remain intact after developing. The result is the original pattern replicated in the form of photoresist. The pattern may then be permanently transferred into the substrate via any number of microfabrication processes, such as etching or lift-off. A single photomask may be used many times to repeatably reproduce a pattern onto different substrates. A "contact aligner" is generally used to perform this operation, so that previous patterns on a substrate may be aligned to the pattern one wants to expose. Upon exiting the photomask-photoresist interface, the image-forming light is subject to near-field diffraction as it propagates through the photoresist. Diffraction causes the image to lose contrast with increasing depth into the photoresist. This can be explained by the rapid decay of the highest-order evanescent waves with increasing distance from the photomask-photoresist interface. This effect can be partly mitigated by using thinner photoresist. Contrast enhancements based on plasmon resonances and lensing films have recently been disclosed. The chief advantage of contact lithography is the elimination of the need for complex projection optics between object and image. The resolution limit in today's projection optical systems originates from the finite size of the final imaging lens and its distance from the image plane. More specifically, the projection optics can only capture a limited spatial frequency spectrum from the object (photomask). Contact printing has no such resolution limit but is sensitive to the presence of defects on the mask or on the substrate. Types of contact masks There are several types of contact lithography masks. The standard binary intensity amplitude mask defines dark and light areas where light is blocked or transmitted, respectively. The dark areas are patterned films consisting of chromium or other metal. The light coupling mask has a corrugated dielectric surface. Each protrusion acts as a localized waveguide. Light is transmitted primarily through the protrusions as a result of this localized guiding effect. Since less contact area is needed, there is less potential for defects. A hybrid nanoimprint-contact mask utilizes both contact imaging and mechanical imprinting, and has been proposed to optimize imaging of both large and small features simultaneously by eliminating imprint residual layer issues. Contact masks have traditionally been fairly large (>100 mm), but it is possible that alignment tolerances may require smaller mask sizes to allow stepping between exposures. As in nanoimprint lithography, the mask needs to have roughly the same feature size as the desired image. Contact