High-performance and Low-cost Capacitive Switches for RF...

HighHigh--performance and Lowperformance and Low--cost Capacitive cost Capacitive Switches for RF ApplicationsSwitches for RF Applications

Bruce Liu

University of California at Santa Barbara

Toyon Research Corporation

DARPA Fame


FrequencyAgileMaterials for Electronics

DARPA



DARPAOutline

• Motivation for microelectromechanical (MEM) switches

• MEMS Switch performances and fabrication issues

• MEMS RF applications

• BST interdigital capacitors (IDCs)

• Low-loss BST phase shifters

• Future work and summary



DARPAOverview

V0 VL

+

-

SeriesSwitch Z0

Z0

V0 VL

+

-Z0

Z0

ShuntSwitch

Ideal switching circuits

V0 VL

+

-Zd Z0

Z0

V0 VL

+

-

Zd

Z0

Z0

Actual switching circuits

++

=−=switchshunt ,2/1log20switch series ,2/1log20

log200

0

0 d

dL

ZZZZ

VVIL



DARPASwitching Technologies

•At present, most commonly used switching devices are PIN diodes, GaAs FETs, and conventional mechanical switches (relays).

I

V

I

V+ -

Vb

Vth

slope = 1/Ron

O ff s ta te

O n sta te C jR s

O n

R on

O ff

Zd=

+

-

V m axVds

Id

V g s

Vd s

+

-

Id

O n s ta te (V gs= 0)

O ff s ta te (V gs< -V po)

Idss

V gs= 0

V gs= -0 .5

V gs= -1

V gs= -V p o

slope ≈ 1/R off

slope ≈ 1/R on

V m in

C ds

rds

C g C grgrg

so urce ga te d ra in

su bstra te

sou rce g a te d ra inrgrg R c

O n sta te (Vg =0 ) O ff s ta te (V g<-V po )

C off

R off

O nR on

O ff

Z d=

PIN diodePIN diode

GaAs FETGaAs FET



DARPAKey Switch Properties

Important figures of merit for switches are:- isolation- insertion loss- power consumption- power handling capability- switching speed- cutoff frequency- cost of fabrication



DARPAMEMS(Micro ElectroMechanical Systems)

MEMS SwitchMEMS Switch

substrate

MEMS bridge dielectric

substrateG GW

g

Lt

Switch up Switch down

air

30

0

827

sp

K gVWLε

=

Con

CoffSwitch up

Switch downOn

Off

Zd=MEMSSwitch

• Outgrowth of “Micromachining”- Creation of unique physical structures through the use of sacrificial

layers resulted in miniature mechanical structures one a substrate



DARPAMEMS Switches in RF Applications

MEMS Switch in a CPW configurationMEMS Switch in a CPW configuration

RF choke

Con/Coff

DC Block DC Block

Vbias

substrate

RF in

RFoutMEMS bridge

coplanarwaveguide

+Vbias

hWwC r

onεε0≈g

WwCoff0ε≈

- acts as RF switch or capacitor (100:1 ratios)- loss dominated by conductor loss- controlled by static DC voltage (10 nJ switching energy)- low cost processing (~4 mask layers)- high cutoff frequency- minimum intermodulation distortion



DARPAWhy RF MEMS?

Comparative tableComparative table

Switch Type Isolation Insertion loss Power handling

Power consumption

Switching speed

Cost

PIN diodes Good Good Good Poor Good Good GaAs FETs Good Good Poor Good Excell Poor MEMS switches

Escell Excell Excell Exell Poor Good

Advantages:Advantages:

•Very good isolation and insertion loss.•Virtually no control circuit power dissipation in either ON or OFF state•With proper design, can be capable of broad band and high power switching.•Switching speed is more than sufficient for RF control circuit applications•Relatively low cost (designed and fabricated by standard processing techniques). Essentially on every substrate.

⇓MEMS technology offers superior performance at lower costs and is

expected to have tremendous impact on microwave systems.



