CA1121666A

CA1121666A - Tin oxide coatings

Info

Publication number: CA1121666A
Application number: CA000313867A
Authority: CA
Inventors: Roy G. Gordon
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-10-20
Filing date: 1978-10-20
Publication date: 1982-04-13
Also published as: FI64128C; GB2033357B; NO144140B; JPS5558363A; NO144140C; NL191210B; BR7806939A; NL191210C; NL7810511A; SE431321B; FI64128B; FR2439240B1; JPS63245813A; IT7869421A0; DE2845782A1; IT1109618B; JPS6361388B2; DE2845782C2; NO783553L; GB2033357A

Abstract

ABSTRACT OF THE DISCLOSURE
Electrically-conductive films of tin oxide are prepared by a novel process utilizing gaseous chemical compounds which react to form a tin-fluorine bond at a temperature which is (1) high enough so that the newly-created tin-fluorine bond-bearing mole-cule remains in the vapor phase; and (2) low enough so that oxida-tion of the molecule occurs only after the indicated re-arrangement.
Films prepared by the process of the invention are characterized by surface resistances as low as 1 ohm per square when the film thickness is as thin as about a micron. These films are also characterized by extremely good reflectance of infrared radiation.

Description

FIELD OF TI~E INVEN'rION

! This invention relates to an improved process for the produc- !
tion of electrically-conductive layers which are highly transparen 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 dis-plays, 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.
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~lZ1666 ~ACKG]~)UNI) Ul llll; INV~NlION

Various metal oxidcs, such as stannic oxide SnO2, indium oxide In2O3, and ca~lmium stannate Cd2SnO4, have been the most ~widely used materials for forming transparent, electrically con-5 1l ductive 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 trans-parent, electrically resistive layers were first made for de-icing ~
10 ,~aircraft windows. However, the spray process produced rather cor- !
rosive by-products, hot chlorine and hydrogen chloride gases, which ~tended to attack the hot glass surface, producing a foggy appear-~ance. U. S. Patent 2,617,745 teaches that this undesirable effect can be mitigated by first applying a coating of pure silica on the 15 ,glass. I-lowever, a silica protective layer is not very effective ion 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 apparatusJ and ithe 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 reproduci-bility in the properties of the coatings. U. S. Patent 2,651,585 Iteaches that better uniformity and reproducibility are obtained 25 'when t}le humidity in the apparatus is controlled. The use of a vapor, rather than a liquid spray, as described for example in German Patent 1,521,239, also results in more uniform and repro-ducible coatings.

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~ S~-UUl Z~6~6 llven ~ith these improvements, more recent studies have been made US:illg vacuum cleposition techniques, such as evaporation and sputtering, in order to achieve cleaner and more rcproducible ! coatings. Despite the much higher cost of these vacuum processes, , 5 l,the reduction of corrosive by-products and unwanted impurities ~introduced by tl~e spray methods is felt to be important particu-larly in applications involving high-purity semiconductors.
The intentional addition of certain impurities is important in these processes, in order to achieve high electrical conducti-10 ,vity and high infrared reflectivity. Thus, tin impurity is incor-porated in indium oxide, while antimony is added to tin oxide (stannic oxide) for these purposes. In each case the function of jthese desirable impurities ("dopants") is to supply "extra" elec-trons which contribute to the conductivity. The solubility of these impurities is high, and they can be added readily using all lof the deposition methods referred to above. Fluorine has an ad-,vantage over antimony as a dopant for tin oxide, in that the trans-parency of the fluorine-doped stannic oxide films is higher than lthat of antimony-doped ones, particularly in the red end of the 20 ,Ivisible 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.l iPossibly this is because fluorine doping has only been demonstratedl 25 l~in the less satisfac~ory spray method, whereas the improved deposi-tion 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 ,

