Abstract:
The present invention is directed at methods and apparatuses for facilitating the establishment of a reference pressure within a reference chamber of a pressure transducer. The transducer has a housing and a cover, the housing defining a reference chamber and an aperture. A meltable sealing material is disposed on at least one of the cover and the housing. The apparatus includes a pressure chamber that is rotatable between a first position and a second position, a pressure source that is connected to the pressure chamber, a guide that is attachable to the transducer near the aperture, and a heater for selectively heating the pressure chamber to a temperature sufficiently high to melt the sealing material. The cover is positioned in an internal space of the guide. The guide is attached to the transducer near the aperture. The transducer, cover and guide are placed in the pressure chamber, the pressure chamber is rotated to the first position and a pressure is generated in the pressure chamber via the pressure source. After a reference pressure has been established in the reference chamber, the pressure chamber is rotated to the second position. Gravity causes the cover to move within the space towards the aperture when the pressure chamber is rotated to the second position. The heater then heats the pressure chamber to melt the sealing material. Upon cooling, the sealing material forms a seal that seals the reference pressure in the reference chamber of the transducer.

Description:
CROSS-REFERENCE  
       [0001]     This application is a divisional application of U.S. patent application Ser. No. 10/960,153, filed Oct. 7, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to capacitive pressure transducers. More specifically, the present invention relates to an improved method and apparatus for forming a reference pressure within a chamber of a capacitive pressure transducer assembly.  
         [0003]      FIG. 1A  depicts a cross-sectional side view of an assembled prior art capacitive pressure transducer assembly  10 .  FIG. 1B  is an exploded view of the upper housing  40 , diaphragm  56  and lower housing  60  of  FIG. 1A . Briefly, capacitive pressure transducer assembly  10  includes a body that defines an interior cavity. A relatively thin, flexible ceramic diaphragm  56  divides the interior cavity into a first sealed interior chamber  52  and a second sealed interior chamber  54 . As will be discussed in greater detail below, diaphragm  56  is mounted so that it flexes, moves, or deforms, in response to pressure differentials in chambers  52  and  54 . Transducer assembly  10  provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure between chambers  52  and  54 . The parameter provided by transducer assembly  10  indicative of the differential pressure is the electrical capacitance between diaphragm  56  and one or more conductors disposed on an upper housing  40 .  
         [0004]     Capacitive pressure transducer assembly  10  includes a ceramic upper housing  40  and a ceramic lower housing  60 . The upper housing  40 , which generally has a circular shape when viewed from the top, defines an upper face  41 , a central lower face  47 , an annular shoulder  42  that has a lower face  42   a  and an annular channel  43  that is located between the central lower face  47  and the annular shoulder  42 . Lower face  42   a  of the annular shoulder  42  is substantially co-planar with central lower face  47 . The upper housing further defines an aperture (or passageway)  48  that extends through the housing  40  from the upper side to the lower side. A metallic conductor  46  is disposed on a center portion of the lower face  47 .  
         [0005]     The diaphragm  56  is generally a circular thin diaphragm that has an upper face  57  and an opposite, lower, face  59 . A metallic conductor  58  is disposed on a center portion of upper face  57  of the diaphragm  56 . The diaphragm  56  and the upper housing  40  are arranged so that the conductor  46  of the upper housing  40  is disposed opposite to the conductor  58  of the diaphragm  56 . Diaphragm  56  is coupled to the upper housing  40  by a high-temperature air-tight seal (or joint)  70 . The seal  70  is located between the lower face  42   a  of the annular shoulder  42  of the upper housing  40  and a corresponding annular portion of face  57  of diaphragm  56 . When sealed, the upper housing  40 , seal  70  and diaphragm  56  define reference chamber  52 . A reference pressure is established and maintained in the reference chamber  52 . Aperture  48  provides an inlet or entry way into reference chamber  52 .  
         [0006]     The lower housing  60 , which generally has a circular shape, defines a central opening  64  and an upwardly projecting annular shoulder  62  that has an upper face  62   a . The upper face  62   a  of shoulder  62  of the lower housing  60  is coupled to a corresponding portion of lower face  59  of diaphragm  56  by a high-temperature air-tight seal (or joint)  76 . Seal  76  can be deposited and fabricated in a manner similar to that of seal  70 . When sealed, the lower housing  60 , seal  76  and face  59  of the diaphragm  56  define process chamber  54 .  
         [0007]     A pressure tube  66  having an inlet passageway  68  is coupled to the lower housing  60  by a seal, for example, so that the inlet passageway  68  is aligned with the opening  64  of the lower housing  60 . Accordingly, the process chamber  54  is in fluid communication, via opening  64  and inlet passageway  68 , with an external environment. In operation, the capacitive pressure transducer assembly  10  measures the pressure of this external environment.  
