Bimorph material/structure designs for high sensitivity flexible surface acoustic wave temperature sensors

Abstract

A fundamental challenge for surface acoustic wave (SAW) temperature sensors is the detection of small temperature changes on non-planar, often curved, surfaces. In this work, we present a new design methodology for SAW devices based on flexible substrate and bimorph material/structures, which can maximize the temperature coefficient of frequency (TCF). We performed finite element analysis simulations and obtained theoretical TCF values for SAW sensors made of ZnO thin films (~5 μm thick) coated aluminum (Al) foil and Al plate substrates with thicknesses varied from 1 to 1600 μm. Based on the simulation results, SAW devices with selected Al foil or plate thicknesses were fabricated. The experimentally measured TCF values were in excellent agreements with the simulation results. A normalized wavelength parameter (e.g., the ratio between wavelength and sample thickness, λ/h) was applied to successfully describe changes in the TCF values, and the TCF readings of the ZnO/Al SAW devices showed dramatic increases when the normalized wavelength λ/h was larger than 1. Using this design approach, we obtained the highest reported TCF value of −760 ppm/K for a SAW device made of ZnO thin film coated on Al foils (50 μm thick), thereby enabling low cost temperature sensor applications to be realized on flexible substrates.

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“…Sensor sensitivity refers to the ratio of the change ∆ y of the system response under static conditions to the corresponding input change ∆ x , that is, the ratio of the dimensions of output and input. When the sensor output and input dimensions are the same, the sensitivity can be understood as the magnification [ 21 , 174 , 175 ]. The temperature coefficient of resistance TCR (TCR, in ) of the common resistance type flexible temperature sensor is expressed in the following expression, , is the relative resistance change ( ) as a function of temperature, where represents TCR.…”

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“…This TCF is 2 orders of magnitude higher than the best TCF reported so far (~0.1% K −1 ) for IR resonant sensors 25 . In addition, the TCF is more than an order of magnitude higher than resonant temperature sensors 45–47 , for which the best reported TCF is 1.7% K −1 .…”

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“…The FEA model was a unit cell taken from the repeated pairs of IDTs of the real device with periodic boundary conditions (see section 3.3), as previously established for both SAW and Lamb waves. 29,30 As with the LFE-TSM waves, a model integrating the actual dimensions of the electrode pads is challenging to build, due to the large aspect ratio between the pad dimensions and the thickness of ZnO thin film. Consequently, we used the same model as for the Lamb waves, with an increased geometry to 600 μm × 600 μm for the fundamental order of the A-TSM.…”