Lithography

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Lithography
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• Lithography in the MEMS context is typically the
  transfer of a pattern to a photosensitive
  material by selective exposure to a radiation
  source such as light.
• A photosensitive material is a material that
  experiences a change in its physical properties
  when exposed to a radiation source. If we
  selectively expose a photosensitive material to
  radiation (e.g. by masking some of the radiation)
  the pattern of the radiation on the material is
  transferred to the material exposed, as the
  properties of the exposed and unexposed regions
  differs

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STEPS IN LITHOGRAPHY

• COATING THE SUBSTRATE WITH PHOTO SENSITIVE
  MATERIAL (PHOTO RESIST)
• FIXING THE MASK WITH THE FEATURES ON THE COAT
• EXPOSURE TO RADIATION
• SPRAY OF DEVELOPER TO OBTAIN EITHER POSITIVE OR
  NEGATIVE
• ETCH OR DEPOSIT
• STRIP THE PHOTO RESIST

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MASK, EXPOSURE DEVELOP
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ETCH(SUBTRACT) or DEPOSIT(ADD)
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CARE TO BE TAKEN

• Alignment
• Exposure

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Alignment

• In order to make useful devices the patterns for
  different lithography steps that belong to a
  single structure must be aligned to one another.
• The first pattern transferred to a wafer usually
  includes a set of alignment marks, which are high
  precision features that are used as the reference
  when positioning subsequent patterns, to the
  first pattern
• Often alignment marks are included in other
  patterns, as the original alignment marks may be
  obliterated as processing progresses.
• It is important for each alignment mark on the
  wafer to be labeled so it may be identified, and
  for each pattern to specify the alignment mark
  (and the location thereof) to which it should be
  aligned.
• By providing the location of the alignment mark
  it is easy for the operator to locate the correct
  feature in a short time. Each pattern layer
  should have an alignment feature so that it may
  be registered to the rest of the layers

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Exposure

• The exposure parameters required in order to
  achieve accurate pattern transfer from the mask
  to the photosensitive layer depend primarily on
  the wavelength of the radiation source and the
  dose required to achieve the desired properties
  change of the photoresist.
• Different photoresists exhibit different
  sensitivities to different wavelengths.
• The dose required per unit volume of photoresist
  for good pattern transfer is somewhat constant

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EFEECTS OF OVER EXPOSURE

• if an image is overexposed, the dose received by
  photoresist at the edge that shouldn't be exposed
  may become significant.
• If we are using positive photoresist, this will
  result in the photoresist image being eroded
  along the edges, resulting in a decrease in
  feature size and a loss of sharpness or corners
• If we are using a negative resist, the
  photoresist image is dilated, causing the
  features to be larger than desired, again
  accompanied by a loss of sharpness of corners.
• If an image is severely underexposed, the pattern
  may not be transferred at all, and in less sever
  cases the results will be similar to those for
  overexposure with the results reversed for the
  different polarities of resist

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Industrial Process steps

• Dehydration bake - dehydrate the wafer to aid
  resist adhesion.
• prime - coating of wafer surface with adhesion
  promoter. Not necessary for all surfaces.
• Resist spin/spray - coating of the wafer with
  resist either by spinning or spraying. Typically
  desire a uniform coat.
• Soft bake - drive off some of the solvent in the
  resist, may result in a significant loss of mass
  of resist (and thickness). Makes resist more
  viscous.
• Alignment - align pattern on mask to features on
  wafers.

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• Exposure - projection of mask image on resist to
  cause selective chemical property change.
• Post exposure bake - baking of resist to drive
  off further solvent content. Makes resist more
  resistant to etchants (other than developer).
• Develop - selective removal of resist after
  exposure (exposed resist if resist is positive,
  unexposed resist if resist is positive). Usually
  a wet process (although dry processes exist).
• Hard bake - drive off most of the remaining
  solvent from the resist.
• Descum - removal of thin layer of resist scum
  that may occlude open regions in pattern, helps
  to open up corners.

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Resolution

• Resolution or the critical dimension is the
  minimum feature size that could be printed
• The ability to project a clear image of a small
  feature onto the wafer is limited by the
  wavelength of the light that is used.
• The minimum feature size that a projection system
  can print is given approximately by
• ? wave length,
• NA numerical aperture, K constant (0.4)

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NUMERICAL APERTURE

• In most areas of optics, and especially in
  microscopy, the numerical aperture of an optical
  system such as an objective lens is defined by
• NA n Sin?
• where n is the index of refraction of the medium
  in which the lens is working (1.0 for air, 1.33
  for pure water, and up to 1.56 for oils), and ?
  is the half-angle of the maximum cone of light
  that can enter or exit the lens

