LIQUID COATING COMPOSITION FOR PRODUCING HIGH QUALITY, HIGH PERFORMANCE FLUORINE-DOPED TIN OXIDE COATINGS

Abstract: A liquid coating composition for producing high quality, high performance fluorine-doped tin oxide coatings by chem­ ical vapor deposition includes a fluorine dopant compound, preferably trifluoroacetic acid, and an organotin compound, preferably monobutyltin trichloride.

Description: LIQUID COATING COMPOSITION FOR PRODUCING HIGH QUALITY, HIGH PERFORMANCE FLUORIDE-DOPED TIN OXIDE COATINGS This invention relates to fluorine-doped in oxide coatings, and more particularly, to a liquid coating composition for producing high quality fluorine-doped tin oxide coating have a low sheet resistance and high visible light transmission. Fluorine-doped tin oxide coatings are known to impart useful properties to many different substrate surfaces, including glass, ceramics, metals and elemental filaments. Such coated substrates find use as heat relective elements, in energy efficient windows and in opto-electronic and semiconductor devices. Several methods have been described in the literature to make such tin oxide coatings, including solution spray, chemical vapor deposition, powder deposition and ion sputtering. These methods all suffer from one or more disadvantages in that the process conditions, or compositions used, or the properties of the resulting for certain commercial applications. Where the product is an energy efficient window, for example, success has been limited by the relatively high sheet resistance and low visible transparency of the coatings, or by the cost of the process. The solution spray method is described, for example, in Japan Kokai 75 61,415. In this method a solution of dibutyltin diacetate and ethyl trifluoroacetate in isopropyl alcohol as a solvent is sprayed onto a glass plate to provide fluorine-doped tin oxide coatings. However, a large amount of solvent is present in the solution, generally about 50 wt Z, which must be removed during the process. Furthermore, the electrical resistivity of the coatings obtained have an unacceptable value of 230 ohms/square. A chemical vapor deposition method is disclosed in European Patent Office Publication Elo. 0 112 780. The process uses a gaseous mixture of butyltin trichloride and dichlorodifluoromethane which is formed from separate gaseous streams of each reactant. The gaseous mixture then is deposited onto a glass surface. However, in this nethod it is inconvenient to control the flow rates of the gaseous streams, the electrical properties of the coating are not adequate. Gordon in U.S. Patent No. 4,265,974 teaches that mixtures of tetramethyltin and M-fluoroalkyl halides can be decomposed to provide fluorine-doped tin oxide films with good optical and electrical properties. However1 the reaction mixture is explosive in air above 1.9 wt Z tetramethyltin, which severely limits the rate of deposition of the coating on sheet glass. In addition, tetramethyltin and its decomposition by-products are very toxic. Yoldas in U.S. Patent No. 4,293,594 discloses a vapor deposition method of forming a highly conductive transparent fluorine-doped tin oxide coating for fluorescent lamps. The patentee uses a gaseous mixture of dimethyltin dichloride and dimethyltin difluoride in an oxygen-containing carrier gas to produce the coating. However, these tin compounds are solids which must be vaporized at high temperatures to provide a gaseous mixture. As will be apparent from the above-described review cf the prior art, there is a need for an improved process of making high quality, high performance fluorine-doped tin oxide coatings. In accordance with the invention, there is provided herein a liquid coating composition for making fluorinedoped tin oxide coatings on substrates such as glass which have a low sheet resistance and high light transmission. The composition comprises: a) 1-30 wt. X of an organic fluorine dopant compound which is trifluoroacetic acid, trifluoroacetic anhydride, ethyl trifluoroacetoacetate, trifluoro ethanol, ethyl trifluoroacetate, or pentafluoro prop ironic acid; and b) 70-99 wt Z of an organotin compound which is an alkyltin trichloride, a dialkyltin dichloride, an alkyldichlorotin acetate, an alkylchlorotin diacetate, an ester tin trichloride, or tin tetrachloride. In a preferred form of the invention, the liquid coating composition includes 2-10 wt Z of the organic fluorine compound, and 90-98 wt % roc the organotin compound. The fluorine dopant preferably is trifluoroacetic acid, trifluoroacetic anhydride or ethyl trifluoroacetoacetate, and the organotin compound is monobutyltin trichloride or tin tetrachloride. The liquid coating composition of the invention may include also a polar organic compound, in an amount of about 1-10 wt % of the composition, which will insure stability of the liquid composition at low temperatures. When the polar organic liquid is present, the liquid coating composition includes 2-10 wt % of the organic fluorine compound, 80-97 wt Z