An Equivalent Circuit Model

1
An Equivalent Circuit Model for a Faraday Cage
Substrate Crosstalk Isolation Structure
Joyce H. Wu (jowu_at_mtl.mit.edu) 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
Silicon nitride
Al
Si substrate
Faraday cage
75-100 µm
Silicon nitride
Ta-Ti-Cu seed
Si
Substrate crosstalk is considered one of the
biggest problems in mixed-signal circuits
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
(5) Cu electro-plating to close bottom of via
(2) DRIE through the substrate
(8) Al e-beam deposition and patterning to form
contact to via
(3) photoresist strip, silicon nitride deposition
from front and backside
(6) Cu electro-plating to fill via
12 µm x 100 µm vias before Cu CMP step (aspect
ratio 8)
Measurements
Test Structures
Substrate-Via Impedance
Faraday Cage Test Structure
Reference
Substrate Via Model
Faraday cage
Im(Z11)
Ground
100 µm
200 µm
100 µm
Re(Z11)
Signal
Rv
70 µm
Z11
Ground
Lv
Transmitter
Receiver
Substrate via
Faraday Cage Substrate Noise Isolation
2
Equivalent Circuit Model
Reference Structure
Reference Structure with center split
Rr
R1
R1
Cpad
Cpad
Cpad
Cpad
R2
R2
R2
R2
C1 2Cr
Cr
C1
C1
R3
C3
R3
C3
R3
C3
R3
C3
Rr 5 k? Cr 3 fF
R1 2.5 k? C1 6 fF
100-µm transmitter-receiver separation
100-µm transmitter-receiver separation
Model unchanged by split
Add series Rv and Lv of via
100-µm transmitter-receiver separation
Faraday Cage Structure
Reference
R1
R1
Rv1 kO Lv500 pH
Cpad
Cpad
Change only Rv and Lv to evolve from reference to
Faraday cage structure
R2
R2
Rv250 O Lv200 pH
Rv70 O Lv70 pH
C1
C1
Faraday Cage
Rv
R3
C3
R3
C3
Rv25 O Lv50 pH
Lv
R1 2.5 k? C1 6 fF
Simulations
Equivalent circuit lumped-element values
Comparison of Measurement and Simulation
Imag
Measured
Simulation
Real
S21

• 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

Conclusions
Increase Tx-Rx separation distance
Increase via spacing

• Developed a simple, lumped-element equivalent
  circuit model
• Model matches experimental data into mm-wave
  regime
• Model will be useful to evaluate substrate noise
  isolation schemes in actual circuits

Rv45 O Lv30 pH
Rv20 O Lv30 pH
70-µm via spacing
10-µm via spacing

• 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

• Increasing pad separation reduces substrate
  noise
• ? R1 and ? C1 to account for greater pad
  separation