Local-Loading Effects for Pure-Boron-Layer Chemical-Vapor Deposition

The so-called local-loading effect is studied for pure boron (PureB) depositions from B2H6 in a chemical-vapor deposition (CVD) reactor. This effect occurs because the boron is not deposited on oxide and this increases the deposition rate (DR) of boron in open Si areas in the oxide. Experiments are performed for wide range of local-oxide ratio (LOR). Three regions can be distinguished. For LOR < 0.1 the oxide areas are too small to have any significant influence on the DR and for 0.1 < LOR < 1 the DR increases as the oxide area increases. An empirical model is developed to describe this behavior in the latter region and it is experimentally verified. This formulation can be used to model the lateral diffusion component of the boron atoms and to develop a comprehensive model to predict the PureB deposition rate on any 2-D uniform or non-uniformly patterned wafer. For LOR > 1, the experimental data deviates from the model, showing saturation of DR as the oxide area increases. In this region the gas phase diffusion of the boron atoms across the oxide is limited by their diffusion length. Conditions that allow reliable deposition of 2-nm-thick PureB layers are found.