Influence of image-force from mask surface on electrons at low energy electron beam lithography

Recently, low energy electron beam projection lithography has begun to attract attention as one of promising candidates of the next - generation lithography tool. It has been accomplished to realize 45nm-wide lines with edge roughness of around 3nm, and the throughput is expected to be over 20 wafers per hour. However, at present in the low-energy electron beam lithography several difficulties and uncertainties, which have not been pointed out as crucial problems for high-energy electron beam lithography system. For example, using low-energy electron beam, the contamination grows quite quickly, and charging problem at the optical system can be serious comparing to the high-energy electron beam system, even if the vacuum condition is the same. Since the sensitivity of resist becomes high for low-energy electrons, the exposure dose can be lowered, but the Coulomb interaction of electrons in the beam will be appreciably large, if the electron density is not so small. In the present paper as one of the problems occur at low-energy electron beam lithography, the image force from the mask surface on electrons is analyzed quantitatively. As the electron energy decreases, this influence becomes large. The material of the mask considered is Si.