FIELD OF THE INVENTION This invention relates to valves and, more particularly, to valves that are provided with a polymeric coating for protection against corrosive fluids. The invention is particularly useful for valves utilized in etching and chemical vapor deposition systems, but is not limited to such applications.

BACKGROUND OF THE INVENTION Conventional gate valves include a valve housing having a fluid conduit and a valve seat, a seal plate that is movable between an open position and a closed position in the fluid conduit, and an actuator mechanism for moving the seal plate between the open and closed positions.

The seal plate in the closed position engages the valve seat and seals the fluid conduit. The seal plate may be moved from the closed position to a retracted position and then moved linearly to the open position.

Gate valves and other types of valves, such as angle valves, are used in a variety of applications, which may involve liquids, gases and vacuum. Some of the liquids and gases may be highly corrosive. An example of such an application is in equipment for processing of semiconductor wafers. In the manufacturing of semiconductor wafers gases such as HCI, HBr, C12, F2, NH3, etc. may be utilized. Such gases are known to be highly corrosive.

The internal portions of the valve, including the fluid conduit and the seal plate, are exposed to the corrosive gases and are subjected to the corrosive action thereof. In addition, such gases may condense within the valve and partially block the valve passages and interfere with the opening and closing of the valve. Thus, such corrosive gases have deleterious effects on the valves with which they are used.

Prior approaches to protection of valves against corrosive gases have included the use of nickel plating, zinc plating and anodizing. These approaches have limited chemical resistance to some corrosive gases and therefore are not entirely satisfactory.

A turbomolecular pump wherein the rotor is provided with a corrosion resistant protective coating formed by a layer of polymeric material is disclosed in U. S. Patent

Nos. 5,904, 469 issued May 18,1999 and 5,938, 406 issued August 17,1999, both in the name of Cerruti.

Accordingly, there is a need for a valve that is resistant to corrosion by a variety of different corrosive materials.

SUMMARY OF THE INVENTION According to a first aspect of the invention, a valve is provided for operation with a corrosive fluid. The valve comprises a valve housing having a fluid conduit and defining a valve seat, a closure member, and a valve actuator for moving the closure member between a closed position in engagement with the valve seat and an open position retracted from the valve seat. At least a portion of the valve exposed to the corrosive fluid has a corrosion resistant coating of a polymeric material.

The corrosion resistant coating is preferably provided on the interior surfaces of the valve that are exposed to the corrosive fluid. The coating may be omitted on surfaces where moving parts of the valve contact each other.

In a preferred embodiment, the polymeric coating comprises poly- (p-xylylene) and has a thickness in a range of about 0.0004 to 0.002 inch. A preferred thickness is about 0.001 inch.

The coating may be selected for resistance to corrosion by gases used in etching and chemical vapor deposition processes, such as gases used in semiconductor wafer processing.

According to another aspect of the invention, a method is provided for protecting a valve against a corrosive fluid. The method comprises the step of coating at least a portion of the valve exposed to the corrosive fluid with a corrosion resistant polymeric coating. Portions of the valve that are subjected to moving or frictional contact during operation of the valve may be masked prior to applying the coating to the valve.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: Fig. 1 is a cross-sectional view of a right angle valve suitable for incorporation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION A cross-sectional view of a right angle valve suitable for incorporation of the present invention as shown in Fig. 1. A right angle valve 10 includes a valve housing 12 that defines a

fluid conduit between an inlet 14 and an outlet 16. The valve housing 12 also defines a valve seat 20. Valve 10 further includes a seal plate 24 and a valve actuator 30. Seal plate 24, which serves as a closure member, is connected to actuator 30 by a shaft 32 and a spring 34. Spring 34 urges seal plate 24 toward valve seat 20. A bellows 36, connected between seal plate 24 and housing 12, surrounds shaft 32 and spring 34 and isolates these elements from a fluid passing through the valve.

In the embodiment of Fig. 1, valve actuator 30 is an air cylinder. Valve actuator 30 moves seal plate 24 between a closed position in engagement with valve seat 20, as shown in Fig. 1, and an open position retracted from valve seat 20 in response to a control signal. In the open position, a fluid may pass through the valve from inlet 14 to outlet 16.

The valve actuator is not limited to an air cylinder. In general, any suitable actuator may be utilized. The valve actuator may be actuated by any valve control signal, such as for example a pneumatic signal or an electrical signal, or may be manually operated.

In accordance with an aspect of the invention, all or a portion of the valve that is exposed to the fluid is provided with a corrosion resistant coating of a polymeric material. In the embodiment of FIG. 1, interior surfaces 40,42 and 44 of valve housing 12 and a surface 46 of seal plate 24 may be provided with a corrosion resistant coating of a polymeric material. In one embodiment, all surfaces of valve 10 that are exposed to the fluid passing through the valve are provided with a polymeric coating. In another embodiment, those surfaces of the valve which are subjected to moving or frictional contact during operation of the valve may have the coating omitted, such as by masking during the coating process. Coating of the exterior surface of the valve, which is not exposed to the corrosive fluid, is optional and depends on the practicalities and economics of the coating process. Thus, for example, exterior surface 50 may or may not be provided with a corrosion resistant coating of a polymeric material. Also, the coating of surfaces inside bellows 36 is optional, since the corrosive fluid does not enter this part of the valve.

The polymeric material is preferably selected for resistance to corrosion by a variety of corrosive materials that are likely to be used with the valve. One application of such valves is in etching and chemical vapor deposition systems and other systems that are used in processing of semiconductor wafers.

A preferred polymeric material for the corrosion resistant coating is a so-called poly- (p- xylylene), which is a polymer of (p-xylylene). However, other polymeric materials that are resistant to one or more corrosive fluids may be utilized.

The coating process may include vaporization of a dimer of (p-xylylene) under vacuum, preferably under a pressure of 100 Pa at a temperature of about 150° C. Then the vapor is passed through a pyrolysis zone at a temperature of about 680° C and a pressure of 50 Pa, thus forming the monomer of (p-xylylene).

The monomer is then admitted into a coating chamber, under a lower pressure, containing the valve which is rotated for better distribution of the coating. The valve is <BR> <BR> substantially at room temperature, i. e. , is cold, with respect to the monomer and this temperature difference causes a condensation with substantially, simultaneous polymerization of the reactive monomer on the valve surface.

A suitable dimer of (p-xylylene) is available from Ausimont under the trade name Galaxyl, or from Union Carbide under the trade name Parylene.

In a preferred embodiment, the thickness of the corrosion resistant coating of polymeric material is in a range of about 0.0004 to 0.002 inch, with a preferred thickness of about 0.001 inch.

Surfaces of the valve where coating is not desired may be masked during the coating process. For example, surfaces that are subjected to frictional or moving contact during valve operation may be masked to prevent coating. If the uncoated surfaces are exposed to the corrosive fluid, these surfaces can be fabricated of stainless steel or other wear resistant and corrosion resistant material.

The corrosion resistant coating of a polymeric material significantly reduces corrosion of valve surfaces by corrosive fluids. In addition, the polymeric coating promotes wetting of the valve surfaces and thereby limits the formation of deposits within the valve.

A right angle valve having a corrosion resistant coating of a polymeric material is described above. It will be understood that the polymeric coating may be utilized on any valve that is exposed to corrosive fluids, including but not limited to gate valves, butterfly valves and other angles valves.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.