Corrosion resistant coating for process gas control valve

A fluid control valve includes a passage block formed with a passage, a valve seat provided in the passage block, and a valve element to be brought into contact with or separated from the valve seat. The fluid control valve is configured to open and close the passage by movement of the valve element with respect to the valve seat to control a fluid. The fluid control valve further includes a shape-changeable part which changes its shape in association with the movement of the valve element. The shape-changeable part has a surface formed with a monomolecular film of fluorosilane material.

BACKGROUND

Technical Field

The present invention relates to a fluid control valve including a passage block formed with a passage, a valve seat provided in the passage block, a valve element which is brought into and out of contact with the valve seat, the fluid control valve being configured to open and close the passage by movement of the valve element with respect to the valve seat in order to control a fluid.

Related Art

A fluid control valve for controlling a fluid is used for example in a semiconductor manufacturing process. In the semiconductor manufacturing process, conventionally, there is widely used a vacuum pressure control valve (one example thereof is disclosed in Patent Document 1) including a valve seat, a valve element which is brought into contact with and separated from the valve seat, a valve stem supporting the valve element to move the valve element, and a bellows protecting the valve stem. This vacuum pressure control valve is disposed in a pipe that connects a vacuum vessel and a vacuum pump and can change an opening degree to regulate vacuum pressure in the vacuum vessel.

In the semiconductor manufacturing process, a corrosive process gas is often used. For preventing corrosion on the surface of the bellows, therefore, a corrosion resistant film or coating having a thickness of about several μm is formed.

On the other hand, for example, the use of a fluorosilane material as a water-repellent coating material on a nozzle of an ink jet printer is disclosed in Patent Document 2.

Patent Document 2 describes that the coating can also be a single molecular layer.

A monolayer film of fluorosilane material is also described in Non-patent Document 1. Further, Patent Document 3 discloses a fluid control valve to control a flow of high-temperature gas (fluid), such as H2gas and Ar gas, heated to about 200° C. in supplying or shutting off the high-temperature gas with respect to a semiconductor manufacturing apparatus and so on in a semiconductor manufacturing process, a liquid crystal panel manufacturing process, and so on.

RELATED ART DOCUMENTS

Patent Documents

Patent Document 2: Japanese translation of PCT International application publication No. 2008-544852

Patent Document 3: Japanese unexamined patent application publication No. 2012-092861

Patent Document 4: Japanese unexamined patent application publication No. 2011-145800

SUMMARY OF INVENTION

Problems to be Solved by the Invention

In recent years, however, a semiconductor manufacturing process tends to use a higher-corrosive process gas. The vacuum pressure control valve in Patent Document 1 therefore causes a problem with corrosion of the surface of the bellows made of stainless steel.

The present inventors estimate the reason as below. Specifically, a bellows largely expands and contracts according to the motion of the valve element. However, the corrosion resistant film is as relatively thick as several μm and thus large expansion and contraction of the bellows result in the generation of cracks, through which the surface of the bellows is corroded.

The above problems have been addressed conventionally by frequently disassembling and cleaning the vacuum pressure control valve or replacing the bellows with a new one. These works lead to a low operation rate of the semiconductor manufacturing process, thus causing a problem of cost increase.

Patent Document 2 discloses that the fluorosilane material is used as a corrosion resistant coating. However, an object to be formed with the coating is a static nozzle. This document does not suggest any formation of the coating on a movable object.

Non-patent Document 1 merely gives a general explanation of a monolayer film of fluorosilane material.

The high-temperature gas control valve disclosed in Patent Document 3, in which a diaphragm valve element formed of a thin metal plate is deformable, also has a problem of corrosion of the diaphragm valve element. It is estimated, as with the bellows, that the diaphragm valve element has a corrosion resistant film that is as relatively thick as several μm, a large change in shape is likely to cause cracks, through which the surface of the diaphragm valve element is corroded.

Further, a valve chamber of a fluid control valve may be sometimes opened and thus an inner wall of the valve chamber is exposed to air. At that time, the gas remaining in the valve chamber may react with air, causing corrosion of the inner wall of the valve chamber. To avoid such defects, the valve chamber inner wall has to be cleaned frequently or replaced with troublesome works. Accordingly, a yield ratio decreases and an operation rate of the apparatus deteriorates, causing a cost increase.

