1. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu How Much Variability Can Designers Tolerate? Andrew B. Kahng ITRS Design ITWG December 1, 2003

2. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu Variation: Across-Wafer Frequency

3. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #1: Designers Handle It Already CD tolerance > 10% not a major issue Sample industry 130nm device model: Leff is nominally 80nm, 68nm for fast, 92nm for slow already see 15% in corner analysis

4. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #2: Vt, Not Leff Vt behavior already dominates gate length for circuit performance and functionality –Gate length tolerance becomes secondary to intradie Vt variation, especially at 65nm and beyond – 10% 3-sigma in linewidth does not carry same timing penalty that it used to Practically: physical gate length tolerance < 30% –Manage capacitive load mismatches –Keep SCE-induced tolerance within non-gate length Vt variation –Intradie gate-length variation could be nearly all of this budget if active Vt control (well biasing) is used to counter mean value of SCE on each chip

5. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu Leakage Variation Subthreshold leakage current varies exponentially with threshold voltage: I exp(-V th ) V th = f(channel length, oxide thickness, doping) –Most affected by variations in gate length ±10% L d ±100% I sub Dennis Sylvester, U. Michigan

6. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu Focus on Leakage Variability Performance fluctuations due to CD will be dominated by leakage power and not by gate delay/speed –Frequency range of 30-35% == leakage spread of 20X –Borkar (Intel), 2002 Speed-binning: fast parts are being tossed out due to power constraints (2-sided yield problem) Adaptive body bias or other techniques may be used to attack high leakage parts by locally raising Vth –Cost, design effort unlikely for mainstream design –But, design techniques have not even scratched the surface of possibilities

7. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu Leakage/Frequency Bins (Borkar, Intel)

8. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu Impact of Variation (Borkar, Intel)

9. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #3: Variability Costs TTM Poor Si correlation of performance analysis tools is partly caused by CD variation –Designers spend much effort fixing non-critical paths Relaxing CD control would increase wasted effort – True critical paths may be overlooked – More steppings, TTM to fully ramp at target bin-split Tighter CD control higher parameterized yield, $$ Process guys own optimization models to determine most profitable CD control level –Must account for: Volume, Bin-Split, $$ paid to equipment suppliers to get incremental control –Expected ASP from CD control Acceptable Variability Designers then deal with this Acceptable Variability –Guardbands, statistical performance analysis, … –Comment: Not a closed loop!

10. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #1 vs. View #2 vs. View #3 View #1: Designers work around issues –Always have, always will –Variability is just another problem (and is already > 10%) View #2: Leakage variability need adaptive biasing or threshold control anyway –Higher variability would be tolerable, especially IF gate length scaling slows AND/OR nominal channel length is set further up on the Vth-Leff roll-off curve –Would reduce impact of L uncertainty on subthreshold leakage and allow more variability to be tolerated View #3: Any variability is bad –Process guys determine acceptable variability –Historical ITRS view, dominated by MPU (high-vol custom) Comments –What is $$$ saved in process with 5% tolerance increase? –Near-zero investment to date in design tools for variability

11. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #4: Design-Mfg Interfaces Increase Design awareness of Manufacturing Restricted layout ground rules improved process window Library-based RET performance convergence Accurate distributions instead of Gaussians instead of corners Awareness of through-pitch component of ACLV tighter performance analysis Awareness of other systematic variation (etch bias...) Many, many techniques not yet deployed –Avoid of risky structures –Understand MEEF, functional impact of geometric var –Lgate biasing for leakage, variability tolerance

12. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu

13. ITRS December 2003, Hsin-Chu Taiwan http://vlsicad.ucsd.edu View #4: Design-Mfg Interfaces Increase Manufacturing awareness of Design Understand designers intent !!! Tolerable per-device CD errors reduced RET effort, mask cost, mask yield Drop deterministic DOF vs. ELAT plots true statistical process windows Functional pattern fidelity e.g., stop wasting effort on OPC of corners, other RET that has no payoff Multilayer and function-aware rules and requirements –Fill, RET are one layer at a time wrong –Rules should distinguish critical vs. non-critical vs. dummy