Low frequency piezoelectric P(VDF-TrFE) micro-cantilevers with a novel MEMS process for vibration sensor and energy harvester applications

Low frequency piezoelectric P(VDF-TrFE) micro-cantilever vibration sensors have been developed for the first time with a novel MEMS process. Design and simulation of micro-cantilevers were carried out using COMSOL Multiphysics based on finite element method. Frequencies and device dimensions were determined based on simulation results. The design was implemented on 110 Si wafer using a specially developed bulk micromachining process. Micro-cantilevers were fabricated with 2.5 μm thick P(VDF-TrFE) co-polymer film deposited by spin coating technique; electrodes for power output were formed by sequential thermal evaporation of Cr-Au thin films. The two critical process steps used for the suspension of P(VDF-TrFE) micro-cantilevers are: (1) bulk micromachining of silicon from the backside using anisotropic wet etchant TMAH to define the micro-cantilever suspension regions, and (2) CHF₃/O₂ based plasma etching of SiO₂ from backside for the final release of P(VDF-TrFE) micro-cantilevers. These devices were operated in longitudinal mode with Cr-Au interdigitated electrodes on P(VDF-TrFE) micro-cantilevers for power extraction. The experimental results obtained with laser Doppler vibrometer for micro-cantilevers with 1000 μm length, 300 μm width and 2.5 μm thickness showed resonant frequency 477.03 Hz and power output 187.4 pW for tip displacement 312.5 μm which are closely in agreement with the simulated values 453.65 Hz and 189 pW for tip displacement 310 μm, respectively. The volume power density of this P(VDF-TrFE) unimorph micro-cantilever is 249.92 nW mm^-3, which is found to be better compared with other polymer piezoelectric cantilevers.