The present invention has been made to solve the above problems and has a purpose to provide a fluid control valve capable of being continuously used for a long time (a long period) without causing corrosion even when high-corrosive process gas is made to flow through the fluid control valve.

To achieve the above purpose, one aspect of the invention provides a fluid control valve including: a passage block formed with a passage; a valve seat provided in the passage block; and a valve element to be brought into contact with or separated from the valve seat, the fluid control valve being configured to open and close the passage by movement of the valve element with respect to the valve seat to control a fluid, wherein the fluid control valve further comprises a shape-changeable part which is deformed in association with the movement of the valve element, the shape-changeable part includes a surface formed with a monolayer film of fluorosilane material.

In particular, the fluid control valve further comprises a valve stem holding the valve element to move the valve element, the shape-changeable part is a bellows connected to the valve element to protect the valve stem from the fluid allowed to flow through the passage, and the fluid control valve is a vacuum pressure control valve to be disposed in a pipe connecting a vacuum vessel and a vacuum pump, and can change an opening degree defined between the valve seat and the valve element to regulate vacuum pressure in the vacuum vessel. Alternatively, the shape-changeable part is a diaphragm valve element and also used as the valve element.

Effects of the Invention

According to the aforementioned configuration, the thickness of the monolayer film of fluorosilane material is for example 10 angstroms (1 nm) or more and 20 angstroms (2 nm) or less. Thus, even when the bellows or the diaphragm valve element formed of the thin metal plate largely expands and contracts, a small internal stress is only generated in the monolayer film formed on the surface of the bellows or diaphragm valve element. Thus, no cracks occur. Furthermore, since the monolayer film of fluorosilane material has a high water repellency, even if a reactive product adheres to the surface of the monolayer film, the reactive product is easily removed by a subsequent flow of process gas. Also at the time of maintenance, the surface of the bellows or the surface of the diaphragm valve element can be easily wiped or cleaned up.

Even when a high-corrosive process gas is made to flow through the fluid control valve, the fluid control valve can maintain durability, and does not need to be frequently disassembled for cleaning and for replacing a bellows or a diaphragm valve element formed of a thin metal plate. This leads to an increased operation rate of the semiconductor manufacturing process and a decreased cost.

DESCRIPTION OF EMBODIMENTS

A detailed description of a preferred embodiment of a fluid control valve embodying the present invention will now be given referring to the accompanying drawings.

First Embodiment

A first embodiment of the present invention will be described first.FIG. 3shows a whole structure of one example of a vacuum pressure control system utilizing a vacuum pressure control valve.

Inside a vacuum chamber (one example of a “vacuum vessel”)11, wafers15are placed in a shelf-like manner. The vacuum chamber11is formed with an inlet13and an outlet14. The inlet13is connected to a supply source of process gas and a supply source of nitrogen gas to clean up the inside of the vacuum chamber11. The outlet14is connected, through a pipe20, to an inlet port18aof a vacuum pressure control valve (one example of a “fluid control valve”)18which is a valve opening-degree proportional valve. An outlet port18bof the vacuum pressure control valve18is connected to a vacuum pump19through a pipe20.

The outlet14is connected to a pressure sensor17through a shutoff valve16. In the present embodiment, a Capacitance Manometer is used as the pressure sensor17.

The structure of the vacuum pressure control valve18will be described below in detail referring toFIG. 2.FIG. 2shows the vacuum pressure control valve18in a closed state. This valve18is broadly divided into a pilot cylinder32on an upper side and a bellows-type poppet valve31on a lower side.

The pilot cylinder32has the following configuration. A piston41is slidably set in a single-acting pneumatic cylinder43. The piston41is biased downward by a return spring42.

An upper end of the piston41is connected to one end of a slide lever48. This slide lever48extends out of the single-acting pneumatic cylinder43and is coupled with an unillustrated rod of a potentiometer50. The rod is connected to a variable resistance in the potentiometer50. With this potentiometer50, accordingly, the position of the piston41is precisely measured.

