Physical sputtering is driven by momentum exchange between the ions and atoms in the material, due to collisions. The incident ions set off collision cascades in the target. When such cascades recoil and reach the target surface with an energy above the surface binding energy, an atom can be ejected. If the target is thin on an atomic scale the collision cascade can reach the back side of the target and atoms can escape the surface binding energy `in transmission'. The average number of atoms ejected from the target per incident ion is called the sputter yield and depends on the ion incident angle, the energy of the ion, the masses of the ion and target atoms, and the surface binding energy of atoms in the target. For a crystalline target the orientation of the crystal axes with respect to the target surface is relevant.
The primary particles for the sputtering process can be supplied in a number of ways, for example by a plasma, an ion source, an accelerator or by a radioactive material emitting alpha particles.
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 (www.srim.org) described in "Stopping and Range of Ions in Solids."
A different mechanism of physical sputtering is heat spike sputtering. This may occur when the solid is dense enough, and the incoming ion heavy enough, that the collisions occur very close to each other. Then the binary collision approximation is no longer valid, but rather the collisional process should be understood as a many-body process. The dense collisions induce a heat spike (= thermal spike), which essentially melts the crystal locally. If the molten zone is close enough to a surface, large amounts of atoms may sputter due to flow of liquid to the surface and/or microexplosions. Heat spike sputtering is most important for heavy ions (say 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, ...). 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 10000 .
Physical sputtering has a well-defined minimum energy threshold which is equal to or larger than the ion energy at which the maximum energy transfer of the ion to a sample atom equals the binding energy of a surface atom. This threshold typically is somewhere in the range 10–100 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, and/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 be AB.
Sputtered atoms ejected into the gas phase 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 usually uses an argon plasma.
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 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 SIMS equipment to enhance the sputter rates. The mechanisms causing the sputtering enhancement are not always well understood, but for instance the case of fluorina etching of Si has been modelled well theoretically.
Sputtering which is 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 temperature 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.
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