The importance of a high vacuum environment for deposition of high quality thin films was introduced in part 1 of the series. Virtually all thin film deposition processes require an initial high vacuum or ultrahigh vacuum in an air tight chamber, which require sophisticated vacuum pumping systems, gas control and pressure control systems reported in Vacuum Technology & Coating Magazine (VT&C). Note that all aspects of thin film deposition, characterization, application design can be found in VT&C.
All physical and laser deposition processes move atoms or molecules from a source and deposit them onto a substrate. These processes generally require that:
- A plasma be used to energize gas atoms (usually an inert gas) and eject atoms or molecules from the source
- Ejected particles are not significantly impeded (scattered) by gas atoms in the deposition chamber. Particles lose energy with each collision with a gas atom.
This can only be accomplished in a high vacuum environment, that is, particles have a long mean free path (MFP) between collisions so they can make it to and be deposited onto the substrate with sufficient energy to adhere and coalesce onto the substrate. Vacuum is essentially the absence of gas molecules. The best vacuum can be found in outer space.
A vacuum pumping system, which may include a series of vacuum pumps and subchambers, must be able to evacuate each chamber volume in a reasonable length of time, and as a result, the throughput and pumping speed of the vacuum pumping system must be designed accordingly. The figure below shows a large deposition chamber that was located at Pacific Northwest National Laboratory. A high vacuum pumping system includes combinations of a roughing pump, high vacuum pump (diffusion pump, cryopump, turbomolecular pump, ion pump), cold trap to freeze out gases, chamber bake out, pressure measurement and control, reactive gas manifold and gas mixing control. The deposition chamber must be leak tight to prevent contaminants and air degrading the vacuum environment and film quality. The inner surface of the chamber must be kept pristine so that contaminants (gases and poorly adhering materials from previous depositions) are not liberated. Leaks and outgassing introduce contaminants into the chamber which can react with materials from the deposition source, deposit loosely bound material onto the substrate and complicate control of gas pressure. To achieve this the inner surfaces are heated during pump down, a process known as degassing.
The Figure below shows a schematic of a vacuum pumping system . The roughing pump is used first to reduce chamber pressure to ~ 100 mTorr. The high vacuum pump takes over at this pressure and completes evacuation of the chamber. Note that the high vacuum pump is not capable of evacuating the chamber starting at atmospheric pressure. If necessary, the walls of the chamber are then baked out. Many chambers have a cold trap positioned before the high vacuum pump to freeze out contaminant gases and improve both vacuum quality and pump down speed. The high vacuum pump is throttled before process and reactive gases are introduced to prevent overloading the pump and for control of chamber pressure during deposition.
The chamber is now ready for deposition of thin films (described in a future blog). The partial pressure of as many as four gases can be controlled by the gas control system. Again, for more information on this equipment, refer to VT&C columns, articles and Product Showcase. Baratron manometers (capacitance manometer) are the heart of pressure control systems. Gases are introduced through flow controllers with flows determined by partial pressure of each gas (measured by Baratrons) and pumping speed. A typical deposition pressure ranges from 1 – 5 mTorr. The controller adds inert and reactive gases as required for the composition of the thin film.
1. Donald M Mattox, Handbook of Physical Vapor Deposition (PVD) Processes, Noyes (1998).