The physics of the light-tip interaction in laser-assisted Atom Probe Tomography
Shedding some light on the coupling of a laser with a nano-tip for atomic scale characterization.
The ultimate dream of a metrologist is to determine the species and position of every atom in any given material. This greatly helps in understanding its (functional or even more exotic) properties, such as mechanical strength, electronic conductivity, magnetism or superconductivity. Atom Probe Tomography (APT) emerged as a very promising technique which can deliver a high spatial 3D resolution (down to a few ångström) and an extremely high element sensitivity.
Figure: a) Operating principle of (laser-assisted) Atom Probe Tomography [Vandervorst et al., MSSP 62, 31 (2017)]. B) Left: Scanning electron micrograph of a FinFET transistor shaped into an APT tip. False-colored regions indicate the material. Right: APT reconstruction of a SiGe FinFet [Melkoyan et al., Ultramicroscopy 179, 100 (2017)].
In a nutshell, APT is based on the concept of controlled field emission of atoms from a specimen, in combination with mass (hence element) identification by time-of-flight, with the aim to determine the original location in the evaporated volume of each evaporated atom (see Figure a). The outcome of such an experiment is a full 3D-compositional analysis with sub-nm resolution (see Figure b). The field evaporation occurs in a substantial electric field (~10V/nm) which can ionize the surface atoms of a tip shaped specimen. A first way to trigger, in a time-resolved fashion, the atom-by-atom evaporation is by voltage pulses (superimposed on a standing (DC) voltage). This method however, is only suitable for conducting specimen. The advent of laser-assisted Atom Probe Tomography (LAPT) enabled to perform atomic scale characterization of poorly or non-conductive samples such as semiconductors, ceramics and oxides, which opened up the field of APT for e.g. semiconductor and energy storage (battery) technologies. In LAPT, the field evaporation is triggered by an ultrashort laser pulse (superimposed on a DC voltage). Notwithstanding the recent experimental successes, the physics of LAPT is still poorly understood [Kelly et al., Cur. Opin. Solid State Mater. Sci. 18, 81 (2014)] and in practice, the analysis suffers from artefacts, which limit the obtained resolution and sensitivity.
The effect of the laser appears to be mainly a thermal effect. Upon irradiation, the laser energy can be adsorbed and thus heats up the APT tip, and as such it thermally assists the field evaporation process. Intriguingly however, field evaporation is even assisted when the bandgap of the material is (significantly) larger than the photon energy, whereby no light absorption and subsequent heating should occur normally. A tentative explanation which has been proposed, is that these materials turn metallic close to the tip’s apex under the high applied DC voltage [Silaeva et al., Nano Lett. 16, 6066 (2014)]. Alternatively, it has been shown that the typical APT tip fabrication techniques generate a nm-thick amorphous shell which can act as a light adsorbing layer [Bogdanowicz et al., J. Quant. Spectr. Rad. Transf. 146, 175 (2014)]. Today, it is still unclear whether these are the only effects which are at play, and to which extent these contribute to the light adsorption. The nanoscale dimensions of the APT tips, comparable to the wavelengths of the used laser, also make that localization and confinements effects come into play [Bogdanowicz et al. , Appl. Surf. Sci. 302, 223 (2014)]. Furthermore, our group recently demonstrated a novel approach to experimentally determine the APT tip apex temperature at the moment of field-evaporation [Kumar et al., J. Appl. Phys. 124, 245105 (2018)], which can be exploited for further unravelling the mechanism underlying LAPT. Finally, it is not excluded that, to some extent, athermal effects need to be taken into account [Kelly et al., Cur. Opin. Solid State Mater. Sci. 18, 81 (2014)].
It is clear that there is an extensive playground to be explored in search of a deeper understanding of the relevant processes in LAPT. As such, the candidate can partake in experimental work (tip fabrication, APT analysis,...), data-analysis (3D reconstruction and analysis,...) and possibly physical modelling (theoretical, simulation,...).
Type of project: Thesis
Duration: 6 months
Required degree: Master of Science
Required background: Nanoscience & Nanotechnology, Physics
Supervising scientist(s): For further information or for application, please contact: Claudia Fleischmann (Claudia.Fleischmann@imec.be)
Imec allowance will be provided