Characterization of nitrided SiO 2 thin films using secondary ion mass spectrometry

Abstract

For the fabrication of MOS devices, the incorporation of nitrogen into thin SiO 2 dielectric films has been shown to improve the electrical and structural integrity of the films. For example, nitrogen present at the SiO 2 Si interface acts to enhance the charge-trapping properties of the oxide. Furthermore, nitrided oxides are more resistant to impurity and dopant diffusion and further oxidation than non-nitrided films. The understanding of the nitriding process has progressed to the point where the distribution and concentration of the nitrogen (the N depth profile) can be controlled, to some extent. The final N depth profile in these films is a critical piece of information to the process engineer. By using different nitriding agents and growing the films using varying temperatures and times, the nitrogen can be ‘concentrated’ at virtually any depth in a very thin oxide (< 100 Å). We have successfully employed secondary ion mass spectrometry (SIMS) for determining the nitrogen distributions in such films. Using an SiO 2 standard, calibrated both in terms of concentration and film thickness, accurate quantification of unknown samples has become straightforward. Through the extensive development of the instrumental protocol, the long-term reproducibility of this measurement has been shown to be better than 9%. The N detection limit using this protocol is approximately 0.001% atomic. We will present recent examples of results using SIMS on dielectric films processed using a variety of oxidation and nitridation conditions.