Abstract:
The disclosed transducer includes a housing, a diaphragm, an inner conductor, an outer conductor, and a first baffle. The housing defines an interior volume. The diaphragm is disposed in the housing and divides the interior volume into a first chamber and a second chamber. The diaphragm flexes in response to pressure differentials in the first and second chambers. The inner conductor is disposed in the first chamber. The outer conductor is disposed in the first chamber around the inner conductor. The first baffle is disposed in the second chamber and defines an inner region, a middle region, and an outer region. The inner region underlies the inner conductor. The middle region underlies the outer conductor. The outer region underlies neither the inner conductor nor the outer conductor. The first baffle defines apertures in at least two of the inner, middle, and outer regions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is related to capacitive pressure transducers. More particularly, the present invention relates to controlling deposition of contaminants in capacitive pressure transducers.  
           [0002]    [0002]FIG. 1A shows a sectional side view of a prior art capacitive pressure transducer  100 . For convenience of illustration, FIG. 1A, as well as other figures in the present disclosure, are not drawn to scale. As shown, transducer  100  includes a housing  102 , a capacitive pressure sensor  106  disposed within housing  102 , an inlet tube  104 , and a filtering mechanism  108 . For convenience of illustration, many details of transducer  100  are omitted from FIG. 1A. However, such sensors are well known and are described, for example, in U.S. Pat. Nos. 5,911,162 and 6,105,436 and U.S. patent application Ser. Nos. 09/394,804 and 09/637,980.  
           [0003]    Briefly, transducer  100  is normally coupled to a gas line  110 , or some other external source of gas or fluid  111  by a coupling  112 . In operation, sensor  106  generates an output signal representative of the pressure of gas  111  (i.e., the pressure within external source  110 ).  
           [0004]    Pressure transducers such as transducer  100  are often used in integrated circuit fabrication foundries, for example, to measure the pressure of a fluid in a gas line that is being delivered to a deposition chamber, or to measure the pressure within the deposition chamber itself. Some of the processes used in integrated circuit fabrication, such as the etching of aluminum, tend to generate a large volume of particles or contaminants. It is generally desirable to prevent such contaminants from entering the sensor  106 . When contaminants do enter, or become built up in, sensor  106 , the accuracy of the pressure measurement provided by transducer  100  is adversely affected. Accordingly, prior art pressure transducers have used a variety of mechanisms to prevent contaminants from reaching the sensor  106 . Such prior art filtering mechanisms are generally disposed between the inlet tube  104  and the sensor  106 , and are indicated generally in FIG. 1A at  108 .  
           [0005]    [0005]FIG. 1B shows a more detailed view of a particular prior art pressure transducer  100  showing both the sensor  106  (which as discussed below includes elements  127   a ,  127   b , and  160 ) and the filtration mechanisms  108  (which as discussed below includes elements  140 ,  150 ). Transducer  100  includes a lower housing  102   a  and an upper housing  102   b , which are separated by a relatively thin, flexible conductive diaphragm  160 . Lower and upper housings  102   a ,  102   b , and diaphragm  160  are normally welded together. Upper housing  102   b  and diaphragm  160  define a sealed interior chamber  120 . Lower housing  102   a  and diaphragm  160  define an interior chamber  130  that opens into inlet tube  104 . Diaphragm  160  is mounted so that it flexes, or deflects, in response to pressure differentials in chambers  120 ,  130 .  
           [0006]    Transducer  100  includes a ceramic electrode  122  disposed within chamber  120 . Electrode  122  is supported within chamber  120  by a support  124 . An inner conductor  127   a  and an outer conductor  127   b  are disposed on the bottom of electrode  122 . FIG. 1C shows a bottom view of electrode  122  showing the geometries of the inner and outer conductors  127   a ,  127   b . As shown, inner conductor  127   a  is circular. Outer conductor  127   b  is annular and surrounds inner conductor  127   a . The area of inner conductor  127   a  is normally selected to be equal to the area of outer conductor  127   b . Conductors  127   a ,  127   b  are generally parallel to and spaced apart from diaphragm  160 . Diaphragm  160  and the conductors  127   a ,  127   b  form two variable capacitors  128   a ,  128   b . More particularly, diaphragm  160  and inner conductor  127   a  form a variable inner capacitor  128   a , which is characterized by an inner capacitance C inner . Similarly, diaphragm  160  and outer conductor  127   b  form a variable outer capacitor  128   b , which is characterized by an outer capacitance C outer .  
