Resist charging in electron-beam lithography

Electron beam exposure of masks and wafers results in charging of the insulating resist film. This charging results in an electric field which deflects incoming electrons and can be a serious source of pattern placement error in electron-beam lithography. In earlier work (Ingino et al. 1992) the surface potential was found to be positive or even zero under certain conditions. In this study, a model is developed to explain this effect and the surface potential is measured by an independent method, a Kelvin probe non-contacting electrostatic voltmeter. This new study confirms qualitatively the findings of the first study. An area of PBS resist measuring a few square millimeters is exposed using a gaussian focused probe and moved under the Kelvin probe immediately after exposure to measure the surface potential. Thicker resist tended to charge more negatively. The model and experiments confirm early studies that the surface potential is a function of resist thickness, and that there may exist a resist thickness where the surface charge is essentially zero.