Traditionally, PCM test devices are measured with a custom setup consisting of a pulse generator, load resistor, and an oscilloscope, with software developed in-house for overall control and data extraction. This approach is straightforward but requires an active probe or floating scope when the load (or sense) resistor is on the high side of the test device. Measuring the voltage drop across this resistor allows for easy calculation of the current flowing, which is why it’s also called the sense resistor. Unfortunately, this I*R voltage drop also complicates the analysis of the results. This I*R drop can be minimized by using a smaller value sense resistor, but this has the drawback of directly reducing the current measure sensitivity, so some compromise must be made. Sometimes, multiple sense resistors are supported, but this also increases the complexity of the system configuration and control software. New testing technology allows users to simultaneously apply pulses to a memory device or material and measure current and voltage with a single instrument.
Modern pulse I-V instrumentation measures the current with a feedback ammeter and a high speed analog-to-digital converter (ADC) to increase sensitivity, while the system provides for high level control, data extraction, and presentation (see figure 2). Multi-pulse waveform generation capability in these systems can be used to characterize the switching mechanism of a memory device in both the transient and I-V domains, using DC-like extractions from the tops of pulses or plotting the measurements taken during pulse transitions. The multi-pulse capability allows for control over each segment of the waveform, delivering precise control of each transition for a single pulse or for hundreds of pulses in a single waveform. Ideal systems further allow users to easily increase the number of channels of synchronized pulse I-V capability. As a result, as the material development using two-terminal test structures transitions into multi-terminal structures, it’s possible to scale the test system to allow for a pulse-per-pin test approach or test multiple devices simultaneously to gather the quantities of data necessary for statistical analysis as a part of modern product and process development.
Figure 2: Model 4225-PMU Ultra-Fast I-V Module and two Model 4225-RPM Remote Amplifier/Switch Module permit the integrated simultaneous measurement of current and voltage on each channel.
Some PCM structures have what is called a select diode or a protection diode integrated in series with the PCM cell. In addition to introducing some issues for the I-V curve, this diode makes the effective fall time shorter. The current flowing through a diode is exponentially dependent on the voltage, so a small decrease in the voltage results in a dramatic reduction in the current flowing through the diode, especially at currents less than 50 µA. Therefore, when testing devices with the series selection diode, the pulse fall time is not as critical for small current test devices and permits testing with standard pulse I-V instrumentation.
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. One way to explain the forming process is that it creates the active area of the PCM cell. The active area is the portion of the chalcogenide material that transitions between the amorphous and crystalline states (see figure 3). The goal of the forming process is reproducible cycling between the SET and RESET states.