Non-classical deposition of silicon thin film during chemical vapor deposition

Abstract

Silicon thin film have been extensively used in electrical devices for thin film transistors (TFTs) and silicon thin film based solar cells. Until now, formation of thin films and nanostructures explained by classical crystal growth based on atomic or molecules growth. However, many puzzling phenomena and problem generated in the growth of films and nanostructure, cannot be explained by this classical growth mechanism. By the reason, non-classical crystallization, where crystals grow by the building block of nanoparticles, has become a significant issue not only in solution but also in the gas phase synthesis such as chemical vapor deposition (CVD). In the various CVD processes, the generation of charged nanoparticles (CNPs) in the gas phase has been persistently reported. Many evidences supporting that these CNPs are the building block of thin films and nanostructures were reported. According this new understanding of non-classical crystallization, many thin films and nanostructures which had been believed to grow by individual atoms or molecules turned out to grow by the building block of CNPs. In this study, the deposition behavior of silicon films by Radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) was studied by the non-classical crystallization, where the growth unit of deposition is a nanoparticles generated in the gas phase of the reactor. According to the non-classical crystallization, the liquid-like property of particles is increased with increasing the amount of charge of particles. To investigate the behavior of particles in the RF-CVD reactor, nanoparticles were observed by transmission electron microscope (TEM) and the deposition rate of films was measured. The behavior of particles which were captured on TEM grid depended by the substrate bias and the conductivity of substrate due to difference of the amount of the charge of particles. Also, the deposition rate of films was changed by conductivity of the substrates and substrate bias. Using the liquid-like property of charged nanoparticles (CNPs), homo-epitaxial growth could be successfully deposited on a silicon wafer at 550 ℃ under the processing condition where multiply CNPs could be selectively deposited. The deposition behavior of silicon films by hot wire chemical vapor deposition (HWCVD) was also approached by non-classical crystallization. Using the liquid-like property of small CNPs, homo-epitaxial growth as thick as ~ 150 nm could be successfully grown on a silicon wafer at 600oC under the processing condition where CNPs as small as possible could be supplied steadily. The size of CNPs turned out to be an important parameter in the microstructure evolution of thin films.