Rossi, Alessandro; (2025) Fabricating the X-ray imaging of the future: innovative fabrication techniques for X-ray phase contrast imaging. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

X-ray phase contrast imaging (XPCI) offers a compelling alternative for revealing details typically hidden in conventional attenuation-based imaging. This capability arises because, in the X-ray regime, the unit decrement of the real part of the refractive index of most objects is significantly larger than the imaginary part. Historically, the application of XPCI in standard laboratories has been made feasible by incorporating phase and absorption gratings into the imaging system. These optical devices are crucial for generating a resolvable signal on the detector, with the field experiencing considerable advancements due to the use of gratings composed of arrays of X-ray transparent lamellae (e.g., Si, graphite, SU8) and opaque septa (e.g., Au). The fabrication of these devices poses challenges due to the required high aspect ratios (often > 20) and density of the gold deposit. In this work, I explore two novel research paths aimed at enhancing contrast in XPCI through nanofabrication of phase and absorption gratings followed by validation by in-house developed wave optics algorithms. First, I demonstrate the adaptation of XPCI to higher energies using a pair of ultra-thick absorption gratings (≃ 500 μm). In this approach, I utilized dry reactive etching and employed Bi-mediated plating to achieve complete bottom-up filling of the trenches. These gratings facilitated imaging at energies exceeding 150 keV, offering higher phase sensitivity compared to thinner gratings and paving the way for higher energy Xray phase contrast imaging. Second, I investigated increasing X-ray phase sensitivity using a pair of submicron-period phase gratings to produce a moiré signal in the far field. Our fabrication process combined photolithography, nanoimprinting lithography, wet etching, and metallization through PVD and electroplating. To prevent the collapse of lamellae post-wet etching, a HMDS-based drying step was introduced. The resulting gratings demonstrated that an achromatic moir´e signal could be generated in the far field, potentially enhancing imaging capabilities.

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