Abstract:
A capacitance diaphragm gauge (CDG) for measuring pressure includes a flush diaphragm mounted to a body structure via a shim or other raised perimeter portion. The diaphragm and the shim are welded to the body structure while the diaphragm is maintained at an elevated temperature. Contraction of the diaphragm as it cools pretensions the diaphragm to substantially reduce hysteresis effects. An electrode advantageously includes two portions with one portion providing excellent bonding characteristics and the other portion having temperature characteristics corresponding to the body structure and the diaphragm. An alternative CDG includes two identical electrodes with a first electrode positioned proximate to the center of the diaphragm and with a second electrode positioned proximate to the perimeter of the diaphragm. The second electrode provides a second capacitance signal that is used to compensate for changes in capacitance between the diaphragm and the first electrode caused by temperature changes.

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
   This application is based on Provisional Patent application No. 60/456,975, filed Mar. 22, 2003 and entitled “CAPACITANCE MANOMETER HAVING A RELATIVELY THICK FLUSH DIAPHRAGM UNDER TENSION TO PROVIDE LOW HYSTERESIS”. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention is in the field of pressure transducers having a variable capacitance between a diaphragm exposed to the pressure and a fixed electrode. 
   2. Description of the Related Art 
   Capacitance diaphragm gauges (CDGs) have been used for many years to measure pressures. CDGs are particularly useful for measuring very low pressures (e.g., much lower than atmospheric pressure) such as pressures in an evacuated system (e.g., a semiconductor fabrication system). A CDG produces an electrical output that represents a measure of a pressure input with respect to a reference pressure. 
   Basically, an exemplary CDG includes at least one electrode that is supported on a suitable support structure. The electrode is positioned in close proximity to a flexible diaphragm in a sealed and evacuated cavity. The diaphragm is positioned in the device so that one face of the diaphragm (the pressure face) is exposed to an unknown pressure to be measured. The electrode is proximate to the opposite face of the diaphragm (the electrode face). The unknown pressure on the pressure face is measured relative to a reference pressure on the electrode face. The reference pressure is substantially constant within the sealed and evacuated cavity. The diaphragm and the electrode comprise the two plates of a variable capacitor that has a capacitance the varies in response to deflections of the diaphragm caused by pressure variations. 
   In many applications, the CDG is positioned within a suitable housing of a pressure-measuring device with the pressure face of the diaphragm exposed to the unknown pressure via suitable passages. Alternatively, the pressure face of the diaphragm may be exposed directly to the unknown pressure. For example, the CDG may be mounted such that the pressure face of the diaphragm is in a gas flow conduit, in which case it is preferable that the diaphragm and other portions of the CDG do not extend into the gas flow to partially block the gas flow or to cause turbulence in the gas flow. If no portion of the CDG extends beyond the pressure face of the diaphragm, the pressure face can be mounted substantially flush with an inner wall of the gas flow conduit. A CDG having such a configuration is called a flush diaphragm design. One skilled in the art will appreciate that a flush diaphragm CDG can be welded into a housing to make a more general device. On the other hand, a CDG that does not have flush diaphragm generally is not convertible to be used in applications requiring a flush diaphragm device because the outer support structures for the diaphragm extend beyond the pressure face of the diaphragm. 
   CDGs having flush diaphragms are known in the art. For example, a first type of flush diaphragm CDG is machined out of a solid block of suitable material to leave a thin layer of material at one end of the block to form the diaphragm. In some cases, the material may be heat treated for certain desired results or because of the properties of the material. 
   Another known type of flush diaphragm CDG is called corrugated diaphragm CDG. The corrugated diaphragm has waves formed into the surface to cause extra material to be present in order to produce more linear deflections in response to the applied pressure. The diaphragm for this type is usually welded into place. 
   A third type of flush diaphragm CDG has a diaphragm formed from a thin material. The thin material is highly tensioned in some manner and is welded in place. 
