Abstract:
An improved bulk fluid distribution for supplying process fluids to semiconductor process tools. The improved system having an alternating pressure vessel engine substantially eliminates pressure fluctuations in the bulk fluid supply line due to head losses from the changing weight of the fluid in the dispensing vessels. The system also enables flexible control of the flow conditions of the fluid in the fluid supply line.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for controlling the pressure of a fluid in a bulk fluid distribution system. More particularly, the present invention provides improved apparatus and methods for controlling pressure of semiconductor process fluids (e.g. ultra-high purity or slurry fluids) in a bulk fluid supply line that supplies process tools used in semiconductor manufacturing or other related applications. 
     BACKGROUND OF THE INVENTION 
     The manufacture of semiconductor devices is a complex process that often requires over 200 process steps. Each step requires an optimal set of conditions to produce a high yield of semiconductor devices. Many of these process steps require the use of fluids to, inter alia, etch, expose, coat, and polish the surfaces of the devices during manufacturing. In high purity fluid applications, the fluids must be substantially free of particulate and metal contaminants in order to prevent defects in the finished devices. In chemical-mechanical polishing slurry applications, the fluids must be free from large particles capable of scratching the surfaces of the devices. Moreover, during manufacturing there must be a stable and sufficient supply of the fluids to the process tools carrying out the various steps in order to avoid process fluctuations and manufacturing downtime. 
     Since their introduction to the semiconductor market in the 1990s, bulk fluid distribution systems having vacuum-pressure engines have played an important role in semiconductor manufacturing processes. Because these systems are substantially constructed of inert wetted materials, such as perfluoroalkoxy (PFA) and polytetrafluoroethylene (PTFE), and because they use an inert pressurized gas as the motive force for supplying the fluids, they do not substantially contribute to particulate and metal contamination of the process fluids. In addition, a single bulk fluid distribution system can provide a continuous supply of process fluid at a sufficient pressure to multiple process tools. Thus, the advent of vacuum-pressure fluid distribution systems served an important need in the semiconductor market. 
     For many reasons, bulk fluid distribution systems (e.g. o-ring failures, valve failures, or contaminated incoming fluid) include filters in the fluid supply line. However, an abrupt change in the flow rate of the fluid through the filters causes hydraulic shock to the filters which results in a release of previously filtered particles into the fluid thereby causing a spike in the particle concentration. Although maintaining a minimum flow rate of the fluid through the filters helps reduce particulate release, the problem is not eliminated. Accordingly, pressure and flow fluctuations of the fluid can result in fluctuations of the particle concentration in the fluid, which may lead to defects in the semiconductor wafers. 
     Moreover, as discussed above, fluid distribution systems often supply many tools. When a tool demands process fluid, the fluid is pumped from the supply line which causes the pressure of the fluid in the supply line to drop by about 5 to about 25 psi. As will be discussed further below, typical fluid distribution systems having vacuum-pressure engines cause pressure fluctuations in the supply line which may adversely affect the flow and purity conditions of the fluid supplied to the tools. Accordingly, there is a need for a fluid distribution system that minimizes or eliminates pressure and flow fluctuations of the fluid in the supply line. 
       FIG. 1   a  depicts a standard vacuum-pressure fluid distribution system used to supply process fluids to semiconductor process tools. Other types of vacuum-pressure fluid distribution systems are described in U.S. Pat. Nos. 5,330,072 and 6,019,250, which are incorporated herein by reference. 
     With reference to  FIG. 1   a , a vacuum-pressure fluid distribution system typically includes two pressure-vacuum vessels  101  and  103 . Each vessel is equipped with at least two fluid level sensors  105 ,  107 ,  109  and  111  (e.g. capacitive sensors). Sensors  105  and  109  monitor a low fluid level condition in vessels  101  and  103 , respectively; and sensors  107  and  111  monitor a high-fluid level condition in vessels  101  and  103 , respectively. The process fluid from fluid source  113  enters vessel  101  through two-way valve  115  and enters vessel  103  through two-way valve  117 . The fluid exits vessel  101  through two-way valve  119  and exits vessel  103  through two-way valve  121 . Upon exiting vessel  101  or vessel  103 , the fluid flows through the bulk process fluid supply line  123 . 
