Sputtering is a method for depositing very thin layers of a material onto a surface by bombarding a source material in a sealed chamber with electrons or other energetic particles to eject atoms of the source as a form of aerosol that then settle onto all surfaces in the chamber. The process can deposit extremely fine layers of films down to the atomic scale, but also tends to be slow and is best used for small surface areas. Applications include the coating of biological samples for imaging in scanning electron microscopes (SEMs), thin film deposition in the semiconductor industry, and depositing coatings for miniaturized electronics. The nanotechnology industry in medicine, computer science, and materials science research often relies on sputtering deposition to design new composites and devices at the nanometer, or one-billionth of a meter, scale.
Several different types of sputter methods are in common use, including gas flow, reactive, and magnetron sputtering. Ion beam and ion-assisted sputtering are also widely used due to the variety of chemicals that can exist in an ionic state. Magnetron sputtering is further broken down into direct current (DC), alternating current (AC), and radio frequency (RF) applications.
Magnetron sputtering works by placing a magnetic field around the source material that will be used for deposition of layers onto the target. The chamber is then filled with an inert gas, such as argon. As the source material is electrically charged with either AC or DC current, ejected electrons are trapped in the magnetic field, and eventually interact with the argon gas in the chamber to create energetic ions composed of both argon and the source material. These ions then escape the magnetic field and impact the target material, slowly depositing a fine layer of source material onto its surface. RF sputtering is used in this case to deposit several varieties of oxide films onto insulating targets by varying the electrical bias between the target and source at a rapid rate.
Ion beam sputtering works without the source needing a magnetic field. Ions that are ejected from the source material interact with electrons from a secondary source so that they bombarded the target with neutral atoms. This makes an ion sputtering system capable of coating both conducting and insulating target material and parts, such as the thin film heads for computer hard drives.
Reactive sputter machines rely on chemical reactions between the target material and gasses that are pumped into a chamber vacuum. Direct control of deposition layers is done by altering the pressure and quantities of gasses in the chamber. Films used in optical components and solar cells are often made in reactive sputtering, as stoichiometry, or chemical reaction rates, can be precisely controlled.