Accurate ultra-low energy SIMS depth profiling of silicon semiconductors
Since the invention of the bipolar transistor in 1947, lateral dimensions of semiconductor devices have reduced by a factor of 4 and in-depth dimensions by some two orders of magnitude. This size reduction is continually making the accurate measurement of the latest generation of semiconductor devices more difficult. Secondary ion mass spectrometry (SIMS) is a highly effective analytical technique, traditionally used to measure concentration depth profiles, due to its high sensitivity and good depth resolution. The development of the floating ion gun (FLIG) at Warwick, allows the routine use of sub-keV beam energies for rapid SIMS depth profiling. The aim of this research project was to find and investigate ultra-low energy SIMS depth profiling conditions, suitable for the accurate analysis of modern silicon semiconductor devices. This work has shown that ultra-low energy ion beams at normal incidence, not only produce the most accurate SIMS depth profiles, at 250 e V the entire depth scale is accurate to within 1.5 nm (2.5 nm at 500 eV), but also for boron with the highest depth resolution. Two significant errors introduced into the depth scale of calibrated boron depth profiles, the surface transient shift and a largely overlooked parameter - the terminal shift, were quantified. Utilising these two shifts a simple universal depth correction procedure has been described, applicable when profiling boron samples using 02+ at normal incidence. The development of surface topography has two detrimental effects, a loss in depth resolution and a variation in the sputter yield, both of which have been quantified. A wide range of analysis conditions were investigated, at 02+ beam energies up to 1 keY, the only angles where ripples do not develop within the top 200 nm of a profile are those using near-normal incidence (θp ≤ 30°) ion bombardment.