Patent Publication Number: US-6656015-B2

Title: Point-of-use fluid regulating system for use in the chemical-mechanical planarization of semiconductor wafers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. Patent Application No. 09/691,805, U.S. Pat. No. 6,431,950 filed Oct. 18, 2000. 
    
    
     TECHNICAL FIELD 
     This invention relates to chemical-mechanical planarization of semiconductor wafers, and more particularly to fluid flow regulating systems used in such machines. 
     BACKGROUND OF THE INVENTION 
     As the level of integration increases on semiconductor wafers, surface irregularities on the wafer have become a serious problem. For example, metallization layers used to form interconnects between the various devices on the wafer may lead to substantial surface irregularities that interfere with the performance of subsequent photolithographic steps on the wafer. In order to flatten these surface irregularities, numerous materials or methods have been developed, such as SOG (Spin on Glass), and reflow. However, since these methods cannot globally planarize the wafer surface and may not sufficiently remove wafer surface irregularities, they have largely given way to the use of polishing techniques to planarize the surface of semiconductor wafers. 
     In one commonly used technique, known as chemical-mechanical planarization, the semiconductor wafer is mounted in a wafer carrier, and a polishing pad is held on a platen that can be rotated. The exposed surface of the wafer is then pressed against the polishing pad with a prescribed down force, and the polishing pad and/or the wafer are then independently rotated while the wafer carrier is translated across the pad surface. The process is continued until the desired degree of surface uniformity on the wafer is attained. In this technique, the abrasive mechanism is generally provided by a planarization fluid that contains abrasive particles in suspension with a combination of chemical etchants that are formulated to etch and dissolve certain materials that comprise the wafer. Alternatively, the planarization fluid may contain only the chemical etchants, with the abrasive elements embedded in a “fixed abrasive” pad. 
     The planarization fluids used in chemical-mechanical planarization are most commonly supplied to wafer manufacturers in a commercially prepackaged form, which may be comprised of two or more parts that are combined prior to planarizing a production run of wafers. Once the components are mixed, the planarization fluid is distributed to wafer planarization machines by a planarization fluid distribution system. Numerous disadvantages are present in planarization fluid distribution systems which are explained more fully with reference to the structure and operation of a typical prior art planarization fluid distribution system  10  which is shown in FIG.  1 . 
     With reference now to FIG. 1, carefully measured volumes of planarization fluid components  130  and  132  are combined in a mixing tank  138  to form a planarization fluid  14 . The mixing tank  138  has a mechanical agitator  136  that is driven by an electric motor  134  to mix the components and to keep the abrasive component of planarization fluid  14  in suspension. After the planarization fluid  14  has been sufficiently mixed, the planarization fluid  14  is transferred to a storage tank  12  through line  120 . The storage tank  12  has an outlet pipe  18  for transferring planarization fluid  14  from the tank  12  to a planarization fluid distribution loop  140 . A peristaltic pump  124  that is driven by a motor  122  pumps planarization fluid around the distribution loop  140 . Planarization fluid distribution branches  160   a - 160   d  allow planarization fluid  14  to be distributed to planarizing machines  126   a - 126   d , and the amount of planarization fluid  14  distributed to the machines  126   a - 126   d  may be controlled by manually actuated valves  150   a - 150   d . Although only four planarization machines are shown for clarity of presentation, a larger number of machines may be present in a typical system. By maintaining constant fluid motion in the distribution loop  140 , abrasive settling in the distribution loop  140  is avoided. Moreover, the constant pumping of planarization fluid  14  from storage tank  12  to the distribution loop  140 , together with the return of the unused portion of the planarization fluid  14  to the storage tank  12  through return pipe  16  may keep the abrasive components of planarization fluid  14  sufficiently agitated. 
     One disadvantage of the prior art fluid distribution system  10  is that it will not permit planarization fluids to be mixed from constituent components close to the machine. The mixing and use of planarization fluid on an as-needed basis is advantageous because the chemical etchants present in the fluid are subject to chemical degradation, and should be used relatively soon after mixing occurs. The combination of fluid components at the machine will generally permit smaller volumes to be mixed which may be more completely consumed in the wafer planarizing process, thus minimizing the waste of planarization fluid. 
