Wrinkling patterns in metal films sputter deposited on viscoelastic substrates

Metal molybdenum (Mo) films have been deposited on viscoelastic gel (for comparison also on elastic PDMS) substrates by magnetron sputtering technique and the wrinkling patterns are investigated by using optical microscopy, atomic force microscopy and scanning electron microscopy. For elastic PDMS substrates, the Mo films can spontaneously form labyrinth wrinkles, whose sizes increase linearly with the film thickness. For viscoelastic gel substrates, two wrinkle patterns with different wavelengths, namely 1st (with small wavelength) and 2nd (with large wavelength) wrinkling, can be observed in the films. They are originated from the surface instability of gel during sputtering and the thermal compression after deposition, respectively. The 2nd wrinkle wavelength increases with the film thickness whereas the 1st wrinkle wavelength is thickness independent. The structural characteristics, morphological evolutions and physical mechanisms of these wrinkling patterns have been discussed and analyzed based on the stress theory.