New electron gun improves imaging resolution of low-voltage scanning electron microscopy

Recent advances in low-voltage (1 kV or less) scanning electron microscopy (LV SEM) has significantly improved the contrasting quality of images, enabling further insight into a material’s surface characteristics. However, image resolution remains restricted, largely because of chromatic distortion caused by the energy spread of the electron beam.

Morishita et al. addressed this problem to significantly improve LV SEM resolution by using a negative electron affinity (NEA) photocathode for a new type of electron gun.

They built the photocathode using a p-type gallium arsenide (GaAs) coating and then initiated the NEA process by depositing cesium and oxygen, which were adsorbed by the GaAs layer. An acceleration voltage was applied to the photocathode, and the excitation light from a laser with a wavelength of 785 nanometers was used to minimize energy spread.

Obtaining high resolution in LV SEM means that a low-voltage electron gun still has to be as bright as the typical Schottky emitter. The Schottky gun relies on the high temperature of the emitter which leads, at low extraction voltages, to the relatively large energy spread and therefore low resolution. To achieve this high-brightness level in their prototype, the researchers increased the intensity of incident light by placing a focusing lens behind the photocathode.

Gold particles on a carbon substrate were observed using the prototype LV SEM electron gun against a conventional Schottky gun at the low acceleration voltage of 1 kilovolt. The researchers found the brightness of their device was as high as the traditional one. The prototype also showed an increase in brightness saturation, which improved the resolution of LV-SEM by 16 percent.

The researchers are exploring how the increased brightness saturation with incident optical power occurred.

Source: “Resolution improvement of low-voltage scanning electron microscope by bright and monochromatic electron gun using negative electron affinity photocathode,” by Hideo Morishita, Takashi Ohshima, Makoto Kuwahara, Yoichi Ose, and Toshihide Agemura, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0005714 .