AVS2010 Session TF2-TuM: Nonvolatile Memory

Resistive-switching Random Access Memories (RRAM) have attracted considerable attention in recent years for future non-volatile memory applications. This resistance switching can be based on a modification of the crystalline structure of a material (PCRAM, Phase Changed RAM). One can also exploits the resistive switching properties of some oxides (OxRRAM, Oxide Resistive RAM) which display a change in resistance upon application of a bias voltage. In this case, oxides materials are deposited in a metal-insulator-metal (MIM) structure. In the last years several different oxides have been studied such as metal oxides (NiO, TiO₂, ZrO₂, Cu_xO…) and perovskites (BaTiO₃, SrTiO₃…) [1]. Hafnium oxide (HfO₂) is among the oxides particularly desirable as far as integration and process compatibility are concerned since it has the advantage of being more mature from a technological point of view.

In this work, HfO₂ RRAM with different thicknesses of HfO₂ films (10 and 20 nm typically) are tested with and without plasma treatment. HfO₂ films are grown at 350°C by atomic layer deposition (ALD) using alternate cycles of H₂O and HfCl₄ precursors (1 Torr) on Pt and TiN electrode materials. It is known that oxygen vacancies are playing a critical role for resistive switching. Different solutions have been proposed to modify the oxygen vacancies concentration in the device: integration of a TiO_X/TiO₂ bilayer films [2], doping a ZrO₂ RRAM by metallic ions [3]. Here hydrogen-based plasma treatments are studied. Hence, several different hydrogen-based (NH₃) plasma annealing treatments of the HfO₂ dielectric are carried out in order to study the influence of the oxygen vacancies or defects on the subsequent switching behaviour before the deposition of the top electrode. The MIM structures are then electrically and physically characterized. I(V) curves are then recorded and switching parameters such the SET voltage are compared for MIM devices with and without plasma treatment. It is for example observed that a 1 min NH₃ plasma treatment of HfO₂ deposited on TiN improved the overall switching properties of the RRAM. The modifications of switching properties are correlated to chemical analysis results, mainly Angle-resolved X-ray Photoelectron Spectroscopy, Attenuated Total Reflexion (ATR) and Spectroscopic Ellipsometry (SE) up to 8 eV, with special attention devoted to metal/oxide interface investigations.

[1] A. Sawa, Materials Today 11 (6) (2008) 28

[2] J.J. Yang, et al, Nature Nanotech. 3 (2008) 429

[3] H. Zhang et al, Appl. Phys. Lett. 96 (2010) 123502

Phase-Change Memory (PCM) is one of the promising candidates for next-generation nonvolatile memory thanks to their low cost, low programming voltage and their excellent scalability to the nanoscale cell size [1]. This technology is based on fast and reversible phase change effect in the chalcogenide materials but suffers of the inherent lack of amorphous state stability which affects archival life of the memory cell. This proble is critical for embedded applications where a high retention time at high working temperature is required. In this case, binary compound GeTe material has been shown to be a good candidate since a phase transition temperature higher than the usual Ge₂Sb₂Te₅ material has been found and 10 years-fail time at 105°C has been estimated [2]. Moreover the reduction of the current pulse needed to change material from crystal phase to amorphous phase implies the confinement of phase change materials at dimensions below 100 nm [3]. In this case the growth of GeTe materials by chemical vapour deposition (CVD) or Plasma Enhanced CVD is required in order to fill confined structures.

For this purpose, this work investigates the deposition of GeTe materials by a pulsed liquid injection Metal Organic CVD system allowing storing the precursor at ambient temperature. The deposition is made in a 200 mm MOCVD tool that can be assisted by a Low Frequency as well as Radio Frequency plasma. The liquid precursors are injected into a heated evaporator where flash evaporation occurs. A sequential injection leads to a precise control of the deposited material stoichiometry. Furthermore deposition in amorphous or crystalline state is performed by setting the substrate temperature. This chamber is connected to a cluster tool which allows quasi in situ analysis of the deposited films crystalline state by Spectroscopic Ellipsometry (SE) and of the growth mechanisms by angle-resolved X-ray Photoelectron Spectroscopy (ARXPS). Impact of process parameters on the films properties are then evaluated in the MOCVD and PECVD mode. In the case of plasma assistance the impact of the Low to Radio Frequency ratio on the thin film deposition is also studied. Direct informations on Ge/Te ratio and carbon contamination are given by the plasma analysis thanks to optical emission spectrometry. In addition to the chemical and physical properties investigations, the phase change performances and the electrical properties of the deposited materials are evaluated.

[1] S. J. Hudgens, J. Non-Crys. Solids, 354, 2748 (2008)

[2] L. Perniola et al., IEEE Electron Device Lett., 31, 5 (2010), pp. 488-490.

[3] S. L. Cho et al., Symp. VLSI Tech. Dig. (2005), p. 96

Patterning of magnetic materials with plasma processes is an integral step in the fabrication of magnetic devices such as magnetic tunnel junctions (MTJs) for magnetoresistive random access memory (MRAM). Obtaining optimal device performance requires preservation of the magnetic material properties throughout the fabrication process, and thus places limitations on the process window (e.g. temperature, gas chemistry) for patterning devices. Due to this, the use of halogenated plasma chemistries, which are commonly used in reactive ion etching (RIE) processing, must be carefully employed when patterning MTJ devices, as they can significantly degrade magnetic properties during the etch. One method used to minimize magnetic degredation is the incorporation of metallic capping layers which potentially isolate the halogen etch chemistry from the critical magnetic layers. However, the optimal thickness and/or properties of these layers have yet to be fully explored. For example, when a Cl₂ based plasma is exposed to a 300 Å Ru capping layer, the magnetic moment/area of the underlying magnetic free layer is reduced 11% from 1.64 to 1.46×10^-4 emu/cm². Therefore in this work, the effects of the plasma etch chemistry on magnetic materials is explored using various halogenated chemistries and capping layer materials to understand the mechanisms by which patterning of magnetic devices can be achieved without degradation of magnetic properties.

Resistive random access memory devices based on transition metal-oxides (TMO) have been considered as the most promising candidates for the next generation nonvolatile memories because of its simple structures and compositions, low voltage operation and process compatibility with CMOS.

In this presentation, stable bipolar resistive switching behaviors of TiN/HfO2/Pt structures will be discussed for the first time. . The HfO₂ films were grown by reactive stutter at room temperature using O₂ and Ar gas, and the subsequent heat treatment was performed in an ambient of O₂ at 300℃. TiN and Pt layers were used as the top and bottom electrode materials, respectively. For the characterization of stable bipolar resistive switching, current-voltage measurements were used in compliance with 1 mA. Excellent memory characteristics including low set/ reset voltages and long retention time were demonstrated without additional electroforming process. The bipolar resistive switching behavior can be explained by the formation of conductive path consisting of oxygen vacancies. We analyzed a composition and chemical bonding of HfO₂ by x-ray photoelectron spectroscopy. In addition, microstructures of the films were analyzed by transmission electron microscopy.