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:
BACKGROUND  
         [0001]    The present invention relates generally to systems and methods for delivering of liquid chemicals, and more particularly, to systems and methods for delivery of liquid chemicals in precise amounts using logic devices and multi-reservoir load cell assemblies.  
           [0002]    The present invention has many applications, but may be best 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.  
           [0003]    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 .  
           [0004]    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.  
           [0005]    During dispense mode, when photoresist is withdrawn by the track tool 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.  
           [0006]    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.  
           [0007]    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.  
           [0008]    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.  
           [0009]    The present invention addresses these problems as well as avoids waste of expensive photoresist, provides a friendly user interface depicting the amount of photoresist remaining in the supply containers, and reduces system capital and operating costs. If, for example, the amount of photoresist 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  
         [0010]    The present invention relates to systems using controllers or logic devices and multi-reservoir load cell assemblies for precision 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  
       [0011]    [0011]FIG. 1 illustrates a chemical delivery system using a single-reservoir and bubble sensors on the supply lines leading to the single-reservoir.  
         [0012]    [0012]FIG. 2A is a front cross-section of a first embodiment of the multi-reservoir load cell assembly of the present invention.  
         [0013]    [0013]FIG. 2B is a top view of the first embodiment of the multi-reservoir load cell assembly.  
         [0014]    [0014]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.  
         [0015]    [0015]FIG. 4A is a front cross-section of a second embodiment of the multi-reservoir load cell assembly.  
         [0016]    [0016]FIG. 4B is a side cross-section of the second embodiment of the multi-reservoir load cell assembly.  
         [0017]    [0017]FIG. 5A is a front cross-section of a third and sixth embodiment of the multi-reservoir load cell assembly.  
         [0018]    [0018]FIG. 5B is a side cross-section of the third and sixth embodiment of the multi-reservoir load cell assembly.  
         [0019]    [0019]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.  
         [0020]    [0020]FIG. 7A is a front cross-section of a fourth embodiment of the multi-reservoir load cell assembly.  
         [0021]    [0021]FIG. 7B is a side cross-section of the fourth embodiment of the multi-reservoir load cell assembly.  
         [0022]    [0022]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.  
         [0023]    [0023]FIG. 9A is a front cross-section of a fifth embodiment of the multi-reservoir load cell assembly.  
         [0024]    [0024]FIG. 9B is a side cross-section of the fifth embodiment of the multi-reservoir load cell assembly.  
         [0025]    [0025]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.  
         [0026]    [0026]FIG. 11A is a front cross-section of a seventh embodiment of the multi-reservoir load cell assembly.  
         [0027]    [0027]FIG. 11B is a side cross-section of the seventh embodiment of the multi-reservoir load cell assembly.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    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.  
         [0029]    In this embodiment, the assembly  200 , constructed of Teflon, SST 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 . The perimeter of reservoir cap  205  seals against the internal surface of the outer housing  212  with the use of an o-ring  203 . 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 air lines for the pneumatic cylinder  226  penetrate the outer Teflon reservoir top  201  through the clearance hole  260 .  
         [0030]    It should be clear that the present invention is not limited to the delivery of 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, such as the delivery of developer or chemical mechanical polishing slurries. 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.  
         [0031]    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. 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.  
         [0032]    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.  
         [0033]    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:  
         [0034]    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 .  
         [0035]    b) The internal sealing shaft  204  is lifted by the pneumatic cylinder  226 , thereby sealing the buffer reservoir  208  from the main reservoir  206 .  
         [0036]    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 .  
         [0037]    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.  
         [0038]    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.  
         [0039]    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 .  
         [0040]    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 .  
         [0041]    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 .  
         [0042]    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.  
         [0043]    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. Pneumic 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 .  
         [0044]    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:  
         [0045]    a) Valve  407  provides control of the fluid path between the main reservoir  406  and the buffer reservoir  408 .  
         [0046]    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 .  
         [0047]    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 .  
         [0048]    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 FIG. 2A and FIG. 3A, respectively.  
         [0049]    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.  
         [0050]    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 .  
         [0051]    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.  
         [0052]    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 .  
         [0053]    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 .  
         [0054]    A sixth embodiment uses the same components of third embodiment shown in FIGS.  5 A- 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 pressure 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 ultrapure chemical output flow.  
         [0055]    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 .  
         [0056]    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. 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.  
         [0057]    In review, the present invention provides at least the following benefits. The output chemical can be maintained at a constant pressure. A track tool never experiences a low pressure chemical line that could prevent a dispense sequence from occurring, therefore the yield of the track 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 are 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.