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Sputtering occurs only when the kinetic energy of the incident particles is much higher than the conventional thermal energy (>> 1 eV).When using direct current (DC sputtering), use a voltage of 3-5 kV. When done using alternating current (RF sputtering), the frequency is around 14 MHz.
Contamination from solid surfaces can be removed by using physical sputtering in a vacuum.Sputter cleaning is commonly used in surface science, vacuum deposition and ion plating. In 1955, Farnsworth, Schlier, George, and Burger reported the use of sputter cleaning in an ultrahigh vacuum system to prepare ultraclean surfaces for low-energy electron diffraction (LEED) studies.Sputter cleaning has become an integral part of the ion plating process.A similar technique, plasma cleaning, can be used when the surface to be cleaned is large.Sputter cleaning has some potential problems such as overheating, gas incorporation of the surface area, bombardment (radiation) damage of the surface area, and surface roughness, especially if done excessively.It is important to have a clean plasma so as not to continually recontaminate the surface during sputter cleaning.Redeposition of sputtered material on the substrate can also be problematic, especially at high sputtering pressures. Sputtering on the surface of a compound or alloy material results in a change in the surface composition.Usually the species with the smallest mass or highest vapor pressure is the one that is preferentially sputtered from the surface.
Thin film deposition
Sputter deposition is a method of depositing thin films by sputtering, which involves etching material from a "target" source onto a "substrate", such as a silicon wafer, solar cell, optical element, or many other possibilities.In contrast, re-sputtering involves the re-emission of the deposited material, for example SiO2 also employs ion bombardment during deposition.The sputtered atoms are ejected into the gas phase, but are not in thermodynamic equilibrium and tend to deposit on all surfaces of the vacuum chamber.A substrate, such as a wafer, placed in the chamber will be coated with a thin film. Sputter deposition typically uses argon plasma because argon is an inert gas and does not react with the target.
Sputtering damage is commonly defined during the deposition of transparent electrodes for optoelectronic devices and usually results from the bombardment of energetic species on the substrate.The main species and representative energies involved in this process can be listed as (values taken from):sputtered atoms (ions) from the target surface (~10 eV), the formation of which depends mainly on the binding energy of the target material;Negative ions (from the carrier gas) formed in the plasma (~5-15 eV), the formation of which depends mainly on the plasma potential;Formation of negative ions (up to 400 eV) on the target surface, the formation of which depends mainly on the target voltage;Positive ions (~15 eV) formed in the plasma, the formation of which mainly depends on the potential drop in front of the substrate at a floating potential;Reflected atoms and neutralizing ions (20–50 eV) from the target surface, the formation of which depends mainly on the quality of the background gas and sputtering elements.As shown in the table above, negative ions formed on the surface of the target and accelerated towards the substrate (e.g. O− and In− from ITO sputtering) acquire maximum energy, which is determined by the potential between the target and the plasma potential. Although the flux of energetic particles is an important parameter, in the case of reactive deposition of oxides, energetic negative O- ions are also the most abundant species in the plasma. However, in some device technologies, the energy of other ions/atoms (such as Ar+, Ar0 or In0) in the discharge may already be sufficient to detach surface bonds or etch soft layers. Furthermore, momentum transfer from the plasma (Ar, oxygen ions) or energetic particles sputtered from the target may affect or even increase the substrate temperature enough to trigger physical (e.g. etching) or thermal degradation of sensitive substrate layers (e.g. thin-film metal halide perovskite).
This affects the functional properties of the underlying charge transport and passivation layers as well as the photoactive absorber or emitter, eroding device performance.For example, due to sputtering damage, unavoidable interfacial consequences such as Fermi-level pinning caused by damage-related interfacial gap states may arise, leading to the formation of Schottky barriers hindering carrier transport.Sputtering damage can also impair the doping efficiency of materials and the lifetime of excess charge carriers in photoactive materials; in some cases, depending on its extent, such damage can even lead to a reduction in shunt resistance.
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