Abstract:
Transducer apparatus and method combining both an absolute pressure sensor for sensing absolute pressure in the load lock chamber and a differential pressure sensor for sensing a pressure difference between ambient atmospheric pressure and pressure in a load lock chamber and provides control signals for opening an interior door from the load lock chamber into a vacuum processing chamber and for opening an exterior door between ambient atmosphere and the load lock chamber. The transducer can also produce signals to control transition from slow to fast vacuum pump-down of load lock chamber pressure at a predetermined pressure set point.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is related generally to load lock controls for vacuum processing chambers and more particularly to a combination differential and absolute pressure transducer for load lock control and a method of controlling load locks with such combination differential and absolute pressure transducer.  
           [0003]    2. State of the Prior Art  
           [0004]    Vacuum processing in reaction chambers is commonly used to deposit thin films of semiconductor materials, metal, dielectrics, and the like onto substrates in the fabrication of semiconductor devices. Typical processes that utilize such vacuum reaction chambers include chemical vapor deposition (CVD) and physical vapor deposition (PVD) and many variations of such processes, as well as etching processes to clean substrates or remove selected portions of materials. Typically, the vacuum process chamber is evacuated with a vacuum pump to a very low pressure, for example down to 10 −4  torr, and, in some processes, much lower, such as 10 −6  or even 10 −7  torr. When the desired vacuum is attained, feed gases are flowed into the process chamber at desired rates and proportions to react and/or deposit desired materials onto substrate wafers. Heat may be used in some processes, but others are performed at room temperature. When deposition of the desired materials is complete, the wafer is removed from the process chamber and another substrate wafer is inserted into the process chamber, where the deposition process is repeated.  
           [0005]    Significant vacuum pumping time is required to pump the process chamber down to the desired pressure, and undesirable contaminants enter the process chamber every time it is opened to atmosphere. Therefore, substantial efforts are made to avoid opening the process chamber to atmosphere and to maintain the process chamber pressure as close to the desired low deposition pressure as possible. Load locks are used, therefore, to facilitate insertion of substrates into the process chambers for deposition and/or etch processing and to remove the wafers from the process chamber while maintaining the vacuum in the process chamber.  
           [0006]    A load lock is, essentially, a second vacuum chamber, often smaller in size than the process chamber, and connected to the process chamber by a passage with an interior “door” or large valve that can be opened for insertion and removal of the wafers into and out of the process chamber. When the interior door is closed, it seals the passage so that no air or gas can flow into or out of the process chamber through the passage. The load lock also has an exterior “door” or large valve, which opens the load lock chamber to the atmosphere to allow insertion or removal of wafers into and out of the load lock chamber. When the exterior door is closed, it seals the load lock so that no air or other gas can flow into or out of the load lock chamber.  
           [0007]    In operation, the process chamber has its pressure maintained at the desired vacuum by a process chamber vacuum pump. With the interior door of the load lock closed, the exterior door is opened to the atmosphere, so one or more wafer substrate(s) can be inserted into the load lock chamber. With the wafer(s) in the load lock chamber, the exterior door is closed, and a load lock vacuum pump draws the air out of the load lock chamber, until the pressure in the load lock chamber is about as low as the pressure in the process chamber. Then, the interior door is opened, so the wafer substrate(s) can be moved from the load lock chamber, through the passage, and into the process chamber. When the wafer(s) are in the process chamber, the interior door can be closed while the wafer(s) are processed in the process chamber, i.e., while feed gas is fed into the process chamber and materials are either deposited on, or etched from, the wafer(s). Alternatively, but not preferably, the interior door could be left open during processing.  
           [0008]    When the processing is complete, the wafer(s) are removed from the process chamber into the load lock chamber. The interior door is then closed to maintain the vacuum in the process chamber, while the pressure in the load lock is brought up to atmospheric pressure by allowing air or an inert gas, such as nitrogen, to flow into the load lock chamber. When the pressure in the load lock chamber is at or near atmospheric pressure, the exterior door is opened to allow removal of the processed wafer(s).  
           [0009]    Some more complex process systems have a central transfer chamber with several process chambers branching out from the transfer chamber. In those circumstances, the load lock is usually connected by the passage and interior door to the transfer chamber.  
