PECVD Silicon Nitride as a Gate Dielectric for Amorphous Silicon Thin Film Transistor Process and Device Performance

Abstract

Plasma-enhanced chemical vapor deposited (PECVD) silicon nitride is a popular gate dielectric for the inverted, staggered thin film transistors (TFTs). In this paper, two subjects have been studied: the low temperature, i.e., 250°C, silicon nitride SiNx deposition process, and the TFT performance based on various SiNx dielectric films. For the PECVD SiNx process, a general mechanism, which includes the coexistence of deposition and etching reactions, is presented. Data from the plasma-phase chemistry, ion bombardment, and film characteristics are used to examine the above model. For TFT applications, device characteristics such as the field effect mobility, the threshold voltage, and the subthreshold slope were shown to be influenced by the deposition process. A relation between the threshold voltage and the nitride characteristics has been observed. The threshold voltage is the lowest when the SiNx layer has a refractive index in the range of 1.85 to 1.90, The exact reason is unknown, but it is possible that the SiNx charge-trapping density is the lowest when the film has a certain ratio of SiH to NH. © 1994, The Electrochemical Society, Inc. All rights reserved.