Photolithography, Next Generation Lithography and Future Lithography 5b

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Photolithography, Next Generation Lithography and
Future Lithography (5b)

• Winter 2009

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Next Generation and Future Lithography

• We categorize emerging lithography tools in next
  generation lithographies (NGLs), i.e.,
  lithographies beyond deep UV lithography (DUV)
  and future lithography approaches in the RD
  stage.

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Next Generation Lithography (NGL)
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Next Generation Lithography (NGL)
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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

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Next Generation Lithography EUV

• Mask fabrication is the most difficult task

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Next Generation Lithography E-Beam

• The advantages of electron lithography are
• (1) Generation of micron and submicron resist
  geometries
• (2) Highly automated and precisely controlled
  operation
• (3) Greater depth of focus
• (4) Direct patterning without a mask

• The biggest disadvantage of electron lithography
  is its low throughput (approximately 5 wafers /
  hour at less than 0.1 µ resolution). Therefore,
  electron lithography is primarily used in the
  production of photomasks and in situations that
  require small number of custom circuits.

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Next Generation Lithography E-Beam

• Diffraction is not a limitation on resolution (l
  lt 1 Å for 10-50 keV electrons)
• Resolution depends on electron scattering and
  beam optics the size of the beam, can reach 5
  nm
• Two modes of operation
• Direct writing with narrow beam
• Electron projection lithography using a mask EPL
  
• Issues
• Throughput of direct writing is very low
  research tool or low pattern density
  manufacturing
• Projection stepper (EPL) is in development stage
  only (primarily by Nikon).
• Mask making is the biggest challenge for the
  projection method
• Back-scattering and second electron result in
  proximity effect reduce resolution with dense
  patterns there is also the proximity effect
• Operates in high vacuum (10-6 10-10 torr) slow
  and expensive

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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

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Next Generation Lithography E-Beam
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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

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Next Generation Lithography E-Beam

• E-beam blur

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Next Generation LithographyE-Beam

• Thermionic emitters
• Electrons boiled off the surface by giving them
  thermal energy to overcome the barrier (work
  function)
• Current given by Richardson-Dushman equation
• 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

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Next Generation LithographyE-Beam
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Next Generation LithographyE-Beam 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

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Next Generation LithographyE-Beam 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

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Next Generation LithographyE-Beam SCALPEL
(SCattering with Angular Limitation Projection
Electron-beam Lithography)
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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 for NGL for
  now

Grenoble Synchrotron
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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

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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

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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.

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Next Generation LithographyIPL

• Ions scatter much less than electrons so a higher
  resolution is feasible
• Problems
• Ion Beam source (e.g. Gallium)
• Mask
• Beam forming
• Not as mature as EPL

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Next Generation LithographyIPL

• 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.

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Next Generation LithographyIPL

• IPL Mask

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Future LithographyOverview

• Proximal Probe Writing Techniques with massive
  parallel writing arrays Using MEMS tools
• Block copolymers
• Zone plate array lithography (ZPAL),
• Quantum lithography (two-photon lithography)
• Lithography with superlenses (Pendrys dream).

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Future LithographyOverview

• SAM and LB films
• Nanoimprint lithography (NIL) and Step-and-Flash
  Imprint Lithography (SFIL)
• It is quite possible that some of the alternative
  lithography tools we treat in the RD category,
  will emerge as serious next generation
  lithographies (NGL) in the coming years.

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Future LithographyProximal Probes

• Proximal probe techniques rely on the use of
  nanoscale probes, positioned and scanned in the
  immediate vicinity of the material surface.
• Proximal probes might involve
• Electrical methods where a scanning tunneling
  microscope (STM) tip generates a local field
  /current that modifies the region directly under
  the tip (e.g., SiH ? Si).
• A second approach involves mechanical methods
  where a scanning force microscope (SFM / AFM) tip
  scrapes, thermally deforms or transfers material
  at the surface, the latter material transfer
  method corresponds to dip-pen lithography (DPL).
• Thirdly it may involve a near-field optical
  scanning microscope (NSOM) tip or apertureless
  near-field scanning optical microscopy (ANSOM),
  that exposes photoresist under the tip only.

