Recent years have seen the full-fledged popularization of SiC power devices—capable of dramatically improving power efficiency in electric vehicles, photovoltaic power generation, industrial equipment power supplies, and other applications. Tsunenobu Kimoto, a professor in the Department of Electronic Science and Engineering at Kyoto University’s Graduate School of Engineering and Faculty of Engineering—a worldwide authority on SiC research—sat down with Kazuhide Ino, a general manager at ROHM Semiconductor’s Power Device Production Headquarters—which leads the world in SiC power device technology development and business—to discuss how to approach the expectations of greater prominence of SiC power devices, and what the future of SiC power devices holds in store.
It’s now been eight years since ROHM became the world’s first mass-producer of SiC MOSFET devices in 2010. In the past, most customer inquiries focused on the value of the technology, as exemplified by the question of, “How much performance can be gained by using SiC?” More recently, however, the inquiries have focused on utilization, such as “How much does it cost,” or “What is ROHM’s production capacity?” It seems that the excellent characteristics of SiC power devices are finally being recognized by engineers looking to develop applications in the power electronics field.
I think you are absolutely right. Compared with the SiC research taking place in the 1990s, the world seems a completely different place. Trains using SiC power devices for inverters have been put to use not only in subways but also along Tokyo’s Yamanote Line, Osaka’s Loop Line, and more. I never expected that we would be able to take advantage of the dramatic 30% power reduction capabilities of SiC devices so quickly. We’ve also seen applications that I had never even thought of coming about, such as using SiC devices for stabilizing power supplies in order to create high-quality sound amplifiers. These are just a few examples of the surprising ways SiC devices have been put to use.
The Time for Mass Utilization of SiC Power Devices is Now
SiC devices have a high potential for use, and the continued development of the technology has resulted in a much wider application scope than we first imagined. All signs now indicate that full-scale adoption is just around the corner, so our expectations and sense of purpose have been on the rise.
For example, we’ve seen SiC power devices being adopted more regularly in solar power conditioners, EV (electric vehicle) charging stations, and server power supplies. Such devices have also become more common in EV and plug-in hybrid vehicle onboard chargers and driving motor inverters. It wouldn’t be surprising to see SiC power devices also becoming more common in various industrial equipment. Recent years have also seen increased interest in electric airplanes and flying taxis, and the characteristics of SiC devices are ideally suited for the weight reduction and high-efficiency output required for bringing such ideas to life.
Personally, I’m really looking forward to the full-scale application of SiC power devices in automobiles. Countries in Europe are already leading the movement away from traditional cars and toward electric vehicles. Elsewhere, the EV market is also expected to grow rapidly in China, the world’s largest automotive market. SiC power devices will help push this movement along thanks to the possibility of dramatic reductions in power loss and of smaller and lighter chargers and inverters. I expect the demand for SiC power devices to increase quite rapidly.
We have been steadily preparing for applications in automobiles. Before mass production began in 2010, we offered samples to automotive industry companies for feedback on points that could be improved. We used that feedback to continue polishing our products. With electric vehicles, using SiC power devices can improve the efficiency of the onboard power system, resulting in longer ranges with the same battery capacity. On the other hand, using SiC devices makes it possible to reduce battery costs without sacrificing current range capabilities. In this way, because reducing costs is always a top priority for the automotive market, using SiC power devices will help increase the value of electric vehicles.
Bringing the Full Potential of SiC Devices to Light
Even though expectations for SiC power devices are increasing, we have to think about what preparations are necessary to maximize potential. Being able to switch out existing silicon devices for SiC devices with no hindrance whatsoever is of course ideal for users. In reality, however, all of the peripheral components and design environments for circuits are based on silicon devices, and circuit designers continue to learn about and hone their skills based on the premise of using silicon devices.
Even though SiC power devices are capable of operating at higher speeds with less loss than silicon devices, a certain level of ingenuity is still necessary to use the devices to their fullest potential. Circuits must be developed that not only utilize the high switching speeds but also reduce the parasitic inductance of module boards and other devices. Also, to make full use of the ability for high-temperature operation, the surrounding boards and packages must also use devices that can withstand such high temperatures.
Right. When we first began offering SiC power devices, our focus was on replacing existing silicon devices. Even though we were able to improve circuit performance in almost all applications, the performance improvements was less than we expected in some situations. This was because, as you pointed out, the technology was not yet capable of utilizing the full potential of the SiC power devices.
