Problems of optics

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Problems of optics for projection nanolithography
N.N.Salashchenko, N.I. Chkhalo Institute for
physics of microstructures RAS Nizhny Novgorod ,
Russia
The international FORUM on NANOTECHNOLOGIEs,
section of Nanodiagnostics" on December, 3-5rd
2008, Moscow, Expocentre
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Outline

• Status and limitations of deep ultraviolet (DUV)
  projection lithography
• Current achievements of extreme ultraviolet
  (EUV) lithography at 13.5 nm
• Main requirements to the projection optics
• Technologi-measuring COMPLEX
• Transmission spectral filters
• Summary

FORUM on NANOTECHNOLOGIEs
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Diffraction limits space resolution of projection
lens

• Wavelength ?193 nm of ArF-laser
• k1 0.30
• NA nsina 1.440.9 1.30
• DOF k2?/(NA2) 100 nm
• Res 44 nm
• Technological limits
• k1 0.25 , NA1.8
• Res 22 (16) nm

Res k1?/(NA) NA nsin ?
FORUM on NANOTECHNOLOGIEs
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Layout of typical EUV scanner
FORUM on NANOTECHNOLOGIEs
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FORUM on NANOTECHNOLOGIEs
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The main problems need to be solved on the way to
competitive EUV scanners

• A high efficient, stable, clean and
  long-lifetime source of EUV (13.5 nm) radiation
• Super precision aspherical mirrors for
  projection objective
• High resolution and sensitive (5-10 mJ/sm2)
  photoresists

FORUM on NANOTECHNOLOGIEs
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Main requirements to the projection optics for
tool with 22-32 nm resolution

• Aspherical objective with a numerical aperture
  up to
• 
  NA 0.4
• Wave front deformations
  rms lt ?/50
• That means for ?13.5 nm
  rms lt 0.27 nm

• Assuming independent character of figure errors
  of different
• mirrors in an objective, for a six mirror
  objectives the
• admissible surfaces shape error of each mirror
  is 0.1 nm

• Optics reflecting, multilayer, aspherical

• Mo/Si stress-free multilayer films with
  reflectivity at 13.5 nm close to theoretical
  limit 74

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Key technologies for producing and
characterization of the optics

• Point diffraction interferometry utilizing as a
  reference a wave appeared due to diffraction of
  visible or EUV light on a pin-hole with a
  diameter d ? (d 50-500 nm)
• Physical methods of surface shape correction with
  sub-nanometer scale accuracy
• Vacuum deposition and precision reflectometry of
  mirrors with curved surfaces and with diameters
  of a few hundreds of mm
• Deformation-free mounting of the mirrors into the
  metallic holders

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Achievements in manufacturing of stress-free
multilayer interference structures in IPM RAS
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The laboratory manufacturing techniques
supersmooth (s ? 0.25-0.3 nm) substrates with
off-axial aspherical and surface profile
differing from set are developed
AFM measurements of the surface roughness
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• Application of the complex
• Manufacturing multielement (mirror, lens,
  mirror-lens) objectives for schemes with the
  ultrahigh spatial resolution, for example
• Projection nanolithography
• DUV-lithography (?193 nm)
• EUV-lithography (?13.5 nm)
• Soft X-ray Lithography (?6.7 nm)
• X-ray microscopy
• Precision optics for astrophysics

Technologi-measuring COMPLEX
Tasks of the complex Manufacturing and
certification optical (X-ray optical) elements
with subnanometer accuracy of the surface shape
and optical systems with deformation of wave
front on subnanometer level.
FORUM on NANOTECHNOLOGIEs
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Measurement of optical elements characteristics

• Measurement of the surface shape of optical
  elements and wave fronts of optical systems
• Measurement of microroughnesses of a surface by
  methods probe microscopy and X-ray diffractometry
• Measurement optical (X-ray) characteristics by
  methods X-ray reflectometry and optical
  spectroscopy

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Precision correction of the surface shape

• Local deposition of short period MLS without
  development of microroughnesses and with an
  opportunity to remove MLS without change of
  parameters of an initial surface
• Local ionic etching in advance deposited MLS
  without development of microroughnesses
• Local plasma-chemical etchings of dielectric
  surfaces (correction of the refracting optics
  elements shape)

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Point diffraction interferometry
The first offer V. Linnik, A simple
interferometer to test optical systems, Proc. of
the Ac. of Sci. of USSR, 1, pp.210-212, 1933.
Spherical wave source pin-hole.

