introduction to rfmems (microelectromechanical...

Microsystems Integration Laboratory

Introduction to RFMEMS (Microelectromechanical Systems)

Yu-Ting Cheng

Microsystems Integration Laboratory (MIL)

Department of Electronics Engineering

National Chiao Tung University


Rf mems components and technology Antennas:


dx

dxdLzdx

zdxdL

ddx

dzdL

x

)(


Capacitive Transducers

• Parallel-plate capacitor:

dg

dCV

A

QW

A

gQ

C

QW(Q)

εA

QECVQdqqVW

g

A

q

2222

0

2

1

2Force,

22 :Energy Potential Stored

:Field Electrical, :Charge ,)( :Energy

C :eCapacitanc

Fixed plate

W g

V

I

L

Movable plate

Z •We can store energy in the capacitor by either fixing the gap and changing the charge Q, or by fixing the charge and changing the gap.


Two-Port Electro-Mechanical Capacitor

• The capacitor has two port as shown in the following figure:

• Electrical parameters: voltage V and current I • Mechanical parameters: Force F and velocity v

W (Q, d)

Capacitor

F V

I=-dQ/dt v=dg/dt

A

QgV

A

QW

A

gQ

C

QW(Q)

2Force

,22

2

22


Sensitivity

g

W

S

S

vs.

dx

dC

CC

SS

dx

dCS

ca

cg

ccr

c

:area overlap varying gap varyingof Comparison

1:ysensitivit Normalized

:ySensitivit

00

From Prof. K.

Najafi’s MEMS Slides


Mechanical-Electrical Equivalent

From Prof. K.

Najafi’s MEMS Slides

Mechanical Electrical

Variable Velocity

Force

Current

Voltage

Lumped Elements Compliance (1/k)

Mass (m)

Damper (c, or b)

Capacitor (C)

Inductor (L)

Resistance (R)


Factors in The Design

• Fringing Fields: around the perimeter of the electrodes in a capacitive device, the electrical field lines have to bend which are called fringing fields. The fringing fields will make the actual area between the two plates appear larger.

• Air Damping: for many capacitive transducers, we see limitation on bandwidth caused by a phenomenon called air damping. Most of these sensors operate at atmospheric pressures, and when the movable electrode moves, it has to push or squeeze air in and out of the very small gap in between electrodes which is called” squeeze film damping”. There is also viscous drag damping when two plates move parallel to another.

)ln(2

g

WW

g

AC


Squeeze Film and Viscous Drag Damping

• Squeeze film damping

• Viscous Drag Damping

)./101.81 :(air fluidambient ofViscosity :

,~

5- smkg

MKA

dQ

air moving out of the gap

Plate moving down

separation Gap :

Length :

width: where

)()/( 3

g

L

W

Lg

WLWfb


RF MEMS Type Switch

Series contact switch:

contacts.between separation physical theis and

capacitor, theof area effective : voltage,applied :

ratio. sPoisson' and modulus sYoung' theare and

ly.respective beam, theof

thicknessand length, width, thware and , , where

2

)1(6

2

2

0

3

332

g

AV

E

tLW

g

AVF

Wt

L

E

LF

Varadan et al., RF

MEMS and Their

Applications , Wiley


Varadan et al., RF MEMS and Their



Hysteresis Behavior




MEMS Switch Factors

MEMS switches, in general, work with very high driving

voltage which is about several tens voltage. Such high

actuation voltage make the switches far beyond the

compatibility of standard IC technology because for RF

applications and microelectronic systems the voltage

should be around 5V.

In order to achieve the goal, several approaches have

been implemented for the design of the RF MEMS

switches: (a)increasing the area of actuation (b)

decreasing the gap between the switch and the bottom

electrode, ( c ) designing the structure with a low spring

constant.


Flexible Structural Design

Varadan et al., RF MEMS

and Their Applications ,

Wiley


Rf mems components and technology SP3T switch


RF MEMS Capacitive Switches

• Voltage actuation For voltage actuation, a dc potential is applied between the bridge and the base

electrode. The force downward on a charged beam above a ground plane is

proportional to the square of the electric field E on the beam.

