Sputter redeposition, sputtering

A number of advantages exist for ion milling compared to plasma etching or RIE. Because of the collimated beam of ions, essentially vertical profiles are possible. Also, profile tapering can be achieved by tilting the substrate relative to the ion beam. In addition, ion milling is performed at pressures at least 100 times lower than those used in plasma etching or RIE. Therefore, redeposition of sputtered material is reduced. 

Unfortunately, since ion milling is a purely physical process, selectivity is generally poor. Indeed, selectivity in such systems depends almost exclusively on differences in sputter yield between materials. Finally, since the etch products are not volatile, redeposition and trenching can be serious limitations (146). 

Ion milling is a more widely applicable etching technique in that all materials may be sputtered away anisotropically. The ion milling rate typically does not vary more than a factor of 100. Redeposition of material from the substrate or surrounding fixtures can lead to undesirable cross-contamination. Hosaka et al. (39) used SIMS to show that redeposition of ionic impurities can occur during ion etching of SiO layers on Si when using a... 

Any redeposition of sputtered wall atoms between burn cycles will make blistering more probable and more rapid. It appears that stainless steel first walls will suffer exfoliation for plasma edge temperatures below 100 eV, at all first-wall temperatures. Blistering can be expected to occur within 103 h of operation of a fusion power reactor. 

Secondary ions return to the sample surface through the influence of the electric field at the cathode. Individual atoms and clusters of atoms undergo collisions that may dissociate the clusters and redeposit material at the surface. A percentage of these sputtered atoms, however, diffuse into the negative glow for subsequent excitation and ionization. 

There has been increased effort in recent years to pin down carbon sources in existing machines i.e., to understand and (possibly) influence material transport toward the divertor and, most important, to elucidate the transport mechanisms of carbon inside the divertor and from the divertor volume towards remote areas or gaps. However, there are still large uncertainties, and further work is needed. Material data uncertainties with carbon include the possible flux dependence, if any, of carbon chemical sputtering, very low energy but non-thermal ( 1-3 eV) hydrocarbon reflection coefficients, and overall properties of redeposited materials [33]. The tools to compare results between different devices are not clearly established and the underlying physics of the transport of the eroded carbon in the SOL by flows and drifts... 

Therefore, whether or not materials removed by sputtering redeposit on the substrate (cathode) surface depends on the relationship between the gap distance and the CDST e2. If the gap distance is equal to or greater than the CDST e2, materials removed by sputtering will not be redeposited on the substrate surface. Otherwise,... 

In the absence of physical confinement a = b), the sputtered particles move and diffuse without any physical barrier to the downstream because the electrode distance b is much higher than the mean free path A, of the sputtered particles. However, when the gap distance decreases, the influence of physical confinement b — d) becomes significant because the path of sputtered particles becomes restricted. When a is smaller than e2, which is a function of mean free path A, many sputtered particles cannot pass easily through the gap and deposit outside of the edge of the glass tube, where lower Ar ions bombard the surface (Figure 17.19). Therefore, redeposition occurs when a is smaller than e2. 

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