Lithography-Part 2

							       Lithography-Part 2

Dr. Marc Madou,
UCI, Winter 2011
Class 3
Content
   Photolithography limits
   Next Generation Lithography
    (NGL)
    –   EUV
    –   X-Ray Lithography
    –   E-beam
    –   Ion-beam
   Other proposed lithography
    scenarios of the future
Photolithography
 Three  ways to
  improve resolution
  Wmin (also R is used
  in the text)
 We will derive this
  expression and
  analyze all the
  different means of
  reducing Wmin(also R)
Photolithography-NA
Photolithography-NA
Photolithography-Diffraction
   At smaller dimensions, diffraction effects
    dominate
   If the aperture is on the order of l, the
    light spreads out after passing through the
    aperture. (The smaller the aperture, the
    more it spreads out.)
   If we want to image the aperture on an
    image plane (resist), we can collect the
    light using a lens and focus it on the image
    plane.
   But the finite diameter of the lens means
    some information is lost (higher spatial
    frequency components).
    Photolithography-Diffraction
   Image formed by a small circular
    aperture (Airy disk) as an example
 Image by a point source forms a circle
    with diameter 1.22lf/d *surrounded by
    diffraction rings (airy pattern)
 Diffraction is usually described in
    terms of two limiting cases
     – Fresnel diffraction - near field.
     – Fraunhofer diffraction - far field.
*If one considers diffraction through a
    circular aperture, then the calculation
    involves a Bessel function -- 1.220 is
    approximately the first zero of the
    Bessel function of the first kind, of
    order one (i.e. J1), divided by π.
Photolithography-Diffraction
    Photolithography-Diffraction
   Rayleigh suggested that a reasonable
    criterion for resolution (R = distance
    between A and B) is that the central
    maximum of one point source lies at the
    first minimum of the Airy pattern of the
    other point (R = diameter of circle)
   The numerical aperture (NA) of a lens
    represents the ability of the lens to collect
    diffracted light and is given by NA = n sin
    a in this expression n is the index of
    refraction of the medium surrounding the
    lens and a is the acceptance angle of the
    lens ( n = 1 for air)
   When a is small, 2sin a =d/f
    Photolithography-Diffraction
   In the latter expression k1 is an
    experimental parameter and
    depends on resist properties and
    the lithography system ( 0.6-0.8)
   You may remember that, for a
    plane wave incident on a grating
    of period d, the angles q at which
    the intensity maxima in the image
    occur are given by: sin q = N l/d,
    where N= 0,1,2,…. (Bragg’s
    equation)---difference is slits vs
    holes
Photolithography-Diffraction
   The angle q in the figure on the top
    right is the maximum angle for which
    diffracted light from the mask will be
    collected for imaging by the lens.
   With sin q = N l/d only those values
    of N for which the term on the right is
    less than sin q are allowed. Thus, as
    the period d gets smaller (l/d gets
    larger), N gets smaller (i.e. lower
    number of diffracted orders gets
    through).
   The middle figure on the right shows
    the spread of the diffracted orders for a
    decrease in relative slit width (b).
   Because of this spreading effect, fewer
    diffracted orders form the image. This
    means that information about the
    pattern is being lost.
     Photolithography-Diffraction
   The figure on the right shows the effect of
    including increasing numbers of diffracted orders
    on the image of a slit of width w. You can think
    of the aperture as truncating these diffracted
    orders at some small number.
   The value of sin a for an optical system is the
    numerical aperture, or NA. If the value of the
    NA is small for a system, fewer orders will be
    imaged, and the grating may not be resolved.
    It has been shown that the depth of focus, DOF,
    or the range of focus for which a feature can be
    resolved, is given by: DOF = ±k2 l/(NA)2
   The R and DOF equations sum up all of the
    problems and the promise of optical lithography
    using projection tools: The way to increase
    resolution is to decrease the wavelength at which
    the machine can operate, and to increase the
    numerical aperture of the lens. However, both of
    these options have the effect of decreasing the
    depth of focus.
Photolithography-DOF
   The defocus tolerance (DOF)




   Much bigger issue in miniaturization
    science than in ICs
                                                   A small aperture was used to ensure the foreground
                                                   stones were as sharp as the ones in the distance.




