Optimizing null ellipsometry for oxidized silicon.

Polarizer imperfections which produce a deviation of the measuring light beam cause systematic errors in the estimation of optical parameters particularly for low absorbing systems such as oxidized silicon. These can be reduced considerably by using the polarizer, specimen, compensator, analyzer (PSCA) instead of the PCSA arrangement. In addition, PSCA measurements can be made nearer to the principal angle where sensitivity is greatest. The discrepancy of Delta measurements between zones is reduced by a factor of 20. Precision is improved by a factor of 10-50 for the PSCA measurements near the principal angle compared with measurements at any angle of incidence for the PCSA arrangement. Results on silicon covered with a natural oxide are compared for illustration.