FI64128B - FOERFARANDE FOER PAOFOERING AV EN TRANSPARENT FLUORDOPAD STANNIOXIDFILM PAO ETT UPPHETTAT SUBSTRAT MED REGLERAD FLUORFOERORENINGSHALT - Google Patents

Description

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Patent and registration authorities Ansöksn utlagd och utl.skriften publkand J J

(32) (33) (31) Requested priority * —Begird prloritet (71) (72) Roy Gerald Gordon, 22 Highland Street, Cambridge, Massachusetts, USA (US) (TM Oy Kolster Ab (SM Method of transparent fluorine as a foreign atom) stanniok »to provide an idiol film for a heated substrate le -Klrfurunde for päföring av en transparent, fluordopad stanni-oxidi.

The invention relates to an improved method for producing electrically conductive layers which are highly transparent to visible light and have a high reflectivity to infrared light, and in particular to suitable coatings prepared by this method. These layers are useful as transparent electrodes for solar electrode photocells, resistance photocells, electro-optical liquid crystal displays, photoelectrochemical cells, and many other types of optical electronic devices. As transparent electrical resistors, these layers are used to remove frost in the windows of airplanes, cars, etc. As heat-reflective transparent coatings on glass, these layers improve the performance of six in solar collectors and building windows, furnaces and sodium vapor lamps, and fiberglass insulation.

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Various metal oxides, such as stannous oxide SnCl 2, indium oxide ln 2 O 2 and cadmium stannate Cd 2 SNO 2, have been the most widely used materials for the production of transparent, electrically conductive coatings and layers.

The oldest methods of applying these coatings were based on spraying a solution of a metal salt (usually chloride) onto a hot surface, such as glass. In this way, satisfactory transparent electric resistance layers were first prepared for de-icing aircraft windows. However, the spraying method yielded relatively corrosive by-products, hot chlorine and hydrogen chloride gas, which tended to corrode the hot glass surface and give it a hazy appearance. U.S. Patent 2,617,745 discloses that this undesired effect can be counteracted by first applying a pure silica coating to the glass. However, the silica protective layer is not very effective with glass having a high alkali content and a high thermal expansion coefficient. as with regular baking soda glass. In addition, these corrosive by-products corrode metal parts in the device, and metal impurities such as iron can then separate into the coating, which has adverse effects on both the electrical conductivity and transparency of the coating.

Another problem has been the lack of uniformity and reproducibility in terms of coating properties. U.S. Patent 2,651,585 discloses that better uniformity and reproducibility can be obtained if the humidity in the device is controlled. The use of steam instead of liquid spraying, as explained, for example, in German Patent 1,521,239, also results in more uniform and reproducible coatings.

Despite these improvements, studies have recently been conducted using vacuum precipitation methods, such as evaporation and cathodic dusting, to provide cleaner and more reproducible coatings. Despite the higher cost of these vacuum processes, it is considered that the reduction of corrosive by-products and unwanted impurities associated with injection methods is an improvement, especially in applications involving high purity semiconductors.

The deliberate addition of certain impurities is important in these methods to achieve high electrical conductivity and infrared reflectivity. Thus, tin is combined as an impurity in indium oxide, while antimony is added to tin oxide (stannous oxide) for these purposes. In all cases, it is the function of the impregnants of these desired impurities to bring in "extra" electrons which contribute to the electrical conductivity. These impurities have a high solubility and can be easily added using all of the above application methods. Fluorine has an advantage over antimony as an impregnating agent for tin oxide in that the transparency of the fluorine-impregnated tin oxide film is higher than that of the antimony-impregnated film, especially at the red end of the visible spectrum. This advantage of using fluorine is significant for potential applications in solar cells and solar thermal collectors. Despite this advantage of fluorine, antimony is used as a impregnating agent for most and possibly all commercially available tin oxide coatings. This may be due to the fact that fluorine impregnation has only been demonstrated in the less preferred spraying method, while improved application methods (chemical vapor deposition, vacuum evaporation and cathodic dusting) are not expected to provide fluorine impregnation. In addition, a recent report by a committee of experts in The American Institute of Physics Conference Proceedings No. 25, page 288 (1975X) concludes that the equilibrium solubility of fluorine in tin oxide is inherently lower than the solubility of antimony. butts disclosed in U.S. Patent 3,677,816 to Gillery.

