The monitor functions on the basis that a built in quartz crystal oscillator
has the characteristic of changing its frequency of oscillation as its mass
changes. As one evaporates either a metal or carbon onto the quartz crystal,
its mass goes up and the frequency of oscillation goes down. This change is
more or less linear and it can be easily calibrated in terms of frequency vs.
deposition thickness.
We provide the unit with a 6" (152 mm) length of high temperature
wire. It is important that the crystal be as close as possible, electrically
, to the external oscillator, to minimize capacitance effects. A longer wire
may result in a unit that does not function properly. We use only the very
highest quality, hermetically sealed BNC feed throughs that money can buy
today.
Originally developed to enhance the possibility for obtaining reproducible
coating thickness, a Quartz Crystal thickness monitor works best with metals
and other samples not prone to off gas surface absorbed (or even absorbed)
moisture. So long as moisture is off-gassing, no metal will deposit onto the
sample, yet during this time, the "crystal" of the monitor is being coated.
Hence in that kind of a situation, the thickness that deposits on the crystal
is not necessarily the same as what is being deposited on the sample.
For moisture sensitive samples, use of the "test" modes in both the SPI
Module Sputter and Carbon Coating systems will produce reproducibility in
coating thickness at least as good if not better than the best quartz crystal
monitor in those circumstances.
However, for semiconductor, thick film, solid metal samples, the SPI Quartz
Crystal Monitor would be an excellent tool to increase the reproducibility of
coating thickness even more. The system is bakeable to at least
105° C, and there is no real reason one could not go higher.
Expected Precision:
The SPI Quartz Crystal Thickness Monitor (QCTM) displays a measurement of film thickness
to a precision of 1 nm. Its resolution can be no better than this. The practical
limitations on resolution are very basic, because what is being measured is the change
in mass of the crystal and displaying the information as the change in thickness of the
film, and in thin films, one does not actually "know" the density of the film; this
assumption, more than anything else, means that the QCTM gives an approximate measurement
and that it is more suitable for assuring that a particular thickness is repeated between
runs, rather than for actual measurement of the film thickness. Most of these uncertainties
can be resolved by diamond knife thin sectioning of actual films on a thin plastic substrate
in order to calibrate the measurements, but the overall uncertainty of the measurement is
several tenths of a nanometer; we have not done rigorous calculations of the uncertainty.
Updates of the data:
There is a problem with updates in that it slows down the display enough that it will show
a particular number long enough to be read; the flickering of the last digit of the display
can be to some users, annoying. The display information is still updated several times per
second even though it might not be reflected in what one sees when viewing the display.
Density and thickness settings:
Density can be set for any value between 0.1 and 99.9 g/cm3.
There is no setting for acoustic impedance. The monitor displays the range 1 to 999 nm
of thickness.
Quartz crystal thickness monitor requires quartz crystals which are a
consumable item used during the operation of the monitor. SPI offers the
highest quality quartz crystals for this application.
Dimensions:
Crystals are 1/2" (12.5 mm) round and with a thickness of 12 mils (305µm/0.
305 mm) thick at the center. The system is shipped with one quartz
crystal, that is, the one that was used for the final testing before
shipment. Additional quartz crystals can be purchased as SPI #11447-AD
(Pack of 3) or #11447-BA (Pack of 10).
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