Derek Wright

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Tutorial 2

• Derek Wright
• Wednesday, January 26th, 2005

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Some Important Units

• 10 Å 1 nm
• Ex) Si-Si bonds are 2.33 Å, or 0.233 nm
• 1 micron 1 ?m
• 1 atm
• Standard Atmospheric Pressure
• 101.3 kPa
• 760 torr
• 1 bar (1 bar 100 kPa)

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Why are we learning about thin film process?

• It is easier to grow nanometer-scale films
  vertically than to mask nanometer-scale patterns
  horizontally.
• Combining thin films with very good lithography
  leads to nanometer-scale devices

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Building Devices
Exposure/ Developing
Deposition/ Growth or Etching
Photoresist Application
Photoresist Etching
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Overview

• Deposition (Growth)
• Good vs. Bad Films
• Physical Methods
• Evaporation
• Pulsed Laser Deposition
• Sputtering
• Chemical Methods
• CVD
• PECVD
• Optical CVD
• Chemical Solution Deposition
• LB Films

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So Whats Good Quality Film?

• A bad film has defects
• Defects are different for crystalline and
  amorphous films
• Crystal Defects
• Vacancies (voids), Interstitials (stuff jammed
  into the lattice), and Dislocations (fractures in
  the lattice)
• Amorphous Defects
• Coordination Defects (dangling bonds)

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Deposition Physical Methods

• Physical deposition means that nm sized chunks of
  material fly at the substrate and stick onto it
• The hotter the substrate, the more easily these
  pieces of material can move around (surface
  mobility)
• They find their point of lowest energy resulting
  in a better film

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Surface Mobility and Sticking
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Evaporation

• Material to be deposited is heated until it
  becomes vapor phase
• The heated material flies into the substrate
• The hotter the substrate, the better the film
  quality
• Can deposit very fast relative to other methods,
  but not always good quality film (up to 200 nm/s
  film growth)

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Evaporation
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Pulsed Laser Deposition

• Similar to Evaporation method, except uses a
  laser to heat the material to be deposited
• Different because the intense energy creates a
  plasma
• Plasmas not only contain inert material, but also
  ions and radicals which could chemically react
  with the surface
• Depends on chemistry of reactants

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Pulsed Laser Deposition
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Sputtering

• The target (material to be deposited) and
  substrate are placed facing each other
• A plasma is ignited between them under vacuum
• A voltage bias between them causes ions from the
  plasma to ram into the target
• The ions eject pieces of the target that
  sputter onto the substrate

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Sputtering
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Magnetron Sputtering

• A big magnet is used to force the electrons into
  spiral paths so that they spend more time
  ionizing neutral gas particles
• This increases the number of ions
• More ions increases the chances of knocking out
  some of the material to be sputtered
• Increases efficiency

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Deposition Chemical Methods

• In chemical deposition, the material being
  deposited on the substrate reacts with the
  surface
• Form bonds with the surface
• Chemical reaction with the surface
• The substrate as well as reactant temperature
  play a role in the rate of reaction

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Chemical Vapor Deposition

• Precursor gas (a gas phase version of what will
  be deposited on the surface) is pumped into the
  reaction chamber
• Its heated until reactive species form
• Ex) SiH4 ? SiH2 H2
• The reactive species chemically interact with the
  surface to stick to (or react with) it
• Surface properties and temperature can determine
  how well something sticks

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Chemical Vapor Deposition
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Plasma Enhanced CVD

• Uses an RF or microwave E-field to strip
  electrons off the precursor gasses
• Since e- are so much lighter than the rest of the
  molecule (ion), they accelerate in the E-field
  faster than the molecules

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Plasma Enhanced CVD

• By the time the E-field changes direction (at RF
  or microwave frequencies) the electron has gained
  a lot of momentum and the remaining molecule
  (ion) has barely started to move
• Thus, the e- have a high temperature and the
  molecules (ions) have a low temperature
• This means that the substrate can have a lower
  temperature, too
• Enables new substrates like glass and plastic
• This is how TFT-LCD displays can be made

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Plasma Enhanced CVD
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Optical CVD

