The Aeronautics Research Mission Directorate at NASA's Glenn Research Center has developed a new silicon carbide differential amplifier integrated circuit chip that may provide benefits to anything requiring long-lasting electronic circuits in very hot environments. The chip exceeded 1,700 hours of continuous operation at 500ºC - a breakthrough that represents a 100-fold increase in what has previously been achieved. Phil Neudeck is the team lead for this work.
NASA Tech Briefs: Can you tell us about the project and the technology that has resulted?
Phil Neudeck: Historically, it's been recognized for a number of years that there's a need for high-temperature electronics in both turbine engines and aerospace applications, as well as automotive applications. So there actually is a high-temperature electronics field that has evolved over the years. There are regularly scheduled professional high-temperature electronics conferences, and things of that nature.
Electronics are already incorporated in automobile engines and jet engines at this point in time, but they're not nearly as high-temperature as what we're developing here. So it's sort of an evolutionary process, if you will, with high-temperature electronics. The more capable the electronics are, the more they get inserted into new places to bring benefits to those systems. It's just that nobody has been able to make semiconductor electronics work at these temperatures [500° C] for much longer than an hour or two until now, so we've expanded the envelope of what's possible here. Parts have to last a long time in order to be useful for most applications. That's what we've done with our accomplishments at 500° C.
Our group has been working with high-temperature electronics for a number of years. Our approach is to use silicon carbide as a semiconductor instead of silicon. Traditionally, silicon is the semiconductor material that all of your computers, cell phones, and every other electronic device are made of. But silicon has its temperature limits; there are physics involved in why silicon can't operate at extremely high temperatures.
Silicon carbide electronics have other applications as well in the area of high-power devices operating at room temperature, RF power devices - there are important applications for silicon carbide other than just high-temperature electronics. Our research group has worked in those areas, too.
NTB: How does heat typically cause an IC to deteriorate and fail?
Neudeck: There are two main failure mechanisms. The first is what I would call semiconductor physics. In the case of silicon, heat will actually turn a semiconductor into a conductor. That's just physics. The temperature at which that happens depends on a fundamental material property called the "bandgap" of the semiconductor. Silicon has a bandgap of about 1 volt and silicon carbide has a bandgap of about 3 volts. The difference in that bandgap actually translates into a big difference in terms of the temperature at which a semiconductor turns into a conductor. Once a semiconductor becomes a conductor, you really can't do the things you need to do with it in terms of making transistors and electronic circuitry. So that's a physics mechanism.
There's also a second mechanism with respect to chemical degradation. Actually, the mechanism we've been fighting with silicon carbide is that silicon carbide has always been a very good semiconductor, even as high as 600°C glowing red-hot. We've always known that you can build a transistor in silicon carbide and it will work fine, but the issue is the chemical degradation of the interfaces and the junctions that you need to actually make a transistor. In the case of silicon, you don't really have that much of a chance, because silicon is much more reactive than silicon carbide. The chemical reactions and diffusions take place and the silicon device will degrade very rapidly.
So, there are two things that prevent silicon from operating at high temperatures, whereas with silicon carbide it's still a semiconductor at high temperatures and we've figured out how to control the interfaces to where they can last a good long time at high temperatures. That's sort of the fundamental discovery, if you will, in silicon carbide – how to control those chemical reactions.
NTB: So that is really the key? Are there any other differences between a conventional IC and one of these high-temperature silicon carbide ICs?
Neudeck: Well, the silicon carbide tends to be more expensive, so you really would only use silicon carbide where you have to use silicon carbide, such as in an environment like this. Silicon is very well commercialized; the chips in these computers are becoming pretty low-price, high-volume commodities. Silicon carbide is not at that stage where it's cheap or much of a commodity. It's definitely much tougher to work with than silicon; that's definitely a difference.
In the end, we're not as far along in the technology maturity process as silicon. For instance, in these integrated circuits that we're talking about for our 500° C demonstration, it's only a couple of transistors and three resistors interconnected together. I've got some other circuits in the lab that haven't run as long as the one we're talking about in the press release. They're at about the four- or five-transistor level. Obviously, the number of transistors in a silicon chip in your computer is on the order of about a million transistors.
The silicon people have had decades to refine their technology, make the individual transistors smaller and faster. That process has gone on in silicon for decades. With silicon carbide, certainly we can follow the path that silicon took in terms of shrinking things and making more complex circuitry. I think we'll be able to make more complex circuitry in silicon carbide more rapidly because we've learned from how silicon did it. But we still have that path to go down.
NTB: What other types of electronic components could this technology be applied to in the future?
Neudeck: That's a really good question. We mentioned in the press release the three obvious applications that have existed for a number of years (aerospace and automotive engine sensors, oil and natural gas exploration, robotic exploration of Venus). The circuits that those people need are certainly going to be the first circuits that get done. In the nearer term, for us here at NASA, we're interested in signal processing and signal conditioning electronics in a sensor for hot environments. One of the things we plan to do is make an operational amplifier and try to integrate it with a pressure sensor to get a good, high-resolution pressure measurement out of an engine ground test that people couldn't do before. That's one of our shorter-term goals.
We're also developing some digital logic circuits. We're looking to try and make some state machines maybe, for doing some harsh environment intelligence or harsh environment control electronics for something that could be used in a Venus probe mission or something like that. Those are all mission-driven. My approach to semiconductors is, you can buy semiconductor chips for your cell phone and you can buy semiconductor chips for your computer, but all the semiconductor chips you can buy are application-driven. That's really the direction we're headed. Now that we've shown we can make an integrated circuit with silicon carbide that lasts a good long time at 500° C, for the people who need specific functions we can now design and build that kind of a circuit nominally. I say nominally because it wouldn't be us; we would do a tech transfer and a company would build it.