   Much emphasis is placed on the hysteresis characteristics of a finished pressure measuring device. Hysteresis refers to the differences between the output of the transducer on approaching a given pressure from different directions (i.e., approaching the given pressure from higher pressures as the unknown pressure is decreasing versus approaching the given pressure from lower pressures as the unknown pressure is increasing). Although the same output value should be generated for the given pressure irrespective of the previous pressure, hysteresis effects may cause the output value to be too high when the given pressure is approached from a higher pressure and may cause the output value to be to low when the given pressure is approached from a lower pressure. 
   The maximum value of the hysteresis error is usually at the midpoint of the pressure excursions. An excursion from zero pressure to full-scale pressure is the maximum normal excursion. Abnormal excursions can cause greater errors. Since hysteresis errors depend at least in part on the magnitude of the pressure excursions, the hysteresis errors are usually unpredictable and are therefore major concerns. In contrast, other errors, such as, for example, linearity or temperature errors, are more correctable because they are repeatable and therefore predictable. 
   A diaphragm subjected to pressure has to carry the pressure load. The difference between the pressures applied on the opposite faces of the diaphragm causes a deflection of the diaphragm. The electrode face of diaphragm acts as one plate of a variable capacitor having the electrode as the other plate of the capacitor. If additional electrodes are included, multiple capacitors are formed with the electrode face of the diaphragm forming one plate of each capacitor. The deflection of diaphragm moves the diaphragm closer to or farther from the electrode, thus varying the capacitance. The capacitance is determined in a suitable conventional manner to provide a measurable quantity responsive to the pressure applied to the pressure face of the diaphragm. 
   In order to produce repeatable measurements of the unknown pressure, the diaphragm deflection should occur with a minimum of hysteresis. That is, when the pressure returns to the previous magnitude, the diaphragm should return to its previous state of deflection regardless of whether the pressure initially increased and then decreased or initially decreased and then increased. 
   Reduction of hysteresis has been accomplished by carrying the load in tension. It has been found that smaller changes in the magnitude of the tension in response to pressure changes results in less hysteresis and thus results in greater measurement accuracy. One problem with high pressure measuring devices is to keep the deflection small enough by having a pretension carrying the load. 
   Many techniques have been used to pretension diaphragms, particularly for diaphragms in low pressure CDGS; however, the techniques used for high pressure diaphragms have proven to be very limited, and as the devices have become smaller, the techniques have become even more limited. One technique that has been used to pretension a diaphragm is to heat the diaphragm prior to welding the diaphragm to the body of the CDG so that when the diaphragm cools, the diaphragm will shrink and develop tension. Previous attempts to do pretension a diaphragm with this technique consisted of placing the diaphragm in contact with a heated platen. This technique causes the whole fixture to become hot and thus causes a significant uncertainty in results as sequential units are processed. Such a technique also presents problems in maintaining good thermal contact between the diaphragm and the platen, which again causes the resulting tension on the diaphragm to be nonrepeatable. 
   The support structure in a typical CDG is formed as one piece with a portion of the structure proximate to the diaphragm providing the function of a shim that spaces the diaphragm away from the electrode in its rest or zero position. Forming the shim as part of the CDG body is a very expensive and unrepeatable way to obtain the spacing between the diaphragm and the electrode. For example, the thin lip of the shim needs to be machined in with great care to provide the tolerances that are necessary to produce a repeatable initial zero capacitance. The shim is under great pressure when the diaphragm deflects. Therefore, the shim needs to be extremely hard. In order to obtain the required hardness with the one-piece design, the part is heat-treated after machining. The heat-treating may cause the part to warp and to lose the spacing accuracy that is required for precision measurements. 
   SUMMARY OF INVENTION 
   Embodiments in accordance with the present invention provide a capacitance diaphragm gauge (CDG) having a flush diaphragm with low hysteresis characteristics. The CDG has a simple structure that can be repeatably manufactured in an affordable manner. 
   One aspect of embodiments in accordance with the present invention is a capacitance diaphragm gauge (CDG) having a flush diaphragm mounted on the body of the CDG by a technique that produces a very high pretension on the diaphragm with a magnitude approximately half the magnitude of the ultimate strength of the diaphragm material. Such a pretension can be shown to be the optimum operating point that minimizes the bending stress of the diaphragm relative to the allowable stress. Since the bending stress on the diaphragm is a primary cause of hysteresis, the hysteresis is minimized by this technique. 
