Secondary ion mass spectroscopy ultrashallow depth profiling for Si/Si1−xGex/Si heterojunction bipolar transistors

Abstract

Starting from a theoretical dopant and composition profile of a Si/Si1−xGex/Si heterojunction bipolar transistor (HBT), the requirements for high depth resolution secondary ion mass spectroscopy (SIMS) measurements are discussed. To characterize the boron out-diffusion from the Si1−xGex base, which can crucially degrade the transit frequency fT, a depth resolution of better than about 3 nm is necessary. SIMS measurements have been performed using low energy oxygen primary beams at normal incidence to characterize sharp B profiles in Si and Si1−xGex layers. Graded Ge concentrations have been measured quantitatively using cesium primary beams and MCs+ secondary ion detection. It is shown that in the limit of low energy primary beams, the leading edges of the SIMS profiles with upslopes of 1–2 nm/decade show sufficient resolution for many of the HBT applications. For better characterization of effects which are located on the trailing edge of the profile, deconvolution procedures are necessary.