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Extreme Ultraviolet Resist Outgassing and Its Effect on Nearby Optics

Rashi GargCollege of Nanoscale Science and Engineering

State University of New York, Albany

June 11, 2008

2008 International Workshop on EUV Lithography

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Outline

• Degradation of EUV projection optics by loss in reflectivity– Mechanisms involved– Sources of this contamination

• EUV Resist Outgassing and eXposure (ROX) system– EUV resist outgassing results with a mass spectrometer

• Contamination results from injection of known resist outgassing species and effect on contamination rate of mirrors during exposure

• Witness plate experiments for resist outgassing measurements– Chamber cleaning results with glow discharge plasma to reduce

amount of contamination due to vacuum chamber

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3http://oemagazine.com/fromTheMagazine/jun02/euv.html

6º

Mo-Si multilayer

Wafer

Reflective mask

Projection optics

0

0.2

0.4

0.6

0.8

12.5 13 13.5 14 14.5

Wavelength (nm)

Ref

lect

ivity

(arb

. uni

ts)

www.cxro.lbnl.gov

EUV Reflective Optics

Capping layer

• Lifetime of optics without capping layer is very short due to oxidation

• Maximum reflectivity of about 70% is achieved with Mo/Si multilayers at 13.5 nm

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Contamination of optics by resist outgassing leads to drop in reflectivity

Contamination from resist outgassing

EUV

Mo-Si multilayer optics

Wafer with resist

Resist Outgassing

Capping layer (Ru, Si, TiO2 )ResisthydrocarbonsH2 O Water molecules and

hydrocarbons from the vacuum chamber

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Optics contamination: Mechanisms

Surface carbon growthReversible

Sub-surface oxidationIrreversible

e- e-e- e-e-e-

EUV EUVCx Hy

H2 O

• Carbon contamination results in surface carbon growth• Water vapor environment results in sub-surface oxidation• Mechanism of contamination may be dominated by either

• Photon dissociation• Secondary electron dissociation

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Role of Secondary Electron Yield (SEY) in optics contamination

• SEY for Ru dominates C around 92.5 eV• If secondary electrons dominate the contamination process, then rate of growth on clean Ru may be faster than for carbon contaminated surface

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−+⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

CRuCCC

RuRuRuCCCD L

DLD

MLMLMLh

exp1exp2 μ

μμνδ

*J. Hollenshead and L. Klebanoff, "Modeling radiation-induced carbon contamination of extreme ultravioletoptics", J. Vac. Sci. Technol. B 24, 64-82 (2006)

hν : incident photon energyµRu , µC : photoabsorption cross-sectionMRu , MC : electron multiplication factorsLRu , LC : secondary electron escape depthD : thickness of growing hydrocarbon

Secondary electron flux from a surface*

∫−

=z

Lz

zzSE dzeMhII μν021

Theoretical results

92.5 eV

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Resist outgassing is one of the a major concerns for optics contamination

Detailed measurement is needed for Rate of outgassing of each species from resist

Detailed understanding is needed for Rate of contamination of each outgassed species

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EUV-ROX System EUV Resist Outgassing and eXposure System

QuadrupoleMass spectrometer

LoadlockZr/Si foil

Injection of calibration species

EUV

EUV Source

QuadrupoleMass Spectrometer

Load Lock

Sample

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Resist outgassing as measured by mass spectrometer

H2 O

CO+N2

CO2

Benzene

(C6 H6 )

Diphenyl sulfide

(C12 H10 S)

CO+N2

Chamber background measured by mass spectrometer

Courtesy Prof. R. Brainard

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What outgases from photoresist?

K. Dean, G. Denbeaux, A. Wüest, R. Garg, “EUV Resist Outgassing: How Much is Too Much?”, Journal of Photopolymer Science and Technology, Vol. 20, pp.393-402 (2007) .

