Abstract:
The present invention relates to chemical delivery systems and methods for delivery of liquid chemicals. In one embodiment, the present invention relates to systems having multi-reservoir load cell assemblies for delivering chemicals used in the semiconductor industry. In one embodiment, the present invention provides a multi-reservoir load cell assembly, including a controller, a buffer reservoir, a main reservoir, one or more load cells, coupled to the assembly and to the controller, operable to weigh the liquid in the reservoir(s), a plurality of supply lines, each supply line having a valve and connecting one of the supply containers to the main reservoir, and a gas and vacuum sources for withdrawing the liquid from the assembly when demanded by the controller and for refilling the assembly from the supply containers.

Description:
[0001]    This application is a continuation of U.S. application Ser. No. 09/968,566, filed on Sep. 29, 2001, which is a continuation of U.S. application Ser. No. 09/870,227, filed on May 30, 2001, now U.S. Pat. No. 6,340,098, which is a continuation of U.S. application Ser. No. 09/568,926, tiled on Feb. 13, 2001, now U.S. Pat. No. 6,269,975, which is a continuing prosecution application of U.S. application Ser. No. 09/568,926, filed on May 10, 2000, now abandoned, which is a divisional of U.S. application Ser. No. 09/224,607, filed on Dec. 31, 1998, now U.S. Pat. No. 6,098,843, which is a continuation of U.S. application Ser. No. 09/222,003, filed on Dec. 30, 1998, now abandoned. This application incorporates by reference each application and each patent listed above. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates generally to systems and methods for mixing and/or delivering of liquid chemical(s), and more particularly, to systems and methods for mixing and delivering liquid chemicals in precise amounts using logic devices and multi-reservoir load cell assemblies.  
           [0003]    The present invention has many applications, but may be explained by considering the problem of how to deliver photoresist to silicon wafers for exposure of the photoresist in the process of photolithography. To form the precise images required, the photoresist must be delivered in precise amounts on demand, be free of bubbles, and be of precise uniform thickness on the usable part of the wafer. The conventional systems have problems as discussed below.  
           [0004]    As shown in FIG. 1, a representative conventional photoresist delivery system includes supply containers  100 ,  102 , typically bottles, which supply photoresist to a single-reservoir  104  by line  117 , which is connected to supply lines  106 ,  108  monitored by bubble sensors  110 ,  112  and controlled by valves V 1  and V 2 . The bottom of the reservoir is connected to a photoresist output line  114  to a track tool (not shown), which dispenses photoresist on the wafer. The space above the photoresist in the reservoir  104  is connected to a gas line  118  which, based on position of a three-way valve V 3 , either supplies nitrogen gas to the reservoir  104  from a nitrogen manifold line  126 , regulated by needle valve  1 , or produces a vacuum in the reservoir  104 . To sense the level of the photoresist in the reservoir  104 , the system employs an array of capacitive sensors  122  arranged vertically on the walls of the reservoir  104 . A two-way valve V 4 , located between the nitrogen gas manifold and the inlet of a vacuum ejector  124 , supplies or cuts off flow of nitrogen to the vacuum ejector  124 .  
           [0005]    The photoresist delivery system must be “on-line” at all times so the track tool can dispense the photoresist as required. Many of the photoresist delivery systems attempt to use the reservoir to provide an on-line supply of photoresist to the track tool, but the photoresist delivery system must still refill the reservoir on a regular basis, which is dependent on timely replacement of empty supply containers. Otherwise, the track tool will still fail to deliver the photoresist when demanded.  
           [0006]    During dispense mode, when the track tool withdraws photoresist from the reservoir  104 , the valve V 3  permits the nitrogen to flow from the nitrogen manifold to the reservoir  104  to produce a nitrogen blanket over the photoresist to reduce contamination and to prevent a vacuum from forming as the photoresist level drops in the reservoir. Once the photoresist in the reservoir  104  reaches a sufficiently low level the system controller (not shown) initiates refill mode, where a set of problems arise.  
           [0007]    During refill mode, the valve V 4  is activated so that nitrogen flows from the manifold line  126  to the vacuum ejector  124 , which produces a low pressure line  170  thereby producing a low pressure space above the photoresist in the reservoir  104 . The bubble sensors  110 ,  112  monitor for bubbles in the supply lines  106 , 108 , presumed to develop when the supply containers  100 , 102 , become empty. If, for example, the bubble sensor  110  detects a bubble, the controller turns off the valve V 1  to supply container  100  and the valve V 2  opens to supply container  102  to continue refilling the reservoir  104 . However, bubbles in the supply line  106  may not mean supply container  100  is empty. Thus, not all of the photoresist in supply container  100  may be used before the system switches to the supply container  102  for photoresist. Thus, although the conventional system is intended to allow multiple supply containers to replenish the reservoir when needed, the system may indicate that a supply container is empty and needs to be replaced before necessary.  
           [0008]    If the supply container  100  becomes empty and the operator fails to replace it and the system continues to operate until the supply container  102  also becomes empty, the reservoir  104  will reach a critical low level condition. If this continues, bubbles may be arise due to photoresist&#39;s high susceptibility to bubbles; if a bubble, however minute, enters the photoresist delivered to the wafer, an imperfect image may be formed in the photolithography process.  
           [0009]    Further, if the pump of the track tool, connected downstream of the chemical output line  114 , turns on when the reservoir is refilling, the pump will experience negative pressure from the vacuum in the single-reservoir pulling against the pump. Several things can happen if this persists: the lack of photoresist delivered to the track tool may send a false signal that the supply containers are empty, the pump can fail to deliver photoresist to its own internal chambers, lose its prime and ability to adequately dispense photoresist, and the pump can even overheat and burn out. The result of each scenario will be the track tool receives insufficient or even no photoresist, known as a “missed shot,” which impacts the yield of the track tool.  
