1. Field of the Invention
This invention relates to deposition processes. In particular the invention relates to deposition processes for depositing polysilicon from silane gas using low pressure chemical vapour deposition techniques.
2. Description of Related Art
Low pressure chemical vapour deposition of polysilicon from silane is an industry-standard technique. In this technique silane gas is passed through a hot walled deposition vessel containing the substrates on which it is required to deposit the polysilicon, such that the reaction of the silane with the heated substrates causes decomposition of the silane such that respective polysilicon films are deposited on the substrates. Typical process parameters using the technique are a deposition vessel temperature of 630.degree. C., and a silane gas flow of between 20 to 60 standard ccs per minute through the vessel, the vessel being held at a pressure of between 10 and 30 Pascals using a rotary or rotary and Root pump system pumping at between 50 to 300 liters per minute.
The grain structure of the polysilicon films produced by such a process is invariably microcrystalline, with a grain size of around 200.ANG..
Such films are not statisfactory for some thin film electronic devices, for example transistors, as grain boundary effects dominate and degrade the performance of the devices. Whilst it is possible to obtain larger grain sizes by increasing the temperature of the substrate this is not possible where, for example, the substrates are made of glass. The use of such glass substrates is becoming of increasing industrial importance for example in matrix addressed large panel displays in which the transistors form switching elements.