An inner circumferential edge of a bellofram51is fixed to a lower surface of the piston41. An outer circumferential edge of the bellofram51is fixed to a chamber inner wall of the single-acting pneumatic cylinder43. The bellofram51is designed to be very thin. The bellofram51is a cylindrical diaphragm having a long stroke and a deep turn-up and having an effective pressure-receiving area maintained constant during operation.

In the present embodiment, the bellofram51is used to isolate both sides of the piston41in the pilot cylinder32in an axial direction. Thus, the piston41can be moved, without causing stick-slip, with high response and accurate positional precision.

An outer appearance of the bellows type poppet valve31is constituted of a first block40and a second block44. A valve stem37is fixedly provided in the center of the piston41and is made to slide up and down within the first block40and the second block44along with movement of the piston41. A lower end of the valve stem37is attached with a poppet valve element33. On an upper surface of the poppet valve element33, a metal bellows38(one example of a “shape-changeable part”, made of stainless steel and so on) is attached at its one end over an entire circumference. The other end of the bellows38is attached to the first block40. Along with upward/downward movement of the poppet valve element33, the bellows38largely expands and contracts. On a lower surface of the poppet valve element33, a valve element34is provided integrally with the poppet valve element33. An O ring35is placed between the poppet valve element33and the valve element34. Along with upward/downward movement of the poppet valve element33, the O ring35is separated or brought into contact with a valve seat44aof the second block44. The second block44is formed with flow passages44band44cthrough which a fluid is allowed to flow.

A structure of a fluorosilane monolayer film formed on a surface38aof the bellows38will be described below. The entire structure is briefly described first and the details of each part are explained later.FIG. 1shows one example of the structure of a fluorosilane monolayer film formed on the surface38aof the metal bellows38.

The monolayer film of fluorosilane material is a monolayer film formed by covalent bond of organic molecules (Self-Assembled-Monolayer (SAM)). This SAM is a monolayer film formed by self-assembling, in which organic molecules are immobilized on a substrate by covalent bond. By using trihalogenated fluorosilane or trialkoxy fluorosilane, each having three reactive functional groups, it is possible to form a fluorosilane SAM film containing molecules assembled with a higher density.

A monolayer film21of fluorosilane material is formed by for example introduction of a precurosor and water vapor into a chemical vapor deposition (CVD) reactor at a low pressure. Specifically, fluorosilane is hydrolyzed in the CVD reactor, so that terminal halogen, methoxy group, and ethoxy group form hydroxy silane. At that time, the surface38aof the metal bellows38(hereinafter, also referred to as a “bellows surface”) may be subjected to an oxygen plasma treatment in advance to form a uniform hydroxyl group on the bellows surface38a.

Hydroxy silane is dehydrated and condensed with the hydroxyl group on the bellows surface38aand immobilized by covalent bond22. Furthermore, adjacent hydroxyl groups are also dehydrated and condensed, forming siloxane bond23and covering over the bellows surface38a.

In each of monolayers, it is terminated at one end with a CF3group24, one molecule forms covalent bond22with the bellows surface38aand creates a network only in a lateral direction, but no more bonds to anything in a vertical direction. Thus, the bellows surface38ais coated with only the monolayer film21.

The thickness of the monolayer film21of fluorosilane material in the present embodiment is about 15 angstroms (1.5 nm).

The details of the structure of the monolayer film21of fluorosilane material will be described below. A monolayer of a fluorosilane material can include one carbon chain or a plurality of carbon chains terminated at one end with a —CF3group. The other end of each carbon chain can be terminated with a SiCl3group. As an alternative, if the relevant molecule is bonded to a silicon (Si) oxide layer, the other end can be terminated with an Si atom which is bonded to an oxygen atom of the Si oxide layer (remaining bonding hands of the Si atom may be bonded to oxygen atoms connected to the terminal Si atoms of adjacent monolayers of the fluorosilane material or may be bonded to OH groups or to both). The carbon chain or chains can be fully saturated or partially unsaturated. For some of the carbon atoms in each carbon chain, the hydrogen atoms can be replaced by fluorine. The number of carbons in each carbon chain can be from 3 to 10. For example, the carbon chain could be (CH2)M(CF2)NCF3, where M≥2 and N≥0, and M+N≥2. In one example in which M=2 and N=7 as shown inFIG. 1, the carbon chain is (CH2)2(CF2)7CF3.

