Field of the Invention

The present invention generally relates to a novel seal for mounting a ceramic valve seat in a metal housing and a method of mounting and more particularly relates to a novel seal of soft metallic material for sealing in between a ceramic valve seat and a metal housing to provide a fluid-tight seal and a method of mounting.

Background of the Invention In many fabrication processes where a corrosive or reactive fluid must be transported, ceramic valves are frequently used. For instance, in a semiconductor fabrication process, reactive gas and liquid are frequently used for the cleaning and doping of semiconductor materials. Ceramic materials are frequently the only candidates for valves that can meet the stringent requirements of corrosion-resistant and non-contaminating. For the purpose of this invention, the term ceramic materials shall include either naturally occurring or artificially fabricated jewel materials such as ruby and sapphire, and other more common ceramic materials such as aluminas and pure silica glass ( 1 00% Si0 ₂ ). Ceramic materials have special characteristics such as anti-corrosion, hardness, dimensional stability, inertness, and high temperature stability. These ceramic materials can be exposed to liquids, gases or vapors that are highly reactive and corrosive with no appreciable loss of their physical properties. For example, ceramic materials are used in mass flow controllers that accurately regulate fluid flow in semiconductor manufacturing processes.

A ceramic valve seat is typically a sapphire, ruby or alumina bearing from an analog watch that is specially lapped to seal tightly against a hard ceramic ball. The lapping process referred to in the present invention is a precision abrading process used to bring a surface to a desired state of refinement or dimensional tolerance by removal of an extremely small amount of material.

Lapping is accomplished by abrading a surface with a fine abrasive grit of diamond rubbed about it in a random manner. Usually less than 0.0005 in. (0.01 2 mm) of the material is removed.

Others have attempted to mount a ceramic valve seat in a metal housing by one of two ways. The first method is the mechanical rolling of the housing material adjacent to the ceramic valve seat. The rollers used are typically balls but can also be other shapes such as cylinders. The rollers deform the housing material and force some of the material locally against the valve seat thus providing a mechanical lock. The deformation caused is normally greater closer to the deforming balls, i.e., closer to the surface of the housing, such that a gap or a pocket is frequently left between the valve seat and the housing. This pocket can trap unwanted materials such as solvents, oils, moisture, etc. The trapped materials bleed out into the fluid stream over a period of time and thus contaminate the fabrication process. The mechanical rolling method is therefore not suitable for use in the semiconductor fabrication process or any other fabrication process where contaminants cannot be tolerated.

Another problem associated with the mechanical rolling method is that the metal housing material may recover, or spring back, after the deformation. This is especially true in stainless steel materials which are frequently used in semiconductor fabrication processes. Such a recovery, or spring back of the metal housing material would greatly endanger the integrity of the valve seat mount.

The mechanical rolling method may be suitable in some applications such as in brass housings. Brass material has less spring back than stainless steel material. Furthermore, trapped volumes are not a problem in applications for watches, meters, and other instruments which employ brass.

In the second method, sealants such as silicones, epoxies, wax, grease or cement are used in between the ceramic valve seat and the metallic housing. This method cannot be used in a semiconductor fabrication process because of the potential problems caused by the outgassing or vaporization of the sealant material, by the moisture absorption of the sealant, and by possible chemical reactions between the sealant and the reactive or corrosive fluids.

It should be appreciated that a silicon solid state device can be adversely affected in its electrical characteristics or its lifetime by impurities or

contaminants on the level as low as a few parts per billion. Moreover, the density of semiconductor devices such as transistors, gates, diodes, and resistors on the surface of a modern integrated circuit has increased to the extent where a 1 6 megabyte DRAM (dynamic random access memory) circuit has about 400 million devices per square inch. As a consequence, minute particles having dimensions as small as 0.05 microns or even molecules of contaminants can cause serious defects in a semiconductor device. This critical requirement obsoletes the established methods for ceramic valve seat containment in a metal housing. It is therefore an object of the present invention to provide a novel seal for a ceramic valve seat mounted in a metal housing and a method of making such mount that do not have any of the drawbacks of the previously used mounting methods.

It is another object of the present invention to provide a novel seal for a ceramic valve seat mounted in a metal housing that does not utilize a mechanical rolling method.

It is a further object of the present invention to provide a novel seal for a ceramic valve seat mounted in a metal housing that does not utilize plastic or elastomeric sealing materials that are prone to outgassing, moisture absorption, or reaction with reactive fluids passing through the valve.

