Information AboutSputtering |
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Sputtering is largely driven by momentum exchange between the ions and atoms in the material, due to collisions. The process can be thought of as atomic billiards, with the ion ( Cue Ball ) striking a large Cluster of close-packed atoms ( Billiard Ball s). Although the first collision pushes atoms deeper into the cluster, subsequent collisions between the atoms can result in some of the atoms near the surface being ejected away from the cluster. The number of atoms ejected from the surface per incident ion is called the ''sputter yield'' and is an important measure of the efficiency of the sputtering process. Other things the sputter yield depends on are the energy of the incident ions, the masses of the ions and target atoms, and the Binding Energy of atoms in the solid. The ions for the sputtering process are supplied by a Plasma that is induced in the sputtering equipment. In practice a variety of techniques are used to modify the plasma properties, especially ion density, to achieve the optimum sputtering conditions, including usage of RF (radio frequency) alternating current, utilization of Magnetic Field s, and application of a bias Volt age to the target. SPUTTER DEPOSITION Sputtered atoms ejected into the gas phase are not in their Thermodynamic Equilibrium state. Deposition of the sputtered material tends to occur on all surfaces inside the vacuum chamber. Sputtering is used extensively in the Semiconductor industry to deposit thin films of various materials in Integrated Circuit processing. Thin Antireflection Coating s on glass for Optical applications are also deposited by sputtering. Because of the low substrate temperatures used, sputtering is an ideal method deposit contact metals for Thin-film Transistor s. 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 Oxide s such as Zinc Oxide , Tin Oxide , or Titanium Dioxide . 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. In contrast with thermal Evaporation techniques one component of the source may have a higher vapor pressure, resulting in a deposited film with a different composition than the source. showing the cathode made of the material to be deposited, the anode counter-electrode and an outer ring meant to prevent sputtering of the hearth that holds the target.]] 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 tolerature the bombardment and are usually coated via evaporation. Types of sputter deposition Sputtering sources are usually Magnetron s that utilise 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 (IBS) is a method in which the target is external to the . The principal drawback of IBS is the large amount of maintenance required to keep the ion source operating. Reactive sputtering refers to a technique where 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 Cell s is made by reactive sputtering. 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 1980's. 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. ANALYSIS
SPACE Sputtering is one of the forms of space weathering, a process that changes the physical and chemical properties of airless bodies, such as asteroids and our moon. It is also one of the possible ways that Mars has lost its atmosphere. EXTERNAL LINKS
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