 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to fluorosilanes which are especially useful as
substrates used in the chemical vapor deposition (CVD) of
silicon-oxy-fluoride films on surfaces. More particularly, the present
invention relates to a process for the preparation of fluorosilanes by the
reaction of hydrogen fluoride on materials containing silicon hydride
bonds.
BACKGROUND OF THE INVENTION
The utility of fluorosilanes, especially alkoxyfluorosilanes, as agents of
CVD is widely known. For example, fluorotriethoxysilane is useful in the
electronics industry for the fabrication of semiconductor devices. The
fluorotriethoxysilane may be used to deposit fluoride containing silicon
oxide film using a variety of techniques including plasma deposition,
e.g., Homma EP 92-305 192; Spin-on-glass Homma, J. Electro. Chem. Soc.,
140, 2046 (1993) and catalytic CVD Homma, J. Electro. Chem. Soc., 140, 687
(1993). The films deposited have excellent step coverage and are useful as
interlayer dielectric films.
A variety of synthetic methods for the preparation of fluorosilanes are
known to those skilled in the art. One general method known is the
conversion of other silicon halides, in particular silicon chlorides to
silicon fluorides by the action of halogen exchange fluorination agents.
This can be achieved using a variety of reagents such as metal fluorides
and hydrogen fluoride. Specific examples in the literature include Booth,
J. Amer. Chem. Soc., 68, 2655 (1946) in which butyltrichlorosilane is
converted to butyltrifluorosilane in low yield by the action of antimony
trifluoride: furthermore, Marans, J. Amer. Chem. Soc., 73, 5127 (1951)
demonstrates the halogen exchange converting triethylchlorosilane to
triethylfluorosilane in 81% yield using 48% aqueous hydrofluoric acid.
Another general method is the substitution of a fluoride for an alkoxy or
aryloxy group bonded to silicon. Examples of this approach include Marans
(as above) wherein, for example, di-n-propyldiethoxysilane is converted to
di-n-propyldifluorosilane in 52% yield by reaction with 48% aqueous
hydrofluoric acid. Tetraethoxysilane may be converted to
fluorotriethoxysilane as described by Peppard, J. Amer. Chem. Soc., 68,
76, 1946 by the reaction of antimony trifluoride catalyzed by antimony
pentachloride; or, as described by Homma (as above) by the reaction of
hydrogen fluoride.
The existing technologies suffer generally from low yields, the use of
excess amounts of fluorinating agents or expensive reagents, and the
generation of reactive by-products such as hydrogen chloride or ethanol.
Thus, there continues to exist the need for a process which gives high
yields of silicon fluorides from commercially available and inexpensive
starting materials.
SUMMARY OF THE INVENTION
According to the present invention, silicon hydride substrates, such as,
for example, alkyl, cycloalkyl, alkoxy, aryl, aryloxy or siloxy silanes
containing at least one silicon hydride bond, are treated with hydrogen
fluoride to give high yields of the corresponding silicon fluorides with
the concomitant generation of hydrogen. In one embodiment of the invention
hydrogen fluoride is contacted with triethoxysilane to give a near
quantitative yield of fluorotriethoxysilane, with surprisingly little
contamination of the desired product by material formed by substitution of
fluoride for ethoxy.
The present invention contemplates therefore a process for producing
fluorosilanes, the process comprising
(a) reacting
(i) a substrate compound having one or more silicon hydride bonds
selectively with
(ii) hydrogen fluoride, alone, or in admixture with an inert gas, under
hydrogen eliminating conditions until the reaction is substantially
complete; and
(b) recovering
(i) a reacted substrate compound having one or more silicon fluoride bonds,
(ii) a mixture of such reacted substrate compounds or
(iii) an oligomeric derivative of (i) or (ii).
In its preferred features, the invention provides such a process in which
the substrate compound is of the formula
(R).sub.4--n --SiH.sub.n
wherein R is an organic group and n is an integer of from 1 to 3, and,
particularly, a process in which the substrate compound is of the formula
R.sub.1 R.sub.2 R.sub.3 --Si--H
wherein R.sub.1, R.sub.2, and R.sub.3, independently, are selected from
alkyl, cycloalkyl, aryl, alkoxy, aryloxy, siloxy or hydrogen, or a mixture
of any of the foregoing; a process in which hydrogen fluoride is contacted
with a silicon hydride containing substrate at temperatures within a range
from about -40.degree. C. to about +200.degree. C. and at pressures from
within a range of about full vacuum to about 200 psig; especially one in
which the reaction temperature is from about +10.degree. C. to about
+100.degree. C.
