CH640276A5 - Process for producing films of stannic oxide on a heated substrate - Google Patents

Abstract

a) Process for producing transparent films of stannic oxide on a heated substrate. b) The process makes it possible to produce electrically conductive films based on tin oxide by employing gaseous chemical compounds which react to form a tin-fluorine bond at a temperature which (1) is sufficiently high for the molecule containing the newly formed tin-fluorine bond to remain in the vapour phase, and (2) sufficiently low for the oxidation of this molecule to take place only after the indicated molecular rearrangement. Means for making use of this process are also described. The films thus prepared have a surface resistance which can go down to 1 ohm per square centimetre when the film thickness is approximately one micron. The films thus obtained reflect infrared radiations remarkably well. The process is employed in particular for the production of transparent conductive coatings for solar cells, photoconductive cells, electrooptical devices with liquid crystals, photoelectrochemical cells and many other optoelectronic devices.

Description

The present invention relates to a method for producing electrically conductive layers which are generally extremely transparent to visible light and which strongly reflect infrared rays, as well as to the particularly advantageous coatings thus formed. Such layers are useful as transparent electrodes for the manufacture of solar or photovoltaic cells or cells, photoconductive cells, electro-optical liquid crystal display devices, photo-electrochemical cells and many others kinds of optico-electronic devices. Such layers are also used as transparent electrical resistors for defrosting windows and, in particular, windshields of aircraft, motor vehicles, etc. Applied on glass, these transparent coatings which reflect heat rays increase the efficiency of solar thermal collectors and windows in buildings, ovens and sodium vapor lamps, as well as the insulation of glass fibers.

Various metal oxides, and in particular stannic oxide Sn02, indium oxide ln203 and cadmium stannate Cd2Sn04, have been widely used to produce transparent and electrically conductive layers and coatings.

The most recent methods of applying these coatings have been to spray a solution of a metal salt (usually chloride) onto a hot glass surface, for example. In this way, transparent layers were first produced satisfactorily forming electrical resistances for de-icing aircraft windows. However, this spraying process has the disadvantage of giving corrosive by-products, in particular hot chlorine and hydrogen chloride gases which tend to attack the hot surface of the glass, thus producing a cloudy or "hazy" appearance. . US Patent No. 2,617,745 teaches that it is possible to overcome this undesirable effect by first applying a coating of pure silica on the glass. However, a protective layer of silica is not very effective on glass when it contains a high percentage of alkali and has a high coefficient of thermal expansion, as is the case with common soda-lime glasses. In addition, these corrosive by-products attack the metallic parts of the device and metallic impurities, for example iron, can then be deposited on the coating, which has harmful effects both on the electrical conductivity and on the transparency of this one.

Another problem which arises is that of the lack of uniformity and reproducibility of the properties of the coatings. US Patent No. 2651585 teaches that better uniformity and reproducibility can be achieved by controlling the humidity in the device. The use of a vapor, instead of a jet of liquid, as described for example in German Patent No. 1521239, also results in more uniform and better reproducible coatings.

However, despite these improvements, recent studies which have been carried out using vacuum deposition techniques, such as spraying and spraying, have shown that it is possible to produce coatings that are cleaner and more reproducible. Despite the much higher cost of these vacuum processes, the reduction in corrosive by-products and unwanted impurities, which are introduced by the spray processes, is justified, especially in applications involving semiconductors extremely pure.

The intentional incorporation of certain impurities is important in these processes in order to obtain a high electrical conductivity and a great power of reflection of the infrared rays. For example, tin is incorporated as an impurity in indium oxide, while antimony is added to tin oxide (stannic oxide) for this purpose.