V: applied voltage

g: the height of the beam above the ground plane

go: is the undeflected height

A : the area

K: spring constant include the beam thickness,

length, and width, the Young's modulus, residual

stress



• Voltage actuation



• Capacitance Ratio


RF MEMS Switch

Top view of the NTU capacitive shunt

switch

Cross section


RF MEMS Capacitors

Feature

• Remove lossy substrate from

beneath

• Tunable with movable,metallic

MEMS structure

Characteristics

• High quality factor (Q)

• Tunable

Suspended capacitor

Suspended metal-insulator-metal capacitor


RF MEMS Capacitors

Top view of the the three-plate polysilicon MEMS varactor

Cross section


High Performance Capacitors

A metal-insulator-metal (MIM) capacitor fabricated on a suspended

membrane shows that the best Q of a 2.6pF capacitor exceeds 100 at

2GHz as compared wit the same capacitor fabricated directly on silicon

which has a Q less than 10.

Varadan et al., RF

MEMS and Their



MEMS Gap-Tuning Capacitor

Structure: the electrodes are 200um by 200um with2 um by 2um holes

spaced 10um apart.

Performance: Q~62. The capacitance varied from 2.11 to 2.46 pF when

applied voltage changed from 0 to 5.5.V, which corresponds to a tuning

range of 16%

Varadan et al., RF

MEMS and Their



Fundamentals of MEMS Gap-Tuning Capacitor

Varadan et al., RF

MEMS and Their



MESA (MicroElevator by Self-Assembly) Capacitor

The 250x250 um2 polysilicon plate is raised above the substrate by four

300um long side supports which are controlled by microactuators. The

capacitance is changed from 500fF to 20fF when the suspended

electrode is raised by 250um.

Varadan et al., RF

MEMS and Their



MEMS Area-Tuning Capacitors

The variable capacitor provides

a capacitance change from

0.035pF to 0.1pF with bias

voltages ranging from 80 to 200V.

The capacitor provides

continuous tuning range of at

least 200% or 3:1 tuning ratio.

Varadan et al., RF

MEMS and Their



RF MEMS Capacitors

Top view of the University of Illinois Wide-tuning-range

varactor

Cross section


PZT (PizeoElectric-actuator) Tuning Capacitor

A MEMS capacitor integrating

with pizeoelectric actuator has

advantages such as low driving

voltages and linear tuning

capacitance

The PZT actuators are fabricated

on silicon substrate and are diced

and bonded to TML on a quartz

substrate using flip-chip bonding

technology.

Low driving voltage ~6V

Varadan et al., RF

MEMS and Their



Planar Inductors

High Q’s (> 30) RF components

are required for high frequency

selectivity in communication

systems especially for the high-

performance low power RF

transceivers.

For the compactness requirement,

on-chip planar inductor and diode

capacitor design becomes the

major approach in the fabrication of

RF front end transceivers.

In general, the Qs of planar spiral

inductors or junction dioide

capacitors are only of the order of

low 10s at higher frequencies.

inductance mutual the:

segments.straight theall

of inductance self theof sum the:

.inductance total the:

0

0

M

L

L

MLL

T

T

Varadan et al., RF

MEMS and Their



Cont.

Varadan et al., RF

MEMS and Their



Cont.

The quality factor Q of an inductor is written as Q=L/R, where is the

operating frequency.

The separation between lines should be as small as possible.

The circular spiral inductor has a shorter conductor then square spiral

and the Q is about 10% higher than that of a square spiral having the

same value of d0

Higher Q can be achieved with increasing the number of turns per unit

area; however, it also lowers the self-resonance frequency as a result of

the increase in capacitance

To avoid parasitic effects, the maximum diameter of the inductor should

be less than /30.


