PORTLAND, Ore.—The surface of Venus is almost 500 degrees Celsius (932 degrees Fahrenheit)—hot enough to melt lead and turn aluminum into a slurry, prompting the National Aeronautics and Space Agency (NASA) to hire high/low temperature specialists, fabless Ozark Integrated Circuits Inc. (Fayetteville, Arkansas)—a spin-off from the University of Arkansas—to build a 500 degree C. process design kit (PDK) with $245,000 from the Small Business Innovation Research Program.
"We are grateful to the National Science Foundation [NSF] to previously fund Ozark ICs to create a 350 degree C. [662 degree Fahrenheit] PDK," Jim Holmes, a veteran from Texas Instruments, who is now chief technology officer at Ozark ICs, told EE Times. "Our success with the NSF PDK has prompted NASA to follow up with a two-phase project to build a 500 degree C. [932 degrees Fahrenheit] PDK for the chips to be used in the Venus Landsailing Rover."
As the most hostile environment in the solar system, NASA has high hopes to outperform the Russian and Japanese probes which have not faired well in their explorations of Venus, lasting at most a few hours before failing. In the first phase of the Ozark IC project, it will prove that it can boost its 350 degree PDK—fabricated at Raytheon Systems Limited (Essex, U.K.), where it is called HiT-SiC CMOS—to 500 degrees C. In particular, NASA wants a reliable ultraviolet imager for planetary composition experiments, in particular to study the environment on Venus. The second part of the phase one award is to build a microcontroller with real-time programmability for the Venus Landsailing Rover (see photo) which professor Alan Mantooth, a EE at University of Arkansas (Fayetteville, Arkansas) who co-founded and serves on its board, will lead the effort with the help of his gaggle of doctoral candidates.
Ozark Integrated Circuits builds silicon-carbide chips for high temperature environments from 350-to-500 degrees Celsius (662-to-932 degrees Fahrenheit).
(Source: Ozark ICs)
If successful, NASA will choose a vendor to use Ozark IC's PDK for the second phase which will produce the mixed-signal sensors, actuators, microcontrollers and other chips that will operate outside the "cool box" in which the Venus Landsailing Rover's main processors will be housed at normal room temperature.
NASA's proposed Venus Landsailing Rover will use winds to push vehicle, or alternatively raise a balloon to float over obstacles.
One key to surmounting the hurdles to 500-degree ICs is the substrate, for which Ozark ICs has chosen silicon carbide, using standard silicon processes atop the wafer to fabricate the transistors and other active devices. However for the interconnect aluminum is too unstable—since it melts at 632 degrees C.—and even copper is unsatisfactory at the high pressures on Venus (9 million Pascals compared to 101 thousand Pascals on Earth). Ozark ICs' Holmes claims to have a solution for the metallization layers of their Venus-compatible ICs, but the company is keeping it a trade secret for the time being.
When asked what the key ingredients were to successfully building the 500 degree C. chips, Mantooth, told EE Times:
There are several key ingredients. The first is a process that has a base semiconductor plus oxides and metallization that can withstand this temperature extreme. We are designing in silicon carbide with special oxides and metals. Next, is having good device models coupled with temperature compensated design methods. We have extended the FET [field-effect transistor] models over temperature using some of our patented temperature scaling methods so that we can predict device performance to very high temperatures. Even so, we must design references and biasing circuits that are temperature compensated using techniques that help to mitigate too much variation over temperature. Properties surely vary at these high temperatures, but remain in regimes where circuits still operate within expected boundaries
Ozark IC, which holds 80 patents on both cryogenic cold and ultra-hot ICs, is working with Mantooth's team at the University of Arkansas to prove-the-concept for the chips that will operate outside of the cool-box, which NASA hopes to maximize, since the size of the cool-box should be as small as possible to save on the power budget needed to keep it cool.
"There are several possibilities including high temperature data converters, memory, logic, and sensor signal processing circuits. So, this involves analog and digital circuitry," Mantooth told us. "This project is focused on a high temperature microcontroller for in-situ instruments and systems. Our initial work will ascertain the bit-width—8-to-32—of this microcontroller for 500 degree C. operations. Of course, part of this activity is memory design too."
Once the team fabricates the proof-of-concept yet chips, defines the PDKs and estimates the costs of building them, NASA will decide who fabricates the actual chips aboard the Venus Landsailing Rover.
"Ozark IC has the possibility of designing actual chips aboard the Venus Rover, but the University of Arkansas team is unlikely to do so," Mantooth told us. "We have been developing models, design kits, and design methods that will likely get transferred to typical NASA contractors such as Boeing and BAE Systems, who then utilize this information to design the actual hardware. It is possible that a company such as Ozark IC could be contracted by one of these 'big boys' to perform design of actual chips, but most likely not my University of Arkansas team. At best we might consult."
The University of Arkansas also has expertise in low- and high-temperature IC packaging at its High Density Electronics Research Center where the high-voltages needed for high-temperature electronics, and the interconnects both on-chip and between chips and boards have been already been designed, modeled and characterized for circuits up to 350 degree C. (662 degree Fahrenheit), which will now be extended to 500 degrees C.
— R. Colin Johnson, Advanced Technology Editor, EE Times