Fatigue Degradation Properties of LIGA Ni Films Using Kilohertz Microresonators

Abstract

A micrometer-scale ultrasonic fatigue characterization technique is introduced to investigate the fatigue degradation properties of LIGA Ni structural films in the very high cycle fatigue regime. Kilohertz microresonators fabricated with the MetalMUMPs process were driven at resonance (~ 8 kHz) in controlled environments (30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C , 50% relative humidity (RH); 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 5% RH; and 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 90% RH) to perform fatigue tests on 20-μm -thick Ni notched beams subjected to fully reversed bending over a wide range of stress amplitudes. It is shown that the fatigue degradation occurring at the notch, observed with scanning electron microscopy, can be quantified with the measured evolution of the microresonators' resonant frequency. The fatigue damage consists of extrusion formation and microcrack nucleation and does not propagate to form fatal cracks due to the extreme stress gradients at the notch. The resonant frequency evolution for fatigue tests performed at 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, 90% RH suggests that stress-assisted oxidation in the presence of humid environments may accentuate fatigue damage formation. This experimental technique brings critical information regarding the long-term fatigue degradation properties of metallic microelectromechanical systems (MEMS) devices. In particular, this technique can allow controlled studies of the effects of fatigue localization and large stress gradients typically encountered in MEMS components.