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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003
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
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 2
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
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 3
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
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 4
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
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 5
=0V
RF-in
shuntswitch
Vg
Zo
RF signalgenerator
RFchoke
DC block RF-out
Zo
DCblock
Vdc
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
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 6
It is the capacitance ratio that counts:
110
1101.0
1.0
−
−>≡spec
spec
IL
I
l
u
up
downC
Cr
ωω
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
ε
εdd
CC
r or
up
down =≡ Conventional capacitive switch
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 7
RF-MEMS electrostatically actuated capacitive switches at IMEC
2000
Shunt bridges
Shunt bridge
RF-MEMS Substrate
Anchor
Signal SlotSlot GNDGND
2002
series
2001
shuntshunt
2002
shunt
2001
shunt
Top floating metalTop floating metal
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 8
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
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 9
ES actuated capacitive shunt switch:.
5 10 15 20 25 30 35 400 45
-1.0
-0.5
-1.5
0.0
w/o top metal
S21
[dB
]
Frequency [GHz]
Insertion Loss [dB] (bridge UP)
simulation
RF
5/02
Down capacitance too low
5 10 15 20 25 30 35 400 45
-20
-10
-30
0
dB(r
8m1b
ddo.
.S(2
,1))
w/o top metal
S21
[dB
]
Frequency [GHz]
Isolation [dB] (bridge DOWN)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 10
ES actuated capacitive shunt switch:
RF
5/02
Introduction floating top metal
floating top metal
5 10 15 20 25 30 35 400 45
-1.0
-0.5
-1.5
0.0
with top metal
w/o top metal
S21
[dB
]
Frequency [GHz]
Insertion Loss [dB] (bridge UP)
simulation5 10 15 20 25 30 35 400 45
-20
-10
-30
0
dB(r
8m1b
ddo.
.S(2
,1))
w/o top metal
with top metal
S21
[dB
]
Frequency [GHz]
Isolation [dB] (bridge DOWN)
Page 6
4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 11
ES actuated capacitive shunt switch:
5 10 15 20 25 30 35 400 45
-20
-10
-30
0
w/o top metal
boosted
with top metal
S21
[dB
]
Frequency [GHz]
Isolation [dB] (bridge DOWN)
ŁŁŁŁ “boosted” switch
simulation
Introduction floating top metal
floating top metal
5 10 15 20 25 30 35 400 45
-1.0
-0.5
-1.5
0.0
boosted
with top metal
w/o top metal
S21
[dB
]
Frequency [GHz]
Insertion Loss [dB] (bridge UP)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 12
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
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 13
C-V measurements shunt switchThe floating metal makes the difference
w/o top floating metal with top floating metal
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 5 10 15 20 25
Voltage (V)
Ca
pa
cit
an
ce
(p
F)
Cdown> 1.1 pF
Cup= 0.20pF
Adown=Aup= 100 x 60 µµµµm2
l = 300 µµµµm
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0 5 10 15 20 25 30 35
Voltage (V)C
ap
acit
an
ce
(p
F)
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)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 14
The road to a high r = Cdown/Cup ratio
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100Cup [fF]
Cdo
wn
[pF]
w/o top metal
with top metal
boosted
Introduce top floating metal
“Boosting”
Conventional
r = 459 r = 102
r = 13
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 15
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):
ε
εd
AC floatr
down =
actupfloatdown AAAA ≈≠=
up
floator
up
downA
A
dd
CC
r •=≡ε
ε
ratio for conventionalcapacitive switch “BOOSTING Factor ββββ”
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 16
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
µ
µ
µ
µ
µ
µ
450µµµµm
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 17
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
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 18
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
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4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 19
0-level packaged RF-MEMS switchesC2W assembly
Glass Cap
Edge of capSwitch BCB
ring (100µµµµm)
CPW
RF feed-through
Capping chip
RF-MEMS Substrate
Metal bridgeRF feedthrough
Sealing ring
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 20
Influence of the cap material and cavity height (planar feedthrough)
2 4 6 8 10 12 14 16 180 20
-0.4
-0.3
-0.2
-0.1
-0.5
0.0
freq, GHz
dB(w
8c9l
10cu
line_
p..S
(2,1
))dB
(w8c
7l8c
ulin
e_p.
.S(2
,1))
dB(w
8c9l
6cul
ine_
p..S
(2,1
))dB
(w8c
9l8c
ulin
e_p.
.S(2
,1))
dB(w
8c5l
9cul
ine_
p..S
(2,1
))dB
(w8c
7l8c
ulin
e..S
(2,1
))
std Si cap (h=85µm)
h=45µm
h=5µm
h=15µmh=25µm
naked CPW
HRSi caps
S21
[dB
]
Frequency [GHz]
CPW Signal line
CPW Ground
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
CPW
Sealing cap
BCBh
Page 11
4th Round Table on MNT for Space20/22 May, 2003 (ESTEC, Noordwijk, NL)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 22
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)
© imec 2003 MCP/EDAS/HT 4th Round Table on MNT for Space, 20/22 May 2003 (ESA-ESTEC, Noordwijk, The Netherlands) 23
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)