High-performance and Low-cost Capacitive Switches for RF ...
High-performance and Low-cost Capacitive Switches for RF ...
High-performance and Low-cost Capacitive Switches for RF ...
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<strong>High</strong>-<strong>per<strong>for</strong>mance</strong> <strong>High</strong> <strong>per<strong>for</strong>mance</strong> <strong>and</strong> <strong>Low</strong>-<strong>cost</strong> <strong>Low</strong> <strong>cost</strong> <strong>Capacitive</strong><br />
<strong>Switches</strong> <strong>for</strong> <strong>RF</strong> Applications<br />
Bruce Liu<br />
University of Cali<strong>for</strong>nia at Santa Barbara<br />
Toyon Research Corporation<br />
DARPA Fame<br />
Toyon Research Corporation<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
Outline<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• Motivation <strong>for</strong> microelectromechanical (MEM) switches<br />
• MEMS Switch <strong>per<strong>for</strong>mance</strong>s <strong>and</strong> fabrication issues<br />
• MEMS <strong>RF</strong> applications<br />
• BST interdigital capacitors (IDCs)<br />
• <strong>Low</strong>-loss BST phase shifters<br />
• Future work <strong>and</strong> summary<br />
DARPA
Toyon Research Corporation<br />
V 0<br />
V 0<br />
Z 0<br />
Z 0<br />
Series<br />
Switch Z 0<br />
Z d<br />
V<br />
IL<br />
= −20log<br />
V<br />
Overview<br />
+<br />
VL -<br />
V 0<br />
Z 0<br />
Ideal switching circuits<br />
Z 0<br />
+<br />
VL -<br />
V 0<br />
Z 0<br />
Actual switching circuits<br />
L<br />
⎧20log1+<br />
Z<br />
= ⎨<br />
⎩20log1+<br />
Z<br />
/ 2Z<br />
/ 2Z<br />
0 0 d<br />
d<br />
0<br />
,<br />
,<br />
Shunt<br />
Switch<br />
Z d<br />
series switch<br />
shunt switch<br />
Z 0<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Z 0<br />
+<br />
VL -<br />
+<br />
VL -<br />
DARPA
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Switching Technologies<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
•At present, most commonly used switching devices are PIN diodes, GaAs FETs, <strong>and</strong><br />
conventional mechanical switches (relays).<br />
V b<br />
+<br />
V gs<br />
-<br />
I d<br />
I<br />
Vds -<br />
V min<br />
+ -<br />
V<br />
+<br />
O ff state<br />
I d<br />
I dss<br />
I<br />
V th<br />
On state (V gs =0)<br />
slope ≈ 1/R on<br />
On state<br />
V max<br />
Off state (Vgs
Toyon Research Corporation<br />
Key Switch Properties<br />
Important figures of merit <strong>for</strong> switches are:<br />
-isolation<br />
- insertion loss<br />
- power consumption<br />
- power h<strong>and</strong>ling capability<br />
- switching speed<br />
- cutoff frequency<br />
- <strong>cost</strong> of fabrication<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
MEMS<br />
(Micro ElectroMechanical Systems)<br />
MEMS bridge dielectric<br />
t<br />
air<br />
g<br />
G W<br />
substrate<br />
G<br />
MEMS Switch<br />
L<br />
substrate<br />
Switch up Switch down<br />
MEMS<br />
Switch<br />
=<br />
Z d<br />
V<br />
p<br />
=<br />
Off<br />
On<br />
3<br />
8Kg<br />
s 0<br />
27ε<br />
WL<br />
0<br />
C off<br />
C on<br />
Switch up<br />
Switch down<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• Outgrowth of “Micromachining”<br />
- Creation of unique physical structures through the use of sacrificial<br />
layers resulted in miniature mechanical structures one a substrate<br />
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V bias<br />
MEMS <strong>Switches</strong> in <strong>RF</strong> Applications<br />
MEMS Switch in a CPW configuration<br />
<strong>RF</strong> choke<br />
DC Block DC Block<br />
C on /C off<br />
coplanar<br />
waveguide<br />
MEMS bridge<br />
<strong>RF</strong> in<br />
V bias<br />
substrate<br />
+<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
ε 0Ww<br />
C<br />
ε 0ε<br />
rWw<br />
off ≈ Con<br />
≈<br />
g<br />
h<br />
- acts as <strong>RF</strong> switch or capacitor (100:1 ratios)<br />
- loss dominated by conductor loss<br />