         [0008]     Conductors  46  and  58  of the capacitive pressure transducer assembly  10  form parallel plates of a variable capacitor C. As is well known, C=Aε r ε 0 /d, where C is the capacitance between two parallel plates, A is the common area between the plates, ε 0  is the permittivity of a vacuum, ε r  is the relative permittivity of the material separating the plates (e.g., ε r =1 for vacuum), and d is the axial distance between the plates (i.e., the distance between the plates measured along an axis normal to the plates). So, the capacitance provided by capacitor C is a function of the axial distance between conductor  46  and conductor  58 . As the diaphragm  56  moves or flexes up and down, in response to changes in the pressure differential between chambers  52  and  54 , the capacitance provided by capacitor C also changes. At any instant in time, the capacitance provided by capacitor C is indicative of the instantaneous differential pressure between chambers  52  and  54 . Known electrical circuits (e.g., a “tank” circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor C) may be used to measure the capacitance provided by capacitor C and to provide an electrical signal representative of the differential pressure. Conductors  46 ,  58  can be comprised of a wide variety of conductive materials such as gold or copper, for example, and can be fabricated via known thin and thick film processes or other known fabrication methods. When thin film processes are utilized, conductors  46 ,  48  may have thicknesses of about 1 μm, for example.  
         [0009]     Diaphragm  56  is often made from aluminum oxide. Other ceramic materials, such as ceramic monocrystalline oxide materials, however, may also be used. Capacitance sensors having ceramic components are disclosed in U.S. Pat. Nos. 5,920,015 and 6,122,976.  
         [0010]     As noted above, changes in the differential pressure between chambers  52 ,  54  cause diaphragm  56  to flex thereby changing the gap between conductor  46  and conductor  58 . Measurement of changes in the gap permits measurement of the differential pressure. The gap, however, can also be affected by factors unrelated to pressure. For example, the gap can be affected by changes in temperature. Since the components of transducer assembly  10  can be made from a variety of different materials, each of which has its own characteristic coefficient of thermal expansion, temperature changes in the ambient environment can cause the diaphragm  56  to move closer to, or further away from, conductor  46 . Fortunately, changes in the gap caused by temperature changes are characteristically different than changes in the gap caused by changes in differential pressure. To compensate for changes in the gap that are caused due to changes in the ambient temperature, it is known to include a second conductor (not shown) that is disposed adjacent to conductor  46  on the lower face  47  of the upper housing  40 . In such an embodiment, conductors  46  and  58  form parallel plates of a variable capacitor C 1  and conductor  58  and the second conductor form parallel plates of a variable capacitor C 2 . The two capacitors, C 1  and C 2 , may be used by known methods to reduce the transducer&#39;s sensitivity to temperature changes.  
         [0011]     The upper housing  40  is positioned so that the lower face  47 , and any conductors disposed thereon, are disposed in a plane that is parallel to the plane defined by the conductor  58  (i.e., diaphragm  56 ) when the pressures in chambers  52 ,  54  are equal. As discussed above, the capacitance defined by the conductors  46 ,  58  depends upon the gap (i.e., axial distance) that exists between these opposing conductors. The gap, which is relatively small (e.g., on the order of 0.0004 inches (10-12 μm)), depends, in part, upon the thickness of the seal  70  and the shape and configuration of the upper housing  40  (e.g., the amount that lower face  42   a  is out of plane, i.e. offset, with lower face  47 , if any).  
         [0012]     In operation, capacitive pressure transducer assembly  10  is normally used as an absolute pressure transducer. In this form, reference chamber  52  is evacuated to essentially zero pressure, e.g., less than 10 −8  Torr, and the reference chamber  52  is then sealed. The reference pressure then serves as a baseline from which a pressure within the process chamber  54  is determined. To maintain the essentially zero pressure within the reference chamber  52 , the transducer assembly  10  includes a tube  80 , a cover  82 , a hold-wire  86 , a screen  88  and a getter element  84 . As is shown in  FIGS. 1A and 1B , the screen  88  supports the getter element  84  within a hollow portion of the tube  80  while the hold-wire  86  maintains the getter element  84  against the screen  88 . The hollow portion of the tube  80  is disposed over the aperture  48  of the upper housing  40  so that the getter element  84  is in fluid communication with the reference chamber  52 . In addition to supporting the getter element  84 , screen  88  also prevents particles from passing into the reference  52  that could adversely affect the operation of the diaphragm  56 .  
         [0013]     The bottom end of the tube  80  is coupled to the upper face  41  of the upper housing  40  around the aperture  48  by a high-temperature air-tight seal  92 , while the cover  82  is coupled to the upper end of the tube  80  by a low-temperature air-tight seal  94 . Seals  92 ,  94  and seal  70 , which is located between the shoulder  42  of the upper housing  40  and the diaphragm  56 , all assist in maintaining the reference pressure that is established in the reference chamber  52 . The high-temperature seal  92  is comprised of a high-temperature glass material while the low-temperature seal  94  is comprised of a low-temperature glass material. To form the high-temperature seal  92 , the high-temperature glass material is deposited on the lower end of the tube  80 , a corresponding sealing area of face  41 , or both. The high-temperature glass material is melted, a force perpendicular to the upper face  41  of the upper housing  40  is applied between the tube  80  and the upper housing  40  and the high-temperature glass material is then allowed to cool (i.e., solidify) thus forming the high-temperature air-tight seal  92 . The low-temperature seal  94  is similarly formed between the upper end of the tube  80  and a corresponding sealing area of the cover  82 . The high-temperature glass material of the high-temperature seal  92  has a melting temperature that is higher than that of the low-temperature glass material of the low-temperature seal  94 . To provide different melting temperatures, the glass materials of the seals  92 ,  94  can be comprised of different materials or have different amounts of a common material. The melting temperature of the high-temperature seal  92  is higher than the melting temperature(s) of the high-temperature seals  70  and  76  and the melting temperature(s) of the high-temperature seals  70  and  76  is higher than the melting temperature of the low-temperature seal  94 .  
