SIGMA SQC-122c - Manual (Page 21)

Chapter 2 Operation
2-7
2.4 Sensor Setup
The theory of operation for a quartz crystal sensor is quite simple. The sensor initially
oscillates at its natural frequency, typically 6MHz. As material is deposited on the
substrate, it is also deposited on the sensor. Depending on the density of the deposited
material, and the amount of material deposited, the sensor’s frequency will drop from its
initial frequency. The rate and thickness can be calculated from this frequency shift.
Sensor setup involves selecting the sensor(s) to be used, setting the Min/Max crystal
frequencies, and adjusting the Tooling Factor. Sensors are assigned to each film, as
described in Section 2.2, Defining a Film. See Section 3.5 for information on setting up
dual shuttered sensors.
In the System Params menu, Max Frequency is the initial frequency of a new crystal,
typically 6.0e+06 Hz (6 MHz). Due to manufacturing tolerances, some crystals may
oscillate above 6MHz initially, which would be detected as a sensor error. Setting the
Max Frequency slightly above the nominal value, to say 6.1MHz, will avoid this problem
with no effect on instrument accuracy.
Min Frequency is the frequency where the SQC-122c will flag a sensor as bad. For a
6MHz crystal, the Min Frequency is typically 5 MHz.
Crystal failure is often predicted by brief periods of “mode hopping,” where the crystal
briefly makes an abrupt change in frequency, or stops oscillating altogether. Some
materials will cause crystals to fail or mode hop well before 5MHz. It is good practice to
set the Min Frequency to a value that indicates crystal failure well before most crystals
actually fail.
To better determine impending crystal failure, Crystal Quality and Stability parameters
are also available for each Film. See Chapter 3, Film Menus for more information on
setting Crystal Quality and Stability.