Contact lithography is still commonly practiced today, mainly in applications requiring thick photoresist and/or double-sided alignment and exposure.
Advanced 3D packaging, optical devices, and micro-electromechanical systems (MEMS) applications fall into this category.
[2][3][4] The exotic dispersion relation of surface plasmon has led to the extremely short wavelength, which helps to break the diffraction limit.
Generally, a photomask is created, which consists of opaque chromium patterns on a transparent glass plate.
Upon exiting the photomask-photoresist interface, the image-forming light is subject to near-field diffraction as it propagates through the photoresist.
This can be explained by the rapid decay of the highest-order evanescent waves with increasing distance from the photomask-photoresist interface.
[3] The chief advantage of contact lithography is the elimination of the need for complex projection optics between object and image.
More specifically, the projection optics can only capture a limited spatial frequency spectrum from the object (photomask).
Reflections from the layer underlying the photoresist also have to be taken into account when absorption and evanescent wave decay are reduced.
[2] By exciting such oscillations under the right conditions, multiple features can appear in between a pair of grooves in the contact mask.
[10] A layer of metal film, has been proposed to act as a 'perfect lens' for amplifying the evanescent waves, resulting in enhanced image contrast.
[2] [13] While silver is the most commonly used metal for demonstrating surface plasmons for lithography, aluminum has also been used at 365 nm wavelength.
[14] While these resolution enhancement techniques allow 10 nm features to be contemplated, other factors must be considered for practical implementation.
The most fundamental limitation appears to be photoresist roughness, which becomes predominant for shorter sub-wavelength periods where only the zeroth diffraction order is expected to propagate.