INFICON
Special Features | 6
074-733-P1A IMM-200 Operating Manual
27 / 67
6.4 Crystal Type
Crystals cut from different crystallographic orientations of quartz can be used with
IMM-200. These are referred to as AT, BT, SC, or IT cut crystals. Thickness shear
mode waves propagating through these different crystal cuts have different acoustic
velocities. Typical Z ratio values commonly found in the literature are normalized for
AT-cut quartz crystals.
For proper thickness calculation it is necessary to select the crystal type to match the
crystals being used.
AT-Cut Crystals
AT-cut crystals are commonly used as gravimetric sensors to monitor film thickness.
These are singly rotated crystals and are less expensive than doubly rotated crystals.
Frequency vs. temperature curves for the family of AT-cut crystals have an inversion
point near room temperature making them ideal to operate on or around room
temperature applications. While the frequency of an AT-cut crystal is sensitive to
thermal radiation impulses and acceleration, the atmospheric loading on its quality (Q
factor) is less than SC or IT-cut crystals.
BT-Cut crystals
BT-cut crystals have a frequency-temperature characteristic that has a second order
polynomial shape without an inversion. These crystals have a stress coefficient that is
equal and opposite to the stress coefficient of AT-cut crystals. In other words, a tensile
film deposited on BT-cut crystals has a frequency shift in the opposite direction to that
of the same film on AT-cut crystals.
IT-Cut Crystals
Like SC-cut crystals, IT-cut crystals are used in OCXOs (oven controlled crystal
oscillators) due to their higher frequency temperature inversion point compared with
other crystal cuts. IT-cut crystals have a frequency temperature inversion around
75°C. Both the IT-cut family and the SC-cut family are ideal for thin film deposition
applications at high temperatures. At higher temperatures, for example 100-300°C,
both the IT and SC-cut crystals have a lower temperature coefficient of frequency
compared to AT-cut crystals.
SC-Cut Crystals
SC-cut crystals are expensive compared to AT-cut crystals. An SC-cut crystal has an
extremely stable resonance frequency. Their frequency-temperature inversion point of
approximately 93°C, being close to the oven temperatures, makes them appealing to
be used in OCXOs. The resonance frequency of this type of crystal has very good
immunity to acceleration. Their acceleration sensitivity is approximately two orders of
magnitude less than AT-cut crystals. TCXOs (temperature compensated crystal
oscillators) (clocks) almost always have these crystals in their resonator circuits.
Содержание IMM-200
Страница 1: ...Operating Manual IMM 200 074 733 P1A ...
Страница 2: ...INFICON Two Technology Place East Syracuse NY 13057 9714 ...
Страница 66: ......
Страница 67: ...074 733 P1A 2020 ...