Transcript (poster)

An Equivalent Circuit Model
for a Faraday Cage
Substrate Crosstalk Isolation Structure
Joyce H. Wu ([email protected]) and Jesús A. del Alamo
Massachusetts Institute of Technology
Technology
Motivation
Faraday Cage Isolation Structure
Substrate-Via Technology
Key Features
Noisy or sensitive devices/circuits
System-on-Chip
Al
A/D
Al
Digital Logic
A/D
Si
substrate
Faraday
cage
Analog/RF
A/D
Silicon
nitride
75-100 µm
Silicon
nitride
Noisy Substrate
Si
Substrate crosstalk is considered
one of the biggest problems in
mixed-signal circuits
Ta-Ti-Cu
seed
Cu
Grounded via
• Deep reactive ion etch
(DRIE)
• Silicon nitride barrier liner
• Electroplated Cu fills via
Cu ground plane
Fabrication
(4) e-beam
deposition of
Ta-Ti-Cu seed
on backside
(1) photoresist
mask patterning
(7) Cu CMP
frontside for a
flush surface
Si
(5) Cu electroplating to close
bottom of via
(2) DRIE through
the substrate
(3) photoresist
strip, silicon
nitride deposition
from front and
backside
(8) Al e-beam
deposition and
patterning to
form contact to
via
(6) Cu electroplating to fill via
Cu
12 µm
12 µm x 100 µm vias
before Cu CMP step
(aspect ratio = 8)
Measurements
Test Structures
Substrate-Via Impedance
Reference
100
Faraday Cage Test Structure
Substrate Via
Model:
Faraday cage
Ground
100 µm
Signal
100 µm
200 µm
70 µm
Z11 (Ω)
10
Im(Z11)
Re(Z11)
Rv
1
Z11
Ground
Lv
0.1
Transmitter Receiver
Simulation
Measured
Substrate via
0.01
0.1
1
Faraday Cage Substrate Noise Isolation
80
Reference
At 100 µm distance,
on average:
32 dB
Faraday Cage
42 dB
-60
-70
Substrate thickness=77 µm
Separation dist.=100 µm
Via separation=10 µm
Via diameter=10 µm
-80
-90
Air
-100
1 GHz: 41 dB
improvement
10 GHz: 30 dB
improvement
50 GHz: 16 dB
improvement
60
L (pH)
|S 21| (dB)
-50
Frequency=10 GHz
Substrate thickness=77 µm
70
18 dB
-40
100
Frequency (GHz)
-20
-30
10
50
40
30
20
Measurement
Theory (Goldfarb)
10
0
0
10
20
30
Frequency (GHz)
40
50
0
10
20
Nominal aspect ratio
30
Equivalent Circuit Model
Reference Structure
Reference Structure with center split
Rr
Cpad
R2
R3
Cpad
R2
Cr
C3
R3
Cpad
Rr
R1 =
2
C1 = 2Cr
C3
R1
R2
R3
Cpad
R2
C1
C3
Rr = 5 k
Cr = 3 fF
C1
R3
C3
R1 = 2.5 k
C1 = 6 fF
-20
-20
100-µm transmitter-receiver separation
100-µm transmitter-receiver separation
Model
unchanged
by split
-30
|S 21| (dB)
|S 21| (dB)
R1
-40
Add series Rv
and Lv of via
-50
0.1
1
10
100
-30
-40
-50
0.1
1
Frequency (GHz)
10
100
Frequency (GHz)
100-µm transmitter-receiver separation
Faraday Cage Structure
-20
Reference
Cpad
R1
R2
R2
C1
R3
Cpad
Rv
C3
|S 21| (dB)
R1
-40
C1
R3
Rv=1 kΩ
Lv=500 pH
Rv=250 Ω
Lv=200 pH
-60
Rv=70 Ω
Lv=70 pH
-80
C3
Change only Rv
and Lv to evolve
from reference
to Faraday cage
structure
Faraday Cage
Rv=25 Ω
Lv=50 pH
Simulation
Measured
Lv
-100
R1 = 2.5 k
C1 = 6 fF
0.1
1
10
100
Frequency (GHz)
Simulations
Comparison of Measurement and Simulation
Equivalent circuit lumped-element values
Imag
-20
Ref.
Cage
Ref.
Cage
Cage
100 µm
100 µm
200 µm
200 µm
200 µm
10 µm
10 µm
10 µm
10 µm
70 µm
Cr
Rr
C1
3 fF
5 kΩ



6 fF
2 fF
6.4 kΩ



5 fF


5 fF
R1

2.5 kΩ

6 kΩ
6 kΩ
R2
200 Ω
200 Ω
250 Ω
250 Ω
250 Ω
C3
17 fF
17 fF
17 fF
17 fF
17 fF
R3
260 Ω
260 Ω
260 Ω
260 Ω
260 Ω
Rv

25-50 Ω

20 Ω
45-85 Ω
Lv

10-130

30-50
30 pH
Tx-Rx
separation
Reference
S21
S21
Measured
-60
Simulation
Faraday
Cage
-80
100-µm pad separation
Simulation
Measured
freq (540.0MHz to 49.05GHz)
-100
0.1
1
10
100
Frequency (GHz)
• Simple model matches data well
(including real and imaginary S21)
• Range of Rv and Lv consistent with
measured values
• Spread of Rv and Lv of substrate via
accounts for spread in S21 of Faraday cage
Increase Tx-Rx separation distance
Increase via spacing
-20
-30
100-µm
Reference
-60
100-µm
Faraday
Cage
200-µm
Simulation
Measured
Cpad
1.4 pF
10-µm via
spacing
-90
-100
-100
0.1
1
10
Frequency (GHz)
• Increasing pad separation reduces substrate noise
•  R1 and  C1 to account for greater pad separation
100
0.1
1
1.4 pF
1.4 pF
1.4 pF
• Developed a
simple, lumpedelement equivalent
circuit model
Rv=20 Ω
Lv=30 pH
70-µm via
spacing
-70
1.4 pF
pH
Conclusions
-60
-80
-80
pH
Rv=45 Ω
Lv=30 pH
-50
200-µm
spacing
Simulation
Measured
-40
|S 21| (dB)
-40
|S 21| (dB)
Real
0.012
0.010
0.008
0.006
0.004
0.002
0.000
-0.002
-0.004
-0.006
-0.008
-0.010
-0.012
|S 21| (dB)
-40
Via
10
100
Frequency (GHz)
• Increasing via spacing reduces substrate noise isolation
• Effectiveness of Rv-Lv shunt is reduced due to fewer vias
• Only need to increase Rv-Lv values for larger via spacing
• Model matches
experimental data
into mm-wave
regime
• Model will be useful
to evaluate substrate
noise isolation
schemes in actual
circuits