Extreme silicon thinning for back side power delivery network: Si thinning stopping on scaled SiGe etch stop layer

Abstract

This paper discusses the challenges relative to the silicon thinning which allows the back side power delivery integration (BSPDN). The back side silicon thinning stopping on a thin Si_0.75Ge_0.25 etch stop layer (ESL) has been investigated as it represents an alternative to the use of SOI wafers. Etch stop layers using 10 nm Si_0.75Ge_0.25 or 10 nm Si_0.75Ge_0.25 boron doped (Si_0.75Ge_0.25:B) have been studied for which different thinning process sequences were considered. All the considered thinning sequences are terminated with a diluted ammonia (NH₄OH) process which provides the selectivity towards the ESL. Considering a 10 nm Si_0.75Ge_0.25:B as an ESL considerably increases the selectivity of the last diluted NH₄OH silicon etching step. It nevertheless induces a risk of device poisoning caused by the diffusion of boron. Considering a 10 nm Si_0.75Ge_0.25 as an ESL has been then demonstrated using different thinning process sequences. Those alternative thinning sequences were optimized with respect to the silicon removal within wafer uniformity.