- controlled by static DC voltage (10 nJ switching energy)<br />
- low <strong>cost</strong> processing (~4 mask layers)<br />
- high cutoff frequency<br />
- minimum intermodulation distortion<br />
<strong>RF</strong>out<br />
DARPA
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Why <strong>RF</strong> MEMS?<br />
Comparative table<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Switch Type Isolation Insertion loss Power Power Switching Cost<br />
h<strong>and</strong>ling consumption speed<br />
PIN diodes Good Good Good Poor Good Good<br />
GaAs FETs Good Good Poor Good Excell Poor<br />
MEMS<br />
switches<br />
Escell Excell Excell Exell Poor Good<br />
Advantages:<br />
•Very good isolation <strong>and</strong> insertion loss.<br />
•Virtually no control circuit power dissipation in either ON or OFF state<br />
•With proper design, can be capable of broad b<strong>and</strong> <strong>and</strong> high power switching.<br />
•Switching speed is more than sufficient <strong>for</strong> <strong>RF</strong> control circuit applications<br />
•Relatively low <strong>cost</strong> (designed <strong>and</strong> fabricated by st<strong>and</strong>ard processing<br />
techniques). Essentially on every substrate.<br />
⇓<br />
MEMS technology offers superior <strong>per<strong>for</strong>mance</strong> at lower <strong>cost</strong>s <strong>and</strong> is<br />
expected to have tremendous impact on microwave systems.<br />
DARPA
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Key Factors of MEMS <strong>Capacitive</strong><br />
<strong>Switches</strong><br />
MEMS switch typical structure<br />
SiN<br />
substrate<br />
MEMS bridge<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Critical parameters to be carefully considered :<br />
-the height of the membrane over the central conductor ( → C OFF , V PD)<br />
-the thickness <strong>and</strong> composition of the top membrane ( → V PD , mechanical properties)<br />
-the size <strong>and</strong> the geometry of the membrane ( → <strong>RF</strong> <strong>and</strong> DC <strong>per<strong>for</strong>mance</strong>s)<br />
-the thickness <strong>and</strong> type of dielectric coating the central conductor ( → C ON , breakdown)<br />
-type of substrate ( → leakage, parasitic effects)<br />
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photoresist<br />
(PMGI)<br />
Typical Fabrication Process<br />
Substrate<br />
Ti/Au<br />
CPW Pattern<br />
Substrate<br />
Sacrificial layer<br />
Substrate<br />
Sacrifical layer removal<br />
Substrate<br />
SiN layer<br />
Substrate<br />
Membrane deposition<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Ti/Al<br />
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PMGI<br />
Water or solvent<br />
Stiction<br />
Stiction<br />
Substrate<br />
Substrate<br />
Substrate<br />
Membrane Release<br />
Ti/Au<br />
Stiction occurred in an air-dried sample<br />
solution<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Critical Point Drying System<br />
<strong>High</strong> pressure chamber<br />
90 o angle view of the released suspended<br />
structure after critical point drier<br />
DARPA
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Membrane:<br />
– gold<br />
– aluminum<br />
– titanium<br />
– platinum<br />
–nickel<br />
Silicon<br />
Si - Si 3 N 4<br />
Substrates & Metals<br />
GaAs<br />
Si - SiO 2<br />
<strong>High</strong> leakage current<br />
<strong>Low</strong> breakdown voltage<br />
Glass<br />
<strong>Low</strong>est leakage current<br />
<strong>High</strong> breakdown voltage<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Strong metal stress (Pt,Ni,Ti) <strong>Low</strong> metal stress (Al,Au)<br />
DARPA
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Important Factors:<br />
Yield <strong>and</strong> Reliability<br />
• compositions of MEMS top membrane � low stress deposition<br />
• reflow temperature <strong>and</strong> surface roughness of sacrificial layer<br />
Conclusions:<br />
