Filtering and dispensing system
This application is a continuation of application Ser. No. 07/835,632, filed Feb. 13, 1992, abandoned. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a pumping and filtering system for use in dispensing precise amounts of filtered liquid at precise rates. In particular, the invention relates to a pump for dispensing liquids such as photoresist liquids which are used to manufacture semi-conductors. 
     U.S. Pat. No. 4,950,134 shows a pump which is useful in practicing the present invention. The pump described in U.S. Pat. No. 4,950,134 is a positive displacement electro-hydraulically operated diaphragm pump particularly useful in applying photoresist liquid during the manufacture of semiconductor wafers. The content of U.S. Pat. No. 4,950,134 is incorporated herein by reference. 
     The need for precision and careful deposition of photoresist liquids during the manufacture of semi-conductors is critical. Wafers, usually about 4 to 8 inches in diameter, are positioned beneath a dispense nozzle. An initial amount of photoresist liquid sufficient to form an initial layer of about 1.5 to 5 milliliters, according to the size of the wafer, is deposited. The wafer is then rotated in order to spread the liquid evenly over the surface of the wafer. The final thickness of the layer of photoresist liquid is measured in thousands of Angstroms. 
     In some systems, photoresist liquid is deposited in the center of the wafer when the wafer is motionless. In such static dispensing systems, a substantial amount of the photoresist liquid is spun from the wafer and discarded, wasting much of the photoresist liquid initially deposited on the wafer. However, a more efficient use of photoresist liquid occurs in dynamic dispensing systems in which the photoresist is deposited on a rotating wafer. The need for precision in the duration, amount and rate of deposition is even more critical in dynamic dispensing as opposed to static dispensing. Motion of the wafer in a dynamic dispense system causes the photoresist liquid to be dispensed in a predetermined pattern. Therefore, dispensing generally, and the dispense rate in particular, must be tightly controlled so that the dispense patter is consistent from one wafer to the next. 
     The liquids which are dispensed with pumps such as the one shown in U.S. Pat. No. 4,950,134 are preferably filtered through filters having very small openings, i.e. on the order of 0.1 to 0.2 microns. As a result, pressure of between about 3 and 7 psi are required to drive the liquid through such filters. However, in precisely dispensing liquids, it has been found advantageous to draw backward or &#34;suckback&#34; liquid at the dispensing nozzle. This is because as flow is stopped, drips, or potential drips, may form at the dispense nozzle. If a drop of photoresist liquid falls on a semi-conductor wafer after the desired amount has been dispensed, the coating on the wafer can become deformed, in which case the wafer may be reprocessed. This problem is present when the wafer remains directly beneath the dispense nozzle as the photoresist is dispersed by rotation of the wafer and during movement of the wafer away from the area beneath the nozzle. 
     The use of a separate line for purposes of conveying suckback liquid allows pressure to be maintained in the main dispensing line. It is particularly important to maintain pressure on the liquid in the area of the filter, because air bubbles, which are unavoidably present in the filter medium, will decompress if pressure is removed. This decompression and the subsequent recompression would, if allowed to occur, distort the measurement of the amount of liquid to be dispensed. 
     It is therefore an object of the present invention to provide a pumping system which is capable of dispensing precise amounts of liquid at predetermined rates. 
     Another object of the present invention is to provide a pumping system that repeatably disperses precise amounts of liquid at precise rates for precise durations. 
     A further object of the present invention is to provide a pumping system which facilitates the suckback of liquid through the system. 
     Still another object of the present invention is to provide a pumping system in which the useful life of the filters used in the system is improved. 
     Yet another object of the present invention is to provide a pumping and filtering system for liquids in which the liquid flows in only one direction through the filter. 
     Still another object of the present invention is to provide a pumping and filtering system in which variations in pressure on the liquid in the area of a filter is minimized. 
     A further object of the present invention is to provide a dispensing system in which the amount and rate of dispensing is repeatable within small tolerances over many cycles. 
     These and other objects of the present invention are achieved with a pumping system in which an electro-hydraulic diaphragm pump is used to withdraw liquid from a source bottle, and, in turn, drive the source liquid through a filter medium to a dispensing port. After a particular amount of the source liquid has been directed through the filter, the pump is used to draw the liquid away from the dispensing nozzle through a suckback line around the filter. This arrangement allows for the maintainence of generally constant pressure on the liquid, thus avoiding decompression and expansion of gases trapped in the filter medium. 
     A better understanding of the present invention will be obtained upon a reading of the following specification read in conjunction with the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the pumping and filtering system of the present invention in an initial mode; 
     FIG. 2 is a schematic diagram of the pumping and filtering system of the present invention in a dispense mode; 
     FIG. 3 is a schematic diagram of the pumping and filtering system of the present invention in the suckback mode; and 
     FIG. 4 is a schematic diagram of the pumping and filtering system of the present invention in a reload mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pumping system of the present invention as shown in each of the drawings 1 through 4 includes a pump 10 in hydraulic communication in a three-way solenoid operated pump valve 12. The pump 10 is connected to the common port 14. A source bottle 18 is in hydraulic communication with the normally open port 15 of the pump valve 12. The normally closed port 16 of the pump valve 12 is connected to the main dispense line 20. A filter 22 is disposed in the main dispense line 20. A dispense line check valve 24 is also located in the main dispense line 20 between the filter 22 and the pump valve 12. The system also includes a three-way solenoid operated filter valve 23, and the main dispense line 20 is connected to the normally closed port 25 of the filter valve 23. A final dispense line 29 is connected to the common port 27 of the filter valve 23. A suckback line 26 is connected to the normally open port 28 of the filter valve 23. The suckback line 26 extends from the filter valve 23 to a point in the main dispense line 20 between the dispense line check valve 24 and the pump valve 12. A suckback check valve 21 prevents flow through the suckback line 26 in the direction of the filter valve 23. 
