Potassium hydroxide polishing of nanoscale deep reactive-ion etched ultrahigh aspect ratio gratings

A fabrication process has been developed to chemically polish the sidewalls of 200 nm-pitch gratings via potassium hydroxide (KOH) etching following the Bosch deep reactive-ion etching (DRIE) process. Previous KOH polishing experiments focused on micron scale features. This work is the first reported combined DRIE-KOH etching process on the nanoscale for ultrahigh aspect ratio structures with feature sizes 30 times smaller than previously published work. The primary application of the gratings is x-ray spectroscopy and requires polished sidewalls for efficient x-ray reflection. Polishing is also critical for increasing the open area by narrowing the grating bars, which increases the throughput of x-rays. The increased open area is also valuable for other applications such as ultraviolet filtration, neutron spectroscopy and biofiltration. Advanced Bosch processes leave approximately 4 nm, root mean square (RMS), of roughness on the sidewalls. This roughness needs to be reduced to below 1 nm to efficiently reflect soft x-rays with wavelengths between 1 and 5 nm. Furthermore, high aspect ratio DRIE can result in bar width variations of approximately a factor of two from the top to the middle of the channel, commonly referred to as bowing. The polishing procedure presented here removes the roughness to below the resolution of the scanning electron microscope, and was measured via an atomic force microscope to be less than 1 nm RMS. The bowing has also been reduced by at least a factor of 3. The polishing process takes advantage of the anisotropy of KOH silicon etching. Specifically, the {111} silicon planes etch approximately 100 times slower than other crystal planes. This anisotropy allows the grating bars to be etched in 50% by weight KOH at room temperature for up to 60 min. Long etches have several key requirements, including 0.2 degree alignment of the grating with respect to the {111} planes, mask roughness below 40 nm and minimal defects in the silicon. If these requirements are not met, the grating will quickly be destroyed by the etch, which etches the non-{111} planes in excess of 1 μm per hour. The fabrication steps of this work are described in detail including a novel technique to align the 200 nm-pitch interference lithography image grating to the {111} planes of a ⟨110⟩ silicon wafer.