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Narrow Band Mo/Si Multilayerswith Thick Si Structures
Tetsuo Harada*, Toshihide Tsuru, Tadashi Hatano and Masaki YamamotoResearch Center for Soft X-ray Microscopy,
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
e-mail: [email protected]
Narrow band M/Si multilayers for 13.5 nm wavelength were designed and fabricated. The design concept for a narrow band multilayers is to increase the total thickness of effective layers in reflection than the needed temporal coherence length. The multilayers have thick Si structures, because Si are more transparent than Mo. The bandwidth of Mo (2.5 nm)/ Si (25.1 nm) multilayers using forth order Bragg reflection was 0.13 nm.
Abstract
Fourth International Extreme Ultra Violet Lithography (EUVL) Symposium
07-09 November 2005San Diego, California
The Interferometer with Laboratory Light Source
IP or CCD
TestMirror Ref.
Mirror
CondenserMirror
LPPtarget Debris
Shutter
B.S.
YAG Laser
Vac.Pump
Vac. Chamber
Our at-wavelength interferometerat λ ~ 13 nm (under developing)
Laser Produced Plasma (LPP) laboratory source
M. Yamamoto et al., Opt. Precis. Eng. 9 (2001) 405.Emission with low Z (atomic number) target
The brilliance of LPP light source is low compared to synchrotron radiation.The coherence is difficult to improve.
Common path interferometer, Fringe analysisThe spatial resolution depends on the density of interference fringes.
Emission with high Z target
J. M. Bridges et al., Appl. Opt., 25 (1986) 2208.
G.Schriever et al., J. Appl. Phys., 83 (1998) 4566.
Twice focused pointstriangular interferometer
( composed with multilayer mirror optics )
The wavelength of a line emission is fixed and depends on target materials.
The wavelength is tunable by a certain procedure of monochromatizing optics.
Li
SnSpectral Continuum
Emission
Line Emission
Li Ly-α line (λ ~ 13.5 nm)λ/∆λ ~ 1900
Multilayer Bandwidth
λ/∆λ ~
The target bandwidth
The multilayer is able to monochromatize its reflection bandwidth.
A reflection spectrum of the standard multilayer
Mo (2.5 nm) / Si (4.4 nm), N = 40, φ = 5 deg
The multilayer is able to separate the line emissions.
For low Z target
For high Z target
The interferometer is composed of multilayer optics with a LPP source.
temporal coherence
temporal coherence
The reflections at the layer interfaces interfere with coherent component of themselves.
A standard Mo/Si multilayer The penetration depth ~ 80 nm
λ/∆λ ~ 30
The total thickness of effective layers in reflection~ temporal coherence length
For our interferometer,three times longer
is necessary !
φ
D
12.5 13.0 13.5 14.0 14.50.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Ref
lect
ance
Wavelength (nm)
∆λ ~ 0.5 nm(FWHM)
100
Emission line width
Reflection bandwidth
Designs of the Narrow Band Multilayer
Standard Mo/Si MLγ ∼ 0.3 – 0.4 Low Z pairs
Low γ design
Si/B4CR ~ 28 %, λ/∆λ ~ 44J. M. Slaughter et al., Optics Letters, 19 (1994) 1786.
Si/Si3N4R ~ 22 %, λ/∆λ ~ 41P. Boher et al., Opt. Eng., 30(1991) 1049.
Temporal coherence
length needed
The basic design idea
Total thickness of effective layers in
reflection >
The optical constants at λ = 13.5 nm (Henke)Si = (1 - 0.001) - 0.0018iMo = (1 - 0.076) - 0.0065iB4C = (1 - 0.036) - 0.0051iSi3N4 = (1 - 0.027) - 0.0093i
Thick Si structure is effective.
Mo layer thickness = dMoSi layer thickness = dSiMo layer thickness ratiodMo / (dMo + dSi) = γPeriod thickness = D
Low γ design
fundamentalm-th Bragg reflection
dSidMo
The fundamental design has same period thickness as standard Mo/Si ML.
The other has a constant dMo and thick dMostructures satisfying with higher order (m-th) Bragg reflection.
most transparent !
