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Claims  |
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What is claimed is:
1. A process for making transparent films of stannic oxide on a heated
substrate using a gaseous mixture initially containing
(1) a first organotin fluorine-bearing compound which is free of any direct
tin-fluorine bond
(2) a oxidizable, tin compound, and
(3) an oxidizing gas said process comprising the steps of:
(a) converting said first organotin fluorine-bearing component of said
gaseous mixture into a second organotin fluoride gaseous compound having a
direct tin-to-fluorine bond;
(b) immediately oxidizing, in immediate proximity to said substrate, the
second fluoride compound to obtain a fluorine dopant in the gaseous
mixture;
(c) and forming a fluorine doped stannic oxide film, on said heated
substrate, by simultaneous deposition thereon of said oxidizable tin
compound and said fluorine dopant.
2. A process as defined in claim 1 wherein said first organotin
fluorine-bearing gaseous compound is formed by heating a gas mixture
containing (a) a gas selected from the group consisting of CF.sub.3 I,
CF.sub.3 Br, and CF.sub.3 SF.sub.5 and homologous alkyl alpha-fluorinated
compounds of said CF.sub.3 I, CF.sub.3 Br and CF.sub.3 SF.sub.5, SF.sub.5
Br, and SF.sub.5 Cl; or mixtures thereof and (b) said oxidizable tin
compound, and wherein (a) and (b) are substantially inert with respect to
one another at temperatures below about 150.degree. F.
3. A process as in claim 2 in which a mixture of (a) and (b) is stable at
about 90.degree. F., but wherein a reaction of (a) and (b) is thermally
initiated and forms an organotin monofluoride vapor; said vapor forming a
source for controlled addition of fluorine impurity to said film of
stannic oxide.
4. A process as in claim 3 in which the fluorine dopant is formed by
reacting trifluoroiodomethane and an organotin compound containing at
least one tin-carbon bond per molecule.
5. A process as in claim 4 in which the fluorine dopant is forming a
reacting trifluoroidomethane gas and tetramethyltin.
6. A process as in claim 5 wherein bromine is substituted for iodine.
7. A process as in claim 4 in which bromine is substituted for iodine.
8. A process as in claim 3 in which the fluorine dopant is formed by
reacting sulfur chloride pentafluoride gas and an organo-tin compound
containing at least one tin-carbon bond per molecule.
9. A process as in claim 8 in which the fluorine dopant is formed by
reacting sulfur chloride pentafluoride gas and tetramethyltin.
10. A process as in claim 3 in which the fluorine dopant is formed by
reacting trifluoromethyl sulfur pentafluoride gas and an organo-tin
compound containing at least one tin-carbon bond per molecule.
11. A process as in claim 10 in which the fluorine dopant is formed by
reacting trifluoromethyl sulfur pentafluoride gas and tetramethyltin.
12. A process as defined in claim 1 wherein said conversion of said first
volatile organotin fluorine-bearing compound which is free of any direct
tin-fluorine bond, into said organotin fluoride gaseous compound having a
direct tin-fluorine bond, takes place on heating by said substrate.
13. A process as defined in claim 1 wherein said substrate to be coated
faces downwardly and said gaseous mixture is directed upwardly toward said
surface.
14. A process as in claim 1 in which tetramethyltin vapor, at
concentrations up to about one percent, is the volatile oxidizable tin
compound; oxygen gas, at partial pressures up to about one atmosphere, is
the oxidizing gas; and said stannic oxide is deposited on a surface heated
at about 500.degree. C.
15. A process as in claim 1 in which said first fluorine-bearing compound
is a volatile tin compound which decomposes on heating to form an
organotin monofluoride vapor.
16. A process as in claim 9 in which said volatile tin compound is
trimethyl trifluoromethyl tin.
17. A process as in claim 9 in which the volatile tin compound is trimethyl
pentafluoroethyl tin.
18. A process as defined in claim 1 wherein the ratio of fluorine dopant
and oxidizable tin compound are selected that the free election
concentration of the films is within about a range of from 10.sup.20
cm.sup.-3 to 10.sup.21 cm.sup.-3.
19. A process as defined in claim 1 wherein fluorine dopant levels in said
stannic oxide film are about 1% to 3% fluorine substituted for oxygen.
20. A process for depositing transparent, fluorine-doped, tin-oxide films,
on a heated substrate said process comprising the steps of
(1) supplying a continuous stream of a reagent gas to the vicinity of said
substrate, said reagent gas containing reagents which are convertible to
an organotin fluoride compound having a direct tin-fluorine bond in the
immediate proximity of said heated substrate, and
(2) depositing said organotin fluoride compound with an oxidizable tin
component of said reagent gas at the surface of said gas and thereby
achieving a fluorine-doped, tin oxide coating upon said surface.
