by Mieke Van Bavel, Ph.D.
Science Editor, IMEC
Wireless sensor nodes are nowadays employed in many different application fields such as healthcare, automotive and predictive maintenance, but these sensors currently provide limited lifetime as they require significant power for operation. Increasing lifetime, preferably unlimited, becomes possible with energy harvesting. One promising way for power generation is vibrations, and an electrostatic transduction method can be employed to transfer mechanical into electrical energy. An electrostatic energy harvester is, in essence, a movable capacitor which needs a high voltage source for power generation. A patterned electret could operate as this voltage source. In its most general meaning, an electret is a material that stores a quasi-permanent electric charge upon its polarization. It can be called the electrical equivalent of a permanent magnet. It is an important component for a MEMS based energy harvester.
Here, it serves as a high-voltage source and can replace both the battery and the high-voltage up-converter. In comparison with batteries, electrets hold the potential of providing much higher polarization voltages (several hundreds of volts) as well as presenting a stable, long-term solution of sustaining high-temperature processes after fabrication and, when micro-sized, of occupying the minimal possible space in the overall microsystem.
Application example of a vibrational energy harvester: IMEC's fully autonomous wireless temperature sensor powered by a vibrational energy harvester.
How can we make patterned electrets and which materials should be used? Several works report their fabrication by using organic electret materials such as CYTOP® (Asahi glass Co, Ltd.), an amorphous perfluoropolymer, deposited on glass substrates, for example. Typically, stable electrets with feature sizes of several hundreds of microns have been demonstrated and implemented in power generators. Alternatively, silicon based inorganic electrets are being studied. Besides their high charge stability, they have the advantage of being compatible with IC and micromachining technologies and therefore are IMEC's preferred material. Fabrication of MEMS devices on top of CMOS is indeed a hot topic and, therefore, a silicon wafer is the main target substrate. Typically, these electrets are made using standard microelectronics layers like silicon oxide and low pressure chemical vapor deposition (LPCVD) grown silicon nitride on top of a silicon substrate. Negative or positive corona charging is performed to charge the samples. Subsequent patterning of the electrets is commonly done by using photolithography and etching of the electret.
For use in practical devices, the stability of charge storage in these electrets must be guaranteed during the device’s term of use, typically for 10 years. The quasi-permanent charge stored in a good electret film can be extremely stable with a charge lifetime of hundreds years or more than a thousand years – a dream for a MEMS device engineer. Unfortunately, for narrow structures patterned using photolithography, electrets show rapid charge decay – the smaller their dimensions, the faster. A common observation is that discharge takes place at the edges of the patterned electrets or electret sample. This puts severe limitations on their further miniaturization.
Therefore, the key factor for making stable, narrow patterned electrets is decreasing both the fringing field at the edges of an electret and the field emission from the silicon substrate. In practice, this can be done by coating the open silicon surface with a dielectric, e.g. a polymer. This however may prohibit using high-temperature processes required for encapsulation, dehydration, wafer stacking and vacuum-tight sealing. An alternative and more robust technology makes use of an silicon oxide/nitride electret in which a charged profile is created. The idea is based on the observed difference in energy between charge traps located in either layer and at their interface. Stability of charge retention in electrets is indeed related to the charge trap energy: the higher their energy, the longer is the lifetime of charge in electrets. Rapid high-temperature anneal of electrets has shown that silicon oxide/nitride interface traps are much deeper than the traps observed in either silicon oxide or silicon nitride. They quite well retain charge even at 400-450°C. This feature allows easy creation of a charge profile in electrets.
Following this principle, the recipe for fabricating a patterned electret looks like this: (1) depositing a thermal silicon oxide layer on a silicon substrate; (2) creating zones with silicon nitride layer on top of silicon oxide using dry etch; (3) leaving the silicon oxide layer non-patterned and (4) charging both types of zones (zones with only silicon oxide and zones with silicon oxide/nitride). Optionally, an anneal step can be added to eliminate charges in silicon oxide and hence to obtain a charge pattern that is better in terms of relative difference in potential between maxima and minima.
Dependence of the potential on line width one year after fabrication. The potential is normalized to the potential remaining in non-patterned zones of the wafer. As a comparison, the charge in silicon oxide patterned in a “classic” way is shown as measured just two days after its fabrication.
The developed process enables fabrication of stable patterned electrets with feature sizes down to 20 µm and probably less. The related research is ongoing at the Holst Centre and imec. The patterned electret structures, even with no encapsulation, show no dependence of charge retention on line width as measured one year after fabrication. The patterned electret can be used in vibration energy harvesters as shown in the picture below with scanning electron micrographs from a typical device. An application for these MEMS based vibration energy harvesters, foreseen in the near future, is an autonomous tire pressure monitoring system. But patterned electrets can also be applied in other MEMS applications, such as micromotors and transducers like MEMS based microphones.
Schematic picture with scanning electron micrographs from a typical energy harvesting device with a patterned electret as a voltage source.
Furthermore, microsystems require packaging, wafer stacking and other high-temperature processes. This technology allows such processes with practically no loss in charge and with no effect of high temperature processes (up to 450°C) on charge retention. The developed technique of electret patterning can be applied to other combinations of electret layers as well, including polymer electrets. With this technique, we have come one step closer to the ultimate self-powered device.
Mieke Van Bavel (firstname.lastname@example.org) received the Ph.D. in Physics from the Katholieke Universiteit Leuven, Belgium in 1995. She joined imec as a science editor in 2001.
Vladimir Leonov received a Ph.D. degree in optical devices from the State Optical Institute, St. Petersburg, where he worked since 1981 on thermal detector arrays as a Senior Scientist. In 1998-2000 he was with SMU, Dallas, TX. Then, he worked as a Principal Scientist at Xenics, Leuven, Belgium, and starting 2003, he is a Senior Scientist at imec. His major interests are concentrated on wearable energy harvesters and electrets for microsystems.
Rob van Schaijk obtained a Master Degree in Applied Physics from the Technical University of Eindhoven (the Netherlands) in 1995 and a Ph.D. from the University of Amsterdam (the Netherlands) in 1999. In 1999 he started senior scientist for Philips Research in the area of silicon processing. In 2006 he started at Philips Semiconductors (nowadays NXP semiconductors) as project leader working on the process development and industrialization of RF-MEMS. Since 2007 he is working for imec and Holst Centre, as principal researcher and program manager in the micropower program. This program focuses on the development of energy harvesters for use in wireless sensor nodes. The main interest is in MEMS based vibrational and thermal energy harvesters.
Copyright 2010 MEMS Investor Journal, Inc.