• risky design if span length longer than 300µm<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• reflow temperature need to be adjusted <strong>for</strong> different types of MEMS switch design<br />
• sputtered aluminum <strong>and</strong> electroplated low-stress nickel are good c<strong>and</strong>idates as<br />
MEMS top membrane<br />
DARPA
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DOWN state<br />
capacitance<br />
2-7pF<br />
DC Measurements<br />
Typical DC measurements<br />
UP state<br />
capacitance<br />
<strong>Capacitive</strong><br />
ratio<br />
0.1-0.5pF 4-70<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Pull-down<br />
voltage<br />
20-100 V<br />
DARPA
Toyon Research Corporation<br />
S21 MAG (dB)<br />
0<br />
-0.5<br />
-1<br />
-1.5<br />
-2<br />
-2.5<br />
-3<br />
-3.5<br />
<strong>RF</strong> Simulations <strong>and</strong> Measurements<br />
-4<br />
-40<br />
0 5 10 15 20<br />
Switch UP<br />
S21 measured<br />
S21 fitted<br />
S21 HFSS<br />
Frequency [GHz]<br />
≈<br />
S11 measured<br />
S11 fitted<br />
S11 HFSS<br />
CPW pad CPW pad<br />
R = 0.5Ω<br />
L = 2 pH<br />
C = 0.075 pF<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
-30<br />
-35<br />
S11 MAG (dB)<br />
S21 MAG (dB)<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
S11 measured<br />
S11 fitted<br />
S11 HFSS<br />
S21 measured<br />
S21 fitted<br />
S21 HFSS<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
-25<br />
0 5 10 15 20<br />
Switch DOWN<br />
Frequency [GHz]<br />
• Good agreements between measurements <strong>and</strong> HFSS simulations<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
CPW pad CPW pad<br />
R = 0.5 Ω<br />
L = 2 pH<br />
C = 2.7 pF<br />
• Instead of 3-D EM simulation, lump-element model can also fit measurements<br />
well, which gives us a simplified way to describe MEMS switch <strong>per<strong>for</strong>mance</strong><br />
≈<br />
S11 MAG (dB)<br />
DARPA
Toyon Research Corporation<br />
More Topics on MEMS <strong>Switches</strong><br />
• How do we improve the switch <strong>per<strong>for</strong>mance</strong> ?<br />
h<br />
A<br />
Switch down<br />
Cdown = ε 0ε<br />
r<br />
A<br />
h<br />
• Increasing switching ratio<br />
(i.e. increasing C DOWN <strong>for</strong> given C UP )<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
<strong>Switches</strong> with Metal Caps<br />
Switch up<br />
Pro: Much higher isolation in DOWN<br />
state than previous design<br />
Con:Direct metal-to-metal contact ����<br />
lower switching speed<br />
Metal cap<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
-30<br />
0 5 10 9<br />
Switch down<br />
1 10 10<br />
UP<br />
DOWN<br />
1.5 10 10<br />
Frequency [Hz]<br />
2 10 10<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
2.5 10 10<br />
DARPA<br />
3 10 10
Toyon Research Corporation<br />
•Advantages of <strong>RF</strong> MEMS<br />
-<strong>High</strong> <strong>per<strong>for</strong>mance</strong>, low bias<br />
power consumption<br />
-Potential low <strong>cost</strong><br />
manufacturing into a variety of<br />
substrates<br />
•Limitations of <strong>RF</strong> MEMS<br />
-Slower switching speed<br />
-Potential lifetime limitations<br />
MEMS <strong>RF</strong> Applications<br />
APPLICATIONS<br />
• Phase Shifters<br />
• Filters<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA<br />
• Reconfigurable Antenna
Toyon Research Corporation<br />
L l<br />
C l<br />
C s<br />
L l<br />
C l<br />
C s<br />
L l<br />
C l<br />
Programmable Delay Lines<br />
Variable Phase Velocity Design<br />
Analog or Digital MEMS <strong>Switches</strong><br />
Switched-Capacitor Variable-Capacitor<br />
C s<br />
L l<br />
C l<br />
C s<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Ll Ll Ll Ll Ll Ll C l<br />
C s<br />
C l<br />
C s<br />
C l<br />
C s<br />
C l<br />
C s<br />
DARPA<br />
4-bit Digital Phase Shifter<br />
<strong>RF</strong> in <strong>RF</strong> out<br />
0-22.5º 0-45º 0-90º 0-180º<br />
V 1 V 2 V 3 V 4
