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In the case of multiple charged projectile ions a special form of electron sputtering can occur, which is called latent sputtering.In these cases, when the ions recombine during impact on the solid surface (the formation of hollow atoms), the potential energy stored in the multiply charged ion (i.e., the ion from which the neutral atom generates that charge state) required energy) is released.This sputtering process is characterized by the strong dependence of the observed sputtering yield on the charge state of the impinging ions and can already occur at ion impact energies well below the physical sputtering threshold. Potential sputtering is only observed for certain target species and requires a minimum potential energy.
Etching and chemical sputtering
Removing atoms by sputtering with an inert gas is called ion milling or ion etching.Sputtering can also play a role in reactive ion etching (RIE), a plasma process using chemically active ions and free radicals, which can significantly increase sputtering yields compared to purely physical sputtering.Reactive ions are often used in secondary ion mass spectrometry (SIMS) equipment to increase sputtering rates.The mechanism leading to the sputtering enhancement is not always clear, although the case of fluorine etching of Si is theoretically well modeled.Sputtering observed to occur below the threshold energy of physical sputtering is also commonly referred to as chemical sputtering.The mechanism behind this sputtering is not always clear and can be difficult to distinguish from chemical etching.At high temperatures, chemical sputtering of carbon can be understood as the weakening of bonds in the sample by incoming ions, which are then desorbed by thermal activation.The observed hydrogen-induced sputtering of carbon-based materials at low temperatures has been explained as H ions entering between C–C bonds and thus breaking them, a mechanism known as fast chemical sputtering.
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 in the surface region, bombardment (radiation) damage in the surface region, and the roughening of the surface, particularly if over done.It is important to have a clean in order to not continually recontaminate the surface during sputter cleaning.Redeposition of sputtered material on the substrate can also give problems, especially at high sputtering pressures. Sputtering of the surface of a compound or alloy material can result in the surface composition being changed.Often the species with the least mass or the highest is the one preferentially sputtered from the surface.
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 energy involved in the process can be listed as (values taken from):
Sputtered atoms (ions) from the target surface (~10eV), 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 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 dissociate 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 material 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 the material and the lifetime of excess charge carriers in photoactive materials; in some cases, depending on its extent, such damage can even lead to reduced shunt resistance.
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