     Another disadvantage of the prior art distribution system  10  is that it cannot accurately regulate the amount of planarization fluid delivered to each machine. Referring again to FIG. 1, a peristaltic pump  124  is used to deliver the planarization fluid  14  to the machines  126   a - 126   d . Since the peristaltic pump  124  is sensitive to changes in the fluid level in the tank  12 , the amount of fluid delivered to machines  126   a - 126   d  will vary as the planarization fluid  14  is used. Consequently, the delivery of planarization fluid to machines  126   a - 126   d  in uniform, precisely regulated amounts cannot be readily accomplished in the prior art system  10 . 
     Still other problems are inherent in the prior art planarization fluid distribution system  10 . For example, the prior art planarization fluid distribution system  10  requires a minimum volume of planarization fluid  14  in order to operate, and depending on the size of the system, this volume may be considerable. With reference again to FIG. 1, it is seen that the planarization fluid distribution system  10  requires that the distribution loop  140  be filled with planarization fluid  14  during operation, and that the storage tank  12  contain a sufficient volume of planarization fluid to permit pumping from the storage tank  12 . Consequently, when all wafer planarization processing is completed, a significant volume of unused planarization fluid is retained within the system  10 . Since the unused planarization fluid loses its effectiveness over time, it cannot be retained for use in planarizing subsequent wafer production runs and is generally discarded. This waste contributes to the overall cost to produce the wafer since commercially available planarization fluid formulations are relatively costly. Still other costs are incurred in discarding the excess planarization fluid, because it must be disposed of as toxic waste. 
     Still other disadvantages are associated with the prior art planarization fluid distribution system  10 . For example, after the removal and disposal of the excess planarization fluid, the entire distribution system is flushed with deionized water to remove the remaining fluid. However, flushing the distribution system presents still other waste disposal problems since the water used to flush the system generally contains significant concentrations of chemical constituents, as well as abrasives. It must therefore be processed to remove these materials before the water can be discharged into a municipal wastewater disposal system. An additional problem associated with flushing the system is that there is usually no way to remove the de-ionized water that is retained in the distribution system after it is flushed and drained. If the distribution system has a significant volume, considerable amounts of water will remain in the system after flushing. Consequently, the water retained by the system will dilute the fresh planarization fluid mixture when it is transported through the system. This diluted planarization fluid may cause inconsistent planarization results in subsequent wafer production runs. 
     Finally, abrasive settling problems are not effectively addressed by the prior art planarization fluid distribution system  10 . Abrasive settling, in particular, is a significant problem in wafer planarization because abrasive-rich mixtures generally form in regions near the bottom of storage vessels, mixing tanks and distribution lines. Once formed, these mixtures may lead to uneven planarization of the wafer, or cause the wafer to be planarized beyond the desired endpoint. Moreover, if the abrasive settling is not controlled, large agglomerations of abrasive particles may ultimately form in the planarization fluid that may lead to surface scratching of the wafer. Although the prior art distribution system  10  uses a distribution loop  140  to inhibit abrasive settling, abrasive particles may still settle in locations that are not subject to recirculation. For example, since wafer planarization generally occurs on a periodic basis, the machines must be stopped in order to remove planarized wafers from the wafer holders and to load unprocessed wafers into the wafer holders. During this period, the flow of planarization fluid  14  from the distribution loop  140  to the machines  126   a - 126   d  is stopped by closing valves  150   a - 150   d , which allows the planarization fluid  14  to remain stationary within the distribution branches  160   a - 160   d , thus allowing the abrasives to settle and agglomerate. Reestablishing movement of the planarization fluid in the distribution lines will not, in general, significantly break up these agglomerations once they have formed. 
     Many of the shortcomings inherent in prior art planarization fluid distribution systems could be eliminated if the fluid could be supplied to the planarization machines individually from a point-of-use planarization fluid distribution system. As used herein, the term “point-of-use” refers to a fluid distribution system that is located in proximity to the planarization machine that supplies planarization fluid to an individual planarization machine. 
     Since the point-of-use system is located in proximity to the machine, the need for long distribution lines and recirculation loops is eliminated. Further, since a point-of-use system supplies planarization fluid to individual planarization machines, the internal volume of the system can be small. Consequently, many of the large volume components associated with the prior art planarization fluid distribution systems, such as recirculating loops, large mixing containers and storage tanks are eliminated. As discussed above, the large volume components comprising the prior art distribution system are generally recognized as significant contributors to planarization fluid waste and system cleaning difficulties. 