           [0010]    In the past, it has been difficult to control the load lock in an efficient manner. Convection pirani pressure sensors, which have absolute pressure measuring capabilities from about 1,000 torr down to about 10 −3  torr (atmospheric pressure at sea level is about 760 torr) have been used in pressure transducers adapted to control opening of the doors in load locks. Such control of load lock doors with that type of pressure transducer has been beneficial, but problems persist. For example, the 10 −3  torr lower pressure measuring limit of the convection pirani sensors is not low enough for effective control of opening the interior door, because the process chambers are usually operated at pressures at least one to three orders of magnitude below that limit, i.e., at 10 −4  torr or even 10 −6  torr or lower. Thus, even when the load lock pressure is pumped down to 10 −3  torr, opening the interior door causes an undesirable rush of gas molecules, along with any particulate impurities and water vapor they carry along, into the process chamber. It puts a greater load on the vacuum pumps of the process and/or load lock chambers, causing larger pump down times after each opening and closing of the interior door, especially in the process chamber to get the pressure pumped back down to the desired process pressure. Such added pumping overhead adds to the processing time and decreases efficiency.  
           [0011]    The problems are even worse on the upper pressure end, i.e., at or near atmospheric pressure (about 760 torr), because density of gas or air molecules is much greater at that pressure than at the vacuum pressures used in vacuum process chambers. Thus, opening the exterior door when pressure inside the load lock chamber is not the same as the ambient atmospheric pressure causes much stronger air currents and is much more contaminating, even when the load lock is in a clean room. Again, convection pirani sensors do have accurate pressure sensing capabilities in the atmospheric range, but it is impossible to set them to control exterior door opening effectively due to constantly changing ambient atmospheric pressure conditions due to weather, altitude, and the like. For example, some manufacturers set the transducer to generate a signal to open the exterior door of the load lock when pressure of the load lock chamber is brought up to 750 torr, thinking it will work for most locations that are slightly above sea level. However, ambient atmospheric pressure in Boulder, Colorado, for example, is about 630 torr, so having a transducer that opens the exterior door when pressure in the load lock chamber reaches 750 torr in Boulder, Colo., would still have adverse gas current and contamination effects. Further, ambient atmospheric pressure at any geographic location varies, such as with different weather conditions or fronts that move into and out of any particular location. Resetting such transducers to generate control signals at different pressures is not easy, may require changing software or control circuits, and is not something that is done by ordinary users.  
           [0012]    Therefore, there is a need for better transducer apparatus and for better control methods for controlling the exterior door openings, especially, and also for controlling interior door openings in load locks.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is a general object of this invention to improve pressure monitoring and control of load locks in semiconductor fabrication process.  
           [0014]    Another general object of this invention is to reduce contamination problems during load lock operations.  
           [0015]    A more specific object of this invention is to provide controls that facilitate opening and closing the interior and exterior doors in load lock operations.  
           [0016]    Another more specific object of this invention is to provide a pressure transducer for load lock control that is accurate and functional over the full range of operation from atmospheric pressures to very low evacuation pressures of 10 −4  or lower.  
           [0017]    Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.  
           [0018]    To achieve the foregoing and other objects, the apparatus of the present invention may comprise, but is not limited to, a combination differential and absolute pressure transducer apparatus for controlling a load lock that facilitates transfer of parts between a room at ambient atmospheric pressure and a vacuum processing chamber maintained at a pressure less than one (1) torr and that has an evacuatable load lock chamber, an exterior door positioned between the load lock chamber and the room, a interior door positioned between the load lock chamber and the processing chamber, a exterior door actuator that is responsive to an exterior door control signal to open or close the exterior door, an interior door actuator that is responsive to an interior door control signal to open or close the interior door, and a vacuum pump connected to the load lock chamber for evacuating the load lock chamber. The combination differential and absolute pressure transducer has a differential pressure sensor that is capable of sensing a pressure difference between ambient atmospheric pressure in the room and pressure in the load lock chamber, and it has an absolute pressure sensor that is capable of sensing absolute pressure in the load lock chamber. The differential pressure sensor is mounted so that a first side of the differential pressure sensor is exposed to ambient atmospheric pressure in the room and so that a second side of the differential pressure is exposed to pressure in the load lock chamber. The absolute pressure sensor is also mounted so that it is exposed to pressure in the load lock chamber. Both the differential pressure sensor and the absolute pressure sensor can be connected in fluid flow relation to the load lock chamber by a common manifold. A differential pressure transducer circuit is connected to the differential pressure sensor and is capable of generating an exterior door control signal at a preset differential pressure value, and an absolute pressure transducer circuit is connected to the absolute pressure sensor and is capable of generating an interior door control signal at a preset absolute pressure value. An exterior door control link connected between the differential pressure transducer circuit and the exterior door is capable of delivering exterior door control signals generated by the differential pressure transducer circuit to the exterior door actuator; an interior door control link connected between the absolute pressure transducer and the interior door is capable of delivering interior door control signals generated by the absolute pressure transducer circuit to the interior door actuator. These links can be any of a variety of devices for transmitting signal, such as a wire or wires, infrared transmitter and receiver, and the like, and can include appropriate input/output components, amplifiers, and other devices as would be understood by persons skilled in the art, once they understand the principles of this invention.  
