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RF MEMS in Energy Harvesting

Prepared By

Aalay D. Kapadia (adk130330)

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DisclaimerThis work was done as part of coursework of course EEMF 6382 under guidance of Prof. Siavash Pourkamali Anaraki (Ph.D.) at University Of Texas At Dallas. Any opinions, findings and conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect those of Prof. Siavash Pourkamali Anaraki (Ph.D.) and University Of Texas At Dallas.

Reference Paper : Energy Harvesting Using RF MEMS By:Yunhan Huang, Ravi Doraiswami, Michael Osterman, and Michael Pecht;Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD 20742, United States

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Introduction

• As early as 1979, microelectromechanical switches have been used to switch low frequency electrical signals.

• Switch designs have utilized cantilever, rotary and membrane topologies to achieve good performance at RF and microwave frequencies.

• Low loss, low power consumption and lack of intermodulation distortion, RF MEMS switches are an attractive alternative to traditional FET or p-i-n diode switches in applications where microsecond switching speed is sufficient.

• Capacitive membrane switches have shown excellent performance through 40GHz and for lifetimes in excess of 1 billion cycles, which allows us to employ them as energy harvesting devices.

• Proposed RF MEMS energy harvesting device is fabricated on a single chip, chip has a small form factor and an excellent integration and performance efficiency.

• Using discontinuous electrostatic charge transfer, allows ions to diffuse and redistribute more evenly and to scale to different sizes of the MEMS.

• These advantages leverage RF MEMS energy harvesting devices to achieve greater scalability and higher efficiency than conventional technologies.

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RF MEMS Design• The capacitive RF MEMS switch here is a co-planar waveguide (CPW) shunt

switch.• The RF MEMS switch operates as a digitally tunable capacitor with two states. • When the membrane is in the up position (switch-off), the signal line sees a small

value of parasitic capacitance, while when the membrane is pulled down by actuated voltage (switch-on), the signal line sees a high value capacitor shunted to ground.

• Operation of a RF MEMS switch & 1-D linear electromechanical model of RF MEMS switch:

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RF MEMS Design• The membrane is subjected to a distributed force across the entire membrane.• The spring constant:

• where E is the Young’s Modulus of the material, and w, t, and l are the width, thickness, and length of the membrane, respectively.

• When voltage is applied between the membrane and the signal line, an electrostatic force is induced on the membrane.

• In order to approximate this force, the membrane over the signal line is modeled as a parallel-plate capacitor.

• Although the actual capacitance is about 20-40% larger due to accumulated fields, this approximation is quite accurate for long, planar systems like the RF MEMS structure.

• Given that the width of the signal line is W, the parallel plate capacitance is

• where g is the height of the membrane above the signal line.4/20/2015

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RF MEMS Design• The electrostatic force induced as charge provides a capacitance, given by

• where V is the voltage applied between the membrane and the signal line.• The electrostatic force is evenly distributed across the section of the membrane

above the signal line. • Therefore, the appropriate spring constant can be used to determine the distance that

the membrane moves under the applied force. • The pull-down voltage can be found from:

• where Vp is pull down voltage, k is the spring constant of membrane, and g is the equivalent gap between membrane and signal line.

• When the RF MEMS is in its initial undisplaced position (switch-off), the gap consists of two dielectric materials, namely air and SiO2.

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RF MEMS Design• This equivalent gap is given by equation:

• the relative permittivity of silicon dioxide, is 3.7. The equivalent gap is found to be approximately 1.94 microns.

• By substituting the parameters listed in Table, modulus of aluminum, E=70 GPa and stiffness k=132.7N/m, the pull-down voltage to be: Vp=40V.

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RF MEMS Fabrication• The membrane is deposited over a sacrificial polymer layer that is released at the

end of the surface micromachining process.• After the sacrificial layer is released, the aluminum membrane is left suspended

over the dielectric layer. Its natural state is in the “up” or unactuated position.• When a sufficient DC electrical potential is applied between the membrane and

electrode, the membrane snapped down into the actuated state.• The metal membrane is fabricated using aluminum because of its high resistance to

fatigue and low electrical resistance.

