Effect of laser annealing on delta-doped boron for super-steep-retrograded well formation using selective Si epitaxy

Abstract

The effect of laser thermal annealing ͑LTA͒ on ␦-doped B has been investigated for the applications of super-steep-retrograde ͑SSR͒ 70 nm n-type metal-oxide-semiconductor field-effect transistors using undoped selective silicon epitaxy. Shallow (р50 Å) melting by LTA was found to suppress the B loss, causing an anomalous lowering and fluctuation of threshold voltage (V t ), upon epitaxial channel growth and rapid thermal annealing ͑RTA͒. Under the laser fluence of 0.42 J/cm 2 , the B profile was also observed to freeze without further diffusion upon RTA at 900°C for 20 s.Significant B loss observed in conventional ␦-doped SSR devices stemmed from the hydrogen ambient at 800°C required for selective silicon epitaxy as well as the rapid B outdiffusion behavior. The laser-induced surface melting decreased the outdiffusion rate by increasing the portion of substitutional B, and also led to a surface roughening which made the B loss from the silicon surface difficult. As a result of LTA, much lower V t fluctuation with reasonable V t was obtained along with improved short channel characteristics.