Local voltage contrast changes in MOSFET using scanning electron microscopy with photoelectron beam technology

Scanning electron microscopy (SEM) is used for metrology and inspection in semiconductor manufacturing. In addition, electrical defects such as short circuits and unintentional insulation appear as contrast differences called voltage contrast (VC) in SEM under low acceleration voltage conditions. Moreover, by using pulsed electron beams from a photocathode, the probe current can be arbitrarily changed by pixel in the SEM image. Using this technology, we succeeded in observing the change in the VC of the drain in the metal-oxide-semiconductor field effect transistor (MOSFET) by changing in electron beam irradiation on the gate only. In this study, to estimate the threshold voltage of n-type MOSFET (nMOS) from VC, we investigated quantitative changes in the specimen current of the drain (I_d) and the gate (I_g) due to gate e-beam irradiation ON/OFF during SEM imaging. The landing energy of the electron beam was set to 0.8 keV, the probe current was 6.3 pA, and the e-beam was irradiated onto only the gate and drain electrodes. I_d and I_g, which showed a positive value at the beginning, decreased with time, and saturated at negative values. When the electron beam irradiation to the gate was turned OFF, the I_d decreased further and reached saturation. When the gate e-beam irradiation was turned ON again, I_g recovered to a positive and then saturated again to a negative value. On the other hand, the drain Id increased when the gate irradiation was turned ON and returned to the same value as before it was turned OFF.