NO144140B - PROCEDURE FOR PREPARING TRANSPARENT FILM TINNOXY MOVIES - Google Patents

Description

The invention relates to a method for producing electrically conductive films which are highly transparent to visible light and which are highly reflective to infrared light. Such films are useful as transparent electrodes for solar cells of the photovoltaic type, photoconductive cells, electro-optical display devices of the liquid crystal type, photoelectrochemical cells and a variety of other types of optoelectronic equipment. As transparent electrical resistors, such films are used for de-icing windows in airplanes or cars, etc. As heat-reflecting, transparent films on glass, these films improve the efficiency of thermal solar energy collectors and of windows in buildings, household furnaces, industrial furnaces and sodium vapor bulbs and of fiber-glass insulation .

Various metal oxides, such as tetravalent tin oxide (SnO 2 ), indium oxide (ln 2 O 3 ) and cadmium stannate (Cd 2 SnO 4 ), have been the most widely used materials for forming transparent electrically conductive films or coatings.

The earliest methods of <p>applying these coatings were

based on spraying a solution of a metal salt (usually the chloride) onto a hot surface, such as a glass surface.

In this way, satisfactory transparent electrical resistance layers were first produced for de-icing aircraft windows. However, the spray process produced relatively corrosive by-products,

hot chlorine and hydrogen chloride gases which tended to attack the hot glass surface forming a foggy appearance. In US patent no. 2617745 it is described that this undesirable effect can be avoided by first applying a coating of pure silicon dioxide to the glass. However, a protective layer of silicon dioxide is not very effective on glass with a high alkali

content and a high thermal expansion coefficient, like normal soda-lime glass. Moreover, these corrosive byproducts attack metal parts in the apparatus, and the metallic contaminants, such as iron contaminants, can then be deposited in the coating with harmful effects both for the coating's electrical conductivity and transparency.

Another problem has been the lack of uniformity and reproducibility of the coatings' properties. In US patent no. 2651585 it is described that a better uniformity and reproducibility is obtained if the humidity in the apparatus is regulated. The use of a steam instead of a liquid shower, as described e.g. in German patent document no. 1521239, also leads to smoother and better reproducible coatings.

Even after these improvements, more recent research has been done using vacuum deposition methods, such as evaporation and sputtering, to achieve cleaner and better reproducible coatings. Despite a much higher cost of these vacuum processes, the reduced formation of corrosive by-products and unwanted contaminants otherwise obtained by the spraying methods is considered to be of importance, especially for applications in connection with pure semiconductors.

The intentional addition of certain contaminants is important for these processes to achieve high electrical conductivity and high reflectivity to infrared light. Tin impurities are thus incorporated into indium oxide, while antimony is added to tin oxide (tetravalent tin oxide) for these purposes. In each case, the effect of these desired impurities ("dopants") is to add "extra" electrons that contribute to the electrical conductivity. The solubility of these pollutants is high, and they can be easily added using all of the deposition methods mentioned above. Fluorine offers an advantage over antimony as a dopant for tin oxide in that films of tetravalent tin oxide that have been doped with fluorine have a higher transparency than films doped with antimony, especially in the red end of the visible spectrum. This advantage of fluorine is important for potential applications for solar cells and solar thermal collectors.

Despite this advantage of fluorine, antimony is used as a dopant for most, and perhaps all, commercially available tin oxide coatings. This is possibly because doping with fluorine has only been demonstrated in connection with the less satisfactory spraying method, while the improved deposition methods (chemical vapor deposition, vacuum evaporation and sputtering) are not believed to

have been proven to provide doping with fluoride. Moreover, an expert committee concludes in a recently published report in the American Institute of Physics Conference Proceedings, No. 25, p. 288 (1975), with the equilibrium solubility of fluorine in tin oxide being inevitably lower than that of antimony. It is nonetheless noted that the tin oxide films with the lowest electrical resistance described in the state of the art are the tin oxide films according to US patent document no. 3677814. Using a spraying method, fluorine-doped tin oxide films with electrical resistances as low as 15 ohm per square unit when using as starting material a compound with a direct tin-fluorine bond. The lowest electrical resistance for commercially available glass

which is coated with tin oxide, is currently approx. 40 ohms per square unit. If it is desired to obtain coating with a so. low electrical resistance such as 10 ohm per square unit, it has so far been necessary to use the far more expensive materials, such as indium oxide.