     During a fill cycle, a vacuum-generating device  125  (e.g. an aspirator or venturi) creates a vacuum in vessel  101  to draw in the fluid. When the fluid flows into vessel  101  during a fill cycle, two-way valves  115  and  127  are open and three-way valve  129  is in position “A”. When the vacuum is operated on vessel  101 , any gas in vessel  101  flows to an exhaust (not shown) as the fluid from the fluid source  113  is drawn into the vessel. When the fluid reaches level sensor  107  (e.g. a capacitive sensor), valves  115 ,  127  and  129  deactivate and the vacuum stops. 
     During a dispense cycle, an inert gas  131 , such as nitrogen, flows through “slave” regulator  133  and through position “B” of three-way valve  129  into vessel  101 . Vessel  101  is initially pressurized to a predetermined value and then valve  119  opens allowing the fluid to flow under the force of the inert gas pressure through valve  119 , through the filters (not shown) and into the bulk fluid supply line  123 . The vessel  101  dispenses the fluid until it reaches low level sensor  105  at which point valve  119  closes and the fill cycle begins again. 
     During operation, vessels  101  and  103  alternate between fill and dispense cycles such that when vessel  101  is filling, vessel  103  is dispensing. During a fill cycle in vessel  103 , valves  117  and  127  are open and valve  137  is in position “A”. During a dispense cycle in vessel  103 , inert gas  131  flows through slave regulator  135  and port “B” of valve  137  to pressurize the fluid in vessel  103  and drive it through valve  121  to supply line  123 . At the end of a dispense cycle in vessel  103 , the vessels switchover so that vessel  103  begins a fill cycle and vessel  101  begins a dispense cycle. Notably, the vacuum-generating device  125  is configured so that the vessels fill faster than they dispense to provide a continuous flow of fluid to the supply line  123 . 
     In the system shown in  FIG. 1   a , a manually-adjustable master regulator  141  is facilitated with a gas, such as compressed dry air, from a high pressure gas source  139 . The master regulator  137  sends a constant gas pilot signal to both slave regulators  133  and  135  which thus provide a constant inert gas pressure to valves  129  and  137 , respectively. The pressure supplied to each valve  129  and  127  is the same. Accordingly, during a dispense cycle of either vessel  101  or  103 , the inert gas pressure supplied to each vessel is constant and the same. 
     A problem with the system of  FIG. 1   a  is that it does not maintain a stable pressure of the fluid in the supply line  123 .  FIG. 1   b  shows a simplified illustration of how the pressure of the fluid in supply line  123  fluctuates over time. Losses due to process tool demands, fittings, piping and other parts present in a complex fluid distribution system were not accounted for in this illustration. During operation of system  100 , as a vessel dispenses from its high sensor to its low sensor, the pressure in the supply line  123  decreases by an amount equivalent to the loss of the head pressure of the fluid between the high and low sensors. The head pressure is defined as the pressure resulting from the weight of the fluid in the vessel acting on the fluid in the supply line. When the vessels switchover the vessel beginning its dispense cycle starts full with fluid up to its high sensor, and the same pressure that was applied to the vessel that just completed its dispense cycle, is applied to the dispensing vessel. Thus, when the vessels switchover the pressure of the fluid in the supply line spikes or increases by an amount equivalent to the head pressure of the newly dispensing vessel. 
     There have been efforts to improve the system of  FIG. 1   a  by actively controlling the pressure of the fluid in the supply line.  FIG. 2   a  shows a modified vacuum-pressure system  200 . System  200  is substantially similar to system  100  except that an electro-pneumatic master regulator  241  is used instead of manually-adjustable regulator  141 . As in system  100 , the electro-pneumatic master regulator  241  of system  200  is facilitated with a gas, such as compressed dry air, from a high pressure gas source  239 . The system of  FIG. 2   a  also includes a sensor  245  to monitor the pressure at a mid-point in the supply line  223 . Like the system of  FIG. 1   a , vessels  201  and  203  alternate between vacuum fill and pressure dispense cycles, and master regulator  241  provides the same pneumatic signal to both slave regulators  233  and  235 . 
     During a dispense cycle, the inert gas pressure applied to the fluid in the dispensing vessel  201  or  203  is adjusted based upon a signal from the pressure indicator  245 . Considering a simplified fluid distribution system with no process tool demands or other pressure losses, the inert gas pressure supplied to the dispensing vessel  201  or  203  while it is dispensing increases to compensate for the loss in head pressure between the high and low sensors ( 207 ,  211  and  205 ,  209 , respectively) of the vessel. 
