Two-layer resist structure for electron-beam fabrication of a submicrometer gate length GaAs device

A two-layer resist structure using EBR-9 and PMMA for fabricating a fine metal line with a mushroom-like cross-sectional profile is reported. The structure provides T-shaped resist cavities with undercut profiles using electron-beam exposure. With the optimum developing condition, the bottom opening is as small as 0.1 µm, and the top opening is wide enough not to require an additional exposure in order to obtain a mushroom-like metal lift-off pattern. A Monte Carlo calculation is carried out in order to analyze the profile of the two-layer resist structure, and it is shown that an undercut T-shaped resist profile with a 0.1-µm bottom opening can be obtained using a high-sensitivity resist on a low-sensitivity resist structure. A 0.15-µn mushroom-like lift-off metal profile has been fabricated on a 0.1-µm recessed GaAs substrate by the use of this resist structure.