of the organotin compound and 1-10 wt Z of the polar organic liquid. The method used herein to produce high quality, high performance fluorine-doped tin oxide coatings from the liquid coating composition of the invention is chemical vapor deposition. Tn this method the liquid composition is vaporized and contacted in an oxygen-containing atmosphere with a substrate maintained at a temperature above the decomposition temperature of said vapor. Preferablv vaporization of the liquid coating composition is carried out bv injecting the liquid into a carrier gas maintained at a temperature sufficient to vaporize said liquid. Deposition is carried out on glass for 45 seconds or less, to produce a fluorine-doped tin oxide coating having a thickness of 160-220 nm, a sheet resistance of less than 40 ohms/sq, a visible light transmission of at least 80 % and an IR reflectivity of 70Z or more. In the preferred form of the invention, the deposition time is 10 seconds or less and the sheet resistance is 30 ohm/sq. or less. In order to better understand the invention reference will be made to the accompanying drawing in which: The FIGURE is a schematic diagram of an apparatus for carrying out a coating process using the liquid coating composition of the invention. Referring now to the FIGURE, there is shown a diagrammatic representation of an apparatus suitable for carrying out the process of the present invention. Accordingly, a carrier gas 10 which includes oxygen, with air being preferred, is metered through a feed line 11 at a predetermined flow rate suitably about 1-30 1/mien., and preferably about 3-15 1/mien., through an air dryer tower 12 to provide a stream 13 of dry air. A separate air stream may be directed through a humidifier 14 containing a suitable quantity of water 15 to provide a wet air stream 16 at a desired relative humidity. Thereby an air stream 17, either dry or wet, may be passed through an evaporator 18 containing vessel 19 for holding the liquid coating composition of the invention. The liquid composition is supplied to evaporator 18 by syringe pump 20 and syringe 21. The air stream is heated from an oil bath (not zhor,) to a desires vaporization temperature. The vaporized liquid coating composition in the air stream 22 travels to a deposition chamber 23 having a coating nozzle 24 in which a substrate 25 is mounted on a heated plate 26. After deposition of the desired coating the gaseous by-products of the deposition are exhausted. The liquid coating composition of the invention is made up of 1-30 wt, 7. of an organic fluorine dopant compound, which is selected from trifluoroacetic acid, trifluoroacetic anhydride, ethyl trifluoroacetoacetate, trifluoroethanol, ethyl trifluoroacetate or pentafluoropropionic acid, and 70-99 wt % of an organotin compound which is an alkyltin trichloride, dialkyltin dichloride, a dialkylchlorotin acetate, a alkylchlorotin diacetate, an ester tin trichloride, or tin tetrachloride. Suitable organotin compound include monobutyltin trichloride, isobutyltin trichloride, methyltin trichloride, dibutyltin dichloride, diisobutyltin dichloride, di-t-butyltin dichloride, butyldichlorotin acetate, butylchlorotin diacetate, carbethoxyethyltin trichloride and tin tetrachloride. Preferably, the liquid coating composition comprises 2-10 wt % of trifluoroacetic acid, trifluoroacetic anhydride or ethyl trifluoroacetoacetate, and 90-98 wt Z of monobutyltin trichloride or tin tetrachloride. The essential characteristic of the coating composition of the invention is that it is liquid at room temperature and includes a predetermined high concentration of the tin component with a sufficient amount of fluorine dopant. The term "alkyl" used herein includes a straight chain and branched alkyl group which has one to six carbon atoms. The term "organotin" compound used herein includes organotins and tin tetrachloride. The liquid coating composition of the invention a include 1-10, of a polar organic compound, such as nethvl- isobutyl ketone, acetic anhydride or ethyl acetate, which will insure that the composition remains stable in one liquid phase below room temperature, e.g. even zt -150C. Thereby the liquid composition may be conveniently stored outdoors or transported during winter weather without becoming hazy which would evidence phase separation. Preferably the liquid coating composition then includes 2-10 wt of the organic fluorine compound, 80-97 wt Z of the organotin compound and 1-10 wt 7 of the polar organic liquid. The vaporization temperature in the process usually ranges from about 100 to about 400 C, and preferably abcut 1500 to 250 C. The substrate temperature ranges from above 400 to about 700 C, preferably about 550 to about 650 C. The carrier gas is an oxygen-containing gas which may be air or a mixture of oxygen and an inert gas, and is preferably air. The carrier air may be dry or wet in the process of the invention, and, accordingly, the relative humidity of the air at 18 C may vary from 0-100%. Preferably the