X-ray lithography

X-ray lithography is a process used in semiconductor device fabrication industry to selectively remove parts of a thin film of photoresist.

It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist, or simply "resist," on the substrate to reach extremely small topological size of a feature.

Having short wavelengths (below 1 nm), X-rays overcome the diffraction limits of optical lithography, allowing smaller feature sizes.

The pattern on the mask is written by direct-write electron beam lithography onto a resist that is developed by conventional semiconductor processes.

Most X-ray lithography demonstrations have been performed by copying with image fidelity (without magnification) on the line of fuzzy contrast as illustrated in the figure.

However, with the increasing need for high resolution, X-ray lithography is now performed on what is called the "sweet spot", using local "demagnification by bias".

The sidewall roughness and slopes are influenced by these secondary electrons as they can travel few micrometers in the area under the absorber, depending on exposure X-ray energy.

[5] Another manifestation of the photoelectron effect is exposure to X-ray generated electrons from thick gold films used for making daughter masks.

On the other hand, the secondaries follow a different trend below ≈30 eV: the lower the energy, the longer the mean free path though they are not then able to affect resist development.

What matters for X-ray lithography is the effective range of electrons that have sufficient energy to make or break chemical bonds in negative or positive resists.

Insulating films like gate oxides and resists have been observed to charge to a positive or negative potential under electron-beam irradiation.

Insulating films are eventually neutralized locally by space charge (electrons entering and exiting the surface) at the resist-vacuum interface and Fowler-Nordheim injection from the substrate.