reliability analysis of flexible electronics: case study of hydrogenated amorphous silicon (a-si:h)...
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Reliability Analysis of Flexible Electronics:
Case Study of Hydrogenated Amorphous Silicon
(a-Si:H) TFT Scan Driver
Tsung-Ching (Jim) Huang
Tim Cheng
Feb. 10th 2007
2
Outline
IntroductionMotivation: Why a-Si:H TFT scan driverReliability of a-Si:H TFT circuits
Electrical degradation and self-recovery
Reliability Analysis for Flexible ElectronicsDevice degradation modelReliability simulation
Conclusion
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Why A-Si:H TFT Scan Driver
Type of TFT Single-Crystal Poly- Si A-Si:H Organic
Proc. Temp. 1000 ºC 450 ºC 200 ºC < 100 ºC
Mobility 270 cm2/Vs 250 cm2/Vs 1 cm2/Vs 0.5 cm2/Vs
Cost/Area High High Medium Low
Maturity High Medium High Low
Flex. Sub. Transfer Transfer Direct Direct
Degrade. N/A Electrical Electrical Elec./Chem.
• Samsung Gen VII display line will produce an area of active electronics equal to ~ 10% of total worldwide IC area per year
• 60,000 1870 mm X 2200 mm panel/month
• Scan driver is used to generate scanning signal to drive TFTs
Ref: Samsung; T.N. Jackson, Penn State Univ.
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Reliability Concern of A-Si:H TFT
Ref: C.-S. Chiang et al, Jap. J. Applied Physics, 1998; AUO Taiwan
a-Si:H(Semiconductor)
Substrate
SiNx (Insulator)
SiNx
Gate
Drain Source
n+ a-Si:H
• Prolonged bias-stress to a-Si:H TFTs will induce electrical degradation which causes threshold voltage (VTH) shift
• Electrical degradation is attributed to bias-induced dangling bonds
• Charge trapping in the SiNx layer and point defect creation in the a-Si:H layer are the major mechanisms
5
Outline
IntroductionMotivation: Why a-Si:H TFT scan driverReliability of a-Si:H TFT circuits
Electrical degradation and self-recovery
Reliability Analysis for Flexible ElectronicsDevice degradation modelReliability simulation
Conclusion
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Electrical Degradation Model
VTH degradation
Pulsed-bias VGS
DC-bias VGS
Pulsed-bias VGS + pulsed-biased VDS
VTH recovery
Reverse pulsed-bias VGS
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Methodology for Reliability Simulation
InputPattern
Device Analyzer
Optimizer
Control Console
Analytical ModelSelection
Device Characterization
FittingParameters
Input SegmentSlicer
Circuit Netlist
HSPICESimulator/Interface
Transient OutputExtractor
Prediction &
Optimization
Model ParameterAdjustment
Degraded Netlist Generator
Ready For Simulation
Analyze
SPICE-level circuit simulationHigh accuracyCompatible with RPI TFT- modelCompatible with Verilog-a behavioral mo
delComprehensive model parameters
Iterative reliability simulationIncrementally change model parameters t
o mimic physical degradation processHigh-accuracy with measured device deg
radation model parameters under various conditions
Simulation time reduction Auto-regressive invariant moving average (ARIMA) model
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Case Study: A-Si:H TFT Scan Driver
A-Si:H TFT scan driver integrated with the LCD pixel circuits on the glass substrate
Save the cost of wire bonding and packaging
Eliminate the need for driver ICsCompatible with low-temperature process
for plastic substrates Device degradation depends on its bias-stress
Degradation profile for each TFT can be obtained by analyzing its bias-stress
Reliability simulation can predict circuit lifetime based on bias-stress analysis
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Comparison of Simulation and Measurement Results
Fig. 1. Before Degradation Fig. 2 After Degradation (33,000s, 85 ºC)
Reliability simulation tool provides a fast and yet accurate way of estimating circuit reliability with a-Si:H TFTs
No physical layout information is required SPICE-compatible Device degradation model and input pattern are needed
Simulation Simulation
Measured Measured
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Conclusion
Flexible electronics are emergingFuture trend in consumer electronicsPotential applications includes:
E-paper, flexible display RFID tags, Implantable IDtags Ubiquitous sensor arrays & rollable solar cells
Reliability analysis is essential Electrical degradation is severe vs. CMOSRobust circuit design and architecture is critical
Our reliability simulation tool shows:Predicting circuit reliability is possible with high accuracy with
in reasonable simulation time