         [0014]     The getter element  84  is comprised of a material that, when activated, acts to effectively absorb any gaseous impurities that may be present within the sealed reference chamber  52 . Thus, when activated, the getter element  84  assists in maintaining the reference pressure at an ultra high vacuum level for long periods of time, e.g., ten or more years.  
         [0015]     Although an ultra high vacuum pressure, i.e., essentially zero pressure, is a convenient and useful reference pressure, other reference pressures can also be used. After the reference pressure has been established in chamber  52 , the pressure tube  66  is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling the pressure tube  66  in this fashion delivers the fluid, the pressure of which is to be measured, to process chamber  54  (and to the lower face  59  of the diaphragm  56 ). The center of diaphragm  56  moves or flexes up or down in response to the differential pressure between chamber  52  and  54  thereby changing the capacitance of capacitor C. Since the instantaneous capacitance of capacitor C is indicative of the position of the diaphragm  56 , transducer assembly  10  permits measurement of the pressure in chamber  54  relative to the reference pressure that is established in chamber  52 .  
         [0016]     The accuracy of the capacitive pressure transducer assembly  10  can depend upon the accuracy at which the reference pressure can be established and maintained in the reference chamber  52 . In other words, as the actual pressure within the reference chamber  52  deviates from an intended and designed reference pressure, the performance of the capacitive pressure transducer assembly  10  will correspondingly suffer.  
         [0017]     The steps of establishing a reference pressure in the reference chamber  52 , activating the getter element  84  and sealing the cover  82  to the tube  80  are typically the last few steps that are performed when fabricating capacitive pressure transducer assembly  10 . Thus, the steps of coupling the upper housing  40  to the diaphragm  56  via the high-temperature seal  70 , coupling the lower housing  60  to the diaphragm  56  via the high-temperature seal  76 , coupling the pressure tube  66  to the lower housing  60  around the opening  64 , and coupling the tube  80  (having the screen  88 , getter element  84  and hold-wire  86 ) to the face  41  of the upper housing  40  around the aperture  48  via the high-temperature seal  92  will usually have already been completed before the reference pressure is established.  
         [0018]     To establish a reference pressure within the reference chamber  52 , the reference chamber  52  is typically subjected to a burn-out and evacuation process and then the cover  82  is sealed to the tube  80 . The reference chamber  52  is “burned-out” by heating the inner surfaces that define the reference chamber  52  (including the surfaces of the cover  82 , tube  80 , housing  40  that are in fluid communication with the reference chamber  52 ), and the chamber  52  is “evacuated” by drawing an ultra-high vacuum on the reference chamber  52 . The burn-out heat vaporizes the contaminants, e.g., volatiles, moisture, that may be present on these inner surfaces while the evacuation vacuum draws the vaporized contaminants and gases out of the reference chamber  52 . Since the cover  82  has not yet been sealed to the tube  80 , the contaminants and gases are sucked out of the reference chamber  52 , the aperture  48  and the hollow portion of the tube  80 . Once the burn-out and evacuation process is completed and while the vacuum pressure is continuing to be maintained, the cover  82  is then sealed to the tube  80  via the low-temperature seal  94  to establish the reference pressure in the reference chamber  52 .  
         [0019]      FIGS. 2A and 2B  illustrate a prior art method and apparatus that is used to establish a reference pressure within the reference chamber  52  of a capacitive pressure transducer assembly  10 .  FIG. 2A  generally depicts the burn-out and evacuation process while  FIG. 2B  generally depicts the process by which the cover  82  is sealed onto the upper end of the tube  80 . The apparatus includes a vacuum housing  93  that defines an interior vacuum chamber  95 . Referring to  FIG. 2A , a low-temperature sealing material  94   a  is deposited on the upper end of the tube  80 . The semi-completed transducer assembly  10 , i.e., one that does not yet have the cover  82  sealed to the tube  80 , is then disposed in the vacuum chamber  95 . [For clarity, the pressure tube  66  has been omitted from  FIGS. 2A and 2B  and some of the other subsequent figures. The pressure tube  66 , however, would typically be coupled to the lower housing  60  prior to the assembly being placed in the vacuum chamber  95 .] After the transducer assembly  10  has been placed in the vacuum chamber  95 , the vacuum housing  93  is placed in an oven (not shown), a vacuum source (not shown) is coupled to the vacuum chamber  95  and the burn-out and evacuation process of the reference  52  is initiated. During the burn-out and evacuation process, which can last for more than 20 hours, the transducer assembly  10  is heated to a temperature of about 250° C. and an ultra-high vacuum pressure of the order of 10 −8  Torr (or less) is generated in the vacuum chamber  95 . In  FIG. 2A , the burn-out and evacuation of reference chamber  52  (and aperture  48  and tube  80 ) is indicated by the arrows which extend from the reference chamber  52 , up through the aperture  48  and up through and out of the top end of the tube  80 .  
         [0020]     After the burn-out and evacuation of the reference chamber  52  is completed, the cover  82  is then coupled to the tube  80  by the low-temperature seal  94 . Cover  82  is attached and sealed to the tube  80  without opening vacuum housing  93  so as to preserve the vacuum in reference chamber  52 . Accordingly, as can been seen in  FIG. 2A , prior to initiating the burn-out and evacuation process, the cover  82  is attached to an end of a rod  96  which penetrates into the vacuum chamber  95  of the vacuum housing  93 . When the burn-out and evacuation process is completed, the rod  96  can be actuated to bring the cover  82  in contact with the low-temperature sealing material  94   a  that is disposed on the upper end of the tube  80 .  