           [0010]    The present invention also may be explained by considering the problems associated with mixing and delivering slurry for chemical mechanical polishing (CMP). In semiconductor manufacturing, a slurry distribution system (SDS) delivers CMP slurry to the polisher. For example,  Handbook of Semiconductor Manufacturing Technology  (2000), which is incorporated by reference, describes delivery of CMP slurries to a polisher and shows an arrangement for a SDS at page 431. In some applications, the SDS needs to mix the components of the slurry in a mix tank. During mixing and handling of the slurry, the SDS must not damage the slurry by subjecting it to too much shear, which may cause aggregation, or too little shear, which may cause settling. A pump may transfer the slurry to a distribution tank when required by the process tool. The SDS should handle a variety of chemistries because a CMP slurry formulation is often tailored to each process. The SDS should introduce precise of amounts of the slurry components into a mix tank so that the slurry mixture is known. At times, there also needs to be a precise flow rate to the process tool and/or delivery at low flow rates. At low flow rates sometime microbubbles form in the dispense lines, which prevents slurry delivery. It would be desirable to clear lines without shut down of the SDS. Of course, reliability for flawlessly daily manufacturing and delivery of the slurry is also desired, as well as ease of regular maintenance to avoid varying slurry composition that may affect process results.  
           [0011]    Flow meters are commonly used to control the flow rates of chemicals. Flow meters are usually only accurate to within 2-3% of the desired flow rate, and are also susceptible to changes due to input pressure. Second, some chemicals will cause the flow meter to plug up and allow no flow, i.e. slurries. Another method for controlling flow is to use a “push” gas to pressurize a reservoir, and then adjust the push gas pressure to adjust the flow rate. This method also will not allow accurate flow rates, due to the potential of the push gas pressure changing, and the flow rate varying as the level within the reservoir changes.  
           [0012]    The present invention addresses these problems as well as avoids waste of chemicals, provides a friendly user interface depicting the amount of chemicals remaining in the supply containers, and reduces system capital and operating costs. If, for example, the amount of chemical in the supply containers cannot be seen, the present invention permits the interface to be provided at a distance by conventional computer network capabilities and the electronics provided.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention relates to systems using controllers or logic devices and multi-reservoir load cell assemblies for precision mixing and/or delivery of liquid chemicals. It also relates to methods of delivering liquid chemicals from supply sources to processes such that the present invention accurately accounts and adjusts for the dynamic supply and use of the liquid chemical to meet process requirements. Finally, the present invention provides multi-reservoir load cell assemblies for monitoring, regulating, and analyzing the liquid supply available to a process.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates a chemical delivery system using a single-reservoir and bubble sensors on the supply lines leading to the single-reservoir.  
         [0015]    [0015]FIG. 2A is a front cross-section of a first embodiment of the multi-reservoir load cell assembly of the present invention.  
         [0016]    [0016]FIG. 2B is a top view of the first embodiment of the multi-reservoir load cell assembly.  
         [0017]    [0017]FIG. 3, a piping and instrument diagram, illustrates embodiments of the chemical delivery system including the multi-reservoir load cell assemblies of FIGS.  2 A- 2 B or  4 A- 4 B.  
         [0018]    [0018]FIG. 4A is a front cross-section of a second embodiment of the multi-reservoir load cell assembly.  
         [0019]    [0019]FIG. 4B is a side cross-section of the second embodiment of the multi-reservoir load cell assembly.  
         [0020]    [0020]FIG. 5A is a front cross-section of a third and sixth embodiment of the multi-reservoir load cell assembly.  
         [0021]    [0021]FIG. 5B is a side cross-section of the third and sixth embodiment of the multi-reservoir load cell assembly.  
         [0022]    [0022]FIG. 6, a piping and instrument diagram, illustrates embodiments of the chemical delivery system including the multi-reservoir load cell assemblies of FIGS.  5 A- 5 B or  11 A- 11 B.  
         [0023]    [0023]FIG. 7A is a front cross-section of a fourth embodiment of the multi-reservoir load cell assembly.  
         [0024]    [0024]FIG. 7B is a side cross-section of the fourth embodiment of the multi-reservoir load cell assembly.  
         [0025]    [0025]FIG. 8, a piping and instrument diagram, illustrates an embodiment of the chemical delivery system including the multi-reservoir load cell assembly of FIGS.  7 A- 7 B.  
         [0026]    [0026]FIG. 9A is a front cross-section of a fifth embodiment of the multi-reservoir load cell assembly.  
         [0027]    [0027]FIG. 9B is a side cross-section of the fifth embodiment of the multi-reservoir load cell assembly.  
         [0028]    [0028]FIG. 10, a piping and instrument diagram, illustrates an embodiment of the chemical delivery system including the multi-reservoir load cell assembly of FIGS.  9 A- 9 B.  
         [0029]    [0029]FIG. 11A is a front cross-section of a seventh embodiment of the multi-reservoir load cell assembly.  
         [0030]    [0030]FIG. 11B is a side cross-section of the seventh embodiment of the multi-reservoir load cell assembly.  
         [0031]    [0031]FIG. 12, a piping and instrument diagram, illustrates an embodiment of the chemical mix and delivery system.  
