242  coupled to the chuck position home sensor  121 ; a vacuum on/off status indicator input  244  coupled to the vacuum on/off sensor  123  on the vacuum chuck  114 ; a centering pin in/out position indicator input  246  coupled to the centering pin position sensor  125 ; and one or more manual positioning command inputs  248  coupled to operator manual positioning switches  126 . 
     The positioning controller  220  outputs include an x-axis servomotor output  226  which is coupled to the x-axis servomotor  154 ; a z-axis servomotor output  228  which is coupled to the z-axis servomotor  164 ; and an elevator motor output  230  which is coupled to the elevator servomotor  115 . 
     The positioning controller  220  discrete digital outputs include a photoresist valve on/off output  254  which is coupled to the photoresist shutoff valve  129 ; a centering solenoid output  256  which is coupled to the centering solenoid  119  which controls the centering pins  138 ; a vacuum solenoid output  258  which is coupled to the vacuum solenoid  131 ; a z-axis motor brake output  260  which is coupled to the z-axis brake  133  in the z-axis positioning table  162 ; an elevator motor brake output  262  which is coupled to the elevator motor brake  135 ; a trigger output  264  to the photoresist dispense pump  127 ; and logical outputs  266  to the spinner controller  280 . 
     The spinner controller  280  runs the coating and spin cycles in response to signals received from the positioning controller  220 . The spinner controller  280  includes a spinner controller processor, a servomotor output, and an encoder (the processor and encoder are well known to those skilled in the art and are not individually illustrated). The spinner controller  280  outputs include a spinner motor output  286  which is coupled to the spinner motor  113 . The output of the spinner controller  280  also includes a simulated encoder signal  288  which is coupled to the positioning controller. The simulated encoder signal  288  allows electronic gearing of the spinner motor  113  speed to control the x-axis positioning of the extrusion head  30  performed by the positioning controller  220 . 
     The extrusion head  30  and the positioning tables  152 ,  162  must be aligned with respect to a wafer  50  mounted on the chuck  114  to obtain reliable coating. Three alignments are required. These alignments will be described with reference to FIGS. 12,  13  and  14 . A first alignment adjusts the path of the extrusion slot  39  so that the extrusion slot  39  passes directly over the center of a wafer  50  mounted on the chuck  114 . This alignment is needed to completely cover the center area of the wafer  50 . The extrusion head  30  is positioned over the center of the wafer  50  by sliding the vertical support plates  134 ,  136  forward or backward over the base plate  132 . The motion of the vertical support plates  134 ,  136  is constrained by a guide on the base plate  132 . Adjustment bolts at the rear of each of the vertical support plates  134 ,  136  allow fine tuning of the position of the vertical support plates  134 ,  136  before the vertical support plates  134 ,  136  are fastened into place. 
     The second alignment adjusts the angle of the x-axis with respect to the wafer surface  51 . This alignment maintains a constant gap between the wafer  50  and the extrusion head  30  as the x-axis positioning table  152  changes position. The angle of the x-axis with respect to the wafer surface  51  can be changed by rotating the cross-support  137  about a first pivot  179  at one end of the cross-support  137 . Fine and coarse adjustment bolts  184 ,  186  allow adjustments of the angle between the x-axis and the wafer surface  51  of 1.64×10 −5  radians per turn of the fine adjustment bolt  184 . The angle of the x-axis with respect to the wafer surface  51  can be determined by scanning across the wafer surface  51  with the optical sensor  174 . During the scan, with the z-axis fixed, measurements of the optical sensor  174  output and the x-position are recorded. A linear regression of these data pairs provides the angle between the wafer surface  51  and the x-axis. 
