Steady-state microbunching

• Comment: This is a lightly reworded version of an originally LLM-generated draft; merely rewording LLM-generated text usually does not resolve the problem of LLM usage. Please remove all text (while keeping the review template) and start over without LLMs. For general advice, see Help:Your first article. Helpful Raccoon (talk) 07:52, 24 September 2026 (UTC)

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Steady-state microbunching (SSMB) is an accelerator-based Synchrotron light source architecture designed to generate high-average-power, continuous-wave (CW) Extreme ultraviolet (EUV) light. To overcome the temporal incoherence of conventional storage ring and the low duty cycles of pulsed linear Free-electron laser (FEL), SSMB synthesizes the strengths of both systems to achieve highly coherent radiation at megahertz repetition rates. It achieves this by utilizing an external high-power laser modulator and magnetic elements to organize a continuously circulating electron beam into regularly spaced, longitudinal microbunches at optical or EUV wavelengths. This externally maintained, periodic structure allows the electrons' electromagnetic fields to add coherently as they pass through an undulator. The primary application for SSMB is multi-kilowatt, 13.5 nm wavelength demands of next-generation high-numerical-aperture (High-NA) semiconductor lithography for sub-3 nm logic nodes. SSMB can achieve average powers exceeding 1 kW, which aims to bypass the physical scaling limitations and theoretical ~500 W ceiling of current laser-produced plasma (LPP) sources. First theorized in 2010 and successfully demonstrated at the Metrology Light Source (MLS) in Berlin in 2021, the technology is now being scaled toward kilowatt-class viability, notably through a Tsinghua University-led prototype facility under construction in Xiong'an New Area, Hebei, China.

Principle

Microbunching and Coherent Radiation generation

The Steady-State Microbunching (SSMB) mechanism generates coherent short-wavelength radiation by creating and maintaining electron microbunches at the wavelength inside the storage ring. The process occurs in three main stages.

Laser modulation

A high-power laser with wavelength ${\lambda }_L$ and wavenumber

$k_L=\frac{2\pi }{{\lambda }_L}$

interacts with the relativistic electron beam in a modulator undulator. The transverse electric field of the laser couples to the electron transverse motion and produces a sinusoidal energy modulation,

$\Delta E=A\sin (k_{L}z+{\varphi }_s),$

where, $A$ is the modulation amplitude, $z$ is the longitudinal coordinate, and ${\varphi }_s$ is the synchronous phase.

Dispersive conversion

The energy-modulated beam then passes through a dispersive magnetic section, such as an achromatic arc or chicane. Because electrons with different energies follow slightly different path lengths, the ring's momentum compaction converts the energy modulation into longitudinal density modulation.

The longitudinal phase-space evolution can be represented by the turn-by-turn map

${\delta }_{n+1}={\delta }_n+A\sin (k_L{z}_n+{\varphi }_s),$

$z_{n+1}=z_n-C_0\eta {\delta }_{n+1},$

where, $\delta =\Delta E/E_0$ is the relative energy deviation, $C_0$ is the ring circumference, and $\eta$ is the momentum-compaction factor.

Microbunching

After the dispersive conversion, electrons accumulate at periodic longitudinal positions separated by the laser wavelength (or its harmonic). This produces microbunches whose density modulation affected by Bessel-function harmonics. At the $h$-th harmonic, the bunching factor can be written schematically as

$b_n=\left|⟨e^{in{k}_{L}z}⟩\right|=|J_n\left(n{k}_L{C}_0\eta A\right)\exp [-\frac{{\left(n{k}_L{C}_0\eta {\sigma }_{\delta }\right)}^2}{2}]|$

where, $J_n$ is the Bessel function, $R_{56}$ is the longitudinal dispersion, and ${\sigma }_{\delta }$ is the beam's natural relative energy spread. Thus, the modulation amplitude and energy spread determine how strongly the beam microbunches. For optimal microbunching, the modulation amplitude must match the phase-slippage factor; in the MLS proof-of-principle experiment, $\eta \approx -2\times 10^{-5}$, and the optimal condition was $A\approx 1.5{\sigma }_{\delta }$ for second-harmonic bunching.

Coherent radiation

The microbunches pass through a Undulator or wiggler and emit coherent EUV radiation at the desired harmonic H:

${\lambda }_R=\frac{{\lambda }_L}{H}$

For EUV lithography at 13.5 nm with a 1064 nm modulator, this requires harmonic generation at $H\approx 79$.