           [0007]    The capacitance of each of the variable capacitors is determined, in part, by the distance d between the diaphragm and the relevant conductor. More specifically, as is well known, C=Ae r e 0 /d, where C is the capacitance between two parallel conductive plates, A is the common area between the plates, e 0  is the permittivity of a vacuum, er is the relative permittivity of the material separating the plates (e 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).  
           [0008]    As diaphragm  160  flexes in response to changes in the differential pressure between chambers  120 ,  130 , the capacitances of the variable capacitors  128   a ,  128   b  change and thereby provide an indication of the differential pressure.  
           [0009]    A reference pressure, which may be near vacuum, is normally provided in chamber  120 , inlet tube  104  is connected via coupling  112  to a gas line  110  containing gas  111 , and transducer  100  provides an electrical output signal indicative of the pressure of gas  111 . In other configurations, a second inlet tube leading into chamber  120  may be provided and connected to a second external source. In such configurations, transducer  100  provides a signal indicative of the differential pressure between the two external sources. Transducers will be discussed herein in the context of measuring the pressure of gas or fluid  111 , but it will be appreciated that they can also be used as differential pressure transducers.  
           [0010]    A capacitive pressure transducer can be built using only a single conductor and a single variable capacitor. However, the output signals generated by such transducers have the undesirable characteristic of varying in response to “planar shifts” of the diaphragm. Such planar shifts can be caused by factors independent of the pressure of gas  111 , such as temperature variations in the ambient environment of the transducer. Different rates of thermal expansion in different parts of the transducer can cause changes in the distance between the diaphragm and the electrode. As is well known, the accuracy and stability of a transducer may be improved by including two variable capacitors in the transducer and by generating the transducer&#39;s output signal according to a function of the difference of the two capacitors (e.g., a function of C inner  minus C outer ). When the pressure of gas  111  increases, diaphragm  160  flexes, or bows, so that the central portion of diaphragm  160  moves closer to electrode  122  than do the outer portions of the diaphragm. This causes both the inner and outer capacitances to change, but the inner capacitance changes by a greater amount. The delta between the inner and outer capacitances gives a good indication of the pressure of gas  111 . However, if the entire diaphragm  160  moves in a direction normal to the diaphragm, either closer to, or further away from, electrode  122  (i.e., if the diaphragm  160  experiences a “planar shift”), the inner and outer capacitance will change by the same amount (as long as the areas of the inner and outer conductors are equal), and the output signal (which is based in the difference between the two capacitances) will be unaffected. Thus, including two variable capacitors can advantageously render the transducer insensitive to planar shifts of the diaphragm.  
           [0011]    As noted above, contaminants (e.g., produced by etching aluminum) are often contained in the gas  111 . When such contaminants become deposited on diaphragm  160 , they can adversely affect the accuracy of transducer  100 . The most common problem caused by contaminant deposition is generally referred to as a “zero shift”. The output signal generated by transducer  100  generally lies in a range between some minimum and maximum values. For example, one popular choice is for the transducer&#39;s output signal to be an analog signal that ranges between zero and ten volts, zero volts representing the minimum limit of pressure detectable by the transducer, ten volts representing the maximum pressure detectable by the transducer, and the signal varying linearly with pressure between zero and ten volts. Electronics (not shown), normally disposed in the transducer outside of chambers  120 ,  130 , normally generate this output signal. When a transducer experiences a zero shift, it will no longer generate an output signal equal to zero volts when the pressure of gas  111  is at the minimum limit of detectable pressure. Rather, when the gas pressure is at this minimum limit, the transducer will generate a non-zero output signal. In an effort to reduce zero shifts and other problems caused by contaminant deposition, prior art transducers have used a variety of filters to prevent contaminants from becoming deposited on diaphragm  160 .  
