Measuring HfO2/Si stacks by secondary ion mass spectroscopy (SIMS) has become a common task in the semiconductor industry. However, when analyzing these stacks with an oxygen beam, a strong impact of the experimental conditions on the Hf+ signal as well on the Si signal can be observed. In order to understand these effects, we investigated the effect of oxygen on the depth profiling of these stacks using an Atomika 4500. In this paper, the ionization probability of Hf sputtered from a HfO2/Si stack as well a Hf implantation in Si is measured for different beam incidence angles and energies. It is shown that basically two physical mechanisms are responsible for the Hf signal intensities. The first part of the signal can be correlated with the impact of oxygen on the Hf ionization degree, in line with the classical bond-breaking theory. The second mechanism is more difficult to define, but appears linked to the emission of Hf into a different (neutral, molecules?) emission channel, thereby reducing the available Hf for detection as Hf+. Finally, a very strong influence of Hf on the Si and Si-cluster ions is reported. (C) 2004 Elsevier B.V. All rights reserved.