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Flare compensation in EUV lithography

Jonathan Cobb, Ruiqi Tian, Robert Boone, Kevin Lucas, Scott Hector, Vladimir Ivin*, Mikhail Silakov*, and George Babushkin*

*SOFT-TEC

J. Cobb et al., 2003 EUV Symposium, Antwerp

Outline

• Description of EUV flare• Calculation details• Compensation strategies• Conclusions

J. Cobb et al., 2003 EUV Symposium, Antwerp

Flare in EUV lithography

• Cause: surface roughness on optics

• Scales as 1/λ2, so more problematic at EUV wavelengths

• Effects:• Scatters light out of bright regions and into dark regions ⇒ reduces contrast

• Couples local light intensity to features 1000’s of mm away ⇒ pattern dependent

• Simple calculations ⇒ 1% (absolute) change in flare causes 0.86 nm CD change

J. Cobb et al., 2003 EUV Symposium, Antwerp

Estimating impact of flare variation on CD control• Statistical simulation parameters (CCI design)

– Average mask transmission: 80–90% – Mean focus error: -0.05–0.05 µm– Cross-slit focus variation (1σ): 0.006–0.008 µm– Mean dose error: ±10%– Cross-slit dose variation (1σ): 2.0–2.6%– Mean flare: 10–15%– Flare variation (1σ): 1–3% (absolute)– 500 calculations per set of conditions

• Constant simulation parameters– 45nm lines on a 110nm pitch– Partial coherence = 0.7– NA = 0.25– Wavefront error = 0.045λ– Absorber stack 120nm thick– Normal incidence

J. Cobb et al., 2003 EUV Symposium, Antwerp

Simulation results

• Flare variation is largest factor influencing CD control• Effect of flare variation is 3.5X the effect of mean flare • Model predicts 1σ flare variation must be less than 1.7%

(absolute) for ±10% CD control at -0.05 µm focus error

Flare_Var(1,3)Def_Err(-0.05,0.05)Dose_Mean(0.27,0.33)Flare_Mean(10,15)Mask_T(80,90)Def_Err(-0.05,0.05)*Flare_Var(1,3)Def_Var(0.02,0.03)Mask_T(80,90)*Flare_Mean(10,15)Def_Var(0.02,0.03)*Dose_Mean(0.27,0.33)Mask_T(80,90)*Dose_Mean(0.27,0.33)

Term-0.04245070.033017230.01629922-0.0122251-0.0108554-0.0083664-0.0074315-0.0037086-0.0029393-0.0029023

Orthog Estimate

J. Cobb et al., 2003 EUV Symposium, Antwerp

Flare calculations, p. 1

Stearns et al. (J. Appl. Phys. 84, 1998):),(),(),(),( 00 yxPSFyxIyxIyxI ⊗+∝

0.1

10

1000

105

107

109

1011

1013

10-8 10-7 10-6 10-5 0.00010.001 0.01 0.1

PSD

(nm

4 )

Spatial Frequency (nm-1)

10-21

10-19

10-17

10-15

10-13

10-11

10-9

0.1 1 10 100 1000 104 105 106

PSF

(nm

-2)

Radial Distance (nm)

r = λZf

PSD on optics PSF in image

J. Cobb et al., 2003 EUV Symposium, Antwerp

10-21

10-19

10-17

10-15

10-13

10-11

10-9

0.1 1 10 100 1000 104 105 106

PSF

(nm

-2)

Radial Distance (nm)

F = <T>•PSF(d)

Flare calculations, p. 2Aberrations

Stearns

)(2 NANr Rλ

=

J. Cobb et al., 2003 EUV Symposium, Antwerp

Flare calculations• Data scaled so that poly

lines are 0.09 µm (appropriate for 0.1 NA ETS)

• Include measured aberrations from ETS field center

• Average flare over 0.09x0.09 µm2 regions inside of a 2x2 mm2

section of mask data

Memory Logic

J. Cobb et al., 2003 EUV Symposium, Antwerp

Calculation results

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300

Per

cent

Fla

re

Distance from Corner (µm)

