Analysis of plasma-induced morphological changes in sputtered thin films over compliant elastomer

Flexible electronics and other polymer-based devices demand effective metallization of thin metal films over soft substrates. However, metallization of thin films over soft elastomers such as polydimethylsiloxane (PDMS) often leads to crack formation and morphological changes in the film as well as over the elastomeric surface offering limited applications in the development of biomedical microdevices. In the present study, optimized sputtering conditions like variations in base vacuum, working pressure, sputtering power and time required for crack-free uniform deposition of nichrome thin film over a PDMS surface are discussed. Analysis of film buckling and cracking under optimized and non-optimized conditions is performed through study of plasma colour, optical microphotographs and scanning electron microscopy images. The Young's modulus of an oxidized PDMS surface under no-crack conditions is estimated through the buckling mechanics of thin films and is found to vary with deposition conditions. The present investigation establishes that interaction of a PDMS surface with plasma modifies the surface properties to the extent that its mechanical properties either become compliant to the overlying deposited film, forming buckles, or become noncompliant, leading to cracked films. Fabrication of microheaters on crack-free nichrome thin film under optimized conditions was carried out to explore its suitability in flexible electronics.