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 (LTA) on δ-doped B has been investigated for the applications of super-steep-retrograde (SSR) 70 nm n-type metal–oxide–semiconductor field-effect transistors using undoped selective silicon epitaxy. Shallow (⩽50 Å) melting by LTA was found to suppress the B loss, causing an anomalous lowering and fluctuation of threshold voltage (Vt), upon epitaxial channel growth and rapid thermal annealing (RTA). Under the laser fluence of 0.42 J/cm2, 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 δ-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 Vt fluctuation with reasonable Vt was obtained along with improved short channel characteristics.