Thin-film thickness monitors, deposition rate controllers, and so on, are a family of instruments used in high and
ultra-high vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
systems. They can measure the thickness of a
thin film
A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...
, not only after it has been made, but while it is still being
deposited, and some can control either the final thickness of the film, the rate at which it is deposited, or both. Not surprisingly, the devices which control some aspect of the process tend to be called controllers, and those that simply monitor the process tend to be called monitors.
Most such instruments use a
quartz crystal microbalance A quartz crystal microbalance (QCM) (also known as ''quartz microbalance'' (QMB), sometimes also as ''quartz crystal nanobalance'' (QCN)) measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The ...
as the sensor. Optical measurements are sometimes used; this may be especially appropriate if the film being deposited is part of a
thin film optical device.
A thickness monitor measures how much material deposited on its sensor. Most deposition processes are at least somewhat directional. The sensor and the sample generally cannot be in the same direction from the deposition source (if they were, the one closer to the source would shadow the other), and may not even be at the same distance from it. Therefore, the rate at which the material is deposited on the sensor may not equal the rate at which it is deposited on the sample. The ratio of the two rates is sometimes called the "tooling factor". For careful work, the tooling factor should be checked by measuring the amount of material deposited on some samples after the fact and comparing it to what the thickness monitor measured.
Fizeau interferometer
A Fizeau interferometerLawson, Peter R. "Principles of Long Baseline Stellar Interferometry." Course notes from the 1999 Michelson Summer School, held August 15–19, 1999. Edited by Peter R. Lawson. Published by National Aeronautics and Space Admi ...
s are often used to do this. Many other techniques might be used, depending on the thickness and characteristics of the thin film, including
surface profilers,
ellipsometry
Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it t ...
,
dual polarisation interferometry and
scanning electron microscopy of cross-sections of the sample. Many thickness monitors and controllers allow tooling factors to be entered into the device before deposition begins.
The correct tooling factor can be calculated as follows:
where F
i is the initial tooling factor, T
i is the film thickness indicated by the instrument, and T
m is the actual, independently measured thickness of the deposited film. If no tooling factor has been preset or used before, F
i equals 1.
References
* {{ISBN, 0-12-524975-6
Measuring instruments
Vacuum systems