WO2000020098A1

WO2000020098A1 - High flow metal membrane gas filter

Info

Publication number: WO2000020098A1
Application number: PCT/US1999/022346
Authority: WO
Inventors: Farhad Saeyan; Robert S. Zeller; Christopher J. Vroman; Iraj Gashgaee
Original assignee: Millipore Corporation
Priority date: 1998-10-05
Filing date: 1999-09-28
Publication date: 2000-04-13
Also published as: TW442317B

Abstract

An ultra-high efficiency, highly porous particulate gas filter capable of operating at gas flows from about 300 slpm to about 25,000 slpm is formed of a highly porous, compact filter sintered metal media. The filters according to the present invention exhibit a filtration efficiency substantially in excess of a 6 log reduction (99.9999+) (6LRV) and preferably better than a 9 log reduction (99.9999999+) (9LRV) at the desired gas flow rates with a flow per unit area of from about 1 to about 4 slpm/sq.cm. These filters are compact, providing a unit which is significantly smaller than those conventionally used in the filtration of ultrahigh process gas streams with the same or greater efficiency of filtration and flow/unit area.

Description

HIGH FLOW METAL MEMBRANE GAS FILTER

This invention relates to a metal membrane based gas filter for high flow applications. More particularly, it relates to a metal membrane based gas filter for high flow applications which is extremely compact in design.

Background of the Invention Semiconductor manufacturing is constrained by the limitations of purity. In chemical vapor deposition of various components used in manufacturing of semiconductors, a critical aspect of the process involves the absence of any particulate impurities. The presence of a minute particle can destroy an entire silicon wafer representing many dollars of potential end product. To that end, an entire industry has developed concerning the filtration of gases which may come into contact with the semiconductor product during its formation. The entire delivery system is carefully designed to minimize contamination of the gas which is being supplied. These gases include silane, arsine, hydrochloric acid and phosphine. The design of the components used in this system endeavors to minimize particle shedding, outgassing and other contamination from the components themselves. One important component in this system is the particulate filter. This filter insures that particulates do not reach the workpiece. Most often such filters are placed directly before the point at which the gaseous component is delivered to the workpiece. These filters are called point of use filters. Such filters must be highly efficient and compact. They are typically made of organic membranes, ceramic membranes or sintered metal fibers or particles. See U.S. 5, 114, 447 and U.S. 5, 487, 771.

These point of use filters are capable of removing almost every particulate contaminant and are rated by their log reduction value (LRV). To be acceptable, they should have an LRV of at least 6, preferably at least 9 (an efficiency of 99.9999 to 99.9999999). They are capable of achieving these values at minimal flow rates, typically from one standard liter per minute (1SLPM) to about seventy five SLPM. There are also filters used before the point of use application. Typically, these filters are used on higher flow applications in the distribution system. Here, flow rates range from 150 SLPM to 25000 SLPM. Due to the higher flow rates, the filters are typically much different than those of the point of use application. Generally, these filters are extremely long tubes of sintered metal particles. For example, one such filter has a length of 36 inches ( 91.44cms) and an outside diameter of from 2 to about 4 inches (5cm to about 10.1 cm). These filters typically have a much more robust membrane which typically has a relatively low porosity. Alternatively, it can be formed of a series of tubes (typically three tubes) nested within each other in order to provide the same filtration area in a somewhat shorter length ( 9 to 12 inches in length)(22.86cm to 30.48cm). This robustness and length has been required by the high flow rates to which these membranes are subjected.

US 5,114,447 cites an example of a disk arrangement similar to one used as a point of use filter. It suggests its use on flows of up to 1400 SLPM. Such a unit was about two feet long (60 cm) and had a diameter of 4 to 5 inches (10 - 12.5 cm). Filters of that design for higher flow applications are not taught, but one can infer from its teachings that they would be larger and incorporate a significantly larger amount of filter material in order to obtain low pressure ratings and filtration efficiency required. What is desired is a high flow, metal membrane gas filter which is compact and capable of providing at least 6 log, preferably 9 log reduction in particulates at gas flow rates of between 150 and 25000 SLPM. The present invention provides just such a device.

Summary of the Invention The present invention comprises a highly porous, high flow filter device. The metal membrane has a porosity of at least 35%, preferably 55 to 80%. It is formed into a compact volume such as may be found with some point of use filters, has a log reduction value of 6 or greater and is capable of operating at gas flows of up to 25000 SLPM.

Brief Description Of The Drawings Figure 1 shows a cross-sectional view of a first embodiment of the present invention. Figure 2 shows a cross-sectional view of a second embodiment of the present invention.

Figure 3 shows a crosssectional, top down view of the manifold of the embodiment of Figure 2. Figure 4 shows a second arrangement of the manifold of Figure 2 in a cross sectional, top down view.

