Adhesion properties of polymer/silicon interfaces for biological micro-/nanoelectromechanical systems applications

Abstract

Polymers are used in biological micro-nanoelectromechanical systems (BioMEMS/NEMS) applications due to their desirable mechanical properties, biocompatibility, and reduced cost relative to silicon microfabrication processes. Understanding the interfacial properties of the films that are used in BioMEMS/NEMS serves as a useful tool in obtaining higher device yield and greater mechanical reliability. In this study, polystyrene (PS) and glycidyl-ether-bisphenol-A novolac polymer (SU8) on silicon substrates were investigated. SU8 is a commonly used material in MEMS/NEMS fabrication, while PS is evaluated for its potential use in BioMEMS/NEMS for interaction with biological cells. The aim is to examine the delamination of the interfaces. Nanoindentation was employed on the PS/Si and SU8/Si film systems coated with a thin metallic layer of Cr to facilitate delamination. The interfacial adhesion energy was determined from measuring the size of the resulting delamination and the contact radius. Scale effects were investigated by comparing the behavior of thin and thick PS and SU8 films, where a thickness dependence on the interfacial adhesion energy was observed. In addition to room temperature testing, film delamination experiments were conducted at 50 and 70°C by fitting the nanoindenter with a heating stage in order to study temperature effects. Nanoindentation-induced delamination is demonstrated for microstrips of PS and SU8 and the measured interfacial adhesion energy is compared to those obtained from films.