Sputtering, physical yield

Lithium is the other primary liquid metal being considered as a plasmafacing material. Lithium has the intriguing property that its melting temperature is 181°C, which means that it can be studied in either its solid or liquid state. Measurements of the physical sputtering yield of lithium across the phase transition (Fig. 14.7 [22]) have verified that no modifications to sputtering theory are needed to predict sputtering from liquid surfaces. One... 

The standard picture of erosion from a chemically inert surface exposed to energetic particles can be divided into two categories. The first, physical sputtering, is independent of surface temperature and proportional to the incident particle flux such that the physical sputtering rate, Rps = Yps Jin (where Yps is the temperature independent physical sputtering yield and J n is the incident particle flux). The second is the sublimation/evaporation rate, JQ, which does not depend on the flux of incident particles and strongly increases with increasing surface temperature. 

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). 

The relevance of chemical erosion became evident when introducing low Z elements, in particular carbon, for plasma facing components in tokamaks. The erosion yields of chemical processes and sputtering can be of the same order of up to a few percent but show significantly different dependencies physical sputtering has a strong j-dependence, whereas the yield Ych of chemical erosion varies with surface temperature Ts and flux density, as is shown in the following. 

For Be and W, experimental data and the fit for the sputtering yield at normal incidence are shown in Fig. 9.f for D ions as a function of incident particle energy. Physical sputtering data are available for both materials from energies close to the threshold energy (9eV for D on Be and 200 eV for D on... 

With the exception of sputtering by C+ ions, the physical sputtering of Be and W appears to be well documented. Carbon bombardment, in general, leads to the deposition of protective surface layers (see Sect. 9.2.2). Only in conditions where carbon self-sputtering exceeds unity, i.e., at grazing incidence or at temperatures above 1800 K, could a few yield data points be obtained for clean W surfaces. 

For all three regimes of carbon erosion outlined above [44] the eroded species were investigated intensively. At room temperature and energies in the keV range, physical sputtering occurs with carbon atoms being eroded, predominantly. At elevated temperatures, chemical erosion increases the erosion yield... 

Fig. 11.12. Energy dependence of the erosion yield Y(Ar+) of physical sputtering of a C H film by Ar+ ions (open symbols) and the yield Y(Ar+ H) for chemical sputtering by a simultaneous flux of Ar+ ions and H atoms (full symbols). The dash-dotted and solid lines are carbon erosion yields from TRIM.SP calculations for the sputtering of carbon by argon ions using a carbon-surface-binding energy of Esb = 0.1 eV and of EBb = 4.5 eV, respectively. The dotted line gives the absolute erosion rate by the applied flux of H atoms only...

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