     The third alignment adjusts the bottom edge of the extrusion head  30 , i.e. the extrusion slot  39 , until it is parallel with the x-axis and the wafer surface  51 . This alignment is crucial for maintaining a constant gap across the width of the extrusion head  30 . The angle between the bottom edge of the extrusion head  30  and the x-axis can be adjusted using a wafer-extruder parallelism adjustment bolt  176 . The wafer-extruder parallelism adjustment bolt  176  pivots the extrusion head support  172  about a wafer-extruder parallelism adjustment pivot  178  at the base of the z-axis positioning table  162 . The angle between the x-axis and the bottom of the extrusion head  30  can be measured using a linear variable differential transformer (LVDT) sensor. The LVDT sensor is secured to the wafer surface  51  with the measurement tip pointing vertically up. Next, the extrusion head  30  is lowered until the lips  41 ,  42  of the extrusion head  30  move the LVTD sensor to a reference position. After the x-axis and z-axis positioning table  152 ,  162  positions are recorded, the procedure is repeated for several other positions along the extrusion head lips  41 ,  42 . The slope of the extrusion head  30  with respect to the x-axis is determined using a linear regression of these data pairs. 
     The optical sensor  174  may be calibrated in a two-step process. First, a voltage offset (i.e., zero-gap bias) voltage is determined by measuring the output voltage of the optical sensor  174  at several small gap distances using precision shims placed between the extrusion head  30  and the wafer surface  51 . A linear regression analysis of the gap distance and sensor voltage data is used to calculate the voltage offset (i.e., sensor voltage at a zero gap). Second, the relationship of the sensor voltage and the height of the extrusion slot  39 , in the linear range of the optical sensor  174 , is determined by raising the extrusion slot  39  in selected increments (e.g., 10 encoder counts equals 12.7 μm) and recording the sensor voltage at each position. A linear regression of the data pair provides the slope of the curve representing sensor voltage versus z-axis position of the extrusion slot  39 . The extrusion head  30  must be aligned with respect to the x-axis and wafer surface, as described above, prior to calibrating the optical sensor  174  so that errors will not arise from the angle between the extrusion head  30  and the wafer surface  51 . 
     The extrusion spin coating process will be described with reference to FIGS. 16-19. The alignment and calibration procedures described above may be performed periodically or prior to a series of runs as determined to be necessary based on experience with the equipment used. 
     Referring to FIG. 16, the vacuum chuck  114  is raised through the cut out  134  in the base plate  132 , and the wafer  50  is placed on the chuck  114 . The wafer  50  is centered on the chuck  114  using the centering pins  138  (FIG.  13 ). The chuck vacuum (not illustrated) is turned on to secure the wafer  50 . The chuck  114  is lowered, lowering the wafer  50  into the coating position, and the extrusion head  30  is lowered into position at the edge of the wafer  50  with the desired gap between the wafer  50  and the extrusion head lips  41 ,  42  as illustrated in FIG.  17 . The chuck  114  is then rotated at an initial rotational speed which is the desired coating speed. The photoresist shutoff valve  129  is opened and the photoresist pump  127  is triggered to begin dispensing photoresist. The extrusion head  30  is moved radially with respect to the wafer  50 . As the extrusion head  30  moves toward the center of the wafer  50 , the rotational speed of the chuck  114  is increased and the extrusion head speed is increased at a rate proportional to the increase in the rotational speed in order to maintain the coating speed of the extrusion head  30  over the wafer  50  constant. When the leading edge of the extrusion head  30  reaches the center of the wafer  50 , illustrated in FIG. 18, the speed of rotation of the wafer  30  is held constant until the trailing edge of the extrusion head  30  reaches the center of the wafer  50 . When the entire wafer  50  is covered with photoresist, the photoresist pump  127  is triggered to stop dispensing photoresist, and the photoresist shutoff valve  129  is closed. Typically, it is necessary to continue extruding photoresist and continue moving the extrusion head  30  until the trailing edge of the extrusion head  30  reaches the center of the wafer  50  in order to cover the entire wafer  50  with photoresist. When the photoresist pump  127  and shutoff valve  129  are triggered to stop dispensing photoresist, a residual amount of photoresist which is already in the extrusion head  30  (and possibly also in tubing leading to the extrusion head  30 ) may continue to flow and be deposited on the wafer  50 . In such cases, the photoresist pump  127  and shutoff valve  129  may be triggered to stop dispensing photoresist a short time prior to covering the entire wafer  50 , thereby allowing such residual photoresist to finish covering the wafer  50 . 
