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Projecting oxide lifetime by a step voltage method using electric field correction [MOS VLSI]

1. ABSTRACT
The validity of a projection method of
TDDB lifetime from the step voltage tests
has been investigated. The conventional
projection method is found not to be able
to project correct lifetime in intrinsic
failure mode. This is caused by a decreased
electric field of gate oxide due to resistance
of gate electrode. This work proposes
a correction method of the electric field
based on Fowler-Nordheim current transport,
which provides the excellent agreement
between projected and measured TDDB lifetimes
in all the failure modes.
2. INTRODUCl'ION
Reliability evaluation of thin oxide is
very important because oxide breakdown becomes
a serious problem with decreasing
device dimensions in integrated circuits[
l]. It is generally known that oxide
breakdown consists of three failure modes,
that is early, random, and intrinsic failure
modes, Device yield, reliability in
device operation and the intrinsic lifetime
can be estimated from early, random and
intrinsic failure modes, respectively.
Therefore, a simple method analyzing all
failure modes is required for not only developing
VLSI devices but also fabricating
them.
Time dependent dielectric breakdown
(TDDB) tests are generally used to estimate
oxide 1 ifet ime. However, oxide 1 ifet ime of
all the failure modes can not be obtained
by TDDB tests simultaneously because of
limited time of measurements, which requires
to evaluate all the failure modes
simply. Hu's group has proposed a evaluat
ion method that projects the TDDB 1 ifetime
from step voltage tests[21[3].
This work studies the validity of the
projecting method. It is found that for
larger gate electrodes, the projected lifetime
is different from the measured lifetime
in intrinsic failure mode. This paper presents
the cause of the difference and the
correct ion method.
 



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