A method of forming the monolayer film21of fluorosilane material on the bellows surface38awill be described below. Firstly, a first explanation is given to sources of the fluorosilane material.

The sources of the fluorosilane material may include for example a source including molecules having molecular ends and attachable ends to the surface38aof the bellows38. For instance, a source including a carbon chain terminated at one end with a —CF3group and at a second end with a —SiCl3group. Concretely, there may be used Perfluoroalkylsilane, such as 1H,1H,2H,2H-Perfluorooctyltrichlorosilane (FOTS) and 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (FDTS), Organomethoxysilane, such as 1H,1H,2H,2H-Perfluorooctyltrimethoxysilane and 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane, and Orgranoethoxysilane, such as 1H,1H,2H,2H-Perfluorooctyltriethoxysilane and 1H,1H,2H,2H-Perfluorodecyltriethoxysilane.

A next explanation is given to the CVD conditions in the present embodiment. Partial pressure of the gas source may be 6.7 Pa to 1.3×102Pa (e.g., 13 Pa to 67 Pa). Partial pressure of H2O may be 6.7 Pa to 2.7×103Pa (e.g., 13 Pa to 2.7×102Pa). A deposition temperature may be set in a range from a room temperature to about 100° C.

It is conceived that when the source (e.g., FOTS or FDTS) in which each molecule includes a —SiCl3end is introduced together with water vapor into the CVD reactor, the source is hydrolyzed, and then the siloxane bond23is generated in which a silicon atom of the —SiCl3group is bonded to an oxygen atom of the —OH group on the surface38aof the bellows38. Thus, a coating of molecules of which non-wetting ends are exposed (for example, a single molecular layer) is formed.

The surface38aof the bellows38can be subjected to an oxygen (O2) plasma treatment prior to the CVD. For instance, the oxygen plasma treatment may be performed with an anode coupling plasma tool.

The bellows38can be put in a vacuum chamber of the plasma tool of which the pressure is decreased close to a vacuum, e.g., less than 1.3×102Pa, e.g., 27 Pa or 1.3×103Pa. Oxygen may be introduced at a flow rate of for example 80 sccm into the chamber. When high-frequency power (e.g., 500 W RF power) begins to be applied, O2plasma is generated. The O2plasma treatment can be performed for example for from 5 minutes to 60 minutes. The bellows outer peripheral surface (the surface38a) can be exposed to the O2plasma.

The O2plasma treatment can increase the density of OH groups on the outer peripheral surface of the bellows38. The increased density of OH groups allows for increased coverage of the monolayer film21of fluorosilane material. In other words, since the material molecules bond to the OH groups of the bellows outer peripheral surface, as the OH group density is higher, the coverage of the monolayer film21of fluorosilane material is further enhanced.

The monolayer film of fluorosilane material has a high water repellency. Specifically, a contact angle of the monolayer film21of fluorosilane material with water is 105° or more, which exhibits a high water repellency.

(1) As described above in detail, the vacuum pressure control valve18in the present embodiment includes the second block44formed with the flow passages44band44c, the valve seat44aprovided in the second block44, the poppet valve element33and the valve element34, which are brought in contact with or separated from the valve seat44a. This vacuum pressure control valve18is configured to open and close the flow passages44band44cby movement of the poppet valve element33and the valve element34with respect to the valve seat44ato thereby control a fluid. The vacuum pressure control valve18further includes the valve stem37holding the poppet valve element33and the valve element34to move these valve elements. Further, the vacuum pressure control valve18includes the bellows38which changes its shape, or is deformed, in association with the movement of the poppet valve element33and the valve element34. The surface38aof the bellows38is formed with the monolayer film21of fluorosilane material. In the present embodiment, for instance, the thickness of the monolayer film21of fluorosilane material is 10 angstroms (1 nm) or more and 20 angstroms (2 nm) or less. Thus, even when the bellows38largely expands and contracts, a small internal stress is only generated in the monolayer film21of fluorosilane material formed on the surface38aof the bellows38. Thus, no cracks will occur.