• A 1-micron, nanoporous SiO2 dielectric isolation layer which separates the membrane and signal line is deposited using a Uniaxis PECD machine.

• The top and cross-sectional view of the RF MEMS.

The RF MEMS switch under microscope. 4/20/2015

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(1) Coplanar waveguide deposition and lithography.(2) Silicon dioxide and wet etching.(3) Pattern Sacrificial Photoresist.(4) Aluminum Deposition and Patterning of Aluminum Bridges. (5) Removal of Sacrificial Polymer.

RF MEMS Fabrication

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RF MEMS Characterization• The electromechanical and RF signal properties of the MEMS are characterized after

fabrication.• A ground-signal- ground (GSG) probe with a 250 micron pitch with appropriate bias tee and

a network analyzer were used to make time domain reflectometry (TDR) analysis of the RF MEMS to extract S11 insertion loss parameters and paracitics.

• The pull-down voltage required to change the capacitance of the switch is 35 volts. This value is less than the analytical calculation, but still on the same order of magnitude.

• The divergence between experimental data and the 1-D model exists because the model used the assumptions that there is no fringing electric field and that the membrane is a flat rigid plate and is subjected to the force across its entire area.

• The resonance frequency of RF MEMS is found to be around 2.5MHz.

The GSG probe was connected to a RF MEMS switch

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Energy Stored by RF MEMS• The novel design replaces the externally applied bias voltage with the charge

generated by a coating of the photosensitive material. • The electric charge is generated by photosensitizing dye; the charge generated is

stored between the gaps of the membrane and signal line of a RF MEMS structure.• By controlling the charging sequence, the electric charge can be stored in the RF

MEMS structures and then discharged as pulsed energy to an external load or rechargeable batteries.

• Thus the output voltage can be adjusted by controlling the on/off frequency of operation of the switch.

• Membrane overlap over the signal isolation layer plays an important role in determining the maximum energy that can be stored per area of the RF MEMS.

• The energy stored is determined by calculating the capacitance of the RF MEMS structure.

• The capacitance was derived: C=0.3 pF (switch off), C= 1 pF (switch on),

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Energy Stored by RF MEMS• For each RF MEMS of 150*200 μm, the charge stored by the can be found by

equation Q = C x V , where V is the voltage level generated by the photosensitive coating.

• Therefore, by using the pull-down voltage and switch-on capacitance, a maximum charge of 35 pC was found to be stored and discharged per cycle per switch.

(1) Diagram of the concept of novel device (2)The structure and operation of the novel energy harvester. By applying a photosensitive coating and transparent electrode, the electric charge can be generated and stored in the RF MEMS structure.

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Reliability Concerns• Mechanical failures such as creep and fatigue are not a big problem because the

vertical displacement of the center point of the membrane (~2 μm) is much smaller than its length (150 μm).

• RF MEMS is subjected to the charge and discharge of electrons generated by photosensitive material, the electrostatic discharge-induced failure (ESD) dominates the RF MEMS’ reliability issues.

• ESD results in charge injection and charge trapping in the interface between the insulation layer and the metal due to the ultrahigh electric strength.

• If RF MEMS is continuously exposed to an electrostatic discharge pulse, both its electromechanical properties (capacitance-voltage curve) and its RF properties (insertion loss, return loss) continue to shift until the point where the charge density reaches a critical value which eventually leads to the breakdown of the insulation layer.

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Conclusion• These devices are in high demand for use in wireless sensor networks, portable

electronics, etc. Unfortunately, there is very little literature available that deals with models for energy harvesting using RF MEMS.

• In order to achieve optimum charge and discharge, it is necessary to compute the membrane pull down voltage. The pull down voltage of the RF MEMS was found to be 35V with a resonant-frequency of 25 MHz .

• For every charge discharge cycle, it was found that 1pF of capacitance corresponds to the switch on capacitance.

• This procedure results in (2.18x10^8) electrons charged and discharged in each cycle. The effect of ESD on the performance and lifetime of the device were also evaluated.

• The choice of the right photosensitive material will result in maximum energy conversion.

• Future work will focus on optimizing RF MEMS design to store electronic charges from constant and pulsed energy charge inputs.

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Q/A

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