Thin-film thickness monitor
Encyclopedia
Thin-film thickness monitors, deposition rate controllers, and so on, are a family of instruments used in high and ultra-high vacuum
systems. They can measure the thickness of a thin film
, 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
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
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
, 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.
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
systems. They can measure the thickness of a thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
, 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
Quartz crystal microbalance
A quartz crystal microbalance measures a mass per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator...
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
Thin-film optics
Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light...
.
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
Fizeau interferometer
A Fizeau interferometer is similar to a Fabry–Pérot interferometer in that they both consist of two reflecting surfaces. In a Fizeau interferometer, however, the two surfaces are usually much less than totally reflecting , so that secondary reflections don't contribute greatly to the fringe...
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
Profilometer
Profilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness.While the historical notion of a profilometer was a device similar to a phonograph that measures a surface as the surface is moved relative to the contact profilometer's stylus, this...
, ellipsometry
Ellipsometry
Ellipsometry is an optical technique for the investigation of the dielectric properties of thin films....
, dual polarisation interferometry
Dual Polarisation Interferometry
Dual polarization interferometry is an analytical technique that can probe molecular scale layers adsorbed to the surface of a waveguide by using the evanescent wave of a laser beam confined to the waveguide...
and scanning electron microscopy
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
of cross-sections of the sample. Many thickness monitors and controllers allow tooling factors to be entered into the device before deposition begins.