         [0032]    [0032]FIG. 13, a flow chart, illustrates a flow rate control system using the diminishing weight of liquid in at least one of the reservoirs.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    In the first embodiment, the present invention includes a multi-reservoir load cell assembly  200  as shown in FIGS.  2 A- 2 B. The assembly  200  can be part of the system shown in FIG. 3, and can replace the problematic single-reservoir  104  and bubble sensors  110 , 112  of FIG. 1. In this embodiment, the assembly  200 , constructed of Teflon, SST, polypropylene or any chemical compatible material, includes an upper compartment  202 , a main reservoir  206 , and a buffer reservoir  208 , all in an outer housing  212 . The buffer reservoir  208  is sealed from the main reservoir  206  by a separator  209 , and an o-ring seal  211  seals the perimeter of the separator  209  to the outer housing  212 . The separator  209  uses a center conical hole  250  that allows an internal sealing shaft  204  to form a liquid and gas-tight seal with the separator  209 . The separator  209  forms a liquid and gas-tight seal to the pneumatic tube  215  with an o-ring seal  210 . The main reservoir  206  contains a middle sleeve  214  that forms a rigid separation between the separator  209  and the reservoir cap  205 . O-ring  203  seals the perimeter of the reservoir cap  205  seals against the internal surface of the outer housing  212 . The reservoir cap  205  seals against the internal sealing shaft  204 , the chemical input tube  217 , and the pneumatic tubes  215  and  218  with a set of o-ring seals  207 ,  220 ,  222 , and  224  (hidden, but location shown in FIG. 2B), respectively. Mounted to the reservoir cap  205  is a spacer  244 , which also mounts to the pneumatic cylinder  226 . The reservoir cap  205  is held in position by the upper sleeve  233  and the middle sleeve  214 . The outer Teflon reservoir top  201  is bolted to the outer housing  212  and forms a mechanical hard stop for the upper sleeve  233  and the pneumatic cylinder  226 . Pneumatic airlines for the pneumatic cylinder  226  penetrate the outer Teflon reservoir top  201  through the clearance hole  260 .  
         [0034]    It should be clear that the present invention is not limited to the delivery of CMP slurries or photoresist on silicon wafers. For example, although the invention shows advantages over the conventional system in this environment, the systems of the present invention can deliver other liquid chemicals for other types of processes. Because the novelty of the present invention extends beyond the nature of the chemical being delivered, the following description refers to the delivery of chemicals to avoid a misunderstanding regarding the scope of the invention.  
         [0035]    As shown in FIG. 3, the multi-reservoir load cell assembly  200  shown in FIGS.  2 A- 2 B is suspended on and weighed by a load cell  412 , preferably such as a Scaime load cell model no. F60X10C610E and a programmable logic controller (PLC)  330 , preferably such as the Mitsubishi FX2N, a computer, or another conventional logic device determines the volume of the chemical in the assembly  200  from the load cell weight and the specific gravity of the chemical. For brevity, we will refer to that logic device as a PLC. As chemical from line  217  is drawn into the main reservoir  206 , the load cell  412  outputs a small mV analog signal  324  proportional to the weight on the load cell  412 . In one embodiment, an ATX-1000 signal amplifier  326  boosts the small signal  324  to the 4-20 millivolt range and sends it to an analog-to-digital converter  328 , such as the Mitsubishi FX2N4-AD, and the output digital signal  332  is sent to the PLC  330 . The PLC  330  can be rapidly programmed by conventional ladder logic. During withdrawal of the chemical, the weight of the assembly  200  decreases until the software set point of the PLC  330  is reached.  
         [0036]    As further shown in FIG. 3, the PLC  330  may control valves V 1 -V 5  using 24 DC Volt solenoid actuated valves, and activate them by an output card such as the Mitsubishi FX2N. Each solenoid valve, when opened, allows pressurized gas from regulator  2  such as a VeriFlow self-relieving regulator, to the pneumatically operated valves V 1 -V 5  to open or close the valves. The sequence of operation of the first embodiment is programmed in the PLC  330  so the components shown in FIGS.  2 A- 2 B and  3  work as described below.  
         [0037]    Once the chemical drops to a certain level, the PLC  330  triggers the system shown in FIG. 3 to begin an automatic refill sequence using the multi-reservoir load cell assembly  200  of FIGS.  2 A- 2 B as follows:  
         [0038]    a) A blanket of preferably low pressure, e.g., one psi inert gas is continuously supplied by the regulator  1 , such as a Veriflow self-relieving regulator, to the main reservoir  206  by the pneumatic tube  218 .  
         [0039]    b) The pneumatic cylinder  226  lifts the internal sealing shaft  204 , thereby sealing the buffer reservoir  208  from the main reservoir  206 .  
         [0040]    c) Once the buffer reservoir  208  is sealed, the main reservoir  206  is evacuated to a vacuum of approximately  28  inches of mercury. As shown in FIGS.  2 A- 2 B, the pneumatic tube  218  from the main reservoir  206  connects to the output side of a three-way valve V 4 . Valve V 4  is actuated so that the tube  218  communicates with the line  316  connected to the vacuum ejector  324  as shown in FIG. 3. The vacuum ejector  324  is powered by compressed gas, which is directed to it by the two-way valve V 5 . Once valve V 5  is on, it allows compressed gas to pass through and the vacuum ejector  324  develops about 28 inches of mercury (vacuum) through the line  316  communicating with the main reservoir  206 .  
         [0041]    d) The vacuum is isolated from the buffer reservoir  208 , which has an inert gas slight blanket above it and continues to supply chemical to the process or tool without exposing the chemical being delivered to the tool to negative pressure or a difference in pressure.  
         [0042]    e) The vacuum generated in the main reservoir  206  creates a low pressure chemical line that is connected to the valves V 1  and V 2 . Assuming that valve V 2  opens, the low pressure line  217  causes chemical from the supply container  102  to flow into the main reservoir  206 . During this period of time the main reservoir  206  refills with chemical until a determined full level is achieved.  
         [0043]    f) The full level is determined by use of the load cell  412  and weight calculations performed by the PLC  330 . For example, one preferred embodiment uses a buffer reservoir  208  with a volume capacity of 439 cubic centimeters (cc) and a main reservoir  206  with a capacity of 695 cc. Using the specific gravity of the chemical, the PLC  330  calculates the volume that the chemical occupies. The PLC  330  then begins a refill sequence once the chemical volume reaches or falls below 439 cc. The refill stops once the chemical volume reaches 695 cc. This sequence allows nearly all of the 439 cc of the chemical in the buffer reservoir  208  to be consumed while refilling the main reservoir  206  with the 695 cc of chemical and prevents overflow of the main reservoir  206  or complete evacuation of chemical from the buffer reservoir  208 .  
