Much like PCM and other NVM materials, FRAM cells require a forming step or process before the cell exhibits reproducible switching behavior. The PUND (Positive, Negative, Up, Down) or FRAM endurance tests can be used to apply the forming pulses. The PUND test is appropriate when a small number of pulses are required for forming, whereas the endurance test is better when a larger number of pulses are required. The hysteresis curve can indicate if the test device was sufficiently formed (see the gap at 0 V in Figure 9). Proper test parameters on a good device should show a complete loop, with the beginning and end at 0 V. The hysteresis test applies positive and negative V-shaped pulses while measuring the current. Because the current is sampled continuously, calculating the total charge is straightforward. Figure 10 shows the test signal applied to the ferroelectric capacitor. Figure 9 shows the hysteresis curve, which was extracted from the data in figure 10.
Click image to enlarge.
Figure 10: Hysteresis
waveform, showing the measurements vs. time (x-axis). This graph shows
the voltage waveform (blue, left y-axis) applied to the ferroelectric
capacitor. The red curve (right y-axis) shows the current flow.
characteristic shape of the hysteresis curve is not only a measure of
the inherent ferroelectric material performance but may also show
degradation due to the semiconductor processing that occurs after the
ferroelectric capacitor fabrication. Additionally, the transient
waveform provides possible insight into the material and process
The PUND test characterizes the polarity change in the
ferroelectric material. It is called PUND because four pulses are
applied sequentially: Positive, Up, Negative, Down (see figure 11). Note
the change in the shape of the current between the two up pulses (red P
and red U) and similarly for the two down pulses (red N and red D). The
first pulse requires additional charge/current to change the
polarization (red P or red N), compared to the second pulse, which has
the capacitive charging only (red U or red D). The difference between
them (P-U or N-D) represents the polarization charge or memory effect. PSW
is the polarization change during the up pulses (PSW
= red P – red U). QSW
is an average of the two charges from the two polarities (two up and two down pulses, QSW
= ((red P – red U) + (red N – red D))/2). Effectively, the U and D
pulses are baseline subtractions to remove any current signal due to the
device or test setup and just provide the signal due to the switching
In addition to fundamental device and material
characterization, the PUND test is used to determine the proper voltages
and timing for the endurance test. Traditional pulse instruments can
output only one pulse amplitude and would require reconfiguration
between the U and N pulses, greatly increasing test time.
Click image to enlarge.
11: PUND waveforms generated and measured by the Model 4225-PMU with
Model 4225-RPMs. This graph shows the applied voltage pulses (blue, left
y-axis) and the current response (red, right y-axis). The values for P,
U, N, and D are extracted from the current waveform.