Ion Sputtering of Solid Surfaces

The animation below shows a Molecular Dynamics computer simulation of the elastic collisions and their consequences when an Argon ion of 5 keV kinetic energy (red) hits the (111) surface of a silver single crystal under normal incidence. The different snapshot images are separated by a time interval of 200 femtoseconds. The particular trajectory shown represents one of the large events, where many atoms are sputtered. When averaged over many different trajectories (corresponding to different impact points on the surface), the average number of sputtered atoms under these conditions amounts to approximately eight. From this single trajectory, a number of observations can be made which are characteristic of the sputtering process :

• The majority of sputtered particles leaves the surface within approximately one picosecond after the impact
• Most of the sputtered atoms originate from the uppermost atomic layer (yellow)
• The sputtered particles travel several nanometers within one picosecond, i. e. the average emission velocity is of the order of 105 cm/s (much faster than evaporated material !)
• The sputtered flux contains not only atoms but also agglomerates of two or more atoms (molecules and clusters). The formation of larger sputtered clusters like the one shown below represents one of our main research topics.

5 keV Ar -----  Ag (111)