Understanding the mechanism of solvent-mediated adhesion of vacuum deposited Au and Pt thin films onto PMMA substrates

The adhesion of vapor deposited Au and Pt thin films onto poly(methyl methacrylate) (PMMA) substrates can be significantly enhanced by either spin-casting or vapor-exposure to hydrohalocarbon solvents prior to metal deposition. X-ray photoelectron spectroscopy (XPS) and evolved gas analysis Fourier transform infrared spectroscopy detect residual halogenated solvent at the PMMA surface which chemically activates the surface. Density functional theory (DFT) calculations show that the solvent molecules form a Lewis acid-base adduct with the ester oxygens in PMMA. DFT predicts that the deposited metal atom (M) inserts into the C-halogen (X) bond on either CHCl₃ or CHBr₃ to form a O-M-X interaction. This is consistent with M-X bonding observed in high resolution XPS. A model is proposed in which the bond energy of the C-X bond of the solvent must be weak enough so that it can be cleaved by the metal atom to form a M-X bond. A negative control of PMMA exposed to CHF₃ is shown to have no effect on Au or Pt adhesion since the bond dissociation energy of the C-F bond is stronger than the C-Cl and C-Br bond energy compared to the metal halide bond energies.