It is yet another object of the present invention to provide a novel seal for a ceramic valve seat mounted in a metal housing such that the sealer material does not emit any particulate or molecular contaminants into the fabrication process, nor react with reactive gases to produce compounds which "poison" or "dope" silicon (potassium, sodium, lithium, phosphorous, boron, etc.).

It is another further object of the present invention to provide a novel seal for a ceramic valve seat mounted in a metal housing that provides a fluid-tight seal and a durable bond between the valve seat and the housing.

It is yet another further object of the present invention to provide a method of mounting a ceramic valve seat in a metal housing by utilizing a metallic sealer material that provides a fluid-tight seal and a durable bond between the valve seat and the metal housing.

Summary of Invention

In accordance with the present invention, a novel seal for use in mounting a ceramic valve seat in a metal housing that is non-contaminating to the fabrication process and capable of providing a fluid-tight seal and a durable bond between the valve seat and the metal housing is provided.

In the preferred embodiment, a novel seal is provided by using a metallic sealer material of nickel in its fully annealed state. The soft annealed nickel is positioned, flowed and hardened in between the valve seat and the receptacle cavity of the metal housing filling the annular space formed between the two members. The seal is inert to reactive and corrosive fluids and is non- contaminating to the fabrication process.

In an alternate embodiment, the outside surface of the valve seat and the inside surface of the receptacle cavity are roughened, scored or grooved to further increase the mechanical bonding between the sealer material and the valve seat and the metal housing.

In yet another alternate embodiment, the outside surface of the valve seat and the inside surface of the receptacle cavity are slanted in the axial direction such that an improved mechanical lock against axial movement is provided between the valve seat and the metal housing by the nickel sealer material. The present invention is further directed to a method of mounting a valve seat in a metal housing for achieving a fluid-tight seal and a durable bond without any possibility of contamination to the fabrication process. In the method, a soft annealed nickel material in a suitable form of a tube or sleeve, is inserted between the valve seat and the receptacle cavity in the metal housing. A predetermined compressive force is then applied axiaily on the seal which causes the nickel material to flow and to almost completely fill the annular space in between the valve seat and the receptacle cavity. The compressive force provides the energy required for the cold working of the nickel material turning it into a hardened sealer and thus providing a fluid-tight seal and a durable bond. This novel method of mounting a ceramic valve seat in a metal housing completely eliminates any possibility of contamination of the fabrication process and thus avoids all the drawbacks and shortcomings of the prior art methods.

Brief Description of the Drawings

Other objects, features and advantages of the present invention will become apparent upon consideration of the specification and the appended drawings, in which FIG. 1 is an enlarged cross-sectional view of a valve seat mounted in a metal housing by a prior art method.

FIG. 2 is an enlarged cross-sectional view of the present invention showing all the elements of a ceramic valve mounted in a metal housing, and

FIG. 3 is an enlarged cross-sectional view of the present invention in which a ceramic valve installed in a metal housing with a novel sealer material installed thereinbetween is shown both before compression and after compression of the sealer.

Detailed Description of the Embodiments Referring initially to FIG. 1 , wherein an enlarged cross-sectional view of a prior art method for mounting a ceramic valve seat in a metal housing is shown.

A ceramic valve seat 10 having an aperture 20 bored therethrough for passage of fluid is mounted in the receptacle cavity 12 of metal housing 22. A typical ceramic valve seat 10 is made of a natural or artificial sapphire, ruby or alumina or pure silica glass material mechanically lapped to seal tightly with a hard ceramic ball (not shown).

For a valve assembly described in the present invention to function properly, all the elements of the valve assembly must be resistant to highly corrosive fluids such as hydrogen fluoride, boron chloride, tungsten fluoride, nitric acid, chlorine gas, boron bromide and other similar corrosive gases and liquids. Furthermore, the materials used for the valve elements must not absorb moisture. This requirement excludes the use of most elastomers and plastics as candidates for sealer materials. The materials must not have permanent deformation after extended use that would cause leakage. The materials for the components must not disintegrate into particles and thus cause contamination. The valve components must also operate with satisfactory linearity over a full range of flow rates. Finally, the materials used for the valve components must have low frictional coefficients so as to eliminate any possible hysteresis loss.

The mechanical rollers 16 (shown in ghost line) which are typically balls but can also be other shapes such as cylinders deform the metallic housing material and force the material locally against the valve seat 10. The deformation is greater closer to the deforming balls 16, i.e., closer to the top surface of the metal housing and therefore leaving a pocket 18 defined by the slanted receptacle cavity wall 14 in metal housing 22 and the outside surface 48 of valve seat 10. Pocket 18 traps processing materials and other contaminants such as solvents, oils, moisture, etc. which can then bleed out into the fluid stream over a period of time and contaminate the fabrication process. This prior art method of mechanical rolling therefore is completely unacceptable to any fabrication processes wherein no contaminants can be tolerated.

