Sandia National Laboratories has developed extremely high aspect ratio, small feature diffraction gratings. Rising interest in non-destructive imaging using X-ray phase contrast imaging (XPCI), has created a need to produce large 100 cm² area, small 2 µm dimension, high aspect ratio (> 50:1) x-ray absorbing, metal diffraction gratings. Lab based XPCI systems have been primarily the interest of the medical community but have excited other industries with the ability to detect defects in low density materials. Imaging higher density materials or thicker samples requires higher x-ray energies. Higher x-ray energies require thicker gratings, increasing the already challenging aspect ratio demands on the three gratings in an XPCI system. Our work investigates anisotropic aqueous etching of monocrystalline silicon, in conjunction with precision electroformed gold coatings to fabricate such gratings. We show methods for determining Si crystalline plane directions and techniques to achieve high alignment accuracy of an etch mask to these crystalline planes. This enables a deep potassium hydroxide etching, which we follow with a precision electroplating process over a ridged Si template to achieve small dimension, deep anisotropic 1D trenches. Several observations are made regarding the difficulty of accurate alignment to silicon crystalline planes for high accuracy, with subsequent electroformed gold coatings, to achieve optimal pitch and grating feature dimensions. Figure 1 shows the result of aspect ratios demonstrated at 80:1, with a precision electroformed gold coating. Improvements to alignment features and etch process are being studied and show promise to continue increasing accuracy for higher aspect ratio gratings to increase energy exposures in XPCI systems.

References

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

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

[3] - D. Kendall, “Vertical etching of Silicon at Very High Aspect Ratios,” in Ann. Rev. Mater. Sci. 1979.9: 373-403.

Sandia National Laboratories is 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. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Figure 1: a) 80:1 High aspect ratio KOH etched silicon trench 159 mm depth b) Magnified SEM image showing 2 mm features with an 8 µm grating pitch c) FIB image of precision electroformed gold coating over ridged silicon template