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Future LithographyProximal Probes

• The use of single proximal probe tips poses a
  serious drawback in terms of processing speed. To
  use these techniques in the actual manufacture of
  ICs and data storage devices, it is necessary to
  devise a scheme for parallel processing by making
  arrays of these proximal probes.

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Future LithographyParallel Writing

• Zlatkin et al., developed an elegant array of
  focused electron writing beams operating at 300
  eV or less. The emitters used are cold
  field-emission (CFE) sharpened tungsten tips,
  although thermionic or Schottky emitters would be
  feasible as well. The emitters are positioned
  several millimeters above the micromachined
  extraction holes of a lens array fashioned in a
  single crystal substrate.
• At Cornells National Nanofabrication Facility
  (NNF) A field emission tip is mounted onto an
  STM the STM feedback principle is used for
  precision x, y, and z piezoelectric alignment of
  the tip to a miniaturized electron lens to form a
  focused probe of electrons

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Future Lithography Proximal Probe Writing
Techniques

• Proximal probe based techniques such as atomic
  force microscopy (AFM), scanning tunneling
  microscopy (STM), dip-pen lithography (DPL) and
  near-field scanning optical microscopy (NSOM),
  and apertureless near-field scanning optical
  microscopy (ANSOM).
• In dip-pen lithography (DPL) a reservoir of ink
  is stored on the cantilever holding the scanning
  probe tip, which is manipulated across the
  surface, leaving lines and patterns behind.
  Lines as thin as 15 nanometers have been drawn.
  The attainable resolution depends strongly on the
  substrate roughness, the writing speed and the
  relative humidity.

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Future Lithography Proximal Probe Writing
Techniques

• A thermal-DPN (tDPN) method was developed by
  Georgia Techs William King and NRLs Lloyd
  Whitman
• By using easily-melted solid inks and special AFM
  probes with built-in heaters writing can be
  turned on and off at will.

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Future Lithography NSOM and ANSOM Proximal Probes

• To obtain a resolution better than Abbes optical
  microscopy limit
• complicated and costly electron (0.1 nm
  resolution) or scanning tunneling microscopes
  (STMs with atomic resolution) are required.
• Unfortunately, these techniques do sacrifice many
  of the advantages associated with traditional
  optical microscopes (non-destructiveness, low
  cost, high speed, reliability, versatility,
  accessibility, ease of use, informative contrast,
  spectroscopy and real time).
• Combining a scanning proximal probe technique
  with optical microscopy in so-called scanning
  near-field optical microscopy, NSOM provides an
  attractive solution to this dilemma.

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Future Lithography NSOM and ANSOM Proximal Probes

• In NSOM, the sample is illuminated by a
  nanoscopic light source located close to the
  surface (10 nm) and the resolution is dictated by
  the source diameter a or
• This is achieved by using nanoscale apertures in
  NSOM or by using aperture-less techniques in
  apertureless near-field scanning optical
  microscopy (ANSOM) or scattering SNOM.

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Future LithographyPlasmon Lithography

• Plasmonics Using local field enhancement
  occurring around metal nanoparticles when they
  are excited at the surface plasmon resonance
  frequency can be used to print nanoscale features
  in thin resist layers.
• Feature sizes below ?/10 were generated in a
  parallel fashion using visible illumination and
  standard g-line photoresist.

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Future LithographyPlasmon Lithography

• Atwater and co-workers from Caltech introduced
  plasmon lithography in 2002.
• Plasmon lithography is based on plasmon resonance
  occurring in nanosized metallic structures,
  allowing for the replication of patterns with a
  resolution limit considerable below the
  diffraction limit.
• Is similar to ANSOM but with the scattering probe
  tip is replaced by nanoparticles.

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Future LithographyBlock Copolymers

• Block copolymers comprise two or more different
  monomer units, strung together in long sequences,
  which can self-assemble into highly-ordered
  lattices with unit cells dimensions of 10-100 nm.
  This length scale reaches well below the limits
  of conventional optical lithography.
• Block copolymer lithography refers to the use of
  block copolymers in the form of thin films in
  which the domain structure provides a template
  for additive or subtractive pattern transfer
  operations.

