In‐situ operation of oxide‐based memories in a transmission electron microscope
Authors: Villepreux Edouard , Cooper David , Saghi Zineb , Roule Anne , Mouis Mirielle 
Published: December 2016

There is a great deal of activity in the development of new memory technologies that can be used to provide the required density and reliability for future generations of data storage [1, 2]. At this time there is a bewildering array of proposed systems each with advantages and disadvantages. One of the problems with the development of these types of materials systems is that it is not clear exactly how these devices function and as a consequence, it is difficult to select the best combinations of materials to provide the best overall performance.

In this presentation we will present results that have been obtained on a range of different TaO and Ta2O5 OxRAM structures that have processed using reactive and RF deposition physical vapour deposition (PVD). The focus on this work is the mapping of oxygen and results will be presented that have been obtained by a range of different techniques including aberration‐corrected high‐resolution annular bright‐field (ABF) scanning transmission electron microscopy (STEM) imaging, EDS (Energy dispersive X‐Ray Spectroscopy) or Electron Energy Loss Spectroscopy (EELS) for the measurement of oxygen concentration (atoms) and electron holography and differential phase contrast (DPC) for the distribution of electrostatic potential caused by the distribution of the oxygen. An example is shown in Figure 1 where a (a) high resolution ABF STEM and (b) HAADF STEM image of a TaO/Ta2O5 stack with TiN top and bottom electrodes can be seen. Figures 1(c) and (d) show EELS spectra that have been acquired for the N, Ti, O and Ta regions. Figure 1(e) and (f) show quantitative maps and profiles respectively. Here the variations in the Ta and O concentrations can be observed across the active region of the device. In this presentation we will highlight the best techniques for measuring the variations of oxygen in the devices and discuss which stacks have the best stoichiometry for memory device applications.

The key to the performance of these devices is the movement of oxygen during switching. As a consequence it is necessary to perform these observations in situ in the TEM to avoid problems with locating the conducting filaments during specimen preparation and additional issues such as device retention and other modifications that could occur during specimen preparation. In this presentation we will present results on specimens that have been switched using a dedicated in‐situ holder in the electron microscope. Figure 2 shows how a movable probe is placed onto a FIB‐prepared specimen with nm‐scale accuracy by using a Nanofactory biasing system. An electrical pulse is then used to switch the specimen in situ in the TEM. In this presentation we will show how to avoid common problems such as device heating and electrical shorting of the specimen from redeposition during preparation by focused ion beam milling. Finally in this presentation we will show results obtained on our TaO specimens that have been switched in situ in the microscope.