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

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 A...