As Intel, Samsung, TSMC, and Japan’s upcoming advanced foundry Rapidus each make their separate preparations to cram more and more transistors into every square millimeter of silicon, one thing they all have in common is that the extreme ultraviolet (EUV) lithography technology underpinning their efforts is extremely complex, extremely expensive, and extremely costly to operate. A prime reason is that the source of this system’s 13.5-nanometer light is the precise and costly process of blasting flying droplets of molten tin with the most powerful commercial lasers on the planet.
But an unconventional alternative is in the works. A group of researchers at the High Energy Accelerator Research Organization, known as KEK, in Tsukuba, Japan, is betting EUV lithography might be cheaper, quicker, and more efficient if it harnesses the power of a particle accelerator.
Even before the first EUV machines had been installed in fabs, researchers saw possibilities for EUV lithography using a powerful light source called a free-electron laser (FEL), which is generated by a particle accelerator. However, not just any particle accelerator will do, say the scientists at KEK. They claim the best candidate for EUV lithography incorporates the particle-accelerator version of regenerative braking. Known as an energy recovery linear accelerator, it could enable a free electron laser to economically generate tens of kilowatts of EUV power. This is more than enough to drive not one but many next-generation lithography machines simultaneously, pushing down the cost of advanced chipmaking.
“The FEL beam’s extreme power, its narrow spectral width, and other features make it suitable as an application for future lithography,” Norio Nakamura, researcher in advanced light sources at KEK, told me on a visit to the facility.
Linacs Vs. Laser-Produced Plasma
Today’s EUV systems are made by a single manufacturer, ASML, headquartered in Veldhoven, Netherlands. When ASML introduced the first generation of these US $100-million-plus precision machines in 2016, the industry was desperate for them. Chipmakers had been getting by with workaround after workaround for the then most advanced system, lithography using 193-nm light. Moving to a much shorter, 13.5-nm wavelength was a revolution that would collapse the number of steps needed in chipmaking and allow Moore’s Law to continue well into the next decade.
The chief cause of the continual delays was a light source that was too dim. The technology that ultimately delivered a bright enough source of EUV light is called laser-produced plasma, or EUV-LPP. It employs a carbon dioxidelaser to blast molten droplets of tin into plasma thousands of times per second. The plasma emits a spectrum of photonic energy, and specialized optics then capture the necessary 13.5-nm wavelength from the spectrum and guide it through a sequence of mirrors. Subsequently, the EUV light is reflected off a patterned mask and then projected onto a silicon wafer.
The experimental compact energy recovery linac at KEK uses most of the energy from electrons on a return journey to speed up a new set of electrons.KEK
It all adds up to a highly complex process. And although it starts off with kilowatt-consuming lasers, the amount of EUV light that is reflected onto the wafer is just several watts. The dimmer the light, the longer it takes to reliably expose a pattern on the silicon. Without enough photons carrying the pattern, EUV would be uneconomically slow. And pushing too hard for speed can lead to costly errors.
When the machines were first introduced, the power level was enough to process about 100 wafers per hour. Since then, ASML has managed to steadily hike the output to about 200 wafers per hour for the present series of machines.
ASML’s current light sources are rated at 500 watts. But for the even finer patterning needed in the future, Nakamura says it could take 1 kilowatt or more. ASML says it has a road map to develop a 1,000-W light source. But it could be difficult to achieve, says Nakamura, who formerly led the beam dynamics and magnet group at KEK and came out of retirement to work on the EUV project.
Difficult but not necessarily impossible. Doubling the source power is “very challenging,” agrees Ahmed Hassanein who leads the Center for Materials Under Extreme Environment, at Purdue University, in Indiana. But he points out that ASML has achieved similarly difficult targets in the past using an integrated approach of improving and optimizing the light source and other components, and he isn’t ruling out a repeat.
