VLSI for 3D Integration: Modeling, Design and Prototyping

1. VLSI for 3D Integration: Modeling, Design and Prototyping Neil Goldsman Bruce Jacob George Metze Omar Ramahi Mike Khbeis, Akin Akturk, Zeynep Dilli, Xi Shao, Latise Parker

2. Problems with Conventional 2D Multi-Chip Integration IC 1 IC 2 Input Output Transmission Line Chip-to-chip PCB integration is limiting: The parasitics of the bond pads, wires and board buses limit speed, driving capability and functionality Transmission Line Bond Wire Bond Pad Pins

3. 3-D Integration 3-D Chip Stacking (die stacking): Bringing bare dies together in a vertical structure. • Advantages: • Enable more mixed signal integration • Increased ratio of active Si substrate area to chip footprint area • Reduction of delay, faster clock speeds and higher bandwidths, through • Less use of pads, smaller pads • Shorter interconnects • Less parasitic impedance • Disadvantages: • Potential increase in heat dissipation problems • Increased (geometric, computational, routing) design complexity. • Research Goals: • Develop prototypes and design tools to understand limits of 2D integration and exploit benefits of 3D integration.

4. Outline • Task I: Modeling and Prototyping Device and Chip Heating for 2D & 3D Integration • Task 2: Modeling and Prototyping for on Chip Electromagnetic Effects • Task 3: Prototyping and Modeling Passive 3D Structures

5. Task I: Coupled Modeling of Time-Dependent Full-Chip IC and Quantum Non-Isothermal Device Operation Pentium III Pentium III Temperature

6. Motivation: • As devices get smaller on-chip thermal effects become increasingly important. • Predictions indicate that chip temperatures will increase exponentially beyond acceptable values. • Thus modeling of full-chip heating is essential for the design of fail-safe architectures in both 2D and 3D. • Objective : • Develop heating models for 2D and 3D IC’s. • Predict circuit and chip performance variations due to chip heating. • Test the model and develop temperature sensors by fabricating specially • designed chips.

7. Functional Blocks in Pentium III % Area % Power 0 Bus Interface Unit 4.25 6.2 1 Clock 1.00 5.2 2 L1 Data Cache 12.5 9.8 3 Memory Order Buffer 3.25 4.7 4 Execute 9.45 13 5 L2 Data Cache 29.75 8.5 6 Register Alias Table 3.25 4.7 7 Issue 9.45 14.1 8 Fetch 12.5 16.8 9 Decode 14.6 17.2 Source: www.intel.com

8. Thermal Network Containing Millions of Nodes KCL-type lumped thermal network Pentium has 40 million nodes MOSFET devices and their thermal connections

9. Schrödinger Eqn. Poisson Eqn. Electron Current Continuity Eqn. Hole Current Continuity Eqn. Heat flow Eqn. Device Equations

10. Integrated Circuit Heat Flow Equation

11. Flowchart Coupled Flowchart

12. Device Simulations MOSFET IV Curves for T=300K and 400K; VGS=0.4, 0.7, 1.0V Temperature profile in the channel of a MOSFET; Far left and right corners are source and drain sides, respectively. Far side is parallel to gate terminal.

13. Chip Simulations & Cooling with Thermal Contacts Maximum chip temperature for different device counts with constant power density. Temperature profile for a 0.5cm IC with uniform device activity throughout the chip: Temperature Isotherms range from 300K at the chip edges to 360K inside chip. Maximum chip temperature as a function of uniformly distributed thermal contacts. Thermal contacts cool chip

14. Calculated Temperature Profile for Pentium III • Calculated Pentium III Temperature Profile with non-uniform device activity throughout the chip but with uniform device activity within each functional block: • Temperature Isotherms range from 300K at the chip edges to 340K inside the clock. • Clock and L2 Cache are the hottest and coolest regions, respectively.

15. Prototyping Temperature Sensor Array (10x10) • Diodes are used as temperature sensors: • Diode current increases exponentially with temperature.

