SECONDARY IONS EMISSION FROM SI(100)

Abstract

The secondary ion intensity of sputtered Si ions has been measured as a function or the emission energy using a calibrated mass analyzer. By assuming lhe Sigmund-Thompson energy distribution for neutrals, lhe ionisation probability R' is inferred. The transmission function or the secondary ion mass spectrometer used has been determined from the trajectories or secondary ions emitted from the target surface to the analyzer through the use of SlMION software. The transmission function (T(E)) is found to have a power law dependence on the emission energies (E) of the secondary ions, in the form T(E) ? E-? with ? = 1.53 ± 0.02. This agrees well with the value or ? = 1.50±0.01 obtained experimentally by sputtering K+ ions from a Cu target. The observation also shows that K atoms are sputtered off mainly as charged particles, a condition that is consistent with the electron tunneling model. The method described in this thesis is suitable for determining the transmission function for both positive and negative secondary ions in any mass spectrometer with an electrostatic or magnetic analyzer. For Si+ ions sputtered from the clean surface, it is found that the behaviour of R+ high emission energies is consistent with a neutralisation process via the electron tunnelling mechanism (resonant electrons tunneling from the substrate to the outgoing ions). The possibility or electronic excitations induced by the collision cascade in Sroubck's model is also considered. Energy spectra of spullcrcd St ions have been measured for different oxygen exposures under Ar+ bombardment. The surface chemical state is monitored by Auger electron spectroscopy (AES). It is found that at low oxygen partial pressure (< 1 x 10-7((1 mbar), which corresponds to a silicon suboxide phase, R' can be well described by two processes - electron tunneling and local Si-O bond breaking. However at high oxygen partial pressure(> 1 x 10-7((1 mbar), the bond breaking mechanism is unable to account for the behaviour of R+ in the high ion energy range. We have also compared our experimental results with that obtained by Passeggi et al using computer simulation or a tight-binding model.