Al and Ti secondary neutral and secondary ion emission from oxide samples in the high-frequency sputtering mode of HF-plasma SNMS

A strong Al+ and a minor Ti+ peak without a proportional increase of the O signal in SNMS high-frequency sputtering mode (HFM) time profiles of an in sulating mu m-thick oxide layer on Ti-48Al-2Cr-2Nb led us to check for a po ssible contribution of positive secondary ions (SI+). SI+ and SI- (negative secondary ions) can be detected in ion energy spectra. This is shown using Al+, O-, AlO-, and AlO2- ions sputtered from massive Al2O3. Similarly, and depending on stoichiometry, also Ti+ from mixed sintered, microscopically inhomogeneous Al2O3-TiO2-SiO2 pellets has been identified to be partly SI+. The subtraction of an assumed contribution of ionized secondary neutrals ( SN+) suggests that SI+ may form several 10% of the detected ions obtained i n the HFM sputtering and plasma processes. However, the positive surface po tential of some 10 V being necessary to cause detectable SI+ contributions does not build up on mu m-thin insulating layers. Therefore, we have to con clude that the Al+ and Ti+ peaks in the sputter time profiles of the mu m-t hick oxide layer on Ti-48Al-2Cr2Nb which are accompanied by an O+ deficienc y cannot have been caused by SI+. Instead, their more probable origin is th e inhomogeneous Al2O3 interlayer itself. Together with the residues of a to pmost TiO2 layer which has strongly been depleted in O by preferential sput tering, the relative O+ deficiency may be explained without assuming SI+ co ntributions.