TUBITAK Project, 2016 - 2019
Sensing element, membrane, is the most critical part of fiber optic micro-electro-mechanical systems (MEMS) microphone. The study involves the design and characterization of a MEMS membrane for a fiber optic microphone, so that the microphone operates in the desired range with desired sensitivity. Microfabrication is done using a commercially available MUMPS (Multi-user MEMS) process. Electrical and optical characterizations of the membrane are done using an impedance analyzer and laser vibrometer, respectively. The transient and steady state analyses of the membrane are utilized, and the temporal and spatial responses of the membrane are obtained. The fundamental resonance of the membrane is 28 kHz. The average peak displacement of 1.5 nm is obtained from vibrometer measurements under 100 mV peak-to-peak voltage and 1 V DC bias condition. The microphone is usable for a wide range of frequency including 20 Hz - 100 kHz. The sensitivity of the membrane is calculated to be around 12 nm / Pa at 28 kHz and the quality factor is calculated as 2.8. The study includes the design and demonstration of the fiber optic MEMS microphone measurement system (Fabry-Perot interferometer). The measurement system setup is designed, constructed and functionalized according to the optimized simulation results. The system is demonstrated with optical components and the membrane, and characteristics of the fiber optic microphone are obtained. The membrane that is utilized in the fiber optic system has characteristics as 15 kHz first resonant frequency and average displacement of 14 nm under 100 mV peak-to-peak voltage and 1 V DC bias. The sensitivity of the membrane is 105 nm / Pa at 15 kHz and the quality factor is 3. With calibrated microphone measurements on fiber optic system, sensitivity of the microphone is optically tested. Signal-to-noise ratio is found as 38 dB in average. Sensitivity of the microphone is found as approximately 50 μV/Pa for the first resonance frequency of 15 kHz and 104.4 μV/Pa in average. This study offers a new aspect for the design of MEMS membrane for optical microphones.