• Not always applicable
• Uses different wavelengths of light to break
  precursor gas bond to form reactive species
• Ex) Cl2 h? (photon) ? 2Cl? (radicals)
• Also enables low temperature deposition

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Chemical Solution Deposition

• Material is deposited on the substrate in the
  liquid state
• Spin Coating Some liquid is placed on the
  substrate and its spun really fast until only a
  thin coating is left
• Dip Coating Dunk the substrate in solution
• Spray Coating Like spray painting the substrate
• Screen Printing Put a stencil on the substrate
  and use a squeegee to pull solution across
• Ink-jet Printing Same as in an ink-jet printer
  for a PC

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Langmuir-Blodgett (LB) Films

• A form of dip coating
• You have a solution with a layer of special
  molecules on the surface
• One side of the molecule is water-soluble, and
  the other is not (like soap)
• Thus all the molecules are aligned on top of the
  solution

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Langmuir-Blodgett (LB) Films

• When you dunk the substrate in, you get a
  monolayer (one layer) of aligned molecules on the
  substrate
• If you keep dunking it youll get a new layer
  each time
• The water soluble side of one layer aligns with
  the water soluble side of the next (alternating
  alignment)

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Langmuir-Blodgett (LB) Films
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Building Devices
Exposure/ Developing
Deposition/ Growth or Etching
Photoresist Application
Photoresist Etching
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Lithography

• When a pattern is applied to the substrate
• The most common is optical lithography where a
  mask is used to expose a pattern onto a substrate
• Like how a transparency on an overhead projector
  works
• The better the lithography, the smaller the
  feature size
• Small feature size ? nanoelectronics

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Technology Nodes
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Optical Lithography

• Resist is spin-coated onto the substrate
• A mask is placed in front of the substrate
• A mask is a clear plate with a pattern on it,
  like an overhead transparency
• A light shines through the uncovered parts of the
  mask and chemically changes the resist (exposure)
• The exposed resist is etched away with a solvent
  (developing)

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Optical Lithography

• Smaller features need smaller wavelengths of
  light
• UV 365nm - 436nm
• Deep UV (DUV) 157nm - 250nm
• Extreme UV (EUV) 11nm - 14nm
• X-ray lt 10nm

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Optical Lithography

• Three types
• Contact The mask is directly against the
  substrate good minimum feature size, bad for
  the mask and substrate to touch
• Proximity The mask is a few ?m away from the
  substrate degrades minimum feature size but
  good for reliability because mask doesnt touch
  substrate
• Projection Lenses are used to focus the masks
  image onto the substrate good minimum feature
  size, good for reliability

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Extreme UV Lithography

• Pretty soon UV lithography will hit the limit in
  terms of minimum feature size
• EUV is the next step
• Few materials allow EUV light to pass through, so
  reflective (instead of transmissive) optics must
  be used
• Mask pattern must be really absorbent to EUV
  light, so heavy metals are used

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X-ray Lithography

• After EUV comes X-ray lithography
• Enables super-high resolution pattern transfer
• There are technical hurdles to overcome before
  x-ray lithography systems are in place
• Synchrotrons need to be further developed as a
  source for x-rays

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E-beam Lithography

• Uses a focused beam of electrons to directly
  write to the substrate
• Works much like a CRT TV an electron gun fires
  electrons and the beam is directed with magnetic
  fields
• There is a limit to how many electrons can be in
  the beam because they will start to repel each
  other and blur the beam

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E-beam Lithography

• Very precise, but very slow method
• Can be accomplished in two ways
• Use a narrow beam and turn it on and off to write
  or not write a pixel
• Use a wide beam and a mask to block the parts
  that shouldnt be written
• Typically very slow and costly good for making
  optical lithographic masks for use in UV, EUV,
  and X-ray

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Nano-imprint Technology

• Much like forging steel, except at a very small
  scale
• A stamp is fabricated at the nm-scale using
  traditional process methods
• A substrate is coated with some kind of polymer
• The polymer is stamped with the nm-scale stamp
• The polymer is either cured with heat or light

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Thank You!

• This presentation will be available on the web.