Jiun -Li Lin, Po- Hsun Wu, and Tsung -Yi Ho

1. A Novel Cell Placement Algorithm For Flexible TFT Circuit With Mechanical Strain And Temperature Consideration Jiun-Li Lin, Po-Hsun Wu, and Tsung-Yi Ho Department of Computer Science and Information Engineering, National Cheng Kung University, Tainan, Taiwan ASPDAC 2013

2. Outline • Introduction • Initial Static Timing Analysis • Static Timing Analysis with Mobility Consideration • Thermal-aware Non-critical Cell Distribution • ILP-based Critical Cell Distribution • Multilevel Global Placement • Row-based Detail Placement • Experimental results • Conclusion

3. Introduction • Flexible thin-film transistor (TFT) is the most popular technology used in flexible electronics due to its low fabrication temperature [11], but its performance is severely affected by the mobility variation. • After extensive experiments have been performed , past researches found that TFT’s mobility will be greatly affected by the mechanical strain [4, 5, 10, 12]

4. Mechanical strain • Mechanical strain is the mathematical expression of the shape changes resulting from mechanical stresses.

5. Mobility variation • Mobility variation makes a great impact on the cell delay of a flexible TFT circuit which further affects the circuit performance. At worst, mobility variation may leads to timing violation and even function failure. • In addition to the impact of mechanical strain, mobility is also sensitive to temperature change [13, 16].

6. Cell placement

7. Algorithm Flow

8. Initial Static Timing Analysis • All critical cells can be further extracted by backtracking procedure in a manner similar to depth-first search (DFS).

9. Static Timing Analysis with Mobility Consideration • The static timing analyzer for flexible TFT circuits (STAF) [6] is performed to determine the mobility influence. • STAF only takes the change of mechanical strain into consideration, it is further modified by replacing equation(1) with equation(3) to consider the change of both mechanical strain and temperature.

10. Static Timing Analysis with Mobility Consideration

11. Thermal-aware Non-critical Cell Distribution • Formulate this problem as a K-way graph partitioning problem to distribute all non-critical cells to different B groups while minimizing total wirelength and the chip temperature.

12. ILP-based Critical Cell Distribution

13. ILP-based Critical Cell Distribution Exclusivity constraint: Distribution constraint:

14. Multilevel Global Placement

15. Multilevel Global Placement

16. Row-based Detail Placement • Initially, it will calculate how many rows can be placed in a bin and partition all cells to different rows based on cell connectivity. • All cells in the same row are classified into hot cells and cool cells, and each cool cell is placed adjacent to the hot cell to reduce the temperature.

17. Experimental results

18. Experimental results

19. Conclusion • The novel cell placement flow and algorithms to minimize the mobility variation caused by the change of both mechanical strain and temperature while minimizing total wirelength.