Abstract:
An improved apparatus for semiconductor wafer bumping utilizes the injection molded solder process and is designed for high volume manufacturing. The apparatus includes equipment for filling patterned mold cavities on a mold structure with solder, equipment for positioning and aligning a patterned surface of a semiconductor structure directly opposite to the solder filled patterned mold cavities of the mold structure, a fixture tool for holding and transferring the aligned mold and semiconductor structures together, and equipment for receiving the fixture tool and transferring the solder from the aligned patterned mold cavities to the aligned patterned semiconductor first surface. The solder transfer equipment include a wafer heater stack configured to heat the semiconductor structure and a mold heater stack configured to heat the mold structure to a process temperature slightly above the solder&#39;s melting point. The fixture tool with the aligned mold and semiconductor structures is inserted between the wafer heater stack and the mold heater stack. A deposition chamber is formed between the wafer heater stack and the mold heater stack by sealing the wafer heater stack and the mold heater stack against the frame.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
   This application claims the benefit of U.S. provisional application Ser. No. 60/888,137 filed Feb. 5, 2007 and entitled “APPARATUS AND METHOD FOR SEMICONDUCTOR WAFER BUMPING VIA INJECTION MOLDED SOLDER”, the contents of which are expressly incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to an apparatus and a method for semiconductor wafer bumping, and more particularly to a solder transfer apparatus for semiconductor wafer bumping. 
   BACKGROUND OF THE INVENTION 
   Injection Molded Solder (IMS) is a process used to produce solder bumps on a semiconductor wafer surface. Referring to  FIG. 1 , the IMS process  30  includes depositing solder into mold cavities ( 34 ), forming a pattern on the semiconductor wafer surface ( 32 ), aligning the filled mold cavities with the patterned semiconductor wafer surface and then transferring the solder from the mold cavities to the semiconductor wafer surface ( 38 ). Solder bumps are formed in a glass mold plate  82  by injecting molten solder into the etched mold cavities. The etched cavities match the pattern of solder bumps required on the semiconductor wafer surface. The process provides fine pitch placement of the solder bumps in the range of 10 to 500 micrometers separation distance between adjacent solder bumps. 
   The IMS process has been tested and applied for laboratory scale applications. It is desirable to provide a scale-up process and a high volume manufacturing (HVM) apparatus designed to optimize the high volume manufacturing process. A critical aspect of the scale-up process involves the solder transfer apparatus and process. It is desirable to provide a sealable deposition chamber with quick heating and cooling cycles while maintaining reliable, high precision and repeatable positioning of the mold plate relative to the semiconductor wafer. 
   SUMMARY OF THE INVENTION 
   In general, in one aspect, the invention features an apparatus for forming solder bumps onto semiconductor structures including equipment for filling patterned mold cavities formed on a first surface of a mold structure with solder, equipment for positioning and aligning a patterned first surface of a semiconductor structure directly opposite to the solder filled patterned mold cavities of the mold structure, a fixture tool for holding and transferring the aligned mold and semiconductor structures together and solder transfer equipment for receiving the fixture tool with the aligned mold and semiconductor structures and transferring the solder from the aligned patterned mold cavities to the aligned patterned semiconductor first surface. The fixture tool includes a frame having a central aperture dimensioned to support a substrate and wherein a second surface opposite to the patterned first surface of the semiconductor structure is brought in contact with a first surface of the substrate. The solder transfer equipment include a wafer heater stack configured to heat the semiconductor structure and a mold heater stack configured to heat the mold structure to a process temperature slightly above the solder&#39;s melting point. The fixture tool with the aligned mold and semiconductor structures is inserted between the wafer heater stack and the mold heater stack so that a second surface opposite to the first surface of the substrate is in contact with the wafer heater stack and a second surface opposite to the first surface of the mold structure is in contact with the mold heater stack. A deposition chamber is formed between the wafer heater stack and the mold heater stack by sealing the wafer heater stack and the mold heater stack against the frame. 