DARPAKey Factors of MEMS Capacitive Switches

substrate

MEMS bridgeSiN

MEMS switch typical structure

Critical parameters to be carefully considered :-the height of the membrane over the central conductor ( → COFF , VPD)-the thickness and composition of the top membrane ( → VPD , mechanical properties)-the size and the geometry of the membrane ( → RF and DC performances)-the thickness and type of dielectric coating the central conductor ( → CON , breakdown)-type of substrate ( → leakage, parasitic effects)



DARPATypical Fabrication Process

Substrate

Sacrifical layer removal

Substrate

Ti/Al

Membrane deposition

Substrate

photoresist(PMGI)

Sacrificial layer

Substrate

SiN layer

Substrate

CPW Pattern

Ti/Au



DARPAMembrane Release

Substrate

Ti/Au PMGI

Water or solvent

Substrate

Stiction

Substrate

Stiction

Stiction occurred in an air-dried sample

solution

90o angle view of the released suspended structure after critical point drier

High pressure chamber

Critical Point Drying System



DARPASubstrates & Metals

Glass

Lowest leakage currentHigh breakdown voltage

Si - SiO2Si - Si3 N4

GaAsSilicon

High leakage currentLow breakdown voltage

Membrane:– gold– aluminum– titanium – platinum– nickel

Strong metal stress (Pt,Ni,Ti) Low metal stress (Al,Au)



DARPAYield and Reliability

Important Factors:• compositions of MEMS top membrane low stress deposition

• reflow temperature and surface roughness of sacrificial layer

Conclusions:• risky design if span length longer than 300µm

• reflow temperature need to be adjusted for different types of MEMS switch design

• sputtered aluminum and electroplated low-stress nickel are good candidates as MEMS top membrane



DARPADC Measurements

2-7pF

DOWN state capacitance

0.1-0.5pF 4-70

UP state capacitance

Capacitiveratio

20-100 V

Pull-down voltage

Typical DC measurements



DARPARF Simulations and Measurements

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-40

-35

-30

-25

-20

-15

-10

-5

0

0 5 10 15 20

S21 measured

S21 fitted

S21 HFSS

S11 measured

S11 fitted

S11 HFSSS21

MA

G (d

B) S11 M

AG

(dB)

Frequency [GHz]

-25

-20

-15

-10

-5

0

-25

-20

-15

-10

-5

0

0 5 10 15 20

S21 measured

S21 fitted

S21 HFSS

S11 measured

S11 fitted

S11 HFSS

S21

MA

G (d

B) S11 M

AG

(dB)

Frequency [GHz]

Switch UP

C = 0.075 pF

R = 0.5Ω

L = 2 pH

CPW pad CPW pad

≈≈≈≈C = 2.7 pF

R = 0.5 ΩSwitch DOWN

L = 2 pH

CPW pad CPW pad

≈≈≈≈

• Good agreements between measurements and HFSS simulations

• Instead of 3-D EM simulation, lump-element model can also fit measurements well, which gives us a simplified way to describe MEMS switch performance



DARPAMore Topics on MEMS Switches

Switch down

A

h

hAC rdown εε0=

• Increasing switching ratio

(i.e. increasing CDOWN for given CUP )

• How do we improve the switch performance ?



DARPASwitches with Metal Caps

Metal cap


-30

-25

-20

-15

-10

-5

0

0 5 109 1 1010 1.5 1010 2 1010 2.5 1010 3 1010

Frequency [Hz]