In a free electron laser, accelerated electrons are subject to alternating magnetic fields, causing them to undulate and emit electromagnetic radiation. The radiation bunches up the electrons, leading to their amplifying only a specific wavelength, creating a laser beam.Chris Philpot
But brightness isn’t the only issue ASML faces with laser-produced plasma sources. “There are a number of challenging issues in upgrading to higher EUV power,” says Hassanein. He rattles off several, including “contamination, wavelength purity, and the performance of the mirror-collection system.”
High operating costs are another problem. These systems consume some 600 liters of hydrogen gas per minute, most of which goes into keeping tin and other contaminants from getting onto the optics and wafers. (Recycling, however, could reduce this figure.)
But ultimately, operating costs come down to electricity consumption. Stephen Benson, recently retired senior research scientist at the Thomas Jefferson National Accelerator Facility, in Virginia., estimates that the wall-plug efficiency of the whole EUV-LPP system might be less than 0.1 percent. Free electron lasers, like the one KEK is developing, could be as much as 10 to 100 times as efficient, he says.
The Energy Recovery Linear Accelerator
The system KEK is developing generates light by boosting electrons to relativistic speeds and then deviating their motion in a particular way.
The process starts, Nakamura explains, when an electron gun injects a beam of electrons into a meters-long cryogenically cooled tube. Inside this tube, superconductors deliver radio-frequency (RF) signals that drive the electrons along faster and faster. The electrons then make a 180-degree turn and enter a structure called an undulator, a series of oppositely oriented magnets. (The KEK system currently has two.) The undulators force the speeding electrons to follow a sinusoidal path, and this motion causes the electrons to emit light.
In an ordinary linear accelerator, injected electrons gain energy from an RF field in a superconducting accelerator. The electrons then enter a free electron laser and are immediately disposed of in a beam dump. In an energy recovery linear accelerator (ERL), the electrons circle back into the RF field and lend their energy to newly injected electrons before exiting to a beam dump.
What happens next is a phenomenon called self-amplified spontaneous emissions, or SASE. The light interacts with the electrons, slowing some and speeding up others, so they gather into “microbunches,” peaks in density that occur periodically along the undulator’s path. The now-structured electron beam amplifies only the light that’s in phase with the period of these microbunches, generating a coherent beam of laser light.
It’s at this point that KEK’s compact energy recovery linac (cERL), diverges from lasers driven by conventional linear accelerators. Ordinarily, the spent beam of electrons is disposed of by diverting the particles into what is called a beam dump. But in the cERL, the electrons first loop back into the RF accelerator. This beam is now in the opposite phase to newly injected electrons that are just starting their journey. The result is that the spent electrons transfer much of their energy to the new beam, boosting its energy. Once the original electrons have had some of their energy drained away like this, they are diverted into a beam dump.
“The acceleration energy in the linac is recovered, and the dumped beam power is drastically reduced compared to [that of] an ordinary linac,” Nakamura explains to me while scientists in another room operate the laser. Reusing the electrons’ energy means that for the same amount of electricity the system sends more current through the accelerator and can fire the laser more frequently, he says.
Other experts agree. The energy-recover linear accelerator’s improved efficiency can lower costs, “which is a major concern of using EUV laser-produced plasma,” says Hassanein.
The Energy Recovery Linac for EUV
The KEK compact energy-recovery linear accelerator was initially constructed between 2011 and 2013 with the aim of demonstrating its potential as a synchrotron radiation source for researchers working for the institution’s physics and materials-science divisions. But researchers were dissatisfied with the planned system, which had a lower performance target than could be achieved by some storage ring-based synchrotrons—huge circular accelerators that keep a beam of electrons moving with a constant kinetic energy. So, the KEK researchers went in search of a more appropriate application. After talking with Japanese tech companies, including Toshiba, which had a flash memory chip division at the time, the researchers conducted an initial study that confirmed that a kilowatt-class light source was possible with a compact energy-recovery linear accelerator. And so, the EUV free-electron-laser project was born. In 2019 and 2020, the researchers modified the existing experimental accelerator to start the journey to EUV light.
