Source: Compiled and translated from Tom Talks about Chip Intelligent Manufacturing.

EUV lithography breaks the limits of traditional lithography with 13.5 nm extreme ultraviolet light, making it a key technology for advanced process nodes. Its high-energy nature dictates that imaging must rely on tin plasma luminescence and multi-layer mirrors, marking a major revolution in the history of semiconductor manufacturing.

Wavelength and Characteristics of EUV Lithography

Wavelength: 13.5 nm (extreme ultraviolet region)

Photon energy: Approximately 92 eV, significantly higher than the 6.4 eV of DUV (ArF, 193 nm)

This high-energy light is sufficient to ionize the outer-shell electrons of atoms of almost all elements, so it cannot be generated using traditional lenses or gas laser methods.

The generation of EUV light requires extremely high energy, hence the adoption of a high-temperature plasma luminescence mechanism.

Luminescence Principle of EUV Light Sources

The core light source mechanism of EUV lithography machines is called:Laser Produced Plasma (LPP)

Its basic process is as follows:

① Generation of Tin Droplets

A high-precision jet system ejects liquid tin (Sn) into tiny droplets with a diameter of approximately 20~30 μm;

The droplet frequency is about 50,000 droplets per second;

The droplets are suspended in a vacuum chamber (no air allowed, as 13.5 nm light would be completely absorbed otherwise).

② High-Power Laser Irradiation of Tin Droplets

Pulsed laser from a CO₂ laser (power: approximately 20~40 kW) is precisely aligned with the tin droplets;

The laser instantly heats the tin to millions of degrees Celsius, fully ionizing it into high-temperature plasma;

At this point, the outer-shell electrons of tin atoms are stripped away, leaving only highly charged ions (Sn⁸⁺～Sn¹⁴⁺).

③ EUV Light Emission from Plasma

Under high-temperature and high-energy conditions, these multi-valent tin ions continuously transition from high-energy states to low-energy states;

When electrons return from outer orbits to inner orbits, photons with specific energy are released;

The energy of these photons is concentrated in the 13.5 nm band (extreme ultraviolet region);

This wavelength band is ideally suited for high-resolution lithography (corresponding to feature sizes <10 nm).

Therefore, EUV light is neither "reflected light" nor "laser light", but rather spontaneous emission from tin plasma.

Collection of EUV Light

EUV light cannot pass through any material (even air absorbs it), so:

1.It must operate in a vacuum;

2.Lenses cannot be used for focusing—only multi-layer mirrors (Mo/Si Bragg Mirrors) can reflect the light.

The light passes through the following components:

Collector Mirror: Converges scattered EUV light;

Intermediate Focus: Forms a uniform light beam;

Projection Optics: Achieves pattern reduction and imaging through multi-layer mirrors.

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