Liquid purification or separation – Filter – Material
Reexamination Certificate
2002-08-09
2004-09-14
Lawrence, Frank M. (Department: 1724)
Liquid purification or separation
Filter
Material
C210S906000, C210S909000, C210S915000
Reexamination Certificate
active
06790358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of purification of fluids, and more specifically to the removal of trace contaminants from inert, non-reactive and reactive gases using solid scavenger adsorption materials. More particularly, this invention provides materials for reducing concentrations of trace contaminants in inert and non-reactive gases to at least parts-per-million (ppm) to sub-parts-per-billion (sub-ppb) levels using an ultra-low emission (ULE) carbon based scavenger without concurrently emitting water vapor or other contaminants into the gas stream, wherein the impurities include carbon monoxide, carbon dioxide, and organic compounds such as hydrocarbons. The ULE carbon material is also capable of reducing contaminants such as hydrocarbons from the liquid form of inert, non-reactive and reactive gases by at least a factor of 5 without concurrently emitting significant amounts of water or other contaminants into the liquid. This invention further provides methods for reducing concentrations of trace impurities in reactive gases and vapors to at least parts-per-million (ppm) to sub-parts-per-billion levels using a preconditioned ultra-low emission carbon based scavenger, wherein the impurities include carbon monoxide, carbon dioxide, and organic compounds such as hydrocarbons without concurrently emitting water vapor or other contaminants into the gas or vapor stream.
2. Description of the Prior Art
Inert and non-reactive gases such as nitrogen, helium, and argon are widely used in the semiconductor industry for the manufacture of microcircuitry devices. In such applications, it is critical that the gases be essentially completely free of impurities such as water and oxygen. For example, in semiconductor fabrication processes, gases such as nitrogen, helium and argon are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductor chips. Such impurities, when introduced onto the semiconductor chip during its manufacture, tend to render the chip deficient or even useless for its intended purpose. Thus, a growing number of industries are now requiring gases having impurity concentrations that do not exceed about 10 parts-per-billion (ppb) levels.
In addition, semiconductor fabrication processes use reactive gases, including dry-etch gases such as hydrogen chloride, hydrogen bromide, chlorine, silicon tetrachloride arsine, phosphine, and ammonia, which is a precursor of nitride semiconductor materials such as gallium nitride, silicon nitride, and indium nitride. These electronic reactive gases are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductors produced or treated by those gases. Specifically, the semiconductor industry requires ammonia gas (NH
3
) to have the purity level of “superammonia,” a term of art used to describe ammonia gas that does not contain more than about 1 ppb level impurities. While moisture is usually the main contaminant in high-purity ammonia, other impurities may also exist in ammonia gas such as oxygen, carbon oxides, and volatile organics—especially lower hydrocarbons such as volatile alkanes. In some cases, ammonia gas may accommodate amines and sulfur-containing molecular impurities. Thus, gas purification systems are widely used in the manufacture of semiconductors to remove process gas impurities to very low, trace concentrations.
Recently, gallium nitride manufacturers have begun using low grade ammonia in conjunction with the use of regenerable purifiers to simulate the desired effects of super ammonia. However the low grade ammonia contains hydrocarbons dissolved in the liquid phase of ammonia that can be problematic for delivery systems that withdraw the ammonia from the liquid phase and externally vaporize the ammonia into the gas phase. The hydrocarbons dissolved in the liquid phase can contaminate the ammonia gas distribution system, possibly causing component failure, and ultimately could affect the quality of the gallium nitride semiconductor devices.
The desire to develop methods to reduce impurities in process gases down to parts-per-billion (ppb) or sub-ppb concentrations and down to parts-per-million (ppm) or sub-ppm levels in liquids is further driven by the present ability to measure impurities at extremely low levels. Modern analytical instrumentation such as Fourier Transform Infrared (FTIR) Spectrometry and Gas Chromatography-Pulsed Discharge Helium Ionization Detector (GC-PDHID) permits the detection of process gas impurities such as carbon monoxide, carbon dioxide, oxygen, and moisture (H
2
O) at sub-ppm concentrations, down to about 10 ppb. Atmospheric Pressure Ion Mass Spectrometry (APIMS) permits detection of contaminants in inert and non-reactive gases such as nitrogen and argon in the 10-100 parts per trillion (ppt) range, and GC-Mass Spectrometry permits detection of hydrocarbon contaminants in liquids.
The advances in the detection of trace levels of hydrocarbons using the above-described analytical instrumentation has motivated researchers to further reduce the levels of these impurities in ultra-pure process gases to below the limits of detection of these ultra-sensitive instrumentations. One challenge has been to develop gas purification materials and techniques that remove hydrocarbon impurities from an ultra-pure gas without adding trace amounts of other impurities.
One known method of gas purification involves the adsorption of process gas impurities on a bed or column of solid scavenger material. In these solid adsorption methods, impurities are caught by the surface of the scavenger material while the process gas preferably passes unaltered through the bed or column. Commonly used solid scavenger adsorption materials include alumina, silica, silica-alumina, other metal oxides such as titania and zirconia, mixed oxides, clays, molecular sieves (e.g., zeolites), and activated carbon. Activated carbon, for example, is used in PSA (Pressure Swing Adsorption) plants and for solvent recovery from air in painting facilities (See, for example, Wood and Stampfer,
Carbon,
30:593 (1992); Wood and Stampfer,
Carbon,
31:195 (1993); Nelson et al.,
Am. Ind. Hyg. Assoc. J.,
33:797 (1972); and Nelson et al.,
Am. Ind. Hyg. Assoc. J.,
52:235 (1991)). However, the use of solid scavenger adsorption materials operating at ambient conditions to reduce high ppb levels of impurities, particularly hydrocarbons, in process gases to low ppb or sub-ppb levels without contaminating the gas stream with other impurities, such as moisture, is not known. Further, the use of solid scavenger adsorption materials operating at ambient conditions to reduce high ppm levels of impurities, particularly hydrocarbons, in liquids to low ppm or sub-ppm levels without severely contaminating the gas stream with other impurities, such as moisture, is not known.
Conventionally activated carbon, for example, is known as a very effective adsorbent for removing hydrocarbon impurities from gases but has not been used to remove impurities from liquids without severely contaminating the liquid with moisture. However, conventionally activated carbon is typically activated at 200° C. to 400° C. in gas streams contaminated with ppm levels of impurities such as moisture and CO
2
. After conventional activation, the carbon material contains trace amounts of water and CO
2
that are either not completely removed during activation or re-adsorbed in the contaminated environment of the treatment process. The carbon material may also produce trace amounts of moisture and CO
2
during thermal activation due to chemical reaction of residual functional groups or adsorbed species, such as by dehydroxylation or decarboxylation reactions. Furthermore, gas impurities such as moisture may be generated upon contacting conventionally activated carbon material with reactive gases, thr
Funke Hans H.
Torres, Jr. Robert
Wyse Carrie L.
Hogan & Hartson L.L.P.
Lawrence Frank M.
Matheson Tri-Gas, Inc.
Petersen, Esq. Steven C.
Smith Sarah J.
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