21. A process for depositing films of fluorine-doped stannic oxide on a
heated substrate, said process comprising mixing
(a) a gaseous, fluorine-bearing component, and
(b) a gaseous, oxidizable tin-bearing component, and
(c) a gaseous, oxygen-bearing component, and, optionally,
(d) inert carrier gas these components being selected so that they remain
in the gas phase at the temperature of mixing and wherein component (a)
forms a compound with a tin-fluorine bond only as the gas mixture is
heated to about the temperature of said heated substrate, said compound
with a tin-fluorine bond and said oxygen bearing component then reacting
to deposit said film of fluorine-doped stannic oxide on said heated
substrate, and wherein said component (a) contains a volatile organotin
fluorine-bearing compound which is free of any direct tin-fluorine bond
but which rearranges on heating to form a direct tin-fluorine bond at
temperatures high enough so that the newly-formed compound with a direct
tin-fluorine bond remains in the vapor phase until it reacts along with
the oxidizable tin compound to deposit a film of fluorine-doped tin oxide.
22. A process as in claim 21, in which component (a) contains a fluoroalkyl
group or substituted fluoralkyl group, bonded to a tin atom.
23. A process as in claim 22 in which component (a) contains trimethyl
trifluoromethyl tin.
24. A process as in claim 23 in which component (b) contains an
organometallic tin compound.
25. A process as in claim 24 in which component (b) contains tetramethyl
tin.
26. A process as in claim 22 in which component (a) contains trimethyl
pentafluoroethyl tin.
27. A process as in claim 22 in which component (b) contains an
organometallic tin compound.
28. A process as in claim 27 in which component (b) contains tetramethyl
tin.
29. A process as in claim 21 in which component (b) contains a compound
containing at least one carbon-tin bond.
30. A process as in claim 29 in which component (b) contains tetramethyl
tin.
31. A process as in claim 29 in which component (b) contains dimethyl tin
dichloride.
32. A process as in claim 21, in which component (b) contains an
organometallic tin compound.
33. A process as in claim 32 in which component (b) contains tetramethyl
tin.
34. A process for depositing films of fluorine-doped stannic oxide on a
heated said process comprising mixing
(a) a gaseous, fluorine-bearing component, and
(b) a gaseous, oxidizable tin-bearing component, and
(c) a gaseous, oxygen-bearing component, and, optionally,
(d) inert carrier gas these components being selected so that they remain
in the gas phase at the temperature of mixing and wherein component (a)
and component (b) react to form a compound with a tin-fluorine bond only
as the gas mixture is heated to about the temperature of said heated
substrate, said compound with a tin-fluorine bond and said oxygen bearing
component then reacting to deposit said film of fluorine-doped stannic
oxide on said heated substrate in which component (a) contains reactive
fluoroalkyl groups and in which component (b) contains an organometallic
tin compound.
35. A process as in claim 34 in which component (a) contains fluoroalkyl
halides, or mixtures thereof.
36. A process as in claim 35 in which component (a) contains a gas selected
from the group consisting of CF.sub.3 Br, CF.sub.3 I and homologous or
substituted fluorinated compounds, or mixtures thereof.
37. A process as in claim 36 in which component (b) contains an
organometallic tin compound.
38. A process as in claim 37 in which component (b) contains
tetramethyltin.
39. A process as in claim 37 in which component (b) contains dimethyl tin
dichloride.
40. A process as in claim 35 in which component (b) contains an
organometallic tin compound.
41. A process as in claim 40 in which component (b) contains tetramethyl
tin.
42. A process as in claim 34, in which component (a) contains reactive
fluorosulfur groups.
43. A process as in claim 34 in which component (b) contains tetramethyl
tin.
44. A process for depositing films of fluorine-doped stannic oxide on a
heated substrate said process comprising mixing
(a) a gaseous, fluorine-bearing component, and
(b) a gaseous, oxidizable tin-bearing component, and
(c) a gaseous, oxygen-bearing component, and, optionally,
(d) inert carrier gas these components being selected so that they remain
in the gas phase at the temperature of mixing and wherein component (a)
and component (b) react to form a compound with a tin-fluorine bond only
as the gas mixture is heated to about the temperature of said heated
substrate, said compound with a tin-fluorine bond and said oxygen bearing
component then reacting to deposit said film of fluorine-doped stannic
oxide on said heated substrate;
in which component (a) contains reactive fluorosulfur groups; and in which
component (b) contains an organometallic tin compound.
45. A process as in claim 44 in which component (b) contains tetramethyl
tin.
