Abstract:
A valve assembly including a main body defining a central axis and a gate. The gate includes a curved surface relative to the central axis and the gate is disposed within the main body. The gate is rotatable about an axis of rotation running along the length of the gate. The valve assembly also includes an actuation assembly for rotating the gate about the axis of rotation between a first position where said valve is open and a second position where said valve is closed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to semiconductor manufacturing equipment and, more particularly, to a gate valve for use with a rapid thermal processing reactor. 
     2. Description of Related Art 
     In the semiconductor industry, to continue to make advancements in the development of semiconductor devices, especially semiconductor devices of decreased dimensions, new processing and manufacturing techniques have been developed. One such processing technique is known as Rapid Thermal Processing (RTP), which reduces the amount of time that a semiconductor device is exposed to high temperatures during processing. The RTP technique typically includes irradiating the semiconductor device or wafer with sufficient power to quickly raise the temperature of the wafer and hold it at that temperature for a time long enough to successfully perform a fabrication process, while avoiding such problems as unwanted dopant diffusion that would otherwise occur at the high processing temperatures. 
     As is widely known in the semiconductor processing industry, processing techniques such as RTP require fabrication clean room space to ensure that the processing is free from contaminants and particles that may reduce manufacturing precision. However, clean room space is expensive both to construct and maintain. Thus, semiconductor wafer processing systems which require large footprints are economically disadvantageous. Accordingly, processing system designers have attempted to construct systems having components with smaller, more compact, and narrower structures. 
     Gate valve structures may be used in the processing system to isolate semiconductor wafers in various chambers, as the wafers are transported between locations of a first pressure to areas of a second pressure. Although, the concept of isolating or sealing a chamber using gate valves is straightforward, the design of such valves can be complicated, especially due to competing design considerations. For example, the gate valve must provide adequate positive closure that can withstand process pressure and vacuum. Most often this need has been met using complicated linkages that typically require both an axial and a lateral sealing action. For example, an apparatus is disclosed in U.S. Pat. No. 4,721,282 where an initial axial motion of a shaft provides for the primary movement of a gate member toward a processing chamber port. A secondary lateral motion provides for movement of the gate member against the port for a positive seal. 
     To conserve clean-room space and provide access to a process chamber, what is needed is a gate valve, which occupies a relatively small volume to maintain a small processor footprint and provides adequate isolation to the process chamber of a processing system during semiconductor processing. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a valve assembly includes a main body defining a central axis and a gate. The gate includes a curved surface relative to the central axis and the gate is disposed within the main body. The gate is rotatable about an axis of rotation running along the length of the gate. The valve assembly also includes an actuation assembly for rotating the gate about the axis of rotation between a first position where said valve is open and a second position where said valve is closed. 
     This invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system that is suitable for use with the described rotary valve. 
     FIG. 2 is a block diagram of an embodiment of an RTP reactor system in the semiconductor wafer processing system. 
     FIG. 3 is a simplified schematic illustration of a reactor chamber. 
     FIGS. 4A-4C illustrate side view and cross-sectional views of a semiconductor wafer processing system incorporating an example of a rotary valve. 
     FIGS. 5A-5E illustrate an embodiment of a rotary gate valve in open and closed positions. 
     FIGS. 6A-6E illustrate another embodiment of a rotary gate valve in open and closed positions. 
     FIG. 7 illustrates an embodiment of a rotary gate valve with inflatable gasket. 
     FIG. 8 illustrates an embodiment of a gate valve main body. 
     FIG. 9 illustrates an embodiment of a rotary gate valve. 
