Detailed Placement for Improved Depth of Focus and CD Control Puneet Gupta 1 Andrew B. Kahng 1,2 Chul-Hong Park 2 1 Blaze DFM, Inc. 2 ECE Department, University of California, San Diego

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

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

SRAF (Sub-Resolution AF) SRAFs enhance process window (focus, exposure dose) Extremely narrow lines  do not print on water Compensate deficiencies of OPC SB2 SB1 SB0 DOF  CD  SB=0 SB=2 SB=1 Active #SB = 0#SB=1#SB= CD (nm) Layout (or Mask ) Design Process Margin Wafer structure (SEM)

SRAFs and Bossung Plots Bossung plot Bossung plot Measurement to evaluate lithographic performance at various exposure levels Measurement to evaluate lithographic performance at various exposure levels Horizontal axis: Depth of Focus (DOF); Vertical axis: CD Horizontal axis: Depth of Focus (DOF); Vertical axis: CD SRAF OPC SRAF OPC Improves process margin of isolated pattern Improves process margin of isolated pattern Larger overlap of process window between dense and isolated lines Larger overlap of process window between dense and isolated lines Bias OPCSRAF OPC

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

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 #SB=1 #SB=2#SB=3#SB=4 Allowable Forbidden

Layout Composability for SRAFs Small set of allowed feature spacings Small set of allowed feature spacings Two components of SRAF-aware methodology Two components of SRAF-aware methodology Assist-correct libraries Assist-correct libraries Inter-device spacing within a standard cells Inter-device spacing within a standard cells Intelligent library design Intelligent library design Assist-correct placement  THIS WORK Assist-correct placement  THIS WORK Intelligent whitespace adjustment between cells Intelligent whitespace adjustment between cells  x+  x   x  Better than

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

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 Cell boundaryForbidden pitch Before AFCorrAfter AFCorr

Minimum Perturbation Approach Objective: Objective: Minimum perturbation of input placement Minimum perturbation of input placement Reduce weighted CD degradation with defocus Reduce weighted CD degradation with defocus Preserve timing Preserve timing Constraint: Constraint: Placement sitemap must be respected Placement sitemap must be respected How: How: One cell row at a time One cell row at a time Solve each cell row by dynamic programming Solve each cell row by dynamic programming

Feasible Placement Perturbations Minimize  i | s.t.  a-1     a +  S a-1 RP + S a LP + (x a – x a-1 – w a-1 )  AF s.t.  a-1     a +  S a-1 RP + S a LP + (x a – x a-1 – w a-1 )  AF w i and x i = width and location of C i  i = perturbation of location of cell C i 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 xaxaxaxa x a-1 S a-1 RP S a LP W a-1

Dynamic Programming Solution  weighted objective function E.g., to account for timing-criticality of cells Slope =  CD /  Pitch = CD degradation per unit space between AF values Slope =  CD /  Pitch = CD degradation per unit space between AF values AF i = closest assist-feasible spacing ≤ HSpace AF i = 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 Overlap_weight = overlap length weighted by relative importance of printability for gate-to-gate, gate-to-field, and field-to-field COST (1,b) = | x 1 -b| // subrow up through cell 1, location b COST (a,b) = (a) |(x a -b)| + MIN {X a -SRCH ≤ i ≤ X a +SRCH} [COST(x a-1,i) + HCost(a,b,a-1,i)] MIN {X a -SRCH ≤ i ≤ X a +SRCH} [COST(x a-1,i) + HCost(a,b,a-1,i)] // SRCH = maximum allowed perturbation of cell location // SRCH = maximum allowed perturbation of cell location HCost = “forbidden-pitch cost” = sum over horiz-adjacencies of [slope(j) |HSpace –AF j | * overlap_weight] [slope(j) |HSpace –AF j | * overlap_weight] s.t. AF j+1 > HSpace  AF j s.t. AF j+1 > HSpace  AF j

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

Experimental Flow 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 Route Post-Placement OPCed GDSs - Delay - # SB - # EPE - # Forbidden pitch - GDSII size - OPC Running Time Experiments

Experimental Setup KLA-Tencor ’ s Prolith KLA-Tencor ’ s Prolith Model generation for OPCpro Model generation for OPCpro Best focus/ worst (0.5 micron) defocus Best focus/ worst (0.5 micron) defocus Calculating forbidden pitches Calculating forbidden pitches Mentor ’ s OPCpro, SBar SVRF Mentor ’ s OPCpro, SBar SVRF OPC, SRAF insertion, OPC simulation OPC, SRAF insertion, OPC simulation Cadence SOC Encounter Cadence SOC Encounter Placement Placement Synopsys Design Complier Synopsys Design Complier Synthesis Synthesis

Experimental Metrics SB Count Total number of scattering bars or SRAFs inserted in the design Higher number of SRAFs implies 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  Better DOF and resist image

Results: Reduced F/P and EPE Forbidden pitch count 81%~100% in 130nm, 93%~100% in 90nm EPE Count 74%~95% in 130nm, 83%~96% in 90nm

Impact on Other Design Metrics Data size  4%, OPC running time  3%, Cycle time  6% Data size  4%, OPC running time  3%, Cycle time  6% Other impacts are negligible compared to large improvement in printability metrics Other impacts are negligible compared to large improvement in printability metrics Utilization(%) Flow:OrigAFCorrOrigAFCorrOrigAFCorr 130nm#EPE R/T (s) GDS (MB) Delay (ns) nm#EPE R/T(s) GDS(MB) Delay(s)

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

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

Thank You!

Notation W = cell width; W = cell width; RP, LP = Boundary poly geometries RP, LP = Boundary poly geometries S = Spacing from boundary poly to cell border S = Spacing from boundary poly to cell border O = Parallel adjacencies between poly features (g-f, g-g, f-f) O = Parallel adjacencies between poly features (g-f, g-g, f-f) Example: S a-1 RP2 + (x a-1 – x a – w a-1 ) + S a LP3 should be assist-correct Example: S a-1 RP2 + (x a-1 – x a – w a-1 ) + S a LP3 should be assist-correct A