The effect of bias-temperature stress on Na+ incorporation into thin insulating films

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: SiO(2), SiO(x)N(y), and Si(3)N(4). For SiO(2), 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 SiO(2)/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 SiO(2) with varying thickness. On the other hand, SiO(x)N(y) and Si(3)N(4) 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.