But now, after eight years since mass production began, effective use of the technology on the customer side has gradually increased. To further encourage such effective use of SiC power devices, we have been offering evaluation boards and simulation models for customers to use. Today, customers are increasingly pursuing more efficient, more compact, and more lightweight electric power systems, all based on a circuit design that uses SiC devices to their full potential.
SiC power devices
ROHM—Growing with Customers toward Effective Use of SiC Devices
As more users actually try out and realize the effectiveness of SiC devices, so have more users taken an increased interest in distinguishing the difference in devices offered by different companies. Going forward, the performance capabilities of SiC power devices themselves will begin to increase. At the same time, the technology being used is also ever-evolving, which opens the door to an upward spiral with increased utilization of the positive effects.
In order to bring about increased use of new devices, developing an easy-to-use support environment is absolutely essential. Such environments should be the responsibility of manufacturers looking to bring advanced devices to the market. Even though ROHM has put a variety of measures in place, I’d say there is still room for improvement. We remain dedicated to providing technologies that further enhance SiC device performance while also focusing on improving the usage environment.
With device technology evolving at one end and application technology evolving at the other, the question of just how close device manufacturers work with customers—and how constructive that relationship is—becomes important. Historically, ROHM is quite adept at meeting custom requests. This strength will give the company a head start on the path toward further evolution of devices and application environments.
SiC Devices: A Key Ingredient to Vertically Integrated Production Systems
As the application market continues to grow, we are seeing an increased need for establishing mass-production technology capable of increased productivity. To that end, I believe the most important tasks are improving the quality of crystals in SiC wafers and expanding production systems.
When it comes to expanding production systems, it seems that wafer manufacturers are only willing to provide conservative capital investments for the time being, resulting in wafer shortages and keeping prices from being lowered. The idea that, because devices are being sold at high prices now, we need only simply increase production with equipment that matches current wafer production methods, will fail to meet sudden future increases in demand.
To make the cost of SiC wafers comparable to silicon wafers, we must be able to make larger diameter ingots efficiently and quickly, which will require a technological breakthrough to form the high-quality thin films in a short time. And to promote the necessary investment decisions and technological developments, device and wafer makers must work together to develop the market under a shared goal.
At ROHM, we’ve adopted an in-house vertically integrated production system for pulling not only SiC but also silicon wafers and ingots. This ensures us a stable supply of wafers. It also allows us to analyze what problems the defects in the wafer crystals cause at the device creation stage, which we can then trace back to the wafer production stage in order to take appropriate countermeasures. Meanwhile, because some wafer crystal defects do not adversely affect device operation, we can avoid wasting time developing technology to correct such benign defects. I would say that, as of right now, this is one of ROHM’s greatest strengths, and not something other companies are doing.
Sometimes defects that appear in the wafer stage lead to defects in device operation, but other times the problem is defects that arise during thin film formation or even during device formation. Analysis of device defects tells us that many cases require countermeasures against defects that arise during wafer production. This is a crucial difference with silicon devices, as it makes procurement of very high-quality wafers possible. Procuring SiC wafers from external sources makes it difficult to establish countermeasures between companies. Controlling all aspects inhouse will make ROHM better equipped to establish effective countermeasures through appropriate processes.
Also, not only have we taken steps to prepare for rapid increases in demand, but we continue to promote advanced capital investment. With a goal of investing 60 billion yen in the SiC devices field by 2025, we are aiming for a rapid expansion in production capacity to 16 times 2017 levels. This may seem like a drastic increase in production, but we believe that full-scale adoption of SiC devices in cars will result in a situation where full-scale operations are required.
In terms of future increases in demand, we can only respond appropriately if we promote both facility improvements and production of larger-diameter wafers. In 2020, ROHM will construct a new SiC plant in Chikugo, Fukuoka, where our main SiC device plant is located. This plant will be ROHM’s first in Japan in 12 years. ROHM began producing 6-inch wafers in 2017, and we plan on increasing the size of our wafers even more at the new plant. By installing devices capable of handling 8-inch wafers now, we will be ready to switch to mass production of 8-inch wafers right away.
New building to be constructed at ROHM Apollo Co., Ltd.’s Chikugo Plant (Fukuoka Prefecture), designed to strengthen SiC power device production capacity
Room yet to Grow in SiC Device Performance
I still believe there is room for growth when it comes to SiC power devices, particularly MOSFET devices. In terms of diodes, we have come close to achieving performance at the theoretical limit. However, the performance of current MOSFET devices is only at the third or fourth stage. This is due to many defects in the interface between the MOS structure semiconductor and the oxide film, and these defects mean we have no choice but to devise the device structure around the defects. The ability to solve such interface defect problems will provide more freedom in designing devices, which will lead to improved performance and reliability.
