Effect of temperature on the retention of 2 keV Cs+ ions in Si

Introduction: Cesium is a commonly used species for SIMS, because it provides a high negative secondary ion yield. For more quantitative modeling of the secondary ion yield during SIMS the precise knowledge of the amount of Cs retained in the near surface region is vital. Its equilibrium concentration can be theoretically described by the implantation process and the material removal by (self) sputtering. However, the experimentally determined concentrations of Cs ions retained in Si [1] are lower than that predicted by TRIDYN or by the model of Schulz and Wittmaack [2]. This indicates more complex interaction of chemical (compound formation) or physical nature (texture), which is not described by the given models. The chemical incorporation of residual gas is thought to be a possible reason (e. g. [3]). Therefore, an IBA set-up including RBS and ERD for simultaneous in-vacuo detection of Cs and O in Si was developed to analyze the incorporation of oxygen during Cs sputtering as a function of sample temperature in order to draw conclusion of their activation threshold.
Results: Fig. 1 shows the saturation of Cs in Si at room temperature. The temporal development is consistent with the model of implantation and self sputter-ing. Regardless the low partial pressure of oxygen, there is also a strong correlation between Cs and O indicating ion induced incorporation pathways on top of the natural oxide on the untreated Si surface.
Fig. 2 shows the steady state areal densities for Cs and O as a function of temperature. Two discrete steady state regimes for Cs are observed at high (> 500°C) and low (< 250°C) temperatures with an transition, where the steady state areal density de-creases almost linearly by a factor of four. The O concentration behaves congruently, while the deviation at temperatures > 600°C might be attributed to thermal oxidation.
Conclusion: Even though no significant influence on the physical conditions of the sputtering processes are expected within the temperature range, the Cs areal density shows two discrete regimes, which, interestingly, correlate closely to the amount of O incorporated from residual gas even at base pressures below 10-7 mbar. The reported recrystallisation by rapid thermal annealing with an app. 10% increase of sputter yield shown by Anderson [4] for Ar in Ge seems to be much to small to explain this strong de-crease of steady state Cs concentration. Instead overlying chemical processes are thought to play a major role, e. g. the Cs enhanced formation of mixed oxides (e. g. Cs2O shown by Michel et al. [5]) can impact sputtering yields and/or thermally desorb in the ob-served temperature range under UHV conditions. The precise nature remains unclear up to now and follow-up experiments at different O partial pressures might provide further insights.
References: [1] Gnaser H. NIMB 267 (2009) 2808–2816. [2] Schulz F. and Wittmaack K. Ra-diat. Eff. 29 (1976) 31-40. [3] Berghmans B. and Van-dervorst W. J. Appl. Phys. 106 (2009), 033509. [4] Anderson G. S. J. Appl. Phys. 38 (1967) 1607-1611. [5] Michel E. G. et al. Phys. Rev. B 38 (1988) 13999-13406.