GB2138843A - Reactive vapour deposition - Google Patents

Abstract

Stoichiometric compound films are deposited on a substrate by a low pressure reactive vapour deposition process eg by evaporating metal from a crucible (1) into a vacuum chamber (2) containing a reactive gas at a reduced pressure, the substrate (4) being placed out of a line of sight with the source. The substrate thereby receives only material that has been scattered by the gas and has therefore reacted to form a stoichiometric compound. <IMAGE>

Description

SPECIFICATION Film deposition This invention relates to methods of and apparatus for the vapour phase deposition of films, e.g. of metal oxides.

The preparation of compounds by reactive evaporation or sputtering is well known. A material such as titanium nitride for example can be prepared by evaporation of the metal into a nitrogen atmosphere in which a glow discharge is excited. A deposit of titanium nitride forms on the substrate by a combination of gas phase reaction of titanium atoms with nitrogen species and reaction at the sustrate of the titanium surface and incident nitrogen species. Similar processes have been used for tin oxide, tantalum oxide/nitride and even carbides (where the gas is methane).

Sputtering may be used instead of evaporation in which case the starting material may conveniently be the oxide or nitride itself.

Even in this case however a reactive gas is found to be necessary if stoiciometry is to be preserved. There is a considerable body of literature on the control of stoichiometry particularly for titanium nitride and indium tin oxide. Generally it is found that careful control of gas flow and evaporation/sputtering rate is required and in some cases post baking in an oxygen atmosphere.

The object of the present invention is to minimise or to overcome the attendant disadvantages of these prior art techniques.

According to the invention there is provided a method of forming a controlled stoichiometry material film on a substrate, the method including dispersing the material from a source into a reactive atmosphere at reduced pressure and condensing the reactand on a substrate, wherein the substrate is disposed away from a line of sight of the source.

The process provides controlled stoichiometry films, and in particular transparent metal oxide films, by the reactive evaporation of metals.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic view of a deposition apparatus.

Referring to the drawing, a metal evaporation source 1, typically an electron beam crucible, provides metal vapour which in the absence of a scattering gas will travel in straight lines from the source to deposit on the wall 2 of the vacuum chamber. The presence of a reactive gas causes some scattering of the metal vapour so that a substrate placed at position 3 receives some scattered metal and some directly deposited metal. This position 3 is the conventional position for reactive deposition. It can be seen that the ratio of the reacted to direct deposition depends on the source to substrate distance, the gas pressure and the evaporation rate. In this mode an increased evaporation rate results in extra gettering of the gas and hence an increase in unreacted metal deposition.With tin metal as the evaporant and oxygen as the gas it is difficult to obtain uniform transparent tin oxide. At position 4, which is out of the line of sight, however only those metal atoms that have suffered scattering by the gas and have thus reacted with the gas can arrive at the substrate. The stoichiometry of the film is then independent of evaporation rate and gas pressure and depends only on the chemistry of the metal/gas scattering reaction. For tin metal evaporated into oxygen it has been found that the film is transparent and has electrical characteristics of fully oxidised films.

Two simple modifications further control the process. First, a shadowing or collimating collar 5 placed around the evaporation source defines the mean angle of scattering by which the reacted metal reaches the substrate 4.

Also the use of mixed gasses enables simple and reproducible control of the deposition stoichiometry. Thus if nitrogen and oxygen are used as the scattering gasses a mixed oxynitride will result. Further, the addition of an inert gas can be used to provide a controlled ratio of unreacted metal to be incorporated in the film.

The resistivity of the tin oxide film produced by this process is a measure of the degree of oxidation in the film. It was found that the resistivity of deposited material at substrate 4 could be increased by incorporating a shield 5 which further reduced the chance of unreacted tin reaching this position.

The following examples illustrate the invention.

Example 1 An electron beam evaporation crucible was charged with 30 grammes of metallic tins.

Substrates were placed at positions 3 and 4 in the vacuum chamber and the system was pumped down to a pressure of 5 X 10-5 Torr.

Oxygen was admitted to raise the pressure in the chamber at a flow rate of 0.8 sccm of oxygen. A radio frequency discharge (13.5 MHz, 50 Watts) was struck in the chamber via a water cooled electrode 6 in order to clean the substrate surfaces. The tin boule was then heated until slow evaporation took place as measured by a film thickness monitor 5 (1 -2A/sec). The electron beam power (controlling the tin evaporation rate) and oxygen flow rate were then adjusted to produce a steady deposition rate in the region of 2 to 3 /sec. The steady state pressure is typically 1.0 X 10-3 Torr as measured by a Pirani gauge in the vacuum chamber.

After 10 minutes deposition the electron beam power was shut off and an removing the substrate from the chamber it was found that substrate 4 had a transparent coating of tin oxide while substrate 3 had a dark brown film containing unreacted tin.

Example 2 Using the same control settings as in example 1 a mixture of 10% by volume oxygen in argon was used. It was found that both substrates 3 and 4 had been coated with a metallic tin mirror since as the argon allows metallic tin to scatter to the substrate at 4.

These examples illustrate the feasibility of the deposition process described herein.

The process may be used in a variety of applications. In particular it may be applied to the formation of transparent conductive tin oxide electrode structures for liquid crystal display cells. It may also be used e.g. in the manufacture of film circuits for the fabrication of inductors or resistors. In a further application the process may be used for providing corrosion protection layers, e.g. of a tantalum nitride, on a surface.

Claims (7)

1. A method of forming a controlled stoichiometry material film on a substrate, the method including dispersing the material from a source into a reactive atmosphere at reduced pressure and condensing the reactand on a substrate, wherein the substrate is disposed away from a line of sight of the source.

2. A method as claimed in claim 1, wherein the reactive atmosphere comprises oxygen, nitrogen or mixtures thereof.

3. A method as claimed in claim 1 or 2, wherein the reactive atmosphere comprises a proportion of an inert gas.

4. A method as claimed in claim 1, 2 or 3, wherein the source material is tin.

5. A method of film deposition substantially as described herein with reference to the accompanying drawing.

6. A substrate supported film prepared by a method as claimed in any one of claims 1 to 5.

7. A liquid crystal display cell provided with an electrode structure by a method as claimed in any one of claims 1 to 5.