1 Department of Micro- and Nanotechnology, Technical University of Denmark2 MEMS-AppliedSensors, Department of Micro- and Nanotechnology, Technical University of Denmark
The objective of the work presented in this Ph.D. thesis is to design and fabricate a miniaturised vibration energy harvester based on screen printed PZT thick film and silicon MEMS processing technology. The vision of the vibration energy harvester is to eliminate the need for batteries by harvesting energy on-site from a vibration source, thereby enabling fully autonomous wireless sensor systems. The vibration harvester is a resonator consisting of a silicon support cantilever with screen printed PZT thick film on top and with an integrated proof mass at the cantilever tip. To achieve matching between the harvesters resonant frequency and vibration sources in the low frequency range, the thickness of the cantilever is required to be in the sub- 100 µm range not to compromise the total dimension of the harvester. Fabricating this challenging and fragile design with the cantilever thickness being two orders of magnitude smaller than the cantilever length and width, is accomplished using the high control and precision of the silicon processing technology. With extensive process development the issue of fragility is minimised, and fabrication yields exceeding 90% are routinely achieved. The final fabrication process features a sequence with screen printing of the PZT thick film at an early stage and cantilever definition by etching at a later stage. Screen printing PZT on a full thickness silicon wafer enables efficient use of a high pressure treatment process with improved performance as a result. Cleanroom contamination issues in the cantilever etching due to the PZT film is solved with a KOH etch where the wafer front side is protected mechanically. From thorough characterisation of the fabricated harvester, it is validated that the power output can be expressed as a power available term and a multiplication factor equal to or less than 1. The available power is proportional to the force acting on the cantilever squared and the inverse of the viscous damping coefficient. The latest fabricated batch of harvesters produced in average 34.5 µW of RMS power over a resistive load of 50 k? with an RMS acceleration of 0.5g at 511 Hz. The best performing devices under similar conditions produced 44.9 µW at 543 Hz. Compared to other state of the art miniaturised vibration energy harvesters, the normalised power density for the harvesters fabricated in this work is 3.5 times higher than the next best harvester.