SiC: More valuable than diamonds?
What is the most valuable material in the world? Most people’s answer would be diamonds. For centuries, diamond has been a very rare material, especially when one was looking for the size and brilliance needed for impressive jewelry. Diamonds were a sure investment.
But these times could be over. Artificially produced diamonds are getting more important every day, leading to falling prices for the natural occurring ones. When first invented, artificial diamonds had a yellowish color and overall did not look as good. They were mostly used in industrial applications. But in recent years the production got much better, the differences between artificial and natural diamonds cannot be seen with the bare eye anymore. Artificial diamonds are used in jewelry today, even in big brilliant rings. With prices being 20% cheaper on average and (at least theoretically) lesser ecological and societal impact, they are slowly starting to replace their natural siblings. Without inflation, the prices for top diamonds have fallen around 80% in the last 30 years. So, diamonds are anything but a sure investment today.
The value of other materials is starting to increase as the demand rises. One example is silicon carbide (SiC), which – in contrast to diamond – is not easy to produce. High purity SiC can also be used as gemstone due to its hardness and refractive index being close to that of diamond. However SiC is in demand not because it looks nice, but because it offers great advantages for countless applications in the semiconductor world. SiC is a wide band gap semiconductor, with 3.2eV bandgap (compared to 1.12eV for silicon). A large breakdown electric field (around 3MV/cm) and a high electron saturation velocity (2·107cm/s) promise contributions to considerable energy savings. Voltage converters based on SiC technology have significant less losses than conventional silicon-based converters and enable much smaller modules, components and systems than silicon.
Yole expects the SiC power semiconductor market to be about $1.5B by 2023 with a compound annual growth rate (CAGR) of 31% for 2017-2023. The material is definitely in demand, but will the supply chain be ready to deliver?
At the moment, the bottleneck is right at the beginning, at the wafer supply. The demand increase is bigger than some suppliers expected, but the main problem is the transition from 4-inch- to 6-inch-wafers: It is not easy to produce high quality SiC wafers with the desired low number of defects in bigger size and meeting the requirements for geometry at the same time.
Securing supply by integrated production
ROHM Semiconductor is a pioneer in the field of SiC. In 2010 ROHM started mass production of SiC power components such as SiC Schottky diodes and SiC planar MOSFETs. In addition, ROHM was the first supplier to produce complete SiC power modules and SiC trench MOSFETs. The company has introduced a vertically integrated production system throughout the group. In order to complete the production chain ROHM acquired the German based SiCrystal GmbH in 2009 – a world leader in SiC substrate manufacturing. Hence, ROHM is covering the entire manufacturing process from SiC wafers through devices to packaging and is already prepared for the steeply rising future demand.
Why is SiC so valuable? – The difficulties of production
Frankly spoken, to describe the production of silicon carbide (SiC) wafers and their corresponding value in a few words is a huge challenge. For that, we want to use a comparison. A comparison between the technology to manufacture silicon substrates and the skills required to produce SiC substrates.
Silicon wafer manufacturing
The starting material for the production of silicon (Si) is quartzite or sand that gets mixed with coke. After heating in a submerged electrode arc furnace, you will have electronic grade Si. This material is processed according to the most dominant technique for manufacturing single crystals today, the Czochralski method. It is especially suitable for the large Si wafers that are used in IC fabrications today.
In the next step silicon is heated to above 1500°C in a furnace, since the Si melting point is 1412°C. A seed crystal with the desired orientation of the wafer, is dipped into the molten Si and slowly withdrawn by the crystal pulling mechanism. The seed crystal is relatively small, compared to the diameter of the final ingot. While it is being pulled, the seed crystal is rotated for example clockwise to ensure uniformity across the surface. The furnace is rotated counterclockwise. The molten Si sticks to the seed crystal and starts to solidify with the same orientation as the seed crystal. The final solidified Si obtained is the single crystal ingot with normally a length of 2m and the target diameter.
The process control, i.e. speed of withdrawal and the speed of rotation of the crystal puller, is crucial to obtain a good quality single crystal.
The art of producing SiC substrates
Typically, the starting point is the production of the SiC source material itself. This can be produced for example from silicon and carbon in top quality. At high temperatures and in an inert gas atmosphere both react to form SiC in extremely high pureness. Besides the already mentioned purity and homogeneity other parameters such as the chemical composition are critical for the right start to produce SiC substrates. To guarantee the best possible SiC starting material SiCrystal GmbH has its own in-house SiC source material preparation.
Now the production of the SiC boule itself starts. For the crystal growth process you can use a process called Physical Vapor Transport (PVT), also referred to as “seeded sublimation growth”. Usually the process is carried out at a temperature above 2200°C in an inductively heated closed graphite crucible surrounded by thermal insulation.
Figure 1: Schematic Illustration of Growth Reactor
At this high temperature and using inert gas the SiC basic material sublimates and deposits on the cooler seed as a single crystal. Compared to conventional silicon ingots which are crystallized from the liquid phase from silicon melt, the growth rate using the sublimation method is slow. Further numerous technologies for crystal growth control are required. SiCrystal's advanced crystal growth development is supported by numerical simulations.
One fundamental point has to be emphasized: The quality of the seed. First, compared to Si, one is required to have a seed which has to have at least the same diameter as the ingot! Second, the seed has to be of very good quality. You can compare it with a simple paper copy for example. Everything parasitic on your original will multiply in your copies. The same effect applies for SiC seeds. To ensure this high quality standard SiCrystal GmbH is producing its own seeds of course.
Enlargement of the diameter
Unfortunately the SiC ingots diameter can be increased only slightly during a growth process. Additionally only the best quality of the grown crystal can be used for the next generation – you remember our example from the paper copy.
Figure 2 shows the process of enlargement of the SiC seeds diameter.
Thus, it is clear that an enlargement of the diameter is very very time consuming.
Figure 2. Enlargement of the SiC seed crystals.
Monocrystalline SiC suitable for a demanding semiconductor industry is not easy to produce. The requirements concerning wafer size and quality are increasing, leading to newly emerging challenges in the production process. Like for diamonds the SiC wafer production is improved continuously and innovations are made to overcome those challenges. With the integrated production from wafer to module, ROHM has the advantage of having its own supply of high quality SiC. And even if the price of SiC will decrease in the future, its value will stay high.