Piezoelectric Zinc Oxide Based MEMS Acoustic Sensor

• Main Authors: Aarti Arora, P. J. George, Anil Arora, V. K. Dwivedi, Vinay Gupta
• Format: Article
• Language: English
• Published: IFSA Publishing, S.L. 2008-04-01
• Series: Sensors & Transducers
• Subjects: Acoustic sensor; Zinc oxide; Rf sputtering
• Online Access: http://www.sensorsportal.com/HTML/DIGEST/april_08/P_260.pdf

An acoustic sensors exhibiting good sensitivity was fabricated using MEMS technology having piezoelectric zinc oxide as a dielectric between two plates of capacitor. Thin film zinc oxide has structural, piezoelectric and optical properties for surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Oxygen effficient films are transparent and insulating having wide applications for sensors and transducers. A rf sputtered piezoelectric ZnO layer transforms the mechanical deflection of a thin etched silicon diaphragm into a piezoelectric charge. For 25-micron thin diaphragm Si was etched in tetramethylammonium hydroxide solution using bulk micromachining. This was followed by deposition of sandwiched structure composed of bottom aluminum electrode, sputtered 3 micron ZnO film and top aluminum electrode. A glass having 1 mm diameter hole was bonded on backside of device to compensate sound pressure in side the cavity. The measured value of central capacitance and dissipation factor of the fabricated MEMS acoustic sensor was found to be 82.4pF and 0.115 respectively, where as the value of ~176 pF was obtained for the rim capacitance with a dissipation factor of 0.138. The response of the acoustic sensors was reproducible for the devices prepared under similar processing conditions under different batches. The acoustic sensor was found to be working from 30Hz to 8KHz with a sensitivity of 139µV/Pa under varying acoustic pressure.