This invention relates to a method of fabricating a sintered body of indium tin oxide.
A film of indium tin oxide is conductive and transparent and therefore such films find wide application where these properties are required, for example as electrodes in liquid crystal displays. Films of indium tin oxide are conventionally deposited by sputtering.
There are at present two principal methods for sputtering a film of indium tin oxide on a substrate. The first method involves sputtering separate targets of indium and tin in an atmosphere of oxygen at low pressure. This method is subject to disadvantage in that it is very difficult to control the proportions of indium and tin in the deposited film. Moreover, since the oxygen pressure must be kept low in order to maintain a reasonable rate of sputtering, the film that is deposited is not in fact indium tin oxide but rather a sub-oxide, which must be converted to indium tin oxide by carrying out a postbake at a temperature of over 400.degree. C. This limits application of this process to deposit of films on substrates that can withstand the postbaking operation.
The second known method of depositing an indium tin oxide film involves sputtering a ceramic target of indium tin oxide, which may be formed by sintering a mixture of SnO.sub.2 and In.sub.2 O.sub.3. This method provides better control over the composition of the deposited film, and the deposited film is generally conductive, although not as conductive as the ceramic target. The conductivity of the film can be improved by postbaking.
In order to avoid large, sharp spatial variations in the thickness of the film it is necessary that the ceramic target be of uniform density, so that it does not have regions of low density. It is desirable that the sputtering target be of high density in order for it to be cohesive and have a reasonable operational life.
A ceramic body of indium tin oxide having a density up to about 84% theoretical density can be made by hot isostatic pressing. However, it has been found that hot isostatic pressing sometimes leads to a target of non uniform composition even though the ingredients have been thoroughly mixed before the pressing takes place.
When granular material is sintered, material diffuses across the grain boundaries with the result that contacting grains are drawn together and merge. A consequence of the sintering operation is shrinkage, so that a sintered body is more dense than the unsintered material was.
It is known to use a sintering agent to promote the sintering operation. A sintering agent is liquid at the sintering temperature. The liquid sintering agent enters the space between grains and promotes diffusion across the grain boundaries. When the grains merge and are bonded together, the sintering agent is displaced.