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 insulating mu m-thick oxide layer on Ti-48Al-2Cr-2Nb led us to check for a possible 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 in the HFM sputtering and plasma processes. However, the positive surface potential 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 conclude that the Al+ and Ti+ peaks in the sputter time profiles of the mu m-thick oxide layer on Ti-48Al-2Cr2Nb which are accompanied by an O+ deficiency cannot have been caused by SI+. Instead, their more probable origin is the inhomogeneous Al2O3 interlayer itself. Together with the residues of a topmost TiO2 layer which has strongly been depleted in O by preferential sputtering, the relative O+ deficiency may be explained without assuming SI+ contributions.