Broad-band and Scalable Circuit-level Models of MSM PD for
Co-design with Preamplifier in Front-end Rx Applications
Ph.D. Defense
Spring, 2004
Cheol-ung Cha
Advisor: Prof. Martin A. Brooke
School of Electrical and Computer Engineering
Georgia Institute of Technology
Atlanta, GA, 30332
1
Outline
 Optical Interconnects and Communications
 MSM Photodetector
 Preamplifier
 Modeling Methodology
 Motivation
 Previous Modeling Work
 Partial Element Equivalent Circuit (PEEC) Model
 Proposed Modeling Method
 Partial Elements (PEs) and Test structures
 Measurement-based PEEC (M-PEEC) Model
 Modeling Procedure
 Case study: Straight Line Modeling
 Calibration
 On-wafer Calibration
 MSM Photodetector Modeling
 Partial Elements (PEs) and Test structures
 M-PEEC Model Extraction
 Optimization Results
 Conclusions
2
Optical Interconnects & Communications
 General OIC system
Electrical
Signal
Tx
MUX
Serializer
TIA
Laser
Driver
Re-timer
Power
Control
PLL
AGC
Transmission
channel
Limiting
Amp.
Decision
Circuitry
DEMUX
Recovered
Signal
Clock
Recovery
Rx
Deserializer
3
MSM PD: What is MSM PD?
 Metal-Semiconductor-Metal Photo-Diode (MSM PD)
 Role : Optical signal
Electrical signal
 Condition : hv >Eg (optical) and reverse voltage bias (electrical)
Lightwave
Interdigitated fingers
B
-
+
E
E
Contact pads
A
Frame
4
MSM PD: Advantages
 Advantages of MSM PD
 Low capacitance
 Broad bandwidth
 Ease of monolithic integration
with FETs
 Ease of alignment
FWHM : 12.46ps
 Low dark current (~ nA scale)
 Drawback of MSM PD
 Low responsivity (about 0.2~0.4)
(Low output current level requires sensitive
preamplifier design)
5
MSM PD: Capacitance
 Capacitance is
 Major parasitic component of the MSM PD
 Main limitation factor for high-frequency (multi-GHz) applications
 Three times smaller than that of PIN PD
(Large detection area enables higher alignment tolerance for packaging)
 Conventional formulas are based on the Microwave theory
 Obtained without illumination of light
 Obtained without considering frame
Ex) Conformal mapping theory only considered interdigitated fingers
without considering the effects of frame and light illumination.
6
MSM PD: Capacitance
 Simulation results with preamplifier with
respect to different capacitance values
: 50, 80, 100 fF
100 fF
3dB
80 fF
50 fF
2.2 3 4.2
7
MSM PD: Capacitance
Pad
Frame
MSM PD w/ & w/o
illumination of light
8
MSM PD: Capacitance
Comparison of measured S22
9
MSM PD: Capacitance
 Interdigitated fingers: Conformal mapping theory
Cfingers   o (1   r )
where

2
0
K (k )  
Depends on size,
finger width,
and spacing
K (k ) 1
(n  1)l
K (k ' ) ( w  s)
d
1  k 2 sin 2  ,
 w 

k  tan2 
 4( s  w)  ,
k '  (1  k 2 )
 Frames: Complete elliptic integral of the second kind
n
C frame   Cx  C1  C2  C3  C4  C5  ..... ,
C x   0 r
x

Where
Lx  2a  2 1  e 2 sin 2 ( ) d , and
0
e
A
Lx
a 2  b 2 (0  e  1)
a
10
MSM PD: Capacitance
 Light illumination: External quantum efficiency
C light
Q