   In particular, in embodiments described herein, heat is applied to the diaphragm prior to welding the diaphragm to the CDG body. After the welding is completed, the diaphragm is pretensioned as the diaphragm shrinks while cooling. 
   In a preferred embodiment, the diaphragm is illuminated with high intensity radiation. For example, the radiation may be provided by a laser or other suitable source. In one particular embodiment, the radiation is generated by a halogen lamp suitably positioned to irradiate a face of the diaphragm. The radiation source is turned on for a few seconds before beginning the welding process and remains on during the welding process. The radiation is caused to selectively heat the diaphragm by raising the emissivity of the diaphragm relative to its surroundings to increase the absorption of the radiation. By increasing the temperature of the diaphragm relative to the surrounding material of the CDG body, the diaphragm expands relative to the surrounding material prior to the welding process. The diaphragm is welded while it is expanded to cause the diaphragm to become pretensioned when it is cooled after the welding is completed. 
   The radiation intensity from the laser, the halogen lamp or other radiation source can be controlled adequately to provide a repeatable temperature so that the pretensioning produces repeatable stress of approximately one half the ultimate stress. 
   An ordinary metal has a very low emissivity and thus has very low absorption. Substantially all of the incident radiation is reflected, and the small amount retained will increase the temperature an inadequate amount. Furthermore, the temperature increase is not likely to produce repeatable results. In accordance with the particularly preferred embodiment, the emissivity of the surface of the diaphragm is increased by coating the surface with carbon or another suitable substance. Preferably, the surface of the diaphragm is coated in a manner that permits the diaphragm to be cleaned easily after the welding process is completed. For example, carbon black (e.g., soot) has been found to be suitable to increase the emissivity and to be easily removed after the processing is completed. In one particular embodiment, the carbon black is applied by exposing the pressure surface of the diaphragm to an oxidizing flame of butane (e.g., from a lighter or the like). The oxidizing flame deposits a thin layer of carbon on the pressure surface. The thin carbon layer absorbs radiation to cause the diaphragm to heat rapidly while the other components remain relatively cool. The carbon layer washes off easily without requiring abrasive cleaning. 
   The techniques described herein are used to produce CDGs having separate, thin unmachined diaphragms. The diaphragms are easily heat treated to the optimum properties in contrast to the very expensive process of machining the diaphragm and support out of one piece and then trying to heat treat the diaphragm after machining without warping the diaphragm. The process described herein allows a diaphragm to be installed on the support in a cost efficient and optimum manner and provides outstanding performance with respect to the deflection characteristics of the diaphragm. In particular, the diaphragm has a low hysteresis. 
   Unlike prior devices with a one-piece body structure having the shim formed as a part of the body structure, embodiments in accordance with the present invention include a separate shim that can be heat treated separately. Like the diaphragm, the shim does not need to be machined. Therefore, the shim does not warp or change its thickness in any way. Thus, optimum performance is obtainable with low-cost parts that are easy to manufacture with repeatable characteristics. As a result, the support structure (e.g., the body of the CDG) in accordance with the embodiments described herein is a simple mass producible part. 
   In order to weld the diaphragm while heated, the diaphragm and the shim are fixed between an upper pressure nose and a lower support surface of a hydraulic arbor press while the heated diaphragm and the shim are welded to the CDG body. In non-flush diaphragm configurations, an outer support ring is also welded during the same process and remains as part of the CDG. In order to obtain a flush diaphragm in accordance with the embodiments described herein, the diaphragm rests on a reusable support jig during the welding process. The support jig is positioned on the lower support surface of the arbor press, and the upper pressure nose is forced against the rear surface of the CDG body. Pressure from the arbor press secures the diaphragm to the CDG body during the welding process. The support jig comprises a high temperature (e.g., refractory) material that does not melt during the welding process and thus does not become attached to the diaphragm. Exemplary refractory materials, such as, for example, tantalum and silicon carbide, are suitable for the support jig. 
