Wavelength very low - lower than a few nm

Resist, usually PMMA, is exposed through an x-ray mask in proximity or in contact with the wafer.

This technique has the advantage that very high aspect ratios can be achieved, since the x-rays are highly penetrating.

The wavelength of an x-ray is given by λ = 1.2/E(keV), so one would expect higher energies to be more desirable. However high energies can mean also damage on the film, special resist, etc.

Blur, caused by the excitation of photoelectrons and the associated shower of secondary electrons limits the patterning ability to about 20 nm.

XRL has been applied to MEMS, where the high aspect ratio is a distinct advantage.

Difficult to make optics for X-ray

Problems: