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Development of a Spectral Purity Filter for CO2 Laser

Produced Plasma based on magnetized plasma

confinement of absorbing gases

Chimaobi Mbanaso1, Gregory Denbeaux1, Alin Antohe1

Horace Bull1, Frank Goodwin2, Ady Hershcovitch3

College of Nanoscale Science and Engineering, University at Albany,

255 Fuller Road, Albany, New York, 12203. USA1

SEMATECH, 257 Fuller Road, Suite 2200, Albany, New York, 12203. USA2

Brookhaven National Laboratory, Upton, New York, 11973. USA3

2010 EUVL Symposium, Kobe, Japan

October 19, 2010

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Content

Introduction

• Infrared(IR) Out-of-Band (OOB) radiation problems in EUVL

• Spectral Purity Filters (SPF) in EUVL

Gas Filter option for spectral filtering of IR light

• IR absorbing gas considerations and measurements

• Confinement methods for IR gas

Summary and Future work

Acknowledgements

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Infrared Out-of-Band (OOB) radiation problems in EUVL

Introduction

• Main Features of CO2 Laser Produced Plasma (LPP) EUV Sources

– 10.6 μm laser light ionizes a target to generate plasma (Tin plasma CE : 2.5 – 4.5%)

– Collector mirrors reflect EUV light and IR light (IR reflectivity from Mo/Si: >90%)

– Drive laser light dominates spectrum at Intermediate focus (IF)

– Undesired heating of optical components beyond IF

CO2 Laser pulse

Target

Intermediate

focus (IF)

Collector mirror

Plasma

Collector mirror

Endo, Akira. “CO2 Laser Produced Tin Plasma Light Source as the Solution for EUV Lithography” ISBN 978-953-307-064-3, pp. 656, February 2010, INTECH, Croatia, downloaded from SCIYO.COM

Endo, Akira et al. EUVA/Gigaphoton “Laser produced plasma source development for EUV Lithography”

~ 20 kW

average

power at

100 kHz

repetition

rate

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• Traditional DUV and VUV SPF’s include foils (Zr, Si, Mo) and reflective gratings (Ru,

Mo)

• Metalized grid-type filter for IR light has shown infrared transmittance at 10.6 μm to be <

0.1% ( Grid thickness typically 2.5 – 10 μm)

• Multilayer mirror coated with diamond-like carbon and silicon layers, show reduced

reflectance in the IR (4.4%)

• EUV transmission limitations - Grid filter showed 74% EUV transmission at normal

incidence and < 50% at 10o incidence, prototype multilayer mirrors with suppressed IR

showed 42.5% EUV reflectance.

• High heat load and debris affects lifetime of filters

Introduction

Spectral Purity Filters in EUVL

Soer et al “Grid Spectral Purity Filters for Suppression of Infrared Radiation in Laser-Produced Plasma EUV Sources,” Proceedings of the SPIE, Volume 7271, pp. 72712Y–72712Y-9 (2009)

W. A. Soer, P. Gawlitza, M. M. J. W. van Herpen, M. J. J. Jak, S. Braun, P. Muys, and V. Y. Banine, "Extreme ultraviolet multilayer mirror with near-zero IR reflectance," Opt. Lett. 34, 3680-3682 (2009)

Kierrey et al, EUV spectral purity filter: optical and mechanical design, gratings fabrication, and testing (2004)

Bibishkin et al, Multilayer Zr/Si filters for EUV lithography and for radiation source metrology (2008)

More filtering options need to be explored to mitigate OOB light

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Gas jets

EUV + OOB light

Absorbing gas

1. Use of flowing gas that

resonantly absorbs IR

2. Gas jet will use directional

momentum to restrict

lateral motion of gaseous

species from target region

3. Use of low EUV

absorbing gases for gas

curtain (Helium or Argon)

Gas jet – Absorbing Gas – Gas jet concept for SPF

Gas Filter Option

IF

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Plasma – Absorbing Gas – Plasma concept as confinement improvement

Hollow Cathode Arcs

IFEUV + OOB light

Absorbing gas

1. Use of flowing gas that

resonantly absorbs IR

2. Plasma discharge will

restrict flow of gaseous

species from target region

with no solid structures

present

3. Allow EUV light to pass

through without any

serious attenuation

Gas Filter Option

Hershcovitch, Ady. Physics of Plasmas Vol. 5 No 5 2130 (1998)

Pinkoski, B.T., Zacharia, I., Hershcovitch, A., Johnson, E. D., Siddons, D. P., “X-ray transmission through a Plasma Window” Physics of Plasmas Vol. 72 No. 3 (2001)

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0 10 20 30 40 50 60 70 80 90 100 110 120

0.0

0.1

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0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Tra

nsm

issio

n

Wavelength (nm)

Gas Filter Option

1. Continuous gas

replenishment to dissipate

heat along beam path

2. Use of neutral species of

low absorption in EUV

may suppress VUV OOB

3. Manufacturing

inconsistencies are largely

avoided

4. Trapping and mitigating

debris

Key advantages of confinement system

Center for X-Ray Optics, http://henke.lbl.gov/optical_constants/

EUV Wavelength – 98% transmission (CXRO)

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Vibrational mode dependence of gases absorbing at CO2 laser lines

Transition(100 – 001) λ(um) Wavenumber(cm-1)

