INCIDENT DOSE MODIFICATION FOR PROXIMITY EFFECT CORRECTION

Electron-beam lithography (EBL) suffers from the effects of scattering of the incident electron beam. For good control of the size of the de veloped features, particularly when the fraction of incident electrons that is backscattered is large, incident dose modification is essenti al. The self-consistent correction technique equalizes the exposure of the resist within the desired features in the pattern, and this algor ithm provides good correction for a wide range of feature shapes and s izes using both positive and negative resist systems. For EBL at the l imits of resolution, the effects of forward scattering must be conside red. The incident dose must be increased to compensate for the forward scattering effects when writing features smaller than about five time s the range of the forward scattering. In order to equalize the averag e dose within these features, the incident doses must be increased by factors of up to 2, and greater increases are required to extend the s ystem to even smaller features. If the backscattered fraction is eta, the incident dose to define an isolated feature must be increased by a s much as 1 + eta. In combination with a correction for forward scatte ring, the range of incident doses required can easily become 3:1. Curv es are shown exhibiting the dose modifications required for different feature sizes and types as the resolution limit is approached, and the exposure contrast available. Sufficient dynamic range must be accommo dated in the design of the electron-beam pattern generator to give suf ficiently fine dose variations over a wide enough range.