Whole-body x-ray dark-field radiography of a human cadaver

Extreme Bottom-up Gold Filling of High Aspect Ratio Features

ConspectusWhere copper interconnects fabricated using superconformal electrodeposition processes have enabled dramatic advances in microelectronics over the past quarter century, gold filled gratings fabricated using superconformal Bi³⁺-mediated bottom-up filling electrodeposition processes promise to enable a new generation of X-ray imaging and microsystem technologies. Indeed, bottom-up Au-filled gratings have demonstrated excellent performance in X-ray phase contrast imaging of biological soft tissue and other low Z element samples even as studies using gratings with inferior Au fill have captured the potential for broader biomedical application. Four years ago, the Bi-stimulated bottom-up Au electrodeposition process was a scientific novelty where gold deposition was localized entirely on the bottoms of metallized trenches 3-μm-deep and 2-μm-wide, an aspect ratio of only 1.5, on centimeter scale fragments of patterned silicon wafers. Today the room-temperature processes routinely yield uniformly void-free filling of metallized trenches 60-μm-deep and 1-μm-wide, an aspect ratio 60, in gratings patterned across 100 mm Si wafers. Four distinctive characteristics of the evolution of void-free filling in the Bi³⁺-containing electrolyte are seen in experimental Au filling of fully metallized recessed features such as trenches and vias: (1) an "incubation period" of conformal deposition, (2) subsequent Bi-activated deposition localized on the bottom surface of features, (3) sustained bottom-up deposition that yields void-free filling, and (4) self-passivation of the active growth front at a distance from the feature opening defined by operating conditions. A recent model captures and explains all four features. The electrolyte solutions are simple and nontoxic, being near-neutral pH and composed of Na₃Au(SO₃)₂ + Na₂SO₃ containing micromolar concentrations of Bi³⁺ additive, the latter generally introduced through electrodissolution from the metal. The influences of additive concentration, metal ion concentration, electrolyte pH, convection, and applied potential have been examined in some depth using both electroanalytical measurements on planar rotating disk electrodes and studies of feature filling, thereby defining and elucidating relatively wide processing windows for defect-free filling. The process control for bottom-up Au filling processes is observed to be quite flexible, with online changes of potential as well as concentration and pH adjustments during the course of filling compatible with processing. Furthermore, monitoring has enabled optimization of the filling evolution, including to shorten the incubation period for accelerated filling and to fill features of ever higher aspect ratio. The results to date indicate that the demonstrated filling of trenches with an aspect ratio of 60 represents a lower bound, a value determined only by the features presently available.

Self-aligned multi-layer X-ray absorption grating using large-area fabrication methods for X-ray phase-contrast imaging

X-ray phase-contrast (XPCi) imaging methods are an emerging medical imaging approach that provide significantly better soft tissue contrast and could function as a viable extension to conventional X-ray, CT, and even some MRI. Absorption gratings play a central role in grating-based XPCi systems, especially because they enable the acquisition of three images in a single exposure: transmission, refraction, and dark-field. An impediment to commercial development and adoption of XPCi imaging systems is the lack of large area, high aspect ratio absorption gratings. Grating technology development, primarily due to technological limitations, has lagged system development and today prevents the scaling up of XPCi system into a footprint and price point acceptable to the medical market. In this work, we report on a self-aligned multi-layer grating fabrication process that can enable large-area X-ray absorption gratings with micron-scale feature sizes. We leverage large-area fabrication techniques commonly employed by the thin-film transistor (TFT) display industry. Conventional ITO-on-glass substrates are used with a patterned film of Cr/Au/Cr that serves as a self-aligned lithography mask for backside exposure. Commonly available SU-8 photoresist is patterned using the backside exposure mask followed by an electroplating step to fill the gaps in the SU-8 with X-ray attenuating material. Consequently, the electroplated patterned material acts as a self-aligned photomask for subsequent SU-8 layer patterning and so forth. The repeatability of the reported process makes it suitable for achieving higher aspect ratio structures and is advantageous over previously reported X-ray LIGA approaches. A prototype three-layer grating, with a thickness of around [Formula: see text], having a visibility of 0.28 at [Formula: see text] with a [Formula: see text] active area was fabricated on a 4-inch glass substrate and demonstrated by modifying a commercially available 3D propagation-based XPCi Microscope. The scalable and cost-effective approach to build larger area X-ray gratings reported in this work can help expedite the commercial development and adoption of previously reported Talbot-Lau, speckle-tracking, as well as coded-aperture XPCi systems for large-area clinical and industrial applications.