Toyon Research Corporation<br />
Transmission-line<br />
Equivalent circuit<br />
Z =<br />
v = 1/<br />
L / C<br />
0 ω <<br />
LC<br />
⎫<br />
⎬<br />
⎭<br />
Loaded<br />
Transmission-line<br />
Distributed Circuit Concepts<br />
2 / LC<br />
Varactor provides variable phase delay:<br />
L<br />
L l<br />
C<br />
C l<br />
L<br />
C s<br />
C<br />
L l<br />
C l<br />
Z 0 , β<br />
L<br />
C<br />
C s<br />
L l<br />
C l<br />
L<br />
C<br />
C s<br />
L l<br />
C l<br />
L<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
C<br />
C s<br />
( )<br />
∆ φ = ω L C + C<br />
l l s<br />
L l<br />
L<br />
DARPA
Toyon Research Corporation<br />
MEMS <strong>Low</strong>-loss Distributed Phase<br />
Shifters<br />
Transmission line sections<br />
Zline , vline Zline , vline Zline , vline<br />
C MEMS<br />
Equivalent Circuit:<br />
L t<br />
C t C MEMS<br />
C MEMS<br />
L sect : Length of transmission line per section<br />
C t : Transmission line capacitance per section<br />
L t : Transmission line capacitance per section<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
C MEMS<br />
Periodic loading of transmission lines with MEMS capacitive switches<br />
creates a structure with a variable phase velocity<br />
DARPA
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Loading Factor:<br />
Min/Max ratio:<br />
Optimization – MEMS Phase Shifters<br />
max<br />
Cvar / Lsect<br />
x =<br />
C<br />
Return Loss: S11 ≤ S11,max<br />
ymin<br />
0.4<br />
0.35<br />
0.3<br />
0.25<br />
0.2<br />
0.15<br />
0.1<br />
0.05<br />
l<br />
⎛1<br />
− S ⎞ 11,<br />
max<br />
a = ⎜ ⎟<br />
⎜1<br />
⎟<br />
⎝ + S11,<br />
max ⎠<br />
min max 1−<br />
a<br />
ymin = Cvar / Cvar≥a− x<br />
2<br />
S 11,max =-20dB<br />
S 11,max =-17dB<br />
S 11,max =-14dB<br />
0<br />
0 2 4 6 8 10<br />
Loading Factor x<br />
Max phase shift/section:<br />
Insertion Loss [dB]<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
( 1+<br />
x − xy )<br />
Lsec<br />
δφ = 2π<br />
f<br />
1<br />
vi<br />
1<br />
fs<br />
=<br />
max<br />
2π<br />
r C<br />
s<br />
var<br />
t +<br />
f<br />
Loss = n<br />
α<br />
5<br />
4<br />
3<br />
2<br />
1<br />
sec t π<br />
2<br />
fs<br />
max<br />
Cvar<br />
Z L + nsect<br />
Lsect<br />
0<br />
0 2 4 6 8 10<br />
Loading Factor x<br />
DARPA<br />
( Z )<br />
i
Toyon Research Corporation<br />
Alternative MEMS Switch<br />
Configurations<br />
Challenge:<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• Not overload the transmission line with an excessively large MEMS<br />
switch capacitance in the DOWN state<br />
C fixed<br />
C var<br />
Solution:<br />
the ‘series’ configuration design<br />
C fixed<br />
Cfixed<br />
C<br />
TOT<br />
Substrate<br />
Cvar<br />
C var C fixed<br />
= 2<br />
C + 2C<br />
if<br />
var<br />
fixed<br />
⎧<br />
⎨<br />
⎩<br />
C<br />
C<br />
var<br />
var<br />
>> C<br />
Toyon Research Corporation<br />
S11 MAG[dB]<br />
Ground<br />
Ground<br />
0<br />
-10<br />
-20<br />
-30<br />
One-bit MEMS Phase Shifters<br />
MEMS capacitors<br />
UP state<br />
S-parameters [dB]<br />
S11<br />
S21<br />
-40<br />
-6<br />
0 5 10 15 20 25 30 35<br />
Frequency [GHz]<br />
0<br />
-1.5<br />
-3<br />
-4.5<br />
S21 MAG[dB]<br />
Differential phase shift [degrees]<br />
350<br />
300<br />
250<br />
200<br />
150<br />
100<br />
50<br />
Simulated<br />
Measured<br />
0<br />
0 5 10 15 20 25 30 35<br />
0<br />
-10<br />
-20<br />
-30<br />
Frequency [GHz]<br />
DOWN state<br />
S-parameters [dB]<br />
-40<br />
-6<br />
0 5 10 15 20 25 30 35<br />
Frequency [GHz]<br />
0<br />
-3<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• The state of the art insertion loss <strong>per<strong>for</strong>mance</strong>, 154°/dB at 25 GHz <strong>and</strong> 160°/dB at<br />
35 GHz, demonstrates the potential <strong>for</strong> the implementation of a very low loss<br />
multi-bit digital MEMS phase shifter.<br />
S11 MAG [dB]<br />
S11<br />
S21<br />
-1.5<br />
-4.5<br />
S21 MAG [dB]<br />
DARPA
Toyon Research Corporation<br />
