Abstract:
The disclosed pressure sensor includes a body, a diaphragm, and a flow defining structure. The body defines an interior volume. The diaphragm divides the interior volume into a first portion and a second portion. At least a first part of the diaphragm moves in a first direction when a pressure in the first portion increases relative to a pressure in the second portion. The first part of the diaphragm moves in a second direction when the pressure in the first portion decreases relative to the pressure in the second portion. The first part of the diaphragm and at least a first part of the body are characterized by a capacitance. The capacitance changes in response to movement of the first part of diaphragm relative to the first part of the body. The flow defining structure provides a fluid flow path from the first portion of the interior volume to a position outside of the internal volume. At least part of the fluid flow path extends from a first location to a second location. The at least part of the fluid flow path is characterized by a total length and a straight line distance. The total length is the shortest distance through the path from the first location to the second location. The straight line distance is the shortest distance between the first location and the second location. The total length is at least two times greater than the straight line distance.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a capacitive pressure sensor. More specifically, the present invention relates to an improved filter for use with a capacitive pressure sensor. 
     FIG. 1A  shows a sectional side view of a prior art ceramic capacitive pressure sensor  100 .  FIG. 1B  shows an exploded view of sensor  100 . Although sensors such as sensor  100  are well known, a brief description of its construction and operation will be provided. Sensor  100  includes a ceramic Pr body  102  (“Pr” representing “reference pressure”), a ceramic Px body  104  (“Px” representing “unknown pressure”), a thin, flexible ceramic diaphragm  106 , and an inlet tube  108 . As shown in  FIG. 1A , when sensor  100  is assembled, Pr body  102  and Px body  104  are bonded together such that diaphragm  106  is clamped between the Pr and Px bodies. Diaphragm  106  may flex or deform in response to the pressure in inlet tube  108 . Consequently, the pressure in tube  108  may be measured by detecting the position of diaphragm  106 . 
   Pr body  102  and Px body  104  are shaped so that when they are bonded together, they define an interior volume. Diaphragm  106  divides this interior volume into an upper chamber  122  and a lower chamber  124  (the terms “upper” and “lower” and similar terms are used herein with reference to the drawings and do not imply any absolute orientation of the sensor). When sensor  100  is assembled, diaphragm  106  and Pr body  102  cooperatively define upper chamber  122 , and diaphragm  106  and Px body  104  cooperatively define lower chamber  124 . Px body  104  defines a central aperture  126 . Inlet tube  108  also defines a central passageway  130 , and passageway  130  is in fluid communication with the central aperture  126  of the Px body. Thus, passageway  130  is in fluid communication with the lower chamber  124 . 
   Diaphragm  106  is a thin flexible ceramic disk onto which a conductive film  140  is deposited. Another conductive film  142  is deposited onto a central portion of Pr body  102  such that film  142  is spaced away from and opposite to the conductive film  140  on diaphragm  106 . The two conductive films  140 ,  142  form two plates of a variable capacitor  144 . As is well known, the capacitance provided by variable capacitor  144  varies with, among other things, the distance between the two plates  140 ,  142 . Sensor  100  also includes conductive pins  150 ,  152 . Pin  150  is electrically connected to the film  140  on diaphragm  106 , and pin  152  is electrically connected to the film  142  on the Pr body  102 . Pins  150  and  152  provide electrical connection to films  140  and  142 , respectively, external to the body of sensor  100 . 
   In operation, a reference pressure (e.g., vacuum) is established in the upper chamber  122  and the inlet tube is connected to a source of gas, the pressure of which is to be measured. Diaphragm  106  flexes, or deforms, in response to changes of pressure within the lower chamber, causing the capacitance provided by variable capacitor  144  to change in accordance with the pressure in inlet tube  108 . Accordingly, the capacitance provided by variable capacitor  144  is indicative of the pressure within inlet tube  108 . 
   As is well known, sensors such as sensor  100  often include additional features, which for convenience of illustration are not illustrated in  FIGS. 1A and 1B . For example, such sensors often include a getter for maintaining a vacuum in the upper chamber  122 . Also, such sensors often include two conductive films disposed on the Pr cover  102  instead of the single illustrated film  142 . As is well known, having two such films allows the sensor to provide two variable capacitors instead of one, and this in turn can be used to improve the temperature stability of the sensor. 
