Vacuum Evaporation

1
Vacuum Evaporation

• Lecture 8
• G.J. Mankey
• gmankey_at_mint.ua.edu

2
Monolayer Time

• The monolayer time is the time for one atomic
  layer to adsorb on the surface t 1 / (SZA).
• At 3 x 10-5 Torr, it takes about one second for a
  monolayer to adsorb on a surface assuming a
  sticking coefficient, S 1.
• At 10-9 Torr, it takes 1 hour to form a monolayer
  for S 1.
• For metals at room temperature S 1, so the
  vapor pressure should be gt10-6 Torr.

Sticking Coefficient S adsorbed / incident
Impingement rate for air Z 3 x 1020 P(Torr)
cm-2 s-1
Area of an adsorption site A 1 Å2 10-16 cm2
3
Vapor Pressure Curves

• The vapor pressures of most materials follow an
  Arrhenius equation behavior
  PVAP P0 exp(-EA/kT).
• Most metals must be heated to temperatures well
  above 1000 K to achieve an appreciable vapor
  pressure.
• For PVAP 10-4 mbar, the deposition rate is
  approximately 10 Å / sec.

4
Physical Evaporation

• A current, I, is passed through the metal boat
  to heat it.
• The heating power is I2R, where R is the
  electrical resistance of the boat (typically a
  few ohms).
• For boats made of refractory metals (W, Mo, or
  Ta) temperatures exceeding 2000º C can be
  achieved.
• Materials which alloy with the boat material
  cannot be evaporated using this method.

Substrate
Flux
Evaporant
Boat
High Current Source
5
Limitation of Physical Evaporation

• Most transition metals, TM, form eutectics with
  refractory materials.
• The vapor pressure curves show that they must be
  heated to near their melting points.
• Once a eutectic is formed, the boat melts and the
  heating current is interrupted.

6
Electron Beam Evaporator

• The e-gun produces a beam of electrons with 15
  keV kinetic energy and at a variable current of
  up to 100 mA.
• The electron beam is deflected 270º by a magnetic
  field, B.
• The heating power delivered to a small (5mm)
  spot in the evaporant is 15 kV x 100 mA 1.5 kW.
• The power is sufficient to heat most materials to
  over 1000 ºC.
• Heating power is adjusted by controlling the
  electron current.

Substrate
e-beam
Flux
Evaporant
Crucible
B
cooling
e-gun
7
Wire Evaporator
substrate

• This is a "mini" version of the electron beam
  evaporator.
• The entire assembly fits through a 2 3/4 " OD
  Flange.
• Electrons from the heated filament bombard a 2 mm
  wire that is held at a large positive bias.
• The power supply is operated in a current
  limiting mode and the heating power is P
  VbiasIemission.

filament
cooling shroud
1-2 kV
0-12V
8
Wire Basket

• Direct or alternating current is passed through a
  pre-fabricated helical wire container.
• Evaporant placed in the helix is heated by
  contact and irradiation.
• Heating power is of the order of 100 W or more
  with a refractory helix with 0.1 - 0.5 mm
  diameter wire.
• Works for Ag, Au, Cu, Cr, Mn, etc.

cooling shroud
1-20 V
9
Knudsen Cell

• The crucible is heated by a coil or heater
  surrounding it.
• Crucibles are usually made of boron nitride,
  alumina, or graphite.
• Since there is a large amount of heat, the device
  is constructed of low outgassing materials and a
  large amount of cooling is necessary.

cooling shroud
1-20 V
10
Measuring and Calibrating Flux

• Many fundamental physical properties are
  sensitive to film thickness.
• In situ probes which are implemented in the
  vacuum system include a quartz crystal
  microbalance, BA gauge, quadrupole mass
  spectrometer, Auger / XPS, and RHEED.
• Ex situ probes which measure film thickness
  outside the vacuum system include the stylus
  profilometer, spectroscopic ellipsometer, and
  x-ray diffractometer.
• Measuring film thickness with sub-angstrom
  precision is possible.

11
Quartz Crystal Microbalance

• The microbalance measures a shift in resonant
  frequency of a vibrating quartz crystal with a
  precision of 1 part in 106.
• fr 1/2p sqrt(k/m) f0(1-Dm/2m).
• For a 6 MHz crystal disk, 1 cm in diameter this
  corresponds to a change in mass of several
  nanograms.
• d m / (rA), so for a typical metal d 10 ng /
  (10 g/cm31 cm2) 0.1 Angstroms.