smart antenna using mems

8/11/2019 Smart Antenna Using MEMS

1/5

Smart Antenna using -MEMS

Georgina Rosas', Roberto Murphy', Wilfrido Moreno

a

Department ofElectronics, National Institute ofAstrophysics, Optics and

Electronics, 72840, Puebla, MEXICO

b

Department of Electrical Engineering, University of South Florida, 33620

Tampa, Florida

[email protected], [email protected], [email protected]

bstract -

This article presents the design of a novel and

compact coplanar antenna using Metamaterials (MTM) and

Micro Electro Mechanical Systems (MEMS). The antenna is

based on coplanar waveguide (CPW) technology; therefore,

the signal and ground are on the same plane, presenting

lower dielectric losses and high signal integrity. The

designed antenna can be tuned in the frequency range from

5.3 to 5.8 GHz by MEMS capacitors, and it is useful for

wireless communications applications, especially beam

steering systems. Finally, the design, 3D full-wave

simulations and MEMS simulations are presented.

n ex

erms

-

Metamaterial (MTM), Transmission Line

(TL), Coplanar (CPW), Composite Right/Left Handed

(CRLH), Micro-Electro-Mechanical Systems (MEMS).

Fig. 1. Proposed antenna using MTM and MEMS

I. INTRODUCTION

II

-MEMS ANTENNA

Nowadays, with the advent

of

RF technology, electronic

products demand more functions and higher performance,

reduced dimensions and higher speeds, and higher output

at a lower cost. The state-of-the-art in this technology

requires the fusion of emerging technologies such as

Metamaterials (MTM) and Micro Electro Mechanical

Systems (MEMS)

[1]-[4].

Together, they can

revolutionize electronics by providing very small and

reliable smart circuits at a minimal cost.

Metamaterials are new artificial materials, that present

unique electromagnetic properties, which are controllable

and are not present in any known natural environment.

Research in this field opens up new ways for innovation

in communications, based on original designs that exploit

singular properties such as simultaneously negative

permittivity e and permeability u , with antiparallel

group velocity (v

g

and phase velocity (v

p

,

and negative

refractive index (n) [5]-[6].

The proposed smart antenna consists

of

a CRLH-MTM

structure with MEMS capacitors and four inductors

connected to ground, as displayed in Figure I. In this

paper, a particular design, determining all quantities of

interest is presented.

The parameters

L

R

,

C

R

, LL,

C

L

of

a composite

right/left handed Metamaterial (CRLH-MTM) structure

are determined using the equations developed by [3] and

following the methodology described in [7]. Using these,

the values listed in Figure

2

are obtained:

LH

Fig. 2. Circuit Equivalent of a basic cell CRLH TL-MTM with

L

R

= 1.437 nH, C

L

= 0.524 pF, C

R=57 7

pF, LL= 1.31nH.

978-1-4244-6689-0/10/ 26 .00 20 10 IEEE


2/5

where ff is the effective stiffness and go is the initial

gap, and all the other variables defined previously.

Figure 3 shows the displacement of the movable

electrode and the associated capacitance variation as a

function of applied voltage, as given by 2). Furthermore,

a detailed electric-mechanical analysis has been

performed using Coventorware [ ]

where eo is the free space permittivity,

e

is the relative

permittivity, W is the design width W=596/lm , L the

length L=596/lm and d is the gap between electrodes

d=1-+5/lm .

Using these values in Equation I , the capacitance

varies from 0.69 to 1.57 pF, while the gap between plates

can vary up to 4 microns.

The structure

of

the variable capacitor is considered

MEMS s techniques such as: dimples, holes, and some

others that support functional stability, operation and a

proper release of the structure [8-10].

a real MEMS capacitor structure, the situation is

more complicated than a simple lumped design. However,

it happens to be a mechanic and elastic element, therefore

it has instability points determined by the pull-in voltage;

in electrostatic MEMS this is called the pull-in instability.

