EEL6935 Advanced MEMS (Spring 2005) Wireless ... · EEL6935 Advanced MEMS 2005 H. Xie 1 ... Digital (dual-mode, dual-band) Analog ... used without deteriorating microwave performance

EEL6935 Advanced MEMS 2005 H. Xie 1

Lecture 13Agenda:

RF MEMS: Introduction

3/7/2005

EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie


Wireless Communications

4G3G2G1G

>2Mb/s to moving vehicles

~384kb/s to moving vehicles

Macro/micro/picocell

Macro cell

GPS (globe positioning system), “messages in space”

Higher speed, improved voice/multimedia mobility, internet

Voice + dataVoice

UMTS (Universal Mobile Telecommunications System)

GSM (Global System for Mobile Communication)

900 MHz

Multistandard + multiband

Multimode, multiband

Digital (dual-mode, dual-band)

Analog (single-band)


Simplified Transceiver Architecture


A 3-Band MEMS Wireless System

G. Rebeiz, RF MEMS: Theory, Design and Technology, John Wiley and Sons, Inc., 2003.


MEMS-Based Wireless Applications

High-Q Passives• Inductors

• Varactors

• Transmission lines

• Switches

• Resonators

Circuits/Systems• Oscillators

• Mixers

• Power amplifiers

• Phase shifters

• Filters

• Switch matrices

• Transceivers


RF MEMS Application Areas

RF MEMS Switches and Varactors• Low loss and low power consumption (0.1 dB up to 120 GHz)• High Isolation (> 30 dB up to 100 GHz)• Potential for low cost fabrication (no epi layers, no 0.15 um litho.)• Very high Q possible with varactors (and large capacitance range)• Very low intermodulation products (> 60 dBm)

RF MEMS Inductors• Very high Q possible (Q > 50 at 1-5 GHz)• Built using MEMS processes but nothing moves (not tunable)

RF MEMS Filters• Very small size possible (1000x reduction in size in element)• Extremely high-Q (5,000 to 50,000)• Compatible with CMOS and potential for low cost


Comparison of Different Switches

Isol

atio

n (d

B)

Insertion Loss (dB)

MEMS: MEMS switchesPIN: GaAs P-I-N DiodesFET: GaAs Transistors

MEMS switches•Good isolation•Low insertion loss



Example RF MEMS Devices

Capacitive Switch (Lincoln Lab)

Pull-down

Metal-contact Switch (Analog Devices)

MEMS Oscillator (U-Michigan)MEMS Inductor (KAIST)


RF Basics

Skin Effect

Transmission Line (t-line)

Microstrip Line

Coplanar Waveguide Transmission Line (CPW)

Smith Chart

Quality Factor


Skin Depth

If a conductor with nonzero resistance is present in a propagating electromagnetic field, the field will penetrate the conductor. The penetration depth depends on the resistivity of the conductor and the frequency of the EM wave.

Skin Depth is defined as the distance at which the field is decayed to e-1 = 36.8% of its value at the air-conductor interface.

1 1f

δπµ σ

= ⋅

σ: conductivity of the conductor; µ: permeability of the medium; and f: signal frequency

Example:σ(Al) = 37.2 MS/m; σ(Cu) = 58 MS/m; Thus, at 2GHz, δ(Al)=1.85µm; δ(Cu)=1.47µm


Skin Depth

Surface resistivity (Ω per surface area)

1s

fR πµδσ σ

= =

Energy Confinement

If the conductor is thin, some energy escapes.

Conductor thickness ∆

∆ = δ 36.8%∆ = 2δ 13.5%∆ = 4δ 1.8%

δ 2δ 3δ

E

x


Transmission Line

http://www.tmeg.com

A signal path connecting a source to a load


Transmission Line

http://www.tmeg.com

Characteristic Impedance

Circuit Model

0forward

forward

VZ

I=


Transmission Line

http://www.tmeg.com

Characteristic Impedance

Free space 00

0

120Zµ

πε

= = Ω

Low-resistive lines0 /Z L C=

Parallel Wires

0276 2log

r

DZdε

=

Coaxial Cable

0138 log

r

DZdε

=


Transmission Line

http://www.tmeg.com

Reflection Coefficient

0

0

reflected load

forward load

V Z ZV Z Z

−Γ = =

+

• Impedance matching is needed to reduce reflection power.

• When the load impedance is greater than the transmission line impedance, a power reflection occurs but no phase shift.

• When the load impedance is smaller than the transmission line impedance, a power reflection also occurs with 180°phase shift.


