Robust Bottom-Up Gold Filling of Deep Trenches and Gratings

A four-grating interferometer for x-ray multi-contrast imaging

BACKGROUND

X-ray multi-contrast imaging with gratings provides a practical method to detect differential phase and dark-field contrast images in addition to the x-ray absorption image traditionally obtained in laboratory or hospital environments. Systems have been developed for preclinical applications in areas including breast imaging, lung imaging, rheumatoid arthritis hand imaging and kidney stone imaging.

PURPOSE

Prevailing x-ray interferometers for multi-contrast imaging include Talbot-Lau interferometers and universal moiré effect-based phase-grating interferometers. Talbot-Lau interferometers suffer from conflict between high interferometer sensitivity and large field of view (FOV) of the object being imaged. A small period analyzer grating is necessary to simultaneously achieve high sensitivity and large FOV within a compact imaging system but is technically challenging to produce for high x-ray energies. Phase-grating interferometers suffer from an intrinsic fringe period ranging from a few micrometers to several hundred micrometers that can hardly be resolved by large area flat panel x-ray detectors. The purpose of this work is to introduce a four-grating x-ray interferometer that simultaneously allows high sensitivity and large FOV, without the need for a small period analyzer grating.

METHODS

The four-grating interferometer consists of a source grating placed downstream of and close to the x-ray source, a pair of phase gratings separated by a fixed distance placed downstream of the source grating, and an analyzer grating placed upstream of and close to the x-ray detector. The object to be imaged is placed upstream of and close to the phase-grating pair. The distance between the source grating and the phase-grating pair is designed to be far larger than that between the phase-grating pair and the analyzer grating to promote simultaneously high sensitivity and large FOV. The method was evaluated by constructing a four-grating interferometer with an 8 µm period source grating, a pair of phase gratings of 2.4 µm period, and an 8 µm period analyzer grating.

RESULTS

The fringe visibility of the four-grating interferometer was measured to be ≈24% at 40 kV and ≈18% at 50 kV x-ray tube operating voltage. A quartz bead of 6 mm diameter was imaged to compare the theoretical and experimental phase contrast signal with good agreement. Kidney stone specimens were imaged to demonstrate the potential of such a system for classification of kidney stones.

CONCLUSIONS

The proposed four-grating interferometer geometry enables a compact x-ray multi-contrast imaging system with simultaneously high sensitivity and large FOV. Relaxation of the requirement for a small period analyzer grating makes it particularly suitable for high x-ray energy applications such as abdomen and chest imaging.

Emission Ghost Imaging: reconstruction with data augmentation

Ghost Imaging enables 2D reconstruction of an object even though particles transmitted or emitted by the object of interest are detected with a single pixel detector without spatial resolution. This is possible because for the particular implementation of ghost imaging presented here, the incident beam is spatially modulated with a non-configurable attenuating mask whose orientation is varied (e.g. via transverse displacement or rotation) in the course of the ghost imaging experiment. Each orientation yields a distinct spatial pattern in the attenuated beam. In many cases, ghost imaging reconstructions can be dramatically improved by factoring the measurement matrix which consists of measured attenuated incident radiation for each of many orientations of the mask at each pixel to be reconstructed as the product of an orthonormal matrix Q and an upper triangular matrix R provided that the number of orientations of the mask (N ) is greater than or equal to the number of pixels (P ) reconstructed. For the N < P case, we present a data augmentation method that enables QR factorization of the measurement matrix. To suppress noise in the reconstruction, we determine the Moore-Penrose pseudoinverse of the measurement matrix with a truncated singular value decomposition approach. Since the resulting reconstruction is still noisy, we denoise it with the Adaptive Weights Smoothing method. In simulation experiments, our method outperforms a modification of an existing alternative orthogonalization method where rows of the measurement matrix are orthogonalized by the Gram-Schmidt method. We apply our ghost imaging methods to experimental X-ray fluorescence data acquired at Brookhaven National Laboratory.

Implementation of a dual-phase grating interferometer for multi-scale characterization of building materials by tunable dark-field imaging

The multi-scale characterization of building materials is necessary to understand complex mechanical processes, with the goal of developing new more sustainable materials. To that end, imaging methods are often used in materials science to characterize the microscale. However, these methods compromise the volume of interest to achieve a higher resolution. Dark-field (DF) contrast imaging is being investigated to characterize building materials in length scales smaller than the resolution of the imaging system, allowing a direct comparison of features in the nano-scale range and overcoming the scale limitations of the established characterization methods. This work extends the implementation of a dual-phase X-ray grating interferometer (DP-XGI) for DF imaging in a lab-based setup. The interferometer was developed to operate at two different design energies of 22.0 keV and 40.8 keV and was designed to characterize nanoscale-size features in millimeter-sized material samples. The good performance of the interferometer in the low energy range (LER) is demonstrated by the DF retrieval of natural wood samples. In addition, a high energy range (HER) configuration is proposed, resulting in higher mean visibility and good sensitivity over a wider range of correlation lengths in the nanoscale range. Its potential for the characterization of mineral building materials is illustrated by the DF imaging of a Ketton limestone. Additionally, the capability of the DP-XGI to differentiate features in the nanoscale range is proven with the dark-field of Silica nanoparticles at different correlation lengths of calibrated sizes of 106 nm, 261 nm, and 507 nm.

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.

Survey of P-Block Metal Additives for Superconformal Cu Deposition in an Alkaline Electrolyte

Catalysis of Cu deposition from a near-neutral Cu2+ complexed electrolyte is examined using Bi3+, Pb2+ and Tl+ additives that were selected based on their known ability to accelerate Au deposition in near neutral pH gold sulfite electrolytes. Where appropriate, the ability of these electrolytes to yield superconformal filling of recessed features is also briefly examined. Voltammetry reveals strong acceleration of Cu deposition by Bi3+ additions while indication of superconformal filling accompanied by unusual microstructural transitions are evident in cross-sectioned specimens examined by scanning electron microscopy. Results are discussed in the context of behaviors observed for the same heavy metal additives in gold sulfite electrolytes.

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.