         [0021]     The low-temperature sealing material  94   a  that forms the low-temperature seal  94  is not melted during the burn-out and evacuation process, i.e., the burn-out temperature is generally set below the melting temperature of the low-temperature sealing material  94   a . Moreover, the burn-out and evacuation process should not compromise the seals that have already been formed in the transducer assembly  10  (e.g., high-temperature seals  70 ,  76  and  92 ) and, thus, the burn-out temperature should not exceed the melting temperatures of these seals.  
         [0022]     A high-temperature dynamic seal  99  (e.g., a gasket) is disposed in the vacuum housing  93  where the rod  96  penetrates the vacuum housing  93 . The high-temperature dynamic seal  99  allows to the rod to travel freely up and down while assisting to maintain the pressure that is present in the vacuum chamber  95  of the vacuum housing  93 .  
         [0023]     Prior to initiating the burn-out and evacuation process, cover  82  is attached to the end of the rod  96  by a low-temperature seal  98 . The melting temperature (i.e., melting point) of the low-temperature seal  98 , which is lower than the melting temperature of the low-temperature sealing material  94   a , is higher than the burn-out temperature and, therefore, does not melt during the burn-out and evacuation process. The rod  96  extends through the high-temperature dynamic seal  99  and, together with the cover  82 , is aligned with the tube  80  of the transducer assembly  10 .  
         [0024]     Referring now to  FIG. 2B , after the burn-out and evacuation process is completed, while the pressure in the vacuum chamber  95  is still being maintained, the rod  96 /cover  82  is lowered until the cover  82  comes into contact with the low-temperature sealing material  94   a . The temperature within the vacuum chamber  95  (as directed by the oven) is then elevated to cause the low-temperature sealing material  94   a  to melt. This increase in temperature also causes the low-temperature seal  98  to melt and causes the getter element  84  to become activated. To form the low-temperature air-tight seal  94  between the cover  82  and the tube  80 , the temperature within the vacuum chamber  95  is decreased until the low-temperature sealing material  94   a  solidifies and, while the low-temperature seal  98  is sufficiently melted, the rod  96  is pulled away from the transducer assembly  10 . Once the low-temperature seal  94  is formed—and the reference pressure in the reference chamber  52  is thus established—the temperature in the vacuum chamber  95  is reduced to ambient temperature, then vacuum source is disconnected and the assembled transducer assembly  10  is removed from the vacuum housing  93 .  
         [0025]      FIG. 3  illustrates the prior art burn-out, evacuation and sealing process of the apparatus and method of  FIGS. 2A and 2B  in more detail. In  FIG. 3 , the x-axis of the process flow represents Time and the y-axis represents Temperature in degrees Celsius. Prior to initiating the burn-out and evacuation process, at Step A of the process flow, the cover  82  is attached to rod  96  via low-temperature seal  98  and the transducer assembly  10 , cover  82  and rod  96  are placed in the vacuum chamber  95  ( FIG. 2A ). During Step A→B, the temperature in the vacuum chamber  95  is raised to a burn-out temperature of 250° C. and the pressure is lowered to an evacuation pressure of 10 −8  Torr. Step A→B is completed in three hours. After the burn-out temperature and evacuation pressure are achieved (Step B), the reference chamber  52  is burned-out and evacuated for 20 hours, Step B→C. Shortly before Step C is reached, the rod  96  and cover  82  are lowered so that the cover  82  comes into contact with the low-temperature sealing material  94   a  that is deposited on the upper end of the tube  80 . Once the burn-out and evacuation step is completed (Step C), the temperature in the vacuum chamber  95  is raised to 475° C., Step C→D, which causes the low-temperature sealing material  94   a  and the low-temperature seal  98  to melt. Step C→D lasts for three hours. The vacuum chamber  95  is then maintained at 475° C. for 30 minutes, Step D→E, to ensure that the low-temperature sealing material  94   a  and the low-temperature seal  98  are sufficiently melted. The temperature in the vacuum chamber  95  is then lowered to 400° C. over the course of two hours, Step E→F, which causes the low-temperature sealing material  94   a  to solidify and form the low-temperature air-tight seal  94 . The melting temperature of the low-temperature seal  98  is below 400° C. and, thus, the low-temperature seal  98  remains melted throughout Step ELF. Shortly before Step F is reached, rod  96  is raised away from the cover  82  ( FIG. 2B ). Lastly, the temperature and pressure in the vacuum chamber  95  are brought to ambient conditions over the course of 4 hours and the assembled pressure transducer assembly  10  is then removed from the vacuum chamber  95  of the vacuum housing  93 , Step F→G. As illustrated in  FIG. 3 , the prior art burn-out, evacuation and sealing process can be completed in 32 hours.  
         [0026]     The method and apparatus described above does not necessarily ensure that an accurate reference pressure has been established within the reference chamber  52  of a capacitive pressure transducer assembly  10 . For example, it is very difficult to establish and maintain an ultra high vacuum of the order of 10 −8  Torr (or less) in a vacuum housing  93  that utilizes a rod  96  and a high-temperature dynamic seal  99  because the pressure integrity of the vacuum housing  93  tends to be compromised by the presence of the high-temperature dynamic seal  99 . It also can be difficult or costly to accurately control the positions and orientations of the cover  82  and the tube  80  during the rod actuating mating process. If the cover  82  is not positioned or oriented properly in relationship to the tube  80  during the mating process, the integrity of the low-temperature seal  94  may be compromised or the low-temperature seal  94  may fail entirely.  