16. Task II: Modeling and Measuring the Effects of EM Parasitics and Coupling in 2D and 3D Microelectronic Circuits metal SiO2

17. Problems with Conventional 2D Multi-Chip Integration IC 1 IC 2 Input Output Transmission Line Chip-to-chip PCB integration is limiting: The parasitics of the bond pads, wires and board buses limit speed, driving capability and functionality Transmission Line Bond Wire Bond Pad Pins

18. Effect of Pads Designed chips with two different ring-oscillators: 1. “External”: Uses bonding pads for stage-to-stage connection 2. “Internal”: Stages directly connected to each other

19. Internal Osc. External Osc. One-stage delay 112 MHz (31-stage) (equivalent to 1.16 GHz for 3 stages) 398 KHz (11-stage) (equivalent to 1.46 MHz for 3 stages) ~330 ps for internal, ~330 ns for external devices Effect of Pads: Results Summary Factor of ~ 800 Faster without Pads & PCB Connections 0.6 m chip, measurements taken by Tektronix oscilloscope with 1 pF-capacitance active probe on the breadboard Speed ratio: 794.5 Load ratio: ~1000 Expecting similar results on a PCB

20. Effects of EM Coupling in IC • Noise Injection: • Capacitive Injection • Hot Electron Injection • Noise Coupling • Resistive Coupling • Inductance Coupling • Noise Reception • Capacitive Reception • Threshold Voltage Modulation n+ n+ n+ n+

21. IC Chip Layout

22. Coupling Measurement Frequency: 50MHz 500MHz 100MHz PhaseNoise: -1.5dBc/Hz -1.8dBc/Hz -2.2dBc/Hz 35.4dBc/Hz 29.1dBc/Hz 12.8dBc/Hz

23. Modeling Interconnects and Coupling: Maxwell’s Equation Metal Substrate Insulator • Challenges: • Skin depth effect in the metal layer. • Substrate current. • How to couple large EM wavelength (mm to cm) scale with fine material structure (of um scale)? • Ans. Develop new EM ADI Maxwell Solver

24. Simulating EM Coupling between Interconnect Lines in Metal-Insulator-Silicon-Substrate (MISS) Structure Adjacent Interconnects X-section Voltage Pulse Coupling Results Results: New simulator allows for resolving large variations in grid points Induced voltage 20% of applied signal even at 20μm apart.

25. Simulations show extensive coupling through substrate currents. Substrate Current: Horizontal x-section Substrate Current: Vertical x-section

26. Task 3: Prototyping and Modeling 3D Structures

27. 3-D Connections Chip-to-chip communication between different chips with vertical vias that require 12m x 12m metal pads Cadence-extracted capacitance for a pad 9.23 fF: Same order of magnitude as inverter load cap To be investigated: Extra capacitive effects of the vertical via in2 out2 out1 in1

28. 3-D Connections: “Symmetric” Chip New chip in fab with structures that can be connected in 3D and planar counterparts for comparison

29. Planar inductor vs. Multilayer inductor Layouts for planar inductor (left) and multilayer inductor (right), in fabrication for probe-station measurements. The total length of the inductors are the same, and the two pictures are of the same scale. Note the much smaller footprint of the multilayer inductor.

30. Modeling Inductance • Net inductance represented by inductance matrix: • Diagonal elements: self-inductances of segments • Off-diagonal elements: mutual inductances between segments Mutual inductance calculation: The vector potential approach

31. Planar Inductor vs. Multilayer Inductor Same net length  same net resistance, but higher inductance. (four-level multilayer)

32. Conclusion: • A methodology was developed to model full-chip heating. • Predictions show thermal coupling between devices causes • the chip heating. • Excessive heating will render circuits inoperable. • Careful placement of micron-scale thermal contacts should • relieve the problem. • To test our methodology and develop new technology, we • designed a chip and submitted it to MOSIS for fabrication.

33. Conclusion: • Prototyped chips show 30dBm in 2D EM noise coupling • An Electromagnetic Solver was developed that is tailored to IC’s • The method can be used to resolve the coupling between large EM wave length and fine material structure, e.g. IC on-chip interconnects. • Show the dispersion and decaying of the signal propagating along the Metal Insulator Semiconductor Substrate (MISS) structure. • Show the detailed structure of the electric and magnetic field inside the metal and substrate. The substrate current can lead to cross-talk and losses. • In the skin-effect mode region, enhancing the silicon substrate doping conforms the shape of propagating signal better.

34. Conclusion: • Prototyping shows bonding pads and I/0 ESD protection circuits can cause 800 times decrease in circuit operation speed. • 3D integration will obviate need for most I/0 circuits reducing delays. • Chip modules for 3D integration developed. • 3D inductors designed and under fabrication. • Modeling indicates 3D inductors yield superior performance to 2D counterparts.

35. Future Work • Incorporate heat sensor into 3D structures and calibrate heat model. • Incorporate heat conduits into 3D structures. • Expand EM calculations to 3D interconnects and more passive structures. • Test fabricated inductor structures to experimentally to verify advantages of 3D. • Perform 3D stacking of 2D MOSIS chips. • Develop state-of-the-art mixed signal 3D IC.