Post on 04-Oct-2021
New technique for aberration diagnostics and alignment
of an extreme ultraviolet Schwarzschild objective
S. Bollanti(*), P. Di Lazzaro, F. Flora, L. Mezi, D. Murra and A. Torre
ENEA, Frascati Research Centre, Technical Unit APRAD-SOR, via E. Fermi 45, 00044 Frascati, Italy - (*) sarah.bollanti@enea.it
R1= 144.23 mm
R2= 45.06 mm
Zo= 340.22 mm Zi= 36.26 mm
ΦΦΦΦ1= 74 mm ΦΦΦΦ2= 12.7 mm
M=1/9.5 NA=n·sinθ=0.23 λ = 14.4 nm
2°International Conference Frontiers in Diagnostic Technologies
INFN-Laboratori Nazionali di Frascati, November 28-30, 2011
Frascati (Rome) Italy
Several factors affect the optics spatial resolution δδδδ.
For each factor the ENEA-SO δδδδ value is given below.
DIFFRACTION
δδδδdiff = k⋅λ⋅λ⋅λ⋅λ / NA
FIGURE
ERROR
OPTICAL ABERRATIONS
( with NA)
e.g. spherical, coma It can be much worse if the
ALIGNMENTis not correct
The objective spatial resolution
δδδδdiff = 38 nm
Aligned SO ⇒⇒⇒⇒ δδδδgeo = 27 nm(in modified configuration)
SO figure error rms = 8 nm
-40 30 20 10 0 10 20 30 400
50
100
150
200
250
300
ZEMAXbest-fit
Longitudinal decentring [μm]
Geo
met
rica
lre
solu
tion
[nm
]
ZEMAXbest-fit
4 3 2 1 0 1 2 3 40
50
100
150
200
250
300
Transverse decentring [μm]
Geo
met
rica
lre
solu
tion
[nm
]
The SO misalignment sensitivity
The ray-tracing program ZEMAX allows to relate the SO mirrors misalignment to the worsening of the geometrical
resolution (defined as the minimum rms diameter of the image of a point source on the optical axis).
The longitudinal decentring of the two mirrors influences the on-axis aberration, i.e. the spherical one.
The transverse displacement of the mirrors’ centres mainly generates the coma aberration, because this
condition corresponds to having an off-axis source. A 1-µm displacement is generated by a 7-µrad tilt of
the concave mirror with respect to the optical axis
2θ2θ2θ2θ
R1
R2
Zo Zi
P Q
Reductor configuration
The aberration diagnostics with the Foucault test
Knife-edge between the marginal (M) and the paraxial (P) foci
In the Foucault test, a knife edge (KE) is scanned across the beam and the pattern of the transmitted light
is observed. Its appearance depends on the scanning plane and on the kind of aberrations.
SPHERICAL
ABERRATION
FOUCAULTGRAMS
M
P
Limiting factors for Schwarzschild Objective spatial resolution and test
When the transverse dimension of the beam is comparable with λ/NA, diffraction effects prevent
any further improvement:
The Schwarzschild Objective (SO)
Foucault test improvement to overcome diffraction limitation
Longitudinal scan and geometrical figures Ultraviolet light and diffraction
Controlled misalignment and interpolation Final SO performances and conclusions
The “classical” transverse KE scan is replaced by a longitudinal scan with a 50% beam cut since:
1) during the scan the transmitted power is almost constant
2) the longitudinal aberrations extend on a wider spatial scale HIGHER SENSITIVITY TO MISALIGNMENT
Pinhole/Source
SO
CCD
XeCl laser
λλλλ=308 nm
KE
z
x
y
λλλλ >> 14.4 nm
50% beam cut in x-direction
+ longitudinal scan
KE before
both foci
KE after
marginal focus
KE half
the way
KE after
paraxial focus
KE
approaching paraxial
focus
(Shadows from mechanical support) (M2 shadow)
Calculated Foucaultgrams for
spherical aberration (by ray tracing)
Calculated Foucaultgrams for coma
aberration (mirror rotation axis || x)
COMA
ABERRATION
KE well
before the focal plane
KE
approaching the focal
plane
KE on
the focal plane
KE well
after the focal
plane
KE slightly
over the focal
plane
Coma LA vs concave mirror tilts
160 nm lines
0 500 1000 1500 2000 2500 3000 3500 400060
70
80
90
100
110
120
130
140
150
90 nm
Hei
gh
t[
a.u
.]
