Nanoimprint technology boosts LED light efficiency

Nanoimprinting is a technique in which a mold having a fine pattern is pressed onto a substrate or the like like a stamp to transfer the pattern in a large amount. Recently, there have been more and more practical cases of this technology.
Mass production of LED molds by R2R The fine pattern formation technology using nanoimprint technology helps to improve the luminous efficiency of LEDs and organic ELs. Toshiba Machine Co., Ltd. has developed a technology that increases the luminous efficiency of LEDs by 20 to 30%, including a dedicated imprinting device. By using a PSS (Patterned Sapphire Substrate) which forms a concave-convex pattern on the surface of the sapphire substrate, the reflectance and the like are improved, and the light-emitting output is improved.

Toshiba said that the problem is how to reduce the number of defects in the mold and how to make the cost more advantageous than using the existing stepper to form a pattern. If the substrate is defective, the LED will not emit light. And if the mold is used repeatedly to reduce costs, the defects will increase.

The company's response to these two problems is to make a one-off product from a resin mold that is largely replicated using the R2R method. The goal of reducing the cost of a 4-inch wafer to less than $5 is already eye-catching. Another advantage of resin molds is that they are suitable for sapphire substrates that are not necessarily flat.

Achieving high-quality GaN crystals has recently emerged with the possibility of using nanoimprint technology to further increase LED efficiency. Furukawa Machinery Metal, Kanazawa Institute of Technology, Toshiba Machine and Associate Professor of Waseda University Shui Yerun's research laboratory developed nano-imprint technology to dislocation of GaN crystals (dislocations: linear defects contained in crystals. Previously GaN crystals The dislocation density is as high as 1109/cm2 or more, which is considered to be a cause of a decrease in luminous efficiency when a large current flows through the LED.
The specific method is as follows: first, a SiO2 film is formed on the original GaN crystal, and a small opening of several tens of nm width is formed by a nanoimprint technique; then the GaN crystal is grown again. Thus, dislocations of the GaN crystal under the SiO2 film do not reach the upper GaN crystal, thereby reducing dislocations of the upper GaN crystal. Professor Mizuno of Waseda University said that we have prototyped LEDs to confirm that this technology can increase output power and extend life. It should also be used for power semiconductors.

Mizuno said that the technology is also expected to reduce the driving voltage of LEDs. Due to the small number of dislocations, GaN crystals that previously had to reach a thickness of 140 m can be greatly reduced to less than 21 m.

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