Crystalline defect formation on aluminum bond pads during CMOS wafer storage and process strategies for defect elimination

Abstract

Crystalline aluminum fluoride defects were observed on aluminum bond pads of wafers from a complementary metal oxide semiconductor process in a wafer fab after less than a month of storage. These defects were formed by reaction of moisture in the front opening unified pod wafer storage container with residual bond pad sidewall fluorocarbon etch polymer releasing gaseous HF which further reacted with nucleation sites on the bond pad aluminum surface, resulting in crystalline aluminum fluoride defects. Elimination of the residual bond pad fluorocarbon polymer using a polymer free etch process or high-efficiency postetch polymer solvent strip resulted in a significant improvement in crystalline defect-free storage time (6 months). However, 12 month storage of sidewall polymer-free wafers showed reappearance of crystalline aluminum fluoride defects. The fluorine source contributing to the appearance of long-term aluminum fluoride crystalline defects was surface fluorine in the top 50 Ǻ of the aluminum bond pad. Lowering the surface fluorine levels from 3%–4% to 1.5% (by Auger electron spectroscopy) resulted in up to 36 months defect-free wafer storage. Incorporating O2/H2 plasma resist strip and high-temperature polymer solvent strip into postetch processing were found to be effective surface fluorine reducing process strategies.