a study of attenuated psm structure for euvl to minimize...

2008 International Workshop on EUV LithographyJune 10-12, 2008, Maui, Hawaii, USA

A Study of Attenuated PSM Structure for E

UVL to Minimize Mask Shadowing Effect

Sangsul Lee1, Chang Young Jeong1, Tae Geun Kim1, Hyun-Duck Shin1, Eun Jin Kim2, Hye-Keun Oh2,

Jinho Ahn1

1Division of Advanced Materials Science and Engineering, Hanyang University2Department of Applied Physics, Hanyang University

June 10-12, 2008, Maui, Hawaii, USA 2008 International Workshop on EUV Lithography Hanyang University

Contents

Introduction-

Requirem

e

nts for EUVL Mask- How to reduce absorber stack thickness?

Attenuated PSM structure for minimizing shadowing effect-

The Sche

matic of attenuated PS

M - The reflectivity & phase shift change with absorber stack-

The characteristics of proposed attenuated PS

M

The Imaging performance of Binary mask & attenuated PSM- The pattern shift with the absorber stack thickness- H-V bias & pattern shift between binary m

ask and attenuated PS

M

Conclusions


The structure of EUVL Mask

Absorber

Multilayer

Substrate

Capping

Buffer

ARC

• To improve DUV contrast for inspection

• Low DUV & EUV reflection• Low sheet resistance for e-beam writing

• To protect ML from absorber-etch damage

• High etch selectivity to absorber• Low stress

• To protect ML from unexpected oxidation

• Prevention of O2 & H2

• To reflect EUV wavelength

•To support mask structure with minimum distortion

• To absorb EUV for image formation

• Patterning characteristics

• High contrast >100:1• Low stress: <100 MPa


Requirements for EUVL Mask

Requirements for high lithographic

performance

-

High image contrast (<1% reflectance on absorber

pattern)

-

Minimized shadowing effect

: Due to reflective mask and oblique illumination (6º?

8º?)

: Correction for shadowing effect should be

considered

-

High flatness control

-

High surface roughness control (Flare)

-

High durability against EUV exposure

-

Low thermal expansion substrate

Requirements for mask manufacturing

-

Easy for mask processing

: writing, etching, cleaning…


The Sche

m

atic of attenuated PSM

Febry-Perot structure

• Phase difference ( φ1 - φ2 )

: 180 ±

6°

• EUV reflectivity R1 : 5~12%

• DUV reflectivity : <5%

• Conventional etch process

• Minimum shadow effect

R1

, φ1

R2

,

φ2

Multilayer

Substrate

■TaN attenuator■Al2 O3 spacer■Mo phase shifter


How to reduce absorber stack thickness m

aintaining high image contrast?

•

Oscillating reflectivity (7 nm period (~1/2 wavelength))

-

>70nm

absorber thickness is needed to achieve high

image

contrast

•

TaN material is a good EUV attenuator

Simulated EUV Reflectivity

1.4%reflectivity

Measured EUV Reflectivity

How about using phase shift concept?

-

to improve image contrast

-

to reduce absorber stack thickness


The DUV reflectivity contour with TaN attenuator and Al2O3 spacer thickness variation

16 18 20 22 24

2

4

6

8

10

Thickness of Al2O3(nm)

Thi

ckne

ss T

aN (n

m)

16 18 20 22 24

2

4

6

8

10

Thickness of Al2O3(nm)

For 257nm wavelengthFor 199nm wavelength

0.5000

5.000

9.500

14.00

18.50

23.00

24.50

• The thickness variation of Al2 O3 is less sensitive than TaN

• The DUV reflectivity (< 5%) could be achieved with the thinnest absorber stack (TaN (3nm) / Al2 O3 (18nm))


The Measured DUV reflectivity with Mo thickness variation

10 15 20 25 300

10

20

30

40

50

Thickness of Mo (nm)

@ 257nm @ 199nm

Effect of Mo thickness variation in TaN(3nm)/Al2

O3

(18nm)/Mo/Ru/ML

Reflectivity does not change

with Mo thickness

4.5% reflectivity

21% reflectivity

•

The Mo thickness variation does not affect the D

UV reflectivity

•

The structure of TaN(3nm)/Al2

O3

(18nm)/Mo/Ru/ML showes a good inspection contrast at 257nm wavelength


The EUV reflectivity and phase shift simulation of TaN(3nm)/Al2

O3

(18nm)/Mo/Ru/ML structure

Conditions: Phase tolerance: ±6

°EUV reflectivity: >5%

20 22 24 26 28 30 32 34 36 38 40-230-220-210-200-190

-180-170-160

-150-140-130

036912

151821

242730

EUV

Ref

lect

ance

(%)

Phas

e shi

ft (d

eg)

Thickness(nm) of Mo

7nm

Effect of Mo thickness variation in TaN(3nm)/Al2 O3 (21nm)/Mo/Ru/ML

• Optimized stack condition of attenuated PSM

- 180±6°

phase shift tolerance and 5~12% of EUV reflectivity

- Large Mo thickness latitude (7nm)

- Thin absorber stack (52nm~58nm)


The m

easured DUV Inspection contrast

200 250 300 350 4000

20

40

60

80

100

Ref

lect

ance

(%)

Wavelength (nm)

Ru capped ML Att-PSM

10nm

Attenuated PSM

structure:

