In the early 2000s the first Silicon Carbide (SiC) based commercial diodes were available in the market; however, it was only in a span of last four years a remarkable increase in demand of Silicon Carbide has been seen. The rise of electric vehicle is one major driving factor for growth of SiC based devices. SiC based components have wider bandgap (3.2 eV), high dielectric breakdown (3.5 MVkm), and high thermal conductivity (4.9 W/cm-K) over conventional Silicon (Si) devices. They have also demonstrated fast reverse recovery and high reverse blocking voltages and excellent high temperature characteristics (case temperatures above 150 C). These characteristics make they apt for power electronics applications such as battery-operated electric vehicles.
A major concern for SiC has traditionally been availability of reliable surface finished for SiC ingots and wafers. SiC has Mohs hardness rating of 9, SiC is third the hardest available material, making the dicing process very difficult. Another major challenge for SiC substrates is various defect types, such as crystalline stacking faults, micropipes, pits, scratches, stains, and surface particles are prominent. Also, the need for testing and monitoring tools to manage the inspection of the wafers remains challenging too.
We analyzed the patents portfolio addressing the manufacturing challenges for SiC, we observed two prominent SiC wafer solution providers have been Wolfspeed formerly known as Cree and Pureon. While Cree has a strong patent portfolio for SiC based process and devices, Pureon does not have any patents registered.
Wolfspeed formerly known as Cree has been supplying 4-inch SiC wafers with zero micro-pipe since 2007. (3) In 2011 they launched first fully qualified commercial silicon carbide power MOSFET. In 2019 Volkswagen Group selected Cree to partner in their launch of electric vehicles.
Wolfspeed have some very early publications ranging 1999 to recent applications in year 2019. One of their early publications teach method of forming high-quality silicon carbide boules from process of gas fed sublimation (GFS), this GFS method demonstrated the ability to provide improved control of the temperature gradients within the crystal growth system. The GFS system comprises graphite crucible and is coated with metal carbide. This system avoids formation of carbon dust.
In their recent two applications recent application filed in 2021 interestingly, they have stepped away from the gas fed sublimation process and in these they teach a method to achieve 150mm crystal with very few detects using oversaturated physical vapor transport.
The method teaches the complete crystal growth process. The fig 1 below will aid in understanding of the process. During the crystal growth stage, a crucible contains source material and the SiC seed; later the source material (this can be Si, C or SiC etc) sublimes on the SiC seed. The SiC crystal is grown to a certain expected height by sublimation of the source material using a physical vapor transport process. It is at this crystal growth stage the crystalline defects are formed and are majorly observed in around the seed region. As the growth progresses the crystalline defects are less in the middle portion of the SiC boule and are more towards the edge portion. Conventionally this very method of growth was observed to generate multiple defects. This suggested solution is use of oversaturated quantities of the source material during the growth stage and achieve a high diameter and lower defects. The process parameters such as temperature, pressure, material purity is so maintained along with supply of an increased vapor flux for deposition of Si and C constituents on the seed. This vapor flux is oversaturated, and parasitic deposition may occur within the crucible away from the seed or some vapor flux may exit through exhaust of the SiC growth system without deposition or sublimation, thus reducing the possibility of defects. The patent claims for SiC wafer of 145 millimeters (mm), a total dislocation density (TDD) of less than or equal to 3000 per square centimeter (cm2) for any annular ring comprising a 1 mm width. The patent has shared the defect value chart as well.
These inventions seem to be facilitating the growth of SiC power modules and discrete components to a larger volume. Thus, addressing one of the major challenges for ingot generations of SiC growth.
Exawatt a technology consulting firm has released a report on SiC in electric vehicles, and the data reveals a high traction from the automakers in SiC based inverter. The blue dots represent Si IGBT used in the electric vehicle and the red dots represents SiC MOSFETs.
Fig 2 The inclusion of SiC components in Automotive
Automakers such as Porsche, Audi and Hyundai have developed in-house SiC inverters and will be implying this in their models, Porsche Taycan, Audi Q6 e-tron, and Hyundai Ioniq 5. Toyota CRDL and Denso began basic research in the 1980s, and now they have developed a silicon carbide (SiC) power semiconductor for use in automotive power control units (PCUs).
Wolfspeed has collaborated with Volksvagen and Lucid Motors for pure electric car. STMicroelectronics provides the Silicon Carbide MOSFETs for Tesla, these are used in Tesla Model 3 for its main inverter. ROHM a Japanese semiconductor provider has collaborated with Formula E team Venturi , Geely automotive and SEMIKRON for SiC based power inverters. These collaborations of automakers and SiC suppliers show a promising future for SiC in automobile domain.
The future holds the answer whether SiC completely replace Si in the automotive power train or will GaN be the alternative.
