Pico-watts Range Uncooled Infrared Detector Based on a Freestanding Piezoelectric Resonant Microplate with Nanoscale Metal Anchors

This paper reports on the first demonstration of an ultra-high resolution (~371 pW/Hz1/2) uncooled infrared (IR) detector based on a high frequency (136 MHz) Aluminum Nitride (AlN) piezoelectric resonant micro-plate completely released from the substrate and supported by two nanoscale Platinum (Pt) anchors. For the first time, fully metallic tethers were employed to support the freestanding vibrating body of a piezoelectric resonator and provide electrical connection to it (the device anchors are conventionally defined in the piezoelectric layer). Such innovative design, with minimum anchor cross section, enabled the implementation of an uncooled resonant thermal detector with ultra-high thermal resistance (~10 K/W) and electromechanical performance (mechanical quality factor, QM≈3133 in air, and electromechanical coupling coefficient, kt≈1.86%). Such unique combination of high sensitivity (~2.1 Hz/nW), low noise performance (~0.78 Hz/Hz1/2) and high resonator figure of merit (FOM=kt⋅Q≈58.3) resulted in the first complete and compelling prototype of a low power (~11 mW) and high performance MEMSCMOS resonant uncooled IR detector with detection limit pushed in ~100s pW/Hz1/2 range. INTRODUCTION The interest in uncooled IR detectors based on Micro/NanoElectro-Mechanical Systems (MEMS/NEMS) resonator technologies is steadily growing due to their potentially ultra-high resolution and unique advantages in terms of size and cost, compared to conventional cryogenically cooled semiconductor photon detectors [1-5]. The most important parameters that ought to be considered for the design and optimization of uncooled and miniaturized thermal detectors are the device sensitivity to absorbed radiation, the noise performance and the ease of readout (especially crucial for focal plane array implementations). All these three fundamental challenges are addressed in this work with the experimental demonstration of a novel micromechanical resonant structure that, by taking advantage of advanced material properties and innovative device engineering, is characterized by a unique set of application enabling features, such as: (a) high sensitivity, due to excellent isolation from the heat sink (enabled by the large thermal resistance associated with the nanoscale metal anchors); (b) ultra-low noise performance, due to the intrinsic high Q of the resonant device (enabled by the improved confinement of acoustic energy in the resonant body of the device by using nanoscale metal anchors); (c) ease of readout, due to the excellent piezoelectric transduction properties of AlN at micro and nano scale which enables the use of a low power and self-sustained CMOS oscillator as direct frequency readout. DESIGN AND FABRICATION A high performance resonant IR detector is composed of a high quality factor, Q, MEMS resonator whose resonance frequency is highly sensitive to IR radiation [4]. When IR radiation is absorbed by the resonant structure, the temperature of the device increases (because of the large thermal resistance of the structure) resulting in a shift in resonance frequency due to the temperature coefficient of frequency (TCF) of the piezoelectric resonator [3]. The temperature rise of the resonator due to the incident IR power is given by: ∆T = ηQp �Gth 2 +ωCth 2 ≈ ηQpRth (1) where η is the absorption coefficient of the resonator, Qp is the incident IR radiation power, Gth is the thermal conductance, Cth is the thermal capacity, and ω is the angular frequency of the incident IR radiation. When the incident IR radiation is constant, or slowly changing over time (ω ≈ 0), the temperature rise of the resonator is directly proportional to the thermal resistance, Rth, between the resonant body and the heat sink. Therefore, the implementation of a resonant structure extremely well isolated from the heat sink is crucial for the achievement of high temperature rise factor, thus high responsivity of the IR detector. The thermal isolation of a resonant thermal detector is mainly determined by the thermal resistance associated with the tethers connecting the freestanding vibrating body of the device to the substrate. In the case of a piezoelectric MEMS resonator, such anchors are conventional composed of a relatively thick piezoelectric layer (directly patterned in the same device layer forming the vibrating body of the resonator) and a relatively thin metal layer employed to route the electrical signal to the electrode placed on top or bottom (depending on the device design) of the freestanding piezoelectric body of the resonator [6]. By completely removing the relatively thick piezoelectric material from the anchors, hence minimizing their thicknesses (ultimately limited by the need of a thin metal layer for electrical routing), maximum thermal isolation of the device resonant body from the heat sink would be readily achieved. Furthermore, the increased acoustic impedance associated with such metal tethers with minimum cross section would act to reduce the energy loss through the device anchors (the acoustic energy would be well confined in the resonant body of the device) which has been identified as a significant source of quality factor, Q, degradation in piezoelectric MEMS resonators operating below 1 GHz [7]. Figure 1: 3-dimensional representation of the AlN micro-plate resonator with nanoscale metallic (Pt) anchors. According to these considerations, a new device concept, based on the use of fully metallic nanoscale tethers to support the resonant body of a piezoelectric MEMS resonator, is introduced in this work as an innovative design solution to maximize the thermal resistance and the electromechanical performance of piezoelectric MEMS resonant thermal detectors. The core of the proposed piezoelectric resonant uncooled thermal detector is an AlN 9781940470016/HH2014/$25©2014TRF 387 Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, June 8-12, 2014 piezoelectric resonant micro-plate (working at a higher order contour-extensional mode of vibration employing a lateral field excitation scheme [6]) completely released from the substrate and supported by two nanoscale Platinum (Pt) anchors. Figure 1 shows the 3-dimensional representation of the proposed AlN micro-plate resonator with fully metallic nanoscale anchors: the freestanding vibrating body consists of a 450 nm thick AlN piezoelectric layer sandwiched between a 100 nm thick gold (Au) film as top electrically floating electrode and a 100 nm thick Platinum (Pt) bottom inter-digital transducer (IDT); two nanoscale Pt tethers (100 nm thick, 3~6 μm wide and 22 μm long) are employed to support the piezoelectric resonant body and provide electrical