A Novel Monochromator with Offset Cylindrical Lenses and Its Application to a Low-Voltage Scanning Electron Microscope.

Low-voltage scanning electron microscopes (LV-SEMs) are widely used in nanoscience. However, image resolution for SEMs is restricted by chromatic aberration due to energy spread of the electron beam at low acceleration voltage. This study introduces a new monochromator (MC) with offset cylindrical lenses (CLs) as one solution for LV-SEMs. The MC optics, with highly excited CLs in offset layouts, has advantageous high performance and simple experimental setup, making it suitable for field emission LV-SEMs. In a preliminary evaluation, our MC reduced the energy spread from 770 to 67 meV. The MC was integrated into a commercial SEM equipped with an out-lens (a conventional objective lens without immersion magnetic or retarding electric fields) and an Everhart­Thornley detector. Comparing SEM images under two conditions with the MC turned on or off, the spatial resolution was improved by 58% at 0.5 and 1 keV. The filtering effect of the MC decreased the probe current with a ratio (i.e., transmittance) of 5.7%, which was consistent with estimations based on measured energy spreads. To the best of our knowledge, this is the first report on an effective MC with higher-energy resolution than 100 meV and the results offer encouraging prospects for LV-SEM technology.

Voltage contrast imaging with energy filtered signal in a field-emission scanning electron microscope.

A new band-pass energy filter (BPF) technique of secondary electron (SE) detection using scanning electron microscope (SEM) was developed to enhance voltage contrast (VC) in SEM images. The energy filtering condition was optimized to enhance VC of dopant distribution using Si p-n structure. The relation between VC and SE energy was investigated by BPF as well as a conventional high-pass filter (HPF). Whereas the p-type regions were always brighter than the n-type region in the case of HPF, the contrast reversal between p region and n region occurred at the low SE energy range in the case of BPF. The variation of signal intensity of BPF against specimen bias voltage can be considered as SE spectrum analysis, and the peak split of the spectra between n-type and p-type regions was obtained. The peak split can be explained with a model with metal-semiconductor contact. This peak split causes the contrast reversal.