Modified double-layer PECVD passivation films for improving nonvolatile memory IC performance

Abstract

Characteristics of PECVD SiO2SiNx passivation layers are modified to improve and optimize electrical performances such as read-write (R/W) cycle and hot carrier reliability of floating-gate nonvolatile memory devices. SiH4/N2O gas mixtures are utilized as precursors for oxide CVD process. Higher SiH4/N2O flow rate ratios render the resulting oxide films more silicon-rich, as manifested by their higher refractive index (R.L) and wet etch rates. These modifications in film characteristics also correspond to longer R/W cycles and lower % hot-carrier linear drain current (I-dlin) degradation. An increase in R.I, from 1.520 to 1.675 translates to a rise in R/W cycles from 17.3K to 32K and a fall in I-dlin from 8.2 % to 4.9 %. Further improvement in device performance is fulfilled by modifying the stoichiometry of the overlying nitride passivation layer. This is achieved by increasing bias power while reducing SiH4/NH3 gas flow rate ratio during the PECVD nitride deposition process. The nitride films deposited herein contain lower hydrogen content and exhibit lower BOE etch rates and compressive stress. Water diffusion from oxide and hydrogen release from nitride are both responsible for hot carrier drain current degradation and loss in read-write cycles. The utilization of high-RI oxide in conjunction with a low hydrogen content nitride would yield the optimal device reliability.