Figure 5 shows a cross-sectional view of a third embodiment of the present invention.

Detailed Description Of The Invention

The present invention provides a highly efficient, compact high porosity metal membrane filter capable of use at high gas flow rates. This makes the filter an ideal high flow rate filter for gas applications such as may be used in the semiconductor, fiber optic and other related industries where space is at a premium.

Referring to Figure 1, the present invention is illustrated in one embodiment. The filter unit comprises a housing 1, which has an inlet 2, an outlet 3 and a casing 4 between the inlet 2 and the outlet 3. As shown in Figure 1 , the inlet 2 is mounted on an end 5 opposite that of the end 6 to which the outlet 3 is mounted. This is typical for an"inline" type device. Alternatively, the inlet and outlet may be located on the same end of the housing. This is used for modular designs as well as offline or 90 degree configurations (not shown).

A filter 7 formed of porous, sintered metal is contained within the casing 4. In this embodiment, the filter 7 is comprised of a series of sintered porous metal disks which are welded to each other. The disk 8A closest to the inlet 2 is continuous across its entire surface. The remainder of the disks,8B, have a central bore 9 through which filtered fluid may pass to the outlet 3.

The filter 7 is secured within the housing such that it forms a fluid seal between the filter 7 and the housing 1 or inlet 2 or outlet 3. Which manner of sealing is not critical to the present invention. As shown in this embodiment, the seal is formed at the opposite end 6 of the housing 1 by welding the last disk 8B in the series of disks to a lug 10 on the inner surface of the housing 1. Additionally, in this embodiment, a spring-biased metal ring 11 is mounted adjacent the end of the filter closest to the inlet 2. This ring 11 acts as a means for mounting and positioning the series of disks in the housing so as to allow for fluid flow between the filter 7 and the housing 1 which ensures that all disks participate in the filtration of the fluid. Alternatively, the outlet may contain a stem 12 (not shown) which can be secured to the filter 7, such as by welding, a screw thread or other such well known means. In another embodiment, the lowermost disk may be of a size which corresponds to that of the inner dimensions of the housing and it is sealed to the housing such as by welding, a spring retainer, metal adhesives, etc. Figure 2 shows a second embodiment of the present invention. In this embodiment the filter 20 is formed of a series of tubes 21 which are mounted to a manifold 22. The manifold may be mounted to the interior surface of the housing 23 so as to form a fluid seal between the housing 23 and the manifold 22 (as shown in this embodiment) or it may be mounted to a stem 24 (not shown) attached to the outlet 25 such that the connection between the stem 24 and the manifold 22 on one end and the outlet 25 on the other end form the fluid seal between the filter elements 21 and the outlet 25. In either configuration, gas is flowed from the inlet 26 into the housing 23, through the filter 20 and out through the outlet 25. Also as shown the tubes are formed from cylindrical sintered stock and have one end 26, the end away from the manifold sealed via an end cap 27 which may be secured in place by a variety of means such as welding, being a part of the sintering process, etc. Alternatively, as is well known in the art, the tube as formed may have an end cap formed of porous sintered metal which is formed as part of the tube during the sintering process. This embodiment is shown with a series of five tubes arranged concentrically around the manifold. One may use more than 5 tubes or less than 5 tubes as desired in order to achieve the desired flow and filtration characteristics. The only limitation on the number of tubes used is the number which can be mounted to a manifold of any given size. Also while shown to be uniformly distributed around the manifold, tubes could also be staggered around the manifold so as to fit more tubes in. For example, Figure 3 shows a top view of the manifold of figure 2. Figure 4 shows a top down view of an alternative arrangement with the staggered tube arrangement. Optionally, a series of tubes could be welded together to form a greater length of tube or a single longer tube may be used instead. Tubes should be as short as practicable with the number and arrangement such that the desired flow and filtration characteristics are met. Typically, they are from about 1 inch (2.54cm) to about 10 inches (25.4cm) in length, preferably from about 3 inches (7.62 cm) to about 7 inches and most preferably from about 4 (11.6cm) to about 6 inches (15.24 cm) in length. They may vary widely in diameter, typically from about .25 inch (outside diameter "O.D." )(.635 cm) to about 3 inches (O.D.)(7.62 cm) and have a wall thickness from about .0625 inch ( .159cm) to about .375 inch (.9525cm). The thickness of the wall may vary upon the flow at which it is to be used, the porosity of the filter material and the pressure drop one wishes or can tolerate.

Figure 5 shows another embodiment of the present invention which is basically a modification of the embodiment of Figure 1 which contains fewer disks and therefore is particularly useful at the lower end of the high flow range, 150 to about 3000 SLPM. As there are fewer elements, the ring of Figure 1 is not needed although it may be used if desired.