     Although system  200  prevents a pressure decrease due to head loss in the dispensing vessel, it does not provide stable pressure control of the fluid in the supply line  223 .  FIG. 2   b  is an illustration of how the pressure in supply line  223  can fluctuate over time in a distribution system free from process tool demands or other pressure losses. During operation, when the vessels switchover the master regulator  241  continues to send the same signal (or pressure requirement) to the vessel beginning its dispense cycle as it was sending to the vessel that just completed its dispense cycle. Accordingly, when the vessels switchover there is a spike in the pressure in the supply line  223  equivalent to the change in head pressure between the high and low sensors of the vessel that just completed its dispense cycle. As a result, the system  200  actively attempts to decrease the pressure of the fluid in the supply line  223  and continues to adjust the pressure until it reaches a predetermined setpoint. Thus, a problem with the system  200  is that the pressure of the fluid in the supply line  223  oscillates until it reaches a steady state as shown in  FIG. 2   b.    
     In addition, another problem with system  200  is that it continually adjusts the pneumatic signal to the slave regulator of the non-dispensing or standby vessel. Thus, the slave regulator for the non-dispensing vessel incurs significant wear and tear on the slave regulator of the standby vessel. 
     Accordingly, there remains a need in the semiconductor industry for improvements to fluid distribution systems including providing stable control of the flow conditions of the process fluid without causing wear and tear on the component parts. 
     BRIEF SUMMARY OF THE INVENTION 
     A method for controlling the pressure of a fluid in a bulk fluid distribution system comprising alternately dispensing fluid from a first vessel and a second vessel to at least one point of use under conditions wherein the pressure of the fluid at the at least one point of use remains substantially constant. 
     A method for controlling the pressure of a fluid in a bulk fluid distribution system having a first vessel and a second vessel for supplying the fluid to a supply line, an inert gas source for supplying an inert gas to the first and second vessels, a controller and a sensor positioned in the supply line comprising the steps of: receiving at the controller a control signal from the sensor; initiating a dispense cycle of the first vessel comprising the steps of: determining a first signal from the control signal and a head pressure of the fluid between a first level and a second level of the second vessel; applying a first pressure to the fluid in the first vessel based upon the first signal; and dispensing the fluid from a first level to a second level of the first vessel; and initiating a dispense cycle of the second vessel comprising the steps of: determining a second signal from the control signal and a head pressure between the first level and the second level of the first vessel; applying a second pressure to the fluid in the second vessel based upon the second signal; and dispensing the fluid from the first level to the second level of the second vessel. 
     An apparatus for controlling the pressure of a fluid in an alternating vessel bulk fluid distribution system comprising: a first vessel having a first pair of sensors for detecting a first level and a second level of the fluid in the first vessel; a second vessel having a second pair of sensors for detecting a first level and a second level of the fluid in the second vessel; an inert gas feed line for supplying an inert gas to the vessels; a first pair of regulators including a first master regulator and a first slave regulator wherein the first slave regulator is adapted to regulate the pressure of the inert gas to the first vessel; a second pair of regulators including a second master regulator and a second slave regulator wherein the second slave regulator is adapted to regulate the pressure of the inert gas to the second vessel; a fluid supply line having a control sensor positioned within the supply line wherein the vessels are adapted to alternately dispense fluid to the supply line; and a controller adapted to receive a control signal from the control sensor, determine a first signal based upon the control signal and a change in head pressure of the fluid between the first and second levels of the second vessel, determine a second signal based upon the control signal and a change in head pressure of the fluid between the first and second levels of the first vessel, and send the first signal to the first master regulator and the second signal to the second master regulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic representation of a prior art vacuum-pressure fluid distribution system. 
         FIG. 1   b  is an illustration of the pressure fluctuations of the fluid in the supply line of the prior art fluid distribution system of  FIG. 1   a.    
         FIG. 2   a  is a schematic representation of a prior art fluid distribution system. 
         FIG. 2   b  is a illustration of the pressure fluctuations of the fluid in the supply line of the prior art fluid distribution system of  FIG. 2   a.    
