Patent Publication Number: US-7897196-B2

Title: Error volume system and method for a pump

Description:
RELATED APPLICATIONS 
     The present Application claims under 35 U.S.C. 119(e) benefit of and priority to U.S. Provisional Patent Application No. 60/742,304 filed Dec. 5, 2005 entitled “Error Volume System and Method” by Cedrone et al., which is hereby fully incorporated by reference herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to fluid pumps. Even more particularly, embodiments of the present invention relate to error correction in a pump. 
     BACKGROUND OF THE INVENTION 
     There are many applications for which precise control over the amount and/or rate at which a fluid is dispensed by a pumping apparatus is necessary. In semiconductor processing, for example, it is important to control the amount and rate at which photochemicals, such as photoresist chemicals, are applied to a semiconductor wafer. The coatings applied to semiconductor wafers during processing typically require a flatness across the surface of the wafer that is measured in angstroms. The rates at which processing chemicals are applied to the wafer has to be controlled in order to ensure that the processing liquid is applied uniformly. 
     Pumps and the related system components for dispensing a fluid to a wafer generally have some amount of compliance. That is, they tend to expand in size based on the amount of pressure asserted on them. Consequently, some amount of work produced by the pump goes to the system compliance rather than moving fluid. If the pump and system compliance is not accounted for, the pump can dispense less fluid than intended or can produce a dispense with poor fluid characteristics. Therefore, there is a need for a system and method to account for the overall compliance of a dispense system. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide systems and methods for reducing the error in the amount of a fluid a pump dispenses. 
     One embodiment of the present invention includes method for compensating for errors in dispense volumes of a dispense pump comprising determining a dispense volume amount from a dispense recipe, determining a value for a fluid property (e.g., viscosity or other property) based on the dispense recipe, determining an error volume amount based on the value of the fluid property from a correlation between the error volume and the fluid property that accounts for compliance in a dispense system and controlling a dispense motor to move a piston in the dispense pump to a position to account for the dispense volume amount determined from the recipe and the error volume amount to dispense the dispense volume amount of fluid from a nozzle. The method can also include compensating for other error volumes, such as user specified volumes. The pump can be controlled to move the piston to a position that accounts for the dispense volume and the error volumes in a time indicated by the recipe to dispense the dispense volume. 
     Another embodiment of the present invention includes a multi-stage pump comprising a pump body defining a dispense chamber, a diaphragm disposed in the dispense chamber, a piston reciprocating in the dispense chamber to move the diaphragm, a motor coupled to the piston to reciprocate the piston, and a controller coupled to the motor (i.e., able to directly or indirectly control the motor). The controller can include a memory storing a correlation between a fluid property and an error volume. Additionally, the controller can be operable to determine a dispense volume amount from a dispense recipe, determine a value for a fluid property based on the dispense recipe, access the memory to determine an error volume amount based on the value of the fluid property from the correlation and control the dispense motor to move the piston to a position associated by the controller with displacing at least the error volume amount and the dispense volume amount. 
     Another embodiment of the present invention comprises a method for compensating for system compliance in a dispense operation performed by a pump that includes portions performed with a test pump installed in a test dispense system and portions performed with a pump installed in a semiconductor manufacturing facility. The pump installed in the semiconductor manufacturing facility can be the same as or different than the test pump. With the test pump, the method can comprise performing a set of test dispenses with corresponding desired dispense volume amounts with a set of test fluids having various values for a fluid property and analyzing a set of actual dispense volume amounts of the test dispenses relative to the desired dispense volume amounts to determine a correlation between the fluid property and the error volume that accounts for compliance in a dispense system (i.e., the pump, tubing and associated components that exhibit compliance when fluid is dispensed from the pump to a site). With the pump installed in a semiconductor manufacturing facility, the method can include determining a desired manufacturing process dispense volume amount from a dispense recipe for dispensing a process fluid, determining a fluid property value for a process fluid based on the dispense recipe, determining an error volume amount based on the fluid property value for the process fluid from the correlation between the fluid property and the error volume and controlling a dispense motor to move a piston to a position to account for the desired manufacturing process dispense volume amount determined from the recipe and the error volume amount to dispense the dispense volume amount of fluid from a nozzle to a wafer. 
     Example steps that can be preformed at the test pump include a) performing test dispenses with a corresponding desired dispense volume amount with a selected test fluid from the set of test fluids, b) determining an average actual dispense volume amount, c) repeating steps a-b for each of a set of additional desired dispense volume amounts, d) repeating steps a-c selecting a new test fluid as the selected test fluid from the set of test fluids, wherein each test fluid has a different value for the fluid property and e) determining a relationship between error volume and the fluid property based on the average actual dispense volume amounts and the corresponding desired dispense volume amounts. 
     Embodiments of the present invention provide advantages over previous pumping systems by increasing the accuracy of a dispense operation. 
     Embodiments of the present invention provide another advantage over previous methods of compensating for error by compensating for compliance in an entire dispense system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
         FIG. 1  is a diagrammatic representation of one embodiment of a pumping system; 
         FIG. 2  is a diagrammatic representation of a multiple stage pump (“multi-stage pump”) according to one embodiment of the present invention; 
         FIGS. 3A ,  3 B,  4 A,  4 C, and  4 D are diagrammatic representations of various embodiments of a multi-stage pump; 
         FIG. 4B  is a diagrammatic representation of one embodiment of a dispense block; 
         FIG. 5A  is a diagrammatic representation of one embodiment of a portion of a multi-stage pump; 
         FIG. 5B  is diagrammatic representation of a section of the embodiment of multi-stage pump of  FIG. 5A  including the dispense chamber; 
         FIG. 5C  is a diagrammatic representation of a section of the embodiment of multi-stage pump of  FIG. 5B ; 
         FIG. 6  is a diagrammatic representation of a motor assembly with a brushless DC motor, according to one embodiment of the invention; 
         FIG. 7  is a diagrammatic representation of a system to determine a correlation between error volume and a fluid property for a dispense system; 
         FIG. 8  is an example chart providing a correlation between error volume and viscosity; 
         FIG. 9  is a flow chart illustrating one embodiment of determining the correlation between error volume and a fluid property; 
         FIG. 10  is a flow chart illustrating one embodiment of a method for controlling a pump; and 
         FIG. 11  is a diagrammatic representation of a single stage pump. 
     
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the present invention are illustrated in the FIGURES, like numerals being used to refer to like and corresponding parts of the various drawings. 
     Embodiments of the present invention are related to a pumping system that accurately dispenses fluid using a multiple stage (“multi-stage”) pump. Embodiments of the present invention provide systems and methods for reducing the error in the amount of a fluid a pump dispenses by factoring in the compliance—that is the change in shape due to pressure—of a dispense system. 
     Generally speaking, in a diaphragm pump, the displacement of a piston in a chamber will displace a particular amount of fluid. In a rigid system, the amount of fluid displaced for a particular piston displacement would not vary regardless of pressure. However, most systems have some amount of compliance (e.g., stretching of parts due to pressure) leading to the problem that the same amount of piston displacement will dispense different amounts of liquid depending on the pressure. The difference between the desired dispense volume and the amount of fluid that a pump actually dispenses is referred to as an error volume. Embodiments of the present invention provide systems and methods to reduce the error volume by providing a mechanism through which the error volume is predicted and taken into account when moving the piston. 
