Magnetron Sputtering
Sputtering relies on a plasma (usually a noble gas, such as Argon) to knock material from a "target" a few atoms at a time. The target can be kept at a relatively low temperature, since the process is not one of evaporation, making this one of the most flexible deposition techniques. It is especially useful for compounds or mixtures, where different components would otherwise tend to evaporate at different rates. Note, sputtering's step coverage is more or less conformal.

Magnetron sputtering is a powerful and flexible technique which can be used to coat virtually any workpiece with a wide range of materials - any solid metal or alloy and a variety of compounds.

Prior to the magnetron sputtering a vacuum of less than one ten millionth of an atmosphere must be achieved. From this point a closely controlled flow of an inert gas such as argon is introduced. This raises the pressure to the minimum needed to operate the magnetrons, although it is still only a few ten thousandth of atmospheric pressure.

When power is supplied to the magnetron a negative voltage of typically -300V or more is applied to the target. This attracts argon ions to the target surface at speed. When they collide with the surface two important processes take place: Atoms are knocked out of the target surface with mean kinetic energies of 4 to 6 eV- this is sputtering. These sputtered atoms are neutrally charged and so are unaffected by the magnetic trap.

These sputtered atoms collide with the substrates to be coated and form an extremely adherent coating. Generally the formation of the coating consists of a four stage process, nucleation, island growth, coalescence and finally continuous growth.

The second important process that occurs when an ion collides with the target surface is that electrons are emitted. The light and negatively charged electrons when they leave the surface are affected by the combination of electrostatic and magnetic forces and execute cycloidal motion around the magnetic field lines.

When an electron of sufficient energy collides with a neutral atom it can knock another electron out of the atom resulting in the creation of a positive ion, ionization. This ion is attracted towards the target surface and the whole sputtering process is repeated. As the electrons are trapped by the magnetic field their path lengths and the degree of ionization is increased. This increased ion density close to the target produces a high deposition rate (>500nm/minute) and the electron trapping provides less free electrons to bombard the substrate resulting in the ability to coat temperature sensitive substrates.

Unbalanced Magnetron
In a "conventional" magnetron a higher substrate ion bombardment, the number of ions hitting the substrate during coating (vital for hard dense coatings) can only be achieved by increasing the power to the target or decreasing the distance from the target. To overcome this problem the unbalanced magnetron was developed. An unbalanced magnetron possesses stronger magnets on the outside resulting in the expansion of the plasma away from the surface of the target towards the substrate to be coated considerably increasing the substrate ion bombardment.

Ion-beam sputtering
Ion-beam sputtering (IBS) is a method in which the target is external to the ion source. A source can work without any magnetic field like in a hot filament ionization gauge . In a Kaufman source ions are generated by collisions with electrons that are confined by a magnetic field as in a magnetron. They are then accelerated by the electric field emanating from a grid toward a target. As the ions leave the source they are neutralized by electrons from a second external filament. IBS has an advantage in that the energy and flux of ions can be controlled independently. Since the flux that strikes the target is composed of neutral atoms, either insulating or conducting targets can be sputtered. IBS has found application in the manufacture of thin-film heads for disk drives. A pressure gradient between the ion source and the sample chamber is generated by placing the gas inlet at the source and shooting through a tube in into the sample chamber. This saves gas and reduces contamination in UHV applications. The principal drawback of IBS is the large amount of maintenance required to keep the ion source operating.

Reactive sputtering
In reactive sputtering, the deposited film is formed by chemical reaction between the target material and a gas which is introduced into the vacuum chamber. Oxide and nitride films are often fabricated using reactive sputtering. The composition of the film can be controlled by varying the relative pressures of the inert and reactive gases. Film stoichiometry is an important parameter for optimizing functional properties.