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This paper reports an acoustic microelectromechanical system (MEMS) resonator structure that features dual-resonant response at 180 and 500 MHz. The MEMS structure uses aluminum nitride as acoustic layer and electrodes with dual design that provide dual-resonance behavior. Each resonant mode operates at the first symmetrical Lamb-wave mode (S0). Due to the large frequency separation between modes, device exhibits differentiated temperature coefficient of frequency) for each mode, which makes this structure suitable for thermometric beat frequency sensing. Reported devices are thus capable to multiply the 20-ppm/°C thermal sensitivity of the individual sensors by one order of magnitude, up to −334 ppm/°C for the thermometric beat frequency sensor. [ABSTRACT FROM PUBLISHER]

Thin-film encapsulation (TFE) is one of the promising wafer-level packaging techniques for microelectromechanical systems (MEMS) devices. One of the drawbacks of TFE is that it is difficult to encapsulate large MEMS devices due to the downward deformation of the cap layer during the release of the TFE. This paper presents a robust bilayer cap made out of aluminum nitride (AlN)/nickel (Ni) layers, which can solve the problem of downward deformation of the cap layer. This bilayer cap provides both strength and flexibility to the TFE. This bilayer cap is simulated using ANSYS and experimentally demonstrated by encapsulating cavities as large as 1200~\mu m \times 1200~\mu \textm with 0.5 \mu \textm AlN/0.7 \mu \textm Ni, whereas even 400~\mu m\,\times \, 400~\mu \textm -sized TFE cannot be fabricated without cracks with a solitary 1- \mu \textm -thick AlN cap layer. Standard microfabrication processes are used for the fabrication of this bilayer cap. [ABSTRACT FROM PUBLISHER]

We present a new plasma etch process optimized for etching piezoelectric aluminum nitride (AlN) film deposited on thin molybdenum (Mo) metal electrode. Such film stack finds application in the integration of AlN-based RF microelectromechanical systems devices. The process is based on Cl2/BCl3/Ar gas chemistry with added buffer gas in inductively coupled plasma reactive ion etching system. The new gas mixture overcomes a generic problem of etched surface roughness without significant drop in AlN etch rate. Using design of experiment, the process window is optimized for improving selectivity to Mo and reducing microtrenching while maintaining smooth etched surface. Finally, an etching rate of 280 nm min−1 with reliable etch stop on Mo electrode and smooth bottom surface is reported. The integration suitability of the developed etch process is tested by etching 2.0 to 5.0 µm size square shaped via holes in 1.0 µm thick (0 0 2) oriented piezoelectric AlN on 0.2 µm thick Mo electrode while integrating contour mode resonators. [ABSTRACT FROM AUTHOR]