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) dept h profiles for characterization of Si/SiGe/Si heterojunction bipolar t ransistors (HBT). We show by device simulation that to achieve transit frequencies of more than 90 GHz for a given HBT, it is necessary to k eep the decay length of the B profile in the SiGe base layer smaller t han 2.5 nm, This formulates stringent requirements for the processing technology and for profile characterization techniques, In time-of-fli ght secondary ion mass spectroscopy (TOF-SIMS) depth profiling (dual-b eam mode), we used a low energy (0.5-3 keV) sputter gun operating with different sputter gases (Ar, O-2, SF6) to achieve a profile decay len gth below 2.5 nm, The full width at half-maximum (FWHM) of 3 nm B dopi ng spikes in HBT structures is well-characterized using an Ar+ ion bea m 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 les s than 2 keV for SF6 and to 0.6 keV for Ar sputtering. The decay lengt h depends linearly on the sputtering ion beam energy in the low energy range. The slope of this dependence is smaller for SF6 sputtering tha n for Ar sputtering. (C) 1998 American Vacuum Society.