Manufacturing-Aware Physical Design

1. Manufacturing-Aware Physical Design Andrew B. Kahng Puneet Gupta (Univ. of Calif. San Diego)

2. Outline • Challenges • “DFM Philosophy” • Manufacturing and Variability Primer • Design for Value • Composability • Performance Impact Limited Fill Insertion • Function Aware OPC • Systematic Variation Aware STA • Futures of Mfg-Aware PD

3. Layout 0.25µ 0.18µ 0.13µ 90-nm 65-nm Printing Figures courtesy Synopsys Inc.

4. Data Volume Explosion Number of design rules per process node MEBES file size for one critical layer vs. technology node

5. 70% RET Layers Explosion Number of TSMC Mask Layers Using OPC/PSM Number of design rules per process node 0% 180nm 150nm 130nm 90 nm Source: TSMC Technology Symposium, April 22 2003

6. Design Rules Explosion Number of design rules per process node

7. Variation: Across-Wafer Frequency

8. Variation: Leakage • Subthreshold leakage current varies exponentially with threshold voltage: I  exp(-Vth) • Vth = f(channel length, oxide thickness, doping) • Most affected by variations in gate length ±100% Isub ±10% Ld Dennis Sylvester, U. Michigan

9. Outline • Challenges • “DFM Philosophy” • Manufacturing and Variability Primer • Design for Value • Composability: PSM and Assists • Performance Impact Limited Fill Insertion • Function Aware OPC • Systematic Variation Aware STA • Futures of Mfg-Aware PD

10. Symptoms: Routing Rules (1) • Minimum area rules and via stacking • Stacking vias through multiple layers can cause minimum area violations (alignment tolerances, etc.) • Via cells can be created that have more metal than minimum via overlap (used for intermediate layers in stacked vias) • Multiple-cut vias • Use multiple-cut vias cells to increase yield and reliability • Can be required for wires of certain widths • Multiple via cut patterns have different spacing rules • Four cuts in quadrilateral; five cuts in cross; six cuts in 2x3 array; … • With wide-wire spacing rules, complicates pin access • Cut-to-cut spacing rules  check both cut-to-cut and metal-to-metal when considering via-to-via spacing

11. Symptoms: Routing Rules (2) • Width- and Length-dependent spacing rules • Width-dependent rules: domino effects • Variant: “parallel-run rule” (longer parallel runs  more spacing) • Measuring length and width: halo rules affect computation • Influence rules or stub rules • A fat wire, e.g., power/ground net, will influence the spacing rule within its surroundings  any wire that is X um away from the fat wire needs to be at least Y um away from any other geometry. • Example: fat wire with thin tributaries • bigger spacing around every wire within certain distance of the thin tributaries • ECO insertion of a tributary causes complications • Strange jogs and spreading when wires enter an influenced area

12. Example: LEF/DEF 5.5, April 2003

13. Example: LEF/DEF 5.5, April 2003

14. Symptoms: Routing Rules (3) • Density • Grounded metal fills (dummy fill*) • Via isodensity rules and via farm rules (via layers must be filled and slotted, have width-dependent spacing rule analogs, etc.) • Non-rectilinear (-geometry) routing • X-Architecture: http://www.xinitiative.org/ • Y-Architecture: http://vlsicad.ucsd.edu/Yarchitecture/ , LSI Logic patents • Landing pad shapes (isothetic rectangle vs.. octagon vs.. circle), different spacings (~1.1x) between diagonal and Manhattan wires, etc. • More exceptions • More non-default classes (timing, EM reliability, …) • Not just power and clock • >0.25um width may be “wide”  many exceptions

15. Symptoms: Routing Rules • Degrade completion rates, runtime efficiency • “Postprocessing” likely no longer suffices • E.g., antennas • There is no chip until the router is done • Must / Should / Can tomorrow’s IC routers “independently” address these issues?

16. Whose Job Is It To Solve: • Mask NRE cost ( runtimes  shapes complexity) • BEOL catastrophic yield loss • Deposited copper  can infer yield loss mechanisms • Open faults more prevalent than short or bridging faults • High-resistance via faults • Cf. “non-tree routing” for reliability and yield? • Variability budget for planarization • Copper is soft dual-material polish mechanisms • Oxide erosion and copper dishing  cross-sectional variability, inter-layer bridging faults, … • Low-k: thermal properties, anisotropy, nonuniformity • Resistivity at small conductor dimensions