• The first realization Sommargren G.E., Laser
  Focus World, 8, 61 (1996).
• Spherical wave source optical fiber
• Estimated accuracy of measurements
  RMS 0.5 nm
• The numerical aperture
  NA0.1

• Interferometer at ?13.4 nm 532 nm Kenneth A.
  Goldberg et al.,
• J. Vac. Sci. Technol. B 20(6), 2002.
• Spherical wave source pin-hole 0.1 µm
• Estimated accuracy of measurements
  RMS lt 0.1 nm
• The numerical aperture
  NA 0.08
• Divergence between optical and EUV
  interferometers RMS0.35 nm

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Accuracy of interferometry measurements basically
is defined by quality of a source of a reference
spherical wave

• The numerical aperture of point diffraction
  interferometer is limited by the phase
  distortions caused by interaction of reference
  radiation with pin-hole wall, also depends on a
  material of the screen.

• Distortions of the pin-hole form ? 10 nm, lead
  to deformation of wave front ? 1 nm.
• The best pin-hole have been made by electron beam
  lithography.

FORUM on NANOTECHNOLOGIEs
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Submicronic source of a reference spherical wave
on the basis of the made narrower metallized
optical fiber
Fiber-Optical Spherical-Wave Source (FOSWS)
SEM image of the spherical wave source. The size
of the source (free part from metallization)
0.2-0.3 µm
The measurements scheme of FOSWS wave fronts
deformations
The measured deformations of wave front in
NA0.1 (RMS) 0. 1 nm ( ?/6000 ) at
repeatability (RMS) 0. 03 nm ( ?/20 000 )
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A source based on a narrowed optical fiber
features a few advantages in comparison with
conventional sources

• The large optical aperture of up to NA 0.3, the
  wide angular range of uniform diffracted-wave
  intensity (? 40), the simplicity of
  adjustment, and the high output intensity are
  typical of an FOSWS. The last feature is related
  to the fact that the radiation is input into the
  fiber using standard high-efficiency methods
  through a 5 µm diameter core
• The well-developed methods for handling optical
  fibers make it possible to easily control the
  polarization parameters of the diffracted
  radiation and implement various schemes for
  interference measurements

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Vacuum diffraction interferometer with a
reference wave based on a single mode optical
fiber with sub-wave exit aperture
Accuracy of interferometry measurements of the
spherical surface
Accuracy at NA0.14 (RMS) 0. 05 nm ( ?/6000 )
Accuracy at NA0.28 (RMS) 0. 1 nm ( ?/3000 )
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Measurement of a sphere surface shape. The sphere
lies free on polished metal surface face side.
True parameters of the detail were PV 12 nm
and RMS 1.3 nm
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Gluing of the detail in the holder by means of
silicon hermetic thin layer
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Features of aspherical surfaces measurements
Interferogram from the spherical wave (R 382.78
mm) and wave front aspherical surfaces. Deviation
of the surface form from spherical one is 6.57
µm.
Application of the compensator which transforms
spherical wave front in aspherical one, close to
the shape of an investigated surface is necessary
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Interferogram from aspherical surfaces (deviation
from sphere 6.57 µm), received with application
of the wave front compensator.
Peak-to-valley (PV) 151.16 nm RMS
22.9 nm
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The following features are typical of the
interferometer