         [0044]    g) Once the main reservoir  206  has refilled, the valve V 5  is turned off, thereby stopping gas flow to and vacuum generation by the vacuum ejector  324 . The three-way valve V 4  is then switched so that the inert gas line  218  communicates with the main reservoir  206  and an inert gas blanket is again formed over the chemical in the main reservoir  206  at the same pressure as the buffer reservoir  208 , since both lines  218 ,  215  receive gas from the same inert gas manifold  318  (see FIG. 3). Also, the valve V 2  is closed which now isolates the supply container  102  from the main reservoir  206 .  
         [0045]    After the main reservoir  206  is full of chemical with an inert gas blanket above, the internal sealing shaft  204  is lowered and allows chemical from the main reservoir  206  to flow into the buffer reservoir  208 . Eventually, the buffer reservoir  208  completely fills along with a majority of the main reservoir  206 . The pneumatic tube  215  connecting the buffer reservoir  208  fills with chemical until the chemical in the tube  215  reaches the same level as the main reservoir  206 , because the pressures in both reservoirs are identical. The internal sealing shaft  204  remains open until it is determined, to once again, refill the main reservoir  206 .  
         [0046]    Because the first embodiment uses load cells instead of bubble sensors for determining the amount of chemical in the supply containers, the present invention provides a number of very useful features. One can accurately determine in real-time the chemical remaining in the supply containers. If the supply containers are full when connected to the system, the PLC can easily calculate the chemical removed (and added to the multi-reservoir load cell assembly) and how much chemical remains in the supply containers. This information can be used to provide a graphical representation of the remaining amount of chemical in the containers. A second feature is that the PLC can determine precisely when a supply container is completely empty by monitoring the weight gain within the system. If the weight of the reservoir does not increase during a refill sequence then the supply container is inferred to be empty. This causes the valve for the supply container to be closed and the next supply container to be brought on line. A related third feature is the load cell technology provides the ability to accurately forecast and identify the trends in chemical usage. Since the exact amount of chemical is measured coming into the reservoir the information can be easily electronically stored and manipulated and transmitted.  
         [0047]    A second embodiment of the multi-reservoir load cell assembly  400  shown in FIGS.  4 A- 4 B, includes a buffer reservoir  408 , fastened and sealed by the o-rings  411  to the bottom cap  410 . The output chemical flows through tube connection  401 . Connected to the buffer reservoir  408  are a pneumatic tube  415 , a chemical valve  407 , a load cell separator  413 , and the load cell  412 . The load cell  412  is securely bolted to the buffer reservoir  408  and the other side is securely bolted to a rigid member (not shown) not part of the multi-reservoir load cell assembly  400 . The outer sleeve  404  slips around the buffer reservoir  408  and rests against the bottom cap  410 . The outer sleeve  404  is machined to allow the load cell  412  to pass through it unencumbered. End  405  of the valve  407  connects to the main reservoir  406  and the other end  409  connects to buffer reservoir  408 . The main reservoir  406  is encapsulated and sealed, by o-rings in the upper cap  403 . The upper cap  403  incorporates a stepped edge along its periphery to secure the outer sleeve  404  to it. Pneumatic line  418  and chemical input line  417  are secured to the upper cap  403 . The outer sleeve  404  provides the mechanical strength for the separate reservoirs  406  and  408 .  
         [0048]    The multi-reservoir load cell assembly shown in FIGS.  4 A- 4 B, and used in the system of FIG. 3, is similar to the first embodiment with the following notable differences:  
         [0049]    a) Valve  407  provides control of the fluid path between the main reservoir  406  and the buffer reservoir  408 .  
         [0050]    b) The outer sleeve  404  provides the mechanical support to form the rigid assembly that supports the main reservoir  406  as well as the buffer reservoir  408 .  
         [0051]    A third embodiment of the multi-reservoir load cell assembly shown in FIGS.  5 A- 5 B, employs two reservoirs  506 ,  508  spaced apart from each other but connected by a flexible fluid line  516 . The third embodiment uses many of the previous components shown in FIGS.  4 A- 4 B, except: (i) it does not use an outer sleeve  404 ; (ii) the buffer reservoir  508  is not mechanically suspended from the main reservoir  506 ; and (iii) the load cell spacer  513  and the load cell  512  are fastened to the bottom of the main reservoir  506 .  
         [0052]    The third embodiment operates like the second embodiment except the load cell  512  only measures the volume of chemical in the main reservoir tank  506  as shown in FIGS.  5 A- 5 B and  6 . The advantage of the third embodiment is the precise amount of chemical brought into the main reservoir  506  is always known and the PLC does not have to infer the amount of chemical that was removed from the buffer reservoir  508  during a refill operation. The third embodiment can be used in the system of FIG. 6 with the control system (i.e., PLC, A/D, signal amplifier, etc.) of FIG. 3. Note, in the application, the lead digit of the part numbers generally indicates which drawing shows the details of the part, while the trailing digits indicate that the part is like other parts with the same trailing digits. Thus, the buffer reservoir  206  and the buffer reservoir  306  are similar in function, and found in FIGS.  2 A and FIG. 3A, respectively.  
         [0053]    A fourth embodiment of the multi-reservoir load cell assembly  700  shown in FIGS.  7 A- 7 B, employs the same components as the third embodiment, however, a second load cell  722  is attached to the buffer reservoir  708 . The assembly  700  is preferably used with the system of FIG. 8 with the control system of FIG. 3 with additional components for the second load cell.  