Smith ChartReflection Coefficient

0

0

reflected load

forward load

V Z ZV Z Z

−Γ = =

+

0 0

0 0

1 11 1

L L L

L L L

Z Z Z Z zZ Z Z Z z

− − −Γ = =

+ + +

11Lz

+ Γ=

− Γ

11

r iL L

r i

jr jxj

+ Γ + Γ+ =

− Γ − Γresistance reactance

Real partImaginary part

rΓ

iΓ


Smith ChartReal part (resistance)

( )

2 2

2 2

11

r iL

r i

r − Γ − Γ=

− Γ + Γ

Imaginary part (reactance)

( )2 2

21

iL

r i

x Γ=

− Γ + Γ

2 2

2 11 1

Lr i

L L

rr r

Γ − + Γ = + +

( )2 2

2 1 11r iL Lx x

Γ − + Γ − =

rΓ

iΓ iΓ

rΓ

Lx

rL

Both imaginary and real parts can be expressed on the same chart EEL6935 Advanced MEMS 2005 H. Xie 18

Smith Chart

The coexistence of complex-impedance and complex-reflection-coefficient information on a single graph allows you to easily determine how values of one affect the other. You can find what complex reflection coefficient would result from connecting a particular load impedance to a system having a given characteristic impedance.

Overlay the two circle clusters on a single 2D plot, resulting in a SMITH CHART.

0iΓ =

rL

xL

0rΓ =


Smith Chart

Example:

Rick Nelson, Test & Measurement World, July 2001http://www.web-ee.com/primers/files/SmithCharts/smith_charts.htm

1 2Z j= +

0.5 0.50.707 / 45

jΓ = += °


Microstrip Line

Conventional Microstrip

Dielectric layer(hundreds microns thick)

w w

Thin-film microstrip lines (TFMSL) are miniaturized microstrip lines (a signal conductor, a dielectric and a ground conductor), located on top of the silicon substrate. The ground metallization shields the line from the silicon substrate effects; therefore low resistivity silicon substrates can be used without deteriorating microwave performance.

Thin-Film Microstrip

Si substrate

Dielectric layer (e.g., BCB)

Small dimensions increase conductor loss. Finite metal conductivity and internal inductance must be taken into account.


Thin-Film Microstrip Line

Equivalent Circuit Modelw

tht

εrR L

C GL: Inductance per unit lengthC: capacitance per unit lengthR: resistance per unit length G: conductance per unit length

• co: free-space light velocity; εr,eff: effective relative dielectric constant• ZLO: characteristic impedance for lossless case with t=0 • weq,0: equivalent signal conductor width without dielectric substrate, which

takes into account the finite thickness of the signal line• G accounts for the dielectric loss of the substrate, described by the dielectric

loss tangent, tan(δε).

,

0 0 0( )r eff

L eq

Cc Z w

ε=

⋅( ),

0 0 0

1tan

1 ( )r eff r

r L eq

G wc Z w ε

ε εδ

ε−

= ⋅ ⋅ ⋅− ⋅


Coplanar Waveguide

• Ease of parallel and series insertion of both active and passive components• High circuit density• The traces of CPW transmission lines can be changed to match component

lead widths while keeping the characteristic impedance constant

S w S


Coplanar Waveguide

, 0( ( ), , )r effw S S H Zξ ε= ⋅

T. Deng, “CAD Model for Coplanar Waveguide Synthesis,” IEEE Transactions On Microwave Theory And Techniques, Vol. 44, No. 10, October 1996

S w S

Hεr

• CPW Trace Width Synthesis

, 0( ( ), , )r effS w w H Zξ ε= ⋅

( ), , 0( , , , )r eff r eff rS H Z Sε ε ε=

( ), , 0( , , , )r eff r eff rS H Z Sε ε ε=

where

CPW Design

• CPW Trace Spacing Synthesis

where


Coplanar Waveguide

T. Deng, “CAD Model for Coplanar Waveguide Synthesis,” IEEE Transactions On Microwave Theory And Techniques, Vol. 44, No. 10, October 1996

( )

( )

0 0

0 0, ,

00

, 0 1

0,

0

00

1 exp exp 14 4 4

2 1( , )

1 exp 2 18

2 1

r eff r eff

rr eff

r eff

r

Z Z

for Z

Z

Z

for Z

η ηπ πε ε

η

εξ ε

ηπ ε

η

ε

−

+ − −

<+

=

− ≥ +

where the free-space impedance 0 377η = Ω


Quality Factor

De Los Santos, RF MEMS Circuit Design, Artech House, 2002

maximum instantaneous energy stored2energy dissipated per cycle

Q π= ×

Resonant Circuits,

0 resonant frequencybandwidth

fQB

= =

0capacitor

CQG

ω=

Capacitor with a shunt parasitic conductance G

Inductor with a series parasitic resistance R

0inductor

LQ

Rω

=

Presence of an external load RL

1 1 1

Ltotal unloaded RQ Q Q= +

0

0L

LR

L

L RQ orR L

ωω

=

series parallel


References


De Los Santos, RF MEMS Circuit Design, Artech House, 2002.

Rick Nelson, How does a Smith chart work?, Test & Measurement World, July 2001. Also available at http://privatewww.essex.ac.uk/~mpthak/smith_charts.htm

Transmission lines, http://www.tmeg.com

Documents

EEL6935 Advanced MEMS (Spring 2005) Wireless ... · EEL6935 Advanced MEMS 2005 H. Xie 1 ... Digital (dual-mode, dual-band) Analog ... used without deteriorating microwave performance