Developing new PCM materials and devices requires characterizing a variety of parameters including:
- Re-crystallization rate—These rates are defined by the PCM material; they can be as short as several tens of nanoseconds, and probably will soon drop still further, making the ability to measure quickly more important than ever.
- Resistance-current (RI) curves—The RI curve is one of the most common parameters collected during PCM characterization (see figure 2). A sequence of pulses is sent through a DUT (see figure 3). The first one, a RESET pulse, sets the resistance of the DUT to the high value. It is followed by a DC-read or MEASURE pulse that’s usually 0.5 V or lower in order to avoid affecting the state of the DUT. This is followed by a SET pulse and another MEASURE pulse. The entire sequence is repeated multiple times, with the amplitude of the SET pulse slowly increased to the value of the RESET pulse.
Figure 2: RI curve in red
Figure 3: Pulse sequence for creating an RI curve includes RESET pulses (tall red curves), SET pulses (shorter red pulses), and resistance (R) measurements (short rectangular black pulses).
- I-V (current-voltage) curves—Here, the voltage sent through a DUT previously RESET to its highly resistive state is swept from low to high (see figure 4). The dynamic switch from a high- to a low-resistive state in the presence of a load resistor produces a characteristic RI curve with a snapback, an area of negative resistance. Snapback is a side effect of the R-load technique that has long been used to obtain both RI and I-V curves.
Figure 4: Example of I-V current voltage sweep.
In the standard R-Load measurement technique, a resistor is connected in series with the DUT, allowing current to be measured across the DUT by measuring the voltage across the load resistor (see figure 5). Active, high-impedance probes and an oscilloscope are used to record the voltage across the load resistor. Current across the DUT will be equal to the applied voltage (VAPPLIED
) minus the voltage across the device (VDEV
), divided by the load resistance. The values of the load resistor usually range from 1 to 3 kΩ. This technique involves a tradeoff. If the load resistance is too high, RC effects and the voltage division between the R-Load and the DUT limit the performance of this technique. If the resistor value is too small, however, it impacts the current resolution.
Figure 5: Standard R-Load technique.
Recently developed ultra-fast pulse source and measure instrumentation, in combination with built-in current-limiting capabilities, eliminate the need for the load resistor and allows for more accurate characterization of low currents in the RI curve. The elimination of the load resistor also eliminates the snapback side effect. This new technique, which takes both I-V and RI curves in a single pulse sweep, uses a high-speed pulse testing solution that can source voltage and simultaneously measure both voltage and current responses with high accuracy, with short rise and fall times (see figure 6).
Figure 6: New current-limiting technique for PCM characterization (top) combines the dual-channel 4225-PMU ultra-fast I-V module and the 4225-RPM remote amplifier/switches to extend sensitivity (bottom). Integrated with a semiconductor characterization system, they not only provide the measurement functions necessary to characterize a PCM device but also the ability to automate the entire testing process.
About the author
Alex Pronin is a lead applications engineer with Keithley Instruments, Inc. in Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. He holds an M.S. in Physics from the Moscow Institute of Physics and Technology and a Ph.D. in Material Science from Dartmouth.