Secondary ion emission from silicon under 8 keV O2 + and Ar+ ion bombardment

Abstract

The energy distributions of the secondary ion yields from silicon under 8 keV O2 + or Ar+ ion bombardment show two distinct peaks when the surface is heavily oxidised. For positive ions, a low energy peak (at between 6 and 8 eV) appears with the formation of SiO2 on the surface. When this peak is dominant in the energy distribution, the exponent N of the high energy tail E-N shows a linear relation with the m/z of the secondary ion. The physically sputtered species (SiO+, Si2O+, etc.) are located on the main trunk while the chemically sputtered species (SiO+, Si2O+, etc.) form branches. In cases where the high energy peak is dominant, no relationship between N and m/z can be observed. The observed peaks are deduced to originate from separate ionisation mechanisms which depend upon surface conditions. Results from the negative secondary ions show that the first-order effect and the surface polarisation model are possible mechanisms of negative ion enhancement. The question of cluster emission is also discussed in relation to the behaviour of N vs m/z.