         FIG. 3   a  is a schematic representation of a fluid distribution system according to the present invention. 
         FIG. 3   b  is a schematic representation of an alternate embodiment of the fluid distribution system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Two embodiments of the present invention are shown in  FIGS. 3   a  and  3   b . The invention is directed to a vacuum-pressure fluid distribution system  300  that provides stable control of the pressure of a fluid in a bulk fluid supply line  323 . The system  300  substantially eliminates all of the pressure fluctuations of the prior art systems shown in  FIGS. 1 and 2 . 
     System  300  has two vessels  301  and  303  each equipped with at least one fluid level sensing device (e.g.  305 ,  306 ,  307 ,  308 ,  309  and  311 ). While vacuum-pressure engines typically employ capacitive sensors as level sensing devices, the present invention additionally contemplates the use of optical sensors, digital sensors, load cells or the like. The system shown in  FIG. 3   a  includes two sensors  305  and  309  for monitoring a low fluid level condition in vessels  301  and  303 , respectively; and sensors  307  and  311  for monitoring a high-fluid level condition in vessels  301  and  303 , respectively. The system shown in  FIG. 3   b  includes two load cells  306  and  308  for monitoring the fluid levels in vessels  301  and  303 , respectively. The fluid from fluid source  313  (e.g. a pump, another chemical distribution system, a pressurized drum or the like) enters vessel  301  through two-way valve  315  and enters vessel  303  through two-way valve  317 . The fluid exits vessel  301  through two-way valve  319  and exits vessel  303  through two-way valve  321 . Upon exiting vessel  301  or vessel  303 , the fluid flows through a filter (not shown) and to the fluid supply line  323 . 
     During a fill cycle, the vessels  301  and  303  can be filled under pressure or vacuum conditions. For example, a pump or the supply line from another fluid distribution system can provide a pressurized supply of the fluid to the vessels  301  and  303 . If a pressurized source is used, then as a vessel is filling, a vent in the vessel (not shown) will open to exhaust residual gas from the vessel. In contrast, when the vessels are filled under vacuum conditions, a vacuum generating device (not shown in  FIG. 3 ), such as an aspirator, will draw the fluid into the vessel as described above and as shown in  FIGS. 1   a  and  2   a.    
     During a fill cycle of vessel  301 , valve  315  is open as fluid flows into the vessel. When the fluid reaches a predetermined high level, as indicated by either a level sensor  307  (e.g. capacitive, optical, digital, or the like) or by a load cell  306 , valve  315  closes. 
     During a dispense cycle of vessel  301 , an inert gas  331 , such as nitrogen, flows through “slave” regulator  333  and valve  329  to pressurize vessel  301  to dispense fluid through valve  319  to supply line  323  until the fluid level in vessel  301  reaches a predetermined “low” level, as detected by a level sensor  305  (e.g. capacitive, optical, digital or the like) or a load cell  306 , at which point valve  319  closes and the vacuum filling sequence begins. 
     During operation, vessels  301  and  303  alternate between fill and dispense cycles such that when vessel  301  is filling, vessel  303  is dispensing. During a dispense cycle in vessel  303 , inert gas  331  flows through slave regulator  335  and valve  337  to pressurize vessel  303  to dispense fluid through valve  321  to supply line  323  until the fluid level in vessel  303  reaches a predetermined “low” level, as detected by a level sensor  309  or a load cell  308 , at which point valve  321  closes and the vacuum filling sequence begins. Notably, the system is configured so that the vessels fill faster than they dispense in order to provide a continuous flow of fluid to the supply line  323 . 
     System  300  uses sensor  345  (e.g. a pressure transducer, flow meter or the like) to monitor a condition of the fluid in the supply line  323  and the system adjusts the inert gas pressure supplied to the vessels to compensate for changes in the condition of the fluid in the supply line  323 . The sensor  345  can be positioned at any point in the supply line  323 , but is preferably positioned at a mid-point in the supply line  323 . In addition, system  300  substantially eliminates any changes in the pressure of the fluid in the supply line  323  resulting from changes in head pressure during dispense cycles of the vessels. 
