For the first time, a piezoelectric harvesting device fabricated by MEMS technology
generates a record of 85µW electrical power from vibrations. A wafer level
packaging method was developed for robustness. The packaged MEMS-based harvester
is used to power a wireless sensor node. Within the Holst Centre program on
Micropower Generation and Storage, imec researchers developed a temperature
sensor that can wirelessly transmit data in a fully autonomous way.
Micromachined vibrational energy harvesters operating in the frequency domain
between 150 and 1000Hz are ideal devices to convert vibrations from machines,
engines and other industrial appliances into electricity. Thanks to their smaller
dimensions, the micromachined devices are the prefered candidates for powering
miniaturized autonomous sensor nodes.
Record and novel material
By using cost-effective, CMOS compatible MEMS processes on 6’ silicon
wafers, imec developed piezoelectric energy harvesters capable of generating
up to 85µW of power.
The harvester consists of a Si mass that is 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 150-1200Hz domain.
Not only the record power output, but also the use of AlN as piezoelectric
layer, is a notable achievement. AlN has several advantages in terms of materials
parameters and ease of processing compared to the commonly used PZT (Lead zirconate
titanate). Just to name two: AlN can be deposited up to three times faster while
composition control is not an issue, thanks to the stoichiometric nature of
Final achievement in the research is the development of a wafer-scale process
to protect the piezoelectric devices by a package. It was shown that the power
output significantly increases by the use of 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
The piezoelectric harvester was connected to a wireless temperature sensor,
built op from of-the-shelf components. After power optimization, the consumption
of the sensor was reduced from 1.5mW to ±10µW, which is an improvement
by three orders of magnitude. When subjected to vibrations at 353Hz at 0.64g
(indicating 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. The result proves the feasibility of building
fully autonomous harvesters for industrial applications.
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 and Holst Centre do not go to market
themselves, but perform the research together with industrial players interested
in commercializing the technology.
The result was obtained within the Micropower Program at Holst Centre, an open-innovation
initiative by imec and TNO. All details of the research are presented during
the 2009 IEEE International Electron Devices Meeting (IEDM) in Baltimore (December
View the release at http://www2.imec.be/imec_com/micromachined-piezoelectric-harvester-with-record-power-output-drives-fully-autonomous-wireless-sensor.php