           [0012]    In the illustrated transducer  100 , the contaminant filtration mechanisms  108  include a particle trap system  140  and a baffle  150 . Trap system  140  includes a baffle  141 , a top view of which is shown in FIG. 2. Baffle  141  includes a central, circular, closed portion  142  and an annular region, defining a plurality of openings  144 , disposed around closed portion  142 . Openings  144  are formed as series of sectors evenly spaced about the baffle  141  in a circumferential direction, and are also arranged at different diameters radially. The diameter of central portion  142  is greater than that of inlet tube  104  and thereby blocks any direct paths from inlet tube  104  to the diaphragm  160 . So, any contaminant in inlet tube  104  can not follow a straight line path all the way to diaphragm  160  and must instead, after traveling the length of inlet tube  104 , then travel in a direction generally perpendicular to the length of inlet tube  104  (the perpendicular direction being generally illustrated in FIG. 1B by the arrow L), enter an annular chamber region  146 , and then pass through one of the peripheral openings  144 . The peripheral openings  144  are sized to prevent relatively large particles (e.g., 250 microns and larger) from passing through the openings. Trap system  140  also includes the chamber  146 , which is defined between baffle  141  and housing member  102   a . Particles that can&#39;t pass through openings  144  tend to accumulate in, or become trapped in, chamber  146 .  
           [0013]    As noted above, transducer  100  also includes a baffle  150  to further reduce the number of contaminants that can reach the diaphragm  160 . Baffle  150  is described in U.S. Pat. No. 6,443,015. FIG. 3 shows a top view of baffle  150 . As shown, baffle  150  is essentially a circular metal plate with a plurality of evenly spaced tabs  152  disposed about the circumference. Housing member  102   a  has stepped regions that come in contact with tabs  152  so as to support baffle  150  in the position shown in FIG. 1B.  
           [0014]    Tabs  152  essentially define a plurality of annular sectors  154  (shown in FIGS. 1B and 3) having a width in the radial direction between the peripheral edge of baffle  150  and housing member  102   a  that is determined by the length of the tabs. Baffle  150  and housing member  102   a  define a region  158  through which any contaminant must flow if it is to travel from inlet tube  104  to diaphragm  160 . The region  158  is annular and is bounded above by baffle  150  and below by either baffle  141  or lower housing member  102   a  (where the terms “above” and “below” are with reference to FIG. 1B, but do not imply any absolute orientation of transducer  100 ). Contaminants may enter region  158  via the peripheral openings  144  and may exit region  158  via the annular sectors  154  (shown in FIGS. 1B and 3) between the peripheral edge of baffle  150  and housing member  102   a.    
           [0015]    Region  158  is characterized by a length L and a gap g. The length L of region  158  (shown in FIG. 1B) is the distance between openings  144  to annular sectors  154 . The gap g of region  154  is the distance between baffle  150  and housing member  102   a . The aspect ratio of region  158  is defined as the ratio of the length L to the gap g. As taught in U.S. Pat. No. 6,443,015, the aspect ratio is preferably greater than 10. The length L is preferably at least 1 cm, and preferably in the range of about 1-4 cm; the gap g is preferably no more than about 0.1 cm, and preferably in a range of about 0.025-0.1 cm.  
           [0016]    When the pressure in chamber  130  is relatively low (e.g., less than 0.02 Torr), movement of material in chamber  130  is characterized by “molecular flow”. In molecular flow, molecules in chamber  130  generally travel in straight line paths until colliding with a solid surface of the transducer. This stands in contrast to behavior in denser gasses in which molecules are unlikely to travel in straight line paths from one surface of the transducer to another and are instead far more likely to rebound off of each other. Under molecular flow conditions, any contaminant traveling through region  158  will likely collide with the surfaces of baffle  150  and housing member  102   a  many times prior to reaching, and passing through, an annular sector  154 . The probability that a contaminant particle will become deposited on, or stuck to, a surface of baffle  150  or housing member  102   a  rather than continuing on through region  158  and passing through an annular sector  154  is an increasing function of the number of collisions the contaminant makes with the surfaces of baffle  150  and housing member  102   a . Selecting the aspect ratio of the length L to the gap g to be greater than 10 ensures that any contaminant traveling through region  158  is likely to become deposited on a surface of either baffle  150  or housing member  102   a  rather than continuing on through region  158 , passing through an annular sector  154 , and ultimately reaching the diaphragm  160 .  
