Contact Lithography - Operating Principle

Operating Principle

Generally, a photomask is purchased/generated, which consists of opaque Chromium patterns on a transparent glass plate. A sample (or "substrate") is coated with a thin film of UV-sensitive photoresist. The sample is then placed underneath the photomask, and pressed into "contact" against it. The sample is "exposed", during which UV light is then shone from the top side of the photomask. Photoresist lying beneath transparent glass is exposed, and becomes able to be dissolved by a developer, while photoresist lying under Chrome did not receive any UV exposure and will remain intact after developing. Thus the pattern may be transferred from the photomask to a sample, in the form of photo-sensitive resist. 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.