## 5 History of MEMS AE Sensors, 1998-2020  
The first example of MEMS application to AE sensing dates back to 1998. [PERSON] and [PERSON] designed a resonating microbeam tuned to 312 kHz fundamental frequency [3]. They deposited piezoresistive material to the edge of a micro beam. [PERSON] et al. [45] advanced the piezoresistive sensing of AE with a silicon cantilever to tuning lengths of individual cantilever elements to different frequencies to increase the frequency bandwidth. Each design still behaved as an individual resonator. The other example of a piezoresistive MEMS AE sensor is n-type piezoresistive area generated on Si layer [46]. Table 2 summarizes the piezoresistive MEMS AE sensors and their specifications. In 2006, [PERSON] et al. demonstrated capacitive MEMS AE sensors made of resonating suspending silicon on a stationary silicon layer [25]. They tuned individual sensors to particular frequencies in the range of 100 to 500 kHz. To reduce the influence of sequence film damping, square etch hole spacing was reduced to 13 \(\upmu\)m spacing. Unit cells of 49 to 100 were combined in series to increase the total capacitance and sensitivity. While the MEMS AE sensors were applied to detect the crack growth successfully, the signal-to-noise ratio was significantly lower than conventional piezoelectric AE sensors. [PERSON] et al. [47] improved the sensitivity of capacitive MEMS AE sensors by introducing rectangular etch holes to reduce the squeeze film damping. Both sensors were manufactured using the surface micromachining method, which limits the maximum thickness of the resonating element to a few micrometers. To address the thickness limitation of the surface micromachining method, [PERSON] and [PERSON] [26] utilized an electroplating technique to build thick metal layer as the resonating layer of a capacitive MEMS AE sensor. They tuned two resonators to 68 and 149 kHz. Similar to [PERSON] et al. (2011), they used rectangular etch holes to reduce the squeeze film damping. They also added silicon nitride (0.35 \(\upmu\)m-thick) between the stationary layer and the resonating layer in addition to 1.1 \(\upmu\)m air, which reduced the stiction problem of two parallel capacitors. The sensitivity of \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Design Geometry** & **Frequency Range** & **AE Demonstration** & **Other** & **Reference** \\ \hline Resonating & & & & \\ polysilicon beam & 312 kHz & PLB stimulation & Proposed tuning each transducer to particular frequency and cascade together to have wideband range. & [3] \\ \hline Silicon cantilever & & & & \\ with different & 216 kHz & Laser excitation & Proposed cantilever array with different resonant frequencies & [45] \\ \hline Circular geometry & Wideband & PLB stimulation & Not sensitive to S\({}_{0}\) mode & [46] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Piezoresistive MEMS AE sensors and their specifications. the capacitive MEMS sensor was greatly improved as compared to earlier versions. [PERSON] et al. [24] studied circular capacitive MEMS AE sensors to design multifrequency individual sensors for improving the sensor bandwidth. They demonstrated the signal-to-noise ratio of 20 dB at the resonance frequency. Table 3 summarizes the capacitive MEMS AE sensors and their specifications. While capacitive MEMS AE sensors have advantages such as flexible geometry design and the integration with on-chip electronics, the need of constant DC bias is a limiting factor for long-term AE sensing, especially wireless applications. With [PERSON] and [PERSON] [4], the research on piezoelectric MEMS AE sensors has started. They built a piezoelectric AE sensor using a corrugated PVDF mechanism. The sensor had wideband frequency range with the sensitivity of 1 mV/nm. The same research group designed a piezoelectric MEMS AE sensor by depositing PZT film on a cantilever sensing structure [48]. A thermoresponsive actuator was added to increase the contact pressure. The application of this sensor was demonstrated on monitoring knee failures [49].