     A point-of-use system capable of precise flow regulation will also eliminate planarization fluid flow regulation problems that stem from the input pressure sensitivity inherent in peristaltic pumps, thereby permitting a more efficient utilization of planarization fluid. Precise flow regulation will additionally permit the components of a multi-component planarization fluid to be combined just prior to depositing the mixture on the planarization pad so that fluid is supplied on an as-needed basis, which greatly reduces waste. 
     Other advantages of the invention will become apparent based upon the description of the invention provided below when read with reference to the drawing figures. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for planarization fluid flow regulation that allows point-of-use distribution of a planarization fluid to a semiconductor wafer planarization machine. In one aspect, the regulating apparatus includes a planarization fluid storage tank with an acoustic fluid level sensor to detect the fluid level within the storage tank. The storage tank is connected to a planarization fluid delivery line that delivers planarization fluid to the storage tank through a flow control valve and delivers a regulated flow of planarization fluid to a planarization machine through a flow measurement device. A gas supply system is connected to the storage tank to provide system pressurization. Regulation of the planarization fluid flow from the regulating apparatus is achieved by a control system in which the flow measurement device and the acoustic fluid level sensing capability comprise feedback elements in a closed feedback system to independently control the pressure in the storage tank and the amount of fluid admitted by the control valve. In an alternate aspect, the fluid level sensor is comprised of an array of capacitive proximity sensors located outside the wall of the storage tank. In another alternate aspect, the fluid level sensor is replaced by a buoyant float that is partially buoyant in the planarization fluid that is adapted to seat in the upper or lower ends of the storage tank when the storage tank is full or empty. Indication of the full and empty tank conditions are obtained from a differential pressure sensor which is suitably located to sense differences in the storage tank pressure when the float is seated in either the upper or lower end of the storage tank. In still another aspect, two or more regulators may be joined in a parallel flow arrangement in order to achieve precise point-of-use mixing and flow rate control of multi-component planarization fluids. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a prior art planarization fluid distribution system. 
     FIG. 2 is a schematic view of a point-of-use planarization fluid distribution system. 
     FIG. 3 is a schematic view of an embodiment of the planarization fluid flow regulator for use in a point-of-use system. 
     FIG. 4 is a schematic view of an alternative embodiment of the planarization fluid flow regulator for use in a point-of-use system. 
     FIG. 5 is a schematic view of an alternative embodiment of the planarization fluid flow regulator for use in a point-of-use system. 
     FIG. 6 is a schematic view of a point-of-use planarization fluid distribution system using an embodiment of the planarization fluid flow regulator. 
     FIG. 7 is a diagram showing a flow for planarizing semiconductor wafers with a point-of-use planarization fluid system operating in the intermittent mode. 
     FIG. 8 is a diagram showing a flow for planarizing semiconductor wafers with a point-of-use planarization fluid system operating in the continuous mode. 
    
    
     In the drawings, like reference numbers identify similar elements or steps. For ease in identifying the discussion of any particular element, the most significant digit in a reference number refers to the Figure number in which the element is first introduced (e.g., element  24  is first introduced and discussed with respect to FIG.  2 ). 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally directed to an apparatus and method of planarization fluid flow regulation that allows point-of-use mixing and distribution of planarization fluid to a planarization machine. Many of the specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 2 through 8 to provide a thorough understanding of such embodiments. One skilled in the art will understand, however, that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. For purposes of the following description, the term planarization fluid may refer either to a planarization fluid with or without abrasive particles, or to a single component of a multi-component planarization fluid that is combined with other components to comprise the planarization fluid. In addition, terms of art such as “slurry” or “polishing slurry” or other similar terms are regarded as equivalent to planarization fluid, as used herein. Moreover, specific dimensions and other physical characteristics related to different embodiments are not to be considered as limiting unless the claims expressly state otherwise. 