Using an injection method, fluorine-impregnated tin oxide films having an ohmic resistance as low as 15 ohms / square were obtained according to said patent using a compound containing a direct tin fluorine bond as a starting material. The lowest resistance in commercially available tin oxide-coated glass is about 40 ohms per square. If it is desired to obtain coatings with a resistance as low as 10 ohms / square, it has hitherto been necessary to use very much more expensive materials, such as indium oxide.

It is an object of the present invention to provide a method for applying a fluorine-impregnated stannous oxide layer or coating having high transparency in the visible range, high electrical conductivity and high reflectivity to infrared light.

Another object of the present invention is that it makes it possible to vary the electrical conductivity in a simple manner during the application of a single such layer and that it makes it possible to achieve a very low specific resistance and surface resistance.

It is an object of the invention to provide a non-corrosive precipitating atmosphere from which such layers of high purity can be easily precipitated without contamination of the substrate by impurities or corrosion of the substrate or device.

It is a further object of the invention to provide gaseous substances for the manufacture of coated products instead of liquids, as described in connection with the invention.

It is a further object of the invention to provide a method which readily provides such layers with highly uniform and reproducible properties over a wide range without the limitations inherent in spraying methods.

It is a further object to make it possible to apply such layers without difficulty inside tubes or bottles or on surfaces of complex shape which cannot be easily spray-coated.

It is a further object of the invention to provide improved products such as solar cells, other semiconductors useful in electrical circuits, heat reflective windows, improved sodium lamps, etc.

It is a further object of the invention to make it possible to apply such layers by standard manufacturing methods in the semiconductor and glass industries.

Other objects and advantages will become apparent from the following description.

A particular feature of the invention is that the reaction components are selected in such a way that the required tin fluorine bond is not formed until precipitation is imminent. Thus, the tin fluoride material remains better in the vapor phase and at such low temperatures that the oxidation of the compound takes place only after rearrangement 1! 5 to 64128 to form a tin fluorine bond. In this way, films formed of fluorine-saturated tin oxide have an exceptionally low electrical resistivity and an excellently high reflectivity to infrared light.

The process of the invention is carried out using a gaseous mixture containing a volatile fluorine-containing organotin compound having no direct tin-fluorine bond. This mixture also contains a volatile oxidizable tin compound as well as an oxidizing gas. This first fluorine compound does not contain a fluorine-tin bond and is converted to a second organotin fluoride compound having such a bond. Immediately after this change, another compound is oxidized to form a fluorine impregnating agent, and this impregnating agent is oxidized together with an oxidizable tin compound to form a stannous oxide film having a controlled amount of fluorine impurity on a solid support.

According to one embodiment of the invention, the organotin monofluoride vapor in the heated precipitation zone is produced by regenerating the vapor of a more volatile compound containing both tin and fluoroalkyl groups bound to tin.

In another preferred embodiment of the invention, organotin monofluoride is formed at or near the gas-substrate interface by reactions comprising organotin vapor and certain fluorine-containing gases containing fluoroalkyl and / or fluorosulfur groups.

In both cases, the product layer is a homogeneous, hard, adherent transparent coating whose electrical conductivity and infra-red reflectivity depend on the concentration of the fluorine-containing impregnating agent.

Figure 1 schematically shows an apparatus suitable for carrying out the process, wherein the fluorine-containing impregnating agent is organotin-fluoroalkyl vapor, which is converted into a vapor phase from a liquid form.

Figure 2 shows a similar diagram of another embodiment according to which a fluorine-containing impregnating agent is formed by reaction with certain fluoroalkyl and / or fluorosulfur gases introduced from a cylinder with a compressed gas.

Figure 3 shows a simplified embodiment of a device for carrying out a first or second embodiment of the invention.

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Figure 4 is a schematic cross-section of a solar cell and illustrates the use of the invention for semiconductors.

Figure 5 shows a window 120 coated with a layer 118 according to the invention.

Figures 6 and 7 are graphs showing how conductivity and reflectivity change as mucus fluoride saturant concentrations. The process of the invention comprises two main steps: (1) preparing a reactive steam mixture which, when heated, yields a compound containing a tin fluorine bond, and (2) applying this steam mixture to a heated surface on which fluorine-saturated tin oxide is precipitated. The embodiments described below differ in terms of the chemical source of the fluorine impregnant in the reactive steam mixture and also in the materials with which the steam mixture is prepared. The second step (precipitation on the heated surface) is approximately the same in both examples.

