S.F. Belykh

MODEL FOR LARGE CLUSTER EMISSION IN ION SPUTTERING OF METALS APPLIED TO ATOMIC AND POLYATOMIC ION BOMBARDMENTS

Abstract We propose a model for the emission of large clusters (N⩾5) under ion bombardment of metals. In the frame of this model, the relative cluster yields have been calculated as a function of the number of atoms in the cluster for the sputtering of Al, Ag, Ta and Nb targets by atomic and polyatomic ions with energies of several keV/atom. The calculated data have shown a very good agreement with experimental results obtained by us and other researchers. On the basis of this agreement we have concluded that larger clusters are emitted as whole agglomerates during the early stages of the collision cascade evolution.

Effect of the electronic subsystem excitation on the ionisation probability of atoms sputtered from metals by atomic and molecular projectiles

In the present work an effect of excitation of the metal electronic subsystem on the ionisation probability of atoms sputtered under fast ion bombardment has been studied. Atomic and molecular primary ions with the same velocity were used to produce different degrees of the electronic excitations. Information on the ionisation probability was obtained from the kinetic energy distributions of Nb+ and Ta+ ions sputtered from the respective clean Nb and Ta targets by Au-m(-) projectiles (1 less than or equal tom less than or equal to3) with the energy of E-0 = 6 keV per atom. It was found that, as compared with the atomic ion bombardment (m = 1), the molecular one (m = 2,3) leads to the increase of the ionisation probability P-m(+) (P-1(+) < P-2(+) < P-3(+)). Such an effect depends on the kinetic energy E of the secondary ions, increasing with decreasing E. It was shown that the bombardment of metals by the molecular projectiles produces non-additive sputtering of atomic ions, which is determined by the joint action of such two factors as non-additive sputtering of atoms and non-additive process of their charge state formation. The results obtained are discussed in the framework of the model where the charge state formation occurs in electron exchange between sputtered atoms and a local surface area excited by the impact of the projectile

Nonadditive sputtering of silicon by keV energy molecular projectiles of heavy and light elements

Abstract In the present work a positive secondary ion emission from a silicon produced by Au m − projectiles (m = 1–3) with energies of E 0 = 9 and 18 keV and by Al m − projectiles (m = 1,2) with energies of E 0 = 6, 9, 12, and 18 keV have been studied. Anomalous high nonadditivity of sputtering as large cluster Si n + ions (n > 4) under molecular Au m − ion bombardment has been found. As compared with heavy (Au m − ) projectiles, the light (Al m − ) projectiles are not effective for sputtering of large cluster ions. For molecular Al m − ion bombardment nonadditivity of sputtering of small cluster Si n + ions (n ≤ 4) increases with decreasing of the energy E 0 from 9 to 6 keV/atom. This effect shows that the efficiency of nonadditive sputtering strongly depends on the penetration depth of molecular projectile and, hence, on the energy density deposited by molecular projectile into subsurface layers of the target from which the cluster ion emission occurs.

Non-additive sputtering of niobium and tantalum as neutral and charged clusters

Abstract An analysis of available literature data on both positive ion emission from Nb and Ta bombarded with 6 keV/atom Aum− atomic and molecular ions (m=1, 2, 3) and positive ionization probabilities of Nbn and Tan neutral clusters sputtered from the same metals by 5 keV Ar+ ions has been conducted. Dependencies of cluster yields Yn,m (regardless of charge state) on the number of atoms n in a sputtered particles were found to follow a power law as Yn,m∼n−σm where σm decreases with an increase of m. A non-linear enhancement of yields for large Nbn+ and Tan+ cluster ions (n>4) appeared to be a result of a non-additive process of sputtering rather than of a non-additive process of their ionization. A manifestation of the non-additive sputtering in kinetic energy distribution of secondary ions was found to be different for atomic and cluster ions.

Effect of the relaxation of the electron subsystem excitation in metals on the ionization probability of sputtered atoms

Abstract A model describing the effect of the relaxation of the electron subsystem excitation in metals on the ionization process of sputtered atoms is proposed. It explains the strong difference in experimental velocity dependence of the ionization probability P+ of sputtered atoms measured for the same Ar+–Cu system.

Effect of the projectile parameters on the charge state formation process in solid sputtering

Abstract An effect of atomic and molecular projectile parameters on the charge state formation process in silicon sputtering has been studied. It was found that the electronic subsystem excitation in a subsurface region of silicon depends on projectile parameters and it affects the ionization probability P+ of sputtered atoms.

Features of non-additive sputtering for various “molecular projectile–solid” systems

Abstract In the present work, a comparative study of non-additive sputtering of secondary positive ions from light and heavy targets bombarded by light atomic and molecular projectiles has been studied. It was discovered that, as compared with a light target, non-additive sputtering of a heavy target is increased strongly when the light molecular ions are used as projectiles. The increase of non-additive sputtering is explained by an effect of the back-scattered projectiles who transfer additional energy into the subsurface layers of the target.