Microbunching matters because of how the fields combine:

• Incoherent radiation: $I_{\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}}\propto N$ (electrons radiate independently)
• Coherent radiation: $I_{\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}}\propto N^2|b{|}^2$

More generally:

$I=I_1[N+N(N-1)|b{|}^2]$

where $I_1$ is the single-electron radiation intensity and quadratic scaling $N^2$ — many electrons contribute to the same electromagnetic mode with correlated phases instead of behaving as independent radiators. The coherent enhancement can't be scaled indefinitely , and the bunching factor, transverse beam size, energy spread, radiator properties, bandwidth, and spatial overlap all affect the output. SSMB therefore requires many electrons whose longitudinal and transverse phase-space properties are sufficiently well controlled that they radiate coherently.

Under the 3D rigid-beam approximation, the coherent radiation power at the H-th harmonic is:

$\hat{P}=\frac{{\pi }^2{K}^2[JJ{]}^2}{2(2+K^2)}\frac{N_u{N}_e^2{b}^{2}m{c}^3{r}_0}{L_u^2}{D}_{\delta }\cdot F{F}_{\perp }$

where $K$ is the undulator parameter, $[JJ]=[J_0(\chi )-J_1(\chi )]$ with $\chi =K^2/(4+2K^2)$, $N_u$ is the number of undulator periods, $b$ the bunching factor, and

$D_{\delta }=\frac{{\mathrm{Erf}}^2(\sqrt{2\pi \xi })}{8\pi {\xi }^2}$

with $\xi =N_u{\sigma }_{\delta ,\mathrm{e}\mathrm{f}\mathrm{f}}$ quantifying the impact of effective energy spread.

The transverse form factor can reduce power due to finite beam at transverse size.

${\sigma }_{\perp }=\sqrt{{ϵ}_{\perp }{\beta }_{\perp }}$

$F{F}_{\perp }(S)=\frac{2}{\pi }[{\tan }^{-1}\left(\frac{1}{2S}\right)+S\ln \left(\frac{(2S{)}^2}{(2S{)}^2+1}\right)],$

Predicted performance: SSMB-EUV with ${\sigma }_{\perp }=5,10,20\mu \mathrm{m}$, theoretical power output can be 92 kW, 14 kW, and 3.5 kW respectively. The on-axis harmonic power gives lower power of 4 kW, 3.6 kW, and 2.6 kW because it neglects red-shifted off-axis radiation. Crucially, kW-level EUV power can be achieved from a 3 mm microbunch train with an average beam current of 1 A at 100% filling factor, corresponding to $N_e\approx 2.2\times 10^4$ electrons per microbunch at ${\lambda }_L=1064\mathrm{n}\mathrm{m}$.

Maintaining the Microbunching (Steady State)

Unlike conventional FELs, where microbunching is destroyed after a single pass, SSMB maintains microbunches indefinitely through a turn-by-turn phase correction mechanism. The laser modulator acts as an optical RF cavity, providing longitudinal focusing akin to synchrotron oscillations in conventional storage rings.

After the beam is modulated and compressed, it circulates around the storage ring and eventually returns to the modulation region. For steady-state operation, the longitudinal phase of the stored beam must remain synchronized with the laser turn after turn. The ring therefore operate close to an isochronous condition in which the electrons with slightly different energies must not acquire large differences in revolution time, otherwise the longitudinal structure gradually slips relative to the laser. This can be identified by the momentum compaction factor ${\alpha }_c$

$\frac{\Delta C}{C}={\alpha }_c\frac{\Delta p}{p}$

In a conventional storage ring, the compaction factor ${\alpha }_c$ affects particles with different energies, those particles have noticeably different revolution frequencies; thus the SSMB requires this effect cancelled or carefully controlled (quasi-isochronous, low-$\alpha$ lattices, with low $\eta$).