         [0054]    The fourth embodiment of the multi-reservoir load cell assembly  700  shown in FIGS.  7 A- 7 B, operates much like the second embodiment except that the load cell  712  only measures the chemical in the main reservoir  706  and the load cell  722  only measures the chemical in the buffer reservoir  708 . The advantage here is the buffer reservoir  708  is constantly monitored so if the downstream process or tool suddenly consumes large amounts of chemical during a refill cycle, the system can stop the refill cycle short to bring chemical into the buffer reservoir  708  from the main reservoir  706  to prevent the complete evacuation of chemical from the buffer reservoir  708 .  
         [0055]    A fifth embodiment of the multi-reservoir load cell assembly  900  shown in FIGS.  9 A- 9 B uses the same components as the third embodiment, except the load cell  912  is attached to the buffer reservoir  908  instead of the main reservoir  906 . The fifth embodiment is preferably used in the system depicted in FIG. 10 with the control system (i.e., PLC, A/D, signal amplifier, etc.) shown in FIG. 3.  
         [0056]    Functionally, the fifth embodiment of the multi-reservoir load cell assembly  900  operates the same as the second embodiment, the only difference is the load cell  912  only weighs the chemical in the buffer reservoir  908 .  
         [0057]    As the process or tool consumes the chemical, the weight of the buffer reservoir  908  remains constant until the main reservoir  906  also becomes empty. Then the weight in the buffer reservoir  908  will start to decrease, indicating that the main reservoir  906  needs to be refilled. At this point the main reservoir  906  is refilled for a calculated period of time. During this sequence the buffer reservoir  908  decreases until the main reservoir  906  has been refilled and the valve  907  has been reopened between the two reservoirs  906 ,  908 .  
         [0058]    A sixth embodiment uses the same components of third embodiment shown in FIG. 5A- 5 B. The only notable difference is that the inert gas blanket (see FIG. 6) of approximately one psi is increased to approximately 80 psi (more or less depending on the type of chemical). The increased inert gas pressure enables the sixth embodiment to use pressure to dispense the chemical at a constant output pressure, which remains unaffected even during the refill cycle. This method would allow very precise non-pulsed output flow of the chemical. This may be a highly critical feature in an ultra high purity application that pumps the chemical through a filter bank. Any pulsation of the chemical can cause particles to be dislodged from the filter bank into the ultra-pure chemical output flow.  
         [0059]    A seventh embodiment uses the same components as the third embodiment with additional components shown in FIGS.  11 A- 11 B, including a main reservoir  1106 , a buffer reservoir  1108 , a second chemical input line  1119  added to the main reservoir  1106  through the valve  1122 , a valve  1123  added to the chemical input line  1117 , and a stir motor  1120  and an impeller assembly  1121 .  
         [0060]    Functionally, the seventh embodiment operates the same as the third embodiment with the added capability of mixing two chemicals in precise proportions before transferring the mixture to the buffer reservoir  1108 . The chemical can be drawn into the main reservoir  1106  through open valve  1123  and the chemical input line  1117  and weighed by the load cell  1112 . When the proper amount has been drawn into the main reservoir  1106 , the valve  1123  is closed and the valve  1122  is opened to allow the second chemical to enter the main reservoir  1106 . When the proper amount has been drawn into the main reservoir  1106 , the valve  1122  is closed and the chemicals are blended via the stir motor  1120  and impeller assembly  1121 . The stirring of the chemicals can be initiated at any time during the above sequence.  
         [0061]    Once the mixing is complete, the valve  1107  opens to allow the chemical to transfer to the buffer reservoir  1108 , which is also connected to gas line  1115 . This is an ideal way to mix time sensitive chemistries and maintain a constant, non-pulsed output of the blended chemicals.  
         [0062]    [0062]FIG. 12, a piping and instrument diagram, illustrates an embodiment of a chemical mixing and delivery system. For clarity we will discuss how the system can be used to mix components together into CMP slurry, but the system can be used to mix other chemicals. FIG. 12 contains many parts, so to avoid clutter we use double-digit part numbers rather than four-digit as the leading and trailing digit convention would require as discussed at page  13 .  
         [0063]    The system includes a main reservoir  69  with DI water lines supplying DI water through a gross fill valve  41  and a flow control valve  43 , and a fine fill valve  42 . In an embodiment, the gross fill valve is a ⅜-inch valve, and the fine fill valve is a ¼-inch valve. As discussed in connection with FIG. 3, the PLC may control valves using 24 DC Volt solenoid actuated valves, and activate them by an output card such as the Mitsubishi FX2N. Each solenoid valve, when opened, allows pressurized gas from regulator such as a VeriFlow self-relieving regulator, to the pneumatically operated to open or close the valves. These actuators will be referred to in the specification, but will not be shown in FIG. 12 to reduce clutter.  
         [0064]    In an embodiment, the PLC sends a signal to such an actuator to open the gross fill valve  41  permitting water to rapidly begin to fill the main reservoir  69 . When the main reservoir  69  contains almost sufficient water, the PLC provides another signal to an actuator to close the gross fill valve  41  and to intermittently open and close the fine fill valve  42 , so called “chatter” the valve. This permits the system to add the precise balance of DI water required for the mixture. Of course, this gross fill and fine fill arrangement can be used for any component but is most useful if there is a major amount of that component in the final mixture. The flow control valve  43  is a manual or automatic controlled valve that compensates for the different water pressures available at a given facility.  
         [0065]    The DI water recirculates through a bypass  40  then back to the DI return. If the velocity of the water recirculating is kept above some level such as seven feet/sec, it will reduce or eliminate bacteria formation. The purpose of the DI water is to dilute Chem A, which represents slurry. The slurry passes through a fine fill valve  44  to the main reservoir  69 , through a bypass  53  and recirculates to the Chem A return, which reduces the settling of abrasives suspended in the Chem A.  