17. The Problem: Evolution • Conflicting goals • Designer: “freedom”, “reuse”, “migration” • EDA: “maintenance mode” • Process/foundry: “enhance perceived value” (= add rules) •  Prisoner’s Dilemma: who will invest in change? • Fiddling: Incremental, linear extrapolation of current trajectory • “GDS-3” • Thin post-processing layers (decompaction, RET insertion, …) • Leads to “dark future” (12th Japan DA Show keynote)

18. DAC-2003 Nanometer Futures Panel:Where should extra R&D $ be spent?

19. The Solution: Co-Evolution • Designer, EDA, and process communities cooperate and co-evolve to maintain the cost (value) trajectory of Moore’s Law • Must escape Prisoner’s Dilemma • Must be financially viable • At 90nm to 65nm transition, this is a matter of survival for the worldwide semiconductor industry

20. Today’s Design-Manufacturing Interfaces Design Rules Device Models Library (Library Team) Litho/Process (Tech. Development) Layout & libs (Corner Case Timing) RET Mask: Dataprep (Mask House) Design (ASIC Chip) Layout (collection of polygons ?) Tapeout Guardbanding all the way in all stages!! (e.g. clock ACLV guardband ~ 30%) • What do we lose ? • Performance  Too much worst-casing • Turnaround time  Huge OPC runtimes, overdesign • Predictability  RET is applied post-design • Mask costs  Overcorrection • Designer’s intent  RET is not driven by design

21. Foundation of the DFM Solution • Bidirectional design-manufacturing data pipe • Fundamental drivers: cost, value • Pass functional intent to manufacturing flow • Example: RET for predictable timing slack, leakage, yield • RETs should win $$$, reduce performance variation •  cost-driven, parametric yield constrained RET • Pass limits of manufacturing flow up to design • Example: avoid corrections that cannot be manufactured or verified  e.g., design should be aware of metrology N.B.: 1998-2003 papers/tutorials: http://vlsicad.ucsd.edu/~abk/TALKS/

22. This Tutorial • Concrete examples of Manufacturing-Driven PD • Deployable today • Topic 1: Composability: PSM and SRAF • Topic 2: Performance impact limited fill insertion • Topic 3: Function Aware OPC • Topic 4: Library-based OPC for predictability • Topic 5: Focus and proximity-effects aware STA • Some ramblings about future: regular layout, robust optimization, leakage saving without multi-Vt • We will start with a “manufacturing primer” …

23. Outline • Challenges • “DFM Philosophy” • Manufacturing and Variability Primer • Lithography, Masks and Process Variations • Design for Value • Composability • Performance Impact Limited Fill Insertion • Function Aware OPC • Systematic Variation Aware STA • Futures of Mfg-Aware PD

24. Photo-Lithographic Process optical mask oxidation photoresist photoresist coating removal (ashing) stepper exposure Typical operations in a single photolithographic cycle (from [Fullman]). photoresist development acid etch process spin, rinse, dry step

25. Lithography Primer: Basics • The famous Raleigh Equation: : Wavelength of the exposure system NA: Numerical Aperture (sine of the capture angle of the lens, and is a measure of the size of the lens system) k1: process dependent adjustment factor • Exposure = the amount of light or other radiant energy received per unit area of sensitized material. • Depth of Focus (DOF) = a deviation from a defined reference plane wherein the required resolution for photolithography is still achievable. • Process Window = Exposure Latitude vs. DOF plot for given CD tolerance

26. Numerical Aperture • NA=nsin  n=refractive index  for air, UB =1. Practical limit ≈ 0.93 • NA increase  DOF decrease • Immersion lithography ?  n>1 (e.g., water) Figures courtesy www.icknowledge.com

27. k1 • k1 is complex process depending on RET techniques, photoresist performance, etc • Practical lower limit ≈ 0.25 • Minimum resolvable dimension with 193nm steppers = 0.25*193/0.93 = 52nm Source: www.icknowledge.com

28. l RET Basics • The light interacting with the mask is a wave • Any wave has certain fundamental properties • Wavelength () • Direction • Amplitude • Phase • RET is wavefront engineering to enhance lithographyby controlling these properties Direction Amplitude Phase Courtesy F. Schellenberg, Mentor Graphics Corp.

29. or Direction: Illumination • Regular Illumination • Many off-axis designs (OAI) • Annular • Quadrupole / Quasar • Dipole +

30. 130 nm lines, printed at different pitchesQuasar illumination NA=0.7 Acceptable Isolated Unacceptable Dense OAI: Impact on PD • Off axis amplifies certain pitches at the expense of the others “Forbidden” pitches • Quasar / Quadrupole Illumination • Amplifies dense 0°, 90 ° lines • Destroys ±45° lines • Dipole Illumination • Prints only one orientation • Must decompose layout for 2 exposures Depth of Focus Pitch (nm) Graph reference: Socha et al. “Forbidden Pitches for 130 nm lithography and below”, in Optical Microlithography XIII, Proc. SPIE Vol. 4000 (2000), 1140-1155.

