Testing resistive memory devices

In part 1 of this article, I explored the basics of resistive random access memory (ReRAM) structures, as well as the test hardware available to characterise them. Now, I will address issues related to characterisation and forming, as well as endurance testing for 1R ReRAM structures.

In Part 1, I noted that the forming process of a ReRAM cell is the most important part of the characterisation because the forming process creates the switching characteristic in the cell, which determines almost all critical memory performance metrics. ReRAM forming results are shown in figure 6a (voltage and current transients—V and I vs. time) and 6b (the same data plotted as current vs. voltage with arrows added to show the time progression of the curve, to simplify comparison with the time-based figure 6a).

Current exponentially grows as voltage reaches a certain value. When the filament is established, the device switches into low resistance state, and the current limit prevents further growth of the filament. In these examples, forming was performed using a Keithley Model 4225-PMU Ultra-Fast I-V Module and two Model 4225-RPM Remote Amplifier/Switch Modules with current limit enabled. Typically, ReRAM researchers use DC instruments (source measure units, or SMUs) with current compliance for forming. When forming is done using a PMU/RPM, it's possible to measure current and power transients during the forming process. However, when it is done with an SMU instrument, the details of the process are completely hidden. (The technical literature on this topic indicates the forming process is controlled mostly by the value of the current limit and the speed of its activation.)

The same test routine used for Characterisation and Forming was applied to an already formed device to collect the "butterfly" curve shown in figure 7a and 7b. Figure 7a shows the current and voltage transient (I and V vs. time); Figure 7b illustrates the current vs. voltage in the shape of butterfly wings. The same routine, reramSweep, with different test conditions, can obtain both DC (SMU) data and pulse (PMU) data. This allows for switching between DC and pulse modes of ReRAM characterisation.