           [0017]    The use of trap system  140  and baffle  150  has been effective at greatly reducing the number of contaminants that reach the diaphragm  160  and in reducing corresponding zero shifts. However, it would nonetheless be advantageous to provide improved control over deposition of contaminants on the diaphragm of a capacitive pressure transducer.  
         SUMMARY OF THE INVENTION  
         [0018]    Prior art filtration techniques attempted to prevent, or reduce as much as possible, contaminants from reaching the diaphragm of capacitive pressure transducers. The present invention provides an alternate approach. Instead of eliminating contaminants, a baffle constructed according to the invention steers contaminants to the diaphragm in an predetermined pattern-so as to minimize the effect that such contaminants can have on performance of the transducer.  
           [0019]    In a transducer with two variable capacitors, contaminants that settle on the diaphragm near a particular conductor, tend to affect the capacitance of that conductor&#39;s associated variable capacitor more than that of the other variable capacitor. Transducers generally generate an output signal according to a function of the difference between the capacitances of two variable capacitors. The effect that contaminants have on the output signal can therefore be minimized by assuring that the contaminants have an equal, or nearly equal, effect on both of the variable capacitors. Baffles constructed according to the invention provide control over where on the diaphragm contaminants may settle and can be configured, for example, to ensure that the amount of contaminants settling on the diaphragm near one conductor are about equal to the amount of contaminants settling on the diaphragm near the other conductor.  
           [0020]    Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]    [0021]FIG. 1A is a sectional side view of a prior art capacitive pressure transducer.  
         [0022]    [0022]FIG. 1B is a more detailed sectional side view of a prior art capacitive pressure transducer.  
         [0023]    [0023]FIG. 1C shows the pattern of conductors deposited onto the bottom of the electrode shown in FIG. 1B.  
         [0024]    [0024]FIG. 2 is a top view of the prior art baffle of the trap system shown in FIG. 1B.  
         [0025]    [0025]FIG. 3 is a top view of the baffle of the transducer show in FIG. 1B.  
         [0026]    [0026]FIG. 4A is a sectional side view of a capacitive pressure transducer constructed in accordance with the present invention.  
         [0027]    [0027]FIG. 4B is an expanded sectional side view of the electrode and diaphragm of the transducer shown in FIG. 4A.  
         [0028]    [0028]FIG. 5 shows a baffle constructed according to the invention aligned with the conductors on the electrode.  
         [0029]    [0029]FIG. 6A shows one embodiment of a baffle constructed according to the invention.  
         [0030]    [0030]FIG. 6B shows the baffle shown in FIG. 6A aligned with the conductors of a pressure transducer constructed according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    [0031]FIG. 4A shows a sectional side view of a capacitive pressure transducer  200  constructed according to the present invention. In addition to the components shown (in FIG. 1B) in prior art pressure transducer  100 , transducer  200  also contains a deposition controlling baffle  250  disposed in internal chamber  130  between diaphragm  160  and baffle  150 . Material (e.g., gas molecules or particle contaminants) entering pressure transducer  200  enters through gas line  110  and passes through trap system  140 , baffle  150 , and finally through deposition controlling baffle  250  before contacting diaphragm  160 . As will be further explained below, baffle  250  redirects the flow of contaminants within chamber  130  so as to control zero shifts within pressure transducer  200 .  
