New technique for aberration diagnostics and alignment of ...

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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 - (*) [email protected] R 1 = 144.23 mm R 2 = 45.06 mm Z o = 340.22 mm Z i = 36.26 mm Φ 1 = 74 mm Φ 2 = 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 δ. For each factor the ENEA-SO δ value is given below. DIFFRACTION δ diff = k⋅λ ⋅λ ⋅λ ⋅λ / NA FIGURE ERROR OPTICAL ABERRATIONS ( with NA) e.g. spherical, coma It can be much worse if the ALIGNMENT is not correct The objective spatial resolution δ diff = 38 nm Aligned SO δ geo = 27 nm (in modified configuration) SO figure error rms = 8 nm -40 30 20 10 0 10 20 30 40 0 50 100 150 200 250 300 ZEMAX best-fit Longitudinal decentring [μm] Geometrical resolution [nm] ZEMAX best-fit 4 3 2 1 0 1 2 3 4 0 50 100 150 200 250 300 Transverse decentring [μm] Geometrical resolution [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 R 1 R 2 Z o Z i 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 F O U C A U L T G R A M S 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 λ=308 nm KE z x y λ >> 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 4000 60 70 80 90 100 110 120 130 140 150 90 nm Height [ 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 C B 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 SO parameters: 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 θ M1 [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.

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Microsoft PowerPoint - Poster 2nd FDT.pptof 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 - (*) [email protected]
R1= 144.23 mm
R2= 45.06 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
For each factor the ENEA-SO δδδδ value is given below.
DIFFRACTION
FIGURE
ERROR
e.g. spherical, coma It can be much worse if the
ALIGNMENT is not correct
The objective spatial resolution
SO figure error rms = 8 nm
-40 30 20 10 0 10 20 30 40 0
50
100
150
200
250
300
4 3 2 1 0 1 2 3 4 0
50
100
150
200
250
300
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
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
F O U C A U L T G R A M S
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:
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
+ longitudinal scan
KE before
both foci
KE after
marginal focus
KE half
the way
KE after
paraxial focus
Calculated Foucaultgrams for
Calculated Foucaultgrams for coma
COMA
ABERRATION
160 nm lines
0 500 1000 1500 2000 2500 3000 3500 4000 60
70
80
90
100
110
120
130
140
150
objective operating at EUV wavelength is
a very critical task
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
projection optics in the EUV MET-Egeria
facility in the ENEA Frascati Research
Centre to print 160-nm-width dense lines
on PMMA photoresist
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
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
P
P’
A
A
C
Observed figures when the KE moves upwards
(at half beam cut)
When possible, the optical element
is adjusted to correct the aberration, looking at the disappearance of the
specific Foucaultgram.
Diffraction forbidden range
Before coma correction
After coma correction
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.