   Implementations of this aspect of the invention may include one or more of the following features. The wafer heater stack is sealed against the frame via a graphite seal ring positioned between an edge of a second surface opposite to the first surface of the substrate and an edge of the central aperture. The mold heater stack is sealed against the frame via a gas bellow seal ring. The apparatus further includes equipment for injecting a process gas into the deposition chamber. The process gas comprises one of formic acid, hydrogen or nitrogen. The gas bellow seal ring comprises a seal frame and a gas bellow placed within a seal frame groove and wherein the seal frame comprises gas feed-through lines for injecting the process gas into the deposition chamber. The seal frame further comprises vacuum lines for evacuating the deposition chamber. The gas bellow comprises a material configured to withstand the process gas and the process temperature. The gas bellow material comprises Perlast. The fixture tool further comprises one or more clamp/spacer assemblies arranged symmetrically around the frame, each clamp/spacer assembly comprising a clamp configured to clamp the mold and semiconductor structures together and a spacer configured to be inserted between the first surface of the semiconductor structure and the first surface of the mold structure and thereby to separate the mold and semiconductor structures by a distance equal to the spacer&#39;s height. The spacer and the clamp within each clamp/spacer assembly are configured to move independent from each other. The clamp/spacer assemblies are configured to move independent from each other. The spacer is configured to be rotated remotely with high precision and repeatability at the process temperature. The apparatus further includes high precision rotary stroke bearings providing the high precision and repeatability of the spacer rotation. The substrate comprises a coefficient of thermal expansion (CTE) matching the semiconductor structure&#39;s CTE. The substrate comprises silicon. The substrate comprises vacuum grooves arranged radially and at the perimeter of concentric circles formed on the substrate and wherein vacuum is drawn through the vacuum grooves for holding the semiconductor structure onto the substrate. The mold heater stack comprises a mold chuck, a mold hot plate being in contact with a first surface of the mold chuck, a mold hot plate cooling flange being in contact with the mold hot plate, a mold ceramic expansion barrier being in contact with the mold hot plate cooling flange and a mold water cooled heat exchanger being in contact with the mold ceramic expansion barrier and wherein a second surface of the mold chuck opposite to the first surface is brought in contact with the second surface of the mold structure. The mold heater stack further comprising mold cooling flange air bellows passing through through-bores formed in the mold water cooled heat exchanger and the mold ceramic expansion barrier and reaching the mold hot plate cooling flange. The mold chuck comprises silicon carbide. The wafer heater stack comprises a wafer hot plate being in contact with a second surface opposite to the first surface of the substrate, a wafer hot plate cooling flange being in contact with the wafer hot plate, a wafer ceramic expansion barrier being in contact with the wafer hot plate cooling flange and a wafer water cooled heat exchanger being in contact with the wafer ceramic expansion barrier. The wafer heater stack further comprising wafer cooling flange air bellows passing through through-bores formed in the wafer water cooled heat exchanger and the wafer ceramic expansion barrier and reaching the wafer hot plate cooling flange. The ceramic expansion barriers comprise a low CTE material. The ceramic expansion barriers comprise Zerodur. An adjustable gap is formed within the deposition chamber between the first surface of the mold structure and the patterned first surface of the semiconductor structure. The gap is adjustable in the range between 0 and 3000 micrometers. The apparatus further includes a drive element configured to move the mold heater stack and the mold plate toward the semiconductor structure and thereby adjusting the gap. The mold chuck comprises vacuum grooves arranged radially and at the perimeter of concentric circles formed on the mold chuck and wherein vacuum is drawn through the vacuum grooves for holding the mold structure onto the mold chuck. 
   In general, in another aspect, the invention features a method for forming solder bumps onto semiconductor structures including the following steps. First, filling patterned mold cavities formed on a first surface of a mold structure with solder. Next, positioning and aligning a patterned first surface of a semiconductor structure directly opposite to the solder filled patterned mold cavities of the mold structure. Next, providing a fixture tool for holding and transferring the aligned mold and semiconductor structures together. The fixture tool comprises a frame having a central aperture dimensioned to support a substrate and wherein a second surface opposite to the patterned first surface of the semiconductor structure is brought in contact with a first surface of the substrate. Next, inserting the fixture tool with the aligned mold and semiconductor structures into solder transfer equipment and transferring the solder from the aligned patterned mold cavities to the aligned patterned semiconductor first surface. The solder transfer equipment includes a wafer heater stack configured to heat the semiconductor structure and a mold heater stack configured to heat the mold structure to a process temperature slightly above the solder&#39;s melting point. The fixture tool with the aligned mold and semiconductor structures is inserted between the wafer heater stack and the mold heater stack so that a second surface opposite to the first surface of the substrate is in contact with the wafer heater stack and a second surface opposite to the first surface of the mold structure is in contact with the mold heater stack. A deposition chamber is formed between the wafer heater stack and the mold heater stack by sealing the wafer heater stack and the mold heater stack against the frame. 
   The solder transferring process includes the following steps. First, preheating the mold and semiconductor structures to a first temperature below the process temperature while maintaining the aligned mold and semiconductor structures within the frame clamped to each other and to the frame via the clamps and separated from each other by the spacers. Next, injecting the process gas into the deposition chamber. Next, holding the semiconductor structure onto the substrate and the mold structure onto the mold chuck by drawing vacuum through the corresponding vacuum grooves and unclamping the clamps and heating the mold and semiconductor structures to the process temperature via the mold hot plate and the wafer hot plate, respectively, thereby melting the solder in the mold cavities. Next, removing the spacers and bringing the mold structure in contact with the semiconductor structure whereby the molten solder transfers from the mold cavities onto the patterned first surface of the semiconductor structure. Next, moving the mold structure away from the semiconductor structure, thereby separating the mold and semiconductor structures and then inserting the spacers between the separated mold and semiconductor structures and maintaining the separated mold and semiconductor structures at the process temperature for a period of time. Next, clamping the mold and semiconductor structures together and onto the frame with the clamps and turning off the vacuum. Finally, cooling down the mold and semiconductor structures to room temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the figures, wherein like numerals represent like parts throughout the several views: 
