INFICON MDC-260 Thin Film Deposition Controller - Manual (Page 129)

MDC-260 DEPOSITION CONTROLLER
THEORY OF OPERATION 10-5
The temperature induced drift is reversible, which means that, even if the
temperature of the crystal is allowed to increase drastically during deposition, as
long as the temperature of the crystal is allowed to return to its pre-deposition
temperature the resulting indicated thickness will contain no error due to crystal
temperature.
There is, however, an upper limit above which the temperature of the crystal
should not be allowed to climb. Quartz has a Curie temperature of about 573° C
(1063° F). At the Curie temperature the crystallographic structure changes and
the quartz loses its piezoelectric nature. The process is irreversible and the
structure which supports piezoelectric behavior does not return when the
temperature is brought back below the Curie temperature. The crystal is thus
irreversibly damaged once the Curie temperature is reached.
The Curie temperature can be significantly lowered by stress so the maximum
crystal temperature should be kept well below the 573° C limit.
10.3.4 CRYSTAL LIFE EXPECTANCY
It is difficult to predict the useful life of a crystal since it depends on many
factors. Some of these factors are:
♦ The quality of the quartz
♦ The amount of deposited material
♦ The stress generated in the crystal due to deposited material
♦ The acoustic losses in the deposited material
♦ The design of the oscillator circuitry
Other aspects that affect the crystal life include the type of the deposited material,
spitting of source material resulting in non- uniform films, film flakes that landed
on the crystal’s active area, and of course, physical damage to the crystal such as
chipping, cracking, or peeling of the electrode, etc.
In general, a sensor crystal can be used until its frequency drops to about 50% of
its uncoated value. However, for the reasons stated above, crystal failures often
occur well before that.
The sensor crystals are considered expendable. However, a crystal may be
reused. In applications where a film is deposited, the crystal can be stripped using
a chemical etchant. Care must be taken so only the deposited material is stripped
and not the crystal electrodes. The amount of times that a crystal can be reused
greatly depends on its condition after each use or stripping. Needless to say,
careful handling and cleaning of the crystal is required to maximize its re-
usability. INFICON does not recommend reusing the crystal once it failed. It is up
to the user to weigh the cost benefit of reusing the crystal versus jeopardizing a
production run to a might-be-marginal crystal.
Noisy or erratic measurement indicates that the crystal is about to fail. It might
even be difficult to obtain a stable baseline. Visually, traces of consumption and
wear can often be seen on the crystal surface. Edges of the sensor crystal might