Ultrashallow secondary ion mass spectroscopy depth profiling of doping spikes and Si/SiGe/Si heterostructures using different primary species

Abstract

We present high resolution secondary ion mass spectroscopy (SIMS) depth profiles for characterization of Si/SiGe/Si heterojunction bipolar transistors (HBT). We show by device simulation that to achieve transit frequencies of more than 90 GHz for a given HBT, it is necessary to keep the decay length of the B profile in the SiGe base layer smaller than 2.5 nm. This formulates stringent requirements for the processing technology and for profile characterization techniques. In time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profiling (dual-beam mode), we used a low energy (0.5–3 keV) sputter gun operating with different sputter gases (Ar, O2, SF6) to achieve a profile decay length below 2.5 nm. The full width at half-maximum (FWHM) of 3 nm B doping spikes in HBT structures is well-characterized using an Ar+ ion beam of 1 keV. To measure B base doping with a decay length of about 1 nm it is necessary to lower the energy of the sputtering ion beam to less than 2 keV for SF6 and to 0.6 keV for Ar sputtering. The decay length depends linearly on the sputtering ion beam energy in the low energy range. The slope of this dependence is smaller for SF6 sputtering than for Ar sputtering.