Elon Musk's potential FEL route could challenge EUV lithography

A blogger has posted that, according to TeraFab's announcement, Elon Musk appears to be pursuing the FEL (Free Electron Laser) route, aiming to disrupt the traditional EUV monopoly.

Subsequently, Elon Musk's reply seemed to confirm this claim.

Of course, this is merely speculation and not factual. However, we believe it's worth discussing.

We must acknowledge that free-electron lasers (FELs) and particle accelerators are not new. For years, companies, research institutions, and universities have owned and operated particle accelerators to produce protons, neutrons, and Quark Extremely small subatomic particles. These systems are typically used in physics and other scientific applications. Free-electron lasers (FELs) have long existed; they are essentially high-power light sources that generate light of different wavelengths using electrons. Particle accelerators are systems that propel charged particles.

From a fundamental perspective, a free-electron laser (FEL) generates laser light by passing electrons traveling near the speed of light through a periodically varying magnetic field. The wavelength of this laser is related to the frequency of the periodic magnetic field variation; by adjusting either the frequency of the magnetic field or the electron beam’s velocity, lasers of different wavelengths can be produced. Therefore, to build an FEL, a high-speed (near-light-speed) electron source is essential. Why near-light-speed electrons? Because only under such conditions does the transverse oscillation of the electrons generate radiation that significantly affects their motion, causing neighboring electrons to cluster into bunches rather than remaining as a uniform electron beam.

Assuming each cluster contains N electrons, the combined energy of the emitted radiation is not simply the sum of the individual electron energies, but rather proportional to N squared—indicating coherent emission, in other words, laser light. The most commonly used source of high-speed electrons today is a linear accelerator (although synchrotron radiation is also used as a free-electron source, its power is insufficient).

In other words, in an FEL, electrons move freely in a vacuum at nearly the speed of light and exchange energy with a co-propagating electromagnetic wave, producing a tunable and exponentially amplified beam of light.

Specifically, it includes the following sections:

1. High-energy electron beam

This process begins with a high-energy electron beam from a particle accelerator.

Microwave amplifiers accelerate electrons to speeds approaching that of light. For example, with an electron beam of 1 GeV (one electron charge multiplied by one gigavolt), the electrons travel only one ten-millionth slower than the speed of light. The higher the energy of the electron beam, the higher the photon energy that the FEL can produce.

2. Radiation from accelerated charges

Any accelerated charged particle emits light. For example:

Bremsstrahlung (German for "braking radiation") light sources generate radiation by directing electrons at a metal wall, causing the electrons to decelerate rapidly; the electron beam dissipates energy by emitting light and releasing heat.

Deflecting magnets can also achieve a similar effect:

If the electron beam is bent into a circular orbit, a synchrotron radiation source is obtained.

By oscillating an electron beam along a sinusoidal path, a wiggler or undulator light source is created.

Synchrotron and wiggler sources produce light with a broad frequency range, whereas undulators are different: under specific conditions, an electron beam passing through an undulator can produce laser emission.

3. Oscillator

An undulator is a device composed of periodically arranged magnets that causes a high-energy electron beam to oscillate along a sinusoidal path (i.e., "wiggle") at a specific frequency. This controlled oscillation of the electron beam is the first step in generating precisely tuned light. The wavelength of the output light is determined by the undulator's period (λᵤ) and the energy of the electron beam.

For this reason, some view FEL as an alternative to EUV lithography.

Traditional EUV lithography uses 13.5 nm because tin plasma can efficiently generate light in this wavelength range.

But FEL operates on a completely different principle: by adjusting the electron beam energy, undulator period, and magnetic field strength, it can change the wavelength of the output light. This means it can theoretically cover wavelengths from soft X-rays and EUV all the way to longer wavelengths. In other words, traditional EUV is like a lithography tool with a fixed focal length, while FEL is more like an optical instrument capable of continuously tuning its wavelength.

To overcome these limitations, some have turned to FEL. The U.S. startup xLight is a strong advocate of this approach. Notably, Pat Gelsinger, former CEO of Intel, now serves as Executive Chairman of xLight.

In xLight’s technology, electrons are first injected into a particle accelerator and then enter a free-electron laser (FEL). xLight states: “The FEL uses electrons from the particle accelerator and passes them through an undulator with a periodic magnetic field to produce a coherent, high-intensity beam.”

In simple terms, EUV light is generated in an accelerator. Then, the EUV light is transmitted from the particle accelerator system to the wafer fab through a device similar to a photon conduit. At this point, the EUV light is directed to a sub-fab, which contains various independent systems known as "flip stations."

