Researchers working within the Micropower Program at Holst Centre, an open-innovation initiative by imec in Belgium and TNO in the Netherlands, have come up with a MEMS-based harvester generating up to 85 μW to power a wireless sensor node.
Micromachined vibrational energy harvesters operating in the frequency domain between 150 and 1 kHz typically convert vibrations from machines, engines and other industrial appliances into electricity. Thanks to their smaller dimensions, the micromachined devices are candidates for powering miniaturized autonomous sensor nodes.
The harvesters use CMOS-compatible MEMS processes on six-inch silicon wafers.The harvester consists of a Si mass suspended on a beam with Aluminum Nitride (AlN) as piezoelectric material. By changing the dimensions of the beam and mass, the resonance frequency of the harvester can be designed for any value in the 150-1.2 kHz domain.
Researchers say the use of AlN as piezoelectric layer is a notable achievement. AlN has several advantages in terms of materials parameters and ease of processing over the commonly used PZT (Lead zirconate titanate). For example, AlN can be deposited up to three times faster while composition control is not an issue, thanks to the stoichiometric nature of the material.
The development of a wafer-scale process to protect the piezoelectric devices in a vacuum package is also noteable. There is significantly more power output with the vacuum package compared to packaging in atmospheric pressure. In a three-step process, glass covers are coated with an adhesive, vacuum bonded on top and bottom of the processed wafer and diced.
The piezoelectric harvester was connected to a wireless temperature sensor, built from off-the-shelf components. When subjected to vibrations at 353 Hz at 0.64g (said to be a realistic amplitude of the vibrations), the system generated sufficient power to measure the environmental temperature and transmit it to a base station with an interval of fifteen seconds.
Once fully mature, the technology can be used to power sensors in industrial applications such as tire-pressure monitoring and predictive maintenance of moving or rotating machine parts.
IMEC put out an announcement about the development: