Metal Impurity Evaluation in Silane Gas from the Qualification of Poly‐Si Layers

Abstract

Metallic contamination in high purity silane gas is tested through the poly‐Si gate process and lifetime measurement of the processed wafers. High purity is used to deposit a 420 Å thick polycrystalline Si film by low pressure chemical vapor deposition on a 100 Å gate oxide under ultraclean conditions. Subsequently, poly‐Si is fully oxidized at 1050°C for 120 min. The minority carrier lifetime, as measured by microwave photoconductive decay (μPCD) for both the reference and the processed wafers are identical, at approximately the bulk lifetime of 900 μs. Thermodynamic analysis of 3d metals behavior at the interface suggests that Fe, Co, Ni, and Cu elements are unlikely to react with the silicon oxide layer and could potentially be contaminates which diffuse into the bulk Si. Moreover, the quality of the interface is probed directly by measuring the effective lifetime, which is found to be about 360 μs. Using a model for the excess carrier decay, it is shown that this effective lifetime value corresponds to a defect state density in the interface below approximately 109 states/cm22. The μPCD sensitivity to metallic traces on the wafer surface is experimentally confirmed by oxidizing the wafers contaminated with an Fe containing solution.