   Alternative embodiments in accordance with the present invention include a two-piece electrode that provides a stable capacitance under variations of temperature in contrast to known single-piece electrode designs in the past. The expansion of an electrode in response to temperature increases the rest capacitance. The increase in rest capacitance may be cancelled by increasing the space between diaphragm and the electrode. The increased space can be provided by making the net expansion of the single electrode smaller than the support path through the shim. This is accomplished in preferred embodiments using a two-piece electrode. A two-piece electrode suitable for high pressure measurements comprises an outer portion comprising titanium or titanium alloy. The titanium or titanium alloy material has high strength bonding characteristics that withstand the great forces of overpressure that are unique to a high pressure CDG. The inner portion of the two-piece electrode is joined to the outer portion by welding (or by another suitable manner that joins the pieces as if they were welded). For example, 300 series stainless steels have been found to be suitable for use as the inner portion. In an embodiment described herein, the inner portion advantageously comprises nickel. Alternatively, suitable performance can be achieved by a single-piece electrode comprising titanium or a titanium alloy. 
   Further embodiments in accordance with the present invention include a second electrode positioned proximate to the perimeter of the diaphragm to compensate for the expansion of the space between the electrode and the diaphragm by providing a second capacitance measurement signal that can be processed to cancel out the effect of the expansion. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other features of embodiments of the present invention are described below in connection with the accompanying drawing figures in which: 
       FIG. 1A  illustrates a front perspective view of an embodiment of a capacitance diaphragm gauge (CDG) in accordance with the present invention, showing the pressure face of a flush diaphragm; 
       FIG. 1B  illustrates a rear perspective view of the CDG of  FIG. 1A  showing the shielded electrode connection, the threaded hole for making electrical connection to the body of the CDG and the pinched-off evacuation tube; 
       FIG. 2A  illustrates a cross section of the CDG of  FIG. 1A  taken along the lines  2 A— 2 A in  FIG. 1A ; 
       FIG. 2B  illustrates an enlarged cross section of the CDG taken along the lines  2 B— 2 B of  FIG. 2A  to show the shim between the diaphragm and the CDG body in more detail; 
       FIG. 3  illustrates an exploded rear perspective view of the CDG of  FIGS. 1A and 1B  showing the electrode, the electrode shield, the insulating glass preforms and the evacuation tube; 
       FIG. 4  illustrates an exploded front view of the CDG of  FIGS. 1A and 1B  showing the relationship between the diaphragm, the shim and the electrode; 
       FIG. 5  illustrates a pictorial depiction in partial cross section of the CDG body, the shim and the diaphragm positioned on a reusable supporting ring in a hydraulic arbor press, which applies pressure while a radiation source applies radiation to heat the diaphragm during a welding process; and 
       FIG. 6  illustrates a cross section of an alternative embodiment in accordance with the present invention in which two electrodes are provided in order to compensate for changes in the spacing between the diaphragm and the center electrode with temperature. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A ,  1 B,  2 A,  2 B,  3  and  4  illustrate an embodiment of a capacitance diaphragm gauge (CDG)  100 . As shown in  FIG. 1A , the CDG  100  includes a body structure  110 , having a front surface  112  (see  FIG. 4 ) and a rear surface  114  (see FIG.  4 ). In the embodiments illustrated herein, body structure  110  is generally cylindrical, and the front surface  112  and the rear surface  114  have circular shapes. In the preferred embodiments, the area of the front surface  112  is smaller than the area of the rear surface  114 , and a forward cylindrical portion  116  of the body structure  110  proximate to the front surface  112  has a smaller diameter than a rearward cylindrical portion  118  proximate to the rear surface  114  such that the body structure  110  has a stepped transition from the front portion  116  to the rear portion  118  that forms a lip  119  around the front portion  116 . The lip  119  may be used when mounting the CDG  100  in certain applications. 
   A flush diaphragm  120  is mounted proximate to the front surface  112  of the body structure  110  and is spaced apart from the front surface  112  by a circular shim  122  (shown more clearly in  FIGS. 2A ,  2 B and  4 ). The diaphragm  120  has a diameter of approximately 1 inch (2.54 cm) and has a thickness that can range from 0.001 inch (0.025 to 0.015 inch (0.38 mm). Preferably, the diaphragm  120  comprises Inconel  750  or another suitable material. 
