１．Overview

Next-generation semiconductor crystal materials (single-crystal SiC, single-crystal GaN, etc.) that can produce high-efficiency power semiconductors are being adopted for electric and hybrid vehicles, trains, etc. However, these materials are extremely difficult to process, and the enormous time and cost required by conventional processing methods have hindered their mass production. single-crystal SiC are one of these materials. Among these, single-crystal SiC undergo a process of cutting, rough machining, medium finishing, and mirror polishing, as shown in Figure 1. Purely mechanical processing other than mirror polishing can be handled as an extension of conventional technology, but mirror polishing has been the last unsolved problem because it is difficult to achieve both processing quality and processing efficiency.

However, this technology has succeeded in achieving highly efficient mirror polishing by using conventional polishing equipment with the aid of potassium permanganate, a strong oxidizing agent. The specific mechanism of this technology is an unconventional and unique technique called abrasive grain generating polishing, in which potassium permanganate is deposited from aqueous solution as fine polishing particles (abrasive-grains) that perform mechanochemical action.

Fig.1 Example of wafer processing

２．Technology

Figure 2 shows a scene of SiC wafer polishing and an SEM photograph of potassium permanganate (KMnO₄) particles deposited on a polishing pad. In loose abrasive polishing using conventional polishing slurry, as shown in Figure 3, the deposited potassium permanganate (KMnO₄) is pressed strongly against the single-crystal SiC by hard mechanical abrasives (abrasive particles) in the oxidizing solution, causing a mechanochemical reaction that oxidizes the single-crystal SiC surface and softens it. This causes the single-crystal SiC surface to be oxidized and softened, resulting in a highly efficient polishing process. The deposited potassium permanganate (KMnO₄) acts as a mechanochemical abrasive grain, so polishing is performed while generating abrasive grains. The efficiency is somewhat low because manganese dioxide (MnO₂) is deposited at this time, which has low polishing efficiency.

Fig2.Images of SiC wafer polishing

Fig.3 Mechanism in loose abrasive polishing

In order to efficiently apply potassium permanganate (KMnO₄), we developed an original abrasive-grain included polishing pad with movable mechanically acting abrasive grains, as shown in Figure 4. We found that increasing the amount of oxygen atoms in the resin of the polishing pad increases the deposition of potassium permanganate (KMnO₄), and adopted this feature for the abrasive-encapsulated polishing pad.

Fig.4 Mechanism in abrasive-grain included polishing pad polishing

３．Conclusion

The development of this technology has greatly promoted the spread of next-generation power devices such as SiC. In the future, we would like to apply this technology to GaN and the next generation of materials, such as diamond, and contribute to the promotion of the use of these wafers.

Makoto Sato
Member, NORITAKE CO., LIMITED (300 Higashiyama, Miyoshi-cho, Miyoshi, Aichi 470-0293, Japan)