assessing chip-level impact of double patterning lithography

Assessing Chip-Level Impact of Double Patterning Lithography

Kwangok Jeong*, Andrew B. Kahng*,**, and Rasit O. Topaloglu***

http://vlsicad.ucsd.edu/

* ECE Dept., UC San Diego** CSE Dept., UC San Diego

*** GlobalFoundries, Inc.

(2)UCSD VLSI CAD Laboratory / GLOBALFOUNDRIES, Inc. - ISQED 2010, March 23, 2010

Outline

• Double Patterning Lithography (DPL)

• Traditional Interconnect Analysis

• Additional Variability in DPL

• Misalignment in Double Patterning

• Analysis in Different DPL Options

• Experiments

• Conclusion


Outline






• Experiments

• Conclusion


Double Patterning Lithography (DPL)• Pattern-doubling:

• ‘2X-resolution’ lithography with ‘1X-resolution’ equipment

• Taxonomy• Resist type: positive /negative• Methods: double exposure (DE) / double patterning (DP) /

spacer double patterning (SDP)• Printed feature: line / space

Target layerResist

1st Exposure

2nd Exposure

Mask1

Mask2

Target layerResist

Mask

1X-resolution

1X

2X-resolution

1X


Traditional Interconnect Analysis• Designers use capacitance

tables from foundries

• 2D/3D field solver with variations Capacitance tables • Major sources of variation:

• Metal/dielectric density-dependent systematic variation

• Random process variation• Results of variation

• Width (W) variation• Metal height (H) variation• Dielectric thickness (D)

variation, etc.

• Traditional interconnect variation analysis1. for (i = -3 ; i 3 ; i=i+1) {2. for (j = -3 ; j 3 ; j=j+1) {3. for (k = -3 ; k 3 ; k=k+1) {4. W=Wnom + iW

5. H= Hnom + jH

6. D= Dnom + kD

7. run field solver over parameterized structure}}}8. Find nominal and worst-case capacitanceM+1

M-1

M W H

D


Additional Variability in DPL• Overlay error

• Causes: • mask misalignment • material stress-impacted

deformations• litho-/etch-impacted

topography• lens aberration, etc.

• Impacts on DPL• Width variation• Space (or pitch) variation

Capacitance variation

• Alignment metric

• Indirect: • Two DPL masks aligned to a

reference layer• Error:

• Direct: • Second DPL mask aligned to

the first DPL mask• Error:

SS

S

S

IndirectAlignment (IA)

Cc

Cg

DirectAlignment (DA)

S2

S


Outline






• Experiments

• Conclusion


Misalignment in Positive DE/DP

1 2 1 2 1

W W

S S

mask2

Positive photoresistDielectric

Space on one side increasesSpace on the other side decreases

Required design of experiments foreach S (-3 ~ 3) mask1 shift by +S/2 mask2 shift by –S/2end

mask1

After exposure + etchCu filling

(misaligned to left)


Misalignment in Negative DE/DP

1 2 1 2 1

W’’ W’

S S

P PS/2

mask1

mask2 (misaligned to left)

Negative photoresistDielectric

After exposure + etch

After filling Cu

Width of one increasesWidth of the other decreasesRequired design of experiments foreach S (-3 ~ 3) mask1 change W by +S shift by S/2 mask2 change W by –S shift by S/2end


Spacer Thickness Variation in Positive SDP

Dielectric


(kind of) Positive photoresist

Primary patternsSpacers (act as if masks)

After filling Cu

Cu

Width and space changeRequired design of experiments foreach S (-3 ~ 3) mask1 change W by 0 mask2 change W by +Send

1 2 1 2 1

WW’’

P P

S S


Spacer Thickness Variation in Negative SDP

1 2 1 2 1

W’ W’

P’’ P’

S S

Primary patterns

Dielectric


After filling Cu

Spacers (act as if masks)(kind of ) Negative photoresist

Cu

Width and space changeRequired design of experiments foreach S (-3 ~ 3) mask1 change W by +S/2 shift by +S/4 mask2 change W by +S/2 shift by –S/4end


Outline





• Experiments

• Conclusion


Photoresist ProcessAlignment

Experiments: Scenarios• We examine impact of misalignment and linewidth

variation across various DPL options

Parallel 5-Interconnect

Structure(TCAD tool)

Interconnectsin a full-chip

(Signoff RCX)

Indirect

Direct

Positive

Negative

DE

DP

SDP

Direct Positive

Negative

DE

DP

SDP


TCAD-Based BEOL Analysis Results• Capacitance variation due to

misalignment in DE/DP• IA shows larger variation than DA• Negative resist processes have

larger variation

• Capacitance variation in different DPL options• SDP has larger variation• Negative resist processes have

larger variation

Cap

acita

nce

(aF/

um)

Cap

acita

nce

(aF/

um)


Design-Level Analysis - Flow• Overlay-aware extraction flow

1. Design GDSTOP.GDS

Initial GDS

AES core with NanGate 45nm Tech.

