High-aspect ratio x-ray absorption gratings are required for hard x-ray or high-resolution soft x-ray phasecontrast imaging (XPCi) applications. X-ray gratings play a central role in grating-based XPCi techniques, where visibility – image quality – is highly dependent on the quality of gratings. Although the fabrication process of x-ray gratings for soft x-ray imaging is well-developed, there are technological challenges in highresolution hard x-ray imaging that impede us from making high-aspect ratio large field-of-view fine x-ray gratings through conventional fabrication processes. Here we present a design and a fabrication process to fabricate highaspect ratio gratings that benefit from a self-aligned hard-mask – a patterned chromium-gold-chromium thin film deposited on a transparent ITO-on-glass substrate – which facilitates both lithography and electroplating processes. The repeatability of the proposed method makes it suitable for achieving high-aspect ratio fine structures as thick as desired through a multi-layer structure without any restraint or limitation on the aspect ratio of features. The multi-layer structure design overcomes the aspect ratio limitation associated with UVlithography. Although this method works well with X-ray lithography, the key advantage of this design is that it enables UV lithography for high-aspect ratio grating fabrication through a reliable yet simple process. To the best of our knowledge, self-aligned multi-layer SU-8 based grating design has not been previously reported. The proposed design and fabrication process help researchers further develop x-ray gratings performance to facilitate high-resolution coded-aperture and Talbot-Lau high-energy x-ray phase-contrast imaging.
X-ray absorption gratings are the heart of coded-aperture and Talbot-Lau x-ray phase-contrast imaging (XPCi) techniques. The quality of imaging – visibility – in the aforementioned techniques is highly dependent on the quality of gratings. SU-8, an epoxy-based photoresist, is valued in Micro-Electro-Mechanical-Systems (MEMS) fabrication due to its excellent mechanical and optical properties. For fabricating x-ray absorption grating following the MEMS process, gold is the material most widely used to stop incident x-rays. One of the main challenges in fabricating a quality x-ray absorption grating is the adhesion between SU-8 molds and gold thin film, where due to poor adhesion of SU-8 to gold thin films and SU-8’s internal high level of stress, SU-8 molds delaminate, crack, or peel off during developing or electroplating steps. This problem becomes more challenging when a greater field of view gratings are required. Employing SU-8 on a gold substrate requires an adhesion layer to assure SU-8’s mechanical stability during fabrication steps. Without this layer, realizing a high-quality grating is challenging. In this work, we have investigated the adhesion quality of SU-8 to gold thin films for different adhesion promoter layers. We have employed a combination of a SU-8 thin film and a metallic-silane-based nanometer-thin film to improve the adhesion quality between SU-8 photoresist and a gold thin film substrate. We have tested the quality of adhesion using tape test and through a harsh (highly acidic) electroplating solution.
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