1 research outputs found
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 (ω), h ∝ ω–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 componentsthe 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 ∼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