According to xLight’s video, each flip station is dedicated to a single EUV tool on the upper fab floor. During operation, EUV light is transmitted to each rotating station in the sub-fab area. Each rotating station then receives the light and directs it to the EUV system located on the upper fab floor, which in turn powers the EUV equipment.

In this scenario, the EUV lithography equipment itself does not include the LPP light source. Instead, EUV light is generated in a particle accelerator and then transmitted to the EUV equipment in the wafer fab. This is a simplified way of describing a complex process.

Nevertheless, xLight’s FEL light source produces four times the power of current LPP systems. xLight states: “By delivering up to 4 times more EUV power, fabs can optimize pattern improvement, increase productivity and yield, generating billions of additional dollars in annual revenue per scanner while reducing cost per wafer by approximately 50%. Additionally, a single xLight system can support up to 20 ASML systems over a 30-year lifespan, reducing capital and operational expenditures by more than threefold.”

Theoretically, xLight’s technology can be applied to low-NA EUV, high-NA EUV, and even ultra-NA EUV. In terms of R&D, ASML is developing 0.75 ultra-NA EUV technology aimed at enabling capabilities for the more distant future.

From Terafab’s elongated factory design and Musk’s response, it is highly likely that Terafab will use a linear accelerator as the high-speed electron source for its FEL. Currently, femtosecond free-electron X-ray lasers are already used in university laboratories for collecting data in protein crystallography.

If you still don’t understand how powerful this is, consider this analogy: if the light source of an AMSL extreme ultraviolet lithography machine were a regular light bulb, then the free-electron laser extreme ultraviolet light source would be like a highly efficient laser capable of easily generating output power in the range of kilowatts to tens of kilowatts, with fully tunable wavelengths.

In contrast, the LPP EUV used by AMSL has a fixed wavelength, and its power is difficult to exceed 1 kW. Moreover, neither the spectral purity nor the coherence of its light can compare. If successfully developed, it would significantly accelerate both the speed and quality of lithography. Another advantage of FEL is its exceptionally high conversion efficiency (energy consumption ratio); the LPP used by AMSL requires approximately 4.4 MW of electrical power to produce just 1 kW of usable EUV light, resulting in an overall efficiency of about 0.05%.

The most advanced energy-recovery linear accelerator-based free-electron laser (ERL-FEL) requires only about 0.7 MW of electrical power to produce 1 kW of EUV light, offering approximately six times higher efficiency, with further improvements possible in the future using advanced superconducting materials.

From a purely technical standpoint, FEL demonstrates clearly superior light source performance, but this does not necessarily mean it is more advanced for industrial lithography, as EUV lithography requires more than just "a beam of 13.5 nm light."

Also required: high power + high stability + high repetition rate + high reliability + extremely low cost + extremely long operating time + compatibility with reflective optical systems.

ASML's current EUV light source, after years of engineering development, has established a complete industrial ecosystem.

FELs typically require large electron accelerators, undulators, vacuum systems, and beam control systems, resulting in enormous equipment size and cost. Therefore, from the perspective of light source physical performance, FELs are clearly more powerful and more versatile.

However, from the perspective of engineering capabilities in semiconductor mass production lithography: the currently mature EUV light sources are more suitable for wafer fabs. FELs have the potential to move EUV light sources beyond the fixed wavelength of 13.5 nm toward more flexible short-wavelength lithography.

For example, if future exploration of next-generation lithography enters the 6.x nm, 5.x nm, or even shorter wavelength ranges, FELs—with their tunable wavelength, high coherence, and high peak brightness—will become highly attractive.

But there is another major issue: as the wavelength becomes shorter, challenges with optical systems, masks, photoresist, reflectivity, photon scattering, and vacuum systems all become increasingly difficult.

Therefore, the true revolutionary potential of FEL lies not merely in "replacing EUV light sources," but in offering an entirely different pathway to high-brightness, short-wavelength light sources.

In addition to FEL, other proposed solutions include high-harmonic generation (HHG), discharge plasma (DPP), and synchrotron radiation. LPP excels in maturity and mass production, FEL emphasizes high brightness, high coherence, and tunable wavelength, while HHG offers potential for miniaturization. The key to future competition lies in balancing power, efficiency, stability, and cost at shorter wavelengths.

More importantly, they are all the same and face even greater challenges, with no definite timeline.

This article is from the WeChat public account "Semiconductor Industry Watch" (ID: icbank), authored by the editorial team.