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7/31/2019 Nano1 Top Down
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Top-down approach for formation of nanostructures:
Lithography with light, electrons and ions
Seminar Nanostrukturierte Festkrper, 30.10.2002
Martin Hulman
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Seminar Nanostrukturierte Festkrper, 30.10.2002Seminar Nanostrukturierte Festkrper, 30.10.2002
Top-down approach for formation of nanostructures:
Lithography with light, electrons and ions
Seminar Nanostrukturierte Festkrper, 30.10.2002
Outline
History
Physical foundations of lithography Overview of lithographic techniques
Resists
Future and perspectives
Lithography in our lab
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LITHOGRAPHY = STONE DRAWING
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A piece of history
invented in 1798
first technique for colorprinting
pictures made by impressing flat embossed
slabs (of limestone), each covered with
greasy ink of a particular color, onto a
piece of stout paper
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SEMICONDUCTOR MANUFACTURING PROCESS
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Lithographic techniques
with electromagnetic waves:
optical
ultra-violet
deep UV
X-ray
with charged particles:
electrons
ions
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Physical basis of lithography
finite resolution of the image-
forming system results in the light
distribution which does not have
clearly defined edges
Diffraction!
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Physical basis of lithography
two ingredients of image formation:
optics
photo-resist
The quality of image is determined by:
resolution power of the optics
focusing accuracy
contrast of the resist process
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Physical basis of lithography:
Diffraction
a circular aperture illuminated by
a point source of light
the light intensity distribution from a point source
projected through a circular aperture
Airy function
x=r d z
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Physical basis of lithography:
the Rayleigh criterion for resolution
two point sources of light
separated by a small angle
the total light intensity is a sum
of individual intensities
The Rayleigh criterion:
maximum of the Airy pattern from
one source falls on the first zero of
the Airy pattern from the other source
the minimum resolved distance dbetween
the peak and the first minimum
of the Airy function
d = 0.61 n sin
n sin is a numerical aperture
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Physical basis of lithography:
typical parameters for optics
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Optical printing lithography techniques
Contact printing:
a photomask is in direct or intimate contact with a resist-covered wafer;
the photomask is pressed against the wafer with a pressure of 0.05 - 0.3 atm;
exposed to light with wavelength of about 400nm;
a high resolution of less than 0.5 m m is possible but the resolution varies across the wafer
the mask used in contact printing is frequently replaced after short period of use
Proximity printing:
there is a typical separation between the mask and the wafer in a range of 20 - 50 m; the defects resulting from proximity printing are not as bad as contact printing ;
its resolution is not as good as compared to that of contact printing ;
the mask used has a longer lifetime
Projection printing: larger separation between mask and wafer;
higher resolution than proximity printing;
the system cost is approximately five times that of contact printing
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Drawbacks of optical systems:
Aberrations
chromatic aberration: inability to focus light over a range of wavelength
distortions: higher resolutions in the center of the fields
astigmatism: points to appear as lines
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Optical Lithography:
the smallest working device -- with 80 nm features
(1999)
a flash memory cell made of silicon
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X - ray lithography
X ray wavelength = 6 14 nm
diffraction effects can be ignored because
of a small wavelength
masks consists of an absorber (Au) ona transmissive membrane
substrate (Si, SiC, Si3N4)
ability to define very high resolution images
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Electron beam lithography
no masks required !
the diameter of the electronbeam as small as 50 nm
electrons with energy
10 50 keV(150 eV => 1 A)
resolution not limited bydiffraction but by scattering
in the resist
masks for optical lithography
aberrations still present
slow compared to optical
lithography
expensive and complicated
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Electron beam lithography
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Ion beam lithography
lithography with charged ions (He+ and Ar+)
at energies up to 200keV
very small particle wavelength ~10-5 nm
electrostatic ion optics with a small
numerical aperture ~ 10-5
resolution down to 50 nm
diffraction limit 3 nm
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Resists
positive resist more soluble after exposing to light,
chemical bonds are destroyed in a photoactivecomponent
negative resist less soluble after exposing to light,
crosslinks between molecules are created
PMMA for UV, deep-UV, X-rayand e-beam lithography
higher resolution is possible with
positive resists in OL
factors limiting resist resolution:
- swelling of the resist in the developer
- index of refraction > 1 (for OL)
- electron scattering (neglible for X-ray)
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Seminar Nanostrukturierte Festkrper, 30.10.2002
Future and perspectives: Moore s Law
Year of introduction Transistors (per IC)
4004 1971 2,250
8008 1972 2,500
8080 1974 5,000
8086 1978 29,000
286 1982 120,000386 processor 1985 275,000
486 DX processor 1989 1,180,000
Pentium processor 1993 3,100,000
Pentium II processor 1997 7,500,000
Pentium III processor 1999 24,000,000
Pentium 4 processor 2000 42,000,000
Violation of the
Moores law ?
Current technology: 0.13 m,down to 0.065 m in 2007
physical limitations
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Future and perspectives: Direct imprint
S. Chu et al., Nature 2002
Resolution down to 10 nm
no masks required !
Lith h i l b
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Lithography in our lab:
Raman microspectroscopy on individual carbon nanotubes
carbon nanotubes on a silicon surface
position of a nanotube with respectto a predefined marker system
AFM images, scale bars 1m
Lith h i l b
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Lithography in our lab:
Raman spectra
150 200 250
0
1
2
3
4
2.41
2.50
2.60
Eexc
(eV)
174.1
214.4
230.7178.0
174.8Intens
ity
(a.u.)
Raman shift cm
-1
150 200 250
0
2
4
6
Eexc
(eV)
2.60
2.50
2.41
2.18
1.92
181.7
206.9
231.9212.2180.6
180.7
Intensity(
a.u.)
Raman shift (cm-1)
Lithograph in o r lab:
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Lithography in our lab:
marker system
masks made by
e-beam lithography
size of letters 1.2 m
Lithography in our lab:
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Lithography in our lab:
Suspended carbon nanotubes
G.T. Kim et al., Appl. Phys. Lett. 80 (2002)