   Pressure sensors such as sensor  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 encountering the diaphragm  106 . When such contaminants build up on diaphragm  106 , the accuracy of the pressure measurement provided by sensor  100  is adversely affected. Accordingly, prior art pressure sensors have used a variety of mechanisms to prevent contaminants from reaching the diaphragm  106 . 
   Although many such filtering mechanisms have been developed, there remains a need for improved methods and structures for preventing contaminants from reaching and settling on the diaphragm. 
   SUMMARY OF THE INVENTION 
   These and other objects are provided by an improved pressure sensor. The pressure sensor includes a body, a diaphragm, and a flow defining structure. The body defines an interior volume. The diaphragm divides the interior volume into a first portion and a second portion. At least a first part of the diaphragm moves in a first direction when a pressure in the first portion increases relative to a pressure in the second portion. The first part of the diaphragm moves in a second direction when the pressure in the first portion decreases relative to the pressure in the second portion. The first part of the diaphragm and at least a first part of the body are characterized by a capacitance. The capacitance changes in response to movement of the first part of diaphragm relative to the first part of the body. The flow defining structure defines, at least in part, a fluid flow path from the first portion of the interior volume to a position outside of the interior volume. At least part of the fluid flow path extends from a first location to a second location. The at least part of the fluid flow path is characterized by a total length and a straight line distance. The total length is the shortest distance through the path from the first location to the second location. The straight line distance is the shortest distance between the first location and the second location. 
   In one aspect, the total length is at least five (5.0) times greater than the straight line distance. In another aspect, the flow defining structure is non-metallic. In another aspect, the at least part of the fluid flow path is curved. In yet another aspect, the flow defining structure is a single, monolithic, structure. 
   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 
     For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings in which the same reference numerals are used to indicate the same or similar parts wherein: 
       FIG. 1A  shows a sectional side view of a prior art ceramic capacitive pressure sensor. 
       FIG. 1B  shows an exploded view of the sensor shown in  FIG. 1A . 
       FIG. 2A  shows a sectional side view of a ceramic capacitive pressure sensor constructed according to the invention. 
       FIG. 2B  shows an exploded view of the sensor shown in  FIG. 2A . 
       FIG. 3A  shows a side view of a turbo sump constructed according to the invention. 
       FIG. 3B  shows a view of the turbo sump taken from the direction indicated by the line  3 B— 3 B as shown in  FIG. 3A . 
       FIG. 3C  shows a perspective view of another turbo sump constructed according to the invention. 
       FIG. 4  shows a magnified view of a turbo sump, a portion of the Px body, and a portion of the inlet tube, of the sensor shown in  FIGS. 2A and 2B . 
       FIG. 4A  shows a view of the same structure shown in  FIG. 4 , without reference characters and with some dimension lines. 
       FIG. 5  shows an abstracted view of a helical channel formed by a sensor constructed according to the invention. 
       FIG. 6  shows an abstracted view of a non-helical channel formed by a sensor constructed according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2A  shows an assembled ceramic capacitive pressure sensor  200  constructed according to the invention.  FIG. 2B  shows an exploded view of sensor  200 . Like prior art sensor  100  ( FIGS. 1A and 1B ), improved sensor  200  includes a Pr body  102 , a diaphragm  106 , an inlet tube  108 , and conductive pins  150 ,  152 . However, unlike prior art sensor  100 , improved sensor  200  also includes a turbo sump  260  and a modified Px body  204 . Turbo sump  260  is disposed within the central aperture  226  defined by the Px body  204 . 
   As will be discussed in greater detail below, turbo sump  260  provides a filtering function. That is, turbo sump  260  filters particles and contaminants and reduces the amount of particles and contaminants that can reach diaphragm  106 . 
     FIG. 3A  shows a magnified view of turbo sump  260  taken from the same vantage as  FIGS. 2A and 2B .  FIG. 3B  shows a view of turbo sump  260  taken from the direction indicated by the line  3 B— 3 B as shown in  FIG. 3A . As shown, tubo sump  260  resembles a threaded screw and includes a central post  310  and a helical thread  320 . The interior portion of thread  320  is attached to the exterior curved surface of central post  310  in the same manner that threads are attached to the central portion of a screw. Turbo sump  260  also defines a disc shaped base  340 . The bottom of central post  310  is attached to a central portion of base  340 , and the bottom of thread  320  merges into base  340 . As shown in  FIGS. 3A and 3B , base  340  defines a plurality of apertures  342 . Turbo sump  260  is preferably made from a single, monolithic, piece of ceramic. 
