Tsung-Ching (Jim) Huang Tim Cheng Feb. 10th 2007

1. 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 • Introduction • Motivation: Why a-Si:H TFT scan driver • Reliability of a-Si:H TFT circuits • Electrical degradation and self-recovery • Reliability Analysis for Flexible Electronics • Device degradation model • Reliability simulation • Conclusion

3. Why A-Si:H TFT Scan Driver • 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.

4. Reliability Concern of A-Si:H TFT • 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 Ref: C.-S. Chiang et al, Jap. J. Applied Physics, 1998; AUO Taiwan

5. Outline • Introduction • Motivation: Why a-Si:H TFT scan driver • Reliability of a-Si:H TFT circuits • Electrical degradation and self-recovery • Reliability Analysis for Flexible Electronics • Device degradation model • Reliability simulation • Conclusion

6. Electrical Degradation Model • VTH degradation • Pulsed-bias VGS • DC-bias VGS • Pulsed-bias VGS + pulsed-biased VDS • VTH recovery • Reverse pulsed-bias VGS

7. Methodology for Reliability Simulation • SPICE-level circuit simulation • High accuracy • Compatible with RPI TFT- model • Compatible with Verilog-a behavioral model • Comprehensive model parameters • Iterative reliability simulation • Incrementally change model parameters to mimic physical degradation process • High-accuracy with measured device degradation model parameters under various conditions • Simulation time reduction • Auto-regressive invariant moving average (ARIMA) model

8. 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 ICs • Compatible 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

9. Comparison of Simulation and Measurement Results Simulation Simulation Measured Measured 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

10. Conclusion • Flexible electronics are emerging • Future trend in consumer electronics • Potential 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. CMOS • Robust circuit design and architecture is critical • Our reliability simulation tool shows: • Predicting circuit reliability is possible with high accuracy within reasonable simulation time

11. Q & A Thank you for your attention !!