Potential sputtering

Kinetic-assisted potential sputtering describes the removal of surface-bound atoms, ions, or molecules, which occurs through various modes of excitation induced as a consequence of the momentum imparted by the incoming ion. 

For the small molecular ions (examples commonly used in SIMS include AUn", Bi ", and SF5" ), energy transfer can also proceed through various modes of electronic excitation. As a result, they are best described as forms of kinetically assisted potential sputtering. The various modes of electronic excitation are often discussed in the form of localized elevated temperatures that quickly dissipate. 

Also of note is the fact that this form of sputtering is insensitive to the charge of the incoming primary ion beam. This is in contrast to the strong charge sensitivity noted for potential sputtering (discussed in Section 3.2.1.2). 

As for the remainder, additional mechanisms/models have been suggested. These are based around varions inelastic modes of energy transfer, i.e. forms of potential sputtering with some being kinetically assisted. These are covered in Section 3.2.1.2. 

As introduced in Section 3.2.1, potential sputtering, whether kinetically assisted or not, results from inelastic energy transfer processes, with electron-phonon interactions playing a part. Cooperative motion describes a kinetic process in which a single primary ion impact induces the movement of a collective body of atoms within the solid. All of the above result in sputter yields (these are covered in Section 3.2.2) that are greater than that expected based on the linear cascade model (see Section 3.2.1.1). 

Kinetically assisted potential sputtering can take several forms depending on the primary ions, the conditions used, and the matrix examined. For dense atomic and the small molecular ion impact (In ", Bi ", Au , SFj" ", etc.), these generally tend to assume the presence of overlapping collision events within the lattice that occur as a result of the same initial collision event (the linear cascade model assumes individual events). This overlap ensues when momentum transfer is constrained within a more localized volume and/or when multiple atoms from the same impacting ion strike the same region. 

Potential sputtering does not require the assistance of knock-on effects. In other words, no momentum transfer is required. As a result potential sputtering can occur even without the primary ion directly colliding with the solid s surface. It only needs to be in close enough proximity to allow for electron transfer. This is realized as any charged particle in close proximity to a solid surface will undergo some form... 

Figure 3.4 Highly simplified pictorial illustration of sputtering believed to occur during kinetically assisted potential sputtering induced on small molecular ion impact. This can also describe the first step in cooperative sputtering from large cluster ion impact. The gray region represents an outward moving dense halo.

Potential sputtering thus serves to explain why sputtering can be noted, albeit at extremely low yields, below the kinetic sputtering threshold (the kinetic sputtering threshold is between 15 and 40 eV depending on the system) that scales with the primary ion charge (Malherbe 1994). 

Figure 3.5 Highly simplified pictorial illustration of the four-step mechanism believed to describe potential sputtering. This multistep process described in the text is initiated through electronic excitations induced as an ion approaches a solid surface.

Except for the interaction of highly charged primary ions, the present consensus tends to favor the defect-mediated sputtering model (Rabalais 1994). For highly charged ions, the intense ultra-fast excitation model appears likely (Aumayr and Winter 1994, 2003). As the probability of inelastic energy loss depends on the overlap of the respective orbital wave functions, simulations of a pure potential sputtering process requires quantum mechanics. As mentioned in Section 3.2.1.3, this is considered outside the scope of this text. 

As sputter rates are more commonly applied in analytical circles, further aspects on sputter rates and potential sputter rate issues noted in depth profiling are covered in Section 5.3.2.4. Details on methods for measuring sputter rates are discussed in Section 5.4.1. 

Lastly, with the exception of the less common form of sputtering referred to as potential sputtering (see Section 3.2.1.2), little dependence of sputtering yields is seen with primary ion charge. 

Secondary ions display little to no dependence on the charge state of the primary ion species. The one exception is noted when potential sputtering is in effect. 

For completeness sake, there also exists a form of sputtering known as potential sputtering. This describes sputtering as occurring through purely inelastic processes, i.e. without the requirement of momentum transfer. As extensive energy transfer is involved, these typically result in atomic emissions alone. These are rare and tend only to occur on specific highly ionic matrices. 

Potential sputtering Removal of atoms/molecules through excitation processes... 

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