Early history of MLD developments in Japan

MLD (Molecular Layer Deposition) is one of the unique ALD technologies. Some researchers use this technique, for instance Self-Assemble Monolayer (SAM) for ASD（Area Selective Deposition), surface treatment of solid Li ion batteries to improve ion conductivity etc. As for mass-production use, MLD is actually used for ink jet heads of our printers. Ink jet heads are made by MEMS (Micro Electro Mechanical System). MLD layer prevent MEMS-made structures from sticking. (3D Silicon structures are easy to stick each other. )

I sometimes heard the early works of MLD had been done in Japan. But I did not know who. In March, 2019, I had a chance to be introduced a book written by Dr. Yoshimura. I started to investigate how MLD was developed initially.

The writer contacted two key figures and heard how MLD has been developed. One contributor is Dr. Atsushi Kubono who is now a professor of Shizuoka University. Another critical figure is Dr. Tetsuzo Yoshimura who is an honorary professor of Tokyo University of Technology.

I here write down the short history of early MLD developments.

October 20, 1987　A young student Kubono (Senior student of Tokyo Institute of Technology) gave oral speech (in Japanese language) at Japan Society of Applied Physics that he discovered monomers are deposited and perpendicularly arrayed using Vacuum Vapor Polymerization. Vapor Polymerization is the technique to send two different monomers into a reactor in order to create polymer. Polymer could not be vaporized so we need to send two vaporized monomers for polymerization. The polymer is randomly arrayed through vapor polymerization, but Kubono discovered that polymer were perpendicularly arrayed by tuning temperature etc. Another remark is that his discovery was self-limited process. Only Material A can be reacted and deposited on Material B (but not on the same Material A) while Material A and B were sent together into a chamber. He used the following materials:
Material A: 1,10-Diaminodecane
Material B: Sebacoyl dichloride
Created: polyamide (poly(decamethlene sebacamide)

Title: “Self-selective epitaxial and arrayed polymerization of Nylon 10,10 through vacuum evaporation (Translated by the writer)”.

October, 1988　Magazine “Hyomen (Surfacce)” published. Dr. N. Okui write the articles of Organic polymerization thin film including Kubono’s work. (Kubono belonged to Dr. Okui lab. of Tokyo Institute of Technology.)

September 13, 1990 Kubono’s first English paper ” HIGHLY ORIENTED POLYAMIDE THIN FILMS PREPARED BY VAPOR DEPOSITION POLYMERIZATION” is published.

March 18, 1991　”Polymer films with monolayer growth steps by molecular layer deposition” was published by Fujitsu R&D team (Leader: Dr. Tetsuzo Yoshimura).
Material A: Benzene-1,4-dicarboxaldehyde
Material B: p-phenylenediamine, (PPD)
According to Dr. Yoshimura, he was inspired by Atomic Layer Epitaxy (ALE) technology. And he developed MLD based on vapor polymerization researches by IBM, Kubono and Ulvac. While Kubono delivered two different materials together in one chamber, Yoshimura separated delivery of Material A and B and add purge times between pulse A and B. He had eventually established this technique and named it “Molecular Layer Deposition”.
After some years, unfortunately Fujitsu R&D had stopped many of advanced research developments including MLD.

April, 2001　Dr. T. Yoshimura became a professor of Tokyo University of Technology and started MLD researches again including polymer arrayed growth on SAM, quantum dots etc made by MLD etc until 2017.

2011　Dr. Yoshimura published a book. “Thin-Film Organic Photonics – Molecular Layer Deposition and Applications”

Comment
Interestingly, a senior student found the root of MLD. According to the very person Dr. Kubono, he tried one small idea to open up this new technology – perpendicularly arrayed growth of polymer. (He was a M1 student when he disclosed his discovery publicly as mentioned above.) I would like to say that young researchers have sometimes done great discoveries like many of Novel Prize winners. And even in ALD community, we have this good example. I wish young ALD researchers (or even aged with young spirits!:)) will do that again and again.

Note
The above dates of papers are when they are received or accepted.

Acknowledgment
I thank you very much for Dr. Yoshimura and Dr. Kubono who sent me their original papers and even explained to me how MLD had been developed. With my appreciation, I am very honored to hear their experiences.
I also like to thank to Dr. N. Hiroshiba, National Institute of Technology, Maizuru College. Without his introduction to Dr. Yoshimura’s book, I would not start this investigation. And I thank Dr. Kuwae, Waseda University for his help to find some academic papers for me.

Reference
Dr. Kubono’s web site: https://wwp.shizuoka.ac.jp/kubono-lab/