DOWN

UP

Pro: Much higher isolation in DOWN state than previous design

Con:Direct metal-to-metal contact lower switching speed



DARPAMEMS RF Applications

•Advantages of RF MEMS

-High performance, low bias power consumption

-Potential low cost manufacturing into a variety of substrates

•Limitations of RF MEMS

-Slower switching speed

-Potential lifetime limitations

APP

LIC

AT

ION

S

• Phase Shifters

• Filters

• Reconfigurable Antenna



DARPAProgrammable Delay Lines

Cs

Ll

Cl

Cs

Ll

Cl

Cs

Ll

Cl

Ll

Cl

Cs

Ll LlLl

ClCs

Ll

ClCs

Ll

ClCs

Ll

ClCs

Variable Phase Velocity DesignAnalog or Digital MEMS Switches

Switched-Capacitor Variable-Capacitor

4-bit Digital Phase ShifterRF in RF out

V1 V2 V3 V4

0-22.5º 0-45º 0-90º 0-180º



DARPADistributed Circuit Concepts

Transmission-line

Equivalent circuit

LCLCv

CLZ /2 /1 /0 <

== ω

Z0, β

L

C

L

C

L

C

L

C

L

C

L

LoadedTransmission-line

Ll

Ll

Cl

Cs

Ll

Cl

Cs

Ll

Cl

Cs

Ll

Cl

Cs

( )l l sL C Cφ ω∆ = +Varactor provides variable phase delay:



DARPAMEMS Low-loss Distributed Phase Shifters

Transmission line sections

CMEMS

Zline , vline Zline , vline Zline , vline

CMEMSCMEMS

Lt Ct CMEMS

Lsect : Length of transmission line per sectionCt : Transmission line capacitance per sectionLt : Transmission line capacitance per section

Equivalent Circuit:

Periodic loading of transmission lines with MEMS capacitive switches creates a structure with a variable phase velocity



DARPAOptimization – MEMS Phase Shifters

( )xyxv

Lfi

t +−+= 112 secπδφ

( )ittLs

t ZLnZCffnLoss απ secsec

maxvar

2

sec +=

maxvar2

1Cr

fs

s π=

Max phase shift/section:maxvar sec/ t

l

C LxC

=

min maxvar var

1min / ay C C ax−= ≥ −

Loading Factor:

Min/Max ratio:

Return Loss: 11 11,maxS S≤2

max,11

max,11

11

+−

=SS

a

0

1

2

3

4

5

0 2 4 6 8 10

Inse

rtio

n Lo

ss [d

B]

Loading Factor x

S11,max=-14dB

S11,max=-17dB

S11,max=-20dB

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 2 4 6 8 10

ymin

Loading Factor x



DARPAAlternative MEMS Switch Configurations

Challenge:• Not overload the transmission line with an excessively large MEMS switch capacitance in the DOWN state

Solution: the ‘series’ configuration design

CfixedCvar

Substrate

C fixed

C var

C fixed

fixed

fixedTOT CC

CCC

22

var

var

+= if

≅→<<≅→>>

varvar

var 2 CCCC

CCCC

TOTfixed

fixedTOTfixed



DARPAOne-bit MEMS Phase Shifters

Ground

MEMS capacitors

Ground

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30 35

SimulatedMeasured

Diff

eren

tial p

hase

shi

ft [d

egre

es]

Frequency [GHz]

-40

-30

-20

-10

0

-6

-4.5

-3

-1.5

0

0 5 10 15 20 25 30 35

UP stateS-parameters [dB]

S11

MA

G[d

B] S21 M

AG

[dB]

Frequency [GHz]

S11

S21

-40

-30

-20

-10

0

-6

-4.5

-3

-1.5

0

0 5 10 15 20 25 30 35

DOWN stateS-parameters [dB]

S11

MA

G [d

B] S21 M

AG

[dB]

Frequency [GHz]

S11

S21

• The state of the art insertion loss performance, 154°/dB at 25 GHz and 160°/dB at 35 GHz, demonstrates the potential for the implementation of a very low loss multi-bit digital MEMS phase shifter.



DARPAThree-bit MEMS Phase Shifters

DC Block

180°°°° 90°°°° 45°°°°

• Designed for 25GHz operation• 3dB max. insertion loss• Phase error less than 8.5° for all switching states

-100

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Diff

eren

tial P

hase

Shi

ft (D

egre

e)

Frequency (GHz)

-20

-15

-10

-5

0

-30

-25

-20

-15

-10

-5

0

5

10

10 15 20 25 30 35

Inse

rtio

n Lo

ss [d

B] R

eturn Loss [dB]

Frequency [GHz]



DARPATopology of 3-Pole Tunable Filters

CPW Ground

CPW Ground

CouplingCapacitors

Loading MEMSCapacitors

Signal

Loaded lineResonator

Input Line50 Ω 50 Ω

Output Line

C12 C23 Cn n+1

Resonator 1 Resonator n

Z1, L1 Z1, L1

CouplingCapacitors

(a)

(b)

C12 C23C01 C34

(a) Topology of capacitively coupled tunable filter based on DMTL resonators. (b) Layout of 3-section tunable filter in coplanar-waveguide form.