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• Photolithography has used ultraviolet light from
  gas-discharge lamps using mercury, sometimes in
  combination with noble gases such as xenon. These
  lamps produce light across a broad spectrum with
  several strong peaks in the ultraviolet range.
  This spectrum is filtered to select a single
  spectral line, usually the "g-line" (436 nm) or
  "i-line" (365 nm).
• CD is 200 to 150nm

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• Current state-of-the-art photolithography tools
  use deep ultraviolet (DUV) light with wavelengths
  of 248 and 193 nm
• which allow minimum feature sizes down to 100 nm

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Immersion lithography

• Immersion lithography is a photolithography
  resolution enhancement technique that replaces
  the usual air gap between the final lens and the
  wafer surface with a liquid medium that has a
  refractive index greater than one. The resolution
  is increased by a factor equal to the refractive
  index of the liquid. (CD 60nm)

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Other issues in photo lithography
                      

• Low depth of field and depth of focus
• Depth of field is a measurement of depth of
  acceptable sharpness in the object space, or
  subject space.
• Depth of focus is a measurement of how much the
  film / substrate can be displaced while an object
  remains in acceptably sharp focus

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Depth of field diagram
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Depth of field and depth of focus
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Depth of focus
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Depth of Focus

• t is the depth of focus,
• N is the f-number of the optical system
• C is the circle of confusion
• v is the distance of the object from lens
• f is the focal length

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maskless lithography

• In maskless lithography, the radiation that is
  used to expose a photosensitive emulsion (or
  photoresist) is not projected from, or
  transmitted through, a photomask. Instead, most
  commonly, the radiation is focused to a narrow
  beam. The beam is then used to directly write the
  image into the photoresist, one or more pixels at
  a time

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FORMS OF MASKLESS LITHOGRAPHY

• Laser (Optical)
• Focused ion beam
• Electron beam

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Multiphoton lithography

• Multiphoton lithography (also known as direct
  laser writing) is a technique for creating small
  features in a photosensitive material, without
  the use of complex optical systems or photomasks.
• By scanning and properly modulating the laser, a
  chemical change (usually polymerization) occurs
  at the focal spot of the laser and can be
  controlled to create an arbitrary two or
  three-dimensional periodic or non-periodic
  pattern.
• This method could also be used for rapid
  prototyping of structures with fine features

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Multiphoton lithography

• In laser physics the numerical aperture is
  defined slightly differently
• The NA of a Gaussian laser beam is related to its
  minimum spot size by
• D beam dia

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Focused ion beam

• Focused ion beam (FIB) systems operate in a
  similar fashion to a scanning electron microscope
  (SEM) except, rather than a beam of electrons and
  as the name implies, FIB systems use a finely
  focused beam of ions (usually gallium) that can
  be operated at low beam currents for imaging or
  high beam currents for site specific sputtering
  or milling

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Why Ions ?

• ions are larger than electrons
• they cannot easily penetrate within individual
  atoms of the sample. Interaction mainly involves
  outer shell interaction resulting in atomic
  ionization and breaking of chemical bonds of the
  substrate atoms.
• The penetration depth of the ions is much lower
  than the penetration of electrons of the same
  energy.

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Why Ions ?

• ions are heavier than electrons
• ions can gain a high momentum. For the same
  energy, the momentum of the ion is about 370
  times larger.
• For the same energy ions move a lot slower than
  electrons. However, they are still fast compared
  to the image collection mode and in practice this
  has no real consequences.
• The magnetic lenses are less effective on ions
  than they would be on electrons with the same
  energy. As a consequence the focused ion beam
  system is equipped with electro-static lenses and
  not with magnetic lenses

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USE of FIB

• Unlike an electron microscope, FIB is inherently
  destructive to the specimen. When the high-energy
  gallium ions strike the sample, they will sputter
  atoms from the surface.
• Gallium atoms will also be implanted into the top
  few nanometers of the surface
• FIB assisted deposition
• the surface will be made amorphous

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FIB

• Because of the sputtering capability, the FIB is
  used as a micro-machining tool, to modify or
  machine materials at the micro- and nanoscale.
• nano machining with FIB is a field that still
  needs developing.
• The common smallest beam size is
• 2.5-6 nm

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ion beam induced deposition

• FIB-assisted chemical vapor deposition occurs
  when a gas, such as tungsten hexacarbonyl
  (W(CO)6) is introduced to the vacuum chamber and
  allowed to chemisorb onto the sample.
• By scanning an area with the beam, the precursor
  gas will be decomposed into volatile and
  non-volatile components the non-volatile
  component, such as tungsten, remains on the
  surface as a deposition.
• From nanometers to hundred of micrometers in
  length, tungsten metal deposition allows to put
  metal lines right where needed.
• Other materials such as platinum, cobalt, carbon,
  gold, etc., can also be locally deposited

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