Immersion Lithography

Immersion lithography

This article describes immersion lithography techniques used to increase the resolution of lithography machines.

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Chip Fabrication: The Evolution of Lithography
For more than half a century, Moore's Law has been driving the development of semiconductor technology, but when the wavelength of the light source of the lithography machine is stuck at 193nm and the chip process is reduced to 65nm, Moore's Law begins to face challenges. Some lithography giants have opted for a conservative strategy, pinning their hopes on dry lithography technology with a wavelength of 157nm for F2 excimer light sources. In 2002, the idea of immersion lithography was proposed, in which the refraction of light in a liquid was further reduced by using water as a medium.

A method for increasing the NA of the numerical aperture
Increasing the resolution of a lithography machine depends on two main factors: the wavelength of the light source (λ) and the numerical aperture (NA) of the projection objective. According to the Rayleigh criterion, the resolution R of the lithography machine can be expressed by the formula R=k1⋅λ/NA, where k1 is the process factor. Therefore, when the wavelength of the light source is fixed, increasing the NA of the numerical aperture becomes the key to improving the resolution. There are two main ways to raise NA: by increasing the objective diameter and by using immersion techniques.

Immersion lithography
At the heart of immersion lithography is the use of a liquid with a high refractive index (typically deionized water) to replace the air gap between the projection objective and the wafer. The wavelength of light in the ArF lithography machine is 193nm, and the refractive index n: air = 1, water = 1.44, which means that the refractive angle of the light emitted from the projection objective lens will be significantly reduced after entering the aqueous medium. This change allows more higher-order diffraction components to be involved in the imaging process, which effectively improves the lithography resolution. Specifically, the original wavelength of 193nm ArF light is changed to 134nm in water, which effectively reduces the wavelength, which is not only lower than the 157nm of F2 excimer light source, but also more compatible with existing manufacturing processes.

Advantageous Resolution Improvement: The resolution of the lithography machine has been significantly improved through immersion technology, making it possible to manufacture chips with smaller feature sizes. Cost-effective: Immersion lithography is less expensive and easier to apply to existing chip fabrications than using shorter wavelength light sources such as F2 excimer light sources. Technology maturity: Immersion lithography technology has been verified by practice for many years, and the technology is more mature and stable.vvvvvvv