Cu-Ag Alloy By Using Chemical Displacement Method for Resistive Random-Access Memory
Authors: Liu Yen-Chun , Li Syuan-Yi , Wang Xin-Jie , Chiu Chan-Yi , Wu Chi-Chang , Yang Wen-Luh 
Published: May 2020

Resistive random-access memory is one of the most promising candidates for the next generation nonvolatile memory due to the simple metal-insulator-metal structure, fast program speed, and compatibility with CMOS technology. It’s switching mechanism is control by metallic ions to form the metal filament between device electrodes, typical CBRAM uses active metal elements as a source of metal ions, such as Copper and Silver. In this study, a bottom electrode of a different proportions Cu-Ag alloy made by chemical displacement technique was compared in terms of resistance switching properties for a silicon dioxide based RRAM device. The result shows the process voltage decrease, stability increase and at least an order of memory window and only a very small percentage of silver is needed as a catalyst to trigger this mechanism.
ReRAM devices are the most excellent in the nonvolatile memory, compare with flash memory devices, it has lower fabrication cost. For ECM-type ReRAM, an active metal, such as copper or silver, is used for anode oxidation, and cathode reduction reaction occurs to establish the conduction filament in the switching layer. Cu is used widely in ECM-type ReRAM because the price is cheaper than Ag. However, Cu is difficult to dry-etch, and its residue after the etching process contaminates the Si device. The Cu etching problem hinders the development of Cu-based ReRAM. In this paper, a bottom electrode of a different proportions Cu-Ag alloy made by chemical displacement technique (CDT) was compared in terms of resistance switching properties for a silicon dioxide based RRAM device, and only a very small percentage of silver is needed to trigger the mechanisms, not only significantly reducing set voltage and reset voltage but also be more stable.

Figure 1 illustrates the fabrication process of the Al/SiO2/Cu-Ag/Si ReRAM device. After RCA cleaning, the wafer is placed in a solution at different concentrations to deposit 60 sec, 120 sec, 180 sec to displace the metal, and the solution formula is as Table 1, then the wafer is cleaned with DI-Water, after cleaning, the wafer surface water stains are removed with nitrogen gun to form a subelectrode to complete the metal lower electrode and then E-Gun deposits a layer of 30 nm thick silicon as an insulation layer, finally, a 100 nm Al as the top electrode layer was deposited by evaporator with metal mask. The diameter of each sample was 100μm. And the evaporator Cu (Ag)/SiO2/TaN/SiO2/Si was prepared as the control sample.

Figure 2 are J–E characteristic curves of Cu Ag alloy-CDT ReRAM devices, the J-E shows that larger operating voltage is required to operate than a CDT-Cu that is not added the silver, since the reset voltage of pure metal is larger than Cu-Ag alloy. The difference between Cu-Ag alloy components and metal components are that decreasing the forming voltage, but as the Ag of the alloy increases, there is no significant proportional change in the forming voltage.
As shown in Figure 3 That in alloy components, silver as the main catalyst to accelerate the formation of Cu filament, chemical displacement method to prepare the alloy electrode components of the assembly operating the electric field is relatively small and stable compare to the Cu electrode assembly, set voltage will also be reduced, because the metal atoms based on the dissociation energy and metal ion react, and Ag dissociation energy is smaller than Cu, represents the Ag atom that is more active than the Cu atom.
In the alloy devices, the Ag atom will capture the Cu atom’s electrons, making the Cu ions more easily dissociation and forming the filament, but need a very small percentage as a catalyst can accelerate the Cu ion dissociates to form the filament, and because of the influence of Ag, resulting in the formation of Cu filament, which needs a lower electric field to fuse. As shown in figure 4, the data retention of pure metal components and Cu-Ag alloy components, respectively as CDT-Cu, although the Ag forming filament is thinner.
In this paper, silicon is replaced with Cu-Ag alloy by chemical displacement method without electroless plating, with the advantages of random blend alloy ratio, can also be controlled the surface roughness of the film by different displacement time. On the other hand, Ag atoms obtain the electrons of the Cu atoms, making it easier for Cu ions to dissote and forming a filament, so adding Ag not only reduces the operating electric field, but also makes the electric field more concentrated, and maintain the data retention at the same order magnitude.

Figure 1