Development of high-resolution depth profiling of ultra shallow dopant implants with SIMS

Secondary Ion Mass Spectrometry (SIMS) is considered a reliable technique for precise and accurate dopant depth profiling in Si with respect to junction depth and implanted dose. The junction depths of source drain extension structures are predicted to be between 19–33nm for the 0.1μm MOSFET generation. Accurate high depth resolution analysis of these ultra-shallow junctions by SIMS can only be provided if atomic mixing caused by energetic primary ion bombardment is minimized and extensive beam induced crater bottom roughening is avoided. For quantitative measurements, the influence of primary ion implantation, sputter rate changes and beam induced crater bottom roughness on secondary ion intensities has to be known. In this work SIMS was used to develop techniques for the accurate analysis of ultra shallow B, P and As implants in Si.; Low energy O₂+ primary ion bombardment was found to give the highest depth resolution for the analysis of B and P in Si, while low energy Cs+ and CsC₆− primary ion bombardment resulted in the highest depth resolution for the analysis of As in Si. To obtain a more accurate profile shape and depth scale it was found to be essential to limit beam induced crater bottom roughness by means of sample rotation, variations of primary ion angle of incidence or change in sample chamber vacuum conditions. Beam induced crater bottom roughness was investigated for low energy O₂+, Cs+ and CsC₆− ion bombardment using atomic force microscopy (AFM) and optical profilometer (OP) measurements. OP was found to be a valuable tool for investigating small changes in sputter rate in the initial stages of a SIMS depth profile. It was shown that dose measurements of ultra shallow implants can be improved by using a correction procedure based on bulk doped standards. SIMS was proven to be a valuable tool for the characterization of ultra shallow implants in Si, but careful consideration of analysis conditions and SIMS artifacts is required for accurate analysis.