Surface terminations critically govern the properties of two-dimensional transition metal carbides and/or nitrides (MXenes), yet a universal strategy to obtain MXenes with uniform and controllable terminations remains elusive. Here we introduce a ‘gas–liquid–solid’ triphasic etching strategy that employs iodine vapour, halide molten salts and MAX phases to produce MXenes with pure and precisely tunable halogen terminations (Cl, Br, I or their combinations). In this process, halide molten salts dissolve iodine via interhalogen anion formation while efficiently transporting etching by-products. The resulting MXenes retain excellent structural integrity, yielding uniformly ordered surfaces. As a representative example, Ti3C2Cl2 shows a 160-fold enhancement in macroscopic conductivity and a 13-fold enhancement in terahertz conductivity relative to conventional Cl/O-terminated Ti3C2, attributed to minimized electron trapping and scattering. Beyond single-halogen terminations, the gas–liquid–solid approach enables dual- and triple-halogen termination control, providing a general platform for tailoring MXene surface chemistry towards advanced (opto)electronic applications. A gas–liquid–solid triphasic etching strategy is demonstrated to obtain MXenes with uniform and tunable halogen terminations. The resulting MXenes have highly ordered structures, enhanced charge transport properties and programmable surface chemistry for advanced (opto)electronic applications.

Linear conducting polymers show ballistic transport, imposed by mobile carriers moving along the polymer chains1,2, whereas conductance in the extended dimension, that is, between polymer strands or layers, remains weak due to the lack of intermolecular ordering and electronic coupling3-5. Here we report a multilayer-stacked two-dimensional polyaniline (2DPANI) crystal, which shows metallic out-of-plane charge transport with high electrical conductivity. The material comprises columnar π arrays with an interlayer distance of 3.59 Å and periodic rhombohedral lattices formed by interwoven polyaniline chains. Electron spin resonance spectroscopy reveals significant electron delocalization in the 2DPANI lattices. First-principles calculations indicate the in-plane 2D conjugation and strong interlayer electronic coupling in 2DPANI facilitated by the Cl-bridged layer stacking. To assess the local optical conductivity, we used terahertz and infrared nanospectroscopy to unravel a Drude-type conductivity with an infrared plasma frequency and an extrapolated local d.c. conductivity of around 200 S cm-1. Conductive scanning probe microscopy showed an unusually high out-of-plane conductivity of roughly 15 S cm-1. Transport measurements through vertical and lateral micro-devices revealed comparable high out-of-plane (roughly 7 S cm-1) and in-plane conductivity (roughly 16 S cm-1). The vertical micro-devices further showed increasing conductivity with decreasing temperature, demonstrating unique out-of-plane metallic transport behaviour. By using this multilayer-stacked 2D conducting polymer design, we predict the achievement of strong electronic coupling beyond in-plane interactions, potentially reaching three-dimensional metallic conductivity6,7.

Microcrack propagation through a BEoL stack is visualized by positioning a miniaturized micro double cantilever beam test in the beam path of a laboratory full-field X-ray microscope. Imaging the propagation of microcracks with sub-100nm resolution, the controlled steering of microcracks in the BEoL stack of dies manufactured in advanced technology nodes is demonstrated. The capability to steer microcrack propagation in a controlled way and to observe this process simultaneously provides a powerful tool for studying the efficacy of guard rings and of upper metallization layers in the BEoL stack, where the Cu plasticity is toughening the region due to size effects, to arrest cracks. In addition, the experiments can provide valuable input for the design of guard rings. This new approach has important implications for risk mitigation strategies in semiconductor industry and for the manufacturing of silicon dies used as chiplets in heterogeneous integration/advanced packaging for reliable and sustainable products.

