| Summary: | The impact of device parameters, including AlN film thickness (<i>h</i><sub>AlN</sub>), number of interdigital transducers (<i>N</i><sub>IDT</sub>), and acoustic propagation direction, on the performance of <i>c</i>-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (<i>λ</i>) of 8 μm, was investigated. The results showed that resonant frequency (<i>f</i><sub>r</sub>) decreased linearly, the quality factor (<i>Q</i>) decreased and the electromechanical coupling coefficient <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mo>(</mo><mrow><msubsup><mi>K</mi><mi>t</mi><mn>2</mn></msubsup></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in <i>h</i><sub>AlN</sub>/<i>λ</i> favored the improvement of TCF. The acoustic propagation direction and <i>N</i><sub>IDT</sub> did not obviously impact the TCF of sensors, but they significantly influenced the <i>Q</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mi>t</mi><mn>2</mn></msubsup></mrow></semantics></math></inline-formula> of the sensors. At all temperatures measured, sensors along the <i>a</i>-direction exhibited higher <i>f</i><sub>r</sub>, <i>Q</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mi>t</mi><mn>2</mn></msubsup></mrow></semantics></math></inline-formula> than those along the <i>m</i>-direction, and sensors with <i>N</i><sub>IDT</sub> of 300 showed higher <i>Q</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>K</mi><mi>t</mi><mn>2</mn></msubsup></mrow></semantics></math></inline-formula> values than those with <i>N</i><sub>IDT</sub> of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher <i>f</i><sub>r</sub> of the sensor along the <i>a</i>-direction than that along the <i>m</i>-direction can be attributed to its larger elastic stiffness <i>c</i><sub>11</sub>, <i>c</i><sub>22</sub>, <i>c</i><sub>44</sub>, and <i>c</i><sub>55</sub> values.
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