Also a critical stability condition dictates that the longitudinal slippage from the beam's natural energy spread ${\sigma }_{\delta }$ must not wash out the microbunch:

$|\eta |C_0{\sigma }_{\delta }\ll \frac{{\lambda }_L}{2\pi }$

At the same time, the microbunching competes with radiation damping and quantum excitation. Synchrotron radiation damps deviations from the equilibrium orbit, while stochastic photon emission produces quantum excitation that diffuses the beam in phase space. SSMB can induce a stable dynamic equilibrium in which the desired longitudinal structure survives this competition between quantum excitation (increasing energy spread) and radiation damping (reducing emittance). The equilibrium longitudinal emittance is set by:

${\sigma }_{z,\min }\approx {\sigma }_{\delta ,s}\sqrt{\frac{⟨F^2⟩-⟨F{⟩}^2}{24}}=\sqrt{\frac{C_q}{J_s}\frac{\rho }{\gamma {\theta }^3}}$

${ϵ}_{z,\min }\approx 2{\sigma }_{\delta ,s}^2\sqrt{\frac{⟨F^2⟩-⟨F{⟩}^2}{24}}=2\sqrt{\frac{C_q}{J_s}\frac{C_q}{{\gamma }^2{\theta }^3}}$

where $C_q=3.8319\times 10^{-13}\mathrm{m}$, $J_s\approx 2$ is the longitudinal damping , $\rho$ is the bending radius, and $\theta$ is the bending angle. Meeting the stability criterion requires specialized quasi-isochronous, low-$\alpha$ magnetic lattices to minimize $\eta$, alongside with the high-power optical enhancement cavities to sustain the modulating laser field continuously.

SSMB for EUV generation

Extending steady-state microbunching (SSMB) from the Visible spectrum or X-ray regime to extreme ultraviolet (EUV) is substantially more difficult because the required longitudinal structure becomes much smaller. The EUV wavelength used for semiconductor lithography is 13.5 nm, corresponding to a photon energy of approximately

$E_{\gamma }=\frac{hc}{\lambda }\approx 91.9\mathrm{e}\mathrm{V}.$

For SSMB, one possible strategy is to use a much longer-wavelength laser to establish the initial periodic energy modulation and then generate shorter wavelength as required through the harmonic microbunching.

Generation of 13.5 nm radiation

A SSMB-EUV typically uses a 1064 nm modulation laser. The ratio between the laser wavelength and the target EUV wavelength is

$\frac{1064}{13.5}\approx \mathrm{78.8.}$

Thus, the 79th harmonic of 1064 nm corresponds to approximately

${\lambda }_{79}=\frac{1064}{79}\approx 13.47\mathrm{n}\mathrm{m},$

which is close to the 13.5 nm wavelength required for EUV lithography.

SSMB-EUV generates this radiation through high-harmonic coherent emission from microbunched electrons. The modulation laser establishes the longitudinal microbunching structure, while the radiation wavelength is selected by the harmonic order and the radiating device. Because of the electrons within each microbunch radiate coherently, the radiated power can scaled to approximately $N_e^2$, rather than linearly, with electron number as in incoherent synchrotron radiation and can produce continuous high power EUV.

To generate high harmonics the electron beam must acquire sufficiently strong and stable density modulation because higher harmonics can become sensitive to the beam's higher energy spread, longitudinal emittance, and imperfections in the phase-space transformation. The coherent radiation spectrum from steady-microbunching can be limited by Fourier-transform. For the Gaussian electron bunch, this relative bandwidth can be related to the bunch duration by

${\left(\frac{\Delta \omega }{\omega }\right)}_{\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}}=\frac{2\sqrt{2}\ln 2}{\pi }\frac{{\lambda }_L}{H(c\Delta t{)}_{\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}}},$

where ${\lambda }_L$ is the modulation-laser wavelength, $H$ is the harmonic number, and $\Delta t$ is the electron-bunch duration. The spectral width should be independent of the harmonic number, whereas the relative bandwidth decreases as per $1/H$.

The resulting coherent spectrum can be narrower than the conventional incoherent undulator in a storage ring , reducing the spectral load on the Mo/Si multilayer mirrors used in the EUV lithography optical system.

The EUV radiation/light is produced when the microbunched electrons interact with a undulator/wiggler. For a planar undulator, the resonant wavelength can be approximated by

${\lambda }_r=\frac{{\lambda }_u}{2{\gamma }^2}(1+\frac{K^2}{2}),$

where ${\lambda }_u$ is the undulator period, $\gamma$ is the relativistic Lorentz factor, and $K$ is the undulator parameter.

