Thin Film Processing

1
Thin Film Processing

• Gary Mankey
• MINT Center and Department of Physics
• http//bama.ua.edu/gmankey/
• gmankey_at_mint.ua.edu

2
Vacuum

• A vacuum is defined as less than 1 Atmosphere of
  pressure.
• 1 Atm 105 Pa 103 mbar 760 Torr
• Below 10-3 Torr, there are more gas molecules on
  the surface of the vessel then in the volume of
  the vessel.
• High Vacuum lt 10-3 Torr
• Very High Vacuum lt 10-6 Torr
• Ultra High Vacuum lt 10-8 Torr

Vacuum
760 mm Hg
ATM
3
Why do we need a vacuum?

• Keep surfaces free of contaminants.
• Process films with low density of impurities.
• Maintain plasma discharge for sputtering sources.
• Large mean free path for electrons and molecules
  (l 1 m _at_ 7 x 10-5 mbar).

l
Mean free path for air at 20 ºC l 7 x 10-3 cm
/ P(mbar)
4
Vacuum Systems

• A vacuum system consists of chamber, pumps and
  gauges.
• Chambers are typically made of glass or stainless
  steel and sealed with elastomer or metal gaskets.
• Pumps include mechanical, turbomolecular,
  diffusion, ion, sublimation and cryogenic.
• Gauges include thermocouple for 1 to 10-3 mbar
  and Bayard-Alpert for 10-3 to 10-11 mbar.

5
Alabama Deposition of Advanced Materials

• All materials are either glass, ceramics,
  stainless steel, copper and pure metals.
• A turbomolecular pump and a cryo pump create the
  vacuum.
• Sputtering sources are used for deposition.
• Characterization methods include RHEED, and Auger
  electron spectroscopy.

6
Bayard-Alpert or Ionization Gauge

• Electrons, e-, produced by the hot filament are
  accelerated through the grid acquiring sufficient
  energy to ionize neutral gas atoms, n.
• The ionized gas atoms, I, are then attracted to
  the negatively, biased collector and their
  current is measured with an electrometer.
• Typical ion gauges have a sensitivity of 1-10 Amp
  / mbar and range of 10-3-10-11 mbar.

Collector
Filament
n
e-
n
Grid
e-
n
I
n
I
n
e-
I
1 cm
e-
n
n
n
-45 V
150 V
Electrometer
6 VAC
7
Residual Gas Analysis

• A quarupole mass spectrometer analyzes the
  composition of gas in the vacuum system.
• The system must be baked at 150 - 200 ºC for 24
  hours to remove excess water vapor from the
  stainless steel walls.
• The presence of an O2 peak at M/Q 32 indicates
  an air leak.
• At UHV the gas composition is H2, CH4, H2O, CO
  and CO2.

8
Monolayer Time

• We define the monolayer time as the time for one
  atomic layer of gas to adsorb on the surface
  t 1 / (SZA).
• At 3 x 10-5 Torr, it takes about one second for a
  monolayer of gas 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 most gases at room temperature Sltlt1, so the
  monolayer time is much longer.

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
9
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.

10
Physical Evaporation

• A current, I, is passed through the 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
11
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.

12
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
B
Crucible
e-gun
13
The Sputtering Process
Electrons (e-) are localized in the plasma by a
magnetic field. The e- collide with argon gas
atoms to produce argon ions. The Ar are
accelerated in an electric field such that they
strike the target with sufficient energy to eject
target atoms. The target atoms, being
electrically neutral, pass through the plasma and
condense on the substrate.
Substrate
1 mTorr Ar
Plasma Discharge
Target
Magnets
14
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, 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.

15
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.

16
Auger / XPS
Electron Energy Analyzer

• An x-ray source produces photoelectrons or a
  electron gun produces Auger electrons.
• The electrons have kinetic energies which are
  characteristic of the material.
• The attenuation of substrate electrons by the
  film is described by Beers law
  I I0 exp(-dcosQ/L).
• Since, L 10 Å, this technique has a high
  sensitivity.

Excitation X-rays or keV Electrons
Photoelectrons Auger Electrons
17
Auger Electron Spectroscopy

• The excitation knocks a core electron out
  producing a core hole.
• To lower the energy of the ion, an electron from
  an upper shell decays nonradiatively into the
  core hole.
• The Auger electron from the upper shell acquires
  an energy equal to the energy difference of the
  core hole and upper shell.
• The kinetic energy of the electrons are measured
  to identify the chemical species of the atoms.

Kinetic Energy
Excitation
EVAC
Upper Shell
Core Hole
18
Secondary Electron Energy Distribution

• The energy distribution is characterized by
• An elastic peak at the incident electron energy.
• A low energy peak which increases as 1/E2 and
  drops off rapidly below 10 eV.
• Auger electrons, which can be measured to
  determine chemical composition.

19
Cu Auger Scan in Pulse Counting Mode

• These transitions correspond to Auger electrons
  ejected from the valence band by a neighboring
  electron filling the L shell or 2p levels.

20
Cu Auger Spectrum for Analog Mode

• In analog mode, the analyzer energy is modulated
  and a lock-in amplifier detects the derivative of
  the number of electrons or dN(e)/dE vs. E.
• The high energy peaks correspond to those on the
  previous slide.

21
The Universal Curve for L

• The Universal Curve describes the dependence of
  electron mean free path, L, on energy for most
  materials.
• In most cases, it is accurate to within a factor
  of two.

L (35 / E)2 0.5 ÖE
with L in Å and E in eV.
22
Reflection High-Energy Electron Diffraction

• 15 keV electrons reflect from the surface and are
  displayed as a spot on a phosphor screen.
• The angle is adjusted such that electrons
  reflecting from adjacent layers interfere
  destructively.
• When only one layer is exposed, the spot is
  bright.
• When the top layer covers half of the surface,
  the spot is extinguished.
• The time between two maxima in the intensity plot
  is the monolayer time.

d atomic spacing
15 keV Electron Gun
Screen
Path difference 2dsinQ (n1/2) l
l 150 / E(eV)1/2
23
Epitaxial Growth

• Epi-Taxi (greek)
• epi meaning on
• taxi meaning arrangement
• in relation to a source of
• stimulation
• The crystal structure of the
• film has a direct relationship
• to that of the substrate

Film
Substrate
24
Growth Modes for Ultrathin Films

• The growing film surface can exhibit different
  behaviors depending on substrate temperature,
  interfacial strain, and alloy miscibility.
• The growth modes must be characterized using a
  combination of chemical tools such as Auger
  electron spectroscopy and structural tools such
  as RHEED and atomic force microscopy.

Stranski-Kastranov
Layer by Layer
Diffusion Limited
Volmer-Weber
Surface Alloy
Surface Segregation