- Capacitively loaded line sets up a slow wave structure

- Resonator electrical length determined by capacitive loading of the line

- Tuning range of the filter determined by the effective capacitance range of varactor

- MEMS varactors have been demonstrated with low loss making them an excellent candidate for filter applications



DARPASimulated Results of Tunable Filters

-30

-25

-20

-15

-10

-5

0

5

10 15 20 25 30

Cvar=12 fFCvar=15 fFCvar=18 fF

S11

(dB

)

Frequency (GHz)

-60

-50

-40

-30

-20

-10

0

10 15 20 25 30

Cvar=12 fFCvar=15 fFCvar=18 fF

S21

(dB

)

Frequency (GHz)

Return Loss Insertion Loss



DARPAMeasured Tunable Filter Performance

-40

-30

-20

-10

0

10

10 15 20 25 30

S11 @ V=0VS11 @ V=50VS11 @ V=60V

S11

[dB

]

Frequency [GHz]

-50

-40

-30

-20

-10

0

10 15 20 25 30

S21 @ V=0VS21 @ V=50VS21 @ V=60V

S21

[dB

]

Frequency [GHz]

Bias Pad

RF IN RF OUT

• 12% bandwidth at 20GHz with 3.6dB insertion loss in passband• Loss is contributed by conductive loss, substrate leakage and radiation. micromachining• 3.8% tuning range, which is limited by the tuning factor of MEMS varactors (~1.3:1)

SEM microphotograph



DARPAReconfigurable Microstrip Grid Antenna --- Toyon Project

• The switch can be used to turn “on” or “off” various conductive paths.

• In this way various virtual antenna states can be realized.

Microstrip structure with switches represents over 1017 possible states.

• Multiple frequency band operation• Possible beam and null steering• Lends itself to multiple feed locations• Polarization diversity

• Reconfigurable antenna systems:

A matrix of RF switches in a metallic grid to control the RF current distribution and hence radiation and impedance characteristics of the antenna structure.



DARPA4-Element Microstrip Grid Antenna

2.5 cm

0.312 cm

SMA connector(CDI p/n 5308-2CC)

0.071 cm

Aluminumhousing

Goldtraces

MEMSswitches4-places

Glasssubstrate(Er = 5.75)

SQUARE RECONFIGURABLE GRID ANTENNA INCORPORATING MEMS CONTROL SWITCHES

1

2

4

0.10 cm1.50 cm

0.40 cm

3/24/00 M. GilbertToyon Research

3



DARPAPreliminary Work on MEMS-based Grid Antenna

-20

-15

-10

-5

0

8 8.5 9 9.5 10 10.5 11 11.5 12

Frequency [GHz]

FULL HFSS MODEL -- ALL MEMS DOWNSQUARE RECONFIGURABLE ANTENNA INCORPORATING MEMS CONTROL SWITCHES

(switches at 3/4 of each leg length, diagonal trace from feed point)

simulated

measured

S11

[dB

]

0

30

60

90

120

150

180

210

240

270

300

330

Radiation Pattern [Electric Field]



DARPABST Thin Film Technology

• MotivationPhased Array Antennas

The Phase-Shifter: A critical component

• Thin-Film BST VaractorsParallel Plate vs. Interdigital Capacitors

Device Characterizations

• Phase-Shifters using Thin-Film BST



DARPAWideband Phased Array

True TimeDelay unit

True TimeDelay unit

True TimeDelay unit

True TimeDelay unit

TTD

TTD

TTD

True TimeDelay unit

TTD

TTD

TTD

Coarse Delay Control

Fine Delay Control

desiredphasefront

True TimeDelay unit

Sub-Array

TTD

TTD

TTD

True TimeDelay unit

12-bitBeam

Control

Feed

AntennaArray

True TimeDelay unit

True TimeDelay unit



DARPABarium Strontium Titanate (BST)

Key BST Properties

- BST thin film properties differ markedly from those of bulk and much work is focused on low-cost deposition of BST thin film for microwave applications.

