Knock-on sputtering

Figure 1.3 Highly simplified pictorial illustration of the knock-on sputtering process (this is the most common form of sputtering as outlined in Section 3.2.1.1). The solid black ball represents an incoming primary ion, the hollow circles refer to the atoms making up the solid surface, the solid arrows refer to incoming ion trajectories, and the gray arrows refer to substrate atom trajectories. Also illustrated are approximate time scales over which the respective processes take place. As outlined in Section 3.3, some percentage of the sputtered population will exist in the ionized state. It is the ionized population (the secondary ion population) that is recorded in SIMS.

Figure 3.1 Highly simplified illustration of a two-step process in which atomic secondary ion emission proceeds via knock-on sputtering resulting from keV primary ion impact (left box) followed by secondary ion formation/survival (right box) from the sputtered population. M in the right box can represent any element with the superscript referring to its associated charge and e" referring to an electron. The ejection of molecular ions appears to follow similar albeit more complicated routes.

Kinetic sputtering describes the removal of surface-bound atoms, ions, or molecules, which occurs purely through the momentum transfer (elastic collisions). This form includes knock-on sputtering and recoil sputtering. 

As electronic excitation is not considered within kinetic sputtering, the collisions can be likened to an atomic-scale billiard ball game that is initiated on primary ion impact. The valence electron shells of the atoms/ions involved would thus represent the billiard ball s surfaces. The linear cascade model, which describes the most prevalent form of ion-induced sputtering, at least that from atomic ions and ions comprising small molecules (common examples used in SIMS include 0 , 02 , Cs, Ar" ", Xe" ", and Ga ), assumes a specific form of kinetic sputtering in which a full isotropic collision cascade is produced close to the surface. This is one form of knock-on sputtering. 

When a full isotropic collision cascade is not formed, the sputtering process becomes more anisotropic. This is noted in recoil sputtering, which is another form of knock-on sputtering. As fewer colhsions occur in this form of sputtering, deviations from the trends implied by the linear cascade model are noted. The linear cascade model is covered in Section 3.2.1.1. 

Linear Cascade Model Sputtering resulting from elastic collisions knock-on sputtering) is the most well understood of all the other forms of... 

Knock-on sputtering proceeds via a sequence of individual elastic collisions (momentum transfer) occurring between atoms and/or ions as they come into close proximity to each other. How close they approach depends on the energies involved. At and below 100 keV, the distance of closest approach can be defined via the Coulombic potential as ... 

The various forms of sputtering not described within the context of knock-on sputtering include the following ... 

Examples of the use of two commonly used packages for modeling knock-on sputtering as applies to energetic atomic ion projectiles are shown in Figures 3.7-3.10. 

Sputtering of intact molecular species initiated via atomic or small molecular projectiles is generally limited to static SIMS conditions. This results from the fact that the damage induced by the kinematics of knock-on sputtering effectively destroys molecular information over the region impacted. 

Quasi-resonance charge transfer will accompany resonance charge transfer if a core hole exists within the departing atomic emission of the substrate. For this to occur, a significant amount of energy must be imparted during the atomic collisions in knock-on sputtering. When present, an additional channel for electron transfer... 

Knock-on sputtering Removal of atoms/molecules through momentum transfer... 

SIMS is inherently damaging to the sample since ion bombardment removes some material from the surface. However, other forms of damage may also occur. These include surface roughening, knock-on effects, preferential sputtering, decomposition, and implantation of source ions [49]. 

On the other hand, SIMS takes advantage of the destructive nature of the ion probe. Atoms can be knocked free (sputtered) from the surface by the bombarding ions and those that become ionized are analyzed by conventional mass spectrometry I70). A large number of different kinds of ions can be emitted from the surface. The resolution is also quite good. Thus, although SIMS is not as surface sensitive as ISS, it does provide more detailed information about the surface chemistry. ISS and SIMS, therefore, complement one another. Furthermore, since the ion probe sputters away the surface that is being analyzed, the change in the chemistry of the surface as a function of depth below the surface can be studied by these techniques. 

The dependence of sputtering yield on ion energy is shown in Fig. 5.3 for different elemental targets. At moderate ion energies in the order of 30 to 1,000 eV, sputtering is characterized by knock on effects where the incident ions collide with a surface atom and these atoms further react with additional atoms. These events may eventually lead to a release of target material atoms. 

This dependence can be described by an extended SLS model including surface transport [107]. In addition to sputter removal from the surface layer, surface atoms can be transferred into deeper layers, also known as the knock-on effect in ion sputtering [108]. 

Sputtering, unlike (thermal) evaporation, is a collision process by which an atom formerly bound to a solid becomes liberated the energy needed to hberate the atoms is provided by binary collisions either with the projectile direct knock-on) or with other target atoms that received their kinetic energy through a sequence of... 

The inability of knock-on mechanisms, inclusive of the linear cascade model, to effectively predict sputter yields in the cases described earlier arises from the fact that such mechanisms describe sputter yields as arising from many individual momentum transfer processes occurring in a linear sequence. However, as outlined in Section 3.2, ejection of atoms/ions or molecules from a solid surface can also occur through ... 

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... 

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