Adsorbent for moisture removal from fluorine-containing fluids

Details Adsorbent for moisture removal from fluorine-containing fluids Adsorbent for moisture removal from fluorine-containing fluids

2002-10-22

2004-03-23

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Inorganic gas or liquid particle sorbed

C095S126000, C095S900000, C096S108000, C096S153000, C423S210000

Reexamination Certificate

active

06709487

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to the removal of impurities contained in fluidic streams. More specifically, the invention relates to an adsorbent, and apparatus comprising same, for the removal of water from a fluorine-containing fluid such as nitrogen trifluoride and methods for making and using same.
In numerous industrial and commercial processes, it may be desirable to provide fluorine-containing fluids such as nitrogen trifluoride (NF
3
) as high purity, anhydrous materials. One area in which high purity nitrogen trifluoride is required in essentially water-free form is in semiconductor manufacturing operations. For example, gaseous nitrogen trifluoride may be used in the plasma etching of wafers. It may also be used as a cleaning gas in chemical vapor deposition (CVD) chambers following oxide deposition. One difficulty in preparing fluorine-containing fluids such as nitrogen trifluoride is that it can form strong hydrogen bonds with water molecules. Because of this, water contamination of nitrogen trifluoride or other fluorine-containing gases may be common.
Water needs to be removed from nitrogen trifluoride because trace amounts of water can significantly reduce the etching rate and thereby increase the consumption of nitrogen trifluoride. Water removal from nitrogen trifluoride and other fluorine-containing gases, however, is difficult because these gases are relatively strong oxidizers and may react with traditionally-used water scavengers at typical purification conditions, i.e., ambient temperature and pressures up to 1450 psig for nitrogen trifluoride. Accordingly, there is a need for a strong yet stable getter or adsorbent for water removal from nitrogen trifluoride and other fluorine-containing gases.
One method used to remove water from nitrogen trifluoride and other fluorine-containing gases is employing moisture-sorptive molecular sieves. In this connection, molecular sieves may be used as a drying agent for dilute nitrogen trifluoride, i.e., up to 50%, at ambient temperature and slightly above ambient pressure (e.g., 5 to 10 psig). However, nitrogen trifluoride can react with sites on the molecular sieves at higher pressures and/or high concentrations thereby generating volatile impurities. This reaction may lead to the breakdown of the sieve substrate. As a result, molecular sieves may not be practical for drying nitrogen trifluoride in semiconductor manufacturing applications such as point-of-use purifiers or built-in purifiers wherein the partial pressure of nitrogen trifluoride is normally as high as 1450 psig (e.g., cylinder pressure at 20° C.).
European Patent Application 0501933 describes an activated zirconium, vanadium, and iron alloy that is used to remove water or oxygen impurities from a noncorrosive, fluorine-containing gas such as CF
4
, CHF
3
, CF
2
Cl
2
, C
2
F
6
, and SF
6
. The zirconium, vanadium and iron alloy is activated by heating the alloy to a temperature of greater than 300° C. The activated zirconium, vanadium and iron alloy operates at an elevated temperature between 100° C. and 150° C. This elevated temperature range allows the alloy to be active towards the water and oxygen impurities within the fluorine-containing gas. One drawback to this approach, however, is that power and additional capital equipment are needed to maintain the fluorine-containing gas at the elevated temperature. Further, the zirconium, vanadium and iron alloy may be unstable in certain corrosive fluorine-containing gases such as nitrogen trifluoride.
U.S. Pat. No. 5,589,148 describes a process for removing water and a hydrogen halide such as hydrogen bromide, hydrogen chloride, or hydrogen fluoride from a halogen-containing gas stream using a purifying agent consisting of a mixture of an alkaline-earth hydroxide and iron oxide. The purifying agent is prepared by mixing the alkaline-earth hydroxide such as strontium hydroxide with iron oxide at an atomic ratio of alkaline earth to iron of 15:1 to 1:12. The iron oxide can consist of tri-iron tetraoxide, iron(II) oxide, iron (II) hydroxide, iron (II) oxide and iron (III) hydroxide. The alkaline-earth hydroxide and the iron oxide are bound together using a polymeric binding agent. A major concern of using this approach for drying nitrogen trifluoride gas is that the purifying agent may react with nitrogen trifluoride which may degrade the purifying agent as well as introduce volatile by-products. As mentioned earlier, volatile by-products are undesirable contaminants in the aforementioned semiconductor manufacturing applications.
U.S. Pat. No. 6,033,460 describes a process and composition for removing moisture from a nitrogen trifluoride or fluoride gas by using an adsorbent consisting of alumina particles coated with aluminum trifluoride (AlF
3
). The AlF
3
coating is formed by reacting alumina particles with an aqueous solution of hydrogen fluoride. This coating may crack thereby exposing the underlying alumina to nitrogen trifluoride. One potential drawback to the '460 adsorbent is that alumina is highly reactive to nitrogen trifluoride which could undermine the effectiveness of the adsorbent as a water scavenger as well generate gaseous by-products that can contaminate the NF
3
. A further drawback is that the surface area of the '460 adsorbent may be reduced due to the volume expansion from the conversion of Al
2
O
3
to AlF
3
. Since AlF
3
is stoichiometrically less dense than Al
2
O
3
, the volume expansion at the surface of the alumina particles may block the open pores of the adsorbent thereby lowering the capacity for water.
U.S. Pat. No. 6,110,258 discloses a method for removing trace moisture from a gas such as nitrogen trifluoride using a zeolite adsorbent having a silica-to-alumina ratio above 10. The zeolite is heated to a temperature above 650° C. prior to contacting the zeolite adsorbent with the gas.
Accordingly, there is a need in the art for an adsorbent for water in nitrogen trifluoride or other fluorine-containing fluid that does not react with the fluid itself which may generate undesirable contaminants. There is a need in the art to provide a adsorbent for water in nitrogen trifluoride or other fluorine-containing fluid that does not require operation at temperatures greater than ambient. Further, there is a need in the art for a commercially viable adsorbent that does not require high activation temperatures.
All references cited herein are incorporated herein by reference in their entirety.
BRIEF SUMMARY OF THE INVENTION
The present invention fulfills some, if not all, of the needs in the art by providing adsorbents for removal of water from a fluorine-containing fluid such as nitrogen trifluoride. The present invention is directed to, in part, an adsorbent comprising an organic substrate having a porosity of 30% or greater and a pore size of 2 &mgr;m or less and a metal fluoride disposed within at least a portion of the substrate. In certain preferred embodiments, the metal fluoride is at least one metal fluoride from the group consisting of CsF, CuF
2
, KPF
6
, AlF
3
, CaF
2
, FeF
2
, ZrF
2
, SbF
3
, LiPF
6
, NaPF
6
, K
2
NbF
7
, or combinations thereof.
In a further aspect of the present invention, there is provided an adsorbent for the removal of water from a fluorine-containing fluid. The adsorbent comprises a fluorine-containing organic substrate comprising a porosity of 30% or greater; and a metal fluoride disposed within at least a portion of the organic substrate wherein the metal fluoride is at least one selected from the group consisting of CsF, CuF
2
, KPF
6
, AlF
3
, CaF
2
, FeF
2
, ZrF
2
, SbF
3
, LiPF
6
, NaPF
6
, K
2
NbF
7
, or combinations thereof. In certain preferred embodiments, the organic substrate comprises polytetrafluoroethylene.
In yet a further aspect of the present invention, there is disclosed a method for preparing an adsorbent for removing water from a fluorine-containing fluid. This method comprises: providing a fluorine-containing organic substrate comprising a porosity of 30% or greater; combining the

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