Film Thickness Monitor (FTM) quartz crystals are used to determine the thickness of material deposited and may be used to terminate coating at a pre-set thickness.
The quartz crystal is placed in the vacuum work chamber. One face of the crystal is exposed towards the deposition source, such that, as material is deposited it will coat the crystal. The system functions as an oscillator, whose output is controlled by the frequency of crystal oscillation. As material is deposited on the crystal, so its frequency is modified.
If coating without using the FTM, place a cover over the crystal to keep it clean.
A straight edge scalpel blade may be used to very gently scrape off previously deposited coatings from the centre part of the crystal. Some coatings can be removed using a cloth with a little alcohol or with some very fine polish. Only the central area needs to be clean.
It is essential to leave the gold contact on the outer edge, as this connects the crystal to the oscillator.
A faulty contact area could be replaced with a new gold coating, but if both sides of the crystal are shorted, then it cannot work .
If a crystal fails it may not be beyond use, but may require improving of the contacts. It is useful to have a new crystal on hand, if only to check that the system is operating properly.