       FIG. 1  is a schematic diagram of a laboratory scale Injection Molded Solder (IMS) process; 
       FIG. 2  is a schematic diagram of a scale-up IMS process according to this invention; 
       FIG. 3  is a block diagram of the scale-up IMS line process flow; 
       FIG. 4  is a schematic diagram of the HVM IMS line equipment system according to this invention; 
       FIG. 5  is a schematic side view diagram of the mold fill process; 
       FIG. 6  is a top view diagram of the mold fill process; 
       FIG. 7A  is a magnified view of a mold area with unfilled cavities; 
       FIG. 7B  is a magnified view of a mold area with solder filled cavities; 
       FIG. 8  is a schematic diagram of the solder transfer process; 
       FIG. 9  depicts the HVM STT equipment system of this invention; 
       FIG. 10  depicts the HVM STT material flow; 
       FIG. 11  is a schematic block diagram of the STT process steps; 
       FIG. 12  is a schematic block diagram of the STT steady-state cycle time; 
       FIG. 13  depicts the HVM aligner module without the fixturing mechanisms; 
       FIG. 14  depicts the HVM aligner module components with the transport fixture (left) and without the transport fixture (right); 
       FIG. 15  depicts the STT mold/wafer transport fixture; 
       FIG. 16  is a front exploded view of the STT mold/wafer transport fixture; 
       FIG. 17  illustrates the clamp and spacer actuators; 
       FIG. 18A  illustrates the STT mold/wafer transport fixture with a 300 mm wafer; 
       FIG. 18B  illustrates the STT mold/wafer transport fixture with a 200 mm wafer; 
       FIG. 19  depicts the HVM STT chamber; 
       FIG. 20  is a cross-sectional view of the HVM STT chamber; 
       FIG. 21  is an exploded view of the HVM STT chamber; 
       FIG. 22  is an exploded perspective view of the STT chamber mold stack; 
       FIG. 23  is a perspective view of the mold heater stack; 
       FIG. 24  is an exploded perspective view of the mold heater stack of  FIG. 23 ; 
       FIG. 25  is an exploded side view of the mold heater stack of  FIG. 23 ; 
       FIG. 26  is a top perspective view of the mold heater stack seal frame; 
       FIG. 27  is a side view of the hot plate cooling flange and the mold chuck fine Z-drive; 
       FIG. 28  is a top perspective view of the wafer heater stack; 
       FIG. 29  is a side exploded view of the wafer heater stack of  FIG. 28 ; 
       FIG. 30  is top view of the transport fixture wafer chuck; 
       FIG. 31  is a side perspective view of the wafer heater stack of  FIG. 28 ; 
       FIG. 32A  is a schematic cross-sectional side view of the STT chamber with the installed transport fixture; 
       FIG. 32B  depicts a cross-sectional side view of the mold stack with the seal down; 
       FIG. 32C  depicts a cross-sectional side view of the mold stack with the seal up. 
       FIG. 33  is a high magnification image of a patterned surface of a semiconductor wafer; 
       FIG. 34  depicts selecting a first target area in the image of  FIG. 33  with a specific wafer pad pattern; 
       FIG. 35  depicts defining a wafer pad area around a pad in the first target area of  FIG. 34 ; 
       FIG. 36  depicts locating and counting all pad positions relative to the wafer target area in the first target area of  FIG. 34 ; 
       FIG. 37  depicts searching all possible target areas that match the pattern of the first target area; 
       FIG. 38  depicts counting the wafer pads in all possible target areas that match the pattern of the first target area; 
       FIG. 39  depicts finding the center coordinates for all wafer pads that were counted in  FIG. 38 ; 
       FIG. 40  depicts a preliminary target area used for training; 
       FIG. 41  depicts a first wafer pad area within the preliminary target area used for training; 
       FIG. 42  depicts measuring the X and Y boundaries of the first wafer pad area of  FIG. 41 ; 
       FIG. 43  depicts building a wafer pad mask covering 90% of the first wafer pad area of  FIG. 41 ; 
       FIG. 44  depicts placing the wafer pad mask of  FIG. 43  on the wafer pads in the preliminary target area of  FIG. 40 ; 
       FIG. 45  depicts a unique wafer target area used for the alignment process; 
       FIG. 46  depicts a block diagram of the process for identifying a unique wafer target for training the alignment system; 
       FIG. 47  depicts a block diagram of the process for identifying a unique mold target for training the alignment system; 
       FIG. 48  depicts a block diagram of the wafer/mold alignment process utilizing the uniquely identified wafer and mold target areas; 
       FIG. 49  is a continuation of the diagram of  FIG. 48 ; 
       FIG. 50  is a continuation of the diagram of  FIG. 49 ; 
       FIG. 51  depicts a block diagram of another embodiment for the process for identifying a unique wafer target for training the alignment system utilizing a mask; and 
       FIG. 52  is a continuation of  FIG. 51 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , the scale-up IMS process  50  includes filling mold cavities with solder ( 34 ), inspecting the filled mold plate ( 86 ), forming a pattern on the semiconductor wafer surface ( 32 ), inspecting the wafer surface ( 75 ) and then transferring the solder from the mold cavities to the semiconductor wafer surface ( 38 ). Referring to  FIG. 3 , the scale-up IMS process  50  includes cleaning of the molds at a mold clean station  60 , filling of the mold cavities with solder and inspecting the filled mold plate at a mold prepare station  80 , and transferring of the solder from the mold cavities onto the patterned semiconductor wafer surface at a wafer bump station  90 . The mold prepare station  80  includes a mold fill tool (MFT)  100 , a mold inspect tool (MIT)  200 , and a mold repair tool  88 . The wafer bump station  90  includes a solder transfer tool (STT)  300  and a wafer loader tool  400 . New molds  61  and previously used molds  62  pass through the mold clean station  60  where they get cleaned with an acid solution  63  and a base solution  64 . The clean molds  82  enter a mold stocker  500  and from there they are introduced into the MFT  100 . After filling the mold cavities with solder, the molds are inspected at the MIT  200  and then transferred to a ready mold stocker  550 . Molds that do not pass inspection are either recycled at the mold clean station  60  or are repaired at the repair tool  88 . Molds that are repaired pass through the MIT  200  again and upon passing the inspection are transferred to ready mold stocker  550 . In some embodiments the mold repair tool  88  is integrated with the MIT  200 . From the ready mold stocker  550  the molds are introduced into the STT  300 . Patterned wafers  74  are introduced into the wafer loader  400  and from there into the STT  300 . After the solder transfer process the bumped wafers  76  exit the wafer bump station  90  and the dirty molds  62   b  are introduced into the mold clean station  60  again. The process repeats until all wafers  74  are bumped. A schematic diagram of the HVM IMS equipment system  52  is shown in  FIG. 4 . It includes the mold clean station  60 , the mold stocker  500 , the MFT  100 , the MIT  200 , the STT  300 , the wafer loader, i.e., front open unified pod (FOUP)  400  and a mold cart  600 . In one example, the HVM system  52  has a capacity of 300 wafer per day (1 wafer every 4 minutes) and 350 molds per day (1 mold every 3.5 minutes). It provides automation of the wafer and mold transfer. The STT can process 200 mm and 300 mm wafers without any hardware changes and each mold carrier can carry up to 25 molds. The molds are identified with a bar code mechanism and the mold stocker/sorter is integrated in the process line. There is also an integrated mold and wafer tracking and management system. The system can accommodate any solder type including no lead/eutectic PbSn (low temperature) at start up and high lead later. 
