Improvement of Data Retention Characteristics By Inserting Ta Adhesion Layer to Ionic Liquid-Supplied Conducting-Bridge Memory
Authors: Sato Hiroshi , Kinoshita Kentaro , Honma Yusei , Shima Hisashi , Naitoh Yasuhisa , Akinaga Hiroyuki , Itoh Toshiyuki 
Published: December 2020

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