Quantitative Secondary Electron Microscopy

Secondary electron imaging (SEI) is widely used in nanoscale material characterization. In such imaging modalities, a focused high-energy charged particle beam (electrons or ions) scans across the sample and generates secondary electrons (SEs). An SE detector maps the intensity of detected SEs from each scan position to a pixel brightness in the final image . Despite ubiquitous use, SEI is not quantitative – the fundamental parameter of interest, i.e. the average number of SEs emitted per incident particle (the SE yield), is not measured directly due to fundamental statistical limitations and lack of knowledge of detector gain and efficiency. The qualitative nature of SEI limits its use in metrology and inefficiencies increase the required imaging dose. We have introduced ion count-aided microscopy (ICAM) as a technique for source shot noise-mitigated nanoscale SE yield mapping, particularly applicable to ion beam imaging. ICAM uses statistical modelling of the SED signal generating process to directly estimate SE yield for every pixel in the scan. We will demonstrate the use of ICAM for pixelwise SE yield metrology in several materials employing a helium ion microscope. ICAM images require lower dose than conventional SE images by up to a factor of 5 for the same image quality. The precise SE yield metrology demonstrated here makes SE images physically meaningful and enables quantitative materials characterization at the nanoscale.