The influence of reaction kinetics between BBr3and O2on the uniformity of base diffusion

The variables affecting the uniformity of sheet resistance in a two step base diffusion system (deposition and drive-in), using BBr3as a source, have been investigated. The uniformity after the drive-in is predominantly dependent on the process variables during the deposition process. Amongst such variables as the design of the deposition system, the total carrier gas velocity, the spacing of wafers on a deposition boat and the reaction rate between BBr3and O2, the latter is the most influential factor in determining the uniformity of diffusion. A uniform diffusion along a certain length of the zone is obtained when the reaction rate between BBr3and O2has reached completion. This is indicated by the lowest sheet resistance (highest surface concentration) obtained and the highest amount of glass built up on the surface of silicon for a given deposition condition. The rate of reaction can be varied by varying the O2concentration or the temperature of reaction (or deposition). Increasing these variables causes an enhanced reaction rate thus shifting the position of the zone of uniform sheet resistance towards the gas inlet of the tube. Decreasing these variables has the reverse effect.