Optimization of Structure and Electrical Characteristics for Four-Layer Vertically-Stacked Horizontal Gate-All-Around Si Nanosheets Devices

Abstract

In this paper, the optimizations of vertically-stacked horizontal gate-all-around (GAA) Si nanosheet (NS) transistors on bulk Si substrate are systemically investigated. The release process of NS channels was firstly optimized to achieve uniform device structures. An over 100:1 selective wet-etch ratio of GeSi to Si layer was achieved for GeSi/Si stacks samples with different GeSi thickness (5 nm, 10 nm, and 20 nm) or annealing temperatures (≤900 °C). Furthermore, the influence of ground-plane (GP) doping in Si sub-fin region to improve electrical characteristics of devices was carefully investigated by experiment and simulations. The subthreshold characteristics of n-type devices were greatly improved with the increase of GP doping doses. However, the p-type devices initially were improved and then deteriorated with the increase of GP doping doses, and they demonstrated the best electrical characteristics with the GP doping concentrations of about 1 × 10¹⁸ cm⁻³, which was also confirmed by technical computer aided design (TCAD) simulation results. Finally, 4 stacked GAA Si NS channels with 6 nm in thickness and 30 nm in width were firstly fabricated on bulk substrate, and the performance of the stacked GAA Si NS devices achieved a larger I_ON/I_OFF ratio (3.15 × 10⁵) and smaller values of Subthreshold swings (SSs) (71.2 (N)/78.7 (P) mV/dec) and drain-induced barrier lowering (DIBLs) (9 (N)/22 (P) mV/V) by the optimization of suppression of parasitic channels and device’s structure.