- 3-i ' 7 __ _ ___ _ -~zi666 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. ~etherthe-less, it is noted that the lowest resistivity tin oxide films reported in the prior art are those of United States Patent 3,677,814 to Gillery. Using a spray method, he obtained fluor-ine-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. r~hen one wishes to ob-tain coatings of as low as 10 ohms per square, one has hereto-fore been forced to use the much more expensive materials like indium oxide.
According to the invention, there is provided a pro-cess 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 a tin fluoride compound having a direct tin-fluorine bond in the immediate proximity of said heated substrate, and (2) depositing said tin fluoride compound along with a reaction product of the oxidizable tin component of said reagent gas at the surface of said substrate and thereby achiev-ing a fluorine-doped, tin oxide coating upon said surface.
The invention also provides an article of manufacture comprising a substrate bearing a coating of fluorine-doped stannic oxide, said substrate being selected from generally transparent substrates and substrates of the type used in semi-conductors, said coating being characterised by having a reflectivity of about 85~ to 10 micron infra-red radiation when the substrate llZ16~6 is generally transparent and by having a resistance of less than about 5 ohms per square and a bulk resistivity in said coating of about 10 4 ohm-cm when the substrate is of the type used in semi-conductors.
~ 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 extraorinarily high reflectivity to infrared wavelengths.

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11216~i6 Pre Fe ra b i~
S~J 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.

k 52-O()l .~ _ llZ1666 In a Eirst form of the invcntion, an organo-tin mono-fluoride vapor is Lormed in the heated deposition region by the reformation ~of the vapor oÇ a more volatile compound containing both tin and fluoroalkyl groups bonded to tin.
~ second advantageous embodiment of the invention utilizes an organo-tin monofluoride formed at or near the gas-substrate inter-face by reactions involving an organo-tin vapor and certain fluorine-containing gases having fluoroalkyl and/or fluorosulfur Igroups.
10 I 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 Tl-IE DI~AWINGS

Figure 1 shows a schematic diagram of an apparatus suitable for carrying out a process in wllich a fluorine dopant is an organo-tin fluoroalkyl vapor, evaporated from its liquid form.
Figure 2 S1lows a similar diagram for the second embodiment, in which the fluorine dopant is formed by reaction with certain 20 ! fluoroalkyl and/or fluorosulfur gases supplied from a compressed gas cylinder.
Figure 3 shows a simplified version of the apparatus for practicing either the first or the second embodiments of the linvention.
Figure 4 is a schematic section of a solar cell and illustrates llone use of the invention in a semicoIlductor application.
~ igure 5 shows window 120 coated with layer 118 according to thc inveIltioll.

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,, ' ~lZ1~66 Figures 6 ancl 7 are graphs illustrative of varying conducti- ¦
vity allcl re~lectivi~y with concelltrations of fluorine dopant. The process of this invelltion has two main steps: (1) forming a re-active vapor mixture whicll will produce, on,heating, a compound 5 Ihaving a tin-fluorine bond, and (2) bringing this vapor mixture to a heated surface, on which fluorine-cloped tin oxide deposits.
Tlle eml)odimellts clescribed below cliffer 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. I`he second step 10 ~I(deposition on the heated surface) is largely the same in each l~example.
i l`he tin is supplied by a volatile, oxidizable tin compound, ~such as tetramethyltin, tetraetllyltin, dibutyltin diacetate, di-j!methyltin dihydride, dimethyltin dichloride, etc. The preferred 15 ~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 T:igures, and an inert carrier gas, such as nitrogen, is bubbled througll the tin compound. ~or the very volatile compouncds, such asl 20 Itetramethyltin and climethyltin 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 l~the instant invention that high-temperature apparatus can be 25 ~avoided and that simple cold-wall supplies can be used.
l`he vapor mixture must contain an oxidizing gas, such as oxygen, nitrous oxide, or the like. Oxygen is the preferred gas, since it is reaclily available and works just as well as the more expensive altcrnate oxiclizers.

