Magnetron sputtering is a plasma-based deposition process in which energetic ions are accelerated towards a target. The ions strike the target and atoms are ejected (or sputtered) from the surface. These atoms travel towards the substrate and incorporate into the growing film.
Magnetron sputtering is a deposition technology involving a gaseous plasma which is generated and confined to a space containing the material to be deposited – the ‘target’. The surface of the target is eroded by high-energy ions within the plasma, and the liberated atoms travel through the vacuum environment and deposit onto a substrate to form a thin film.
In a typical sputtering deposition process, a chamber is first evacuated to a high vacuum to minimize the partial pressures of all background gases and potential contaminants. After base pressure has been reached, sputtering gas which comprises the plasma flows into the chamber and the total pressure is regulated – typically in the milliTorr range – using a pressure control system.
To initiate plasma generation, high voltage is applied between the cathode – commonly located directly behind the sputtering target – and the anode – commonly connected to the chamber as an electrical ground. Electrons which are present in the sputtering gas are accelerated away from the cathode causing collisions with nearby atoms of sputtering gas. These collisions cause an electrostatic repulsion which ‘knock-off’ electrons from the sputtering gas atoms, causing ionization. The positive sputter gas atoms are now accelerated towards the negatively charged cathode, leading to high energy collisions with the surface of the target. Each of these collisions can cause atoms at the surface of the target to be ejected into the vacuum environment with enough kinetic energy to reach the surface of the substrate. In order to facilitate as many high energy collisions as possible – leading to increased deposition rates – the sputtering gas is typically chosen to be a high molecular weight gas such as argon or xenon. If a reactive sputtering process is desired, gases such as oxygen or nitrogen can also be introduced to the chamber during film growth.
A magnetron sputtering source takes advantage of the above phenomena by using very strong magnets to confine the electrons in the plasma at, or near the surface of the target. Confining the electrons not only leads to a higher density plasma and increased deposition rates but also prevents damage which would be caused by the direct impact of these electrons with the substrate or growing film.
Magnetron sputter deposition does not require melting and evaporation of the source material, leading to many advantages over other PVD technologies: first, nearly all materials can be deposited by magnetron sputtering regardless of their melting temperature; second, sources can be scaled and positioned anywhere in the chamber based on the requirements of the substrate and the coating; finally, films of alloys and compounds can be deposited while maintaining similar composition to that of the source material. Successful magnetron sputter deposition requires the correct choice of power delivery system.