Differential Phase Shift (Degree)<br />
600<br />
500<br />
400<br />
300<br />
200<br />
100<br />
0<br />
Three-bit MEMS Phase Shifters<br />
DC Block<br />
180° 90° 45°<br />
• Designed <strong>for</strong> 25GHz operation<br />
• 3dB max. insertion loss<br />
• Phase error less than 8.5° <strong>for</strong> all switching states<br />
-100<br />
0 5 10 15 20 25 30 35<br />
Frequency (GHz)<br />
Insertion Loss [dB]<br />
0<br />
-5<br />
-10<br />
-15<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
-20<br />
-30<br />
10 15 20 25 30 35<br />
Frequency [GHz]<br />
10<br />
5<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
Return Loss [dB]<br />
DARPA
Toyon Research Corporation<br />
Topology of 3-Pole Tunable Filters<br />
50 Ω Z1 , L1 Z1 , L1 50 Ω<br />
Input Line<br />
Output Line<br />
CPW Ground<br />
Signal<br />
CPW Ground<br />
C 12 C 23 C n n+1<br />
C01<br />
Coupling<br />
Capacitors<br />
Resonator 1 Resonator n<br />
(a)<br />
C 12<br />
Coupling<br />
Capacitors<br />
Loaded line<br />
Resonator<br />
(b)<br />
Loading MEMS<br />
Capacitors<br />
(a) Topology of capacitively coupled tunable filter based on DMTL resonators.<br />
(b) Layout of 3-section tunable filter in coplanar-waveguide <strong>for</strong>m.<br />
- <strong>Capacitive</strong>ly loaded line sets up a slow wave structure<br />
- Resonator electrical length determined by capacitive loading of the line<br />
C23<br />
C 34<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
- Tuning range of the filter determined by the effective capacitance range of varactor<br />
- MEMS varactors have been demonstrated with low loss making them an excellent<br />
c<strong>and</strong>idate <strong>for</strong> filter applications<br />
DARPA
Toyon Research Corporation<br />
S11 (dB)<br />
5<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
Simulated Results of Tunable Filters<br />
Cvar=12 fF<br />
Cvar=15 fF<br />
Cvar=18 fF<br />
-30<br />
10 15 20 25 30<br />
Frequency (GHz)<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Return Loss Insertion Loss<br />
S21 (dB)<br />
0<br />
-10<br />
-20<br />
-30<br />
-40<br />
-50<br />
Cvar=12 fF<br />
Cvar=15 fF<br />
Cvar=18 fF<br />
-60<br />
10 15 20 25 30<br />
Frequency (GHz)<br />
DARPA
Toyon Research Corporation<br />
S11 [dB]<br />
10<br />
0<br />
-10<br />
-20<br />
-30<br />
Measured Tunable Filter<br />
Per<strong>for</strong>mance<br />
S11 @ V=0V<br />
S11 @ V=50V<br />
S11 @ V=60V<br />
-40<br />
10 15 20 25 30<br />
Frequency [GHz]<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
<strong>RF</strong> IN <strong>RF</strong> OUT<br />
Bias Pad<br />
S21 [dB]<br />
0<br />
-10<br />
-20<br />
-30<br />
-40<br />
S21 @ V=0V<br />
S21 @ V=50V<br />
S21 @ V=60V<br />
-50<br />
10 15 20 25 30<br />
Frequency [GHz]<br />
DARPA<br />
• 12% b<strong>and</strong>width at 20GHz with 3.6dB insertion loss in passb<strong>and</strong><br />
• Loss is contributed by conductive loss, substrate leakage <strong>and</strong> radiation.���� micromachining<br />
• 3.8% tuning range, which is limited by the tuning factor of MEMS varactors (~1.3:1)<br />
SEM microphotograph
Toyon Research Corporation<br />
• Reconfigurable antenna systems:<br />
Reconfigurable Microstrip Grid<br />
Antenna --- Toyon Project<br />
A matrix of <strong>RF</strong> switches in a metallic grid to<br />
control the <strong>RF</strong> current distribution <strong>and</strong> hence<br />
radiation <strong>and</strong> impedance characteristics of<br />
the antenna structure.<br />
• Multiple frequency b<strong>and</strong> operation<br />
• Possible beam <strong>and</strong> null steering<br />
• Lends itself to multiple feed locations<br />
• Polarization diversity<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA<br />
• The switch can be used to turn<br />
“on” or “off” various conductive<br />
paths.<br />
• In this way various virtual antenna<br />
states can be realized.<br />
Microstrip structure with switches<br />
represents over 10 17 possible states.