   Referring to  FIG. 5  and  FIG. 6 , the mold fill process  34  includes melting bulk solder (wire, shots, slugs) in a reservoir  81 . Reservoir  81  is heated above the melting point of the solder and is slightly pressurized. An injector head  83  communicates with the reservoir  81  and is in contact with the mold plate  82 . Mold plate  82  is scanned under the injector head  83  in the scan direction  87  and molten solder is injected through a solder slot  89  formed at the bottom of the injector head  83  and fills the empty cavities  85   a  in the mold  82 . The filled mold plate is then cooled and inspected at the MIT  200 .  FIG. 7A  depicts a glass mold plate  82  with unfilled cavities  85   a  and  FIG. 7B  depicts a glass mold with filled cavities  85   b . Cavities  85  are etched on the glass mold  82  according to the required bump pattern. The glass mold  82  has a thermal expansion coefficient (CTE) similar to the CTE of the semiconductor wafer  72 . 
   Referring to  FIG. 8 , the solder transfer process  38  includes bringing together a wafer  74  patterned with under bump metallurgy (UBM) structures  73  with a mold plate  82  having solder filled cavities  85   b  ( 92 ). Next, heating the mold  82  and the wafer  74  to a temperature of 20 degrees higher than the solder melting point ( 94 ) and then bringing the mold  82  and wafer  74  in close proximity (about 20 micrometers) or soft contact so that the solder wets the UBM structures  73  ( 96 ). The solder bumps from the cavities  85   b  are transferred to the UBM structures  73  and stay on the wafer  74  after the mold  82  separates from the wafer  74  ( 98 ). A critical aspect of this process is the alignment of the mold plate  82  relative to the semiconductor wafer  74  so that the solder bumps  85   b  are transferred to precise UBM structures  73 . The alignment needs to be maintained during the transport of the aligned mold-wafer system from station to station and during the actual solder transfer process at the required temperature, atmosphere and pressure. 
   Referring to  FIG. 9 , the HVM STT equipment system  300  includes a mold/wafer aligner  800 , a mold/wafer transport fixture  900  and the solder transfer tool (STT) chamber unit  301 . Referring to  FIG. 10 ,  FIG. 11  and  FIG. 12 , the HVM STT process  100  includes the following steps. First the patterned wafer  74  enters the wafer FOUP unit  410  ( 101 ) and the filled mold  82  enters the mold pod unit  420  ( 106 ). Next, a robot end effector module (EFEM)  850  transfers the wafer  74  and the mold  82  in the transfer station  856  where the wafer  74  is optically characterized ( 102 ), pre-aligned ( 103 ) and flipped ( 104 ). The filled mold  82  is pre-aligned ( 107 ) and the mold identifying barcode is read and entered in the computer ( 108 ). Next, a robot end effector module (EFEM)  850  transfers the wafer  74  and the mold  82  in the aligner module  800 . In the aligner the wafer and mold are placed in the mold/wafer transport fixture  900  so that the mold  82  is positioned under the wafer ( 105 ,  109 ) and images of the wafer and the mold are taken ( 110 ,  112 ). The Wedge Error Compensation process is performed at this point ( 111 ). Wedge Error Compensation describes the action of “floating” the mold on the aligner chuck so that it evenly contacts all the fixture spacer flags (which in turn sit on the wafer edge). Once in even contact, the mold chuck locks and the parallelism (no wedge error) of mold to wafer is set. Next, the mold and the wafer are aligned ( 113 ) and the aligned mold and wafer are locked in the transport fixture  900  ( 114 ). Transport fixture  900  with the aligned mold  82  and wafer  74  is then transferred in the STT chamber unit  301 , where the transfer of the solder bumps takes place. The aligned mold and wafer are purged with nitrogen ( 115 ) and then are preheated ( 116 ). In one example, the temperature is increased from room temperature to 180 degrees C. in 2 minutes. At the temperature of at least 180 degrees C. the mold  82  and wafer  74  are scrubbed with an acid ( 117 ) and then the temperature is increased from 180 degrees to 280 degrees ( 118 ) in 3 minutes. Next, the mold is brought into contact (under controlled pressure) with the wafer ( 119 ) and the solder bumps  85   b  transfer from the mold cavities  85  onto the UBM pads  73  of the patterned wafer  74 . After the solder transfer the mold is separated from the wafer in a controlled way ( 120 ). The separated wafer and mold are kept at the temperature of 280 degrees C. for about 10 minutes ( 121 ) so that a good inter-metallic bond is formed between the solder bumps and the conducting patterned lines on the wafer surface (inter-metallic dwell). Next, the temperature is ramped down to 200 degrees C. in 3 minutes and the solder bumps solidify on the wafer surface ( 122 ) and then the temperature is ramped down to 60 degrees in 8 minutes. The fixture  900  with the separated mold and wafer are purged with air ( 124 ) and the alignment lock is released ( 125 ). Next the wafer is flipped  126  and then the empty mold  82  is transferred to the mold port  430  ( 127 ) and the bumped wafer  74  to the wafer FOUP unit  440  ( 128 ). The process repeats itself for the next mold/wafer pair. In one example, the solder transfer process time is 29 minutes and the alignment time is 65 sec. The STT system is designed to have more than one STT chamber units  301  to increase the production throughput. In the example of  FIG. 10 , there