llZ16~;6 'l`he pressures of the gases are Lixed by the regulators 25, and thc ~low ratcs of the oxygen from tank 20, and of the carrier gas rom tank 21, are controlled by metering vales 30, and measured by 1Owllleters 40. The gas streams then pass through one-way check 5 Ivalves 50 into a mixing tube 60 and funnel-shaped chamber 70. A l, tin oxide film deposits on the hottest surface 80, which is heated ¦
by the heater 90, typically to temperatures about 4000 to 6000C.
The general type of process just described is commonly known lin the art as chemical vapor deposition. Various modifications, 10 Isuch 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 jiapplication.
15 ~ Rotation of the substrate is recommended in order to best move ,i ,the sample through any convection currents which may occur in the apparatus and thereby best assure the uniformity of the deposited layers. Ilowever, it has now been discovered that, by placing the iheated 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 by product formed by homogeneous 'nucleation in the gas, does not fall onto the growing film.
25 ! An invention described herein is an improved process by which !
controlled amounts of fluorine impurity may be introd-lced 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 ;

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112~..6ti6 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 form-ing 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 advanta-geously occurs at temperatures high enough (e.g. > 100C) so that the tin fluoride thus formed remains in the vapor phase, but also low enough (e.g. < 400C) so that the oxidation of the compound occurs only after the rearrangement. One example of such a com-pound is trimethyl trifluoromethyltin, (CH3)3SnCF3. On heating to a temperature of about 150C in a heated zone adjacent to the deposition surface 80, this compound rearranges to form a direct tin-fluorine bond, in (CH3)3SnF vapor, which then reacts as the fluorine donor or dopant. Other compounds which undergo similar rearrangements at temperatures which will, of course, differ some-what from compound to compound, have the general formula R3SnRF, where R is a hydrocarbon radical, and RF is a fluorinated hydro-carbon 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 .

K 5~-U01 .~ ._ 11~16~6 at room temperature. Illis simplifies thc desigll of thc apparatus, as sllown in ligure 1, by elimillatillg the need for maintaining a warm zone between tlle bubblcr 15 and the reaction chamber 70, to keep the 1uorine dopallt in the vapor phase. Thus the apparatus 5 Ican be o the type wllicll is usually called a "cold-wall chemical vapor deposition reactor," which is widely used, for example, in ¦ the semicon~uctor industry to deposit silicon, silicon dioxide, silicon nitride, etc. Anotller important feature of the "cold-wall reactor" for semiconductor applications is that it minimizes un-10 Iwanted 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 470C for Isoft gIass. In this way, highly uniform films can be achieved in 15 Ithe normal glass-production equipment.
The preferred compound for use in the embodiment of Figure 1 is (Cl13)3SnCF3, since it is more volatile than the compounds with more carbon atoms. It is a stable, colorless, non-corrosive liquid, W]liC]I does not decompose in air at room temperature, and only reacts extremely slowly with water.
A particular advantageous second embodimellt of the invention luses a fluorine-containing gas which reacts with an organo-tin vapor ! on heating, to produce a tin fluoride vapor. For example, ,;~-fluoroalkyl halides, preferably wherein the alkyl group has 4 carbons or fewer, such gases as iodotrifluoromethane, CF3I, CF3CF2I, C3F7I, and tlle like, can be mixed Wit]l organo-tin vapors such as tetrametllyltin vapor ~CH3)4Sn, at room temperature, i.e.
to 90F, and more preferably to temperatures of 150F, without any reaction. Moreover, fluoroalkyl bromides like C~3Br, C2~sBr i .