The invention aims to provide a method for depositing a film or a coating of fluorine-doped tetravalent tin oxide with high visible transparency, high electrical conductivity and high reflectivity to infrared light.

The invention also aims to easily vary the electrical conductivity during deposition. such- simple film and to make this, able to get very low volume resistance:. and surface resistance.

The invention also aims to provide

a non-corrosive deposition atmosphere from which such high purity films can be easily deposited and without contaminating the substrate with contaminants or without corrosive attack on the substrate or apparatus.

The invention also aims to provide a gaseous, instead of liquid, agent for the production of the coated products as described herein.

The invention also aims to provide a method by means of which it is possible to easily produce such films "with very uniform and highly reproducible properties over large areas, without the limitations that always apply to spraying methods.

The invention also aims to enable a

simple deposition of such films inside pipes or bulbs e'1'léf over the surface of complicatedly shaped objects which cannot be easily coated by spraying.

The invention also aims to provide improved objects, such as solar cells, other semiconductors that are usable in electrical circuits, heat-reflecting windows or improved sodium lamps, etc.

The invention also aims to enable the deposition of such films using standard manufacturing processes within the semiconductor industry and the glass industry.

A special feature of the invention is to choose the reactants in such a way that the necessary tin-fluorine bond is not formed until just before the deposition is to be carried out. The tin fluoride material should thus preferably be kept in the vapor phase and at sufficiently low temperatures that oxidation of the compound will only occur after the rearrangement to form a tin-fluorine bond. Films of fluorine-doped tin oxide which have been prepared in this way have an exceptionally low electrical resistance and an exceptionally high reflectivity towards infrared wavelengths.

The invention thus relates to a method for producing transparent films of tetravalent tin oxide with such a concentration of fluorine dopant that 1-3% oxygen is replaced by fluorine, on a heated substrate using a gaseous mixture which initially contains (1) a first fluorine-containing organotin compound free of any direct tin-fluorine bond,

(2) an oxidizable organotin compound and

(3) an oxidizing gas, and the method is characterized in that

(a) the first fluorine-containing organotin component in the gaseous mixture is converted into another gaseous organotin fluoride compound with a direct tin-fluorine bond by the application of heat of short duration from the heated

substrate immediately before the compound contacts the substrate, (b) the second organotin fluoride compound is oxidized immediately and in close proximity to the substrate to form a fluorine dopant in the gaseous mixture, and (c) a fluorine-doped tetravalent tin oxide film is formed on the heated substrate by simultaneous deposition on this of the oxidizable organotin compound and the fluorine dopant.

The product film is a smooth, hard, adherent, transparent film with an electrical conductivity and reflectivity to infrared radiation which are dependent on the concentration of the fluorine-containing dopant.

Of the drawings, Fig. 1 shows a schematic diagram of an apparatus which is suitable for carrying out a method where a fluorine dopant is constituted by an organotin-fluoroalkyl vapor which has been evaporated from a liquid state. Fig. 2 shows a simplified embodiment of an apparatus for carrying out the present method, Fig. 3 shows a window 120 which is coated with a film 118 according to the invention, and Figs. 4 and 5 are curves showing how the electrical conductivity and the reflectivity varies with the concentration of fluorine dopant.

The present method comprises two main steps:

(1) formation of a reactive vapor mixture which, when heated, will give a compound with a tin-fluorine bond, and (2) transfer of this vapor mixture to a heated surface on which fluorine-doped tin oxide will be deposited.

