Sputtering is a common technique for Physical Vapor Deposition (PVD), one of the methods of producing thin film coatings. Standard sputtering uses a target of whatever pure material is desired, and an inert gas, typically argon.
If the material is a single pure chemical element, the atoms simply come off the target in that form and deposit in that same form. However, it is also possible to use a non-inert (reactive) gas, such as oxygen or nitrogen, either in place of, or more commonly, in addition to, the inert gas. This process is known as Reactive Sputtering using sputtering equipment.
When this process is used, the ionized non-inert gas reacts chemically with the target material vapor cloud, producing a molecular compound, which then becomes the deposited film. For example, a silicon target reactively sputtered with oxygen gas can produce a silicon oxide film, or with nitrogen gas, can produce a silicon nitride film.
There are numerous variables to consider during this process. One important factor is the distribution of the reactive gas inside the chamber. Whereas the inert gas atoms can be re-used – ionized, neutralized, and ionized again in the plasma – the reactive gas is chemically consumed to form the desired product compound. In order to produce a consistently reacted film, it is often necessary to include a specific process of injecting this gas uniformly through the entire plasma around the cathode.
This technique typically utilizes a “gas ring” surrounding the cathode. This would be a tube or channel with small holes spaced along it, each allowing a portion of the gas to bleed into the plasma at intervals along the perimeter of the target. Since the reactive gas ionizes and is accelerated into the target by the electrical field, it does sputter material off the target as does the inert gas.
It is also possible for some of those reactive gas ions to react chemically with the atoms on the surface of the target without sputtering them off of the target. This is referred to as “poisoning” the target surface, changing it from a plate of pure original material into one which includes some percentage of already reacted molecules. Those molecules will behave differently in terms of conductivity, sputter yield, and so forth.
For this reason, it can be necessary to periodically recondition the surface by sputtering off this top layer with just the inert gas to get back to original material. If the original target material is suitably conductive, the relatively simple and inexpensive technique of DC Sputtering can be considered. Additionally, if dielectric coatings (insulative materials which can take on a charge that can result in arcs that spew droplets into the plasma, disrupting thin film quality control) are being deposited, the use of a Pulsed DC Sputtering power supply can help reduce the need for this regenerative step, which is otherwise an interruption in normal processing.
Molecular films can also be produced by inert sputtering of molecular material targets. For example, the previously mentioned silicon oxide or silicon nitride, can be the result of argon sputtered from targets made of those molecular materials. It is important to know that ions slamming into a target with sufficient energy to sputter whole molecules off the surface, are also capable of tearing some of those molecules apart. For this reason, even if the Reactive Sputtering process is not the basic process, it can still be useful to include some amount of background reactive gas to restore stoichiometry for these dissociated fragments.
One of the more useful accessories to have on sputter systems meant for Reactive Sputtering is a Residual Gas Analyzer (RGA) or Process Gas Analyzer (PGA), in order to monitor the actual partial pressures of gas species involved in the process. These can then be controlled by means of gas Mass Flow Controllers (MFC’s), along with a pumping throttle valve and suitable control instrumentation. Reactive Sputtering can be a dynamic, rather than static, environment, with these partial pressures being a key parameter for process optimization.
Reactive Sputtering can be a very useful tool for creating molecular thin films, especially if there is a need to tinker some with stoichiometry in the film in order to control properties such as resistivity for semiconductor circuits or index of refraction for optical or glass deposition applications. As with all such things, there will be several details to consider when establishing the ideal procedure for your end product.
Norm Hardy is a Process Engineer at Semicore Equipment Inc., a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high-tech industries. Please let our helpful support staff answer any questions you have regarding “What is Reactive Sputtering?” and how to implement the best techniques and equipment for your specific needs by contacting us at sales@semicore.com or by calling us at 925-373-8201.
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