The pull-in voltage equation can be defined as: [10]:

2

1.8

1.6

iL

1.4

o

1.2

Q)

o

1

2

0.8 = =-0

o 0.6

0.4

0.2

0

0 5

5

4.5

-=ellc 4 0

3

=-- -

=

2.5

2

1.5

1

0.5

o

10 15 20 25 30

Voltage [V]

To avoid collapse, dimples were defmed for the second

level. Furthermore, these give the structure a more robust

mechanical stability. The dimples were designed using

the criteria for SUMMIT V [12] and 3D full-wave

electromagnetic field simulations for radio frequency

RF), to obtain optimal results.

Figure 4 a) ilustrates a schematic of the MEMS

capacitor with all integrated elements dimples and

holes).

Figure 4 b) shows stress simulations along the y axis.

The different shades in Figure 4 b) indicate the

distribution of mechanical stresses along the length of the

positive mobile) electrode.

The MEMS capacitor was simulated under electrostatic

actuation, as is shown in Figure 4 c). The electrostatic

analysis was performed sweeping the applied voltage

from 0 to 24V, obtaining a variation of 0 to 3 microns

approximately. The effective stiffuess eg} can be

obtained from for mechanic-electrostatic analysis. This

has been extract from Coventorware simulations, and

also, was calculated in accordance to [9] and [10].

Fig. 3. Displacement of mobile electrode and capacitance as a

function of applied voltage.

a)

I

2)

A. MEMS Capacitor

The MEMS capacitor consists of two parallel plates

two electrodes, one mobile-positive and one fixed

negative), whose capacitance can be varied using the

electrostatic principle.

The MEMS capacitor can be fabricated using the

surface micromachining technique, and it can be

integrated in a CMOS chip. The process under

consideration here is composed of four materials and five

levels of masks on a silicon wafer 100 orientation,

p>4000 Q.cm) acting as the mechanical support.

Titanium Ti) and Gold Au) are used as structural

materials, with one suspended level for mechanical

structures, and SU8 is used as the sacrificial material.

Silicon dioxide SiOz and BenzoCyclobutene BCB are

used as dielectrics.

The capacitance between parallel plates is determined

by:


3/5

b)

c)

Fig. 4. Coventorware simulations a) Sketch of MEMS

capacitor with dimples and holes, b) MEMS capacitor stress

simulations along the

axis, and c) Simulations along the z

axis of a parallel-plate capacitor with an applied voltage of24V.

III. SIMULATION RESULTS

In this paper, the smart antenna was simulated in HFSS

vlO from Ansoft Corporation using it as a 3D full-wave

electromagnetic field solver, using a high resistivity

silicon substrate and a gold conducting layer with a

thickness

of

3

J m

to reduce losses. The layout for the

antenna only without an integrated DC bias line) using

an MTM-MEMS CRLH-TL basic cell

of

dimensions

1.397mm X 2.022mm, is shown in Fig. 5 a). It has a

1/37.02 0 x 1/25.5 0 footprint and is among the smallest

in the literature, where

0

is the free space wavelength.

Figure 5 b) shows the details of the MEMS capacitor as a

tuning element. The dimple size is 15 x 15

J m

for the

mobile electrode and is of 25 x 25 J m for the support

structure of the fixed electrode . The dimple base is

isolated. The hole size is 15 x 15 J m . Each MEMS

capacitor has ten holes.

The antenna, consisting

of

two double MEMS

capacitors and four inductors connected to ground,

presents the following characteristics: The capacitor uses

an area of

596J 1m

x

596J 1m

and presents a nominal value

of 1.048pF. The spiral inductor dimension are 200f lm X

200J 1m , with a strip width of 20J 1m and a line spacing of

10J 1m, and provides an inductance value

of

1.31nH. This

structure was designed for a central frequency of 5.8GHz.

The coplanar line CPW) is designed for a characteristic

impedance

of

500 The line spacing is 50J 1m and the

signal line width is

78J 1m

.