~and ~lle like are llsoruL as ~luorille-~olltaillillg gases. Ihey are :Iess re.lctive al~(l abo~ o 2~ tilnes nlolc alc re(~ e(l il~ tlle re-actant gas, I)ut they are mucll less expensive. lhis is particularly surprising because of tlle reputed inertness for such compounds.
Iluoroal~yl chlorides are not favored for use because their reacti-vity is substantially lower than even the bromides.
IVhen such a vapor mixture approaches the heated surface, re-action takes place in the gas phase to, event~ually, produce the desired tin-fluoriJIe bonds. Although the reaction sequence is complex, it is believed to begin by reactions such as CF3I + R~SII ~ 3SIlCF3 + RI
to yield the organo-tin fluoroalkyl R3SnCF3 vapors in the region near the inter-face of the hot surface, where they serve as fluorine dopants for the growing tin oxide film, just as in the first embodi!-ment.
Certain other fluorine-containing gases also function in this second embodiment of the invention. For example, sulfur chloride pentafluoride, SF5Cl, is an effective fluorine donor gas, as is sulfur bromide pentafluoride SF5Br.
20 1 In a similar way, tri fluoromethyl sulfur pentafluoride CF3SF5 gas acts to form tin-fluoricle bonds by gas phase reactions.
I}le advantage of this second embodiment is that the fluorine donor is a gas, and the process is further shown in Figure 2. The Ipreferred gases are CF3I and CF3Br, which are non-corrosive, non-25 ~flammable, not appreciably toxic, and readily available commerciall?.S1-5Cl and SF5Br and highly toxic, and thus are less desirable for use. Cl3SF5 is noll-toxic, but somewhat less reactive thaJI CF3I.

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llZ1666 l`llc dcpos:itioll process may be rurther simplified, as shown ;n l~igure 3, if thc gas mixtures are pre-mixed and stored in a compresse(l gas cylinder 19. For safe storage and use, the oxidiz-able compoulld must of course be kept at a concentration such that S 'it cannot form an explosive mixture. For e~am~le, the lower explo-`sion limit Or~ tetramethyltin in air is about . The concentra-tions whicll have used for the chemical vapor depositions are less than a ~ ~ o-f this level. In addition, the use of CF3I
or CF3Br as a fluorine dopant incidentally acts as a flame suppres-sant.
Films prepared according to the invention are found to haveinfrared reflectivities of 90% and more measured, as is known in the art, at the convention 10-micron wave length of light which is characteristic of thermal infrared radiation at room temperature.
Tllis 90% reflectivity is to be compared to the 80~ reflectivity ,whicll is heretofore achieved using tin oxide coatings. In usual practice, these inLrared 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 wavelengtll 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 ~ree Carriers in SnO2 l.ayers," Zeitschrift fur Naturforschung, Volume 179, pages 7~9 to 793 ~1962), values were obtained for the free electron concentratioll in the films. The values obtained were in the range from 102 cm~3 to 1021 cm~3, and increased regularly with incrcasing concentrations of the fluorine (~opant. Theoreti-cally, OIIC free clectron should be released for each fluorine atom ~ ~ 2 () ~) 1 112i666 wlli.c]l replaces an oxyge]l atom in the lattice. This hypothesis wasveriied by ~uger l.lectron Spectroscopic measurements of the total fluorinc concentratioll in some oE the films, which gave fluorine concentrations in agreement with the free electron concentrations,;
to within the experimental uncertainties. This agreement is indi- , cative that most of the incorporated fluorine is electrically active.
The infrared reflectivity at 10 microns and also the bulk lelectrical conductivity of the films, were found to be maximum at 10 Il`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 .Ol in the film) is most desirable. I~owever, this optimum will vary somewhat, depend- , ling on the spectral distribution of interest in a given applica-tion. By varying the fluorine dopant concentration, routine ex-perimentation 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 25 ,results had not exceeded 1%, and the opinion, cited above, was that this was the solubility limit for fluorine. I~hile such high doping levels are not needed to produce optimum infrared reflecti-vity or electrical conductivity, the gray films produced at doping levels of 2% or more may be useful on architectural glass, for -l3 : -.,, . -llZi666 ~limitillg solar heat gain in air-conditioned buildings. In such applica~ions, the doping level at the surface of the film advan-tageously is reduced to about 2~, in order to have maximum infra-red reflectivity.
` 5 ' Using the measured electron concentrati~ons and electrical ~conductlvities, the electron drift mobilities can be obtained.
~For various films, values from 50 to 70 cm2/Volt-sec were calcula-ted in this way. Previously obtained mobilit~ values for tin oxide Ifilms have ranged from 5 to 35 cm2/Volt-sec. It is believed the lO linstant films are the first to have suc]l mobilities exceeding i40 cm2/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 15 Iuse 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. - j l`he most advantageous mode of the invention is that wherein the organo-tin fluQride compound having a tin-fluorine bond is decom-posed at the substrate immediately after form~tion. I`his decompo- !
sition is preferably in a narrow reaction zone which is largely heated to the decomposition temperature by heat from the substrate itself.
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llZ16~i6 I ILLU~'l`Rl\'~'IVl~ Ol)[~ll.N'I' Ol 'l'lll. INVI~N'I'ION

