Stress engineering of polycrystalline aluminum nitride thin films for strain sensing with resonant piezoelectric microbridges

Abstract

For an optimized performance of micro electromechanical systems (MEMS) double-clamped bridge-type resonators for mechanical strain sensing, a modified sputter process was developed which exploits the influence of varying sputter pressure during deposition on the intrinsic stress component of functional thin films. In detail, four different, polycrystalline aluminum nitride layers, synthesized with different sputter parameter sets were characterized related to their microstructure with techniques, such as X-ray diffraction, scanning electron microscopy and atomic force microscopy, respectively. Furthermore, the intrinsic thin film stress and the longitudinal piezoelectric coefficient were evaluated. The best performing layers were integrated in the fabrication process of two MEMS resonant strain sensor devices to study the stress-related impact on the resonant device performance. The initial static buckling of the sensor devices was studied by white light interferometric measurements, whereas the frequency response as a function of externally applied as well as intrinsic strain was analyzed by laser Doppler vibrometry. The behavior of the sensor devices was compared to theoretical predications and the influence of intrinsic thin film stress on the resonance frequency as a function of strain was studied. With a precise tailoring of the intrinsic film stress, a responsivity of ∼17000 is measured, representing an improvement by a factor of ∼5 compared to state-of-the-art resonant strain sensors.