         [0066]    Chemicals B-D represent other components used in small amounts such as stabilizers, surfactants, and pad conditioners supplied through fine fill valves  46 - 48  into the main reservoir  69 . The PLC sends control signals to admit Chem A through Chem D sequentially so that the load cells  12  and  13  of the main reservoir  69  can weigh each component accurately. The two load cells shown in FIG. 12 may permit higher accuracy than one load cell, but the number of load cells is not essential to the invention. The PLC also sends control signals to the engage the main mixer motor  20 , which rotates the shaft  24  and impeller  21 , which stirs the components into CMP slurry. Process requirements will define the best time period and rpm for the impeller  21 . The impeller  21  will continuously stir certain CMP slurry formulations.  
         [0067]    As shown at the top of FIG. 12, an inert gas supply provides inert gas through a regulator, a safety pressure relief valve  33 , and a check valve  35  to an inert gas humidifier. For some CMP slurries nitrogen is preferred, but other chemicals require other gases. One of ordinary skill will know what inert gas is suitable for a given CMP formulation. For brevity we will discuss the inert gas as being nitrogen, which is bubbled through a tube in the DI water to humidify the nitrogen. This reduces the caking of the CMP slurry mixture inside the main and buffer reservoirs. The humidified nitrogen is supplied through the main reservoir pressure regulator  51 , an inlet pressure valve  50 , and to the main reservoir  69 . The vent valve  49  is a safety valve, which is normally open (NO) when not actuated. As known, the set of check valves  16 ,  35 ,  37 ,  39 ,  76 ,  86 ,and  99  prevent backflow on the associated lines.  
         [0068]    The main reservoir  69  transfers the mixed CMP slurry to buffer reservoir(s). In one embodiment, the main reservoir  69  holds two liters so that it can effectively service each of two buffer reservoirs  71 ,  92 , holding one liter each. The transfer of the CMP slurry passes through a main reservoir outlet valve  58 , through a line, then to a buffer reservoir inlet valve  60 . Likewise, the main reservoir  69  transfers the CMP slurry initially through a main reservoir outlet valve  57 , through a line, then to a buffer reservoir inlet valve  97 . The process tool determines when the buffer reservoirs  71  and  92  deliver the CMP slurry through dispense lines  1  and  2 . Manual valves  84  and  85  are associated with the dispense lines  1  and  2  lines for safety.  
         [0069]    Buffer reservoirs  71  and  92  each include a proportional valve block, which will be used by the PLC to control the pressure in each buffer reservoir. The PLC sends control signals to the proportional valve block to maintain the pressure in the buffer reservoirs that is necessary to achieve a desired flow rate of CMP slurry from the buffer reservoirs. For example, the pressure transducer, PT in FIG. 12, reads the pressure in the buffer reservoir  71  and sends a signal indicative of that pressure to the PLC. Based on the measured pressure and the pressure set point, the PLC will send signals to the proportional valve block to either open a buffer control inlet valve  80  to increase the buffer reservoir pressure  71  or open a buffer control outlet valve  81  to decrease the buffer reservoir pressure  71 . Likewise, the pressure transducer of buffer reservoir  92  reads the pressure and sends signals to the proportional valve block to maintain the pressure necessary for a desired flow rate of CMP slurry from the buffer reservoir  92 . Based on the PLC signals, the proportional valve block will either open a buffer control inlet valve  56  to increase or open buffer control outlet valve  52  to decrease the pressure of the buffer reservoir  92 . The buffer reservoir  92  also includes an optional buffer manifold  90 , which can be used as a mounting surface to connect multiple buffer outlet valves, but is not required for a single buffer outlet valve  87  as illustrated. The buffer reservoir  71  is shown with a buffer manifold  72 , which is also not required for a single buffer outlet valve  73 .  
         [0070]    A pinch valve is located downstream of the buffer outlet valve  73 , and another downstream of buffer outlet valve  87 . FIG. 12 shows one suitable control arrangement for the pinch valve of buffer reservoir  71 , which can be used for other buffer reservoirs such as the buffer reservoir  92 . In this arrangement, the PLC connects to an air actuator  78 , which controls the flow rate of clean dry air (CDA) passing through a pressure regulator  79 . Although not depicted in FIG. 12, it should be evident that the same communication channels, clean dry air source, and CDA lines can be used as one embodiment for the control of the pinch valve of the buffer reservoir  92 . The signal amplifier  77 , the A/D converter, and the load cells shown in FIG. 12 can be the same parts and have the same operation described in the earlier embodiments. The mixer motor  93  rotates shaft  94  and impeller  95  in the buffer reservoir  92 , and the mixer motor  71  rotates shaft  65  and impeller  66  in the buffer reservoir  71 . The buffer reservoirs  71  and  92  include buffer reservoir vent valves  62  and  53 , which are normally open to release pressure when not in service as safety features.  
         [0071]    The parts described can be obtained from the following vendors. Partek, A Division of Parker Corporation located in Tucson, Ariz. can provide suitable gross fill valves, part no. PV36346-01, fine fill valves, part no. PV106324-00, valve manifolds  70 ,  72 , and  90 , part no. CASY1449, and check valves part number CV1666. Another suitable PLC is the Mitsubishi AG05-SEU3M. A suitable proportional valve block is part no. PA237 manufactured and/or sold by Proportion Aire, Inc. located in McCordsville, Ind. A suitable inert gas humidifier part no. 43002SRO1, and pinch valve, part no. PV-SL-.25, are manufactured and/or sold by Asahi America located in Maiden, Mass.  
         [0072]    In operation, the chemical mix and delivery system has different modes. The initial mode is a fill or refill sequence where the system adds and mixes together the components in the main reservoir  69 . In one embodiment, the fill or refill sequence can be implemented as follows:  
         [0073]    1. The PLC sends control signals to open the DI line and Chem A-D lines to supply components to the main reservoir  69 . Although not the only arrangement, it is preferred to admit these chemical components sequentially to the main reservoir  69  so that the load cells  12  and  13  directly indicate the weight of each component in the final mixture.  