Lumped Model of An Planar Spiral

Varadan et al., RF

MEMS and Their



Substrate Effect

Simulation Results:

Varadan et al., RF

MEMS and Their



Varadan et al., RF

MEMS and Their



Air Gap Design

Varadan et al., RF

MEMS and Their



Spiral Inductor with Air Gap

Varadan et al., RF

MEMS and Their



Resistance Effect


MEMS Type Inductor Design

Varadan et al., RF

MEMS and Their



Cont.

Varadan et al., RF

MEMS and Their



Summary

Varadan et al., RF MEMS and Their Applications , Wiley


Cont.




Cont.

Varadan et al., RF MEMS and

Their Applications , Wiley


Evolution of MEMS Type Capacitors




Filters

Low-pass filter, high-pass filter, bandpass filter, and bandstop filter are

very important components in RF circuit which are designed to accept or

reject frequencies containing desired information.

Varadan et al., RF

MEMS and Their



The Micromachined Suspended Inductor and The Reliability Issue


The Process Flow of The Optimum Design

Un-Deformed Spiral Inductor

ANSYS Simulator for Mechanical Analysis

Node Solution and Node Coordinate

Deformed Spiral Inductor with Node Coordinate

Re-building the Deformed Spiral Inductor in HFSS Simulator

Equivalent Circuit Model Parameters Extraction


The Inductance Variation of the Suspended Inductor Due to the Accelerative

Disturbance

•Morphology deformation would result in large inductance variation


The Suspended Inductor with the Blanket Membrane Supporting


The Reliability Issue of The Blanket Membrane Inductor


The Fields Analysis and The Mechanical Analysis of the Micromachined Inductor

The surrounding magnetic

fields

Orthogonal Magnetic Fileds

Weak Magnetic Coupling

Parallel Magnetic Fields

Strong Magnetic Coupling

The surrounding magnetic

fields


The Thermal Stress Analysis of the Optimized Micromachined Inductor


The Fabrication Procedure of the Optimized Micromachined Inductor

The proposed optimum inductor.

The process flow of the optimum inductor.


Result & Discussion -Temperature Variation

The inductance

variation is

increased with

the increasing

frequency.

The increasing

trend of the

inductance

variation is

shifted to the

higher

frequency.

Due to the

unstable

suspended

structure and the

complex motion,

the inductance

variation is not

related with

frequency directly.

The thermal stress can

be released by the

corner region

The deformation can be

shifted to the corner.

Low restrictive region


The Comparison of the Performance Between the Suspended, Blanket Membrane

and Cross Membrane Inductors


The Modeling of Micromachined Inductor

Ls Rs

Cs

Ls : physics-based closed-form expression [1]

Cs : Distributed capacitance model (DCM) [2]

Rs :

depthskintheist

w

lRs

,

exp1

freq (2.000GHz to 12.00GHz)

S(2

1,2

1)

S(2

1,2

2)

S(2

5,2

5)

S(2

5,2

6)

S(2

4,2

4)

S(2

4,2

3)

3 4 5 6 7 8 9 10 112 12

-10

-8

-6

-4

-2

-12

0

freq, GHz

dB

(S(2

1,2

1))

dB

(S(2

5,2

5))

dB

(S(2

4,2

4))

dB[S11]

Simulated by equivalent model

Simulated by HFSS

Measured result

Compare the model, simulation, and measured results

of inductor @2.3nH for frequency 2~12 GHz.

dB

S11

S11

S12

Complete optimized design


Physics-Based Closed-Form Inductance Expression

l

nd

l

nd

nd

l

nd

lnn

twn

llLp

441

441ln147.02.0ln

2

22

0

1231123

i

ii

Nn

NNnswd

Distributed Capacitance Model (DCM)

1

1

2)]1(1[4

1n

k

kmms kdkdlCC

kk hhhhkd 121

kkk

k lllllll

h 121, .

1ln

0)(

tw

sC

effr

mm

where w is the metal width, s is the spacing between segments, n is the number of turns,

Ni is the integer part of n, and l is the total length of inductor

ococps LLLL , is the contribution of inductance of the bridge.

, the capacitance per unit length between adjacent metal tracks.