     For context,  FIGS. 1-6  provide examples of dispenses systems and a multi-stage dispense pump for which error volume compensation can be implemented. Additional embodiments of multi-stage pumps are described in U.S. Provisional Patent Application No. 60/742,435, entitled “SYSTEM AND METHOD FOR MULTI-STAGE PUMP WITH REDUCED FORM FACTOR”, by Inventors Cedrone et al., filed Dec. 5, 2005 and U.S. patent application Ser. No. 11/602,464, entitled “SYSTEM AND METHOD FOR A PUMP WITH REDUCED FORM FACTOR”, by Inventors Cedrone et al., filed Nov. 20, 2006. It should, however, be understood that embodiments of the present invention can be implemented in other systems and pumps.  FIG. 1  is a diagrammatic representation of a pumping system  10 . The pumping system  10  can include a fluid source  15 , a pump controller  20  and a multi-stage pump  100 , which work together to dispense fluid onto a wafer  25 . The operation of multi-stage pump  100  can be controlled by pump controller  20 , which can be onboard multi-stage pump  100  or connected to multi-stage pump  100  via a one or more communications links for communicating control signals, data or other information. Additionally, the functionality of pump controller  20  can be distributed between an onboard controller and another controller. Pump controller  20  can include a computer readable medium  27  (e.g., RAM, ROM, Flash memory, optical disk, magnetic drive or other computer readable medium) containing a set of control instructions  30  for controlling the operation of multi-stage pump  100 . A processor  35  (e.g., CPU, ASIC, RISC, DSP or other processor) can execute the instructions. One example of a processor is the Texas Instruments TMS320F2812PGFA 16-bit DSP (Texas Instruments is Dallas, Tex. based company). In the embodiment of  FIG. 1 , controller  20  communicates with multi-stage pump  100  via communications links  40  and  45 . Communications links  40  and  45  can be networks (e.g., Ethernet, wireless network, global area network, DeviceNet network or other network known or developed in the art), a bus (e.g., SCSI bus) or other communications link. Controller  20  can be implemented as an onboard PCB board, remote controller or in other suitable manner. Pump controller  20  can include appropriate interfaces (e.g., network interfaces, I/O interfaces, analog to digital converters and other components) to controller to communicate with multi-stage pump  100 . Additionally, pump controller  20  can include a variety of computer components known in the art including processors, memories, interfaces, display devices, peripherals or other computer components not shown for the sake of simplicity. Pump controller  20  can control various valves and motors in multi-stage pump to cause multi-stage pump to accurately dispense fluids, including low viscosity fluids (i.e., less than  100  centipoise) or other fluids. An I/O interface connector as described in U.S. Patent Application Ser. No. 60/741,657, entitled “I/O INTERFACE SYSTEM AND METHOD FOR A PUMP,” by Cedrone et al., filed Dec. 2, 2005 and U.S. patent application Ser. No. 11/602,449, entitled “I/O SYSTEMS, METHODS AND DEVICES FOR INTERFACING A PUMP CONTROLLER”, by Inventors Cedrone et al., filed Nov. 20, 2006, which are hereby fully incorporated by reference herein, can be used to connected pump controller  20  to a variety of interfaces and manufacturing tools. 
       FIG. 2  is a diagrammatic representation of a multi-stage pump  100 . Multi-stage pump  100  includes a feed stage portion  105  and a separate dispense stage portion  110 . Located between feed stage portion  105  and dispense stage portion  110 , from a fluid flow perspective, is filter  120  to filter impurities from the process fluid. A number of valves can control fluid flow through multi-stage pump  100  including, for example, inlet valve  125 , isolation valve  130 , barrier valve  135 , purge valve  140 , vent valve  145  and outlet valve  147 . Dispense stage portion  110  can further include a pressure sensor  112  that determines the pressure of fluid at dispense stage  110 . The pressure determined by pressure sensor  112  can be used to control the speed of the various pumps as described below. Example pressure sensors include ceramic and polymer pesioresistive and capacitive pressure sensors, including those manufactured by Metallux AG, of Korb, Germany. According to one embodiment, the face of pressure sensor  112  that contacts the process fluid is perfluoropolymer. Pump  100  can include additional pressure sensors, such as a pressure sensor to read pressure in feed chamber  155 . 
     Feed stage  105  and dispense stage  110  can include rolling diaphragm pumps to pump fluid in multi-stage pump  100 . Feed-stage pump  150  (“feed pump  150 ”), for example, includes a feed chamber  155  to collect fluid, a feed stage diaphragm  160  to move within feed chamber  155  and displace fluid, a piston  165  to move feed stage diaphragm  160 , a lead screw  170  and a stepper motor  175 . Lead screw  170  couples to stepper motor  175  through a nut, gear or other mechanism for imparting energy from the motor to lead screw  170 . According to one embodiment, feed motor  170  rotates a nut that, in turn, rotates lead screw  170 , causing piston  165  to actuate. Dispense-stage pump  180  (“dispense pump  180 ”) can similarly include a dispense chamber  185 , a dispense stage diaphragm  190 , a piston  192 , a lead screw  195 , and a dispense motor  200 . Dispense motor  200  can drive lead screw  195  through a threaded nut (e.g., a Torlon or other material nut). 
     According to other embodiments, feed stage  105  and dispense stage  110  can be a variety of other pumps including pneumatically or hydraulically actuated pumps, hydraulic pumps or other pumps. One example of a multi-stage pump using a pneumatically actuated pump for the feed stage and a stepper motor driven hydraulic pump is described in U.S. patent application Ser. No. 11/051,576, entitled “PUMP CONTROLLER FOR PRECISION PUMPING APPARATUS”, by Inventors Zagars et al., filed Feb. 4, 2005. The use of motors at both stages, however, provides an advantage in that the hydraulic piping, control systems and fluids are eliminated, thereby reducing space and potential leaks. 
     Feed motor  175  and dispense motor  200  can be any suitable motor. According to one embodiment, dispense motor  200  is a Permanent-Magnet Synchronous Motor (“PMSM”). The PMSM can be controlled by a digital signal processor (“DSP”) utilizing Field-Oriented Control (“FOC”) or other type of position/speed control known in the art at motor  200 , a controller onboard multi-stage pump  100  or a separate pump controller (e.g. as shown in  FIG. 1 ). PMSM  200  can further include an encoder (e.g., a fine line rotary position encoder) for real time feedback of dispense motor  200 &#39;s position. The use of a position sensor gives accurate and repeatable control of the position of piston  192 , which leads to accurate and repeatable control over fluid movements in dispense chamber  185 . For, example, using a 2000 line encoder, which according to one embodiment gives 8000 pulses to the DSP it is possible to accurately measure to and control at 0.045 degrees of rotation. In addition, a PMSM can run at low velocities with little or no vibration. Feed motor  175  can also be a PMSM or a stepper motor. It should also be noted that the feed pump can include a home sensor to indicate when the feed pump is in its home position. 
     During operation of multi-stage pump  100 , the valves of multi-stage pump  100  are opened or closed to allow or restrict fluid flow to various portions of multi-stage pump  100 . According to one embodiment, these valves can be pneumatically actuated (i.e., gas driven) diaphragm valves that open or close depending on whether pressure or a vacuum is asserted. However, in other embodiments of the present invention, any suitable valve can be used. 
