Fabrication of X-ray Gratings for Interferometric Imaging by Conformal Seedless Gold Electroplating

• Main Authors: Konstantins Jefimovs, Joan Vila-Comamala, Carolina Arboleda, Zhentian Wang, Lucia Romano, Zhitian Shi, Matias Kagias, Marco Stampanoni
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Micromachines
• Subjects: X-ray interferometry; phase contrast imaging; grating; high aspect ratio; deep reactive ion etching; Bosch process
• Online Access: https://www.mdpi.com/2072-666X/12/5/517

We present a method to produce small pitch gratings for X-ray interferometric imaging applications, allowing the phase sensitivity to be increased and/or the length of the laboratory setup to be minimized. The method is based on fabrication of high aspect ratio silicon microstructures using deep reactive ion etching (Bosch technique) of dense grating arrays and followed by conformal electroplating of Au. We demonstrated that low resistivity Si substrates (<0.01 Ohm·cm) enable the metal seeding layer deposition step to be avoided, which is normally required to initiate the electroplating process. Etching conditions were optimized to realize Si recess structures with a slight bottom tapering, which ensured the void-free Au filling of the trenches. Vapor HF was used to remove the native oxide layer from the Si grating surface prior to electroplating in the cyanide-based Au electrolyte. Fabrication of Au gratings with pitch in the range 1.2–3.0 µm was successfully realized. A substantial improved aspect ratio of 45:1 for a pitch size of 1.2 µm was achieved with respect to the prior art on 4-inch wafer-based technology. The fabricated Au gratings were tested with X-ray interferometers in Talbot–Laue configuration with measured visibility of 13% at an X-ray design energy of 26 keV.