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AMRITA SCHOOL OF ENGINEERING, COIMBATORE
SPUTTERING is a widely used and highly versatile vacuum coating
system used for the deposition of a variety of coating materials.
A plasma at higher pressure is used to “knock "metal atoms out of a
“target”. These energetic atoms deposit on a wafer located near the
target. The higher pressure produces better step coverage due to more
random angled delivery. The excess energy of the ions also aids in
increasing the surface mobility (movement of atoms on the surface)
On the left side, sputter off an Al atom.
On the right side, generate secondary electrons,
which are accelerated across the sheath region
and 1) ionize/excite an Ar; or 2) ionize an
impurity atom, here O, to generate O- (for Ar,
always positive ion Ar+). This O- is accelerated
toward substrate and may go into the film
After collision ionization, there
are now TWO free electrons.
This doubles the available
electrons for ionization.
This ongoing doubling process is
called "impact ionization”, which
sustains a plasma.
Energy of each incoming ion is 500-1000eV. Energy of sputtered atoms is
Thus, the sputtering process is very inefficient from the energy point of
view, 95% of incoming energy goes to target heating & secondary
High rate sputter processes need efficient cooling techniques to avoid
target damage from overheating (serious problem).
The sputtered species, in general, are predominantly neutral.
The energy of the ejected atoms shows a Max-wellian distribution with a
long tail toward higher energies.
The energies of the atoms or molecules sputtered at a given rate are about
one order of magnitude higher than those thermally evaporated at the
same rate, which often lead to better film quality.
However, since sputtering yields are low and the ion currents are limited,
sputter-deposition rates are invariably one to two orders of magnitude
lower compared to thermal evaporation rates under normal conditions.
The fundamental mechanism of sputtering is the collision
interaction between the impinging ions and the lattice
atoms of the target. The ion dissipates it's energy in
collision cascades. The ejection depth of the target atoms
is approx. 1 nm.
For quantitative statements one has to apply the
formalism of transport theory to the mechanism of a
At ion energies Ei < 1keV the Born-Mayer potential may
At Ei > 1 keV the Thomas-Fermi potential is valid.
CHARACTERISTICS OF SPUTTERING
Emissions from cathode include neutral atoms, ions (both positive
and negative), electrons, neutral clusters of atoms and charged
clusters of atoms. Of these, the vast majority are neutral atoms.
These atoms have kinetic energies approximately 50 to 100 times
that of neutral atoms generated from thermal evaporation sources.
This additional energy is thought to be the reason for the greater
adhesion often observed for sputter deposited films over thermally
Due to the relatively high pressure in an operating sputter deposition
chamber, the mean-free path of sputtered species is short. The
numerous gas-phase collisions which the sputtered material suffers
between the target and substrate tend to reduce the amount of kinetic
energy the depositing species have upon arrival. the density and
crystal structure of the thin film are affected.
Elastic energy transfer
E2 is greatest for M1=M2.
There is also inelastic energy transfer, which
leads to secondary electrons emission…
• Sputter yield Y: the number of sputtered atoms per impinging ion.
• Obviously, the higher yield, the higher sputter deposition rate.
• Sputter yield is 1-3: not too much difference for different materials.
• The sputter yield depends on: (a) the energy of the incident ions; (b) the masses of the
ions and target atoms; (c) the binding energy of atoms in the solid; and (d) the incident
angle of ions.
• The yield is rather insensitive to the target temperature except at very high temperatures
where it show an apparent rapid increase due to the accompanying thermal evaporation.
Magnetron sputtering is the most
commonly used method for a sputter
. It usually utilizes a strong electric and
magnetic fields to trap electrons close to the
surface of the magnetron, which is known as
The electrons follow helical paths around
the magnetic field lines undergoing more
ionizing collisions with gaseous neutrals
near the target surface than would otherwise
The extra argon ions created as a result of
these collisions leads to a higher deposition
rate. It also means that the plasma can be
sustained at a lower pressure.
The sputtered atoms are neutrally charged
and so are unaffected by the magnetic trap.
ION BEAM SPUTTERING
Ion beam sputtering utilizes an ion source
to generate a relatively focused ion beam
directed at the target to be sputtered.
The ion source consists of a cathode and
anode with a common central axis.
Applying a high voltage field of 2-10 kV
to the anode creates an electrostatic field
inside the ion source, confining electrons
around a saddle point in the center of the
When argon gas is injected into the ion
source, the high electric field causes the gas
to ionize, creating a plasma inside the
The ions are then accelerated from the
anode region to the exit aperture (cathode)
creating a “collimated” ion beam.
The resulting ion beam impinges upon a target material and, via momentum
transfer between the ion and the target, sputters this material onto the sample.
Able to deposit a wide variety of metals, insulators, alloys
Replication of target composition in the deposited films.
Capable of in-situ cleaning prior to film deposition by
reversing the potential on the electrodes .
Better film quality and step coverage than evaporation.
Can use large area targets for uniform thickness over large
Sufficient target material for many depositions.
No x-ray damage.
Substrate damage due to ion bombardment or UV
generated by plasma.
Higher pressures 1 –100 m torr ( < 10-5 torr in
evaporation), more contaminations unless using ultra clean
gasses and ultra clean targets.
Deposition rate of some materials quite low.
Most of the energy incident on the target becomes heat,
which must be removed.
Difficult to deposit uniformly on complex shapes such as
High performance thick coatings are hard to produce due
to high internal residual stress levels
COMPARISON OF TYPICAL THIN FILM
Aerospace & Defense:
Heads-up cockpit displays
Jet turbine engines
Mirrors for optical and x-ray telescopes
Night vision equipment.
Dies and molds
Tool and drill bit hardening.
Auto headlights and tail-lights
Auto trim components
Drive train bearings and components
Wheels and rims
Optical filters for achromatic lenses