Toyon Research Corporation<br />
4-Element Microstrip Grid Antenna<br />
SQUARE RECONFIGURABLE GRID ANTENNA INCORPORATING MEMS CONTROL SWITCHES<br />
Aluminum<br />
housing<br />
0.10 cm<br />
Gold<br />
traces<br />
1<br />
MEMS<br />
switches<br />
4-places<br />
4<br />
2.5 cm<br />
2<br />
3<br />
1.50 cm<br />
Glass<br />
substrate<br />
(Er = 5.75)<br />
0.40 cm<br />
0.071 cm<br />
0.312 cm<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
SMA connector<br />
(CDI p/n 5308-2CC)<br />
3/24/00<br />
M. Gilbert<br />
Toyon Research<br />
DARPA
Toyon Research Corporation<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
Preliminary Work on MEMS-based<br />
Grid Antenna<br />
FULL HFSS MODEL -- ALL MEMS DOWN<br />
SQUARE RECONFIGURABLE ANTENNA INCORPORATING MEMS CONTROL SWITCHES<br />
(switches at 3/4 of each leg length, diagonal trace from feed point)<br />
S11 [dB]<br />
simulated<br />
measured<br />
8 8.5 9 9.5 10 10.5 11 11.5 12<br />
Frequency [GHz]<br />
180<br />
150<br />
210<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Radiation Pattern [Electric Field]<br />
120<br />
240<br />
90<br />
270<br />
60<br />
300<br />
30<br />
330<br />
DARPA<br />
0
Toyon Research Corporation<br />
• Motivation<br />
BST Thin Film Technology<br />
Phased Array Antennas<br />
The Phase-Shifter: A critical component<br />
• Thin-Film BST Varactors<br />
Parallel Plate vs. Interdigital Capacitors<br />
Device Characterizations<br />
• Phase-Shifters using Thin-Film BST<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
Feed<br />
12-bit<br />
Beam<br />
Control<br />
Coarse Delay Control<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
Wideb<strong>and</strong> Phased Array<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
Sub-Array<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
True Time<br />
Delay unit<br />
Fine Delay Control<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
TTD<br />
Antenna<br />
Array<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
desired<br />
phasefront<br />
DARPA
Toyon Research Corporation<br />
Barium Strontium Titanate (BST)<br />
Key BST Properties<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
� Large field dependent permittivity --- Compact tunable circuits<br />
� Intrinsically fast field response --- Fast switching speeds<br />
� <strong>High</strong> breakdown fields, >3x10 8 V/cm --- <strong>High</strong> power h<strong>and</strong>ling<br />
capability<br />
� <strong>Low</strong> drive currents (dielectric leakage) --- <strong>Low</strong> prime power<br />
requirement<br />
� Simple fabrication --- <strong>Low</strong> <strong>cost</strong><br />
- BST thin film properties differ markedly from those of bulk <strong>and</strong> much<br />
work is focused on low-<strong>cost</strong> deposition of BST thin film <strong>for</strong> microwave<br />
applications.<br />
DARPA
Toyon Research Corporation<br />
Loss (dB) <strong>for</strong> 360 degrees at 20 GHz<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
0.05<br />
Influence of Loss <strong>and</strong> Tunability on<br />
Phase Shifter Per<strong>for</strong>mance<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Tunability: how much we can vary capacitance of film<br />
C max /C min<br />
0.04<br />
Distributed Circuit Phase Shifter<br />
BST on Silicon, Tunability: 2.5<br />
0.03<br />
Total Loss<br />
CPW conductor Loss<br />
0.02<br />
0.01<br />
Effective Device Loss Tangent at 20 GHz<br />
0<br />
Device loss <strong>and</strong> tunability are important<br />
Loss (dB) <strong>for</strong> 360 degrees at 20 GHz<br />
8<br />
7<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
BST Loss Tangent=0.05<br />
BST Loss Tangent=0.02<br />
BST Loss Tangent=0.01<br />
DARPA<br />
0<br />
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2<br />
BST film tunability<br />
Silicon Substrate
Toyon Research Corporation<br />
BST Device Considerations<br />
Desirable Features of a Viable Thin-Film Varactor<br />
Technology<br />
� Reproducibility<br />
� Inexpensive substrates<br />
� St<strong>and</strong>ard growth/processing steps<br />
� <strong>Low</strong> loss tangent (tan δ < 0.01)<br />
� <strong>High</strong> tunability (>2:1)<br />
� Compatible with low-<strong>cost</strong> packaging<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
