At present, it is considered that the sputtering phenomenon is a direct result of the elastic collision, and the sputtering is completely an exchange process of kinetic energy. When positive ions bombard the cathode target and an incident ion initially strikes atoms on the surface of the target, an elastic collision occurs. It directly transfers its kinetic energy to an atom or molecule on the surface of the target, and this surface atom obtains kinetic energy and then transfers the energy to internal atoms of the target. After a series of cascaded collisions, when one of the atoms or molecules obtains momentum that points outside the target surface, and has energy to overcome the surface barrier (binding energy), it can be separated from other atoms or molecules nearby and escape the target surface to become the sputter atoms.
Fig.1. Schematic Diagram Cascade Collision In Solid-State Sputtering Process
It can be seen that the sputtering process is the process that energy exchange by the incident ions through a series of collisions. The energy that the incident ions transferred to the sputtered atoms is only about 1% of the original energy. Most of the energy is consumed in the surface layer of the target by cascade collision and then converted into vibration of the crystal lattice. Most of the sputtered atoms come from the superficial layer, and it can be considered that the atoms begin to peel off from the surface when the target is sputtered. If the energy of the bombardment ion is insufficient, the atoms on the surface of the target can only vibrate without sputtering. If the bombardment ion energy is high, the ratio of the number of sputtered atoms to the number of bombarded ions will decrease, because the ionization phenomenon occurs as the bombardment ion energy is too high.