         [0027]     A need therefore exists for a method and apparatus for accurately establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly.  
       SUMMARY OF THE INVENTION  
       [0028]     The present invention is directed to methods and apparatuses for establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly.  
         [0029]     The pressure transducer includes a cover and a housing that defines a reference chamber and an aperture. A meltable sealing material is disposed on at least one of the cover and the housing. The apparatus includes a pressure chamber that is rotatable between a first position and a second position and a guide that is attachable to the transducer near the aperture. The guide defines an internal space. A cable can be used to rotate the pressure chamber between the first and second positions. An actuator motor and an actuator rod can alternatively be used to rotate the pressure chamber. A pressure source connected to the pressure chamber can establish a desired pressure within the pressure chamber while a heater (e.g., oven) can selectively heat the pressure chamber to a temperature sufficiently high to melt the sealing material.  
         [0030]     The cover is positioned in the space of the guide and the guide is attached to the transducer near the aperture. The transducer, cover and guide are placed in the pressure chamber, the pressure chamber is rotated to the first position and a pressure is generated in the pressure chamber via the pressure source and the chamber is heated to bake out unwanted materials. After a reference pressure has been established in the reference chamber, the pressure chamber is rotated to the second position wherein gravity thereby causes the cover to move towards the aperture within the space. The heater then heats the pressure chamber to melt the sealing material. Upon cooling, the sealing material forms a seal that seals the reference pressure in the reference chamber of the transducer.  
         [0031]     The apparatus may also include a weight, such as a ball, that is disposed within the space of the guide.  
         [0032]     By utilizing an apparatus that has a guide and a rotatable pressure chamber, the methods and apparatuses of the present invention are capable of accurately locating and orienting the cover during the reference chamber sealing process. The methods and apparatuses of the present invention, moreover, do not require the use of a high-temperature dynamic seal to maintain the pressure in the pressure chamber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     Various objects, features, and advantages of the present invention can be more fully appreciated with reference to the following detailed description of the invention when considered in connection with the following drawing, in which like reference numerals identify like elements. The following drawings are for the purpose of illustration only and are not intended to be limiting of the invention, the scope of which is set forth in the claims that follow.  
         [0034]      FIG. 1A  shows a cross-sectional view of a prior art capacitance sensor.  
         [0035]      FIG. 1B  shows partial, expanded cross-sectional view of the prior art capacitance sensor of  FIG. 1A .  
         [0036]      FIGS. 2A and 2B  illustrate a prior art method and apparatus used to establish a reference pressure within a reference chamber of a capacitive pressure transducer assembly.  
         [0037]      FIG. 3  depicts a process flow for establishing a reference pressure within a reference chamber of a transducer assembly in accordance with the prior art method and apparatus of  FIGS. 2A and 2B .  
         [0038]      FIG. 4A  shows a side view of an apparatus constructed in accordance with the invention for establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly.  
         [0039]      FIG. 4B  shows a front view of the apparatus of  FIG. 4A .  
         [0040]      FIG. 5  illustrates a partial, cross-sectional, side-view of the apparatus of  FIGS. 4A and 4B  that shows the internal components of the apparatus and how the pressure transducer assembly is disposed therein.  
         [0041]      FIG. 6A  shows a frame assembly constructed in accordance with the invention.  
         [0042]      FIG. 6B  shows a cross-section view of the frame assembly of  FIG. 6A .  
         [0043]      FIG. 7A  illustrates one step in an exemplary method of establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly in accordance with the invention.  
         [0044]      FIG. 7B  is a close-up view that further illustrates how the step of  FIG. 7A  is to be performed.  
         [0045]      FIG. 8A  illustrates another step in an exemplary method of establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly in accordance with the invention.  
         [0046]      FIG. 8B  is a close-up view that further illustrates how the step of  FIG. 8A  is to be performed.  
         [0047]      FIG. 9  illustrates a process flow for establishing a reference pressure within a reference chamber of a transducer assembly in accordance with the method and apparatus of the present disclosure.  
     
    
     DETAILED DESCRIPTION  
       [0048]     The present invention is directed to methods and apparatuses for accurately establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly. The present invention is capable of establishing an ultra-high vacuum in a vacuum chamber for facilitating the burn-out and evacuation process of a transducer assembly and is capable of controlling the delivery and mating of an aperture cover during the reference chamber sealing process. Moreover, the present invention does not utilize a high-temperature dynamic seal to maintain the ultra-high vacuum in the vacuum chamber.  
         [0049]      FIG. 4A  depicts a side view of an exemplary apparatus  100  constructed in accordance with the invention.  FIG. 4B  depicts a front view of the apparatus  100 . Apparatus  100  is comprised of a vacuum housing  110  and a support assembly  120 . The vacuum housing  110  defines an internal vacuum chamber, which is discussed in more detail below. A capacitive pressure transducer assembly  10  that is to be burned-out, evacuated and sealed is secured within the internal vacuum chamber of the vacuum housing  110 . The support assembly  120  supports the vacuum housing  110  when the capacitive pressure transducer assembly  10  is being burned-out, evacuated and sealed and, more specifically, allows the vacuum housing  110  and the capacitive pressure transducer assembly  10  that is disposed therein to be rotated while these processing steps are being performed.  
