Molecules
move independently of one another
SOME OF THE UNITS OF PRESSURE
1 atmospheric pressure=760 m.m. of Hg.=760
Torr
1 atmospheric pressure=
1013 mbar
1 mbar = 9.8 x 10-4 atm.
VACUUM MEASUREMENTS
760-25 Torr - Low vacuum
25- 10-3
Torr - Medium Vacuum
10-3-10-6
Torr - High Vacuum
10-6-10-9
Torr - Very High Vacuum
Less than 10-9
Torr - Ultra High Vacuum
VACUUM SYSTEMS AND COMPONENTS
The performance of a vacuum system with
a chamber or bell jar is the single most important consideration for vacuum
deposition techniques. This
consideration arises from the fact that the ultimate vacuum and residual gases
and their partial pressures may influence the structure and properties of a
film depending on the material. It is
necessary to employ as good a vacuum condition as possible. However several factors like cost and
pumping time limit the generally employed vacuum in the range 10-5
to 10-8 Torr.
A VACUUM DEPOSITION SYSTEM
A vacuum evaporation
system/apparatus can be divided into three main parts.
i)
Pumping
system
ii)
Coating
chamber apparatus and
iii)
The
electrical services
The pumping
arrangement of a vacuum evaporation system suitable for routine film production
(similar to the one in our laboratory) consists of a water-cooled oil diffusion
pump backed by a rotary mechanical oil pump.
The arrangement
of the coating chamber cannot be generalized since it depends on the nature of
the coating application in hand.
VACUUM PUMPS
Oil sealed Rotary Pump
An eccentrically positioned rotor is fitted
tightly inside a stator. Two spring
loaded vanes sliding diametrically opposite slots in the rotor press against
the inner surface of the stator.
Friction and wear and tear are minimized by a thin oil film, which
lubricates all parts of the pump and also seals the minute gap at the
seat. When turned on the rotor draws
air from the inlet side into the pump.
The volume is then compressed and forcing the outlet valve to open and
permitting the gas to be discharged.
Fluids in rotary pump are either mineral oils or diphenyl ethers.
Used for
obtaining “rough” vacuum (10-3 Torr)- which is the lower limit of
the viscous flow regime.
Safety considerations
Trap vapors
entering pump to prevent degradation of pump oil and seals
Do not pump on
atmosphere. The pump will over
heat. It is not designed to be an air
compressor.
Types of Oil
used in these pumps:
Molecular Distilled Oil
Properties: Lubricants, doesn’t mix with water, highly
resistant to corrosive gases.
Help: Better yield; better pump life and good recovery.
Water-cooled Diffusion Pump
A
boiler heats the work fluid, and its hot vapor rises in a chimney. The direction of flow is reversed at the jet
cap so that the vapor issues through the nozzle pointing away from the high
vacuum side. Gas molecules from the
high vacuum side diffuse through the throat and directed towards the fore
vacuum side by colliding with the molecules of the working fluid. Thus a zone of reduced gas pressure is
generated in the vicinity of the nozzle and more gases diffuse from the high
vacuum side towards this region.
Consequently this creates a region of increased pressure in the lower
part of the pump from which the accumulated gas load must be removed by backing
pump.
This pump is used for obtaining
high vacuum.
The working range is 10-2
to 10-6 Torr. The pump works
in the molecular flow regime.
Diffusion pump oils:
Silicon fluids, Poly Phenyl ether fluids
Properties:
Not oxidized by air at working temperatures, not hydrolyzed by water vapors
Safety considerations:
Rotary oil pump must be on to
reduce the pressure to 10-2 Torr – otherwise hot oil could combust
in the presence of oxygen.
Diffusion pump must be cooled
with water to prevent overheating.
Vacuum line must be protected
with a trap to prevent migration of diffusion pump oil.
OTHER PUMPS
Turbo Molecular Pump:
Ultra-fast fan blades knock molecules out of vacuum system.
Cryo Pump: Molecules are frozen
out.
Sorption Pump: Molecules
diffuse into absorbing material.
Sputter Ion Pump: Molecules are
ionized and buried.
FIGURE-1
SCHEMATIC OF A TYPICAL WATER COOLED DEPOSITION
SYSTEM (USED IN THE LABORATORY)
FAMILIARIZATION OF
THE SWITCHES/CONTROLS/VALVES – USED IN THE DEPOSITION OF OUR LABORATORY
VACUUM
GAUGES
PIRANI GAUGE
A resistive wire enclosed
in glass or metal envelope is connected to the system where the pressure is to
be measured while being part of a wheatstone’s bridge. An identical wire in a similar but
thoroughly evacuated sealed enclosure serves as the reference. Both the wires are heated by a voltage
source.