   The circular shim  122  comprises Inconel  750  formed as a thin ring having an outer diameter of approximately 1 inch (2.54 cm) and an inner diameter of approximately 0.98 inch (2.49 cm). In a preferred embodiment, the shim  122  has a thickness of approximately 0.003 inch (0.08 mm). Thus, the diaphragm  120  is spaced from the front surface  112  by approximately 0.003 inch. In the preferred embodiment, the shim  122  is a separate unit as illustrated in the figures. The shim  122  forms a raised perimeter portion that bounds the flat front surface  112  of the body structure  110 . In alternative embodiments, the shim  122  can be formed as part of the body structure  110  by machining or other suitable technique to form a raised perimeter portion around a substantially flat central portion of the front surface  112 . The raised perimeter portion has an effective thickness measured perpendicular to the central portion of the front surface  112  that corresponds to the thickness of the shim  122 , as discussed above. 
   As shown more clearly in  FIGS. 2A and 4 , a cylindrical bore  124  extends through the body structure  110  from the front surface  112  to the rear surface  114  and is generally centered with respect to both surfaces. An electrode assembly  130  extends through the cylindrical bore  124 . The electrode assembly  130  comprises a cylindrical electrode  132  surrounded by a concentric electrode shield  134 . The electrode assembly  130  is positioned through the cylindrical bore  124  so that a front surface  136  of the electrode  132  and a front surface  138  of the electrode shield  134  are substantially flush with the front surface  112  of the body structure  110 , as shown in  FIGS. 2A and 4 . 
   As shown in  FIG. 2A , the electrode  132  is electrically insulated from the electrode shield  134  by a first concentric insulator  140  positioned between the electrode  132  and the electrode shield  134 . Similarly, the electrode shield  134  is electrically insulated from the wall of the bore  124  and is thus insulated from the body structure  110  by a second concentric insulator  142  positioned between the electrode shield  134  and the wall of the cylindrical bore  124 . 
   As illustrated in  FIG. 3 , the first concentric insulator  140  is advantageously formed by placing a first plurality of ring-shaped glass preforms  140   a ,  140   b  around a portion of the electrode  132 , positioning the electrode shield  134  over the first plurality of glass preforms  140   a ,  140   b . The second concentric insulator  142  is advantageously formed by placing a second plurality of glass preforms  142   a ,  142   b ,  142   c ,  142   d  around the electrode shield  134  and then positioning the electrode shield  134  within the cylindrical bore  124 . The glass preforms  140   a ,  140   b  are sized to generally center the electrode  132  within the electrode shield  134 , and the glass preforms  142   a ,  142   b ,  142   c ,  142   d  are generally sized to center the electrode shield  134  within the cylindrical bore  124 . 
   The components are positioned as described in an alignment fixture (not shown). The front surface  136  of the electrode  132  advantageously includes a small opening  144  that is engageable with a pin (not shown) in the alignment fixture. Similarly, a hole (not shown) in the alignment fixture is engageable with a contact pin  146  extending from a rear surface  148  of the electrode  132 . The opening  144  and the pin  146  maintain the electrode  132  in a substantially concentric position within the electrode shield  134  until the glass preforms  140   a ,  140   b ,  142   a ,  142   b ,  142   c ,  142   d  have been heated sufficiently to flow around the electrode  132  and the electrode shield  134  and have subsequently cooled. In certain preferred embodiments, the glass preforms,  140   a ,  140   b ,  142   a ,  142   b ,  142   c ,  142   d  advantageously comprise borosilicate glass that softens sufficiently at approximately 700° C. to flow around the components and form a permanent insulating bond. 
   After the body structure  110  has cooled, the front surface  112  of the body structure  110  is smoothed by lapping or other suitable method so that the front surface  136  of the electrode  132  is flush with the front surface  112 . 
   In the preferred embodiment, the electrode  132  comprises a front portion  132   a  and a rear portion  132   b . The rear portion  132   b  advantageously comprises titanium, which has a low coefficient of expansion in response to temperature. Thus, as the temperature is increased to cause the glass preforms to flow and subsequently decreased to form the permanent bond, the diameter of the rear portion  132   b  remains sufficiently constant that the glass bond formed around the rear portion  132   b  remains intact as the glass hardens. 
