Research On MEMS Energy Harvester Based On Piezoelectric Thick Film

With the rapid development of microelectronics, micro-machining technology andwireless sensing technology, the embedded systems, radio frequency identification systemsand wireless sensor network have been widely used in daily life. The power consumptionof these devices is very low and even with the level of microwatts. However, their powersupply should be featured with small/micro volume, long life, replacing needless andself-service. Traditional battery can’t satisfy all the demands apparently for its largevolume, high weight, the limited energy supply time and other shortcomings. Energyharvester can convert vibrational energy from environment into electrical energy andreplace the conventional power source for micro devices. On the other hand, the vibrationenergy harvester based on MEMS technology can be well integrated with manymicro-electronic chips, micro-sensors and micro-actuators, and supply micro-devices withlasting, stable and clean electric energy. At present, the MEMS piezoelectric energyharvester has become the research hotspot of many international research groups due to itsbroad application.This dissertation mainly focuses on piezoelectric MEMS energy harvester based onpiezoelectric thick film. On the basis of domestic and international research work, a designscheme of MEMS vibration energy harvester based on piezoelectric thick film is presentedin this work. The electromechanical coupling model of piezoelectric vibration energyharvester is established and the structural parameters of the device are optimized. Inaddition, piezoelectric thick film with superior performance is prepared by using bondingand thinning techniques and then the energy harvester prototype is fabricated by MEMSprocess. Finally, the performance of the fabricated prototype is characterized and analyzedtotally. The detailed work and the conclusion of this dissertation are as following:1. A design scheme of MEMS vibration energy harvester based on piezoelectric thick film is presented. Based on the design requirement of MEMS piezoelectric energyharvester, the structural type, material selection and operating mode are analyzed. On thisbasis, three types of the piezoelectric energy harvester with cantilever/mass compositestructure are proposed, namely, copper-based trapezoidal PZT cantilever, silicon-basedPZT and PMNT cantilevers.2. The electromechanical coupling model of the piezoelectric vibration energyharvester is created. The effect of the structural parameters on the natural frequency andpower density output is analyzed. These main parameters include the length ofpiezoelectric cantilever beam and proof mass, the thickness of supporting layer andpiezoelectric layer and the width of cantilever beam and proof mass. In addition, the effectof epoxy resin as the bonding layer is also discussed and structural stability of the device isanalyzed. Finally, the energy harvester structure dimensions are determined based on theanalytical results and the feasibility of fabrication techniques.3. The piezoelectric thick film with excellent piezoelectric properties is developedbased on bonding and thinning technique and its patterning technique is proposed.Conductive epoxy resin is used as the intermediate adhesive layer for a low temperaturePZT (PMNT)-Si bonding. A bonding strength of more than15Mpa has been obtainedunder the optimum bonding parameters with0.1Mpa bonding pressure and175°C bondedcuring temperature. The mechanical lapping and wet-etching combined method isproposed to thin bulk PZT and the thickness of PZT and PMNT film can be controlled atthe range of10~100μm and5~100μm, respectively. The fabricated piezoelectric thickfilms are characterized and the testing results show that the ferroelectric and dielectricproperties of the PZT thick films obtained here is close to that of bulk materials. Inaddition, wet etching and mechanical dicing methods are deployed to pattern the preparedpiezoelectric thick film.4. The piezoelectric vibration energy harvester prototypes are fabricated by MEMSmicromachining technique. The involved technologies include piezoelectric thick filmpreparation, electrode deposition, optical lithography, and etching process, UV-LIGA basedSU8process, micro-electroforming, etc. The detail process parameters are studied, andrelated process control and improvement issues are discussed.5. The output performance of the energy harvester prototype is measured by the testing platform including excitation system, vibration monitoring system and electricaltest system. The natural frequency, output voltage, maximum output power andrectification and capacitance storage experiment are measured or carried out. Goodagreement between theory analysis and testing results is obtained. The maximal outputpower and power density for the prototype with Cu trapezoidal beam structure are1.43μWand7889.7μW/cm~3under1.0g acceleration and1010Hz resonant frequency, respectively.The output voltage, power and power density of the silicon-based PZT cantilever prototypeare5.04VP-P,11.56μW and28856.7μW/cm~3under1.0g acceleration and514.1Hz resonantfrequency, respectively. The output voltage, power and power density of silicon-basedPMNT cantilever prototype are2.08VP-P,2.704μW and5352.3μW/cm~3under the vibrationexcitation of1.0g acceleration and237.4Hz, respectively.6. The output performance of piezoelectric energy harvester prototype in liquidenvironment (silicone oil) is studied. The testing results show that the damping ratio of thedevice in the silicone oil environment is about two times larger than that in the airenvironment at the vibration excitation of1.0g acceleration, which results in lower outputvoltage, resonant frequency and broad bandwidth. The resonant frequency and outputpower of the device in the liquid environment is decreased by28.16%and85.89%respectively, while the effective bandwidth is increased by52.98%, compared with that ofthe device in the air environment. So the liquid environment can be used to realize thewideband operation of energy harvester.