Manufacturing Bits: Oct. 5

EUV lithography storage ring
At the recent SPIE Photomask Technology + EUV Lithography conference, Japan’s High Energy Accelerator Research Organization (KEK) presented a paper on its latest efforts to develop a free-electron laser (FEL) storage ring source power unit for extreme ultraviolet (EUV) lithography.

KEK has demonstrated a proof-of-concept EUV-FEL, which has been in R&D. EUV-FEL promises to provide the power of more than 1 kilowatts for multiple EUV scanners situated on a storage ring. This in turn could overcome unwanted variations, known as stochastics, in EUV lithography. It could also achieve higher throughputs.

EUV-FEL technology is still unproven and it’s unclear if they will ever appear. KEK is one of many entities developing new and alternative source power technologies for EUV.

Used for chip production in fabs today, EUV lithography makes use of a giant scanner that patterns tiny features in chips at advanced nodes. Based on a 13.5nm wavelength, the EUV scanner generates photons, which eventually interact with a light-sensitive photoresist material on a wafer. This process patterns precise features in chips at advanced nodes.

Chipmakers are in production using ASML’s EUV scanners. Incorporating a 0.33 numerical aperture lens with a 13.5nm wavelength, the system has 13nm resolutions. The scanner also integrates ASML’s 246-watt source power unit, enabling a throughput from 135 to 145 wafers per hour.

Using EUV, chipmakers have developed millions of leading-edge chips. But EUV is far from perfect. In EUV, photons hit the resist, causing a reaction. The process is repeated several times. At each event, there might be a new and different reaction, due to the unpredictable and random nature of these processes.

Thus, EUV is prone to what’s called stochastics, which describes events that have random variables. These variations, called stochastic effects, sometimes cause unwanted defects and pattern roughness in chips. Both can impact the performance of a chip or even cause a device to fail.

To mitigate these problems, chipmakers use resists with a higher dose. But higher doses impact or slow down the throughput of the scanner. Ultimately, chipmakers want to print fine patterns with EUV using a lower dose. This in turn requires higher source power units for EUV.

“The required EUV power is more than 1.5 kilowatt for the 3nm technology node and more than 2.8 kilowatt for the 2nm node,” said Norio Nakamura, a professor and a leader of the beam dynamics and magnet group in the Accelerator Division VI (Light Source Division) at KEK, in a presentation at the SPIE Photomask/EUV conference.

The industry is nowhere close to those figures. In R&D, ASML and others are developing conventional higher power sources beyond 246 watts.

In 2018, meanwhile, KEK began working on a different approach. It is developing a giant storage ring using an energy recovery linac (ERL) EUV-FEL technology. The storage ring is 200 meters long and 20 meter wide. It can support several EUV scanners with a power greater than 1 kilowatt. The line can be upgraded to support a light source with 6.7nm wavelengths.

KEK has demonstrated a proof-of-concept of EUV-FEL. As part of those efforts, it has developed a mid-infrared FEL within a compact ERL. “In this light source, the electron beam is accelerated up to about 800 MeV by the superconducting linac and then recirculated to generate the EUV-FEL light by undulators,” Nakamura said during the presentation. “The beam is returned back to the main linac again in deteriorating phase, decelerated down to 10.5 MeV for energy recovery, and finally dumped at the beam dump. By this energy recovery scheme, the electron bunch is repeated at more than 100 megahertz, and the high average current of 10 million or more can be achieved for the high FEL power of 10 kilowatt or more.”

Over time, KEK has made some improvements to the compact ERL beam quality and the FEL system. In the proof-of-concept, researchers installed undulators or periodic magnetic structures in the compact ERL. It also deployed a self-amplified spontaneous emission FEL (SASE FEL) without a resonant oscillator.

Construction of the compact ERL IR-FEL was completed in May of 2020. “For the high-power operation, the compact ERL dump line was reconstructed in autumn 2020. The energy acceptance is improved by more than 70%, with the purpose of suppressing the beam loss in the dump line. High power operation will be planned for 2022,” Nakamura said.

Rotating target EUV source
EUV Labs and the Institute of Spectroscopy from the Russian Federation provided more details about its source power technology for EUV lithography.

The technology, called TEUS, is a high-brightness EUV laser produced plasma (LPP) light source based on a fast rotating target. The TEUS-S100 and S400 models employ 100W and 400W of average laser power, respectively. At a separate event, EUV Labs and the Institute of Spectroscopy recently disclosed the details and some slides here about the technology.

“Like any other LPP light source, it uses a laser, which is a solid-state neodymium laser, and target delivery system,” said Mikhail Krivokorytov from EUV Labs in a presentation at SPIE Photomask/EUV. “We use tin as the plasma fuel and under appropriate focus and conditions of the laser, it provides relatively high conversion efficiency.”

What’s different about the technology? The target is based on a liquid metal, which located in a fast rotating crucible. “A fast rotating target has several advantages but the main ones are excellent inherent spatial stability of the source and the fact that fast rotation provides a redirection of droplets away from the optical elements, keeping them clean,” Krivokorytov said during the presentation.

The source power units provide 100 watts, 200 watts, and 400 watts of laser power. “The low power modification, which is TEUS-S100, provides 8.5 milliwatts of in-band EUV irradiation. And the most powerful modification, TEUS-M400, provides more than 100 milliwatts of in-band EUV irradiation,” Krivokorytov said. “The plasma size is 60 microns. And it is the same for all modifications. So EUV source brightness varies from 60 watts per millimeter square per steradian for a 100 watt drive laser up to 240 watts per millimeter squared per steradian for a 400 watt drive laser.”

For EUV source power units, collector mirror lifetime and maintenance are also key. Lifetime is more than 12 months for the TEUS-S100. “And it is linearly decreasing with pulse frequency. So for more powerful systems, the lifetime is smaller,” Krivokorytov said.

“And to increase the collector-mirror lifetime, TEUS can be equipped with an additional debris mitigation technique. It is debris filter in form of thin membrane made of CNT-based material,” he added.