Contact lithography
Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.
The first integrated circuits had features of 200 micrometres which were printed using contact lithography. This technique was popular in the 1960s until it was substituted by proximity printing, where a gap is introduced between the photomask and the substrate. Proximity printing had poorer resolution than contact printing (due to the gap allowing more diffraction to occur) but generated far less defects. The resolution was sufficient for down to 2 micrometre production. In 1978, the step-and-repeat projection system appeared. The platform gained wide acceptance due to the reduction of the mask image and is still in use today.
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. In addition, the contact platform is the same as used in imprint processes.
Recently, two developments have given contact lithography potential for comeback in semiconductor lithography. First, surface plasmon resonance enhancements including the use of silver films as lenses have been demonstrated to give resolution of less than 50 and even 22 nm using wavelengths of 365 and 436 nm. The exotic dispersion relation of surface plasmon has led to the extremely short wavelength, which helps to break the diffraction limit. Second, nanoimprint lithography has already gained popularity outside the semiconductor sector (e.g., hard-drive, biotechnology) and is a candidate for sub-45 nm semiconductor lithography, driving defect reduction practices and uniformity improvement for masks in contact with the substrate. Step-and-flash imprint lithography (SFIL), a popular form of nanoimprint lithography which involves UV curing of the imprint film, essentially uses the same setup as contact lithography.
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.
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