Further, since the monolayer film21of fluorosilane material has a high water repellency, even if a reactive product adheres to the surface of the monolayer film21, the reactive product is easily removed by a flow of process gas. Also at the time of maintenance, the surface38aof the bellows38can be easily wiped or cleaned up.

Consequently, even when a high-corrosive process gas is made to flow through the vacuum pressure control valve18, the vacuum pressure control valve18can maintain durability, and does not need to be frequently disassembled for cleaning and for replacing the bellows38. This leads to an increased operation rate of the semiconductor manufacturing process and a decreased cost.

(2) In the vacuum pressure control valve18described in (1), the bellows38is made of metal (stainless steel and so on) and the fluorosilane material includes hydroxysilane. This hydroxysilane is an intermediate product which is generated by hydrolysis of the fluorosilane material, and is dehydrated and condensed with a hydroxyl group in the surface of the substrate (metal, such as stainless steel), and thus immobilized on the substrate. Further, adjacent molecules form siloxane bond.

Specifically, the fluorosilane material produces hydroxysilane by hydrolysis. This hydroxysilane is dehydrated and condensed with a hydroxyl group on the substrate38aof the bellows38, and thus is immobilized by the covalent bond22. Further, hydroxyl groups of adjacent hydroxysilanes are dehydrated and condensed to form the siloxane bond23.

Conventionally, a Teflon® film is applied by baking onto a surface. Such a baking causes troubles such as low adhesive strength and partial peel-off. In the present embodiment, in contrast, the monolayer film21of fluorosilane material is bonded to the surface by the covalent bond22resulting from chemical reaction with the metal surface (stainless steel and so on), so that high adhesive strength is achieved and the monolayer film21is less likely to come off the surface38aof the bellows38. Further, the monolayer film21can have high lateral bonding strength. This can prevent the generation of cracks or pin holes.

(3) In the vacuum pressure control valve18described in (1), the fluorosilane material may be terminated with the CF3group24. The CF3group24is a functional group which has a lowest critical surface tension and is able to maintain the water repellency of the bellows surface38a. Specifically, since a reactive product less adheres to the surface38aof the bellows38, the surface38ais less likely to be corroded even if contacting with high-corrosive process gas. Thus, the surface38acan be effectively protected through the monolayer film21of fluorosilane material.

(4) In the vacuum pressure control valve18described in (1), the monolayer film21of fluorosilane material may be formed by the CVD method. The monolayer film21of fluorosilane material can be formed with a uniform thickness (e.g., 15 angstroms) on a largely rough or irregular surface38aof the bellows38made of metal (stainless steel and so on). Since a uniform thin film is formed on the bellows38, the internal stress generated in the monolayer film21when the bellows38largely expands or contracts will be dispersed uniformly. No cracks will occur.

(5) In the vacuum pressure control valve18described in (4), the bellows38is made of metal (stainless steel and so on). Prior to formation of the monolayer film21of fluorosilane material, the surface38aof the bellows38may be subjected to the oxygen plasma treatment. This treatment promotes chemical reaction between the surface38aof the bellows38and the fluorosilane material. Accordingly, the surface38aof the bellows38and the monolayer film21of fluorosilane material can be bonded by strong chemical bond.

(6) In the vacuum pressure control valve18described in (1), the inner wall surface45aof the valve chamber of the vacuum pressure control valve18is formed with the monolayer film21of fluorosilane material. Specifically, the fluorosilane material is self-assembled on the metal surface, forming a monolayer film21. This film forms physically strong siloxane bond with the metal surface and provides a high water repellency and a high chemical resistance to thereby prevent corrosion of the metal surface. Even when a reactive product adheres to such a high-water-repellent film, the product is easily removed, enabling repeated reuse. In some cases, the valve chamber45of the vacuum pressure control valve18is opened, allowing the inner wall of the valve chamber45to be exposed to air. At that time, in the present embodiment in which the valve chamber inner wall45ahas the monolayer film21of fluorosilane material formed thereon, even if remaining gas in the valve chamber45reacts with air, the valve chamber inner wall is less likely to be corroded. Even when a reactive product adheres to the inner wall, such a product is easily removed. Further, there is no need to frequently clean up and replace the valve chamber inner wall. This can prevent deterioration of a yield and decrease in operation rate of the apparatus, and therefore does not cause cost increase.