Figure 6 a) shows the simulation results for the SII

dispersion parameter. The simulated return losses at the

point m, 5.8GHz is 16 .10dB with a 4 bandwidth

efined by

ISIII

lOdB). This full wave simulation

considers the design aspects for the MEMS capacitors,

such as dimples and holes. This simulation shows greater

losses than the previous simulation shown in Figure 8.

Figure 6b shows the radiation pattern for the antenna

resonating at 5.8GHz.

a)

Mobile Electrode

Holes

b)

Fig. 5. a) Layout of the smart antena of a cell MTM and b)

Details distribuition of dimples and holes.

The dispersion curve of the MTM-MEMS cell antenna

is plotted in Figure 7 for a transition frequency of

5.8GHz, with Beta

=

O

It

shows the typical characteristics

of

a structured metamaterial; the negative sign of the

slope demonstrates the existence of the negative phase

velocity.

The dispersion curve is obtained from a zero-resonator

structure using the unwrapped phase ofS

21

The CPW RF choke is designed for a central frequency

of

5.8GHz and has a length

of

2.58mm, and it is loaded

with a capacitance ofO.845pF, as shown in Figure 9 a). It


4/5

/

/

RH

l .. /

1

:

Transition

-

I

IFrequency

-

LHt = [

I -

I I

I I I I I I I I

20

18

16

14

N

I 12

9

10

8

6

4

2

o

is implemented on chip in the same fabrication processes

that the smart antenna. The CPW transmision line is of

the meander type, with a length of 90 electric degrees,

which makes it possible to achieve high impedance ( >

1 500n

at 5.3 to 5.8GHz) bias line which does not impact

on the microwave performance

of

the device, as shown in

Figure 9(b). The coplanar bias-T line is designed to have

a characteristic impedance of 50n . The line spacing is

50 m, and the signal line width is 13 m.

00 0

o

20 40 60 80 100 120 140 160 180 200

beta

rP qu

ocv[ ltl}

5-

-s oo

iir1500

.20.00

I

I

v.

r

L a. lU .UU lL. -J

a . U

-J

LU.

Fig. 7. Dispersion Diagram.

It

shows a frequency transition at

5.8GHz and a beta =

0

-10-

;:

-15-

(a)

Ui

-20-

o

I I I I I I I I I

2 4 6 8 10 12 14 16 18 20

a i n o t a l

1.0737e 00 2

1 .easse-eez

9, 4018e

12J3

8, 7343e-e0S

8.

066g

e 12J3

7, S9gSe e0S

6 .732

1e ee

S

6

.0647e 003

5 ,S973 e 0e 3

If. 7298e-C0S

If 06

24e 003

3 .S9S0e -003

2 ,7276e -003

2.060

2e

00 3

1 . S927e -003

7.2533e 004

5 , 7 9

11e 0

05

(b)

Fig. 6. (a) Simulated return loss of the antena with the following

resonance frequencies m,

=

5.8 GHz, and (b) Radiation pattern

for the antenna resonating at 5.8 GHz.

Figure 8 shows the simulation results for the 8

11

dispersion parameter, considering the change in

capacitance. The simulated return losses at points m, =

5.3GHz and m2

=

5.8GHz are 13 .747 dB and - 20.15dB,

respectively. This implies that the response of the antenna

is very good at this range. The behavior can be improved

overall by increasing the number of cells.

freq GHz

Fig.8. Simulated return loss of the smart antena with the

following resonance frequencies m,

=

5.3GHz,

z=

5.8GHz.

U

(a)


5/5

Fr

eq oency

(GHzj

000

Fr

eq oency

lGHzj

ACKNOWLEDGEMENT

REFERENCES

its on-going support in the fabrication process

of

the

device.

