Physical Vapor Deposition
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Transcript Physical Vapor Deposition
Physical Vapor
Deposition
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
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
Evaporation
http://www.ee.byu.edu/cleanroom/metal.parts/vaporpressure.jpg
Mean Free Path
~ 63% of molecules undergo a collision in a
distance less than l and ~0.6% travel more
than 5 l.
1
l
2d o2 n
PV nRT (IdealGas Law)
0.05
l
P(in torr)
– where do is the diameter of the evaporatant and n is
the concentration of gas molecules in the chamber
Evaporation
The vacuum is usually < 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
Thermal Evaporation
http://www.lesker.com/newweb
/Deposition_Sources/ThermalEv
aporationSources_Resistive.cfm
E-beam Evaporation
http://www.fen.bilkent.edu.tr/~aykutlu/msn551/evaporation.pdf
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
PVD
PVD
For a source that emits only upwards, = 2
– The number of atoms that hit the area Ak of the surface
is
Revap
Fk
cos k
2
r
– The deposition velocity is the above expression divided
by the density (N) of the material
v
Revap
Nr
2
cos k
Evaporation
1/ 2
m
Revap 5.8310 AS Pe
T
2
–
–
–
–
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
PVD
PVD
The velocity can be normalized to the
velocity at the center of the wafer
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
v
Revap
Nr
cos
cos
k
i
2
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
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
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
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.
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
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.
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
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.
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
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
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.
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
Salicide
http://www.research.ibm.com/journal/rd/444/jordansweet.html