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Fil-Tech, Inc.
Technical Bulletin No. 1
What Exactly is a Quartz Crystal Sensor?
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Fil-Tech is a worldwide supplier of replacment quartz crystals
for most popular thin film
monitors and controllers. As many thin film process engineers and operators know, the quartz
crystal is the key to a successful deposition run. Quartz crystals open a window on the coating
process, reveal the thickness of material that is being deposited on the
substrates and convey the
rate, or thickness per second, that is being evaporated. This information is in turn used to control
the power supply to the evaporation source allowing precise control of the coating process from
beginning to end. When the crystal fails, however, the window abruptly shuts leaving the
operator in the dark and potentially terminating the coating run. With such a potentially
destructive impact from such a small inconspicuous item the question begs to be asked, "What
exactly is a quartz crystal sensor?"
The thin circular disc that is eventually placed in the
sensor head originates as a
multifaceted bar of quartz that is shaped like a sparkling six-sided rod. Through a series of
machining and milling this bar is eventually converted to a stack of thin (approximately 10
thousandths of an inch thick) circular wafers. Each wafer is then contoured on one side and
cleaned. Finally, the wafer is coated with a thin metal film, edge-to-edge on one side and a
keyhole pattern on the backside. After final inspection to determine electrical properties, the
crystals are packaged in an indexable container and shipped to the end user.
The real mystery, however, is how this thin tiny disc actually
works. It was first discovered
that certain crystalline materials, like quartz, would develop an electrical charge like a battery,
when pressed or squeezed. This property came to be known as the piezoelectric effect
(pronounced "pea-a-zo"). Conversely, if a battery were connected to a crystal, the crystal
would
change shape by stretching or compressing. If the battery were then turned on and off quickly, in
rapid succession, the crystal would vibrate.
In the 1950’s a German scientist named George Sauerbrey,
showed that the vibration of a
quartz crystal could be slowed down by depositing a thin coating on the crystal’s surface. It was
found that the change in vibration or “frequency," was a function of the thickness and the
density
of the coating. Using sophisticated electronics, this could be calculated many times per second
giving a real time measurement of the thickness of coating being deposited on the crystal or any
object in its vicinity.
And with that, the thin film monitor was born.
Tel 800-743-1743 FIL-TECH, INC. 617-742-0686 Fax
COPYRIGHT FIL-TECH, INC. 1998
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