News: Optoelectronics
23 June 2025
OKI and NTT establish mass-production technology for high-power terahertz devices by heterogeneous material bonding
Tokyo-based Oki Electric Industry Co Ltd, in collaboration with Japan-based NTT Innovative Devices Corp, has established mass-production technology for high-power terahertz devices using its proprietary crystal film bonding (CFB) technology for heterogeneous material bonding to bond indium phosphide (InP)-based uni-traveling carrier photodiodes (UTC-PD) (a PiN junction photodiode that selectively uses electrons as active carriers) onto silicon carbide (SiC) with what are claimed to be excellent heat dissipation characteristics for improved bonding yields. UTC-PDs could operate faster and with much wider output linearity simply by excluding the hole transport contribution to the diode operation.
Terahertz devices are expected to play a core technology role in supporting high-capacity low-latency communications for the next-generation 6G communication standard and high-precision non-destructive inspection for improved safety. Based on these results, both companies are working on product development, aiming to start mass production in fiscal year 2026.
Terahertz waves are electromagnetic waves in the range between radio waves and visible light, having both the penetrating characteristics of radio waves and the straightline propagation of light. Due to their non-invasiveness to the living body, which is a problem with x-ray inspection technology, terahertz waves are expected to be developed for applications including non-destructive inspection and security. In wireless communication applications, higher carrier frequencies contribute to increased communication capacity. On the other hand, terahertz waves have the drawback of being significantly attenuated in the atmosphere, creating the need for the development of high-power terahertz devices. Establishing mass-production technologies is also essential for real-world implementation.
To address these challenges, NTT Innovative Devices has been working to improve the performances (output power, output spectrum etc) of the UTC-photomixers (the module that applies UTC-PD to THz signal generation). Particularly in wireless communications, to propagate standard multi-level modulation signals over long distances, it is essential to achieve high output power at 1dB compression. To achieve the high output power at 1dB compression, NTT Innovative Devices and a team at The University of Osaka, Kyushu University and The University of Tokyo — under commission from Japan’s National Institute of Information and Communications Technology (NICT) through the ‘Beyond 5G R&D Promotion Program’ (JPJ012368C-00901) — focused on the heat dissipation characteristics of the device and studied the technology of bonding InP-based UTC-PDs directly onto SiC with high heat dissipation characteristics (Note 7). This paved the way to realize UTC-photomixers offering an approximately ten-fold performance increase (the output power at 1dB compression exceeding 1mW) compared with conventional devices.
In wafer bonding, due to the large bonding area, even a minute bonding defect at one location can cause a bonding failure over a large area. Therefore, more advanced bonding technology is required for mass production. For UTC-PD on SiC chip, material cost (effective use of materials) is also a demanding improvement item, because the required InP area is less than 10% of the area in the chip.
OKI applied CFB technology to divide the InP-based crystal films on the InP-based epitaxial wafers at the device level, selectively picking up only the portions necessary for device operation before bonding them to the SiC wafers by heterogeneous material bonding. CFB technology, OKI’s proprietary heterogeneous material bonding technology developed in the printer market and refined over about 20 years of mass production, has already established high yields.
The process also offers high efficiency, since InP-based crystal films divided at the device level are bonded all together at wafer-size scales.
The results of evaluating the yield of devices bonded using CFB technology show dramatically higher yields in the bonding process, with the bonding yield improving from about 50% to nearly 100% compared with conventional processes.
Additionally, dividing the crystal films at the device level and selectively bonding the devices has made it possible to make effective use of the crystal films that were previously discarded with conventional processes, helping to reduce costs and environmental impact by improving material utilization efficiency.
NTT Innovative Devices developed chips by forming UTC-PDs in the device process on SiC wafers with crystal films bonded using CFB technology. The results of device evaluations following chip development showed an output power at 1dB compression exceeding 1mW in a single device, demonstrating high output and excellent linearity. Compared with devices produced using conventional bonding processes, dark current was reduced to about one-third, confirming that the process using CFB technology is capable of bonding while effectively maintaining the characteristics of InP-based crystal films.
This co-creation work has established mass-production technology for high-power terahertz devices and made real-world implementation a reality.
Moving forward, both companies aim to build on the results of this joint research to start mass production of terahertz devices in fiscal year 2026 and strengthen collaboration with industry and academia to focus on commercializing 6G communication technologies and the broad application of non-destructive sensing technologies. Both companies will also draw on the jointly developed technology to accelerate efforts to contribute to a next-generation society, communicating to the world advanced technologies for both Japanese and global markets.
NTT Innovative Devices is exhibiting this technology in booth B2.331 at Laser World of Photonics 2025 in Munich, Germany (24–27 June).
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