Effect of RF or VHF Plasma on Nanocrystalline Silicon Thin Film Structure: Insight from OES and Langmuir Probe Measurements

Abstract

ABSTRACTOptical emission spectroscopy (OES) and Langmuir Probe were used to characterize RF and VHF plasma properties under conditions leading to nanocrystalline silicon film deposition. Films deposited by RF plasma at low pressure (3 Torr), even with high crystalline volume fraction, show weak X-ray diffraction signals, suggesting small grain size, while RF films at higher pressure (8 Torr) and VHF films at both high and low pressure have larger grain sizes. The preferential growth orientation is controlled by the H2/SiH4 ratio with RF plasma, while the film deposited by VHF shows primarily (220) orientation independent of H-dilution ratio. Langmuir Probe measurements indicate that the high energy electron population is reduced by increasing pressure from 3 Torr to 8 Torr in RF plasma. Compared with RF plasma, the VHF plasma shows higher electron density and sheath potential, but lower average electron energy, which may be responsible for the larger grain size and crystal orientation. The growth rate and crystalline volume fraction of the film is correlated with OES intensity ratio of SiH* and Hα/SiH* for both RF and VHF plasmas.