Development of a non-vacuum process to achieve high quality functional thin film fabrication


Specialized field

Chemical engineering, electronics, growth technology, thin film formation reaction theory, ultrasonic technology, spray technology

Seeking a non-vacuum process to reduce environmental burden

The electronic devices in our life use a wide variety of electronic components built around functional thin films. Most of these functional thin films are fabricated via a process that involves working in a vacuum environment, but this process has been criticized for the large amount of energy required to maintain the vacuum state. In order to dramatically reduce the environmental burden of thin film fabrication, there has been an extensive search for an alternative approach. In that scenario, associate professor Dr. Kawaharamura has been focusing on the merit of a 'non-vacuum process' which can fabricate thin film at atmospheric pressure.

Dr. Kawaramura says, "Since a non-vacuum process would not involve a vacuum state, energy consumption could be greatly reduced. There would be a number of other advantages, including low operating cost, simple system structure, and easy maintenance, so there is significant merit to converting to a non-vacuum process."

However, in non-vacuum processes, side reactions and disturbances are rather large, so advanced control methods would be required. The development of technology for strict control at atmospheric pressure has not yet been realized.

In order to solve these technical problems, Dr. Kawaharamura has developed mist Chemical Vapor Deposition (mist CVD), a new thin film production system based on the use of mist. In this method, a solution is atomized in some way and transported by a carrier gas to produce thin films by thermal decomposition. Since this system would not require special parts or a vacuum environment, a simple configuration would possible, and since safe materials could be used, the system is highly versatile and shows promise as an energy saving method.

Dr. Kawaharamura continues, "The use of mist is highly significant here. Mist droplets generated by ultrasonic waves are extremely small, so they can stay suspended in the air, which means it is possible to move and rectify them. I realized that the characteristics of mist make this method most suitable for making homogeneous thin films."

He thought that it would be possible to produce homogeneous thin films by utilizing this characteristics to contrive rectification technology.

Dr. Kawaharamura explains, "Heat convection is a serious problem when we handle gas in the atmosphere. When gas is directed at a heated substrate, it is pushed back by heat convection and as a result it is difficult to achieve homogeneous flow."

When he was a student, Dr. Kawaharamura devised a method called 'collisional mixing' which decreases pressure and rectifies material by colliding two streams of mist-bearing gas in order to stabilize the flow of mist. Subsequently he invented a device that produces a uniform mist flow, and succeeded in controlling material flow. Furthermore, it was possible to keep the ambient temperature constant within this equipment, which made possible a technology which could solve all the remaining problems at once.

Elucidation of the mechanism that is key to homogeneous thin film fabrication

Another important key to fabrication of high quality functional thin films by the mist method is the Leidenfrost phenomenon. In the Leidenfrost phenomenon, when the surface of a liquid is covered with a vapor film with low thermal conductivity resulting from rapid heating, the droplets are suspended. This a phenomenon somewhat similar to the behavior when we drop water on a hot skillet and the droplets jump. When a gas containing mist flows inside the reaction zone, the droplets enter the Leidenfrost state due to the sudden change in the field.

Dr. Kawaharamura says, "Now we can say with certainty that this phenomenon occurs during the process of the mist method. For a long time it was not known whether thin film growth occurs after the droplets directly touch the substrate, or after the droplets evaporate without reaching the substrate."

In order to elucidate this enigma, in 2011 Dr. Kawaharamura developed a technique called mist etching. He changed the solution in the mist method to etchant such as acid or alkali, and applied experimental etching to zinc oxide thin film on a glass substrate by the thin film etching method. He observed irregularities on the etching side surface at 105°C and 120°C, indicating that the droplets had contacted the thin film directly, whereas at 130°C the etching side surface was smooth and the droplets did not contact the substrate. That suggested that some or all of the droplets had vaporized and that the etching had occurred in the gas phase.

He reflects, "This experimental result convinced me that the Leidenfrost phenomenon did occur in the mist method. My demonstration of this occurrence led to the elucidation of a principle governing the fabrication of homogeneous thin films and was a trigger to great developments in this research."

The Leidenfrost phenomenon occurs in the mist method process. What is the impact of that on the fabrication of high quality functional thin films?

"In ordinary gas as a fluid flows from one source, the upstream concentration is high the while the downstream concentration is lower, which gives rise to film unevenness. But if the fluid is made into a mist, then when droplets of raw material move close to the heated substrate, they enter a state where they run on the substrate under the Leidenfrost effect and thus supply a uniform volume of gas. Therefore, it is expected that we will be able to fabricate homogeneous thin films, unlike in the usual process. This is the greatest feature of mist Chemical Vapor Deposition (mist CVC), discovered after about 10 years of research from a wide range of perspectives."

Dr. Kawaharamura has developed a technology for high rectification under the conventional mist method and has established a non-vacuum process, but he also elucidated a mechanism for homogeneous thin film fabrication. This will make it possible to fabricate high quality thin films which are uniform at the atomic level over a large area, without the use of special equipment.

A new era when everyone can not only use new thin film technology but also create it

Dr. Kawaharamura started this research when he was in the first year of his master's degree studies. When his laboratory began a project to create functional thin films from zinc oxide as a raw material by atmospheric ultrasonic atomization, he saw a great opportunity and joined the project.

Dr. Kawaharamura says, "The principle of ultrasonic atomization is used in ordinary devices such as humidifiers, but I was interested in the fact that the process was still mysterious physically. Simply exposing water to ultrasonic waves generates mist. I was fascinated by the behavior of mist."

Originally the raw material for thin films was zinc oxide, but now all kinds of materials can be used, including metals, organic substances and sulfur compounds. Dr. Kawaharamura says he would like to use this technology to contribute to industrial development.

"The fact that few researchers are working in this area is a big advantage for me. My work is the only study to contribute to the understanding of the principle of highly controlled thin film formation in a complicated process where disturbance and side reactions occur at atmospheric pressure. I would like to fully complete this technology with my own hands and disseminate it to the world."

What kind of future will we see when this technology is put into practical use?

Dr. Kawaharamura replies, "Currently devices using functional thin films are very familiar, but if we can develop compact devices that can make thin films using this technology, it would enable us to provide experiments in which children could develop thin films in elementary and junior high school chemistry classes. I think that eventually people will be able to make these films easily at home. That will be the beginning of the era when we can each make electronic devices such as mobile phones ourselves."

This new non-vacuum thin film fabrication technology appears to be pointing towards unimaginable changes in our lives.