                         What you need here is a use a telephoto lens at its widest aperture.
Photolithography-DOF
    Photolithography- MTF
   Another useful concept is the
    modulation transfer function or
    MTF
   MTF is the ratio between image
    intensity modulation over the
    object intensity modulation
   This parameter qualifies the
    capability of an optical system
Photolithography- MTF

   Function describes
    contrast as a function of
    size of features on the
    mask
   Generally, MTF needs to
    be > 0.5 for the resist to
    resolve features
Photolithography- Coherence
   Only point sources are
    completely coherent (light
    waves impinging perpendicular
    on the mask)
   In reality, light sources do have
    a finite size resulting in partially
    coherent light
   The definition for coherence is
    S= NAc/NAo or also S =s/d (see
    Figure)
   MTF depends on S : An S of
    0.5-0.6 is typical design trade-
    off
Photolithography- Coherence
Photolithography-OAI and Kohler
   “Off-axis illumination” also
    allows some of the higher order
    diffracted light to be captured
    and hence can improve
    resolution (by decreasing k1).
Photolithography-OAI and Kohler
   Kohler illumination systems
    focus the light at the entrance
    pupil of the objective lens. This
    “captures” diffracted light
    equally well from all positions
    on the mask.
   This method improves the
    resolution by bringing k1 down.
Photolithography- OPC
   Optical Proximity Correction
    (OPC) can be used to
    compensate somewhat for
    diffraction effects.
   Sharp features are lost
    because higher spatial
    frequencies are lost due to
    diffraction. These effects can
    be calculated and can be
    compensated for. This
    improves the resolution by
    decreasing k1.
Photolithography-Phase Shift Masks
   Extends resolution capability of
    current optical lithography
   Takes advantage of the wave
    nature of light
   PSM changes the phase of light
    by 180° in adjacent patterns
    leading to destructive
    interference rather than
    constructive interference
   Improves MTF of aerial image
    on wafer. Making k1 smaller.
Photolithography- Phase Shift Masks
   A number of companies
    now provide OPC and
    phase shifting software
    services.
    The advanced masks
    which these make possible
    allow sharper resist
    images and/or smaller
    feature sizes for a given
    exposure system.
Photolithography-l
Photolithography-NA
   At the same time that exposure
    wavelengths have been reduced,
    improvements in lens design has
    led to improvements in the NA of
    exposure systems lens, see figure .
    In the mid eighties an NA value of
    approximately 0.4 was typical,
    today 248nm exposure systems
    are available with an NA greater
    than 0.8. The physical limit to NA
    for exposure systems using air as
    a medium between the lens and
    the wafer is 1, the practical limit
    is somewhere around 0.9, with
    recent reports suggesting that an
    NA as high as 0.93 may be
    possible for ArF systems in the
    future .
Photolithography- k1
   The third element in the Rayleigh
    equation is k1. k1 is a complex factor
    of several variables in the
    photolithography process such as
    the quality of the photoresist and the
    use of resolution enhancement
    techniques such as phase shift
    masks, off-axis illumination (OAI)
    and optical proximity correction
    (OPC). While exposure wavelengths
    have been falling and NA rising, k1
    has been falling as well, see figure .
    The practical lower limit for k1 is
    thought to be about 0.25.
Photolithography-Immersion Litho
   From the discussion to this point, the resolution limit for 193nm exposure
    systems may be calculated using the Rayleigh equation with, l = 193nm,
    NA = 0.93 and k1 = 0.25 or


   From the above a highly optimized ArF exposure system has an absolute
    maximum resolution of 52nm, sufficient for 65nm linewidths forecast in
    2005, but not capable of meeting the 45nm linewidths forecast in 2007.
   The technical challenges with 157nm and shorter wavelength exposure
    systems make any technique that can improve the resolution of the 193nm
    exposure systems and delay the need to move to shorter wavelengths an
    important development.
    Photolithography-Immersion litho
   NA is determined by the acceptance angle of the lens and the index of refraction
    of the medium surrounding the lens. The physical limit for an air based system is
    clear, but what if a medium with a higher index of refraction is substituted for air?
    Microscopy has for years used oil between the lens and the sample being viewed
    for resolution enhancement and it is somewhat surprising that the semiconductor
    industry has taken this long to seriously consider the merits of replacing air with
    an alternative.
    Photolithography-Immersion Litho




   The medium between the lens and the wafer being exposed needs to have an
    index of refraction >1, have low optical absorption at 193nm, be compatible
    with photoresist and the lens material, be uniform and non-contaminating.
    Surprisingly, ultrapure water may meet all of these requirements. Water has an
    index of refraction n = 1.47, absorption of <5% at working distances of up to
    6mm, is compatible with photoresist and lens and in it’s ultrapure form is non-
    contaminating.
Photolithography-Immersion Litho




Quiz: what does immersion litho do to DOF?
Next Generation Lithography (NGL)
Next Generation Lithography (NGL)
Next Generation Lithography : EUV