There have been
D1
D1
m × D1
The Design Parameters and Deposition
1500.0911408D10.0551.72.58th91040.1310404D10.0925.12.54th8790.1732403D10.1218.22.53rd7560.2445402D10.1811.32.52nd6960.149120D10.036.70.21st5670.2136100D10.096.30.61st4530.265180D10.145.91.01st3400.345360D10.205.51.41st2290.476040D1 (6.9 nm)0.364.42.51st1
λ/ ∆λ∆λ (nm)R (%)NDγdSi (nm)dMo (nm)mNo.
#1, standard, D = 6.9 nm (D1 ), dMo = 2.5 nm #2 – #5, fundamental low γ, D = D1, dMo = 0.2 – 1.4 nm#6 – #9, m-th Bragg low γ, D = m × D1, dMo = 2.5 nm constant
An Ion beam sputtering system (TOYAMA MST-4)
Base pressure 2 × 10-4 PaAr pressure 5 × 10-3 PaAccelerator voltage 1400 V
Two ECR ion guns using Ar sputtering gas (ELIONIX EIG-240)Deposition rates
Si ~ 3.4 nm/minMo ~ 2.0 nm/min
Design rules
Table 1 The detailed design parameters and experimental results.
12.8 13.0 13.2 13.4 13.6 13.8 14.0 14.20.0
0.1
0.2
0.3
0.4
0.5
0.6
N40, φ = 5 degdMo = 2.5 nm
#1 #6 #7 #8 #9
Wavelength (nm)
p-R
efle
ctan
ce
12.8 13.0 13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.80.0
0.1
0.2
0.3
0.4
0.5
0.6
#1 #2 #3 #4 #5
Fundamental DesignD = 6.9 nm, φ = 5 deg1st Bragg reflection
Wavelength (nm)
p - R
efle
ctan
ce
The reflectance of low γ Mo/Si multilayers ( m > 1 ) and the standard Mo/Si multilayer.
Soft X-ray Performance
The reflection spectrum were measured with synchrotron radiation at BL-12A, the Photon Factory using p-polarized light.
γ = 0.36
0.140.09
0.03
m = 1st
2nd
3rd
4th
8th
The reflectance of the fundamental low γ Mo/Si multilayers ( m = 1 ).
The wavelength of peak reflection were not constant for using the same deposition rate as the sample #1. The structure of the interfaces are not the same as
standard multilayer (sample #1).
The wavelength of peak reflection were almost the same as calculations, the remaining difference
being due to the instability of the deposition rate.
0.20
0
10
20
30
40
50
60
70
0 20 40 60 80 100 120 140 160
Si/Si3N4
#9, 8th, 0.05
#8, 4th, 0.09
#7, 3rd, 0.12
#6, 2nd, 0.18
#5, 1st, 0.03
#3, 1st, 0.14
#4, 1st, 0.09
#2, 1st, 0.20
λ/∆λ
Ref
lect
ance
(%)
#1, 1st, 0.36
Si/B4C
The sample No. #, m, γ
Discussion
The relation of the resolving power λ/∆λ and reflectance of the narrow band multilayers, the fundamental low γ multilayers (●), the low γ multilayers of constant Mo layer thickness (∆) and the
multilayers of low Z pair (■).
R ~ 9 %, λ/ ∆λ ~ 96dMo ~ 0.2 nm (too thin !)The island structure could be generated.
The samples #5, #8, #9 have good resolving power λ/ ∆λ.
Conclusion• The low γ multilayers were designed and deposited.• The low γ multilayer of m = 1, γ = 0.03, λ/ ∆λ has too thin Mo layer thickness.• The low γ multilayer of m = 4 has a good resolving power of λ/ ∆λ ~ 100. The layer interface structures will be stable, because the Mo layer thickness was 2.5 nm same as standard multilayer.
Target λ/ ∆λ
sample #5
sample #8
R ~ 10 %, λ/ ∆λ ~ 104The reflectance would be high.(comparison of the samples #7, #9)
sample #9R ~ 11 %, λ/ ∆λ ~ 150The coating was peeled for its stress.Si layers are under a strong compressive stress.
P. B. Mirkarimi, Opt. Eng. 38 (1999) 1246.
