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Improved Focused Ion Beam Sample Preparation Techniques for Transmission Electron Microscopy and Failure Analysis of Memristor Devices

The memristor is a two-terminal device derived from materials that exhibit resistance switching, wherein the resistance of the material is tunable by an applied electric field [1]. This switching process is reversible, and is also non-volatile, so that the change in resistance is maintained for a long period of time even after the applied field is removed. These devices offer distinct advantages over existing flash memory architecture, due to higher switching speed, lower power consumption, better scalability and higher endurance [2]. However, realizing their potential for more advanced applications, as in neuromorphic computing, has been hampered due to device failure associated with variability in current-voltage (I-V) characteristics between individual memristor devices integrated into a single chip. This variability may be attributed to difference in their switching behavior due to factors such as morphology and thickness of individual dielectric layer, and chemistry of the electrode/dielectric interfaces. In this contribution we perform cross-sectional transmission electron microscopy (TEM) to investigate microstructural factors that contribute to device failure. A key aspect in this study is the improvements made to focused ion beam milling (FIB) techniques used for sample preparation, by incorporating procedures used in traditional methods employing Ar+ ion milling. The memristor device design investigated in this study consisted of a SiO2 dielectric layer sandwiched between Ti-W top electrodes and W bottom electrode. In the present study we present results from two devices: one that exhibited the expected switching behavior and the other, a failed device that exhibited poor resistance values. These devices are labeled normal and shorted device, respectively. The samples were examined by TEM convention bright-field