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 mu m 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/O-2 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 mu m length, 300 mu m width and 2.5 mu m thickness showed resonant frequency 477.03 Hz and power output 187.4 pW for tip displacement 312.5 mu m which are closely in agreement with the simulated values 453.65 Hz and 189 pW for tip displacement 310 mu m, 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.