         [0050]     The vacuum housing  110  includes a metal lower flange  112  and a metal upper housing  114 . The vacuum housing  110  also includes left and right pins  136  that are coupled to the upper housing  114  and a vacuum port (not shown) that can be connected to one end of a vacuum line  138 . The other end of the vacuum line  138  is connected to a vacuum pump (not shown) that is capable of drawing an ultra-high vacuum. The pins  136  define a rotational axis  240  through which the vacuum housing  110  can rotate when supported by the support assembly  120 . The vacuum port is located near the left pin  136 , i.e., near the rotational axis  240 , so that the vacuum line  138  is subjected to a minimum amount of displacement and flexure when the vacuum housing  110  is rotated. The vacuum housing  110  also includes a cable (or wire)  116  having an end that is coupled to the backside of the lower flange  112 . When the vacuum housing  110  is secured in the support assembly  120 , i.e., via the pins  136 , the cable  116  can be operated to rotate the vacuum housing  110  forward to a downwardly-slanted position ( FIGS. 7A and 7B ) and backwards to an upright position ( FIGS. 8A and 8B ).  
         [0051]     The support assembly  120  includes a base  126 , left and right support brackets  132 , two lower supports  122  and two upper supports  124 . The support brackets  132 , lower supports  122  and upper supports  124  are all mounted on a face of the base  126 . The base  126  includes a front edge, a back edge and opposite side edges. As can be seen in  FIGS. 4A and 4B , the support brackets  132  are located near the opposite side edges of the base  126 , the upper supports  124  are located inboard of the support brackets  132  near the back edge of the base  126  while the lower supports  122  are located inboard of the upper supports  124  near the front edge of the base  126 . Each support bracket  132  has a slot (or hole)  134  that can accommodate a pin  136 . Each lower support  122  has a distal end  122   a  and each upper support  124  has a distal end  124   a . The vacuum housing  110  is secured in the support assembly  120  by mounting the pins  136  of the upper housing  114  into the slots  134  of the support brackets  132 . The slots  134  can be slotted and indexed to accommodate the pins  136  and to facilitate the rotation and loading and unloading of the vacuum housing  110 .  
         [0052]     After the capacitive pressure transducer assembly  10  has been placed in the vacuum housing  110  and the vacuum housing  110  has been secured to the support assembly  120 , the apparatus  100  is placed in an oven (not shown) and the vacuum line  138  is coupled to the vacuum port. To operate the cable  116  at a location that is external to the oven, the opposite end of the cable  116  is routed between the support brackets  132  and out through an access port that is provided in the oven.  
         [0053]     As is discussed in more detail below, when the vacuum housing  110  is in an upright position (as shown in  FIGS. 4A and 4B ), the lower flange  112  of the vacuum housing  100  rests upon the distal ends  124   a  of the upper supports  124 . However, when the vacuum housing  110  is rotated forward (as shown in  FIGS. 7A and 7B ), the upper housing  114  then comes to rest on the distal ends  122   a  of the lower supports  122 .  
         [0054]      FIG. 5 , which depicts a cross-sectional, side-view of the vacuum housing  110 , shows some additional components of the apparatus  100  and illustrates how the capacitive pressure transducer assembly  10  is secured in the vacuum housing  110 . As can be seen in  FIG. 5 , the vacuum housing  110  also includes a copper sensor support  210  that secures the transducer assembly  10  that is to be burned-out, evacuated and sealed. The transducer assembly  10  can be secured to the sensor support  210  by tightening screws (not shown) or by a wide variety of other types of fastening means that are suitable for temporarily securing the transducer assembly  10  to the sensor support  210 . The transducer assembly  10  that is to be secured to the sensor support  210  generally has a low-temperature sealing material  94   a  deposited on the upper end of the tube  80  and on the corresponding sealing surface of the cover  82 .  
         [0055]     The apparatus  100  further includes a cylindrical guide assembly  300 , a ball  320  and copper wool  330 . The ball  320  is disposed within a hollow portion of the guide assembly  300 . As is discussed in more detail below, the guide assembly  300  is temporarily coupled to the tube  80  and, together with the ball  320 , guides the cover  82  towards the upper end of the tube  80  during the sealing process. The ball  320  is comprised of a high-temperature, high-density material such as Tungsten Carbide or Silicon Nitride, for example. The copper wool  330 , which is disposed between the guide assembly  300  and the upper housing  114 , provides a thermal conductive pathway between the upper housing  114 , the guide assembly  300  and the transducer assembly  10 .  
         [0056]     After the transducer assembly  10  has been secured in the sensor support  210 , the sensor support  210  is coupled to the lower flange  112 , the ball  320 , guide assembly  300  and copper wool  330  are installed and the lower flange  112  is then coupled to the upper housing  114 . When assembled, the lower flange  112  and upper housing  114  define an interior vacuum chamber  200 .  
         [0057]     To ensure that the vacuum chamber  200  is air-tight, a temporary air-tight copper seal is provided between the lower flange  112  and the upper housing  110 . The vacuum port (not shown) provides fluid communication between the vacuum line  138  and the vacuum chamber  200 . During the burn-out, evacuation and sealing steps, the external vacuum pump evacuates the vacuum chamber  200  to an ultra-high vacuum pressure via the vacuum line  138  and vacuum port.  