31. Amplitude: OPC • Optical Proximity Correction (OPC)modifies layout to compensate for process distortions • Add non-electrical structures to layout to control diffraction of light • Rule-based or model-based

32. Dense CD window Exposure Iso CD window Defocus OPC: Assist Features • SRAF = Sub-Resolution Assist Feature ≡ SB = Scattering Bar ≡ Assists • SRAFs make isolated lines “behave” as dense • SRAF are not supposed to be printed on wafer but exist on mask Process Overlap Window Iso-window after SRAF insertion

33. Phase shifter Phase: PSM • Phase Shifting Masks (PSM) etch topography into mask • Creates interference fringes on the wafer Interference effects boost contrast Phase Masks can make extremely small gates phase shifting mask conventional mask glass Chrome Electric field at mask Intensity at wafer

34. 0 + = 180 180 Double-Exposure Bright-Field PSM

35. shifters The Phase Assignment Problem • Assign 0, 180 phase regions such that critical features with width < B are induced by adjacent phase regions with opposite phases 0 180 <B

36. Key: Global 2-Colorability • Odd cycle of “phase implications” ® layout cannot be manufactured • layout verification becomes a global, not local, issue ? 180 0 180 180 0 180

37. Phase Assignment for Bright-Field PSM • PROPER Phase Assignment: • Opposite phases for opposite shifters • Samephase for overlapping shifters Overlapping shifters

38. Critical features: F1,F2,F3,F4 F2 F4 F1 F3

39. F2 F4 F1 F3 Opposite-Phase Shifters (0,180)

40. S3 F2 S4 S8 F4 S7 S1 F1 S2 S5 F3 S6 Shifters: S1-S8 PROPER Phase Assignment: • Opposite phases for opposite shifters • Same phase for overlapping shifters

41. Phase Conflict S3 F2 S4 S8 F4 S7 S1 F1 S2 S5 F3 S6 Phase Conflict Proper Phase Assignment is IMPOSSIBLE

42. Conflict Resolution: Shifting S3 F2 S4 S8 F4 S7 S1 F1 S2 S5 F3 S6 Phase Conflict feature shifting to remove overlap

43. Conflict Resolution: Widening S3 F2 S4 S8 F4 S7 S1 F1 S2 F3 Phase Conflict feature widening to turn conflict into non-conflict

44. Minimum Perturbation Problem • Layout modifications • feature shifting • feature widening  area increase, slowing down  manual fixing, design cost increase • Minimum Perturbation Problem: Find min # of layout modifications leading to proper phase assignment. [Kahng et al. ASPDAC 2001]

45. OPC Fracture Mask Mask Costs(1) Design Mask Cost  Data Volume OPC, PSM, Fill  increased feature complexity  increased mask cost Figure courtesy Synopsys Inc.

46. Mask Costs(2) Half of all mask sets used for < 570 wafers (< 100K parts) Vector scan: Write cost proportional to feature complexity Difficult to inspect, verify masks!

47. Manufacturing Yield • IC manufacturing process affected by random disturbances • different silicon dioxide growth rates, mask misalignment, drift of fabrication equipment operation, etc…. • These disturbances are often uncontrollable and affect the circuit performance • Yield: percentage of manufactured products that pass all performance specifications • Parametric yield (process variations) • What is the performance of the manufactured chips? • Catastrophic or functional yield (defects) • How many chips work?

48. Process Variation Taxonomy • Spatial scale: • Die-to-Die or Inter-Die. E.g. Focus, etch • Within-Die or Intra-Die. E.g. lens aberration, diffraction effects • Nature: • Random. E.g. batch-to-match material variation • Systematic. E.g. diffraction-based proximity effects • Systematic but difficult to model variations  random

49. Process Variation Sources • Wafer: topography, reflectivity • Reticle: CD error, proximity effects, defects • Stepper: Lens heating, focus, dose, lens aberrations • Etch: Power, pressure, flow rate • Resist: Thickness, refractive index • Develop: Time, temperature, rinse • Environment: Humidity, pressure

50. Simulation of Variation • Value X for a given parameter for a device i in path j in the kth Monte-Carlo run is given by • RAN-WID: Random within-die variation • RAN-DTD: Random die-to-die variation • SYS-WID: Systematic within-die variation • SYS-DTD can not be accounted for at die-scale