Baffle for a capacitive pressure sensor

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 radial 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. US Patent No. 5, 811,685 describes a baffle and also some previous baffle designs. The patent further discusses reasons for and benefits of a baffle in such a sensor. US Patent No. 4,920,805 also discloses a pressure sensor having a baffle protecting a diaphragm from direct exposure to inward flow of measurement fluid. Summary of the Invention According to one aspect of the invention, there is provided a capacitive pressure sensor comprising: a housing with an inlet having a first diameter in a plane; a flexible diaphragm parallel to the plane and connected to the housing; and a baffle member parallel to the plane and positioned between the diaphragm and the inlet, the baffle member being arranged to allow fluid to pass in a direction perpendicular to the plane, characterized by a particle trap with a closed central portion parallel to the plane and having a second diameter greater than the first diameter, and peripheral openings for allowing particles smaller than the openings to pass through, the baffle member having a third diameter greater than the second diameter, and a periphery spaced from the housing to allow said fluid to pass. Preferably, a radial path between the openings of the particle trap and the periphery of the length of the path to a gap between the housing and the baffle has an aspect ratio greater than 10, and more preferably greater than 50. The baffle is preferably positioned close to the diaphragm so that there is a small volume between them, preferably less than 1.6cm 3 , 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 will become apparent from the following detailed description of a preferred embodiment of the invention. Brief Description of the Drawings Fig. 1 is a cross-sectional view of one half of a sensor according to an embodiment of the present invention. Fig. 2 is a plan view of the particle trap. Fig. 3 is a plan view of the baffle according to an embodiment of 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 US-A-5,811,685. 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 I 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 13.3 Pa (100mT). 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 steel 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.

Claims: A capacitive pressure sensor (10) comprising: a housing (12,18) with an inlet (14) having a first diameter in a plane; a flexible diaphragm (16) parallel to the plane and connected to the housing; and a baffle member (30) parallel to the plane and positioned between the diaphragm and the inlet, the baffle member being arranged to allow fluid to pass in a direction perpendicular to the plane, characterized by a particle trap (28) with a closed central portion (34) parallel to the plane and having a second diameter greater than the first diameter, and peripheral openings (36) for allowing particles smaller than the openings to pass through, the baffle member having a third diameter greater than the second diameter, and a periphery spaced from the housing to allow said fluid to pass. A pressure sensor according to claim 1, wherein a radial path between the openings of the particle trap and the periphery has an aspect ratio greater than 10. A pressure sensor according to claim 2, wherein the aspect ratio is greater than 50. A pressure sensor according to any preceding claim, wherein the housing has an annular opening (38) for allowing particles to travel from the inlet to the openings in the particle trap. A pressure sensor according to any preceding claim, wherein the volume between the baffle member and diaphragm is less than 1.6 cm 3 .

Inventors: GRUDZIEN CHRIS P, HINKLE LUKE D, POULIN JAMES, GRUDZIEN, CHRIS P, HINKLE, LUKE D, POULIN, JAMES
Applicants: MKS INSTR INC, MKS INSTRUMENTS, INC.
Application year: 2000
Main classification: G01L19
IPC codes: G01L19