Kramers Kronig Relations

• Anomalous Dispersion

• Standing Wave B.C.’s

• Compton Scattering

• Inductance

'

'Im''

2Re

0 220

dP

'

'Re'

2Im

0 22

0

dP

rh

qrE ˆ

12csc2

4sin8

4

1)(

22

0

))(Re1(02

dI

dvBHL


• Electron kinetic energy in sample metal

Determination of Self-Resonant Frequency


• Scattering potential in corners

Determination of Self-Resonant frequency

inductor) (circular for , 7.24

24

4

for , 2

csc8

4sin8

4

2

3/2

2

2

22

32

2

eff

effC

NV

q

NV

q

E

• Self-resonant frequency

CLr ENE


• Frequency-depended form of inductance

Determination of Inductance

1

2

0

3

0 rec 1exp 2

TkAn

lnL

B

rrtotale

1

2

0

3

0 oct 1exp 5

3

TkAn

lnL

B

rrtotale

1

2

0

3

0 cir 1exp

TkAn

lnL

B

rrtotale


Model Validation

Comparisons (n=3.5)

SRF based on the model (GHz)

SRF based on HFSS (GHz)

Rectangular 23.9 22.9

Octagonal 24.9 23.6

Circular 25.8 24.6

Number of Turns 1.5 2.5 3.5 4.5 5.5

ωr for HFSS (GHz) 39.5 27.1 22.9 20.6 19.4

ωr for the model 38.6 28.6 23.9 21.4 19.9

ωr for G model X X X X X

L for HFSS @ 3GHz(nH) 1.58 2.94 4.27 5.31 6.01

L for the model @ 3GHz 1.18 2.65 4.13 5.32 5.95

L for G model @ 3GHz 1.60 3.02 4.28 5.18 5.60

L for HFSS @ 5GHz 1.58 2.99 4.38 5.49 6.21


L for HFSS @ 9GHz 1.63 3.22 4.92 6.31 7.23


Less CPU processing time 6400 times difference (Ansoft HFSS)

*3.4GHz double CPUs and 2048MB DDR2 RAMs.


RF MEMS Inductors

Feature

• Remove substrate from

beneath coil

• Achieve 3D coil

geometry

Characteristics

• High quality factor (Q)

• High resonant frequency

• Compact size

Micromachined inductors on thin

membranes.

The resonant frequency 70 GHz for

the 1.1 nH inductor.


RF MEMS Inductors

An on-chip tunable micro

inductor with moveable

magnetic core in the 3D on-

chip solenoid coil

Schematic of an on-chip tunable micro inductor

http://mems.utdallas.edu/pictures/rfid/index.htmhttp://mems.utdallas.edu/pictures/rfid/index.htm


RF MEMS Inductors

An on-chip 3D arch-shape

solenoid inductor

A 3D on-chip toroidal

inductor



RF MEMS Inductors

Micromachined 3D copper

spiral inductors integrated on

a TSMC-fabricated

RFIC chip

A 3D on-chip solenoid

inductor with nano magnet wire

core integrated in it



RF MEMS Transmission Lines

Feature

• Substrate removed from

between or beneath

coplanar waveguide

transmission lines

Characteristics

• Greatly reduced

dielectric loss

• Transmission is

conductor loss limited

• Reduction in unwanted

moding of CPW

W-band micromachined coupled-line filter

Cross-section of the filter structure


MEMS Distributed Phase Shifters

Periodic loading of transmission lines with MEMS

capacitive switches creates a structure with a variable phase

velocity

• Circuit schematic of the phase shifter

• Equivalent Circuit


MEMS Distributed Phase Shifters

• Photograph of the fabricated one bit phase shifter

• Photograph of the fabricated three bit phase shifter


MEMS Switched delay line Phase Shifters

The switches of the phase shifters in a cascaded are

configured in different lines which can be selectively

controlled for the propagation of RF signals

• Circuit schematic of the phase shifter


MEMS Switched delay line Phase Shifters

• Photograph of the fabricated switched

delay line phase shifter


Phase Array Antenna

Principle of a phased array antenna using phase shifters


MEMS Resonator

Vertically-Driven Micromechanical Resonator

• To date, most used design to achieve VHF frequencies


MEMS Resonator

• Photograph of the fabricated

HF MEMS Resonator

• Extracted Q =8,000

(vacuum)