Tin is fed as a volatile, oxidizable tin compound, for example tetramethyltin, tetraethyltin, dibutyltin diacetate, dimethyltin dihydride, dimethyltin chloride, etc. The preferred compound is tetramethyltin, which is readily volatile, is not sufficiently volatile at room temperature, is not volatile at room temperature. This volatile tin compound is passed to a pulping apparatus, indicated by 10 in the figures, and an inert carrier gas, for example nitrogen gas, is pulped through the tin compound. For highly volatile compounds, for example tetramethyltin and dimethyltin dihydride, the pulping device may be kept at room temperature, but for other less volatile compounds, the pulping device and tubes must be suitably heated as will be apparent to those skilled in the art. One advantage of the present invention is that the use of high temperature devices can be avoided and that simple cold wall devices can be used.

The vapor mixture must contain an oxidizing gas, for example oxygen, N2O, etc. Oxygen is the preferred gas because this gas is readily available and works just as well as more expensive alternative oxidants.

The gas pressure is determined by regulators 25 and the oxygen gas flow rate from tank 20 and the carrier gas flow rate from tank 21 or 7 64128 are controlled by control valves 30 and measured by flow meters 40. with a heating device 90, usually to a temperature between about 400 and 600 ° C.

The general type of method described above is well known in the steam precipitation technique. Various variations, for example that the substrate surface is arranged vertically or rotatably or below the reaction chamber and rotatable, are well known to those skilled in the art and may be particularly suitable for use depending on the geometry of the substrate or the conditions affecting a particular use.

Rotation of the substrate is recommended to best move the sample through any convection currents that may be present in the device and thus to ensure the uniformity of the applied layer in the best possible way. Furthermore, according to the invention, it has been found that by bringing the heated substrate upside down, very homogeneous coatings can be obtained in a simpler manner without rotation, since no disturbing convection currents occur in the gas when heated from above. Another advantage of keeping the substrate above the reactive vapors is that any dust and dirt or powder formed as a by-product due to homogeneous nucleation in the gas does not fall down on the growing film.

The invention described herein is an improved method by which controlled amounts of fluorine as an impurity can be combined with a growing tin oxide film. In its simplest form, the fluorine impregnating agent is steam containing one tin fluorine bond in each molecule. The other three valencies of tin are saturated with organic groups and / or halogens other than fluorine. A typical example of such compounds is tributyltin fluoride. It has been found that fluorine bound in this way and made available for the hot surface in the form of steam does not cleave from the tin during oxidation in the vicinity of the hot surface.

Unfortunately, not all previously known compounds having such a direct tin-fluorine bond are substantially volatile near room temperature.

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A particular advantage is achieved by the present invention by forming a fluorine impregnating agent from volatile compounds which do not have the required tin-fluorine bond, but in which reaction or rearrangement occurs on heating to form a direct tin-fluorine bond. This regrouping preferably takes place at a temperature high enough (e.g. -100 ° C) so that the tin fluoride thus formed remains in the vapor phase, but also so low (e.g. <400 ° C) that the oxidation of the compound takes place after regrouping. An example of such a compound is trimethyltrifluoromethyltin, (CH 2 Cl 2). When heated to a temperature of about 150 ° C in a heated zone near the precipitation surface 80, this compound regroups to form a direct tin fluorine bond in (CH 2) SnF vapor, Other compounds which undergo similar rearrangements at temperatures which, of course, vary somewhat from one compound to another have the general formula R 1 SnRF, where R is a hydrocarbon group and RF is a fluorinated hydrocarbon group having at least one fluorine atom which is a fluorine donor, i.e. a saturant. The main advantage of these fluorine impregnants is that they are volatile liquids so that they can easily give a sufficient vapor pressure when evaporated at room temperature.This simplifies the design of the device, as shown in Figure 1, by eliminating the need to maintain a hot Zones, are compatible e pulping between the device 15 and the reaction chamber 70 to keep the fluorine impregnant in the vapor phase. The apparatus may thus be of a type commonly referred to as a "cold wall type chemical vapor deposition reactor" and widely used, for example, in the semiconductor industry for the precipitation of silicon, silica, silicon nitride, etc. Other essential features of a "cold wall reactor" for semiconductor applications are that it reduces the amount of unwanted impurities to a low level in both the substrate and the applied film. Similarly, in the manufacture of glass, the gas mixture can be added to the annealing and cooling furnace at a stage when the glass has a suitable temperature, for example about 470 ° C for soft glass. In this way, very uniform films can be obtained in a conventional glass manufacturing apparatus.