The microbunching and radiation therefore be designed together: the longitudinal structure determines how coherently the electrons radiate, while the radiator determines the resonant wavelength and other properties of the emitted radiation.

In an SSMB-EUV, the stored bunch length, microbunch spacing, harmonic wavelength, and emitted radiation wavelength are related but are notidentical.

Storage ring and laser system

Appropriate parameters of the prototype SSMB-EUV facility under construction by Tsinghua University.

Subsystem	Function	Key Parameters
Photocathode gun	Electron production	High-brightness injector
Linac	Acceleration to 400 MeV	Energy spread < 0.05%
Stretching ring	Beam conditioning	Large $R_{56}$, ~200 turns
SSMB main ring	Steady-state microbunching	Circumference ~100–150 m
Laser modulator + OEC	Turn-by-turn phase locking	1064 nm, ~1 MW intracavity
Radiator undulator	EUV generation	13.5 nm, ~meter scale
Extraction beam dump	Safe beam termination	Radiation shielding

This shows that the SSMB-EUV storage ring provide a combination of properties that are unusual compared to a conventional synchrotron light source ring. It requires a very low momentum compaction so that longitudinal phase relationships can survive many revolutions in the storage ring, with the low longitudinal and transverse emittances to maintain microbunching and provide strong coupling to the radiation stage.

The storage-ring lattice therefore becomes an essential part of the light source. In the storage ring dipole magnets bend the electron beam and quadrupoles magnets control the transverse optics. Also specialized lattice sections manipulate the longitudinal phase space and provide strong focusing required for microbunch formation.

SSMB requires a small global phase-slippage factor, with the momentum-compaction factor reduced to approximately $\eta \approx -2\times 10^{-5}$,. This is achieved through careful low-alpha or quasi-isochronous lattice design, including sextupole correction of nonlinear momentum-compaction. Reducing the momentum compaction decreases the sensitivity of revolution time to the electron energy spread and allows shorter equilibrium bunch lengths than those of conventional storage rings.

1. Longitudinal Strong Focusing (LSF): Uses a low-alpha ring (~100 nm bunch) combined with an Angular Dispersion Modulation (ADM) section to compress to ~3 nm. This requires hundreds of MW pulsed laser power (1% duty factor).

2. Hybrid Insertion: in a normal storage ring with ADM plus longitudinal strong focusing achieves ~3 nm microbunches with ~1 MW CW laser power via an optical enhancement cavity. This is the baseline for the EUV lithography source, projecting >1 kW average EUV power at 1 A average current.

Another related approach is generalized longitudinal strong focusing (GLSF). Rather than relying on the laser system alone to generate an extremely short longitudinal structure, GLSF uses transverse–longitudinal coupling within the storage-ring lattice to assist bunch compression and longitudinal phase-space control.

The optical enhancement cavity (OEC) is therefore critical for sustaining the modulating laser. A prototype two-mirror cavity has been demonstrated with

Finesse: 16,518 ± 103

Average intracavity power: 55 kW (target: 1 MW)

Mirror absorption coefficient: ~12 ppm

Cavity Q: ~10,000

Length: 5 m

Though D-shaped mirrors suppress modal instabilities, a validated cavity power coupling model ensures a stable operation.

To compensate for the synchrotron radiation losses in the ring, an induction linac operating at 1 MHz repetition rate with 1 kV voltage and 100 ns flat-top is used for energy recovery.

EUV power and photon flux

The relative fluctuation of turn-by-turn coherent radiation power is given by:

$\frac{\mathrm{Var}[P(\theta ,\phi ,\omega )]}{{⟨P(\theta ,\phi ,\omega )⟩}^2}=\frac{1}{⟨N_{\mathrm{p}\mathrm{h}}⟩}+\frac{2}{N_e}(\frac{1+{|b(\theta ,\phi ,2\omega )|}^2}{{|b(\theta ,\phi ,\omega )|}^2}-2)+O\left(\frac{1}{N_e^2}\right)$

For a Gaussian microbunch with ${\sigma }_z=3\mathrm{n}\mathrm{m}$ and $N_e=2.2\times 10^4$, the relative power fluctuation at 13.5 nm is approximately 2% sufficiently stable for lithographic exposure.

