RF-MEMS metal contact capacitive switches

transcript

4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)

RFRF--MEMS metal MEMS metal contactcontactcapacitive switchescapacitive switches

4th Round Table onMicro/Nano Technologies for Space

ESTEC Conference Centre, Noordwijk, The Netherlands 20/22 May 2003

X. Rottenberg, A. Jourdain, P. Fiorini, R. Mertens, W. De Raedt and H. A. C. TilmansIMEC v.z.w., Division MCP, Kapeldreef 75, B-3001 Leuven, BelgiumB. NauwelaersK.U. Leuven, ESAT-TELEMIC, Leuven, BelgiumF. Deborgies, L. MarchandESA - European Space & Technology Centre, Noordwijk, The Netherlands

OUTLINE

Introductionn Target specs

n Configurations and choice

Capacitive RF-MEMS switchesn Top floating metal (”metal contact” capacitive switch)

n Boosted switch

0-level packagingn Technology

n RF performance

Reliability

Conclusions

Target specifications RF-MEMS switch for communications switching matrix

n Frequency range 1-30 GHz

n Insertion loss < 0.4 dBn Isolation > 50 dBn Return loss > 20 dB

n Actuation voltage < 50 Vn Switching time “small”n Power consumption “minimal”

n Operating ambient -25 to +75 °Cn Life time 106 cycles

Configuration & Choice of switching device meeting specs

Circuit configuration:

series or shunt

Switching contacts:

resistive or capacitive

Actuation mechanism:

Electrostatic (ES), electromagnetic, electrodynamic, electrothermal, ….

Driving configuration:

switch or relayPosition of the armature

(with respect to TL):

inline or broadside

• low power consumption• “easy” and well-known technology• spec allows up to 50 Vdc bias

• compatible with ES actuation• switching of DC signals not required• allows switch configuration• expected better reliability• basic technology available (with Ta2O5)

• simple structure• compact (more robust)• pin compatibility with FET and PIN diodes

shuntswitch

RF signalgenerator

RFchoke

DC block RF-out

DCblock

SIDE VIEWS

TOP VIEW

GND GND

dielectric

Flexiblemetal bridge

ON-state

“small” C

Highly resistive substrate

Shunt capacitive RF-MEMS switchimplemented on a CPW line

RF+DC in

RF out

=0V=20V

RF+DC in

RF out

OFF-state

“large” C

Highly resistive substrate

Flexiblemetal bridge

It is the capacitance ratio that counts:

1101.0

−>≡spec

IL<ILspec AND I>Ispec

in the frequency band <ωωωωl,ωωωωu> requires that:

Capacitance ratio determined by process (not geometry):

Switch modeled as lumped capacitorApplies for series and shunt switches

down =≡ Conventional capacitive switch

RF-MEMS electrostatically actuated capacitive switches at IMEC

Shunt bridges

Shunt bridge

RF-MEMS Substrate

Anchor

Signal SlotSlot GNDGND

series

shuntshunt

Top floating metalTop floating metal

5/6-Mask Fabrication Process“Metal Surface Micromachining”

Metal bridge

Ground(thick metal)

Sacrificial layerAir gap

Dielectric

RF-MEMS SubstrateSignal line (thin metal)

Ground(thin metal)

Top floating metal

Metal bridge

Ground(thick metal)

Sacrificial layerAir gap

Dielectric

RF-MEMS SubstrateSignal line (thin metal)

Ground(thin metal)

Top floating metal

ES actuated capacitive shunt switch:.

5 10 15 20 25 30 35 400 45

w/o top metal

Frequency [GHz]

Insertion Loss [dB] (bridge UP)

simulation

Down capacitance too low

5 10 15 20 25 30 35 400 45

w/o top metal

Frequency [GHz]

Isolation [dB] (bridge DOWN)

ES actuated capacitive shunt switch:

Introduction floating top metal

floating top metal

5 10 15 20 25 30 35 400 45

with top metal

w/o top metal

Frequency [GHz]

simulation5 10 15 20 25 30 35 400 45

w/o top metal

with top metal

Frequency [GHz]

ES actuated capacitive shunt switch:

5 10 15 20 25 30 35 400 45

w/o top metal

boosted

with top metal

Frequency [GHz]

ŁŁŁŁ “boosted” switch

simulation

Introduction floating top metal

floating top metal

5 10 15 20 25 30 35 400 45

boosted

with top metal

w/o top metal

Frequency [GHz]

ES actuated capacitive series switch:

With floating top metal (100µµµµm)

Wo floating top metal

With floating top metal (100µµµµm)

Wo floating top metal With floating top metal(100 – 200 – 300 µµµµm)

w/o floating top metal

With floating top metal300µµµµm

200µµµµm

100µµµµmWo floatingtop metal

Lgap Lactuation Lfloat

DC bias

CPW lineCPW line

A A’

RF in RF out

Introduction floating top metal ŁŁŁŁ “boosted” switch

Floating top metal

Lgap Lactuation Lfloat

DC bias

CPW lineCPW line

A A’

