Lyle E. Levine

Ultra-small-angle X-ray scattering at the Advanced Photon Source

The design and operation of a versatile ultra-small-angle X-ray scattering (USAXS) instrument at the Advanced Photon Source (APS) at Argonne National Laboratory are presented. The instrument is optimized for the high brilliance and low emittance of an APS undulator source. It has angular and energy resolutions of the order of 10−4, accurate and repeatable X-ray energy tunability over its operational energy range from 8 to 18 keV, and a dynamic intensity range of 108 to 109, depending on the configuration. It further offers quantitative primary calibration of X-ray scattering cross sections, a scattering vector range from 0.0001 to 1 A−1, and stability and reliability over extended running periods. Its operational configurations include one-dimensional collimated (slit-smeared) USAXS, two-dimensional collimated USAXS and USAXS imaging. A robust data reduction and data analysis package, which was developed in parallel with the instrument, is available and supported at the APS.

Dislocation nucleation during nanoindentation of aluminum

Through multiscale simulations, we explore the influence of both smooth and atomically rough indenter tips on the nucleation of dislocations during nanoindentation of single-crystal aluminum. We model the long-range strain with finite element analysis using anisotropic linear elasticity. We then model a region near the indenter atomistically and perform molecular dynamics with an embedded atom method interatomic potential. We find that smooth indenters nucleate dislocations below the surface but rough indenters can nucleate dislocations both at the surface and below. Increasing temperature from 0 to 300 K creates prenucleation defects in the region of high stress and decreases the critical depth.

High-energy Ultra-Small Angle X-ray Scattering Instrument at the Advanced Photon Source

This paper reports recent tests performed on the Bonse–Hart-type ultra-small-angle X-ray scattering (USAXS) instrument at the Advanced Photon Source with higher-order reflection optics – Si(440) instead of Si(220) – and with X-ray energies greater than 20 keV. The results obtained demonstrate the feasibility of high-energy operation with narrower crystal reflectivity curves, which provides access to a scattering q range from ∼2 × 10−5 to 1.8 A−1 and up to 12 decades in the associated sample-dependent scattering intensity range. The corresponding size range of the scattering features spans about five decades – from less than 10 A to ∼15 µm. These tests have indicated that mechanical upgrades are required to ensure the alignment capability and operational stability of this instrument for general user operations because of the tighter angular-resolution constraints of the higher-order crystal optics.

Application of Finite Element, Phase-field, and CALPHAD-based Methods to Additive Manufacturing of Ni-based Superalloys

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni-Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

Synchrotron-based measurement of the impact of thermal cycling on the evolution of stresses in Cu through-silicon vias

One of the main causes of failure during the lifetime of microelectronics devices is their exposure to fluctuating temperatures. In this work, synchrotron-based X-ray micro-diffraction is used to study the evolution of stresses in copper through-silicon via (TSV) interconnects, “as-received” and after 1000 thermal cycles. For both test conditions, significant fluctuations in the measured normal and shear stresses with depth are attributed to variations in the Cu grain orientation. Nevertheless, the mean hydrostatic stresses in the “as-received” Cu TSV were very low, at (16 ± 44) MPa, most likely due to room temperature stress relaxation. In contrast, the mean hydrostatic stresses along the entire length of the Cu TSV that had undergone 1000 thermal cycles (123 ± 37) MPa were found to be eight times greater, which was attributed to increased strain-hardening. The evolution in stresses with thermal cycling is a clear indication that the impact of Cu TSVs on front-end-of-line (FEOL) device performance will change through the lifetime of the 3D stacked dies, and ought to be accounted for during FEOL keep-out-zone design rules development.

Development of ultra‐small‐angle X‐ray scattering–X‐ray photon correlation spectroscopy

This paper describes the development of ultra-small-angle X-ray scattering–X-ray photon correlation spectroscopy (USAXS–XPCS). This technique takes advantage of Bonse–Hart crystal optics and is capable of probing the long-time-scale equilibrium and non-equilibrium dynamics of optically opaque materials with prominent features in a scattering vector range between those of dynamic light scattering and conventional XPCS. Instrumental parameters for optimal coherent-scattering operation are described. Two examples are offered to illustrate the applicability and capability of USAXS–XPCS. The first example concerns the equilibrium dynamics of colloidal dispersions of polystyrene microspheres in glycerol at 10, 15 and 20% volume concentrations. The temporal intensity autocorrelation analysis shows that the relaxation time of the microspheres decays monotonically as the scattering vector increases. The second example concerns the non-equilibrium dynamics of a polymer nanocomposite, for which it is demonstrated that USAXS–XPCS can reveal incipient dynamical changes not observable by other techniques.

Backstress, the Bauschinger Effect and Cyclic Deformation

Backstresses or long range internal stresses (LRIS) in the past have been suggested by many to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The heterogeneities include edge dislocation dipole bundles (veins) and the edge dipole walls of persistent slip bands (PSBs) in cyclically deformed materials and cell and subgrain walls in monotonically deformed materials. The existence of long range internal stress is especially important for the understanding of cyclic deformation and also monotonic deformation. X-ray microbeam diffraction experiments performed by the authors using synchrotron x-ray microbeams determined the elastic strains within the cell interiors. The studies were performed using, oriented, monotonically deformed Cu single crystals. The results demonstrate that small long-range internal stresses are present in cell interiors. These LRIS vary substantially from cell to cell as 0 % to 50 % of the applied stress. The results are related to the Bauschinger effect, often explained in terms of LRIS.

Homogenization kinetics of a nickel-based superalloy produced by powder bed fusion laser sintering

Additively manufactured (AM) metal components often exhibit fine dendritic microstructures and elemental segregation due to the initial rapid solidification and subsequent melting and cooling during the build process, which without homogenization would adversely affect materials performance. In this letter, we report in situ observation of the homogenization kinetics of an AM nickel-based superalloy using synchrotron small angle X-ray scattering. The identified kinetic time scale is in good agreement with thermodynamic diffusion simulation predictions using microstructural dimensions acquired by ex situ scanning electron microscopy. These findings could serve as a recipe for predicting, observing, and validating homogenization treatments in AM materials.

Nondestructive Measurement of the Residual Stresses in Copper Through-Silicon Vias Using Synchrotron-Based Microbeam X-Ray Diffraction

In this paper, we report a new method for achieving depth resolved determination of the full stress tensor in buried Cu through-silicon vias (TSVs), using a synchrotron-based X-ray microdiffraction technique. Two adjacent Cu TSVs were analyzed; one capped with SiO2 (0.17 μm) and the other without. The uncapped Cu TSV was found to have higher stresses with an average hydrostatic stress value of 145 ± 37 MPa, as compared with the capped Cu TSV, which had a value of 89 ± 28 MPa. Finite element-based parametric analyses of the effect of cap thickness on TSV stress were also performed. The differences in the stresses in the adjacent Cu TSVs were attributed to their microstructural differences and not to the presence of a cap layer. Based on the experimentally determined stresses, the stresses in the surrounding Si for both Cu TSVs were calculated and the FinFET keep-out-zone (KOZ) from the Cu TSV was estimated. The FinFET KOZ is influenced by the microstructural variations in their neighboring Cu TSVs, thus, it should be accounted for in KOZ design rules.

Full elastic strain and stress tensor measurements from individual dislocation cells in copper through-Si vias

A ground breaking new capability for measuring complete strain and stress tensors nondestructively from deeply buried, sub-micrometre sample volumes within microstructurally complex and multicomponent specimens is presented. The method is demonstrated on technologically important copper through-Si vias that are used in advanced three-dimensional microelectronics.