High-speed modulation of probe current using scanning electron microscope with photocathode technology

Semiconductor inspection technology using e-beam-based voltage contrast (VC) observation provides information on the electrical state and characteristics of terminals with less than 100 nm size, such as the source, drain, and gate in a MOSFET. In VC observation, a nanometer-sized electron beam causes charge accumulation in the target electrode, which is detected as bright or dark according to its electrical state and characteristics. For reproducibility of VC observations for semiconductor inspection, it is essential to release the charge accumulation due to electron beam irradiation of the terminal via leakage currents around the terminal surface. For VC observation of target terminals in the field of view in SEM imaging, electron irradiation time can be used for charge accumulation and unirradiated time for release of charge accumulation within the electron beam scanning. Therefore, it is possible to repeatedly observe VC changes by appropriate selection of the irradiated electron beam current and scanning speed, in addition to the selection of the electron energy. In addition, more fine modulation of the irradiated electron beam current is essential to obtain VC observations with a higher dynamic range, but such modulation should not result in changes in the position or image quality of the SEM image. In this study, an SEM equipped with a photocathode electron gun (PC-SEM) was used to investigate the effect of beam current modulation by photoemission on the SEM image quality of position and focus. Furthermore, in VC observations integrating the local beam current modulation technology of the PC-SEM, the irradiation current was modulated only for the gate plugs in an nMOSFET, and changes in the contrast of the drain plugs were investigated.