     FIG. 4  shows a magnified view of turbo sump  260  disposed in the central aperture  226  of the Px body  204  of sensor  200 . As shown, central aperture  226  of Px body  204  is defined by an inner wall  228  of the Px body  204 . When turbo sump  260  is installed in sensor  200 , the outer edges of thread  320  fit closely to the inner wall  228 , but do not make contact. The small gap G between the outer edge of thread  320  and the inner wall  228  is shown best in  FIG. 4A . The solid central post  310  of turbo sump  260  occupies the central portion of aperture  226 . Thus, the turbo sump  260  and the Px body  204  cooperate to define a helical channel  350  that extends from the bottom to the top of the central aperture  226 . Apertures  342  in the base  340  of turbo sump  260  provide fluid communication between the channel  130  defined by inlet tube  108  and the helical channel  350 . The top of helical channel  350  opens into the lower chamber  124 , which is defined by the bottom of diaphragm  106  and the upper surface of the Px body  204 . Thus, helical channel  350  provides fluid communication between channel  130  (defined by inlet tube  108 ) and the diaphragm  106 . Particles or contaminants traveling from channel  130  towards diaphragm  106  pass through apertures  342  and helical channel  350  before reaching the diaphragm  106 . 
   The apertures  342  defined by the base  340  of turbo sump  260  are configured to preclude particles of a selected size from being able to travel from channel  130  into helical channel  350 . That is, apertures  342  act as a filter that prevents particles that are too big from entering channel  350 . As shown in  FIGS. 3A and 3B , the apertures  342  are generally elongated and are characterized by a long dimension L ( FIG. 3B ) and a shorter dimension W ( FIG. 3A ). In one preferred embodiment, the shorter dimension W is 0.010 inches, and the longer dimension L is between 0.0042 and 0.063 inches. Since most of the particles found in the gas, the pressure of which is being measured, are generally spherical, the apertures  342  filter out particles characterized by a radius greater than or equal to W. It will be appreciated that the apertures  342  may be configured in a similar manner as the apertures in the baffle shown at FIG. 4 of U.S. Pat. No. 5,811,685 (entitled FLUID PRESSURE SENSOR WITH CONTAMINANT EXCLUSION SYSTEM, and which is assigned to the assignee of the present invention). 
     FIG. 3B  shows the apertures  342  being disposed in four groups around approximately one third of the area of base  340 . That is, in the embodiment illustrated in  FIG. 3B , no apertures are defined in approximately two thirds of base  340 . However, as shown, for example, in  FIG. 3C , in other embodiments, the apertures  342  may be distributed over the entire base  340 . 
   Sensor  200  can be used to measure low fluid pressures (e.g., less than 0.02 Torr). When the pressure in channel  350  is below about 0.02 Torr, movement of material in channel  350  is characterized by “molecular flow”. With reference to  FIG. 4 , in molecular flow, molecules in channel  350  generally travel in straight-line paths until colliding with a solid surface of the sensor (e.g., wall  228 , a wall of thread  320 , or the exterior wall of central post  310 ). 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 sensor to another and are instead far more likely to rebound off of each other. Under molecular flow conditions, any contaminant traveling through channel  350  will likely collide with the surfaces that define channel  350  (wall  228 , walls of thread  320 , or a wall of central post  310 ) many times prior to passing through channel  350  and reaching diaphragm  106 . The probability that a contaminant particle will become deposited on, or stuck to, a surface of sensor  200  rather than continuing on through channel  350  and into chamber  124  is an increasing function of the number of collisions the contaminant makes with the surfaces of sensor  200 . The helical shape of channel  350  insures that contaminants passing from channel  130  towards chamber  124  will collide with the surfaces of sensor  200  (that define channel  350 ) many times before it can reach chamber  124 . This significantly reduces the likelihood that any contaminant can actually pass through channel  350  and reach the diaphragm  106 . 
   The path taken by a molecule passing from channel  130 , through helical channel  350 , to chamber  124  is shown generally in  FIG. 4  by arrows  360 ,  370 . Arrows  360  illustrate molecules traveling from channel  130  through apertures  342  into helical channel  350 . Arrows  370  show the helical path generally followed by molecules traveling through helical channel  350  to chamber  124 . However, it will be appreciated that arrows  370  represent only the general, or average, path taken by such molecules. Since molecules in a molecular flow regime travel in straight line paths, they require many, many straight line paths and collisions to achieve the average flow shown by curved arrow  370 . 
