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).