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Novolaks are phenolic polymers prepared by the condensation of phenols or substituted phenols with aldehydes in acidic reaction medium. Because of their many favorable characteristics, novolaks are the polymer most widely used in commercial positive photoresists. The non-swelling aqueous alkali solubility behavior of novolaks is a key property required to produce high resolution resist images. Novolak dissolution rates and development induction can be altered by changes made in their chemical composition, structure and molecular weight. The relationship of some novolak chemical compositions and their aqueous alkali solubility behavior is discussed in this paper. Positive photoresist solutions formulated with relatively high dissolution rate novolaks resolved submicron images with unique profiles, provided higher photosensitivities and plasma etch resistance than lower dissolution rate systems. Although this conclusion was confined to specific cresol novolaks investigated in this work, it is believed that many other novolak compositions show similar correlations. This behavior was attributed to the presence of a protective surface layer or "skin" on top of the resist film. This "skin" was formed during the softbaking of the resist coating and has a sufficiently different dissolution rate than the resist bulk. Detailed DRM studies provided evidence of development induction on the resist surface caused by the "skin". This effect may also be viewed as a surface contrast enhancement effect during the development. Resist systems requiring more aggressive developers due to their lower dissolution rates exhibited total or partial removal of the "skin" or micro peeling in some cases as observed by scanning electron microscopy. The resist thickness and softbaking conditions were found to affect the thickness of the "skin". On the other hand, the removal or preservation of that layer depended largely on the resist composition and the developing process. The exact chemical composition of the "skin" is not known, however, the literature suggests several possibilities. Novolak sensitizer reactions in absence of trace water, novolak oxidation and the changes in resist surface morphology as the solvent is removed are all different mechanisms that can occur during the softbaking of the resist, particu-larly on the surface.
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