Quartz crystals expand when heated, influencing frequency. When the temperature is regulated, stability increases. Temperature fluctuations will have a reduced affect on stability when temperature remains within the crystal’s operating range. When process temperature closely matches the crystal’s minimum operating temperature, stability and accuracy increase due to minimal shifts in the crystal’s frequency vs. temperature curve. The frequency vs temperature curve for the AT cut crystal is at about 25°C (depending on the cut). Using the correct crystal cut for the specified temperature, along with maintaining the temperature with cooling at that specific point, increases the accuracy of the readings. As long as temperature is maintained within its specified range, the AT crystal has very little change in frequency due to temperature. This means that if the temperature shifts several degrees, frequency change at this temperature is reduced due to its FvsT curve. If, however, the temperature is regulated and consistent throughout the deposition, frequency due to temperature remains unchanged and the readings are extremely accurate at any temperature within the crystal’s operating range.
The graph illustrates that at approximately 25°C, the AT crystal’s frequency experiences less drifting due to changes in temperature. If, however, the temperature of the sensor remains unchanged during the deposition, the crystal frequency will also remain unchanged, causing increased accuracy for thickness readings. In short, the more stable the temperature, the more accurate the readings.