         [0032]    [0032]FIG. 4B shows an expanded view of a portion of transducer  200 . More specifically, FIG. 4B shows an expanded sectional side view of electrode  122  and diaphragm  160 . As shown, diaphragm  160  may be thought of as being segmented into three different regions based on proximity to the inner conductor  127   a  and the outer conductor  127   b . An inner region I of diaphragm  160  is proximal to the inner conductor  127   a . A middle region M of diaphragm  160  is proximal to outer conductor  127   b . Finally, an outer region O of diaphragm  160  lies outside of the outer conductor  127   b . There is some flexibility in defining the extent of regions I, M, and O. In FIG. 4B, the boundaries of the inner and middle regions I, M are determined by lines extending from the edges of the conductors in a direction normal to the conductors. Alternatively, regions I and M could be described as “underlying”, the inner and outer conductors, respectively, and region C could be described as extending around and not underlying the outer conductor (the word “underlying” being with reference to the orientation shown in FIGS. 4A and 4B and not implying any absolute orientation of the transducer). However, it will be appreciated that the regions need not be defined with such precision. The inner and middle regions I, M may be thought of as simply being proximal to the inner and outer conductors, respectively, and outer region O may be thought of as lying outside of middle region M. Outer region O may alternatively be thought of as lying outside the “active area” of the diaphragm since this region of the diaphragm does not contribute principally to the capacitance of the inner and outer capacitors.  
         [0033]    Deposition of contaminants in inner region I tends to affect the capacitance of the inner capacitor  128   a  (i.e., the capacitor defined by diaphragm  160  and the inner conductor  127   a ). Deposition of contaminants in middle region M tends to affect the capacitance of the outer capacitor  128   b  (i.e., the capacitor defined by diaphragm  1160  and the outer conductor  127   b ). Deposition of contaminants in outer region O does not significantly affect the capacitance of either of the variable capacitors  128   a ,  128   b . Contaminant deposition in inner region I may either increase or decrease the capacitance of inner capacitor  128   a . Similarly, contaminant deposition in the middle region M may either increase or decrease the capacitance of outer capacitor  128   b . Whether contaminant deposition increases or decreases the capacitance of the relevant variable capacitor depends on factors such as surface tension in the deposited contaminant layer, composition of the contaminants, etc. Some processes, such as etching of aluminum, may cause contaminant deposition that increases capacitance. Other processes tend to cause contaminant deposition that decreases capacitance. Unless explicitly specified, the discussion below will assume that the contaminants are of the variety that cause increases in capacitance.  
         [0034]    Returning to FIG. 1B, when prior art transducer  100  is operated in contaminant rich environments, baffle  150  tends to ensure that most contaminants that reach, and become deposited on, diaphragm  160 , are deposited in outer region O of the diaphragm. Contaminants deposited in outer region O (or outside the active region) do not affect the capacitance of the variable capacitors as much as contaminant deposition in regions I and M. However, over time baffle  150  permits sufficient contaminants to reach middle region M so as to increase the capacitance of the outer capacitor  128   b . When this occurs, the transducer  100  experiences a “negative zero shift”. Since the transducer&#39;s output signal is generated according to a function of the inner capacitance minus the outer capacitance, an artificially caused increase of the outer capacitor (i.e., an increase caused by factors other than pressure variations in gas  111  such as contaminant deposition) tends to reduce the value of the output signal generated by transducer  100  in response to any given gas pressure. When the pressure of gas  111  is at the minimum limit measurable by transducer  100 , and when the output signal generated by transducer  100  should be zero volts, a contaminant induced increase in the outer capacitance causes transducer  100  to generate an output signal that is below zero volts, or an output signal that is “negative zero shifted”.  
         [0035]    Negative zero shifts represent a potentially serious problem for many users of transducer  100 . Transducer  100  normally generates an analog output signal representative of the pressure of gas  111 . The analog output signal can generally assume values below the expected minimum value (e.g., it can assume negative values when zero volts is the expected minimum value), and it can also assume values above the expected maximum value. However, many users of transducer  100  read the transducer&#39;s output signal via an analog-to-digital converter that is incapable of producing output values below an expected minimum. For example, if the expected minimum value for the output signal is zero volts, many analog-to-digital converters will translate a negative analog output signal (i.e., a signal below zero volts) into a digital zero, thus rendering a negative zero shift invisible, or undetectable.  
