Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2000-04-05
2002-05-28
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S126000, C095S132000, C095S902000, C502S078000, C502S079000
Reexamination Certificate
active
06395070
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to methods for removing water and impurity metals from gases, in particular acid gases, using a zeolite purifier with a high silica-to-alumina ratio, and with low impurity metal levels, especially low titanium levels, which has been heated to at least about 350° C. but preferably to at least about 650° C. The invention also relates to methods for preparing zeolites with low levels of impurity metals.
2. BACKGROUND OF THE INVENTION
A wide range of hydridic, halide and bulk gases are used in processes for manufacture of semiconductor devices and materials. As semiconductor geometries have become smaller and devices more sophisticated, the purity of these gases has become more crucial to the viability and success of semiconductor manufacture.
Water contamination in acid gases used in the production of semiconductors is particularly disadvantageous for a number of reasons. Even trace amounts of water in acid gases such as hydrogen chloride (HCl) and hydrogen bromide (HBr) cause corrosion of the piping, valves and flowmeters used to handle the gases in semiconductor manufacture. The presence of water in these gases can also cause the walls of the cylinders used to store the gases to corrode. Such corrosion leads to the generation of metal particulate contaminants which can become incorporated into the semiconductor device during manufacture. In addition, certain processes used in semiconductor manufacture result in the decomposition of water present in the process gas into H
2
and O
2
. The presence of these gases can result in formation of additional contaminants, particularly oxides, which can also become incorporated into the semiconductor device. Contamination of semiconductor devices with metal particulate and oxide impurities is severely detrimental to the performance of the devices, and often renders the devices deficient or even useless for their intended purpose. Moreover, the corrosion caused by the presence of water in these gases necessitates frequent replacement of expensive piping, manifolds, valves and other gas handling equipment.
Metallic impurities in acid gases can also have a detrimental effect on semiconductor manufacturing processes. Metallic impurities, such as titanium, iron, magnesium, zinc, calcium or aluminum, can become incorporated into the semiconductor devices during manufacture. Contamination of semiconductor devices with such metallic impurities can cause “shorts” or “opens” in the microelectronic circuit, rendering the semiconductor device inoperative. In addition, metallic contamination in acid gases can cause “haze” or “spikes” on wafers after polishing.
A number of materials have been developed for the removal of moisture from acid gases. One such material is a chlorosilylated alumina which is effective for removal of trace moisture from hydrogen chloride, hydrogen bromide, chlorosilanes and chlorine. This material comprises an octahedral alumina substrate with Al—O—Al linkages, which is functionalized with chlorosilyl groups. The material removes water from the gas by an irreversible chemical reaction of the surface chlorosilyl groups with water, and is capable of removing moisture to levels below 0.1 ppm.
There are a number of disadvantages associated with the use of chlorosilylated alumina for removal of trace moisture from acid gases. The preparation of this material is complex and expensive, involving treatment with silicon tetrachloride (SiCl
4
), which is a corrosive material. Moreover, chlorosilylated alumina is only suitable for applications using low pressure HCl, i.e., about 50 psig or less. At high pressure, the HCl reacts with the alumina, producing aluminum trichloride (AlCl
3
or the dimer, Al
2
Cl
6
) which contaminates the purified gas stream. In the case of HBr, contamination with the aluminum halide occurs even at low pressure since HBr is more reactive than HCl and AlBr
3
(Al
2
Br
6
) is more volatile than AlCl
3
by about an order of magnitude. The leaching of aluminum from chlorosilylated alumina purifiers in this manner causes the structure of the chlorosilylated alumina to degrade, resulting in particulate contamination of the gas, and necessitating frequent replacement of this solid purifier. Moreover, the material requires a preconditioning step with the halide acid gas during which water is initially generated, with a concomitant temperature increase to 120-150° C. This preconditioning step is time consuming and requires the use of a significant quantity of costly halide acid gas. Furthermore, in many applications, the preconditioning must be conducted off-line, so that critical downstream components are not damaged by the initial surge of moisture from the purifier.
Alumino-silicate zeolites, in particular, molecular sieves of the Zeolite A family such as the 3A, 4A and 5A zeolites, are well known moisture adsorbents. However, the Zeolite-A molecular sieves have proved to be unsuitable for drying acid gases such as HCl and HBr. See, e.g., Barrer, R. M. and Kanellopoulos, A. G., 1970, “The Sorption of Ammonium Chloride Vapor in Zeolites. Part I. Hydrogen Chloride and Ammonia,” J.
OF THE
C
HEM
. S
OC
. (A):765 (decomposition of 4A molecular sieves was observed upon exposure to hydrogen chloride at a pressure of 228 mm Hg for 18 hours at 50° C.). The stability of the alumino-silicate zeolites to hydrogen chloride has been found to relate to the silica-to-alumina ratio. The higher the silica-to-alumina ratio, the more stable the zeolite is to hydrogen chloride, with zeolites having silica-to-alumina ratios of 10 and above being considered sufficiently stable to HCl. In contrast, the Type A and Type X (synthetic faujasite) zeolites have silica-to-alumina ratios of 2 and 2.5, respectively, which do not provide them with sufficient stability towards hydrogen chloride.
One type of zeolite with a high silica-to-alumina ratio which is used to remove trace water from acid gases is known as the type AW-300 molecular sieve, which is commercially available from UOP. AW-300 is a natural mordenite-type zeolite, which has the structure M
2
O.Al
2
O
3
.10SiO
2
.6H
2
O, M being an alkali metal such as Na; a silica-to-alumina ratio of 10, and a pore size of 4 angstroms. This type of mordenite has been reported as useful for removing water from gas mixtures containing hydrogen chloride, such as reformer recycle hydrogen, flue gas, chloroform, trichloroethylene, vinyl chloride, and chlorine. Collins, J. J., “A Report On Acid-Resistent Molecular Sieve Types AW-300 and AW-500,” Molecular Sieves Product Data Sheet, Union Carbide International Co., 270 Park Avenue, New York, N.Y. 10017. Regeneration of the zeolite is accomplished by desorbing the water by purging with a hot gas at 300-600° F. (150-315° C.). Id See also “Method for Dehydrating Butadiene-Hydrogen Chloride Mixture,” Japanese Kokai 77 89,602 (Cl. C07C11/16) Jul. 27, 1977 [c.f. CA 87:202855q]. Activated synthetic mordenite has also been reported to be useful for drying hydrogen chloride. “Purification of Acidic Gases By Synthetic Mordenite,” Japanese Kokai Tokyo Koho JP 61 54,235 [86 54,235] [c.f. CA 105:8642t]; “Zeolite For Purification of Chlorine or Hydrogen Chloride for Semiconductor Use,” Japanese Kokai 77 65,194 (cl. C01B7/02), May 30, 1977 [c.f. CA:87:103913a].
The acid-resistant mordenite-type zeolites such as AW-300 have an advantage over chlorosilylated alumina purifiers in that they are stable against alumina leaching due to the fact that the zeolite structure contains isolated tetrahedral AlO
2
units residing within a tetrahedral silica matrix. These units create water adsorption sites that are related to the ion exchange properties and capacity of the zeolite. In contrast, the alumina of chlorosilylated alumina is octahedral and has Al—O—Al chemical linkages which are more vulnerable to attack and destruction by acid gases.
While the high silica mordenites have certain advantages over chlorosilylated aluminas, they are not without disadvantages. Chlorosilylated alumina
Bhadha Paul M.
Fraenkel Dan
Watanabe Tadaharu
Matheson Tri-Gas, Inc.
Spitzer Robert H.
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