An Alternate Approach to Backside Thinning: A Doping Selective Silicon Wet Etch

Backside illuminated technologies have been used in complementary metal–oxide–semiconductor image sensors for years and are finding a renewed interest and are being investigated for imagers and detectors in light detection and ranging applications. Coupled with the improved performance facilitated through 3D integration, the thinning of bonded wafers has become a crucial processing step. In this study, an alternate and potentially more cost-effective method has been explored using an HNA ( hydrofluoric , nitric, acetic)wet etch for backside thinning of bonded epitaxial P+/P- silicon wafers. By utilizing a dopant concentration gradient, P+ silicon is selectively etched and P- silicon can be effectively used as an etch stop. Several HNA concentrations were explored to determine ratios that yield high P+ silicon etch rates while maintaining high P+/P- selectivity. Furthermore, bath life, spiking scheme, and repeatability were examined using a large wafer sample size with the goal of developing a full process flow.