Materials Science Forum Vols. 778-780

Abstract: Silicon carbide (SiC) is a promising semiconductor material for high-temperature, high-frequency, high-power, and energy-saving applications. However, because of the hardness and chemical stability of SiC, few conventional machining methods can handle this material efficiently. A plasma chemical vaporization machining (PCVM) technique is an atmospheric-pressure plasma etching process. We previously proposed a novel style of PCVM dicing using slit apertures for plasma confinement, which in principle can achieve both a high removal rate and small kerf loss, and demonstration experiments were performed using a silicon wafer as a sample. In this research, some basic experiments were performed using 4H-SiC wafer as a sample, and a maximum removal rate of approximately 10 μm/min and a narrowest groove width of 25 μm were achieved. We also found that argon can be used for plasma generation instead of expensive helium gas.

Abstract: In this paper, a machining energy control slicing method for cylindrical shaped ingots and forty-wire electrical discharge slicing (EDS) technology are investigated. Our recent study in [4], ten-wire EDS was applied to 100 mm-square polycrystalline SiC material. Applying this technology to ingot slicing, an appropriate process technology for cylindrical shaped SiC materials which are the same as an actual ingot is required. The slicing of cylindrical shaped SiC using conventional multi-wire EDS causes the increase of sori, or wire break with unstable machining process since wasteful machining power is not controlled as a function of machining length. To resolve this problem, we applied the machining energy control method which varies machining power with machining position. Using proposed method, ten-simultaneous slicing of cylindrical shaped SiC material is obtained with 80 μm/min in machining speed. The sori of machined surface is 34 μm, and TV5 is 28 μm as a result. Moreover, forty-wire EDS technology is applied to SiC slicing for improving wafer productivity. We successfully verified forty-simultaneous slicing of 100 mm-square poly-crystal SiC material without wire break. The sliced 39 thin plates are obtained 385 μm in average thickness, and 16 μm in maximum thickness variation.

Abstract: Recently, ingots of silicon carbide have been adapted to be sliced by the wire-cut electrical discharge machining. Fast slicing, and the reduction in the loss are important for slicing of the wafer. In this paper, characteristic features of the electric discharge machining in the ion-exchange water and the fluorine-based fluid were compared for these improvement. The discharge was caused by a pulse voltage applied to a ingot of silicon carbide and the wire in machining fluid, and the slicing was proceeded. As a result, improvement of surface roughness and kerf loss was confirmed, for the first time. In addition, the improving methods for fast slicing were considered.

Abstract: Development of high efficient and high accuracy slice processing technology is required for realizing the high quality and low cost large SiC wafer. Our target of high speed slicing is slicing a 6 inch SiC single crystal ingot in about 9 hours. This slicing speed is about 10 times higher than the loose abrasive slurry sawing and about 4 times higher than the current technology of diamond wire sawing. The slicing speed and the slicing accuracy are in the relationship of trade-off. Therefore, in this research, we have studied the high speed slicing technique of 3 inch and 4 inch SiC single crystal ingot aiming at reduction of sliced wafers SORI. Moreover, we have extracted subjects to scale-up for the high speed slicing of the 6 inch SiC single crystal ingot.

Abstract: In order to slice the larger size ingot toward 6 inch of silicon carbide (SiC), we are developing Multi-wire Electric Discharge Machining (EDM). To prevent wire break during slicing, we have developed the electric discharge pulse control system. So far, with 10 multi-wires, we have succeeded in slicing of 4 inch SiC balk single crystal without wire break. High quality slicing surface (e.g. small value of around 10 μm of SORI for 3 inchi wafer) was also achieved. By polishing methode, EDM-sliced wafer was estimated to have the uniform thickness of damaged layer over the entire surface. We confirmed that the wafer sliced by EDM can be processed in the later process, by grinding the 3 inch wafer. And it was confirmed that 6 inch ingot can be sliced with 10 multi-wire EDM, by slicing the boule of SiC poly crystal. For the larger diameter ingot than 4 inch, Multi-wire EDM will be practically used by the effective removal of machining chips from the machining clearance between the wire and work.

Abstract: We fabricated electrostatically actuated single-crystalline 4H-SiC microcantilever resonators. To realize a narrow gap between cantilevers and substrate, we etched a thin p-type SiC layer in n/p/n multilayer structure by doping-selective electrochemical etching. The resonant characteristics of the fabricated 4H-SiC microcantilevers were investigated under a vacuum condition. Electrostatic actuation of microcantilevers was successfully performed by applying 10 mV_rms ac voltage with 20 mV dc bias. The quality factor of 4H-SiC microcantilevers was above 100,000, which is about ten times higher than the quality factor of Si cantilevers with the same structure. Resonant characteristics were almost identical for mechanical actuation and electrostatic actuation.

Abstract: The multi-wire electrical discharge slicing (multi-wire EDS), which is a brand-new method for fabricating wafers, is expected to considerably reduce the production cost of SiC wafers by decreasing in the width of kerf and kerf loss. We evaluated, for the first time, the influences of a wire electrical discharge machining (WEDM) on the SiC wafers based on experiments using WEDM equipped with a power supply of EDS. Although the analyses by transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) revealed that the WEDM influenced layer consists of a contamination layer including several kinds of metals and a layer having crystal defects was certainly formed near the wafer surfaces, the width of the influenced layers was only 3μm, and the layer could be easily removed by the grinding process. Furthermore, characteristics of Schottky barrier diodes (SBDs) fabricated with removing the influenced layer formed by WEDM are comparable to those fabricated with using conventional wafers.

Abstract: Edge termination guaranteeing high breakdown voltage and robustness in its fabrication are required in SiC power devices. We newly employed the VLD edge termination for 3.3 kV-rated SiC SBDs, which was formed by Al ion implantation using a resist mask having a varying thickness. The breakdown voltage is recorded to be over 96% of the parallel-plane breakdown voltage, and the reverse bias characteristics are well accorded with the result of TCAD simulation.

Abstract: The static performance of different active and termination area designs for SiC-based Schottky diodes, suitable for 3.3kV applications, were investigated by means of extensive numerical simulations. We found quantitatively that the high electric field of SiC close to avalanche-breakdown is shielded most effectively from the Schottky interface by a trench-based design. Moreover, we conclude that the edge termination design with junction termination extension and four implanted p⁺ guard rings is most robust against oxide interfacial charge.

Abstract: 4H-SiC JBS diode with breakdown voltage higher than 4.5 kV, has been successfully fabricated on 4H-SiC wafers with epitaxial layer. In this paper we report the design, the fabrication, and the electrical characteristics of 4H-SiC JBS diode. Numerical simulations have been performed to select the doping level and thickness of the drift layer and the effectiveness of the edge termination technique. The epilayer properties of the N-type are 55 μm with a doping of 9×10¹⁴ cm⁻³. The diodes were fabricated with a floating guard rings edge termination. The on-state voltage was 4V at J_F =80 A/cm²