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Fil-Tech, Inc.
Technical Bulletin No. 4
Crystal Sensor Head Design and Failure
Modes
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In order to use a quartz crystal in the
coating chamber, a device had to be developed that
could be electrically activated to vibrate the crystals in vacuum, allow only a portion of the crystal
to be coated with evaporated material, and permit the crystal to be changed easily. Furthermore,
the device would have to shield the crystal from the high temperatures inherent in the coating
processes. Thus, the “sensor head” was created.
In currently available sensor head designs,
the crystal rests in the “crystal holder” a
metal cap with a hole drilled through the center of the cap. The holder serves as one half of the
electrical circuit that causes the crystal to vibrate. This hole allows the evaporating film to
coat a
circular region of approximately 0.3" diameter (7.6 mm) on the crystal face. The crystal is held
in
place in the holder by a spring. In some models, the spring is located around the inside of the
holder and presses against the crystal edge. In other designs, a “ceramic retainer" with a
gold-coated spring in the center presses against the outer back edge of the crystal.
In both designs, the crystal holder is
inserted into the sensor head (a metal block), where
a second gold coated spring presses against the back of the crystal or retainer to complete the
crystal electrical circuit. The head often contains another spring for holding the crystal holder in
place and providing electrical contact between the holder and the head.
On the outside of the sensor head is
a screw-on miniature connector, a “Microdot
Connector”. At this point, a thin microcoaxial (two wires in one body) cable connects the
sensor
head to the feedthrough, interfacing the inside of the chamber and atmosphere. The feedthrough
then connects in atmosphere to the oscillator and on to the thin film monitor.
The sensor head also has two small
metal tubes, “water lines” extending from one ento
to provide water cooling to the crystal. The water lines feed either an internal passageway drilled
through the sensor head or simply connect to a tubing bend that rests in the back of the head.
Both water lines connect to the feedthrough.
When new, a sensor head allows easy
insertion of the crystal snug mounting of the
crystal holder, uninterrupted electrical connection to the feedthrough and unimpeded water
cooling. After repeated use, however, this system can break down and lead to erratic or failed
crystal readings. Several of the most common modes of failure are:
- The contact springs in the crystal holder
or sensor head break or bend sufficiently,
interrupting the electrical circuit;
- The wires inside the sensor head connecting
to the microcoaxial cable fitting break,
interrupting the electrical circuit;
- The microcoaxial cable connecting the
sensor head to the feedthrough cracks or loosens,
interrupting the electrical circuit; and ,
- The water lines clog due to high mineral
content of the water, causing the sensor head to
overheat.
Tel 800-743-1743 FIL-TECH INC. 617-742-0686
Fax
COPYRIGHT FIL-TECH INC. 1998
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