     The following provides a summary of various stages of operation of multi-stage pump  100 . However, multi-stage pump  100  can be controlled according to a variety of control schemes including, but not limited to those described in U.S. Provisional Patent Application No. 60/742,168, entitled “SYSTEM AND METHOD FOR VALVE SEQUENCING IN A PUMP,” by Gonnella et al., filed Dec. 2, 2005; U.S. patent application Ser. No. 11/602,465 entitled “SYSTEM AND METHOD FOR VALVE SEQUENCING IN A PUMP”, by Inventors Gonnella, et al., filed Nov. 20, 2006; U.S. Provisional Patent Application No. 60/741,682, entitled “SYSTEM AND METHOD FOR PRESSURE COMPENSATION IN A PUMP” by Inventors Cedrone et al., filed Dec. 2, 2005; U.S. patent application Ser. No. 11/602,508 entitled “SYSTEM AND METHOD FOR PRESSURE COMPENSATION IN A PUMP” by Inventors Cedrone et al., filed Nov. 20, 2006; U.S. Provisional Patent Application No. 60/741,657, entitled “I/O Interface System and Method for a Pump,” by Cedrone et al., filed Dec. 2, 2005; U.S. patent application Ser. No. 11/602,449, entitled “I/O SYSTEMS, METHODS AND DEVICES FOR INTERFACING A PUMP CONTROLLER”, by Inventors Cedrone et al., filed Nov. 20, 2006, U.S. patent application Ser. No. 11/502,729 entitled “SYSTEMS AND METHODS FOR FLUID FLOW CONTROL IN AN IMMERSION LITHOGRAPHY SYSTEM” by Inventors Clarke et al., filed Aug. 11, 2006, Provisional Patent Application No. 60/741,681, entitled “SYSTEM AND METHOD FOR CORRECTING FOR PRESSURE VARIATIONS USING A MOTOR” by Gonnella et al., filed Dec. 2, 2005; U.S. patent application Ser. No. 11/602,472, entitled “SYSTEM AND METHOD FOR CORRECTING FOR PRESSURE VARIATIONS USING A MOTOR” by inventors Cedrone et al., filed Nov. 20, 2006; U.S. patent application Ser. No. 11/292,559 entitled “SYSTEM AND METHOD FOR CONTROL OF FLUID PRESSURE” by Inventors Gonnella et al., filed Dec. 2, 2005; U.S. patent application Ser. No. 11/364,286 entitled “SYSTEM AND METHOD FOR MONITORING OPERATION OF A PUMP” by Inventors Gonnella et al., filed Feb. 28, 2006, each of which is fully incorporated by reference herein, to sequence valves and control pressure. According to one embodiment, multi-stage pump  100  can include a ready segment, dispense segment, fill segment, pre-filtration segment, filtration segment, vent segment, purge segment and static purge segment. During the feed segment, inlet valve  125  is opened and feed stage pump  150  moves (e.g., pulls) feed stage diaphragm  160  to draw fluid into feed chamber  155 . Once a sufficient amount of fluid has filled feed chamber  155 , inlet valve  125  is closed. During the filtration segment, feed-stage pump  150  moves feed stage diaphragm  160  to displace fluid from feed chamber  155 . Isolation valve  130  and barrier valve  135  are opened to allow fluid to flow through filter  120  to dispense chamber  185 . Isolation valve  130 , according to one embodiment, can be opened first (e.g., in the “pre-filtration segment”) to allow pressure to build in filter  120  and then barrier valve  135  opened to allow fluid flow into dispense chamber  185 . According to other embodiments, both isolation valve  130  and barrier valve  135  can be opened and the feed pump moved to build pressure on the dispense side of the filter. During the filtration segment, dispense pump  180  can be brought to its home position. As described in U.S. Provisional Patent Application No. 60/630,384, entitled “System and Method for a Variable Home Position Dispense System” by Laverdiere, et al. filed Nov. 23, 2004 and PCT Application No. PCT/US2005/042127, entitled “System and Method for Variable Home Position Dispense System”, by Applicant Entegris, Inc. and Inventors Laverdiere et al., filed Nov. 21, 2005, the home position of the dispense pump can be a position that gives the greatest available volume at the dispense pump for the dispense cycle, but is less than the maximum available volume that the dispense pump could provide. The home position is selected based on various parameters for the dispense cycle to reduce unused hold up volume of multi-stage pump  100 . Feed pump  150  can similarly be brought to a home position that provides a volume that is less than its maximum available volume. 
     At the beginning of the vent segment, isolation valve  130  is opened, barrier valve  135  closed and vent valve  145  opened. In another embodiment, barrier valve  135  can remain open during the vent segment and close at the end of the vent segment. During this time, if barrier valve  135  is open, the pressure can be understood by the controller because the pressure in the dispense chamber, which can be measured by pressure sensor  112 , will be affected by the pressure in filter  120 . Feed-stage pump  150  applies pressure to the fluid to remove air bubbles from filter  120  through open vent valve  145 . Feed-stage pump  150  can be controlled to cause venting to occur at a predefined rate, allowing for longer vent times and lower vent rates, thereby allowing for accurate control of the amount of vent waste. If feed pump is a pneumatic style pump, a fluid flow restriction can be placed in the vent fluid path, and the pneumatic pressure applied to feed pump can be increased or decreased in order to maintain a “venting” set point pressure, giving some control of an other wise un-controlled method. 
     At the beginning of the purge segment, isolation valve  130  is closed, barrier valve  135 , if it is open in the vent segment, is closed, vent valve  145  closed, and purge valve  140  opened and inlet valve  125  opened. Dispense pump  180  applies pressure to the fluid in dispense chamber  185  to vent air bubbles through purge valve  140 . During the static purge segment, dispense pump  180  is stopped, but purge valve  140  remains open to continue to vent air. Any excess fluid removed during the purge or static purge segments can be routed out of multi-stage pump  100  (e.g., returned to the fluid source or discarded) or recycled to feed-stage pump  150 . During the ready segment, inlet valve  125 , isolation valve  130  and barrier valve  135  can be opened and purge valve  140  closed so that feed-stage pump  150  can reach ambient pressure of the source (e.g., the source bottle). According to other embodiments, all the valves can be closed at the ready segment. 
     During the dispense segment, outlet valve  147  opens and dispense pump  180  applies pressure to the fluid in dispense chamber  185 . Because outlet valve  147  may react to controls more slowly than dispense pump  180 , outlet valve  147  can be opened first and some predetermined period of time later dispense motor  200  started. This prevents dispense pump  180  from pushing fluid through a partially opened outlet valve  147 . Moreover, this prevents fluid moving up the dispense nozzle caused by the valve opening, followed by forward fluid motion caused by motor action. In other embodiments, outlet valve  147  can be opened and dispense begun by dispense pump  180  simultaneously. 
     An additional suckback segment can be performed in which excess fluid in the dispense nozzle is removed. During the suckback segment, outlet valve  147  can close and a secondary motor or vacuum can be used to suck excess fluid out of the outlet nozzle. Alternatively, outlet valve  147  can remain open and dispense motor  200  can be reversed to such fluid back into the dispense chamber. The suckback segment helps prevent dripping of excess fluid onto the wafer. 
       FIG. 3A  is a diagrammatic representation of one embodiment of a pump assembly for multi-stage pump  100 . Multi-stage pump  100  can include a dispense block  205  that defines various fluid flow paths through multi-stage pump  100  and at least partially defines feed chamber  155  and dispense chamber  185 . Dispense pump block  205 , according to one embodiment, can be a unitary block of PTFE, modified PTFE or other material. Because these materials do not react with or is minimally reactive with many process fluids, the use of these materials allows flow passages and pump chambers to be machined directly into dispense block  205  with a minimum of additional hardware. Dispense block  205  consequently reduces the need for piping by providing an integrated fluid manifold. 
     Dispense block  205  can include various external inlets and outlets including, for example, inlet  210  through which the fluid is received, vent outlet  215  for venting fluid during the vent segment, and dispense outlet  220  through which fluid is dispensed during the dispense segment. Dispense block  205 , in the example of  FIG. 3A , does not include an external purge outlet as purged fluid is routed back to the feed chamber (as shown in  FIG. 4A  and  FIG. 4B ). In other embodiments of the present invention, however, fluid can be purged externally. U.S. Provisional Patent Application Ser. No. 60/741,667, entitled “O-Ring-Less Low Profile Fitting and Assembly Thereof” by Iraj Gashgaee, filed Dec. 2, 2005, and U.S. patent application Ser. No. 11/602,513, entitled “O-RING-LESS LOW PROFILE FITTINGS AND FITTING ASSEMBLIES”, by Inventor Gashgaee, filed Nov. 20, 2006, which are hereby fully incorporated by reference herein, describe an embodiment of fitting that can be utilized to connect the external inlets and outlets of dispense block  205  to fluid lines. 
