Sheldon Wayman, Mohamed Rinzan, INFICON, East Syracuse, NY
Quartz crystal microbalance (QCM) technology has been used for decades to control deposition rate and thickness for the most complex processes seen in the ophthalmic, optical, display, and solar markets. INFICON has recently made a new advancement in QCM technology to address a problem seen across all of these industries related to QCM temperature effects. Thermal shock can cause QCM thickness errors which can decrease yield and increase manufacturing costs if temperature is not accounted for.
A complete QCM system consists of a thin film deposition controller, oscillator, and sensor which houses the QCM and provides the electrical connection. Quartz is a piezoelectric material, meaning when a voltage potential is applied across the two electrodes of the quartz crystal, the quartz will physically deform. With the help of an oscillator, the QCM can resonate at its fundamental frequency and be measured using a thin film deposition controller. As material is deposited onto the surface of the QCM, the mass on the crystal increases and the frequency decreases proportionally.
Temperature also impacts the frequency measurement and can create false mass readings. Thin film deposition controllers currently on the market have no good way of distinguishing frequency shifts related to mass from frequency shifts related to temperature, resulting in thickness errors and poor PID control. This can be detrimental to today’s complex coating processes due to the low deposition rates and incredibly thin layers required. Thermal shocks can occur at any point in a deposition process and can cause unnecessary PID-loop correction, triggering non-uniform deposition in respect to time. This means that the quality throughout the bulk of the material is inadequate. For very thin films, the thickness termination may not be at the real intended thickness because the process time window is small compared to the time allowed for a QCM to recover from a thermal shock event.
INFICON has patented a new temperature compensation technique for SC-cut crystals to remove the effects of temperature variation on the QCM without the need for additional hardware or custom and expensive sensors. INFICON temperature compensation is also instantaneous unlike competing methods that use a thermocouple embedded inside the QCM sensor. By removing temperature from the thickness calculation, the thickness accuracy and deposition rate stability can significantly improve. Traditionally, AT-cut crystals have been used for most coating applications, however there are two additional advantages of SC-cut crystals compared to AT-cut. SC-cut crystals have 100 times reduced sensitivity to acceleration induced frequency changes compared to AT-cut. For tools that have QCM sensors mounted on robotic arms, acceleration jerks can induce a frequency change impacting the rate measurement. The second benefit to using SC-cut crystals is that SC-cut crystals have lower rate noise compared to AT-cut crystals when exposed to organic deposition material. Lower rate noise allows for better PID control with higher measurement accuracy to support technology transitions across several industries and meet the sensitivity and precision requirements of the latest product generations.