humidity ranges from 10-SOZ, where the rate of deposition is enhanced without introducing unnecessary haze into the coating. The air flow preferably ranges from about 1 to about 20 limit. with the preferred rate being about 3-15 llmin. The substrate to be coated may be glass, ceramics, solid state materials, metals, elemental filaments and the like. The process conditions can be varied to provide a hard, resistant fluorine-doped tin oxide coating on many substrates with acceptable transmittance, reflectance and conductivity properties. For example, if the substrate to be coated is glass made by the floatation process, the sheet resistance of the coating at a thickness of about 160 to 250 nn will be 40 ohm/sq. or less, and, in a preferred form o' the invention, less than 30 ohrns/sq. Thicker films above 250nm will have lower sheet resistances but the light transmittance of the glass will be reduced. Below 160run thickness the conductivity of the glass will be too low and the film will permit too great a heat loss from a room protected by such coated glass. The sheet resistance (ohms/sq) of the tin oxide film is measured with a conventional four point probe according to ASTM standard method F374-81. The film thickness is measured by the beta-back-scatter method according to British Standards Institution method BS5411: Part 12, 1981, ISO 3543-1981. The infrared reflectivity is measured by a specular reflectance technique described by J. Stewart in "Infrared Spectroscopy", Marcel Dekker, Inc., N.Y. 1970 p. 539 ff. The visible transmittance is measured on a UV/vis spectrophotometer over the 400-800nm region, versus air and the 7. Tvis is averaged over the wavelengths. The bulk conductivity was determined from the sheet resistance and the film thickness according to the well known relationship. 1 &alpha;= R@ # t where equals conductivity in (ohm cm) RD equals sheet resistance in ohms/sq. t equals film thickness in centimeters. Films prepared according to the preferred embodiments of the invention have infrared reflectivities greater than 70% at the conventional 10 micron wavelength of light which is characteristic of thermal infrared radiation at room temperature, visible transmittance of 80% or greater, sheet resistances of less than 40 ohm/sq. and conductivities greater than 1250 (ohm cm) 1for films 1600-2500nm thick. The films show a very light orange color in transmitted light, a light blue iridescence in reflected light, and are substantially haze-free. The advantages of this invention can be more readilv appreciated by reference to the following examples. It should be noted that in each example the results shown in the Tables are the best achievable over a range of process conditions. In these examples the process parameters are 1) the rate of addition of the liquid composition at a 2) predetermined vaporization temperature given as the temperature of the carrier air, 3) the flow rate of the carrier air, 4) the relative humidity of the carrier air, 5) the deposition temperature, which is given as the temperature of the heating block, and 6) the time of deposition. The properties of the coatings are 1) sheet resistance, 2) film thickness and 3) film conductivity. The components of the liquid coating composition of the invention are presented in the Tables as follows: LIQUI@ COATING COMPOSITION Organic Fluorine Dopant Compound TFA = trifluoroacetic acid TFAA = trifluoroacetic anhydride ETFAA = ethyl trifluoroacetoacetate ETFA = ethyl trifluoroacetate PFPA = pentaeluoropropionic acid TFE = trifluoroethanol Organotin Compound MBTC = monobutyltin trichloride ESTC = carbethoxyethyltin trichloride BDTA = butyldichlorotin acetate BCTA = butylchlorotin diacetate IBTC = isobutyltin trichloride DIBTC = diisobutyltin dichloride DBTC = dibutyltin dichioride DTBTC = di-t-butyltin dichloride MTC = methyltin trichloride TT = tin tetrachloride Polar Organic Compound ACAN = acetic anhydride ETAC = ethyl acetate MIBK = methylisobutyl ketone TABLE I Example No. 1 2 3 4 5 6* 7* 8* 9* 4/5% TFA 4.6% TFA 4.5% TFA 4.5% TFA 5.7% TFA 6% TFA 4.8% TFAA 4.5% ACAN 2.3% ETAC 4.5% ACAN 4.5% ACAN Composition (% by wt.) 94.3% MBTC 94% MBTC 95.2% MBTC 91.0% MBTC 93.1 MBTC 91.0 MBTC 91.0% MBTC PROCESS CONDITIONS Rate of addition of liquid composition (ml/br) 9.7 19.4 19.4 14 6.5 17.8 19.4 20.5 29.5 Temperature of carrier air ( C) 140 150 150 180 150 150 150 167 150 Flow rate of carrier air (1/min.) 10 10 10 4.6 10 10 10 3.7 4.2 Relative humidity of air at 18 C (%) 14 100 0 14 47 47 47 14 14 Temperature of heating block ( C) 630 560 560 625 560 655 690 650 650 Deposition time (sec) 9 6 32 10 14 5.3 4 8 7 PROPERTIES OF THE COATING Sheet resistance R (ohm/sq) 28 37 36 37 26 22 22 35 32 Film thickness (nm) 187 198 194 192 200 200 185 207 200 Conductivity (ohm cm)-1 1910 1365 1432 1406 1932 2270 2460 1380 1562 * liquid stays clear at -15 C. TABLE II Example No. 10 11 12 13 14 15 16 17 18 7.8% TFA 8.3% TFA 7% TFA 25.6% TFA 5% TFA 18% TFA 10% TFA 5% TFA 9.8% ACAM 8.3% ACAM 10% ACAM 4% TFA Composition (% by wt.) 74.4% ESTC 95% BDTA 82% BCTA 90% IBTC 95% DIBTC 82.4%DBTC 83.4%DTBTC 83% MTC 96% TT PROCESS CONDITIONS Rate of addition of liquid composition (ml/hr) 14.4 30.6 30.6 30.6 30.6 22.3 22.3 15.5 6.5 Temperature of carrier air ( C) 200 204 200 150 150 200 175 125 150 Flow rate of carrier air (1/min.) 