The effect of bias-temperature stress on Na⁺ incorporation into thin insulating films
2011
Krivec, Stefan | Buchmayr, Michael | Detzel, Thomas | Froemling, Till | Fleig, Juergen | Hutter, Herbert
The action of Na⁺ incorporation into thin insulating films and transport therein under influence of a bias voltage and temperature (BT stress) is the subject of this work. Deposited onto highly n-doped Si wafers, the insulators get BT stressed and subsequently investigated by means of time-of-flight-secondary ion mass spectrometry (ToF-SIMS). A thin PMMA film, spin-coated onto the insulator, serves as host matrix for a defined amount of Na⁺, provided via sodium triflate. Combining BT stress and ToF-SIMS depth profiling enables the unambiguous detection of Na⁺, incorporated into the insulating material. The insulators of interest vary in their nitride content: SiO₂, SiOxNy, and Si₃N₄. For SiO₂, it is shown that once a threshold BT stress is exceeded, Na⁺ gets quantitatively incorporated from PMMA into the underlying insulator, finally accumulating at the SiO₂/Si interface. A quantitative assessment by combination of Butler-Volmer kinetics with hopping dynamics reveals activation energies of E a = 1.55 − 2.04 eV for Na⁺ transport in SiO₂ with varying thickness. On the other hand, SiOxNy and Si₃N₄ films show a different Na⁺ incorporation characteristic in this type of experiment, which can be explained by the higher coordination of nitrogen and hence the reduced Na⁺ permeability within these insulators.
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