Backside metallization affects residual stress and bending strength of the recast layer in laser-diced Si

Abstract

Thin Silicon dies separated by laser dicing form a thin layer via redeposition of ablated silicon known as recast layer. This work analyzed the influence of the recast layer microstructure and nanoscale residual stress gradients on the bending strength of bare and metalized silicon dies <100 μm. Scanning and transmission electron microscopy revealed an intricate microstructure of ablated silicon and elements of the wafer backside metallization within the recast layer. Refined silicon grains decorated by nanoscopic metallic precipitates at their grain boundaries were observed. Cross-sectional synchrotron X-ray nanodiffraction revealed that the altered microstructure increased the tensile residual stress from 200 to 295 MPa for bare and metalized dies, respectively. Additionally, the metalized die exhibited gradients in residual stress and grain size between the die front- and backside. Despite their similar frontside bending strengths of ∼340 MPa, observed in 3-point bending experiments, a considerable strengthening of the backside from 425 up to 957 MPa was measured for bare and metalized die, respectively. The origins of the tensile residual stress and the influence of the backside metallization on the die bending strength are discussed.