Overcoming Ambient Drift and Negative-Bias Temperature Instability in Foundry Carbon Nanotube Transistors

TL;DR

This paper presents methods to enhance the stability and reliability of carbon nanotube transistors in integrated circuits by addressing ambient drift and bias temperature instability through encapsulation and pulsed operation techniques.

Contribution

It introduces novel encapsulation and AC operation strategies to significantly improve CNFET stability and reliability, which were previously underexplored.

Findings

Silicon Nitride encapsulation reduces threshold voltage drift by ~8x over 90 days.

AC/pulsed operation extends NBTI time-to-failure by over 10,000x compared to DC operation.

AC at 10 MHz with 20% duty cycle improves CNFET lifespan under stress.

Abstract

Back-end-of-line (BEOL) logic integration is emerging as a complementary scaling path to supplement front-end-of-line (FEOL) Silicon. Among various options for BEOL logic, Carbon Nanotube Field-Effect Transistors (CNFETs) have been integrated within commercial silicon foundries, and complex CNFET circuits (e.g., RISC-V core, SRAM arrays) have been demonstrated. However, there lacks comprehensive studies that analyze the ambient drift (i.e., air-stability) and reliability of CNFETs. Here, for the first time, we thoroughly characterize and demonstrate how to overcome ambient drift and negative bias temperature instability (NBTI) in CNFETs using the following techniques: (1) Silicon Nitride encapsulation to limit ambient atmosphere induced threshold voltage shift (~8x reduction of median VT shift over 90 days) and (2) AC/pulsed operation to significantly improve CNFET NBTI vs. DC operation across a wide frequency range (e.g., 20% duty cycle AC operation at 10 MHz could extend CNFET NBTI time-to-failure by >10000x vs. DC for a target VT shift tolerance < 100 mV with gate stress bias VGS,stress = -1.2 V at 125 C).