           [0019]    The absolute pressure sensor preferably comprises a pirani sensor with a resistivity that varies as a function of the pressure in the load lock chamber, and the absolute pressure transducer circuit can include a pirani bridge circuit that incorporates the pirani sensor as a resistive element in the bridge circuit. An analog process circuit connected to the pirani bridge circuit adjusts voltage across the pirani sensor as pressure in the load lock chamber varies and thereby keeps the bridge circuit in balance. A relay control circuit monitors voltage across the pirani sensor and generates the interior door control signal when the voltage across the pirani sensor is at a value that corresponds with the preset absolute pressure value.  
           [0020]    The differential pressure preferably comprises a capacitance manometer pressure sensor in which a diaphragm is positioned with the load lock chamber pressure on one side of the diaphragm and ambient atmospheric pressure of the room on another side of the diaphragm so that the diaphragm flexes one way or the other, with the direction and magnitude of such flexing dependent on the direction and magnitude of the differential pressure across the diaphragm. Capacitance between the diaphragm and an adjacent plate varies as a function of differential pressure across a diaphragm. A sensor control circuit, converts the capacitance to a voltage that corresponds in value to the magnitude of the differential pressure across the diaphragm. A relay control circuit monitors the voltage from the sensor control circuit and generates the exterior door control signal when the voltage of the sensor control circuit corresponds with the preset differential pressure value.  
           [0021]    To further achieve the foregoing and other objects, the invention may also comprise, but is not limited to, a method of automatically controlling such a load lock, including predetermining both a desired differential pressure value at which to open the external door and a desired absolute pressure value at which to open the internal door. The method then includes sensing actual differential pressure between the load lock chamber and the ambient pressure in the room, comparing the actual differential pressure to the predetermined differential pressure value, and, when the actual differential pressure equals the predetermined differential pressure value, producing and delivering an exterior door control signal to the exterior door actuator. The method also includes sensing actual absolute pressure in the load lock chamber, comparing the actual absolute pressure to the predetermined absolute pressure value, and, when the actual absolute pressure equals the predetermined absolute pressure value, producing and delivering an interior door control signal to the interior door actuator.  
           [0022]    The method of this invention may also comprise, but is not limited to, transducing the sensed differential pressure to a voltage that is indicative of, or corresponds in value to, the sensed differential pressure, producing a differential pressure reference voltage that corresponds in value to the voltage that is transduced from the differential pressure when the differential pressure is at a desired differential pressure value for opening the exterior door, comparing the differential pressure reference voltage to such transduced voltage, and, when the transduced voltage equals the differential pressure reference voltage, producing and delivering the exterior door control signal to the exterior door actuator. This method may further include transducing the sensed absolute pressure to a voltage that is indicative of, or corresponds in value to, the absolute pressure, producing an absolute pressure reference voltage that corresponds in value to the voltage that is transduced from the absolute pressure when the absolute pressure is at a desired absolute pressure for opening the interior door, comparing the absolute pressure reference voltage to such transduced voltage, and, when the transduced voltage equals the absolute pressure reference voltage, producing and delivering the exterior door control signal to the interior door actuator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the descriptions serve to explain the principles of the invention.  
