Reliability Aspects of Thin Dielectric Films Used in NVM ICs
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write/erase operation mode. In WRITE - mode tunnel current tends to the value a-C12 and, hence electric oxide field E is stabilized: (I) . T + V. - Vth (T)), -- ' -• (oc. E =dC1 d CY where ct-ramp rate of write voltage, T-time duration of write ramp voltage, VO -threshold voltage with uncharged floating gate, Vth (T) -write threshold voltage, d-dioxidhe thickness, C 12 capacitance between control and floating gates and Cx-total capacitance of floating gate. Changing cc, we have obtained oxide I-V 2 characteristics with a good t Fowler Nordheim ==1 approximation in the wide Id, current range: 10-16_10-7 A (see &.C12 Fig.3). A similar procedure 1ý can be applied in ERASE mode. In this mode drain Id- .......... current is also stabilized, but on the level exceeded cuC 12. Id-=-[(d.-C 12+IssmYax ) Ismax .................... '- , t It can be explained from the Fig. 1.Scheme of measuring in W/E modes, dynamic drain and well known model of e-h pairs generation by energetic tunnel substrate currents versus time. electrons8 on the drain and, hence an appearance of additional electrons in drain and holes in substrate. The observations of dynamic currents, threshold voltage window and low-field conductance using idea of current stabilization are useful for the degradation study. Vg
Vg t
write
erase
77U .P
RESULTS Charge state of tunnel dioxide As W/E cycling is repeated, V0h window is modified and the beginning of current stabilization (see Fig. 1) is monotonically drifted. Due to the injection current stabilization, and as a consequence the injection field stabilization, an equation of charge trapping kinetics is simplified. From our measurements of the threshold voltage window drift versus cycles, we have received the different types of holes and electrons traps. As opposed to cross-section, the density of traps and their centroids notably depend on the technology. Table I. Common types of traps for the dioxide thickness 130 A in FLOTOX-tr.: type of traps: hole traps electron traps electron traps
cross-section (cm 2): 10"
- 10"16
centroid location: in the vicinity of injection interface
probable reason:
(60 - 100) A
-SiOO*Si-
commonly close to Si0 2 -poly Si
SiO-
i017-10"18
10-20
--Si.
Simple method of the tunnel current stabilization on different levels in W/E mode allows us to vary a tunneling barrier distance and, hence, to take into account an influence of charge trapped very close to the injecting interface. At low injection current (field) mode (10"16_1013 A, 6 - 8 MV/cm) after 1-10 W/E cycles a positive charge is removed from tunnel dioxide entirely. It allows to determine separately the effects of positive and negative charge on oxide degradation. 28
Quantum Yield of electron impact ionization in silicon and Localization In Erase mode of FLOTOX- transistor, i.e. when the high negative voltage is applied to the control gate, drain and substrate are grounded, F.N. tunnel electrons become "hot" when entering the n' Si drain, with a minimum energy of qelb (the potential drop from Si0 2 to Si & 3.2eV)
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