Sputtering

Sputtering is a physical vapor deposition, PVD process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions. It is commonly used for thin-film deposition, as well as analytical techniques (see below).

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Physics and chemistry of sputtering

Standard physical sputtering is driven by momentum exchange between the ions and atoms in the material, due to collisions (Behrisch 1981, Sigmund 1987). The process can be thought of as atomic billiards, with the ion (cue ball) striking a large cluster of close-packed atoms (billiard balls). Although the first collision pushes atoms deeper into the cluster, subsequent collisions between the atoms can result in some of the atoms near the surface being ejected away from the cluster. The number of atoms ejected from the surface per incident particle is called the sputter yield and is an important measure of the efficiency of the sputtering process. Other things the sputter yield depends on are the energy of the incident ions, the masses of the ions and target atoms, and the surface binding energy of atoms in the solid.

The primary particles for the sputtering process are supplied either by a plasma that is induced in the sputtering equipment, or an ion or electron accelerator. In the plasma sputtering devices, a variety of techniques is used to modify the plasma properties, especially ion density, to achieve the optimum sputtering conditions, including usage of RF (radio frequency) alternating current, utilization of magnetic fields, and application of a bias voltage to the target.

Numerous variations of the process are possible. The one described above is strictly speaking standard physical sputtering in the linear cascade regime . This means that the sputtering is induced by a sequence of binary collisions between atoms with kinetic energies clearly higher than the cohesive energy of the solid (>> 10 eV). It can be qualitatively well understood with the billiard ball analogy described above, although in reality the interatomic interactions are not the same as for hard balls. This kind of sputtering can be well predicted theoretically with statistical collision algorithms which simulate the sequence of atomic collisions. The sputtering yield in this regime is directly related to the stopping power (particle radiation).

Another variety 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 locally essentially melts the crystal. 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 (Ghaly and Averback 1994). 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 (Bouneau 1982).

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 target is bombarded. 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 solid will during prolonged bombardment become enriched in the B component thereby increasing the probability that B is sputtered such that the composition will approach AB again.

Chemical sputtering is not a very well defined concept. It is variously used to describe several effects. Since preferential sputtering is related to the chemical composition of the sample, it is sometimes called chemical sputtering (even though the sputtering mechanism may be purely physical linear cascade sputtering).

One important variety of chemical sputtering is reactive ion sputtering. This means sputtering carried out with chemically active ions, 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 (Schoolcraft and Garrison 1991)

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 (Küppers 1995). 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 (Salonen 2001).

The term electronic sputtering can mean either sputtering induced by energetic electrons, or sputtering due to very high-energy or highly charged heavy ions which lose energy to the solid mostly by electronic stopping power, where the electronic excitations cause sputtering (Schenkel 1997).

In the case of multiply charged projectile ions a particular form of electronic sputtering can take place which has been termed potential sputtering (Neidhart 1995, Sporn 1997). In these cases the potential energy stored in multiply charged ions (i.e., the energy necessary to produce an ion of this charge state from its neutral atom) is liberated when the ions recombine during impact on a solid surface (formation of hollow atoms). This sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can already take place at ion impact energies well below the physical sputtering threshold . Potential sputtering has only been observed for certain target species (Aumayr 2004) and requires a minimum potential energy (Hayderer 1999).

For film deposition

Main article: Sputter deposition

Sputter deposition is a method of depositing thin films by sputtering, i.e. eroding, material from a "target," e.g., SiO₂, which then deposits onto a "substrate," e.g., a silicon wafer. Resputtering, in contrast, involves re-emission of the deposited material, e.g., SiO₂, during the deposition also by ion bombardment.

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.

For analysis

Another application of sputtering is to etch away the target material. One such example occurs in Secondary Ion Mass Spectroscopy (SIMS), where the target sample is sputtered at a constant rate. As the target is sputtered, the concentration and identity of sputtered atoms are measured using Mass Spectroscopy. In this way the composition of the target material can be determined and even extremely low concentrations (20 µg/kg) of impurities detected. Furthermore, because the sputtering continually etches deeper into the sample, concentration profiles as a function of depth can be measured.

In space

Sputtering is one of the forms of space weathering, a process that changes the physical and chemical properties of airless bodies, such as asteroids and our moon. It is also one of the possible ways that Mars has lost its atmosphere.

References

(Behrisch 1981) R. Behrisch (ed.), Sputtering by Particle bombardment I (Springer, Berlin) 1981

(Sigmund 1987) P. Sigmund, Nucl. Instr. Meth. Phys. Res. B 27 (1987) 1.

(Ghaly and Averback 1994) Mai Ghaly and R. S. Averback, Physical Review Letters 72 (1994) 364

(Schoolcraft and Garrison 1991) T. A. Schoolcraft and B. J. Garrison, Journal of the American Chemical Society 113 (1991) 8221.

(Küppers 1995) J. Küppers, Surface Science Reports 22 (1995) 249.

(Salonen 2001) E. Salonen et al, Physical Review B 63 (2001) 195415.

(Schenkel 1997) T. Schenkel et al, Physical Review Letters 78 (1997) 2481

(Neidhart 1995) T. Neidhart et al, Physical Review Letters 74 (1995) 5280

(Sporn 1997) M. Sporn et al, Physical Review Letters 79 (1997) 945

(Hayderer 1999) G. Hayderer et al, Physical Review Letters 83 (1999) 3948

(Aumayr 2004) F. Aumayr and HP. Winter, Philosophical Transactions of the Royal Society London A 362 (2004) 77