     Dispense block  205  routes fluid to the feed pump, dispense pump and filter  120 . A pump cover  225  can protect feed motor  175  and dispense motor  200  from damage, while piston housing  227  can provide protection for piston  165  and piston  192  and, according to one embodiment of the present invention, be formed of polyethylene or other polymer. Valve plate  230  provides a valve housing for a system of valves (e.g., inlet valve  125 , isolation valve  130 , barrier valve  135 , purge valve  140  and vent valve  145  of  FIG. 2 ) that can be configured to direct fluid flow to various components of multi-stage pump  100 . According to one embodiment, each of inlet valve  125 , isolation valve  130 , barrier valve  135 , purge valve  140  and vent valve  145 , is at least partially integrated into valve plate  230  and is a diaphragm valve that is either opened or closed depending on whether pressure or vacuum is applied to the corresponding diaphragm. In other embodiments, some of the valves may be external to dispense block  205  or arranged in additional valve plates. According to one embodiment, a sheet of PTFE is sandwiched between valve plate  230  and dispense block  205  to form the diaphragms of the various valves. Valve plate  230  includes a valve control inlet for each valve to apply pressure or vacuum to the corresponding diaphragm. For example, inlet  235  corresponds to barrier valve  135 , inlet  240  to purge valve  140 , inlet  245  to isolation valve  130 , inlet  250  to vent valve  145 , and inlet  255  to inlet valve  125  (outlet valve  147  is external in this case). By the selective application of pressure or vacuum to the inlets, the corresponding valves are opened and closed. 
     A valve control gas and vacuum are provided to valve plate  230  via valve control supply lines  260 , which run from a valve control manifold (covered by pump cover  263  or housing cover  225 ), through dispense block  205  to valve plate  230 . Valve control gas supply inlet  265  provides a pressurized gas to the valve control manifold and vacuum inlet  270  provides vacuum (or low pressure) to the valve control manifold. The valve control manifold acts as a three way valve to route pressurized gas or vacuum to the appropriate inlets of valve plate  230  via supply lines  260  to actuate the corresponding valve(s). 
       FIG. 3B  is a diagrammatic representation of another embodiment of multistage pump  100 . Many of the features shown in  FIG. 3B  are similar to those described in conjunction with  FIG. 3A  above. However, the embodiment of  FIG. 3B  includes several features to prevent fluid drips from entering the area of multi-stage pump  100  housing electronics. Fluid drips can occur, for example, when an operator connects or disconnects a tube from inlet  210 , outlet  215  or vent  220 . The “drip-proof” features are designed to prevent drips of potentially harmful chemicals from entering the pump, particularly the electronics chamber and do not necessarily require that the pump be “water-proof” (e.g., submersible in fluid without leakage). According to other embodiments, the pump can be fully sealed. 
     According to one embodiment, dispense block  205  can include a vertically protruding flange or lip  272  protruding outward from the edge of dispense block  205  that meets top cover  263 . On the top edge, according to one embodiment, the top of top cover  263  is flush with the top surface of lip  272 . This causes drips near the top interface of dispense block  205  and top cover  263  to tend to run onto dispense block  205 , rather than through the interface. On the sides, however, top cover  263  is flush with the base of lip  272  or otherwise inwardly offset from the outer surface of lip  272 . This causes drips to tend to flow down the corner created by top cover  263  and lip  272 , rather than between top cover  263  and dispense block  205 . Additionally, a rubber seal is placed between the top edge of top cover  263  and back plate  271  to prevent drips from leaking between top cover  263  and back plate  271 . 
     Dispense block  205  can also include sloped feature  273  that includes a sloped surface defined in dispense block  205  that slopes down and away from the area of pump  100  housing electronics. Consequently, drips near the top of dispense block  205  are lead away from the electronics. Additionally, pump cover  225  can also be offset slightly inwards from the outer side edges of dispense block  205  so that drips down the side of pump  100  will tend to flow past the interface of pump cover  225  and other portions of pump  100 . 
     According to one embodiment of the present invention, wherever a metal cover interfaces with dispense block  205 , the vertical surfaces of the metal cover can be slightly inwardly offset (e.g., 1/64 of an inch or 0.396875 millimeters) from the corresponding vertical surface of dispense block  205 . Additionally, multi-stage pump  100  can include seals, sloped features and other features to prevent drips from entering portions of multi-stage pump  100  housing electronics. Furthermore, as shown in  FIG. 4A , discussed below, back plate  271  can include features to further “drip-proof” multi-stage pump  100 . 
       FIG. 4A  is a diagrammatic representation of one embodiment of multi-stage pump  100  with dispense block  205  made transparent to show the fluid flow passages defined there through. Dispense block  205  defines various chambers and fluid flow passages for multi-stage pump  100 . According to one embodiment, feed chamber  155  and dispense chamber  185  can be machined directly into dispense block  205 . Additionally, various flow passages can be machined into dispense block  205 . Fluid flow passage  275  (shown in  FIG. 4C ) runs from inlet  210  to the inlet valve. Fluid flow passage  280  runs from the inlet valve to feed chamber  155 , to complete the pump inlet path from inlet  210  to feed pump  150 . Inlet valve  125  in valve housing  230  regulates flow between inlet  210  and feed pump  150 . Flow passage  285  routes fluid from feed pump  150  to isolation valve  130  in valve plate  230 . The output of isolation valve  130  is routed to filter  120  by another flow passage (not shown). These flow paths act as a feed stage outlet flow path to filter  120 . Fluid flows from filter  120  through flow passages that connect filter  120  to the vent valve  145  and barrier valve  135 . The output of vent valve  145  is routed to vent outlet  215  to complete a vent flow path while the output of barrier valve  135  is routed to dispense pump  180  via flow passage  290 . Thus, the flow passage from filter  120  to barrier valve  135  and flow passage  290  act as feed stage inlet flow path. Dispense pump, during the dispense segment, can output fluid to outlet  220  via flow passage  295  (e.g., a pump outlet flow path) or, in the purge segment, to the purge valve through flow passage  300 . During the purge segment, fluid can be returned to feed pump  150  through flow passage  305 . Thus, flow passage  300  and flow passage  305  act as a purge flow path to return fluid to feed chamber  155 . Because the fluid flow passages can be formed directly in the PTFE (or other material) block, dispense block  205  can act as the piping for the process fluid between various components of multi-stage pump  100 , obviating or reducing the need for additional tubing. In other cases, tubing can be inserted into dispense block  205  to define the fluid flow passages.  FIG. 4B  provides a diagrammatic representation of dispense block  205  made transparent to show several of the flow passages therein, according to one embodiment. 
     Returning to  FIG. 4A ,  FIG. 4A  also shows multi-stage pump  100  with pump cover  225  and top cover  263  removed to show feed pump  150 , including feed stage motor  190 , dispense pump  180 , including dispense motor  200 , and valve control manifold  302 . According to one embodiment of the present invention, portions of feed pump  150 , dispense pump  180  and valve plate  230  can be coupled to dispense block  205  using bars (e.g., metal bars) inserted into corresponding cavities in dispense block  205 . Each bar can include on or more threaded holes to receive a screw. As an example, dispense motor  200  and piston housing  227  can be mounted to dispense block  205  via one or more screws (e.g., screw  312  and screw  314 ) that run through screw holes in dispense block  205  to thread into corresponding holes in bar  316 . It should be noted that this mechanism for coupling components to dispense block  205  is provided by way of example and any suitable attachment mechanism can be used. 