In each case, the function of these useful impurities (also called “dopants”) consists in supplying additional electrons which contribute to the conductivity. These impurities are very soluble, so that they can be easily incorporated into the desired substance, whatever the deposition process used. Fluorine has an advantage over antimony to boost tin oxide in that fluoride-doped stannic oxide films are more transparent than antimony-doped films, especially in the red end of the visible spectrum . This advantage of fluorine is important, in particular in certain applications such as solar cells and solar thermal collectors. Despite this advantage of fluorine, most and probably all commercial tin oxide coatings use antimony as a dopant. This is explained by the fact that fluorine was only used in the less satisfactory spraying process, while the more sophisticated deposition processes (chemical vapor deposition, evaporation and spraying under vacuum) are said to be unable to operate as dopants with fluorine. In addition, a recent report of a committee of experts published in "The American Institute of Physics Conference Proceedings", No. 25, p. 288 (1975), concludes that the equilibrium solubility of fluorine in tin oxide is inherently lower than that of antimony. Nevertheless, it should be noted that the tin oxide films having the lowest resistivity, reported in the prior art, are those of US Patent No. 3,677,814 to Gillery. Using a sputtering process, he obtained fluorine doped tin oxide films having resistances of only 15 Ci2 using as a starting material a compound having a direct tin / fluorine bond. However, the lowest resistance of tin oxide coated glasses currently found on the market is of the order of about 40 μl. Consequently, when one wishes to obtain coatings having only a resistance of 10 pounds 22, one is forced to have recourse to much more expensive substances such as indium oxide.

One of the aims of the present invention is to provide a method for depositing a layer or coating of fluorine doped stannic oxide which is extremely transparent to visible light, which has

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a high electrical conductivity and which strongly reflects infrared rays.

Another object of the invention is to allow the electrical conductivity to be varied during the deposition of such a layer and to have the possibility of achieving very low mass resistivities and surface resistances.

Another object of the invention is to produce a non-corrosive deposit atmosphere, from which extremely pure layers can be easily deposited, without contaminating the substrate with impurities and without attacking it or the apparatus used. by corrosive chemicals.

The invention also proposes to use gas phases rather than liquid phases to produce the coatings described in the present application.

The present invention also contemplates a process which allows extremely uniform and reproducible layers to be easily produced over relatively large areas, without encountering the limitations inherent in spraying processes.

Another object of the invention is to make it possible to deposit the layers in question inside tubes or ampoules or on surfaces having complicated contours, where spraying can hardly be used.

Another object of the invention is to manufacture improved products, in particular solar cells, semiconductors which can be used in electrical circuits, heat reflecting windows, improved sodium lamps, etc.

Another objective of the present invention is to allow the layers in question to be deposited by conventional manufacturing methods, commonly used in the semiconductor industry and in that of glass.

A particular feature of the process of the invention is to select the reactants so that the required tin / fluorine bond does not form before the deposition is imminent. Thus, the tin fluoride used is better maintained in the vapor phase and at temperatures low enough for the oxidation of this compound to take place only after a rearrangement of its molecules leading to the formation of a tin / fluorine. The fluorine-doped tin oxide films thus formed have an extraordinarily low electrical resistivity and an unusually high reflectance of infrared rays.

The method of the invention is implemented using a gas mixture containing a volatile organotin compound containing fluorine, but which contains no direct tin / fluorine bond. This mixture also contains an oxidizable volatile tin compound and an oxidizing gas. This first fluorine compound, which does not contain a fluorine / tin bond, is transformed into a second organotin fluoride compound having such a bond. Immediately after this conversion, this second compound is oxidized to form a fluorinated dopant and this dopant is oxidized at the same time as the oxidizable tin compound to form a solid substrate, a film of stannic oxide containing a controlled amount of fluorine as an impurity. .

In a first embodiment of the invention, a vapor of an organotin monofluoride is formed in the hot deposition region by reconstitution of the vapor of a more volatile compound containing both tin and fluoroalkyl groups bound to tin.

In a second advantageous embodiment of the invention, an organotin monofluoride is used formed at or near the interface between the gas and the substrate by reactions involving an organotin vapor and certain gases containing fluorine having fluoroalkyl and / or fluorosulfur groups.

In each case, a layer is thus produced constituting a uniform, hard and adherent transparent coating, the electrical conductivity and the reflectance of infrared rays depending on the concentration of the dopant containing fluorine.

Other characteristics and advantages of the invention will emerge from the description which follows, given solely by way of nonlimiting example, with reference to the appended drawing, in which:

- fig. 1 is a schematic view of an apparatus suitable for implementing a first form of the process of the invention in which the fluorinated dopant is a vapor of an organotin containing fluoroalkyl groups which has been vaporized from its liquid form;

- fig. 2 is a similar schematic view of a second embodiment in which the fluorinated dopant is formed by reaction with certain fluoroalkyl and / or fluorosulfurized gases which come from a bottle in which they are compressed;

- fig. 3 illustrates a simplified variant of the apparatus for implementing both the first and the second embodiment of the invention;

- fig. 4 is a schematic sectional view through a solar cell and illustrates the use of the invention in the field of semiconductors, and

- fig. 5 shows a window 120 comprising a layer 118 according to the invention.