• Freq. influenced by dc-

bias and Anchor effect


Transceiver Front-End Architecture Using Vibrating Micromechanical Signal Processers

•Proposed and Developed by Professor Nguyen at UM

•Courtesy by Professor Nguyen


MEMS Filter

• Photograph of the fabricated HF Spring-

Coupled Micromechanical Filter


MEMS-Based Transceiver Architecture

Micromechanics are shaded in green


RF MEMS Circuit

C. T. C. Nguyen, Proceeding,

1998, Sensors Expo. 447~455.


Wine-Glass Micromechanical-Disk Reference Oscillator -Nguyen


Wine-Glass Micromechanical


Wine-Glass Micromechanical-Disk Reference Oscillator -Nguyen


Packaging Object

• MEMS packaging: Object: a system

IC +Mechanical+Optical+Bio...

Function:

protect system and provide

special environment for inside

devices

Cost:

More than 30% of product

expense in IC than MEMS cost

more!!!

ADXL50 by Analog Devices. Inc.


Packaging Challenge

• MEMS Packaging Challenge : no universal solutions, cost could be very high, multi-disciplines required but very few research effort.

Hermetic/vacuum package

Low temperature package

High density IC interconnects (with IC integration)

Chemical resistant

Special requirements: high precision, high g shock, and high melting point…….

• Solution: Using existing IC packaging solutions:

MCM, BGA, Flip chip techniques, 3-D stacking….etc..

Developing several unit processes for MEMS packaging applications

Wafer to wafer bonding using anodic or fusion bonding

Localized heating and bonding (LHB) technique

High precise splicing using micropipes

Metal composite for 3-D through via Interconnects


• Post-process packaging method: lid sealing, wafer bonding, and microshell encapsulation

More process flexibility in the system

Hermetic Encapsulation Approach

• Integrated encapsulation:

DC V

AC V

DC V

AC V

DC V

AC V

DC V

AC V


Localized Heating

• Finite element analysis shows isotherms around a 5 m wide microheater capped with a pyrex glass substrate

5m


mechanical Resonators with On-chip Resistive heaters

• Design layout • Close SEM View

• SEM of µ-mechanical resonators before vacuum encapsulation

Aluminum(Si) Solder

Folded-Beam, Comb Drive

µMechanical Resonator

• SEM of the resonator after breaking glass cap

Ground Pad Bias Pad

Sense Pad

Drive Pad

Anneal Pad

Aluminum/Si Solder


Device Fabrication Processes


Scheme of Vacuum Encapsulation

Device Substrate

Glass Cap

Pressure

Heating Pad

PECVD

Vacuum ~ 25 mtorr,

input power: 3.4 W,

bonding time: 10 mins

1st Step : resonator and glass cap fabrication

2nd Step : alignment and contact

3rd Step : vacuum and bonding


Vacuum Encapsulated High Q resonator

•Transmission spectrum with 120 mins. waiting time

-80

-65

-50

-35

-20

57.68 57.73 57.78 57.83 57.88

Frequency [KHz]

Tra

ns

mis

sio

n [

dB

] Q=9600

•Q factor vs. Pressure Q factor vs. Pressure

0

2000

4000

6000

8000

10000

12000

0.0001 0.01 1 100 10000Torr

Q f

ac

tor

Before

Annealing

After

Annealing

Before

Annealing

with higher Q


Reliability

•IEEE/ASME, JMEMS, Oct. , 2002, ~54 weeks


Wafer Level Chip Scale Packaging Design


What is SOC?

• SOC means “System on a Chip”. It is defined that the system contains embedded CPU, memory, interface circuits, such as USB, PCI, Internet, and mixed-signal blocks..etc.

• In fact, it should contains more and more functional chips including MEMS sensors and actuators in the future.