Tin is supplied in the form of a volatile, oxidizable, organic tin compound, such as tetramethyltin, tetraethyltin, dibutyltin diacetate, dimethyltin dihydrogen or dimethyltin dichloride, etc. The preferred compound is tetramethyltin as it is sufficiently volatile at room temperature, non-corrosive, stable and easy to clean. This volatile tin compound is filled in a bubble apparatus 10 according to the drawing, and an inert carrier gas, such as nitrogen, is bubbled through the tin compound. When using the highly volatile compounds, such as tetramethyltin and dimethyltin hydride, the bubble apparatus can be kept at room temperature, while the bubble apparatus and the tube must be heated to a suitable temperature when using the other less volatile compounds, as a person skilled in the art will understand. It is an advantage of the present invention that the use of a high-temperature apparatus can be avoided and that simple "cold-wall" supply devices can be used.

The steam mixture must contain an oxidising gas, such as oxygen or dinitrogen oxide etc. Oxygen is the preferred gas as it is easily available and gives just as good results as the more expensive, also usable oxidising agents.

The gas pressures are maintained using the regulators 25,

and the flow rates for the oxygen from the tank 20 and for the carrier gas from the tank 21 are regulated by means of dosing valves 30 and measured by means of flow meters 40. The gas flows then pass through one-way valves 50 and into a mixing tube 60 and a funnel-shaped chamber 70. A tin oxide film is deposited on the hottest surface 80 which is heated by means of a heater 90, usually to a temperature of 400-600°C.

The general type of process described above is commonly known in the art as chemical vapor deposition. Various changes, such as a vertical arrangement of the substrate surfaces with rotation or an arrangement under the reaction chamber with rotation, are known to the person skilled in the art and may be particularly suitable depending on the geometric design of the substrate or on other conditions affecting a given application.

It is recommended to rotate the substrate in order to best move the sample through any convection currents that may occur in the apparatus and thereby best ensure that the deposited layers are even. According to the invention, however, it has been shown that if the heated substrate is placed so that it faces downwards,

highly even coatings can be obtained more easily without rotation because the gas, when heated from above, does not cause troublesome convection currents to form. Another advantage of having the substrate arranged above the reactive vapors is that any dust or dirt or powdery by-product formed by homogeneous nucleation in the gas does not fall onto the growing film.

According to the invention, this includes an improved method where regulated amounts of fluorine pollution can be introduced into the growing tin oxide film. According to the simplest embodiment of the invention, the fluorine dopant consists of a vapor containing one tin-fluorine bond in each molecule. The other three tin valencies are satisfied by organic groups and/or halogens other than fluorine. Typical of such compounds is tributyltin fluoride.

According to the invention, it has been shown that the fluorine bound in this way and made available in vapor form to a hot surface, does not split off from the tin during oxidation on a hot surface.

Unfortunately, all known compounds with such a direct tin-fluorine bond are not particularly volatile at temperatures close to room temperature.

It is a particular advantage of the invention that with this the fluorine dopant is formed from volatile compounds which do not have the necessary tin-fluorine bond, but which will rearrange upon heating to form a direct tin-fluorine bond. This rearrangement advantageously takes place at sufficiently high temperatures (e.g. above 100 C), so that the tin fluoride thus formed remains in the vapor phase, but also at sufficiently low temperatures (e.g. below 400°C), so that the oxidation of the compound only takes place after the rearrangement. An example of such a compound is trimethyltrifluoromethyltin, (CH-^^SnCF-j. When heated to a temperature of about 150°C in a heated zone near the deposition surface 80, this compound will rearrange to form a direct tin-fluorine bond to (CH^J-jSnF-

vapor which then reacts as a fluorine donor or dopant.

