Top-down approach for formation of nanostructures: Lithography with light, electrons and ions
Martin Hulman
Seminar Nanostrukturierte Festkrper, 30.10.2002
Top-down approach for formation of nanostructures: Lithography with light, electrons and ions
Outline
History Physical foundations of lithography Overview of lithographic techniques Resists Future and perspectives Lithography in our lab
Seminar Nanostrukturierte Festkrper, 30.10.2002 Seminar Nanostrukturierte Festkrper, 30.10.2002
LITHOGRAPHY = STONE DRAWING
Seminar Nanostrukturierte Festkrper, 30.10.2002
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
Seminar Nanostrukturierte Festkrper, 30.10.2002
SEMICONDUCTOR MANUFACTURING PROCESS
Seminar Nanostrukturierte Festkrper, 30.10.2002
Lithographic techniques
with electromagnetic waves: optical ultra-violet deep UV X-ray with charged particles: electrons ions
Seminar Nanostrukturierte Festkrper, 30.10.2002
Physical basis of lithography
Diffraction!
finite resolution of the imageforming system results in the light distribution which does not have clearly defined edges
Seminar Nanostrukturierte Festkrper, 30.10.2002
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
Seminar Nanostrukturierte Festkrper, 30.10.2002
Physical basis of lithography: Diffraction
Airy function
x=r d / z
a circular aperture illuminated by a point source of light
the light intensity distribution from a point source projected through a circular aperture
Seminar Nanostrukturierte Festkrper, 30.10.2002
Physical basis of lithography: the Rayleigh criterion for resolution
two point sources of light separated by a small angle
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 d between the peak and the first minimum of the Airy function d = 0.61 / n sin n sin is a numerical aperture
the total light intensity is a sum of individual intensities
Seminar Nanostrukturierte Festkrper, 30.10.2002
Physical basis of lithography: typical parameters for optics
Seminar Nanostrukturierte Festkrper, 30.10.2002
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
Seminar Nanostrukturierte Festkrper, 30.10.2002
Drawbacks of optical systems: Aberrations
distortions: higher resolutions in the center of the fields
astigmatism: points to appear as lines
chromatic aberration: inability to focus light over a range of wavelength
Seminar Nanostrukturierte Festkrper, 30.10.2002
Optical Lithography: the smallest working device -- with 80 nm features (1999)
a flash memory cell made of silicon Seminar Nanostrukturierte Festkrper, 30.10.2002
X - ray lithography
X ray wavelength = 6 14 nm diffraction effects can be ignored because of a small wavelength ability to define very high resolution images masks consists of an absorber (Au) on a transmissive membrane substrate (Si, SiC, Si3N4)
Seminar Nanostrukturierte Festkrper, 30.10.2002
Electron beam lithography
the diameter of the electron beam as small as 50 nm electrons with energy 10 50 keV(150 eV => 1 A) resolution not limited by diffraction but by scattering in the resist aberrations still present no masks required ! masks for optical lithography slow compared to optical lithography expensive and complicated
Seminar Nanostrukturierte Festkrper, 30.10.2002
Electron beam lithography
Seminar Nanostrukturierte Festkrper, 30.10.2002
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
Seminar Nanostrukturierte Festkrper, 30.10.2002
Resists
positive resist more soluble after exposing to light, chemical bonds are destroyed in a photoactive component negative resist less soluble after exposing to light, crosslinks between molecules are created PMMA for UV, deep-UV, X-ray and 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)
Seminar Nanostrukturierte Festkrper, 30.10.2002
Comparison of various lithographic techniques
Seminar Nanostrukturierte Festkrper, 30.10.2002
Future and perspectives: Moore s Law
Current technology: 0.13 m, down to 0.065 m in 2007
physical limitations
Year of introduction 4004 8008 8080 8086 286 386 processor 486 DX processor Pentium processor Pentium II processor Pentium III processor Pentium 4 processor 1971 1972 1974 1978 1982 1985 1989 1993 1997 1999 2000 Transistors (per IC) 2,250 2,500 5,000 29,000 120,000 275,000 1,180,000 3,100,000 7,500,000 24,000,000 42,000,000
Violation of the Moores law ?
Seminar Nanostrukturierte Festkrper, 30.10.2002
Future and perspectives
trends for technology for the scaling into deep nanometer regime
Seminar Nanostrukturierte Festkrper, 30.10.2002
Future and perspectives: Direct imprint
Resolution down to 10 nm
no masks required !
S. Chu et al., Nature 2002
Seminar Nanostrukturierte Festkrper, 30.10.2002
Lithography in our lab: Raman microspectroscopy on individual carbon nanotubes
AFM images, scale bars 1m
position of a nanotube with respect to a predefined marker system
carbon nanotubes on a silicon surface Seminar Nanostrukturierte Festkrper, 30.10.2002
Lithography in our lab: Raman spectra
4
214.4
Eexc (eV)
2.41
174.1
Intensity (a.u.)
178.0
230.7
2.50 1
174.8
2.60 200 250
-1
150
Raman shift (cm )
6 180.7
Eexc (eV)
4 Intensity (a.u.) 180.6 212.2 206.9 2 231.9 2.50 2.41 2.60
181.7
2.18 1.92 200 250
-1
0 150
Raman shift (cm )
Seminar Nanostrukturierte Festkrper, 30.10.2002
Lithography in our lab: marker system
masks made by e-beam lithography
size of letters 1.2 m
Seminar Nanostrukturierte Festkrper, 30.10.2002
Lithography in our lab: Suspended carbon nanotubes
G.T. Kim et al., Appl. Phys. Lett. 80 (2002)
Seminar Nanostrukturierte Festkrper, 30.10.2002