P(14) 10.53 949

P(16) 10.55 948

P(18) 10.57 946

P(20) 10.59 944

P(22) 10.61 942

P(24) 10.63 941

Molecule Fundamental Frequency close

to 10.6 um

Associated vibrational motion

Ethylene (C2H4) υ7 near 949 cm-1

υ8 near 943 cm-1

Wag

Wag

Sulfur Hexafluoride (SF6) υ3 near 948 cm-1 Stretching

Ammonia (NH3) υ2 near 950 cm-1 Deformation

Cantrell, C.D. “Multiple-Photon Excitation and Dissociation of Polyatomic Molecules” Springer-Verlag Berlin Heidelberg, 1986

PNNL - http://vpl.astro.washington.edu/spectra/allmoleculeslist.htm

CO2 Laser wavelengths commonly used and their corresponding P branch transitions

Some IR absorbing molecules with vibrational modes close to 10.6 μm

IR Gas Filter Options

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Characteristics of Sulfur Hexafluoride (SF6)

IR Gas Filter Options

1. Inert, colorless and odorless gas

2. Octahedral symmetric structure with 15

vibrational degrees of freedom

3. 2 infrared active modes (υ3 and υ4)

4. υ3 infrared active mode is resonantly

excited by CO2 laser infrared photons

5. Absorption is dependent on population

distribution among vibrational energy

levels

6. Exhibits multi-photon absorption

phenomena

7. Absorption enhancement in the

presence of non absorbing partners The overall effect of temperature on the population of energy levels

Energy

Cantrell, C.D. “Multiple-Photon Excitation and Dissociation of Polyatomic Molecules” Springer-Verlag Berlin Heidelberg, 1986.

Jovanovic-Kurepa et al “Multiple absorption and relaxation processes in SF6-CH4 mixtures: an experimental study” Chemical Physics 211 (1996) 347-358

McDowell et al, “The modern evolution in infrared spectroscopy” Los Alamos science

Lyman et al “Multiple-Phton Excitation” Los Alamos science

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What is the maximum infrared absorption that can be achieved?

Absorption limits for SF6

For only 10% EUV absorption, then areal density of SF6 is 4.8 x 1015 molecules/cm2

(for example 150 mTorr , 1 cm wide)

Limited to 30 photons absorbed per SF6 molecule before vibrational dissociation

Use an inert gas for collisional relaxation of SF6 between laser pulses

Assume the gas filter is used in a location with an EUV beam diameter of 6 cm

Then, at most the filter can absorb 70 mJ/pulse

At 100 kHz drive laser frequency, this is at most 7 kW, with energy transfer at the molecular

speed of 200 m/s

If more SF6 areal density and more EUV absorption allowed, this rises

If more area for the interaction region, this rises

If lower drive laser frequency, this is reduced

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Tunable Merit-G

CO2 Laser

Monochromator

ZnSe Beamsplitters

Gas cell

Aluminum mirror

Thermopile

detectors

CO2 laser enclosure

Laser beam

Infrared Absorption System Design

IR Gas Filter Options

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10.53 10.55 10.57 10.59 10.61 10.63 10.651.0x10

-17

2.0x10-17

3.0x10-17

4.0x10-17

5.0x10-17

6.0x10-17

7.0x10-17

8.0x10-17

9.0x10-17

1.0x10-16

IR absorption cross sections

EUV absorption cross section (13.5 nm)

Absorp

tion c

ross s

ection (

cm

2 p

er

mole

cule

)

Wavelength ( m)

0 10 20 30 40 50 60 70 80 90 100 110 1200.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Tra

nsm

issio

n

Wavelength (nm)

Absorption cross section comparison

IR Gas Filter Options

Center for X-Ray Optics, http://henke.lbl.gov/optical_constants/

EUV Wavelength

Transmission (CXRO)

lNeTT – Transmission

σ – Absorption cross section (cm2

per molecule)

l – Path length (cm)

N – Gas density (molecules per

cm3)P(18) P(20) P(22) P(24)P(16)

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Characteristics and operation of Hollow Cathode Arc

• Low pressure arc discharge

• Use of refractory metal such as tantalum with

large thermionic emission as cathode

• Thermionic electrons from hot cathode ionize gas

atoms flowing through hot cathode

• Confinement of external plasma column by axial

magnetic field

Haas et al “Diagnostics of Effusing Plasmas” 1985

Confinement methods for IR gas

Magnetic

Field coils

Tantalum

Electrodes

Pump

Argon gas feed

Plasma

column

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Experimental Configuration to measure diffusion across Hollow Cathode Arc

Plasma vacuum

chamber

Plasma conditionsArgon flow rate – 0.8 TL/s

Confining magnetic Field – 280 Gauss

Background pressure – 150 mTorr

Confinement methods for IR gas

Pump

Argon gas feed

Plasma

column

Position

adjustable

probe

connected to

Mass

spectrometer

chamber

Gas line to mass

spectrometer chamber SF6

molecules

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Confinement methods for IR gas

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

Pre

ssu

re (

To

rr)

Position (cm)

Argon flow only

Plasma

Confinement method Factor Improvement

Argon gas curtain 740

Plasma (Hollow Cathode

Arc)

1200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

10-9

10-8

10-7

10-6

Pre

ssu

re (

To

rr)

- L

og

sca

le

Position (cm)

Argon flow only

Plasma

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Summary

• Gas filters have potential to reduce unwanted radiation

from EUV sources

• Infrared absorbing gases such as SF6 can suppress 10.6

μm radiation

• Confinement of SF6 can be accomplished using gas jets or

with more improvement with low density plasma arcs