Everything about X-ray Lithography totally explained
X-ray lithography is a next generation
lithography that has been developed for the
semiconductor industry. Batches of
microprocessors have already been produced.
The short
wavelengths of 0.8 nm X-rays overcome
diffraction limits in the resolution of otherwise competitive
optical lithography. The X-rays illuminate a mask placed in proximity to a resist-coated wafer. No lenses are used, and only rudimentary
collimating mirrors. The X-rays are broadband, typically from a compact
synchrotron radiation source, allowing rapid exposure.
Deep X-ray lithography uses yet shorter wavelengths, about 0.1 nm with modified procedures, to fabricate deeper structures, sometimes three dimensional, with reduced resolution.
The mask consists of an X-ray absorber, typically of gold or compounds of tantalum or tungsten, on a membrane that's transparent to X-rays, typically of silicon carbide or diamond. The pattern on the mask is written by direct write electron beam lithography onto a resist that's developed by conventional semiconductor processes. The membrane can be stretched for overlay accuracy.
Most X-ray lithography demonstrations have been performed by copying with image fidelity, for example without magnification, 1x, on the line of fuzzy contrast as illustrated in the figure. But with the increasing need for high resolution, X-ray lithography is now performed on the Sweet Spot, using local “demagnification by bias .” Dense structures are developed by multiple exposures with translation. Many advantages accrue from the application of 3x “demagnification": the mask is more easily fabricated; the mask to wafer gap is increased; and the contrast is higher. The technique is extensible to dense 15 nm prints. The resulting printing has high contrast.
X-rays generate secondary electrons as in the cases of
extreme ultraviolet lithography and
electron beam lithography. While the fine pattern definition is due principally to secondaries from Auger electrons with a short path length, the primary electrons will sensitize the resist over a larger region than the X-ray exposure. While this doesn't affect the pattern pitch resolution (determined by wavelength and gap), the image exposure contrast
is reduced since the pitch is on the order of the primary photo-electron range. The sidewall roughness and slopes are influenced by these secondary electrons as they can travel few microns in the area under the absorber; depending on exposure x-ray energy. Several prints at about 20 nm have been published.
Another manifestation of the photoelectron effect is exposure to X-ray generated electrons from thick gold films used for making daughter masks. Simulations suggest that photoelectron generation from the gold substrate may affect dissolution rates.
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