     FIG. 2 is a schematic representation of a point-of-use planarization fluid distribution system  20  that is capable of precise point-of-use mixing and flow rate control. With reference to FIG. 2, the components of a multi-component planarization fluid are stored in fluid dispensers  21   a - 21   c . For clarity of presentation, three fluid dispensers are shown, although more than three may be present, or only a single dispenser may be used. The fluid dispensers  21   a - 21   c  may be the original containers in which the components of the planarization fluid were shipped from the manufacturer, or they may be other containers adapted to retain planarization fluid components. Additionally, other dispensers may be present to contain solutions that are used exclusively to recondition the pad, or to contain deionized water. The fluid component streams  22   a - 22   c  originate at the fluid dispensers  21   a - 21   c  and are directed to a plurality of planarization fluid regulating devices  23   a - 23   c  as unregulated streams by a gravity feed system. Alternatively, fluid component streams  22   a - 22   c  may be pumped to the regulating devices  23   a - 23   c  by pumps (not shown). The regulating devices  23   a - 23   c  are capable of precisely regulating the fluid component streams  22   a - 22   c  to provide a precisely selected planarization fluid flow rate at the regulated output streams  24   a - 24   c . Associated with each of the regulating devices  23   a - 23   c  are fluid sensing systems  230   a - 230   c  that sense fluid motion and fluid accumulation within the regulating devices  23   a - 23   c , and fluid command systems  240   a - 240   c  that admit fluid to each of the regulating devices  23   a - 23   c  and pressurize the fluid within the regulating devices  23   a - 23   c . The fluid sensing systems  230   a - 230   c  and fluid command systems  240   a - 240   c  act cooperatively with a control system  200 , which receives control inputs from the fluid sensing systems  230   a - 230   c  through lines  28   a - 28   c  and transmit control outputs to fluid command system  240   a - 240   c  through lines  29   a - 29   c  to regulate the fluid component streams  22   a - 22   c . The reception and transmission of control inputs and outputs between the control system  200  and the fluid sensing systems  230   a - 230   c  and fluid command systems  240   a - 240   c  may utilize any available data transmission method, and do not do not need to be physically connected. For example, the fluid sensing systems  230   a - 230   c  and fluid command systems  240   a - 240   c  may communicate with the control system  200  by means of radio frequency energy. Additionally, the regulating devices  23   a - 23   c  may be individually controlled by control systems which are integral to the regulating devices  23   a - 23   c.    
     Subsequent to regulation by the devices  23   a - 23   c , the regulated output streams  24   a - 24   c  are then directed to a mixing unit  25  for combination before emerging as a regulated stream  26  for distribution onto the planarizing pad of a semiconductor wafer planarization machine  27 . The mixing unit  25  may comprise a simple mixing manifold for combining fluid streams, or it may include active mixing devices such as mechanical agitators. However, in order to attain a point-of-use system of limited internal volume, the internal volume of the mixing unit  25  should be limited, preferably, to a fraction of the combined internal volumes of the regulating devices  23   a - 23   c.    
     A point-of-use planarization fluid distribution system that is capable of flow rate control of a single component planarization fluid is obtained when a single unregulated fluid stream is regulated. For example, referring to FIG. 2, single component regulation is obtained when a single fluid dispenser  21   a  is present, having a single unregulated stream  22   a . Since no mixing of planarization fluid components is required, unregulated stream  22   a  is controlled to a specified flow rate by the regulating device  23   a  to achieve a regulated output stream  24   a . As before, the regulated output stream  26  is then ultimately directed to a semiconductor wafer planarization machine  27  for use in wafer planarizing. Since no mixing of components is required when a single component planarization is used, a mixing unit  25  is generally not required. 
     In order to achieve the advantages of point-of-use operability, it is preferable that the fluid dispensers  21   a - 21   c , the regulating devices  23   a - 23   c  and the mixing unit  25  be located in close proximity to the semiconductor wafer planarization machine  27  so that fluid volumes associated with the delivery lines for fluid component streams  24   a - 24   c  and the regulated output stream  26  are minimized. 
     As previously described, an important aspect of the present invention resides in the apparatus used to regulate planarization fluids at the point-of-use. In the foregoing discussion, various embodiments of a regulating device applicable to a point-of-use planarization distribution system will be described. 
     FIG. 3 is a schematic representation of an embodiment of a planarization fluid regulating apparatus  30  according to the invention. As shown therein, a gas supply system  33  includes a pressure source  34  which may be a high-pressure bottle or a centralized gas supply facility, to supply pressurization to the system  30 . Preferably, nitrogen is used as the pressurization source  34 , although a wide variety of gases and gas mixtures may be used. A pressure regulator  36  is connected to the pressure source  34  to reduce and regulate the source pressure to moderate pressures compatible with the operation of the regulating apparatus  30 . The pressure regulator  36  is preferably a device that will permit the outlet pressure from regulator  36  to be set and controlled based upon a control input received from a control system  310  along line  35 . The control system  310  will be described in greater detail below. The control input to set the pressure at regulator  36  may be a digital signal, or alternatively, an analog voltage level. An example of a pressure regulator that is responsive to an analog input voltage that may be used with this embodiment is the SMC Series ITV200 E-P Regulator manufactured by SMC Pneumatics, Inc. of Indianapolis, Ind., although other alternatives exist. 