Figs. 6 and 7 represent curves which illustrate the variation of the conductivity and the reflecting power as a function of the concentration of fluorine. The process of the invention comprises two main stages, namely: 1) formation of a reactive vapor mixture which, when heated, produces a compound having a tin / fluorine bond, and 2) bringing this into contact mixture of vapors with a hot surface on which is deposited a layer of tin oxide doped with fluorine. The embodiments described below differ in the chemical source providing the fluorine used for doping in the reactive vapor mixture and also in the means by which this vapor mixture is produced. The second step, i.e. the step of depositing on the hot surface, is to a large extent identical in the different examples.

Tin is provided in the form of an oxidizable volatile tin compound such as tetramethyltin, tetraethyltin, dibutyltin diacetate, dimethyltin dihydride, dimethyltin dichloride, etc. The preferred compound is tetramethyltin since it is sufficiently volatile at room temperature, non-corrosive, stable and easy to purify. This tin compound is placed in a bubbler referenced 10 in the figures and an inert gaseous vehicle, for example nitrogen by bubbling, is passed through the tin compound. When using highly volatile compounds, such as tetramethyltin and dimethyltin dihydride, the bubbler can operate at room temperature but, for other less volatile compounds, the bubbler and tubing must be suitably heated, as will be understood easily all knowledgeable technicians. One of the advantages of the present invention is that it makes it possible to avoid the use of devices operating at high temperatures and that simple supply devices with cold walls can be used.

The vapor phase mixture must contain an oxidizing gas, such as oxygen, nitrous oxide or the like. Oxygen is the preferred gas since it is readily available and gives as good results as other more expensive oxidizing gases.

The pressures of the gases are fixed by regulators 25, while the flow rates of oxygen from the tank 20 and from the gaseous vehicle from the tank 21 are adjusted by means of metering valves 30 and are measured by flow meters 40. The gas streams then pass through the unidirectional valves 50 to gain a mixing tube 60 and a funnel-shaped chamber 70. A film of tin oxide is thus deposited on the hottest surface 80, which is carried by a heating element 90 at a temperature generally between about 400 and 600 ° C.

The process which has just been described is known in the art under the name of chemical vapor deposition process. Various modifications, such as the use of vertical and rotating support surfaces or those located under the reaction and rotating chamber, are known to technicians and may be specific5

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ment adapted for certain applications according to the configuration or conformation of the substrate or according to other conditions inherent to the application considered.

It is recommended to rotate the substrate in order to better move the sample through the convection currents that can develop in the device and thus ensure better uniformity of deposits. However, it has been found that by placing the heated substrate face down, extremely uniform coatings can be obtained in a simpler, non-rotating manner, because the gases, when heated from above, do not create troublesome convection currents. Another advantage which results from the fact that the substrate is placed above the reactive vapors is that the dust or dirt, or else the powdery by-products formed by the homogeneous nucleation in the gas, do not fall on the growing film. .

The invention described opposite is an improved process by means of which certain quantities of fluorine, constituting a doping impurity, can be introduced into the tin oxide film in formation. In the simplest aspect of the invention, the dopant is in the vapor phase and contains a tin / fluorine bond in each molecule. The other three valences of tin are satisfied by organic groups and / or by halogen atoms other than the fluorine atom. A typical example of these compounds is tributyltin fluoride. It has been discovered that the fluorine thus bound, and which is made available to a hot surface in the form of vapor, does not separate from the tin during oxidation at this surface.

Unfortunately, none of the known compounds having such a direct tin / fluorine bond is non-volatile in the vicinity of room temperature.

