Recoil sputtering

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. 

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. 

Sputtering under conditions that are insufficient to induce a full isotropic coUi-sion cascade can still result in measurable sputter yields. These occur also under the same processes described earlier, but with far fewer collisions involved. As a result, a more energetic sputtered population will be noted that displays a more anisotropic angular distribution. This is sometimes referred to as recoil sputtering. 

Recoil sputtering Sputtering resulting from a single collision event... 

Fig. 1. The ballistic interactions of an energetic ion with a sohd. Depicted are sputtering events at the surface, single-ion /single-atom recoil events, the development of a collision cascade involving a large number of displaced atoms, and the final position of the incident ion. ° = normal atom ...

The surface structures of ionic liquids have been studied by direct recoil spectrometry. In this experiment, a pulsed beam of 2-3 keV inert gas ions is scattered from a liquid surface, and the energies and intensities of the scattered and sputtered (recoiled) ions are measured as a function of the incident angle, a, of the ions. Figure 4.1-16 shows a scheme of the process for both the scattered and sputtered ions. 

The incident ions cause recoil in the surface atoms. In studies of ionic liquids, only direct recoil - that is, motion in the forward direction - was measured. Watson and co-workers [56, 57] used time-of-flight analysis with a pulsed ion beam to measure the kinetic energies of the scattered and sputtered ions and therefore determine the masses of the recoiled surface atoms. By relating the measured intensities of the... 

As described in Sec. 3.1, an energetic ion will produce a collision cascade upon surface impact. Recoiled surface atoms within the nascent hot spot may escape into the vacuum. As the sputtered atoms emerge from the surface, they can capture or lose an electron to the surface. On metal surfaces where conduction band electrons are delocalized, secondary ions are formed when sputtered atoms undergo resonant electron transfer along their outgoing trajectory. The relations expressed by Eqs. (12)-(14) in Sec. 3.2 approximate the charge transfer probability more sophisticated models have also been developed. ... 

The sputtering yield is proportional to the number of displaced or recoil atoms. In the linear cascade regime, which is a reasonable approximation for medium mass ions (such as Ar), the number of recoils is proportional to the energy deposited per unit depth in nuclear energy loss. The sputtering yield, Y, for particles incident normal to the surface is then expressed as (Sigmund 1981)... 

The SRIM program, a binary collision Monte Carlo approach, has been used to predict sputtering yields (Fig. 12.2). The incident ions and the recoil atoms are... 

One of the unsolved problems in the interaction of low-energy ions with surfaces is the mechanism of charge transfer and prediction of the charge composition of the flux of scattered, recoiled and sputtered atoms. The ability to collect spectra of neutrals plus ions and only neutrals provides a direct measure of scattered and recoiled ion fractions. SARIS images can provide electronic transition probability contour maps which are related to surface electron density and reactivity along the various azimuths. 

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