Microscopy study of the conductive filament in HfO 2 resistive switching memory devices S. Privitera a,⇑ , G. Bersuker b , B. Butcher b,c , A. Kalantarian b,d , S. Lombardo a , C. Bongiorno a , R. Geer c , D.C. Gilmer b , P.D. Kirsch b a Institute for Microelectronics and Microsystems (IMM), National Research Council (CNR), Zona Industriale VII Strada 5, Catania 95121, Italy b SEMATECH, Albany, NY, USA c College of Nanoscale Science and Engineering (CNSE), Albany, NY, USA d Stanford University, 450 Serra Mall, Stanford, CA 94305, USA article info Article history: Available online 3 April 2013 Keywords: Resistive switching Conductive filament Electron energy loss spectroscopy abstract A detailed physical analysis of the conductive filament electrically formed in HfO 2 -based resistive switch- ing memory devices with both Hf and Ti metal oxygen exchange layers is presented. The filament, observed by applying transmission electron microscopy (TEM), scanning TEM (STEM), and electron energy loss spectroscopy (EELS) techniques to 50  50 nm 2 cells, is a cone-shaped metal-rich region in the HfO 2 dielectric of the resistive switching device. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Resistive switching (RS) random access memory (RRAM) based on the resistance change of transition metal oxides, such as HfO 2 , has attracted significant interest due to its low power operation, switching speed, high endurance and dense integration. The con- ductive filament formation mechanism in RRAMs is not yet fully understood, although it has been shown that the RS properties strongly depend upon the metal electrodes. Proposed models [1– 6] (primarily based on electrical characterization) agree that the switching phenomenon is due to formation and rupture of a con- ductive filament. However, the filament physical properties remain a controversial issue, in part due to a complexity associated with locating a filament in the device, and preparing a sample for TEM study without affecting the filament composition. Direct micro- scopic observation of the conductive filament in the memory cell is thus critically important to support RS models. In this study, we have employed scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to observe electrical stress-formed conductive filaments in the HfO 2 -based crossbar devices with Hf or Ti top metal gettering or oxygen ex- change layers (OEL), and TiN electrodes. 2. Experimental procedure Crossbar devices with size of 50 nm  50 nm have been manu- factured using either Hf or Ti metal layer over the HfO 2 dielectric film as an oxygen exchange layer for oxygen gettering. TiN bottom electrodes have been employed in both type of devices. Thickness of HfO 2 layer is 5 nm. Current–voltage (I–V) characteristics for both fresh and formed devices have been measured as a function of temperature in the À50 °C to 100 °C range using an Agilent HP4156B parameter ana- lyser. The conductive filament formation (the forming operation) has been achieved by DC I–V voltage sweep at room temperature. The structural characterization of the conductive filament by the Transmission Electron Microscopy (TEM) technique has been performed using a JEOL JEM2010 equipped with the electron en- ergy loss spectroscopy (EELS) and with scanning TEM (STEM) imaging capabilities. To study the morphology of the formed con- ductive filaments, two different techniques have been employed: STEM in dark field configuration and EELS at low energy. The for- mer is sensitive to the local average atomic number while the lat- ter is strictly related to the plasmon losses, determined by the local chemical composition and phase. Resulting micrographs have a lat- eral resolution of about 1 nm, mainly determined by the STEM electron beam size. 3. Results Fig. 1(a) shows the I–V characteristics measured from À50 °C to 100 °C, for a fresh (prior to forming) 50 nm  50 nm crossbar de- vice. The fresh cell exhibits low conductivity in the entire temper- ature range, with its conductance (dI/dV) exponentially increasing as a function of temperature (see Fig. 1(b)). Extracted activation energy for the conductance is 0.11 eV. 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.145 ⇑ Corresponding author. Tel.: +39 0955968233. E-mail address: stefania.privitera@imm.cnr.it (S. Privitera). Microelectronic Engineering 109 (2013) 75–78 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee