As described in [95, 96], a constant base-level CP measurement with
is performed using a gradually increasing pulse amplitude
.
Until the desired stress level is reached, starting from
down to
,
the pulse slopes have to be kept constant to obtain comparable results. Constant
pulse slopes ensure that the upper and lower energy boundaries of the active
energy interval remain unchanged when
increases [50]. Due to a
constant pulse slope the amplitude of
is proportional to the pulse
rising (often referred to as leading) and falling (trailing) time. Given the
additional requirement of a constant duty cycle, the rise and fall times
have to be adapted at every voltage step within the CP measurement to
obtain the proper charge pumping current
. Since it is inevitable to
change both the pulse width and also the rise and fall times one has to
ask for the potential pitfalls: Are OFIT-data obtained during stress and
relaxation comparable? If that is not the case, is there some possibility to
correct this nonconformity? These questions will be examined in the
following.
Starting with Fig. 5.13 the two large arrows pointing up and down reveal
some important aspects of the temporal evolution of the pulses during a CP
measurement. The charge pumping current at stress conditions
(
) differs a lot when compared to that obtained during relaxation
(
). The higher the NBTI stress conditions, the larger the
-signal becomes. This can be partly attributed to the desired effect of using
the measurement setup to also stress the device. However, it cannot fully account
for the observed behavior.