A particular advantage of the invention results from the fact that the fluorinated dopant is produced from volatile compounds which do not have the necessary tin / fluorine bond, but which rearrange under the action of heat so as to form a bond direct tin / fluorine. This rearrangement occurs at a sufficiently high temperature (for example 100 ° C.), so that the tin fluoride thus formed remains in the vapor phase but which, however, is quite low (for example <400 ° C.), so that the oxidation of the compound takes place only after the rearrangement. An example of such a compound is trimethyltrifluoromethyltin (CH3) 3SnCF3. By heating it to a temperature of around 150 ° C in the vicinity of the heated deposition surface 80, this compound rearranges to form a direct tin / fluorine bond, in the form of a vapor of (CH3) 3SnF, which then reacts as a fluorine donor or fluorinated dopant. Other components which undergo similar rearrangements at temperatures which, obviously, are slightly different from one compound to another, correspond to the general formula R3SnRF, in which R is a hydrocarbon radical and RF a fluorinated hydrocarbon radical comprising , at least, a fluorine atom linked to the carbon atom which is itself linked to tin. The main advantage of these fluorinated dopants is that they are volatile liquids, so that they can easily develop sufficient vapor pressure when they are vaporized at room temperature. This simplifies the construction of the device, as shown in fig. 1, by eliminating the need to maintain a hot zone between the bubbler 15 and the reaction chamber 70 in order to maintain the fluorinated dopant in the vapor phase. The apparatus used is of the type known as a cold wall vapor deposition reactor which is commonly used, for example, in the semiconductor industry to produce deposits of silicon, silicon dioxide, silicon nitride, etc. An important feature of this cold-walled reactor for manufacturing semiconductors is that it minimizes unwanted impurities both on the substrate and in the deposited film. Likewise, in the manufacture of glass, the gas mixture can be added to the annealing and cooling furnace when the glass is at an appropriate temperature, for example around 470 ° C. for soft glass. In this way, extremely uniform films can be obtained in normal glass making equipment.

The preferred compound for use in the embodiment of FIG. 1 is (CH3) 3SnCF3, since it is more volatile than the compounds having a greater number of carbon atoms. It is a colorless, stable and non-corrosive liquid, which does not decompose in air at room temperature and which reacts only very slowly with water.

A second particularly advantageous embodiment of the invention uses a fluorine-containing gas which reacts with a gaseous organotin under the action of heat to produce a tin fluoride in the form of vapor. Thus, for example, that α-fluoroalkyl halides, preferably those in which the alkyl group has 4 carbon atoms or less, in the form of gases such as iodotrifluoromethane, CF3I, CF3CF2I, C3F7I and others , can be mixed with organotin (CH3) 4Sn vapors at room temperature, i.e. at about 30 ° C or, better still, at temperatures of around 65 ° C, without reacting . In addition, some fluoroalkyl bromides, such as CF3Br, C2F5Br and others, are useful fluorinated gases. They are less reactive and it takes about 10 to 20 times more, but they cost much less. This is particularly surprising, given the reputed inertia of these compounds. Fluoroalkyl chlorides are not advantageous to use because they are still much less reactive than bromides.

When such a mixture of vapors approaches a hot surface, there takes place a reaction which, possibly, produces the desired tin / fluorine bonds. Although the reaction steps are complex, it is assumed that they start with reactions such as:

CF3I + R4Sn- »R3SnCf3 + RI

to give organo-tin fluoroalkyl R3SnCF3 vapors in the region close to the interface of the hot surface, thus serving as fluorinated dopants during the development of the tin oxide film, just as in the first embodiment.

Certain other fluorinated gases also function in the second embodiment of the invention. Thus, for example, sulfur pentafluoride chloride SF5C1 is an effective fluorinated gaseous donor, as is sulfur pentafluoride bromide SFsBr.

Similarly, sulfur trifluoromethylpentafluoride CF3SF5 is a gas which acts to form tin / fluoride bonds through gas phase reactions.

The advantage of this second embodiment is that the fluorinated donor is a gas, the process used being shown in FIG. 2. The preferred gases are CF3I and CF3Br which are non-corrosive, non-flammable, practically non-toxic and commercially available. SFsCl and SFsBr are extremely toxic and therefore less suitable. CF3SF5 is non-toxic, but slightly less reactive than CF3I.

The deposition process can be further simplified,

as shown in fig. 3, by preparing the gas mixtures in advance and keeping them compressed in a bottle or cylinder 19. To be able to be used and stored without danger, it is obviously necessary that the concentration of the oxidizable compound be kept low enough so as not to form an explosive mixture. Thus, for example, the lower explosion limit of tetramethyltin in air is about 1.9%. The concentrations used by the licensee to form the chemical deposits are less than half of this percentage. In addition, the use of CF3I or CF3Br as a fluorinated dopant plays, incidentally, the role of extinguishing agent.