The technology translates to a lot of other applications. We, for the first time, have shown that we can make a 500° C integrated circuit chip and have it last an unusually long time. Now people will say, "Hey, I can use this circuit in my 500° C application." So what we've done is we've shown the building blocks of how to do it and the applications are the ones that we know of. I think there are applications we don't know of, and that's where it goes after this, in my opinion.
NTB: In the future, when the price comes down and the technology matures, could you see this technology being used to eliminate things like heat sinks and cooling devices in everyday things like personal computers, or would this be too exotic to apply to those types of devices?
Neudeck: Well, that's a long way away! My train of thought is not necessarily on a personal computer but just to get the cost low enough and mass fabrication high enough to get into the automotive market. That's a very cost-sensitive market. There are places on an automobile where I think this technology will come in a lot sooner than I think it will come into a PC. Again, silicon is awfully cheap and it just seems like it's a long way before silicon carbide gets cost competitive with it in a room-temperature ambient application.
NTB: Is this technology compatible with radiation hardening?
Neudeck: It absolutely is an inherently radiation-hard technology. That is another potential application. To put it in perspective, it's my opinion that our advancement hasn't really helped radiation hardness that much because as long as you weren't operating in a really hot thermal environment, people could've made silicon carbide chips five years ago that would've been extremely radiation-hard. So we've pushed the thermal end of things, in my opinion.
NTB: Just to give our readers some perspective, what's the projected cost differential between the silicon carbide components and your average silicon components?
Neudeck: I would say right now it's a factor of about a hundred probably, just for the starting semiconductor wafer material. For finished parts, like room-temperature power diodes, the cost difference is much less, but still significant.
You have to need that special capability in order to justify the extra cost of silicon carbide over silicon. I have a review article on our Web site about the role of silicon carbide in high-temperature electronics and one of the statements I make in this paper is that a hundred dollars worth of high-temperature electronic component parts enables a system capability that's worth millions of dollars to some people. So this is a very leveraged technology. In other words, the person that's manufacturing the silicon carbide part may only make a few hundred dollars on it, but to the industry person, the new capability that it brings can be worth an awful lot over the lifetime of their product.
So even though the silicon carbide may be more expensive by a factor of a hundred, it's a pretty good investment when you consider what the chip is doing in a big million-dollar or larger system.
NTB: Your test sample ran continuously for more than 1,700 hours at a temperature of 500°C. Do you think that's the limit, or do you think you can improve on that with some design modifications based on what you've learned to date?
Neudeck: Well, I definitely think we have some improvements to make in terms of the density of the circuitry and whatnot. In terms of the thermal performance, this thing is still cranking today. We're at hour 1,800 and it's still going. This is actually part of the test where we're trying to learn what the limits are for this part. So far the device has been operating continuously. It's basically in the lab, in an oven, and the computer keeps collecting data, and everything is performing well. I don't know how long it's going to go, and I don't know where the limits are on this device.
We're kind of in a hurry to try and get some more parts packaged and hooked up and tested so that we can find those limits. If we can get another set of parts packaged, we'll probably throw one in the oven, crank it up to a higher temperature, and see if we can accelerate failure a little bit. In the meantime, with this one at 500°C, we're just going to let it run and see how long it lasts.
NTB: Will the effects of thermal cycling be different for the silicon carbide chips than they would be for normal silicon chips?
Neudeck: Probably not from a purely electrical perspective. Thermal cycling tends to attack the packaging of the chip, so you tend to lose the die attach, or you tend to lose the wire bonds. So it's not the electronics part of the chip; it's really more the mechanical packaging of the chip that I think the thermal cycling will attack. But being a scientist, I keep an open mind. We are going to run thermal cycling on these things and see how they last.
Of course, we're talking about putting this in aerospace environments where there's also vibration. These are issues our high-temperature packaging experts need to tackle. They don't worry about this kind of a total cycle from 500° C all the way down to room temperature with silicon because they can't operate over that broad a temperature range. So we've looked ahead and tried to anticipate some things. You can simulate things and learn a lot, but I am a firm believer in running it in the laboratory to see if it actually works.
NTB: Has anyone from the commercial sector expressed an interest in licensing this technology yet?
Neudeck: I would say yes and no. I had actually put in a couple of proposals with industry a few years ago and they were interested in the technology before this accomplishment. Certainly it will be interesting to see who gets on line as they hear more about this accomplishment. So there has been some general interest, but nobody's come forward and put out a license agreement or anything just yet.
NTB: Any ideas about potential applications for this in the commercial sector?
Neudeck: Well, I think the obvious ones have all been mentioned. One of the interesting things with new technology is there's always somebody out there in the commercial sector who sees a news release and they think of a new way to use it that I never thought of. So it's going to be really interesting to see what does come out.
I believe if you push the envelope and you show a new capability, people will find new ways to use it. But my feeling is certainly that the application where it's first going to be used is one that's less cost sensitive, which would be aerospace. Then it's going to work its way down the cost sensitivity chain to larger and larger uses as the technology becomes more commercialized, more understood, and more trusted.
The big issue with the commercial applications is you have to show very good reliability. Nobody likes it when their cell phone stops working. We've shown now that we can run a circuit for an awfully long time at 500°C, but we've only shown a handful of them. It's obviously going to take more work by us and by the commercial people that we transfer this technology to [for them to] be comfortable enough to put their name on a product.