Chapter 1 Silica, silicon nitride and oxynitride thin films: An overview of fabrication techniques, properties and applications

Abstract

Abstract of Chapter 1 - Silica, silicon nitride and oxynitride thin films - An overview of fabricationtechniques, properties and applications. Although thermal oxide remains an insulator of choice for silicon-based microelectronic devices, thin nitride and oxynitride films possess superior qualities for specific applications. In this chapter we describe both the main techniques used to manufacture thin nitrided insulator films and the main physical and electrical properties of the obtained films. The direct nitridation of silicon in ammonia, by thermal means, is difficult because it requires high temperature (> 950°C) and the obtained films are very thin ( Since the network of amorphous silicon nitride is denser than that of silica, it resists chemical etching (by HF solutions) better and behaves as a better barrier against diffusion. These properties are exploited in the LOCOS process. Unlike the oxide defects, the defects contained in nitrides behave mostly as electron traps. Thin oxynitride films can be fabricated by nitriding thin silica films in ammonia at high temperature (> 800°C). Nitridation may be performed at low pressure (P(NH3) The introduction of nitrogen atoms in silica, their concentration and their distribution modify in a complex fashion the physical and electrical properties of this material. The presence of nitrogen increases the dielectric strength of silica but increases the density of electron traps and introduces a hole conduction mode. It is also possible to fabricate thin oxide or nitride films, or to nitride thin silica films by using Rapid high Temperature Processes (based on the lamp heating of single wafers). The short process times used allow one to fabricate films whose properties are superior to those grown in furnace processes. To minimize the problems associated with the presence of hydrogen atoms introduced in nitrides during processing in ammonia, one can reoxidize these nitrided films. It is also possible to obtain virtually hydrogen-free nitride films by processing silicon surfaces directly in gaseous N2O or NO. The films thus obtained possess electrical properties which are superior to those of usual nitride films, and especially a greater dielectric strength and a better resistance to electrical stresses. It is also possible to fabricate nitride and oxynitride films at lower temperatures (and even at room temperature) by enhancing the reactivity of the species involved, by non-thermal means. These processes, briefly reviewed, use photons, electrons, ions or plasmas as an enhancement method, which allows one to reduce considerably the “thermal budget”. Thin nitride and oxynitride films have found numerous applications in microelectronic devices.