Evaluation of Double Spacer Local Oxidation of Silicon (LOCOS) Isolation Process for Sub-Quarter Micron Design Rule

Double Spacer local oxidation of silicon (LOCOS) with shallow recess of silicon (DS-LOCOS) is described. The process has two spacers, a thin nitride spacer and a medium temperature chemical vapor deposition (CVD) oxide spacer. The process does not have intentional silicon recess etching step but achieves the shallow recess of silicon through nitride overetchings. It has been found that the key processes of the DS-LOCOS are both isolation etching and spacer etching, which critically affect 2ΔW and V t roll-off behaviors of active transistor and junction characteristics. The DS-LOCOS achieves physical bird's beak length of below 0.03 µ m/side, field oxide volume ratio over 80%, and superior planar surface. The DS-LOCOS also gives no degradation in punchthrough voltage down to 0.20 µ m isolation spacing and in gate oxide reliability. These results show that the DS-LOCOS is a simple and promising isolation technology for sub-quarter micron design rule.