2. Split GDS

Base GDS Sub-GDS1

Sub-GDS2

DPL layersNon-DPL layers 3. PatternDecomposition

Sub-GDS1-1

Sub-GDS1-2

Sub-GDS2-1

Sub-GDS2-2

ILP-based min cost coloring(Kahng et al. ICCAD08)

Coloring and Splitting

SUB2.GDSSUB1.GDS

4. Shift and Merge(Cadence Virtuoso)

Shifting and Merging

SUB1 (x1, y1)TOP.GDS

SUB2 (x2, y2)

TOP.GDS

TOP.GDS

5. Resize and Extraction(Synopsys Hercules, Star-RCXT) Resizing


• Overlay error can cause more than +/- 10% capacitance variation within a die, for all DPL options

Large on-chip variation Increase of timing optimization difficulty

Cap

acita

nce

Varia

tion

(%)

Design-Level Capacitance Variation

M2 M3 M4 M5


• A net having maximum crosstalk delay (17um long)• SDP shows more sensitivity tighten overlay spec• P-DE/DP shows least sensitivity lessen overlay spec

Maximum Crosstalk-Induced Delay

P-DE/DP(Space on one side)

N-DE/DP(Width)

P-SDP(Spaces on both sides)

N-SDP(Space & width)

M4

|S| |S|/2 |S|/2 |S|/2

M2 M4 M2 M4

w/o metal fill w/ metal fill


Total Negative Slack Variation• SDP, especially for lower layer (smaller feature),

shows more sensitivity tighter overlay spec

0%

2%

4%

6%

8%

10%

12%

1 2 3

TNS

Varia

tion

(%)

Overlay error S variation (sigma)P-DE/DP M2 P-DE/DP M3 P-DE/DP M4 P-DE/DP M5N-DE/DP M2 N-DE/DP M3 N-DE/DP M4 N-DE/DP M5P-SDP M2 P-SDP M3 P-SDP M4 P-SDP M5N-SDP M2 N-SDP M3 N-SDP M4 N-SDP M5

P-SDP

N-SDPM2

M4

TNS

Varia

tion

(%)


Outline





• Experiments

• Conclusion


Summary of Observations

• Overlay error with indirect alignment (IA) results in higher

capacitance variations compared to direct alignment (DA)

• Capacitance can vary > 10% due to misalignment

Large OCV increase timing optimization difficulty

• Timing can be degraded significantly, e.g., > 10% worse TNS

• P-DE/DP may be the most favorable option for BEOL DPL • With the same 3 overlay control, the variation in P-DE/DP is

50% of N-DE/DP or P-SDP, and 25% of N-SDP Overlay control spec for P-DE/DP can be relaxed by 2X compared to others


Conclusion and Ongoing Work• We provide a variational interconnect analysis

framework for double patterning lithography• We analyze mechanisms of interconnect variations due to

misalignment and spacer thickness variation in DPL• We provide both interconnect and design-level RC-

extraction framework reflecting interconnect variation in a 45nm DPL process

• We compare the impact of overlay error in different DPL options

• Ongoing work• Development of timing analysis and optimization

methodology considering interconnect variation in DPL• Incorporation of statistical techniques to target pessimism

reduction

Thank You!


Impact of Misalignment on FEOL• Standard cell decomposition

• Experimental setup• 10nm 3 misalignment is assumed between layers• Design of experiments (all permutation: 3*3*3*3 = 81 cases)

• P1: -10nm (L) / 0nm (C) / +10nm (R)• P2: -10nm (L) / 0nm (C) / +10nm (R)• M: -10nm (L) / 0nm (C) / +10nm (R)• C: -10nm (L) / 0nm (C) / +0nm (R)

