New SIMS method to characterize hydrogen in polysilicon films

A new analysis protocol for profiling the hydrogen (H) concentration depth distributions in polysilicon (poly-Si) thin films on Si oxide was developed by using secondary ion mass spectrometry (SIMS). Traditional SIMS determination of H concentrations in poly-Si films is to monitor and collect the atomic H−, but this analysis method presents some challenges for providing stable and repeatable H depth profiles, especially, for shallow profiling in thinner poly-Si films on thicker Si oxide films. In this paper, we present a thorough study of the SIMS characterization of H levels in poly-Si thin films on Si oxide. We describe an approach for the use of cluster SiH− instead of atomic H− to determine the H concentrations. The utilization of the SiH− cluster ion minimizes both H adsorption effects on the film surface regions and sample surface charging effects arising from the Si oxide layer underneath, thus enhancing the secondary ion signal stability in poly-Si films, as discussed with secondary ion energy distribution on the samples. This method differs from the conventional SIMS analysis of H in thin films and significantly improves the data quality and accuracy. Fourier transform infrared spectroscopy analysis was used to study the Si–H chemical bonding information for investigating the nature of the SIMS collected SiH− cluster ions. Both the H implant sample with no significant Si–H bonding and the poly-Si samples with high levels of H and the weak Si–H bonding exhibited strong SiH− secondary cluster ion intensities in our studies. This indicates the cluster ion signals arise from either the combination of Si and H ions through the physics formation process or weak chemically bonded Si and H.