• Simplicity of adjustment and maintenance favoring
  its use both in a laboratory and at a plant
• The possibility of testing individual optical
  components and complex reflection and refraction
  optical systems
• The possibility of using wave-front correctors
  (special lenses), which is important for studying
  aspherical concave and any convex surfaces and
  components of peculiar sizes

• The currently achieved accuracy of 0.3 nm for
  measuring surface shapes meets the contemporary
  demands on the optics for EUV projection
  lithography and X-ray microscopy

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Correction of the quasi-spherical surface by
deposition through local masks of MLS with given
thicknesses
NA0.24 Before correction P-V 42.6 nm RMS 7.3
nm
The scheme of correction process
Magnetron sputtering facility for MLS deposition
on substrates with a diameter up to 400 mm
After 12-th corrections P-V 6.7 nm RMS 0.6 nm
Maps of deviations of the substrate surface shape
from "ideal" sphere
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Correction of the surface shape by the method of
local ion-beam etching
Facility of local ion-beam etching The angle
between a surface and an ion beam changes from 15
º up to 45º. Energy of ions 300 1500 eV
At energy of ions 500 eV and the angle of ions
(Ar) incidence of 14 deg. etching up to depth 55
nm does not lead to development of the surface
microroughness.
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The stand for measurements of multilayer mirrors
and filters characteristics in spectral area
0.6-270 nm
Goniometer with the investigated sample
(multilayer spherical mirror in diameter of 270
mm)
Goniometer has five degrees of freedom for moving
the investigated sample and two degrees of
freedom for the detector, allows to study samples
with the aperture up to 300 mm with any shape of
a surface.
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The basic results on development of the COMPLEX

• The main result is the development of physical,
  engineering, and technological principles of
  subnanometer-precision metrology and correction
  of large-numerical-aperture surfaces of optical
  components for superhigh-resolution objectives
• The developed techniques for measurement and
  correction spherical and aspherical substrates
  and testing wave-front deformations of complex
  optical systems permit to start development of
  Russian-made super-high-resolution mirror
  objectives for projection lithography and X-ray
  microscopy and astronomy
• The developed methods can be interesting for
  optical industry and can be used for development
  and characterization of state quality standards
  for reference glasses and optical-interferometers

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The nearest plans on perfection of the COMPLEX

• Further development of this work will be aimed
  improving the accuracy of the interferometric
  measurements up to RMS 0.1 nm

• At the same time, methods for correction the
  surfaces, in particular, plasma-chemical etching,
  should be improved.
• This will permit to correct not only reflective,
  but also refractive optics with subnanometer
  accuracy.

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• Requirements to the spectral filter
• Transparency of the filter on working wavelength
  (13.5 nm) up to 75-80 ,
• Attenuation of UV, visible and IR radiation up
  to 100 times,
• Long duration withstand influence of thermal
  flow with power density up to 1 W/cm2 (3-4
  W/cm2).

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The spectral filter based on diffraction grating
H.Kiereya, K.Heidemanna, B.Kleemanna, et.al. EUV
spectral purity filter optical and mechanical
design, gratings fabrication, and testing. Proc.
of SPIE Vol. 5193
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Transmission spectral filters
The technique of manufacturing free stading
transmission filters with the transparency up to
T78 in the field of 13.5 nm is
developed. Filters block radiation ? ? 0.08-10.6
?m.

• Scopes of filters apllication
• Plasma diagnostics
• Projection EUV-lithography
• X-ray microscopy
• X-ray astronomy

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T?? 76.87 (? 0.05)
P?? 5.7 W/?m2
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Summary

• Projection XEUV lithography has NO real
  ALTERNATIVE for HIGH VOLUME MANUFACTIRING of new
  generation of microelectronic devices with
  topology beyond 22 nm
• At present in Russia originate an unique
  capabilities for qualitative progress in
  microelectronic industry by development
  EUV-lithography.
  The capabilities are supported by
  achievements of domestic institutes in the key
  technologies necessary for developing the EUV
  scanner.

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Many thanks for attention