Imaging buried structures with photoelectron emission microscopy

Abstract

Imaging of devices buried under oxides up to 0.5 μm thick is demonstrated with the photoelectron emission microscope. Specifically, we have observed contrast reversal of entrenched Ti lines, time-dependent oxide charging effects, and visibility of biasing effects when a SiO2 overlayer is present. An internally consistent explanation for all observations is that transport of electrons photoinjected from the buried structures to accessible states in the near-surface region allows for observation of material contrast. The observed contrast reversal is most likely the result of the formation of a TiO2–SiO2 interface, which increases the threshold for photoinjection into the oxide overlayer. Gradual accumulation of charge in trap states in the oxide under continuous UV exposure provides a consistent explanation for the observed time-dependent effects. Contrast effects that accompany biasing of the buried structure are due to the resulting lateral electric field, consistent with known field-effect contrast mechanisms.