INFICON Cygnus Thin Film Deposition Controller - Manual (Page 200)

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Cygnus Operating Manual
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.
9.1.1 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
Figure
9-1 has a frequency response spectrum that is schematically shown in Figure 9-2 .
The ordinate represents the magnitude of response, or current flow of the crystal,
at the specified frequency.
Figure 9-1 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 9-3 . 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 9-1 is the result of several significant
improvements from the square crystals with fully electroded plane parallel faces
that were first used. The first improvement 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. Contouring dissipates the energy of the traveling acoustic wave
before it reaches the edge of the crystal. Energy is not reflected back to the center
where it can interfere with other newly launched waves, essentially making a small
crystal appear to behave as though it is infinite in extent. With the crystal’s
vibrations restricted to the center, it is practical to clamp the outer edges of the