Physical Vapor Deposition

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Physical Vapor Deposition
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PVD

• Physical methods produce the atoms that deposit
  on the substrate
• Evaporation
• Sputtering
• Sometimes called vacuum deposition because the
  process is usually done in an evacuated chamber
• PVD is used for metals.
• Dielectrics can be deposited using specialized
  equipment

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Evaporation

• Rely on thermal energy supplied to the crucible
  or boat to evaporate atoms
• Evaporated atoms travel through the evacuated
  space between the source and the sample and stick
  to the sample
• Few, if any, chemical reactions occur due to low
  pressure
• Can force a reaction by flowing a gas near the
  crucible
• Surface reactions usually occur very rapidly and
  there is very little rearrangement of the surface
  atoms after sticking
• Thickness uniformity and shadowing by surface
  topography, and step coverage are issues

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Evaporation
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http//www.ee.byu.edu/cleanroom/metal.parts/vaporp
ressure.jpg
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Mean Free Path

• 63 of molecules undergo a collision in a
  distance less than l and 0.6 travel more than 5
  l.
• where do is the diameter of the evaporatant and n
  is the concentration of gas molecules in the
  chamber

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Evaporation

• The vacuum is usually lt 10-5 torr
• 4x10-6 torr, l 18 inches
• The source heater can be
• Resistance (W, Mo, Ta filament)
• Contaminants in filament systems are Na or K
  because they are used in the production of W
• E-beam (graphite or W crucible)
• E-beam is often cleaner although S is a common
  contaminant in graphite
• Top surface of metal is melted during evaporation
  so there is little contamination from the
  crucible
• More materials can be evaporated (high
  melting-point materials)
• A downside of e-beam is that X-rays are produced
  when the electron beam hits the Al melt
• These X-rays can create trapped charges in the
  gate oxide
• This damage must be removed by annealing

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Thermal Evaporation
http//www.lesker.com/newweb/Deposition_Sources/Th
ermalEvaporationSources_Resistive.cfm
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E-beam Evaporation
http//www.fen.bilkent.edu.tr/aykutlu/msn551/evap
oration.pdf
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PVD

• At sufficiently low pressure and reasonable
  distances between source and wafer, evaporant
  travel in straight line to the wafer
• Step coverage is close to zero
• If the source is small, we can treat it as a
  point source
• If the source emission is isotropic, it is easy
  to compute the distribution of atoms at the
  surface of the wafer

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PVD
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PVD

• For a source that emits only upwards, ? 2?
• The number of atoms that hit the area Ak of the
  surface is
• The deposition velocity is the above expression
  divided by the density (N) of the material

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Evaporation

• Pe is the equilibrium vapor pressure of the melt
  (torr)
• m is the gram-molecular mass
• T is the temperature (K)
• As is area of source
• The vapor pressure depends strongly on the
  temperature (Claussius-Clapeyron equation)
• In order to have a reasonable evaporation rate
  (0.1-1 ?m/min), the vapor pressure must be about
  1-10 mtorr

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PVD
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PVD

• The velocity can be normalized to the velocity at
  the center of the wafer

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PVD

• Corrections can be applied if the source is a
  small, finite area
• If we now move the center of the wafer from the
  perpendicular position, but tile it with respect
  to the source, an extra term must be added

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Planetaries

• Wafer holders that rotate wafer position during
  deposition to increase film thickness uniformity
  across wafer and from one wafer to another.
• Wobbling wafer holders increase step coverage

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PVD

• Nonuniformity of evaporatant can occur when
  angular emission of evaporant is narrower than
  the ideal source
• Crucible geometry
• Melt depth to melt area ratio
• Density of gas atoms over the surface of the melt

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Evaporation

• Evaporating alloys is difficult Because of the
  differing vapor pressures.
• Composition of the deposited material may very
  different from that of the target material
• The problem can be overcome by
• Using multiple e-beams on multiple sources
• This technique causes difficulties in sample
  uniformity because of the spacing of the sources
• Evaporating source to completion (until no
  material is left)
• Dangerous to do in e-beam system

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Evaporation

• Compounds are also hard to evaporate because the
  molecular species may be different from the
  compound composition
• Energy provided may be used to dissociate
  compound.
• When evaporating SiO2, SiO is deposited.
  Evaporation in a reactive environment (flowing O2
  gas near crucible during deposition) helps
  reconstitute oxide.