The preferred compound for use in the embodiment of Figure 1 is (CH 2 Cl 2, as this is more volatile than compounds having more than 1 64 64128 carbon atoms. This compound is a stable, colorless, non-corrosive liquid that does not decompose in air at room temperature). and which reacts only extremely slowly with water.

In another particularly preferred embodiment of the invention, a fluorine-containing gas is used which reacts with the organotin vapor when heated to form tin fluoride vapor. Thus, for example, N-fluoroalkyl halides, especially those having 4 alkyl atoms or less in the alkyl group, gases such as iodotrifluoromethane, CFI, CF3CF2I, C3F7I, etc., can be mixed with organotin vapors, for example tetramethyltin vapor, at room temperature, Up to 32 ° C, especially up to 65 ° C without reaction Further fluoroalkyl bromides, such as CF 3 Br, C 2 CF 2 Br, etc., are useful as fluorine-containing gases, are less reactive and about 10 to 20 times more are required for the reactor gas, but they are much cheaper.This is particularly surprising since such compounds have been considered inert.Fluoroalkyl chlorides are not preferred for use because their reactivity is substantially lower than that of bromides.

As such a steam mixture approaches the heated surface, a reaction occurs in the gas phase so that the desired tin-fluorine compounds are eventually formed. Although the sequence of reactions is complex, it is assumed that this begins with reactions such as CF3J + R4Sn -> R3SnCF3 + R1 to form organotinafluoroalkyl R3SnCF3 in vapor form near the interface with the hot surface in which they act as fluorosaturants in the same manner as the growing tin oxide.

Certain other fluorine-containing gases are also effective in accordance with this second embodiment of the invention. Thus, for example, sulfur chloride pentafluoride SF 2 Cl is an efficient fluorine donor gas as is sulfur bromide pentafluoride SF 2 Br.

Similarly, trifluoromethyl sulfur pentafluoride CF3SF5 acts in the form of a gas to form tin fluoride bonds through a gas phase reaction.

The advantage in this second embodiment is that the fluorine generator is a gas and the process is further illustrated in Figure 2. Preferred gases are CF 2 Br and CF 2 Br, which are non-corrosive, non-combustible, substantially non-toxic and commercially readily available. available. SF ^ Cl and SF ^ Br are highly toxic and are therefore less desirable for use.

CF ^ SF ,. is non-toxic but somewhat less reactive than cf3i.

The precipitation process can be further simplified as shown in Figure 3 if the gas mixtures are premixed and stored in a compressed gas cylinder 19. For safe storage and use, the concentration of oxidizable compound must of course be kept such that it cannot form an explosive mixture. For example, tetramethyltin has a lower explosion limit in air of about 1.9%. Concentrations used for chemical vapor deposition are less than half of this concentration. In addition, the use of CF 3 I and CF 3 I as a fluorine impregnant also acts as an anti-fire agent.

The films made according to the invention have been shown to have an infrared reflectance of 90% or more, as measured in the prior art, at a conventional wavelength of 10 μm, which is characteristic of thermal infrared radiation at room temperature. This 90% reflectance can be compared to the 80% reflectance achieved so far using tin oxide coatings. In normal use, these infrared reflecting layers have a thickness between about 0.2 and 1 and thicknesses between 0.3 and 0.5 are typical.

To characterize fluorine impregnant concentrations more quantitatively in the films, infrared reflectance was measured in the wavelength range from 2.5 μm to 40 μm. By placing these values in theoretical curves such as R, Groth, E. Kauer, and PCvan den Linden, in detail in their in SnO2 Layers ", Zeitschrift fur Natur Forschung, Volume 179, pages 789-793 (1926), values were obtained for the concentration of free electrons in 20-332 -3 films. The values obtained ranged from 10 cm to 10 cm and increased regularly as the concentration of fluorine impregnant increased. Theoretically, one free electron h 11 64128 should be donated for each fluorine atom, which is replaced by an oxygen atom in the lattice. This hypothesis was confirmed by "Auger Electron Spectroscopic" measurements of the total fluorine content in certain membranes, which gave fluorine concentrations consistent with the free electron concentrations within the experimental accuracy. This concordance indicates that a greater proportion of the combined amount of fluorine is electrically active.

