Gas separation: processes – Electric or electrostatic field – With addition of solid – gas – or vapor
Reexamination Certificate
2000-06-23
2002-08-20
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Electric or electrostatic field
With addition of solid, gas, or vapor
C095S060000, C095S073000, C095S074000, C096S028000, C096S074000, C096S088000, C096S097000
Reexamination Certificate
active
06436170
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention is directed to removing particles from high purity gas systems. In particular, the present invention is directed to a process and apparatus for removing particles from high purity gas cylinders and flowing high purity gas systems.
Methods for measuring suspended particles in high purity specialty gas systems for the electronics and semiconductor industries have been developed. However, the sources of particulate contamination in the gases are not currently controlled. Consequently, levels of particulate contamination in recently filled gas cylinders can substantially exceed normally accepted levels for semiconductor processing gases. As used herein, the term “particle” is intended to refer to any unwanted discrete solid or liquid contaminant of any size.
Particle measurements performed on recently filled gas cylinders reveal the following deficiencies. First, the cylinder filling process produces high suspended particle concentrations immediately after fill. Second, the cylinder filling process produces high variability in particle concentrations immediately after fill. Finally, gravitational and diffusive particle settling in recently filled cylinders is very gradual with time. For example, a certification of less than 10 particles per standard cubic foot (≧0.16 micrometer in size) cannot be achieved in a practical time period following uncontrolled fill. Settling periods on the order of months may be required to achieve such specifications.
The suspended particles in a gas cylinder immediately after fill can originate from four principal sources. First, they may originate in the gas fill system and enter the cylinder suspended in the gas. Second, in the case of reactive gases, they may form within the cylinder through reaction with residual impurities, or by cylinder corrosion followed by particle dislodgment from internal surfaces. Third, they may be released from the cylinder valve during actuation. Fourth, they may be released from the valve and other internal cylinder surfaces by the hydrodynamic shear forces occurring during the fill process. Such shear forces are generally highest at points of flow restriction, such as the cylinder valve, where gas velocities are at the maximum.
Particles originating in the gas fill system can be controlled only through expensive and difficult means, such as clean-up or reconstruction of complete electronics cylinder preparation areas and gas fill systems, and complete revision of all specialty gas fill procedures. Such changes would substantially increase specialty gas production costs and may, in some cases, be economically impractical.
Difficulties with respect to on-site specialty gas distribution systems are as follows.
Certain process gas distribution systems, e.g., gas distribution systems for WF
6
, SiCl
4
, BCl
3
and HF, among other gases, located at, for example, semiconductor processing facilities are prone to substantial contamination by damaging particles following reaction with residual impurities, such as H
2
O and O
2
, or following particle release from mass flow controllers and other in-line components (shedding). In addition, such low vapor pressure gases, or other gases stored as liquids under their own vapor pressure (e.g., NH
3
, HCl, CHF
3
, C
2
F
6
, C
3
F
8
and SF
6
) are subject to vigorous liquid boiling in supply cylinders, especially when gas is withdrawn from the cylinder at a high flow rate, as indicated in Wang, Udischas and Jurcik, “Measurements of Droplet Formation in Withdrawing Electronic Specialty Gases From Liquefied Sources” Proceedings, Institute of Environmental Sciences, 1997, p.6-12. Such high flow rate withdrawal to multiple processing tools is common at, for example, modern semiconductor facilities. Low vapor pressure gases are also subject to droplet formation following pressure reduction or cooling in the distribution system. These liquid droplets have been found to be highly stable, and are easily transported through a gas distribution system at near ambient temperature. Furthermore, any evaporated droplets may produce solid or otherwise non-volatile residue particles, which remain suspended in the flowing gas.
However, due to the low source pressure of certain cylinder gases (typically less than 20 psia for WF
6
, SiCl
4
, BCl
3
, and HF, among other gases) such systems require low resistance flow components. Therefore, although compatible filters exist for such chemically reactive gases, any high resistance in-line components would tend to restrict the available flow rate of gas to the semiconductor processing equipment. Filters can also clog under substantial particle or droplet loading, resulting in a progressive restriction of flow through the system and a consequent reduction in operational reliability of the gas system. In-line filtration of these gases is therefore undesirable in most circumstances. Consequently, damaging particles or droplets having highly variable concentrations may be transported to sensitive semiconductor substrates located in the downstream processing tool. Particles and droplets can also reduce the operational lifetimes of mass flow controllers, and other in-line components. Droplets are also responsible for flow fluctuations, severe corrosion, and premature failure of flow delivery components.
Likewise, difficulties in high purity gas cylinders exist. Due to the detrimental effect of particles on, for example, the microchip fabrication process, semiconductor manufacturers require processing gases to meet strict particle specifications (e.g., less than 10 particles per standard cubic foot larger in size than 0.1 micrometer). Such specifications require routine particle testing of flowing bulk gas systems. Current industry trends are toward similar particle specifications on specialty gases packaged in pressurized cylinders. Particle tests are therefore required in pressurized specialty gases after cylinder fill. Depending upon the process gas, such cylinders may contain a single gaseous phase, or combined gaseous and liquid phases, and may have an internal pressure ranging from less than 0 psig to more than 3000 psig.
Methods for measuring particle concentrations in gas cylinders after fill have been developed. These methods permit measurement of suspended particles larger than 0.16 micrometer directly from the gas cylinder at full pressure; no pressure reduction or filtration of the gas is performed in the test.
Although methods for measuring suspended particles in filled gas cylinders have been developed, the sources of particulate contamination in the gas are not currently controlled. Consequently, as described above, levels of particulate contamination in recently filled gas cylinders substantially exceed normally accepted levels for semiconductor processing gases. Also, as described above, the suspended particles in a gas cylinder immediately after fill can originate from several principal sources, and these particle sources can be controlled only through expensive and difficult means. Such changes would substantially increase specialty gas production costs and may in some cases be economically impractical.
There have also been numerous previous attempts to solve the above difficulties. First, with respect to gas cylinder fill systems in flowing high purity gas systems, particles originating in the gas fill system can be controlled using bulk filtration of the entire gas system or at the point-of-fill for each cylinder. However, in some cases, multiple cylinders are filled rapidly from a single source. Flow rates into the cylinders during fill can be high. Therefore, this method requires installation of large capacity filters in the cylinder gas fill manifold. However, due to their substantial pressure drop, under-sized filters may restrict the rate of flow to the cylinders, and therefore increase the required cylinder fill time. An under-sized filter may also be p
McDermott Wayne Thomas
Ockovic Richard Carl
Air Products and Chemical, Inc.
Chase Geoffrey L.
Chiesa Richard L.
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