Other compounds subject to similar rearrangements

at temperatures which will of course vary somewhat from compound to compound, has the general formula R^SnRF, in which R denotes a hydrocarbon radical and RF a fluorinated hydrocarbon radical with at least one fluorine atom bound to the carbon atom bound to the tin. The main advantage of these fluorine dopants is that they are volatile liquids, so that they can easily produce a sufficient vapor pressure when evaporated at room temperature. This simplifies the construction of the apparatus, as shown in Fig. 1, in that the need to maintain a hot zone between the bubble apparatus 15 and the reaction chamber 70 to thereby keep the fluorine dopant in the vapor phase is avoided. The apparatus can thus be of the type usually referred to as a "cold wall reactor for chemical vapor deposition" which is widely used, e.g. within the semiconductor industry to deposit silicon, silicon dioxide or silicon nitride, etc. Another important feature of the "cold wall reactor" for use within the semiconductor industry is that when it is used, the content of unwanted contaminants is lowered to a minimum both in the substrate and in the deposited film. Within glass production, the gas mixture can be added to the annealing and cooling furnace in a similar way at the stage when the glass has the appropriate temperature, e.g. about. 4 70°C, for soft glass.

In this way, very even films can be obtained by using durable glass production equipment.

The preferred compound for use

according to Fig. 1 is (CH^J^SnCF^ as this compound is more volatile than the compounds with more carbon atoms. It is a stable, colorless, non-corrosive liquid which does not decompose in air at room temperature and which only reacts very slowly with water.

Films produced according to the invention have been shown to have a reflectivity towards infrared radiation of 90% or more when measured, as is well known in the relevant art, at the usual wavelength for light of 10 µm which is typical for thermal infrared radiation at room temperature. This reflectivity of 90% must be compared with the reflectivity of 80% which has hitherto been achieved using tin oxide coatings. In normal practice, these layers which are capable of reflecting infrared radiation will have a thickness of 0.2-1 pm, with thicknesses of 0.3-0.5 µm being typical.

To more quantitatively characterize the fluorine dopant concentrations in the films, the infrared reflectance was measured for the wavelength range 2.5-40 µm. By fitting these data to theoretical curves, as described in detail by R. Groth,

E. Kauer and P.C. van den Linden, “Optical Effects of Free

Carriers in SnC^ Layers,” Zeitschrift fur Naturforschung, vol

179, pp. 789-793 (1962), values were obtained for the concentration of free electrons in the films. The values obtained were in the range 10 20 cm -3 to 10 21 cm -3 and increased steadily with increasing concentration of the fluorine dopant. Theoretically, a free electron should be released for each fluorine atom that replaces an oxygen atom in the lattice. This hypothesis was confirmed by electron spectroscopic measurements with an electron spectroscope of the Auger type of the overall fluorine concentration in a part of the films, and these measurements gave fluorine concentrations in accordance with the concentrations of free electrons, within the limits of experimental error. This agreement indicates that the main part of the incorporated fluorine is electrically active.

The infrared reflectivity at 10 pn and also the electrical volume conductivity of the films proved to be highest at a dopant concentration of 1.5-2% fluorine which had replaced oxygen. The maximum areas are very wide, and films with 1-2.5% fluorine exhibit almost maximum conductivity and reflectivity. There is also a weak, broad absorption within the entire visible wavelength range, and this increases directly with the fluorine concentration. For the production of films with high electrical conductivity and high visible transparency, a fluorine concentration in the film of approx. 1%

(ie a fluorine:oxygen ratio of 0.01 in the film) is strongly preferred. However, this optimum will vary somewhat depending on the spectral distribution that is of interest for a given application. By

vary the concentration of fluorine dopant, the optimum percentage for any given application can easily be determined by means of routine experiments.

Fluoride dopant concentrations of over 3% can easily be achieved

in the films using the methods according to the invention. Previous results have not exceeded 1%, and the opinion prevailed, as stated above, that this was the solubility limit for fluorine. Although such high concentrations of dopant are not necessary to obtain optimal infrared reflectivity or optimal electrical conductivity, the gray films formed at dopant concentrations of 2% or more may be useful for architectural glass to limit solar heat gain in air-conditioned buildings . For such applications, it is advantageous to lower the concentration of dopant at the film's surface to approx. 2% to achieve a maximum infrared reflectivity.

