4th Round Table on MNT for Space
20/22 May, 2003 (ESTEC, Noordwijk, NL)
RF-MEMS metal contact
capacitive switches
X. Rottenberg, A. Jourdain, P. Fiorini, R. Mertens, W. De Raedt and H. A. C. Tilmans
IMEC v.z.w., Division MCP, Kapeldreef 75, B-3001 Leuven, Belgium
B. Nauwelaers
K.U. Leuven, ESAT-TELEMIC, Leuven, Belgium
F. Deborgies, L. Marchand
ESA - European Space & Technology Centre, Noordwijk, The Netherlands
4th Round Table on
Micro/Nano Technologies for Space
ESTEC Conference Centre, Noordwijk, The Netherlands 20/22 May 2003
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OUTLINE
Introduction
n
Target specs
n
Configurations and choice
Capacitive RF-MEMS switches
n
Top floating metal (
n
Boosted switch
”metal contact” capacitive switch)
0-level packaging
n
Technology
n
RF performance
Reliability
Conclusions
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Target specifications RF-MEMS switch
for communications switching matrix
n
Frequency range
1-30 GHz
n
Insertion loss
Isolation
Return loss
< 0.4 dB
> 50 dB
> 20 dB
Actuation voltage
Switching time
Power consumption
< 50 V
“small”
“minimal”
Operating ambient
Life time
-25 to +75 °C
106 cycles
n
n
n
n
n
n
n
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Configuration & Choice of switching
device meeting specs
Circuit configuration:
series or shunt
• compatible with ES actuation
• switching of DC signals not required
• allows switch configuration
• expected better reliability
• basic technology available (with Ta2O5)
• low power consumption
• “easy” and well-known technology
• spec allows up to 50 Vdc bias
Switching contacts:
resistive or capacitive
Actuation mechanism:
Electrostatic (ES), electromagnetic, electrodynamic, electrothermal, ….
Driving configuration:
Position of the armature
(with respect to TL):
switch or relay
• simple structure
• compact (more robust)
• pin compatibility with FET and PIN diodes
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inline or broadside
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4th Round Table on MNT for Space
20/22 May, 2003 (ESTEC, Noordwijk, NL)
Shunt capacitive RF-MEMS switch
implemented on a CPW line
“small” C
RF out
“large” C
GND
GND
Highly resistive substrate
OFF-state
ON-state
SIDE VIEWS
Flexible
metal bridge
Vdc=0V
=20V
RF
choke
dielectric
DC block
TOP VIEW
DC
block RF-out
Zo
V
g
RF+DC in
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RF-in
MCP/EDAS/HT
shunt
switch
Zo
RF signal
generator
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It is the capacitance ratio that counts:
IL<ILspec AND I>Ispec
in the frequency band <ωl,ωu> requires that:
C
ω
r ≡ down > u
Cup
ωl
0.1I spec
−1
0.1ILspec
−1
10
10
Switch modeled as lumped capacitor
Applies for series and shunt switches
Capacitance ratio determined by process (not geometry):
r≡
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C down ε r d o
=
Cup
dε
Conventional capacitive switch
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4th Round Table on MNT for Space
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RF-MEMS electrostatically actuated
capacitive switches at IMEC
shunt
shunt
series
2001
shunt
2002
2000
shunt
2001
Shunt bridge
GND
Slot
RF-MEMS Substrate
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Anchor
Signal Slot
Top floating metal
GND
2002
Shunt bridges
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5/6-Mask Fabrication Process
“Metal Surface Micromachining”
Top floating metal
Sacrificial
layer
Air gap
Dielectric
Metal bridge
Ground
(thin metal)
Ground
(thick metal)
RF-MEMS Substrate
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Signal line
(thin metal)
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ES actuated capacitive shunt switch:
.
