Electronics provides the foundation for future developments in megatrends such as mobility, the energy supply, and communications. When it comes to putting electronics to efficient use, one key driver is power electronics. Depending on the specific application, this can efficiently switch back and forth between direct and alternating current, thus increasing running times or ranges, reducing charging times, and cutting costs. The latest generation of power electronics is used, for example, in wireless and fast charging for mobile end devices, and in powertrains for electric vehicles such as cars and trains, as well as in charging stations.
Two highly promising materials for the power electronics of the future are silicon carbide (SiC) and gallium nitride (GaN). Both materials convert electricity far more efficiently than does silicon (Si). Currently still the most widely used material, silicon loses more heat when converting direct current into alternative current, thus reducing the amount of usable energy. This means that components made of silicon carbide and gallium nitride also reduce the need for expensive and bulky cooling systems.
Silicon carbide alone enables the energy losses to be nearly halved while increasing the switching times by a factor of ten. Compared with silicon, it has ten times more dielectric strength and three times higher thermal conductivity. These properties can be used, for example to reduce the volume and weight of important electronics components for electric vehicles by up to 50%. This makes vehicle batteries correspondingly smaller and cheaper. The only drawback of these materials is the fact that chips made of silicon carbide and gallium nitride are still significantly more expensive than their silicon-based counterparts.
When silicon carbide and gallium nitride components are used in power control systems, the substantial benefits they offer compared with the currently still prevalent material of silicon mean that the cost differential can already be offset today in some cases. Experts estimate that the use of power electronics would make it possible to save up to 35 percent of current energy requirements. Based on European energy consumption of 4,000 TWh in 2020, this would equate to the capacity of 115 large power plants.
"Experts estimate that the use of power electronics would make it possible to save up to 35 percent of current energy requirements."
One major challenge presented by silicon carbide and gallium nitride, however, is that of manufacturing either material inexpensively. Compared with silicon, the crystals produced are around 100 times more likely to have defects. That makes it far more complicated to develop robust semiconductor layers. And that in turn means that the production of wafers – circular discs around one millimeter thick made of mono-crystalline or multi-crystalline blanks that serve as the substrate (baseplate) for electronics components – is more laborious and challenging.
Producing a wafer (substrate) requires semiconductor crystals with as few defects as possible. Mono-crystals are therefore produced in complex processes. Unlike natural crystals, which comprise numerous crystal grains grown together, each mono-crystal only consists of one single crystal.
Dr. Frank Wischmeyer, Vice President Marketing & Business Development Power Electronics at AIXTRON, explains: “For silicon, this process can be managed very well – more than 1,000 wafers can be cut from blocks of up to two meters in length. For silicon carbide, on the other hand, we can currently only produce mono-crystal blocks with edge lengths of up to 10 cm. That only results in around 70 wafers.”
Synthetic crystals are then produced on these substrates using epitaxy, the customary method used to grow semiconductor materials. Consisting of layers as thin as one atom of various chemical elements, these then form the increasingly important compound semiconductors – silicon and carbon (silicon carbide) or gallium and nitrogen (gallium nitride).
And this crucial point is where our innovative and unique production technology comes in. That is because AIXTRON systems facilitate secure and efficient processes to manufacture high-quality epitaxy wafers. Depending on the chip size and device structure, this can produce high chip yields of between 80% and 95%.
The secret of power electronics is to be found in the manufacturing process – metal organic chemical vapor deposition (MOCVD). As technology leaders, we have played a major role in the further development of this technology for more than 30 years now. It involves vaporizing the components of organometallic compounds, and then inserting these, together with other high purity gases, in extremely fine doses into the precisely at high temperature heated reaction chambers of our equipment.