     System  300  includes a controller  343  which receives a control signal from sensor  345 . The controller is connected to master regulators  341  and  342  (e.g. electro-pneumatic regulators), which control slave regulators  333  and  335  (e.g. dome loaded pressure regulators), respectively. Master regulators  341  and  342  are facilitated with gas from a high-pressure gas source  339 . The sensor  345  and master regulators  341  and  342  may be connected to the controller  343  by analog cables, digital cables (e.g. Ethernet cables), or wireless connections. The slave regulators  333  and  335  control the pressure of inert gas supplied to each vessel  301  and  303 , respectively. 
     To eliminate pressure fluctuations of the fluid in the supply line  323  resulting from changes in head pressure in the vessels during dispense cycles, the controller biases the signal sent to each vessel at the beginning of a dispense cycle. The following example illustrates the operation of the invention to eliminate fluctuations due to changes in the head pressures. 
     Example 1 
     Assume Vessel  301  has completed a fill cycle by filling the vessel with fluid to its high level ( 307  as shown in  FIG. 3   a ) and is standing by while vessel  303  completes its dispense cycle by dispensing fluid to its low level ( 309  as shown in  FIG. 3   a ). 
     During the dispense cycle of vessel  303 , the controller  343  is periodically or continuously receiving a signal from sensor  345  and adjusting the inert gas pressure supplied to vessel  303  to maintain a predetermined flow condition (e.g. pressure, flow rate or the like) in the supply line  323 . As vessel  303  dispenses from its high level ( 311  as shown in  FIG. 3   a ) to its low level ( 309  as shown in  FIG. 3   a ) the head pressure of the fluid decreases between level h 1,303  and level h 2,303  in accordance with the following equation for the change in head pressure of a fluid in a vessel: ΔP 303 =P 1,303 −P 2,303 =pg(h 1,303 −h 2,303 ) (where p=density of the fluid and g=9.8 m/s 2 ). 
     Consequently, to prevent a decrease in the pressure of the fluid in the supply line  323 , the controller  343  sends a signal (e.g. a 4-20 mA signal) to master regulator  342  to increase the inert gas pressure, controlled by slave regulator  335 , to the vessel  303 . Notably, the sensor  345  may detect other changes in the pressure due to tool demands or pressure losses through the pipes and fittings in the fluid distribution system, but for the purposes of this example, these losses will not be considered. When the fluid in vessel  303  reaches the low level, the vessels switchover and vessel  301  begins a dispense cycle while vessel  303  begins a fill cycle. 
     While vessel  303  is dispensing, the controller is independently determining or calculating a first signal to be sent to the regulators controlling the inert gas pressure to vessel  301  when it begins its dispense cycle. In this example, the controller monitors the control signal sent by sensor  345  and determines the first signal by reducing the control signal by an amount correlating to the change in head pressure of vessel  303 . Thus, when vessel  301  begins its dispense cycle, the inert gas pressure applied to the fluid in vessel  301  is reduced by an amount equivalent to the change in head pressure of the fluid in vessel  303 . Without this reduction, the pressure applied to the vessel would be too high and cause the pressure in the supply line  323  to spike. 
     After the beginning of its dispense cycle, the controller  343  adjusts the inert gas pressure supplied to vessel  301  in the same manner as described above with respect to vessel  303  in order to maintain the predetermined flow condition of the fluid in the supply line  323 . 
     The system  300  of the present invention provides improved pressure control of the process fluid over the prior art systems  100  and  200 . Indeed, depending on the placement of the sensors, (i.e. the vertical distance between them), the invention may provide pressure control of the fluid in the supply line to about ±0.2 psi to about ±1.5 psi of a predetermined setpoint with continuous adjustment to maintain steady state conditions whereas system  200  at best offered control from 1.5 to 3 psi of a predetermined setpoint. 
     Another advantage of the present invention is that the pair of regulators  333 , 341  and  335 , 342  can be independently controlled. This enables more flexibility in the control process and reduces wear and tear on the slave regulators so that the slave regulator for the non-dispensing vessel does not have to continually adjust. 
     In addition, as noted above, the system  300  can compensate for other pressure or flow condition changes (monitored by sensor  345 ) resulting from inter alia changes in tool demand, pressure losses across filters, and frictional losses from piping and other system components. Thus, the system  300  of the present invention offers much more stable control of flow conditions of the fluid supplied to points of use than other prior art systems. 
     It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in light of the and variations likewise be included within the scope of the invention as set forth in the following claims.