Chapter 2 | Operation |
2.4Sensor 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
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.