And in 13.5 nm EUV bandwidth, each photon has an energy of ~ 92 electron volt (eV). Therefore it's corresponding photon rate is:

${\dot{N}}_{\gamma }=\frac{P_{\mathrm{E}\mathrm{U}\mathrm{V}}}{E_{\gamma }}.$

For example, an ideal 1 kW source at 13.5 nm would emit approximately

${\dot{N}}_{\gamma }\approx 6.8\times 10^{21}$

photons per second before accounting for the spectral bandwidth or losses in the beamline.

Development

Original proposal

The SSMB concept was first proposed by Daniel F. Ratner and Alexander W. Chao in 2010. The framework established that by replacing the RF cavity of a conventional storage ring with a laser modulator, the electron bunches could be maintained in a microbunched state indefinitely through turn-by-turn phase locking. This mechanism was recognized as a way to combine the high repetition rate of storage rings with the coherent N² power scaling of Free electron lasers.

Experimental demonstration

A proof-of-principle experiment was conducted in 2021 with the help of Tsinghua University and German physicist at the Metrology Light Source (MLS) in Berlin, Germany. The experiment in which a 250 MeV electron beam in a quasi-isochronous ring ($C=48\mathrm{m}$, $\eta \approx -2\times 10^{-5}$) was modulated by a 1064 nm laser and after one revolution, second harmonic (532 nm) was detected, in the coherent undulator/wiggler confirming:

• Sub-μm microbunching formation
• Turn-by-turn phase correlation at sub-laser-wavelength precision
• Fourier-transform-limited coherent spectrum

This experiment provided the first validation of the SSMB mechanism and established the foundation for scaling to EUV wavelengths.

Development toward EUV

Following the MLS demonstration, the project advanced through multiple phases:

• Phase I: Single-turn modulation with coherent radiation detection (completed at MLS)

• Phase II: Multi-turn modulation (hundreds to thousands of turns) with phase-locked laser.

• Phase III: True steady-state operation with optical enhancement cavity and induction linac energy recovery.

Proposed SSMB-EUV sources

Started in early 2025, a prototype SSMB-EUV developed by Tsinghua University is being constructed in Xiong'an New Area, Hebei Province, China. It's Key parameters are:

Parameter	Design value
Electron energy	400 MeV
Ring circumference	~100–150 m
Modulation laser wavelength	1064 nm
Target radiator wavelength	13.5 nm (79th harmonic)
Average beam current	1 A
Microbunch length	~3 nm
Average EUV power	>1 kW
Repetition rate	MHz (CW)
Optical enhancement cavity power	~1 MW intracavity
Energy recovery	1 MHz induction linac

The facility is designed as a large-scale light source serving multiple lithography tools from a centrally housed accelerator via relay optics, rather than integrating the source into individual exposure tools.

Challenges

The fundamental SSMB mechanism has been experimentally demonstrated, but turning it into a practical high-power EUV source requires solving several problems simultaneously. The main problems are-

Beam dynamics

1. Low-Alpha Lattice Design

The electron beam must maintain an exceptionally small and well-controlled longitudinal phase-space distribution while carrying a high average current. Also the global phase slippage factor (momentum compaction) must be precisely controlled to η ≈ −2 × 10⁻⁵ to prevent a rapid longitudinal phase slippage and the lattice to retain adequate transverse stability, dynamic aperture, and chromatic control for injection and stored-beam stability.

2. Longitudinal Emittance and Bunch Compression

The equilibrium bunching length should be compressed from ~100 nm (low-alpha ring) to ~3 nm at the radiator. This requires precise control of the ADM (alpha damping mechanism) and LSF (longitudinal strong focusing) parameters, to keep the longitudinal emittance exceptionally small throughout the storage ring.

3. Energy Spread Management

In a SSMB the effective energy spread should be minimized to preserve the bunching factor. This demands a careful balance of these competing effects:

• Radiation damping and quantum excitation establish the equilibrium distribution, but the stochastic nature of photon emission can disturb the fine longitudinal structure needed for strong microbunching.

• Laser heating must also be controlled, since it directly inflates the energy spread.

4. Nonlinear Momentum Compaction

When the required bunching length becomes extremely small, the linear momentum-compaction coefficient alone no longer fully describes the longitudinal dynamics. Higher-order terms cause different particles to experience different longitudinal path-length variations, distorting the microbunching, so that nonlinear effects should be compensated in the lattice design.