   Uses very short 13.4 nm light
   All reflective optics (at this
    wavelength all materials
    absorb!)
   Uses reduction optics (4 X)
   Step and scan printing
   Optical tricks seen before all
    apply: off axis illumination
    (OAI), phase shift masks and
    OPC
   Vacuum operation
   Laser plasma source
   Very expensive system
Next Generation Lithography : EUV
 Mask fabrication is the
 most difficult task
Next Generation Lithography: E-Beam
 oDiffraction is not a limitation on resolution (l < 1 Å for 10-50 keV electrons)
 oResolution depends on electron scattering and beam optics the size of the beam, can

 reach ~ 5 nm
 oTwo modes of operation:

      oDirect writing with narrow beam

      oElectron projection lithography using a mask :EPL

 oIssues:

      oThroughput of direct writing is very low : research tool or low pattern density

      manufacturing
      oProjection stepper (EPL) is in development stage only (primarily by Nikon).

      oMask making is the biggest challenge for the projection method

      oBack-scattering and second electron result in proximity effect –reduce

      resolution with dense patterns there is also the proximity effect
      oOperates in high vacuum (10
                                      -6 –10-10 torr) –slow and expensive
Next Generation Lithography: E-
Beam
                                         The biggest disadvantage of
   The advantages of electron
                                          electron lithography is its low
    lithography are:
                                          throughput (approximately 5
    (1) Generation of micron and
                                          wafers / hour at less than 0.1 µ
    submicron resist geometries
                                          resolution). Therefore, electron
    (2) Highly automated and
                                          lithography is primarily used in
    precisely controlled operation
                                          the production of photomasks
    (3) Greater depth of focus
                                          and in situations that require
    (4) Direct patterning without a
                                          small number of custom
    mask
                                          circuits.
Next Generation Lithography: E-Beam

    Electron scattering in resist and
     substrate
    The scattered electrons also
     expose the resist
    Interaction of e-and substrate +
     resist leads to beam spreading
      – Elastic and in-elastic
         scattering in the resist
      – Back-scattering from
         substrate and generation of
         secondary e-
      – 100 Å e-beam become 0.2
         µm line
Next Generation Lithography: E-Beam
Next Generation Lithography: E-Beam
     Pattern directly written into
      resist by scanning e-beam
     Device is just like an SEM with
       – On-off capability
       – Pixelation
       – Accurate positioning
       – E-beam blur
Next Generation Lithography: E-
Beam
   E-beam blur
Next Generation Lithography:E-Beam
   Thermionic emitters:
     – Electrons “boiled” off the surface
       by giving them thermal energy to
       overcome the barrier (work
       function)
     – Current given by Richardson-
       DushmanEquation
   Field Emitters:
     – Takes advantage of the quantum
       mechanical properties of electrons.
       –Electrons tunnel out when the
       surface barrier becomes very
       narrow
     – Current given by Fowler-
       Nordheim equation
   Photo Emitters:
     – Energy given to electrons by
       incident photons
     – Only photo-electrons generated
       close to the surface are able to
       escape
SCALPEL® (SCattering with Angular Limitation Projection
Electron-beam Lithography)

   EPL is e-beam with a mask for high-throughput
   The aspect of SCALPEL which differentiates it from
    previous attempts at projection electron-beam
    lithography is the mask. This consists of a low
    atomic number membrane covered with a layer of a
    high atomic number material: the pattern is
    delineated in the latter. While the mask is almost
    completely electron-transparent at the energies used
    (100 keV), contrast is generated by utilizing the
    difference in electron scattering characteristics
    between the membrane and patterned materials. The
    membrane scatters electrons weakly and to small
    angles, while the pattern layer scatters them strongly
    and to high angles.
   An aperture in the back-focal (pupil) plane of the
    projection optics blocks the strongly scattered
    electrons, forming a high contrast aerial image at the
    wafer plane
    SCALPEL® (SCattering with Angular Limitation Projection
    Electron-beam Lithography)


