SiC material

Silicon carbide (SiC) is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests. Electronic applications of silicon carbide as light-emitting diodes (LEDs) and detectors in early radios were first demonstrated around 1907, and today SiC is widely used in high-temperature/high-voltage semiconductor electronics. Large single crystals of silicon carbide can be grown by the Lely method; they can be cut into gems known as synthetic moissanite. Silicon carbide with high surface area can be produced from SiO2 contained in plant material.

Natural Occurence

Naturally occurring moissanite is found in only minute quantities in certain types of meteorite and in corundum deposits and kimberlite. Virtually all the silicon carbide sold in the world, including moissanite jewels, is synthetic. Natural moissanite was first found in 1893 as a small component of the Canyon Diablo meteorite in Arizona by Dr. Ferdinand Henri Moissan, after whom the material was named in 1905. Moissan's discovery of naturally occurring SiC was initially disputed because his sample may have been contaminated by silicon carbide saw blades that were already on the market at that time.

While rare on Earth, silicon carbide is remarkably common in space. It is a common form of stardust found around carbon-rich stars, and examples of this stardust have been found in pristine condition in primitive (unaltered) meteorites. The silicon carbide found in space and in meteorites is almost exclusively the beta-polymorph. Analysis of SiC grains found in the Murchison meteorite, a carbonaceous chondrite meteorite, has revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system; 99% of these SiC grains originate around carbon-rich asymptotic giant branch stars. SiC is commonly found around these stars as deduced from their infrared spectra

Production

Because of the rarity of natural moissanite, most silicon carbide is synthetic. It is used as an abrasive, and more recently as a semiconductor and diamond simulant of gem quality. The simplest manufacturing process is to combine silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1,600 °C and 2,500 °C. Fine SiO2 particles in plant material (e.g. rice husks) can be converted to SiC by heating in the excess carbon from the organic material. The silica fume, which is a byproduct of producing silicon metal and ferrosilicon alloys, also can be converted to SiC by heating with graphite at 1,500 °C.

Progress on Materials

The last decade has seen tremendous improvements in single crystal SiC materials. Device manufacturing was originally established on 2” diameter wafers featuring more than 20 device-killing defects, such as micropipes and epi defects, per cm2. Today 100 mm diameter substrates are commercially available in volumes and qualities suitable for manufacturing. Since more than 5 years SiC substrate material is available with micropipe density below 1 cm-2 on more than 90% of the wafer area. The total extended defect density (after epi), including prominent defects like triangles, carrots, comets, and downfall particles, averages to 1-2 cm-2. These material improvements led to improved yield and reduced cost in the manufacturing of devices with voltage ratings of up to 1700 V and current ratings of up to 20 A. A further reduction in defect density is necessary for the cost-effective manufacturing of devices with higher current ratings of 50-100 A. It is estimated that a total defect density of less than 0.2 cm-2 is required to achieve 80% yield for large area (1 cm2) devices. 

SiC substrate manufacturers are currently working hard to increase the wafer diameter to 150mm. The transition to 150mm diameter wafers in SiC device fabrication is expected to happen during the next 2-3 years leading to a further reduction in manufacturing cost. However, advanced devices working at very high voltage levels and increased current densities make it necessary to focus not only on the wafer size, but also on reducing defects such as dislocations. Dislocations have so far not severely limited the performance of commercial devices, but could limit the performance of 10 kV-rated devices