Enhancement of Signal-to-Noise Ratio in Piezoelectric Microphone Using Highly Scandium-Doped Aluminum Nitride

This study reveals that charge trapping in scandium-doped aluminum nitride (ScAlN) films is the main factor responsible for the degradation of the expected performance in piezoelectric microphones employing ScAlN. The charge trapping effect was found to reduce the direct piezoelectric coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{d}_{31}$</tex> of ScAIN. A significant improvement in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{d}_{31}$</tex>, from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$7.2 \text{pm} / \mathrm{V}$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$11.5 \text{pm} / \mathrm{V}$</tex>, was achieved by introducing an insulating layer between the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Sc}_{0.4} \text{Al}_{0.6} \mathrm{N}$</tex> film and the electrode, which suppressed charge trapping. Furthermore, this high direct piezoelectric coefficient ScAlN film was applied to a piezoelectric microphone, and its signal-to-noise ratio (SNR) reached <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$74.3 ~\text{dB}(\mathrm{A})$</tex>, the highest SNR reported so far.