         [0024]    In the Drawings:  
         [0025]    [0025]FIG. 1 is a perspective view of the combination differential and absolute pressure transducer of this invention;  
         [0026]    [0026]FIG. 2 is a diagrammatic view of a process chamber equipped with a load lock and illustrating the use of the combination differential and absolute pressure transducer of this invention with the load lock;  
         [0027]    [0027]FIG. 3 is a function block diagram of the combination differential and absolute pressure transducer of this invention.;  
         [0028]    [0028]FIG. 4 is a vertical cross-sectional view of a convection pirani pressure sensor used to implement this invention;  
         [0029]    [0029]FIG. 5 is a diagrammatic cross-sectional view of a differential pressure sensor use to implement this invention; and  
         [0030]    [0030]FIG. 6 is an electric circuit diagram of an electric circuit used to implement this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    The combination differential and absolute pressure transducer  10  according to this invention is shown in FIG. 1. In general, it comprises an absolute pressure sensor  20  and a differential pressure sensor  30 , each of which is connected in gas flow relationship to a common manifold  40 . The manifold  40  has a connector  42 , such as a pipe fitting, for connecting the manifold  40  to a load lock chamber, which will be discussed below. A circuit board  12  with signal processing and control circuitry, which will be discussed in more detail below, is shown mounted to the absolute pressure sensor  20 . A housing  14  containing the absolute pressure sensor  20  and differential pressure sensor  30  is fastened by a mounting block  15  to the manifold  40 . A J1 connector  16  is provided in the housing  14  to accommodate connecting the circuit board  12  to an outside power source, to control actuators (not shown) for the load lock doors (discussed below), and the like. A J2 connector  18  is used to connect the differential pressure output signals to circuit board  12  components.  
         [0032]    Referring now to FIG. 2 in combination with FIG. 1, a load lock  60  is shown connected to a vacuum processing chamber  70  by a passage  61  with an interior door  62 . The processing chamber  70  has a vacuum pump  71  to maintain a vacuum, usually in the range of about 1 to 10 −8  torr. A platform  72  is usually provided to support one or more wafers  73  during processing, such as deposition of semiconductor thin films derived from feed gas sources  74 ,  75 ,  76 . The load lock  60  also has a vacuum pump  65  to pump down pressure in the load lock chamber  60 . A source of gas  63 , such as nitrogen, or sometimes air, is used to bring the pressure in the load lock chamber  60  back up to ambient, so the exterior door  64  can be opened to remove and/or insert a wafer  73  from and/or into the load lock chamber  60 .  
         [0033]    The combination differential and absolute pressure transducer  10  is shown connected in fluid-flow relation to the load lock chamber  60 , so that the manifold  40 , thus also the absolute pressure sensor  20  and the differential pressure sensor  30 , are effectively at the same pressure as the load lock chamber  60 . An outside electric power source  82  is shown connected to the circuit board  12  via the J1 connector  16 . A process control link  83  between the circuit board  12  and the exterior door  64 , via the J1 connector  16 , carries control signals from the electric circuit  80  (shown in FIG. 6) to a suitable actuator (not shown) or actuator circuit (not shown), such as a solenoid or motor actuator (not shown) to control opening and/or closing the exterior door  64 . Such actuators or actuator circuits for opening and/or closing exterior doors  64  on load lock chambers and how a control signal or signals can be used to operate such actuators or actuator circuits are well-known to persons skilled in the art and need not be described here for an understanding or enablement of this invention. A process control link  84  between the circuit board  12  and the interior door  62 , via the J1 connector  16 , carries control signals from the electric circuit (shown in FIG. 6) to a suitable actuator (not shown) or actuator circuit (not shown), such as a solenoid or motor actuator (not shown) to control opening and/or closing the interior door  62 . Again, such actuators or actuator circuits for opening and/or closing an interior door  62  of a load lock chamber and how a control signal or signals can be used to operate such actuators or actuator circuits are well-known to persons skilled in the art and need not be described here for understanding or enablement of this invention. Another optional process control link  68 , shown in broken lines, can be used to control the effective pumping speed of the vacuum pump  65  by controlling a throttle valve  66 . By partially closing the throttle valve  66  and thereby slowing down the effective pumping speed, turbulence is reduced in the pumping line  67  and in the load lock chamber  60 , thus reducing the particle contamination inside the load lock chamber  60  from particles and contaminants that could otherwise be stirred up in the pumping line  67  and load lock chamber  60 . When the load lock chamber  60  is evacuated and most of the air or gases are removed, turbulence is not so much of a problem. Therefore, when the vacuum in the load lock chamber  60  gets pumped down to a certain threshold pressure, the process control link can be used to re-open the throttle valve  66 , thereby stepping the effective pumping speed of the vacuum pump  65  up to full speed.  
         [0034]    The process control links  68 ,  83 , and  84  can be any type of components or devices that are known in the art for transmitting signals from one component to another. For example, they can include simple wire conductors, infra-red transmitters and receivers, any associated input/output components, amplifiers, and the like, as would be understood by persons skilled in the art.  
