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Photoresists

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This technology was adopted by the integrated circuit (IC) industry, where the demands on resolution are very hard to meet. State of the art devices have photodefined regions that are less than 100 nm. Note that a human hair is about 100 microns in diameter! The photoresist is spin coated on a silicon wafer to form a very thin film, less than 1 micron thick. After photopatterning, the remaining resist is used to allow underlying SiO2 to be etched away by aqueous HF or a gas phase etchant that converts it to SiF4. Then one can modify the properties of the exposed silicon by further processing; the oxide is a very robust material.

The exposed areas of negative resists do not dissolve in the developer but they do absorb solvent, which causes some swelling that degrades the image. They are not used for IC production.

Sixty years ago a new form of resist was invented by Dr. Suss (not the writer) in Germany. He started with a nice film-forming polymer based on phenolic monomers, called a Novolac. Phenols are weak acids and these polymers are easily soluble in alkali. He added a second material that was both photosensitive and hydrophobic to make the resulting film insoluble in alkali. However, upon exposure to UV light from a mercury arc lamp, the photosensitive component is converted to a carboxylic acid. Thus both components of the film can be dissolved by aqueous alkali to form an image. Since the material that remained represented the unexposed regions, the combination is called a positive resist. These materials are capable of extremely high pattern resolution.

Chemical Scheme associated with conventional positive photoresists

Chemical Scheme associated with conventional positive photoresists. Reproduced from Annu. Rev. Mater. Sci. 1993. Reichmanis, E. and Novembre, A. E., Lithographic Resist Materials Chemistry, 23:11-43. www.annualreviews.org/aronline

In order to achieve ever finer resolution, photolithographers have resorted to shorter wavelengths for exposure. Unfortunately, the positive resists described above absorb light so strongly that even a thin film does not allow enough light to penetrate to the bottom. A new technology was needed.

The new film-forming polymer begins with a polyhydroxystyrene material that has had the phenolic groups protected by esterification (tBOC). The modified material is thus no longer acidic and is insoluble in alkali. The ester group is a special one that is sensitive to protic acid. A second component in the film at low concentration is photosensitive and generates acid on exposure to deep UV light (248 nm). When the film is heated briefly to 100° C the protective groups are easily removed. This is a catalytic process; one proton can deprotect up to 1,000 phenolic groups. Since it is hard to get high intensity light at the short wavelength used for exposure, this “chemical amplification” is very important.

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