And I believe it is the responsibility of university research to develop the guidelines for solving such problems. For the past several decades, research has been focused on finding ways to reduce defects by varying the production conditions. Going forward, I feel that we have come to the point where we need to look at the interface at the atomic level through science and computational science, and then develop countermeasures from there.
In terms of the evolution of SiC power devices, things are just getting started. From here on we will begin focusing on continually improving wafer quality and refining device structures for performance improvements. This year, ROHM will introduce its fourth-generation SiC MOSFET. These fourth-generation devices are being developed with a lower on-resistance compared with previous generations, resulting in twice the performance. ROHM expects to introduce new generation devices every three or four years. With the next generation, we are hoping to lower the on-resistance even more by improving the overall device structure. After that, however, we are anticipating the need to revamp our approach by reducing interface defects in order to increase mobility. To do so, the results of university-level research will become invaluable, so I am very much looking forward to the progress made on that front.
Opening the Door to Applications Not Possible with Silicon
SiC boasts many excellent physical properties not found in silicon. By utilizing these unique features, I believe we can develop applications that would not be possible with silicon semiconductors.
Along with that pursuit of even more possibilities with SiC devices, we are also interested in developing power devices capable of ultra-high voltages in excess of 10 kV, which have not yet been put to practical use with silicon.
We don’t yet have a clear picture of what the market holds for unique applications in the range of 6 kV and 10 kV. But the possibility of using only one SiC device in an application that not even numerous low withstand voltage devices connected in series could handle, is certainly useful in terms of developing power electronics.
It is possible that high-power electric power facilities controlled through large, heavy transformers, mechanical switches, and other devices will become semiconductor-based very quickly. This could, for instance, enable more precise network-based control of power grids, and open the door to the possibility of transforming social infrastructures.
I think that putting the unique physical properties of SiC to use can open the door to applications beyond power devices. We could very well see surprising new applications coming to light in the future.
And university-based research will become essential for bringing such undeveloped applications to light.
I would like to see close collaboration between companies and universities, with ideas developed at the university level being refined by the company for quick utilization.
It is a very interesting and exciting time for SiC researchers. The dedication at the academic level will help to solve many problems with practical devices, which in turn will open the door to new applications, so the societal contribution of such efforts will be clearly visible. Ideas developed from the research may very well become highly influential patents. I truly hope that today’s younger researchers—armed with fresh perspectives and ideas—find interest in this research.
On that note, I look forward to expanding into new SiC power device applications. Anyone with an idea for using SiC devices to solve previously unsolvable problems, we look forward to hearing from you. At ROHM, we make it our goal to make devices the meet the expectations of the individuals looking to make such ideas reality.
Helping to Save Energy in High-Voltage Systems in High-Temperature/High-Humidity Environments
The high energy savings of SiC devices has resulted in expanding utilization in automobiles and industrial equipment. ROHM’s BSM250D17P2E004 full SiC power module—rated at 1700 V and 250 A—was developed for use in inverters and converters for outdoor power generation systems and other industrial equipment power supplies. The energy savings made possible through the application of 1000 V in direct current under high-temperature/high-humidity environments ensure an increased level of reliability.
Up to now, adoption of SiC devices has mainly been for products with a withstand voltage of 1200 V. However, higher-voltage systems have continued to appear as application equipment continues to become more sophisticated, resulting in an increased demand for 1700 V withstand voltages. Nevertheless, commercialization of SiC devices with a withstand voltage of 1700 V is not easy in terms of reliability, leaving silicon IGBT as the only option.
ROHM’s newly developed module incorporates a new coating material and a new construction method to prevent dielectric breakdown and prevent increased leakage currents. The result is greater reliability that does not lead to dielectric breakdown even when exceeding 1000 hours of high-temperature/high-humidity bias testing (HV-H3TRB). Adopting this new module with the BSM250D17P2E004 makes it possible to safely use SiC devices with withstand voltages of 1700 V even under high-temperature/high-humidity environments. Even after applying 1360 V for over 1000 hours in a high-temperature/high-humidity environment (85°C/85%), operation has been confirmed as normal with no malfunctions detected.
Installation of a ROHM SiC MOSFET and SiC Schottky barrier diode (SBD) in the module and optimization of the internal module structure and device placement provides a 10% improvement in resistance performance over comparable SiC products. This also leads to greater energy savings in applications.
ROHM is dedicated to expanding solutions that take advantage of the high energy savings of SiC devices in even more applications in the future.