         [0035]    As illustrated in the function block diagram in FIG. 3, a power supply  91  for the absolute pressure transducer function  90  and a power supply  101  for the differential pressure transducer function  100  are connected to an external power source  82 . In the absolute pressure transducer function  90 , the absolute pressure sensor  20 , such as a standard pirani sensor, which can sense absolute pressure accurately from about 100 torr down to about 10 −4  torr, senses pressure in the load lock chamber  60 . A pirani bridge circuit  92  produces a voltage signal that is indicative of the absolute pressure sensed by the pirani sensor  20 . An analog process circuit  93  drives the bridge circuit  92  and amplifies and conditions the voltage signal from the bridge circuit  92 . The relay control circuit  94  utilizes the voltage signal from the process circuit  93  to generate control signals to the interior door  62  actuator to open the door  62  when the pressure in the load lock chamber  60  reaches a certain minimum pressure to match or at least get close to the pressure at which the process chamber  70  (FIG. 2) is operated. Alternatively, the control signal from relay control circuit  93  can be used to prevent the interior door  62  from being opened until the minimum threshold pressure in the load lock chamber  60  is reached. Also, as mentioned above, the relay control circuit  94  can also be configured to output a control signal to the throttle valve  66  controller (not shown) to speed up the vacuum pump  65  when the pressure in the load lock chamber gets pumped down to some selected intermediate pressure threshold so as to keep gas flow in the load lock chamber  60  to a minimum during pump down of the load lock chamber  60  when pressure is relatively high. Other absolute pressure sensors could also be used in this invention instead of the pirani sensor  20  described.  
         [0036]    The differential pressure transducer function  100  shows the differential pressure sensor  30 , which can be, for example, a capacitance manometer pressure sensor. The differential pressure sensor  30 , senses differential pressure between the ambient atmospheric pressure and the pressure in the load lock chamber  60 , as will be explained in more detail below. The capacitance manometer sensor  30  has a capacitance that varies as a function of the pressure difference between the atmospheric pressure and the load lock chamber  60  pressure, as will be explained in more detail below. The sensor control circuit  103  senses the capacitance of the capacitance manometer sensor  30  and converts the capacitance variance into a voltage signal that is indicative of the pressure differential. The relay control circuit  104  utilizes the voltage signal from the sensor control circuit  103  to output a control signal to the exterior door  64  actuator to either open the exterior door  64  when a certain pressure differential is reached or to prevent the exterior door  64  from opening until a certain pressure differential is reached. For example, the exterior door  64  could be opened when the differential pressure between the atmosphere and the load lock chamber  60  is zero, i.e., when the load lock chamber  60  pressure and the atmospheric pressure are equal. At such zero differential pressure, there would be very little, if any, flow of air or gas (other than diffusion) either into or out of the load lock chamber  60  when the exterior door  64  is opened. Of course, the relay control circuit  104  could be set to output a control signal at a differential pressure other than zero, if desired.  
         [0037]    An example pirani absolute pressure sensor  20  is shown in FIG. 4. The pirani sensor  20  comprises a filament  21  enclosed by a tubular container  22 , which is connected to the load lock chamber  60  via the manifold  40  (not shown in FIG. 4, but indicated by arrow  23 ). Therefore, the density of gas molecules in the tubular container  22  and surrounding the filament  21  is substantially the same as the density of gas molecules in the load lock chamber  60 , which increases as pressure rises and decreases as pressure lowers, An electric current I running through the filament  21  heats the filament  21 , and heat dissipation from the filament  21  is a function of the gas density in the tubular container  22  surrounding the filament  21 . Specifically, the current I required to maintain the filament  21  at a constant temperature is directly relative to the thermal conductivity and pressure of the gases present in the tubular container  22 , thus in the load lock chamber  60 . Therefore, as pressure decreases, the voltage VF across the filament  21  has to be decreased in order to maintain a constant filament  21  temperature. Conversely, as pressure in the load lock chamber  60 , thus in the tubular container  22 , increases, the voltage VF required to maintain the filament  21  at a constant temperature increases. The electric leads  24 ,  25  of the pirani sensor  20  are connected to the bridge circuit  92  (shown in FIGS. 3 and 6), which, along with a bridge driver circuit in the analog processing circuit  93  (FIGS. 3 and 6), adjust the voltage VF as required to maintain the filament  21  at a constant temperature as the pressure in the tubular container  22  thus load lock chamber  60 , varies up or down. The voltage V F , therefore, is indicative of the absolute pressure in the tubular container  22 , thus of the absolute pressure in the load lock chamber  60 , within a range of about 100 torr to 10 −4  torr, as mentioned above. Thus, this voltage V F  can be used by the relay control circuit  94  (FIGS. 3 and 6) to generate and output a signal via process control link  84  at a particular voltage V F , i.e., at a particular pressure in the load lock chamber  60 , to the interior door  62  actuator to open the door  62  or to allow interior door  62  to be opened.  
