Deposition and characterization of polysilicon films deposited by rapid thermal processing

Abstract

Low-pressure chemical vapor deposition (LPCVD) of polycrystalline silicon in a cold-wall, lamp-heated, rapid-thermal processor was investigated. Blanket polysilicon films were obtained by the pyrolysis of a 10% silane–argon gas mixture onto (100) Si wafers which were capped with a thermal SiO2 layer. The depositions were performed at a total pressure ranging from 1 to 5 Torr and in a temperature range from 575 to 850 °C. It was found that the deposition of films was controlled by a surface limited reaction at temperatures below ∼780 °C, and an activation energy of 39±2 kcal/mol was measured for this reaction. Above 780 °C, a decrease in activation energy was observed. To meet the throughput requirement of single wafer processing, deposition temperatures higher than 700 °C are needed. In this temperature range, deposition rates exceed 1000 Å/min as compared to 20–300 Å/min in conventional LPCVD furnaces. The structural characteristics of the films were assessed by ultraviolet surface reflectance, Raman spectroscopy, and transmission electron microscopy. The transition temperature from amorphous silicon to polycrystalline silicon occurs at ∼600 °C. Processing windows, which result in smooth films with a root-mean-square roughness of less than 100 Å, are defined. Polysilicon films (∼2500 Å in thickness) have a columnarlike grain structure, and the grains span distances as little as ∼100 Å near the SiO2 interface to ≥1000 Å as they approarch the upper surface. A correlation between average projected grain size and surface roughness is demonstrated. Oxygen levels were derived from secondary ion mass and Auger electron spectroscopic data. The oxygen content of the polysilicon films is estimated to be in the range of 3%–4%. We speculate that relatively high oxygen levels in the films may be responsible for smooth films at high temperatures. If this is true, it may be possible to obtain smooth polysilicon films at high deposition temperatures by adding measured amounts of oxygen to the gas stream.