Biaxial testing of nanoscale films on compliant substrates: Fatigue and fracture

Two problems of technological importance for microelectromechanical systems (MEMS) and microelectronics industry are addressed: fatigue of thin films and nanoscale film cracking. A device is described that can (1) conduct biaxial fatigue tests on thin films and (2) be utilized to study fracture patterns in nanoscale coatings under biaxial stress state. Thin-film specimens, in the form of circular membranes, are exposed to cyclic pressures between two fixed pressure limits. Corresponding pressure and specimen deflection are measured. Experimental results, including hysteresis loops spanning deflections of 15–50 μm are presented for 4.6-μm-thick polyimide films. Furthermore, the evolution of crack patterns in a 150-nm-thick Al film deposited on a polyimide substrate is studied. Critical mode I stress intensity factor for Al is extracted from experimental results.