         [0038]    As mentioned above, this invention could be implemented with other kinds of absolute pressure sensors in place of the pirani sensor  20  describe above, such as a thermocouple sensor (not shown) or a convection pirani sensor (not shown). However, the regular pirani sensor  20  described above has advantages in this application. For example, the regular pirani sensor  20  described above is more accurate than a thermocouple sensor, measures over a wider pressure range, and responds more rapidly to pressure changes. A convection pirani sensor is similar to the regular pirani sensor  20  described above, but has a larger tubular container to accommodate gas convection currents around the filament. The gas convection currents increase the range of measurement in higher pressures, but has little effect at lower pressures. For example, where a regular pirani sensor  20  has an accurate pressure measurement range of about 100 torr down to 10 −4  torr, a convection pirani sensor has a range of about 1,000 torr down to 10 −3  torr. In the present invention, the lower measurement range of the regular pirani sensor  20 , i.e., down to about 10 −4  torr, is more important than the higher measurement range of a convection pirani sensor, because the present invention takes care of the higher pressure range control of the exterior door  64  in a different way, as explained below. Specifically, to avoid problems associated with use of an absolute pressure transducer for controlling the exterior door  64 , such as variations of atmospheric pressure at different altitudes and by changing weather patterns, which cause increased risk of contamination of the load lock chamber  60  with each opening of the exterior door  64 , thus eventual contamination of the process chamber  70  with subsequent opening of the interior door  62 , the pressure transducer  10  of the present invention has a differential pressure sensor  30  for controlling the exterior door  64  opening. Therefore, the higher absolute pressure measuring capability of a convection pirani sensor, i.e., up to 1,000 torr, is not needed.  
         [0039]    There are many types of differential pressure sensors, as is well-known in the art, many of which could be used with this invention, including, but not limited to, piezo pressure sensors. However, a particularly useful differential pressure sensor  30  for this application is a capacitance manometer pressure sensor, because it is relatively simple, durable, and very accurate. As mentioned above, atmospheric pressure at sea level is about 760 torr, which varies with weather patterns, and atmospheric pressures at higher elevation locations are significantly lower than 760 torr, such as about 630 torr in Boulder, Colo., which also vary with weather patterns. Therefore, any setting of a particular absolute pressure for opening the exterior door hardly ever matches actual ambient atmospheric pressure, thus almost invariably cannot avoid an air inrush or outrush into or out of the load lock chamber  60  upon opening of the exterior door  64 .  
         [0040]    In contrast, the differential pressure sensor  30  can generate a control signal to open the exterior door  64 , or to allow exterior door  64  to be opened, only when the pressure in the load lock chamber  60  equals the ambient atmospheric pressure, regardless of what such ambient atmospheric pressure may be at any particular time or at any particular location or elevation. Thus, the regular pirani pressure sensor  20  described above enables accurate and effective opening of interior door  62  at specific absolute pressures in the load lock chamber  60  down to 10 −4  torr to match, or at least get reasonably close to, the absolute pressure being maintained in the process chamber  70 , which is quite constant and known during processing, while the differential pressure sensor  30  enables accurate and effective opening of the exterior door  64  when the load lock chamber  60  pressure matches the ambient atmospheric pressure, regardless of variations of ambient atmospheric pressure from time to time and from one location to another.  
         [0041]    A diagrammatic representation of a capacitance manometer differential pressure sensor  30  in cross-section is shown in FIG. 5. Essentially, an enclosed manometer chamber  31  is connected in fluid flow relation to the load lock chamber  60  via the manifold  40  (FIGS. 1 and 3) as indicated by arrow  32 , so that the pressure in the manometer chamber  31  is substantially the same as the pressure in the load lock chamber  60 . One wall  33  of the manometer chamber  31  is thin enough to flex or deform as a diaphragm, as indicated by broken line  33 ′, when atmospheric pressure, indicated by arrow  34 , is greater than pressure in the manometer chamber  31 , which is indicated by arrow  35 . If the atmospheric pressure  34  is equal to the manometer chamber  31  pressure  35 , then there will be no flexure or deformation  33 ′ of the wall or diaphragm  33 . If the atmospheric pressure  34  is less than the manometer chamber  31  pressure  35 , the thin wall or diaphragm  33  will flex outwardly, as indicated by broken line  33 ″. The extent of flexure of deformation  33 ′ or  33 ″ is proportional to the magnitudes of pressure differential between atmospheric pressure  34  and manometer chamber  31  pressure  35 . Therefore, measurement of the amount of flexure  33 ′,  33 ″, is indicative of pressure differential between atmospheric pressure  34  and manometer chamber  31  pressure  35 .  