5. Transverse–Longitudinal Coupling

Coupling between transverse and longitudinal motion must be carefully controlled:

• Horizontal emittance, the Curly-H function, and betatron tune should be managed to prevent emittance exchange that would destroy microbunching.

• In advanced designs such as GLSF, this coupling is intentionally exploited to assist bunch compression, but it must be precisely designed, unless it will create uncontrolled phase-space distortion.

Laser and optical system

The optical enhancement RF cavity faces several engineering challenges:

• Laser–electron synchronization: The modulation laser should be synchronized with the circulating electron beam to a precision compatible with the optical wavelength. For example a 1064 nm laser, an uncontrolled timing error can be correspond to a significant fraction of the wavelength, that can change the phase of the modulation; over many rotations, this can reduce stability of the microbunching.

• Mirror damage thresholds: Sustaining 1 MW Continuous intracavity power at 1064 nm requires ultra-low-absorption mirrors (<12 ppm) with better thermal management.

• Cavity length stabilization: The cavity length must be locked to the modulation laser wavelength with sub-nanometer precision to maintain resonance.

• Modal instabilities: Higher-order transverse modes must be suppressed; D-shaped mirrors thus fur have successfully used for this purpose.

• Laser power scaling: The external laser must deliver sufficient power to compensate for cavity losses and maintain the modulation amplitude A ≈ 1.5σ_δ.

• Cavity stability requirements: An optical enhancement cavity can reduce the external laser power required to create a strong intracavity field, but it introduces additional requirements for cavity length stability, laser frequency stability, optical mode matching, mirror quality, and thermal control.

• Thermal management at EUV power levels: At the power levels for EUV generation, the optical system should also dissipate the heat produced by absorption of the laser and maintain stable operation for long periods. Thus the laser and optical cavity therefore becomes a part of the overall accelerator stability problem rather than merely an auxiliary component.

Collective effects

High-current electron beams interact with themselves and with the surrounding accelerator structures. This collective behavior becomes increasingly challenging at high current, especially when preserving nanometer-scale microbunches and must be included in any complete high-power design. The challenges are:

• Intra-beam scattering (IBS):

IBS causes emittance growth, energy-spread growth, and bunch lengthening, making it more difficult to preserve short longitudinal structures. It is particularly problematic for maintaining nanometer-scale microbunches.

• Space-charge effects:

Significant at high peak current within the microbunches. Space charge can modify longitudinal and transverse dynamics, potentially detuning the lattice and causing tune shifts.

• Wakefields:

Wakefields is generated by vacuum-chamber components, can modify the longitudinal and transverse dynamics. Long-range and short-range wakefields can drive longitudinal instabilities or emittance growth. Additionally, the coherent radiation produced by the microbunched beam can itself affect subsequent electrons, because the radiation field is coherent, its interaction with the beam can be much more strong than an ordinary incoherent radiation field. In particular, the interaction between electron bunches and recirculating coherent undulator radiation in the OEC can drive longitudinal single-bunch multi-turn instabilities, with growth rate:

$\frac{1}{\tau }=\frac{N_e{e}^{2}c{R}_{56}}{2{\mu }_0{E}_{0}C}\sum _{p=1}^{\mathrm{\infty }}W{\text{'}}_{\parallel }(pC)\sin (p{\mu }_0)$

where, $W{\text{'}}_{\parallel }$ is the derivative of the longitudinal wakefield and ${\mu }_0$ is the synchrotron phase advance.

• Other adverse effects:

Coherent synchrotron radiation, resistive-wall effects, Touschek scattering, and coupled-bunch instabilities can also influence beam stability and lifetime.

• Collective instabilities and operational challenges:

High-current operation demands precise control of beam energy, emittance, bunch length, current, and synchronization with the modulation laser. It also creates practical challenges for vacuum systems, beam diagnostics, feedback control, and component lifetime.

Comparison with Laser-produced plasma (LPP)

The most established industrial method for generating 13.5 nm extreme ultraviolet (EUV) radiation uses laser-produced plasma (LPP). In an LPP source,a droplet generator ejects ~25 μm diameter molten tin droplets at ~70 m/s through a vacuum chamber. A dual-pulse Carbon-dioxide (Cow) laser operating at a (wavelength at 10.6 μm, pulse duration of ~15–25 ns and intensity of ~10¹¹ W/cm²) then targets each tin droplet. First a low-energy "pre-pulse" strikes the droplet, expanding it into a low-density pancake-like cloud of tin vapor. Then a high-energy "main pulse" strikes this vapor, ionizing it into a high-temperature plasma with electron temperatures of 20–50 eV and electron densities of 10²⁰–10²¹ cm⁻³.