The infrared reflectance at 10 μm as well as the electrical mass conductivity of the films turned out to be at their highest at the saturation level where 1.5-2% oxygen had been replaced by fluorine. These maxima are very broad and have near-maximum conductivity and reflectivity on films with 1-2.5% fluorine. There is still a weak, general absorption over the entire visible wavelength range, which increases directly with the fluorine content. Therefore, for the production of films with high electrical conductivity and high visible transparency, a fluorine content of about 1% in the film (i.e. a ratio of fluorine: acid in the film of 0.01) is particularly desirable. However, this optimum varies somewhat depending on the book distribution that is of interest for a given use. By varying the concentration of the fluorine saturant, the optimal concentration for a particular use can be readily determined by routine experimentation.

Concentrations of fluorine impregnating agent in excess of 3% can be readily achieved in films using the methods of the invention. The results by previously known methods have not exceeded 1% and, as previously understood, as discussed above, this is considered to be the solubility limit for fluorine. Although such high levels of impregnant are not required to achieve optimal infrared reflectance or electrical conductivity, gray films made with impregnant contents of 2% or more may have use in glasses intended for architectural use in buildings to limit the import of solar heat. In such use, the impregnation concentration on the surface of the film is suitably reduced to about 2% to achieve maximum infrared reflectance.

Using the measured electron concentrations and the values of electrical conductivity, the mobility of the electrons is obtained. Values between 50-70 cm / volt-second were calculated for different membranes in this way.

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Previously obtained mobility values for tin oxide films have varied between 5 and 35 cm / volt-second. It is assumed that the films according to the invention are the first films with mobility values 2 exceeding 40 cm / volt-second. These values illustrate in another way the better quality of the method according to the invention and of the films produced with it.

The method according to the invention is also very useful in the manufacture of new devices, for example those with electron-conducting layers in the manufacture of semiconductors (for example integrated circuits, etc.) and also in the manufacture of heat-reflecting transparent objects such as windows.

The most preferred embodiment of the invention is an embodiment in which an organotin fluoride compound having a tin fluorine bond is decomposed on a substrate immediately after formation. This decomposition preferably takes place in a narrow reaction zone, which is mainly heated to the decomposition temperature by the heat obtained from the substrate itself.

In order to further illustrate the invention, exemplary embodiments of the method according to the invention and the products prepared therefrom are given below.

Unless otherwise indicated, the following examples were performed using the following general procedure:

Example 1

An example of the process is an experiment using the apparatus of Figure 1 to produce a gas stream containing 1% tetramethyltin (CH 2 Cl 2, 0.02% trimethyltrifluoromethyltin (CH 2 Cl 2), 10% nitrogen as a carrier gas and the remainder oxygen. the stream is passed over a 15 cm diameter pyrex glass plate and maintained at a precipitation time of about 5 minutes at 500 ° C. The gas flow rate is about 400 cm 2 / min, this gas flow in funnel 70 corresponds to about one gas exchange in 2 minutes. A transparent film with a thickness of about 1 μm is precipitated, the electrical resistance of the film is 2 ohms / square, which corresponds to a volume specific resistance of 0.0002 ohm-cm This film is measured to have a fluorine / oxygen ratio of about 0.017 and a driving mobility of about 50 cm / volt second.

i3 641 28

Example 2

By repeating the procedure of Example 1 using a sodium-free silicon substrate, the resistance value is reduced to about 1 ohm / square, i.e., about half the value of the resistivity achieved with the sodium-second substrate.

Example 3

A preferred embodiment of the method can be illustrated by a process using the device shown in Figure 2. The resulting gas mixture comprises 1% tetramethyltin (CH3) 4Sn, 0.2% iodotrifluoromethane SF3I, 20% nitrogen carrier gas and the remainder oxygen. The films deposited on the Pyrex glass substrate had the same electrical properties as in Example 1.

Example 4 A simplified apparatus according to Figure 3 was used to prepare the mixture described in Example 3 in a compressed gas cylinder 19. The results are similar to those obtained according to Example 3. After one month of storage in a gas cylinder, the experiment was repeated to give similar results. This indicates the stability and shelf life of the mixture.

Example 5

The experiment of Example 3 was repeated with the difference that the precipitation is stopped when the thickness of the stannous oxide film is Q.5M. The infrared reflectivity of the obtained stannous oxide film is about 90%.

Examples 3-13

The gases listed below were all used to replace, in equimolar amounts, CF3I in the process of Example 3 (with the exception that the fluorine impregnant concentration was increased 15-fold according to Examples 6, 7, 8 and 13). Very good conductivity and reflectivity to infrared radiation are obtained.

