INFICON STM-2 USB Thin Film Rate/Thickness Monitor - Manual (Page 92)

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STM-2 Operating Manual
A crystal with a starting frequency of 6.0 MHz will display a reduction of its
frequency by 2.27 Hz when 1 angstrom of Aluminum (density of 2.77 g/cm
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is added to its surface. In this manner, the thickness of a rigid adlayer is inferred
from the precise measurement of the frequency shift of the crystal.
The quantitative knowledge of this effect provides a means of determining how
much material is being deposited on a substrate in a vacuum system, a
measurement that was not convenient or practical prior to this understanding.
8.1.1 Monitor Crystals
No matter how sophisticated the electronics surrounding it, the essential device of
the deposition monitor is the quartz crystal. The quartz crystal shown in
Figure 8-1
has a frequency response spectrum that is schematically shown in Figure 8-2 .
The ordinate represents the magnitude of response, or current flow of the crystal,
at the specified frequency.
Figure 8-1 Quartz resonator
The lowest frequency response is primarily a “thickness shear” mode that is called
the fundamental. The characteristic movement of the thickness shear mode is for
displacement to take place parallel to the major monitor crystal faces. In other
words, the faces are displacement antinodes as shown in
Figure 8-3 on page 8-4 .
The responses located slightly higher in frequency are called anharmonics; they
are a combination of the thickness shear and thickness twist modes. The response
at about three times the frequency of the fundamental is called the third
quasiharmonic. There are also a series of anharmonics slightly higher in
frequency associated with the quasiharmonic.
The monitor crystal design depicted in Figure 8-1 is the result of several significant
improvements from the square crystals, with fully electroded plane parallel faces,
that were first used.
The first improvement implemented the use of circular crystals. The resulting
increase in symmetry greatly reduced the number of allowed vibrational modes.
The second set of improvements was to contour one face of the crystal and to
reduce the size of the exciting electrode. These improvements have the effect of
trapping the acoustic energy.
1.4 cm (0.55 in.)
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