Diatoms are widely studied biological objects because of their large variety of geometric shapes and their unique physical and chemical properties. They survive widely in nature within moisture. Imaging the diatoms three dimensionally in moisture and correlating their mechanical behavior is an interesting and challenging topic. Here, the morphology and mechanical properties of diatoms were studied in wet state and then in dry state. A customized sample holder was integrated into a laboratory transmission X-ray microscope to image the morphology changes and volume shrinkage of the diatom while transitioning from the wet to the dry state. The measured volume shrinkage of a single diatom cell of Actinocyclus sp. is about 0.16. By performing an in-situ micromechanical experiment in both states, the maximal loading force of a single Actinocyclus sp. was determined until cracking appeared and compared in both states. This value is in the range of several hundred µN in the wet state and single-digit mN in the dry state. The normalized stiffness of the studied diatoms is significantly higher in the dry state than in the wet state. 2D radiograph and 3D tomography imaging of the diatoms reveal the different locations for crack propagation in both states. Our study supplies the important imaging method, the structure and functional information of the diatoms for future studies on diatoms in moisture but also in dry state. This information can help design bio-inspired materials and even in the development of bio-sustainable materials.

2D polymers have emerged as a highly promising category of nanomaterials, owing to their exceptional properties. However, the understanding of their fracture behavior and failure mechanisms remains still limited, posing challenges to their durability in practical applications. This work presents an in-depth study of the fracture kinetics of a 2D polyimine film, utilizing in situ tensile testing within a transmission electron microscope (TEM). Employing meticulously optimized transferring and patterning techniques, an elastic strain of ≈6.5% is achieved, corresponding to an elastic modulus of (8.6 ± 2.5) GPa of polycrystalline 2D polyimine thin films. In step-by-step fractures, multiple cracking events uncover the initiation and development of side crack near the main crack tip which toughens the 2D film. Simultaneously captured strain evolution through digital image correlation (DIC) analysis and observation on the crack edge confirm the occurrence of transgranular fracture patterns apart from intergranular fracture. A preferred cleavage orientation in transgranular fracture is attributed to the difference in directional flexibility along distinct orientations, which is substantiated by density functional-based tight binding (DFTB) calculations. These findings construct a comprehensive understanding of intrinsic mechanical properties and fracture behavior of an imine-linked polymer and provide insights and implications for the rational design of 2D polymers.

High-quality patterning determines the properties of patterned emerging two-dimensional (2D) conjugated polymers which is essential for potential applications in future electronic nanodevices. However, the suitable patterning method for 2D polymers is yet concluded because it’s still challenging to gain comprehensive understanding of their damage mechanisms by visualizing the structural modification during patterning process. Here, the damage mechanisms during patterning of 2D polymers, induced by various patterning methods, are unveiled based on a systematic study of structural damage and edge morphology on an imine-based 2D polymer (polyimine). Patterning using focused electron beam, focused ion beam (FIB) and mechanical carving is evaluated. Focused electron beam successively introduces sputtering effect, knock-on displacement damage and massive radiolysis effect as increasing the electron dose from 9.46×107 e-/nm2 to 1.14×1010 e-/nm2. The successful pattering is enabled by knock-on damage while impeded by carbon contamination when beyond a critical sample thickness. FIB creates current-dependent edge morphologies and extensive damage from the ion implantation caused by the tail of unfocused beam. A precisely controlled tip can tear the polyimine film through grain boundaries and in hence create the patterning edge with suitable edge roughness for certain application senarios when the beam damage is avoided. Taking structural damage and the resulting quantitative edge roughness into consideration, this study provides a detailed instruction on the proper patterning techniques for 2D crystalline polymers and paves the way for tailored intrinsic properties and device fabrication using these novel materials.

High-resolution imaging of Cu/low-k on-chip interconnect stacks in advanced microelectronic products is demonstrated using full-field transmission X-ray microscopy (TXM). The comparison of two lens-based laboratory X-ray microscopes that are operated at two different photon energies, 8.0 keV and 9.2 keV, shows a contrast enhancement for imaging of copper nanostructures embedded in insulating organosilicate glass of a factor of 5 if 9.2 keV photons are used. Photons with this energy (Ga-Kα radiation) are generated from a Ga-containing target of a laboratory X-ray source applying the liquid-metal-jet technology. The 5 times higher contrast compared to the use of Cu-Kα radiation (8.0 keV photon energy) from a rotating anode X-ray source is caused by the fact that the energy of the Ga-Kα emission line is slightly higher than that of the Cu-K absorption edge (9.0 keV photon energy). The use of Ga-Kα radiation is of particular advantage for imaging of copper interconnects with dimensions from several 100 nm down to several 10 nm in a Cu/SiO2 or Cu/low-k backend-of-line stack. Physical failure analysis and reliability engineering in the semiconductor industry will benefit from high-contrast X-ray images of sub-μm copper structures in microchips.