In or~er to point out more fully the nature of the present invelltlon, the following examples arc given as illustrative embodi-lments of the present process and products p~oduced thereby.
' Unless otherwise specified, the specific examples disclosed below are carried out according to the following general procedure:

.I~.xa T}le process is exempli~ied by an cxperiment using the appa-l'ratus of ~igure 1 to produce a gas stream which contains 1%
¦tetrametllyltin ~CI-l3)4Sn, o.n2OO trimcthyl trifluoromethyltin (Cll3)3snc~3~ 10o nitrogen carrier gas, and balance oxygen gas.
~he resulting stream is passed over a pyrex glass platc which iS
~15 Clll in diameter and maintained at 500C for about a 5 minute ideposition period. Ihe 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 al)out once each two minutes. A transparent film ahout l micron thick is de~osited. It shows electrical res-istance I

of 2 ohms per square, corresponding to a volullle resistivity o-f l0.0002 ohm-clll. Ihis film is measured to have a Iluorine to oxygen ratio o~ abollt .nl7 and a drift mobility of about 50 cm2/Volt-sec.
_xamplc 2 IVhell the process of Example 1 is repeated using a sodium free silicon substrate, the resistance value dropped to about 1 ohm per Isquare, i.e. about one-hal~ the value of the resistivity achieved Wit}l a sodium-bearing substrate.
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11216~6 lc 3 ~ n adValltagCOUS pl`OCCSS is illustratcd by a process utilizing ¦
th~ ap1);1r~1tl1s o~ urc 2. Thc rcsulting gas mixturc consists of l~ tetr11mcthylti1l (C113)4Sn, 0.2% iodotrifluoromethane C~31, 20~ i nitrogen carrier gas, balance oxygen. T:ilms grown on pyrex glass substrates showed the same clectrical charac~eristics as in Examp]e l.
Example 4 1he simplified apparatus in Figure 3 is ~sed by forming the mixture describecl in Example 3, in a compressed gas cylinder l9.
T11e results are identical to those of Example 3. After a month of ~storage in tlle gas cylinder, the experiment was repeated, giving ~ identical results. This demonstrates the stability and shelf life ; of this mixture.
, i Exam~lc 5 Example 3 is repeated, except that when the stan1lic oxide film is 0.5 microns thick, deposition is stopped. The re- ¦
sulting stannic oxide fil1n llas an infra-red reflectivity Or otlt 90%.
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~ ~ 20 ~xamples 6-13 , ~ , ~ ~ I ` lhe following gases each are substituted, in equi-molecular , .
portions, for Cl3I in the procedure of Example 3 (excepting that the concentration of fluorine dopants is incrcased l5 times in examples 6, 7, 8 and 13.) Excellent conductivity and infra-red - 25 relÇectivity are achieved: 1 . i ,, ~ . I

