E. Cadel

Phosphorus segregation in nanocrystalline Ni–3.6 at.% P alloy investigated with the tomographic atom probe (TAP)

Abstract The microstructures of electroless plated and thermally aged nanocrystalline nickel–3.6 at.% phosphorus layers were investigated on an atomic scale with a tomographic atom probe (TAP). After heat treatments at 250 and 400°C, a continuous P-segregation in the grain boundaries of the nanocrystalline structure was directly proved for the first time. This segregation effect explains the comparatively high thermal stability of the material. Assuming the existence of a metastable equilibrium, a simple mass balance calculation, which uses experimentally determined data exclusively, makes it possible to predict grain sizes of other NiP alloys within the thermal stability region.

Nanometre-scale evidence for interfacial dissolution–reprecipitation control of silicate glass corrosion

Silicate glasses are durable solids, and yet they are chemically unstable in contact with aqueous fluids-this has important implications for numerous industrial applications related to the corrosion resistance of glasses, or the biogeochemical weathering of volcanic glasses in seawater. The aqueous dissolution of synthetic and natural glasses results in the formation of a hydrated, cation-depleted near-surface alteration zone and, depending on alteration conditions, secondary crystalline phases on the surface. The long-standing accepted model of glass corrosion is based on diffusion-coupled hydration and selective cation release, producing a surface-altered zone. However, using a combination of advanced atomic-resolution analytical techniques, our data for the first time reveal that the structural and chemical interface between the pristine glass and altered zone is always extremely sharp, with gradients in the nanometre to sub-nanometre range. These findings support a new corrosion mechanism, interfacial dissolution-reprecipitation. Moreover, they also highlight the importance of using analytical methods with very high spatial and mass resolution for deciphering the nanometre-scale processes controlling corrosion. Our findings provide evidence that interfacial dissolution-reprecipitation may be a universal reaction mechanism that controls both silicate glass corrosion and mineral weathering.

The Role of the Atom Probe in the Study of Nickel-Based Superalloys

Abstract Nanostructural features of nickel-based superalloys as revealed by Field-Ion Microscopy, Atom Probe (APFIM), and 3D Atom Probe are reviewed. The unique and original information provided by these techniques is discussed on the basis of an extended and almost exhaustive analysis of bibliography over the last 30 years. Atom Probe techniques are shown to be able to measure the composition of tiny γ′ precipitates, a few nanometer in size, to detect ordering or subtle clustering effects within the γ solid solution or γ′ particles. Plane-by-plane analysis of (001) planes of the γ′ phase makes it possible to estimate the degree of order as well as the preferential sites of various addition elements included in superalloys. Due to its ultrahigh depth resolution, the microchemistry of interfaces and grain boundaries can also be characterized on an atomic scale. Most salient results will be reviewed, and the specific role of 3D APFIM will be highlighted with selected examples.

The spatial resolution of 3D atom probe in the investigation of single-phase materials

The resolution of three-dimensional atom probe (3DAP) is known to be mainly controlled by the aberrations of the ion trajectories near the surface of the specimen. A model has been developed to compute the ion trajectories in 3D near a sharp hemispherical electrode defined at the atomic scale. Simulations were applied on one-phase binary alloys. The influence of the evaporation fields of chemical species is studied. Simulated desorption images are consistent with experiments in both ordered alloys and random solid solution. An extra loss in the lateral resolution is observed in disordered alloys as compared to pure metals. The predicted order of evaporation provided by this model is in excellent agreement with experiments. The stacking sequence of atomic planes reconstructed from simulated data is shown to be disturbed in a similar way as observed in real experiments with 3DAP.

Depth resolution function of the laser assisted tomographic atom probe in the investigation of semiconductors

The investigation of boron delta layers by tomographic atom probe (3DAP) is used to demonstrate that a depth profiling resolution of 0.9 nm (full width at half maximum) can be achieved. Results are compared with measurements provided by secondary ion mass spectrometry. The steepness is found to be below 1 nm/decade. In addition, silicon atomic planes are resolved in the real space demonstrating an in-depth spatial resolution of the 3DAP below 0.2 nm.