Within this plasma, tin ions in charge states Sn IX through Sn XIV undergo 4p⁶4dⁿ → 4p⁵4dⁿ⁺¹ + 4dⁿ⁻¹4f transitions, producing strong emission lines clustered around 13.5 nm with a 2% full-width at half-maximum (FWHM) bandwidth. The source operates at a repetition rate of ~50 kHz and conversion efficiency of ~5–6%. Current-generation LPP sources deliver approximately 250 W of in-band EUV power at intermediate focus with plasma size of ~100 μm and an etendue below 1 mm²·sr.

Thus SSMB represent a fundamentally different approach. It does not rely on a hot plasma as the primary radiation source. Instead, it use relativistic electrons circulating in a storage ring and extract coherent radiation from their interaction with an undulator/wiggler. A FEL also provides another accelerator-based light source. In an FEL, a high-quality electron beam passes through an undulator and develops microbunching through its collective interaction with the electromagnetic field. SSMB instead seeks to establish and preserve the microbunching before radiation extraction and to reuse the electron beam over many turns. This enables a combination of advantages such as coherence, high repetition rate, and high average power. Its main disadvantage at present is technological maturity: LPP is an established industrial technology, while SSMB remains in research and development phase.

Parameter	LPP (ASML NXE:3400C)	SSMB-EUV (Target)
Average EUV power @ IF	~250–350 W	1,000 W
Wavelength	13.5 nm	13.5 nm (tunable)
Bandwidth (FWHM)	~1–2%	<0.4% (coherent)
Waveform	Pulsed (~50 kHz)	Quasi-CW (MHz)
Source cleanliness	Tin debris, collector erosion	Clean (no debris)
Optical system	11 mirrors (>30% loss each)	Fewer mirrors possible
Footprint	Tool-integrated	Campus-scale facility
Scalability to <3 nm	Limited (~500 W max)	Theoretically >10 kW

The comparison illustrates the different source architectures. While LPP provides a mature, high-power industrial solution, but its plasma-based operation introduces tin debris, collector degradation and a broad spectral emission that must be filtered and collected. SSMB, by contrast, aims to generate narrow-band, coherent EUV radiation from a circulating electron beam, potentially enabling quasi-continuous operation, higher average power and reduced source debris. Its storage-ring architecture, however, would require a much larger accelerator facility than an integrated LPP source.

SSMB currently not considered as a replacement for established LPP EUV source. Its significance lies in the possibility of providing an alternative EUV source in the future, if the beam-dynamics, radiation-extraction and engineering problems can be solved.

Outlook

SSMB has progressed from a theoretical proposal to an experimentally demonstrated method for producing and maintaining coherent microbunching in a storage ring. The 2021 experiment established a conceptual transition from theoretical to practical feasibility demonstrating that a storage ring can maintain optical-scale microbunching to demonstrating that it can do so at high current, at sufficiently short wavelength, with sufficiently high bunching factor, and for sufficiently long periods to serve as a practical light source.

The ongoing prototype development at Xiong'an, combined with advances in high-power optical enhancement cavities, low-alpha lattice design, and induction linac technology, positions SSMB as a viable next-generation EUV source. If successful, SSMB could enable high-volume manufacturing(HVM) at the "2 nm process" and beyond, while also serving as a high-flux, energy-tunable photon source for condensed-matter physics, ultrahigh-resolution ARPES, and other scientific applications.

Though significant engineering challenges remain, like the electron beam must have extremely well-controlled longitudinal and transverse phase space, the storage ring must minimize detrimental longitudinal phase slippage, the laser must maintain precise synchronization, and collective effects must remain controlled at high current demonstrating steady-state operation over extended periods.

See also

• Free electron laser (FEL)
• EUV lithography
• Semiconductor industry in China
• Huawei
• SiCarrier
• Shanghai Micro Electronics Equipment (SMEE)
• Advanced Micro-Fabrication Equipment (AMEC)
• Naura Technology
• Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP)

References

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