Preferred embodiments of the invention embrace such processes in which the
hydrogen generated is either removed from the reaction mass physically or
chemically, or is contained within the reaction; those wherein the
reactants are contacted either in a batch operation or in a continuous
reactor; those which are carried out with a reaction mixture consisting
essentially of the silicon hydride substrate (a)(i) and hydrogen fluoride,
alone, or in admixture with an inert gas (a)(ii); those carried out with a
reaction mixture comprising the silicon hydride substrate (a)(i), hydrogen
fluoride, alone or in admixture with an inert gas (a)(ii), and an inert
liquid medium (a)(iii), at a temperature of between about the freezing
point of the medium to about the boiling point of the medium.
Special mention is made of a process as defined above which comprises
contacting hydrogen fluoride with triethoxysilane with the introduction of
said hydrogen fluoride either in the gas or liquid phase to generate as
the principal product fluorotriethoxysilane; as defined above which
comprises contacting triethylsilane with hydrogen fluoride to generate
triethylfluorosilane and hydrogen; a process which comprises contacting a
silicon hydride containing substrate and hydrogen fluoride in the presence
of the product mixture from the reaction of a tetraalkoxysilane or
alkylalkoxysilane such that the conversion to a fluorosilane is enhanced;
and a process which comprises contacting triethoxysilane with pyridinium
poly(hydrogen fluoride) to generate fluorotriethoxysilane.
DETAILED DESCRIPTION OF THE INVENTION
We have found surprisingly that silanes of the general formula R.sub.1
R.sub.2 R.sub.3 SiH where R.sub.1, R.sub.2 and R.sub.3 can be alkyl,
cycloalkyl, aryl, alkoxy, aryloxy, siloxy or hydrogen can be treated with
hydrogen fluoride to produce the corresponding fluorosilane R.sub.4
R.sub.5 R.sub.6 SiF where R.sub.4, R.sub.5, R.sub.6 are alkyl, cycloalkyl,
aryl, alkoxy, aryloxy, siloxy or fluoride in high yield with little or no
by-product formation other than the generation of hydrogen.
R.sub.1, R.sub.2 and R.sub.3 may comprise hydrocarbyl groups where R.sub.1
and/or R.sub.2 and/or R.sub.3 are aliphatic or cycloaliphatic alkyl
wherein R.sub.n is C.sub.1 -C.sub.30 ;R.sub.1 and/or R.sub.2 and/or
R.sub.3 is alkoxy or alkyl alkoxy with R.sub.n C.sub.1 -C.sub.10 ; R.sub.1
and/or R.sub.2 and/or R.sub.3 may also be aryl or aralkyl or aryloxy or
aralkyloxy with R.sub.n C.sub.6 -C.sub.30 or siloxy. The substrates may
also contain more than one silicon hydride bond, so R.sub.1 and/or R.sub.2
and/or R.sub.3 may represent hydrogen radicals. The substrates may be
partially fluorinated so R.sub.1 and/or R.sub.2 and/or R.sub.3 may
represent fluoride radicals.
Specific examples of silanes useful as hydrogen fluoride reactive
substrates in accordance with the present invention include, but are not
limited to, trimethylsilane, triethylsilane, tripropylsilane,
methyldiethylsilane, triphenylsilane, phenyldimethylsilane,
dimethylsilane, diethylsilane, diphenylsilane, methylphenylsilane,
trimethoxysilane, triethoxysilane, methyldimethoxysilane,
phenyldimethoxysilane, phenoxysilane, dimethylphenoxysilane,
6-chlorohexyldimethylsilane, 4-chlorobenzyldimethylsilane, tris
(methoxyethoxy)silane, pentamethyldisiloxane,
1,1,2,2-tetramethyldisiloxane, tris(trimethylsiloxy)silane, and the like,
mixtures of any of them and the like, alone, and in admixture with dimers,
trimers, and other oligomers thereof.
The selectivity of the reaction forming the basis of the invention is
particularly surprising in the case where R.sub.1 and/or R.sub.2 and/or
R.sub.3 is alkoxy or aryloxy. In these cases it would be expected for
hydrogen fluoride to react with the alkoxy or aryloxy groups to generate
the fluorosilane and corresponding alcohol. However, we have found that
the silicon hydride bond reacts preferentially with hydrogen fluoride
under mild reaction conditions with the generation of relatively few
fluoride for alkoxy substituted products. In fact, it is one preferred
embodiment of this invention to improve, for example, the reaction of
tetraethoxysilane with hydrogen fluoride by the subsequent addition of
triethoxysilane to remove unreacted hydrogen fluoride from the system
enhancing process yields and ease of purification.
The hydrogen fluoride utilized in the reaction may be substantially
anhydrous or be added as an aqueous solution. The hydrogen fluoride is,
however, preferably anhydrous. The hydrogen fluoride may be contacted with
the silane in the gas or liquid phase. The hydrogen fluoride may also be
introduced as a salt, for example an amine salt, or as a stabilized liquid
such as pyridinium poly(hydrogen fluoride).