     Back plate  271 , according to one embodiment of the present invention, can include inwardly extending tabs (e.g., bracket  274 ) to which top cover  263  and pump cover  225  mount. Because top cover  263  and pump cover  225  overlap bracket  274  (e.g., at the bottom and back edges of top cover  263  and the top and back edges pump cover  225 ) drips are prevented from flowing into the electronics area between any space between the bottom edge of top cover  263  and the top edge of pump cover  225  or at the back edges of top cover  263  and pump cover  225 . 
     Manifold  302 , according to one embodiment of the present invention can include a set of solenoid valves to selectively direct pressure/vacuum to valve plate  230 . When a particular solenoid is on thereby directing vacuum or pressure to a valve, depending on implementation, the solenoid will generate heat. According to one embodiment, manifold  302  is mounted below a PCB board (which is mounted to back plate  271  and better shown in  FIG. 4C ) away from dispense block  205  and particularly dispense chamber  185 . Manifold  302  can be mounted to a bracket that is, in turn, mounted to back plate  271  or can otherwise be coupled to back plate  271 . This helps prevent heat from the solenoids in manifold  302  from affecting fluid in dispense block  205 . Back plate  271  can be made of stainless steel, machined aluminum or other material that can dissipate heat from manifold  302  and the PCB. Put another way, back plate  271  can act as a heat dissipating bracket for manifold  302  and the PCB. Pump  100  can be further mounted to a surface or other structure to which heat can be conducted by back plate  271 . Thus, back plate  271  and the structure to which it is attached act as a heat sink for manifold  302  and the electronics of pump  100 . 
       FIG. 4C  is a diagrammatic representation of multi-stage pump  100  showing supply lines  260  for providing pressure or vacuum to valve plate  230 . As discussed in conjunction with  FIG. 3 , the valves in valve plate  230  can be configured to allow fluid to flow to various components of multi-stage pump  100 . Actuation of the valves is controlled by the valve control manifold  302  that directs either pressure or vacuum to each supply line  260 . Each supply line  260  can include a fitting (an example fitting is indicated at  318 ) with a small orifice. This orifice may be of a smaller diameter than the diameter of the corresponding supply line  260  to which fitting  318  is attached. In one embodiment, the orifice may be approximately 0.010 inches in diameter. Thus, the orifice of fitting  318  may serve to place a restriction in supply line  260 . The orifice in each supply line  260  helps mitigate the effects of sharp pressure differences between the application of pressure and vacuum to the supply line and thus may smooth transitions between the application of pressure and vacuum to the valve. In other words, the orifice helps reduce the impact of pressure changes on the diaphragm of the downstream valve. This allows the valve to open and close more smoothly which may lead to increased to smoother pressure transitions within the system which may be caused by the opening and closing of the valve and may in fact increase the longevity of the valve itself. 
       FIG. 4C  also illustrates PCB  397 . Manifold  302 , according to one embodiment of the present invention, can receive signals from PCB board  397  to cause solenoids to open/close to direct vacuum/pressure to the various supply lines  260  to control the valves of multi-stage pump  100 . Again, as shown in  FIG. 4C , manifold  302  can be located at the distal end of PCB  397  from dispense block  205  to reduce the affects of heat on the fluid in dispense block  205 . Additionally, to the extent feasible based on PCB design and space constraints, components that generate heat can be placed on the side of PCB away from dispense block  205 , again reducing the affects of heat. Heat from manifold  302  and PCB  397  can be dissipated by back plate  271 .  FIG. 4D , on the other hand, is a diagrammatic representation of an embodiment of pump  100  in which manifold  302  is mounted directly to dispense block  205 . 
       FIG. 5A  illustrates a side view of a portion of multi-stage pump  100  including dispense block  205 , valve plate  230 , piston housing  227 , lead screw  170  and lead screw  195 .  FIG. 5B  illustrates a section view A-A of  FIG. 5A  showing dispense block  205 , dispense chamber  185 , piston housing  227 , lead screw  195 , piston  192  and dispense diaphragm  190 . As shown in  FIG. 5B , dispense chamber  185  can be at least partially defined by dispense block  205 . As lead screw  195  rotates, piston  192  can move up (relative to the alignment shown in  FIG. 5B ) to displace dispense diaphragm  190 , thereby causing fluid in dispense chamber  185  to exit the chamber via outlet flow passage  295  or purge flow passage  300 . It should be noted that the entrances and exits of the flow passages can be variously placed in dispense chamber  185 .  FIG. 5C  illustrates a section of  FIG. 5B . In the embodiment shown in  FIG. 5C , dispense diaphragm  190  includes a tong  395  that fits into a grove  400  in dispense block  200 . The edge of dispense diaphragm  190 , in this embodiment, is thus sealed between piston housing  227  and dispense block  205 . According to one embodiment, dispense pump and/or feed pump  150  can be a rolling diaphragm pump. 
     It should be noted that the multi-stage pump  100  described in conjunction with  FIGS. 1-5C  is provided by way of example, but not limitation, and embodiments of the present invention can be implemented for other multi-stage pump configurations. 
     As discussed above, feed pump  150  according to one embodiment of the present invention can be driven by a stepper motor while dispense pump  180  can be driven by a brushless DC motor or PSMS motor.  FIG. 6  below describe an embodiment of a motor assembly usable according to various embodiments of the present invention. 
       FIG. 6  is a schematic representation of a particular embodiment of a motor assembly  600  with a motor  630  and a position sensor  640  coupled thereto, according to one embodiment of the invention. In the example shown in  FIG. 6 , a diaphragm assembly  610  is connected to motor  630  via a lead screw  620 . In one embodiment, motor  630  is a permanent magnet synchronous motor (“PMSM”). Embodiments of a control schemes for a PMSM motor are described in U.S. Provisional Patent Application No. 60/741,660, entitled “SYSTEM AND METHOD FOR POSITION CONTROL OF A MECHANICAL PISTON IN A PUMP”, by inventors Gonnella et al., filed Dec. 2, 2005, U.S. Provisional Patent Application No. 60/841,725, entitled “SYSTEM AND METHOD FOR POSITION CONTROL OF A MECHANICAL PISTON IN A PUMP”, by inventors Gonnella et al., filed Sep. 1, 2006, and U.S. patent application Ser. No. 11/602,485, entitled “SYSTEM AND METHOD FOR POSITION CONTROL OF A MECHANICAL PISTON IN A PUMP”, by Inventors Gonnella et al., filed Nov. 20, 2006, which are hereby fully incorporated by reference herein. In a brush DC motor, the current polarity is altered by the commutator and brushes. However, in a PMSM, the polarity reversal is performed by power transistors switching in synchronization with the rotor position. Hence, a PMSM can be characterized as “brushless” and is considered more reliable than brush DC motors. Additionally, a PMSM can achieve higher efficiency by generating the rotor magnetic flux with rotor magnets. Other advantages of a PMSM include reduced vibration, reduced noises (by the elimination of brushes), efficient heat dissipation, smaller foot prints and low rotor inertia. Depending upon how the stator is wounded, the back-electromagnetic force, which is induced in the stator by the motion of the rotor, can have different profiles. One profile may have a trapezoidal shape and another profile may have a sinusoidal shape. Within this disclosure, the term PMSM is intended to represent all types of brushless permanent magnet motors and is used interchangeably with the term brushless DC motors (“BLDCM”). 
