Frequency-specific highly sensitive acoustic sensor using a piezoresistive cantilever element and parallel Helmholtz resonators

Acoustic sensors for specific-frequency detection are used in various applications, and their sensitivity improvement has been attracting significant attention. Mechanical acoustic resonator technology is an effective method because of its simple structure and low power consumption. A Helmholtz resonator (HR) is a typical mechanical resonator; however, previous HRs for an acoustic sensor were attached to the rear of the sensing element, which was unsuitable for a differential pressure (DP) sensor that blocks the pressure wave propagating through the sensor chip. This paper proposes a micro-electro-mechanical system (MEMS)-based acoustic sensor, enhanced by a front-mounted parallel HR array, which is suitable for a DP sensor-type element. A piezoresistive cantilever-type DP sensor was used as the sensing element because of its high sensitivity to acoustic waves in its mechanical resonant frequency (RF) range. Its sensitivity was enhanced by attaching an HR array to the DP sensor, like a cap. We designed and fabricated a piezoresistive cantilever with dimensions 80 × 80 × 0.2 µm and an RF of 4.5 kHz, and four HR arrays with an RF close to that of the cantilever. The sensor responses to acoustic waves with and without the HRs were evaluated. The developed sensor with the HR realized three times higher sensitivity than those without the HR at the RF. The obtained acoustic pressure resolution was approximately 4 mPa.