Ion sources can be used for a variety of applications in thin film deposition and etching. For example, they can be used for substrate pre-cleaning, reactive deposition of oxide and nitrides (ion beam assisted deposition), and ion beam milling. When selecting and designing a deposition system that uses an ion source, many factors come into play. Some design features are dictated by the application, and some can be balanced against cost and performance. Below are five factors to keep in mind as you select an ion beam source.
Gas flow rate—Gas flow rate is determined by the size and type of ion source. Large ion sources require higher gas flow rates, and that may dictate a higher pumping speed for the vacuum chamber. Most ion sources are made to work in the mid-10-4 Torr range, although some sources can work at a pressure a decade higher.
Gridded ion sources—Gridded ion sources come in two basic types: hot filament and RF. The hot filament design uses standard thermionic emission from a W filament to provide electrons to create plasma from which the ions are extracted. RF sources use 13.56 MHz excitation to create the plasma. Hot filament sources are limited in the types of gasses that can be used. They cannot be used to produce oxygen ions. RF sources are more expensive and really can only be used in applications with non-conductive materials, but they can run in pure argon, oxygen, nitrogen, and other reactive gasses. Gridded ion sources can typically operate from about 50 to 1,200 eV and are available with focused, collimated, or divergent grid sets. Ion energy distribution is usually low for such sources.
Non-gridded sources—Non-gridded sources, typically called end-Hall sources, use a permanent magnet and electric field to produce a localized plasma within the source. These sources typically have a broad energy and flux distribution, making them good for large-area applications, such as pre-cleaning. When run at high power, water cooling of the source may be required. These sources can typically run from about 75 to 350 eV.
Ion beam neutralization—All ion sources need some sort of neutralization to prevent the substrate from building up too much charge. Hot filaments, hollow cathode neutralizers (HCN), or RF plasma generators can all be used. The HCN and RF sources require gas flow which adds to the pump load.
Ion source mounting—Ion sources can come flange-mounted or remote-mounted. Depending on the application, either mounting technique may be used. However, remote mounting will be required in some cases. Remote mounting gives the flexibility to change the angle of the ion source or the ability to move it within the deposition chamber.
Choosing the right ion source requires a good understanding of the application and the various ion source capabilities. Because ion sources are relatively expensive, it is important to select the right source for your application up front, as retrofitting a different ion source into an existing chamber can be very costly or even impossible.
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