Vacuum-Packaged 30%-Doped ScAlN Resonators With AlN Absorbers for IR Spectroscopy

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Abstract
This paper presents a high-performance infrared sensor based on a Scandium Aluminum Nitride (ScAlN) laterally vibrating resonator integrated with plasmonic absorbers for spectrally selective IR absorption. The proposed technology enables fast, compact, and high-resolution IR detection, making it ideal for the implementation of mobile gas-spectroscopy systems for environmental monitoring over large areas for extended periods. The sensor was designed to exhibit high IR absorptance at $4.5~\mu $ m, matching the absorption peak of N2O. The resonant sensor experimentally shows an outstanding mechanical quality factor ( $Q_{m}$ ) of 2000, a 2.7% electromechanical coupling ( $k_{t}^{2}$ ), a high responsivity of 0.9 Hz/nW, a fast response of 6 ms and a low NEP of 320 pW/ $\mathrm {Hz}^{1/2}$ . While we focus on $4.5~\mu $ m in this system, the absorption wavelength is lithographically tunable, enabling the fabrication of multi-gas sensors on the same chip without using additional masks. A key innovation in this work compared to prior resonant IR detectors is the use of a novel material stack: Aluminum Nitride (AlN) as the dielectric in the Metal–Insulator–Metal plasmonic absorbers, combined with 30%-doped Scandium Aluminum Nitride (ScAlN) for the acoustic resonant cavity. This combination enhances sensor performance by taking advantage of the higher thermal resistance of ScAlN, as well as the higher temperature coefficient of frequency (TCF) of AlN compared to silicon dioxide (SiO2). SiO2 is traditionally used for IR absorption due to its optical properties; however, it is also characterized by a positive TCF, thereby degrading the thermal response of the resonator. In this work, the influence of geometrical parameters is experimentally investigated in hundreds of devices. The effect of tethering configurations and anchors’ width-to-length ratios on the sensor’s quality factor, thermal isolation, time response, and TCF is reported. Following optimization, the best-performing sensors are vacuum-packaged to enhance thermal isolation and improve response to IR. The device’s IR response is tested both in open-loop configuration with a vector network analyzer and in closed-loop within an oscillator circuit optimized for reduced phase noise. [2025-0145]
Funding Information
  • NanoSI Institute and was supported by the Advanced Research Projects Agency–Energy

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