     Still referring to FIG. 3, the outlet pressure of the regulator  36  may be optionally connected to a bubbler  38 , which introduces moisture into the regulated gas by bubbling the gas through a volume of deionized water  37 . The introduction of moisture by means of the bubbler  38  may be preferred in cases where the pressurized gas obtained from the pressurized source  34  has low moisture content. The regulated gas may be optionally isolated from the system using a valve  336 . This may be preferred when the storage tank  32  and delivery line  31  are rinsed, as will be discussed below. 
     Still referring to FIG. 3, a storage tank  32  that is capable of internal pressurization is used to contain a volume of a planarization fluid  328 . The storage tank  32  also contains an internal gas space  329  that is pressurized by gas from the gas supply system  33 , to impart fluid pressure to the planarization fluid  328 . The storage tank  32  is preferably comprised of TEFLON™, but other suitably non-contaminating and non-reactive materials may be used. In addition, composite structures may be used, such as a stainless steel tank that contains a thin surface coating of TEFLON™, or other materials. The storage tank  32  preferably has a conical top surface  332  and a conical bottom surface  330  to allow the interior surface of the conical top surface  332 , and the interior surfaces of storage tank  32  to be rinsed more effectively. In addition, a conical bottom surface  330  is preferable because it will inhibit the formation of concentrations of abrasive particles or other abrasive agglomerations. Other surface shapes are available as alternatives to the conical top surface  332  and the conical bottom surface  330 . For example, hemispherical or concave shapes may be used. 
     Rinsing of the interior surfaces of storage tank  32  may be required when it becomes necessary to change to a planarization fluid of different composition. Accordingly, a source of deionized water  335  may be provided to the storage tank  32  through a valve  334 . The gas supply system  33  may be isolated from the rinse water supply by closing valve  336 . 
     The storage tank  32  is further comprised of a fluid level sensor  322  located on the conical top surface  332  to continuously monitor the planarization fluid level  325  within the storage tank  32 . In this embodiment, the fluid level sensor  322  is an ultrasonic level detector that can directly sense the location of the fluid surface  325  within tank  32 . An example of an ultrasonic level detector that may be used is the Sonic OMNI-BEAM™ ultrasonic proximity detector manufactured by Banner Engineering Corporation of Minneapolis, Minn. However, other alternative continuous level sensing devices and methods are available, which are interchangeable with the ultrasonic level detector. One such alternative is a magnetostrictive level sensor, such as the LEVEL PLUS™ magnetostrictive fluid level detector manufactured by MTS Systems Corporation of Cary, N.C. 
     With reference still to FIG. 3, a planarization fluid delivery line  31  is connected to the lower end of storage tank  32  at the bottom surface location  331  by a fluid exchange line  391 . The planarization fluid delivery line  31  is preferably comprised of TEFLON™, although other non-contaminating and non-reactive materials may be used. The planarization fluid delivery line  31  also includes a planarization fluid inlet  340  that is connected to an unregulated source of planarization fluid. Fluid may be pumped into the planarization fluid delivery line  31  by external pumping means (not shown), or the fluid may be introduced into the delivery line  31  from a gravity feed system (not shown) or line  31  may optionally have a pump  338  driven by motor  250  to transport planarizing fluid through the line  31 . In order to avoid contamination of the planarizing fluid, the pump  338  is preferably a peristaltic pump, although other pumps could be used interchangeably. Flow admitted to the planarization fluid delivery line  31  is controlled by a flow control valve  312  that has a continuously variable valve opening which is set by a valve actuator. Alternatively, the flow control valve  312  may provide only an on/off capability. The flow control valve  312  is responsive to a control input received from the control system  310 , which will be described in greater detail below. The control input to set the valve position at the flow control valve  312  may be either a digital signal, or a voltage level. An example of a TEFLON™-lined control valve that is responsive to either a digital or an analog input that could be employed in this embodiment is the Tylan MDV motor driven valve manufactured by the Millipore Corporation of Bedford, Mass. 