By using the measured electron concentrations and electrical conductivities, the electron migration mobilities can be calculated. For various films, values of 50-70 cm 2 /V-second were calculated in this way. Previously obtained mobility values for tin oxide

films have ranged from 5 to 35 cm 2 /V-second. It is believed that the present films are the first to have such mobilities exceeding 40 cm 2 /V-second. These values show in another way the superior quality of the present method and of the films produced by means of it.

The present method is also very useful for use in the manufacture of new devices, such as those with electrically conductive layers, in the manufacture of semiconductors (e.g. integrated circuits or similar devices), and also in the manufacture of heat-reflective, transparent objects, such as windows.

It. ^eii::strongly advantageous according to the invention that

. org.anoti-iin^f-l-ucfridf arbindels.one who has a tin-

fluorine bond, is cleaved on the substrate immediately after it has been formed. This cleavage preferably takes place within a narrow reaction zone which is essentially heated to the cleavage temperature by heat from the substrate itself.

examples by means of typical embodiments of the present method and of the products produced by means of it.

If nothing else is specified, the special examples given below are carried out in accordance with the following general method:

Example 1

The present method is exemplified here by means of an experiment using the apparatus according to Fig. 1 for the production of a gas stream containing 1% tetramethyltin, (CH3)4Sn, 0.02%' trimethyltrifluoromethyltin, (CH3)3SnCF3, 10% nitrogen carrier gas and the rest oxygen gas. The current obtained is passed over a plate of refractory glass (Pyrex") with a diameter of 15 cm and which is maintained at a temperature of 500°C for a deposition period of approximately 5 minutes. The flow rate of the gas stream is approximately 400 cm/min This flow rate is such that the gas exchange rate in funnel 70 is about once every 2 minutes. A transparent film about 1 µm thick is deposited. It has an electrical resistance of 2 ohms per square unit which corresponds to a volume resistance of 0.0002 ohm-cm This film is found by measurement to have a fluorine:oxygen ratio of about 0.017 and a migration mobility ("drift mobility") of about 50 cm o/V-second.

Example 2

When the method according to example 1 is repeated using a sodium-free silicon substrate, the resistance value drops to approx. 1 ohm per square unit, i.e. to approx. half the value for resistance obtained with a sodium-containing substrate.

Conventional photoelectric silicon cells (solar cells) have so far had typical surface resistances of 50-100 ohms per square unit. In order to achieve an acceptably low total cell resistance, a metal grid with a distance of 1 or 2 mm is deposited on the silicon surface. By depositing a fluorine-doped tin oxide layer with a sheet resistance of approx. 0.5 ohm per square unit (approx. 2 pm £ykt) on the cell surface, the metal grid spacing can be increased to approx. 10 mm with a corresponding reduction in the costs for the grid. The grid size can possibly be kept small, and the cell will then be able to work effectively even if the sunlight has been concentrated by a factor of approx. 100, provided that sufficient cooling of the cell is maintained.

A schematic section 100 through such a cell is shown on

Fig. 2, where a 2 pm thick layer 102 of n-SnO 2 (the fluorine-doped material according to the invention is used), a 0.4 pm thick layer 104

of n-silicon (with phosphorus doped silicon as known in the art), a 0.1 mm thick p-silicon layer 106 (with boron doped silicon as known in the art) is bonded to an aluminum layer 108 which serves as a electrode. The metal grids 110 are arranged at a distance of 10 mm from each other.

An excellent result is again obtained.

The deposited films can be used for the production of other semiconductor objects, e.g. electrical conductors or resistors. Tin oxide coatings have previously been used in this way for integrated circuits. The improved electrical conductivity will allow a more extensive use of this material in the future. Not only has the sheet resistance range been extended to far lower values (e.g. approx. 5 ohms per square unit or less) than hitherto possible, but also deposition of films can be obtained in the same apparatus used e.g. to grow epitaxial silicon. This removes the costly and time-consuming emptying, cleaning and filling steps between deposits.

