Sven Kayser oral presentation (SN1-Mon2-3-4)
Large O-cluster ions as sputter beam for ToF-SIMS depth profiling of alkali metals in thin SiO2 films
1 ION-TOF GmbH, Heisenbergstr. 15, 48149 Muenster, Germany
2 Infineon Technologies Austria AG, Siemensstraße 2, 9500 Villach, Austria
3 Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria
For many years, the acquisition of time of flight - secondary ion mass spectrometry (ToF-SIMS) depth profiles of alkali metals (mainly Li, Na, K) in insulating materials has been a challenge due to the presence of artefacts arising during the measurement procedure. For depth profiling of inorganic materials, the sample erosion is commonly carried out using positive ions as O2+ or Cs+, depending on the desired polarity of detected secondary ions. This approach, however, has an inherent drawback, since the impinging ions within the sputter cycles are generating positive charges in the near-surface region of the insulating sample. In combination with a grounded counter electrode, as for instance the sample holder, an electric field arises, in particular prominent in thin insulating films. Fast diffusors as positively charged alkali metals with small ion radii will immediately react on this electric field by migration towards the cathode, causing a distorted depth profile.
In literature, several approaches and countermeasures have been proposed as for instance O2+ depth profiling at low temperatures or the use of a sputter beam consisting of C60 cluster ions [1,2]. However, up to now there is no efficient method which delivers artefact free depth profiles for alkali metal distributions in thin insulating films while preserving a high secondary ion yield by the signal enhancing effect of O2+ and having a reasonable erosion rate.
The use of large positively charged O-clusters as sputter beam combines all those advantages, enabling an effective acquisition of depth profiles with artefact free alkali metal distributions. On the one hand, the average energy per incoming sputter particle is reduced heavily due to the high number of O atoms per cluster ion. On the other hand, the net charge introduced in the insulator decreases in comparison to e.g. O2+, allowing a more efficient compensation of surface charges. In this presentation, the use of large O-cluster ions is demonstrated using the example of thin SiO2 layers (200 nm), deposited on a Si substrate and implanted by alkali metal ions. Such films are frequently in use for semiconductor products or coatings of a variety of materials, indicating the strong need for a fast and effective way to record artefact free alkali metal distributions in insulators.
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