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Evaporation

• Advantages
• Little damage to the wafer
• Deposited films are usually very pure
• Limited step coverage

• Disadvantages
• Materials with low vapor pressures ae very
  difficult to evaporated
• Refractory metals
• High temperature dielectrics
• No in situ precleaning
• Limited step coverage
• Film adhesion can be problematic

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Step-coverage

• Evaporation technique is very directional due to
  the large mean free paths of gas molecules at low
  pressure.
• Shadowing of patterns and poor step coverage can
  occur when depositing thin films.
• Rotation of the planetary substrate holder can
  minimize these effects.
• Heating substrate can promote atom mobility,
  improve step coverage and adhesion.
• Shadow masking and lift-off are processes where
  poor step coverage is desirable.

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Other PVD Techniques

• Other deposition techniques include
• Sputter deposition (DC, RF, and reactive)
• Bias sputtering
• Magnetron sputtering
• Collimated and ionized sputter deposition
• Hot sputter deposition

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Sputtering

• Sputter deposition is done in a vacuum chamber
  (10mTorr) as follows
• Plasma is generated by applying an RF signal
  producing energetic ions.
• Target is bombarded by these ions (usually Ar).
• Ions knock the atoms from the target.
• Sputtered atoms are transported to the substrate
  where deposition occurs.

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Sputtering

• Wide variety of materials can be deposited
  because material is put into the vapor phase by a
  mechanical rather than a chemical or thermal
  process (including alloys and insulators).
• Excellent step coverage of the sharp topologies
  because of a higher chamber pressure, causing
  large number of scattering events as target
  material travels towards wafers.
• Film stress can be controlled to some degree by
  the chamber pressure and RF power.

http//www.knovel.com
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Deposition conditions

• Temperature Room to higher
• Pressure 100mtorr
• compromise between increasing number of Ar ions
  and increasing scattering of Ar ions with neutral
  Ar atoms
• Power
• Heating of target material
• Low temperature metals can melt from temperature
  rise caused by energy transfer from Ar ions

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Sputter sources

• Magnetron
• Magnetic field traps freed electron near target
• Move in helical pattern, causing large number of
  scattering events with Ar gas creating high
  density of ionized Ar
• Ion beam
• Plasma of ions generated away from target and
  then accelerated toward start by electric field
• Reactive sputtering
• Gas used in plasma reacts with target material to
  form compond that is deposited on wafer
• Ion-assisted deposition
• Wafer is biased so that some Ar ion impact its
  surface, density the deposited film. May sputter
  material off of wafer prior to deposition for
  in-situ cleaning.

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Sputtering

• Advantages
• Large-size targets, simplifying the deposition of
  thins with uniform thickness over large wafers
• Film thickness is easily controlled by fixing the
  operating parameters and simply adjusting the
  deposition time
• Control of the alloy composition, step coverage,
  grain structure is easier obtained through
  evaporation
• Sputter-cleaning of the substrate in vacuum prior
  to film deposition
• Device damage from X-rays generated by electron
  beam evaporation is avoided.

• Disadvantages
• High capital expenses are required
• Rates of deposition of some materials (such as
  SiO2) are relatively low
• Some materials such as organic solids are easily
  degraded by ionic bombardment
• Greater probability to introduce impurities in
  the substrate because the former operates under a
  higher pressure 

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Salicide
http//www.research.ibm.com/journal/rd/444/jordans
weet.html