Applications of Rutherford backscattering spectrometry to refractory metal silicide characterization

Abstract

Rutherford backscattering spectrometry (RBS) provides a unique combination of sensitivity and accuracy for characterization of thin films. Applications include analyses of composition, contamination levels, and determination of the thickness-density product of both silicide layers and adjacent films. The large mass differences present in refractory metal silicide films make them ideally suited for RBS analysis. Excellent sensitivity, typically to tens of ppm for heavy elements, allows use of RBS for analyses of diffusion of refractory metals into other films, and study of cross-contamination effects. This paper reviews the elements of Rutherford backscattering spectrometry which affect its applicability to refractory metal silicide analysis, and gives specific examples of these applications. The use of RBS analysis for determination of composition and contamination levels is demonstrated, including an estimation of the statistical uncertainty in such calculations. An example of trace refractory metal presence in an adjacent oxide layer is quantitatively calculated to be in the hundred ppm range. Further, the depth resolution generally accessible with normal angle detection is evaluated for various disilicides as a function of depth, and is found to be on the order of tens of nanometers. In addition, the development of two types of automated RBS data analysis is described. One type, a nonlinear least squares data analysis, provides accurate values for the average composition and density-thickness product of multiple films. The value of such a program lies in its general applicability to any laterally uniform sample. The second method takes advantage of the separation of elemental signals, characteristic of thin film refractory metal silicide RBS spectra, to rapidly generate a composition depth profile.