Working with the Korea Institute of Science and Technology (KAIST), NASA is pioneering the development of tiny spacecraft made from a single silicon chip that could slash interstellar exploration times.
On Wednesday at the International Electron Devices Meeting in San Francisco, NASA’s Dong-Il Moon will present new technology aimed at ensuring such spacecraft survive the intense radiation they’ll encounter on their journey.
If a silicon chip were used as a spacecraft, calculations suggest that it could travel at one-fifth of the speed of light and reach the nearest stars in just 20 years. That’s one hundred times faster than a conventional spacecraft can offer.
Twenty years in space is still too long for an ordinary silicon chip, because in addition to the frailties it suffers on earth, such as swings in temperature, it is bombarded by radiation of very high energy. This radiation leads to the accumulation of positively charged defects in the chip’s silicon dioxide layer, where they degrade device performance. The most serious of the impairments is an increase in the current that leaks through a transistor when it is supposed to be turned off, according to Yang-Kyu Choi, leader of the team at KAIST, where the work was done. However, there are also other issues, such as a shift in the voltage at which the transistor turns on.
Two options for addressing chip damage are to select a path through space that minimizes radiation exposure and to add shielding. But the former leads to longer missions and constrains exploration, and the latter adds weight and nullifies the advantage of using a miniaturized craft. A far better approach, argues Moon, is to let the devices suffer damage but then to add a an extra contact to the transistors, and use this contact to heal the devices with heating.
“On-chip healing has been around for many, many years,” says Jin-Woo Han, a member of the NASA team. Milestones including the revelation in the 1990s— by a team at the National Microelectronics Research Centre in Cork, Ireland— that heating could drive the recovery of radiation sensors, and far more recently, heat-induced healing of flash memory by Macronix of Taiwan. The critical addition made now, Han says, is the most comprehensive analysis on radiation damage.
This study uses KAIST’s experimental “gate-all-around” nanowire transistor. Gate-all-around nanowire transistors use nanoscale wires as the transistor channel instead of today’s fin-shaped channels. The gate, the electrode that turns on or off the flow of charge through the channel, completely surrounds the nanowire. Adding an extra contact to the gate allows you to pass current through it. That current heats the gate and the channel it surrounds, fixing any radiation-induced defects.
Nanowire transistors are ideal for space, according to KAIST, because they have a relatively high degree of immunity to cosmic rays and because they are very small, with dimensions in the tens of nanometers. “The typical size for [transistor-dimensions on] chips devoted to spacecraft applications is about 500 nanometers,” says Choi. “If you can replace 500 nanometer feature sizes with 20 nanometers feature sizes, the chip size and weight can be reduced.” Costs fall too.
The gate-all-around device may not be that well known today, but production is expected to rocket in the early 2020s, when silicon foundries will use it in place of the today’s FinFET for producing circuits featuring transistors with gate lengths smaller than 5-nm.
KAIST’s has been used to form three key building blocks for a single-chip spacecraft: a microprocessor, a DRAM memory for supporting this, and a flash memory that can serve as a hard disk.
Repairs to radiation-induced damage can be made many times, with experiments showing that flash memory can be recovered up to around 10,000 times and DRAM returned to its pristine state 1012 times. With logic devices, an even higher figure is expected. These results indicate that a lengthy interstellar space mission could take place, with the chip powered down every few years, heated internally to recover its performance, and then brought back to life.
Adding a second gate for heating is not ideal, because it modifies chip design and demands the creation of a new transistor library, which escalates production costs. To address this, those at KAIST are investigating the capability of a junctionless transistor that heats the channel during normal operation when current flows through it. Separately, at NASA researchers are developing on-chip embedded microheaters that are compatible with standard circuits.
Cutting the costs of self-healing tech is critical to the future of the program. It will help to increase the appeal of the technology, which will require many more years of investment if the launch of the first silicon-chip spacecraft is to get off the ground.
