Sputter deposition

Sputter deposition is a physical vapor deposition PVD method of depositing thin films by sputtering, i.e. eroding, material from a "target," i.e., source, which then deposits onto a "substrate," e.g., a silicon wafer. Resputtering, in contrast, involves re-emission of the deposited material during the deposition process also by ion bombardment.

Sputtered atoms ejected into the gas phase are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum chamber. A substrate (such as a wafer) placed in the chamber will be coated with a thin film. Sputtering usually uses an argon plasma.

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Uses

Sputtering is used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing. Thin antireflection coatings on glass for optical applications are also deposited by sputtering. Because of the low substrate temperatures used, sputtering is an ideal method to deposit contact metals for thin-film transistors. Perhaps the most familiar products of sputtering are low-emissivity coatings on glass, used in double-pane window assemblies. The coating is a multilayer containing silver and metal oxides such as zinc oxide, tin oxide, or titanium dioxide. Sputtering is also used to metalize plastics such as potato chip bags. A large industry has developed around tool bit coating using sputtered nitrides e.g. titanium nitride creating the familiar gold colored hard coat. Sputtering is also used as the process to deposit the metal (Aluminum) layer during the fabrication of CD and DVD discs.

Hard disk surfaces use sputtered CrOx and other sputtered materials.

Comparison with other deposition methods

One important advantage of sputtering as a deposition technique is that the deposited films have the same composition as the source material. The equality of the film and target stoichiometry might be surprising since the sputter yield depends on the atomic weight of the atoms in the target. One might therefore expect one component of an alloy or mixture to sputter faster than the other components, leading to an enrichment of that component in the deposit. However, since only surface atoms can be sputtered, the faster ejection of one element leaves the surface enriched with the others, effectively counteracting the difference in sputter rates. This is in contrast to thermal evaporation techniques, where one component of the source may have a higher vapor pressure, resulting in a deposited film with a different composition than the source.

Sputter deposition also has an advantage over molecular beam epitaxy (MBE) due to its speed. The higher rate of deposition results in lower impurity incorporation because fewer impurities are able to reach the surface of the substrate in the same amount of time. Sputtering methods are consequently able to use process gases with far higher impurity concentrations than the vacuum pressure that MBE methods can tolerate. During sputter deposition the substrate may be bombarded by energetic ions and neutral atoms. Ions can be deflected with a substrate bias and neutral bombardment can be minimized by off-axis sputtering, but only at a cost in deposition rate. Plastic substrates cannot tolerate the bombardment and are usually coated via evaporation.

Types of sputter deposition

Sputtering sources are usually magnetrons that utilize strong electric and magnetic fields to trap electrons close to the surface of the magnetron, which is known as the target. The electrons follow helical paths around the magnetic field lines undergoing more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. The sputter gas is inert, typically argon. The extra argon ions created as a result of these collisions leads to a higher deposition rate. It also means that the plasma can be sustained at a lower pressure. The sputtered atoms are neutrally charged and so are unaffected by the magnetic trap. Charge build-up on insulating targets can be avoided with the use of RF sputtering where the sign of the anode-cathode bias is varied at a high rate. RF sputtering works well to produce highly insulating oxide films but only with the added expense of RF power supplies and impedance matching networks. Stray magnetic fields leaking from ferromagnetic targets also disturb the sputtering process. Specially designed sputter guns with unusually strong permanent magnets must often be used in compensation.

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 like the stress in SiNx and the index of refraction of SiOx. The transparent indium tin oxide conductor that is used in optoelectronics and solar cells is made by reactive sputtering.

Ion-assisted deposition

In ion-assisted deposition (IAD), the substrate is exposed to a secondary ion beam operating at a lower power than the sputter gun. Usually a Kaufman source like that used in IBS supplies the secondary beam. IAD can be used to deposit carbon in diamond-like form on a substrate. Any carbon atoms landing on the substrate which fail to bond properly in the diamond crystal lattice will be knocked off by the secondary beam. NASA used this technique to experiment with depositing diamond films on turbine blades in the 1980s. IAS is used in other important industrial applications such as creating tetrahedral amorphous carbon surface coatings on hard disk platters and hard transition metal nitride coatings on medical implants.

High-target-utilization sputtering

High-target-utilisation sputtering "HiTUS" is specialized commercial process. More information may be found http://www.plasma-quest.com/hitus-technical-benefits.html The process based upon the remote generation of a high density plasma. The plasma is generated in a side chamber opening into the main process chamber, containing the target and the substrate to be coated. As the plasma is generated remotely, and not from the target itself (as in conventional magnetron sputtering), the ion current to the target is independent of the voltage applied to the target.

High Power Impulse Magnetron Sputtering (HIPIMS)

HIPIMS is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. HIPIMS utilises extremely high power densities of the order of kWcm^-2 in short pulses (impulses) of tens of microseconds at low duty cycle of < 10%.

References

Magnetron sputtering theory

The Foundations of Vacuum Coating Technology by D. Mattox

External links

• Carnegie Mellon Nanofabrication Facility (14 Sputtering Systems & Over 200 Sputtering Targets)