46. A process for depositing films of fluorine-doped stannic oxide on a
heated substrate said process comprising mixing
(a) a gaseous, fluorine-bearing component, and
(b) a gaseous, oxidizable tin-bearing component, and
(c) a gaseous, oxygen-bearing component, and, optionally,
(d) inert carrier gas these components being selected so that they remain
the gas phase at the temperature of mixing and wherein component (a) and
component (b) react to form a compound with a tin-fluorine bond only as
the gas mixture is heated to about the temperature of said heated
substrate, said compound with a tin-fluorine bond and said oxygen bearing
component then reacting to deposit said film of fluorine-doped stannic
oxide on said heated substrate;
in which component (a) contains reactive fluorosulfur groups selected from
SF.sub.5 Cl, SF.sub.5 Br, or SF.sub.5 CF.sub.3, and homologous or
substituted compounds of mixtures thereof; and in which component (b)
contains organometallic tin compound.
47. A process as in claim 46 in which component (b) contains tetramethyl
tin. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to an improved process for the production of
electrically-conductive layers which are highly transparent to visible
light and highly reflective to infrared light, and to the particularly
advantageous coatings formed therewith. Such layers are useful as
transparent electrodes for solar photovoltaic cells, photoconductive
cells, liquid crystal electro-optical displays, photoelectrochemical
cells, and many other types of optical-electronic devices. As transparent
electrical resistors, such layers are used for defrosting windows in
airplanes, automobiles, etc. As heat-reflecting transparent coatings on
glass, these layers enhance the efficiency of solar thermal collectors and
of windows in buildings, ovens, furnaces, and sodium-vapor lamps, and of
fiberglass insulation.
BACKGROUND OF THE INVENTION
Various metal oxides, such as stannic oxide SnO.sub.2, indium oxide
In.sub.2 O.sub.3, and cadmium stannate Cd.sub.2 SnO.sub.4, have been the
most widely used materials for forming transparent, electrically
conductive coatings and layers.
The earliest methods of applying these coatings were based on spraying a
solution of a metal salt (usually the chloride) on a hot surface, such as
glass. In this way, satisfactory transparent, electrically resistive
layers were first made for de-icing aircraft windows. However, the spray
process produced rather corrosive by-products, hot chlorine and hydrogen
chloride gases, which tended to attack the hot glass surface, producing a
foggy appearance. U.S. Pat. No. 2,617,745 teaches that this undesirable
effect can be mitigated by first applying a coating of pure silica on the
glass. However, a silica protective layer is not very effective on glass
with a high alkali content and high thermal expansion coefficient, such as
common soda-lime glass. In addition, these corrosive by-products attack
metal parts of the apparatus, and the metallic impurities, such as iron,
may then be deposited in the coating, with deleterious effects on both the
electrical conductivity and transparency of the coating.
Another problem has been a lack of uniformity and reproducibility in the
properties of the coatings. U.S. Pat. No. 2,651,585 teaches that better
uniformity and reproducibility are obtained when the humidity in the
apparatus is controlled. The use of a vapor, rather than a liquid spray,
as described for example in German Pat. No. 1,521,239, also results in
more uniform and reproducible coatings.
Even with these improvements, more recent studies have been made using
vacuum deposition techniques, such as evaporation and sputtering, in order
to achieve cleaner and more reproducible coatings. Despite the much higher
cost of these vacuum processes, the reduction of corrosive by-products and
unwanted impurities introduced by the spray methods is felt to be
important particularly in applications involving high-purity
semiconductors.
The intentional addition of certain impurities is important in these
processes, in order to achieve high electrical conductivity and high
infrared reflectivity. Thus, tin impurity is incorporated in indium oxide,
while antimony is added to tin oxide (stannic oxide) for these purposes.
In each case the function of these desirable impurities ("dopants") is to
supply "extra" electrons which contribute to the conductivity. The
solubility of these impurities is high, and they can be added readily
using all of the deposition methods referred to above. Fluorine has an
advantage over antimony as a dopant for tin oxide, in that the
transparency of the fluorine-doped stannic oxide films is higher than that
of antimony-doped ones, particularly in the red end of the visible
spectrum. This advantage of fluorine is important in potential
applications to solar cells and solar thermal collectors. Despite this
advantage of fluorine, most -- and perhaps all -- commercially available
tin oxide coatings use antimony as a dopant. Possibly this is because
fluorine doping has only been demonstrated in the less satisfactory spray
method, whereas the improved deposition methods (chemical vapor
deposition, vacuum evaporation and sputtering) are not believed to have
been shown to produce fluorine doping. In addition, a recent report by a
committee of experts in the American Institute of Physics Conference
Proceedings No. 25, p. 288 (1975), concludes that fluorine equilibrium
solubility in tin oxide is inherently lower than that of antimony.
