Photoresist and the photoresist/wafer interface with a local thermal probe

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Abstract
This paper reports the development of local differential thermal analysis and ellipsometry to measure the glass transition temperature (Tg) of thin films of photoresist. We apply the techniques to measure the glass transition temperature as a function of the film thickness of polystyrene and poly(methyl methacrylate) (PMMA). We also study the effect of the duration of post apply bake and the extent of reaction on the Tg of films of a negative photoresist, SAL605. Measurements of the Tg as a function of film thickness of polystyrene cast on native silicon oxide substrates primed with hexamethyldisiloxane show that the Tg is depressed incrementally as thickness decreases to a maximum of Tg - 20 degree(s)C for film thicknesses below 80 nm. Films of PMMA cast on native silicon oxide show a similar depression in the Tg of 10 degree(s)C for film thicknesses less than 70 nm. Our study of SAL605 photoresist finds that the Tg is not influenced by the condensation reaction between the crosslinker and resin. We find that there is a strong plasticizing effect by the residual solvent in SAL605 over short PAB times that can change the Tg by as much as 15 degree(s)C. The Tg reaches a steady value after 30 seconds of baking at 90 degree(s)C.

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