Electroformed Fabrication of Extremely High Aspect Ratio Diffraction Gratings for X-Ray Phase Contrast Imaging

Abstract

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.