While embodiments are shown with screw thread type fittings, other fittings, which are well known to one of ordinary skill in the art, may be used, including but not limited to O-rings, compression seals or simple unthreaded ends known in the industry as a butt weld tubes The present invention is selected with appropriate number of elements of the desired style, either disk or tube and the appropriate flow rating and pressure drop. It is inserted into the gas stream and gas is flowed through the device in order to purify the stream of particles.

The filter elements of the present invention, whether in the form of a disk or a tube, are preferably formed of a substantially homogeneous porous sintered metal material. This material may be a very fine particle metal or preferably a dendritic metal powder such as is disclosed in U.S. patent 5, 487, 771. The metal selected may include stainless steel such as Grade 316, nickel such as Inco 255 powder, nickel alloys such as Hastelloy C-22, chromium, chromium alloys or blends, titanium, palladium or other such metals.

The method by which the element is made is well known in the art such as are taught by US Patents 5,114,557 and US 5,487,771 the teachings of which are herein in their entirities. Typically, the element is formed in either a dry or wet state, with or without a binder. Preferably, it is formed without a binder, in a dry state by an airlaying technique as taught by US 5,487,771. The form is typically compressed into its desired shape and is sintered either in its compressed form within a mold or without a mold if it is in a form-stable shape. The element is typically sintered in a hydrogen atmosphere at a temperature and time sufficient to provide a form-stable element. Minimal sintering is preferred as this tends to lead to a highly porous structure which provides for higher flows and filtration efficiency. Typical sintering temperatures for nickel range from about 500 C to about 1000C, preferably from about 650C to about 750 C. Temperatures for stainless steel, nickel alloys, chromium and titanium are generally higher, from about 900C to about 140C.

Times for sintering vary depending upon the temperature and the desired porosity and strength of the sintered product. Typically, these times will make from about 3 minutes to about 20 minutes. Preferably about 5 to about 10 minutes, at temperatures from about 500C to about 750C for nickel are preferred.

Filtration efficiency is measured by a process described in the article by Rubow, et. al. "Characteristics of Ultra-High Efficiency Membrane Filters in Gas Applications," Journal of Environmental Sciences, Volume 31, pages 26-30 (May, 1988). In this test, the filter is tested under one or more test conditions and at the most penetrating particle size for that filter under those conditions. For the present invention, a filtration efficiency of at least 6 log reduction at the most penetrating particle size is desired. Preferably, a log reduction of at least 9 at the most penetrating particle size is most desired.

Another value for determining the efficiency of a filter is the flow/unit area of filtration for a stated LRV. For example one filter according to the present invention has a filter area of 140 square centimeters and achieves a LRV of 9 at a flow of 300 slpm resulting in a flow/unit area of 2.1 slpm/sq.cm. The same filter has a LRV of 6 at a flow of 750 slpm resulting in a flow/unit area of 5.3 slpm/sq.cm.. Preferably, the flow/unit area is measured at 9LRV and for a filter according to the present invention should be from about 1 to about 4slpm/sq.cm. at 9LRV. More preferably, a value of from about 1.5 to about 3 slpm/sq.cm. at 9 LRV is obtained for a filter of the present invention. This range represents a balance between the desired flow with the desired porosity, pressure drop and mechanical strength needed and desired in such a product.

The following examples are meant to be illustrative of the invention and are not meant to limit its scope in any manner. Example 1:

A filter module according the teachings of the present invention and having a configuration similar to that of Figure 1 was made. The housing was all stainless steel (316L), having a length of 8.86 inches, a diameter of 2.5 inches, as measured at the mid point of the casing, an inlet and outlet having a central bore of 7 inches and a series of 35 porous, sintered metal membrane disks welded together and the disk closest to the outlet was welded to the lug so as to form a fluid seal.

The disks were formed of nickel membranes, about 2 inches (5cm) in diameter and 0.060 inches (0.1524cm) in thickness. These membranes were made by Millipore Corporation according to the teachings of U.S. 5, 487, 771, sintered at 825C for 90 minutes. The membranes had a porosity of about 38 - 42%. Air was flowed through the device at 1500 SLPM at inlet pressures of 30 psi, 50 psi and at 70 psi. Pressure drop for the device at those inlet pressures were measured accordingly as 10 psi, 5.0 psi and 3.5 psi respectively. The particle filtration efficiency at its most penetrating particle size (0.1 microns) was measured for each of these inlet pressures and in each case was determined to be greater than log 9 reduction. Example 2 :

A device according to be present invention was made containing 7 stacked disks in an arrangement similar to that shown in Figure 5. The disks were nickel filter elements made according to U.S. Patent 5, 487, 771. The membranes had a porosity of about 38-42%, a diameter of about 2 inches (5cm) and a thickness of 0.060 inches(0.1524cm).

