Nanofabricated Reflection Gratings

Grazing-incidence x-ray reflection gratings are an important component of advanced highresolution spectrometers and other x-ray optics. These have traditionally been fabricated by diamond scribing with a ruling engine or by interference lithography followed by ion etching. These methods result in gratings which suffer from a number of deficiencies, including high surface roughness and poor groove profile control, leading to poor diffraction efficiency and large amounts of scattered light.

We are developing improved methods for fabricating blazed x-ray reflection gratings which utilize special (111) silicon wafers, cut 0.5-10 degrees off the (111) plane. Silicon anisotropic etching solutions, such as potassium hydroxide (KOH), etch the (111) planes very slow compared to other crystallographic directions, resulting in the desired super-smooth groove surface. Previous work used similar off-cut (111) silicon substrates to fabricate blazed diffraction gratings, but utilized a second KOH etch step that compromised the grating facet flatness and is unsuitable for small grazing-angle x-ray diffraction.

Gratings are patterned using interference lithography with the lambda=351.1 nm wavelength, and transferred into substrates using tri-level resist processing, reactive-ion etching (RIE), and silicon-nitride masking during the KOH etch. The narrow (~100 nm) ridge of silicon which supports the nitride mask is removed using a novel chromium lift-off step followed by a CF4 RIE. The result is extremely smooth sawtooth patterns, which, after applying a thin evaporative coating of Cr/Au, are suitable for x-ray reflection.

We are also developing UV and thermal nanoimprint lithography for low cost production of gratings. We have demonstrated the extremely high fidelity of the nanoimprint replication process, where added surface roughness is < 0.2 nm.

Potential applications of these improved gratings are for materials science research with synchrotron radiation and satellite-based high-resolution x-ray spectroscopy for planned NASA missions such as Constellation X. We are also exploring other applications for this technology, including telecom devices and atom microscopy.