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Reactive sputtering is a variation of the plasma sputtering process used to deposit a thin film onto a substrate material. In this process, a target material, such as aluminum or gold, is released into a chamber with an atmosphere made of a positively charged reactive gas. This gas forms a chemical bond with the target material and is deposited on a substrate material as a compound.
While normal plasma sputtering takes place in a vacuum chamber that has been voided of an atmosphere, reactive sputtering takes place in a vacuum chamber with a low pressure atmosphere made up of a reactive gas. Special pumps on the machine remove the normal atmosphere, which is made of carbon, oxygen and nitrogen among other trace elements, and fill the chamber with a gas, such as argon, oxygen or nitrogen. The reactive gas in the reactive sputtering process has a positive charge.
The target material, such as titanium or aluminum, is then released into the chamber, also in the form of a gas, and exposed to a high intensity magnetic field. This field turns the target material into a negative ion. The negatively charged target material is attracted to the positively charged reactive material, and the two elements bond before settling on the substrate. In this way, thin films can be made of compounds such as Titanium-Nitride (TiN) or Aluminum-Oxide (Al2O3).
Reactive sputtering greatly increases the rate at which a thin film can be made out of a compound. While traditional plasma sputtering is appropriate when creating a thin film out of a single element, compound films take a long time to form. Forcing the chemicals to bond as a part of the thin film process helps speed the rate at which they settle on the substrate.
The pressure inside the reactive sputtering chamber must be carefully managed in order to maximize the growth of the thin film. At low pressures, the film takes a long time to form. At high pressures, the reactive gas can “poison” the target surface, which is when the target material receives its negative charge. This not only decreases the rate of growth for the thin film on the substrate below, but also increases the rate of poisoning; the fewer negative particles there are, the fewer chemical bonds they can form with the positively charged reactive gas and thus, the more reactive gas there is to poison the target surface. Monitoring and adjusting the pressure in the system helps to prevent this poisoning and allows for the thin film to grow quickly.