Chemical foundations in understanding the step coverage problem in chemical vapour deposition using silane chemistry

Abstract

The correlation between deposition kinetics and the step coverage problem (in relation to the deposition of thin films over the walls of a rectangular trench) in chemical vapour deposition using silane chemistry are investigated. For low-pressure chemical vapour deposition (LPCVD) of polysilicon, the near-unity value of the step coverage parameter (defined as the ratio of the film thicknesses at the bottom and at the top of the trench) is consistent with the near-zero order of the surface reaction with respect to silane, which indicates saturation of the surface with silicon-containing species. Although the LPCVD and plasma-enhanced chemical vapour deposition processes for SiO 2 deposition in a trench, dimensions of around 1 μm, are characterized by a free molecular flow regime (the atmospheric pressure chemical vapour deposition process is described by a transitional flow regime), the step coverage parameter of the SiO 2 thin films for all these processes presents similarly poor values (lower than 1). This behaviour can be interpreted in terms of the surface reaction kinetics, described by a reaction of order 0.5 with respect to silane, which indicates that the film surface is not saturated with silicon-containing species and that the flux of these species is reduced towards the bottom of the trench.