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A technique has been developed for making comparative evaluations of the adhesion of photoresists to various types of substrates. The technique is based on the use of a fluoride-containing etchant which exaggerates the rate of undercutting of photoresist on silicon dioxide (SiO2) surfaces and thus permits direct microscopic measurement of the relative rates of undercutting that result with various types of SiO2 surfaces and with various types of photoresist materials. Using the special etchant, the benefits of adhesion promoters such as hexamethyldisilazane are readily demonstrated. An empirical equation is given which approximately describes the shape of the undercut oxide edges, thus allowing a numerical measure of the relative adhesion (that is, resistance to etchant undercutting) of photoresist/SiO2 composites.
A variation of this test can be used to detect nonuniform adhesion across a wafer surface. A photomask with a very-fine-geometry, highly recurrent pattern is used to define the photoresist, after which the wafer is etched. By using an optical microscope, the resultant dielectric/silicon substrate can be inspected very rapidly for localized adhesion variation because of the distinct color difference between areas which have or have not been undercut.
photoresist, adhesion, undercutting, semiconductors (materials), fluoride etchants, adhesion promoter, hexamethyldisilazane, evaluation
Member of the technical staff, RCA Laboratories, Princeton, N. J.