- Integrated monolithic capacitors using sputtered/MOCVD material<br />
on low-<strong>cost</strong> substrates<br />
- Sapphire is a <strong>cost</strong> effective wide area substrate that has excellent<br />
microwave <strong>and</strong> rf properties<br />
DARPA
Toyon Research Corporation<br />
Substrate<br />
Monolithic BST Device Structures<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Parallel Plate vs. Interdigitated Capacitor (IDC)<br />
w w w<br />
• Vertical polarization<br />
• <strong>High</strong> tunability, efficient use of BST<br />
• <strong>Low</strong> breakdown voltage<br />
• Complex fabrication<br />
l<br />
w<br />
3w<br />
SiN<br />
BST<br />
Pt<br />
• Horizontal polarization<br />
• <strong>Low</strong> tunability, inefficient use of BST<br />
• <strong>High</strong> breakdown voltage<br />
• Easier fabrication<br />
DARPA
Toyon Research Corporation<br />
Capacitance (pF)<br />
7<br />
6.5<br />
6<br />
5.5<br />
5<br />
4.5<br />
4<br />
Tunability<br />
Width=2um, Spacing=1um<br />
Width=2um, Spacing=2um<br />
3.5<br />
-20 0 20 40 60 80 100<br />
DC Bias (V)<br />
DC Measurements<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Interdigitated Structure<br />
DARPA<br />
-1000Å film<br />
-70-80% of material tunability<br />
-0-100 V control<br />
-Device tunability limited by<br />
finger-to-finger spacings<br />
-Ti/Au metalization<br />
-3-mask process<br />
Good choice <strong>for</strong><br />
low-<strong>cost</strong> circuits
Toyon Research Corporation<br />
<strong>RF</strong> Parameters Extraction Method<br />
Layout <strong>for</strong> Parameters Extraction:<br />
Open Short Load<br />
Equivalent Circuit Expression:<br />
Y op<br />
Y P<br />
Y S<br />
Y sh<br />
Y P<br />
Y S<br />
(YL-Yop )(Ysh-Yop )<br />
≡<br />
Ysh-YL Q= ωC/G<br />
YD<br />
Y L<br />
Y P<br />
≡ G+iωC<br />
Y S<br />
Y D<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
Capacitance [fF]<br />
Quality Factor<br />
200<br />
150<br />
100<br />
50<br />
0<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
w=2µm, s=0.5µm<br />
w=2µm, s=1µm<br />
4 8 12 16 20<br />
Frequency [GHz]<br />
w=2µm, s=0.5µm<br />
w=2µm, s=1µm<br />
5 10 15 20 25 30 35 40<br />
Frequency [GHz]<br />
Quality Factor Limits<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
Total device loss consists of conductor<br />
loss <strong>and</strong> BST material loss:<br />
1 1 1<br />
= +<br />
Qdevice Qcond Qbst<br />
•Qbst , inherent to deposited BST film<br />
•Q cond , determined by R metal <strong>and</strong> C device<br />
DARPA<br />
• Devices with different finger to finger<br />
spacings but same other dimension<br />
parameters are fabricated on the same<br />
wafer<br />
•Q device stays the same while Q cond<br />
varies � Q bst dominates the total<br />
device Q device
Toyon Research Corporation<br />
Quality Factor<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Ba/Sr=50/50<br />
Quality Factor of BST Interdigital<br />
Capacitors<br />
Ba/Sr=30/70<br />
5 10 15 20 25 30 35 40<br />
Frequency [GHz]<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
•100nm (Ba,Sr)TiO 3 thin films<br />
deposited on sapphire substrate<br />
DARPA<br />
•Quality factor rolls off as frequency<br />
increases<br />
•Stoichiometry of deposited thin<br />
film determines its loss tangent<br />
•Q>35 in X-b<strong>and</strong> <strong>for</strong> (Ba 30 ,Sr 70 )TiO 3<br />
thin film<br />
T Surface = 700 °C: <strong>RF</strong> power = 2* 150 W ( 3.3 W/cm 2 ): 90/10 Ar/O 2 (sccm): 100 nm
Toyon Research Corporation<br />
Capacitance [fF]<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Tuning Factors of BST Interdigital<br />
Capacitors<br />
0V<br />
40V<br />
100V<br />
4 8 12<br />
Frequency [GHz]<br />
16 20<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
•Finger spacing = 1µm<br />
•100nm (Ba 30 ,Sr 70 )TiO 3<br />
thin film on sapphire<br />
substrate<br />
•~2:1 tuning range<br />
•0-100V DC control<br />
T Surface = 700 °C: <strong>RF</strong> power = 2* 150 W ( 3.3 W/cm 2 ): 90/10 Ar/O 2 (sccm): 100 nm<br />
DARPA
Toyon Research Corporation<br />
15mm<br />
X-b<strong>and</strong> BST Phase Shifters<br />
15mm<br />
0<br />
-5<br />
-10<br />
-15<br />
-20<br />
-25<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
0V<br />