The semiconductor industry is continuing the scaling down of both device and on-chip interconnect features, for performance and economic reasons. This trend has implications for the design of guard ring structures, i.e. metallic non-functional structures in the back-end-of-line (BEoL) stack designed to be efficient to stop microcracks. In this work, we present a sample design for an in situ experiment to study mechanical degradation and failure mechanisms of crack stop structures in the BEoL stack, to ensure the mechanical robustness of microchips for future technology nodes. Additional finite element method (FEM) simulations provide supplementary understanding of the crack kinetics. To examine the effects of mechanical loading on crack stop elements of the BEoL stack, a novel sample geometry for an in situ fatigue experiment using x-ray microscopy was developed. The x-ray microscope (ZEISS Xradia 800 Ultra) enables high-resolution imaging of the 3D-patterned sample structures and defects such as microcracks. The tailored sample geometry allows the application of a tensile load to a BEoL specimen by a lever mechanism. The feasibility of the sample design is shown by mode-I loading of a pure interconnect sample. Post-mortem analysis by scanning electron microscopy (SEM), confirming planar microcrack propagation from the notch through the dielectric layer with small deflections of the crack path near cone shaped copper vias. FEM simulations focusing on the stress-strain fields around a crack tip indicate the beginning of copper plasticity as major mechanism starting the redirection of cracks due to resulting material compression in front of the obstacle.

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High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics—a side-by-side Montel (or nested Kirkpatrick–Baez) condenser optic and a high aspect-ratio multilayer Laue lens—results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid–liquid interdiffusion (SLID) bonding.

Surface terminations profoundly influence the intrinsic properties of MXenes, but existing terminations are limited to monoatomic layers or simple groups, showing disordered arrangements and inferior stability. Here we present the synthesis of MXenes with triatomic-layer borate polyanion terminations (OBO terminations) through a flux-assisted eutectic molten etching approach. During the synthesis, Lewis acidic salts act as the etching agent to obtain the MXene backbone, while borax generates BO2- species, which cap the MXene surface with an O-B-O configuration. In contrast to conventional chlorine/oxygen-terminated Nb2C with localized charge transport, OBO-terminated Nb2C features band transport described by the Drude model, exhibiting a 15-fold increase in electrical conductivity and a 10-fold improvement in charge mobility at the d.c. limit. This transition is attributed to surface ordering that effectively mitigates charge carrier backscattering and trapping. Additionally, OBO terminations provide Ti3C2 MXene with substantially enriched Li+-hosting sites and thereby a large charge-storage capacity of 420 mAh g-1. Our findings illustrate the potential of intricate termination configurations in MXenes and their applications for (opto)electronics and energy storage.

The anion-intercalation chemistries of graphite have the potential to construct batteries with promising energy and power breakthroughs. Here, we report the use of an ultrathin, positively charged two-dimensional poly(pyridinium salt) membrane (C2DP) as the graphite electrode skin to overcome the critical durability problem. Large-area C2DP enables the conformal coating on the graphite electrode, remarkably alleviating the electrolyte. Meanwhile, the dense face-on oriented single crystals with ultrathin thickness and cationic backbones allow C2DP with high anion-transport capability and selectivity. Such desirable anion-transport properties of C2DP prevent the cation/solvent co-intercalation into the graphite electrode and suppress the consequent structure collapse. An impressive PF 6 − -intercalation durability is demonstrated for the C2DP-covered graphite electrode, with capacity retention of 92.8% after 1000 cycles at 1 C and Coulombic efficiencies of > 99%. The feasibility of constructing artificial ion-regulating electrode skins with precisely customized two-dimensional polymers offers viable means to promote problematic battery chemistries.