It is found that the films prepared by the process of the invention reflect infrared rays at 90% or more, these percentages being measured, in a conventional manner, at the conventional wavelength of 10 | t which is the wavelength characteristic of thermal infrared radiation at room temperature. This reflecting power of 90% is to be compared with the reflecting power of 80% previously obtained with tin oxide coatings. In current practice, these reflective layers of

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infrared have a thickness of about 0.2 to 1 (i; thicknesses between 0.3 and 0.5 | i are typical.

To better determine quantitatively the fluorine doping levels of the films, their power of reflection in the infrared was measured at wavelengths between 2.5 and 40 p. By plotting these results on theoretical curves, such as those described in detail by R. Groth, E. Kauer and PC van den Linden in "Optical Effects of Free Carriers in Sn02 Layers", Zeitschrift für Naturforschung, vol. 179, pp. 789 to 793 (1962), values representing the concentration of free electrons in the films were obtained. The values obtained are between 1020 cm-3 and 1021 cm-3 and increase regularly with the concentration of the fluorinated dopant. Theoretically, a free electron should be created by a fluorine atom replacing an oxygen atom in the network. This hypothesis was verified by measurements of the total concentration of fluorine, carried out by means of an electronic Auger spectroscope in certain films, the results obtained with these measurements being in agreement with theory, within the limits of experimental uncertainties. This agreement indicates that most of the fluorine incorporated in the films is electrically active.

The reflectivity of infrared rays at 10 p. and also the electrical conductivity of the films were found to be maximum for a doping level corresponding to approximately 1.5 to 2% of fluorine replacing oxygen. This maximum is very wide and conductivities and reflection powers corresponding almost to the maximum are obtained with films containing 1 to 2.5% of fluorine. Films also have a low absorption band relatively wide in the field visible wavelengths, absorption which increases directly with the fluorine concentration. Therefore, to prepare films with high electrical conductivity and high transparency to visible light, it is preferable that the fluorine concentration of the film is about 1% (i.e. the fluorine ratio / oxygen is about 0.01 in the film). However, this optimal ratio can vary to some extent, depending on the spectral distribution of interest in a given application. By varying the concentration of the fluorinated dopant, routine experiments easily make it possible to establish the percentage which is most suitable for a given application.

Fluorine doping levels exceeding 3% can be easily obtained using the methods of the present invention. The results of the prior art did not exceed 1% and the opinion, cited above, was that this represented the limit of solubility of fluorine. Although such high doping levels are not necessary to achieve optimal infrared reflectance or electrical conductivity, gray films produced when the doping level reaches or exceeds 2% may be useful on some architectural glasses, for example to limit heating by the sun's rays in certain air-conditioned buildings. In such applications, it is advantageous to reduce the level of doping of the surface of the film to around 2% to reflect the infrared rays as much as possible.

Using the electron concentrations and the measured electrical conductivities, the mobility of the electrons can be determined. For various films, values between 50 and 70 cm2 / V-s were calculated in this way. The mobility values previously obtained for tin oxide films were between 5 and 35 cm2 / V-s. It is assumed that these films are the first in which these mobilities exceed 40 cm2 / V-s. These values illustrate, from another angle, the quality of the process of the invention and the superiority of the films prepared using it.

The method of the invention is also extremely advantageous for producing new devices, for example electroconductive layers during the manufacture of semiconductors (for example of integrated circuits and others) and also for the manufacture of transparent objects reflecting the heat, such as window panes.

The most advantageous mode of implementation of the invention is that in which the fluorinated organotin compound having a tin / fluorine bond decomposes on the substrate immediately after its formation. Preferably, this decomposition takes place in a narrow reaction zone strongly heated to the decomposition temperature by the heat of the substrate itself.

The following examples, which of course have no limiting character, will make the particular features of the invention better understood.