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Future Lithography Zone plate array lithography

• Zone-Plate-Array Lithography (ZPAL), requires no
  masks, but rather uses arrays of individually
  targetable optical beams.
• Beamlets of photons, which can be rapidly turned
  on and off, are projected through diffractive
  Fresnel-zone-plate lenses allowing myriad complex
  shapes to be fabricated.
• The zone plates are made using lithography
  techniques and the shutters under each beamlet
  are micromechanical.
• ZPAL was originally developed at MITs
  nanostructures laboratory in the mid to late
  1990s. Each zoneplate is responsible for one unit
  cell of photoresist exposure corresponding to the
  diameter of a one individual zoneplate. The
  writing with the shutters on and off results in a
  dot-matrix type lines and moving the wafer in a
  serpentine fashion under the focused beamlets
  results in a full pattern.

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Future Lithography Zone plate array lithography

• Zone plates use constructive interference of
  light rays from adjacent zones (Fresnel Zones) to
  form a focus. The zones are spaced so that
  diffracted-light constructively interferes at the
  desired focus. The Fresnel zone plate is a
  relative of the pinhole camera in that it does
  not use mirrors or lenses for its imaging
  properties.
• The zone plate is especially useful in the
  ultraviolet and x-ray regions of the spectrum,
  for which other imaging devices are hard to find.
  Self-supporting gold zone plates have been
  manufactured for these spectral regions.

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Future Lithography Quantum Lithography

• It has been demonstrated that quantum-lithography
  with entangled N-photon states beats the Rayleigh
  diffraction limit by a factor of N, and in
  two-photon lithography the resolution is thus
  improved by a factor of 2 (as if one used a
  classical source with wavelength ?/2).
• Einstein, Poldosky and Rosen described the
  entangled two-particle state according to the
  principle of quantum superposition in 1935 and
  they pointed out a surprising consequence the
  momentum ?Px (position ?x) for neither photon is
  known
• If one particle is measured to have a certain
  momentum (position), the momentum (position) of
  its twin is known with certainty, despite the
  distance between them (this is known as the EPR
  paradox).
• The entangled photon pairs come out from a point
  of the object plane, undergo two-photon
  diffraction, and result in twice narrower point
  spread function on the image plane.

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Future Lithography Superlens

• A poors man near-field superlens (e lt1 and m1)
  was demonstrated. Zhang et al imaged objects as
  small as 40-nm across with their superlens, which
  is just 35-nm thick
• This superlens images 10 nm features with 365 nm
  light N. Fang, H. Lee, C. Sun, and X. Zhang,
  Sub-diffraction-limited optical imaging with a
  silver superlens, Science 308, 534-537 (2005).
• Pendrys dream.

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Future LithographyStamp Lithography

• Soft lithography (Whitesides)
• 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

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Future Lithography Nanoimprint Lithography

• Nanoimprint lithography (NIL) and Step-and-Flash
  Imprint Lithography (SFIL) are techniques that
  use hard molds instead of the soft molds used in
  Soft Lithography.
• Stephen Chou at Princeton University invented
  nanoimprint lithography (NIL) in 1994, with the
  aim of overcoming the diffraction limited minimal
  feature sizes obtained in semiconductor
  manufacturing based on DUV lithography
  (http//www.princeton.edu/chouweb/).

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Future Lithography Nano-Imprint Technology (NIL)

• 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 Si or a
  metal substrate and excellent uniformity over 1
  square inch.

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Future Lithography Nanoimprinting (NIL)
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Future LithographyStep-and-Flash Imprint
Lithography (SFIL)

• The University of Texas (UT)-Austin developed its
  version of nanoimprint lithography, i.e.,
  step-and-flash imprint lithography (SFIL), in
  1998.
• The SFIL method is distinct from the original NIL
  in its use of UV-assisted nanoimprinting that
  molds photocurable liquids in a step-and-repeat,
  die-by-die fashion rather than by heat-assisted
  molding of full, polymer-coated wafers.