V
 AP  
where Q  q   ext  o  and    (1  )(1  e d ) s 
ext
i
 hc 
 s  w
 Total capacitance
CTotal  C fingers  C frame  Clight
11
MSM PD: Capacitance
 20/1/2 MSM photodetector
Theory
Measurement
Capacitance of interdigitated
electrodes (Cfingers)
By conformal mapping
10 fF
NA
Capacitance of Frames (Cframe)
By proposed formula
5.5 fF
6 fF
Capacitance from
illumination of light (Clight)
By proposed formula
2.7 fF
By subtraction
3 fF
Superposition
(CTotal)
18.2 fF
18 fF
What makes this huge difference?
12
MSM PD: Transit time & BW
 Transit time
 The time for a carrier to take to travel through the active region and collected by contacts.
 Low mobility of hole causes a long tail in the impulse response and small bandwidth
in the frequency response.
Depends on
finger spacing
 The transit time is
 tr 
where
vsat
d
vsat
is the saturated carrier velocity and d is the distance of travel.
 Bandwidth (BW)
 Two main factors that limit the speed is “capacitance” and “transit time”
 Trade off between capacitance and transit time (size, finger spacing, and width).
 The BW is
RC time const.
f3 dB 
1
2 ( RC ) ( tr )
2
2
Transit time const.
13
MSM PD: Bandwidth
 Bandwidth of Square MSM PDs
3dB freq. of
transit time const.
3dB freq. of
RC time const.
3dB freq. of
total time const.
14
MSM PD: Bandwidth
 Total bandwidth of MSM PDs (Trade off between RC and transit time const.)
20x20 MSM PDs
40x40 MSM PDs
60x60 MSM PDs
80x80 MSM PDs
15
MSM PD: Lumped Equivalent-circuit Model
Equivalent-circuit model of pad and MSM PD
10 Gbps
20 Gbps
30 Gbps
16
Preamplifier: Performance Metrics
 Key performance metrics of optical receiver
 Bandwidth, Sensitivity, Noise, and Gain
 Mainly determined by front-end (preamplifier and photodetector)
 TransImpedance Amplifier (TIA)
 Convert low-level photocurrent to usable voltage signal
 Feedback in preamplifier
 Extending BW
 Reducing noise (Good sensitivity)
 Controlling input and output impedance
 The close-loop gain is
Ao ( )
1
AC ( ) 

1  Ao ( )  
xs
+
Ao(ω)
xo
- x
f
β
where Ao ( ) the open-loop gain.
17
Preamplifier: Eye Diagrams
 MSM PD with commercial TIA ( Maxim 2.5 Gbps TIA)
BERT
Modulator
Laser
1.2 Gbps
Oscilloscope
2.5 Gbps TIA
MSM PD
2 Gbps
50 Ohm
Matched
The output current of
MSM PD (60/1/2) is too
weak to be detected by
oscilloscope
3Gbps
6Gbps
18
Outline
 Optical Interconnects and Communications
 MSM PD
 Preamplifier
 Modeling Methodology
 Motivation
 Previous Modeling Work
 Partial Element Equivalent Circuit (PEEC) Model
 Proposed Modeling Method
 Partial Elements (PEs) and Test structures
 Measurement-based PEEC (M-PEEC) Model
 Modeling Procedure
 Case study: Straight Line Modeling
 Calibration
 On-wafer Calibration
 MSM Photodetector Modeling
 Partial Elements (PEs) and Test structures
 M-PEEC Model Extraction
 Optimization Results
 Conclusions
19
Motivation: Higher Performance
 Demand for higher bandwidth and speed requires well-designed
front-end (preamplifier with photodetector) of optical Rx.
 Front-end is a dominant component in a Rx because the sensitivity of the Rx is mainly
determined by the noise factor of the front-end.
F
SNRinput
SNRoutput
,
F  F1 
F2  1 F3  1

 ....
G1
G1G2
 Reduction in bandwidth comes from the parasitic capacitance of a photodetector and pad.
 The capacitance of bond-pad is typically 10–50 fF (significant for GHz circuitry).
- Flip-chip bonding techniques can be used to reduce parasitics at the interface
between InGaAs and CMOS.
 The capacitance of commercial PIN and avalanche photodiode is 200–900 fF.
- Using MSM PDs, this value can be reduced up to 50-300 fF.
(The reduced capacitance would allow enough budgets for circuit design)
Solution
Co-design of photodetector with preamplifier is a solution
: when a circuit designer design circuitry, he/she can choose proper
device specifications such as device size, finger spacing and width,
and thickness of active layer to satisfy the requirements.
20
Motivation: Modeling Method
 Modeling methodology for co-design should be
 Easy to use (Needs to be integrated into existing circuit design environment such as HSPICE
and ADS.
- This approach circumvents the inconvenient, iterative interface between
a photonic device simulator and a circuit design tool.
 Fast
- The finite-element methods need long simulation time and huge memory resource
 Accurate
- Existing analytical equation-based methods are not accurate.
 Scalable
- Modeling method can predict the model of different dimensional device.
21
Modeling Methodology Tree
Analytical (Equation-based)
Improved in this research
for the capacitance modeling
of the MSM PD
Numerical (EM full wave-based)
Time domain
Frequency domain
Differential equation
Integral equation
(Grids on whole area)
(Grids only on conductors)
Empirical (Measurement-based)
Measurement-based Partial Element
Equivalent Circuit (M-PEEC)
Proposed in this research
Finite Methods
(Discretization)
Finite Difference
Time Domain
Method of
Moments (MoM)
Finite Element
(Spatial discretization)
Electric Field
Integral Equation
Partial Element
Equivalent Circuit
(Discrete Approx. of EFIE)
Finite Element
Equivalent Circuit
22
Previous Modeling Work
 Earlier work in high frequency component modeling mainly
originated from the microwave engineering community.
 Three fundamental methodologies
 Analytical equation-based modeling method
Direct derivation from first physical principles
- very few, available only for very simple structures
 Generally difficult and time consuming to develop
 Not very flexible
 Not accurate