On-chip interconnect stacks (Back End of Line, BEoL) in modern heterogeneous microelectronic products are exposed to various micromechanical loads. Each can lead to different failure modes of the whole system. To locally probe the micromechanical robustness of BEoL stacks, it is necessary to precisely control the mechanical loading conditions there. Three micromechanical BEoL robustness evaluation designs are presented, enabling a load and failure mode-adapted damage induction and identification. Those enable to test BEoL structures below and in the vicinity of Cu-pillars in compression, shear, and tensile mode. Acoustic Emission sensing capabilities are implemented to detect the very early stages of mechanical failures.

A novel experimental setup for testing interface properties of single copper interconnect structures is presented. The method is based on in-situ SEM nanoindentation experiments, utilized to probe customized copper structures manufactured by the Dual Damascene process. In this way the testing structures resemble product-like length-scales and properties. For the investigation of interface properties, the experimental load-displacement data is reviewed. Focused ion beam (FIB) cross sections are performed to validate the delaminated interface. FEM-simulations, based on the measured load-displacement data are used to determine the stress state in the test structures and to derive values for the interface fracture strength of the investigated interface.

Corrosion of steel reinforcements in concrete constructions is a worldwide problem. To assess the degradation of rebars in reinforced concrete, an accurate description of electric current, potential and concentrations of various species present in the concrete matrix is necessary. Although the concrete matrix is a heterogeneous porous material with intricate microstructure, mass transport has been treated in a homogeneous material so far, modifying bulk transport coefficients by additional factors (porosity, constrictivity, tortuosity), which led to so-called effective coefficients (e.g., diffusivity). This study presents an approach where the real 3D microstructure of concrete is obtained from high-resolution X-ray computed tomography (XCT), processed to generate a mesh for finite element method (FEM) computations, and finally combined with a multi-species system of transport and electric potential equations. This methodology allows for a more realistic description of ion movements and reactions in the bulk concrete and on the rebar surface and, consequently, a better evaluation of anodic and cathodic currents, ultimately responsible for the loss of reinforcement mass and its location. The results of this study are compared with a state-of-the-art model and numerical calculations for 2D and 3D geometries.

The monitored tomographic reconstruction (MTR) with optimized photon flux technique is a pioneering method for X-ray computed tomography (XCT) that reduces the time for data acquisition and the radiation dose. The capturing of the projections in the MTR technique is guided by a scanning protocol built on similar experiments to reach the predetermined quality of the reconstruction. This method allows achieving a similar average reconstruction quality as in ordinary tomography while using lower mean numbers of projections. In this paper, we, for the first time, systematically study the MTR technique under several conditions: reconstruction algorithm (FBP, SIRT, SIRT-TV, and others), type of tomography setup (micro-XCT and nano-XCT), and objects with different morphology. It was shown that a mean dose reduction for reconstruction with a given quality only slightlyvaries with choice of reconstruction algorithm, and reach up to 12.5 % in case of micro-XCT and 8.5 % for nano-XCT. The obtained results allow to conclude that the monitored tomographic reconstruction approach can be universally combined with an algorithm of choice to perform a controlled trade-off between radiation dose and image quality. Validation of the protocol on independent common ground truth demonstrated a good convergence of all reconstruction algorithms within the MTR protocol.

For micromechanical robustness evaluation methods, it is advantageous if the mechanical loading conditions applied can be controlled as precisely as possible. For microchips, this is required to determine the robustness under specific conditions, e.g. during assembly or characteristic application/usage scenarios. In this work, three different micromechanical BEoL (Back End of Line) robustness evaluation methods are presented which should enable a more precise and flexible mechanical load induction and damage identification. They have been subsequently developed. Three main aspects characterize the customization of the developed approaches:•The design and testing of customized micro-tools to precisely apply mechanical load to individual Cu-pillars.•The implementation of an AE (Acoustic Emission) monitoring approach to detect minor damages during mechanical loading. This strategy also enabled the development of sub-critical loading experiments for which AE signals served as a damage indicator and mechanical loading was aborted upon the detection of AE events.•The development of a new measurement setup and approach to enable the solder attach of individual Cu-pillars to a mechanical testing system. The applications of these approaches should enable the induction of customized mechanical loading conditions and the identification of failure modes and damage initiation locations.