Toward accurate characterization of nitrogen depth profiles in ultrathin oxynitride films

Good accuracy in depth profile analyses of nitrogen in ultrathin oxynitride films is desirable for process development and routine process monitoring. Low energy SIMS is one of the techniques that has found success in the accurate characterization of thin oxynitride films. This work investigated the artifacts in a typical depth profile analysis of nitrogen with the current SIMS technique and the ways to improve the accuracy by selecting optimal analytical conditions. It was demonstrated that surface roughness developed rapidly in a SiO2/Si stack when it was bombarded with an O-2(+) beam at 250 eV and angle of incidence from 70 to 79 degrees. The roughness caused distortion in the measured depth profiles of nitrogen and the major component elements. However, the above roughness and the distortion in the depth profiles can be eliminated by using a 250 eV O-2(+) beam at an angle of incidence above 80 degrees. Depth profile analyses with a 250 eV 83 degrees O-2(+) beam exhibited minimal surface roughening and insignificant variation in the secondary ion yield of SiN- from SiO2 bulk to the SiO2/Si interface, facilitating an accurate analysis of nitrogen distribution in a SiO2/Si stack. In addition, depth profiles of the major component elements such as O-18(-) and Si-28(-) delivered clear information on the location of the SiO2/Si interface. Using the new approach, we compared nitrogen distribution in thin SiNO films with the decoupled-plasma nitridation (DPN) at various powers. Copyright (c) 2008 John Wiley & Sons, Ltd.