     PMSM  630  can be utilized as feed motor  175  and/or dispense motor  200  as described above. In one embodiment, pump  100  utilizes a stepper motor as feed motor  175  and PMSM  630  as dispense motor  200 . Suitable motors and associated parts may be obtained from EAD Motors of Dover, N.H., USA or the like. In operation, the stator of BLDCM  630  generates a stator flux and the rotor generates a rotor flux. The interaction between the stator flux and the rotor flux defines the torque and hence the speed of BLDCM  630 . In one embodiment, a digital signal processor (DSP) is used to implement all of the field-oriented control (FOC). The FOC algorithms are realized in computer-executable software instructions embodied in a computer-readable medium. Digital signal processors, alone with on-chip hardware peripherals, are now available with the computational power, speed, and programmability to control the BLDCM  630  and completely execute the FOC algorithms in microseconds with relatively insignificant add-on costs. One example of a DSP that can be utilized to implement embodiments of the invention disclosed herein is a 16-bit DSP available from Texas Instruments, Inc. based in Dallas, Tex., USA (part number TMS320F2812PGFA). 
     BLDCM  630  can incorporate at least one position sensor to sense the actual rotor position. In one embodiment, the position sensor may be external to BLDCM  630 . In one embodiment, the position sensor may be internal to BLDCM  630 . In one embodiment, BLDCM  630  may be sensorless. In the example shown in  FIG. 6 , position sensor  640  is coupled to BLDCM  630  for real time feedback of BLDCM  630 &#39;s actual rotor position, which is used by the DSP to control BLDCM  630 . An added benefit of having position sensor  640  is that it proves extremely accurate and repeatable control of the position of a mechanical piston (e.g., piston  192  of  FIG. 2 ), which means extremely accurately and repeatable control over fluid movements and dispense amounts in a piston displacement dispense pump (e.g., dispense pump  180  of  FIG. 2 ). In one embodiment, position sensor  640  is a fine line rotary position encoder. In one embodiment, position sensor  640  is a  2000  line encoder. Using a 2000 line encoder, it is possible to accurately measure to and control at 0.045 degrees of rotation. 
     BLDCM  630  can be run at very low speeds and still maintain a constant velocity, which means little or no vibration. In other technologies such as stepper motors it has been impossible to run at lower speeds without introducing vibration into the pumping system, which was caused by poor constant velocity control. This variation would cause poor dispense performance and results in a very narrow window range of operation. Although a particular motor assembly is shown, embodiments of the present invention can be implemented using a variety of motor assemblies for the feed and/or dispense motors. 
     Typically, dispense operations require dispensing fluid at a specified flow rate for a specified time so that a correct volume of fluid is dispensed during the time period. The flow rate of a fluid in a dispense system depends on the viscosity of the fluid and the pressure asserted on the fluid. In addition to dispensing a particular amount of fluid in a specified amount of time, it is desirable that the fluid dispenses as a fairly uniform column. An “good” dispense can be visualized as a straight column of fluid with perhaps some tapering at the ends as the outlet valve opens and closes, but without discontinuities, drips or significant deformations to the column. 
     Returning to  FIGS. 2 and 3A , in a perfectly rigid system dispense piston  192  would always move the same amount to displace a particular volume of fluid with a good shape, regardless of the viscosity of the fluid. In actuality, however, dispense pump  100  and other components of the dispense system exhibit compliance. That is, the various components of the dispense system tend to stretch or expand under pressure, with the amount of compliance depending on the pressure. As dispense piston  192  moves, some of the movement goes into the compliance of the system. When dispense piston  192  stops moving, the components can contract, returning to their original volume. This can create problems with the quality of the column of dispensed fluid as the last part of the column is moved by the components returning to their unstrained (or less strained) states. As an example, assume a piston moves x distance, corresponding to a 1 mL dispense. Some of the volume of fluid will be dispensed, say 0.9 mL, while some of the volume of fluid, say 0.1 mL, takes up the additional volume caused by compliance. When the piston stops moving (and if the outlet valve is not closed), the additional 0.1 mL will dispense as the tubing, diaphragm and other components contract. While the proper 1 mL may be dispensed, the last 0.1 mL will typically not have a good shape as there may be discontinuities, drips or waves in the fluid column. Embodiments of the present invention can compensate for this by moving the piston further and closing the outlet valve when the proper amount of fluid has been dispensed to achieve a good dispense (e.g., a dispense with a substantially uniform fluid column). 
     An error volume can be determined for a dispense system including multi-stage pump  100  based on the viscosity of the process fluid (or other parameters). The error volume is a volume added to (or subtracted from) the dispense volume to compensate for the difference between a programmed dispense amount and the amount of fluid dispense pump  100  would dispense in the absence of factoring in an error volume (e.g., assuming that the outlet valve closes at the same time in either case). The error volume may be the result of the physical or control characteristics of pump  100 , process variables or the system to which pump  100  is connected. The error volume can be translated into an additional amount the motor must move to provide the desired dispense amount. The pump controller can control the dispense motor to move the piston to a position that accounts for the dispense volume and the error volume. For example, if the dispense volume is 1 mL and the error volume is 0.1 mL, the pump controller can control the dispense motor to move the piston to a position that, according to the controller, corresponds to a 1.1 mL dispense. Due to compliance in the system, only 1 mL is actually dispensed in the time period. 
     Various methods can be used to determine the compliance of the pump and/or overall dispense system during a dispense operation. According to one embodiment, a length of tubing of known diameter and compliance is connected to outlet  210  and extended vertically. Dispense chamber  185  is filled with fluid so that a column of fluid fills a portion of the tubing and any air in chamber  185  is vented. The position of the top of the fluid column at atmospheric pressure is marked. Pressure can then be applied to the end of the tubing distal from the pump, thereby pressurizing the liquid column and the liquid in dispense chamber  185 . This will cause the column of liquid to move down the tube. By measuring the difference between the position of the top of the column of fluid at the start and the position of the top of the column of fluid after the pressure is applied, the volumetric change based on pressure can be determined because the diameter of the tube is known (i.e., a drop of 1 millimeter will correspond to a particular number of cubic centimeters of fluid, based on the diameter of the tube). This volumetric change is caused by the compliance of the tube and the pump. The volumetric change due to the known compliance of the tube can be subtracted out to determine the compliance of just the pump. 
     The volumetric error caused by compliance of the pump can be added to a desired dispense volume to more accurately achieve the desired dispense volume. By way of example, if a pump has an error of 0.02 milliliters at a pressure of 5 psi above atmospheric and a dispense recipe requires a dispense of 1 milliliter of fluid at a particular flow rate that corresponds to a dispense pressure of 5 psi above atmospheric, the pump controller will move piston  192  an amount that, at atmospheric pressure (or in a perfectly rigid system) would cause the pump to dispense 1.02 milliliters of fluid. Put another way, the pump controller will cause dispense motor  200  to move extra distance to make up for the compliance of the pump at 5 psi. 
     A pump is rarely used in isolation, however, and methodologies that simply-account for the compliance of the pump do not adequately compensate for the compliance of the overall dispense system including the pump and additional components. Additionally, the above method does not account of the fact that a rolling diaphragm may have different compliances at the same pressure at different stages in movement. Furthermore, methods such as the one described above that rely on simply asserting a pressure on the fluid in a dispense chamber do not account for the fact that the valve timings and other control processes may reduce the pump compliance during dispense. Embodiments of the present invention provide a method to better determine the error volume caused by compliance in the overall system (including the pump) in a dispense operation to accurately dispense fluid in manufacturing facility. According to one embodiment, a pump can be calibrated in a test system designed to simulate the environment in which the pump will operated. The data generated from the calibration can be stored in a pump controller and used to determine the appropriate error volume for a given process recipe for dispensing a process fluid in a semiconductor manufacturing facility. 