Large field dependent permittivity --- Compact tunable circuits

Intrinsically fast field response --- Fast switching speeds

High breakdown fields, >3x108 V/cm --- High power handling capability

Low drive currents (dielectric leakage) --- Low prime power requirement

Simple fabrication --- Low cost



DARPAInfluence of Loss and Tunability on Phase Shifter Performance

Tunability: how much we can vary capacitance of filmCmax/Cmin

Device loss and tunability are important

0

1

2

3

4

5

6

00.010.020.030.040.05

Loss

(dB

) for

360

deg

rees

at 2

0 G

Hz

Effective Device Loss Tangent at 20 GHz

Total Loss

CPW conductor Loss

Distributed Circuit Phase ShifterBST on Silicon, Tunability: 2.5

0

1

2

3

4

5

6

7

8

1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2

BST film tunability

Loss

(dB

) for

360

deg

rees

at 2

0 G

Hz

BST Loss Tangent=0.05



Silicon Substrate



DARPABST Device Considerations

Reproducibility

Inexpensive substrates

Standard growth/processing steps

Low loss tangent (tan δδδδ < 0.01)

High tunability (>2:1)

Compatible with low-cost packaging

Desirable Features of a Viable Thin-Film Varactor Technology

- Integrated monolithic capacitors using sputtered/MOCVD material on low-cost substrates

- Sapphire is a cost effective wide area substrate that has excellent microwave and rf properties



DARPAMonolithic BST Device Structures

Parallel Plate vs. Interdigitated Capacitor (IDC)

• Vertical polarization• High tunability, efficient use of BST• Low breakdown voltage• Complex fabrication

• Horizontal polarization• Low tunability, inefficient use of BST• High breakdown voltage• Easier fabrication

S u b s t r a te

w w w

w

l

3 w

B S T

S iN

P t



DARPA

3.5

4

4.5

5

5.5

6

6.5

7

-20 0 20 40 60 80 100

Width=2um, Spacing=1umWidth=2um, Spacing=2um

Cap

acita

nce

(pF)

DC Bias (V)

DC Measurements

Interdigitated Structure

-1000Å film-70-80% of material tunability-0-100 V control-Device tunability limited by finger-to-finger spacings

-Ti/Au metalization-3-mask process

Good choice forlow-cost circuits

Tunability



DARPARF Parameters Extraction Method

Layout for Parameters Extraction:

Open Short Load

Equivalent Circuit Expression:

YL

YP

YS

YD

Yop

YP

YS

Ysh

YP

YS

DY(YL-Yop)(Ysh-Yop)≡ Ysh-YL

≡ G+iωC

Q= ωC/G



DARPAQuality Factor Limits

0

20

40

60

80

100

5 10 15 20 25 30 35 40

w=2µm, s=0.5µmw=2µm, s=1µm

Qua

lity

Fact

or

Frequency [GHz]

0

50

100

150

200

4 8 12 16 20

w=2µm, s=0.5µmw=2µm, s=1µm

Capa

cita

nce

[fF]

Frequency [GHz]

Total device loss consists of conductor loss and BST material loss:

1 1 1

device cond bstQ Q Q= +

•Qbst, inherent to deposited BST film

•Qcond, determined by Rmetal and Cdevice

• Devices with different finger to finger spacings but same other dimension parameters are fabricated on the same wafer

• Qdevice stays the same while Qcondvaries Qbst dominates the total device Qdevice



DARPAQuality Factor of BST Interdigital Capacitors

0

10

20

30

40

50

60

5 10 15 20 25 30 35 40

Qua

lity

Fact

or

Frequency [GHz]