     FIGS. 10A-10E illustrate an alternative embodiment of a gate valve assembly. 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method and apparatus are disclosed for determining robot alignment of semiconductor wafers and wafer-like objects contained in a carrier or container. The invention may be used in a variety of applications including the manufacture of semiconductor devices, hard disks, and liquid crystal displays. 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system  10  that establishes a representative environment of the present invention. The representative system is fully disclosed in co-pending U.S. patent application Ser. No. 09/451,677 now U.S. Pat. No. 6,410,455, which is herein incorporated by reference for all purposes. Processing system  10  includes a loading station  12  which has multiple platforms  14  for supporting and moving a wafer cassette  16  up and into a loadlock  18 . Wafer cassette  16  may be a removable cassette which is loaded into a platform  14 , either manually or with automated guided vehicles (AGV). Wafer cassette  16  may also be a fixed cassette, in which case wafers are loaded onto cassette  16  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  16  is inside loadlock  18 , loadlock  18  and transfer chamber  20  are maintained at atmospheric pressure or else are pumped down to a vacuum pressure using a pump  50 . A robot  22  within transfer chamber  20  rotates toward loadlock  18  and picks up a wafer  24  from cassette  16 . A reactor or thermal processing chamber  26 , which may also be at atmospheric pressure or under vacuum pressure, accepts wafer  24  from robot  22  through a gate valve  30 . Optionally, additional reactors may be added to the system, for example reactor  28 . Robot  22  then refracts and, subsequently, gate valve  30  closes to begin the processing of wafer  24 . After wafer  24  is processed, gate valve  30  opens to allow robot  22  to pick-up and place wafer  24  into cooling station  60 . Cooling station  60  cools the newly processed wafers before they are placed back into a wafer cassette in loadlock  18 . In accordance with an embodiment of the present invention, reactors  26  and  28  are RTP reactors, such as those used in thermal anneals. In other embodiments, reactors  26  and  28  may also be other types of reactors, such as those used for dopant diffusion, thermal oxidation, nitridation, chemical vapor deposition, and similar processes. Reactors  26  and  28  are generally horizontally displaced though they may be vertically displaced (i.e. stacked one over another) to minimize floor space occupied by system  10 . Reactors  26  and  28  are bolted onto transfer chamber  20  and are further supported by a support frame  32 . Process gases, coolant, and electrical connections may be provided through the rear end of the reactors using interfaces  34 . 
     A simplified block diagram of an RTP reactor system is shown in FIG.  2 . In one example, reactor system  200  may include a reactor chamber  210 , a controller  212 , a process control computer  214 , a gas network  216 , a rotary gate valve assembly  218 , and a pump assembly  220 . A microprocessor or process control computer  214 , generally controls the processing of a semiconductor wafer placed in the RTP reactor and may be used to monitor the status of the system for diagnostic purposes. In one embodiment, process computer  214  provides control signals to controller  212  in response to temperature data received from temperature sensors (not shown) in chamber  210 . Process computer  214  may also direct pressure setpoints to pump assembly  220  as well as gas and plasma inlet flow signals to mass-flow controllers in gas network  216 . For example, controller  212  may be a real-time Proportional Integral Derivative (PID), multi-zone controller, available from Omega Corporation. Controller  212  provides control signals to a SCR-based phase controlled power supply  221 , which provides power to the resistive heating elements provided in chamber  210 . In operation, the multi-zone controller receives temperature sensor outputs via sensing line  222  from chamber  210 , as well as the desired wafer temperature setpoint from computer  214  via line  224  and delivers controlled power setpoints to the heating element power supply  221 . The heating elements increase or decrease their energy output in response to the increase or decrease in power supplied from power supply  221 . 
     FIG. 3 shows an alternative embodiment of reactor chamber  210 , which may help to maintain the structural integrity of quartz tube  230  during high temperature processing. An external cavity  240  may be formed around tube  230  and filled with air, N 2 , O 2  or other process gases. Using pure gases to fill the external cavity may help to extend the usage life of other components, such as heating elements, which may be housed in cavity  240 . External cavity  240  may be maintained having at least an equal or lower pressure than interior cavity  232  (P1≦P2). In one embodiment, tube  230  may be in communication with loadlock  18 , typically through rotary gate valve  218 , such that the pressure in tube  230  may be equal to the pressure in loadlock  18  (P2=P3). In this embodiment, the pressure differential between external cavity  240  and tube  230  creates a force on the internal walls of tube  230 . To create the pressure differential external cavity  240  is evacuated directly at orifice  234  and through pump pipe  237 . Tube  230  is evacuated through loadlock  18  at orifice  235  and through loadlock pipe  236 . Pump pipe  237  and loadlock pipe  236  meet at tube intersection  238  and proceed as one pipe  239  to pump assembly  220 . Since the combined volume of loadlock  18  and tube  230  is greater than the volume of external cavity  240 , it follows that the pressure in external cavity  240  can be less than that in the combined loadlock  18  and tube  230  configuration. In this manner, the internal pressure in tube  230  can be used to fortify tube  230  against failure, and ensures that the structural integrity of tube  230  is maintained. 