Atom probe tomography investigation of the microstructure of superalloys N18

Abstract Microstructural features of nickel base superalloys N18 have been investigated by transmission electron microscopy (TEM), field emission gun-scanning electron microscopy (FEG-SEM), field ion microscopy (FIM), one-dimensional atom probe (APFIM) and three-dimensional atom probe (3DAP). Heat treatments conducted at 700 and 800°C lead to the precipitation of a large volume fraction of γ′ precipitates (57%) that are Al and Ti enriched. Cr, Co and Mo preferentially partition to the γ solid solution. Small Cr-enriched particles (θ) were discovered in the core of largest γ′ particles. These contain more Cr and less Mo than the γ solid solution but are thought to be tiny metastable γ particles that were formed in the inner region of γ′ particles during cooling from 800°C. Intergranular Ni-depleted borides arranged in “clusters” of particles very close to each other were observed using 3DAP. Their stoichiometry is M2B with M=Cr, Mo . One nanometer-thick Ni and Co enriched films (compared to borides) separate adjoining boride particles. Grain boundary (GB) segregation of B, Mo and to some extent Cr has been observed. The thickness of the segregated layer wetting GB is close to 1 nm.

First stages of the formation of Ni silicide by atom probe tomography

Atom probe tomography assisted by femtosecond laser pulses has been performed on a Ni(Pt) film on (100)Si. Two phases with different compositions were found to form during deposition at room temperature: a NiSi layer with a relatively constant thickness of approximately 2nm and a particle of Ni2Si. The shape of the Ni2Si particle is in accordance with nucleation followed by lateral growth formation. This confirms the growth model deduced from calorimetric measurement of silicides and intermetallics and from atom probe tomography studies of the Al∕Co system. A nonuniform redistribution of Pt was also observed.

3D analysis of advanced nano-devices using electron and atom probe tomography.

The structural and chemical properties of advanced nano-devices with a three-dimensional (3D) architecture have been studied at the nanometre scale. An original method has been used to characterize gate-all-around and tri-gate silicon nanowire transistor by combining electron tomography and atom probe tomography (APT). Results show that electron tomography is a well suited method to determine the morphological structure and the dimension variations of devices provided that the atomic number contrast is sufficient but without an absolute chemical identification. APT can map the 3D chemical distribution of the atoms in devices but suffers from strong distortions in the dimensions of the reconstructed volume. These may be corrected using a simple method based on atomic density correction and electron tomography data. Moreover, this combination is particularly useful in helping to understand the evaporation mechanisms and improve APT reconstructions. This paper demonstrated that a full 3D characterization of nano-devices requires the combination of both tomography techniques.

Atom probe study of Cu-poor to Cu-rich transition during Cu(In,Ga)Se2 growth

Atomic scale chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin film has been characterized at key points of the 3-stage process using atom probe tomography. 3D atom distributions have been reconstructed when the layer is Cu-poor ([Cu]/([Ga] + [In])   1), and at the end of the process. Particular attention has been devoted to grain boundary composition and Na atomic distribution within the CIGSe layer. Significant variation of composition is highlighted during the growing process, providing fundamental information helping the understanding of high efficiency CIGSe formation.

Growth of Si nanowires on micropillars for the study of their dopant distribution by atom probe tomography

This article reports on the growth of Au islands on the Si(111) surface as a function of the Au evaporation rate and the temperature of the surface in ultrahigh vacuum. By controlling the density of the Au islands and their size, it is possible to subsequently grow single vertically oriented Si nanowires on top of (111)-oriented silicon micropillar and analyze their chemical composition at the atomic scale with the femtosecond laser assisted tomographic atom probe. Three-dimensional images of the atom distribution in the nanowire, in particular, the distribution of boron impurities, are obtained and compared to the intended impurity concentration.