RF
5/02
Insertion Loss [dB] (bridge UP)
S21 [dB]
-0.5
Isolation [dB] (bridge DOWN)
0
w/o top metal
-1.0
dB(r8m1bddo..S(2,1))
S21 [dB]
0.0
-1.5
w/o top metal
-10
-20
Down capacitance too low
-30
0
5
10
15
20
25
30
35
40
45
0
5
10
Frequency [GHz]
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15
20
25
30
35
40
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Frequency [GHz] simulation
9
ES actuated capacitive shunt switch:
Introduction floating top metal
floating top metal
RF
5/02
Insertion Loss [dB] (bridge UP)
S21 [dB]
-0.5
w/o top metal
with top metal
-1.0
-1.5
0
5
10
15
20
25
30
35
40
45
Frequency [GHz]
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Isolation [dB] (bridge DOWN)
0
dB(r8m1bddo..S(2,1))
S21 [dB]
0.0
w/o top metal
-10
-20
with top metal
-30
0
5
10
15
20
25
30
35
40
45
Frequency [GHz] simulation
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ES actuated capacitive shunt switch:
Introduction floating top metal Ł “boosted” switch
floating top metal
Insertion Loss [dB] (bridge UP)
0.0
Isolation [dB] (bridge DOWN)
0
boosted
w/o top metal
w/o top metal
with top metal
-1.0
-10
S21 [dB]
S21 [dB]
-0.5
boosted
-20
with top metal
-1.5
-30
0
5
10
15
20
25
30
35
40
45
0
Frequency [GHz]
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5
10
15
20
25
30
35
40
45
Frequency [GHz] simulation
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ES actuated capacitive series switch:
Introduction floating top metal Ł “boosted” switch
Floating top metal
RF in
RF out
CPW line
CPW line
A
Lgap
Lactuation
Lfloat
A’
DC bias
Withtop
floating
µm)
With floating
metaltop metal (100µ
300µ
µm
Wo
w/o floating top metal
200µ
µm
100µ
µm
Wo floating top metal Wo floating
top metal
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With
Withfloating
floatingtop
topmetal
metal (100µ
µm)
(100 – 200 – 300 µm)
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C-V measurements shunt switch
The floating metal makes the difference
w/o top floating metal
1.8
45.0
Adown=Aup= 100 x 60 µ m2
1.6
1.4
Cdown> 1.1 pF
1.2
1.0
0.8
0.6
0.4
0.2
Cdown= 39 pF
40.0
l = 300 µm
Capacitance (pF)
Capacitance (pF)
with top floating metal
Cup= 0.20pF
35.0
30.0
25.0
20.0
15.0
Adown=Afloat =70x450 µm2
10.0
5.0
Cup= 0.15pF
0.0
0.0
0
5
10
15
20
0
25
Aup= 100x20 µm2
5
l = 300 µm
10
15
25
30
• Measured Cdown<< Predicted Cdown
• Measured Cdown= Predicted Cdown
• Cdown increases with increasing bias
• Cdown independent of bias (>Vpull-in)
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Voltage (V)
Voltage (V)
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13
The road to a high r = Cdown/Cup ratio
9
“Boosting”
r = 459
8
r = 102
Cdown [pF]
7
6
5
Introduce top
floating metal
4
3
w/o top metal
2
Conventional
with top metal
1
r = 13
boosted
0
0
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20
40
Cup [fF]
60
80
100
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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:
Cdown =
ε r A float
dε
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
Adown = A float ≠ Aup ≈ Aact
Area difference in up and down state can be used to amplify the
capacitance area r (boosted concept):
n
r≡
C down ε r d o A float
=
•
Cup
dε
Aup
ratio for conventional
capacitive switch
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“BOOSTING Factor β ”
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Broadband switching (1-30 GHz)
A. Single switch:
n
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)
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 >100 (e.g. if
Aup= 20x20µm, then Afloat =4x104µm 2, or for W float = 70µm, Lfloat > 0.6mm)
Is feasible
450µ
µm
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µ
µ
µ
µ
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µ
µ
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4th Round Table on MNT for Space
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Broadband switching (1-30 GHz)
B. Combination switch:
Series-shunt-shunt-series
(with boosted constituents)
s
IDLE-state
RF In
sh
sh
s
RF Out
All the bridge in the air
Series open – shunt closed
Series closed – shunt open
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Need for on-wafer MEMS packaging
MEMS wafer
Capping chip
Bonding
layer
MEMS device
MEMS substrate
Standard wafer sawing will destroy fragile MEMS
Needed for on-wafer protective envelope
0-LEVEL PACKAGE
wafer saw
Cap chip
wafer saw
Cap chip
wafer saw
Cap chip
Cap chip
MEMS wafer
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0-level packaged RF-MEMS switches
C2W assembly
Switch
Edge of
cap
BCB
ring
(100µ
µm)
CPW
RF feedthrough
Sealing ring
Capping chip
RF feedthrough
Metal bridge
Glass Cap
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RF-MEMS Substrate
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Influence of the cap material and
cavity height (planar feedthrough)
CPW Ground
Sealing cap
CPW
BCB
h
CPW Signal line
CPW Ground
dB(w8c7l8culine ..S (2,1))
dB(w8c5l9culine _p..S (2,1))
dB(w8c9l8culine _p..S (2,1))
dB(w8c9l6culine _p..S (2,1))
dB(w8c7l8culine _p..S (2,1))
S21 [dB]
dB(w8c9l10culine
_p..S (2,1))
AF45 glass substrate
0.0
CPW:
naked CPW
(25/100/25µ
µm
-0.1
h=45µm
h=25µm
h=15µm
-0.2
HRSi caps
-0.3
h=5µm
-0.4
Cu 3µ
µm thick
2.3 mm long)
AF45 substrate
Cap material:
standard Si (1-10Ω
Ω cm)
std Si cap (h=85µm)
HRSi (>4000Ω
Ω cm)
-0.5
0
2
4
6
8
10
12
14
16
18
Frequency
[GHz]
fre q, GHz
20
BCB bond (5µ
µm thick)
Transducers’03/AJ
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Conclusions(2)
© imec 2003
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, “twoterminal 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 a
single 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)
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Acknowledgements
Financial support from ESA-ESTEC
under 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)
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