Cryogenic systems are key to improving vacuum deposition by reducing moisture contamination and optimizing vacuum chamber performance.
Goal of vacuum systems
The purpose of a vacuum is to reduce the density of free gas molecules in a chamber, thereby reducing the pressure to create the environment for thin film deposition processes. The lower the pressure, the longer the mean free path and the fewer material-gas collisions.
Gas behavior in the molecular flow regime
The vacuum minimizes reactive gases, preventing collisions and oxidation of process materials. Gas molecules are in a continual state of rapid motion. Different gases (H2O, N2, O2) have different masses and move at different speeds. Gas motion in vacuum (~<3mTorr) is considered to be in the “Molecular Flow Regime”. This equates to no flow viscosity, and the direction of travel for gas molecules are independent from each other. Collision to a cold trapping surface is random (dependent only on exposure or conductance to that surface).
Water vapor contamination
The most reactive contaminant in a vacuum chamber with a pressure of less than 10-3 Torr is typically water vapor. The humidity is the enemy of vacuum, both in the chamber and in the substrates. For this reason, to prevent water in the atmosphere from being absorbed by the plastic part it is recommended that the coating process be carried out immediately after the part leaves the moulding machine.
Importance of the cryogenic pump
Cryochiller comes into play here: as the vacuum chamber begins pumping down, the cryochiller cools the cryotrap which condenses and rapidly captures water vapor improving cycle time by 25% to 75% due to higher pumping speed and keeping it constant cycle by cylce.
Defrost operations and system optimization
During the deposition process, the cryochiller remains in the Cool mode to greatly reduce water vapor contamination and increase product yield. After a certain number of cycles selected by the process engineer, the cryochiller quickly heats the cryocoil to defrost and sublime the captured water vapor.
Depending on the selected model and design of cryo-trap installed, it provides water vapor pumping speeds up to 400,000 l/sec.
For many decades, Arzuffi has been installing cryochillers on its equipment, mostly Polycold ® MaxCool unit by Edwards. By installing Polycold on the equipment, the overall cycle time decreases, the cycle time remain constant and repeatable during all working hours, the tool throughput increases with a consequent increase in income.