   The functions of contrast generation
    and energy absorption are thus
    separated between the mask and the
    aperture. This means that very little
    of the incident energy is actually
    absorbed by the mask, minimizing
    thermal instabilities in the mask. It
    should be noted that, although the
    membrane scatters electrons weakly
    compared to the scatterer, a
    significant fraction of the electrons
    passing through the membrane are
    scattered sufficiently to be stopped
    by the SCALPEL aperture.
   Mask easier/simpler than EUV
SCALPEL® (SCattering with Angular Limitation Projection
Electron-beam Lithography)
Next Generation Lithography: x-
Rays
   X-ray lithography employs a shadow
    printing method similar to optical
    proximity printing. The x-ray
    wavelength (4 to 50 Å) is much shorter
    than that of UV light (2000 to 4000 Å).
    Hence, diffraction effects are reduced
    and higher resolution can be attained.
    For instance, for an x-ray wavelength of
    5 Å and a gap of 40 µ, R is equal to 0.2
    µ.
   Became very important in MEMS:
    LIGA
   Despite huge efforts seems abandoned       Grenoble Synchrotron
    for NGL for now
    Next Generation Lithography: x-
    Rays
   Types of x-ray sources:
    – Electron Impact X-ray
      source
    – Plasma heated X-ray source
        » Laser heated
        » E-beam heated
    – Synchrotron X-ray source
Next Generation Lithography: x-Rays
   Mask: Needs a combination of
    materials that are opaque
    (heavy element, e.g. Au) and
    transparent (low atomic mass
    membrane, e.g. BN or S3N4) to
    x-rays
   Mask written by e-beam
   Diffraction is not an issue
    (shadowing is, see next
    viewgraph)
   Masks difficult to make due to
    need to manage stress
   Dust less of a problem because
    they are transparent to x-rays
    Next Generation Lithography: x-Rays
   On account of the finite size of
    the x-ray source and the finite
    mask-to-wafer         gap,       a
    penumbral effect results which
    degrades the resolution at the
    edge of a feature.
   An additional geometric effect
    is the lateral magnification error
    due to the finite mask-to-wafer
    gap and the non-vertical
    incidence of the x-ray beam.
    The projected images of the
    mask are shifted laterally by an
    amount d, called runout. This
    runout       error    must      be
    compensated for during the
    mask making process.
Next Generation Lithography:IPL
   Ions scatter much less than                          Ion source

    electrons so a higher                                    Ion beam

    resolution is feasible                                            Mask

   Problems:
    – Ion Beam source (e.g.
      Gallium)
                                                                Electrostatic
    – Mask                                                      lens system
                                                                (4:1 reduction)
    – Beam forming
                                  Reference
    – Not as mature as EPL             plate

                                                                Step-and-scan
                                                                 wafer stage



                                               Vacuum chamber
    Next Generation Lithography:IPL
   Ion lithography can achieve higher resolution than optical, x-ray, or electron
    beam lithographic techniques because ions undergo no diffraction and scatter
    much less than electrons. In addition, resists are more sensitive to ions than to
    electrons. The Figure below depicts the computer trajectory of 50 H+ ions
    implanted at 60 keV. As illustrated, the spread of the ion beam at a depth of
    0.4 µ is only 0.1 µ. There is also the possibility of a resistless wafer process.
    However, the most important application of ion lithography is the repair of
    masks for optical or x-ray lithography, a task for which commercial systems
    are available.
Next Generation Lithography:IPL
 IPL   Mask                          e-beam writing

                                                  PMMA 1m
                                                   Au 100A
                                                  2 m Si3N4
                                                  Si wafer
                            develop




                    electroplating and etch off




                          backside etch
Future Lithography: Massive
Parallel Writing Arrays
   High-throughput direct-write
    electron beam lithography.
    Addressable arrays of negative
    electron affinity cathodes have
    been advanced as an approach
    to improve throughput for
    electron beam direct - write
    applications.
   Massively parallel arrays of
    atomic force microscopes (
    AFMs ). Perhaps the ultimate
    device in lithography might be
    achieved by using amorphous
    Si as a resist in conjunction
    with a large array of AMFs.
Future Lithography:Microcontact
Printing
   Soft lithography:
    – Replication of a “master-
      pattern” using PDMS
      (stamp)
    – Inking the stamp with
      molecules (thiols,
      thioethers, alkoxysilanes,
      chlorosilanes, etc.)
    – Contact the stamp with the
      substrate surface
    – Monolayer formation at
      regions of contact
    Future Lithography: Nano-Imprint
    Technology
   Nanoimprintlithography patterns a
    resist by deforming the resist shape
    through embossing (with a mold),
    rather than by altering resist chemical
    structures through radiation (with
    particle beams). After imprinting the
    resist, an anisotropicetching is used to
    remove the residue resist in the
    compressed area to expose the
    underneath substrate. 10nm diameter
    holes and 40nm pitch in PMMA can
    be achieved on Sior a metal substrate
    and excellent uniformity over 1 square
    inch.
Future Lithography: Nanoimprinting
Dip Pen Lithography
Scanning AFM Nanostencil
 Cantilever   tip with holes
Scanning AFM Nanostencil
Nanopatterning Methods Compared
Homework
1.   How does one derive a diameter of 1.22lf/d for the imaging of a small
     aperture ?
2.   What does immersion lithography do to DOF?
3.   Demonstrate that F = (1+M)f=1/2NA