The resistance values obtained for the silicon substrates with deposited tin oxide doped with fluorine are approx. 10 -4 ohm cm which are comparable to the resistance values for evaporated tantalum metal which is occasionally used for connections in integrated circuits. The good match between the thermal expansion coefficients for tin oxide and silicon enables the deposition of thick layers without significant stresses.

In Fig.4, the electrical conductivity of the fluorine-doped films of tetravalent tin oxide is shown as a function of the measured fluorine:oxygen ratio in the films at deposition temperatures of 480°C and 500°C. In Fig.5, the infrared reflectivity of the fluorine-doped films of divalent tin oxide is shown as a function of the measured fluorine:oxygen ratio in the films for deposition temperatures of 480°C and 500°C.

On Fig.4 and 5 is also (1) the electrical conductivity for

the expensive indium oxide materials known in the art and as described in Philips Technical Review, vol. 29, p. 17 (1968), by van Boort and Groth, and (2) the best claimed known values for the electrical conductivity and reflectivity of doped coatings of divalent tin oxide shown.

Claims (11)

1. Process for the production of transparent films of tetravalent tin oxide with such a concentration of fluorine dopant that 1-3% oxygen is replaced by fluorine, on a heated substrate using a gaseous mixture initially containing (1) a first fluorine-containing organotin compound that is free of any direct tin-fluorine bond; (2) an oxidizable organotin compound and (3) an oxidizing gas, characterized in that (a) the first fluorine-containing organotin component in the gaseous mixture is converted into another gaseous organotin-fluoride compound with a direct tin-fluorine bond by the application of heat of short duration from the heated substrate immediately before the compound comes into contact with the substrate, (b) the second organotin fluoride compound is oxidized immediately and in close proximity to the substrate to form a fluorine dopant in the gaseous mixture, and (c) a fluorine-doped tetravalent tin oxide film is formed on the heated substrate by simultaneous deposition on this of the oxidizable organotin compound and the fluorine dopant. 2. Method according to claim 1, characterized in that such a ratio is used between the fluorine dopant and the oxidizable organotin compound that the free electron concentration for the films will lie within a range of 10 cm 21 -3 10 cm, as the free electron concentration increases with increasing amount of the fluorine dopant. 3. Method according to claim 1 or 2, characterized in that a volatile component is used as the first fluorine-containing organotin component which is free of a direct tin-fluorine bond, but which rearranges upon heating to form a direct tin-fluorine bond at temperatures that are sufficient high enough that the newly formed compound with a direct tin-fluorine bond will remain in the vapor phase until it reacts with the oxidizable organotin compound during deposition of the film of fluorine-doped tin oxide. 4. Method according to claims 1-3, characterized in that a component containing er is used as the first fluorine-containing organotin component. fluoroalkyl group or substituted fluoroalkyl group attached to a tin atom. 5. Method according to claim 4, characterized in that a first fluorine-containing organotin component consisting of trimethyltrifluoromethyltin is used. 6. Method according to claim 4, characterized in that trimethylpentafluoroethyltin is used as the first fluorine-containing organotin component. 7. Method according to claims 1-6, characterized in that a compound containing at least one carbon-tin bond is used as the oxidizable organotin compound. 8. Method according to claim 7, characterized in that tetramethyltin is used as the oxidizable organotin compound. 9. Method according to claim 7, characterized in that dimethyltin dichloride is used as the oxidizable organotin compound. 10. Method according to claims 1-8, characterized in that tetramethyltin vapor in a concentration of up to 1% is used as the oxidizable tin compound, that oxygen gas at a. partial pressure of up to 1 atmosphere is used as the oxidizing gas, and that the tetravalent tin oxide is deposited on a surface that is heated to approx. 500°C. 11. Method according to claims 1-10, characterized in that a solid material with its surface facing downwards is used as the substrate to be coated, and that the gaseous mixture is directed upwards towards the surface.