         [0074]    2. The PLC sends control signals to an actuator to shut off the inlet pressure valve  50  which would otherwise admit nitrogen to the main reservoir  69  and to open the normally open vent valve  49  so any residual gases can vent from the main reservoir  69 .  
         [0075]    3. The PLC sends control signals to start the mixer motor  20 . In one embodiment, the mixer motor  20  starts when the impeller  21  is covered with DI water or Chem A, but the time is process dependent and not part of the invention. It could start before, during, or after the time Chem A-D and DI water enter the main reservoir  69 .  
         [0076]    4. The PLC sends control signals to an actuator to open the inlet pressure valve  50  to increase the nitrogen pressure to a sufficient pressure, e.g., 20 psig, determined by the flow rate and process requirements.  
         [0077]    The PLC or logic device(s) will also send control signals to prepare the inert gas humidifier for service as follows:  
         [0078]    1. The PLC sends control signals to an actuator to close the DI drain valve  36 , which is normally open, of the inert gas humidifier.  
         [0079]    2. The PLC sends control signals to open the DI inlet valve  38  so that DI water begins to fill the inert gas humidifier.  
         [0080]    3. The HI sensor associated with the inert gas humidifier will subsequently detect a high DI water level and send signals to the PLC to send control signals to close the DI inlet valve  38 . Separately, the HI HI sensor functions to send an alarm signal if the DI water fills beyond the operational level.  
         [0081]    4. The PLC sends control signals to an actuator to open the valve  34 , which admits nitrogen to bubble up through the DI water to humidify the nitrogen that flows from the inert gas humidifier. The valve  34  is either all the way open or all the way closed. It is normally closed (NC) so that when the system is powered down, that is, out of operation, valve  34  closes preventing introduction of the inert gas, e.g., nitrogen into the inert gas humidifier.  
         [0082]    5. The inert gas humidifier feeds the nitrogen through the lines up to the inlet pressure valve  50  and the buffer inlet control valves  56  and  80 . It should be noted that the pressure supplied through the inlet pressure valve  50  is used to pressure the CMP slurry out of the main reservoir  69  at the desired flow rate.  
         [0083]    The system transfers the CMP slurry mixed from the main reservoir to the buffer reservoir as follows:  
         [0084]    1. The PLC sends a signal to open main reservoir dispense valve  58  and to open buffer reservoir inlet valve  60 .  
         [0085]    2. The PLC also sends signals to control the proportional valve block to maintain the desired pressure in the buffer reservoir(s), that is, the set point stored in the PLC. In one embodiment, the set point pressure may be 5-12 psig when the pressure of the main reservoir  69  is held at 20 psig. In one example, a pressure transducer labeled PT in FIG. 12 provides the pressure in the buffer reservoir  71  and the buffer control outlet valve  81  opens if the pressure is too high or the buffer control inlet valve  80  opens if the pressure is too low compared to the set point for buffer reservoir  71 .  
         [0086]    3. The mixer motor  93  or  64  of the buffer reservoir  92  or  71  stirs the components into a mixture. Again, the start time and time period and rpm depend on the process.  
         [0087]    In one embodiment, the process tool, for example, polisher, triggers when the dispense valve outlet  87  and  73  of respectively the buffer reservoir  92  and  71  open and close.  
         [0088]    The load cells  91  and  96  of the buffer reservoir  92  send signals to the PLC, which will be used to control the main reservoir outlet valve  57  and the buffer inlet valve  97  for transfer of CMP slurry between the main reservoir  69  and the buffer reservoir  92 . The buffer reservoir  71  operates by a similar arrangement as shown in FIG. 12.  
         [0089]    The load cells  12  and  13  associated with the main reservoir  69  will indicate when to add new components to make another batch of CMP slurry. The main reservoir dispense valves  57  and  58  are closed when the components are added to the main reservoir  69  so that the load cells  12  and  13  accurately indicate the weight of each component added to the main reservoir  69 .  
         [0090]    To clean and/or flush out the main reservoir  69 , the PLC can send control signals to close the main reservoir dispense valve  58 , open the gross fill valve  41  to admit DI water, open the main reservoir dispense valve  57 , close the buffer reservoir inlet valve  97 , open the main reservoir drain valve  99  so that the DI water passes from the main reservoir  69  bypassing the buffer reservoir  92 . A similar sequence can be used with the buffer reservoir  71 .  
         [0091]    To clean and/or flush out the buffer reservoir  92 , the DI water can pass through opened main reservoir dispense valve  57 , opened buffer reservoir inlet valve  97 , and closed main reservoir outlet valve  89 . The buffer reservoir  71  can be cleaned and/or flushed by a similar arrangement as shown in FIG. 12.  
         [0092]    In another embodiment, a fixed orifice pinch valve can be employed in cases where ultra-low flow rates are unattainable due to the properties of the mechanical components and the physical attributes of the mediums being dispensed. This pinch valve uses a flexible flow path that is compressed to a determined set point to create a fixed orifice. This will allow the desired restriction in the flow path necessary to increase or maintain a pressure for the “push” to control very low flow rates. The pinch valve can be actuated to open the flexible flow path to its maximum orifice to allow full flow during a flush sequence and then return to the desired determined set point. For example, a ¼-inch valve with a ¼-inch orifice controls the push pressure to the buffer reservoir to dispense the chemical. The output valve for the fluid is also ¼-inch. As the flow rate decreases the pressure required to push also decreases. In the case of ultra-low flow rate the inherent properties of the valves controlling the push pressure limit the repeatability of the precise volume of gas required to push. By installing the fixed orifice pinch valve and increasing the restriction on the dispense flow path, the push pressure can operate at higher levels resulting in precise repeatable control of ultra-low flow rates.  
