Evaporation is a form of physical vapor deposition (PVD) where material is heated to a high vapor pressure, often in molten state. The vapors are then condensed on to a substrate to form a desired thickness of a thin film. In our case, a thin film precious metal coating. The heating is typically accomplished via resistive heating or by E-beam (electron beam).
Why Use Materials Engineered Specifically for Evaporation?
Engineered pellets or slugs are manufactured with specific form factors intended to vaporize at known rates. Often during evaporation processes, “spitting” results in liquid droplet material splattering on to the substrate. Engineered pellets are made with specified metal purities and processes intended to minimize incorporated gases and impurities to mitigate “spitting” in process.
A QCM or quartz crystal microbalance is widely used to monitor deposition thickness during process. A QCM is based on a reverse piezoelectric effect such that a mechanical strain or deflection occurs when an electric field (voltage) is applied to the quartz. In the case of a QCM, the electric field is alternating in voltage, thus causing the crystal to vibrate at its resonant frequency inherent to quartz. As a thin film coating builds on the quartz crystal, its mass changes. This results in a change in the vibration or frequency of oscillation. Two additional factors come into place in order to correct the QCM. The QCM is designed to be in the chamber aside from the substrate, otherwise one or the other would shadow the deposition of the other.
For purposes of process calibration, this brings about 2 important factors in addition to the deposited material’s density: