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Jordi Agustí Batlle

Departament d’Enginyeria Electrònica

Universitat Autònoma de Barcelona

Spain

Jordi.Agusti@uab.cat

RF energy harvester based on MEMS

NiPS Summer School 2010Summer School: Energy Harvesting at micro and nanoscale, August 1‐6, 2010

NiPSWorkshop:Noise indynamical systemsat themicro and nanoscale,August 6‐8, 2010La Tenuta dei Ciclamini, Avigliano Umbro (TR) - Italy

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Outline

Jordi Agustí

• Introduction to Energy Harvesting

• Research background of our group

• RF energy harvester based on MEMS

�RF-MEMSTENNA concept

�Fabrication process

�Prototype test methods

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What does the concept of Energy Harvesting mean?

Is the capacity to extract energy from an ambient source and convert it to electrical

energy.

Introduction to Energy Harvesting

Energy sources:

Kinetic (vibrations, random fluctuations, wind, flow, …), Thermal gradients, Electromagnetic sources (solar, RF, beta

radiation, …), others

Transduction methods:

Electrostatic transducers, Piezoelectric transducers, Electromagnetic transducers and Thermoelectric transducers

Thomas, J., M. Qidwai, and J. Kellogg, Energy scavenging for small-scale unmanned systems. Journal of Power Sources, 2006. 159(2): p. 1494-1509.

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Research background of our group

Extracted from G. Murillo STIMESI Workshop talk.

2000 2002 2004 2006 2008 2010Date (years)

Working frequency

SCAVENGINGMEMS

3kHz

300kHz

30MHz

3GHz

30Hz

2012

SENSING MEMS

RF MEMS

Power Managment Circuitry

Energy Storage Element

Sensor ULP Controller

µGenerator

RF Transceiver

Energy

Scavenging

Research

WSN diagram:

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“State of the art” or …

… where did the idea came from?

Jensen, K., et al., Nanotube radio. Nano Lett, 2007. 7(11): p. 3508‐3511.

Pros:

- Broadband RF energy harvester (i.e. from 4.8 to 8 GHz)

- The designed can be tuned to harvest energy from certain bands

between the MHz and GHz range

- Efficiencies up to 90%

- Cheap fabrication process

Cons:

- Dimensions on the order of cm

- Non-integrableHagerty, J., et al., Recycling ambient microwave energy with broad-band rectenna arrays. IEEE Transactions on Microwave Theory and Techniques, 2004.

Rectenna concept:

Rectifier + Antenna = Rectenna → Made to harvest energy from the RF electromagne�c spectrum

Nanotube Radio:

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- The source could be a theoretical free and with unlimited power (we don’t care where

does it come from) or…

… it could be an specific one

What are we looking for?

RF energy harvester based on MEMS

NEW DEVICE!

- Using MEMS or NEMS because they…

… are integrable & cheap

… so they can be used to power present and future micro and nano devices (specially

future Ultra Low Power Wireless Sensing Nodes, ULP-WSN)

- Expecting to harvest an amount of energy between pW and µW

- Harvest energy from the radiofrequency electromagnetic spectrum (3 kHz – 300 GHz), …

… focusing on the bands which have more available power (i.e. ISM bands: TV, radio,

WLAN, GSM…)

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The name accounts for the two concepts involved:

MEMS & anTENNA = MEMSTENNA.

The idea is to have a mechanical structure, such as a cantilever or a bridge, that has a certain

quantity of trapped charge incrusted in a specific part of its movable structure. The structure

would be able to generate electricity through an integrated thin film piezoelectric transducer

due to the interaction of its incrusted charge with an incidence RF electric field.

mm

RF wave+

-

λ/2 dipole

RECTENNA CONCEPT

RF waveqPiezo

μm MEMSTENNA CONCEPT

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RF-MEMSTENNA concept

An array of this devices would be used in order to harvest a reasonable amount of energy.

The solution is like:

• The cantilever as a linear oscillator:

If we consider that the mechanical structure is a 1D

harmonic oscillator without losses:

• Taking into account the losses and the incident electric field:

RF-MEMSTENNA concept

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Now our system is a 1D forced harmonic oscillator, where

the electrostatic force is the excitation source.

In order to enhance the induced movement the incident

wave should have a frequency equal to the resonance

frequency of the mechanical structure

2

2

( )( ) 0eff eff

x tm k x t

t

∂ ⋅ + ⋅ =∂

2

2

( )( ) sin(2 )

2 12eff n

n n nVeff

k t Yx t A f f f

m l

κππ ρ⋅= ⋅ ⋅ ⋅ → = → = ⋅ ⋅⋅ ⋅l

22

0 02

( ) ( )2 ( ) e

e f

x t x tx t F

t tF Q E

ξ ω ω∂ ∂+ ⋅ ⋅ ⋅ + ⋅ =∂ ∂

= ⋅

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Is it feasible to implant a charge in the tip of the cantilever?

Is this charge going to last forever?

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Trapped charge… how?

Microphone ELECTRET concept:

Yu-Chong Tai - A Hig Performance MEMS Thin-Film Teflon Electret Microphone

Achievable stable charge densities 1·10-5 C/m2 to 8·10-4 C/m2

τdecay = 10 ‘s - 100’s years

COMSOL Multiphysics modeling

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We are coupling these physics:

- Structural mechanics - Piezoelectric materials

- Electric currents - Electronic circuits

Using COMSOL Multyphysics we are able to simulate our RF-MEMSTENNA with an

standard interface circuit:

… and much more.

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Fabrication process similar to the one used in the VIBES (Vibration Energy Scavenging)

project. The piezoelectric transducer would be made of Aluminum Nitride.

In order to incrust the trapped charge an electrec fabrication process is proposed:

Fabrication process

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Jacobs, H. and G. Whitesides, Submicrometer patterning of charge in thin-film electrets. Science, 2001. 291(5509): p. 1763.

Marzencki, M., et al. A MEMS piezoelectric vibration energy harvesting device. 2005.

• Optic method

If the prototype does not have the piezoelectric

transducer, an optical characterization of the

RF-MEMSTENNA must be done. With this kind

of setup one would be able to characterize the

mechanical properties of the fabricated device.

• Electric method:

If the prototype has the piezoelectric

thin film transducer, the device could

be tested with an electronic circuitry.

Then we could do an electric

characterization of the sample.

Prototype test methods

Agustí, J., et al., Optical vibrometer for mechanical properties characterization of silicalite-only cantilever based sensors. Microelectronic Engineering, 2009.

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Jordi Agustí Batlle

Departament d’Enginyeria Electrònica

Universitat Autònoma de Barcelona

Spain

Jordi.Agusti@uab.cat

RF energy harvester based on MEMS

NiPS Summer School 2010Summer School: Energy Harvesting at micro and nanoscale, August 1‐6, 2010

NiPSWorkshop:Noise indynamical systemsat themicro and nanoscale,August 6‐8, 2010La Tenuta dei Ciclamini, Avigliano Umbro (TR) - Italy