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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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    LITHOGRAPHY = STONE DRAWING

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    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

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    SEMICONDUCTOR MANUFACTURING PROCESS

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    Lithographic techniques

    with electromagnetic waves:

    optical

    ultra-violet

    deep UV

    X-ray

    with charged particles:

    electrons

    ions

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    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

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    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 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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    Physical basis of lithography:

    typical parameters for optics

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    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

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    Optical Lithography:

    the smallest working device -- with 80 nm features

    (1999)

    a flash memory cell made of silicon

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    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

    masks consists of an absorber (Au) ona transmissive membrane

    substrate (Si, SiC, Si3N4)

    ability to define very high resolution images

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    Electron beam lithography

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    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

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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    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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    Seminar Nanostrukturierte Festkrper, 30.10.2002

    Lithography in our lab:

    Suspended carbon nanotubes

    G.T. Kim et al., Appl. Phys. Lett. 80 (2002)


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