B. Deconihout

The tomographic atom probe: A quantitative three‐dimensional nanoanalytical instrument on an atomic scale

The physical architecture and the performance of a quantitative three‐dimensional atom probe recently constructed are described. The development of such an instrument relies on the design of a multi‐impact position sensitive detector. The multidetection system that we have developed is based on the use of a 10×10 anode array placed behind a two microchannel plate assembly in a chevron arrangement. The spread of charge between the microchannel plate and the multianode is used to derive the position of ion striking the detector. Spatial coordinates can be calculated for multiple and simultaneous time‐of‐flight events. The procedure used for the derivation of ion positions from charge measurements is given. Specific experiments were carried out in order to determine the intrinsic spatial resolution of the multidetector. Three‐dimensional reconstruction of two‐phase materials are provided and illustrate the performance of this new apparatus. The reconstructed images demonstrate that atoms are positioned with ...

Estimation of the tip field enhancement on a field emitter under laser illumination

We report the experimental evidence of controlled field evaporation of atoms from the surface of a tip-like-shape specimen with subwavelength dimensions by means of subpicosecond laser pulses. It is shown that the evaporation is assisted by the intrinsic laser electric field without any significant thermal activation. The single-atom detection sensitivity of the field ion microscope is used to get an accurate measurement of the electric field enhancement factor at the tip apex as a function of the wave polarization. The absence of thermal diffusion of atoms at the tip surface prior to field evaporation, demonstrates the feasibility of a laser assisted three-dimensional atom probe.

Estimation of the cooling times for a metallic tip under laser illumination

The temperature evolution at the apex of a sharply pointed needle submitted to ultrafast pulsed-laser irradiation was determined using a pump-probe method. The laser pulse acts as a pump pulse whereas the probe pulse is a fast high-voltage pulse. Then cooling times are consistent with a heating zone of a few microns with a laser beam polarized along the tip axis and a spot size of 0.8mm.

Advance in multi-hit detection and quantization in atom probe tomography

The preferential retention of high evaporation field chemical species at the sample surface in atom-probe tomography (e.g., boron in silicon or in metallic alloys) leads to correlated field evaporation and pronounced pile-up effects on the detector. The latter severely affects the reliability of concentration measurements of current 3D atom probes leading to an under-estimation of the concentrations of the high-field species. The multi-hit capabilities of the position-sensitive time-resolved detector is shown to play a key role. An innovative method based on Fourier space signal processing of signals supplied by an advance delay-line position-sensitive detector is shown to drastically improve the time resolving power of the detector and consequently its capability to detect multiple events. Results show that up to 30 ions on the same evaporation pulse can be detected and properly positioned. The major impact of this new method on the quantization of chemical composition in materials, particularly in highly-doped Si(B) samples is highlighted.

Field evaporation mechanism of bulk oxides under ultra fast laser illumination

The controlled field evaporation of single atoms from an oxide surface assisted by ultra fast laser pulses has recently been demonstrated. When UV light is used, a photoionization mechanism was proposed. However, experimental results observed when the laser intensity and wavelength are changed cannot be explained by this mechanism. Instead, a thermal assisted evaporation mechanism characterized by two evaporation times is proposed. The fast and slow evaporation rates are associated to two cooling processes inside the tip sample. Experiments are carried out on TiO2 and MgO field emitter tips to check the dependence of the evaporation process on structural properties of the oxide. A good agreement between the predictions of our model and the experimental data is found.

Ultrafast laser-triggered field ion emission from semiconductor tips

We study experimentally and theoretically the controlled field evaporation of single atoms from a semiconductor surface by ultrafast laser-assisted atom probe tomography. The conventional physical mechanisms of field evaporation cannot explain the experimental results recently reported for such materials. A new model is presented in which the positive dc field leads to band bending with a high density of laser-generated holes near the surface of the sample. The laser energy absorption by these holes and the subsequent energy transfer to the lattice considerably increase the tip temperature. We show that this heating plays an important role in the field ion emission process. In addition, experiments are carried out for germanium and silicon tips to check the role of the dc field in the absorption processes, as well as the heating of the tip and the following evaporation. Good agreement between the predictions of our model and the experimental data is found.

Energy deficit of pulsed-laser field-ionized and field-emitted ions from non-metallic nano-tips

The energy deficit of pulsed-laser field-evaporated ions and field-ionized atoms of an inert gas from the surface of a non-metallic nano-metric tip is reported as a function of the laser intensity, ion current, and temperature. A new model is proposed to explain these results, taking into account the resistive properties of non-metallic nano-tips. A good agreement between the theoretical predictions and the experimental results is obtained for all parameters investigated experimentally. This model is also used to discuss the evaporation behavior of oxides analyzed in laser-assisted atom probe tomography. New insight into the contribution of the electrostatic field and the laser illumination on the evaporation process of non-metallic materials is given.

Performance of an energy compensated time-of-flight mass spectrometer

Abstract This paper describes the performance of an energy compensated time-of-flight atom probe. The compensating system is of the Poschenrieder type with a large acceptance angle. The mass resolution is measured as a function of the acceptance angle. The role of the fringe field correction electrodes is studied. The observed mass resolution is compared to that predicted by the second order Poschenrieder theory.

Reneutralization time of surface silicon ions on a field emitter

In this work, the lifetime of silicon (Si) ions generated through photoionization of Si surface atoms from a field emitter was measured. Under low-intensity fs laser pulse illumination, a linear dependence of the number of evaporated ions per pulse on the laser intensity was observed. A simple model was developed to explain this linear dependence and to estimate the rate of success of the field evaporation process. It is shown that the number of evaporated ions per pulse depends on the standing field applied to the Si surface, demonstrating the existence of an ionic energy barrier for Si ions. The lifetime of these ions was estimated to be 0.5 ps.

Conditions to cancel the laser polarization dependence of a subwavelength tip

Using laser assisted atom probe tomography, we investigate the polarization dependence of the absorption coefficient of a subwavelength Al tip illuminated by an ultrashort laser pulse. In practice, we find an equilibrium condition as a function of the incident wavelength for which the atom evaporation rate becomes independent of the wave polarization. It is experimentally shown that this condition only depends on the ratio between the tip radius and the laser wavelength. Furthermore, our calculations demonstrate that a transverse local plasmon polariton mode can be resonantly excited at the tip apex.