Vacuum Depositions
Some Vacuum History and Technology
might give one a nice overview before beginning to study Vacuum Depositions..
Before performing a vacuum deposition it is important to understand the instruments
you are working with.
Pumps
Mechanical Pumps
Mechanical pumps allow gas to enter a pump cavity, and then seal the cavity
and compress the gas. The compressed gas is expelled into the atmosphere,
and then the cycle repeats with more gas entering the cavity. Mechanical
pumps can get to pressures of 10-2 to 10-3 Torr.
A good diagram of a mechanical pump is available
here
.
Diffusion Pumps
Before using a diffusion pump it is usually necessary to pre-evacuate your
vessel with a mechanical pump, referred to as your "roughing pump."
Then the diffusion pump allows hot oil vapors to push gas molecules to the
bottom of the pump, creating an increase in pressure in the lower pump.
The gas in the lower pump is removed with another mechanical pump, usually
referred to as the "backing pump." Diffusion pumps can get to pressures
of 10-4 to 10-7 Torr.
For a short list of more pump types,
click here
.
Gauges
Thermocouple Gauge
A thermocouple consists of two different adjacent metals. When a current
is sent through the metals, the voltage is linearly dependent on temperature,
and the temperature is inversely dependent upon the pressure, because at
higher pressures there are more gas molecules nearby to exchange energy to,
thus cooling the couple. Therefore, by measuring the voltage across
the thermocouple, we can measure the pressure of the system. Thermocouple
gauges can read pressures from .5 to 500 mTorr. For more information
see here
.
Ionization Gauge
A hot filament ionizes the gas nearby and produces positive ions. The
number of ions produces is proportional to the pressure of the gas.
Therefore, by collecting the positive ions at a negative electrode and measuring
the current at the electrode, we can measure pressures from 10-3
to 10-11 Torr.
For more information on pressure gauges,
click here.
Pump Behavior
Pump Behavior is dependent upon what type of flow is occurring.
Viscous Flow
In viscous flow the movement of gasses is governed by the Poiseuille law,
which states that the rate of flow is proportional to the area of tubing and
pressure difference, and is inversely proportional to the length of the tubing
and temperature. This type of flow occurs at higher pressures, when
gas is moved due to collisions with other gas molecules.
Molecular Flow
In molecular flow gas molecules are moved more due to collisions with the
sides of the tubing. Here the rate of flow is proportional to the pressure
difference and length but inversely proportional to the pressure and radius
cubed. Molecular flow is dominant at lower pressures. Programs
have been developed to emulate this type of flow, for an example,
click here.
Transitional Flow
As there is not a point where viscous flow ends and molecular flow begins,
transitional flow refers to when there are elements of both viscous and molecular
flow occurring in a sample.
See here
for some related Vacuum theory
Deposition
A vacuum deposition is a way to deposit a thin film onto a glass slide.
Depositions of the correct fraction of a wavelength of appropriate materials
result in
dielectric/non reflectance
or
reflective
coatings.
Depositions are made on class slides in a vacuum when a boat of substance
is evaporated onto the slides. Since they are in a vacuum the vaporized
particles will on average not hit other particles in the air on the way to
the gas, as they would in a higher pressure system.
See
here
for an example of how to perform a deposition with a diffusion pump.
Depositions can be professionally ordered from companies such as
Allied Coatings
, for use in specific experimental applications.
For more information, see the Society of Vacuum
Coaters (SVC)
, or American Vacuum Society
.
For information on vacuum equipment, try the
Association of Vacuum Equipment Manufacturers International (AVEM).