General description
Water Cooled Crystal Holder
The Water Cooled Crystal Holder is used in conjunction with a crystal, an Oscillator Unit and a
film thickness monitor {such as the Edwards FrM5) as shown in Figure 2. You can mount the
Water Cooled Crystal Holder in a vacuum chamber of a coating system to monitor the rate and
thickness of deposition from a deposition source.
The crystal is held in a water cooled crystal head to shield it from radiated heat and to remove
heat gained from the deposited layer. The crystal is fully shrouded and you may use it in systems
using thermal, electron beam or sputter deposition techniques without modification. An
aperture in the crystal head defines the area of crystal exposed to the deposited layer.
The cooling-water is delivered to the crystal head by coiled copper tubes. The copper tubes
support the crystal head and are semi-flexible to allow you to position the crystal head in position
for operation. The tubes are long enough to be used in a large chamber and flexible enough to
be bent into a complete loop in a small chamber if required. Because the internal diameter of
the coiled copper tubes is small, they must be connected in parallel with the main cooling-water
supply for the coating system. A filter is included in the water supply tube to prevent the crystal
head from becoming clogged with water-borne debris.
Water and electrical connections to the vacuum side of the Water Cooled Crystal Holder are
made through a single vacuum lead through which will fit a standard Edwards baseplate
lead through hole or other baseplate leadthrough holes.
The electrical connection from the Oscillator Unit to the crystal is made through coaxial cable
and a BNC plug. On the vacuum side of the leadthrough, coaxial cable is used to connect the
lead through to the crystal head. The crystal has two connections, 'live' and 'earth'. The centre
conductor of the coaxial cable is live and the sheath is earthed. The side of the crystal which
receives the deposited is earthed.
Oscillator Unit
The Oscillator Unit is a sealed device, used with the Water Cooled Crystal Holder and a film
thickness monitor such as the FrMS. The unit contains an oscillator circuit, the oscillating
frequency of which is controlled by the natural frequency of the crystal. As deposited layer forms
on the crystal, the natural oscillating frequency decreases, together with the output frequency
of the oscillator. The change in frequency is detected by the thickness monitor, which converts
the frequency change into deposition rate and thickness information.
2
Water Cooled Crystal Holder
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