Precision Au Electrocoating of Double Sided Aligned Si Gratings for High Energy X-Ray Phase Contrast Imaging
Authors: Hollowell Andrew E , Arrington Christian L. , Finnegan Patrick Sean , McClain Jaime , Musick Kate , Resnick Paul , Volk Steven , Amber Dagel L 
Published: February 2020

State of the art grating fabrication currently limits the maximum source energy that can be used in lab based x-ray phase contrast imaging (XPCI) systems. In order to move to higher source energies, and image thicker and more dense materials, new grating fabrication methods are needed. In previous work we developed a differentiating mold construction technique that replaces photodefinable organic templates1 with high aspect ratio silicon (Si) deep reactive ion etching (DRIE) followed by precision electrodeposition of gold (Au)2. Combining DRIE with precision electro-coating enabled the fabrication of un-supported Au absorption gratings with aspect ratios as high as 50:1 and grating areas as large as 100 cm2. Further pushing this grating fabrication to enable the use of higher energy x-ray sources, we developed a new modality for grating fabrication that involves precision alignment of etched features on both sides of a substrate, effectively doubling the thickness of the. This double sided grating fabrication approach was applied to make a phase grating out of DRIE Si. In this work, we couple double sided Si DRIE with precision gold (Au) electrodeposition to fabricate absorption gratings for high energy lab based XPCI.

A 250 µm deep, 2 µm wide, and 4 µm pitch absorption grating was fabricated by aligning and etching 125 µm deep grating features on both sides of a Si wafer. The Si grating features are patterned and etched on an 8 µm pitch with the width of the Si features patterned at 2 µm. Fabricating uniform, high aspect ratio features across large areas on both sides of a substrate is difficult due to the need to temporarily protect one side of the grating while processing on the opposite side of the substrate. After both sides are processed, this temporary support material must be removed without damaging the gratings on either side of the substrate. At these feature sizes, surface tension forces after wet processing are strong enough to collapse the grating features and must be carefully avoided.

Our integration approach for accommodating these challenges is illustrated in Figure 1. Alignment features are first patterned and etched into both sides of the substrate. The front side gratings are then patterned with reference to the front side field image alignment marks and etched into the Si. Plasma enhanced tetraethylorthosilicate (PETEOS) SiO2 is then deposited to protect the front side gratings during backside processing. The backside gratings are similarly aligned using the alignment marks on the backside of the substrate. After the gratings on both sides of the substrate are etched, a selective wet etch is used to remove the supportive oxide and a critical point dryer is used to prevent surface tension forces from damaging the Si bars. A conformal platinum (Pt) plating seed metal is deposited by atomic layer deposition (ALD) and Au is precisely electroplated at a uniform thickness of 2 μm throughout the depth of the Si grating. The Au serves as a high x-ray absorbing material finalizing the fabrication process. In this work, we provide details on our fabrication approach and measurements of our final grating geometries.

Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

[1] - High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging: J. Mohr, T. Grund, D. Kunka, J. Kenntner, J. Leuthold, J. Meiser, J. Schulz and M. Walter, International Workshop on X-Ray and Neutron Phase Imaging with Gratings, Tokyo, Japan, 2012;

[2] - Extensively long high aspect ratio gold analyzer gratings: A. E., Hollowell, C. L. Arrington, J. J. Coleman, P. S. Finnegan, A. M. Rowen, and A. L. Dagel, X-ray and Neutron Phase Imaging with Gratings, Bethesda, MD, 2015

[3] – Double Sided Grating Fabrication for High Energy X-Ray Phase Contrast Imaging: A. E. Hollowell, C. L. Arrington, P. Finnegan, K. Musick, P. Resnick, S. Volk, and A. L. Dagel, Material Science in Semiconductor Processing, 2018, under review.

Figure 1