The keys to extending anisotropic KOH silicon etching to higher aspect ratio features

Anisotropic etching silicon (Si) is an active area of research for applications including energy storage (Enovix), energy conversion (photovoltaics), and now for x-ray phase contrast imaging (XPCI) diffraction gratings. Previously, a lack of control over precise alignment of the etch pattern to crystal planes and the constant evolution of hydrogen bubbles inhibited uniformity and limited the potential for higher aspect ratios. Sandia National Laboratories has made significant advancements in anisotropic silicon etching, including improvements to accurate crystal alignment using new equipment capabilities and methods of liberating hydrogen bubbles trapped in deep trenches. The possibility of reaching aspect ratios of 600:1 using anisotropic wet etching in Si have been cited in literature, but we have found no evidence of such aspect ratios being achieved. Our process is focused on improvements to yield, better anisotropy and uniformity, enabling gratings with aspect ratios as high as 170:1. The well-defined sharp edges and deep trenches that can be achieved using this technique make it a suitable method for optical grating fabrication. Deeper trenches support pushing XPCI to higher x-ray energies, which will allow access to imaging thicker or denser samples, or improved image contrast at lower energies. Higher aspect ratios in the gratings will better improve sensitivity and enable higher energy systems.