Editing Sputtering (section)

==Physics==
When energetic ions collide with atoms of a target material, an exchange of [[momentum]] takes place between them.<ref name="Behrisch1981"/><ref name=Sigmund1987>{{cite journal| author= P. Sigmund, Nucl. Instrum. Methods Phys. Res. B |volume=27 |issue=1 |year=1987| pages=1–20|doi = 10.1016/0168-583X(87)90004-8 |title=Mechanisms and theory of physical sputtering by particle impact |journal=Nuclear Instruments and Methods in Physics Research Section B|bibcode = 1987NIMPB..27....1S }}</ref><ref name=Behrisch2007>{{cite book| editor-first1=Rainer |editor-last1=Behrisch |editor-first2= Wolfgang |editor-last2=Eckstein|title= Sputtering by Particle bombardment: Experiments and Computer Calculations from Threshold to Mev Energies |publisher=Springer, Berlin| year=2007}}</ref>

[[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 cascade]]s 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|crystal]] 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 (physics)|plasma]], specially constructed [[ion source]]s, [[particle accelerator]]s, 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.<ref name=Thompson>{{cite journal|author=M.W. Thompson|journal= Phil. Mag. |volume=18|page=377|year= 1962|doi=10.1080/14786436808227358|title=Energy spectrum of ejected atoms during the high- energy sputtering of gold|bibcode = 1968PMag...18..377T|issue=152 }}</ref> An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program [[Stopping and Range of Ions in Matter|TRIM]].<ref name=Ziegler1984>{{cite book| author=J. F. Ziegler, J. P, Biersack, U. Littmark|title=The Stopping and Range of Ions in Solids," vol. 1 of series Stopping and Ranges of Ions in Matter| publisher= Pergamon Press, New York| year=1984|isbn=978-0-08-021603-4}}</ref>

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 [[Collision cascade#Heat spikes (thermal spikes)|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.<ref name=Ghaly1994>{{cite journal|author1=Mai Ghaly  |author2=R. S. Averback |name-list-style=amp | journal= Physical Review Letters|volume=72|year=1994|pages=364–367| title=Effect of viscous flow on ion damage near solid surfaces|pmid=10056412|doi = 10.1103/PhysRevLett.72.364| issue= 3| bibcode=1994PhRvL..72..364G}}</ref> 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.<ref name=Bouneau1982>{{cite journal|author1=S. Bouneau |author2=A. Brunelle |author3=S. Della-Negra |author4=J. Depauw |author5=D. Jacquet |author6=Y. L. Beyec |author7=M. Pautrat |author8=M. Fallavier |author9=J. C. Poizat |author10=H. H. Andersen  |name-list-style=amp |title=Very large gold and silver sputtering yields induced by keV to MeV energy Au<sub>n</sub> clusters (n=1–13)|journal= Phys. Rev. B |volume=65|page= 144106 |year=2002|doi=10.1103/PhysRevB.65.144106|bibcode = 2002PhRvB..65n4106B|issue=14 |s2cid=120941773 |url=http://hal.in2p3.fr/in2p3-00011005/document}}</ref> For animations of such a process see "Re: Displacement Cascade 1" in the [[#External links|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|eV]].

''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.