Secondary Electron Metrology for Enhanced Charged Particle Microscopy

Scanning electron microscopy and helium ion microscopy are widely used for nanoscale imaging. In each case, a high energy beam of electrons or ions is raster scanned over the sample. At each pixel, the beam generates secondary electrons (SEs) often detected by an Everhart-Thornley detector (a scintillator at high voltage followed by a light pipe-photomultiplier couple). The detector produces a voltage pulse whose magnitude is expected to be proportional to the number of detected SEs. The ideal image would be a pixelwise map of the yield of SEs, i.e., the average number of SEs emitted per incident particle. However, this goal is not realized because the gains and efficiencies of detector components are highly variable and generally unknown. In this work, we present methods for SE yield mapping by accurate characterization of the response of the detector to single SEs using two approaches. First, we present a model that assumes a Gaussian detector response and demonstrate model parameter extraction by imaging samples with a range of SE yields. Second, we describe how the detector response can be obtained empirically by imaging a sample with low SE yield. Finally, we discuss pixelwise estimators for the SE yield and their experimental implementation. Emphasis on quantitative electron detection, combined with advanced statistical analyses is expected to enable new limits of resolution and material analysis.