Nevertheless, it is noted that the lowest resistivity tin oxide films
reported in the prior art are those of U.S. Pat. No. 3,677,814 to Gillery.
Using a spray method, he obtained fluorine-doped tin oxide films with
resistances as low as 15 ohms per square by utilizing a compound, as a
starting material, which has a direct tin-fluorine bond. The lowest
resistance in a commercially available tin-oxide coated glass is presently
in the range of about 40 ohms per square. When one wishes to obtain
coatings of as low as 10 ohms per square, one has heretofore been forced
to use the much more expensive materials like indium oxide.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide a process for
depositing a layer or coating of fluorine-doped stannic oxide having a
high visible transparency, high electrical conductivity and high infrared
reflectivity.
Another object of the present invention is to allow the electrical
conductivity to be varied easily during the deposition of a single such
layer and to have the ability to achieve very low volume resistivities and
surface resistances.
Another object of the present invention is to provide a noncorrosive
deposition atmosphere, from which such layers of high purity may be
deposited easily, and without contamination of the substrate by impurities
or corrosive attack on the substrate or apparatus.
Still another object of the invention is to provide gaseous, rather than
liquid, means for making coated products as described herein.
A further object of the present invention is to provide a process which
easily produces such layers with highly uniform and reproducible
properties over large areas without limitations inherent in spraying
procedures.
Another object is to permit easy deposition of such layers inside tubes or
bulbs, or over the surface of complicated shapes not easily sprayed.
Still other objects of the invention are to provide improved articles such
as solar cells, other semiconductors useful in electrical circuitry,
heat-reflective windows, improved sodium lamps and the like.
A further object of the present invention is to permit deposition of such
layers with standard manufacturing processes in the semiconductor industry
and glass industry.
Further objects and advantages will become apparent as the following
description proceeds.
A particular feature of the invention is to select the reactants in such a
way that the required tin-fluorine bond is not formed until the deposition
is imminent. Thus, the tin fluoride material is better maintained in the
vapor phase and at temperatures low enough that oxidation of the compound
occurs only after the rearrangement to form a tin-fluorine bond. Films of
fluorine-doped tin oxide, thus formed, have extraordinarily low electrical
resistivity and extraordinarily high reflectivity to infrared wavelengths.
The process of the invention is carried out utilizing a gaseous mixture
containing a volatile, organotin, fluorine-bearing compound which is free
of any direct tin-fluorine bond. This mixture also contains a volatile
oxidizable tin compound and an oxidizing gas. This first fluorine compound
which is free of a fluorine-tin bond is converted into a second organotin
fluoride compound having such a bond. Immediately after such conversion
this second compound is oxidized to form a fluorine dopant and the dopant
is oxidized along with the oxidizable tin compound to form a stannic oxide
film with a controlled amount of fluorine impurity on said solid
substrate.
In a first form of the invention, an organo-tin mono-fluoride vapor is
formed in the heated deposition region by the reformation of the vapor of
a more volatile compound containing both tin and fluoroalkyl groups bonded
to tin.
A second advantageous embodiment of the invention utilizes an organo-tin
monofluoride formed at or near the gas-substrate interface by reactions
involving an organo-tin vapor and certain fluorine-containing gases having
fluoroalkyl and/or fluorosulfur groups.
The product layer in each case is a uniform, hard, adherent, transparent
coating whose electrical conductivity and infrared reflectivity depend on
the concentration of the fluorine-containing dopant.
IN THE DRAWINGS
FIG. 1 shows a schematic diagram of an apparatus suitable for carrying out
a process in which a fluorine dopant is an organo-tin fluoroalkyl vapor,
evaporated from its liquid form.
FIG. 2 shows a similar diagram for the second embodiment, in which the
fluorine dopant is formed by reaction with certain fluoroalkyl and/or
fluorosulfur gases supplied from a compressed gas cylinder.
FIG. 3 shows a simplified version of the apparatus for practicing either
the first or the second embodiments of the invention.
FIG. 4 is a schematic section of a solar cell and illustrates one use of
the invention in a semiconductor application.
FIG. 5 shows window 120 coated with layer 118 according to the invention.
FIGS. 6 and 7 are graphs illustrative of varying conductivity and
reflectivity with concentrations of fluorine dopant.