RF in RF out

C-V measurements shunt switchThe floating metal makes the difference

w/o top floating metal with top floating metal

0 5 10 15 20 25

Voltage (V)

Cdown> 1.1 pF

Cup= 0.20pF

Adown=Aup= 100 x 60 µµµµm2

l = 300 µµµµm

0 5 10 15 20 25 30 35

Voltage (V)C

Adown=Afloat =70x450 µµµµm2

Aup= 100x20 µµµµm2

l = 300 µµµµm

Cdown= 39 pF

Cup= 0.15pF

• Measured Cdown<< Predicted Cdown

• Cdown increases with increasing bias

• Measured Cdown= Predicted Cdown

• Cdown independent of bias (>Vpull-in)

The road to a high r = Cdown/Cup ratio

0 20 40 60 80 100Cup [fF]

w/o top metal

with top metal

boosted

Introduce top floating metal

“Boosting”

Conventional

r = 459 r = 102

r = 13

Conclusions (1):The trick of the top floating metal

n Down capacitance Cdown is predictable and is accurately defined by the area of the top floating metal:

n Area in the down state Adown can be chosen “independent” from the area in the up state Aup and from the actuation area Aact:

n Area difference in up and down state can be used to amplify the capacitance area r (boosted concept):

AC floatr

down =

actupfloatdown AAAA ≈≠=

floator

r •=≡ε

ratio for conventionalcapacitive switch “BOOSTING Factor ββββ”

Broadband switching (1-30 GHz)

A. Single switch:n IL<0.4dB and I>50dB over 1-30 GHz requires down/up capacitance ratio r

of: r=Cdown/Cup > 30,000 (“impossible” for conventional technology)

n For a switch technology with εr=25, do=2.5µm, dε=0.2µm, the capacitance ratio is r ≈≈≈≈ 300 Ł boosted design requires a boosting factor β β β β >100 (e.g. ifAup= 20x20µm, then Afloat=4x104µm2, or for Wfloat = 70µm, Lfloat > 0.6mm)

Is feasible

450µµµµm

Broadband switching (1-30 GHz)

B. Combination switch:Series-shunt-shunt-series(with boosted constituents)

s sshsh

Series closed – shunt open

Series open – shunt closed

RF In RF Out

All the bridge in the air

IDLE-state

Need for on-wafer MEMS packaging

Standard wafer sawing will destroy fragile MEMS

Needed for on-wafer protective envelope

0-LEVEL PACKAGE

Cap chip Cap chip Cap chipCap chip

MEMS wafer

MEMS substrate

MEMS deviceBondinglayer

Capping chip

wafer saw wafer saw wafer saw

MEMS wafer

0-level packaged RF-MEMS switchesC2W assembly

Glass Cap

Edge of capSwitch BCB

ring (100µµµµm)

RF feed-through

Capping chip

RF-MEMS Substrate

Metal bridgeRF feedthrough

Sealing ring

Influence of the cap material and cavity height (planar feedthrough)

2 4 6 8 10 12 14 16 180 20

freq, GHz

std Si cap (h=85µm)

h=45µm

h=5µm

h=15µmh=25µm

naked CPW

HRSi caps

Frequency [GHz]

CPW Signal line

CPW Ground

Transducers’03/AJ

CPW:(25/100/25µµµµmCu 3µµµµm thick2.3 mm long)

AF45 substrateCap material:

standard Si (1-10ΩΩΩΩcm)HRSi (>4000ΩΩΩΩcm)

BCB bond (5µµµµm thick)

AF45 glass substrate

Sealing cap

Conclusions(2)

n The implementation of the top floating metal is an easy and viable way to build capacitive switches with predictive RF (and C-V) characteristics.

n The boosted capacitive switch approaches the behaviour of the resistive relay, while retaining the advantageous features that are characteristic for a switch configuration (compact, simple, “two-terminal device”).

n The specifications IL<0.4dB and I>50dB requires a capacitance ratio better than 30,000; this seems feasible for a single switch, but the ultimate limitation of satisfying the spec is the parasitic resistance and inductance in series with the switch capacitance.

n Combination switches improve the RF specifications compared to asingle switch design, but this goes at the expense of a higher complexity, increased size (implying higher resistive losses).

n The influence of the 0-level package on the RF characteristics can be kept negligibly small.

n Critical issues remaining to be solved are:n packaging (hermeticity, controllability, cost)

n reliability (stiction, life cycles, choice of materials)

Acknowledgements

Financial support from ESA-ESTECunder contract 14627/00/NL/KW

IMEC, team CRO (Rita van Hoof & Co) (Clean Room Operations)

IMEC, MSR group (Ingrid De Wolf & Co.)(Micro Systems Reliability)

IMEC, team MEMS packaging (Piet De Moor & Co)

Henri Jansen, Un. Of Twente (NL) Hocine Ziad (AMIS, Oudenaarde, B)R. Puers, J De Coster (KU Leuven, B)

RF-MEMS metal contact capacitive switches

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