Materials Science Forum Vols. 717-720

Abstract: We report on the initial demonstration of a tungsten-nickel (75:25 at. %) ohmic contact to silicon carbide (SiC) that performed for up to fifteen hours of heat treatment in argon at 1000 °C. The transfer length method (TLM) test structure was used to evaluate the contacts. Samples showed consistent ohmic behavior with specific contact resistance values averaging 5 x 10-4 Ω-cm². The development of this contact metallization should allow silicon carbide devices to operate more reliably at the present maximum operating temperature of 600 °C while potentially extending operations to 1000 °C.

Abstract: In this study, investigations on MAX phase Ti3SiC2 formation to n-type 4H-SiC substrates and its ohmic-behaved are reported. Ti-Al layers were deposited onto SiC substrates at room temperature by magnetron sputtering in high vacuum system. Thermal annealing at 1000°C in Ar atmosphere were performed to allow interdiffusion processes. X-ray diffraction and High Resolution Transmission Electron Microscopy reveal that a Ti3SiC2 contact, in perfect epitaxy with 4H-SiC substrate, is so-obtained. In situ annealing experiment underlines the evolution of Ti-Al contact microstructure versus temperature. The evolution of contact system from Schottky to Ohmic behaved is observed by I-V measurements for annealing temperatures larger than 700°C.

Abstract: The characteristics of Ga-doped zinc oxide (GaZnO) thin films deposited at different substrate temperatures (TS~250 to 550oC) on 4H-SiC have been investigated. Structural and electrical properties of GaZnO thin film on n-type 4H-SiC (100)were investigated by using x-ray diffraction, atomic force microscopy (AFM), Hall effect measurement, and Auger electron spectroscopy (AES). Hall mobility is found to increase as the substrate temperature increase from 250 to 550 oC, whereas the lowest resistivity (~3.3 x 10-4 Ωcm) and highest carrier concentration (~1.33x1021cm-3) values are observed for the GaZnO films deposited at 400 oC. It has been found that the c-axis oriented crystalline quality as well as the relative amount of activated Ga3+ Introduction ions may affect the electrical properties of GaZnO films on SiC.

Abstract: A new high-temperature die attachment system that is cost effective has been strongly desired for SiC power applications in electric vehicles and consumer electronics. This paper presents preliminary results for SiC/Zn-Al/Cu-SiN die attachments using eutectic Zn-Al solder (m.p. = 356°C), focusing on preparation and die-shear reliability. Superior wettability and reproducibility were achieved in the soldering process. It was found that the attachments were viable at least for short-term application in a temperature range up to 300°C. Reliability test results revealed that they could withstand storage for 1500 hours at 200°C and thermal cycle stress of 1500 cycles between –40°C and 200°C.

Abstract: We investigated the effect of the metal work-function and doping concentration on the barrier height of Ni-contacts with embedded nano-particles (NPs) on 4H-SiC surfaces. Both n-type epitaxial layers with ND=1×1016 cm-3, and layers doped by phosphorous implantation to a doping concentration of ~1×1019 cm-3 are used. The barrier height is reduced with increasing doping concentration and the silver (Ag) nano-particles (R~18.5 nm) further enhances the local electric field of the electrical contacts to 4H-SiC in comparison to gold (Au) nano-particles (R~20.2 nm). In the case of ion-implanted samples, the barrier height of the fabricated SiC diode structures with embedded Ag-NPs was significantly reduced by ~0.09 eV and ~0.25 eV compared to the samples with Au-NPs and the sample without NPs, respectively.

Abstract: A new method based on electric discharge machining (EDM) was developed for cutting a silicon carbide (SiC) ingot. The EDM method is a very useful technique to cut hard materials like SiC. By cutting with the EDM method, kerf loss and roughness of sample are generally smaller than those obtained by cutting with a diamond saw. Moreover, the warpage is smaller than that by the diamond saw cutting, and the cutting speed can be 10 times faster than that of the diamond saw at the present time. We used wires of 50 mm and 100 mm diameters in the experiments, and the experimental results of the cutting speed and the kerf losses are presented. The kerf loss of the 50 mm wire is less than 100 mm, and the cutting speed is about 0.8 mm/min for the thickness of a 6 mm SiC ingot. If we can maintain the cutting speed, the slicing time of a 2 inches diameter ingot would be about seven hours.

Abstract: Silicon carbide (SiC) is a promising semiconductor material for high-temperature, high-frequency, high-power, and energy-saving applications. However, it is so hard and chemically stable that there are few efficient conventional machining methods for it. We have developed plasma chemical vaporization machining (PCVM), an atmospheric-pressure plasma etching process, and investigated its application to the processing of SiC substrates. In this paper, the cutting characteristics of a SiC substrate by PCVM with a wire electrode are described. We found that increasing the rf power and reactive gas concentration increases the etch rate and that the etch width can be reduces by increasing the SF6 concentration. The maximum etch rate was 2.1 μm/min and the minimum etch width was 220 μm. It was also demonstrated that a SiC wafer prethinned to 100 μm can be successfully cut without breaking or cracking.

Abstract: Relationship between the chemical reactivity and the orientation of SiC substrates was investigated. Thermal etching of 4H-SiC in the mixed gas of oxygen and chlorine was carried out as the chemical reaction. The etching rate did not change monotonously with the increase of the off angle in 4H-SiC (000-1) C substrate. By the use of such tendency in the thermal etching, the three dimensional structure with the specific pyramidal plane was able to be obtained.

Abstract: A novel abrasive-free planarization method “called catalyst-referred etching (CARE)” has been invented. After the CARE process, a flat and well-ordered surface is obtained as observed by atomic force microscopy (AFM). To determine the atomic structure at the topmost surface, in this study, CARE-processed surfaces of a standard commercial 2-inch n-type 4H-SiC (0001) wafer cut 8^o off-axis toward the [1-100] direction were observed by high-resolution transmission electron microscopy (HRTEM). The HRTEM images showed alternating wide and narrow terraces and a single-bilayer step height. The relationship between the width of the terraces and the 4H-SiC crystal structure has been clarified.

Abstract: In this article, we report a new cleaning method for silicon carbide (SiC) wafers. We found that the dipping treatment in hydrogen fluoride (HF) solution damages the SiC in the “RCA cleaning process”, so we have designed a new cleaning method that does not use HF and reduced the cleaning process to three steps. The characteristic factor of this new method is using a transition metal complex. Generally, no metals have been used for wafer cleaning, but we deliberately used metal and found it could clean the wafer surface very well. After cleaning, the atomic force microscope (AFM) and “Candela” images showed that the particles on most parts of the SiC surface had been removed and the contact angle for ultra-pure water became very low.