     Pump assembly  220  may include any suitable pump for creating the required process pressures within chamber  210 . Pump assembly  220  may also serve other purposes. For example, pump assembly  220  may be used to pump down or create a vacuum in process chamber  230 , such that the cool down rate within the chamber can be controlled. An exemplary pump assembly may include mechanical pump model HC-60B available from Kashiyama Industries Ltd. 
     As shown in FIG. 3, the processing chamber section may generally include a closed-end process chamber or tube  230 , which defines an interior cavity  232 . In one embodiment, tube  230  may be constructed with a substantially rectangular cross-section, having a minimal internal volume surrounding a wafer  170 . Wafer  170  may be made of conventional materials commonly used in the industry, such as silicon, gallium arsenide, or other similar compound or the wafer may be a semiconductor wafer, made from quartz or glass. In this embodiment, the volume of tube  230  is usually no greater than 5000 cm 3 , preferably the volume is less than about 3000 cm 3 . One result of the small volume is that uniformity in temperature is more easily maintained. Additionally, the small tube volume allows reactor chamber  210  to be made smaller, and as a result, system  10  may be made smaller, requiring less clean room floor space. The smaller reactor size, in conjunction with the use of the robot loader, allows multiple reactors to be used in system  10  by vertically stacking the reactors as shown in FIG.  1 A. Tube  230  is made of quartz, but may be made of silicon carbide, Al 2 O 3 , or other suitable material. To conduct a process, quartz tube  230  should be capable of being pressurized. Typically, tube  230  should be able to withstand internal pressures of about 0.001 Torr to 1000 Torr, preferably between about 0.1 Torr and about 760 Torr. 
     Referring to FIGS. 4A-4C, the loading/unloading section of reactor chamber  110  includes gate valve assembly  218 . Chamber  110  also includes a hot plate support  140 , a plurality of hot plates  145 , and wafer support standoffs  160  located on each hot plate. Hot plate  145  may be used for the heat treatment of semiconductor wafers, as well as the baking or heating of wafers. Housed within gate valve main body  100  and aligned along the valve central axis  101  are a plurality of rotary gates  104 , and a rotary actuator  126  for each rotary gate  104 , all assembled together to provide gate valve assembly  218 . In one example, valve main body  100  defines a plurality of ports  117 . Each port  117  has a first end  121 , which provides initial access to reactor  110  through gate valve  218 , and a second end  123 , which has an aperture or opening  102  configured to mate with aperture  138  of quartz tube  230 . The geometry and dimensions of valve aperture  102  generally correspond to those of reactor aperture  138 , so that valve aperture  102  and reactor aperture  138  can be used together to provide a seal, which maintains a selected vacuum or pressurized environment within tube  230  or isolates cavity  232  during wafer processing operations. Standoffs  160  may be formed from any high temperature resistant material, such as quartz. Standoffs  160  may have a height of between about 50 μm and about 20 mm. In order to monitor the temperature of wafer  170  during processing, at least one thermocouple may be embedded into at least one standoff  160 . 
     Aperture  138  at one end of tube  230  provides access for the loading and unloading of wafer  170  before and after processing. Aperture  138  may be a relatively small opening, but with a height and width large enough to accommodate a wafer of between about 0.5 to 2 mm thick and up to about 300 mm (˜12 in.) in diameter, and robot arm  22  passing therethrough. The height of aperture  138  is no greater than between about 18 mm and 50 mm, and preferably, no greater than 20 mm. The relatively small aperture size helps to reduce radiation heat loss from tube  230 . Also, the small aperture size reduces the number of particles entering cavity  230  and allows easier maintenance of the isothermal temperature environment. In one embodiment, during a processing procedure, an edge of wafer  170  may be no less than 50 mm from aperture  138  when the wafer is placed on standoffs  160 . 