Example Gas Example Gas

6 CF3Br 10 C3F7I

7 C2F5Br 11 SF ^ Br 8 C3F7Br 12 SF5C1 9 C2F5 13 CF3SF5 64128 14

Typical surface resistance values for conventional silicon electrolyte photocells ("solar cells") have been 50-100 ohms / square. To obtain an acceptably low total cell resistance, a metal lattice is deposited on the silicon surface at 1 or 2 mm intervals. By depositing a fluorine-saturated tin oxide layer with a layer resistance of about 0.5 ohms / square (about 2 μm thick) on the cell surface, the gaps in the metal lattice can be increased to about 10 mm with a corresponding reduction in lattice costs. Alternatively, the lattice size can be kept small and the cell can then operate efficiently even when the sunlight is concentrated by a factor of about 100, provided that a satisfactory cooling of the cell is maintained.

A schematic cross-section 100 of such a cell is shown in Fig. 4, in which a layer 102 having a thickness of 2 μm and made of n-SnO 2 (using a fluorine-impregnated substance according to the invention), a layer 104 having a thickness of 0.4 μm and is n-silicon (phosphorus-saturated silicon of a previously known type), a layer 106 having a thickness of 0.1 mm and which is p-silicon (a known type of boron-saturated silicon) are connected to an aluminum layer 108 which acts as an electrode. The metal gratings 110 are arranged at internal distances of about 10 mm. Nevertheless, a very good result is obtained.

The applied layers can be used in the manufacture of other semiconductor products, such as conductors or resistors. Tin oxide coatings have been used for this purpose in integrated circuits. Improved conductivity allows wider use of this material. Thus, not only can the layer resistance range be extended to much lower values (e.g., about 5 ohms / square or smaller) than has been possible so far, but layer deposition can also be achieved in the same device used for further growth of epitaxial silicon, for example. This eliminates the costly and cumbersome step of emptying, cleaning and transporting between precipitation work steps.

The resistivity values obtained for fluorine-saturated -4 tin oxide on a silicon substrate are about 10 ohm-cm, which is comparable to the value for vaporized tantalum metal used in certain cases for connections in integrated circuits.

Good agreement between the coefficients of thermal expansion of tin oxide and silicon allows the deposition of thick layers without substantial stresses.

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Figure 6 shows the electrical conductivity of a fluorine-saturated stannous oxide film as a function of the measured fluorine / oxygen ratio in the film for precipitation temperatures of 480 and 500 ° C.

Figure 7 shows the infrared reflectance of fluorine-saturated stannous oxide films as a function of the measured fluorine / oxygen ratio in the film at precipitation temperatures of 480 and 500 ° C.

Figures 6 and 7 also show (1) the conductivity of previously known expensive indium oxides and, as described by van Boort and Groth in Philips Technical Review, Volume 29, page 17 (1968), and (2) the best known values for impregnated conductivity and reflectivity of stannous 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 modifications may be made without departing from the spirit or scope of the appended claims.

Claims (9)

A process for preparing a transparent fluorine-doped stannous oxide film with controlled fluorine contamination content on a heated substrate, characterized in that a gas mixture is used initially containing (a) a first fluorine-containing compound free from direct tin-fluorine bonding, (b) an oxidizable tin compound, and (c) an oxidizing gas, and perform the following steps: (1) converting the first fluorine-containing compound in the gas mixture into a second gaseous fluorine-containing compound with a direct tin-fluorine bond, (2) immediately oxidizing this second fluorine-containing compound in the immediate vicinity of the substrate such that a fluorine dopant is obtained in the gas mixture, and (3) prepares a fluorine-doped stannic oxide film on the heated substrate by co-application thereto of the oxidizable tin compound and fluorine dopant. Process according to Claim 1, characterized in that a continuous stream of the gas mixture containing components (a), (b) and (c) is applied adjacent to the substrate such that the fluorine-containing compound with direct tin-fluobinding is formed immediately adjacent to the heated the substrate, and the fluorine-doped stannic oxide film is deposited on the substrate. 3. A process according to claim 1, characterized in that the first fluorine-containing gaseous compound is prepared by heating a gas mixture containing (a) a gas selected from the group consisting of CF 1, CF 2 Br and with CF 1 and CF 2 Br homologous. O (-fluorinated alkyl compounds, CF 3 SF,. SF 5 Br and SF (.Cl or mixtures thereof), and (b) the oxidizable tin compound, wherein components (a) and (b) are substantially inert relative to each other. at temperatures below 65 ° C. 4. A process according to any of the preceding claims, characterized in that the oxidizable tin compound is tetramethyltin, dimethyltin dichloride or dimethyltin dihydride.