R&D Report 1
Deep UV-LEDs for
Quick Water Disinfection
In 2014, professor Isamu Akasaki of Meijo University and professor Hiroshi Amano of Nagoya University et al. were awarded the Nobel Prize in Physics for blue LEDs. The professors had already begun research into "LEDs to succeed blue LEDs." They are deep UV-LEDs. The deep UV-LEDs create light of a shorter wavelength that cannot be obtained by conventional LED light sources, allowing the effects of disinfection, air purification, etc. In the past mercury lamps had been used for such applications. However, mercury lamps require high power consumption, and are harmful to the human body and environment. On the contrary, deep UV-LEDs have low power consumption, a long life, and no harmful effect on the human body. Nikkiso's development staff took on the challenge of productization for this technology.
Epi Development Group
Manufacturing and Manufacturing Engineering Department
Package Manufacturing Group
UV-LED Business Development Department
Module Development Group
PHASE 1To produce high-power deep UV-LEDs
The biggest task in developing deep UV-LEDs was, how to achieve high-power light. Yusuke Matsukura, who is in charge of developing chips in the UV-LED Business, stated that, "Since the deep UV-LEDs were different from regular LEDs, it was necessary to make the structure of a chip that matched the deep UV-LEDs." Normally, LED light is emitted by making a PN junction where the n-type layer and p-type layer are joined and the current flows through there. However, in the case of deep UV-LEDs, the chip is manufactured mainly using AlGaN material and growing the crystals.
"Because making the p type AlGaN* is very difficult, GaN, which has been also used in blue LEDs, is used for the p-type layer. In the case of blue LEDs, since the emitted light is not absorbed even when a GaN is used for the p-type layer, the light can be effectively removed. However, in the case of deep UV-LEDs, since generated light is absorbed in the GaN layer if the GaN is used for the p-type layer, it has been necessary to change the structures of both the chip and wafer to make high-powered LEDs." (Matsukura)
Although there was a case where the "wafer glowed tremendously" (Matsukura) in the experiment, the wafer did not glow even afterwards even when created with a method, and then trial production and analysis was repeated for about half year. "Although several months passed before we identified the cause, we happened to take notice of the idea that the cause might be an influence of a certain material that our company newly introduced, and we tried to create wafers by changing the conditions in various ways." (Matsukura) As a result, the glowing wafer was reproduced. Thus, development of the High-powered deep UV-LEDs took a step forward.
* Making the p type AlGaN denotes adding an impurity that operates as the p type to the AlGaN, and creates the p-type semiconductor.
PHASE 2Achieving a high-powered, long lasting package
Although the method make the deep UV-LED light steady was known, the next development theme was "How to get that light out." The group leader of the Package Manufacturing Group, Keiji Ichinokura addressed the task. In order to enhance the light, a resin sealing package is used for ordinary LEDs. However, in the case of deep UV-LEDs, similar conventional material cannot be used because the resin material itself deteriorates under the influence of the deep ultraviolet rays (DUV). Then, the Ichinokura Group selected a package that doesn't use resin.
At the same time, a size smaller than 8 mm square was targeted and a reduction to 3.5 mm square was possible as a result. At this time, the life of LEDs became the issue. The 8 mm package was driven with a current of 100 mA. However, in the case of the 3.5 mm size, it is necessary to drive with a current of 350 mA. "Although product life of LEDs is reduced when the current is increased, we desired a product life (durability) of at least 10,000 hours in order to be used by customers without worry. Thus, we investigated the mechanism regarding deterioration progression in deep UV-LEDs through various kinds of trial manufacturing and discussions with the development team." (Ichinokura)
As a result, the target product life of 10,000 hours was achieved, and the high-power package, compactness, and long service life were complete. "I think, finally the full-fledged package, which is desired by customers, has been created." (Ichinokura)
PHASE 3Aiming for solutions that are useful on a long-term basis
The package for deep UV-LEDs with a size of 3.5 mm received very high evaluations from many customers. Around that time, a certain client consulted with us as to whether deep UV-LEDs would be applicable to water purifying plants, instead of mercury lamps. The Module Development Group, including Hiroki Kiuchi, responded to this consultation. It was said that the mercury lamps have a risk of adversely affecting the human body, etc., and the international treaty, called the "Minamata Convention on Mercury" was concluded in 2013. "Because 'Mercury free' fields were targeted, naturally applications for water disinfection were included in the plan. However, in the case of large quantities of water such as water purifying plants, the method for enabling certain and effective water disinfection must be considered." (Kiuchi)
Although the water disinfection module of "2 L/min," applicable to drinking water at home, etc. was already at the end of the development stage, the ability to process even larger amounts of water, namely 50 to 100 L/min, is required in the case of water purifying plants. For that purpose, Kiuchi and other members made an array of multiple deep UV-LEDs to enhance disinfection ability and improve the light's use efficiency via the structure of the package. As a result, disinfection ability for 120 L/min or more was confirmed.
"There are still so many items to be considered, such as what we will do if it fails, what to do during repairs, and so on. We are aiming for a solution so that customers can use the deep UV-LEDs with a sense of security for a long period." (Kiuchi)
PHASE 4Future target and vision
Presently, demand for safe and clean water is increasing throughout the world, and interest in deep UV-LEDs is increasing as the alternative of mercury lamps, notably in Europe, the United States, and Asia. In addition, regarding Nikkiso's deep UV-LEDs, which are expected to have applications in various fields such as industry, medical treatment, environment, etc., inquiries about the future were posed to three persons.
"Mercury lamps also emit light with a wavelength that is intrinsically not necessary. However, in the case of LEDs, when light with an arbitrary wavelength of, for example, only 270 nanometers is necessary, it is possible to design it so that only light of 270 nanometers is emitted. By taking advantage of such features, we hope to create products that match the use applications even better." (Matsukura)
"As for the present package, only about 60% of the light emitted from the chip can be taken outside. By improving it, we hope to create the magic package in which the light emitted from the chip enters into the package and 100% of the light can exit it. If it can be created, I feel we can also advance to the next step." (Ichinokura)
"It is also said that there are no water purifying plants near Japanese villages deep in the mountains. We would like to develop modules that can be used even such places. If such modules can be developed, I think those can be accepted also in Asian countries, etc." (Kiuchi)