are four STT chamber units  301 ,  302 ,  303  and  304 . The STT system of  FIG. 10  has a throughput of 150 wafer/day and an approximate production rate of one wafer every 8 minutes. This parallel process cycle is shown schematically in  FIG. 12 . As shown, a first mold/wafer pair positioned in a fixture (n) is loaded in the aligner ( 151 ), aligned ( 152 ) and then the aligned fixture is transferred from the aligner to the process station ST 1  ( 153 ), where is processed ( 154 ). The processed fixture (n) is then unloaded from process station ST 1  to the fixture unloader/transfer station ( 156 ) and from there the EFEM unloads fixture (n) to the corresponding mold port  430  or wafer unit  440  ( 159 ). The total processing time fixture (n) is 2051 seconds and is distributed as follows: 131 seconds for loading fixture (n) in the aligner (step  151 ), 65 seconds for aligning fixture (n) (step  152 ), 41 seconds for transferring the aligned fixture (n) to process station ST 1  (step  153 ), 1742 seconds for the solder transfer process of fixture (n) (step  154 ), 25 seconds for unloading fixture (n) from ST 1  to transfer station (step  156 ), 47 seconds for moving mold and wafer to their ports (step  159 ). In the next staggered parallel process a second mold/wafer pair in fixture (n+1) is loaded in the transfer station ( 149 ), then loaded in the aligner ( 155 ), aligned ( 157 ) and then the aligned fixture is transferred from the aligner to the process station ST 2  ( 161 ), where is processed ( 162 ). The processed fixture (n+1) is then unloaded from process station ST 2  to the fixture unloader/transfer station ( 163 ) and from there the EFEM unloads fixture (n+1) to the corresponding mold port  430  or wafer unit  440  ( 164 ). The total processing time for fixture (n+1) is 2288 seconds, is distributed as above and includes 42 second for waiting in the transfer station (step  149 ). Similarly, a third mold/wafer pair in fixture (n+2) ( 158 ) is processed in the next staggered process starting at step  158  and a fourth mold/wafer pair in fixture (n+3) is processed in the next staggered process starting at step ( 177 ). 
   Referring to  FIG. 15  and  FIG. 16 , the mold/wafer transport fixture  900  includes a square frame  910  having a central aperture  911  and four clamp/spacer assemblies  930   a - 930   d . A circular ceramic chuck  920  is mounted in the central aperture  911  of the frame and a seal ring  922  is placed at the interface between the front edge of the ceramic chuck and the backside edge of the central aperture  911 . Clamp/spacer assemblies  930   a - 930   d  are mounted at the centers of each side of the square frame  910   a - 910   d , respectively. Each clamp/space assembly  930   a  includes a spacer  932   a  and a clamp  934   a . Spacer  932   a  and clamp  934   a  are independently remote controlled with actuators  832 , shown in  FIG. 17 . The motion of spacer  932   a  and clamp  934   a  is very precise and repeatable both at room temperatures and at the high temperatures where the solder transfer process takes place. In the embodiment of  FIG. 15 , spacer  923   a  and clamp  934   a  are configured to rotate around an axis perpendicular to their elongated body and passing through an end or the center of their body. The high precision and repeatable rotation of the clamps and the spacer is accomplished by using high precision rotary stroke bearings  834  along the rotation shaft, shown in  FIG. 17 . In one example, rotary stroke bearings  834  are purchased from Mahr International Co, Goettingen, Germany. In other examples linkages, cam followers, or linear slides are used to provide repeatable high precision motion of the spacers and clamps. Spacers  932   a - 933   d  and clamps  934   a ,  934   d  are dimensioned and arranged so that the transport fixture  900  can accommodate both a 300 mm and a 200 mm wafer, as shown in  FIG. 18A  and  FIG. 18B , respectively. In operation, a wafer  74  is loaded onto the silicon chuck  920  and spacers  932   a - 932   d  are placed on top of the wafer  74 . Next, a mold  82  is placed on top of the spacers  932   a - 932   d  and then clamps  934   a - 934   d  are moved over the wafer/mold stack to clamp the stack together. The clamping force is applied through the spacers and this arrangement prevents the introduction of stresses or torque on the wafer or the mold, damage of the wafer and mold surfaces, contact between the mold and wafer and helps maintain the high accuracy alignment between the wafer and mold. The ceramic chuck  920  has circular and radial grooves  922  through which vacuum (vacuum grooves) is drawn to hold the wafer  74  in contact with the chuck  920 . A vacuum pump line connects to the transport fixture  910  via the vacuum pass through elements  935 . The ceramic chuck  920  has the same CTE as the wafer  74 . In one example the chuck  920  is made of silicon and the seal ring  922  is made of graphite. In one example, the transport fixture frame  920  is made of aluminum or other thermally stable alloy and has a width of 420-430 millimeters, length of 430-440 millimeters, and a height of 40 millimeters. The central aperture  911  of the base  910  has a diameter of at least 300 millimeters to be able to accommodate substrates and wafers having diameter up to 300 millimeter. As shown in  FIG. 30 , the ceramic chuck  920  also has a raised edge  921  that contacts the graphite seal ring  922  to seal against the inner edge of the back surface of the transport fixture frame  910 . 
