Effective KOH Etching Prior to Modified Secco Etching for Analyzing Defects in Thin Bonded Silicon on Insulator (SOI) Wafers

The modified Secco etching which was developed for detecting threading dislocations in separation by implanted oxygen (SIMOX) wafers showed etch pits with a density of 103–106/ cm2 when it was applied to bonded silicon on insulator (SOI) wafers thinner than 1 µ m produced by the plasma assisted chemical etching (PACE) process including touch polishing. When these pits were observed, a group of pits appeared as a scratch pattern. Also the density of pits was dependent on the remaining SOI thickness after the first diluted Secco etching in the modified Secco etching process. These results indicated that the density of defects had a certain distribution in the SOI thickness direction and that the defects were different from threading dislocations which reached buried oxides from the surface. In order to observe the distribution of defect density in the SOI depth direction, KOH etching was utilized prior to the modified Secco etching. Using this method, it was found that surface defects were predominant and bulk defects were also present in thin bonded SOI layers.