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2015: The new mechanism of sputtering with cluster ion beams

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MSU Physicists (Prof. V.S. Chernysh, Dr. A.E. Ieshkin, Faculty of Physics, and Senior Researcher Yu.A. Ermakov, Scobeltsyn Nuclear Physics Research Institute, MSU) discovered a new mechanism of sputtering with cluster ion beams.

A cluster is an assembly of atoms or molecules which quantity is in range from a few up to a few tens of thousands units per cluster. These formations can exist in vacuum as individual particles or particle beams. Recent decades, interaction of cluster ions with solids has been under extensive investigation.

Processes of cluster-solid interaction and monomer-solid interaction are significantly different. Principal difference is that a large number of atoms of a cluster simultaneously collide with at least the same number of atoms of a solid. Furthermore, considering initial cluster and monomer energies are the same, depth of cluster atoms penetration into the solid is less than path of the monomer. So, under cluster ion beam irradiation a larger amount of energy is deposited onto an elementary surface area.

These features were utilized by our colleagues from Japan and USA to develop an industrial technique of surface smoothing and ultra-shallow implantation. Though cluster ion beams are already widely used for practical purposes, processes of their interaction with surfaces are not quite understood.

In our laboratory, angular distributions of matter sputtered under irradiation of targets with 10keV argon cluster ions were studied experimentally. The experiments were conducted using the first Russian gas cluster ion beam accelerator, created in the joint laboratory of Physical Faculty of MSU, SINP MSU and JSC “Tensor”. It was found that under irradiation of targets with a high modulus of elasticity (Mo, W), a dominating small-angle component of angular distribution appears, while under irradiation of soft targets, secondary emission occurs mainly under large emission angles (so-called lateral sputtering, known before).

To explain the discovered phenomenon, a new mechanism is proposed, which considers elastic properties of targets. Estimations show that transfer of momentum from a 1000-atomic argon cluster to atoms of target creates pressure up to a few tens Mbars in the surface layers. It causes compression of target. Since binding energy in a cluster is small (about 10 meV), the cluster rapidly collapses. In this process, nonlinear cascades of atomic collisions are formed, which are responsible for lateral flux of sputtered atoms. Then, relaxation of elastic compression after cluster collapse causes emission of sputtered particles along the surface normal. Thus, the distinctive feature of the sputtering mechanism found is sputtering as a result of interaction of a cluster as an integral object with the target.

The results of this work have been published in the paper: V.S. Chernysh, A.E. Ieshkin, Yu A. Ermakov. The new mechanism of sputtering with cluster ion beams. Appl. Surf. Sci., 326, 285 (2015).