FABRICATION AND CHARATERIZATION OF PIEZOELECTRIC ENERGY HARVESTERS

Abstract

The applications of low power autonomous electronic devices such as wireless sensors, transducers, and implantable medical devices have been increasing over the last decade. These devices are powered by using conventional batteries, which have their own limitations. This is particularly a major issue for low power electronic devices. Moreover, though the size of the electronic devices has been reducing, batteries limit the size and weight of the sensor nodes and integrated circuits. To overcome these issues, vibrational energy harvesting/scavenging is a promising solution, as a power source to drive these compact electronic devices. The self powered electronics devices have applications such as: structural health monitoring, medical devices, transport-tracking, and internet of things (IoT), etc. In this dissertation, we focus on design, modeling, optimization, and fabrication of two types of piezoelectric energy harvesters (PEHs). First, a micromachined PEH using piezoelectric aluminum nitride (AlN) film with integrated silicon proof mass is presented for silicon on chip (SOC) based applications. Second, for large-area applications, a low-cost flexible substrate based PEH using polyvinylidene fluoride trifluoroethylene (PVDF-TrFE) film is presented. The PVDF TrFE based PEH is capable of harvesting power from water bodies, human motions, and wind, etc. We start with developing an analytical model of PEH of given specifications such as: frequency, dimensions, and materials. We obtain the characteristics of generated output like voltage and power from the PEH, with frequency and load at particular acceleration. Moreover, using the developed model, we optimize the PEH for: (i) cantilever to proof mass length and (ii) cantilever to proof mass width ratios for the maximum output power from PEH. The fabrication of PEH using AlN as a piezoelectric material requires the growth of c-axis AlN film on a metal electrode. However, it is challenging to grow AlN in c-axis orientation on a metal electrode, due to lattice mismatch and the difference in the coefficient of thermal expansion. In this study, we have investigated and further optimized the effect of several process parameters, such as the plasma power, N2 flow concentration, quality of bottom electrode, and oxygen content, for enabling the growth of c-axis textured AlN film. To this end, highly c-axis oriented AlN thin films are sputtered on the molybdenum (Mo) electrode at the low substrate temperature (300°C). The deposited c-axis oriented AlN thin film at optimized process parameters has extremely low full width at half maximum (FWHM) value of 0.62°. The fabricated metal i insulator-metal (MIM) test structure using the optimized AlN film has a dielectric constant of 8.89, which makes it suitable for use in the applications in PEH. Further, we fabricated CMOS compatible micromachined PEH using the process guideline and optimized parameters of the AlN film discussed above. The PEH is fabricated using a novel integration scheme for the bottom electrode with Au as an interlayer. The Au interlayer is used to grow a good quality of AlN film. It is found that in order to achieve the optimum output from PEH, the fractional length and width occupied by cantilever are 28-40% and 38-45%, respectively. The optimized designs are fabricated using a CMOS compatible process using challenging deep reactive ion etching (DRIE) process. The maximum power density measured from the fabricated PEH is found to be 9.36 μW/mm3. The optimized design and fabricated PEH outperforms the similarly reported AlN based PEHs in terms of power density. Finally, the output voltage across the resistive load and corresponding output power are measured and compared with the model. The optimized and compact low power PEHs reported in this work has high potential to be integrated with the system on chip (SOC) and the other wireless sensor applications. For large area applications, a low-cost, flexible substrate-based PEH is fabricated using the PVDF-TrFE film. We first optimize the deposition parameters to grow the β-phase of PVDF TrFE film on polyethylene terephthalate (PET) substrate. Using an optimized β-phase PVDF TrFE film, a rectangular shaped PEH is fabricated on a flexible substrate and characterized. The PEH is fabricated using Mo and Al as the bottom and top electrodes, respectively. The maximum measured power density from the flexible substrate based PEH is found to be 0.312 μW/mm3, which is better than the similar reported studies. Moreover, we implemented the developed analytical model to predict output power from the flexible substrate based PEH. The fabricated and model characteristics such as voltage and power with frequency and load are shown to match well with mechanical and induced electrical damping as model parameters. The designed methodology and fabricated PEH reported in this work have a high potential for low cost and large area applications, such as monitoring of water bodies and plant health in agriculture, etc.