 Numerical EM-full wave based modeling method
Accurate
 Highly flexible
 Very slow and requiring huge memory resource, so it’s not practical
for complex geometry system analysis
23
Previous Modeling Work
 Two dominant methods exist (continued)
- The Finite Element Method (FEM)
• FEM yields high accuracy for 3 dimensional structures.
• Grids structure into many small pieces, and solves Maxwell’s Equations
- The Method of Moment (MoM)
• MoM is a 2 1/2-D method with less accuracy in 3 dimensions.
• Assumes a conductor height of zero.
• Grids structure into many small pieces, and solves Green’s Function
 Measurement-based modeling method
Measured data from time or frequency domain can be fit to a circuit model
using optimization techniques
 Non-ideal processing effects can be considered
 The method allows for statistical modeling
 Very accurate for measured structures
 Not very flexible

Improved measurement-based, scalable, and flexible modeling method
24
Partial Element Equivalent Circuit (PEEC) Model
 Three dimensional partial element equivalent circuit (PEEC) model was
originated from high-speed interconnect modeling in 1970s[Ruehli].
 The PEEC method is based on Maxwell’s integral equation that is interpreted
in terms of RLC elements and their couplings.
 Maxwell’s Electric Field Integral Equation (EFIE)
    A  r, t     r, t   0
J r, t

t
The advantages of the PEEC method are
 The output of the PEEC analysis is spice-like equivalent-circuit model
(it can be easily integrated with other circuit models such as transistor
models into a conventional circuit simulation tools such as SPICE).
 The PEEC models work equally well in the time and frequency domains.
 The PEEC analysis can reduce simulation time by using Maxwell’s integral equation.
 The PEEC models include cross coupling terms.
25
Partial Element Equivalent Circuit (PEEC) Model
Square Partial Element (PE)
Pad Partial Element (PE)
CS15
CP15
CS13
LS22
CS11
CS35
RS22
RS44
CS33
LS24
CP13
LS44
LP22
CS55
CP11
CP35
RP22
RP44
CP33
LP44
CP55
LP24
26
Partial Element Equivalent Circuit (PEEC) Model
 Primitive PEEC cell
Capacitive cell_1
 i2 (t )
 i (t )
 Lp 24 4   3 (t )  1 (t )  0
t
t
 i (t )
 i (t )
i4 (t ) R 44  L p 24 2  L p 44 4   4 (t )   3 (t )  0
t
t
Capacitive cell_3
Capacitive cell_5
i2 (t ) R 22  Lp 22
c11
c
 13
c15
c13
c33
c35
c15   1 (t )   q1 (t ) 
c35   3 (t )   q3 (t ) 
c55   5 (t ) q5 (t )
n
j 1
 i j (t )
t
Inductive cell_2
5
Inductive cell_4
C15
3
C13
1
L22

3
w
d
 In the general case, the ith circuit equations
of n inductive and m capacitive cells are
ii (t ) R22   L p i j
1
C35
R22
R44
5
L44