   Referring to  FIG. 19 ,  FIG. 20  and  FIG. 21 , STT chamber unit  301  includes a top frame  306 , a bottom frame  308 , frame Z-guide rods  309   a - 309   d , a mold heater stack  310  supported on the intermediary frame  307 , and a wafer heater stack  330  supported on the top frame  306 . 
   Referring to  FIG. 22  mold heater stack  310  includes a cooling flange gas manifold  311 , a water cooled heat exchanger  312 , a ceramic expansion barrier  313 , cooling flange air bellows  314 , a hot plate cooling flange  315 , mold hot plate  316 , mold chuck  317 , bellows gas seal  320  and a seal frame  319  for the formic acid injection. The cooling flange gas manifold  311  is positioned below the water cooled heat exchanger  312 . The heat exchanger  312  is made of a good thermal conducting material and is positioned below the ceramic expansion barrier  313 . In one example, the heat exchanger  312  is made of aluminum. The ceramic expansion barrier  313  is made of a material with a low CTE and is positioned below the hot plate cooling flange  315 . In one example barrier  313  is made of Zerodur®, a glass ceramic composite with a very low CTE, manufacture by Schott AG, Duryea, Pa., USA. The low CTE ceramic expansion barrier is capable of accommodating the high temperatures (about 300° C. or more) of the hot plate on one side while the other side is in contact with the water cooled heat exchanger which is at room temperature. Cooling flange air bellows  314  pass through through-bores formed in the water cooled heat exchanger  312  and the ceramic expansion barrier  313  to reach the cooling flange  315 . The mold hot plate  316  is placed on top of the cooling flange  315  and the mold chuck  317  is placed on top of the hot plate  316 . In one example mold chuck  317  is made of silicon carbide and hot plate  316  is a ceramic heater plate. 
   Referring to  FIG. 28  and  FIG. 29 , the wafer heater stack  330  includes a water cooled heat exchanger  331 , a ceramic expansion barrier  332 , cooling flange air bellows  333 , a hot plate cooling flange  334 , and wafer hot plate  335 . As was described above, the ceramic expansion barrier is made of a low CTE material capable of accommodating the high temperatures (about 300° C. or more) of the hot plate on one side while the other side is in contact with the water cooled heat exchanger which is at room temperature. The wafer heater stack  330  is arranged mirror imaged to the mold heater stack  310 . The wafer chuck corresponding to the mold chuck  317  is provided by the wafer chuck  920  of the transport fixture  900 , shown in  FIG. 30 . As shown in  FIG. 30 , the wafer chuck  920  has a raised edge  921  that contacts a graphite seal ring  922  to seal against the inner edge of the back surface of the transport fixture frame  910 , shown in  FIG. 32A . 
   For the solder transfer operation, the transport fixture  900  with the aligned filled mold  82  and wafer  74  is placed in the solder transfer unit  301  between the mold heater stack  310  and the wafer heater stack  330 . The transport fixture  900  is oriented between the mold stack  310  and the wafer stack  330  so that the back side of the wafer chuck  915  (shown in  FIG. 15 ) is positioned to be in contact with the wafer hot plate  335  of the wafer stack  340  and the back side of the mold plate  940  (shown in  FIG. 18B ) is positioned to be in contact with the mold chuck of the mold heater stack  310 . A temporary deposition chamber  350  is formed by bringing together the mold heater stack  310 , the transport fixture  900  and the wafer heater stack  330 , as shown in  FIG. 32A . The bottom of the temporary deposition chamber  350  is formed by the mold hot plate  316 , the mold chuck  317  and mold  82 . Mold  82  is positioned so that its back surface  940  is in contact with the mold chuck  317  and the front surface with the filed cavities faces up. The top of the temporary deposition chamber  350  is formed by the wafer hot plate, wafer chuck  920  and wafer  74 . Wafer  74  is positioned so that the patterned surface faces down directly opposite to the solder filled mold cavities. The top and bottom of the temporary chamber  350  are sealed together with the mold heater stack seal frame  319  and seal ring  320  and with the graphite seal  922  between the front raised edge of the wafer chuck  920  and the back side of the transport fixture frame  910 . As shown in  FIG. 32A ,  FIG. 32B  and  FIG. 32C , seal ring  320  is mounted on the seal frame  319  and is brought into contact with the front surface of the transport fixture frame  910  to seal the sides of the temporary chamber  350 . During the solder transfer process the wafer  74  and mold  82  are unclamped from the fixture  900  and the wafer  74  is held in contact with the wafer chuck  920 , which in turn is held by the wafer hot plate  335  and the mold  82  is held by the mold chuck  317  which in turn is held by the mold hot plate. An adjustable gap  352  is formed between the wafer  74  and the mold  82 . Gap  352  is adjustable in the range between 0 and 3000 micrometers. Gap  352  is sealed on the sides with the seal ring  320  and graphite seal  922  and a process gas  354  is injected through the openings of the seal frame into the gap area  352 . In one example, the process gas is formic acid. The process steps of  FIG. 8  take place in this temporary deposition chamber  350 . 
