CONTROL OF PREFERENTIAL ORIENTATION IN POLYCRYSTALLINE SILICON FILMS PREPARED BY PLASMA-ENHANCED CHEMICAL VAPOR-DEPOSITION

Undoped polycrystalline Si (poly-Si) films were prepared on a fused quartz substrate as a function of the RF power (0-30W), hydrogen dilution ratio (HySiEL = 0 to 8), and deposition temperature, Td (620°–770°C), by low-pressure chemical vapor deposition (LPCVD) at RF power = OW, and plasma-enhanced CVD (PECVD) using the same fabrication system. The preferential orientation to a random, <100> or <110> crystal axis can be selected by changing some of these preparation conditions. The effect of plasma supply on the structure is equivalent to a decrease in H2/SiH4 or a decrease in Td, and is interpreted in terms of both an increase in deposition rate and a decrease in a surface-diffusion coefficient of the SiH-related adsorbates responsible for the film growth, due to a reduction of H atoms covering the surface with an increase in RF power. The plasma supply also has the essential effect of smoothing the film surface, in addition to its effect on structure. The effect on the roughness is attributed to sputtering due to the plasma shower on the surface. The characteristics of thin-film transistors (TFTs) fabricated on the PECVD and LPCVD poly-Si films were investigated. The PECVD film exhibiting a dominant <110> texture is the best TFT material, while a randomly oriented LPCVD film is the worst material. These results are discussed in terms of the crystallite size, preferential orientation, and surface roughness. © 1990, The Electrochemical Society, Inc. All rights reserved.