Mechanism of Bismuth Stimulated Bottom-up Gold Feature Filling

The mechanism underlying Bi ³⁺-stimulated bottom-up Au filling and self-passivation in trenches and vias in slightly alkaline Na ₃ Au(SO ₃)₂ + Na ₂ SO ₃ electrolytes is explored. The impacts of electrolyte components Na ₃ Au(SO ₃)₂, Na ₂ SO ₃ and Bi ³⁺ and potential-dependent kinetic factors on the rate of Au electrodeposition are quantified experimentally. Derived parameters are applied within the surfactant conservation Curvature Enhanced Accelerator Coverage model to simulate the filling of high aspect ratio trenches. It is observed that Bi adsorption accelerates the Au deposition rate with a non-linear dependence occurring around a critical coverage. Further, the impact of electrolyte composition is such that gradients of A u ( S O 3 ) 2 3 - and S O 3 2 - derived from reduction of A u ( S O 3 ) 2 3 - during deposition accentuate deposition farther from the feature opening. These factors and surface area reduction at the bottoms of filling features localize active deposition to feature bottoms. Ultimately, weakening of the concentration gradients and associated kinetics as bottom-up feature filling progresses reduces the Bi coverage on the growth front below the critical value and bottom-up growth terminates. Good agreement is observed with key experimental features including the incubation period of conformal deposition, transition to bottom-up growth, subsequent bottom-up filling and finally self-passivation as the growth front nears the field.

Robust Bottom-Up Gold Filling of Deep Trenches and Gratings

This work extends an extreme variant of superconformal Au electrodeposition to deeper device architectures while exploring factors that constrain its function and the robustness of void-free processing. The unconventional bottom-up process is used to fill diffraction gratings with trenches 94 μm deep and 305 μm deep, with aspect ratios (height/width) of just below 20 and 15, respectively, in near-neutral 0.16 mol·L^-1 Na₃Au(SO₃)₂ + 0.64 mol·L^-1 Na₂SO₃ electrolyte containing 50 μmol·L^-1 Bi³⁺. Although the aspect ratios are modest compared to previously demonstrated void-free filling beyond AR = 60, the deepest trenches filled exceed those in previous work by 100 μm - a nearly 50 % increase in depth. Processes that substantially accelerate the start of bottom-up deposition demonstrate a linkage between transport and void-free filling. Final profiles are highly uniform across 65 mm square gratings because of self-passivation inherent in the process. Electron microscopy and electron backscatter diffraction confirm the fully dense Au and void-free filling suggested by the electrochemical measurements. X-ray transmission "fringe visibility" average more than 80 % at 50 kV X-ray tube voltage across the deeper gratings and 70 % at 40 kV across the shallower gratings, also consistent with uniformly dense, void-free fill across the gratings.

Edge-illumination x-ray phase-contrast imaging

Although early demonstration dates back to the mid-sixties, x-ray phase-contrast imaging (XPCI) became hugely popular in the mid-90s, thanks to the advent of 3rd generation synchrotron facilities. Its ability to reveal object features that had so far been considered invisible to x-rays immediately suggested great potential for applications across the life and the physical sciences, and an increasing number of groups worldwide started experimenting with it. At that time, it looked like a synchrotron facility was strictly necessary to perform XPCI with some degree of efficiency-the only alternative being micro-focal sources, the limited flux of which imposed excessively long exposure times. However, new approaches emerged in the mid-00s that overcame this limitation, and allowed XPCI implementations with conventional, non-micro-focal x-ray sources. One of these approaches showing particular promise for 'real-world' applications is edge-illumination XPCI: this article describes the key steps in its evolution in the context of contemporary developments in XPCI research, and presents its current state-of-the-art, especially in terms of transition towards practical applications.