Block-level Designs of Die-to-Wafer Bonded 3D ICs and Their Design Quality Tradeoffs

1. Block-level Designs of Die-to-Wafer Bonded 3D ICs and Their Design Quality Tradeoffs KritAthikulwongse, Dae Hyun Kim, MoongonJung, and Sung KyuLim School of Electrical and Computer Engineering, Georgia Institute of Technology National Electronics and Computer Technology Center Cadence Design Systems ASPDAC’13

2. Outline • Introduction • Preliminaries • Block-Level 3D IC Design • Design Evaluation • Experimental Results

3. Introduction • Three-dimensional integrated circuits (3D ICs) are built in various design styles and at different levels. • Homogeneous 3D integration technology : Compact and high-degree of logic integration. • Heterogeneous 3D integration technology : Allows various electronic components such as analog circuits, memory elements, logic, and sensors in the same 3D-IC stack

4. Introduction • Bonding • Wafer to wafer • Lower cost • Die to wafer • more practical • Die to die

5. Introduction • This paper study how to design block-level 3D ICs, where the footprint of the dies in the stack are different. • Focus on a two-tier 3D IC, where the bottom die has a larger footprint. • Configuration : • Both dies are facing down so that the heat sink is located above the back-side of the top die, and C4 bumps are below the front-side (top metal layer) of the bottom die. • better power delivery and potentially better cooling if the top die consumes more power.

6. Introduction • Depending on the location of through-silicon vias (TSVs) in the bottom die, a redistribution layer (RDL) is necessary on the back-side of the bottom tier to connect the two dies. • Three different ways to place TSVs in the bottom die • TSV-farm • TSV-distributed • TSV-whitespace

7. Preliminaries • Die Bonding and Redistribution Layers • For die-to-wafer bonding in 3D ICs, three methods have been proposed: • Back-to-back • Face-to-face • Face-to-back Metal Substrate face Back Src: Sung KyuLim, TSV-Aware 3D Physical Design Tool Needs for Faster Mainstream Acceptance of 3D ICs [DAC47]

8. Preliminaries • Die Bonding and Redistribution Layers • For die-to-wafer bonding in 3D ICs, three methods have been proposed: • Back-to-back • Signal should go through Two TSVs when it is transmitted from one die to its adjacent die. • TSVs have non-negligible capacitance, transferring a signal through two TSVs might degrade the delay and the signal integrity of the net. • Face-to-face • Face-to-back Metal Substrate

9. Preliminaries • Die Bonding and Redistribution Layers • For die-to-wafer bonding in 3D ICs, three methods have been proposed: • Back-to-back • Face-to-face • Face-to-face bonding does not utilize TSVs because the interconnect between the dies is established using metal layers only. • Face-to-back Metal Substrate

10. Preliminaries • Die Bonding and Redistribution Layers • For die-to-wafer bonding in 3D ICs, three methods have been proposed: • Back-to-back • Face-to-face • Face-to-back • Face-to-back bonding is utilized between two dies with different die sizes, redistribution-layer (RDL) routing on the back side of the bottom die is required in some cases. face Back

11. Preliminaries • Die Bonding and Redistribution Layers Redistribution Layers • Wires on the RDL are wide, possibly as wide as wires on the topmost metal layers. • Parasitic capacitance is much higher than local metal wires, and causes timing degradation and dynamic power overhead If some TSVs in the bottom die are outside the footprint area of the top die, RDL routing is necessary to connect the TSVs to the bonding pads of the top die. Upper die face Lower die Back

12. Preliminaries The Goal of This Work • Insert all TSVs inside the footprint area of the top die. • Limits TSV locations • Does not require RDL wires. • TSV - farm • Insert TSVs wherever they are needed and perform RDL routing to connect them in the bottom die to the bonding pads in the top die. • Higher degree of freedom on TSV locations • TSV - distribute • TSV - whitespace

13. Preliminaries The Goal of This Work • Three different ways to place TSVs in the bottom die TSV-farm TSV-distributed TSV-whitespace

14. Block-Level 3D IC Design • Partitioning • TSV Insertion and Floorplanning • Bonding Pad Assignment and RDL Routing

15. Block-Level 3D IC Design Partitioning • Cut size • #cut 3D nets between the two dies = #TSVs • Assigning low-power blocks to the bottom die and high-power blocks to the top die • Reduces temperature • The top die is closer to the heat sink than the bottom die • Assigning thermally-sensitive blocks (such as memory blocks) to the top die and thermally-insensitive blocks to the bottom die increases predictability and reliability of the 3D IC. • Partitioning are manually performed with all these factors considered.

