(Invited) High Speed AFM Observation of PEG Additive Behavior Depending on Electrode Potential
Authors: Matsushima Hisayoshi , Yoshioka Taiki , Yasuda Yohei , Ueda Mikito 
Published: May 2020

1. Introduction

Copper electrodeposition has still attracted many attention, since it is applied for the miniaturization of integrated circuits (ICs) and micro electro mechanical systems (MEMS). A damascene process is employed there and a copper wiring controlled on a nanoscale is made. By using a several kinds of additives which is classified into inhibitors, accelerators and levelers, void-free filling electrodeposition is possible. Each additive works by adsorbing on the substrate surface, and their synergistic effect enables the complete filling in the trench. Many studies have been reported on the adsorption mechanism of additives, which is analyzed by in-situ techniques under electrochemical environment [1]. However, since the adsorption of additives occurs at high speed in a fine region, it is very difficult to capture the phenomenon. We could observe the nucleation phenomenon of copper electrodeposition by using high-speed atomic force microscope (HS-AFM) [2, 3]. In this presentation, we will focus on polyethylene glycol (PEG), which is widely used as a copper plating inhibitor, and observe the adsorption / desorption behavior by using HS-AFM in order to investigation the suppression mechanism depending on electrode potential.

2.
Experimental

Experiments were conducted in a three-electrode cell. The working electrode was an Au(100) single crystal (ϕ =3mm, MaTecK), the counter electrode was a platinum wire and the reference electrode was a palladium wire occluded with hydrogen. Prior to each experiment, the single crystal was annealed with a butane burner for a few seconds and then cooled in the atmosphere. The electrolyte was a mixture of 300 ppm PEG aqueous solution (Mw
avg ~ 3000, Fujifilm Wako Chemicals) with 1.2 mM hydrochloric acid and 50mM sulfuric acid. Experiments were conducted with an electrochemical measurement system (HZ-7000, Hokuto Denko Co.). The constant potentials from − 0.5 V to 0.1 V were applied at room temperature. The ultra-short cantilevers (NanoWorld AG) were used as the tips for HS-AFM observation. In the present experiment, the scan area was in the range of 500-1000 nm2 and the scanning speed was 1 frame s-1. During the observation, the electrode potential was kept constant value, and the morphological change of the electrode surface was recorde

3.

Results and discussion

Fig. 1 shows HS-AFM images of PEG adsorption when the electrode potential was -0.10V. Initially at open circuit potential, smooth surface was observed after annealing [Fig. 1 (a)]. PEG adsorption occurred immediately after applying cathodic potential, and line-like PEG structure was formed along the steps edge after 5 s [Fig. 1 (b)]. After 10 s, the adsorbed PEG grew in one direction (as indicated by the arrow) and formed film shape [Fig. 1 (c)]. The growth rate was calculated to be 3.7 nm s-1 from image analysis. This rate was found to be very slow compared to the results of copper deposition in a copper sulfate solution without PEG. After that, the electrode surface was covered with PEG thin film [Fig. 1 (d)]. From the height analysis, the thickness of the PEG film was approximately 4 nm or less, and it was suggested that PEG was adsorbed with a certain thickness by comparing with the literature value [4]. The PEG film was stable at −0.10 V and did not become thicker even after keeping the cathodic potential.

REFERENCES

[1] Moffat T. P., Wheeler D., Kim S. K., Josell D., J. Electrochem. Soc, 153 (2006) C127.

[2] Yoshioka T., Matsushima H., Ueda M., Electrochim. Acta, 302 (2019) 422.

[3] Yoshioka T., Matsushima H., Ueda M., Electrochem. Commun., 92 (2018) 29.

[4] Walker M. L, Richter L. J, Moffat T. P., J. Electrochem. Soc, 152 (2005) 403.

Figure 1