A Comparative Study of AC Positive Bias Temperature Instability of Germanium nMOSFETs With GeO₂/Ge and Si-cap/Ge Gate Stack

Rui Gao,Jigang Ma,Weidong Zhang,Zhigang Ji,Hong Yang,Xiaowen Zhang,Yun Huang,Xiaoling Lin,Jianfu Zhang,Yunfei En,Guoguang Lu

doi:10.1109/jeds.2021.3078540

Abstract

AC positive bias temperature instability (PBTI) of germanium nMOSFETs with GeO2/Ge and Si-cap/Ge gate stack was investigated in this brief. AC-DC-AC alternating PBTI stress tests were conducted on both types of devices, the experiment data shows the inserted DC stress phase has little impact on the following AC stress kinetics on GeO2/Ge nMOSFETs but introduce a significant “additional DC generation” on Si-cap/Ge devices. The “additional DC generation” is ascribed to the existence of energy alternating defects (EAD) according to previous studies. Energy distribution under DC and AC stress further demonstrate that EAD are significant on Si-cap/Ge but negligible on GeO2/Ge devices. Effective lifetime prediction is carried out and compared under DC stress after discharge (with a purposely introduced measurement delay) and AC stress on both GeO2/Ge and Si-cap nMOSFETs. The results show GeO2/Ge nMOSFETs’ effective lifetime exhibits no difference under two stress modes, while Si-cap/Ge nMOSFETs’ effective lifetime is underestimated using DC stress after discharge approximation without considering the EAD-induced “additional DC generation”. An extra 0.14V 10-year Vdd design margin can be obtained for Si-cap/Ge nMOSFETs to gain higher performance by taking “additional DC generation” into account. The conclusion is beneficial for process optimization and PBTI reliability improvement of Ge nMOSFETs.

Highlights

Owning to the higher bulk mobility of both hole and electron, germanium (Ge) possesses great potential to replace silicon (Si) in the channel of CMOS to enhance the carrier transport and to achieve higher drive currents and switching speeds [1]
AC-DC-AC alternating positive bias temperature instability (PBTI) stress tests and energy distribution results clearly reveal that: GeO2/Ge nMOSFETs have negligible Energy Alternating Defects (EAD) and EAD-induced “additional DC generation”, while Si-cap nMOSFETs have significant EAD and exhibit clear “additional DC generation” phenomenon under DC after discharge stress compared to AC stress, resulting in an underestimation of AC effective lifetime if using conventional industry-adopted DC stress after discharge approximation
We have recently reported that this hypothesis is valid on SiON pMOSFETs but not applicable on GeO2/Ge and Si-cap/Ge pMOSFETs subject to NBTI stress

Summary

INTRODUCTION

Owning to the higher bulk mobility of both hole and electron, germanium (Ge) possesses great potential to replace silicon (Si) in the channel of CMOS to enhance the carrier transport and to achieve higher drive currents and switching speeds [1]. A comparative study of AC PBTI of germanium nMOSFETs with GeO2/Ge and Si-cap/Ge gate stack was carried out. AC-DC-AC alternating PBTI stress tests and energy distribution results clearly reveal that: GeO2/Ge nMOSFETs have negligible EAD and EAD-induced “additional DC generation”, while Si-cap nMOSFETs have significant EAD and exhibit clear “additional DC generation” phenomenon under DC after discharge stress compared to AC stress, resulting in an underestimation of AC effective lifetime if using conventional industry-adopted DC stress after discharge approximation. Previous studies reveal that GeO2/Ge devices offer higher mobility for both p and n MOSFETs but suffer from poor reliability [19]–[21], while Si-capped devices exhibit a better NBTI reliability compared to Si counterpart [19]. Whether EAD exists on GeO2/Ge nMOSFETs, and how they impact the AC PBTI lifetime of Ge nMOSFETs, have not been studied yet

DEVICES AND EXPERIMENTS

CONCLUSION