Examination of Reducing Agents Used for Aluminum Electroless Deposition
Authors: Makino Kohei , Marumoto Daisuke , Koura Setsuko 
Published: December 2020

1. Introduction
Because of its properties such as low electrical resistance, simple to structure in dry etch processes, aluminum is widely used for wiring material in the field of semiconductors. However, it is necessary to use large instruments such as sputtering and glow plasma methods to deposit aluminum on the silicon substrate. If the technique of electroless plating of aluminum is established, it will be able to obtain the thin film on the silicon substrate easily without electricity. We reported an electroless plating of aluminum on the glass by using a room-temperature ionic liquid composed of aluminum chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (EMIC) with lithium hydride (LiH) or di-isobutyl aluminum hydride (DIBAH) as reducing agents1).
In this research, we tried electroless plating of aluminum on silicon substrate coated with tungsten (W) or with titanium nitride (TiN) which has not been reported so far. And the possibility of shortening the time of electroless plating was investigated by dissolving various new reducing agents in the plating solution.
2. Experimental
To perform the electroless aluminum plating experiment, the silicon substrate coated with W or with TiN were degreased, pickled, and catalyzed. Thereafter, reducing agents (DIBAH or various new reducing agents) were added to a plating solution of 66.7 mol% AlCl3 + 33.3 mol% EMIC ionic liquid heated to 70 ℃, and the substrate was immersed in the solution to perform electroless aluminum plating. The linear sweep voltammetry measurements were employed, a three-electrode cell consisting of a tungsten working electrode, an aluminum counter electrode, and an aluminum reference electrode by the scan rate was 5 mV/s. An aluminum wire (99.99%, φ:2mm) immersed in a 66.7 mol% AlCl3 – 33.3 mol% EMIC melt in a Pyrex tube separated by a G4 glass frit was used as the reference electrode. The electrochemical behavior of reducing agents in 50mol% AlCl₃ + 50mol%EMIC ionic liquid was investigated. All experimental operations were performed inside a glove box under an inert gas atmosphere.
3. Results and Discussion
A plating film was provided on silicon substrate coated with W and with TiN from the 66.7 mol% AlCl₃ + 33.3 mol% EMIC ionic liquid with the DIBAH reducing agent. The characteristics of the plated films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM). The observed X-ray diffraction peaks of the plated film agreed with those of aluminum in the literatures. However, it took more than 6 hours to get few ㎛ thickness of aluminum plating. We tried to find other reducing agent to shorten the time of electroless plating. The self-potential of reducing agents in the 50mol% AlCl₃ + 50mol% EMIC ionic liquid were measured. Some reducing agents showed less-novel potential, indicating that they can be used as a reducing agent as well as DIBAH for electroless Al plating. It was confirmed that electroless plating was performed by using those reducing agents. Especially, reducing agent A shortens the plating time to about 1-2 minutes to get few ㎛ thickness. Also, it was found that when the concentration of the reducing agent E was high, the reactivity became too strong, and the liquid decomposition occurred rapidly, so that plating could not be performed. From these results, we could say that the reducing agent A was very useful, but it was important to control the reactivity by adjusting the concentration.
The electroless plating mechanism was studied by the polarization curves of a tungsten electrode in room-temperature ionic liquids. It was concluded that the electroless plating reaction consisted of anodic oxidation reaction of reducing agent and cathodic reduction of Al2Cl7-.
4. Conclusion
Electroless Al plating is possible for W-coated silicon substrate and TiN-coated silicon substrate from 66.7 mol% AlCl₃ + 33.3mol% EMIC ionic liquid with DIBAH reducing agent. The results of self-potential measurement showed that reducing agents with less-novel potential could be used as reducing agents for Al electroless plating. The results of electroless plating experiments suggested that the use of reducing agent A could shorten the plating time significantly.
5. References
1) Setsuko Koura, Souta Ooshima, Daiki Kudo, Electroless deposition of Aluminum from a Room Temperature Ionic Liquid Electrolyte, 2016 / 230th ECS Meeting Electrochemical Society (USA and Japan) (2016)