Thermal Cycling Effects on Critical Adhesion Energy and Residual Stress in Benzocyclobutene-Bonded Wafers

Abstract

The effects of thermal cycling on critical adhesion energy and residual stress at the interface between benzocyclobutene (BCB) and silicon dioxide coated silicon wafers were evaluated by four-point bending and wafer curvature techniques. Wafers were bonded using BCB in an established (baseline) process, and the films were deposited by plasma-enhanced chemical vapor deposition (PECVD). Thermal cycling was done between room temperature and a peak temperature. In thermal cycling performed with 350 and peak temperatures, the critical adhesion energy increased significantly during the first thermal cycle. The increase in critical adhesion energy is attributed to relaxation of residual stress in PECVD , which in turn is attributed to condensation reactions in those films. Thermal cycling also cures the BCB beyond the achieved in the baseline process, and the residual stress in the BCB is reset at a glass transition temperature corresponding to the increased BCB cure conversion. As more thermal cycles are performed, stress hysteresis in the BCB decreases as the cure stabilizes at 94-95%.