• Driving forces: demanding application, shrinking product cycles, process technology, and the need for mass productivity with lower cost.


Is Hermetic Package Necessary to MEMS?

• Wafer level process cost • Structure release or dicing first?

•Failed capacitive accelerometer.

(a) washed out during the dicing process.

(b) stuck to the bottom electrode during the dicing process.


Low temperature wafer level hermetic encapsulation

• Low temperature wafer level hermetic encapsulation using UV curable adhesive (IEEE 54TH ECTC, 2004)

Figure 10. Four inch wafer level bonding.

Air Trapping

600μm

Figure 11. A packaged overlapping parallel capacitor.

Capacitor

Wafer

Contact Pad

Glass Protection Cap

•Encapsulated accelerometer


Packaging Process

Figure 2. UV curable adhesive wafer level bonding scheme

Transparent Vacuum Chuck

Contact Pads

UV Curable Adhesive

Wafer


Device

Separation Cut

Die Cut

Figure 4. The removal of glass over contact pads.

Contact Pad

Dummy

Clip

Glass Cap

Vacuum Chuck Windshield

Double Side Tape

Figure 7. Second spin setup.

(a)


Silicon Substrate

UV Curable

Adhesive

Residue

Bonding

Region

Glass Protection

Cap

Silicon Substrate

Bonding

Region


Systems on a Package (SOP)

•Goal: With the same performance as SOC (Systems on a Chip) using standard Si processes but with lower cost and higher manufacture yield.

SCC

DRAM

DRAM DRAM

DRAM

CPUs with e-DRAM

MSC

Transceivers

Transducers SCC SCD SCD MSC

•Sze et al.,

•2-D MCM


SoC: From Board to Chip


SoC (System-on-chip)


Trends of Process Technology


Cu Interconnects

Moore’s Law: the number of transistors per square inch on integrated

circuitry had doubled every year since the integrated circuit was invented.

Although in subsequent years, the pace slowed down a bit, but data

density has doubled approximately every 18 months, and this is the current

definition of Moore's Law.

Chip Area (mm2)


Cu Interconnects

The speed limitations of circuitry caused by interconnects can be

simply estimated based on the delay time:

linect interconne theofy resistivit theis and field fringing the todue isK

oxide, theofconstant dielectric theis K where

1189.089.0

I

ox

2

0

sox

oxILWLHx

LKKRC

Ref. Havemann et al. IEEE,

2001


Capacitive Delay of Interconnections

An interconnection can be treated as a capacitor which

can either be charged and discharged using the inverter

circuit as shown in the following since the capacitance

arises in part from the metal conductors of the

interconnect and ground circuitry. The stray capacitance is

also introduced by the physical proximity of other

interconnects.


國立交通大學電子與資訊研究中心 National Chiao-Tung University

Microelectronics and Information Systems Research Center

110

2003 ITRS Roadmap


Challenges in the Present and Future System Integration

• Data communication speed limited

• Process limited; hard to optimize system performance

• High testing cost and thermal problem in a complex

SOC system

• Signal Integrity


DRIE for Packaging Applications

Recent 3-D Packaging Structure; Sharp.


3-D Stacked Chip Concept


3-D Stacked Chip Concepts

RPI Fraunhofer Institute


Packaging Technology Roadmap


Si Based Heterogeneous Integration

Performance Cost Form Factor Process Complexity

Conventional SOC

(System-on-Chip)

Excellent Low

(Batch Process)

Small Complicate

Silicon Based

SOP

(System-on-Package)

Could be Similar to SOC

Could be the Same (Batch Process)

Could be Same Area to SOC but thicker structure

Could be Simpler


MEMS Challenges

• From device fabrication point of view: the challenge is to integrated and process all of these heterogeneous materials together in a complex microsystems. Sometimes it has to include planar, nonplanar fabrication technologies, such as IC processes, micromilling, laser micromachining, wafer bonding…etc.,

• From product point of view: there is no reliable method yet that would qualify as a versatile post-process packaging for MEMS with the rigorous process requirements of low temperature, hermetic sealing, and long-term stability….etc.

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