Mathematical Evaluation of a-Si:H Film Formation in rf-PECVD Systems

Abstract

Radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) is an efficient technique for preparing hydrogenated amorphous silicon (a-Si:H) layers used in thin film silicon solar cells. The most important parameters in a PECVD system are the chamber pressure, substrate temperature, partial gas flow, plasma power, electrode spacing, and deposition time, by which the physical and electrical parameters of the deposited layers, such as phase, quality, thickness, doping concentration, energy gap, defect density, and mobility can be controlled. Moreover, some of the film parameters are indirectly related to the others. As a result, it is always difficult to describe the relations between the deposition parameters and the specifications of the deposited layer in closed-form equations, therefore, prediction of the properties of the deposited films remains an issue. In addition, the helpful facilities for in-situ monitoring of the film under deposition are very limited. In this research, based on a broad experimental data reported in the literature, it has been found that in many cases the performance of a PECVD system can be described in equation or graph forms, and effective estimations may be established for prediction of its performance. A flowchart is also presented to explain the steps required for adjusting the input parameters of the system for the fabrication of a desired layer, which can reduce the complexity of interrelations among the variables, leading to a more accurate and flexible process design.