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llZ1666 EY~ t' (,-ls 1.. ~ )1c (,~s 6 Cl~31~r lO c3l7 7 C2l~5]~1 11 Sl7513r 8 C31:713r 12 S~5Cl 9 C2J:5 r . 13 C~3S~5 Conventional silicon photovoltaic cells,("Solar cells") have heretoore comprise~ typical surface resistances of 50 to 100 ¦
ohms per s~!uare. In order to have an acceptably low total cell resistallce, a metallic gri~ witll a spacillg oE.l or 2 millimeters is deposite~l on tlle silicon surface. By ~epositing a fluorine-~ope~ tin-oxide layer witll 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 lO millimeters, with a correspollding reduction in the cost of t]le grid. Alternatively, the grid size can be kept small, and the cell is able to function eEEiciently even when t]le sunlight has been concentrated by a factor of ahout lO0, provided aclequate cooling of the cell is maintained.
A schematic section lO0 of such a cell is s11own in Figure 4 wherein a 2-~icron layer 102 of n-Sn~2 (the fluorine-doped material of the invention is used), a 0.4 micron layer 104 ol` n-silicon ~phospllorous-dope~ silicon as ~nown to the art), a O.lmm p-silicon layer lOG (~oron-doped silicon as known to the art) are joined with an aluminum layer 108 serving as an electrode.
iMetallic grids llO are speced about lO millimeters apart. Yet an excellent performance is achievod.
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11;~ ;6 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 un-lO ; loadlng, cleaning, and loading steps between depositions.
The resistivity values obtained for the fluorine-doped tin-oxide on silicon substrates is about 10 4 ohm-cm, which is ;~
comparable to that of evaporated tantalum metal, which is sometimes .
used for connections in integrated circuits. The good match be-tween thermal expansion coefficients of tin-oxide and silicon allows deposition of thick layers without significant strains.
Figure 6 shows the electrical conductivity of the fluorine-doped stannic oxide films as a functïon of measured fluorine to oxygen ratio in the films, for deposition temperatures of 480C and 500C.
Figure 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 480C
and 500 & .
Also indicated on Figures 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.

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` `-`.~ 11216~;6 ~ lthougll scvcral emho(l~ cllts o~ tllc prescnt invention havc beell ~Icscrihccl an~l illustratc~l, it will he apparent to those skille(l in thc art that various cllangcs and further modiEications may be ma(lc thereill Wit]lout departure from the s~irit of the in-vention or from the scol~e of the appen~e~ claims.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 a tin fluoride compound having a direct tin-fluorine bond in the immediate proximity of said heated substrate, and (2) depositing said tin fluoride compound along with a reaction product of the oxidizable tin component of said rea-gent gas at the surface of said substrate and thereby achieving a fluorine-doped, tin oxide coating upon said surface.

2. A process according to claim 1 wherein the reagent gas contains (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.

K52-001 Claims

3. process as defined in Claim 2 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 CF3I, CF3Br, and homologous alkyl alpha-fluorinated compounds of said CF3I, CF3Br, and CF3SF5, SF5Br, and SF5Cl; 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°F.

4. A process as defined in Claim 2 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.

5. A process as defined in Claim 2 wherein said substrate to be coated faces downwardly and said gaseous mixture is directed upwardly toward said surface.

K52-001 Claims

6. A process is in Claim 2 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°C.

7. A process as in Claim 2 in which said first fluorine bearing compound is a volatile tin compound which decomposes on heating to form an organotin monofluoride vapor.

8. A process as in Claim 7 in which said volatile tin compound is trimethyl trifluoromethyl tin.

9. A process as in Claim 7 in which the volatile tin compound is trimethyl pentafluoroethyl tin.

10. A process as in Claim 3 in which a mixture of (a) and (b) is stable at about 90°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.

K52-001 Claims

11. A process as in Claim 10 in which the fluorine dopant is formed by reacting trifluoroiodomethane and an organotin compound containing at least one tin-carbon bond per molecule.

12. A process as in Claim 11 in which the fluorine dopant is formed by reacting trifluoroiodomethane gas and tetramethyltin.

13. A process as in Claim 11 in which bromine is substituted for iodine.

14. A process as in Claim 12 wherein bromine is substituted for iodine.

15. A process as in Claim 10 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.

K52-001 Claims

16. A process as in Claim 15 in which the fluorine dopant is formed by reacting sulfur chloride pentafluoride gas and tetramethyltin.

17. A process as in Claim 10 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.

18. A process as in Claim 17 in which the fluorine dopant is formed by reacting trifluoromethyl sulfur pentafluoride gas and tetramethyltin.

