Assembly and Interconnection will be the Issue of the Future

Limited Availability of SiC Raw Wafers and Gallium Oxide

Let’s turn to another hot topic: the limited availability of SiC raw wafers. This could inhibit the proliferation of this technology. Do you agree? What can be done about it?

This is a big issue indeed. Basically, we are dealing here with a chicken-and-egg problem: The manufacturers of raw wafers would have to invest massively in expanding their manufacturing capacity today, without having the corresponding sales volumes. It’s also about wafer diameters. Currently, most of these have a diameter of 150 millimeters, but it’s necessary to switch to 200 millimeter wafers. This is a time-consuming process with silicon carbide – not only in order to reduce the defect densities, so that the chip yield is commercially justified. Silicon carbide is slowly grown from the vapor phase in an energy- and thus cost-intensive process. Silicon, on the other hand, is grown relatively quickly from a melt. This means that the wafer price for silicon carbide will never be the same as for silicon. So who is making the first step?

This is why semiconductor companies like Infineon and STMicroelectronics have signed long-term supply agreements with Cree or acquired SiC raw wafer manufacturers such as Rohm with SiCrystal or STMicroelectronics with Norstel.

This is a good way – the only one, that’s making sense to me. You have to send out a strong signal to the manufacturers of raw wafers about the sales volumes they can expect, so that they can ramp up their manufacturing capacities accordingly.

But there are so many issues still left unsolved with silicon carbide. No one dares to take the full risk of investing on a large scale. Currently, the market is still very small. In addition, we still have not enough basic data on lifetime reliability. We need completely novel assembly and interconnection technologies compared to what we currently use for silicon. So there are still many issues to be addressed in order to exploit the full potential of silicon carbide devices. In this context we should not forget that we have to understand and be able to control these small, extremely fast switching chips both in their package and in the system. This affects thermal management, chip contacting techniques to increase load cycle resistance, fast transitions in current and voltage in the switching cell, as well as extremely high switching frequencies at the consumer, especially when inductive loads or filters are involved.

Another wide bandgap material is currently causing excitement: At the end of 2018, Japanese researchers presented a complete ion implantation process for gallium oxide (Ga₂O₃). What is your assessment of the future prospects of this semiconductor material?

Obviously, this is an interesting approach. And the Japanese are working hard on this technology. But you have to keep in mind that the band gap at gallium oxide is 4.8 electron volts and the breakdown field strength is eight megavolts! For comparison: with silicon carbide, it is »only« 3.3 electron volts and 3 megavolts per centimeter, respectively. This means that it would be possible to build an 800-volt device in gallium oxide, whose drain and source terminals are just one micrometer apart. But this is an utopian idea, because the semiconductor has a passivation applied to it – and that passivation as well must be able to withstand these extremely high field strengths. How do I get the field plates over it? How can I control the electric field? These questions are still totally unanswered.

This means, even though it may soon be possible to manufacture such devices, the enormous potential of the material cannot be exploited at all. Even field strengths of 3.5 megavolts per centimeter cannot be absorbed by passivation today. For safety reasons, the structure sizes of GaN devices are currently some ten micrometers and larger.

But this is definitely an interesting way to go. I don't want to belittle that. I was in Kyoto back in January and took a look at it. But there is still a long way to go before the enormous potential of this material can be fully exploited.

Professor Lorenz, thank you very much for taking the time.

The interview was conducted by Ralf Higgelke.

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1. Assembly and Interconnection will be the Issue of the Future
2. Novel Power Module Design
3. Processing on 300-mm Wafers and Special Sessions at PCIM
4. Limited Availability of SiC Raw Wafers and Gallium Oxide