Improving Patterning Resolution Via Solid-State Anodic Dissolution at the Polymer Electrolyte Membrane/Cu Interface Under Optimal Electrolytic Conditions
Authors: Tsuji Atsuki , Murata Junji 
Published: December 2024

Copper (Cu) is used for metal interconnects in integrated circuits and other applications because of its high electrical conductivity and resistance to electromigration. In addition, patterned Cu has been applied as a metasurface for flexible design of refractive indices, reflectance, and transmittance,[1] and as a graphene growth catalyst for graphene transistors,[2] which provide high carrier mobility. Other applications include current collectors to improve the performance of lithium-ion batteries[3] and transparent conductive electrodes used in optoelectronic devices such as solar cells and displays.[4] Micro and nanoscale patterning of Cu surface is conventionally complex and has a high environmental impact. Photolithography and nanoimprinting are widely employed in the fabrication of semiconductor devices because of their high pattern resolution and are used for nanopatterning of Cu surfaces. However, these patterning techniques, which require resists and harsh chemicals, are complex processing processes and increase processing costs. There are also direct patterning techniques based on electrochemical approaches, such as electrochemical micromachining, however, these techniques have low resolution and a high environmental impact due to the treatment of electrolytic solutions. We previously demonstrated Cu direct patterning without using a liquid electrolyte by utilizing solid-state anodic dissolution at the polymer electrolyte membrane (PEM)/Cu interface.[5] In this approach, a PEM stamp with a patterned surface attached to Cu (anode) and electrochemical treatment in anode/PEM stamp/cathode sandwich electrochemical system enables the pattern of the PEM stamp to be transferred to the Cu surface. Although this method has been demonstrated to form patterns with a resolution of 500 nm, higher-resolution nanoscale pattern formation has not been achieved. Therefore, this study analyzed the reaction at the PEM/Cu (anode) interface using electrochemical measurements and attempted to form high-resolution Cu patterns by optimizing the electrolytic conditions. Cyclic voltammetry (CV) was performed on a potentiostat using a two-electrode electrochemical system consisting of Cu (anode)/PEM stamp/Pt(cathode) with Cu as the anode (working electrode) and Pt as the cathode (which serves both the counter and the reference electrode). The voltage range was set to -1 to +1 V to avoid the influence of oxygen and hydrogen generation, and the scanning speed was 5 mV/s. PEM stamps were prepared via hot embossing and immersed in deionized water before use, and the experiments were conducted at room temperature and pressure. The CV curve exhibited peaks at a voltage of around 500 mV (Fig. 1(a)). The standard electrode potentials for the Cu ionization reaction are 337 and 520 mV for Cu2+ and Cu+, respectively, and can be attributed to peaks at around 500 mV. This result indicates that at voltage after 500 mV, Cu+ reactions occur in addition to Cu2+. The CV curves also exhibited peaks in the negative bias region, suggesting that the Cu ions in PEM were reduced at the PEM/Cu interface. Although previous studies have discussed the problem of degradation of PEM etching performance due to the deposition of Cu ions inside the PEM by repeated use, this negative bias reduction was demonstrated to restore the PEM etching performance to its pre-use state. To demonstrate nanoscale patterning, PEM stamps with 100 nm scale Line & Space (L&S) patterns were used to fabricate patterns on a 200 nm Cu thin film deposited on a silicon (Si) substrate by electrolysis at a constant voltage of 1 V and 500 mV for 1 min, respectively. The scanning electron microscopy (SEM) image shown in Fig. 1(b-1), the pattern width was non-uniform under the 1 V electrolysis condition, suggesting excessive etching. In contrast, under the 500 mV electrolysis condition, the pattern was uniform, and a 100 nm scale L&S pattern was fabricated with high resolution (Fig. 1(b-2)). Therefore, the electrolysis time was changed from 1 min to 10 s under the 1 V electrolysis condition, resulting in the formation of a high-resolution pattern as well as 500 mV electrolysis condition. These results suggest that the amount of charge is an important factor in the application of our technology to nanopatterning. Thus, by adjusting the electrolysis conditions, such as 10 s of 1 V electrolysis, ultrafast patterning can be achieved, resulting in improved processing efficiency.
References
[1] K. Sato and T. Suzuki, Nanophotonics 12, 2537 (2023).
[2] M. Wang et al., Scientific Reports 3, 1238 (2013).
[3] S.-H. Wang et al., Advanced Materials 29, 1703729 (2017).
[4] Myung-Gyu Kang et al., Solar Energy Materials and Solar Cells 94, 1179 (2010).
[5] A. Tsuji, E. Morimoto, M. Takizawa and J. Murata, Advanced Materials Interfaces 11, 2300896 (2024).

Figure 1