       FIG. 7  illustrates one embodiment of a setup for determining an error correction based on viscosity for a pump. It should be noted that the dimensions provided are provided by way of example and not limitation. Embodiments of the present invention can be implemented in a wide variety of test systems. The inlet and vent of multistage pump  100  are put in fluid communication with a fluid source  700  through tubing (in this example, 76 inches (193.04 centimeters) of tubing for the inlet and 36 inches (91.44 centimeters) of tubing for the vent, both ¼ inch OD×0.156 inch (0.396 centimeter) ID tubing). The outlet of multi-stage pump  100  is routed to an outlet valve  147  and suckback valve  704  through 15 feet of ¼ inch (0.635 centimeter) OD×0.157 inch (0.399 centimeter) ID tubing. From outlet valve  147  and suckback valve  704 , pump  100  is in fluid communication with a calibrated balance (e.g., scale) (not shown) through 55 inches (139.7 centimeters) of 4 mm OD×0.3 mm ID tubing and a nozzle. At the end of the 55 inches (139.7 centimeters) of 4 mm OD tubing is a 2 mm ID nozzle. 
     A solenoid valve  706  (e.g., an SMC VQ11Y-5M solenoid valve from SMC Corporation of America of Indianapolis, Ind., USA) provides pressure to suckback valve  704  (e.g., needle valve part no. CKD AS1201FM of CKD USA Corp. of Rolling Meadows, Ill., USA and suckback valve CKDAMDSZO-XO388) and outlet valve  147  through 15 inches of 4 mm OD×2.5 mm ID tubing. Solenoid valve  706  regulates 60 psi of pressure to outlet valve  147  and suckback valve  706  to open or close these valves. Additionally, 20 in Hg vacuum and 38-40 psi pressurized gas are provided to pump  100  to open close the various valves in valve plate  230  as described above. 
     According to one embodiment, pump  100  is primed with 4 cP viscosity standard, measure density of fluid and the dispense rate is set to 1.0 mL/sec. The dispense cycle is set to dispense 1 mL of fluid. The fluid is dispensed onto a calibrated balance (i.e., a scale) and the mass of 5 dispenses is recorded to find the average mass. The dispense volume is then changed 2 mL of fluid. Again, 5 dispenses are performed to a calibrated balance and the average mass is found. The process of finding the average mass dispensed for five dispenses is repeated for settings 4, 6, 8, and 10 mL dispense volumes. The process of finding the average mass of 5 dispenses for each set dispense volume (e.g., 1, 2, 4, 6, 8 and 10 mL) is repeated for 23, 45, 65 and 100 viscosity fluids. While specific examples of dispense amounts and viscosities are provided above, these are provided by of example and not limitation. 
     The viscosity based error volume (e.g., the difference between the average volume actually dispensed and the dispense volume setting) is plotted as a function of viscosity and a curve fit performed. This curve fit represents the error between a user defined dispense volume and the amount the pump would actually dispense. The curve (or a table representing the curve) can be saved in the firmware of pump  100 . When a user sets up a dispense cycle, the user can enter the viscosity of the process fluid so that the pump can apply the appropriate error correction. Additional tables or curves can be developed if it is anticipated that dispenses will occur at different dispense rates. The calibration data generated using a particular pump can be installed in a set of pumps having common characteristics. 
     The embodiment of  FIG. 7  illustrates one embodiment of a system that can be used for determining the correlation between viscosity (or other parameter) and error volume. Components of the test setup can be selected to approximate components in the anticipated manufacturing environment. For example, the outlet tubing from the pump  100  to outlet valve  147  (stop valve) can be 4-5 meters of 5-6.5 mm OD, 4-4.35 ID tubing. Outlet valve  147  can be a separate outlet valve or combination outlet valve, suckback valve such as a CKDAMDSZOX0388 by CKD USA Corp. of Rolling Meadows, Ill., USA. The tubing from outlet valve  147  (or the suckback valve) can be 4 mm OD, 2 mm ID tubing of approximately 1 to 1.5 meters long. Again, it should be noted that the various sizes and parts are provided by way of example and not limitation. 
       FIG. 8  is a graph plotting volume error as a function of viscosity. It can be seen from this example graph that the error volume is approximately linear based on the viscosity of the process fluid. Thus, for example, if a user sets a dispense of 5 mL of 10 cP fluid, pump  100  can factor in the volume error of 0.052106 mL for 10 cP fluid. On the other hand, if the user sets a dispense of 5 mL of 20 cP fluid, pump  100  can factor in the volume error of 0.088935 mL. 
     It should be noted that other embodiments of the present invention can include different test setups (e.g., different lengths and diameters of tubing, different parts and different operating conditions). Additionally, testing can be performed using more or less dispense volumes and viscosities of fluids. Other schemes of determining the volume error can also be implemented. 
     When the pump is installed in the manufacturing facility, a user can enter a recipe (e.g., dispense amount, dispense time or flow rate, fluid viscosity or other parameters). Based on the fluid viscosity (or other fluid property), the pump controller can determine the appropriate error volume based on the correlation between the fluid property and error volume (e.g., through calculation, lookup table or other mechanism). Using the graph of  FIG. 8 , if the user enters a recipe for a fluid with a viscosity of 2 cP, a dispense volume of 2 mL and a flow rate of 1 mL/sec, the pump controller can automatically add 0.05211 mL to the 2 mL dispense. During dispense, the pump controller can cause dispense motor  200  to move piston  192  to a position to account for the dispense volume of 2 mL and the error volume of 0.05211 μL. Because of the compliance in the dispense system (including the pump  100 ), the amount dispensed will be approximately 2 mL. 
     The actual dispense system in which pump  100  is installed may differ from the test system in which the correlation between error volume and viscosity or other fluid property is developed. Therefore, even applying the error volume according to  FIG. 8  may leave some small amount of error between the desired dispense and the actual dispense. According to one embodiment, the user can be given the option to specify a user specified error volume that is added to the dispense volume in addition to the error volume determined from the correlation (e.g., in addition to the viscosity based error volume). During dispense the pump controller can control dispense motor  200  to move piston  192  to a position, that according to the pump controller, accounts for the dispense volume, the viscosity based error volume and the user defined error volume. 
     If the pump is moved at the same velocity to a position that accounts for the dispense volume and the error volume(s) as it would move to just displace the dispense volume, the actual dispense rate will be below that specified in the recipe and the dispense time too long because the piston is traveling a longer distance at the same speed. To compensate for this, the pump controller can control dispense motor  200  to move to the appropriate position to account for the error volume(s) in the time prescribed by the recipe. Using the previous example, the pump controller can control dispense motor  200  to move piston  192  to a position to account for the 2 mL dispense volume, the 0.05211 mL viscosity error volume and the user specified error volume in 2 seconds based on the 2 cc dispense at 1 cc/sec specified in the original recipe. Consequently, the correct amount of fluid is dispensed in the correct amount of time. In any case, according to an embodiment, the outlet valve can be closed when piston  192  reaches the appropriate position so that additional fluid is not dispensed by contraction of system components. 
       FIG. 9  is a flow chart illustrating one embodiment of a method for determining an error volume for a pump. The steps of  FIG. 9  can be performed in a test system designed to simulate expected manufacturing dispense systems. A test pump can be used to develop the correlation between a fluid property and error volume and the correlation propagated to multiple pumps, which may include the test pump, to be installed at a semiconductor manufacturing facility. At step  900  a pump is installed in a test dispense system that reasonably simulates an intended dispense environment. The controller of the test pump can initially be configured such that a particular position of the piston (e.g., based on actual position or displacement relative to a starting position) corresponds to a particular dispense volume. At step  902 , a recipe including a dispense volume is programmed into the pump. The pump, at step  904 , runs a dispense according to a recipe to dispense a volume of fluid. During the dispense, the pump controller can control the dispense motor to move the piston a distance corresponding to the dispense volume (i.e., the distance the controller is configured to associate with the dispense volume). At step  906 , the dispensed fluid is measured to determine the volume of fluid actually dispensed. For example, when using a scale, the mass is determined and the mass divided by the density to determine the volume. 
