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 ) Cij1 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