Quantification of germanium and boron in heterostructures Si/Si 1− xGe x/Si by SIMS

Abstract

The use of secondary ions mass spectrometry (SIMS) generally leads to very high sensitivity and depth resolution for silicon-based components. Low concentration elements such as dopants, are easy to quantify in silicon, whereas high concentration elements are subject to “matrix effects”, i.e. the intensity does not vary linearly with the concentration. Matrix effects are presently studied in SiGe alloys using two primary beams O 2 + and Cs + and a CAMECA IMS4F instrument. Rapid thermal chemical vapor deposition grown Si/Si 1− x Ge x /Si multilayer structures with x varying from 0 to 23.5% are preliminarily characterized by Rutherford backscattering spectrometry (RBS) in thickness and atomic composition. The linear variation of the SIMS intensity ratio I Ge +/ I Si + with the RBS concentration ratio x/(1− x) is confirmed for both O 2 + and Cs + beams. However, the linearity does not imply the absence of matrix effects. A variation of the ionization yields with Ge concentration and a similar behavior for τ Ge+ and τ Si+ actually lead to an exact compensation of the intensity ratio variation. At any energy, O 2 + experiments systematically over-estimate the real Ge concentration, whereas matrix effects under Cs + beam are weak. For polyatomic MCs + ions, strong matrix effects are observed in Si 1− x Ge x alloys. A quantitative analysis of major constituents and a dopant (boron) is used to determine the depth profiles of a thin base of a SiGe HBT heterostructure. Two different procedures are used depending on the primary beam. Consistent results are obtained for the concentrations of the major elements. The fully autonomous SIMS analysis with the Cs + beam is still penalized by the lack of sensitivity for boron in the secondary positive mode. © 1997 Elsevier Science S.A. All rights reserved.