Electroless Deposition of Aluminum from a Room Temperature Ionic Liquid Electrolyte
Authors: Koura Setsuko , Ooshima Sota , Kudo Daiki , Koura Nobuyuki 
Published: February 2020

The development of the technology to produce the aluminum thin film and aluminum wiring with low cost has been earnestly desired. Such a technology has profound consequence across many electronic machinery industries including semiconductor, electronic circuit, etc. Since aluminum is quite less noble metal, it is impossible to obtain the electrodeposition from aqueous solution. On the other hand, we could developed aluminum deposition technology from the room temperature ionic liquid. Moreover, we developed first the revolutionary technique of the electroless deposition of aluminum based on using the ionic liquid as electrolyte. There are many use of aluminum thin films including aluminum wiring as stated above, but it is necessary to use large instruments such as sputtering and glow plasma methods in order to deposit aluminum on the insulating substrate of silicon and glass. If the technique of electroless deposition of aluminum is established, it will be able to obtain the thin and thick film coating on the substrates that are insulating material and have quite complicated structure, without electricity.

At our laboratory, electrodeposition of aluminum with the room temperature ionic liquid composed of aluminum chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (EMIC) have been developed. Therefore, the purpose of this study is to develop electroless deposition method of aluminum with room temperature ionic liquid. In this study, various substrates were attemted electroless deposition of aluminum and deposits were evaluated. In addition, the reaction mechanism of the aluminum electroless deposition was figured out by the electrochemical consideration.

First, LiH was added as the reducing agent into the AlCl3-EMIC (2:1) melt. At the same time, various substrates (glass, silicon, etc.) were immersed in the electrolyte, then the electroless deposition of aluminum started. The plated specimen was qualitatively identified by X-ray diffractmeter and its composition was determined by inductively coupled plasma atomic emission spectrometry. The observed diffraction peaks of the plated film agreed with those of aluminum in the literatures. The data from ICP measurement also showed that the purity of the deposited aluminum was more than 99%. These indicated that the deposited film from the electrolyte with LiH was aluminum. The elemental mapping result by SEM-EDX also indicated that the plating film consisted of aluminum only, and the aluminum was homogeneously distributed on the surface of the film.

Next, the mechanism of the aluminum electroless plating was tried to find out. The change of the ionic apecies was analyzed by using laser Raman spectrophotometer. It suggested that Al2Cl7
-, which is ionic species for aluminum deposition in the acidic AlCl3-EMIC melt, had such a reaction shown below and the aluminum was deposited from the electrolyte.

4Al2Cl7
- + 3e- → 7AlCl4
-+ Al (1)

It was also considered that the reducing agent, LiH, had a reaction indicated below in order to cause the reaction (1).

LiH → Li+ + 1/2H2+ e- (2)

Hydride composits such as LiH are good reducing agent, however, those also have some dangerous of explosion. We tried to find other reducing agent instead of hydride.

In this meeting, we will show the results of electroless deposition of aluminum from room temperature ionic liquid with other reducing agents.