750 IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 6, JUNE 2013 Leakage Current–Forming Voltage Relation and Oxygen Gettering in HfO x RRAM Devices Kristina G. Young-Fisher, Gennadi Bersuker, Brian Butcher, Andrea Padovani, Luca Larcher, D. Veksler, and David C. Gilmer Abstract— We observe a trend between initial leakage currents in polycrystalline HfO x resisitive random access memory (RRAM) cells (before forming) and the forming voltages. This trend points to the dominant role played by conduction paths located at grain boundaries, which is promoted by the oxygen deficiency in HfO x . One of these paths is then converted into the conductive filament responsible for nonvolatile resistance switching. In addition, we find that by engineering the RRAM stack, the forming voltage can be tuned-up to meet specific RRAM requirements, such as lower power and forming-less operations. Index Terms— Forming, HfO x , nonvolatile memory, RRAM. I. I NTRODUCTION R ESISTIVE random access memory (RRAM) is currently being explored for embedded and high-density mem- ory applications. HfO 2 -based RRAM with TiN electrodes is promising because of its short switching times (∼1 ns), good endurance (>1 B), retention, materials simplicity, and use of existing process tools [1]–[4]. The resistance switching in HfO 2 -based RRAM is controlled by a conductive filament formed in the dielectric as a result of dielectric breakdown [5]. In stoichiometric HfO 2 , the leakage current is low and the forming voltage is usually rather high, which may result in poor switching characteristics (large I reset , nonunifor- mity of high resistance state, and so on). This is partially because of the current overshoot (when the current exceeds the compliance limit), which may occur even in a 1T1R (i.e., a transistor in series with the RRAM cell) configuration. To reduce the forming voltage, which may lower opera- tion currents and decrease variability [6], a substoichiometric HfO x is routinely used [3], [4]. To form substoichiometry in a hafnia film grown by the atomic layer deposition (ALD), which is a process of choice for 3-D integration scheme, a layer of metal [oxygen exchange layer (OEL)] having high oxygen affinity (e.g., Ti, Hf, Zr) is deposited on top of the ALD HfO 2 followed by anneal. Although, it is generally understood that the oxygen extraction by the overlaying metal film results in a gradual profile of the oxygen vacancy concentration in Manuscript received January 30, 2013; revised March 22, 2013; accepted March 27, 2013. Date of publication May 3, 2013; date of current version May 20, 2013. The review of this letter was arranged by Editor D. Ha. K. G. Young-Fisher is with SEMATECH and GLOBALFOUNDRIES, Albany, NY 12203 USA (e-mail: Kristina.Young-Fisher@GlobalFoundries. com). G. Bersuker, D. Veksler, and D. C. Gilmer are with SEMATECH, Albany, NY 12203 USA. B. Butcher is with SEMATECH, College of Nanoscale Science and Engi- neering, University at Albany, Albany, NY 12208 USA. A. Padovani and L. Larcher are with DISMI, Università di Modena e Reggio Emilia, Reggio Emilia 42100, Italy. Digital Object Identifier 10.1109/LED.2013.2256101 1.E-12 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 0 1 2 3 Current (A) Voltage (V) Fig. 1. Correlation between leakage current and forming voltage: x HfO 2 / yTi OEL 1T1R 1 × 1 μm 2 devices (x and y are thicknesses). Inset: subsequent ten switching cycles of a representative device. hafnia [7], obtaining specific details of the vacancy distribu- tion and its relation to film morphology present significant challenges. To address this issue, this letter explores the relationship between initial leakage and the forming voltage observed in ALD HfO 2 -based devices of various OEL/HfO 2 ratios. The findings suggest that the oxygen extraction may proceed effectively along the grain boundaries (GBs) of the polycrystalline dielectric film, creating multiple leakage paths of widely varying resistivity. During forming a single, most leaky, path is transformed into a conductive filament responsible for the device electrical properties and switching characteristics. II. EXPERIMENT AND MODELING The RRAM devices are fabricated using TiN electrodes and ALD HfO 2 dielectric, which, at the thicknesses of ∼35–50 Å used, is found (according to HR–TEM data) to be crystal- lized as grown with a wide distribution of grain sizes from 2 to 10 nm depending on film thickness. A PVD OEL of titanium metal of different thicknesses (∼4–6 nm) is inserted between the HfO x and the top electrode. Incorporating the OEL to getter oxygen from HfO 2 during the postdeposition anneal is a practical and fabrication-friendly approach to manipulate dielectric stoichiometry. Device sizes ranged from 50 nm × 50 nm to 30 μm × 30 μm. Forming curves capturing the current–voltage ( I -V ) characteristics of the devices are considered for all devices with identical sweep rates and compliance currents. Forming I- V curves suggest a relationship between leakage current and forming voltage: lower leakage corresponds to higher forming voltages, as shown by an example in Fig. 1. The plot of the leakage currents ( I ) versus forming volt- ages (V f ), collected on a variety RRAM stacks of different HfO 2 thicknesses, OEL thicknesses, and device dimensions 0741-3106/$31.00 © 2013 IEEE