     As seen in FIGS. 5A-5E, rotary gate  104  is an elongated cylinder mounted along valve main body  100 . Rotary gate  104  may be made of quartz, ceramics or any metallic material such as aluminum, stainless steel or Al 2 O 3 . Rotary gate valve  104  may be machined or formed by extrusion. Each rotary gate includes a slot  105  aligned along an axis  103  with aperture  102  and aperture  138  when gate  104  is in an open position. The number of slots  105  is generally equal to the number of ports  117 . Slot  105  may be machined into gate  104 ; sized and shaped so as to allow robot  22  to pass a wafer  170  from transfer chamber  20  through port  117  to chamber  110  where wafer  170  is placed upon wafer support standoffs  160 . Slot  105  is roughly oblong-shaped and cut through gate  104  along a cross-sectional diameter of gate  104 . Slot  105  may be rectangular or any shape suitable for a wafer  170  to be passed through it by robot  22 . The length of slot  105  is less than that of the elongated cylinder which forms gate  104 . The elongated cylinder is well suited for sealing slot-type openings, such as valve aperture  102 . The geometry of gate  104  may be changed to accommodate differently shaped openings. Preferably, as shown in FIGS. 4A-4C, gate  104  may be circular in cross-section. In one example, outer surface  113  of gate  104  may have a highly polished surface or may be coated with a heat/radiation reflective coating, such as gold, silver, polished aluminum Ni, Molybdenum, or other metal with a high melting point relative to the process temperatures. A SiN coating may be placed on top of the reflective coating, e.g. when the reflective coating is a silver layer, for U.V. protection. The reflective coating may be a thin-film coating (e.g., less than one micron in thickness). The reflective surface may reflect radiation energy, which may leak through valve aperture  102 , back into tube  230 . Each rotary gate  104  further includes a drive shaft  107  which is connected to the rotary actuator  126  for that rotary gate  104 . A rotary valve allows a smaller process chamber to be used. Space savings allow a user to stack additional process chambers. 
     By way of example, rotary actuator  126  rotates rotary gate  104  between a closed position and an open position, as seen in FIGS. 5B and 5C, respectively. The width w of slot  105  is sufficient to allow robot  22  to pass wafer  170  through slot  105  into and out of chamber  110 . Rotary valve  104  is in a closed position (FIG. 5B) as wafer  170  is brought to end  121 . Rotary actuator  126  is activated and rotates rotary valve  104  along axis of rotation  116 . When rotary valve  104  achieves open position (FIG.  5 C), slot  105  is now positioned to permit robot  22 , which is carrying wafer  170 , to pass through ends  121 ,  123  into chamber  110 . Wafer  170  is placed on wafer supports  160  and robot  22  is withdrawn through ends  121 ,  123 . 
     Rotary actuator  126  is air-driven or electro-magnetically-driven. In one example, to move drive shaft  107 , actuator  126  is activated by software implemented by the computer. Actuator  126  opens/closes gate  104  as necessary. Before wafers may be moved into the process chamber, gate  104  is opened by actuator  126 . Robot  22  then inserts/transfers the wafers to the process chamber. After insertion of the wafers, robot  22  is retracted to the transport module and actuator  126  moves gate  104  into a closed position. When wafer processing is completed, software instructs actuators  126  to move gate  104  into an open position. Robot  22  enters the process chamber to remove the processed wafers. When the wafers are removed, software again activates to close gate  104 . 
     As shown in FIGS. 5D-5E, when rotary valve  104  is closed, port  117  is sealed to prevent contamination of chamber  110  from the outside environment during processing and vice versa. As seen in FIG. 5E, a number of inflatable gaskets  120  may be placed along the surface  118  of port  117 . Gaskets  120  extend along the length of rotary valve  104  and are typically longer than the length of slot  105 . Gaskets  120  may be made of U.V resistant or high temperature resistant rubber. When rotary valve  104  is rotated to a closed position (FIG.  5 E), slot  105  is no longer aligned with ends  121 ,  123 . Gaskets  120  inflate and press against surface  113  of rotary valve  104 , sealing the respective spaces between ends  121 ,  123  and slot  105 . Each gasket  120  on surface  118  contacts surface  113  of rotary valve  104  with a force F to create a positive seal, which isolates process chamber  110 . Gaskets  120  can be pressurized by a pump or compressed air. Pressurized air comes from a tank or compressor which is part of the entire assembly or separate. In order to deflate gaskets  120 , either a pump may be used to remove the gas or air within each gasket  120  or a valve may be opened for air to bleed out of each gasket  120 . In the alternative, gaskets  120  retractable such that when not activated, deflated gaskets  120  are recessed under surface  118 . When activated, gaskets  120  inflate and extend above surface  118  to contact surface  113  of rotary gate valve  104 , providing a seal, as described above. 
