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

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

OPC (Optical Proximity Correction) Before OPC After OPC C.-H. Park et al., SPIE 2000 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

SRAF (Sub-Resolution AF) Process Margin (180nm) Layout (or Mask ) Design SB=0 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.0 0.3 0.4 0.5 0.6 SB2 SB1 SB0 DOF CD Active SB=1 SB=2 Wafer structure (SEM) #SB = 0 #SB=1 #SB=2 CD (nm) 160 177 182 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

SRAFs and Bossung Plots Bias OPC SRAF OPC 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

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

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

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

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

AFCorr: SRAF-Correct Placement Before AFCorr After AFCorr Forbidden pitch Cell boundary 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

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

Feasible Placement Perturbations SaLP gate field Sa-1RP xa xa-1 Wa-1 Minimize  | di | s.t. da-1 + da + Sa-1RP + SaLP + (xa – xa-1 – wa-1)  AF 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

Vertical Forbidden Pitches Cell under consideration Row i Row i-1 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

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 = perturbation weight 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

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

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

Experimental Setup KLA-Tencor’s Prolith Mentor’s OPCpro, SBar SVRF 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

Experimental Metrics SB Count Forbidden Pitch Count EPE 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

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

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

Impact on Other Design Metrics Utilization(%) 90 80 70 Flow: Orig AFCorr 130nm #EPE 4890 4721 5975 562 4276 15 R/T (s) 7821 7902 7876 7934 7913 7973 GDS (MB) 48.9 48.8 48.2 48.4 Delay (ns) 4.2 4.6 4.5 4.7 90nm 7523 1262 4813 532 2131 107 R/T(s) 6211 6327 6322 6431 6482 6499 GDS(MB) 43.1 43.3 43.2 Delay(s) 2.7 2.6 2.4 2.5 Impact : Data size < 1%, OPC run time < 2%, Cycle time < 4% Other impacts are negligible compared to large improvement in printability metrics

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

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

Thank You!

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