Characterization of ultra-variable-pressure detector for secondary electrons in low-vacuum SEM

Scanning electron microscope (SEM) observation in low vacuum can overcome the issue of charge-up at the specimen surface, allowing for the observation of insulating samples without sample pretreatment. The ultra-variable-pressure detector (UVD) was developed as a secondary electron (SE) detector for the low-vacuum observation in SEM. It works by collecting the light signal released from the collision between SEs and gas molecules. In this study, we propose a simple method using a stainless-steel sphere to characterize the feature of UVD signal in low-vacuum SEM and compare it with the traditional Everhart-Thornley (E-T) detector in normal SEM. The UVD signal showed characteristic features, namely a two-round-peak feature in the profile, which is different from that of E-T detector. Through experiment and simulation, we revealed that at higher vacuum levels (as a few Pa), SEs provide the primary contribution to the UVD signal, exhibiting a profile similar to that of the E-T signal. As the vacuum deteriorates, as 30 Pa, the main contribution to the UVD signal shifts from SEs to low-energy backscattered electrons (BSEs). Our finding indicates that by tuning the chamber pressure, we can vary the UVD image between SE and low-energy BSE features.