  l (i ) (t )   k (i ) (t )  0
C15
C15
C15
m
 i (t )   Cij1 Q j (t )
j 1
L24
where l (i ) and k (i) are the index of the capacitive cells connected to inductive cell i.
27
Outline
 Optical Interconnects and Communications
 MSM PD
 Preamplifier
 Modeling Methodology
 Motivation
 Previous Modeling Work
 Partial Element Equivalent Circuit (PEEC) Model
 Proposed Modeling Method
 Partial Elements (PEs) and Test structures
 Measurement-based PEEC (M-PEEC) Model
 Modeling Procedure
 Case study: Straight Line Modeling
 Calibration
 On-wafer Calibration
 MSM Photodetector Modeling
 Partial Elements (PEs) and Test structures
 M-PEEC Model Extraction
 Optimization Results
Conclusions
28
Partial Elements (PEs) & Test Structures
 If we can accurately model individual parts of a structure, then we can
predictively model any structure comprised of those parts accurately.
 Those individual parts are called “Partial Elements” (PEs).
 “Test structures” are designed, fabricated, and measured to extract the
equivalent circuit models, which are called “ Measurement-based partial
element equivalent circuits (M-PEECs).”
 Partial elements must have enough sensitivity within a test structure
in order to be deembedded.
 Initial guesses are derived from measured S-parameters.
 Optimized M-PEEC models, which are resulted from one test structure, are
used in extracting other M-PEEC models for subsequent test structures.
 Models of different geometry structures can be created by combining
M-PEEC models of partial elements.
29
Measurement-based PEEC (M-PEEC) Model
 The M-PEEC models have these advantages:
 The
M-PEEC models are accurate because they are derived from test structures and
measurements that automatically include unexpected processing effects such as
processing fluctuations, uneven depositions, and non-ideal material properties.
 The M-PEEC models can be generated easily and simulated very quickly in a standard
and conventional circuit simulator.
 The M-PEEC models can be applicable to both electrical and optical devices (passive
and active devices) and interconnects modeling which are electrically long and short
structures. (In case of optical devices modeling, iterative and inconvenient interface
between optical device and electrical circuit simulators can be overcome).
 The M-PEEC models are independent of technology or the process in which the
structures are fabricated because changed and modified factors are automatically taken
into account in the measurements.
 The M-PEEC models are scalable and predictive since equivalent-circuit models of
different dimensional devices can be constructed from obtained several M-PEEC models.
 The M-PEEC models can take into account statistical information in the models.
30
Modeling Procedure
 Design and Modeling Flow
What structure to be considered?
Generate Design Rule Library
Define Partial Elements (PEs)
Design Desired
Device
Design & Fab. Test Structures
Calibration & Measurement
Extract M-PEEC models
using optimization
Design Rule
Checking
Pass
Fail
Co-simulation with
Circuitry in SPICE-type
Simulator
Accurate simulation results
31
Case Study: Straight Line Modeling

Straight line is meshed into 20 square PEs and pads
by commercial EM simulator (MoM in ADS)
20 square PEs
Coplanar
waveguide
32
Case Study: Straight Line Modeling

Straight line is meshed into 20 square PEs and 2 pads
by the proposed modeling method.
Square Partial Element (PE)
Square
M-PEEC
Pad Partial Element (PE)
Pad
M-PEEC
33
Case Study: Straight Line Modeling

Two PEs and their parameter values of M-PEECs
Square Partial
Element (PE)
Pad Partial
Element (PE)
34
Case Study: Straight Line Modeling

S11 comparison: measured data, MoM model, and M-PEEC model.
Measured data
Mom model
M-PEEC model
35
Case Study: Straight Line Modeling

S21 comparison: measured data, MoM model, and M-PEEC model.
Measured data
Mom model
M-PEEC model
36
Outline
 Optical Interconnects and Communications
 MSM PD
 Preamplifier
 Modeling Methodology
 Motivation
 Previous Modeling Work
 Partial Element Equivalent Circuit (PEEC) Model
 Proposed Modeling Method
 Partial Elements (PEs) and Test structures
 Measurement-based PEEC (M-PEEC) Model
 Modeling Procedure
 Case study: Straight Line Modeling
 Calibration
 On-wafer Calibration
 MSM Photodetector Modeling
 Partial Elements (PEs) and Test structures
 M-PEEC Model Extraction
 Optimization Results
Conclusions
37
On-wafer Calibration
 Calibration : Defining the ends of a measurement
system and the begins of a DUT
Reference plane
38
On-wafer Calibration
 SOL on-wafer calibration
 SOL (Short-Open-Load)
 On-wafer : Calibration structures are on the same substrate with DUT
Short
Open
NiCr Resistors
Original
Load
Trimmed
Load
39
On-wafer Calibration
29.286 Ohm
28.809 Ohm
 Un-trimmed load
 Designed for 25 Ohm.
 NiCr is used.
49.873 Ohm
50.025 Ohm
 Laser-trimmed load
 Optimized for 50 Ohm.
 NiCr is used.
40
Outline
 Optical Interconnects and Communications
 MSM PD
 Preamplifier
 Modeling Methodology
 Motivation
 Previous Modeling Work
 Partial Element Equivalent Circuit (PEEC) Model
 Proposed Modeling Method
 Partial Elements (PEs) and Test structures
 Measurement-based PEEC (M-PEEC) Model
 Modeling Procedure
 Case study: Straight Line Modeling
 Calibration
 On-wafer Calibration
 MSM Photodetector Modeling
 Partial Elements (PEs) and Test structures
 M-PEEC Model Extraction
 Optimization Results
Conclusions
41
Partial Elements (PEs) and Test structures