     FIG. 1 shows the system in an &#34;initialize&#34; mode. In the initialize mode, the pump valve 12 is not energized, which places the pump 10 in communication with the source bottle 18. Source fluid is withdrawn from the source bottle 18 into the pump 10. 
     FIG. 2 shows the system in a &#34;dispense&#34; mode. In the dispense mode, the pump valve 12 is energized, placing the pump 10 in fluid communication with the main dispense line 20. The suckback check valve 21 prevents flow of the source fluid through the suckback line 26. The driving forces of the pump 10 cause the source liquid to flow through the main dispense line 20 past the dispense check valve 24 and through the filter 22. In the dispense mode, the filter valve is energized, causing the main dispense line 20 to be in fluid communication with the final dispense line 29. As a result, the source liquid is driven through the filter valve 23 and through the final dispense line 29 out the dispense nozzle 31. In the system of the present invention, the nozzle may be a separate fitting, but need not be. The nozzle may simply be the end of a piece of tubing used to form the final dispense line 29. 
     FIG. 3 shows the system in a &#34;suckback&#34; mode. In the suckback mode, the pump valve 12 remains energized, whereas the filter valve 23 is de-energized so that the final dispense line 29 is in fluid communication with the suckback line 26. In the suckback mode, the pump 10 is reversed, drawing the source liquid away from the dispense nozzle through the final dispense line 29 and through the suckback line 26 and through the suckback check valve 21. In the suckback mode, the dispense check valve 24 prevents flow of liquid through the filter 22. Because substantial pressure, between 3 and 7 psi, is required to drive the source liquid through the filter 22 in the dispense mode, there is a short delay to achieve transition from the dispense mode to the suckback mode. The delay is due to the fact that the pump 10 must reverse and move in the negative direction to reduce the pressure in the main dispense line 20 and achieve a negative 0.5 psi, which is required to open the suckback check valve 21. The delay is on the order of about 0.5 seconds. 
     FIG. 4 shows the system in a &#34;reload&#34; mode. In the reload mode, the pump valve 12 is de-energized to allow the pump 10 to be in fluid communication with the source bottle 18 as it was in the initialized mode. 
     The pumping and filtering system of the present invention includes a filter housing 35 and a filter medium 37, shown schematically. The filter housing 35 and the filter medium 37 are preferably cylindrical elements, the filter housing 35 being generally hollow, and the filter medium 37 being a replaceable filter cartidge, such as the 0.1, 0.2 and 0.5 micron filter cartridges sold by Pall and Millipore. The filter housing 35 includes an airvent stopcock 30 to allow removal of air which inadvertently enters the system. The filter housing 35 also includes a drainstop stopcock 32 which allows removal of source liquid from the filter housing so that the filter 22 can be replaced. 
     The ability of the pumping and filtering system of the present invention to drawback fluid through the suckback line away from the dispensing nozzle without having to draw or suck fluid through the filter 22 is an important feature of the present invention. The ability to quickly draw fluid away from the dispense nozzle by use of the suckback line of the present invention allows for more precise delivery of liquids without the danger of damage to the semi-conductor wafer coating due to post-deposition dripping of liquid which is not intended to be deposited. Relieving pressure in the main dispense line 20 to perform suckback would potentially create enlarged air bubbles in the filter. In the system of the present invention, pressure is maintained on fluid in the main dispense line in the area of the filter 22 and fluid flows from the dispense nozzle through the final dispense line 29 and through the suckback line 26 around the filter 22 upon reversal of the pump 10. The system of the present invention results in improved repeatability and accuracy of dispensing by avoiding the decompression and recompression of fluid in the filter 22. 
     A further advantage of the present invention is the fact that the fluid in the portion of the main dispense line 20, which extends between the dispense check valve 24 and the filter valve 23, remains under relatively constant pressure. Small pockets of air are practically unavoidable with filter media used to filter photoresist liquids. Filters, such as the Millipore and Pall brand 0.1 and 0.2 micron filters, have inherent design features which result in small pockets of entrapped air, despite efforts to eliminate such pockets. 
     Because air is substantially more compressible than photoresist liquid, changes in pressure in lines containing such filters can cause error in measurements of volume in those lines. The system of the present invention maintains the liquid in the filter under generally constant pressure. This is achieved by timing the activation and de-activation of the filter valve 23 with the pumping action of the pump 10. When a dispense cycle is completed, the solenoid controlling the filter valve 23 is deactivated, flow from the main dispense line 20 to the final dispense line 29 ceases, and liquid is trapped between the dispense check valve 24 and the filter valve 23 and maintained at a pressure which is generally equal to the pressure at which the liquid is dispensed. When dispensing is again required, the pump 10 is activated in the forward direction just prior to re-activation of the solenoid controlling the filter valve 23. The slight delay between the time when activation of the pump 20 in the driving direction occurs and the time when the main dispense line 20 and the final dispense line 29 are in fluid communication allows for the pump 10 to build up pressure in the main dispense line 20 between the pump 10 and the dispense check valve 24. This is necessary because that portion of the main dispense line is in fluid communication with the suckback line 26, which, during the suckback mode, has a negative pressure. This negative pressure is overcome and raised to a positive pressure equal to the pressure of liquid in the filter just as the main dispense line 20 and the final dispense line 29 are opened to each other by activation of the solenoid controlling the valve 23. 
     While a specific embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that numerous alternatives, modifications, and variations of the embodiment shown can be made without departing from the spirit and scope of the appended claims.