The effect of bias-temperature stress on Na+ incorporation into thin insulating films
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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.
KeywordsMobile ions Bias-temperature stress ToF-SIMS Barrier layers
This work was jointly funded by the Federal Ministry of Economics and Labor of the Republic of Austria (contract 98.362/0112-C1/10/2005) and the Carinthian Economic Promotion Fund (KWF; contract 18911|13628|19100). The authors thank R. Grilz from Infineon Technologies Austria AG for production of the wafers. Furthermore, A. Limbeck (instrumental analytical chemistry group; TU Vienna) is acknowledged for conducting F-AAS measurements and A. Henriksson (inorganic chemistry division; TU Vienna) for the assistance in terms of ellipsometry.
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