Vibrational piezoelectric energy harvesters based on thinned bulk PZT sheets fabricated at the wafer level
We present a complete wafer level microfabrication process for the production of unimorph MEMS energy harvesters based on thinned bulk piezoelectric ceramic, lead zirconate titanate (PZT), sheets. This process eliminates the need for individual bonding of PZT pieces and proof masses at the chip level while still benefitting from the excellent properties of bulk PZT. With the process presented in this paper, 20 piezoelectric energy harvesters have been fabricated in parallel at the wafer level by bonding a single bulk (PZT) sheet onto a silicon-on-insulator (SOI) wafer using a low-temperature process and structuring the bonded stack with standard microfabrication techniques including thinning of the bulk PZT sheet using mechanical grinding as well as electrodeposition to deposit a thick nickel proof mass on the tip of each cantilever. A single fabricated harvester with an effective volume of 47.82 mm(3) is capable of generating a normalized power density of 3346 mu W cm(-3) g(-2) with an average power of 1.6 mu W under an excitation of 0.1 g (1 g = 9.81 m s(-2)) at a resonant frequency of 100 Hz through an optimal resistive load of 11.8 k Omega Thinned bulk PZT exhibits high power and a useable voltage while maintaining a low optimal resistive load, demonstrating the potential of high performance piezoelectric MEMS energy harvesters using bulk PZT sheets fabricated at the wafer level. (C) 2014 Elsevier B.V. All rights reserved.