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 10 Relative humidity of air (%) 25 75 100 50 50 50 50 50 47 Temperature of heating block ( C) 650 650 650 650 650 650 650 650 560 Deposition time (sec) 40 7 30 45 18 27 7.5 7 18 PROPERTIES OF THE COATINC Sheet resistance R (ohm/sq) 24 35 39 38 32 30 31 34 30 Film thickness (nm) 206 201 198 203 188 201 205 211 200 Conductivity (ohm cm)-1 2022 1421 1302 1296 1661 1658 1574 1394 1667 TABLE III Example No. 19 20 21 22 23 7% ETFAA 5% ETFAA 10% ACAN 25% ETFA 16% TFE 3.5% PFPA Composition (% by wt.) 95% MBTC 83.9%MTC 75% MBTC 84% MBTC 96.5% MBTC PROCESS CONDITIONS Rate of addition of liquid composition (ml/hr) 15.1 22.3 26 9.7 6.5 Temperature of carrier air ( C) 150 150 150 150 150 Flow rate of carrier air (1/min) 3.7 3.7 10 10 10 Relative humidity of air (%) 100 50 0 0 0 Temperature of hesting block ( C) 650 650 620 560 625 Deposition time (sec) 4 7.5 18 41 33 PROPERTIES OF THE COATING Sheet resistance R (ohm/sq) 29 37 24 25 35 Film thickness (nm) 1960 220 200 224.4 190 Conductivity (ohm cm)-1 1759 1228 2083 1786 1540 TABLE I, Example 1-9 In these examples a glass substrate is coated by chemical vapor deposition using a liquid coating composition of trifluoroacetic acid or trifluoroacetic anhydride, and monobutyltin trichloride, within predetermined compositional ranges. In Examples 6-9 a small amount of acetic anhydride, ethyl acetate or methylisobutyl ketone is included in the composition as a low temperature phase stabilizer. The result of Examples 1-9 demonstrate that trifluoroacetic acid and trifluoroacetic anhydride (from which trifluoroacetic acid is formed in situ in a humid air atmosphere! provide fluorine-doped tin oxide coatings having high performance properties, including a sheet resistance below 40 ohm/sq. at a film thickness of about 200 nm and a deposition time below 35 seconds. In Examples 6 and 7, an advantageous sheet resistance of 22 ohm/sq is obtained. Typically, the infrared reflectivity of the tin oxide coatings produced in the Examples was greater than 70% as measured at 10 microns. The visible light transmission of the coatings in the Examples was 80% or greater. With respect to process conditions, it is apparent, that at a given addition rate, the presence of some moisture during deposition is desirable to reduce the deposition time. The presence of small amounts of a polar organic liquid in the composition (Ex.6-9) provides useful coatings which are stable against phase separation at temperatures as low as -15 C. TABLE II, Examples 10-18 In Table II there is shown the results of coating a glass substrate by chemical vapor deposition from a liquid coating composition of liquid trifluoroacetic acid and various organotin compounds other than butyltin trichloride. These organotin compounds generally require a larger percentage of TFA to form 2 liquid coating composition, and/or a higher rate to provide deposition times and sheet resistances comparable to Examples 1-9. Tin tetrachloride in combination with TFA provides fluorine-doped tin coatings having excellent properties under advantageous reaction conditions. TABLE III, Examples 19-23 In Table III there is shown results with fluorine dopants other than TFA and TFAA in combination with MBTC or MTC. It is seen that ethyl trifluoroacetoacetate, ETFAA, provides an excellent coating even at low concentrations of the dopant in the composition. ETFA and TFE dopants are suitable but both require a higher concentration of dopant. Furthermore these dopants require a higher rate of addition of the composition into the deposition chamber to provide reasonable deposition times. BFPA dopant provides a liquid coating composition at a low dopant concentration but both the film resistance and the deposition times are at the high end of the desired ranges. Example 24 The effect of a change in Relative Humidity on deposition rate of the coating was evaluated, using the compositions of Examples 7, 8, 11 and 14 to deposit the film. The results show that an increase in RH will generally increase the deposition rate. The relative magnitude of the increase, however, is dependent on other parameters, such as addition rate and air flow. Example 25 The effect of air flow on deposition rate was evaluated using the composition of Example 19. The results show that. for a fixed tin concentration, the deposition rate is directly dependent on air flow rate. Example 26 The effect of deposition temperature on the deposition rate was studied. It was found that, generally, as temperature was increased to about 600"C, the deposition rate increased. Above about 600"C the glass temperature had no effect on deposition rate at the vapor velocities shown.