The system is housed in an all-concrete room to protect researchers from the intense electromagnetic radiation produced during operation. The room is some 60 meters long and 20 meters wide with much of the space taken up by a bewildering tangle of complex equipment, pipes, and cables that snakes along both sides of its length in the form of an elongated racetrack.
The accelerator is not yet able to generate EUV wavelengths. With an electron beam energy of 17 megaelectronvolts, the researchers have been able to generate SASE emissions in bursts of 20-micrometer infrared light. Early test results were published in the Japanese Journal of Applied Physics in April 2023. The next step, which is underway, is to generate much greater laser power in continuous-wave mode.
To be sure, 20 micrometers is a far cry from 13.5 nanometers. And there are already types of particle accelerators that produce synchrotron radiation of even shorter wavelengths than EUV. But lasers based on energy-recovery linear accelerators could generate significantly more EUV power due to their inherent efficiency, the KEK researchers claim. In synchrotron radiation sources, light intensity increases proportionally to the number of injected electrons. By comparison, in free-electron laser systems, light intensity increases roughly with the square of the number of injected electrons, resulting in much more brightness and power.
For an energy-recovery linear accelerator to reach the EUV range will require equipment upgrades beyond what KEK currently has room for. So, the researchers are now making the case for constructing a new prototype system that can produce the needed 800 MeV.
An electron gun injects charge into the compact energy recovery linear accelerator at KEK.KEK
In 2021, before severe inflation affected economies around the globe, the KEK team estimated the construction cost (excluding land) for a new system at 40 billion yen ($260 million) for a system that delivers 10 kW of EUV and supplies multiple lithography machines. Annual running costs were judged to be about 4 billion yen. So even taking recent inflation into account, “the estimated costs per exposure tool in our setup are still rather low compared to the estimated costs” for today’s laser-produced plasma source, says Nakamura.
There are plenty of technical challenges to work out before such a system can achieve the high levels of performance and stability of operations demanded by semiconductor manufacturers, admits Nakamura. The team will have to develop new editions of key components such as the superconducting cavity, the electron gun, and the undulator. Engineers will also have to develop good procedural techniques to ensure, for instance, that the electron beam does not degrade or falter during operations.
And to ensure their approach is cost effective enough to grab the attention of chipmakers, the researchers will need to create a system that can reliably transport more than 1 kW of EUV power simultaneously to multiple lithography machines. The researchers already have a conceptual design for an arrangement of special mirrors that would convey the EUV light to multiple exposure tools without significant loss of power or damage to the mirrors.
Other EUV Possibilities
It’s too early in the development of EUV free-electron lasers for rapidly expanding chipmakers to pay it much attention. But the KEK team is not alone in chasing the technology. A venture-backed startup xLight, in Palo Alto, Calif. is also among those chasing it. The company, which is packed with particle-accelerator veterans from the Stanford Linear Accelerator and elsewhere, recently inked an R&D deal with Fermi National Accelerator Laboratory, in Illinois, to develop superconducting cavities and cryomodule technology. Attempts to contact xLight went unanswered, but in January, the company took part in the 8th Workshop EUV-FEL in Tokyo, and former CEO Erik Hosler gave a presentation on the technology.
Significantly, ASML considered turning to particle accelerators a decade ago and again more recently when it compared the progress of free-electron laser technology to the laser-produced plasma road map. But company executives decided LLP presented fewer risks.
And, indeed, it is a risky road. Independent views on KEK’s project emphasize that reliability and funding will be the biggest challenges the researchers face going forward. “The R&D road map will involve numerous demanding stages in order to develop a reliable, mature system,” says Hassanein. “This will require serious investment and take considerable time.”
“The machine design must be extremely robust, with redundancy built in,” adds retired research scientist Benson. The design must also ensure that components are not damaged from radiation or laser light.” And this must be accomplished “without compromising performance, which must be good enough to ensure decent wall-plug efficiency.”
More importantly, Benson warns that without a forthcoming commitment to invest in the technology, “development of EUV-FELs might not come in time to help the semiconductor industry.”
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