At high pressures, the temperature of the sensing wire
decreases s the thermal conductivity of the gas increases. Consequently the resistance of the wire
decreases and the current through the unbalanced bridge indicates the change in
pressure.
The applicable range is 10-3 to 10-1
Torr
PENNING GAUGE
Practically
all pressure measurements below 10-3 are based on the phenomenon of residual
gas ionization. To induce ionization,
electrons with energies greater than the ionizing potential of the gas, usually
above 50eV are injected. If they
collide with the gas particles the latter becomes ionized. Electron energies of about 150eV, which
yield the highest ionization rate are employed in most ionization gauges.
Generally
in ionization gauges, ionization is obtained by a glow discharge and the
positive gas ions generated by the electron collisions are accelerated toward a
collector and the resulting current is taken as an indication of the particle
density in the gauge.
VACUUM VALVES (WE WILL COME ACROSS IN THE DEPOSITION
SYSTEM OF THE LABORATORY)
VALVES
TWO WAY VALVE (V1)
When the
diffusion pump is kept continuously at working temperature it is necessary to
reduce the chamber to a fraction of m.m. of mercury before opening the high
vacuum valve. Otherwise the diffusion
pump fluid would be blown as vapor into the backing space.
The coating chamber can be “roughed” or pre-exhausted by
means of the rotary mechanical pump, which is also used for backing
purposes. The rotary pump must be
connected via a two-way valve to the coating chamber and to the backing side of
the diffusion pump.
HIGH VACUUM /BAFFLE VALVE (V2)
Care
must be taken to see that the diffusion pump is adequately baffled against
direct entry of oil vapor molecules into the vacuum chamber. The diffusion pump is fitted with a combined
baffle and a high vacuum valve. When
the sealing plate of the valve is raised from it’s seating, the back streaming
oil molecules must either collide with the sealing plate or be condensed on the
water-cooled valve shell. The cooling
water lead into the baffle valve prior to entry into the diffusion pump so as
to ensure that the valve is the coldest region between the coating chamber and
the diffusion pump.
CHAMBER AIR ADMITTANCE VALVE (V3)
We cannot evacuate the chamber with this valve in “open”
condition.
Once the chamber is evacuated access to the interior of
the chamber is possible only when this valve is “opened”.
BASIC REQUIREMENTS OF A HIGH VACUUM SYSTEM
- High
pumping speeds- to ensure fast removal of gas liberated by the vapor
source and collector during coating.
- Minimum
back-streaming into the coating chamber of vapors from diffusion pump and
rotary pump working fluids, which would contaminate the substrate.
- De-mountable
systems for easy maintenance.
CONSTRUCTION
AND USE OF VAPOR SOURCES
WIRE/FILAMENT SOURCES
Materials of construction are the refractory metals,
which have high melting points and low vapor pressure. Most commonly used are Tungsten, Molybdenum
or Tantalum.
The
sources shown in figure A, B are commonly made from 0.02 to 0.06 inches
diameter tungsten wire. Their utility
is limited to evaporants, which can be affixed to the source typically in the
form of wire. Upon melting, the
evaporant must wet the filament and be held by its surface tension. From the symmetry of these arrangements it
follows that evaporation should take place uniformly in all directions.
Hairpin and Helical tungsten filaments Tungsten
filament basket
Wire baskets as
shown in figure C are used to evaporate pellets or chips of metals, which do
not wet the wire material upon heating.
If wetting occurs, the turns of the basket are shorted and the
temperature of the source drops.
Metal foils as shown in figure D & E have capacities
of grams and are the most universal types of sources for small evaporant
quantities. They are fabricated from
sheets of tungsten, molybdenum or tantalum.
The dimdpled sources have reduced widths in the center to concentrate
the heating in the area of the evaporant.
The recessed circles are only 1/8 or 1/4 in deep since refractory metals
are not easily drawn to greater depths.
Canoe or boaat sources may be made in the laboratory by bending metal
sheets into the desired shape. This is
readily done with tantalum and not too difficult with Molybdenum. Tungsten however is very brittle and breaks
if it is bent at room temperature.
