Microphone based on polyvinylidene fluoride (PVDF) micro-pillars and patterned electrodes

Piezoelectric materials have the ability to transfer energy between the electric and mechanical domains. Polyvinylidene fluoride (PVDF) exhibits higher piezoelectricity than other polymer materials such as nylon and polyvinyl chloride. PVDF is a superior material for sensors because its stress constant, the ability to convert stress into electrical energy, is more than 20 times higher than that of lead zirconate titanate. Nonetheless, there is significant interest in improving the effective stress constant of PVDF devices beyond the intrinsic sensitivity of the material. Significant research has focused on improvements in material properties, such as increasing beta phase ratio or artificially introducing defects, and processing, such as optimizing stretch ratio and poling temperature or applying a high electric field. This research is focused on improving the stress constant, or sensor sensitivity, by means of design.;The acoustic sensor presented in this dissertation exploits the key advantages of PVDF as a sensor material by means of two key design elements aimed at increasing the charge and decreasing the effective device capacitance. The first design element is a stress amplification mechanism through the area ratio between the overall surface exposed to acoustic waves and the area of an array of PVDF micro-pillars. Because PVDF responds to stress, this mechanism increases the amount of charge for a given pressure level. The second design element is top and bottom electrodes selectively patterned to form an overlapping active area determined by the micro-pillars. Excluding the capacitance of the other inactive area, the design with patterned electrodes reduces the capacitance of the sensor and hence increases the voltage generated by the sensor.;The small size, high stiffness, and reduced mass of MEMS sensors are of great interest because such devices can significantly improve both the temporal and spatial measurement bandwidth. The sensor realization requires micro-fabrication process and this technology is available at the Biomedical Engineering Center of The Ohio State University. Previously patterned polydimethylsiloxane (PDMS) stamps molded from photolithographically fabricated masters are used in the production of individual and interconnected PVDF micro-pillar arrays. Taking advantage of the thickness or "33" mode, the developed PVDF micro-pillar sensor has a frequency bandwidth of at least 20 Hz-100 kHz and maximum response of up to a few MHz (depending on specific sensor dimensions).;A PVDF micro-pillar sensor with patterned electrodes and gap ratio of 5.82 was developed and various acoustic tests were performed on this sensor. The sensitivity calibration test shows that the developed sensor has a sensitivity of 189.3 muV/Pa, which is 60.39x greater than that of commercial solid PVDF with the same footprint and thickness. The measured stress constant of the sensor is -19.93 V/m/Pa, which is 60.39x larger than that of commercial solid PVDF (g33 =-0.33 V/m/Pa). The measured stress constant amplification ratio is in good agreement with the predicted amplification ratio of 59.19, thus confirming the performance advantages of the micropillar sensor.