The process of this invention has two main steps: (1) forming a reactive
vapor mixture which will produce, on heating, a compound having a
tin-fluorine bond, and (2) bringing this vapor mixture to a heated
surface, on which fluorine-doped tin oxide deposits. The embodiments
described below differ in the chemical source of the fluorine dopant in
the reactive vapor mixture, and also in the means by which the vapor
mixture is made. The second step (deposition on the heated surface) is
largely the same in each example.
The tin is supplied by a volatile, oxidizable tin compound, such as
tetramethyltin, tetraethyltin, dibutyltin diacetate, dimethyltin
dihydride, dimethyltin dichloride, etc. The preferred compound is
tetramethyltin, since it is sufficiently volatile at room temperature,
non-corrosive, stable and easily purified. This volatile tin compound is
placed in a bubbler marked 10 in the Figures, and an inert carrier gas,
such as nitrogen, is bubbled through the tin compound. For the very
volatile compounds, such as tetramethyltin and dimethyltin dihydride, the
bubbler can be at room temperature, while for the other less volatile
compounds, the bubbler and the tubing must be heated appropriately, as
will be understood by those skilled in the art. It is one advantage of the
instant invention that high-temperature apparatus can be avoided and that
simple cold-wall supplies can be used.
The vapor mixture must contain an oxidizing gas, such as oxygen, nitrous
oxide, or the like. Oxygen is the preferred gas, since it is readily
available and works just as well as the more expensive alternate
oxidizers.
The pressures of the gases are fixed by the regulators 25, and the flow
rates of the oxygen from tank 20, and of the carrier gas from tank 21, are
controlled by metering vales 30, and measured by flowmeters 40. The gas
streams then pass through one-way check valves 50 into a mixing tube 60
and funnel-shaped chamber 70. A tin oxide film deposits on the hottest
surface 80, which is heated by the heater 90, typically to temperatures
about 400.degree. to 600.degree. C.
The general type of process just described is commonly known in the art as
chemical vapor deposition. Various modifications, such as having the
substrate surfaces vertical and rotating or below the reaction chamber and
rotating, are known to those skilled in the art, and may be particularly
suitable for use depending upon the geometry of the substrate or other
conditions affecting a given application.
Rotation of the substrate is recommended in order to best move the sample
through any convection currents which may occur in the apparatus and
thereby best assure the uniformity of the deposited layers. However, it
has now been discovered that, by placing the heated substrate facing
downwardly, highly uniform coatings may be obtained more simply without
rotation, because the gas, when heated from above, does not set up
troublesome convection currents. Another advantage of having the substrate
above the reactive vapors is that any dust or dirt, or powder byproduct
formed by homogeneous nucleation in the gas, does not fall onto the
growing film.
An invention described herein is an improved process by which controlled
amounts of fluorine impurity may be introduced into the growing tin oxide
film. In the simplest aspect of this invention, the fluorine dopant is a
vapor containing one tin-fluorine bond in each molecule. The other three
tin valences are satisfied by organic groups and/or halogens other than
fluorine. Typical of such compounds is tributyltin fluoride. It has been
discovered that the fluorine thus bound, and made available to a hot
surface in vapor form, is not cleaved from the tin during oxidation at a
hot surface.
Unfortunately, all known compounds with such a direct tin-fluorine bond are
not significantly volatile near room temperature.
A particular advantage of the invention is achieved by forming the fluorine
dopant from volatile compounds which do not have the required tin-fluorine
bond, but which will rearrange on heating to form a direct tin-fluorine
bond. This rearrangement advantageously occurs at temperatures high enough
(e.g. > 100.degree. C.) so that the tin fluoride thus formed remains in
the vapor phase, but also low enough (e.g. < 400.degree. C.) so that the
oxidation of the compound occurs only after the rearrangement. One example
of such a compound is trimethyl trifluoromethyltin, (CH.sub.3).sub.3
SnCF.sub.3. On heating to a temperature of about 150.degree. C. in a
heated zone adjacent to the deposition surface 80, this compound
rearranges to form a direct tin-fluorine bond, in (CH.sub.3).sub.3 SnF
vapor, which then reacts as the fluorine donor or dopant. Other compounds
which undergo similar rearrangements at temperatures which will, of
course, differ somewhat from compound to compound, have the general
formula R.sub.3 SnRF, where R is a hydrocarbon radical, and RF is a
fluorinated hydrocarbon radical having at least one fluorine atom bonded
to that carbon atom which is bonded to the tin. The main advantage of
these fluorine dopants is that they are volatile liquids, so that they can
easily supply sufficient vapor pressure when evaporated at room
temperature. This simplifies the design of the apparatus, as shown in FIG.
