Detailed Placement for Improved Depth of Focus and CD Control

1. Detailed Placement for Improved Depth of Focus and CD Control Puneet Gupta1 (puneet@blaze-dfm.com) Andrew B. Kahng1,2 Chul-Hong Park2 1 Blaze DFM, Inc. 2 ECE Department, University of California, San Diego

2. Outline • OPC and SRAF: An Introduction • The AFCorr Methodology • AFCorr Placement Perturbation • Experiments and Results • Summary

3. C.-H. Park et al., SPIE 2000 Before OPC After OPC OPC (Optical Proximity Correction) • Gate CD control is extremely difficult to achieve • Min feature size outpaces introduction of new hardware solutions • OPC = one of available reticle enhancement techniques (RET) to improve pattern resolution • Proactive distortion of photomask shape  compensate CD inaccuracies

4. 0.22 0.2 0.18 0.16 0.14 CD 0.12 0.1 0.08 DOF 0.06 0.04 0.0 0.1 0.2 0.3 0.4 0.5 0.6 SB2 SB0 SB1 SRAF (Sub-Resolution AF) Process Margin (180nm) Layout (or Mask ) Design SB=0 Active SB=1 SB=2 Wafer structure (SEM) • SRAF = Scattering Bar (SB) • SRAFs enhance process window (focus, exposure dose) • Extremely narrow lines  do not print on water • More SBs helps to enhance DOF margin and to meet the target CD #SB = 0 #SB=1 #SB=2 CD (nm) 160 177 182

5. Bias OPC SRAF OPC SRAFs and Bossung Plots • Bossung plot • Measurement to evaluate lithographic manufacturability • Maximize the common process window • Horizontal axis: Depth of Focus (DOF); Vertical axis: CD • SRAF OPC • Improves process margin of isolated pattern • Larger overlap of process window between dense and isolated lines

6. Outline • OPC and SRAF: An Introduction • The AFCorr Methodology • AFCorr Placement Perturbation • Experiments and Results • Summary

7. #SB=2 #SB=3 #SB=4 #SB=1 Allowable Forbidden Forbidden Pitches • Forbidden pitch lowers printability, DOF margin and exposure margin • Typically based on tolerance of +/- 10% of CD •  Must avoid forbidden pitches in layout

8. Layout Composability for SRAFs Better than x+dx  x  • Small set of allowed feature spacings • Two components of SRAF-aware methodology • Assist-correct libraries • Library cell layout should avoid all forbidden pitches • Intelligent library design • Assist-correct placement  THIS WORK • Intelligent whitespace adjustment in the placer

9. Outline • OPC and SRAF: An Introduction • The AFCorr Methodology • AFCorr Placement Perturbation • Experiments and Results • Summary

10. AFCorr: SRAF-Correct Placement • By adjusting whitespace, additional SRAFs can be inserted between cells • Resist image improves after assist-aware placement adjustment • Problem: Perturb given placement minimally to achieve as much SRAF insertion as possible Before AFCorr After AFCorr Forbidden pitch Cell boundary

11. Minimum Perturbation Approach • Objective: • Reduce forbidden pitch violation • Reduce weighted CD degradation with defocus • Minimum perturbation: preserve timing • Constraint: • Placement site width must be respected • How: • One standard cell row at a time • Solve each cell row by dynamic programming

12. field gate gate field Feasible Placement Perturbations SaLP Minimize  | di | s.t. da-1+da +Sa-1RP + SaLP + (xa– xa-1– wa-1)  AF Sa-1RP xa xa-1 Wa-1 wi and xi = width and location of Ci i= perturbation of location of cell Ci AF = set of allowed spacings RP, LP = boundary poly shapes with overlapping y-spans - Overlap types: g-g, g-f, f-f S = spacing from boundary poly to cell border

13. Vertical Forbidden Pitches • Handled in a way similar to horizontal overlap • Usually field poly • Typically, #vertical forbidden pitches < #horiz. F.P. • Due to restricted design rules like single orientation poly Cell under consideration Row i Row i-1

14. Dynamic Programming Solution COST (1,b) = | x1-b| // subrow up through cell 1, location b COST (a,b) = l(a) |(xa -b)| + MIN{Xa-SRCH ≤ i ≤ Xa+SRCH} [COST(xa-1,i) + αHCost(a,b,a-1,i) + βVCost(a,b)] // SRCH = maximum allowed perturbation of cell location HCost = horizontal “forbidden-pitch cost” = sum over horiz- adjacencies of [slope(j) |HSpace –AFj| * overlap_weight] s.t. AFj+1 > HSpace  AFj VCost = vertical forbidden pitch cost • l = perturbationweight • α, β = weights for horizontal vs vertical forbidden pitches • Slope = CD / Pitch = CD degradation per unit space between AF values • AFi = closest assist-feasible spacing ≤ HSpace • Overlap_weight = overlap length weighted by relative importance of printability for gate-to-gate, gate-to-field, and field-to-field

15. Outline • OPC and SRAF: An Introduction • The AFCorr Methodology • AFCorr Placement Perturbation • Experiments and Results • Summary

16. Lithography model generation (Best & Worst DOF) Benchmark design Placement Post-Placement Forbidden pitch Route Route Assist Corrected GDS Experiments Typical GDS • Delay • GDSII size • OPC Run Time • # Forbidden pitch • # SB • # EPE SB OPC OPCed GDSs • SB Insertion • Model-based OPC • (Best DOF model) Experimental Flow

17. Experimental Setup • KLA-Tencor’s Prolith • Model generation for OPCpro • Best focus/ worst (0.5 micron) defocus • Calculating forbidden pitches • Mentor’s OPCpro, SBar SVRF • OPC, SRAF insertion, ORC (Optical Rule Check) • Cadence SOC Encounter • Placement & Route • Synopsys Design Complier • Synthesis

18. Experimental Metrics • SB Count • Total number of scattering bars or SRAFs inserted in the design • Higher number of SRAFs indicates less through-focus variation and is hence desirable • Forbidden Pitch Count • Number of border poly geometries estimated as having greater than 10% CD error through-focus • EPE Count • Number of edge fragments on border poly geometries having greater than 10% edge placement error at the worst defocus level

19. Results: Increased SB Count • SB count increases as utilization decreases due to increased whitespace • #SB increases after AFCorr placement

20. Results: Reduced F/P and EPE • Forbidden pitch count (border poly only) • 81%~100% in 130nm, 93%~100% in 90nm • EPE Count (border poly only) • 74%~95% in 130nm, 83%~96% in 90nm

21. Impact on Other Design Metrics • Impact : Data size < 1%, OPC run time < 2%, Cycle time < 4% • Other impacts are negligible compared to large improvement in printability metrics

22. Outline • OPC and SRAF: An Introduction • Forbidden Pitch Extraction • The AFCorr Methodology • Experiments and Results • Summary

23. Summary • AFCorr is an effective approach to achieve assist feature compatibility in physical layout • Up to 100% reduction of forbidden pitch and EPE • Relatively negligible impacts on GDSII size, OPC runtime, and design clock cycle time • Compared to huge improvement in printability • Ongoing research • Developing “correct-by-construction" standard-cell layouts which are always AFCorrect in any placement

24. Thank You!

25. A Notation • W = cell width; • RP, LP = Boundary poly geometries • S = Spacing from boundary poly to cell border • O = Parallel adjacencies between poly features (g-f, g-g, f-f) • Example: Sa-1RP2 + (xa-1– xa– wa-1) + SaLP3 should be assist-correct