Resistance change in memory structures integrating CuTCNQ nanowires grown on dedicated HfO 2 switching layer Ch. Muller a,⇑ , D. Deleruyelle a , R. Müller b , M. Thomas c , A. Demolliens c , Ch. Turquat c , S. Spiga d a im 2np, Institut Matériaux, Microélectronique et Nanosciences de Provence, UMR CNRS 6242, Aix-Marseille Université, IMT Technopôle de Château Gombert, F-13451 Marseille Cedex 20, France b imec, Interuniversity MicroElectronics Center, Kapeldreef 75, B-3001 Leuven, Belgium c im2np, Institut Matériaux, Microélectronique et Nanosciences de Provence, UMR CNRS 6242, Université du Sud Toulon Var, BP 20132, F-83957 La Garde Cedex, France d Laboratorio MDM, IMM-CNR, via C. Olivetti, I-20041 Agrate Brianza, Italy article info Article history: Received 29 June 2010 Received in revised form 11 October 2010 Accepted 17 October 2010 Available online 13 November 2010 The review of this paper was arranged by Dr. Y. Kuk Keywords: Resistive switching Memory devices CuTCNQ complex Conductive-AFM Redox process abstract The present paper deals with the bipolar resistive switching of memory elements based on metal–organic complex CuTCNQ (copper-7,7’,8,8’-tetracyanoquinodimethane) nanowires grown on a dedicated HfO 2 oxide switching layer. Switching characteristics are explored either at millimeter scale on pad-size devices or at nanoscale by using conductive atomic force microscopy. Whatever the investigation scales, the basic memory characteristics appear to be controlled by copper ionic transport within a switching layer. This latter corresponds to either HfO 2 layer in pad-size devices or nanogap formed at nanoscale between the atomic force microscopy conductive tip and CuTCNQ surface. Depending upon the observa- tion scale, the switching layer (either HfO 2 oxide or nanogap) acts as a matrix in which copper conductive bridges are formed and dissolved thanks to redox processes controlled in alternating applied bias voltages. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Last ten years have seen the emergence of new memories labeled with the acronym RRAM for ‘‘Resistive Random Access Memories’’ [1–4], based on various mechanisms of resistance switching excluding crystalline-amorphous phase transition as used in PCM (Phase Change Memories) [4–6]. In its simplest form, RRAM memory element relies on a MIM structure (Metal/Insula- tor/Metal) whose conductivity can be electrically switched be- tween high (state ‘‘0’’) and low (state ‘‘1’’) resistive states. RRAM memory cells are gaining interest for (i) their intrinsic downscaling characteristics compared to devices based on a polysilicon floating gate; (ii) their potential small size; and (iii) their ability to be orga- nized in dense multi-layered crossbar arrays. Different ‘‘flavors’’ of RRAM memory elements were disclosed depending on the nature of the bi-stable material and mechanism involved in the resistance change [1,4]. Among them, those inte- grating metal–organic complex CuTCNQ (copper-7,7’,8,8’-tetracya- noquinodimethane) attract much attention due to (i) their low cost process; (ii) their compatibility with Back-End of Line (BEOL) with integration in small interconnect structures [7]; and (iii) their large discrimination window between high and low resistance states. However, thermogravimetric analysis has shown that CuTCNQ nanowires thermally decompose at temperatures below 300 °C [8]. This signifies that a low temperature encapsulation process (for example at 250 °C) is required for CuTCNQ integration into BEOL wafers instead of the traditional process taking place around 400–450 °C. Investigations of the switching mechanism of CuTCNQ-based memory elements have shown that the resistance change is not intrinsic to the metal–organic complex itself, but due to a combi- nation of CuTCNQ and a dedicated ‘‘switching layer’’ (SL) [9–13]. Complex CuTCNQ is an ionic copper (I) salt which likely plays the role of a solid ionic conductor and provides mobile copper cat- ions (Cu + ) [14,15]. The ‘‘switching layer’’ is a porous ion-permeable layer in which electrochemical processes involving the Cu + cations provided by CuTCNQ lead to formation/dissolution of conductive metallic Cu channels [11,12]. When the switching layer is at least partially bridged by a conductive channel the memory element is in low resistance state (LRS) whereas it is in high resistance state (HRS) as the channels are at least partly dissolved. This mechanism is in agreement with a bipolar resistive switching requiring polar- ization reversal for changing memory state. In pioneering works, the switching was attributed to the formation/dissolution of Cu bridges through a native oxide layer spontaneously formed at the 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.10.006 ⇑ Corresponding author. Tel.: +33 491054779; fax: +33 491054782. E-mail address: christophe.muller@im2np.fr (Ch. Muller). Solid-State Electronics 56 (2011) 168–174 Contents lists available at ScienceDirect Solid-State Electronics journal homepage: www.elsevier.com/locate/sse