[I]

C. Caloz and T.

Itoh,

Applicationof the transmissionline

theoryofleft-handed (LH) materialsto the realizationof a

microstrip LH transmission line, in Proc. IEEE-AP-S

USNCIURSI National Radio ScienceMeeting, vol. 2, San

Antonio, TX, pp.412-415, June 2002.

[2] C. Caloz and T.

Itoh,

Novel microwave devices and

structures based on the transmission line approach of

meta-materials, in IEEE-MTT InCI Symp., vol. I,

Philadelphia,PA, pp. 195-198, June2003.

[3] Christophe Caloz and Tatsuo Itoh , Electromagnetic

Metamaterials: TransmissionLine Theory and Microwave

Applications , Copyright2006, JohnWiley Sons, Inc.

[4] Wei Tong, Zhirun Hu, Hong Chua, Philip Curtis, et al

Left-Handed Metamaterial Coplanar Waveguide

Components and Circuits in GaAs MMIC Technology ,

IEEETransactionson MicrowaveTheory andTechniques,

Vol 55, No.8, August2007.

[5] V.G. Veselago. The electrodynamics of substanceswith

simultaneouslynegativevaluesof

e

and

u,

SovietPhysics

Uspekhi, vol. 10, no. 4, pp. 509-514, Jan., Feb. 1968.

[6] R. A Shelby, D. R. Smith, and S. Schultz. Experimental

verification of a negative index of refraction, Science,

vol. 292, pp. 77-79, April 2001.

[7] G. Rosas, R. Murphy and A Corona, Metamaterial

MEMS Reconfigurable Transmission Line , XV

Workshop Iberchip, Buenos Aires-Argentina, March

2009.

[8] A Dec. K. Suyama, Micromachined Electro

Mechanically Tunable capacitors , IEEE Transactions

IEEETransactionson MicrowaveTheory andTechniques,

Vol46, No.12, December 1998.

[9] S. Pamidighantam, R. Puers, et ai, Pull-in Analysis of

Electrostatically Actuated Beam Structures with Fixed

Fixed and Fixed-Free end Conditions ,

1.

of

Micromechanics and Mircoengineering 12 , pp. 458-464,

Jun(2002).

[10] David A Czaplewski, Christopher W. Dyck, A Soft

Landing Waveformfor Actuationof a Single-PoleSingle-

Throw Ohmic RF MEMS Switch , Journal

Microelectromechanical Systems,

Vo1 15

No.6,

December2006.

[I 1] Coventorware,

http://www.coventor.com/coventorware.html

2] SandiaNational Laboratories,

http://www.mems.sandia.gov/

I

I

- Impedance

I

I

f

I

-

I

I

LO

ow

o

W

Rkturn Ldss

ow

c r OU

Fig. 9. (a) Layoutof DC

bias-T

line for the antenna and (b) SII

and Zin simulationresults of a CPW bias-T line.

IV. CONCLUSION

This paper has shown the design

of

a Smart MTM

antenna, using MEMS capacitors and considering the

most

important factors in order to achieve mechanical

s tabi li ty in the antenna performance using dimples and

holes. Additionally, the MEMS capacitor is a radiant

element, as well as a MTM structure. In the fabrication

process, the metal has been considered thicker in order to

avoid conductor losses by the skin effect at low

frequencies. This work has shown that MTM MEMS

technologies allow to simultaneously add variabi li ty,

small size (much less than -/4), broader bandwidth, low

losses and design flexibility. These simulations encourage

the fabrication

of

the structure, which will be undertaken

in the near future.

The authors wish to acknowledge CONACyT Mexico,

for the partial support of this work through Grant 83774

Y. Georgina Rosas also thanks CONACyT for the

scholarship to undertake doctoral studies, Number

102735 and for the support in carrying out this research.

Special recognition to the Nanomaterials

Nanomanufacturing Research Center (NNRC) at USF for

oog

Oog

2500

-2000

I

o

lD

in

150000

'000

500

200000

N

;

50000

smart antenna using mems

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