Original MP1 P2 C BASE


Experimental Results on FEOL• Flow

• Impact of misalignment on cell delay is negligibly small (< 2%)• Capacitance variation due

to misalignment << gate capacitance

• Measured Delay Variation (%)P1 P2 M C

L C R

L

LL 0.91 1.19 0.89C 0.89 1.15 0.87R 0.87 1.13 0.83

CL 0.06 0.23 0.45C 0.04 0.19 0.4R 0.04 0.17 0.38

RL -1.45 -0.68 -0.64C -1.47 -0.72 -0.66R -1.47 -0.74 -0.7

C

LL 0.72 1.02 0.7C 0.7 0.98 0.68R 0.68 0.96 0.64

CL -0.15 0.04 0.23C -0.15 0 0.21R -0.17 -0.02 0.17

RL -1.68 -0.89 -0.87C -1.68 -0.94 -0.89R -1.7 -0.96 -0.91

R

LL 0.51 0.85 0.51C 0.49 0.81 0.49R 0.47 0.79 0.47

CL -0.38 -0.15 0.02C -0.38 -0.19 0R -0.4 -0.21 -0.02

RL -1.94 -1.11 -1.09C -1.94 -1.15 -1.11R -1.96 -1.15 -1.13

Tr-level RC-ExtractionSTAR-RCXT

Circuit SimulationHSPICE


DPL Options• Double Exposure • Double Patterning • Spacer-DP

• Photoresist • Printed Featuremask

positive resistnegative resist

After exposure & etch

Dielectric

After Cu filling

(a) Positive-tone (b) Negative-tone

Cu interconnect

mask

positive resist

After exposure & etch

Dielectric

(a) Spaces (Trench-First) (b) Lines

Cu interconnect

Poly

Poly

Target layer

ResistHardmask

Buffer oxideHardmask

Target layer1st Litho-etch

Spacer formation Oxide depo. CMP

Spacer removal 2nd etch

Mask

Target layer

ResistHardmask

1st Litho-etch

2nd Litho-etch

Mask1

Mask2

Target layerResist

1st Exposure

2nd Exposure

Mask1

Mask2


Mask Coloring and Layout Examples in DPL• Mechanism of misalignment-induced variation

(a) DE and DP Process

(b) SDP Process

12

3

4

5

6 12

3

4

5

6 14

5

2

3

6

Original patterns

Original patterns

Coloring Patterns 1 Patterns 2

Coloring Spacer formation(Large spacer)

Trim & repair (dark gray)

S S

Narrow spaceW

12

3

4

5

6

W”

Dummy for pattern 6

14

5

2

3 6

Spacer (gray) a b


Design-Level Analysis - DOE• Design of Experiments for DE/DP with DA

1. foreach layer { M2, M3, M4, M5 }2. decompose layer into layermask1 and layermask2

3. foreach S { -3/2, -2/2, -/2, 0, /2, 2/2, 3/2} 4. shift layermask1 by S5. shift layermask2 by –S6. end7. layer layermask1 + layermask2

8. foreach W { -3/2, -2/2, -/2, 0, /2, 2/2, 3/2}9. resize layer by W 10. end11. merge with other layers12. RC-Extraction and Timing Analysis13. end


Impact on Capacitance Variation• Total interconnect capacitance: maximum C(%)

• Among top 20% high capacitance nets• Impact of overlay < impact of width

• Sum of capacitance in the most critical path• Critical path has short interconnects impact of BEOL

variation significantly reduces• Impact of overlay < impact of width

-3 +3Min Avg Max Min Avg Max

Overlay -7.7% 1.4% 9.2% -7.3% 1.4% 9.7%Width -22.2% 4.7% 7.1% -3.6% 5.3% 28.6%

Interconnect only Interconnect + GateMin Max Min Max

Overlay -0.08% 0.47% -0.04% 0.25%Width -1.87% 2.59% -0.99% 1.38%


Impact on Crosstalk-Induced Delay• Maximum coupling induced delay change

• PrimeTime-SI (Synopsys) is used to find a net that is mostly affected due to crosstalk• Temporal/functional filtering is performed• Selected net structure

• A net with relatively small length (~17um) can have >10% delay changes due to overlay error

Cc (pF) Cg (pF) Delay@Min @Max @Min @Max Min Max

Overlay 1.842 1.863 1.089 1.053 -0.47% 13.1%Width 1.840 1.906 1.089 1.120 -0.82% 15.4%

M2 segment:1.604um

M3 segment:0.78um

M4 segment:14.788um

Capacitance when Delay is minimumCapacitance when Delay is maximum


Impact on Timing• Longest path and total negative slack (TNS)

• Impact of overlay << impact of width• Longest path delay changes negligibly• However, overall timing (TNS) can change significantly

Longest path delay TNSMin Max Min Max

Overlay -0.06% 0.98% 3.2% 3.8%Width -1.22% 2.00% -34.3% 49.4%

Tota

l Neg

ativ

e Sl

ack

(ns)

-60

-50

-40

-30

-20

-10

0-3s -2s -1s 0s 1s 2s 3s

Interconnect varaition

OverlayWidth

assessing chip-level impact of double patterning lithography

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