PEEM imaging and modeling of dopant-concentration variation in Si devices

Abstract

In photoelectron emission microscopy (PEEM), there are several contrast mechanisms that may be useful for dopant profiling, failure analysis, or as a processing diagnostic of Si. In particular, contrast arises from the reduction of the effective photothreshold when Si is heavily to degenerately doped. We report on a quantitative investigation of doping-induced contrast in PEEM images of Si devices. Additionally, we have performed electron optics simulations of the imaging properties of PEEM in order to understand how contrast mechanisms individually and in combination produce a PEEM image. Device structures were fabricated using standard photolithography and focused ion beam (FIB) writing. The devices consist of p-type (B) striped of different dopant concentrations (1018–1020 cm−3) and various line separations written on n-type (P1014 cm−3)Si(001) substrates. Using a near-threshold light source, we find that the signal intensity increases monotonically with doping over a range of doping from 1×1017 to 2×1020 cm−3. The measured intensity ratios are in good agreement with a calculation based on photoemission from the valence band. A numerical simulation program based on the Surface Charge Method is being used to assist in PEEM image interpretation. Model samples with regions of different photothreshold, vertical structure and lateral fields are being constructed. In combination, the results will be used to investigate how doping information may be extracted from PEEM images of simulated device samples.