Unless otherwise specified, the specific examples described below were carried out according to the following general procedure:

Example 1:

The process is carried out by means of an experiment using the apparatus of FIG. 1 to produce a gas stream which contains 1% tetramethyltin (CH3) 4Sn, 0.02% trimethyltrifluoromethyltin (CH3) 3SnCF3, 10% nitrogen as the vehicle, the remainder being oxygen. The resulting current is passed over a Pyrex glass plate 15 cm in diameter, maintained at 500 ° C., for approximately 5 min. The flow rate of the gas stream is adjusted to approximately 400 cm3 / min. This flow rate corresponds to a rotation of the gas of approximately once every 2 min in the funnel 70. Under these conditions, a transparent film of approximately 1 n is deposited on the plate. This film has an electrical resistance of 2 £ i2, corresponding to a volume resistivity of about 0.0002 £ 2 / cm. Measurements have shown that, in this film, the fluorine / oxygen ratio is approximately 0.017 and that the mobility of the electrified carriers is approximately 50 cm2 / V-s.

Example 2:

If the method of Example 1 is repeated, using a silicon substrate which does not contain sodium, the resistance drops to about 1122, i.e. to about half of the resistivity obtained with a substrate containing sodium.

Example 3:

An advantageous method consists in using the apparatus of FIG. 2. The resulting gas mixture comprises 1% tetramethyltin (CH3) 4Sn, 0.2% iodotrifluoromethane CF31.20% nitrogen as the vehicle, the remainder being oxygen. The films produced on Pyrex glass substrates have the same electrical characteristics as in Example 1.

Example 4:

The simplified apparatus of fig. 3 with the mixture described in Example 3 contained, in the compressed state, in a cylinder or a bottle 19. The results are identical to those of Example 3; after a storage of 3 months in the bottle, the experiment is repeated with the same results. This demonstrates the stability and the preservability of this mixture.

Example 5:

The procedure is as in Example 3 except that, when a 0.5 (i thick) stannic oxide film has been obtained, the deposition is stopped. The resulting stannic oxide film has a ray reflection power about 90% infrared.

Examples 6 to 13:

The following gases are replaced each time in CF3I of Example 3 in equimolar proportions (except that the concentration of fluorinated dopants is increased by 15 times in Examples 6,7, 8 and 13). Films are thus obtained having excellent conductivity and excellent infrared reflection power:

Example

Gas

Example

Gas

6

cf3Br

10

c3f7i

7

C2F5Br

11

sf5Br

8

c3f7 Br

12

sf5c1

9

c2f5i

13

cf3sf5

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Traditional silicon photovoltaic cells (solar cells) currently have typical surface resistances of 50 to 100 Q2. To obtain an acceptable total resistance, a metal grid is deposited, the meshes of which are spaced 1 or 2 mm apart on the surface of the silicon. By depositing a layer of fluorine-doped tin oxide having a surface resistance of approximately 0.5 Cl2 approximately 2 n thick on the surface of the cell, the grid spacing of the grid can be increased to approximately 10 mm with a corresponding reduction thereof. Alternatively, the mesh dimensions of the grid can be kept small and the stack can be able to operate efficiently, even when the sunlight has been concentrated so as to multiply it by about 100, provided that adequate cooling is maintained. the battery.

A schematic sectional view 100 of such a cell or cell is shown in FIG. 4, this cell comprising a layer of 2 (i 102 15 of n-SnO2 (using the fluorine-doped material of the invention), a layer of 0.4 d 104 of n-silicon (silicon doped with phosphorus so known), a 0.1 mm layer of p-silicon 106 (boron-doped silicon, in known manner), connected to an aluminum layer 108 serving as an electrode. The metal grids 110 are spaced about 10 apart. Nevertheless, excellent results are obtained.

The layers deposited by the process of the invention can be used for the manufacture of other semiconductor components, for example conductors or resistors. Tin oxide coatings have been used in this way before in integrated circuits. The better conductivity obtained by the invention offers a wider field of application to this material in the.

future. Not only has the surface resistance been extended to much lower values (for example around 5 Ci2 or less)

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that it was previously impossible to reach, but, moreover, the layer can be formed with the same apparatus as that used, for example, for the epitaxial growth of silicon. This eliminates the costly and time consuming steps of unloading, cleaning and reloading between depositions.

The resistivity obtained with fluorine-doped tin oxide on silicon substrates is about 10 ~ 4 fl-cm, i.e. comparable to that of vaporized tantalum, which is sometimes used to establish connections in some integrated circuits. The good adaptation between the coefficients of thermal expansion of tin oxide and silicon makes it possible to deposit relatively thick layers without resulting appreciable stresses.