Partial Elements (PEs) and Test structures for MSM PD modeling
Interdigitated PE
Pad PE
Line PE
Test structures
42
Partial Elements (PEs) and Test structures
 “MSM PD” is comprised of “interdigitated partial elements” and couplings
Coupling
Inductance
Coupling
Capacitance
Interdigitated partial
element (PE)
43
Step I: Pad M-PEEC Model Extraction
Pad
Extracting
Circuit model
This obtained “Pad M-PEEC” is
used for “Line M-PEEC” extraction.
“Line M-PEEC” modeling
44
Step II: Line M-PEEC Model Extraction
Line
Pad
Line
Line
Line
Line
This obtained
“Line M-PEEC”
is used for
“Interdigitated M-PEEC”
extraction.
45
Step III: Interdigitated M-PEEC Model Extraction
Pad
Line
M-PEEC
Interdigitated
M-PEEC
Line
M-PEEC
This obtained
“Interdigitated M-PEEC”
is used for “MSM PDs”
modeling.
46
M-PEEC Model Extraction : Parameters

Three PEs and their parameter values of M-PEECs
Pad Partial
Element (PE)
Line Partial
Element (PE)
Interdigitated Partial
Element (PE)
47
Optimization Results: Scalable Model
Pad
PE
Line
PE
Interdigitated PE
Coupling
Inductance
Coupling
Capacitance
48
Optimization Results : Test Structures
49
Optimization Results : Scalable Model
 40/1/1 um MSM Photodetector
S21 (Lin) of MSM Photodetectors(40um)
S21 (Pha) of MSM Photodetectors(40um)
0.068
90
Measured
M-PEEC
Equation
0.066
Meadured
M-PEEC
Equation
88
86
0.062
S21(Pha)
S21(Lin)
0.064
0.06
0.058
84
82
0.056
80
0.054
0.052
0
2
4
6
Frequency
8
10
12
x 10
9
78
0
2
4
6
Frequency
8
10
12
x 10
9
50
Optimization Results : Scalable Model
 40/1/1 um MSM Photodetector
S22 (Lin) of MSM Photodetectors(40um)
S22 (Pha) of MSM Photodetectors(40um)
0.98
0
Measured
M-PEEC
Equation
0.97
Measured
M-PEEC
Equation
-5
S22(Pha)
S22(Lin)
0.96
0.95
-10
-15
0.94
-20
0.93
0.92
0
2
4
6
Frequency
8
10
12
x 10
9
-25
0
2
4
6
Frequency
8
10
12
x 10
51
9
Optimization Results : Scalable Model
 60/1/1 um MSM Photodetector
S21 (Lin) of MSM Photodetectors(60um)
S21 (Pha) of MSM Photodetectors(60um)
0.075
90
Measured
M-PEEC
Equation
0.07
88
86
S21(Pha)
0.065
S21(Lin)
Measured
M-PEEC
Equation
0.06
84
82
0.055
80
0.05
0.045
78
0
2
4
6
Frequency
8
10
12
x 10
9
76
0
2
4
6
Frequency
8
10
12
x 10
52
9
Optimization Results : Scalable Model
 60/1/1 um MSM Photodetector
S22 (Lin) of MSM Photodetectors(60um)
S22 (Pha) of MSM Photodetectors(60um)
1
0
Measured
M-PEEC
Equation
0.98
-10
0.96
-20
S22(Pha)
S22(Lin)
Measured
M-PEEC
Equation
0.94
0.92
-30
-40
0.9
-50
0.88
0.86
-60
0
2
4
6
Frequency
8
10
12
x 10
9
0
2
4
6
Frequency
8
10
12
x 10
53
9
Conclusions
 An
improved measurement-based modeling method has been
proposed and developed for co-design
 The main features of developed M-PEEC method are
Accurate
 Fast
 Scalable and predictive
 Process independent
 Implementable within existing EDA frameworks such as SPICE
 Applicable to 2 and 3-D electrical and optical structures

54
Acknowledgement
 Gratitude
to:
 Dr. Brooke and Dr. Jokerst
 Committee members: Dr. Hasler, Dr. Rhodes,
Dr. Chang, and Dr. Kohl
 Group members
55
Questions and Answers
56