Design Article
Alternative NVM technologies require new test approaches, Part 1
Peter Hulbert, Keithley Instruments Inc.
11/13/2012 9:00 AM EST
Characterizing with the I-V curve
Figure 6 shows how the waveform measurements map to the R-I results. The R-I curve is generated by increasing the SET pulse height and making the measurements shown. This is a common test, but because the waveform is specified by voltage and time segments, other types of sweeps are possible, such as a SET pulse fall-time sweep.
In addition to the R-I curve shown in figure 4, we can characterize PCM behavior using an I-V curve. The I-V sweep test uses a single inverted-V-shaped pulse to capture the PCM switching characteristics. Because this test uses a pulse with V and I sampling, there are two ways to view the data. Figure 7 shows the waveform data, with I and V plotted versus time. This is not the typical way to display the characteristics but shows how the test is performed. Usually, this test is performed in the DC realm with SMUs. With SMUs, this test can take tens to hundreds of milliseconds. Note that the time scale in Figure 7 is tens of microseconds.
State-of-the-art test equipment provides a fast, simple method for characterizing PCM devices using a single instrument to both pulse and measure the response of the material. In part 2 of this article, we will discuss testing of FRAM, another alternative to floating gate flash memory.
About the author
Peter Hulbert is a product and applications development engineer for Keithley Instruments (Cleveland, Ohio), which is part of the Tektronix test and measurement portfolio. He holds a bachelor’s degree in physics from Washington State University. His career in measurement instrumentation has overlapped a good portion of the electromagnetic spectrum—from ionizing radiation to the far infrared.
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Figure 6 shows how the waveform measurements map to the R-I results. The R-I curve is generated by increasing the SET pulse height and making the measurements shown. This is a common test, but because the waveform is specified by voltage and time segments, other types of sweeps are possible, such as a SET pulse fall-time sweep.
Click image to enlarge.
Figure 6: Various measurements of the waveform (figure 4) help generate the R-I curve. The process involves sweeping the SET curve while measuring the RESET resistance (red M), SET current (green M), and SET resistance (blue M).
In addition to the R-I curve shown in figure 4, we can characterize PCM behavior using an I-V curve. The I-V sweep test uses a single inverted-V-shaped pulse to capture the PCM switching characteristics. Because this test uses a pulse with V and I sampling, there are two ways to view the data. Figure 7 shows the waveform data, with I and V plotted versus time. This is not the typical way to display the characteristics but shows how the test is performed. Usually, this test is performed in the DC realm with SMUs. With SMUs, this test can take tens to hundreds of milliseconds. Note that the time scale in Figure 7 is tens of microseconds.
Click image to enlarge.
Figure 7: Transient I-V waveform for a PCM cell shows the applied voltage (blue curve, left axis) and the resulting current flowing through the PCM test device (red curve, right axis), plotted versus the time on the x-axis.
The second way to view the data is by plotting current versus voltage, with a dual (up and down) sweep (see figure 8). The data plot shows the switching from high resistance (amorphous) to low (crystalline) states around 1 V for the rising edge of the pulse (green curve). The falling edge of the triangle pulse (blue curve) shows the PCM material remaining in the low resistance state.
Click image to enlarge.
Figure 8: Plot of current versus voltage for a PCM device shows parameters as the material switches from the high resistance (amorphous) to low resistance (crystalline) states around 1 V for the rising edge of the pulse (green curve).
State-of-the-art test equipment provides a fast, simple method for characterizing PCM devices using a single instrument to both pulse and measure the response of the material. In part 2 of this article, we will discuss testing of FRAM, another alternative to floating gate flash memory.
About the author
Peter Hulbert is a product and applications development engineer for Keithley Instruments (Cleveland, Ohio), which is part of the Tektronix test and measurement portfolio. He holds a bachelor’s degree in physics from Washington State University. His career in measurement instrumentation has overlapped a good portion of the electromagnetic spectrum—from ionizing radiation to the far infrared.
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R G.Neale
11/15/2012 4:29 AM EST
Peter did you mean this ""Much like other types of NVM technologies, a PCM cell must be formed before it displays the consistent switching necessary to be a memory element.""
While PCM has a number of problems, the need to “form” the device should no longer be necessary. Imagine the problems, cost and time, of forming each bit of an 8G-bit array. Unless the author knows something that we are not being told forming effects should have gone away with the use of the crystallized active material as one electrode in the PCM structure.
In the distant past, when PCM devices were fabricated with the active material in the amorphous state then there is a first switching (or forming) pulse. This is because the state of disorder, which determines the threshold voltage, of the as deposited film, differs from that of the same material after the first set/reset cycle, also there may have been some element separation that may have an impact on the value of the threshold voltage. While it is still necessary to empirically establish the optimum operating parameters by an iterative process for each new PCM device structure I think the author may have inadvertently confused that process with a need for forming. Once the optimum operating conditions are established with crystallized active electrodes there should be no need for forming.
One other question have the annotations for Current and Voltage been accidentally transposed in Figure 7 or is this a power PCM device??
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Kristin Lewotsky
11/16/2012 6:13 PM EST
Good catch, Ron -- the axes labels on Figure 7 were indeed transposed; look for a corrected version to appear shortly.
Kristin
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resistion
11/16/2012 7:23 PM EST
When you have a resistance-based memory like PCM or STT MRAM and it needs to scale to lower currents, there don't seem to be any chip array testers (not individual cell probes) that can go down to nano amps for read currents. Hope this situation will change.
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