METAL IMPURITY EVALUATION IN SILANE GAS FROM THE QUALIFICATION OF POLY-SI LAYERS

Metallic contamination in high purity silane (SiH4) gas is tested thro ugh the poly-Si gate process and lifetime measurement of the processed wafers. High purity SiH4 is used to deposit a 420 angstrom thick poly crystalline Si film by low pressure chemical vapor deposition on a 100 angstrom gate oxide under ultraclean conditions. Subsequently, poly-S i is fully oxidized at 1050-degrees-C for 120 min. The minority carrie r lifetime, as measured by microwave photoconductive decay (muPCD) for both the reference and the SiH4 processed wafers are identical, at ap proximately the bulk lifetime of 900 mus. Thermodynamic analysis of 3d metals behavior at the SiO2/Si interface suggests that Fe, Co, Ni, an d Cu elements are unlikely to react with the silicon oxide layer and c ould potentially be contaminates which diffuse into the bulk Si. Moreo ver, the quality of the Si/SiO2 interface is probed directly by measur ing the effective lifetime, which is found to be about 360 mus. Using a model for the excess carrier decay, it is shown that this effective lifetime value corresponds to a defect state density in the Si/SiO2 in terface below approximately 10(3) states/cm2. The muPCD sensitivity to metallic traces on the wafer surface is experimentally tally confirme d by oxidizing the wafers contaminated with an Fe containing solution.