   Referring to  FIG. 26  seal frame  319  includes gas feed-through connectors  323   a ,  323   b  and vacuum connectors  324   a ,  324   b  connecting to formic gas lines  321  and vacuum lines  322 . Gas flow is controlled with Reed valves  325 . Seal frame  319  includes a grove  326  where the bellows gas seal (seal ring)  320  is placed. Gas seal  320  is designed to withstand the harsh chemical and thermal environment of the deposition chamber  350 . In one example gas seal  320  is made of Perlast® manufactured by Perlast Ltd, San Jose, Calif. 
   The mold  82  is transferred to the mold chuck  317  by handing off positive control of the mold  82  from the transport fixture  900 . The hand-off is performed by pinning the mold against the spacers  932   c  with the mold transfer pins  982   a ,  982   b ,  982   c  and then actuating the fixture unclamping action with the fixture indexer assemblies  980   a  (shown in  FIG. 31 ). The mold stack  310  is then driven to contact the mold, and the mold chuck  317  vacuum is actuated to mount the mold  82  to the mold chuck  317 . 
   Referring to  FIG. 13  and  FIG. 14 , the aligner module  800  includes an alignment support frame  801 , alignment stage  802  for supporting the mold/wafer transport fixture  900 , two microscope XYZ stages  804 , two microscopes  806 , a wedge compensation system  808  and alignment stage XYT drives  809 . The mold/wafer alignment process includes a mechanical pre-alignment and a pattern based image alignment. The final alignment is locked in the transport fixture  900  and is retained throughout the solder transfer process in the STT unit  301 . For the alignment process, first the transport fixture  900  is loaded in the alignment stage  802  and then the mold  82  and the wafer  74  are mechanically pre-aligned in the transport fixture  900  using fiduciary markers. The wafer  74  includes a notch in its radial periphery and three marking points on its surface that are aligned with three motorized alignment pins. The mold includes three marking points that are also aligned with the three motorized alignment pins. After the mechanical pre-alignment, the wafer patterned surface and the mold surface with the solder filled cavities are imaged and the images of the wafer and mold surfaces are aligned using a pattern recognition methodology. 
   Prior art pattern recognition methodologies utilize unique features on object surfaces. One example of such a prior art pattern recognition methodology is the Patmax® program available from Cognex Co, Natick Mass. However, in the present case, the wafer  74  and mold  82  have homogeneous distributions of uniform (circular-shaped) and homogeneous UBM structures (pads)  73  and solder bumps  85   b , respectively, and the prior art pattern recognition methodologies cannot be applied. A new process is used to define a unique wafer target area on a patterned wafer and a unique mold target area on a mold and the aligner system is trained to identify these unique wafer and mold target areas with the Patmax® program. These trained unique wafer and mold target areas are used to align the wafer/mold pairs. 
   Referring to  FIG. 46  and  FIG. 33  to  FIG. 40 , a microscope training process  600  for identifying a unique wafer target includes the following steps. First a prealigned wafer is loaded in the aligner module  800  ( 601 ). Next the microscope  806  is positioned and focused onto an area  813  of the wafer with a possible unique target in the field of view (FOV) ( 602 ), as shown in  FIG. 33 . The wafer search area  813  in the FOV is defined relative to the position of the microscope ( 603 ). Within the wafer search area  813 , a wafer target area  813   a  (training target) is defined around a unique pattern of pads, as shown in  FIG. 34 , and the system is trained with this unique wafer target ( 604 ). Within this training wafer target  813   a , a wafer pad area  812   a  around a single pad  812  is defined, as shown in  FIG. 35 , and the system is trained with this pad area ( 605 ). Next, a search is performed to locate the wafer target position within the wafer search area ( 606 ) and then all pad positions are searched and located relative to the wafer target area ( 607 ), as shown in  FIG. 36 . The relative wafer pad positions and number are saved to the corresponding wafer target area ( 608 ). The microscope is then moved in the X and Y directions to center the identified unique wafer target area  813   a  in the FOV and the position is saved relative to the wafer ( 609 ). The microscope position is locked in the X-Y plane ( 610 ). Next, the training process  600  continues with the definition of a unique mold target area on the mold and the system is trained with this mold target. Referring to  FIG. 47 , the microscope is lowered in the Z-direction and the mold stage is centered below the wafer ( 611 ). Next, a mold is loaded and prealigned on the mold stage between the wafer and the microscope ( 612 ) and the microscope is raised in the Z-direction and focused on the mold with a possible unique mold target in the FOV ( 613 ). The mold stage is moved in the X, Y directions and rotated around an axis perpendicular to its surface by an angle Theta (T) to center the unique mold target in the FOV ( 614 ). The mold search area in the FOV is defined relative to the microscope and stage positions ( 615 ) and a mold target area around a unique pattern of solder bumps is defined within the search area and the system is trained with this mold target ( 616 ). Within the mold target area, a mold solder bump area around a single solder bump is defined and the system is trained with this mold bump ( 617 ). Next, a search is performed to locate the mold target position within the mold search area ( 618 ) and then all mold bump positions are searched and located relative to the mold target area ( 619 ). The relative mold bump positions and number are saved to the corresponding mold target area ( 620 ). The mold stage is then moved in the X and Y directions to center the unique mold target area in the FOV and the mold stage offset position relative to zero is saved ( 621 ). 