   As noted above, in addition to defining helical channel  350 , the sump  260  and the Px body  204  also define a small gap G (shown best in  FIG. 4A ) between the outer edge of thread  320  and inner wall  228 . This gap G is provided to facilitate assembly of the sensor  200  (i.e., to facilitate inserting the relatively brittle ceramic sump  260  into the aperture  226  defined in the Px body  204 ). In theory, molecules traveling from channel  130  towards chamber  124  can follow the path through helical channel  350  (indicated generally by arrows  370 ) or can take “short cuts” by passing through one of the gaps G between the outer edge of thread  320  and inner wall  228 . It will be appreciated however, that the small gap G between the inner wall  228  and the outer edge of the thread  320  is much smaller than channel  350 . The conductance through this gap G is therefore much smaller than that of channel  350  almost eliminating particle and molecular flow through the gap G. Also, any contaminant that actually enters the gap G is likely to rebound off the surfaces of thread  320  and wall  228  many, many times while in the gap G and thereby become stuck in the gap G (i.e., become deposited on one of the surfaces defining the gap G). Accordingly, if sensor  200  is used to measure the pressure of a contaminant containing gas or fluid, the gap G will likely eventually become plugged, or sealed off, by contaminants that have become stuck in the gap G. 
   The gap G can be eliminated or reduced during assembly, for example, by providing a glass seal between the outer edge of thread  320  and the inner wall  228 . However, the presence of gap G does not degrade the performance of sump  260  or sensor  200 , and it is therefore considered unnecessary to remove the gap G. 
   By insuring that any contaminant must collide with the surfaces of sensor  200  many times before the contaminant can reach the diaphragm, the turbo sump  260  (and the helical channel  350  formed by sump  260 ) provides a function similar to that of the chamber described in U.S. Pat. No. 6,443,015 (entitled BAFFLE FOR A CAPACITIVE PRESSURE SENSOR, and which is assigned to the assignee of the present invention), which is characterized by a high aspect ratio of length to width. However, turbo sump  260  provides this function in a more compact geometry and advantageously assists in production of very small, compact, pressure sensors. Referring to  FIG. 4A , in one preferred embodiment, the outer diameter D 1  of the turbo sump  260  is 0.248 inches, the diameter of aperture  226  defined by Px body is 0.248 inches, and the gap G is on average 0.001 inches. In this embodiment, the outer diameter D 3  of the base  340  is 0.29 inches, the height H 1  of the central post  310  is 0.16 inches, the total height H 2  of the sump  260  is 0.18 inches, and the height H 3  of the base  340  is 0.02 inches. Also in this embodiment, the outer diameter D 4  of sensor  200  ( FIG. 2B ) is 1.500 inches and the height H 4  of sensor  200  ( FIG. 2A ) is 0.400 inches. 
   In summary, turbo sump  260  provides two distinct types of mechanical filtering. First, apertures  342  prevent particles of a certain size from entering channel  350 . Second, the configuration of channel  350  prevents many of the contaminants that enter channel  350  (which are small enough to pass through apertures  342 ) from ever reaching the diaphragm  106 . 
   In addition to the mechanical filtering functions described above, turbo sump  260  also provides a thermal filtering function. Sensor  200  can be used to measure the pressure of hot gasses or fluids (e.g., 200 degrees Celsius). Sensor  200  can be heated so that the sensor is at or near the temperature of the gas, the pressure of which is being measured. Heating sensor  200  can reduce the amount of condensation that forms on interior surfaces of sensor  200  and can also improve the accuracy of pressure measurements provided by sensor  200 . In operation, inlet tube  108  is generally connected to a source of gas, the pressure of which is to be measured. The gas source can be, for example, a pipe, valve, or chamber. When the gas, the pressure of which is being measured, is at a high temperature, the gas source to which inlet tube  108  is connected can appear to sensor  200  as a source of thermal radiation. Turbo sump  260  blocks the line of sight path from the gas source to the diaphragm  106  and thereby provides a filter for thermal radiation. That is, turbo sump  260  prevents thermal radiation emitted from the gas source from being directly incident on the diaphragm  206 . 
