Silicon nanowire pH sensors fabricated with CMOS compatible sidewall mask technology

Abstract

Silicon nanowire (SiNW) field-effect transistors (FETs) have been developed as various bio/chemical sensors due to their high sensitivity as a result of the huge surface-to-volume ratios. This paper presents a CMOS compatible sidewall mask technology (SMT) for top-down fabrication of SiNWs and a highly sensitive pH sensor based on SiNW FETs. The essence of SMT is to use stand-alone vertical sidewalls of a conformal thin film, fabricated by anisotropically etching away the planar part of the thin film deposited on a thin mesa, as hard masks for SiNW etching. As the feature size of the sidewalls, depending on the film thickness rather than lithography, can be controlled readily to tens to hundreds of nanometers by tailoring the film thickness, SMT allows high output and low cost fabrication of SiNWs without need of advanced lithography facilities. Complete SMT processes have been developed for fabrication of SiNW FETs, and pH sensors using SiNW FETs have been realized. Measurement results show that the pH sensors have good static characteristics and transient performance, and the sensitivity reaches 54.5 mV/pH for a wide range of pH values from 1 to 12, approaching the theoretical Nernstian limit.