Methodology for Measuring 1/f Noise in Thin Film Piezoelectric Devices for Energy Harvesting and Sensor Applications

Polyvinylidene difluoride-trifluoroethylene (PVDF–TrFE) thin films have been utilized as a flexible sensing and energy harvesting material due to their piezoelectric capabilities [1]. A common use of PVDF-TrFE is in the development of acoustic sensors. Previous literature has reported applications as a microphone for cochlear implants [2] and as piezoelectric energy harvesters [3]. For these applications, it is important to understand the electrical noise generated by the device to accurately determine its noise floor, as it is the primary factor in determining critical sensor metrics like limit of detection and signal to noise ratio (SNR).<br/>The primary challenge in measuring noise in an acoustic MEMS system is the normalization of the noise floor. PVDF thin films are constantly vibrating, even without external input, due to its flexible and deformable nature. Small fluctuations in the acoustic and electronic environment can then make drastic changes in the noise floor, impacting the accuracy of the overall sensor or system.<br/>In this work, we are interested in characterizing the noise current generated by the sensor due to the underlying properties of the material in the absence of external interference. This measurement is useful in providing a more accurate noise budget and electrical model to aid in the design of interfacing circuitry. Proper characterization of noise allows for the rigorous quantification of SNR and minimum detectable signal amplitude which are important metrics for any sensor.<br/>The metrology system has three main components, 1) an insulated, anechoic chamber made with thick metal walls to reduce acoustic and electromagnetic interference 2) an SRS low-noise pre-amplifier to amplify the generated noise current 3) a spectrum analyzer, to measure the noise spectral density of signals in the range 40 Hz to 25 kHz. Together, these components provide a low-noise measurement environment suitable to study intrinsic noise generation.<br/>To test the efficacy of the system described above, a printed PVDF-TrFE device described in [4] was measured. Before any noise measurements were taken, the test device’s capacitance was measured. Likewise, its output voltage over time when exposed to acoustic stimulation was recorded using an ADMET Universal Testing System with readout electronics designed in [4]. These measurements affirm that the device acts as a capacitive piezoelectric acoustic sensor. Finally, once the devices were characterized, they were placed in the chamber and the current noise measurements were taken in the 40 Hz - 25 kHz band.<br/>[1] B. Stadlober, M. Zirkl, and M. Irimia-Vladu, “Route towards sustainable smart sensors: Ferroelectric polyvinylidene fluoride-based materials and their integration in flexible electronics,” Chemical Society Reviews, vol. 48, no. 6, pp. 1787–1825, 2019.<br/>[2] S. Park, X. Guan, Y. Kim, F. (P. Creighton, E. Wei, I. J. Kymissis, H. H. Nakajima, and E. S. Olson, “PVDF-based piezoelectric microphone for sound detection inside the cochlea: Toward totally implantable cochlear implants,” Trends in Hearing, vol. 22, 2018.<br/>[3] D. Vatansever, R. L. Hadimani, T. Shah, and E. Siores, “An investigation of energy harvesting from renewable sources with PVDF and PZT,” Smart Materials and Structures, vol. 20, no. 5, p. 055019, 2011.<br/>[4] C. K. McGinn, K. A. Kam, M.-M. Laurila, K. Lozano Montero, M. Mäntysalo, D. Lupo, and I. Kymissis, “Formulation, printing, and Poling method for piezoelectric films based on PVDF–TrFE,” Journal of Applied Physics, vol. 128, no. 22, p. 225304, 2020.