     Still referring to FIG. 3, a flow sensor  318  is included in planarization fluid delivery line  31  to measure the rate of planarization fluid flow, which can transmit a control output to the control system  310  along a line  319 . Since the flow sensor  318  must indicate the total amount of flow issuing from the regulating apparatus  30 , it must be located in the planarization fluid delivery line  31  at a location downstream from the location where fluid exchange line  391  joins line  31 , and preferably, near the regulated fluid output location  360 . A flow sensor of the variable area type may be used as flow sensor  318  in this embodiment, although other alternative flow sensor technologies, such as ultrasonic flow sensors, thermal-pulse flowmeters, vortex-shedding flowmeters, or laminar element flowmeters may be used. An example of a TEFLON™-lined variable area flow sensor that could be employed in this embodiment is the Model 4400 Flow meter manufactured by NT International, Inc. of Minneapolis, Minn. 
     Other flow conditioning components may be optionally included in planarization fluid delivery line  31 . For instance, a pressure damping device  320  may be used to dampen the periodic pressure pulsations that are generated by the pump  338 , which may be objectionably pronounced when a peristaltic pump is used. The device  320  is preferably an accumulator having a closed vertical fluid column with a trapped gas space filled with nitrogen. A plurality of accumulators  320  may be used at different locations along the planarization fluid delivery line  31  as required. Additional dissipation of pressure pulsations may be attained through the optional use of a flow restrictor  314  at a location downstream of the flow control valve  312  and an additional flow restrictor  316  downstream of the flow sensor  318 . Although two flow restrictors are depicted in this embodiment, more than two may be used, and may be placed in other locations along the planarization fluid delivery line  31  as required. Additionally, an optional flow shut off valve  361  may be located near the fluid output location  360 . 
     The control system for the planarization fluid regulating apparatus  30  will now be described in detail. Referring to FIG. 3, the control system  310  operates as a multi-input, multi-output (MIMO) closed loop control system with the fluid level sensor  322  and the fluid flow sensor  318  acting as feedback elements. The control system  310  provides control output signals to the pressure regulator  36  and the flow control valve  312  so that the flow of planarization fluid that issues from the planarization fluid outlet  260  is uniformly maintained. The control algorithm employed by the control system  310  should provide at least a proportional-integral (PI) capability, however, a proportional-integral-differential (PID) algorithm is preferred. Additional input and output means  311  are provided to allow the operation of the flow regulating system  30  to be continuously monitored and to allow the entry of commands. The control system  310  may be a programmable digital computer with stored instructions to execute the control algorithm, with analog to digital (A/D) interfaces to communicate with the control devices, or it may be a self-contained programmable logic controller (PLC) capable of MIMO operation. One example of a PLC that is capable of MIMO operation that uses a PID algorithm is the Keyence KV Series PLC manufactured by the Keyence Corporation of America, Woodcliff Lake, N.J. 
     Turning now to FIG. 4, an alternative embodiment of the planarization fluid regulating apparatus  40  is shown. In this embodiment, the location of planarization fluid level  325  is determined by an array of proximity sensors  400   a - 400   d  that are located adjacent to the outer surface of storage tank  32 . Although four proximity sensors are shown for clarity of presentation, a larger number of proximity sensors may be present. The proximity sensors  400   a - 400   d  are preferably capacitive proximity sensors that detect the presence of planarization fluid  328  by a change in capacitance. The sensors  400   a - 400   d  may be connected to a multiplexer  410  that sequentially interrogates the sensors  400   a - 400   d  in order to provide an input control signal to the control system along line  420 . The use of capacitive proximity sensors has certain advantages over the level sensing method used in the previous embodiment. For example, since a number of sensors are arrayed along the exterior surface of the storage tank  32 , a single failure of a sensing element is unlikely to render the level sensing feedback element inoperative. In addition, since the capacitive proximity sensors  400   a - 400   d  can sense the presence of planarization fluid through a nonmetallic container, no penetrations through the wall of storage tank  32  are required. Although many types of proximity sensors are available which may be used successfully with this embodiment, an example of a proximity sensor that may be used is the Type 53 capacitive proximity sensor manufactured by the Cutler-Hammer Corporation of Milwaukee, Wis. 
