Silicon-to-silicon wafer bonding using evaporated glass

Anodic bending of silicon to silicon 4-in. wafers using an electron-beam evaporated glass (Schott 8329) was performed successfully in air at temperatures ranging from 200 degrees C to 450 degrees C. The composition of the deposited glass is enriched in sodium as compared to the target material. The roughness of the as-deposited films was below 5 nm and was found to be unchanged by annealing at 500 degrees C for 1 h in air. No change in the macroscopic edge profiles of the glass film was found as a function of annealing; however, small extrusions appear when annealing above 450 degrees C. Annealing of silicon/glass structures in air around 340 degrees C for 15 min leads to stress-free structures. Bonded wafer pairs, however, show no reduction in stress and always exhibit compressive stress. The bond yield is larger than 95% for bonding temperatures around 350 degrees C and is above 80% for bonding temperatures higher than 225 degrees C. Purl testing revealed maximum bond strengths larger than 50 N/mm(2) and an average bond strength around 25 N/mm(2) for bonding temperatures above 300 degrees C. Structures bonded at temperatures lower than 300 degrees C show a near-linear decrease of the bond strength from 25 N/mm(2) to 0 N/mm(2) at 200 degrees C. A weak dependence on feature size was observed. For bonding temperatures higher than 300 degrees C fracture occurs randomly in the bulk of the silicon, whereas for bonding temperatures lower than 300 degrees C fracture always occurs at the bonding interface. Fracture of the glass itself was not observed. (C) 1998 Elsevier Science S.A. All rights reserved.