Magnetic properties of Ni80Fe20 films exposed to HF for integration in MEMS devices

Abstract

One of the greatest challenges in the monolithic integration of micro-magnets in Si-based microelectromechanical systems (MEMS) is the release of suspended structures through exposure to hydrofluoric acid. Ni80Fe20 (Py) has been mainly used in prototypes of these hybrid devices, essentially because of its high resistance to exposure to hydrofluoric acid (HF). However, a detailed investigation of the physical mechanisms leading to its resistance to corrosion, as well as a characterization of the impact of HF exposure on the magnetic properties of Py, is still missing. Here, we present a combined surface science and magnetic investigation of Py films exposed to HF vapors in the typical conditions of MEMS release. X-ray photoemission spectroscopy and scanning Auger microscopy measurements show the tendency of Py to self-passivation by the creation of a surface fluoride layer (a few nm thick), accompanied by the formation of blisters with diameter on the order of 10–20 μm. A decrease in the average static magnetization is observed by vibrating sample magnetometry upon HF exposure, compatible with the creation of an interfacial fluoride capping behaving as a magnetic dead layer. A slight increase in the Gilbert damping (from 0.0079 to 0.0095) is found by vector network analyzer-ferromagnetic resonance, which could be compatible with blistering. Overall, this analysis confirms that Py displays excellent intrinsic resistance to HF so that it can be effectively used for integration of thick magnetic elements in MEMS without protective layers. For thin films to be used in spintronic/magnonic devices, instead, an additional capping layer should be used to fully preserve dynamic magnetic properties.