   In the preferred embodiment, the diaphragm  120 , the shim  122  and the body structure  110  comprise Inconel  750  or other suitable material. The front portion  132   a  of the electrode  132  advantageously comprises nickel. The front portion  132   a  has a similar coefficient of expansion in response to temperature as the body structure  110 , the diaphragm  120  and the shim  122 . Thus, the front portion  132   a  expands and contracts substantially in proportion to the other components to thereby maintain a relatively fixed spacing with respect to the diaphragm  120 . The electrode shield  134  also advantageously comprises nickel in order to have a similar coefficient of expansion. 
     FIG. 5  illustrates a system for mounting the diaphragm  120  and the shim  122  to the body member  110 . After the electrode  132  and electrode shield  134  are bonded to the each other and to the body structure  110 , as described above, the shim  122  and the diaphragm  120  are welded to the front surface  112  of the body structure in a manner that pretensions the diaphragm  120 . In particular, the shim  122  is positioned on the front surface  112  such that the outer perimeter of the shim  122  substantially conforms to the outer perimeter of the front surface  112 . The circular diaphragm  120  is then positioned on the shim  122 . A reusable, ring-shaped tooling jig (support jig)  170  is then positioned over the diaphragm  120 . 
   The body structure  110 , the shim  122 , the diaphragm  120  and the tooling jig  170  are positioned in a hydraulic arbor press  172 , a portion of which is shown in  FIG. 5  in partial cross section. The tooling jig  170  rests on a cylindrical lower support surface  174  of the arbor press  170  with the diaphragm  120 , the shim  122  and the body structure  110  resting on the tooling jig  170 . A cylindrical upper pressure nose  176  of the arbor press  172  is positioned on the rear surface  114  of the body structure  110 . A varying force is applied to the pressure nose  176  of the arbor  172  by hydraulic cylinders (not shown) or other conventional equipment to thereby squeeze the diaphragm  120  and the shim  122  between the perimeter of the front surface  112  and the tooling jig  170 . 
   As further shown in  FIG. 5 , a source  180  of radiant energy is positioned below the diaphragm  120 . For example, a halogen lamp  180  advantageously provides the radiant energy in the illustrated embodiment. The radiant energy is directed toward the diaphragm  120  to heat the diaphragm and cause the diaphragm to expand. 
   Since the diaphragm  120  comprises Inconel, which has a generally high reflectivity, a substantial portion of the radiant energy incident on the diaphragm  120  from the lamp  180  would ordinarily be reflected. In order to enhance the absorption of the radiant energy, the diaphragm is coated with a high emissivity material since a high emissivity material also readily absorbs radiant energy. On the other hand, many high emissivity coatings are difficult to remove from a surface. Any contaminating material remaining on the exposed surface of the diaphragm  120  would likely affect the performance of the diaphragm. In preferred embodiments, the exposed surface of the diaphragm  120  is coated with lamp black (e.g., soot)  182 . For example, in one embodiment, the lamp black  182  is formed on the diaphragm  120  by positioning a butane flame (not shown) proximate the exposed surface. After permanently fixing the shim  122  and the diaphragm  120  to the body structure  110 , as described below, the lamp black  182  is easily removed from the diaphragm with water or a mild cleaning solution without using abrasives or force that might damage the diaphragm  120 . 
   Initially, a sufficient pressure is applied to the rear surface  114  of the body structure  110  to maintain the relative positions of the body structure  110 , the shim  122  and the diaphragm  120  while the diaphragm  120  is heated by the radiant energy absorbed by the lamp black  182 , thus causing the diaphragm  120  to expand. Full pressure is then applied to the assembled components to restrain the diaphragm  120  in the expanded configuration. 
   A welding head  190  is activated to fuse the diaphragm  120  and the shim  122  to the front surface  112  of the body structure  110 . The welding head  190  revolves about the perimeter of the diaphragm in a conventional manner (e.g., electrical arc welding, laser welding, electron beam welding, or other suitable bonding processes) to form a continuous weld around the entire perimeter of the diaphragm  120 . The diaphragm  120  and the shim  122  are secured to the body structure  110  to thereby form a sealed cavity between the inner surface of the diaphragm and the front surface  112  of the body structure. 