19. An article of manufacture comprising a substrate bear-ing a coating of fluorine-doped stannic oxide, said substrate being selected from generally transparent substrates and sub-strates of the type used in semi-conductors, said coating being characterised by having a reflectivity of about 85% to 10 micron infra-red radiation when the substrate is generally transparent and by having resistance of less than about 5 ohms per square and a bulk resistivity in said coating of about 10-4 ohm-cm when the substrate is of the type used in semi-conductors.

20. An article of manufacture comprising generally trans-parent substrate such as glass and a fluorine-doped stannic oxide film thereon said object having a reflectivity of about 85% to 10 micron infra-red radiation.

21. An article of manufacture according to claim 20 wherein said film is characterized by a maximum surface resistance of about 5 ohms per square.

22. An article of manufacture according to claim 20 or 21 wherein said film has a fluorine-to-oxygen ratio of between about 0.007 and 0.03.

23. A semiconductor article of the type used in electronic circuitry comprising a coating of fluorine-doped stannic oxide which is characterized by a resistance of less than about 5 ohms per square and bulk resistivity in said coating of about 10-4 ohm-cm.

24. A process as defined in claim 1 wherein the ratio of fluorine dopant and oxidizable tin compound are selected that the free electron concentration of the films is within about a range of from 1020cm-3 to 1021cm-3.

25. 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.

26. A process according to claim 1, 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.

27. A process according to claim 1, 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.

28. A process as in Claim 27 in which component (a) contains fluoro-alkyl halides, or mixtures thereof.

K52-001 Claims

29. A process as in Claim 28 in which component (a) contains a gas selected from the group consisting of CF3Br, CF3I
and homologous or substituted fluorinated compounds, or mixtures thereof.

30. A process as in Claim 26, in which component (a) contains reactive fluorosulfur groups.

31. A process as in Claim 30, in which component (a) contains SF5Cl, SF5Br or SF5CF3 and homologous or substituted compounds of mixtures thereof.

32. A process according to claim 1, said process com-prising 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 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.

52-001 Claims

33. A process as in Claim 32, in which component (a) contains a fluoroalkyl group of substituted fluoralkyl group, bonded to a tin atom.

34. A process as in Claim 33 in which component (a) contains trimethyl trifluoromethyl tin.

35. A process as in Claim 33 in which component (a) contains trimethyl pentafluoroethyl tin.

36. A process as in Claim 32 in which component (b) contains a compound containing at least one carbon-tin bond.

37. A process as in Claim 36 in which component (b) contains tetramethyl tin.

38. A process as in Claim 36 in which component (b) contains dimethyl tin dichloride.

39. A process as in Claim 27 in which component (b) contains an organometallic tin compound.

40. A process as in Claim 39 in which component (b) contains tetramethyl tin.

41. A process as in Claim 23 in which component (b) contains an organometallic tin compound.

42. A process as in Claim 41 in which component (b) contains tetramethyl tin.

43. A process as in Claim 29 in which component (b) contains an organometallic tin compound.

44. A process as in Claim 43 in which component (b) contains tetramethyltin.

45. A process as in Claim 30 in which component (b) contains an organometallic tin compound.

46. A process as in Claim 45 in which component (b) contains tetramethyl tin.

E52-001 Claims

47 . A process as in Claim 31 in which component (b) contains an organometallic tin compound.

48. A process as in Claim 47 in which component (b) contains tetramethyl tin.

49. A process as in Claim 32 in which component (b) contains an organometallic tin compound.

50. A process as in Claim 49 in which component (b) contains tetramethyl tin.

51. A process as in Claim 33 in which component (b) contains an organometallic tin compound.

52. A process as in Claim 51 in which component (b) contains tetramethyl tin.

53. A process as in Claim 34 in which component (b) contains an organometallic tin compound.

?52-001 Claims

54. A process as in Claim 53 in which component (b) contains tetramethyl tin.

55. A process as in Claim 43 in which component (b) contains dimethyl tin dichloride.

56. A process as in Claim 26 in which component (b) contains oxygen gas.

57. A process as in Claim 26 in which component (b) contains nitrogen or argon.

58. A process as in Claim 26 wherein said substrate to be coated is a solid which faces downwardly and said gaseous mixture is directed upwardly toward said surface.