| Summary: | 碩士 === 國立交通大學 === 電子工程學系 === 85 === In this thesis, we proposed a novel two-phase subthreshold
transient current measurement method and an analytical model to
characterize the stress induced low-level leakage current (SILC)
in a flash EEPROM cell. The associated steady-state and
transient characteristics of the read-disturb degradation was
evoluated. In order to investigate the oxide charge effect on
the read-disturb degradation, a specially designed experiment
with twoerase methods were performed. The measurement method
consists of two alternating phase; one is the read-disturb phase
and the other is the subthreshold current measurement phase. By
varying the gate bias in the read-disturb phase, the field
dependence of the SILC and the read-dsiturb mechanism can be
obtained. An analytical model relating a subthreshold current
transient to the charge variations in the tunnel oxide and in
the floating gate has been derived. Since in the model the
subthreshold current exhibits an exponential dependence on oxide
charge and floating gate charge, this method can provide a
better resolution than the traditional threshold voltage method.
Two erase methods, a conventional method and a modified method,
were employed in this work. Our study reveals that the transient
component of the SILC is attributed to oxide charge trapping/
detrapping while the steady-state component of the SILC is due
to trap-assisted sequential tunneling. Threshold voltage shifts
resulting from these two components were characterized
separately. During read-disturb, in spite of the large initial
transient, thethreshold voltage shift is mainly affected by the
steady-state component.However, in the application of a low-
voltage operation, the transient component may appear to be a
dominant read-disturb degradation mechanism. Our study shows
that the transient component can be significantly suppressed by
using a modified erase method. Furthermore, by using the two-
phase subthreshold transient current technique, an extremely low
leakage current (about 1E-19 Amp) can be measured.
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