         [0036]    As noted above, in transducer  200 , baffle  250  redirects the flow of contaminants within chamber  130  so as to control zero shifts within pressure transducer  200 . Baffle  250  tends to eliminate zero shifts, and to the extent that zero shifts occur, baffle  250  tends to ensure that the zero shifts are “positive zero shifts” instead of “negative zero shifts”. Since positive zero shifts (e.g., shifts in which the transducer&#39;s output signal is above an expected minimum value when the pressure of gas  111  is at the minimum detectable limit) are generally more easily detectable by users of transducer  100 , and can be dealt with by recalibration, it is advantageous to ensure that any zero shifts occurring in transducer  200  are positive rather than negative.  
         [0037]    [0037]FIG. 5 shows a view of deposition controlling baffle  250  aligned with the inner and outer conductors  127   a ,  127   b  taken in the direction of arrow  5 - 5  as shown in FIG. 4B. For convenience of illustration, diaphragm  160 , which is disposed between baffle  250  and the inner and outer conductors  127   a ,  127   b , is not shown in FIG. 5. As shown, deposition controlling baffle  250  is divided up into three regions: an inner region I, and middle region M, and an outer region O. The inner region I of baffle  250  lies inside dashed circle  262 . The middle region M of baffle  250  lies between dashed circles  264  and  262 . The outer region O of baffle  250  lies outside of dashed circle  264 . Apertures (not shown in FIG. 5) are provided in the inner region I, the middle region M, and the outer region O of baffle  250 .  
         [0038]    The baffle  250  is disposed proximal to diaphragm  160  such that: (a) the majority of contaminants passing through apertures in the inner region I of baffle  250  tend to deposit on the inner region I of the diaphragm; (b) the majority of contaminants passing through apertures in the middle region M of baffle  250  tend to deposit on the middle region M of the diaphragm; and (c) the majority of contaminants passing through apertures in the outer region O of the baffle  250  tend to deposit on the outer region O of the diaphragm. Due to the random motion of particles in a gas (even a low pressure gas characterized by molecular flow), some contaminants passing through apertures in the inner region I of the baffle  250  will become deposited on the diaphragm outside of the inner region I. Similarly, some of the contaminants passing through the middle region M and the outer region O of baffle  250  will become deposited on the diaphragm  160  outside of the middle region M and the outer region O, respectively, of the diaphragm  160 . However, since the majority of particles passing through any particular region of baffle  250  become deposited in a corresponding region of the diaphragm, baffle  250  provides control over the location of contaminant deposition as contaminants accumulate on the diaphragm.  
         [0039]    Prior art filtration techniques attempted to prevent all, or as many as possible, contaminants from reaching the diaphragm  160 . Deposition controlling baffle  250  uses a different strategy. Instead of relying on eliminating or trapping contaminants, deposition controlling baffle  250  instead controls where on the diaphragm contaminants will become deposited. Deposition controlling baffle  250  provides a degree of control not previously available in the prior art.  
         [0040]    The apertures of deposition controlling baffle  250  may be arranged in a variety of patterns to achieve desired affects. For example, in one configuration, the total area of all the apertures in the inner region I of baffle  250  is selected to be exactly equal to the total area of all the apertures in the middle region M of baffle  250 . In theory, such a selection of aperture areas (and the corresponding balancing of contaminant deposition) will prevent the transducer  200  from ever experiencing a zero shift. This is so because any increase in the capacitance of the inner capacitor  128   a  (caused by deposition of contaminants passing through inner region I of the baffle  250 ) will be exactly matched by a corresponding increase in the capacitance of the outer capacitor  128   b  (caused by deposition of contaminants passing through middle region M of the baffle).  
         [0041]    However, since it may be difficult to achieve a perfect balancing of contaminant deposition, and since positive zero shifts are preferred over negative zero shifts, it is preferable to make the total area of all the apertures in inner region I of the baffle  250  to be slightly larger than the total area of all the apertures in middle region M of the baffle  250 . Such a selection of aperture areas tends to minimize zero shifts (since the amount of contaminants that reach inner region I of the diaphragm will be roughly equal to the amount of contaminants that reach middle region M of the diaphragm), but also ensures that any zero shifts that do occur will be positive rather than negative (i.e., because the amount of contaminants reaching inner region I of the diaphragm will be slightly larger than the amount of contaminants reaching middle region M of the diaphragm).  
