Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2000-01-19
2002-03-26
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C095S008000, C095S143000, C210S677000, C210S690000
Reexamination Certificate
active
06361696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention herein relates to the removal of contaminants from fluid streams. More particularly it relates to the production of substantially contaminant free supercritical and liquid carbon dioxide (CO
2
) fluid streams.
2. Description of the Prior Art
A supercritical fluid is a fluid which is in a state above its critical temperature and critical pressure where gas and liquid phases resolve into a single medium, in which density can vary widely without a phase transition. This allows, for instance, substances which normally act as solvents primarily for inorganic or polar substances to also become efficient solvents for organic or nonpolar materials. Because of these unique properties, supercritical fluids are used in a wide variety of industries, particularly for solvent extraction; examples include polymer and pharmaceutical manufacturing, food processing, environmental processes and precision cleaning in manufacture of products where high purity and cleanliness is required, such as semiconductor manufacturing, including removal or organic photoresist materials. For the most part, achieving supercriticality requires rasing gaseous or liquid compounds to quite high temperatures and pressures.
In contrast, a compound which finds numerous uses in its supercritical state is carbon dioxide, which reaches supercritical state under moderate conditions at a critical point (CP) of 31.3° C. (88.3° F.) and 74 bar (1070 psi). The supercritical region for carbon dioxide is shown in the upper right hand portion of the P/T graph of FIG.
7
. Another fluid phase, liquid carbon dioxide, exists at temperatures in the range shown substantially in the center of the P/T graph of FIG.
7
. Essentially the liquid region is bounded by the supercritical region above 31.3° C. (88.3° F.) and 74 bar (1070 psi), the pressure/temperature curve between the liquid and gaseous phase regions defining distillation Of CO
2
, running from the triple point (TP) at −56.6° C. (−69.9° F.) and 5.2 bar (75.1 psi) to the critical point, and the liquid/solid curve defining solidification and running from the triple point at substantially constant temperature and increasing pressures. Carbon dioxide is therefore liquid at moderate pressures and temperatures (including “room temperature” of 25° C. [77° F.] and about 68 bar [985 psi]).
For brevity herein the term “fluid carbon dioxide” or “fluid CO
2
” will be used as the comprehensive term to refer to both liquid and supercritical carbon dioxide, where the text discusses information applicable to both. Where one or the other phase is expressly intended, it will be so identified. Further, supercritical carbon dioxide will also sometimes be referred to as “SC CO
2
”.
In the manufacture of semiconductors and wafers, it is critical that at each stage of manufacture the materials be extremely clean. The presence of any significant amount of contamination will usually render the final product unusable. Manufacturing specifications therefore commonly require that concentrations of contaminants in each stage of manufacturing be maintained in the sub-ppb (parts per billion) range. A common class of contaminant in the manufacture of semiconductors and wafers is hydrocarbons, which occur as residues from use of hydrocarbon solvents earlier in the manufacturing process, from hydrocarbon skin oils left by workers handling the materials, from lubricants used in the compressors and pumps which move the high pressure gases through the production chambers, and from lubricants used on other manufacturing equipment.
Fluid carbon dioxide, especially SC CO
2
, has been found to be particularly useful for cleaning of semiconductors and wafers, where it is used in the form of “snow” or solid carbon dioxide flakes. The snow is formed when the fluid carbon dioxide is “flashed” or sprayed at high flow rate from nozzles into the cleaning chamber. In one mode of decontamination, the carbon dioxide flakes traveling at high speed impact against the contaminants on the surface of the semiconductor or wafer and cause the contaminants to be ejected into the moving carbon dioxide gas stream. In another mode the fluid carbon dioxide absorbs the contaminants from the semiconductor or wafer surface, in a manner akin to solvent extraction. The solid carbon dioxide snow then sublimes into the gas stream and is carried away for an environmentally safe recovery.
Of course for this cleaning system to work effectively the fluid carbon dioxide itself must be contaminant free. While there have been prior systems to decontaminate carbon dioxide, such prior art systems (such as distillation and cryogenic processes) have all been cumbersome, time-consuming and of limited effectiveness. Further, many processes focus on decontaminating the carbon dioxide as a gas prior to its being raised to liquid or supercritical conditions. This creates a potential for recontamination during the conversion to the fluid state, since oils and lubricants used in the compressors and pumps used to achieve the liquid and supercritical pressures often contaminate the CO
2
as it is being compressed. To make fluid carbon dioxide cleaning as efficient as possible, it is important to reduce contaminants in the fluid carbon dioxide itself, and, consistent with the other specified system contaminant levels, particularly to reduce them to a ppb level. Prior art systems to have been unable to reach this level with fluid carbon dioxide on a consistent basis or in an economically viable manner.
Many processes exist to decontaminate gases by passing them through beds of conventional zeolites, silica, alumina and other oxides, metals, etc. Commercial products produced by the assignee of this patent application, Aeronex, Inc., of San Diego, Calif., have incorporated high silica content zeolites for the removal of water from corrosive gas streams; patents have been applied for for such products and the decontamination methods they employ. None of these prior art systems, however, has been used to decontaminate a gaseous compound while it is in its liquid or supercritical state.
In addition to the Aeronex product and method mentioned above, zeolites have also been used in other contaminant removal processes, but primarily as carriers or substrates for various impregnated metal getters or dehydrating or decontaminating catalysts. In this regard they have merely been substitutes for conventional silica, alumina and carbon substrates.
Further, none of the prior art processes has had the ability to decontaminate fluid CO
2
on a continuous basis. To accomplish that, one must have the ability to regenerate some of the decontaminant capacity while operating the remaining capacity for decontamination. A process for continuous removal of water and CO
2
from specialty gases in a two-vessel system is shown in U.S. Pat. No. 5,833,738, but that process is not self-regenerating, since it uses nitrogen from a source outside the system for the regeneration, and passes purified gas from the purification vessel to the regeneration vessel only for a short initial period to equilibrate the vessels.
SUMMARY OF THE INVENTION
We have now developed a unique and highly effective process for the removal of contaminants from fluid (liquid and supercritical) carbon dioxide down to essentially a 1 ppb concentration. Intermediate levels which can readily be reached are 100 ppb and 10 ppb. This decontamination process can be operated for long periods of time, since the critical material used is not susceptible to degradation in the carbon dioxide liquid phase and supercritical phase temperature and pressure regimes. The process also provides for self-regeneration of the deactivated bed of one vessel with purified gas from the operating bed of the other vessel. This permits continual production of purified fluid CO
2
from the process, by alternating use of the vessels with the zeolite bed of one vessel decontaminating the fluid CO
2
while the zeolite bed of the other vessel is being regenerat
Alvarez, Jr. Daniel
Cook Joshua T.
Shogren Peter K.
Spiegelman Jeffrey J.
Aeronex, Inc.
Hruskoci Peter A.
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