Fig. 6 represents the electrical conductivity of fluoride doped stannic oxide films as a function of the ratio measured between fluorine and oxygen for deposition temperatures of 480 and 500 ° C.

Fig. 7 shows the reflectance of the infrared rays of fluorine doped stannic oxide films as a function of the measured ratio of fluorine to oxygen for deposition temperatures of 480 and 500 ° C.

We also indicated in fig. 6 and 7 (1) the conductivity of known expensive materials based on indium oxide which are described in "Philips Technical Review", vol. 29, p. 17 (1968) by Van Boort and Groth, and (2) the best conductivity and reflection values claimed by the prior art of doped tin oxide coatings.

It goes without saying that numerous modifications can be made to the embodiments shown and described, without however departing from the scope of the invention.

R

2 sheets of drawings

Claims (42)

640,276 2 1. Method for producing transparent films of stannic oxide on a heated substrate using a gas mixture initially containing: 1) a first fluorine-containing compound which is free of any direct tin / fluorine bond; 2) an oxidizable tin compound, and 3) an oxidizing gas, characterized in that: a) converting said first compound containing fluorine from said gas mixture into a second gaseous compound containing fluorine and comprising a direct tin / fluorine bond; b) this second fluorine-containing compound is immediately oxidized, in the immediate vicinity of said substrate, to obtain a fluorinated dopant in said gaseous mixture, and c) a stannic oxide film doped with fluorine is formed on said substrate heated by a deposit simultaneous thereon of said oxidizable tin compound and said fluorinated dopant. 2. Method according to claim 1, characterized in that said first gaseous compound containing fluorine is formed by heating a gaseous mixture containing a) a gas chosen from the group comprising CF3I, CF3Br and the alkyl a-fluorinated compounds homologous to said compounds CF3I, CF3Br and CF3SF5, SF5Br and SFsCl, or mixtures thereof, and b) said oxidizable tin compound, components a) and b) being substantially inert to each other at lower temperatures at 65 ° C. 3. Method according to claim 1, characterized in that said conversion of said first volatile compound containing fluorine free of direct tin / fluorine bond into a gaseous fluorinated compound having a direct tin / fluorine bond results from heating by said substrate. 4. Method according to one of the preceding claims, characterized in that the substrate to receive the coating is turned downwards and in that said gas mixture is directed towards this surface. 5. Method according to one of the preceding claims 1, characterized in that the volatile oxidizable tin compound used is a tetramethyltin vapor at a concentration which can amount to 1%, the oxidizing gas being oxygen at a partial pressure of up to 1 atm, and in that said stannic oxide is deposited on a surface heated to about 500 ° C. 6. Method according to claim 1, characterized in that the first fluorine-containing compound is a volatile tin compound which decomposes under the action of heat to form a vapor of organotin monofluoride. 7. Method according to claim 6, characterized in that said volatile tin compound is trimethyltrifluormethyltin. 8. Method according to claim 6, characterized in that the volatile tin compound is trimethylpentafluoroethyltin. 9. Method according to claim 2, characterized in that the mixture of component a) and component b) is stable at a temperature of approximately 32 ° C and in that a reaction is initiated between components a) and b ) by the action of heat, thus forming a vapor of organotin monofluoride, this vapor forming a source for a controlled addition of an impurity of fluorine in said film of stannic oxide. 10. Method according to claim 9, characterized in that the fluorinated dopant is formed by reacting trifluoro-iodomethane and an organotin compound containing, at least, one tin / carbon bond per molecule. 11. Method according to claim 10, characterized in that the fluorinated dopant is formed by reacting trifluoro-iodomethane in the gas phase with tetramethyltin. 12. Method according to one of claims 10 or 11, characterized in that the bromine is substituted for iodine. 13. Method according to claim 9, characterized in that the fluorinated dopant is formed by reacting a gas of penta- chloride sulfur fluoride SFsCl and an organotin compound containing at least one tin / carbon bond per molecule. 14. Method according to claim 13, characterized in that the fluorinated dopant is formed by reacting a gas of sulfur penta-fluoride chloride and tetramethyltin. 15. The method of claim 9, characterized in that the fluorinated dopant is formed by reacting a sulfur trifluoromethyl-pentafluoride gas CF3SFs and an organotin compound containing at least one tin / carbon bond per molecule. 