Claims: Claims: 1. A liquid coating composition for making high quality fluorine-doped tin oxide coatings having a low sheet resistance and high visible light transmission characterized by comprising: a) 1-30 wt. % of an organic fluorine dopant compound which is selected from trifluoroacetic acid, trifluoroacetic anhydride, ethyl trifluoroacetate, trifluoroethanol, ethyl trifluoroacetate or pentafluoropropionic acid; and b} 70-99 wt z of an organotin compound which is selected from an alkyltin trichloride, a dialkyltin dichloride, an afkyldichlorotin acetate, a dialkylchlorotin diacetate, an ester tin trichloride, or tin tetrachloride. 2. A liquid coating composition according to claim 1 in which said fluorine dopant is selected from trifluoroacetic acid; trifluoroacetic anhydride or ethyl trifluoroacetate and said organotin compound is selected from monobutyltin trichloride; tin tetrachloride; disobutyltib dichioride butyldichlorotin acetate. 3. A liquid coating composition as in either of claims 1 or 2 characterized in the said fluorine dopant is 2-10 wt %, and said organotin compound is 90-98 wt %, of the composition. 4. A liquid coating composition according to anyone of claims 1-3, CB also including 1-10 wt % of a polar organic compound. 5. A liquid coating composition according to anyone of claims 1, 3 or 4 CIT said fluorine dopant is selected from trifluoroacetate or trifluorodaulic acid and said organotin compound is selected from di-t-butyltin dichloride and methyltin trichloride. 6. A liquid coating composition according to anyone of claims 1 or 3 CB comprising 2-10 wt 8 of trifluoroacetic acid, 80-97 wt % of monobutyltin trichloride and 1-10 wt % of a polar organic liquid. 7. A liquid coating composition according to claim 4, CIT the polar organic compound is selected from methylisobutyl ketone, acetic anhydride or ethyl acetate. 8. A method of producing a high quality fluorine- doped tin oxide coating having a low sheet resistance and high visible light transmission CB comprising: a) forming a liquid coating composition of b) vaporizing said liquid composition at 100 - 4000C., and i) 1-30 wt. % of an organic fluorine dopant compound which is selected from trifluoroacetic acid, trifluoroacetic and; ; ii) 70-99 wt % of an organotin compound which is selected from an alkyltin trichloride, a dialkyltin dichloride, an alkyldichlorotin acetate, a dialkylchlorotin diacetate, an ester tin trichloride, or tin tetrachloride c) contacting said vapor in an oxygen containing atmosphere with a substrate maintained at a temperature above 4000 and below 7000C., said deposition is carried out for 45 seconds or less to produce a coating having a thickness of 160-220 nm, a sheet resistance of less than 40 ohms/square, an infrared reflectivity of greater than 70% and a visible light transmission of at least 80% 9. A method of producing a high quality fluorine-doped tin oxide coating according to claim 8 CIT said oxygen-containing atmosphere is air. 10. A method according to claim 8, CIT said substrate is glass.

Inventors: LINDNER, GEORG HEINRICH, RUSSO, DAVID ALAN
Applicants: M & T CHEMICALS, INC.
Application year: 1985
Main classification: C03C17
IPC codes: C03C17, C23C22