KAIST was established by the Korean government with the help of American policymakers in 1971 as the nation’s first research oriented science and engineering institution. KAIST has approximately 10,200 full-time students and 1,140 faculty researchers and had a total budget of US$765 million in 2013, of which US$459 million was from research contracts. From 1980 to 2008, the institute was known as the Korea Advanced Institute of Science and Technology. In 2008, the name was shortened to KAIST.
KAIST ranked 60th globally and 12th in Asia in the QS World University Rankings (2013/14) while it was the 6th best university in the independent regional QS Asian University Rankings (2013), both of which considered it 2nd within South Korea. Moreover, it was placed 24th in Engineering and Technology by QS of the same year.
Korea Advanced Institute of Science and Technology research articles from Innovation Toronto
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A research team led by Professor Keon Jae Lee from the Korea Advanced Institute of Science and Technology (KAIST) and by Dr. Jae-Hyun Kim from the Korea Institute of Machinery and Materials (KIMM) has jointly developed a continuous roll-processing technology that transfers and packages flexible large-scale integrated circuits (LSI), the key element in constructing the computer’s brain such as CPU, on plastics to realize flexible electronics.
Professor Lee previously demonstrated the silicon-based flexible LSIs using 0.18 CMOS (complementary metal-oxide semiconductor) process in 2013 (ACS Nano, “In Vivo Silicon-based Flexible Radio Frequency Integrated Circuits Monolithically Encapsulated with Biocompatible Liquid Crystal Polymers”) and presented the work in an invited talk of 2015 International Electron Device Meeting (IEDM), the world’s premier semiconductor forum.
Highly productive roll-processing is considered a core technology for accelerating the commercialization of wearable computers using flexible LSI. However, realizing it has been a difficult challenge not only from the roll-based manufacturing perspective but also for creating roll-based packaging for the interconnection of flexible LSI with flexible displays, batteries, and other peripheral devices.
To overcome these challenges, the research team started fabricating NAND flash memories on a silicon wafer using conventional semiconductor processes, and then removed a sacrificial wafer leaving a top hundreds-nanometer-thick circuit layer. Next, they simultaneously transferred and interconnected the ultrathin device on a flexible substrate through the continuous roll-packaging technology using anisotropic conductive film (ACF). The final silicon-based flexible NAND memory successfully demonstrated stable memory operations and interconnections even under severe bending conditions. This roll-based flexible LSI technology can be potentially utilized to produce flexible application processors (AP), high-density memories, and high-speed communication devices for mass manufacture.
Professor Lee said, “Highly productive roll-process was successfully applied to flexible LSIs to continuously transfer and interconnect them onto plastics. For example, we have confirmed the reliable operation of our flexible NAND memory at the circuit level by programming and reading letters in ASCII codes. Out results may open up new opportunities to integrate silicon-based flexible LSIs on plastics with the ACF packing for roll-based manufacturing.”
Dr. Kim added, “We employed the roll-to-plate ACF packaging, which showed outstanding bonding capability for continuous roll-based transfer and excellent flexibility of interconnecting core and peripheral devices. This can be a key process to the new era of flexible computers combining the already developed flexible displays and batteries.”
A Korean research team has created semi-transparent perovskite solar cells that demonstrate high-power conversion efficiency and transmit visible light while blocking infrared light, making them great candidates for solar windows.
Modern architects prefer to build exteriors designed with glass mainly from artistic or cost perspectives. Scientists, however, go one step further and see opportunities from its potential ability to harness solar energy. Researchers have thus explored ways to make solar cells transparent or semi-transparent as a substitute material for glass, but this has proven to be a challenging task because solar cells need to absorb sunlight to generate electricity, and when they are transparent, it reduces their energy efficiency.
Typical solar cells today are made of crystalline silicon, but it is difficult to make them translucent. Semi-transparent solar cells under development use, for example, organic or dye-sensitized materials, but compared to crystalline silicon-based cells, their power-conversion efficiencies are relatively low. Perovskites are hybrid organic-inorganic halide-based photovoltaic materials, which are cheap to produce and easy to manufacture. They have recently received much attention as the efficiency of perovskite solar cells has rapidly increased to the level of silicon technologies in the past few years.