   The tooling jig  170  comprises a refractory metal or other suitable material (e.g., tantalum or silicon carbide) having a much higher melting temperature than the Inconel  750  material used for the body structure  110 , the shim  122  and the diaphragm  120 . Thus, the tooling jig  170  is not affected by the welding process and does not fuse with the other components. The welded components are readily removable from the tooling jig  170 , and the same tooling jig  170  can be used multiple times. 
   When the lamp  180  is turned off, the diaphragm  120  gradually cools and contracts. However, since the outer perimeter of the diaphragm  120  is firmly secured to the body structure  110 , which was not heated to any significant extent by the radiant energy, the surface of the diaphragm  120  effectively stretches and becomes pretensioned as it cools. 
   Because of the pretensioning introduced by the foregoing assembly method, the diaphragm  120  has very little hysteresis. When used in a pressure-sensing application, the pretensioning of the diaphragm  120  causes the diaphragm to return to its initial undeflected position after being deflected by pressure variations. 
   As further illustrated in  FIGS. 1B and 3 , a smaller through bore  150  extends from the front surface  112  to the rear surface  114 . During assembly of the CDG  100 , an evacuation tube  152  is mounted into the bore  150 . After the CDG  100  is fully assembled, a very low pressure is applied to the evacuation tube  152  to remove any residual gases within a cavity formed between the front surface  112  and the diaphragm  120 . The evacuation tube  152  is then pinched to form a cold weld and the excess portion of the evacuation tube  152  is removed to form a stub as shown in FIG.  1 B. 
   The rear surface  114  further includes a threaded bore  160  that extends a selected depth into the body structure  110  but does not extend to the front surface  112 . When the CDG  100  is installed in a pressure sensing application, an electrical connection (not shown) is attachable to the body structure  110  by engaging the threaded bore  160  with a screw (not shown) to thereby complete an electrical circuit to the diaphragm  120  via the body structure  110  and the shim  114 . Thus, a first electrical connection is made to one plate of the variable capacitor formed by the diaphragm  120  and the front surface  136  of the electrode  132 . A second electrical connection is made to the electrode  132  by engaging the pin  146  with the center contact of a coaxial connector (not shown). The shield contact of the coaxial connector engages the electrode shield  134 . 
   Note that the cross section in  FIG. 2A  is selected so that the through bore  150 , the evacuation tube  152  and the threaded bore  160  are not shown. 
   In some embodiments, an additional through bore (not shown) may be included to allow installation of a conventional getter can (not shown) to chemically remove any residual gas remaining after the evacuation process. 
   The structure of the CDG  100  and the method of pretensioning the diaphragm  120  permits CDGs to be manufactured with a wider range of pressure-sensing capabilities. For example, a diaphragm  120  having a diameter of approximately 1 inch (2.54 cm) and having a thickness of approximately 0.001 inch (0.025 mm) can be manufactured to measure pressures in a range extending from 0.0001 Torr to 1 Torr up to a range extending from 0.001 Torr to 10 Torr. A diaphragm  120  having a similar thickness and a diameter of approximately 2 inches (5.08 can be manufactured to measure pressures in a range extending from 0.00001 Torr to 0.1 up to a range extending from 0.001 Torr to 10 Torr. 
   The structure of the CDG  100  and the method of pretensioning the diaphragm  120  is particularly advantageous for manufacturing CDGs for measuring higher ranges of pressures using much diaphragms that are proportionately thicker with respect to their diameters. 
   Heretofore, CDGs having pretensioned flush diaphragms with very low hysteresis and having sufficient thicknesses to measure higher pressure ranges were not available at reasonable costs. The structure and method of the embodiments described herein provide low cost, very accurate flush diaphragms that can be manufactured for use in a variety of applications. For example, a diaphragm  120  having a diameter of approximately 0.75 inch (1.9 cm) and a thickness of 0.001 inch (0.025 mm) can be manufactured to measure pressures in a range extending from 0.01 Torr to 100 Torr. A diaphragm  120  having a diameter of approximately 0.75 inch (1.9 cm) and a thickness of 0.003 inch (0.076 mm) can be manufactured to measure pressures in a range extending from 0.1 Torr to 1,000 Torr. A diaphragm  120  having a diameter of approximately 0.75 inch (1.9 cm) and a thickness of 0.01 inch (0.254 mm) can be manufactured to measure pressures in a range extending from 1 Torr to 10,000 Torr. 