1, by eliminating the need for maintaining a warm zone between the bubbler
15 and the reaction chamber 70, to keep the fluorine dopant in the vapor
phase. Thus the apparatus can be of the type which is usually called a
"cold-wall chemical vapor deposition reactor," which is widely used, for
example, in the semiconductor industry to deposit silicon, silicon
dioxide, silicon nitride, etc. Another important feature of the "cold-wall
reactor" for semiconductor applications is that it minimizes unwanted
impurities at a low level in both the substrate and the deposited film.
Similarly, in glass manufacture, the gas mixture can be added to the
annealing and cooling oven at the stage when the glass is at the
appropriate temperature, e.g. about 470.degree. C. for soft glass. In this
way, highly uniform films can be achieved in the normal glass-production
equipment.
The preferred compound for use in the embodiment of FIG. 1 is
(CH.sub.3).sub.3 SnCF.sub.3, since it is more volatile than the compounds
with more carbon atoms. It is a stable, colorless, non-corrosive liquid,
which does not decompose in air at room temperature, and only reacts
extremely slowly with water.
A particular advantageous second embodiment of the invention uses a
fluorine-containing gas which reacts with an organo-tin vapor on heating,
to produce a tin fluoride vapor. For example, .alpha.-fluoroalkyl halides,
preferably wherein the alkyl group has 4 carbons or fewer, such gases as
iodotrifluoromethane, CF.sub.3 I, CF.sub.3 CF.sub.2 I, C.sub.3 F.sub.7 I,
and the like, can be mixed with organo-tin vapors such as tetramethyltin
vapor (CH.sub.3).sub.4 Sn, at room temperature, i.e. to 90.degree. F., and
more preferably to temperatures of 150.degree. F., without any reaction.
Moreover, fluoroalkyl bromides like CF.sub.3 Br, C.sub.2 F.sub.5 Br and
the like are useful as fluorine-containing gases. They are less reactive
and about 10 to 20 times more are required in the reactant gas, but they
are much less expensive. This is particularly surprising because of the
reputed inertness for such compounds. Fluoroalkyl chlorides are not
favored for use because their reactivity is substantially lower than even
the bromides.
When such a vapor mixture approaches the heated surface, reaction takes
place in the gas phase to, eventually, produce the desired tin-fluorine
bonds. Although the reaction sequence is complex, it is believed to begin
by reactions such as
CF.sub.3 I + R.sub.4 Sn .fwdarw. R.sub.3 SnCF.sub.3 + RI
to yield the organo-tin fluoroalkyl R.sub.3 SnCF.sub.3 vapors in the region
near the interface of the hot surface, where they serve as fluorine
dopants for the growing tin oxide film, just as in the first embodiment.
Certain other fluorine-containing gases also function in this second
embodiment of the invention. For example, sulfur chloride pentafluoride,
SF.sub.5 Cl, is an effective fluorine donor gas, as is sulfur bromide
pentafluoride SF.sub.5 Br.
In a similar way, tri fluoromethyl sulfur pentafluoride CF.sub.3 SF.sub.5
gas acts to form tin-fluoride bonds by gas phase reactions.
The advantage of this second embodiment is that the fluorine donor is a
gas, and the process is further shown in FIG. 2. The preferred gases are
CF.sub.3 I and CF.sub.3 Br, which are non-corrosive, nonflammable, not
appreciably toxic, and readily available commercially. SF.sub.5 Cl and
SF.sub.5 Br and highly toxic, and thus are less desirable for use.
CF.sub.3 SF.sub.5 is non-toxic, but somewhat less reactive than CF.sub.3
I.
The deposition process may be further simplified, as shown in FIG. 3, if
the gas mixtures are pre-mixed and stored in a compressed gas cylinder 19.
For safe storage and use, the oxidizable compound must of course be kept
at a concentration such that it cannot form an explosive mixture. For
example, the lower explosion limit of tetramethyltin in air is about 1.9%.
The concentrations which I have used for the chemical vapor depositions
are less than a 1/2 of this level. In addition, the use of CF.sub.3 I or
CF.sub.3 Br as a fluorine dopant incidentally acts as a flame suppressant.
Films prepared according to the invention are found to have infrared
reflectivities of 90% and more measured, as is known in the art, at the
conventional 10-micron wave length of light which is characteristic of
thermal infrared radiation at room temperature. This 90% reflectivity is
to be compared to the 80% reflectivity which is heretofore achieved using
tin oxide coatings. In usual practice, these infrared reflective layers
will be from about 0.2 to 1 micron in thickness; thicknesses of 0.3 to 0.5
microns are typical.
