New hybrid (e-beam/x-ray) exposure technique for high aspect ratio microstructure fabrication

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Abstract
A new lithographic technique will be described in this paper which makes it possible to increase the height-to-width (aspect) ratio of electron-beam-exposed patterns in the resist layer, without significantly increasing proximity effects. The technique consists of applying two resist layers on the substrate, separated by a thin metal film. The pattern is electron-beam exposed on the top resist layer, and after development, a thin (2000–3000 Å) gold or other x-ray absorbing material is used to form an in situ x-ray mask to expose a much thicker bottom resist layer in an x-ray exposure system. It is shown that with this technique, lines of 0.25-μm wide with vertical walls, can be obtained in 2-μm-thick PMMA resist or an aspect ratio of eight can be easily achieved. It is also shown that the in situ mask can be registered to a previous level on the workpiece with accuracy better than 0.1 μm.

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