     Turning now to FIG. 5, still another embodiment of the planarization fluid regulating apparatus  50  is shown. This embodiment uses a float  52  and a differential pressure transducer  54  to determine when the fluid level within the storage tank  32  has reached the maximum level. The float  52  moves within the storage tank  32  as the planarization fluid level  325  rises or falls. A differential pressure transducer  54  has a high pressure sense port  520  in communication with the internal gas space  329 , and a low pressure sense port  530  connected to the pressurization gas source at a location  531 . An indication of the maximum level  510  in tank  32  is generated when the level of fluid in tank  32  rises to a maximum level  510 , and the spherical float  52  seats in the conical top surface  332 . At this point, the high pressure sense port  520  is exposed to a higher pressure than the pressure established at low pressure sense port  530  so that the differential pressure transducer  54  indicates a significantly non-zero and positive value for the differential pressure. An input control signal is then transmitted to the control system  310  along a line  540  to indicate that the planarization fluid level  325  has risen to the maximum level  510 , and that the flow control valve  312  should be commanded to close. Determination of the minimum level in tank  32  may only be inferred from the known flow rate measured by flow meter  318  and the elapsed time of planarization. Although the level sensing system described in this embodiment may reflect only the full fluid level state, it is significantly simpler than the previous embodiments, since it relies on a simple differential pressure transducer as the level sensing feedback element. Moreover, since the differential pressure transducer  54  only needs to discriminate between a zero and a significantly non-zero differential pressure condition, a relatively low-cost transducer may be used. 
     Turning now to FIG. 6, an embodiment of the invention is shown that permits planarization fluid components of a multi-component planarization fluid to be accurately combined at the point of use. As shown therein, two planarization fluid regulators,  601  and  602  are jointly operated in a parallel arrangement. Although this embodiment illustrates a system applicable to a two-component planarization fluid, additional regulators may be added in parallel for planarization fluids that are comprised of more than two components. 
     To achieve precise mixing of the planarization fluid components, a first planarization fluid component is supplied to regulator  601  from an unregulated source connected to planarization fluid inlet  340   a . Similarly, a second planarization fluid component is supplied to regulator  602  from an unregulated source connected to planarization fluid inlet  340   b . The regulators  601  and  602  may be connected to a common pressurization source  34 , or alternatively, may be attached to separate pressurization sources. The first and second planarization fluid components are then independently regulated by regulators  601  and  602  according to set points input to control systems  600  and  610 . The set points for the regulators  601  and  602  reflect the relative proportions of the first and second components to be combined, and the flow rate of planarization fluid that must be delivered. As an alternative, the control systems  600  and  610  may be combined into a single control system to jointly control the regulators  601  and  602 . 
     With the planarization fluid regulators  601  and  602  operating as described above, a regulated output of the first planarization fluid component is obtained at planarization fluid outlet  260   a , and similarly, a regulated output of the second component is obtained at outlet  260   b . The regulated outputs at  260   a  and  260   b  are then combined in a mixing tank  640  to achieve complete mixing of the first and second planarization fluid components to achieve fluid  656 . The fluid  656  may then be deposited on a planarization pad  654  of machine  650  through an optional distribution device  642 . 
     Numerous advantages are associated with the point of use mixing apparatus described above. For example, since the components of the planarization fluid remain separated until they are combined in the mixing unit  640  problems associated with planarization fluid degradation are minimized. Furthermore, since planarization fluid regulators  601  and  602  have relatively small system volumes, the problems associated with large volume mixing and distribution systems is avoided. 
     A point-of-use fluid flow regulating system according to the disclosed embodiments may be configured to process semiconductor wafers in either an intermittent or a continuous mode. Briefly, when operating in the intermittent mode, the pressure supplied to the storage tank  32  is the only element controlled by the control system  310 , in response to a feedback signal from flow sensor  318 . Flow control valve  312  remains closed while wafer planarization occurs so that no additional volume of fluid is admitted to the storage tank  32 . Operation of the point-of-use system  20  in the intermittent mode is advantageous when small batches of wafers are to be processed. The continuous mode of operation uses both the pressure supplied to the storage tank  32  and the flow control valve  312  as elements controlled by the control system  310 . Since the flow control valve  312  may open during wafer planarization, the planarization fluid volume  328  may be continuously replenished from fluid source  340  while wafers are being planarized. These two modes of operation are discussed more fully below. 
