Abstract:
A baffle is positioned near a diaphragm of a capacitive pressure sensor to have a small volume therebetween. The baffle creates a high aspect ratio path to create molecular flow for molecules to travel before reaching the diaphragm. The path encourages contaminants to stick to the baffle or housing before reaching the diaphragm. The sensor further includes a particle trap between an inlet and the baffle.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a capacitive pressure sensor that senses changes in capacitance between a diaphragm and an electrode. 
     Capacitive pressure sensors are used in a number of applications, including climate control systems and semiconductor processing. A pressure sensor has a housing, an input in the housing for receiving a fluid (gas or liquid) whose pressure is being sensed, a conductive flexible diaphragm and an electrode next to the diaphragm. The diaphragm and electrode have a capacitance between them. The sensor senses changes in the capacitance as the diaphragm moves relative to the electrode in response to pressure from the fluid. 
     It can be desirable to have a baffle between the inlet for the fluid whose pressure is being sensed, and the flexible diaphragm. The baffle helps prevent contaminants from contaminating the diaphragm. U.S. Pat. No. 5,811,685, which is hereby incorporated by reference for all purposes, describes a baffle and also some previous baffle designs. The patent further discusses reasons for and benefits of a baffle in such a sensor. 
     SUMMARY OF THE INVENTION 
     A capacitive pressure sensor of the present invention has a housing with an inlet for receiving a fluid, a diaphragm that flexes in response to pressure in the fluid, and a baffle designed to provide a high aspect ratio radial path for the gas molecules to flow between the inlet and the diaphragm. Preferably, the invention also includes a particle trap system (which is also a baffle), positioned such that the baffle is between the trap system and the diaphragm. The aspect ratio (the ratio of the radial length to the gap) of a path formed by the baffle is greater than 10, and preferably greater than 50. 
     In another aspect, the capacitive pressure sensor has a housing with an inlet for receiving a fluid, a diaphragm that flexes in response to pressure in the fluid, and a baffle between the inlet and the diaphragm, with the baffle designed so that a path taken by molecules from the inlet to the diaphragm creates molecular flow, as opposed to laminar flow. 
     The baffle is preferably positioned close to the diaphragm so that there is a small volume between them, so that the sensor responds quickly to changes in gas pressure. 
     The higher aspect ratio path formed by the baffle increases the likelihood that molecules will stick to a surface of the baffle or the housing before reaching the flexing portion of the diaphragm, thereby protecting the diaphragm from deposition. The small volume between the baffle and diaphragm does not reduce response time, as a large volume would. Other features and advantages will become apparent from the following detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one half of a sensor according to the present invention. 
     FIG. 2 is a plan view of the particle trap. 
     FIG. 3 is a plan view of the baffle according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a cross-sectional view of a symmetrical half of a portion of a sensor  10 . Sensor  10  has a housing member  12  with an inlet  14  for receiving a fluid to be sensed. A diaphragm  16  is welded between housing member  12  and another housing member  18 . A chamber  20  is enclosed by diaphragm  16  and housing member  18  and has a desired pressure, e.g., zero pressure. Next to diaphragm  16  is an electrode  22  supported by a post  24 . While shown here as a single conductive piece, the electrode may be one or more conductive films formed on a dielectric (ceramic) disk. The diaphragm and electrode have a capacitance between them. Diaphragm  16  flexes in response to a change in pressure in the fluid at inlet  14 , thus changing the capacitance between the diaphragm and the electrode. 
     In the path between the inlet and the diaphragm is a trap system  28 , and between trap system  28  and diaphragm  16  is a baffle  30 . The design of trap system  28  is described in more detail in the incorporated patent. Referring also to FIG. 2 herein, trap system  28  has a central portion  34  with a diameter greater than that of inlet  14  to block a direct path from the inlet to the diaphragm. Around central portion  34  are a number of peripheral openings  36 . These openings are formed as a series of sectors evenly spaced about the trap system in a circumferential direction, and also arranged at different diameters radially. 
     Referring to FIG. 1, particles from inlet  14  pass through an annular region  38  between housing member  12  and trap system  28 , and then through openings  36  (unless the particles are too small to fit through the annular region or the openings). Annular region  38  and openings  36  are sized to prevent relatively large particles (e.g., 250 microns and larger) from passing through. 
     Referring also to FIG. 3, baffle  30  is essentially a circular metal plate with a plurality of evenly spaced tabs  40  about the circumference. These tabs essentially define a number of annular sectors  42  with a width in the radial direction between baffle  30  and housing member  12  being the same as the length of the tabs. The design of the baffle and its positioning relative to the housing thus define openings as annular sectors through which the fluid can pass. 
     Baffle  30  is positioned next to housing member  12  to define a radial path  44  for the gas including any contaminant species component to flow between openings  36  and annular sectors  42 , and then to diaphragm  16 . The aspect ratio of this radial path is defined as the ratio of a radial length l of the path from openings  36  to annular sectors  42 , to the gap d representing the gap between baffle  30  and housing member  12 . The aspect ratio according to the present invention is greater than 10, and is preferably greater than 50. The length is preferably at least 1 cm, and preferably in the range of about 1-4 cm; the gap is preferably no more than about 0.1 cm, and preferably in a range of about 0.025-0.1 cm. 
     A contaminant molecule traveling in such a high aspect ratio path will likely collide with surfaces of baffle  30  and housing member  12  hundreds of times while traversing this path. The probability for such a molecule to be deposited on a surface of baffle  30  or housing member  12  is a function of the number of these collisions and a sticking coefficient. With an increasing number of collisions, the likelihood of the molecule being deposited goes up dramatically. If the number of collisions is greater than 100, even a low sticking coefficient will result in a high probability of deposition on the baffle and the housing surfaces, thereby protecting the diaphragm from the depositing species. The path thus is designed so that there is a significant likelihood of deposition of contaminants. 
     The path creates a molecular flow at normal vacuum processing conditions, i.e., less than 100 mT. Such molecular flow causes multiple collisions with the surfaces for molecules traversing the gap, as opposed to laminar flow in which a boundary layer of flow is around and isolates a center stream. Molecular flow, as opposed to laminar flow, is flow in which the mean free path of a molecule is much greater than the dimension of the path (here, the gap); i.e., a molecule hits the walls of the path more often than it hits other molecules. 
     If there is a large volume between the baffle and the diaphragm, the response time for the diaphragm decreases because of the resultant pneumatic time constant, defined as V/C, where V is the volume behind a pathway with conductance C. With the present invention, however, the volume between the baffle and the diaphragm is small, so that the response is fast even though the small gap imposes a lower conductance due to a restriction on flow. The gap g between baffle  30  and diaphragm  16  is preferably in the range of about 0.025-0.1 cm, and the internal volume  50  between baffle  30  and diaphragm  16  is preferably less than 0.1 cubic inches (1.6 cm 3 ), and more than 0.05 cubic inches (0.8 cm 3 ). 
     Trap system  28  is preferably made of a corrosion-resistant, non-contaminating material, such as stainless steal or an alloy such as Inconel. Baffle  30  is also preferably made of metal. 
     The baffle of the present invention thus provides an advantageously long path for allowing molecular collisions and sticking before reaching the diaphragm, while simultaneously providing an advantageously small volume between the baffle and the diaphragm. 
     Having described embodiments of the present invention, it should be apparent that modifications can be made without departing from the scope of the invention as defined by the appended claims. For example, the baffle system could be used with a differential capacitor sensor.