40V<br />
100V<br />
DARPA<br />
-30<br />
-60<br />
0 2 4 6 8 10<br />
Frequency [GHz]<br />
Photograph of fabricated X-b<strong>and</strong> BST phase shifter S-parameter measurements of distributed<br />
BST phase shifter<br />
Return Loss [dB]<br />
0<br />
-10<br />
-20<br />
-30<br />
-40<br />
-50<br />
Insertion Loss [dB]
Toyon Research Corporation<br />
Differential Phase Shift [Degree]<br />
600<br />
500<br />
400<br />
300<br />
200<br />
100<br />
0<br />
0V<br />
40V<br />
100V<br />
Measured Differential Phase Shift<br />
-100<br />
0 2 4 6 8 10<br />
Frequency [GHz]<br />
Differential phase shift at 0V, 40V <strong>and</strong> 100V<br />
bias voltage<br />
Phase Shift per dB Loss [Degree/dB]<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
10<br />
0 2 4 6 8 10<br />
Frequency [GHz]<br />
Phase shift per dB insertion loss at 100V bias<br />
voltage<br />
• 0º-360º phase shift @ 8.2GHz with 5dB insertion loss<br />
• Return loss is better than –10dB from DC to 10GHz<br />
DARPA
Toyon Research Corporation<br />
State of Progress<br />
What have been done so far?<br />
� Developed a novel design of MEMS devices<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
� MEMS-based <strong>RF</strong> circuits (i.e. phase shifters, filters)<br />
� BST interdigital capacitors <strong>and</strong> phase shifters<br />
What the plan <strong>for</strong> future work?<br />
� BST-MEMS switches<br />
� MEMS-based reconfigurable microstrip grid antenna<br />
� MEMS single-controlled single-pole-double-throw<br />
(SPDT) switches<br />
DARPA
Toyon Research Corporation<br />
BST-MEMS <strong>Switches</strong><br />
Barium Strontium Titanate (BST)<br />
Switch up<br />
BST<br />
Switch down<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
• <strong>High</strong> dielectric constant of BST (>250)<br />
DARPA<br />
• >15dB more isolation than conventional<br />
MEMS switch in DOWN state<br />
What we will do?<br />
• BST/Pt/Au/Ti/Sapphire<br />
• Reduce pull-down voltage, avoid<br />
breakdown in BST thin film
Toyon Research Corporation<br />
Future Work on Reconfigurable<br />
Antenna --- Toyon Project<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
� Demonstrate the 4-element microstrip grid antenna<br />
using st<strong>and</strong>ard MEMS fabrication techniques<br />
� <strong>RF</strong> measurements <strong>and</strong> simulations accounting <strong>for</strong><br />
the parasitic effects in the model<br />
� Finally exp<strong>and</strong> this design to a 16-element switching<br />
reconfigurable antenna array<br />
DARPA
Toyon Research Corporation<br />
INPUT<br />
Control 1<br />
Control 2<br />
OUTPUT 1<br />
OUTPUT 2<br />
Port 1<br />
MEMS Single-Controlled SPDT<br />
<strong>Switches</strong><br />
Signal IN<br />
λ/4<br />
MEMS<br />
Switch 1<br />
λ/4<br />
λ/4<br />
CDOWN<br />
(a) Switch Down<br />
λ/4<br />
Coplanar Waveguide<br />
MEMS<br />
Switch 2<br />
Coplanar Waveguide<br />
Port 2<br />
Port 3<br />
Isolated<br />
λ/4<br />
λ/4<br />
Port 1<br />
λ/4<br />
(b) Switch Up<br />
Signal OUT 2<br />
Signal OUT 3<br />
Port 2<br />
Isolated<br />
Port 3<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
DARPA
Toyon Research Corporation<br />
Summary<br />
Frequency<br />
Agile<br />
Materials <strong>for</strong><br />
Electronics<br />
•MEMS based switches promise superior <strong>per<strong>for</strong>mance</strong> relative to<br />
conventional devices. Through its superior <strong>per<strong>for</strong>mance</strong> characteristics,<br />
the MEMS switches are developed in a number of existing circuits,<br />
including switches, phase shifters <strong>and</strong> filters.<br />
•BST thin films have been characterized <strong>and</strong> used to demonstrate a 0º-<br />
360º X-b<strong>and</strong> phase shifter with only 5dB insertion loss.<br />
•Replace Si 3 N 4 with high dielectric constant BST thin film in MEMS switch<br />
is expected to further improve the switch <strong>per<strong>for</strong>mance</strong>.<br />
•MEMS technology offers the potential <strong>for</strong> building a new generation of<br />
low loss high-linearity reconfigurable antenna arrays <strong>for</strong> radar <strong>and</strong><br />
communications applications.<br />
DARPA
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