16. The method of claim 15, characterized in that the fluorinated dopant is formed by reacting a gas of sulfur trifluoromethyl-pentafluoride and tetramethyltin. 17. Method according to one of claims 1 to 16, characterized in that the ratio of the fluorinated dopant and the oxyfable tin compound is chosen so that the concentration of free electrons in the films produced is between 1020 cm-3 and 1021 cm-3. 18. The method of claim 17, characterized in that the doping levels of said tin oxide film are between 1 and 3% of fluorine replacing oxygen. 19. The method of claim 1, which consists in mixing: a) a fluorinated gaseous component, b) an oxidizable gaseous component containing tin, c) a gaseous component containing oxygen and, optionally, d) an inert gaseous vehicle, these components being chosen to remain in the gas phase at the mixing temperature, component a) and component b) reacting to form a compound having a tin / fluorine bond only when this gaseous mixture is heated approximately to the temperature of said heated substrate, and the compound comprising the tin / fluorine bond and the oxygen-containing component then reacting to cause the deposition of said fluorine-doped stannic oxide film on the heated substrate. 20. The method of claim 19, characterized in that component a) contains reactive fluoralkyl groups. 21. The method of claim 20, characterized in that component a) contains fluoralkyl halides or mixtures thereof. 22. Method according to claim 21, characterized in that component a) contains a gas chosen from the group comprising CF3Br, CF3I and the homologous or substituted fluorinated compounds or mixtures thereof. 23. The method of claim 19, characterized in that component a) contains reactive fluorosulfuric groups. 24. The method of claim 23, characterized in that component a) contains SFsCl, SFsBr or SF5CF3 and homologous or substituted compounds or mixtures thereof. 25. The method of claim 19, characterized in that component a) contains a volatile organotin compound containing fluorine which is free of any direct tin / fluorine bond, but whose molecules rearrange under the action of heat to form a direct tin / fluorine bond at temperatures high enough for the newly formed compound containing a direct tin / fluorine bond to remain in the vapor phase until it reacts with an oxidizable tin compound to deposit on said substrate a fluorine doped tin oxide film. 26. The method of claim 25, characterized in that component a) contains a fluoroalkyl group or a substituted fluoralkyl group, linked to a tin atom. 27. The method of claim 26, characterized in that component a) contains trimethyltrifluoromethyltin. 28. The method of claim 26, characterized in that component a) contains trimethylpentafluoroethyltin. 29. Method according to claim 25, characterized in that component b) contains a compound containing at least one carbon / tin bond. 30. The method of claim 29, characterized in that component b) contains tetramethyltin. 5 10 15 20 25 30 35 40 45 50 55 60 65 3 640,276 31. Method according to claim 29, characterized in that component b) contains dimethyltin dichloride. 32. Method according to one of claims 20, 21, 23, 24, 25, 26 or 27, characterized in that component b) contains an organometallic tin compound. 33. Method according to claim 32, characterized in that component b) contains tetramethyltin. 34. Method according to claim 32, characterized in that component b) contains dimethyltin dichloride. 35. Method according to one of claims 19 to 34, characterized in that component b) contains oxygen. 36. Method according to one of claims 19 to 35, characterized in that component b) contains nitrogen or argon. 37. Method according to one of claims 19 to 36, characterized in that the substrate called to receive the coating is a solid body whose face is turned towards the base and in that said gaseous mixture is directed at the top towards this surface . 38. Product obtained by the method according to claim 1, having a reflectance of infrared rays of about 90%. 39. Product according to claim 38, characterized in that the stannic oxide film has a maximum surface resistance of 5 Q2. 40. Product according to claim 39, characterized in that it has a reflection capacity of infrared rays of approximately 90% and that the stannic oxide film doped with fluorine has a fluorine / oxygen ratio of between 0.007 and 0, 03. 41. Product according to one of claims 38 or 39, characterized in that the substrate is made of glass. 42. Use of the product according to claim 38, having a resistance equal to or less than 5 fi2 and a mass resistivity of approximately 10 ~ 4 ß-cm, as a semiconductor component usable in electronics.