Announcement of Development of Mosaic Crystals for 2-inch Wafer Production

Our company's primary advantage lies in our ability to produce large diamond single crystals, and we have been advancing the development of wafers for diamond device fabrication. We launched 1-inch single-crystal wafers (25mm diameter) in April 2025; however, 2-inch (50mm diameter) wafers are essential for manufacturing semiconductor devices, and we have continued development aiming for their practical application. As a result, we have successfully developed mosaic crystals for producing 2-inch mosaic wafers.

1. Introduction
Our company was established with the goal of applying the excellent physical properties of diamond to various applications, supplying single-crystal materials, and creating that market. In particular, since the advantages of using diamond as a semiconductor, such as low loss, are considered to have great potential for energy conservation, we have aimed to commercialize the necessary wafers.

In semiconductor device fabrication, using disk-shaped wafers allows for the use of established process equipment, enables the simultaneous production of numerous devices on a single wafer, and reduces device production costs. However, increasing the size of diamond single crystals is extremely difficult with the current growth process, the microwave plasma CVD method, and we have not yet been able to produce large single-crystal wafers.

To address this challenge, we have aimed to develop large wafers by connecting single crystals laterally to create mosaic crystals consisting of a small number of single crystals. Although the grain boundaries, which are the connection points between crystals, are not suitable for semiconductor device fabrication, the interior of the grain boundaries can be treated as a single crystal, allowing for the fabrication of numerous devices through the process as a single wafer. We have already expanded the area of our mosaic crystals and have commercialized 38x38mm mosaic crystals.

In the device fabrication process, silicon has led the way in achieving larger wafers, and process equipment has been developed accordingly. Currently, 12-inch (300mm diameter) is the maximum size used for various device fabrications. On the other hand, 4-inch (100mm diameter) wafers are used for small-scale device production or as semiconductor pilot lines. Since some process equipment supports 2-inch wafers, 2-inch (50mm diameter) is considered the realistic minimum size.

If 2-inch wafers are realized in this way, they can be utilized for the development of diamond semiconductor devices, and small-scale production of some devices will become possible. We have aimed for 2-inch wafer development as the first stage of our diamond wafer development. In the diamond wafer roadmap announced in November 2024, we decided to develop 2-inch wafers by December 2025. To achieve this, we thought we could produce 2-inch wafers by developing single crystals with an area of 25x25mm or more, connecting four of them laterally to produce mosaic crystals of 50x50mm or more, and cutting a 50mm diameter disk from them. Although 30x30mm single crystals were launched in February 2025, we were unable to solve the technical issues in our efforts to produce 50x50mm or larger mosaic crystals by the December 2025 target. Subsequently, we set a new target for the end of March 2026, but we were still unable to resolve the technical issues.

This problem was a phenomenon where the mosaic crystal would break or crack due to stress generated near the grain boundaries when joining the single crystals. This phenomenon was not a major issue when producing existing mosaic crystals, but it became apparent as the size increased. By overcoming this problem, we have now succeeded in developing mosaic crystals for 2-inch wafers.

Also, polishing such large-area mosaic crystals is a first, and we could not polish the entire surface under the same conditions as the 38x38mm mosaic crystals, which were the largest until now, so we added several improvements. As a result, we confirmed that the mosaic crystal in the photo achieved a surface roughness of about 5nm on almost the entire surface and can be used as a parent crystal.

2. Contents of the Developed 2-inch Mosaic Crystal
(1) Size: 53x53x1.2mm
(2) Number of constituent single crystals: 4 (individual crystals have an area of 25x25mm or more)
(3) Surface polishing: One side polished, Ra ≈ 5nm

3. Mass Production Plan for 2-inch Wafers
Now that this mosaic crystal is complete, we will produce child crystals for 2-inch wafers by utilizing our proprietary replication technology using ion implantation. Then, we will finish them into 2-inch wafers by cutting them to a diameter of 50mm with a laser. We believe these processes can be smoothly commercialized as we have a track record in producing many substrates and wafers.

First, we will produce many parent crystals and start mass production. If this preparation proceeds smoothly, we believe that a mass production system will be in place in the second half of this fiscal year.

A challenge in advancing mass production is that polishing takes a very long time for a 2-inch area. It takes about 10 times longer than the polishing time for 1-inch, and this improvement is an urgent task. We believe that measures such as increasing the size of the polishing equipment currently in use are necessary, and we will aim to realize such equipment development through future fundraising.

4. Future Development
We will continue to advance the development of 4-inch mosaic wafers in accordance with the roadmap we have already disclosed. To do this, we will develop single crystals of 50x50mm or more and use them to produce mosaic crystals of 100x100mm or more. In the development of this 2-inch mosaic crystal, many technical issues were discovered in the production and polishing of mosaic crystals connecting large crystals. We will promote various developments to solve these issues.

In addition, as risk management for when mosaic wafers cannot be developed as expected, we will also consider further area expansion of single crystals and "bonded wafer" technology, in which small diamond substrates are attached to wafers other than diamond.