Detailed Placement for Detailed Placement for Improved Depth of Focus and Improved Depth of Focus and

CD ControlCD Control

Puneet GuptaPuneet Gupta11

Andrew B. KahngAndrew B. Kahng1,21,2

Chul-Hong ParkChul-Hong Park2 2

(chpark@vlsicad.ucsd.edu)(chpark@vlsicad.ucsd.edu)

1 Blaze DFM, Inc.2 ECE Department, University of California, San Diego

http://vlsicad.ucsd.edu

OutlineOutline

OPC and SRAF: An IntroductionOPC and SRAF: An Introduction The AFCorr MethodologyThe AFCorr Methodology AFCorr Placement PerturbationAFCorr Placement Perturbation Experiments and ResultsExperiments and Results SummarySummary

OPC (Optical Proximity Correction)OPC (Optical Proximity Correction)

Gate CD control is extremely difficult to achieveGate CD control is extremely difficult to achieve Min feature size outpaces introduction of new hardware solutionsMin feature size outpaces introduction of new hardware solutions

OPC = one of available reticle enhancement techniques OPC = one of available reticle enhancement techniques (RET) to improve pattern resolution(RET) to improve pattern resolution Proactive distortion of photomask shape Proactive distortion of photomask shape compensate CD compensate CD

inaccuraciesinaccuracies

Before OPC After OPCC.-H. Park et al., SPIE 2000

SRAF (Sub-Resolution AF)SRAF (Sub-Resolution AF)

SRAFs enhance process window (focus, exposure dose) SRAFs enhance process window (focus, exposure dose) Extremely narrow lines Extremely narrow lines do not print on water do not print on water Compensate deficiencies of OPCCompensate deficiencies of OPC

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.0 0.1 0.2 0.3 0.4 0.5 0.6

SB2 SB1 SB0

DOF

CD

SB=0

SB=2SB=1

Active

#SB = 0 #SB=1 #SB=2160 177 182CD (nm)

Layout (or Mask ) Design Process Margin

Wafer structure (SEM)

SRAFs and Bossung PlotsSRAFs and Bossung Plots

Bossung plotBossung plot Measurement to evaluate lithographic performance at various Measurement to evaluate lithographic performance at various

exposure levelsexposure levels Horizontal axis: Depth of Focus (DOF); Vertical axis: CDHorizontal 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 linesLarger overlap of process window between dense and isolated lines

-20

20

60

100

140

180

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

DOF (um)

CD

(n

m)

12

11.5

11

10.5

10

9.5

Bias OPC SRAF OPC

-20

20

60

100

140

180

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

DOF (um)

CD

(n

m)

12

11.5

11

10.5

10

9.5

OutlineOutline

OPC and SRAF: An IntroductionOPC and SRAF: An Introduction The AFCorr MethodologyThe AFCorr Methodology AFCorr Placement PerturbationAFCorr Placement Perturbation Experiments and ResultsExperiments and Results SummarySummary

Forbidden PitchesForbidden Pitches

Forbidden pitch lowers printability, DOF margin Forbidden pitch lowers printability, DOF margin and exposure marginand exposure margin

Typically based on tolerance of +/- 10% of CD Must avoid forbidden pitches in layout

-30

10

50

90

130

170

100 300 500 700 900 1100 1300 1500

pitch (nm)

CD

(n

m)

W/O OPC(Best DOF)W/O OPC(Defocus)

Bias OPC(Defocus)SRAF OPC (Defocus)

#SB=1 #SB=2 #SB=3 #SB=4

Allowable

Forbidden

Layout Composability for SRAFsLayout Composability for SRAFs

Small set of allowed feature spacingsSmall set of allowed feature spacings Two components of SRAF-aware methodologyTwo components of SRAF-aware methodology

Assist-correct librariesAssist-correct libraries Inter-device spacing within a standard cellsInter-device spacing within a standard cells Intelligent library designIntelligent library design

Assist-correct placement Assist-correct placement THIS WORK THIS WORK Intelligent whitespace adjustment between cellsIntelligent whitespace adjustment between cells

x+x x

Better than

OutlineOutline

OPC and SRAF: An IntroductionOPC and SRAF: An Introduction The AFCorr MethodologyThe AFCorr Methodology AFCorr Placement PerturbationAFCorr Placement Perturbation Experiments and ResultsExperiments and Results SummarySummary

AFCorrAFCorr: SRAF-Correct Placement: 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 AFCorr After AFCorr

Minimum Perturbation ApproachMinimum Perturbation Approach

Objective:Objective: Minimum perturbation of input placementMinimum perturbation of input placement Reduce weighted CD degradation with defocusReduce weighted CD degradation with defocus Preserve timingPreserve timing

Constraint:Constraint: Placement sitemap must be respectedPlacement sitemap must be respected

How:How: One cell row at a timeOne cell row at a time Solve each cell row by Solve each cell row by dynamic programmingdynamic programming

Feasible Placement PerturbationsFeasible Placement Perturbations

Minimize Minimize i i ||

s.t. s.t. a-1a-1a a ++SSa-1a-1RP RP + S+ Saa

LPLP + (x + (xaa – x – xa-1a-1 – w – wa-1a-1) ) AF AFwi and xi = width and location of Ci

i = perturbation of location of cell Ci

AF = set of allowed spacingsRP, LP = boundary poly shapes with overlapping y-spans

- Overlap types: g-g, g-f, f-f

S = spacing from boundary poly to cell border

xxaaxxa-1a-1

SSa-1a-1RPRP

SSaaLPLP

WWa-1a-1

Dynamic Programming SolutionDynamic Programming Solution

weighted objective function E.g., to account for timing-criticality of cells

Slope = Slope = CD / CD / Pitch = CD degradation per unit space Pitch = CD degradation per unit space between AF values between AF values

AFAFii = closest assist-feasible spacing ≤ HSpace = closest assist-feasible spacing ≤ HSpace Overlap_weight = overlap length weighted by relative Overlap_weight = overlap length weighted by relative

importance of printability for gate-to-gate, gate-to-field, and importance of printability for gate-to-gate, gate-to-field, and field-to-fieldfield-to-field

COST (1,b) = | xCOST (1,b) = | x11-b| -b| // subrow up through cell 1, location b// subrow up through cell 1, location b

COST (a,b) = COST (a,b) = (a) |(x(a) |(xaa -b)| + -b)| +

MINMIN{X{Xaa-SRCH ≤ i ≤ X-SRCH ≤ i ≤ Xaa+SRCH}+SRCH} [COST(x [COST(xa-1a-1,i) + HCost(a,b,a-1,i)],i) + HCost(a,b,a-1,i)]

// SRCH = maximum allowed perturbation of cell location// SRCH = maximum allowed perturbation of cell locationHCost = “forbidden-pitch cost” = sum over horiz-adjacencies ofHCost = “forbidden-pitch cost” = sum over horiz-adjacencies of [slope(j) |HSpace –AF[slope(j) |HSpace –AF jj| * overlap_weight]| * overlap_weight] s.t. AFs.t. AFj+1j+1 > HSpace > HSpace AF AFjj

OutlineOutline

OPC and SRAF: An IntroductionOPC and SRAF: An Introduction The AFCorr MethodologyThe AFCorr Methodology AFCorr Placement PerturbationAFCorr Placement Perturbation Experiments and ResultsExperiments and Results SummarySummary

Experimental FlowExperimental Flow

Forbidden pitch

SB OPC

- SB Insertion- Model-based OPC (Best DOF model)

Lithography modelgeneration

(Best & Worst DOF)

Benchmark design

Placement

Assist CorrectedGDS

Route

Typical GDS

Route

Post-Placement

OPCed GDSs

- Delay - # SB - # EPE - # Forbidden pitch- GDSII size- OPC Running Time

Experiments

Experimental SetupExperimental Setup KLA-Tencor’s KLA-Tencor’s ProlithProlith

Model generation for Model generation for OPCproOPCpro Best focus/ worst (0.5 micron) defocusBest focus/ worst (0.5 micron) defocus

Calculating forbidden pitchesCalculating forbidden pitches Mentor’s Mentor’s OPCproOPCpro, SBar , SBar SVRFSVRF

OPC, SRAF insertion, OPC simulationOPC, SRAF insertion, OPC simulation Cadence Cadence SOC EncounterSOC Encounter

PlacementPlacement Synopsys Design ComplierSynopsys Design Complier

SynthesisSynthesis

Experimental MetricsExperimental 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 CountResults: Increased SB Count

0

50000

100000

150000

200000

90 80 70 60 50

Utilization(%)

# t

ota

l SB

0

5000

10000

15000

20000

25000

30000

35000

40000

# S

B D

iffe

ren

ce

SB difference (130)

SB difference (90)

SB w/o AFCorr(130)

SB w AFCorr(130)

SB w/o AFCorr(90)

SB w AFCorr(90)

SB count increases as utilization decreases due to increased whitespace

Better DOF and resist image

Results: Reduced F/P and EPEResults: 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

60

70

80

90

100

90 80 70 60 50

Utilization(%)

Red

uct

ion

(%

)

EPE (130)EPE (90)

F/Pitch (130)F/Pitch (90)

Impact on Other Design MetricsImpact on Other Design Metrics

Data size Data size 4%, OPC running time 4%, OPC running time 3%, Cycle time 3%, Cycle time 6% 6% Other impacts are negligible compared to large improvement Other impacts are negligible compared to large improvement

in printability metricsin printability metrics

Utilization(%)Utilization(%) 9090 8080 7070

Flow:Flow: OrigOrig AFCorrAFCorr OrigOrig AFCorrAFCorr OrigOrig AFCorrAFCorr

130nm130nm #EPE#EPE 87728772 22672267 59755975 962962 49764976 274274

R/T (s)R/T (s) 67216721 67326732 68396839 68996899 68786878 69326932

GDS (MB)GDS (MB) 42.942.9 41.941.9 41.841.8 42.342.3 42.242.2 42.242.2

Delay (ns)Delay (ns) 4.214.21 4.494.49 4.5474.547 4.4444.444 4.5014.501 4.3714.371

90nm90nm #EPE#EPE 75237523 12621262 48134813 532532 21312131 107107

R/T(s)R/T(s) 48354835 50115011 54515451 55355535 55295529 56325632

GDS(MB)GDS(MB) 41.141.1 42.342.3 41.241.2 43.243.2 42.242.2 42.342.3

Delay(s)Delay(s) 2.4782.478 2.3052.305 2.4582.458 2.6022.602 2.5222.522 2.472.47

OutlineOutline

OPC and SRAF: An IntroductionOPC and SRAF: An Introduction Forbidden Pitch Extraction Forbidden Pitch Extraction The AFCorr MethodologyThe AFCorr Methodology Experiments and ResultsExperiments and Results SummarySummary

SummarySummary AFCorr is an effective approach to achieve assist

feature compatibility Up to 100% reduction of forbidden pitch and EPEUp to 100% reduction of forbidden pitch and EPE Relatively negligible impacts on GDSII size, OPC Relatively negligible impacts on GDSII size, OPC

runtime, and design clock cycle timeruntime, and design clock cycle time Compared to huge improvement in printabilityCompared to huge improvement in printability

Ongoing researchOngoing research Considering forbidden pitches of field poly of “vertically” Considering forbidden pitches of field poly of “vertically”

adjacent cellsadjacent cells Developing “correct-by-construction" standard-cell layouts

which are always AFCorrect in any placement

Thank You!

NotationNotation

W = cell width; W = cell width; RP, LP = Boundary poly geometriesRP, LP = Boundary poly geometries S = Spacing from boundary poly to cell borderS = 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:Example: S Sa-1a-1

RP2 RP2 + (x+ (xa-1a-1 – x – xaa – w – wa-1a-1) + S) + SaaLP3LP3 should be assist-correct should be assist-correct

A