         [0093]    A PLC and/or operator can adjust the pinch valve&#39;s minimum and wide-open orifice size. The wide-open orifice setting can be used to clear obstructions in dispense lines. Turning the pinch valve wide-open is referred to as burping the line. This feature is important for CMP slurries because microbubbles form during low flow rates. The PLC can control the pinch valve so that pressure builds up to permit ultra-low flow rates and burped after a process cycle to clear any obstructions. The time period of the burp may be short such as  0 . 5  seconds and performed after delivery of the CMP slurry. A typical process only requires delivery of CMP slurry for up to 1.5 minutes and microbubbles may not appear in some CMP slurries for about 5 minutes so post-process burping may suffice. If not, the dispense lines can be burped more frequently without unduly affecting the flow rate over a given process cycle.  
         [0094]    [0094]FIG. 13 illustrates the functional blocks of the PLC that will be associated with one embodiment of the flow rate control system, which uses the diminishing weight of CMP slurry or other chemical mixture in at least one buffer reservoir. The load cells constantly monitor the weight of the CMP slurry and generate analog signals indicative of the weight of the CMP slurry in the buffer reservoir. An A/D converter converts those analog signals into digital format then sends them to the PLC. The PLC stores the specific gravity of each component to calculate component volumes. In parallel or serially with this activity, the user inputs through a keyboard, keypad, or touch screen a desired volumetric flow rate such as 200 ml/min. The PLC can convert the flow rate to rate per second. Next, the PLC directs that the buffer inlet pressure valves open to apply pressure to the buffer reservoir to produce the desired flow. The PLC monitors the declining weight in a time period and compares the current flow rate to the desired flow rate. If the flow rate is too low, the PLC sends signals to the proportional valve block to increase pressure, and if the flow rate is too high, the PLC sends signals to the proportional valve block to decrease the pressure. If the volumetric flow rate is within a predetermined tolerance, the PLC send signals to the proportional valve block that neither increase nor decrease the pressure to the buffer reservoir.  
         [0095]    In other words, a “push” gas is supplied to the buffer reservoir by either a proportional control valve or valves and pressure is monitored via a pressure transducer or transmitter. The desired flow rate is entered, and a calculation is performed to determine the required weight loss from the reservoir during the course of a certain period of time. The PLC causes a signal to be sent to the proportional valve to adjust the push gas pressure, to adjust the weight loss within the reservoir to meet the flow rate requirements. The weight loss can be monitored over the course of time varying from 0.1 seconds to 60 seconds (or higher) depending on the accuracy of the flow rate required. For example, a flow rate of 180 milliliters per minute equals a flow rate of three milliliters per second. The PLC monitors the weight change within each buffer reservoir. If the weight loss is less than three milliliters per second, the pressure is increased. If the weight loss within the buffer reservoir is greater than three milliliters per second, the pressure is decreased. To achieve greater accuracy, the time frame can be shortened to the weight loss achieved during the course of ½ second, or even as low as 0.1 second or lower. The determining factor may be the resolution of the load cells associated with the buffer reservoir. If the load cell is able to resolve  0 . 1  gram, tighter controls can be implemented.  
         [0096]    This embodiment requires no additional components to control flow. Since no additional devices are used, the problems of plugging are eliminated. Because the pressure to the reservoir is controlled by the PLC, varying input pressures are accounted for and proper adjustments are made to keep the flow at the desired rate. The PLC can be used to determine the average pressure utilized to maintain the proper flow rate. Once the level within the buffer reservoir reaches a point to where it needs to be refilled, the average pressure can be utilized to maintain the flow rate while the buffer reservoir is refilling. The volume level is also monitored real time, which alleviates any requirement for additional components to detect level. The buffer reservoir will dispense chemical as required to satisfy a request command transmitted from the process tool. The volume level is replenished when a low volume set point is triggered as the declining weight is monitored. In like manner the volume being replenished is stopped when a high set point is triggered as the increase in weight is monitored. The present invention provides at least the following benefits. The output chemical can be maintained at a constant pressure. A process tool never experiences a low-pressure chemical line that could prevent a dispense sequence from occurring; therefore the yield of the tool is increased. A multitude of containers and sizes can be connected to the reservoir system as chemical supply containers. If the fluid volume of the supply containers is known before they are connected, the computer can calculate very accurately the amount of chemical that has been removed from the container and therefore present the information to a display for a visual, real time indication of the remaining amount of chemical. The graphical interface communicates to the operator at a “glance” the condition of the supply containers. The load cells can determine when the supply container is completely empty since there will not be a continued weight increase during a refill sequence. This indicates the supply container is empty and that another container should be brought on line. In one embodiment, data logging of chemical usage can be provided since the chemical in the reservoir(s) is continuously and accurately weighed by load cell(s) which give an input signal to the PLC or other logic device which outputs real time, accurate information as to the amount of chemical available in the reservoir. The load cell is an inherently safe sensing device since failure is indicated by an abnormally large reading or an immediate zero reading, both of which cause the PLC or other logic device to trigger an alarm. The invention can also prevent bubbles that occur during a supply container switching operation from passing through to the output chemical line, can provide constant, non-varying pressure dispense with multiple supply containers, can refill itself by vacuum or by pumping liquid to refill the reservoir or refill with different chemicals at precise ratios and mix them before transferring the mixture to the buffer reservoir, which may be important for time dependent, very reactive chemistries.  
         [0097]    The invention can provide precise flow control of fluids, chemistries, and compound mixtures utilizing pressure reservoirs fitted with valves, tubing, weight sensors (load cells), and a control system. The invention can also monitor and control volumetric level replenishment utilizing pressure reservoirs fitted with valves, tubing, weight sensors, and a control system. The invention can also replace the commonly used methodologies to control precise flow, such as manually set throttle valves and flow meters. The invention when fitted with the pinch valve can control very precise low flow rate.