         [0042]    It is also preferable to steer some of the contaminants to outer region O of the diaphragm (where they theoretically have only a minimal effect on zero shifts) by providing apertures in outer region O of the baffle  250 . In theory, transducer  200  could avoid zero shifts entirely by providing all the apertures of baffle  250  in outer region O (and thereby steering all contaminants to outer region O of the diaphragm). However, since some of the contaminants passing through outer region O of the baffle  250  will inevitably become deposited in regions of the diaphragm other than the outer region O, such a configuration would be likely to eventually produce a zero shift. Accordingly, in the most preferred embodiments, apertures are provided in all three regions I, M, and O of baffle  250 . In one embodiment, the outer region O contains forty percent of the total area of apertures in the baffle  250 , the middle region M contains twenty nine percent of the total area of apertures in the baffle  250 , and the inner region I contains the remaining thirty one percent of the total area of apertures in the baffle. It will be appreciated that considerable variation of these numbers is possible. However, it is generally preferred for the total area of apertures in the inner region I to be slightly larger than the total area of apertures in the middle region, and it is further preferred for the outer region O to contain a non-zero portion of the total aperture area. The desired configuration of apertures in deposition controlling baffle  250  may alternately be expressed in terms of the I:M ratio, or the ratio of the total area of apertures in the inner region I of baffle  250  to the total area of apertures in the middle region M of baffle  250 . The I:M ratio is preferably nearly equal to one, but slightly greater than one. And again, the outer region O of baffle  250  preferably includes a non-zero portion of the total aperture area in baffle  250 .  
         [0043]    [0043]FIG. 6A shows a view of one embodiment of deposition controlling baffle  250  taken in the direction of arrow  5 - 5  as shown in FIG. 4B. In this embodiment, baffle  250  defines a plurality of apertures  252 , and the apertures  252  are arranged in five concentric rings  253 ,  254 ,  255 ,  256 ,  257 . The apertures  252  are arc shaped such that all the apertures  252  in any given ring form a ring-like, or annular perforated area. Adjacent apertures  252  are separated by ribs  258  that extend radially (or spoke like) through baffle  250 . The ribs  258  are formed by portions of baffle  250  that have not been perforated to define the apertures  252 .  
         [0044]    [0044]FIG. 6B, which is again taken in the direction of the arrow  5 - 5  shown in FIG. 4B, shows the alignment of baffle  250  and the inner and outer conductors  127   a ,  127   b , the conductors being shown with cross-hatching. For convenience of illustration, diaphragm  160 , which is disposed between baffle  250  and the inner and outer conductors  127   a ,  127   b , is not shown in FIG. 6B. As shown, the three innermost rings  253 ,  254 ,  255  underlie the inner conductor  127   a ; the fourth ring  256  is centered under the outer conductor  127   b ; and the outermost ring  257  is disposed outside of the outer conductor  127   b . Referring again to FIG. 4B, it will be appreciated that most contaminants that pass through rings  253 ,  254 ,  255  will become deposited in inner region I of the diaphragm  160 ; most contaminants that pass through ring  256  will become deposited in middle region M of the diaphragm  160 ; and most contaminants that pass through ring  257  will become deposited in outer region O of the diaphragm  160 .  
         [0045]    Deposition controlling baffle  250  is preferably welded to shoulder  148  (shown in FIG. 4A) of lower housing  102   a . Baffle  250  may be spot welded at locations  259  (some of which are indicated in FIG. 6A) that are at the outer periphery of baffle  250  and are evenly spaced between adjacent ribs  258 . Baffle  250  and housing  102  may be made from the same metal (e.g., Inconel). The gap between baffle  250  and diaphragm  160  may be for example 0.03 centimeters. Baffle  250  is preferably used in combination with baffles  150 ,  140 . However, baffle  250  may also be used as the sole mechanism for controlling contaminant deposition.  