To satisfy the high standards our industrial and research customers have in terms of precision and the reproducibility of deposition, AIXTRON has permanently enhanced the flow system of its reference equipment. This way, the input gases can be introduced into the center of the chamber in a targeted manner and then flow very evenly over the hot wafers on which epitaxial deposition occurs. The gaseous compounds then split, as a result of which only the desired atoms are deposited on the wafer surfaces. To achieve optimal uniformity in this deposition, the wafers are moved through the reactor, in this cases on rotating planetary orbits around the gas inlet.
Furthermore, the absolutely steady flow of the gas mixture into the processing chamber makes it possible to achieve the finest transitions between individual layers of the compound semiconductor and thus optimally control deposition rates for nanometer-sized semiconductor layers of the utmost quality. The result: layers with the precision of a few atoms suitable for use in highly efficient power electronics.
Minor errors, even affecting just one layer of the package, which itself often has several hundred layers, can therefore have a major impact on the performance of the component which they are then used to manufacture. Our engineers aim to optimize the system technology to such an extent that defects in the substrate are not continued in the layered stack. After all, as Dr. Frank Wischmeyer explains: “The performance capacity of the electronics components made from these layers depends on this first production step. That is because the quality of the semiconductor crystals determines the performance capacity of the subsequent chips”.
"We are working intensively with our partners from the worlds of science and industry."
To maintain this head start, we are working intensively with our partners from the worlds of science and industry. When it comes to enhancing and optimizing production processes for larger, and thus more viable, silicon carbide wafers, AIXTRON is working closely together with the Fraunhofer Institute for Integrated Systems and Device Technology (IISB) in Erlangen. Together with 29 partners in the European research project UltimateGaN, AIXTRON is also working to develop the next generation of energy-saving chips based on gallium nitride. The aim here is to further miniaturize the chips while ensuring the utmost quality and competitive costing.
"In the past, SiC and GaN technology was developed in customer-specific projects. To meet customers’ ever higher expectations in these process technologies, we are now setting out on new paths”, adds Dr. Frank Wischmeyer to summarize AIXTRON’s approach.
These cooperation projects mean that power component production equipment is being permanently enhanced, always with the aim of making the compound semiconductors ever more competitive compared with established silicon-based materials. In recent years, AIXTRON developed equipment types specifically for use in the production of power electronics components made of gallium nitride and silicon carbide. To achieve the necessary increase in production temperature, the silicon carbide equipment works with the “warm wall” concept. In the AIX G5 WW C system, for example, all walls facing the wafer are made of graphite. Due to heat radiated from the actively heated susceptor (process temperature: 1,600°C), the walls are kept at maximum temperature so as to guarantee an optimal process workflow.
The next generation of AIX G5 WW C is currently already in trial operation. “For the first time ever in power electronics manufacturing, this system will enable data such as wafer surface temperature to be recorded during the process itself, thus facilitating enhanced process control. Digitalization is opening up new opportunities to develop our systems technology. Moreover, we introduce true automation capabilities in order to further close the gap towards silicon type fabrication processes”, comments Dr. Frank Wischmeyer. This way, we are setting a further milestone and ensuring that we are always one step ahead of industry expectations.
"Power electronics is helping to make core components smaller, lighter and less expensive."
Efficient gallium nitride-based power electronics already helps to prevent servers at data centers from overheating, for example, and thus reduces the need for complex cooling systems and their running costs. State-of-the-art power electronics is demonstrating its benefits in everyday life as well. It increases the range of electric vehicles by around 20% and significantly reduces their charging times. In electric cars such as Tesla’s Model 3, most of the power electronics used in the powertrain is already manufactured on a silicon carbide basis. Volkswagen plans to bring around 70 new electric vehicle models to market in the next ten years – with power modules based on silicon carbide. At the same time, power electronics is helping to make core components smaller, lighter and, in the case of batteries – the centerpiece of e-mobility – less expensive.
Having said this, numerous smaller devices are also benefiting from these technical advances and becoming lightweights capable of mobile use. Smartphones can already be charged on a wireless basis, while laptop chargers will in future be no larger than a bank card.