   Turbo sump  260  has been described within the context of a ceramic capacitive pressure sensor. However, it will be appreciated that turbo sump  260  can be used in other types of sensors as well. For example, turbo sump  260  can be made of metal and used in metallic sensors. It will be appreciated that, since metal is less brittle than ceramic, in such sensors it is relatively easier to eliminate or reduce the gap G between the outer edge of the thread and the inner wall of the aperture  226 . For example, in such sensors the outer diameters of the sump can be made slightly larger than the diameter of the aperture within which the sump fits, and the (larger) sump can be press fit into the (smaller) aperture. 
   Also, turbo sump  260  has been described as having a helical thread  320  (which in turn creates a helical channel  350 ). However, it will be appreciated that neither the thread  320  nor the channel  350  must be perfectly helical. As long as the channel  350  formed by the turbo sump and the Px body is circuitous or serpentine, the sump will provide the desired contaminant filtering function (by insuring that a contaminant must contact surfaces of the sensor many times before the contaminant can reach the diaphragm, at least when the pressure within the channel is low enough to provide for molecular flow).  FIG. 5  shows an abstracted view of a helical channel  500 . Channel  500  has an inlet  502  and an outlet  504 . It will be appreciated that channel  500  as shown in  FIG. 5  is an abstracted representation of the channel  350  (shown, e.g., in  FIG. 4 ). That is, inlet  502  corresponds to apertures  342  and outlet  504  corresponds to the junction of channel  350  and the chamber  124 .  FIG. 6  shows an abstracted view of another circuitous channel  510 , that has an inlet  512  and an outlet  514 . Channel  510  is not helical, but it is circuitous, or serpentine. A sump constructed according to the invention could provide a channel shaped as shown in  FIG. 6  instead of a helical channel. Although channels  500  and  510  look quite different, they share some important common features. Neither channel provides a straight line path from the inlet to the outlet. Rather, any particle traveling through either channel  500  or  510 , will change direction several times before it can travel from the inlet to the outlet. Both channels  500 ,  510  are characterized by a total length LT (i.e., the length that extends through the channel from the inlet to the outlet) and a straight line distance DS between the inlet and the outlet. In both channels, the total length length LT of the channel is significantly larger than the straight line distance DS. 
   Although sensors using channels  500  or  510 , or other circuitous channels, may be constructed according to the invention, the helical channels  500 , or  350 , may be optimal. This is because in any channel that has no straight portions and is instead constantly curving (such as in a helical channel), there is no significant portion of the length of the channel that can be traversed by a single straight line path. On the other hand, channels such as channel  510  do include sections that could be traversed by a single straight line path. For example, channel  510  could, at least in theory, be traversed by a molecule in a molecular flow regime that made only about twenty collisions (i.e., one collision for every right angle in the channel). On the other hand, many more collisions would be required for a molecule, flowing in a molecular flow regime, to traverse a constantly curving channel of similar total length. Also, of all the constantly curving channels, a helical channel is the most geometrically compact for any given total length. Accordingly, helical shaped channels may be optimal. 
   In channels constructed according to the invention, the total length of the channel LT is preferably at least two (2.0) times longer than the straight line distance DS between the inlet and the outlet. It is more preferable for the total length of the channel LT to be at least five (5.0) times longer than the straight line distance DS between the inlet and the outlet. It is more preferable for the total length of the channel LT to be about six (6.0) times longer than the straight line distance DS between the inlet and the outlet. Also, the channels are preferably characterized by a circuitous, or serpentine path from the inlet to the outlet. In the embodiment of turbo sump  260  for which dimensions D 1 –D 3  and H 1 –H 3  were provided above (in connection with  FIG. 4A ), the shortest total length of the channel LT (i.e., a path that winds tightly around the central post  310 ) is about 1.2 inches, whereas the total height H 2  of the sump (which is close to the straight line distance DS) is 0.18 inches. 
   Turbo sump  260  has been described as having a single thread  320 . It will be appreciated that turbo sump  260  can alternatively be built with several threads instead of just a single thread as has been described. In such embodiments, the sensor defines a plurality of circuitous, or serpentine, channels instead of a single such channel. Also, the sump may be provided with obstacles (e.g., “fins”) that extend from the thread and further occlude channel  350  thereby further increasing the likelihood that contaminants will not reach the diaphragm. Such obstacles preferably do not substantially lower the conductance of the channel  350 . Also, turbo sump  260  has been described as being disposed within an aperture defined in the body of the sensor. Alternatively, the sump can be disposed within the inlet tube. 
   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.