Reactive sputtering is a process that uses a positively charged reactive gas to deposit a thin film onto a substrate material. The target material is released into a vacuum chamber with a low-pressure atmosphere consisting of a reactive gas, and exposed to a high-intensity magnetic field. This process greatly increases the rate at which a thin film can be made from a compound, but the pressure within the chamber must be managed carefully to maximize thin film growth.
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 consisting of a positively charged reactive gas. This gas forms a chemical bond with the target material and is deposited on a substrate material such as a compound.
While normal plasma sputtering occurs in a vacuum chamber that has been emptied of an atmosphere, reactive sputtering occurs in a vacuum chamber with a low-pressure atmosphere consisting of a reactive gas. Special pumps on the machine remove the normal atmosphere, which is made up 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 transforms the target material into a negative ion. The negatively charged target material is attracted to the positively charged reactive material and the two bind before depositing on the substrate. In this way thin films of compounds such as Titanium Nitride (TiN) or Aluminum Oxide (Al2O3) can be made.
Reactive sputtering greatly increases the rate at which a thin film can be made from a compound. While traditional plasma sputtering is appropriate when creating a thin film from a single element, composite films take a long time to form. Forcing chemicals to bond as part of the thin film process helps speed up the rate at which they are deposited on the substrate.
The pressure within the reactive sputtering chamber must be managed carefully in order to maximize thin film growth. At low pressures, the film takes a long time to form. At high pressures, the reactive gas can “poison” the surface of the target, which is when the target material receives its negative charge. This not only reduces the growth rate of the thin film on the underlying substrate, but also increases the poisoning rate; the fewer negative particles there are, the fewer chemical bonds they can form with the positively charged reactive gas and therefore, the more reactive gas there is to poison the target surface. Monitoring and regulating the pressure in the system helps prevent this poisoning and allows the thin film to grow rapidly.
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