Horizontal position [nm]
PMMA printing• The alignment of a Schwarzschild
objective operating at EUV wavelength is
a very critical task
• The attainable spatial resolution is
strictly related to a correct alignment
• We demonstrated that aberrations diagnostics and correction using a wavelength that is ~20× the operating one
to align a SO are possible through the
described procedure
• The aligned SO has been used as the
projection optics in the EUV MET-Egeria
facility in the ENEA Frascati Research
Centre to print 160-nm-width dense lines
on PMMA photoresist
Mirrors longitudinal decentring of ~ -350 µm
20-µm KE z-course
• The effect of the “long”-wavelength-light diffraction in the plane of KE cutting smears out the foucaultgrams calculated in geometrical approximation
• The observed images are blurred and put a limit on the minimum detectable aberrations
Experiment
Model (multiple ray-tracing by transverse KE shift within Airy disk)
Experiment
Mirrors transverse decentring of ~ 120 µm(corresponding to a 0.84-mrad tilt of M1)
20-µm KE z-course
The obtained
edge response
is 90 nm
• The optimum position of the SO mirrors can be found by means of interpolation for each parameter.
• The SO has 3 degrees of freedom for alignment: the distance between mirrors and the rotations of the concave mirror around two mutually orthogonal axes crossing each other at the mirror vertex.
• The longitudinal aberrations (LA) are measured as function of these parameters outside the diffraction forbidden region, looking at the foucaultgrams obtained varying the parameters one at a time.
• The procedure is cyclically repeated for the 3 degrees of freedom until the results are reproducible.
A
A
P
Spherical LA vs mirrors distance
P
P’
A
A
C
CB
B
B
A
C
D
D
D
Knife-edge on the focal plane
Observed figures when the KE moves upwards
(at half beam cut)
Observation plane
Observation plane
When possible, the optical element
is adjusted to correct the aberration, looking at the disappearance of the
specific Foucaultgram.
ENEA SOparameters:
LA
[µm
]
z [mm]
-50
-40
-30
-20
-10
0
10
20
30
40
50
-0,4 -0,2 0 0,2 0,4 0,6 0,8 1
Diffraction forbidden range
Before coma correction
After coma correction
θθθθM1 [mrad]
LA
[µm
]
-50
-40
-30
-20
-10
0
10
20
30
40
50
-3 -2,5 -2 -1,5 -1 -0,5 0
M1 vertical tilt
M1 horizontal tilt
Diffraction forbidden range
A.Budano, F.Flora, L.Mezi, Appl. Opt. 45, 4254-4262 (2006)S.Bollanti, P.Di Lazzaro, F.Flora, L.Mezi, D.Murra, A.Torre, Appl. Phys. B 85, 603–610 (2006)
Model (multiple ray-tracing by transverse KE shift within Airy disk)
S.Bollanti, P.Di Lazzaro, F.Flora, L.Mezi, D.Murra, A.Torre, EPL, 84 (2008) 58003
P.Di Lazzaro, S.Bollanti, F.Flora, L.Mezi, D.Murra, A.Torre, IEEE Trans. Plasma Sci. 37, 475-480 (2009)
S.Bollanti, P.Di Lazzaro, F.Flora, L.Mezi, D.Murra, A.Torre, Appl. Phys. B 91, 127–137 (2008)
Schwarzschild objectives (SO) are widely used in the extreme ultraviolet (EUV) and soft X-ray spectral
regions both as magnification and reduction optics, e.g. for microscopy and small-field projection
lithography, respectively. The ENEA SO is used in the second configuration.
A SO consists of two spherical mirrors (one concave, the other convex) put in concentric configuration. It is
possible to determine a pair (P,Q) of conjugated points on the optical axis where the aberrations are dramatically reduced and the attainable spatial resolution from a geometric point of view is comparable
with the diffractive one.