December 14, 2009 - At this year’s International Electron Devices Meeting (IEDM), IMEC and partners TNO (a Netherlands-based research group) and the Holst Center (IMEC-TNO joint center set up in 2005), disclosed their latest work in creating a MEMS-based piezoelectric energy harvesting device with record power generation, and a "world-first" organic transponder circuit with bit rate of 50kbits/s, nearing requirements for Electronic Product Coding (EPC) standards.
New mark in MEMS piezoelectric energy harvesting
Micromachined devices to harvest energy from vibrations typically operate in a range of 150-1000Hz, ideally used to convert energy from vibrations in machines, engines, and other industrial appliances. Their tiny size also makes them useful for powering miniaturized autonomous sensor nodes.
In work within the Holst Center’s program on micropower generation and storage, IMEC researchers created a temperature sensor that can wirelessly and autonomously transmit data — a wafer-level-packaged MEMS-based harvester, generating a record 85µW electrical power from vibrations. The harvester is a Si mass suspended on a beam, built used CMOS-compatible MEMS processes on 6" silicon wafers. Changing the dimensions of the beam and mass can modify the harvester’s resonance frequency for any value in the 150-1200Hz range.
Among the achievements IMEC noted in its work:
- Aluminum nitride is used instead of lead zirconate titanate as the piezoelectric material; AlN enables more favorable materials parameters and ease of processing, e.g. up to 3× faster deposition and better composition control due to AlN’s stoichiometric nature.
- A wafer-scale process was developed to protect the piezoelectric devices in a package: glass covers coated with an adhesive, vacuum-bonded on top and bottom of the process wafer, and diced. Power output was shown to increase significantly using a vacuum package vs. packaging in atmospheric pressure.
The harvester was connected to a wireless temperature sensor built from off-the-shelf components. After power optimization, the sensor’s energy consumption was reduced from 1.5mW to ~10µW, a three-orders-of-magnitude improvement. Subjected to vibrations at 353Hz at 0.64g (a realistic amplitude) the system generated sufficient power to measure and transmit environmental temperature to a base station with 15sec interval.
Fully autonomous wireless temperature sensor powered by a vibrational energy harvester. (Source: IMEC)
The achievement proves the feasibility of building fully autonomous energy harvesters for industrial applications, IMEC says. Once it is developed to maturity (by industry, not IMEC or Holst), the technology could power sensors for applications such as tire-pressure monitoring systems (TPMS) and predictive maintenance of moving or rotating machine parts.
"World’s first" 50kbit/s organic transponder
Another declaration at IEDM was the debut from IMEC, TNO, and Holst of the world’s first organic transponder circuit with 50kbits/sec bit rate, which approaches requirements for Electronic Product Coding (EPC) standards, which support the use of radio-frequency identification (RFID).
Flexible circuits are attractive for both manufacturing as well as final products in applications such as plastic RFID tags, but would need to adhere to EPC specs for item-level tagging, which requires 50kb/s bit rate. The Holst Center, wtih IMEC and TNO, have developed an 8-bit flexible transponder circuit on foil using pentacene as the semiconductor material and a high-k gate dielectric. The device’s current drive extends well beyond previous efforts with 1-2kbits/s bitrates, pushing all the way to >50kbits/s data rate, "which compares favorably" with such EPC specs.
RFID is already being used in high-volume logistics applications, e.g. pallet-level logistics; the next step is to use EPC tags at the package level, and eventually on individual items (item-level tagging). Organic electronic technology offers the promise of, and is being explored to be used for, high-volume and low-cost manufacturing of simple electronic circuits. "The new results demonstrate that the technology is now on the way to reach EPC compatibility," IMEC said in a statement.