Detailed Placement for Improved Depth of Focus and CD Control

Detailed Placement for Improved Depth of Focus and CD Control Puneet Gupta1 Andrew B. Kahng1,2 Chul-Hong Park2 (chpark@vlsicad.ucsd.edu) http://vlsicad.ucsd.edu 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

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

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 Layout (or Mask ) Design SB=0 Active SB=1 SB=2 Wafer structure (SEM) • SRAFs enhance process window (focus, exposure dose) • Extremely narrow lines  do not print on water • Compensate deficiencies of OPC #SB = 0 #SB=1 #SB=2 CD (nm) 160 177 182

Bias OPC SRAF OPC SRAFs and Bossung Plots • Bossung plot • Measurement to evaluate lithographic performance at various exposure levels • 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

#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

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

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

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

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

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) + HCost(a,b,a-1,i)] // SRCH = maximum allowed perturbation of cell location HCost = “forbidden-pitch cost” = sum over horiz-adjacencies of [slope(j) |HSpace –AFj| * overlap_weight] s.t. AFj+1 > HSpace  AFj • l = weighted objective function • E.g., to account for timing-criticality of cells • 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

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

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, OPC simulation • Cadence SOC Encounter • Placement • Synopsys Design Complier • 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% • 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 • 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 • 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!

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

Detailed Placement for Improved Depth of Focus and CD Control