Self-assembled folding of a biaxially compressed film on a compliant substrate

Amorphous carbon films grown by ion beam deposition from hydrocarbon precursors on compliant polymer substrates are shown here to undergo spontaneous self-assembled folding during growth. When deposited up to 30 min, the deposition-induced stretch strain of an amorphous carbon film on poly(dimethylsiloxane) (PDMS) with a Young's modulus of 1-2 MPa reached more than 50%, which is much higher than usually observed compressive mismatch strains of approximately 1-2% on silicon. During deposition of the carbon film, compliant PDMS substrates allowed large amplitude film buckling to let in lateral growth of the film with the significant compressive mismatch strain. The film wrinkled at a low strain of approximately 1% at an early stage of deposition. Then, the wrinkled film was observed to transform its configuration through two different nonlinear modes; formations of ridges and asymmetric localized folds. Due to the biaxial nature of the deposited thin film, the wrinkled film showed herringbone or labyrinth patterns for strains less than 10%, while the folds were made in random orientations to create asymmetric disordered tessellation for strains more than 30%.