New Insights into Spin Coating of Polymer Thin Films in Both Wetting and Nonwetting Regimes

Spin coating is a common method for fabricating polymer thin films on flat substrates. The well-established Meyerhofer relationship between film thickness (h) and spin rate (w), h proportional to w-1/2, enables the preparation of thin films with desired thickness by adjusting the spin rate and other experimental parameters. The 1/2 exponent has been verified by previous studies involving organic thin films prepared on silicon wafers. In this study, 88% and >99% hydrolyzed poly(vinyl alcohol) (PVOH) polymers were adsorbed and spin-coated from an aqueous solution onto four different substrates. The substrates were prepared by covalently attaching poly(dimethylsiloxane) (PDMS) of different molecular weights onto silicon wafers (SiO2). Atomic force microscopy images indicate that the PVOH films transitioned from stable on SiO2, to metastable, and then to unstable as PDMS molecular weight was increased. Notably, none of the polymer-substrate systems studied here exhibited the thickness-spin rate profile predicted by the Meyerhofer model. Based on the experimental results, a more general adsorption-deposition model is proposed that decouples the total spin coated thickness into two components -the adsorbed thickness (h1) and the spin-deposited thickness (h2). The former accounts for polymer-substrate interactions, and the latter depends on polymer concentration and spin rate. In unstable systems, the exponents were found to be similar to 0 because slip takes place at the solution-substrate interface during spin and the spin-deposited thickness is 0. In metastable and stable systems, a universal relationship between spin-deposited thickness and spin rate emerged, independent of the substrate type and polymer concentration for each polymer examined. Our findings indicate the importance of film stability and polymer-substrate interactions in the application of spin coating.

Automated summary

Spin coating is a common method for fabricating polymer thin films, but the relationship between film thickness and spin rate does not fully conform to previous theoretical models. In this study, a more general adsorption-deposition model was proposed, which decomposed the film thickness into two components: adsorbed thickness and spin-deposited thickness. A universal relationship between spin-deposited thickness and spin rate was found, independent of substrate type and polymer concentration.