     FIGS. 6A-6D illustrate an alternative embodiment of a rotary gate valve. Rotary gate valve  114  operates in a manner substantially similar to that described above with respect to rotary gate  104  (FIGS.  5 A- 5 D). Rotary gate  114  is an elongated cylinder mounted along valve main body  100  and includes a slot  115  aligned along an axis  103  with aperture  102  and aperture  138 . Rotary gate  114 , like rotary gate  104 , is roughly circular in cross-section. However, rotary gate valve  114  differs from rotary gate valve  104  in that slot  115  is formed in the elongated cylinder such that only three sides of slot  115  are bounded by the elongated cylinder. As seen in FIGS. 6A-6D, a cross-section of rotary gate  114  along the length of the slot appears to be less than a half-moon. 
     As seen in FIGS. 6A &amp; 6B, gate  114  may further include an exhaust port  223 , which allows for controlling pressure within the process chamber. Exhaust port  223  may be connected to an access passage  227  drilled or otherwise formed on one end of drive shaft  107  such that gas may be exhausted through passage  227  from the process chamber. In the alternative, passage  227  may be used to pass coolant through gate  114  in order to cool gate  114 . Also, cooling fluid ports  124  are provided (FIG.  6 B), which allow a coolant to flow so as to reduce the external temperature of the gate valve main body during RTP. 
     FIG. 7 illustrates an alternative embodiment of rotary gate valve  104 . A gasket tube  128  is placed in a groove formed around the periphery of each opening of slot  115  in gate  104 . Gasket  128  may be formed of the same materials as the gaskets described above. When rotary gate valve  104  is in a closed position, gasket  128  is inflatable to form a seal between slot  115  and the openings to the process chamber  110  and transfer chamber  20 . An access passage  127  can be drilled or otherwise formed on one end of drive shaft  107 . Passage  127  is operationally connected to inflatable gasket  128  such that gasket  128  is pressurized and inflates when air or any other suitable gas is passed into passage  127  as gate valve  104  is in a closed position. In the alternative, access passage  127  may be used to pass coolant through gate  104  in order to cool gate  104 . In the alternative, gate  104  may further include an exhaust port  223 , which allows for controlling pressure within the process chamber. Exhaust port  223  may be connected to access passage  127  drilled or otherwise formed on one end of drive shaft  107  such that gas may be exhausted through passage  127  from the process chamber. 
     As seen in FIG. 8, the gate valve assembly may further include an exhaust port  122 , which allows for controlling pressure within the process chamber pressure. Also, cooling fluid ports  124  are provided, which allow a coolant to flow so as to reduce the external temperature of the gate valve main body during RTP. 
     FIG. 9 illustrates an alternative embodiment of a rotary gate valve  134  which operates in a manner substantially similar to that described above with respect to rotary gate  104  (FIGS.  5 A- 5 D). Rotary gate  134  includes a draft shaft  107  connected at each end of an elongated hemispherical portion. The hemispherical portion fits generally within a hemispherical portion of port  117  such that when gate valve  134  is in an open position, a robot  22  carrying wafer  170  is able to pass unobstructed through ends  121 ,  123  into chamber  110 . Outer surface  133  of gate  134  may have a highly polished surface or may be coated with a heat/radiation reflective coating in a manner as described above. When gate  134  is in a closed position, surface  133  is exposed to process chamber  110  and inflatable gaskets  120  located at end  123  may be used so as to seal process chamber  110  from transfer chamber  20 . 
