Self-Assembled Monolayers as High-Resolution Electron Beam Resists
This work represents the study of a new class of ultra-thin films as high-resolution electron beam resist layers. The self-assembled monolayer (SAM) films form high-coverage, high-uniformity organic layers of a few nanometer thickness on a variety of surfaces useful for semiconductor processing. Octadecylsiloxane (ODS) on silicon and some metals and octadecylthiol layers (ODT) on GaAs are used as the SAM resists. Highly-focused electron beams are used to modify the SAMs, and the resulting patterns are observed with scanning force microscopy. Imaging the direct patterning in the film layers demonstrates that the chemical modifications can be confined to size scales of ~5 nm and represents an upper bound on the resolution limits of these films. The specific chemical changes introduced by the electron beam irradiation were studied using a variety of spectroscopic analysis techniques on large irradiated SAM layers. The irradiated films consist of a densely packed, cross-linked residue resulting from a large hydrogen loss during irradiation. To demonstrate the resist properties of the films, the SAMs were used as masking layers for both wet and plasma etching. For wet etching, the increase in surface energy caused by electron beam exposure makes the irradiated areas more susceptible to chemical attack producing a positive resist. The most useful etching strategy found for SAMs consists of wet etch patterning of a very thin silicon oxide layer that is subsequently used as a plasma etch mask. SAMs can also be used directly as masking layers for plasma etching. Irradiated ODT layers on GaAs are less susceptible to plasma etching, but irradiated ODS layers on the metals are more susceptible to etching. The role of the electron beam exposure and the underlying substrate are very important in understanding the masking characteristics. Finally, simple devices are constructed using SAM lithographic technology. GaAs-based heterostructures and thin-film polycrystalline silicon wires are fabricated and studied by optical and electrical measurements, respectively.
- Pub Date:
- Engineering: Electronics and Electrical; Physics: Condensed Matter