Mapping the Phase-Change Parameter Space of Hot-Wire CVD Si:H Films Using In-Situ Real Time Spectroscopic Ellipsometry

In-situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during film growth. We have used in-situ RTSE to characterize the film morphology and crystallinity of hot-wire CVD (HWCVD) Si:H films as a function of hydrogen dilution R=[H]/[H+SiH4], substrate temperature Ts, and film thickness db. Transitions from one mode of film growth to another are indicated by abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the bulk dielectric function. We have studied the growth parameter space consisting of R from 0 to 12, Ts from 150°C to 550°C, and db from 0 to 1 um. For each set of R and Ts values, the structural evolution of the film can be characterized by the shape of the surface roughness thickness ds versus bulk thickness db curve. In contrast to studies done by Collins et al on PECVD growth of Si:H films, our studies of HWCVD growth find no conditions where ds remains constant after coalescence of the initial nucleation centers. Most of the films grown within the range of parameters studied exhibit a secondary nucleation and coalescence signature. The transition between a-Si:H and uc-Si:H growth is near the R=3 to R=4 dividing line. Initial coalescence of purely uc-Si:H material doesn't occur until R>8. We have verified the RTsE crystallinity classification using ex-situ Raman scattering.