In order to characterize more quantitatively the fluorine doping levels in
the films, the infrared reflectivity was measured over the wavelength
range of 2.5 microns to 40 microns. By fitting these data with theoretical
curves, as described in detail by R. Groth, E. Kauer and P. C. van den
Linden, "Optical Effects of Free Carriers in SnO.sub.2 Layers,"
Zeitschrift fur Naturforschung, Volume 179, pages 789 to 793 (1962),
values were obtained for the free electron concentration in the films. The
values obtained were in the range from 10.sup.20 cm.sup.-3 to 10.sup.21
cm.sup.-3, and increased regularly with increasing concentrations of the
fluorine dopant. Theoretically, one free electron should be released for
each fluorine atom which replaces an oxygen atom in the lattice. This
hypothesis was verified by Auger Electron Spectroscopic measurements of
the total fluorine concentration in some of the films, which have fluorine
concentrations in agreement with the free electron concentrations, to
within the experimental uncertainties. This agreement is indicative that
most of the incorporated fluorine is electrically active.
The infrared reflectivity at 10 microns, and also the bulk electrical
conductivity of the films, were found to be maximum at a doping level of
about 1.5-2% fluorine substitution for oxygen. The maxima are very broad,
and almost maximum conductivities and reflectivities are shown by films
with 1% to 2.5% fluorine. There is also a weak, broad absorption
throughout the visible wavelength range, which increases directly with
fluorine concentration. Therefore, to prepare films with high electrical
conductivity and high visible transparency, a fluorine concentration in
the film of about 1% (i.e., fluorine to oxygen ratio 0.01 in the film) is
most desirable. However, this optimum will vary somewhat, depending on the
spectral distribution of interest in a given application. By varying the
fluorine dopant concentration, routine experimentation can easily
establish the optimum percentage for any given application.
Fluorine doping levels exceeding 3% can easily be achieved in the films,
using the methods of the instant invention. Prior art results had not
exceeded 1%, and the opinion, cited above, was that this was the
solubility limit for fluorine. While such high doping levels are not
needed to produce optimum infrared reflectivity or electrical
conductivity, the gray films produced at doping levels of 2% or more may
be useful on architectural glass, for limiting solar heat gain in
air-conditioned buildings. In such applications, the doping level at the
surface of the film advantageously is reduced to about 2%, in order to
have maximum infrared reflectivity.
Using the measured electron concentrations and electrical conductivities,
the electron drift mobilities can be obtained. For various films, values
from 50 to 70 cm.sup.2 /Volt-sec were calculated in this way. Previously
obtained mobility values for tin oxide films have ranged from 5 to 35
cm.sup.2 /Volt-sec. It is believed the instant films are the first to have
such mobilities exceeding 40 cm.sup.2 /Volt-sec. These values illustrate,
in another way, the superior quality of the process of this invention and
of the films prepared therewith.
The process of the invention is also highly desirable for use in making
novel devices such as those having electroconductive layers in
semiconductor manufacture (e.g. integrated circuits and the like), and
also the manufacture of heat-reflective transparent objects like windows.
The most advantageous mode of the invention is that wherein the organo-tin
fluoride compound having a tin-fluorine bond is decomposed at the
substrate immediately after formation. This decomposition is preferably in
a narrow reaction zone which is largely heated to the decomposition
temperature by heat from the substrate itself.
ILLUSTRATIVE EMBODIMENT OF THE INVENTION
In order to point out more fully the nature of the present invention, the
following examples are given as illustrative embodiments of the present
process and products produced thereby.
Unless otherwise specified, the specific examples disclosed below are
carried out according to the following general procedure:
EXAMPLE 1
The process is exemplified by an experiment using the apparatus of FIG. 1
to produce a gas stream which contains 1% tetramethyltin (CH.sub.3).sub.4
Sn, 0.02% trimethyl trifluoromethyltin (CH.sub.3).sub.3 SnCF.sub.3, 10%
nitrogen carrier gas, and balance oxygen gas. The resulting stream is
passed over a pyrex glass plate which is 15 cm in diameter and maintained
at 500.degree. C. for about a 5 minute deposition period. The flow rate of
the gas stream is about 400 cc per minute. This flow rate is such that the
gas turnover rate in funnel 70 is about once each two minutes. A
transparent film about 1 micron thick is deposited. It shows electrical
resistance of 2 ohms per square, corresponding to a volume resistivity of
0.0002 ohm-cm. This film is measured to have a fluorine to oxygen ratio of
about 0.017 and a drift mobility of about 50 cm.sup.2 /Volt-sec.
EXAMPLE 2
When the process of Example 1 is repeated using a sodium free silicon
substrate, the resistance value dropped to about 1 ohm per square, i.e.
about one-half the value of the resistivity achieved with a sodium-bearing
substrate.
