Ect' 16 Thin Film deposition

1
Ect. 16 Thin Film deposition

• Thin-film deposition is any technique for
  depositing a thin film of material onto a
  substrate or onto previously deposited layers.
  "Thin" is a relative term, but most deposition
  techniques allow layer thickness to be controlled
  within a few tens of nanometers, and some
  (molecular beam epitaxy) allow single layers of
  atoms to be deposited at a time.
• Deposition techniques fall into two broad
  categories, depending on whether the process is
  primarily chemical or physical.
• Chemical deposition Here, a fluid precursor
  undergoes a chemical change at a solid surface,
  leaving a solid layer. Since the fluid surrounds
  the solid object, deposition happens on every
  surface, with little regard to direction thin
  films from chemical deposition techniques tend to
  be conformal, rather than directional.
• Physical deposition uses mechanical or
  thermodynamic means to produce a thin film of
  solid.. Since most engineering materials are held
  together by relatively high energies, and
  chemical reactions are not used to store these
  energies, commercial physical deposition systems
  tend to require a low-pressure vapor environment
  to function properly most can be classified as
  Physical vapor deposition or PVD.
• The material to be deposited is placed in an
  energetic, entropic environment, so that
  particles of material escape its surface. Facing
  this source is a cooler surface which draws
  energy from these particles as they arrive,
  allowing them to form a solid layer. The whole
  system is kept in a vacuum deposition chamber, to
  allow the particles to travel as freely as
  possible. Since particles tend to follow a
  straight path, films deposited by physical means
  are commonly directional, rather than conformal.

2
Thin Film deposition

• Physical vapor deposition (PVD) is a general term
  used to describe any of a variety of methods to
  deposit thin films by the condensation of a
  vaporized form of the material onto various
  surfaces (e.g., onto semiconductor wafers).
• The coating method involves purely physical
  processes such as high temperature vacuum
  evaporation or plasma sputter bombardment rather
  than involving a chemical reaction at the surface
  to be coated as in chemical vapor deposition.
• Variants of PVD include, in order of increasing
  novelty - Evaporative deposition -- In which
  the material to be deposited is heated to a high
  vapor pressure by electrically resistive heating
  in "high" vacuum. - Electron Beam Physical
  Vapor Deposition -- In which the material to be
  deposited is heated to a high vapor pressure by
  electron bombardment in "high" vacuum. -
  Sputter deposition -- In which a glow plasma
  discharge (usually localized around the "target"
  by a magnet) bombards the material sputtering
  some away as a vapor.- Cathodic Arc Deposition
  -- In which a high power arc directed at the
  target material blasts away some into a vapor.
• Pulsed laser deposition - In which a high power
  laser ablates material from the target into a
  vapor.

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Thin Film deposition
Wafer

• Thermal evaporation (Metals)
• the source material is placed in a crucible,
  which is radiatively heated by an electric
  filament. Alternatively, the source material may
  be hung from the filament itself, filament
  evaporation.
• The process starts by increasing the filament
  temperature till the metal is melted and wets the
  filament.

Shutter
Hot boat

• Filament temperature is then raised to evaporate
  the metal from the filament and the metal is
  condensed on the wafer.
• Disadvantages 1) contamination by the filament
  material. 2) Alloys are not easily controlled
  3) Thick films are not easily
  obtained.

• Electron beam evaporation (Metals)
• In a high vacuum system, a high intensity beam of
  electrons (up to 15 KeV) is focused on the source
  material (placed in crucible).
• The energy of the electrons melts a region of the
  target.
• The metal evaporates and covers the wafers
  mounted above the source facing downwards.
• Several crucibles are used for different
  materials.
• Dual e-beam systems can be used for combined
  materials.
• Disadvantage X-ray radiation can cause radiation
  damage to pattern on the wafer.

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Thin Film deposition

• Sputtering (Metal)
• The principle is very similar to ion-milling.
• Ar ions are formed in the plasma and directed to
  bombard the target surface and knock the loose
  atoms off the surface.
• These atoms transport to the wafer.

• Pulsed laser deposition (PLD)
• A high power pulsed laser beam is focused inside
  a vacuum chamber to strike a target of the
  desired composition.
• Material is then vaporized from the target and
  deposited as a thin film on a substrate, such as
  a silicon wafer facing the target.
• This process can occur in ultra high vacuum or
  in the presence of a background gas, such as
  oxygen which is commonly used when depositing
  oxides to fully oxygenate the deposited films.

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Lect 16 Thin film deposition

• An important factor in thin film deposition is
  the deposition rate.
• Estimating the deposition rate would require some
  basic understanding of radiometry.
• Assume a hypothetical material point source that
  radiates in every direction.
• Assume that a source emits Me cm/s of the
  material in all directions equally. Then the rate
  of material deposited on a sphere of radius r per
  unit area can be defined as below

As
?
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Thin film deposition

• If a wafer is placed at a distancer from the
  point source oriented normal to the radiation,
  the deposition rate is then
• If the wafer surface is tilted with an angle ?
  then the area seen by the flux is Ascos ? , and
  the deposition rate can be re-written as
• In practice, the source has a finite surface
  area, Ae. The surface area can then be
  dividedinto smaller areas, dAe, each emitting
  with solid angle d??.
• It is now useful to define a new quantity,the
  radiance L, defined as the emissionrate per unit
  source area per unit solid angle.

Ms Me
Ae
7
Thin film disposition

• If the source is normal to the direction of
  radiation, then the radiance is defined as
• If the source is tiled with an angle ?? with
  respect to the radiation, then the effective
  source area is reduced to Aecos?, and the
  radiance is re-written as
• The deposition rate over a hemisphere with
  radius r is Me .

dAs
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Thin film deposition

• If a source with a surface area emits at a rate
  Me, the rate of deposition on a wafer located at
  a distance r from the source, oriented with an
  angle ? with respect to the normal to the source,
  and tilted with an angle ? with respect to the
  direction of radiation, is
• The deposition rate Me depends on the density ?
  (g/cm3) and mass evaporation rate mt (g/sec) of
  the source and the area of the wafer.
• The dependence of deposition rate on the wafer
  location and alignment can be eliminated using
  the configuration on the right

?
As
?
Ae
9
Thin film disposition

• To estimate a value of the deposition ratio, we
  first need to review the kinetic theory and the
  behavior of gases.
• For gas molecules with velocity v, the number of
  molecules that collides with the surface during a
  time period ?t can be calculated as follows.
• Any molecule within a distance will
  collide with the surface.
• Assume molecular density of N atoms/cm3 and a
  surface area of As, then the volume
  will contain n molecules.
• If the probability of having gas molecules with
  velocity v is f(v), the number of atoms collide
  with the surface for all positive values of v is

m
As
10
Thin film disposition

• For the first law of Kinetic theory
  
  where P is the equilibrium pressure of the
  crucible material . And KB is Boltzman constant.
  n is the number of molecules, N is molecule
  density, and m is the molecule mass.
• Molecules have kinetic energy

Reference http//www.webelements.com/
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Thin film deposition
Reference http//www.tf.uni-kiel.de/kr/thinfilms
/tsf_i_chpt_3_1.pdf
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Thin film deposition
?
x

• Deposited film thickness is then the deposition
  rate multiplied by the deposition time

h
r
?
Change of the deposited thickness of Ag on 4
wafer for different wafer hieghts.
13
Thin film deposition
Reference http//www.uccs.edu/tchriste/courses/P
HYS549/549lectures/gasses.html