TaN/Al2

O3

/Mo/Ru/ML

ML

Ru(2nm)

Mo(28nm)

Al2

O3

(21nm)

TaN(3nm)

R1 DUV (%)

R2 DUV (%)

Contrast(%)

@257nm 46.38% 1.72% 92.84%

DUV (at 257 nm) reflectivity measurement with favorable condition

of att-PSM structure( TaN 3nm/Al2

O3

21nm/Mo 28nm/ Ru 2 nm/ML )

TEM imageMeasured 257nm DUV Reflectivity


The condition of aerial image simulation

Simulation condition

List of mask structure

Type Structure Thickness of absorber stack (nm)

BIM Al2O3(20nm)/TaN(27nm)/Ru(2nm)/ML40 47nm





Type Structure Thickness of absorber stack (nm)






Parameter Value

Median wavelengthTemporal bandwidthNAMagnification factorSpatial coherence factor, sigmaIncident anglePattern size

13.5nm2% FWHM0.2540.5

6°22nm Iso, 22/32nm L/S

Parameter Value


13.5nm2% FWHM0.2540.5

6°22nm Iso, 22/32nm L/S

TaN

substratesubstrate

Mo/SiMo/Si

Al2O3

TaN

substratesubstrate

Mo/SiMo/Si

Al2O3

Mo: 2.7nmSi: 4.15nm

This stack was proposed in SPIE 2006 by Seung Yoon Lee (Hanyang University)


The pattern shift with absorber stack thickness at 22n

m isolated line

Pattern shiftAerial image

•

As the absorber stack thickness increased, the

pattern shift increased

•

Using 52nm absorber stack reduced pattern

shift(~1nm) compared to 75nm absorber stack

52nm absorber thickness


The pattern shift with absorber stack thickness at 22/32nm L/S

• As the absorber stack thickness increased, the pattern shift increased

• Using 52nm absorber stack reduced pattern shift(~1nm) compared to 75nm absorber stack

Pattern shiftAerial image

52nm absorber thickness


The condition of aerial image simulation

Simulation condition

List of mask structure

TaN

substratesubstrate

Mo/SiMo/Si

Al2O3

Mo

R1

, φ1 R

2

,

φ2

Attenuated PSM

TaN

substratesubstrate

Mo/SiMo/Si

Binary mask

Parameter Value


13.5nm2% FWHM0.2540.5

6°32nm L/S, 45nm L/S

Parameter Value


13.5nm2% FWHM0.2540.5

6°32nm L/S, 45nm L/S

Type Structure Absorber thickness (nm) REUV(abs) (%)

Att-PSM

TaN(3nm)/Al2O3 (21nm)/Mo(28nm)/Ru(2nm)/ML 52nm 9.5 %

BIM TaN(52nm)/Ru(2nm)/ML 52nm 0.2%

Type Structure Absorber thickness (nm) REUV(abs) (%)

Att-PSM

TaN(3nm)/Al2O3 (21nm)/Mo(28nm)/Ru(2nm)/ML 52nm 9.5 %

BIM TaN(52nm)/Ru(2nm)/ML 52nm 0.2%


The aerial image of horizontal &

vertical pattern between binary and attenuated PSM

-0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.080.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Position

BIM_Vertical BIM_Horizontal PSM_Vertical PSM_Horizontal

• The Attenuated PSM shows steeper edge profile and higher image contrast compared to the binary mask

• The Horizontal pattern shows higher image contrast and negligible pattern shift compared to vertical pattern

45nm L/S pattern32nm L/S pattern

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.060.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Inte

nsi

ty

Position

BIM_Vertical BIM_Horizontal PSM_Vertical PSM_Horizontal

Positi

on

Positi

on


The H-V CD bias and pattern shift with defocus

• The H-V CD bias increased and pattern shifted with defocus

• The attenuated PSM shows less H-V CD bias compared to the binary mask

-> Larger process window than binary mask

• 32nm pattern shows larger H-V CD bias compared to 45nm pattern

• The Pattern shift is less sensitive than the H-V CD bias to the optical property of absorber

-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15123456789

1011

BIM_32nm_L/S BIM_45nm_L/S PSM_32nm_L/S PSM_45nm_L/S

H-V

bia

s

Defocus

DoF: 0.20um

-0.15 -0.10 -0.05 0.05 0.10 0.15

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Pa

ttern

Sh

ift

Defocus

BIM_32nm L/S BIM_45nm L/S PSM_32nm L/S PSM_45nm L/S

0

DoF: 0.20um

Defoc

us

Defoc

us

H-V

bia

s


Conclusions

Proposed optimized attenuated PSM structure

-

TaN(3nm)/Al2

O3

(21nm)/Mo(28nm)/Ru/ML: TaN (attenuator), Al2

O3

(spacer), Mo (phase shifter) - Total thickness: 52 nm

: Minimized shadowing effect

- High DUV contrast for m

a

sk inspection : > 90% @ 257nm

- Large Mo thickness latitude

The Imaging performance of BIM & attenuated

PSM

-

The attenuated P

S

M show

s steeper edge profile and higher image contrast co

mpared to the binary m

ask -

The attenuated P

S

M show

s less H

-

V C

D bias co

m

pared to the binary mask-

The pattern shift is less sensitive than the H

-V CD bias with the optical property of absorber

- Larger process window than binary m

ask

a study of attenuated psm structure for euvl to minimize...

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