         [0058]      FIG. 6A  shows the cylindrical guide assembly  300  in more detail, while  FIG. 6B  shows a cross-section view of the guide assembly  300  and how the ball  320  is disposed within the hollow portion of the guide assembly  300 . The cylindrical guide assembly  300  defines a hollow cylindrical interior space  316  having a closed distal end  312  and an open proximal end  314 . The ball  320  is disposed within the space  316  of the guide assembly  300  and, depending upon the orientation of the guide assembly  300 , can move freely towards or away from the distal end  312  and the proximal end  314  of the guide assembly  300 . To prevent excessive side-to-side motions (i.e., motions that are perpendicular to a line that is drawn between the distal end  312  and the proximal end  314 ) of the ball  320  within the space  316 , the diameter of the ball  320  is closely matched to the diameter dimension of the space  316 , i.e., the diameter of the ball  320  is slightly less than the diameter of the space  316 . In one exemplary embodiment, for example, the diameter of the ball  320  is 0.5000±0.0001 inches and the diameter of the space  316  is 0.505±0.002 inches. The diameters of the ball  320  and space  361  are appropriately sized to account for any thermal expansion effects that may occur during the burn-out and evacuation process.  
         [0059]     The interior space  316  of the guide assembly  300  is also sized and configured to accommodate the cover  82  and tube  80  that are temporarily disposed within the space  316 . The tube  80  and cover  82  generally have the same radial dimension. The radial dimension of the space  316  is, therefore, established to be slightly larger than the radial dimensions of the cover  82  and tube  80 .  
         [0060]     The cylindrical guide assembly  300  further includes a set of holes  310  that are arranged radially throughout the guide assembly  300  and a set of tightening screws  318  that are disposed towards the proximal end of the guide assembly  300 . The tightening screws  318  are used to temporarily secure the guide assembly  300  (with the ball  320  disposed therein) to the tube  80  during the burn-out, evacuation and sealing steps. The holes  310  provide a fluid pathway between the interior space  316  of the guide assembly  300  and the vacuum chamber  200 . Thus, during the burn-out and evacuation process, i.e., when the cover  82  has not yet been sealed on the tube  80 , fluid pathways exist between the reference chamber  52  and the vacuum chamber  200  via the aperture  48 , hollow portion of the tube  80  and the holes  310 .  
         [0061]      FIG. 7A  is a side view that illustrates how the vacuum housing  110 , guide assembly  300  and ball  320  of the apparatus  100  are oriented during the burn-out and evacuation process.  FIG. 7B  shows a close-up, side view that more accurately depicts the orientation and arrangement of the tube  80 , cover  82 , guide assembly  300  and ball  320  of  FIG. 7A . As previously discussed, prior to securing the transducer assembly  10  into the sensor support  210 , low-temperature sealing material  94   a  is deposited onto the upper end of the tube  80  and the corresponding sealing area of the cover  82 . After the transducer assembly  10  is secured in the sensor support  210  and the vacuum chamber  200  has been sealed and secured in the support assembly  120 , the vacuum housing  110  is then rotated in a counterclockwise direction (as shown in  FIG. 7A ), i.e., forward, until the vacuum housing  110  comes to rest on the distal ends  122   a  of the lower supports  122 . The distal ends  122   a  are located such that, upon rotation, the ball  320  and cover  82  which are located within the interior space  316  of the guide assembly  300  travel away from the tube  80  towards the distal end  312  of the guide assembly  300 . Thus, by sufficiently rotating the vacuum housing  110 , one can ensure that a gap (i.e., a fluid pathway) between the cover  82  and the tube  80  is present during burn-out and evacuation process. Once the transducer assembly  10  has been brought up to the desired burn-out temperature and an ultra-high vacuum pressure has been established and is being drawn in the vacuum chamber  200 , the burn-out and evacuation processing of the transducer assembly  10  is then initiated. As is indicated by the arrows in  FIG. 7B , the reference chamber  52  of the transducer assembly  10  is evacuated by drawing the contaminants and gases out of the assembly  10  and into the vacuum chamber  200  via the aperture  48  (not shown), the tube  80  and the holes  310  of the guide assembly  300 . The contaminants and gases are then further drawn out of the vacuum chamber  300  by the external vacuum pump via the vacuum port and vacuum line  138 .  
         [0062]     The cable  116  can be manipulated to cause the vacuum housing  110  to rotate counterclockwise. The vacuum housing  110 , for example, can be weighted so that a slackening of the cable  116  causes the vacuum housing  110  to rotate counterclockwise, i.e., forward.  
         [0063]     Once the burn-out and evacuation process has been completed, the reference pressure in the reference chamber  52  is then locked in by sealing the cover  82  to the tube  80 .  FIG. 8A  is a side view that illustrates how the vacuum housing  110 , guide assembly  300  and ball  320  of the apparatus  100  are oriented during the cover sealing process.  FIG. 8B  shows a close-up, side view that more accurately depicts the orientation and arrangement of the tube  80 , cover  82 , guide assembly  300  and ball  320  of  FIG. 8A . To seal the cover  82  onto the tube  80 , the vacuum housing  110  of the apparatus  100  is rotated in a clockwise direction (as shown in  FIG. 8A ), i.e., backwards, to an upright position by pulling the cable  116  that is attached to the backside of the lower flange  112 . Cable guides (not shown), such as pulley wheels or other types of devices or guides, can be utilized to facilitate the operation of the cable  116 . When the vacuum housing  110  is pulled into its upright position, the lower flange  112  of the vacuum housing  110  will come to rest on the distal ends  124   a  of the upper supports  124  of the support assembly  120 . The upright position need not be exactly vertical. Instead, it may be advantageous to position the distal ends  124   a  of the upper supports  124  so that, upon rotation, the vacuum housing  110  leans slightly backwards. That way, if the tension in the cable  116  slackens, the vacuum housing  110  is less likely to inadvertently rotate forward towards the distal ends  122   a  of the lower supports  122 .  