Second Embodiment

Next, a second embodiment of the present invention will be described.FIG. 4is an explanatory view of a gas valve30(one example of a “fluid control valve”) in a valve closed state. The gas valve30includes a diaphragm valve element4(one example of a “shape-changeable part”, which is made of nickel alloy and so on) formed of a thin metal plate, a passage block7formed with an input flow passage5and an output flow passage6, and a valve seat member8provided in the passage block7. The diaphragm valve element4is brought into contact or separated from the valve seat member8, thereby controlling a flow of high-temperature gas.

The gas valve30is broadly constituted of an air control section2and a valve control section3as shown inFIG. 4. The air control section2will be first described.

The air control section2in the present embodiment includes two cylinders (a first cylinder10and a second cylinder52), two pistons (a first piston53and a second piston54), a partitioning member55, and others, all of which are made of metal, such as stainless steel. The first cylinder10is formed with an operation port56, a first exhaust port57, and a second exhaust port58. The first cylinder10and the second cylinder52are integrally connected by threaded engagement. The portioning member55is interposed between the first cylinder10and the second cylinder52to partition the inside defined by the first cylinder10and the second cylinder52into a first pressurizing chamber25and a second pressurizing chamber26.

The first piston53is placed between the first cylinder10and the partitioning member55. The second piston54is placed between the second cylinder52and the partitioning member55. Inside the first cylinder10and the second cylinder52, an upper part of a piston rod60extending in an axial direction AX is located. The upper part of the piston rod60is bored with a pilot hole60H extending in the axial direction AX. Further, two through holes; a first through hole61H and a second through hole62H, are formed intersecting with the pilot hole60H and extending in a radial direction CR. A lower part of the piston rod60is located in the valve control section3which will be described below.

The valve control section3is explained below. This valve control section3includes the passage block7and the diaphragm valve element4, both of which are made of metal, such as stainless steel. The passage block7has the input flow passage5and the output flow passage6as shown inFIG. 4. Between the input flow passage5and the output flow passage6, there is placed the valve seat member8, which is a separate part from the passage block7. The diaphragm valve element4and the valve seat member8are placed in a valve chamber27.

The valve seat member8has a ring shape. The valve seat member8in the present embodiment is made of fluorine resin, particularly PFA (Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer).

At least a surface4awhich is a lower surface of the diaphragm valve element4formed of a thin metal plate is formed with a monolayer film21of fluorosilane material. Further, an inner wall surface27aof a valve chamber of the gas valve30is formed with a monolayer film21. The structure of the monolayer film21of fluorosilane material and a method of forming the monolayer film21are the same as in the first embodiment and herein omitted.

As described in detail above, the gas valve30includes the passage block7formed with the input flow passage5and the output flow passage6, the valve seat member8provided in the passage block7, and the diaphragm valve element4to be brought into contact with or separated from the valve seat member8. The diaphragm valve element4is operated with respect to the valve seat member8to open or close the input flow passage5and the output flow passage6to control a fluid. The surface4aof the diaphragm valve element4is formed with the monolayer film of fluorosilane material. In the present embodiment, for example, the thickness of the monolayer film21of fluorosilane material is 10 angstroms (1 nm) or more and 20 angstroms (2 nm) or less. Therefore, even when the diaphragm valve element4largely changes its shape, or is deformed, a small internal stress is only generated in the monolayer film21of fluorosilane material formed on the surface4aof the diaphragm valve element4. Thus, no cracks occur.

Third Embodiment

A third embodiment of the present invention will be described below.FIG. 5is an explanatory view of a proportional solenoid valve70(one example of a “fluid control valve”) in a valve closed state.

