Broad-band and Scalable Circuit-level Models of MSM PD for Co-design with Preamplifier in Front-end Rx Applications

1
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

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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

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Optical Interconnects Communications

• General OIC system

Transmission channel
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MSM PD What is MSM PD?

• Metal-Semiconductor-Metal Photo-Diode (MSM PD)
• Role Optical signal Electrical
  signal
• Condition hv gtEg (optical) and reverse voltage
  bias (electrical)

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MSM PD Advantages

• Advantages of MSM PD
• Low capacitance
• Broad bandwidth
• Ease of monolithic integration
• with FETs
• Ease of alignment
• Low dark current ( nA scale)
• Drawback of MSM PD
• Low responsivity (about 0.20.4)
• (Low output current level requires sensitive
• preamplifier design)

• FWHM 12.46ps

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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.

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MSM PD Capacitance

• Simulation results with preamplifier with
• respect to different capacitance values
• 50, 80, 100 fF

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MSM PD Capacitance
Pad
Frame
MSM PD w/ w/o illumination of light
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MSM PD Capacitance

• Comparison of measured S22

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MSM PD Capacitance

• Interdigitated fingers Conformal mapping theory

Depends on size, finger width, and spacing
where
,
,

• Frames Complete elliptic integral of the second
  kind

,
Where
, and
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MSM PD Capacitance

• Light illumination External quantum efficiency

where
and

• Total capacitance

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MSM PD Capacitance

• 20/1/2 MSM photodetector

Measurement
Theory
NA
By conformal mapping 10 fF
Capacitance of interdigitated electrodes
(Cfingers)
6 fF
By proposed formula 5.5 fF
Capacitance of Frames (Cframe)
What makes this huge difference?
By subtraction 3 fF
By proposed formula 2.7 fF
Capacitance from illumination of light (Clight)
18 fF
18.2 fF
Superposition (CTotal)
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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.
• The transit time is

Depends on finger spacing

• 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.
Transit time const.
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MSM PD Bandwidth

• Bandwidth of Square MSM PDs

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MSM PD Bandwidth

• Total bandwidth of MSM PDs (Trade off between RC
  and transit time const.)

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MSM PD Lumped Equivalent-circuit Model

• Equivalent-circuit model of pad and MSM PD

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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

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Preamplifier Eye Diagrams

• MSM PD with commercial TIA ( Maxim 2.5 Gbps TIA)

The output current of MSM PD (60/1/2) is too weak
to be detected by oscilloscope
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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

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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.

• Reduction in bandwidth comes from the parasitic
  capacitance of a photodetector and pad.
• The capacitance of bond-pad is typically 1050
  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 200900 fF.
• - Using MSM PDs, this value can be
  reduced up to 50-300 fF.
• (The reduced capacitance would allow
  enough budgets for circuit design)

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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.

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Modeling Methodology Tree
Frequency domain
Time domain
Measurement-based Partial Element Equivalent
Circuit (M-PEEC)
Improved in this research for the capacitance
modeling of the MSM PD
Differential equation (Grids on whole area)
Integral equation (Grids only on conductors)
Proposed in this research
Electric Field Integral Equation
Finite Methods (Discretization)
Method of Moments (MoM)
Finite Element (Spatial discretization)
Finite Difference Time Domain
Partial Element Equivalent Circuit (Discrete
Approx. of EFIE)
Finite Element Equivalent Circuit
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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
  its not practical
• for complex geometry system analysis

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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 Maxwells 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 Greens 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
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Partial Element Equivalent Circuit (PEEC) Model

• Three dimensional partial element equivalent
  circuit (PEEC) model was
• originated from high-speed interconnect
  modeling in 1970sRuehli.
• The PEEC method is based on Maxwells integral
  equation that is interpreted
• in terms of RLC elements and their couplings.
• Maxwells Electric Field Integral Equation
  (EFIE)
• 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 Maxwells integral equation.

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Partial Element Equivalent Circuit (PEEC) Model
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Partial Element Equivalent Circuit (PEEC) Model

• Primitive PEEC cell

• In the general case, the ith circuit equations
• of n inductive and m capacitive cells are

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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

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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.

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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.

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Modeling Procedure

• Design and Modeling Flow

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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
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Case Study Straight Line Modeling

• Straight line is meshed into 20 square PEs and 2
  pads
• by the proposed modeling method.

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Case Study Straight Line Modeling

• Two PEs and their parameter values of M-PEECs

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Case Study Straight Line Modeling

• S11 comparison measured data, MoM model, and
  M-PEEC model.

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Case Study Straight Line Modeling

• S21 comparison measured data, MoM model, and
  M-PEEC model.

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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

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On-wafer Calibration

• Calibration Defining the ends of a
  measurement
• system and the begins of a DUT

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On-wafer Calibration

• SOL on-wafer calibration
• SOL (Short-Open-Load)
• On-wafer Calibration structures are on the
  same substrate with DUT

NiCr Resistors
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On-wafer Calibration
28.809 Ohm
29.286 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.

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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

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Partial Elements (PEs) and Test structures

• Partial Elements (PEs) and Test structures for
  MSM PD modeling

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Partial Elements (PEs) and Test structures

• MSM PD is comprised of interdigitated
  partial elements and couplings

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Step I Pad M-PEEC Model Extraction
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Step II Line M-PEEC Model Extraction
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Step III Interdigitated M-PEEC Model Extraction
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M-PEEC Model Extraction Parameters

• Three PEs and their parameter values of M-PEECs

48
Optimization Results Scalable Model
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Optimization Results Test Structures
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Optimization Results Scalable Model

• 40/1/1 um MSM Photodetector

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Optimization Results Scalable Model

• 40/1/1 um MSM Photodetector

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Optimization Results Scalable Model

• 60/1/1 um MSM Photodetector

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Optimization Results Scalable Model

• 60/1/1 um MSM Photodetector

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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

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Acknowledgement

• Gratitude to
• Dr. Brooke and Dr. Jokerst
• Committee members Dr. Hasler, Dr. Rhodes,
  Dr. Chang, and Dr. Kohl
• Group members

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Questions and Answers
Thank you! Questions