     Steps  904  and  906  can be repeated any number of times with the same recipe and fluid. At step  908 , the dispense volume and the results of measuring the actual dispense volumes can be analyzed to determine an error volume for the fluid. For example, the desired dispense volume specified in the recipe can be subtracted from the average dispense volume for a number of dispenses, say five dispenses, to determine the error volume under a particular set of conditions. Steps  902 - 906  can be repeated for a recipe having a new desired dispense volume and steps  902  through  908  can be repeated using a new fluid having a different value for the fluid property for which the correlation is being developed. At step  910 , a correlation between error volume and viscosity (or other property of the fluid) determined. It should be noted that the correlation between error volume and fluid property can be done in terms of any measure corresponding to volume, such as an actual volume measure, a measure piston displacement distance, a mass, or other measure that corresponds to volume. 
       FIG. 10  illustrates one embodiment of a method for operating a pump to account for error volume. It is assumed, for purposes of  FIG. 10 , that the pump is installed in a semiconductor manufacturing facility and is programmed with the correlation(s) between error volume and fluid property as described above. At step  1000 , a user can enter a recipe including, for example, a dispense volume (or information from which the dispense volume can be derived), a dispense time (or flow rate), and a fluid type (or viscosity). Based on the recipe, the pump controller, at step  1002 , can determine a dispense volume amount, a value for the fluid property (e.g., viscosity) and, based on the correlation between error volume and fluid property, an error volume amount. This can be done, for example, through the use of a lookup table, calculations or other mechanism that utilizes the error volume correlations. It should be noted that in determining the dispense volume amount and error volume amounts can be any measure that corresponds to volume including a volume measure, a distance measure (e.g., the error volume amount can be a measure of how far to move the piston to displace a particular volume), or other measure that corresponds to volume. 
     If there are multiple correlation curves or sets of correlation data, the pump can select the correlation that best fits the recipe provided by the user. As another example, if the pump includes a correlation curve between viscosity and error volume for a 1 cc/sec dispense and for a 10 cc/sec dispense, the pump can select the correlation that more closely fits the recipe parameters. According to yet another embodiment, the pump controller can interpolate correlation data for recipe if the correlation data does not match a particular recipe. For example, if the pump controller has correlation data between viscosity and error volume for a 1 cc dispense and for a 10 cc dispense, but the recipe calls for a 7 cc/sec dispense, the pump controller can interpolate the relationship between viscosity and error volume for the 7 cc/sec dispense. 
     At step  1004 , the pump controller can receive an additional error volume that can be user specified. A user, for example, can run a dispense that accounts for the error volume known to the pump controller (i.e., based on the correlations) and determine that the pump is still slightly under-dispensing fluid. This can occur if the actual dispense system or recipe varies significantly from the conditions under which the correlation data is developed. The user can provide the appropriate additional error volume to the pump controller. 
     At step  1006 , the pump can perform a dispense. In the dispense, the pump controller can control the dispense motor to move to a position that, according to the controller, accounts for the dispense volume plus the error volume(s). In other words, the pump controller can convert the dispense volume plus the error volume(s) to a position or displacement (if not already measured as positions or displacements) and can control the dispense motor accordingly to move the piston to a particular position. However, because of compliance in the system, only the dispense volume is actually dispensed to the wafer. According to one embodiment, the controller can control dispense motor such that the dispense of fluid occurs in the time specified by the recipe. This can include controlling the dispense motor to move at a higher velocity to cover the greater distance required by the error volumes. 
     Various steps of  FIGS. 9 and 10  can be implemented as computer instructions (e.g., computer instructions  30  of  FIG. 1 ) stored on a computer readable medium (e.g., computer readable medium  27  of  FIG. 1 ). The steps of  FIGS. 9 and 10  can be repeated as needed or desired. 
     Although described in terms of a multi-stage pump, embodiments of the present invention can also be utilized in a single stage pump.  FIG. 11  is a diagrammatic representation of one embodiment of a pump assembly for a pump  4000 . Pump  4000  can be similar to one stage, say the dispense stage, of multi-stage pump  100  described above and can include a rolling diaphragm pump driven by a stepper, brushless DC or other motor. Pump  4000  can include a dispense block  4005  that defines various fluid flow paths through pump  4000  and at least partially defines a pump chamber. Dispense pump block  4005 , according to one embodiment, can be a unitary block of PTFE, modified PTFE or other material. Because these materials do not react with or are minimally reactive with many process fluids, the use of these materials allows flow passages and the pump chamber to be machined directly into dispense block  4005  with a minimum of additional hardware. Dispense block  4005  consequently reduces the need for piping by providing an integrated fluid manifold. 
     Dispense block  4005  can include various external inlets and outlets including, for example, inlet  4010  through which the fluid is received, purge/vent outlet  4015  for purging/venting fluid, and dispense outlet  4020  through which fluid is dispensed during the dispense segment. Dispense block  4005 , in the example of  FIG. 23 , includes the external purge outlet  4010  as the pump only has one chamber. U.S. Patent Application Ser. No. 60/741,667, entitled “O-RING-LESS LOW PROFILE FITTING AND ASSEMBLY THEREOF” by Iraj Gashgaee, filed Dec. 2, 2005, and U.S. patent application Ser. No. 11/602,513, entitled “O-RING-LESS LOW PROFILE FITTINGS AND FITTING ASSEMBLIES”, by Inventor Iraj Gashgaee, filed Nov. 20, 2006, which are hereby fully incorporated by reference herein, describes an embodiment of fittings that can be utilized to connect the external inlets and outlets of dispense block  4005  to fluid lines. 
     Dispense block  4005  routes fluid from the inlet to an inlet valve (e.g., at least partially defined by valve plate  4030 ), from the inlet valve to the pump chamber, from the pump chamber to a vent/purge valve and from the pump chamber to outlet  4020 . A pump cover  4225  can protect a pump motor from damage, while piston housing  4027  can provide protection for a piston and, according to one embodiment of the present invention, be formed of polyethylene or other polymer. Valve plate  4030  provides a valve housing for a system of valves (e.g., an inlet valve, and a purge/vent valve) that can be configured to direct fluid flow to various components of pump  4000 . Valve plate  4030  and the corresponding valves can be formed similarly to the manner described in conjunction with valve plate  230 , discussed above. According to one embodiment, each of the inlet valve and the purge/vent valve is at least partially integrated into valve plate  4030  and is a diaphragm valve that is either opened or closed depending on whether pressure or vacuum is applied to the corresponding diaphragm. In other embodiments, some of the valves may be external to dispense block  4005  or arranged in additional valve plates. According to one embodiment, a sheet of PTFE is sandwiched between valve plate  4030  and dispense block  4005  to form the diaphragms of the various valves. Valve plate  4030  includes a valve control inlet (not shown) for each valve to apply pressure or vacuum to the corresponding diaphragm. 
     As with multi-stage pump  100 , pump  4000  can include several features to prevent fluid drips from entering the area of multi-stage pump  100  housing electronics. The “drip proof” features can include protruding lips, sloped features, seals between components, offsets at metal/polymer interfaces and other features described above to isolate electronics from drips. The electronics and manifold and PCB board can be configured similarly to the manner described above to reduce the effects of heat on fluid in the pump chamber. 
     Thus, embodiments of the present invention can include a method for compensating for errors in dispense volumes of a pump comprising determining a dispense volume amount from a dispense recipe, determining a value for a fluid property based on the dispense recipe, determining an error volume amount based on the value of the fluid property from a correlation between the error volume and the fluid property that accounts for compliance in a dispense system and controlling a dispense motor to move a piston in the dispense pump to a position to account for the dispense volume amount determined from the recipe and the error volume amount to dispense the dispense volume amount of fluid from a nozzle. 
     Although the present invention has been described in detail herein with reference to the illustrative embodiments, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments of this invention and additional embodiments of this invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the scope of this invention as claimed.