         [0042]    There are many ways to detect and measure the amount of flexure  33 ′,  33 ″, such as with strain gauges, optically, and others that are well-known to persons skilled in the art. In the case of the capacitance manometer sensor  30  illustrated in FIG. 5, the flexure  33 ′,  33 ″ of the wall or diaphragm  33  is measured by detecting capacitance between the wall or diaphragm  33  and an adjacent metal plate  36 . As is well known in the art, two metal plates, such as the metal wall or diaphragm  33  and the plate  36 , when separated by a dielectric or an empty space, have a capacitance C when a voltage is applied between them across the dielectric or empty space and that the capacitance C changes when the distance between the plates changes. Therefore, as the differential pressure across the diaphragm  33  causes the diaphragm  33  to flex, either as indicated by  33 ′ or  33 ″, the distance between the diaphragm  33  and the plate  36  changes, and such distance changes result in capacitance C changes. Therefore, the capacitance C between the diaphragm  33  and the plate  36  is indicative of, and corresponds to, the differential pressure across the diaphragm  33 . Persons skilled in the art also know how to measure capacitances C and changes in capacitance C with a sensor control circuit  103  (FIG. 3), since it is well-known that capacitance C is a function of voltage potential between the plate  36  and diaphragm  33  and that such voltage is easy to measure and control. Such capacitance manometers  30  are well-known and readily available to persons skilled in the art. Therefore, the differential pressure between the load lock chamber  60  pressure  35  and the ambient atmospheric pressure  34 , if any, can be measured by measuring the capacitance C between the wall or diaphragm  33  and the plate  36 . As mentioned above, the sensor control circuit  103  can be configured to produce a voltage that is indicative of, or that corresponds to, the capacitance C, thus is also indicative of, or corresponds to, the differential pressure, and such voltage is used by the relay control circuit  104  (FIG. 3) to generate and output a signal via process control link  83  when such voltage corresponds to a preset differential pressure, e.g., when the differential pressure is zero (no flexure  33 ′or  33 ″ in FIG. 5) to open the exterior door  64  or to allow exterior door  64  to be opened. For example, but not for limitation, a constant reference voltage can be produced and preset to correspond with the voltage output that would be produced by the sensor control circuit  103  when the differential pressure is at the desired value for opening the exterior door  64 . Then, a common voltage comparator circuit can be used to compare the actual voltage produced by the sensor control circuit  103  with the reference voltage to actuate a relay and generate the exterior door control signal on control link  83  when the actual voltage from the sensor control circuit  103  matches the reference voltage. Of course, persons skilled in the art will also understand that such comparison of pressure, capacitance C, or voltage values to trigger generation of the exterior door control signals can be accomplished in myriad ways with analog or digital signal processing, software, and the like. The relay control circuit  104  could also be set to generate and output such a control signal to exterior door  64  when the differential pressure is some desired discrete amount more or less then zero, for example, by setting the reference voltage described above to correspond with the voltage produced by the control circuit  103  at such discrete differential pressure.  
         [0043]    Thus, it does not matter in this invention what the specific load lock chamber  60  absolute pressure  35  is or what the particular ambient atmospheric pressure  34  is. When the differential pressure between them is some specific amount, such as zero or some other desired set value, the exterior door  64  will open or be allowed to open.  