   After the training process the system is ready to use the stored unique wafer target and unique mold target for the alignment of the wafer/mold pairs. Referring to  FIG. 48  and  FIG. 49 , the alignment process  810  includes the following steps. First, a prealigned wafer is loaded in the aligner ( 809 ), and the microscope is positioned and focused in the trained position ( 811 ). The wafer viewing area is searched for all possible patterns  813   b ,  813   c ,  813   d  matching the unique wafer target pattern ( 816 ), as shown in  FIG. 37 . Next, the pads in all pattern match areas are searched and counted ( 817 ), as shown in  FIG. 38 . If the number of pads in the selected pattern does not equal the number of pads in the training target area  813   a , the pattern is eliminated from consideration ( 818 ) and the next possible pattern is searched. If the number of pads in the selected pattern equals the number of pads in the training target area, the pad locations within the pattern are searched ( 819 ). For each target area whose pattern and number of pads matches the pattern and number of pads of the training target  813   a , the X-Y location coordinates of the centers of all pads are identified and compared to the X-Y location coordinates of the centers of the pads of the training target  813   a , as shown in  FIG. 39 . If the X-Y location coordinates of the pad centers do not match the X-Y location coordinates of the pad centers of the training target area, the searched patterned target area is eliminated and the next patterned target area is searched ( 820 ). If the X-Y location coordinates of the pad centers match the X-Y location coordinates of the pad centers of the training target area, the number of pattern matches found is increased by one ( 821 ). If the pattern matches found equals one, then a unique wafer target pattern is in view ( 822 ) and the microscope position is locked in the X-Y directions ( 823 ), as shown in  FIG. 40 . Next, the microscope is lowered in the Z-direction and the mold stage is centered directly below the wafer ( 824 ). The mold is loaded and prealigned onto the mold stage between the wafer and the microscope ( 825 ) and the microscope is raised in the Z-direction and focused on the mold view ( 826 ). Next, the mold stage is moved to the trained X-Y and T positions ( 827 ) and the mold viewing area is searched for all possible mold pattern matches ( 828 ) and each mold pattern match area is searched first for the correct number of solder bumps ( 829 ) and then for the correct solder bump center locations ( 831 ). If the number of solder bumps does not equal the number of solder bumps in the training target the pattern is eliminated and the next possible pattern match is searched ( 830 ). If the number of solder bumps equals the number of solder bumps in the training target the pattern is searched for the correct solder bump center locations ( 831 ). For each solder bump pattern if the center of each solder bump does not match the center of the trained solder bump, the pattern is eliminated and the next pattern is searched ( 832 ). For each solder bump pattern if the solder bump locations match the trained solder bump locations, the number of pattern matches found is increased by one ( 833 ). If the pattern matches found equals one, then a unique mold pattern is in view ( 834 ). Finally the mold stage is moved in the X-Y directions and rotated by an angle Theta in order to bring the center of the unique mold target area in line with the center of the unique wafer target area ( 835 ). 
   In another embodiment, an improved wafer/mold pattern recognition utilizes an automated mask generation for identifying ambiguous circular-shaped pads or solder bumps. Referring to  FIG. 51-52 , the process of training the system with a patterned mask  840  includes the following steps. First the microscope is positioned onto a wafer (or mold) area to define a search location containing the desired pattern ( 841 ), as was shown in  FIG. 33 . Within this pattern area, the pad target area  813   a  is defined ( 842 ), and the pad search area is set to match the pad training area ( 843 ), as shown in  FIG. 41 . Next the pads positioned within the pad training area  813   a  are identified ( 844 ) and the edges of each pad are located ( 845 ), as shown in  FIG. 42 . Next, the pad diameters in the X and Y directions are measured ( 846 ), and the pad dimensions are used to design a mask image based on the pad&#39;s center position and radius ( 847 ). Next, the pad target is retrained with the new mask  855  eliminating 90% of the pads center area ( 848 ), as shown in  FIG. 43 . Next, the search area is set to match the target pattern training area ( 849 ), the pads within the pattern target are located and counted ( 850 ) and an array of pad positions is obtained ( 851 ), as shown in  FIG. 44 . Next, a pattern mask image is built based upon the training area, the pad positions and the pad radii ( 852 ) and then the pattern target is retrained using the pattern mask ( 853 ), as shown in  FIG. 45 . The pattern mask image  813   u  is used for the image alignment process, as was described above. 
   Among the advantages of this invention may be one or more of the following. The mold chuck and wafer chuck provide uniform heat transfer to the mold and wafer respectively. Mold heater plate and wafer heater plate are made of ceramics that have a CTE matching the CTE of the mold and the wafer, respectively. A thermal expansion barrier material prevents warping of the mold stack and heater stack materials and ultimately of the molds and wafer. The cooling flanges prevent heat from escaping to the frame. The mold/wafer alignment is set at the aligner and then retained throughout the entire solder transfer process. 
   Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.