Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Free carbon containing
Reexamination Certificate
2002-10-02
2004-03-23
Lawrence, Frank M. (Department: 1724)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Free carbon containing
C502S439000, C502S519000, C073S029010, C053S432000
Reexamination Certificate
active
06710012
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 gases and reactive fluids using solid scavenger adsorption materials, without concurrently emitting water vapor or other contaminants into the gas stream. More particularly, this invention provides methods for reducing concentrations of trace contaminants in inert and non-reactive gases to parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb) levels using an ultra-low emission carbon based scavenger, wherein the impurities include carbon monoxide, carbon dioxide, and organic compounds such as hydrocarbons. This invention further provides methods for reducing concentrations of trace impurities in reactive fluids to parts-per-billion and subparts-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.
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 and silicon tetrachloride, and production gases such as arsine and 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 (NH3) 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.
The desire to develop methods to reduce impurities in process gases down to sub-part-per-million (sub-ppm) or sub-ppb concentrations is further driven by the present ability to measure impurities at extremely low levels. Modem analytical instrumentation such as Fourier Transform Infra Red Spectrometry (FTIR) 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.
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 (197); 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 low parts-per-million (ppm) or high parts-per-billion (ppb) levels of impurities, particularly hydrocarbons, to low ppb or sub-ppb levels without 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. 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, through reactions of the reactive gas with surface impurities in the carbon. The residual water and CO
2
in the conventionally activated carbon material are then released in small quantities into a gas stream during a gas purification process, thereby causing significant contamination of the gas and rendering the effluent gas useless for high purity applications. In some cases, conventionally activated carbon is characterized as “hydrophobic” (repels or fails to adsorb water), even though in some cases activated carbon has been shown to weakly adsorb moisture upon exposure of a gas containing several hundreds to several thousands of ppm of moisture (see, for example, Barton et al.,
Carbon,
22:22 (1984), However, this adsorbed moisture is also easily released into a process gas stream during purification of the gas. Thus, reducing hydrocarbon impurities in a process gas to sub-ppb levels while maintaining very low levels of water vapor and CO
2
has proven extremely difficult.
Among the methods utilized in the prior art for removing water from ammonia is the use of moisture-sorptive molecular sieves. The difficulty of employing such method for the production of high-purity ammonia for semiconductor applications is that ammonia is competitive with water for the adsorption sites on the molecular sieves. As a result, it is not possible to obtain the necessary low residual water values, on the order of part-per-billion concentrations of water in the effluent, using conventionally activated molecular sie
Fraenkel Dan
Funke Hans H.
Houlding Virginia H.
Hogan & Hartson LLP
Lawrence Frank M.
Matheson Tri-Gas, Inc,
O'Rourke Sarah S.
Petersen Steven C.
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