A feature of the reaction, in all cases, is the generation of hydrogen
which may preferably be removed continuously from the reaction. The
reactions can be run successfully at subatmospheric or superatmospheric
pressure, however, the reactions are generally run at atmospheric
pressure.
The reaction is generally conducted at ambient to moderate temperatures,
although reduced temperatures down to -40.degree. C. or temperatures of up
to 200.degree. may be utilized, depending upon the physical properties and
reactivity of the substrate.
Solvents useful in this invention are inert solvents that do not degrade
under the reaction conditions. Solvents such as toluene, xylene or heptane
may be used. The reaction is most preferably carried out in the absence of
solvent in the cases where the substrate to be treated with hydrogen
fluoride is a liquid.
Preferably the reaction contents are agitated to maintain a well mixed
solution and the hydrogen fluoride is fed into the reaction to maintain
control of the heat generated. External cooling may be applied to the
reaction vessel. The process may be run either in a batch manner or in a
continuous manner, such as the concurrent feed of the substrate and
hydrogen fluoride through a static mixer. The products can be isolated
using conventional purification techniques such as distillation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following examples illustrate the present invention, but the claims are
not to be construed as limited thereto.
EXAMPLE 1
A 1-gallon stirred stainless steel reactor was loaded with 2550 g of
triethoxysilane. Gaseous anhydrous hydrogen fluoride, 311 g, was then
added subsurface during a period of 11/2 hours. The temperature was
controlled to less than 25.degree. C. by recirculation of ice water
through internal cooling coils. After stirring for 16 hours at room
temperature analysis by Gas Chromatography (GC) showed the composition to
be 2.2% ethanol, 2.9% fluorodiethoxysilane, 3.6% difluorodiethoxysilane,
8.6% triethoxysilane, 80.2% fluorotriethoxysilane and 1.6%
tetraethoxysilane. After heating to 40.degree.-50.degree. C. for about 80
hours the composition was 0.6% difluorodiethoxysilane, 0.25%
triethoxysilane, 89.1% fluorotriethoxysilane and 10.1% tetraethoxysilane.
EXAMPLE 2
The same reactor was charged with 800 g of tetraethoxysilane. Gaseous
anhydrous hydrogen fluoride, 77 g, was then added in an equivalent manner
to example 1. After stirring for 16 hours at 50.degree. C. analysis by GC
showed the composition to be 26.6% ethanol, 10.7% difluorodiethoxysilane,
40.7% fluorotriethoxysilane, 14.5% tetraethoxysilane and 7.39% siloxane
oligomers. Triethoxysilane, 600 g was then added portionwise over a period
of two hours. After heating to 50.degree. C. analysis by GC showed the
composition to be 0.17% fluorodiethoxysilane, 0.22%
difluorodiethoyxsilane, 3.8% triethoxysilane, 43.7% fluorotriethoxysilane,
44.5% tetraethoxysilane and 7.64% siloxane oligomers.
EXAMPLE 3
The same reactor was charged with 500 g of triethylsilane. Gaseous
anhydrous hydrogen fluoride, 86 g, was then added in an equivalent manner.
After stirring at 50.degree. C. for 18 hours analysis by GC showed the
composition to be 42.1% triethylsilane and 56.2% fluorotriethylsilane (97%
yield based upon 100% conversion of the starting material).
EXAMPLE 4
The same reactor was charged with 300 g of diethylsilane. Gaseous anhydrous
hydrogen fluoride, 50 g, was then added in an equivalent manner. A GC
showed the composition to be 62.8% diethylsilane, 21.0%
fluorodiethylsilane and 8.6% difluorodiethylsilane.
EXAMPLE 5
A 500 ml Teflon flask was charged with 10 g of triethoxysilane. Aqueous
hydrofluoric acid (50%), 1 ml, was added very slowly to control the
exotherm to less than 60.degree. C. Analysis by GC showed the composition
to be 32.4% ethanol, 4.0% fluorodiethoxysilane, 43.2% triethoxysilane and
9.2% fluorotriethoxysilane.
The patents, patent applications and publications cited above are
incorporated herein by reference.
While there have been described what are presently believed to be preferred
embodiments of the invention, it will be apparent to a person skilled in
the art that numerous changes can be made in the ingredients, conditions
and proportions set forth in the foregoing embodiments. For example, the
silicon hydride can be dissolved in a solvent, such as toluene, xylene or
heptane and treated with gaseous hydrogen fluoride, an amine salt of
hydrogen fluoride, or a stablized liquid form of hydrogen fluoride, such
as pyridinium poly(hydrogen fluoride). The reaction can be carried out
continuously in a loop reactor comprising cooling zones for controlling
the heat of reaction. All such obvious modifications can be employed
without departing from the invention as described herein and as defined in
the appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
