Surface potential distribution of resist exposed by electron beam and the non-charging exposure conditions

In this study, we experimentally analyzed the charging phenomenon when an insulating resist film on a conductive layer formed on bulk glass is irradiated by electron beams (EBs). To quantify the charging potential induced, an electrostatic force microscope device was installed in the scanning electron microscope sample chamber, and potential distributions formed under various exposure conditions were obtained. Based on the results obtained, a model for charge accumulation within the sample, explaining positive and negative charging and their transitions, was developed. At an EB acceleration voltage of 30 kV, the following observations were made: 'global charging' could be avoided by applying -5 V to the sample. Regarding 'local charging' near the exposure area of the EB, at low exposure doses, emission of secondary electrons from the sample surface induced positive charging, while the accumulation of incident electrons within the sample induced negative charging. At exposure doses where the effects of both are balanced, the sample exhibited zero potential, revealing the appearance of the 'first zero-cross exposure dose'. At higher exposure doses, the sample transitions from negative to positive as the exposure dose increases due to the electron-beam-induced conduction, resulting in the so-called second zero-cross exposure dose. The exposure dose dependence of the charging potential distribution at various acceleration voltages was obtained. In particular, we found that at an acceleration voltage of 0.6 kV, the sample surface is not charged even when exposed to small to very large doses of EBs.