         [0044]    A schematic diagram of the electric circuit on the circuit board  12  (FIGS. 1 and 2) is shown in FIG. 6 with the portions of the circuit that comprise pirani sensor  20 , pirani bridge circuit  92 , analog process circuit  93 , relay control circuit  94 , and switching power supply  91  outlined with broken lines. Persons skilled in the art will readily understand this electric circuit from the functions and features described, but several salient features can be mentioned. The basic Wheatstone bridge comprises essentially, the pirani filament  21  between voltage nodes V 0  and V 2 , the resistor R 11  between voltage nodes V 0  and V 1 , the resistor R 10  between voltage node V 1  and ground node G, and the parallel resistors R 12 , R 13 , R 14 , and R 15  between the voltage node V 2  and the ground node G. As pressure in the load lock chamber  60  (FIGS. 2 and 3), thus gas pressure around the filament  21 , decreases, conduction of heat by gas molecules from the filament  21  decreases. Decrease in heat dissipation from the filament  21  would, in the absence of an adjustment, cause temperature of the filament  21 , thus resistance of the filament  21 , to increase. An increase in resistance of the filament  21  would change current flow in the bridge circuit  92  and cause the bridge voltages V 1  and V 2  to become unbalanced, i.e., V 1  would not equal V 2 , which is detected by a voltage comparator  110  in the analog process circuit  93 . In response, the transistor controller  112  in the analog process circuit  93  lowers the voltage V 0  in the bridge circuit  92 , which lowers the voltage VF across the filament  21 , thus lowers current flow I through the filament  121 . The lower current I in filament  21 , lowers heat production in the filament  21 , because production of heat requires power, and power equals I 2 R. Less heat production means temperature of the filament  21  comes back down, thus resistance of the filament  21  comes back down, which readjusts current flow in the bridge circuit  92  back in balance, i.e., V 1 =V 2  again. Conversely, when load lock chamber  60  pressure, thus pressure around the filament  21 , increases, more gas molecules conduct more heat away from the filament  21 , which, in the absence of an adjustment, would lower temperature, thus resistance, of the filament  21 . Lower resistance in filament  21  would change current flow in the bridge circuit  92 , thus causing the bridge circuit  92  to become unbalanced, i.e., V 1  would not equal V 2 . Again, such imbalance is detected by the voltage comparator circuit  110 , which causes the transistor controller  112  to increase V 0 . The increased V 0  increases V F  across the filament to increase current I in the filament, which increases power (I 2 R) to raise the temperature, thus resistance, of filament  21 , to put the bridge circuit  92  back into balance, i.e., V 1 =V 2 . Consequently, with these adjustments of the voltage V 0 , the temperature of the filament  21  is kept constant. Further, such decreases and increases of the voltage V 0  required to maintain the filament  21  temperature constant, as explained above, are indicative of changes in load lock chamber  60  pressure. Thus, the voltage V 0  can be monitored electronically and used to actuate the relay control circuit  94  to generate and output a control signal on link  84  (FIGS. 2 and 3) to open the interior door  62 , or to allow the interior door  62  to be opened, at some selected minimum load lock chamber  60  pressure level that matches or is near the pressure maintained in the process chamber  70 . Optionally, as mentioned above, the voltage V 0  could also be used to actuate the relay control circuit  94  or another relay control circuit (not shown) to generate and output a control signal on link  68  to the throttle valve  66  (FIGS. 2 and 3) to increase the effective pumping speed of the vacuum pump  65  after the load lock chamber  60  pressure is drawn down to some desired intermediate load lock chamber  60  pressure threshold.  
         [0045]    A potentiometer  114  in the relay control circuit  94  is used to set the voltage level at which V 0  will actuate a transistor  120  to actuate the relay  130  to generate the control signal on link  84  (FIGS. 2 and 3) to open the inner door  62  or to allow the inner door  62  to be opened (optionally to increase speed of the vacuum pump  65 ). A voltage comparator  1   8  compares V 0  from the analog process circuit  93  to a voltage on lead  116  set by the potentiometer  114  to actuate the switch  120 , thus actuating the relay  130 . A failsafe circuit  122  also monitors the bridge voltage, such as V 2 , and, if it is not within a proper range or level, such as would happen if the filament  21  would break, a transistor switch  124  is actuated to prevent the relay  130  from being actuated.  
         [0046]    The switching power supply  91  provides power at  134  for the pirani sensor  20 , the bridge circuit  92 , the analog processing circuit  93 , and relay control circuit  134 . The J1 and J2 pins marked “TO MKS SWITCH” in FIG. 6 show the connections of the capacitance manometer  30  (FIGS. 1 and 4) to the circuit board  12  (FIG. 1). Since, as mentioned above, capacitance manometers that can be set to trip at pressures in relation to atmospheric pressure have been well-known and available commercially for many years (e.g., BARATRON™ Atmospheric Switches manufactured by MKS Instruments, Inc., Andover, Mass. 01810-2449), a detailed description of power supply  101 , sensor control circuit  103 , or relay control circuit  104  shown in the function block diagram of FIG. 3 is not necessary to the understanding or implementation of this invention.  
         [0047]    The foregoing description is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. Accordingly, resort may be made to all suitable modifications and equivalents that fall within the scope of the invention. The words “comprise,” “comprises,” “comprising,” “include,” “including,” and “includes” when used in this specification are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups thereof. The term “about”, when used in relation to pressure, means within a range of plus or minus 100 torr.