     FIGS. 10A-10E illustrate an alternative embodiment of gate valve assembly  218  of FIG.  3 . Housed within gate valve main body  300  and aligned along the valve central axis  301  are stacked sliding gate  304 , bellows  306 , plumbing interface  308 , linear drive shaft  310 , and actuator  326 , all assembled together to provide gate valve assembly  218 . In one example, valve main body  300  defines a port  317 . Port  317  has a first end  321 , which provides initial access to the reactor through gate valve  218 , and a second end  323 , which has an aperture or opening  302  configured to mate with aperture  238  of quartz tube  230 . The geometry and dimensions of valve aperture  302  generally correspond to those of reactor aperture  238 , so that valve aperture  302  and reactor aperture  238  can be used together to provide a seal, which maintains a selected vacuum or pressurized environment within tube  230  or isolates cavity  232  during wafer processing operations. 
     Gate  304  is an elongated plate mounted at an upper end of drive shaft  310 . The elongated plate is well suited for sealing slot-type openings, such as valve aperture  302 . The geometry of gate  304  may be changed to accommodate differently shaped openings. As shown in FIGS. 10A-10E, gate  304  maybe made of quartz, ceramics or any metallic material such as aluminum, stainless steel or Al 2 O 3 . Gate valve  304  may be machined or formed by extrusion. Each gate  304  includes a plurality of slots  315  aligned along an axis  303  with aperture  302  and aperture  338  when gate  304  is in an open position. The number of slots  315  is generally equal to the number of ports  317 . Slots  315  may be machined into gate  304 ; sized and shaped so as to allow robot  22  to pass a wafer  170  from transfer chamber  20  through port  317  to tube  230  where wafer  170  is placed upon wafer support standoffs  160 . Each slot  115  is roughly oblong-shaped and cut through gate  304  from a first side  313  to a second side  316 . Slot  315  may be rectangular or any shape suitable for a wafer  170  to be passed through it by robot  22 . The length L (FIG. 10C) of slot  315  is less than that of the width W of gate  304 . The geometry of gate  304  may be changed to accommodate differently shaped openings. In one embodiment, the surface of side  313  may have a highly polished surface or may be coated with a heat/radiation reflective coating, such as gold, silver, Ni, Molybdenum, or other metal with a high melting point relative to the process temperatures. The reflective surface may reflect radiation energy, which may leak through valve aperture  302 , back into tube  230  (FIGS.  10 A- 10 E). 
     By way of example, when drive shaft  310  is moved up into main body  300 , gate  304  is moved upward into an open position (FIGS.  10 B and  10 E). Drive shaft  310  is moved up and/or down through linear guide  319  by a linear action created using actuator  326 . In one embodiment, to move drive shaft  310 , actuator  326  is supplied at plumbing interface  308  with a conventional incompressible fluid, such as water or alcohol. The supply of fluid causes drive shaft  310  to move linearly through linear guide  319  into main body  300 . 
     By moving linear shaft  310  vertically upward, as described in FIGS. 10B and 10E, linear shaft  310  drives the expansion of bellows  306 . Bellows  306  surrounds shaft  310  along axis  301 . In this embodiment, bellows  306  establishes a vacuum seal between actuator  326  and main body  300  to ensure that tube  230  is not contaminated from the outside environment during opening and closing of the valve. 
     As shown in FIG. 10B, when linear shaft  310  reaches the end of its effective travel length, each port  315  is in an open position, allowing wafers  170  to be moved into and/or out of tube  230 . To close valve  304  from the open configuration, gate  304  is lowered along bore  317  when drive shaft  310  is moved down and out from main body  300  through linear guide  319  (FIGS. 10A and 10D) by actuator  326 , thus closing gate  304  for processing of wafer  170  in tube  230 . 
     In the alternative, access passage  327  may be used to pass coolant through gate  304  in order to cool gate  304 . A plurality of passages  327  run through gate  304  generally parallel to slots  315 , with passages  327  located above and below each slot  315 . As coolant passes through passage  327 , heat is absorbed from gate  304 . A plurality of cooling fluid tubes  324  are located at opposite sides  326 ,  328  of gate  304 . Each cooling fluid tube  324  operationally connects two passages  327 , in order to allow coolant to flow between the passages  327 , where each passage  327  is located either above or below the passage to which it is connected through tube  324 . Additionally in the alternative, gate valve  304  may also include an exhaust port (not shown) which allows for controlling pressure within the process chamber. 
     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as falling within the true spirit and scope of this invention.