Appendix C
Troubleshooting
failure. Depending on the material, crystals may fail well before their typical value. If
you find that crystals consistently fail early, you may want to set Min. Frequency to a
higher value.
If the frequency reading is zero or unstable, check the InVac cable from the feedthrough
to the sensor. Also check that the crystal is seated properly in the sensor head.
While not depositing, observe the frequency readings for each sensor. The value
should be stable /- .3Hz
When the frequency reading is stable, reconnect the source supply. Start the
deposition process in Manual mode with 0% Power. The frequency readings should
remain stable.
In Manual mode, slowly raise the % Power until a Rate reading is displayed on the
screen. As material is deposited on the crystal, the frequency should drop slowly and
consistently. If not, check your source supply for erratic output. Also assure that the
sensor is not too close to the source (particularly in sputtering). An increasing
frequency indicates that the sensor is not adequately cooled.
Incorrect Rate or Thickness Measurement:
First, complete the procedures in the previous section to assure reliable sensor
operation.
Set the crystal Tooling as described in Section 3. Incorrect crystal Tooling values will
cause consistently low or high rate/thickness values for every material.
Verify that the Density and Z-Factor values match those in the Material Parameters
Appendix. If the material is not listed, check a materials handbook. Density has a
significant effect on rate/thickness calculations.
Z-Factor corrects for stresses as a crystal is coated. If readings are initially accurate,
but deteriorate as crystal life drops below 60-70%, you need to adjust the Z-Factor or
replace crystals more frequently. The relationship between Z-Factor and Acoustic
Impedance is discussed in Appendix A.
Poor Rate Stability:
First, be sure that a stable rate can be achieved in Manual mode, as explained in the
previous sections. Once a stable rate is achieved in Manual mode, set the PID control
parameters as described in Section 2. Also set the Slew rate to limit the amount of
power that can change per second. Adjust these parameters as necessary to achieve
desired performance.