16. Block-Level 3D IC Design TSV Insertion and Floorplanning • TSV-farm • An array of TSVs (and bonding pads) are placed in the middle of the bottom die. • Treated TSVs as obstacles during floorplanning. • Blocks are placed around the TSV farm. If all blocks are highly connected, the TSV-farm design style causes significant wirelength overhead.

17. Block-Level 3D IC Design TSV Insertion and Floorplanning • TSV-distributed • TSVs are preplaced in an array and treated as obstacles during floorplanning. • TSVs are placed all over the bottom die. • Large blocks have very limited locations for their positions . • Degrade wirelength, timing, and power. • Expected to show low temperature and small TSV stress.

18. Block-Level 3D IC Design TSV Insertion and Floorplanning • TSV-whitespace • A 3D floorplanner is used to obtain 3D-IC layouts. • TSVs are manually inserted into whitespace existing between blocks close to a pin of each 3D net. • TSVs are irregularly placed. • When TSVs are inserted, the current floorplan is perturbed by moving blocks to create or expand whitespace. • Is expected to optimize the wirelengthbetter than all other design styles since a 3D floorplanner is used.

19. Block-Level 3D IC Design Bonding Pad Assignment and RDL Routing • TSV farm • Locations of the bonding pads in the top die are duplicated from the locations of the TSVs in the bottom die. • TSV-distributed and the TSV-whitespace • Locations of the bonding pads in the top die are determined by recursive bipartitioning before floorplanning of the top die. • Minimize the wirelength. • RDL routing is performed after floorplanning of the top die.

20. Block-Level 3D IC Design

21. Design Evaluation • methodology to evaluate 3D-IC layouts • Traditional Metrics( STA & Power analysis) • Cadence QRC Extraction (Extract parasitic resistance and capacitance) • Synopsys PrimeTime • Thermal Analysis • Ansys FLUENT • Mechanical Stress Analysis • ABAQUS

22. Experimental Results • For the experiments, 45-nm technology [7] is used. • An opensource hardware IP core [8] is synthesized using an open cell library [9]. • The thickness of bottom die and top die is 30 μm and 530 μm, respectively.

23. Experimental Results • Baseline • The number of TSVs depends on the partitioning of blocks and area ratio of the two dies. • Use the same partitioning, thus same number of TSVs, in all the three styles for fair comparison. • The TSV size is 10 μm, and TSV pitch is 30 μm.

24. Experimental Results • Use the same area and footprint for all the three styles LPD : Longest Path Delay Design Style (TSV size, TSV pitch)

25. Experimental Results The decrease in TSV capacitance (size) improves timing slightly. • Use the same area and footprint for all the three styles Design Style (TSV size, TSV pitch) LPD : Longest Path Delay TSV arrays distributed all over the bottom die obstruct optimal block placement. Without timing optimization, none of the designs meets the target delay (1.25ns)

26. Experimental Results • Power and Temperature The design in TSV-distributed and TSV-whitespace styles can only operate at slower speed than the design in TSV-farm style.

27. Experimental Results • Power and Temperature low speed Decrease in longest path delay The design in TSV-distributed and TSV-whitespace styles can only operate at slower speed than the design in TSV-farm style.

28. Experimental Results

29. Experimental Results Area with Stress > 10 MPa

30. Experimental Results highest average stress With decrease in TSV size, the stress decreases. Decreasing TSV pitch, stress from each TSV starts overlapping each other again. ( Stress ) Stress interference on a TSV coming from horizontal and vertical directions has opposite impacts, their effect partially cancels each other [12]

31. Experimental Results

32. Conclusion • TSV-distributed style has the worst wirelength because TSV arrays interfere with block placement. • However, it has the lowest temperature because TSVs distributed across the die help reduce temperature. • Because of the lack of RDL wiring, TSV-farm style has the best timing. • The design in this style has the highest average stress, but the area impacted by stress is smallest.

33. Conclusion