Ba/Sr=30/70

Ba/Sr=50/50

•100nm (Ba,Sr)TiO3 thin films deposited on sapphire substrate

•Quality factor rolls off as frequency increases

•Stoichiometry of deposited thin film determines its loss tangent

•Q>35 in X-band for (Ba30,Sr70)TiO3thin film

TSurface = 700 °C: RF power = 2* 150 W ( 3.3 W/cm2): 90/10 Ar/O2 (sccm): 100 nm



DARPATuning Factors of BST Interdigital Capacitors

0

10

20

30

40

50

60

70

80

4 8 12 16 20

0V40V100V

Capa

cita

nce

[fF]

Frequency [GHz]

•Finger spacing = 1µm

•100nm (Ba30,Sr70)TiO3thin film on sapphire substrate

•~2:1 tuning range

•0-100V DC control

TSurface = 700 °C: RF power = 2* 150 W ( 3.3 W/cm2): 90/10 Ar/O2 (sccm): 100 nm



DARPAX-band BST Phase Shifters

-30

-25

-20

-15

-10

-5

0

-60

-50

-40

-30

-20

-10

0

0 2 4 6 8 10

0V

40V

100V

0V

40V

100V

Ret

urn

Loss

[dB

] Insertion Loss [dB]

Frequency [GHz]

Photograph of fabricated X-band BST phase shifter S-parameter measurements of distributed BST phase shifter

15mm

15mm



DARPAMeasured Differential Phase Shift

-100

0

100

200

300

400

500

600

0 2 4 6 8 10

0V40V100V

Diffe

rent

ial P

hase

Shi

ft [D

egre

e]

Frequency [GHz]

10

20

30

40

50

60

70

80

0 2 4 6 8 10

Phas

e Sh

ift p

er d

B L

oss

[Deg

ree/

dB]

Frequency [GHz]

Differential phase shift at 0V, 40V and 100V bias voltage

Phase shift per dB insertion loss at 100V bias voltage

• 0º-360º phase shift @ 8.2GHz with 5dB insertion loss

• Return loss is better than –10dB from DC to 10GHz



DARPAState of Progress

What have been done so far?

Developed a novel design of MEMS devices

MEMS-based RF circuits (i.e. phase shifters, filters)

BST interdigital capacitors and phase shifters

What the plan for future work?

BST-MEMS switches

MEMS-based reconfigurable microstrip grid antenna

MEMS single-controlled single-pole-double-throw (SPDT) switches



DARPABST-MEMS Switches

Barium Strontium Titanate (BST)Barium Strontium Titanate (BST)


BST

• High dielectric constant of BST (>250)

• >15dB more isolation than conventional MEMS switch in DOWN state

What we will do?

• BST/Pt/Au/Ti/Sapphire

• Reduce pull-down voltage, avoid breakdown in BST thin film



DARPAFuture Work on Reconfigurable Antenna --- Toyon Project

Demonstrate the 4-element microstrip grid antenna using standard MEMS fabrication techniques

RF measurements and simulations accounting for the parasitic effects in the model

Finally expand this design to a 16-element switching reconfigurable antenna array



DARPAMEMS Single-Controlled SPDT Switches

Signal IN

Signal OUT 2MEMSSwitch 2

MEMSSwitch 1

Coplanar Waveguide

Signal OUT 3

Coplanar Waveguide

Port 2

λ/4

CDOWNPort 1

Port 3Isolated

(a) Switch Down

λ/4

Port 1

Port 3

IsolatedPort 2

(b) Switch Up

Control 1

Control 2INPUT

OUTPUT 1

OUTPUT 2

λ/4

λ/4λ/4λ/4λ/4

λ/4λ/4λ/4λ/4



DARPASummary

•MEMS based switches promise superior performance relative to conventional devices. Through its superior performance characteristics, the MEMS switches are developed in a number of existing circuits, including switches, phase shifters and filters.

•BST thin films have been characterized and used to demonstrate a 0º-360º X-band phase shifter with only 5dB insertion loss.

•Replace Si3N4 with high dielectric constant BST thin film in MEMS switch is expected to further improve the switch performance.

•MEMS technology offers the potential for building a new generation of low loss high-linearity reconfigurable antenna arrays for radar and communications applications.

High-performance and Low-cost Capacitive Switches for RF...

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