WHY THIS MATTERS IN BRIEF
- Many regions of space are still off limits to us because of the extreme distances involved but new self-healing, nano sized spacecraft that can travel at a fifth of the speed of light will help us unlock more mysteries of the universe
Working with the Korea Institute of Science and Technology (KAIST), NASA is pioneering the development of tiny nano sized spacecraft that are made from a single silicon chip that could slash interstellar exploration times, and costs.
Last Wednesday at the International Electron Devices Meeting in San Francisco, NASA’s Dong-Il Moon presented the teams new technology aimed at ensuring the miniature spacecraft can survive the intense radiation they’ll encounter on their journey.
Using a silicon chip as a spacecraft it’s estimated that it could travel at one fifth of the speed of light and reach our nearest stars in just 20 years – that’s hundreds of times faster than today’s best spacecraft. But twenty years in the harsh environment of space is much more than any ordinary silicon chip can endure because, asides from the huge swings in temperatures and space dust and debris that could pummel them, they’d also be bombarded by ginormous doses of high energy radiation.
These doses of radiation would lead to the accumulation of positively charged defects in the chip’s silicon dioxide layer, where over time they’d degrade the devices performance.
One of the most serious consequences for the miniature spacecraft would be when an increase in the current, caused by a radiation spike, leaks through a transistor when it is supposed to be turned off. But there are also other issues, such as a shift in the voltage at which the transistor turns on – all seemingly small details but fatal to a chip.
Ordinarily the team would have two options, the first would be to select a path through space that minimizes radiation exposure and the second would be to add shielding. But the former leads to longer missions and constrains exploration, and the latter adds weight and eliminates the advantage of using a miniaturised spacecraft in the first place.
A far better approach, argues Moon, is to let the devices suffer damage but then to add 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 included the revelation in the 1990s – by a team at the National Microelectronics Research Centre in Ireland – that heating could repair radiation sensors, and far more recently, Macronix demonstrated that heat could be used to heal flash memory.”
As a consequence the team have turned to KAIST’s latest experimental self-healing technology – a “Gate-all-around” nanowire transistor. Gate-all-around nanowire transistors differ from regular transistors in that they use nanoscale wires, often made from graphene 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 and if you add an extra contact to the gate you can pass current through it and it’s that current that will heat the gate, and the channel it surrounds, and fix it. And there it is – a self-healing chip.
Nanowire transistors are ideal for space because they have a relatively high degree of immunity to cosmic rays 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, and 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 new technique has so far been used as the foundation for three key building blocks of the single chip spacecraft – the microprocessor, a DRAM memory module to support it and a flash memory module that will serve as the nanocrafts hard disk.
The new technique will let the team repair any radiation damage many times over with experiments showing that flash memory can be recovered up to around 10,000 times and DRAM can be returned to its pristine state 1012 times – the figure is even higher with logic devices and all of these results indicate that a long interstellar space mission could be feasible, especially if the chip is powered down every few years, heated internally to recover its performance, and then brought back to life.
While adding a second gate for heating isn’t ideal, because it modifies chip design and demands the creation of a new transistor library, which escalates production costs it could be the technology that makes the mission possible.
To address the extra costs though the team are looking into junctionless transistors that could heat the channels during normal operation when current flows through them. And separately, at NASA, researchers are developing on-chip embedded micro-heaters that are compatible with standard circuits.
Cutting the costs of self healing technology is crucial to the future of the program and it’s hoped that as the teams discover more about the potential of the technology more people will pile into the space, at which point the launch of the first silicon chip spacecraft, the first man made object to potentially enter into orbit around our next nearest star, won’t be too far away.