Alternative NVM technologies require new test approaches, part 2

Testing FRAM
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.






The 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 effects.

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. P_SW is the polarization change during the up pulses (P_SW = red P – red U). Q_SW is an average of the two charges from the two polarities (two up and two down pulses, Q_SW = ((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 effect.

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.