Using perovskites, a Korean research team led by Professor Seunghyup Yoo of the Electrical Engineering School at the Korea Advanced Institute of Science and Technology (KAIST) and Professor Nam-Gyu Park of the Chemical Engineering School at Sungkyunkwan University developed a semi-transparent solar cell that is highly efficient and, additionally, functions very effectively as a thermal-mirror.
The team has developed a top transparent electrode (TTE) that works well with perovskite solar cells. In most cases, a key to success in realizing semi-transparent solar cells is to find a TTE that is compatible with a given photoactive material system, which is also the case for perovskite solar cells. The proposed TTE is based on a multilayer stack consisting of a metal film sandwiched between a high refractive-index (high-index) layer and an interfacial buffer layer. This TTE, placed as a top-most layer, can be prepared without damaging ingredients used in perovskite solar cells. Unlike conventional transparent electrodes focusing only on transmitting visible light, the proposed TTE plays the dual role of passing through visible light while reflecting infrared rays. The semi-transparent solar cells made with the proposed TTEs exhibited average power conversion efficiency as high as 13.3% with 85.5% infrared rejection.
The team believes that if the semi-transparent perovskite solar cells are scaled up for practical applications, they can be used in solar windows for buildings and automobiles, which not only generate electrical energy but also enable the smart heat management for indoor environments, thereby utilizing solar energy more efficiently and effectively.
This result was published as a cover article in the July 20, 2016 issue of Advanced Energy Materials. The research paper is entitled “Empowering Semi-transparent Solar Cells with Thermal-mirror Functionality.” (DOI: 10.1002/aenm.201502466)
The team designed the transparent electrode (TE) stack in three layers: A thin-film of silver (Ag) is placed in between the bottom interfacial layer of molybdenum trioxide (MoO3) and the top high-index dielectric layer of zinc sulfide (ZnS). Such a tri-layer approach has been known as a means to increase the overall visible-light transmittance of metallic thin films via index matching technique, which is essentially the same technique used for anti-reflection coating of glasses except that the present case involves a metallic layer.
Traditionally, when a TE is based on a metal film, such as Ag, the film should be extremely thin, e.g., 7-12 nanometers (nm), to obtain transparency and, accordingly, to transmit visible light. However, the team took a different approach in this research. They made the Ag TE two or three times thicker (12-24 nm) than conventional metal films and, as a result, it reflected more infrared light. The high refractive index of the ZnS layer plays an essential role in keeping the visible light transmittance of the proposed TTE high even with the relatively thick Ag film when its thickness is carefully optimized for maximal destructive interference, leading to low reflectance (and thus high transmittance) within its visible light range.
The team confirmed the semi-transparent perovskite solar cell’s thermal-mirror function through an experiment in which a halogen lamp illuminated an object for five minutes through three mediums: a window of bare glass, automotive tinting film, and the proposed semi-transparent perovskite solar cell. An infrared (IR) camera took thermal images of the object as well as that of each window’s surface. The object’s temperature, when exposed through the glass window, rose to 36.8 Celsius degrees whereas both the tinting film and the cell allowed the object to remain below 27 Celsius degrees. The tinting film absorbs light to block solar energy, so the film’s surface became hot as it was continuously exposed to the lamp light, but the proposed semi-transparent solar cell stayed cool since it rejects solar heat energy by reflection, rather than by absorption. The total solar energy rejection (TSER) of the proposed cell was as high as 89.6%.
Professor Yoo of KAIST said, “The major contributions of this work are to find transparent electrode technology suitable for translucent perovskite cells and to provide a design approach to fully harness the potential it can further deliver as a heat mirror in addition to its main role as an electrode. The present work can be further fine-tuned to include colored solar cells and to incorporate flexible or rollable form factors, as they will allow for greater design freedom and thus offer more opportunities for them to be integrated into real-world objects and structures such as cars, buildings, and houses.”