1 monitor crystals – INFICON IQM-233 Thin Film Deposition Controller PCI-Express Card Operating Manual User Manual
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IQM-233 Operating Manual
when 1 angstrom of Aluminum (density of 2.77 g/cm
3
) is added to its surface. In
this manner the thickness of a rigid adlayer is inferred from the precise
measurement of the crystal’s frequency shift. 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.
7.1.1 Monitor Crystals
No matter how sophisticated the electronics surrounding it, the essential device of
the deposition monitor is the quartz crystal. The quartz resonator shown in
has a frequency response spectrum that is schematically shown in
The ordinate represents the magnitude of response, or current flow of the crystal,
at the specified frequency.
Figure 7-1 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
.
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 is also a series of anharmonics slightly higher in frequency
associated with the quasiharmonic.
The monitor crystal design depicted in
is the result of several significant
improvements from the square crystals with fully electroded plane parallel faces
that were first used.
The first improvement was to use circular crystals. This increased 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. Reducing the electrode diameter limits the excitation to the central area.
1.4 cm (0.55 in.)
R
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