EXAMPLE 3
An advantageous process is illustrated by a process utilizing the apparatus
of FIG. 2. The resulting gas mixture consists of 1% tetramethyltin
(CH.sub.3).sub.4 Sn, 0.2% iodotrifluoromethane CF.sub.3 I, 20% nitrogen
carrier gas, balance oxygen. Films grown on pyrex glass substrates showed
the same electrical characteristics as in Example 1.
EXAMPLE 4
The simplified apparatus in FIG. 3 is used by forming the mixture described
in Example 3, in a compressed gas cylinder 19. The results are identical
to those of Example 3. After a month of storage in the gas cylinder, the
experiment was repeated, giving identical results. This demonstrates the
stability and shelf life of this mixture.
EXAMPLE 5
Example 3 is repeated, except that when the stannic oxide film is 0.5
microns thick, deposition is stopped. The resulting stannic oxide film has
an infra-red reflectivity of about 90%.
EXAMPLES 6-13
The following gases each are substituted, in equi-molecular portions, for
CF.sub.3 I in the procedure of Example 3 (excepting that the concentration
of fluorine dopants is increased 15 times in examples 6, 7, 8 and 13.)
Excellent conductivity and infra-red relfectivity are achieved:
______________________________________
Example Gas Example Gas
______________________________________
6 CF.sub.3 Br
10 C.sub.3 F.sub.7 I
7 C.sub.2 F.sub.5 Br
11 SF.sub.5 Br
8 C.sub.3 F.sub.7 Br
12 SF.sub.5 Cl
9 C.sub.2 F.sub.5 I
13 CF.sub.3 SF.sub.5
______________________________________
Conventional silicon photovoltaic cells ("Solar cells") have heretofore
comprised typical surface resistances of 50 to 100 ohms per square. In
order to have an acceptably low total cell resistance, a metallic grid
with a spacing of 1 or 2 millimeters is deposited on the silicon surface.
By depositing a fluorine-doped tin-oxide layer with a sheet resistance of
about 0.5 ohms per square (about 2 microns thick) on the cell surface, the
metallic grid spacing can be increased to about 10 millimeters, with a
corresponding reduction in the cost of the grid. Alternatively, the grid
size can be kept small, and the cell is able to function efficiently even
when the sunlight has been concentrated by a factor of about 100, provided
adequate cooling of the cell is maintained.
A schematic section 100 of such a cell is shown in FIG. 4 wherein a
2-micron layer 102 of n-SnO.sub.2 (the fluorine-doped material of the
invention is used), a 0.4 micron layer 104 of n-silicon (phosphorous-doped
silicon as known to the art), a 0.1mm p-silicon layer 106 (boron-doped
silicon as known to the art) are joined with an aluminum layer 108 serving
as an electrode. Metallic grids 110 are spaced about 10 millimeters apart.
Yet an excellent performance is achieved.
The deposited layers can be used in manufacture of other semiconductor
articles, e.g. conductors or resistors. Tin-oxide coatings have been so
used in integrated circuits before. The improved conductivity will allow
wider application of this material in the future. Not only is the sheet
resistance range extended to much lower values (e.g. about 5 ohms per
square or less) than herefore possible, but also deposition of the layer
can be achieved within the same apparatus which is used, for example, to
grow epitaxial silicon. This eliminates the costly and cumbersome
unloading, cleaning, and loading steps between depositions.
The resistivity values obtained for the fluorine-doped tin-oxide on silicon
substrates are about 10.sup.-4 ohm-cm, which is comparable to that of
evaporated tantalum metal, which is sometimes used for connections in
integrated circuits. The good match between thermal expansion coefficients
of tin-oxide and silicon allows deposition of thick layers without
significant strains.
FIG. 6 shows the electrical conductivity of the fluorine-doped stannic
oxide films as a function of measured fluorine to oxygen ratio in the
films, for deposition temperatures of 480.degree. C. and 500.degree. C.
FIG. 7 shows the infra-red reflectivity of the fluorine-doped stannic oxide
films as a function of measured fluorine to oxygen ratio in the films, for
deposition temperatures of 480.degree. C. and 500.degree. C.
Also indicated on FIGS. 6 and 7 are (1) the conductivity of the expensive
indium-oxide materials known to the art and as described in Philips
Technical Review, Vol. 29, Page 17 (1968) by van Boort and Groth and (2)
the best alleged prior art values for conductivity and reflectivity of
doped stannic oxide coatings.
Although several embodiments of the present invention have been described
and illustrated, it will be apparent to those skilled in the art that
various changes and further modifications may be made therein without
departure from the spirit of the invention or from the scope of the
appended claims.
* * * * *
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