         [0064]     When vacuum housing  110  is rotated to its upright position, gravity causes the ball  320  to move towards the proximal end  314  of the guide assembly  300  which thereby causes the cover  82  to engage the tube  82  and, more specifically, causes the low-temperature sealing material  94   a  that is disposed on the bottom-side of the cover  82  to come into contact with the low-temperature sealing material  94   a  that is disposed on the upper end of the tube  80 . As situated, the weight of the ball  320  and the weight of the cover  82  thus provide a contact force between the cover  82  and the tube  80  in the area of the low-temperature sealing material  94   a  interface. To seal the cover  82  onto the tube  80 , i.e., to form the low-temperature seal  94 , while the ultra-high vacuum is still being maintained in the vacuum chamber  200  the temperature in the oven is elevated to cause the two layers of low-temperature sealing material  94   a  to melt and fuse together. This increase in temperature also serves to activate the getter element  84  that is disposed in the tube  80 . After the layers of low-temperature sealing material  94   a  have sufficiently melted and fused together, the temperature is lowered below the melting point of the low-temperature sealing material  94   a  and, upon cooling, the low-temperature air-tight seal  94  is thus formed between the cover  82  and the tube  80  ( FIG. 8B ). By blocking the last fluid pathway that existed between the reference chamber  52  and the external environment, i.e., the vacuum chamber  200 , the reference pressure in the reference chamber  52  is thus established when the seal  94  is formed.  
         [0065]     Once the seal  94  is formed, the oven and vacuum pump can be turned off and the completed transducer assembly  10  can be removed from the vacuum housing  110  and the guide assembly and ball  320  can be removed from the transducer assembly  10 . The apparatus  100  can then be used to process another transducer assembly  10 .  
         [0066]     The apparatus  100  can be configured to process more than one transducer assembly  10  at a time. Instead of the cable  116 , it may be advantageous to utilize an actuator rod(s) with an actuator motor to control the rotational orientation of the vacuum housing  110 . Additionally, while the method and apparatus described herein have been directed to a transducer assembly  10  that measures an absolute pressure and utilizes a getter element, etc., the method and apparatus of the present invention can also be used to establish a reference pressure in a reference chamber of a wide variety of other gauge-type pressure transducer assemblies.  
         [0067]      FIG. 9  illustrates the burn-out, evacuation and sealing process of the present disclosure in more detail. In  FIG. 9 , the x-axis of the process flow represents Time and the y-axis represents Temperature in degrees Celsius. Prior to initiating the burn-out and evacuation process, at Step A of the process flow, the cover  82 , ball  320 , guide assembly  300  and pressure transducer assembly  10  are arranged in the vacuum chamber  200  of the vacuum housing  110  and the vacuum housing  110  is rotated counterclockwise (forward) as shown in  FIGS. 7A and 7B . During Step A→B, over the course of three hours, the temperature in the vacuum chamber  200  is raised to a burn-out temperature of 250° C. and the pressure is lowered to an evacuation pressure of 10 −8  Torr. After the burn-out temperature and evacuation pressure are achieved (Step B), the reference chamber  52  is burned-out and evacuated for 20 hours, Step B→C. Shortly before Step C is reached, the vacuum housing  100  is rotated clockwise (backwards) to the upright position as shown in  FIGS. 8A and 8B . When rotated to the upright position, the movement of the ball  320  causes the cover  82  to move towards tube  80  and the two layers of low-temperature sealing material  94   a  to come into contact with each other. Once the burn-out and evacuation step is completed (Step C), the temperature in the vacuum chamber  200  is raised to 475° C., Step C→D, which causes the two layers of low-temperature sealing material  94   a  to melt. Step C→D lasts for three hours. The vacuum chamber  200  is then maintained at 475° C. for 30 minutes, Step D→E, to ensure that the layers of low-temperature sealing material  94   a  sufficiently melt together. Lastly, over the course of 4½ hours, the temperature and pressure in the vacuum chamber  200  are brought to ambient conditions and the assembled pressure transducer assembly  10  is then removed from the vacuum chamber  200  of the vacuum housing  110 , Step E→G.  
         [0068]     By eliminating the intermediate temperature ramp down portion of the prior art method (Step E→F of  FIG. 3 ), which is necessary forming the low-temperature seal  94  while maintaining the low-temperature seal  98  in a melted state, the burn-out, evacuation and sealing process of the present disclosure can be completed in as little as 31 hours. Thus, in addition to accurately establishing a reference pressure within a reference chamber, the present disclosure can also advantageously shorten the time that is required to perform the burn-out, evacuation and sealing process of the pressure transducer assembly  10 .  
         [0069]     Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise any other varied embodiments that incorporate these teachings.