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In physics, sputtering is a phenomenon in which microscopic of a solid material are ejected from its surface, after the material is itself bombarded by energetic of a plasma or gas. (1981). 9783540105213, Springer, Berlin. ISBN 9783540105213 It occurs naturally in outer space, and can be an unwelcome source of wear in precision components. However, the fact that it can be made to act on extremely fine layers of material is utilised in science and industry—there, it is used to perform precise etching, carry out analytical techniques, and deposit thin film layers in the manufacture of , semiconductor devices and nanotechnology products. It is a physical vapor deposition technique.
[[File:linearcollisioncascadesput.png|right|thumb| Sputtering from a linear collision cascade. The thick line illustrates the position of the surface, with everything below it being atoms inside of the material, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material. The purple circle is the incoming ion. Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively. Two of the atoms happen to move out from the sample, i.e. they are sputtered.]]
These ions, known as "incident ions", set off collision cascades in the target. Such cascades can take many paths; some recoil back toward the surface of the target. If a collision cascade reaches the surface of the target, and its remaining energy is greater than the target's surface binding energy, an atom will be ejected. This process is known as "sputtering". If the target is thin (on an atomic scale), the collision cascade can reach through to its back side; the atoms ejected in this fashion are said to escape the surface binding energy "in transmission".
The average number of atoms ejected from the target per incident ion is called the "sputter yield". The sputter yield depends on several things: the angle at which ions collide with the surface of the material, how much energy they strike it with, their masses, the masses of the target atoms, and the target's surface binding energy. If the target possesses a crystal lattice structure, the orientation of its axes with respect to the surface is an important factor.
The ions that cause sputtering come from a variety of sources—they can come from plasma, specially constructed , particle accelerators, outer space (e.g. solar wind), or radioactive materials (e.g. alpha radiation).
A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model. An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program TRIM.J. F. Ziegler, J. P, Biersack, U. Littmark (1984). 9780080216034, Pergamon Press, New York. ISBN 9780080216034
Another mechanism of physical sputtering is called "heat spike sputtering". This can occur when the solid is dense enough, and the incoming ion heavy enough, that collisions occur very close to each other. In this case, the binary collision approximation is no longer valid, and the collisional process should be understood as a many-body process. The dense collisions induce a heat spike (also called thermal spike), which essentially melts a small portion of the crystal. If that portion is close enough to its surface, large numbers of atoms may be ejected, due to liquid flowing to the surface and/or microexplosions. Heat spike sputtering is most important for heavy ions (e.g. Xe or Au or cluster ions) with energies in the keV–MeV range bombarding dense but soft metals with a low melting point (Ag, Au, Pb, etc.). The heat spike sputtering often increases nonlinearly with energy, and can for small cluster ions lead to dramatic sputtering yields per cluster of the order of 10,000. For animations of such a process see "Re: Displacement Cascade 1" in the external links section.
Physical sputtering has a well-defined minimum energy threshold, equal to or larger than the ion energy at which the maximum energy transfer from the ion to a target atom equals the binding energy of a surface atom. That is to say, it can only happen when an ion is capable of transferring more energy into the target than is required for an atom to break free from its surface.
This threshold is typically somewhere in the range of ten to a hundred electron-volts.
Preferential sputtering can occur at the start when a multicomponent solid target is bombarded and there is no solid state diffusion. If the energy transfer is more efficient to one of the target components, or it is less strongly bound to the solid, it will sputter more efficiently than the other. If in an AB alloy the component A is sputtered preferentially, the surface of the solid will, during prolonged bombardment, become enriched in the B component, thereby increasing the probability that B is sputtered such that the composition of the sputtered material will ultimately return to AB.
Sputtering can also play a role in reactive-ion etching (RIE), a plasma process carried out with chemically active ions and radicals, for which the sputtering yield may be enhanced significantly compared to pure physical sputtering. Reactive ions are frequently used in secondary ion mass spectrometry (SIMS) equipment to enhance the sputter rates. The mechanisms causing the sputtering enhancement are not always well understood, although the case of fluorine etching of Si has been modeled well theoretically.
Sputtering observed to occur below the threshold energy of physical sputtering is also often called chemical sputtering. The mechanisms behind such sputtering are not always well understood, and may be hard to distinguish from chemical etching. At elevated temperatures, chemical sputtering of carbon can be understood to be due to the incoming ions weakening bonds in the sample, which then desorb by thermal activation. The hydrogen-induced sputtering of carbon-based materials observed at low temperatures has been explained by H ions entering between C-C bonds and thus breaking them, a mechanism dubbed swift chemical sputtering.
Sputtered atoms are ejected into the gas phase but 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 deposition usually uses an argon plasma because argon, a noble gas, will not react with the target material.
As seen in the list above, negative ions (e.g., O− and In− for ITO sputtering) formed at the target surface and accelerated toward the substrate acquire the largest energy, which is determined by the potential between target and plasma potentials. Although the flux of the energetic particles is an important parameter, high-energy negative O− ions are additionally the most abundant species in plasma in case of reactive deposition of oxides. However, energies of other ions/atoms (e.g., Ar+, Ar0, or In0) in the discharge may already be sufficient to dissociate surface bonds or etch soft layers in certain device technologies. In addition, the momentum transfer of high-energy particles from the plasma (Ar, oxygen ions) or sputtered from the target might impinge or even increase the substrate temperature sufficiently to trigger physical (e.g., etching) or thermal degradation of sensitive substrate layers (e.g. thin film metal halide perovskites).
This can affect the functional properties of underlying charge transport and passivation layers and photoactive absorbers or emitters, eroding device performance. For instance, due to sputter damage, there may be inevitable interfacial consequences such as pinning of the Fermi level, caused by damage-related interface gap states, resulting in the formation of Schottky-barrier impeding carrier transport. Sputter 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 reduced shunt resistance.
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