     FIG. 6  illustrates a cross section of an alternative embodiment of a CDG  200  in accordance with the present invention in which two electrodes are provided in order to compensate for changes in the spacing between the diaphragm and the center electrode responsive to temperature variations. The embodiment of  FIG. 6  is particularly advantageous for improving the performance of CDGs having larger diameter diaphragms (e.g., diameters on the order of 2 inches). The structure of the CDG  200  is similar to the structure of the CDG  100  described above, and like elements not specifically discussed below are not numbered in FIG.  6 . 
   The CDG  200  includes a body structure  210  comprising Inconel  750 . The body structure  210  is generally circular as was illustrated above for the body structure  110  of the CDG  100 . The body structure  210  has a diameter of approximately 2 inches (5.08 cm). The body structure  210  has a front surface  212  and a rear surface  214 . 
   A diaphragm  220  is positioned proximate to the front surface  212  and is spaced from the front surface  212  by a circular shim  222 . The diaphragm  220  and the shim  222  are constructed as described above; however, the diameters are larger (e.g., 2 inches (5.08 cm) to correspond to the diameter of the body structure  210 . 
   A first bore  224   a  extends through the body structure  210  from the center of the front surface  212  to the center of the rear surface  214 . A second bore  224   b  extends through the body structure  210  in parallel to the first bore  224   a . The second bore  224   b  is located near the perimeter of the front surface  212 . 
   A first electrode assembly  230   a  is positioned within the first bore  224   a , and a second electrode assembly  230   b  is positioned within the second bore  224   b . Each of the electrode assemblies  230   a ,  230   b  is advantageously constructed in the manner described above with respect to the electrode assembly  130 . In particular, the first electrode assembly  230   a  includes a first electrode  232   a  that has a first electrode front surface  236   a , and the second electrode assembly  230   b  includes a second electrode  232   b  that has a second electrode front surface  236   b.    
   The body structure  210  advantageously includes a through bore to accommodate a evacuation tube and a threaded bore to receive an electrical connection. These elements are not shown in  FIG. 6 ; however, the elements correspond to like elements shown in FIG.  3 . 
   The CDG  200  is assembled as described above in connection with the CDG  100  so that the diaphragm  220  is pretensioned across the front surface  212  of the body structure  210 , and the cavity between the inner surface of the diaphragm  220  and the front surface  212  is evacuated and sealed. 
   The inclusion of the second electrode assembly  230   b  in the CDG  200  is particularly advantageous when a larger diameter diaphragm is used. As the temperature increases around the CDG  200 , the shim  222  will tend to expand to cause the diaphragm  220  to move away from the front surface  212  proximate to the front surface  236   a  of the first electrode  232   a . Thus, the capacitance between the first electrode  232   a  and the diaphragm  220  will change with temperature. Since the change in capacitance caused by temperature may not be readily distinguished from the change in capacitance caused by pressure, the measured capacitance may not accurately indicate the pressure. 
   Since the second electrode assembly  230   b  is located near the perimeter of the diaphragm  220  where the diaphragm  220  is secured to the front surface  212  via the shim  222 , the spacing between the portion of the diaphragm  220  and the front surface  236   b  of the second electrode  232   b  changes very little in response to pressure changes. However, the spacing between the diaphragm  220  and the front surface  236   b  of the second electrode  232   b  changes substantially the same as the spacing between the diaphragm  220  and the front surface  236   a  of the first electrode  232   a  in response to temperature changes. Thus, the change in capacitance caused by the change in temperature is substantially the same for both electrodes. Therefore, the capacitance measurement taken between the diaphragm  220  and the first electrode  232   a  and the capacitance measurement taken between the diaphragm  220  and the second electrode  232   b  are used to compensate for the effect of temperature when the pressure is determined. 
   This invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of the invention is indicated by the following claims rather than by the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered within their scope.