Air was flowed through the device at 300 SLPM at inlet pressures of 30 psi, 60 psi and 70 psi with pressure drops of 5.5 psi, 3.5 psi and 3.0 psi respectively. The particle filtration efficiency at its most penetrating particle size( 0.1 microns) was measured for each of these inlet pressures and in each case was determined to be greater than log 9 reduction. Example 3:

A device according to the present invention was made in accordance to the arrangement shown in Figure 2. The five tubes are sintered nickel tubes, made by Millipore Corporation according to US Patent 5,487,771 having a solid nickel end cap on one end welded to the top of the tube so as to close one end. The tubes were 2.28 inches in length(5.8cm), 0.635 inches (1.61 cm) in outside diameter with a wall thickness of 0.065 inches (0.1651cm). The porosity was about 65%. The total membrane area was 21.8 square inches (140 square cm). They were welded to a carousel which in turn was welded to the interior of the housing in a fluid tight arrangement. The overall dimensions of the device were a length of 5 inches including the fittings and an external diameter of 2.4 inches.

Air was flowed through the device at flow rates varying from 0 to 500 SLPM with inlet pressures varying from 30 psi, 60 Psi and 90 psi with measured pressure drops at 500 SLPM of 7.8 psi, 4.2 psi and 3.7 psi respectively. The particle filtration efficiency at its most penetrating particle size of 0.1 microns at 300 SLPM was greater than log 9 reduction and at 500 SLPM was greater than log 6 reduction.

What is claimed:

Claims

1. A porous metal membrane containing filter for high flow gas applications comprising a metal housing having a casing for retaining a porous metal membrane, said housing having an inlet and an outlet, a porous metal membrane located within said housing between said inlet and said outlet and being sealably connected to either said housing or said inlet or said outlet to thereby defined a filter flow path through said housing, said metal membrane having a porosity of from about 35% to about 70% and which is capable of providing at least a 6 log reduction in particle filtration at its most penetrating particle size at flow rates of from about 150 SLPM to about 25000 SLPM.

2. The filter of claim 1 wherein the metal membrane is a sintered membrane formed of particles selected from the group consisting of dendritic particles, fibrous particles and blends thereof and wherein the metal is selected from the group consisting of stainless steel, nickel, nickel alloys, chromium, chromium alloys, chromium blends titanium, palladium and mixtures thereof.

3. The filter of claim 1 wherein the log reduction in particle filtration is at least 9 log reduction.

4. The filter of claim 1 wherein the inlet is formed on the housing at an end opposite that of the outlet.

5. The filter of claim 1 wherein the membrane is formed as a series of filter members stacked therein in such a manner so as to provide a porous surface area at least about five times the volume occupied by the filter element.

6. The filter of claim 1 wherein the membrane is formed of a series of tubes having one end sealed, the tubes are connected to the outlet via a manifold and wherein the manifold is mounted to the housing so as to create a fluid tight seal between the manifold and the housing on the side adjacent the inlet .

7. The filter of claim 1 wherein the housing is formed of a metal selected from the group consisting of stainless steel, nickel, chromium, nickel alloys and chromium alloys.

8. The filter of claim 5 wherein series of filter members are arranged as pairs of welded disks, the disk closest to the inlet is continous, the remaining disks having a central core which communicate with the outlet, the disks being attached to the housing via a first metal retention lug mounted to a portion of the housing adjacent the inlet and a second retention lug mounted to a portion of the housing adjacent the outlet so as to create a fluid tight seal between the housing and the disks.

9. The filter of claim 8 wherein the first retention lug is a spring biased ring having one or more arms projecting outwardly against the housing and the second retention lug is a ring having a central opening which ring is welded at its outer circumference to the housing.

10. The filter of claim 9 wherein the second retention lug is a ring which is also welded to the last filter element disk.

11. The filter of claim 1 wherein the inlet and outlet are located on the same side of the housing.

12. The filter of claim 1 wherein the inlet and outlet are adjacent to each other on the same side of the housing.

13. A porous metal membrane filter comprising a housing having an inlet and outlet and a casing there between, a porous, sintered metal membrane positioned between the inlet and outlet and secured in such a manner so as to create a fluid seal between the membrane and the outlet so that all fluid entering the inlet must flow through the membrane before reaching the outlet, and wherein the filter is capable of providing at least a 6 log reduction in particle filtration at flow rates of from about 150 SLPM to about 25000 SLPM.

14. The filter of claim 1 wherein the flow per unit area at a 9LRV is from about 1 to about 4slpm sq.cm.

15. The filter of claim 1 wherein the flow per unit area at a 9LRV is from about 1.5 to about 3slpm/sq.cm.

16. The filter of claim 13 wherein the flow per unit area at a 9LRV is from about 1 to about 4slpm/sq.cm.

17. The filter of claiml3 wherein the flow per unit area at a 9LRV is from about 1.5 to about 3slpm/sq.cm.