Understanding the R-I curve
Long-term cycling, or endurance, is a key attribute for any memory candidate. One of the challenges associated with testing a new or unknown cell is to determine the appropriate pulse parameters (amplitude, rise/fall, width) for the RESET and SET pulses. This is usually an iterative process, first starting with a reasonable RESET pulse, then optimizing the SET pulse. Just as with flash memory, it is possible to over-stress the cell and permanently damage it. Because the RESET voltage is the largest, the search for the appropriate RESET voltage must be done with care.
Figure 3: In a GST-based PCM memory device, application of heat by a resistor converts the crystalline material into the amorphous phase (orange half-circle) in what is known as the forming process.
The R-I curve is a typical phase-change NVM device measurement. The SET pulse voltage is increased while the RESET and SET resistance values are being measured. Figure 4 shows a four-pulse waveform for one point in the R-I curve, which was generated and measured with a single instrument. Note that the entire four-pulse test waveform, which is one step in a SET sweep, requires only tens of microseconds. Some test systems may use SMUs for the resistance measurements, but this requires additional switching and much longer test times. The newest pulse I-V instrumentation available measures the voltage and current simultaneously at multiple points in the test, providing flexibility while ensuring that the proper measurements are made. Here, spot means are taken (green boxes in Figure 4) to capture the measurements needed to produce the resulting R-I curve.
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Figure 4: R-I curve is generated by extracting measurements from this four-pulse waveform. The RESET-measure-SET-measure pulse waveform includes four pulses: the RESET pulse, which puts the material into a high resistance, amorphous state; the pulse that measures the resistance of the RESET state; the SET pulse, which puts the material into a low resistance, crystalline state; and the pulse that measures the resistance of the material in the SET state.
The SET current of the R-I curve usually has the characteristic U shape usually known as a “bathtub” curve (see blue curve in figure 5). Note that the RESET resistance is traditionally displayed with the SET resistance, even though the RESET pulse is not changing throughout the sweep, causing the RESET R to be a straight line. Plotting the RESET R does indicate if the RESET pulse is adequately resetting the material after each SET pulse.
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Figure 5. R-I curve shows the resistance variation of the SET state resistance, based on the data captured from the tops of the pulses shown in Figure 4. The SET curve (blue) displays the characteristic “bathtub” shape.