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10 | Measurement Theory INFICON
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10.2 Monitor Crystals
No matter how sophisticated the electronics surrounding it, the essential device of the
deposition monitor is the quartz crystal.
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.
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 above 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