Memory

Logic

J. Cobb et al., 2003 EUV Symposium, Antwerp

Proposed compensation strategies (Krautschik et al.)• Selective sizing

– Resize lines according to known d(CD)/dF response– Apply global resizing in middle of mask where flare

variation assumed to be small– Apply local resizing in corners where variation is largest– Iterate to convergent solution

• Dummification (i.e., tiling)– Reduce flare variation by reducing pattern density

variation– Add dummy features to areas of low pattern density– Dummy features must not interfere with circuit function

J. Cobb et al., 2003 EUV Symposium, Antwerp

No place to apply global resizing

0

5

10

15

20

0 500 1000 1500 2000 2500 3000

Per

cent

Fla

re

Distance from Corner (µm)

J. Cobb et al., 2003 EUV Symposium, Antwerp

Effects of sizing on process windowTFFyxIyxI ×+−×= )1(),(),( 0

NA = 0.25, Θi = 6°, σ = 0.7, λ = 13.5 nm, <T> = 75%

0

0.2

0.4

0.6

0.8

1

1.2

-0.06 -0.04 -0.02 0 0.02 0.04 0.06

Rel

ativ

e In

tens

ity

Horizontal Position (µm)

Scalar, no flare, no aberrations

5% flare17.5% flare

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2

CD

(µm

)

Defocus (µm)

17.5% flare,common dose

17.5% flare with bias

5% flare and 17.5%, best dose

J. Cobb et al., 2003 EUV Symposium, Antwerp

Overlapping process window for dense and isolated lines possible with biasing but smaller window than without flare variation

0.0250.0270.0290.0310.0330.0350.037

-0.2 0 0.2

Defocus (microns)A

eria

l im

age

linew

idth

(m

icro

ns)

30 nm isolated line,10% flare31 nm dense line,10% flare30 nm isolated line,15% flare31 nm dense line,15% flare

35 nm in resistDense: 90 nm pitchIsolated: 270 nm pitch

25% average pattern density

J. Cobb et al., 2003 EUV Symposium, Antwerp

MEEF effects minimal, and mean-to-target CD control not critical for biasing

0.981

1.021.041.06

0 10 20Mean flare (%

absolute)

MEE

F Bestfocus-50 nmdefocus50 nmdefocus

• MEEF varies with flare and focus

– Effect of focus larger but only 1-2%

• Linear behavior implies MTT CD control not critical

35 nm in resist; 90 nm pitch

0.025

0.03

0.035

0.04

25 35

Mask linewidth (nm)

Aer

ial i

mag

e lin

ewid

th

(mic

rons

) Bestfocus50 nm ofdefocus-50 nm ofdefocus

J. Cobb et al., 2003 EUV Symposium, Antwerp

Various tiling algorithms tried

• Origins in CMP processes• Rule-based

– Insert dummy features in all appropriate empty space– Increases pattern density uniformity over short length scales

• Model-based CMP– Insert tiles according to empirical model that relates pattern

density and polish uniformity– Considers weighted pattern densities over mm length scales

• Model-based EUV– Place subresolution tiles– Minimize pattern density variation with optimization calculation

that attempts to consider all relevant length scales in PSF– Extend tile placement into borders with model-based CMP

algorithm

J. Cobb et al., 2003 EUV Symposium, Antwerp

Tiles from EUV algorithm

Circuitfeatures

Tiles

J. Cobb et al., 2003 EUV Symposium, Antwerp

Effects of tiles on flare

0

5

10

15

20

0 50 100 150 200 250 300

Per

cent

Fla

re

Dis t ance from Corne r (µm)

Uncompensated

Rule-based tiling

EUV tiling

J. Cobb et al., 2003 EUV Symposium, Antwerp

Conclusions

• Barring significant improvements in EUV optical fabrication technology, mask compensation will be required to reduce flare variation

• Selective sizing is feasible but is computationally expensive and reduces the focus latitude

• Tiling also reduces variation but certain features are not tiling-friendly

• Both selective sizing and tiling will likely be required for full compensation