FIG. 2 shows the present invention ceramic valve seat mounted in a metal housing in a cross-sectional view. A ceramic valve seat 24 is mounted in metal housing 42 by filling the annular space 40 in between the valve seat 24 and the receptacle cavity wall 44 of the metal housing with a novel metallic sealer 38.

Process fluid enters conduit 34 and the aperture 36 in valve seat 24 pushes up the ceramic ball 26 such that the process fluid enters chamber 46 through gap 32 in between ball 26 and valve seat 24. Diaphragm 28 is used to contain the ceramic ball 26 on top of aperture 36 in valve seat 24. The upward motion of diaphragm 28 is restricted by actuator 30 which typically receives its signal from a transducer such as a flow or pressure sensor in a closed loop controller.

In a typical application of the present invention, the ceramic hard ball and the ceramic valve seat can seal to about 0.1 seem (standard cubic centimeters/minute) of nitrogen with 25 — 50 psi differential pressure across them as tested by using a calibrated mass flow meter.

A leak-proof tightness between ball 26 and valve seat 24 is achieved by mechanical lapping of the parts. Such process and materials used in the process are traditionally used by the Swiss jewel industry. Commonly suitable ceramic materials are synthetic ruby, sapphire, high purity aluminum oxide, or pure silica glass. These materials can be finished to extreme precision without pitting, chipping or cracking. Typically, round or spherical dimensions can be held to five millionths of an inch on parts at fairly low cost.

Some special problems encountered in sealing of ceramic valve seats in metal housings are the mismatching of coefficients of expansion, the difficulty of fusing ceramic and metal together, the difficulty in maintaining intimate leak-

free contacts between the materials, the difficulty in avoiding mechanical force through stress transfer that cracks the ceramic, and the differences in the moduli of elasticity between metals and ceramics which frequently result in the spring back of the metal. In our novel invention, the sealer material employed is an annealed nickel such as Inco ^® Alloy 209, 201 or 301 supplied commercially by Inco Alloys International. These materials are 93-99% pure nickel. They have the following desirable properties that are essential to the present invention, a superior corrosion resistance to semiconductor processing fluids, a low hardness in the annealed state, a rapidly work hardenability as the result of small deformations, a low moisture absorption, and a coefficient of thermal expansion reasonably compatible with ceramic materials.

The novel method provided by the present invention to mount a ceramic valve seat in a metallic housing is shown in FIG. 3. A short hollow tube 38 of annealed nickel is machined from bar stock or cut off from an extruded tube and inserted into the annular space 40 between the ceramic valve seat 24 and the receptacle cavity 44 of metal housing 22. This is shown in FIG. 3(A). In the machining process, the nickel material should preferably be in a work-hardened state because it is very difficult to machine fully annealed nickel. High purity nickel can only be hardened by cold working. The machined part is then annealed preferably to its fully annealed state resulting in rockwell B hardness of 50 or less before being used in the present invention. The wall thickness and the length of tube 38 are selected such that the annular space 40 between the valve seat 24 and the receptacle cavity 44 can be almost completely filled by the subsequent compression on the seal.

Nickel seal 38 is compressed axially so that the material flows radially to fill the annular space 40 between ceramic valve seat 24 and receptacle cavity 44 in metal housing 22. This is shown in FIG. 3(B). The amount of compression and the compressive force required to seal is determined experimentally and is accurately repeatable in a production process. The pure nickel material employed in the present invention instantly becomes hard upon compression and does not relax in subsequent service, at least over the temperature range of between approximately 70° to approximately 1 50° F.

In an alternate embodiment, an improved sealing strength and integrity can be obtained for use in conditions of high pressure, extreme temperature, or

extreme thermal cycling. This is achieved by roughening, scoring or grooving the outside surface of the ceramic valve seat and the inside surface of the receptacle cavity which the nickel sealing material contacts after compression. The sealing material flows into the grooves or the roughened surface during compression and provides an improved mechanical bond.

In another alternate embodiment, the inside surface of the receptacle cavity in the metal housing and the external surface of the valve seat can be slanted in the axial direction so that the seal provides additional axial restraint by a mechanical lock. In yet another alternate embodiment, when sealing a deep cavity where the cross-section of the cavity has a length many times its diameter, it is desirable to use two seals with one on top of the other to achieve uniform seal deformation.

While this invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation.

Furthermore, while this invention has been described in terms of a preferred embodiment and two alternate embodiments thereof, it is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: