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

CD ControlCD Control

Puneet GuptaPuneet Gupta11

(puneet@blaze-dfm.com)(puneet@blaze-dfm.com)

Andrew B. KahngAndrew B. Kahng1,21,2

Chul-Hong ParkChul-Hong Park22 1 Blaze DFM, Inc.

2 ECE Department, University of California, San Diego

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)

SRAF = Scattering Bar (SB)SRAF = Scattering Bar (SB) 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 More SBs helps to enhance DOF margin and to meet the target CDMore SBs helps to enhance DOF margin and to meet the target CD

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 (180nm)

Wafer structure (SEM)

SRAFs and Bossung PlotsSRAFs and Bossung Plots

Bossung plotBossung plot Measurement to evaluate lithographic manufacturability Measurement to evaluate lithographic manufacturability Maximize the common process windowMaximize the common process window 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 Library cell layout should avoid all forbidden pitchesLibrary cell layout should avoid all forbidden pitches Intelligent library designIntelligent library design

Assist-correct placement Assist-correct placement THIS WORK THIS WORK Intelligent whitespace adjustment in the placerIntelligent whitespace adjustment in the placer

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: Reduce forbidden pitch violationReduce forbidden pitch violation Reduce weighted CD degradation with defocusReduce weighted CD degradation with defocus Minimum perturbation: preserve timingMinimum perturbation: preserve timing

Constraint:Constraint: Placement site width must be respectedPlacement site width must be respected

How:How: One standard cell row at a timeOne standard 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

gategate

field

field

Vertical Forbidden PitchesVertical Forbidden Pitches

Handled in a way similar to horizontal overlapHandled in a way similar to horizontal overlap Usually field polyUsually field poly Typically, #vertical forbidden pitches < #horiz. F.P.Typically, #vertical forbidden pitches < #horiz. F.P.

Due to restricted design rules like single orientation polyDue to restricted design rules like single orientation poly

Row i-1

Row i

Cell under consideration

Dynamic Programming SolutionDynamic Programming Solution

perturbationweight αα, , ββ = weights for horizontal vs vertical forbidden pitches = weights for horizontal vs vertical forbidden pitches Slope = Slope = CD / CD / Pitch = CD degradation per unit space between AF values Pitch = CD degradation per unit space between AF values AFAFii = closest assist-feasible spacing ≤ HSpace = closest assist-feasible spacing ≤ HSpace Overlap_weight = overlap length weighted by relative importance of printability Overlap_weight = overlap length weighted by relative importance of printability

for gate-to-gate, gate-to-field, and field-to-fieldfor gate-to-gate, gate-to-field, and field-to-field

COST (1,b) = | xCOST (1,b) = | x11-b| -b| // subrow up through cell 1, location b// subrow up through cell 1, location bCOST (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) + ,i) + ααHCost(a,b,a-1,i) HCost(a,b,a-1,i)

+ + ββVCost(a,b)]VCost(a,b)] // SRCH = maximum allowed perturbation of cell location// SRCH = maximum allowed perturbation of cell location

HCost = horizontal “forbidden-pitch cost” = sum over horiz-HCost = horizontal “forbidden-pitch cost” = sum over horiz- adjacencies of 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 AFj j

VCost = vertical forbidden pitch costVCost = vertical forbidden pitch cost

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 - GDSII size- OPC Run Time- # Forbidden pitch- # SB - # EPE

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, ORC (Optical Rule Check)OPC, SRAF insertion, ORC (Optical Rule Check) Cadence Cadence SOC EncounterSOC Encounter

Placement & RoutePlacement & Route 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 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 CountResults: Increased SB Count

SB count increases as utilization decreases due to increased whitespace

#SB increases after AFCorr placement

0

50000

100000

150000

200000

250000

300000

90 80 70 60 50

Utilization(%)

# T

ota

l SB

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

# 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)

Results: Reduced F/P and EPEResults: 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

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

Impact : Data size < 1%, OPC run time < 2%, Cycle time < Impact : Data size < 1%, OPC run time < 2%, Cycle time < 4%4%

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 48904890 47214721 59755975 562562 42764276 1515

R/T (s)R/T (s) 78217821 79027902 78767876 79347934 79137913 79737973

GDS (MB)GDS (MB) 48.948.9 48.948.9 48.848.8 48.948.9 48.248.2 48.448.4

Delay (ns)Delay (ns) 4.24.2 4.64.6 4.54.5 4.74.7 4.54.5 4.64.6

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

R/T(s)R/T(s) 62116211 63276327 63226322 64316431 64826482 64996499

GDS(MB)GDS(MB) 43.143.1 43.343.3 43.243.2 43.343.3 43.243.2 43.343.3

Delay(s)Delay(s) 2.72.7 2.72.7 2.62.6 2.62.6 2.42.4 2.52.5

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 in physical layout Up to 100% reduction of forbidden pitch and Up to 100% reduction of forbidden pitch and

EPEEPE Relatively negligible impacts on GDSII size, Relatively negligible impacts on GDSII size,

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

Ongoing researchOngoing research 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