Electrochemical deposition processes have been widely applied to form functional thin films and micro/nano structures featuring their controllability and capability to fabricate uniform deposits to wide, non-flat surfaces. For achieving further precise control, we have attempted molecular-level analysis of the deposition processes and reaction mechanisms through theoretical and experimental approaches. In this paper, we will introduce these approaches with some of our resent results.

For the theoretical analysis to quantitatively understand the reaction mechanisms, we have applied density functional theory (DFT) calculations focusing on the surface species such as additives, reaction intermediates, reducing agents (for electroless deposition), and so on[1]. Effects of solvation and catalytic activity of the surfaces, etc., were also investigated and behaviours of these species, which are unique to the deposition at solid/liquid interfaces, were elucidated. While these theoretical analysis could predict rigorous mechanisms of the deposition reaction processes, acquiring such molecular-level data through experimental approach is quite challenging since the amount of these surface species are trace and they are “buried” under the electrolyte. For this, we focused on surface enhanced Raman scattering (SERS) equipped with confocal microscopy and developed plasmonic sensors with patterned nanostructures to achieve very high enhancement effect, and we applied them to analyze the electrochemical deposition processes such as the behaviour of additives.

For the plasmonic sensors, we have developed “reflection” type and “transmission” type; the “reflection” type sensors consist of metals active for plasmonic enhancement such as Ag, Au, and Cu with centrifuge-grooved nanostructures or nanodot arrays for maximizing the enhancement, which can be directly used as the substrates for the deposition[2]. The “transmission” type sensors, which are single or array of the microlens coated with discrete nanoparticles of the plasmon-active metals, can be applied to various processes regardless of the substrate materials[3]. By using these sensors, we analyzed behaviours of the additives and reductants at the deposition sites. In addition, we have also developed a system for in situ analysis of the processes inside the microscopic pores such as through silicon via (TSV) [4]. A model-pore was fabricated using the sensor (to be a part of “side-wall” of the pore) with the metal nanodot array covered with the structure made of poly(dimethylsiloxane) (PDMS). It was confirmed that Raman signal of the additives could be detected in situ in high sensitivity without interference of the signals originated from PDMS, and their behaviour was investigated in detail. Furthermore, attempts on measuring local temperature as well as pH in the vicinity of the electrode were made by using the SERS measurements, which will be able to provide new insights for precise analysis of the behaviours of surface species during the deposition processes.

This work was financially supported in part by “Development of Systems and Technology for Advanced Measurement and Analysis,” Japan Science and Technology Agency, and Grant-in-Aid for Scientific Research, MEXT, Japan.

[1] For example, T. Homma, Electrochemistry, 83, 680 (2015).

[2] B. Jiang, M. Kunimoto, M. Yanagisawa, T. Homma, J. Electrochem. Soc., 160, D366 (2013).

[3] M. Yanagisawa, M. Saito, M. Kunimoto, T. Homma, Appl. Phys. Express, 9 , 122002 (2016).

[4] T. Homma, A. Kato, M. Kunimoto, M. Yanagisawa, Electrochem. Commn., submitted.