We demonstrate a new memristive device (IL-Memristor), in which an ionic liquid (IL) serve as a material to control the volatility of the resistance. ILs are ultra-low vapor pressure liquids consisting of cations and anions at room temperature, and their introduction into solid-state processes can provide new avenues in electronic device fabrication. Because the device resistance change in IL-Memristor is governed by a Cu filament formation/rupture in IL, we considered that the Cu filament stability affects the data retention characteristics. Therefore, we controlled the data retention time by clarifying the corrosion mechanism and performing the IL material design based on the results. It was found out that the corrosion of Cu filaments in the IL was ruled by the comproportionation reaction, and that the data retention characteristics of the devices varied depending on the valence of Cu ions added to the IL. Actually, IL-Memristors involving Cu(II) and Cu(I) show volatile and non-volatile nature with respect to the programmed resistance value, respectively. Our results showed that data volatility can be controlled through the metal ion species added to the IL. The present work indicates that IL-memristor is suitable for unique applications such as artificial neuron with tunable fading characteristics that is applicable to phenomena with a wide range of timescale.

Ionic liquids (ILs), non-volatile liquids composed of cations and anions, have various attractive properties for electronic devices, such as wide potential windows. Combining ILs with electronic devices is presumed to be able to provide new options for realizing a sustainable internet of things society because such liquid-solid hybrid devices have the capability to act as a key in realizing further possibilities that cannot be achieved with all-solid-state devices. In this paper, we describe the development of ILsupplied conducting-bridge random access memory (IL-CBRAM) whose operating mechanism is the Cu filament formation/rupture caused by redox reactions in ILs as an electrochemical reaction field. Although the introduction of liquids into solid-state processes is challenging, we successfully demonstrated the reproducible memory operation of IL-CBRAM with a Cu/SiO2/Pt structure and a microfabricated pore filled with IL in the SiO2 layer. We also improved the wettability of the IL exposing Ar plasma to SiO2, which was essential not only to obtain an IL thin film from the droplet but also to ensure pore filling by the IL before Cu deposition. The present device fabrication process for IL-CBRAM is highly reliable and compatible with conventional vacuum processes.

Previously, we reported on the memory characteristics of ionic liquid-supplied conducting-bridge random access memory (IL-CBRAM)1. IL-CBRAM is a kind of CBRAM devices in which ionic liquid (IL) is confined in an artificially formed pore in the SiO2 layer of Cu/SiO2/Pt stuck structure. The resistive switching phenomenon is caused by formation and rupture of Cu filaments consisting of electrochemically eluted Cu from the Cu electrode (EL). Although IL-CBRAM has excellent features such as reduction of operating voltages and their variations, there is a serious problem that IL addition makes data retention in low resistance state (LRS) poor. When two metals with different EL potentials (EPs) are in contact in the presence of solvents, it is known that the corrosion of the metal with lower EP proceeds. Therefore, we considered that the abovementioned problem of poor data retention might come from the configuration of LRS itself, that is, the configuration that Cu filaments with low EP directly contact to Pt-EL with high EP. In this study, we demonstrated that data retention characteristics in LRS were drastically improved by inserting the Ta adhesion layer between the SiO2 and Pt-EL. It was suggested that the Ta layer that had lower EP than those of both Cu and Pt provided electrons to Pt-EL, suppressing the corrosion of Cu filaments. Figure 1(a) shows the schematic diagram of IL-CBRAM device. IL-CBRAM has a stack structure of Cu (50 nm)/Cu- or Ag-IL (30 nm)/Pt (20 nm), and IL was supplied to an artificially introduced pore with the diameter of 1 μm in SiO2 layer. Here, Cu- and Ag-IL are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([bmim][Tf2N]) solution containing 0.4 M Cu(Tf2N)2 and 0.4 M Ag(Tf2N), respectively2. The Ta layer with the thickness, d, of 1 nm or 5 nm is inserted as an adhesion layer between the SiO2 and Pt layers. Figures 1(b) and (c) show schematics when filaments are formed in IL-CBRAM at d = 1 and 5 nm, respectively. Cu filaments were observed to be formed on the wall surface of the pore that was introduced in the SiO2 layer1. Since the hole penetrates not only the SiO2 layer, but also the Ta layer, the Cu filament contacts both the Pt and Ta layers. Figure 2 shows data retention characteristics of IL-CBRAM in LRS, for d = 1 and 5 nm. The compliance current to prevent excessive growth of filament was set to 200 μA. Data retention time is less than 103 s for d = 1 nm, whereas more than 104 s for d = 5 nm (still working). After formation of Cu filaments connecting the Cu- and Pt-ELs, the corrosion of Cu filaments with lower standard EP (SEP) should proceed, because of large difference of SEPs between Cu (0.34 V) and Pt (1.2 V). The drastic improvement of the retention characteristics for d = 5 nm suggests that the insertion of the Ta layer that has lowest SEP of -0.81 V provides electrons to Pt instead of Cu, suppressing the corrosion of Cu filaments. On the other hand, the Ta layer with d = 1 nm is considered to be oxidized completely during the SiO2 deposition (Fig. 1(b)) in contrast to a case of thicker d of 5 nm (Fig. 1(c)). The completely oxidized Ta layer no longer provide electrons and cannot contribute to an improvement of data retention. It is also important that the surface of Ta layer except for the interface with Pt is oxidized for the 5 nm thick Ta layer. This prevents corrosion of the Ta layer itself. Acknowledgement This work was supported by NAGASE & CO., LTD. Colors & Advanced Processing Dept. Reference [1] H. Sato, et. al., 14th IEEE NMDC, Stockholm, Sweden, program booklet, pp.171-172, 1698674, Oct. 27-30, 2019. [2] A. Harada, et. al., J. Mater. Chem., 4, pp. 7215-7222 (2016). Figure 1

We fabricated an ionic-liquid conductive-bridge memory (IL-CBRAM) in which the solid-state electrolyte in a conventional CBRAM sandwich structure of Cu- and Pt-electrodes was replaced with IL. To stabilize the Cu-filaments in IL, an additional metal layer with a standard electrode potential (SEP) lower than that of Cu was inserted into the memory cell. We found Ta to be promising for this purpose, because the SEP in IL exhibited the Ta < Cu < Pt relation. Actually, inserting the Ta-adhesion layer increased the data-retention time. The data-retention characteristics control through cell design considering the SEP arrangement of metals was successfully demonstrated.