     Referring to FIG. 7, a flow diagram for planarizing semiconductor wafers using a point-of-use planarization fluid distribution system operating in the intermittent mode is shown. At step  1 , unprocessed wafers are loaded onto a planarizing machine, and a set point command corresponding to a desired planarization fluid flow rate is input into the control system  310  by an operator using the input-output means  311 . The control system  310  then transmits a control signal to the flow control valve  312  along line  313  that opens valve  312 , to admit a volume of planarization fluid  328  to the storage tank  32  through planarization fluid delivery line  31 . The fluid level sensor  322  continuously monitors the volume of fluid admitted, so that the storage tank  32  is filled to a maximum permissible volume, or alternatively, to an operator-prescribed volume that is less than a maximum permissible volume. After the planarization fluid  328  has been admitted to the storage tank  32 , flow control valve  312  is commanded closed by control system  310 . At step  2 , regulation of the planarization fluid flow to the machine is established when the control system  310  transmits a control signal to the pressure regulator  36  to pressurize the internal gas space  329  in storage tank  32 . In response, planarization fluid enters the planarization fluid delivery line  31  and proceeds along delivery line  31  to the fluid flow sensor  318 , which monitors the rate of fluid flow delivered by the planarization fluid regulating apparatus  30 . In order to maintain the prescribed rate of fluid flow emanating from the planarization fluid outlet  260 , the gas pressure in the internal gas space  329  is continuously adjusted by the control system  310  in response to flow rate information received from flow sensor  318 . At step  3 , the wafers are planarized in the conventional manner while a regulated flow of planarization fluid is deposited on the planarization pad. As the wafer planarization proceeds, however, fluid is continuously removed from the tank. Ordinarily, the wafer planarization endpoint will be reached before the fluid is depleted, since the storage tank  32  is generally sized to accommodate a volume of planarization fluid that is sufficient to complete the wafer planarization. However, if an insufficient amount of fluid remains in the storage tank  32 , the low fluid level condition will be detected by the fluid level sensor  322 . Alternatively, the fluid level may be inferred from the known flow rate as measured by flow sensor  318  multiplied by the elapsed time since fluid distribution started. If a low fluid level is detected, an appropriate alert signal may be sent to the control system  310 , which, in turn, will provide an appropriate advisory message to an operator, as shown in step  4 . Step  5  concludes the process with the unloading of processed wafers from the machine. At this step, the process may be terminated, or repeated by returning to step  1 . 
     With reference now to FIG. 8, a flow diagram for planarizing semiconductor wafers using a point-of-use planarization fluid distribution system operating in the continuous mode is shown. At step  1 , unprocessed wafers are loaded onto the planarization machine and a set point command is input into the control system  310  using the input-output means  311 . The control system  310  then transmits a control signal to the flow control valve  312  to open, admitting a volume of planarization fluid  328  to the storage tank  32  through planarization fluid delivery line  31 . When the maximum level of planarization fluid has been attained in storage tank  32 , the flow control valve  312  is commanded to close. At step  2 , the control system  310  transmits a control signal to the pressure regulator  36  to pressurize the internal gas space  329  in storage tank  32 , to apply a pressure to the planarization fluid  328 . In response, planarization fluid enters the planarization fluid delivery line  31  and proceeds along delivery line  31  to the fluid flow sensor  318  to establish a regulated flow of planarization fluid from the outlet  360 . At step  3 , semiconductor wafers are planarized in the conventional manner in the presence of the continuously regulated flow. As the wafer planarization process consumes the fluid in storage tank  32 , the control system  310  continuously adjusts the gas pressure in the internal gas space  329  to maintain the prescribed flow rate of planarization fluid while monitoring the position of planarization fluid level  325  in storage tank  32 . In response to the fluid level detected within storage tank  32 , the control system  310  modulates the position of flow control valve  312  to ensure that a sufficient amount of fluid is admitted at inlet  340  to keep the storage tank  32  sufficiently filled with planarization fluid. As a result, the wafers may be continuously planarized until an endpoint is reached. At step  4  of the process, the regulated flow of fluid is interrupted, either by the optional valve  361  attached to the outlet  360 , or by releasing the pressure in the storage tank  32  and closing the flow control valve  312 , but preferably, valve  361  is used, since waste of the planarization fluid would be minimized. The processed wafers may now be unloaded from the machine. At step  5 , unprocessed wafers are loaded onto the machine, and the regulated flow is reestablished at step  2  to planarize the wafers. When the regulated flow is reestablished, the control system  310  may either impose the regulated flow rate used for the previous planarization cycle, or alternatively, an updated flow rate may be established. An updated planarization flow rate is particularly advantageous, since it may be used to compensate for changes that occur to the wafer planarization pad as successive batches of wafers are planarized. 
     The above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed. While specific embodiments of, and examples of, the invention are described in the foregoing for illustrative purposes, various equivalent modifications are possible within the scope the invention, as those skilled in the relevant art will recognize. Moreover, the various embodiments described above can be combined to provide further embodiments. Accordingly, the invention is not limited by the disclosure, but instead the scope of the invention is to be determined entirely by the following claims.