     The chuck  114  then lowers the wafer  50  into the catch cup  116 , and the extrusion head  30  is raised from the coating area as illustrated in FIG.  19 . The wafer  50  is then spun at high speed to remove excess photoresist and achieve the desired coating uniformity. The chuck  114  stops spinning and is raised through the center cut out  134  in the base plate  132 . The vacuum is turned off and the wafer  50  removed from the chuck  114 . 
     FIG. 20 is a diagram which illustrates certain parameters of extrusion spin coating motion according to the invention. In FIG. 20, a wafer  50 , has a radius R, and is rotating about its center at an angular velocity of Ω. An extrusion head  30  is above the wafer  50 , with the extrusion slot  39  radially aligned with respect to the wafer  50 . The extrusion slot  39  has a width w, and is moving radially with respect to the wafer  50  at a velocity u. The distance between the center of the wafer  50  and the trailing edge of the extrusion head  30  is r. 
     The tangential velocity of any point on the surface of the wafer  50 , at a distance r from the axis of rotation shown in FIG. 20 is: 
     
       
         
           v=Ωr 
         
       
     
     With the trailing edge of the extrusion head  30  at a distance r from the axis of rotation, a spiral extrusion pattern can be made by moving the extrusion head  30  inward one length of the extrusion slot  39  for each revolution of the wafer  50 , The extrusion head  30  speed along the diameter of the wafer  50  is then:        u   =       Ω                 w       2      π                              
     Solving for Ω and substituting yields:        u   =     wv     2      π                 r                              
     For radially inward motion, u=−dr/dt, and a differential equation for the position of the extrusion head can be obtained as follows:               r          t       =     -     wv     2      π                 r                                
     Integrating this equation using the initial condition r=r 0  at time t=0 yields:        r   =       (       r   0   2     -     wvt   π       )       1   /   2                              
     The wafer rotation speed can be expressed as a function of time as:        Ω   =     v       (       r   0   2     -     wvt   π       )       1   /   2                                
     and the head speed can be expressed as a function of time as:        u   =     wv     2          Π        (       r   0   2     -     wvt   π       )         1   /   2                                  
     FIG. 21 illustrates an extrusion spin coating spiral pattern  202  according to one aspect of the invention. The spiral pattern  202  results from the extrusion head  30  starting at the outer edge  52  of the wafer  50  and moving radially inward toward the center of the wafer  50 . A first shaded region  204  represents wasted photoresist at the outer edge of the wafer  50 , and a second shaded region  206  represents a double thickness of photoresist extruded in the center region of the wafer  50 . It is necessary to start the process with the extrusion head  50  just off the outer edge  52  of the wafer  50  to cover the entire outer edge  52  with the extruded spiral pattern  202  without unnecessary overlap or double thickness around the outer edge  52  of the wafer  50 . This results in the first shaded region  204  of wasted photoresist. Likewise, it is necessary to continue to extrude photoresist after the leading edge of the extrusion head  30  reaches the center of the wafer  50  until the entire wafer  50  is covered. Typically, it will be necessary to continue the process until the trailing edge of the extrusion head  30  reaches the center to cover the entire center region of the wafer  50 . The overlap in the second shaded region  206  at the center of the wafer  50  is inevitable because of the finite width of the extrusion head  30 . However, the amount of wasted and excess photoresist is relatively small, and the efficiency of the extrusion spin coating process far exceeds the efficiency of prior spin coating processes. 
     FIG. 21 illustrates an extrusion spin coating spiral pattern which results from starting the extrusion head at the outer edge of the wafer and, while spinning the wafer, moving the extrusion head radially inward toward the center of the wafer. The method and apparatus of the invention may instead start the extrusion head at the center of the wafer and move the extrusion head radially outward toward the outer edge of the wafer. 
     It will be readily apparent to those skilled in the art that this invention is not limited to the embodiments described above. Different configurations and embodiments can be developed without departing from the scope of the invention and are intended to be included within the scope of the claims.