         [0046]    An alternative to the baffle configuration shown in FIGS. 6A and 6B is to fabricate deposition controlling baffle  250  from a sintered metal element such as the type commercially available from Pall Corporation of East Hills, N.Y. and also from Mott Corporation of Farmington, Conn. Such elements are mesh-like and define a plurality of very small apertures. Also, such elements may be fabricated in disk shapes so as to fit into transducer  200  as shown-generally in FIG. 4A at  250 . Each unit area of such an element defines substantially the same area of apertures as any other unit area. Since the area of inner conductor  127   a  is substantially equal to the area of outer conductor  127   b , using such a sintered metal element for baffle  250  essentially guarantees that the total area of apertures proximal to (or underlying) inner conductor  127   a  is substantially equal to the total area of apertures proximal to (or underlying) outer conductor  127   b . So, such a baffle again tends to reduce or eliminate zero shifts from the transducer even while allowing contaminants to pass through to the diaphragm.  
         [0047]    Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not a limiting sense. For example, transducers have been described herein as using an inner circular conductor and an outer annular conductor to form two variable capacitors. It will be appreciated that a variety of shapes for the conductors may be used, and also that more than two conductors may be used, in transducers constructed according to the invention. Further, a deposition controlling baffle may be constructed according to the invention to match any particular configuration of the conductors. As another example, referring to FIG. 5, the inner region I of baffle  250  is shown as having a greater area than that of the inner conductor  127   a ; and similarly, the middle region M of baffle  250  is shown as having a greater area than that of outer conductor  127   b . It will be appreciated that there is considerable flexibility in defining the boundaries of the inner, middle, and outer regions of baffle  250 . For example, the inner and middle regions I, M of baffle  250  can be defined as being coextensive with conductors  127   a ,  127   b , respectively, such that the entire inner region I exactly underlies inner conductor  127   a  and such that the entire middle region M exactly underlies the outer conductor  127   b . Alternatively, as suggested by FIG. 6B, the inner region I of baffle  250  can entirely underlie and be smaller than inner conductor  127   a . Similarly, the middle region M of baffle  250  can entirely underlie and be smaller than outer conductor  127   b . As yet another example, baffles having an I:M ratio of slightly greater than one have been discussed in the context of contaminants that cause an increase in capacitance. However, if transducers constructed according to the invention are used in environments in which contaminant deposition causes a decrease in capacitance, it can be desirable for the deposition controlling baffle to have an I:M ratio of slightly less than one (i.e., for the total area of apertures in the inner region I to be slightly less than the total area of apertures in the middle region M). Such a ratio will again tend to minimize zero shifts and to the extent that zero shifts occur, it will tend to ensure that the zero shifts are positive rather than negative. As yet another example, deposition controlling baffle  250  has been discussed as defining apertures in the inner region I, the middle region M, and the outer region O. However, deposition controlling baffles may be constructed and used in pressure transducers according to the invention that define apertures in only two out of three of the inner region I, the middle region M, and the outer region O. For example, such a deposition controlling baffle may define apertures in the inner region I and the outer region O and may define no apertures in the middle region M. Such a baffle can, for example, direct (a) most of the contaminants that pass through the baffle to the outer region O of the diaphragm, where they theoretically have little or no effect on the capacitance of any of the variable capacitors, and (b) a small portion of the contaminants that pass through the baffle to the inner region I of the diaphragm to ensure that any zero shift resulting from contaminant deposition is positive rather than negative. Similarly, a baffle that defines apertures in only the middle region M and the outer region O (and not in the inner region I) can be useful to ensure that any zero shifts resulting from contaminant deposition are positive rather than negative when the contaminants passing through the baffle are of the variety that lower, rather than raise, capacitance. Finally, baffles that define apertures in the inner region I and the middle region M, and do not define any apertures in the outer region O, may also be useful to for example balance all contaminant deposition in the active areas of the transducer. As yet another example, transducers have been discussed herein as having a conductive, or metallic, diaphragm  160 . However, it will be appreciated that the diaphragm itself need not be conductive and may instead be made from non-conductive material such as ceramic. When such a non-conductive diaphragm is used, a conductive film is disposed on the diaphragm and the variable capacitors are formed by the conductive film on the diaphragm and by the conductors disposed proximal to the diaphragm.