Dimpled foil boats (Molybdenum/tantalum)
Electrical connections to wire and foil sources are made
by attaching their ends to heavy stainless-steel or copper clamps and are
connected to a pair of electrical feedthroughs. Since the electrical resistance of wire and foil source is small,
low voltage power supplies at 1 to 3 kW are required. Typical arrangements consist of a step down transformer (5 to 20 Volts) whose primary side is
connected to a variable transformer.
This is necessary to raise the operating voltage as the resistance of
the refractory metals increase strongly with temperature.
CLEANING OF SUBSTRATES
When an insulating material such as glass is polished, the
surface develops an electromagnetic charge, which firmly holds minute particles
on the cleaned surface. These particles
are subsequently responsible for pinholes in the films. To obtain the most durable and adherent
coatings on glass the surface must be free from contaminant films such as
grease, adsorbed water etc.
Glass surfaces contaminated with grease layers can be
readily cleaned by immersion in water to which a detergent has been added. Adherent contaminating layers can be removed
by rubbing with cotton wool impregnated with the detergent. A lukewarm, ultrasonically agitated ionic
detergent removes gross contaminants.
The glass is then rinsed thoroughly several times in water (de-ionized)
and later subjected to vapor degreaser using pure alcohol. Cleaned glasses can be stored immersed in
pure alcohol and occasionally agitated ultrasonically. Before use , the glass is dried by blowing
with dry nitrogen.
The most important aspect of chemical cleaning is to
obtain a uniform surface finish, i.e. complete freedom from oily streaks and
watermarks etc.
THICKNESS MEASUREMENTS
Thickness is the single most significant film
parameter. It may be measured either by
in-situ monitoring of the rate of deposition or after the film is taken out of
the deposition chamber.
Electrical methods
This is applicable to metallic and low resistivity
semiconductor films. This technique is
based on the fact that the resistance is related to the film thickness and the
number of charge carriers. The
situation is complex in ultrathin, structurally discontinuous films so that no
reliance can be put on the resistance method in this region. The resistivity of semi-conductor films is
very sensitive to deposition conditions and as a consequence the resistance
method is applicable only for comparison of film thickness rather than for
absolute measurements.
Mechanical Method (Stylus)
Variations in the movements of a
mechanical stylus can be amplified electronically so that step heights and
surface irregularities of approximately 10A0 can be measured.
The stylus in this case consists of
a diamond with a rounded (approximately 0.7 to 1.3 micron diameter) or
four-sided pyramid tip fastened to a lever arm. The arm is delicately balanced so that the load on the stylus is
very small. The vertical movement of
the stylus is detected with a transducer, amplified to about 100 times and then
fed to a recorder.
Under
the pressure of the stylus, the films of soft materials deform
considerably. But a suitable choice of
the tip can surmount this difficulty.
Optical Interference Methods
If
optical constants are known, the thickness can be calculated. Among these the photometric,
Spectrophotometric method and Interference fringes technique methods are based on
the optical Interference phenomenon.
Some of
these techniques can be used for in-situ monitoring and controlling of the
deposition of films and some are suited for scanning the film and thus
determining the thickness and surface roughness. The methods to be used depend on the type of deposit, deposition
technique and particular use of the film.
Quartz crystal thickness monitors (used in our laboratory’s deposition
system)
Microprocessor controlled digital thickness
monitor uses a quartz crystal as the basic transducing element. The crystal is excited into mechanical
motion by an external oscillator. The
unloaded crystal vibrates in the thickness shear mode at a frequency of
approximately 6 megahertz. The
frequency at which the quartz crystal oscillates is lowered by the addition of
material to its surface. The film
thickness is then calculated from the change in crystal frequency due to the
additional mass onto its surface.
Deposition Parameters
- Tooling
Factor
The tooling factor parameter compensates
for geometric factors in the deposition system which result ina difference
between the deposition rate on the substrates and the rate on the sensing
crystal. This parameter is entered in
percent units and 100% corresponds to equal rates at the substrate and at the
sensing crystal.
- Density
The density parameter provides
the monitor with the density of the material being deposited so that it can
calculate and display the physical film thickness.
- Acoustic
Impedance
The shear wave acoustic impedance
of the deposited film is required by the monitor in order to accurately
establish the sensor scale factor when the sensor crystal is heavily loaded.
Reference: Detailed manual is
available in the laboratory for more information on this thickness monitor.
REFERENCES
THINFILM PHENOMENA, K L CHOPRA
VACUUM DEPOSITION OF THIN FILMS, L HOLLAND
HAND BOOK OF THIN FILM TECHNOLOGY, L MAISSEL AND R GLANG