The proportional solenoid valve70includes a bobbin72on which a coil71is wound, and a rod-shaped fixed core73fixed in a hollow part72aof the bobbin72. The proportional solenoid valve70further includes a holder74supporting the coil71, the bobbin72, and the fixed core73, a movable core75having a substantially circular plate-like shape and being placed in correspondence with a leading end (an end face) of the fixed core73and attractable by the fixed core73, a valve element76fixed to the center of a leading end (an end face) of the movable core75, and a metal plate spring77(one example of a “shape-changeable part”, which is made of stainless steel and so on) fixed to the leading end (the end face) of the movable core75. Still further, the proportional solenoid valve70includes a body78connected with the holder74, a valve chamber79formed in the body78, in which the valve element76is placed, and a valve seat80formed in the body78and located in the valve chamber79in correspondence with the valve element76. The body78is provided with a lead-in passage81through which a fluid flows in the valve chamber79, a lead-out passage82through which the fluid flows out of the valve chamber79via the valve seat80, and an orifice83formed just downstream of the valve seat80.

The fixed core73consists of a first core73A mostly accommodated in the hollow part72aof the bobbin72and a second core73B fixed to a leading end portion of the first core73A. The holder74includes a bonnet86having a rectangular cross section having an open side, and a core87mounted in a mounting hole formed in one end (a lower end) of the bonnet86. The other end of the bonnet86is formed with a screw hole86b. With a screw88tightened in the screw hole86b, one end of the fixed core73is fixed to the bonnet86. The movable core75is located inside the core87. The movable core75is provided to face the first core73A and be slightly displaceable in an axial direction of the first core73A. The holder74having the core87fit in a stepped portion78aof the body78is integrally provided with the body78.

The shape of the plate spring77is shown inFIG. 6. This plate spring77is formed of a circular thin plate having a central hole77aand cutouts77beach having a predetermined shape. The plate spring77is fixed in such a manner that a central part is fixed to the leading end (the end face) of the movable core75around the valve element76by weld spots91, while an outer peripheral portion is sandwiched between the body78and the holder74(the core87). Herein, a reinforcing plate is provided over a part of the plate spring77formed with the weld spots91. Specifically, the annular reinforcing plate92is put on the central part of the plate spring77and then the plate spring77and the reinforcing plate92are bonded to the movable core75by the weld spots91. The plate spring77is made of stainless steel and so on.

A surface77cof the plate spring formed of a thin metal plate is formed with a monolayer film21of fluorosilane material. The structure and manufacturing method of the monolayer film21of fluorosilane material are the same as in the first embodiment and herein omitted.

As described in detail above, the proportional solenoid valve70includes the bobbin72on which the coil71is wound, the fixed core73fixed in the hollow part of the bobbin72, the holder74supporting the coil71, the bobbin72, and the fixed core73, the movable core75placed in correspondence with the end face of the fixed core73and attractable by the fixed core73, the valve element76fixed to the end face of the movable core75, the body78connected with the holder74, the valve chamber79formed in the body78, in which the valve element76is placed, the valve seat80formed in the body78and placed in the valve chamber79in correspondence with the valve element76, the lead-in passage81formed in the body78to allow a fluid to flow in the valve chamber79, the lead-out passage82formed in the body78to allow a fluid to flow out of the valve chamber79via the valve seat80, and the plate spring77urging the valve element76in a direction to bring the valve element76into contact with the valve seat80. The movable core75is displaced by balance between the attraction force of the fixed core73generated when the coil71is excited by energization and the urging force of the plate spring77, thereby adjusting the position of the valve element76relative to the valve seat80. The surface77cof the plate spring77is formed with the monolayer film21of fluorosilane material. Further, the inner wall surface79aof the valve chamber of the proportional solenoid valve70is formed with the monolayer film21.

In the present embodiment, for example, the thickness of the monolayer film21of fluorosilane material is 10 angstroms (1 nm) or more and 20 angstroms (2 nm) or less. Therefore, even when the plate spring77is largely deformed, a small internal stress is only generated in the monolayer film21of fluorosilane material formed on the surface77cof the plate spring77. Thus, no cracks occur.

The foregoing embodiments are mere examples and give no limitation to the present invention. The present invention may be embodied in other specific forms without departing from the essential characteristics thereof.

For instance, the first embodiment is applied to the vacuum pressure control valve18which is a pressure proportional valve. As an alternative, the invention is applicable to a simple on-off valve in which a vacuum pressure control valve includes a bellows.

REFERENCE SIGNS LIST

4Diaphragm valve element

18Vacuum pressure control valve

21Monolayer film of fluorosilane material