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Chosen species for injection and exposure of mirrors to measure contamination

Intended to represent known or similar structures that may outgas from resists

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Contamination Studies of Injected Species

• We have directly injected a few species known to outgas from resist at high concentrations of approximately 1x10-6 Torr (about 100x higher pressure than during outgassing experiments)– Benzene– Tert-butanol– Diphenyl Sulfide

• Then, we exposed a mirror to >30 J/cm2 (8 hours) in these high hydrocarbon environments

• At these high pressures and modest doses, we can not measure reflectivity loss above the measurement accuracy

We have yet to identify any of the outgassed species from resist that contribute significantly to optics contamination!

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Reflectivity results due to contamination from these species

No significant reflectivity loss for these species at these pressures and doses

Chamber Conditions

Chamber Pressure (Torr)

Exposure time (hours)

Total Dose (J/cm2)

Number of pulses (millions)

Reflectivity drop (ΔR/R%)

Clean(background)

2.5 x 10-8 8 29 36 0.35

Benzene 1 x 10-6 8 29 36 0.35

Tert-Butanol 3 x 10-6 8 11.5 36 -0.09

Diphenyl Sulfide 1 x 10-6 4.2 15 19 0.1

Diphenyl Sulfide 1 x 10-6 3.6 13 16 -0.23

Diphenyl Sulfide 1 x 10-6 2.9 42 13 0.1

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Zr/SiFilter

Optics: Mo/Si mirror at 6 degrees to the incident

light

VacuumChamber

Resist sample

Energetiq Xenon Plasma EUV Source

• Two set of experiments done:

• Witness plate: The optics exposed to EUV in presence of resist sample

• Control witness plate: The optics exposed to EUV with no resist sample

Optics contamination: Experimental configuration

EUV

Resist sample

Optics

Witness PlateControl Plate

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Glow Discharge Plasma Chamber Cleaning

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 185 193

Mass number (amu)

Part

ial P

ress

ure

(Tor

r)

Before Cleaning 1.7e-8 Torr After Cleaning 1.9e-8 Torr

H2OCO,N2

CO2

69

43

100

58

Argon oxygen plasma cleaning at ~20 mTorr for 1 hour caused a drop in the mass spectrometer scan for high mass species

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Witness Plate and Control Mirror Results

-0.035

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

0 20 40 60 80 100 120

Dose on mirror (J/cm2)

Ref

lect

ivity

Los

s dR

/R

High outgassing resistPre Control - Bad filter

Post Control

After Chamber Clean Pre ControlAfter Chamber Clean Post Control

After Chamber Clean Resist A

Prior to chamber cleanAfter chamber clean

•Before chamber cleaning, there were large reflectivity losses and a wide spread in results•After chamber cleaning, the results were improved•The effect of the resist was subtle compared to chamber effects

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After Exposure and Reflectivity Loss, XPS Shows Primarily Carbon

Unexposed Witness Plate Exposed Witness PlateShows primarily an increase in carbon

XPS with sputtering to look at materials through sample thickness

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10Thickness (nm)

Ato

mic

%

Si Mo

O

C

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10Thickness (nm)

Atom

ic %

Si Mo

O

C

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Conclusions• Outgassing with a mass spectrometer works routinely

– However, without an understanding of which species are likely (if any) to contaminate optics, interpretation of the results for each resist is a challenge

• Witness plate work will provide a more direct understanding of the danger of each resist to the optics– However, the current test has a low signal from the resist and a

relatively high level of contamination due to the chamber – so it is slow and challenging

• The hydrocarbon species injected into the system directly (so far) do not show large contamination

• There are bad components and there are bad chambers, but it is a challenge to see any effect of resist outgassing causing the contamination– Either by injecting the outgassed species– Or by witness plate exposures of mirrors near exposed resist

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AcknowledgementsCNSE Chimaobi Mbanaso, Justin Waterman, Leonid Yankulin, Alin Antohe,

Yu-Jen Fan, Warren Montgomery, Robert Brainard, Greg Denbeaux

SEMATECH Kim Dean, Kevin Orvek, Andrea Wüest

ASML Bill Pierson, Thomas Laursen, Sang-In Han, Robert Routh, Kevin Cummings

Qimonda Yayi Wei

AMD Obert Wood

IBM Chiew-Seng Koay

CXRO, Berkeley Eric Gullikson, Andy Aquila

NIST Charles Tarrio, Steven Grantham

DESY Saša Bajt