The physics of light-tip interaction in laser-assisted Atom Probe Tomography

Leuven
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More than two weeks ago

Shedding light on the coupling of a laser with a nano-tip for atomic scale characterization.

The performance of contemporary and future nanomaterials and -devices is now tailored by the structure and chemical composition at the atomic level. In other words: just a few misplaced atoms can make the difference. Thus, having 3D metrology with close to atomic precision is not only a dream - it is a requirement! Atom Probe Tomography (APT) emerged as a very promising solution which can deliver a high spatial 3D resolution (down to a few ångström) and an extremely high element sensitivity.

Atom Probe

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 materials. Right: APT reconstruction of SiGe FIN field-effect transistor [Melkoyan et al., Ultramicroscopy 179, 100 (2017)].

In a nutshell, APT is based on the concept of controlled field emission of atoms from a needle-shaped 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). To facilitate the atom-by-atom field evaporation in a time resolved manner, voltage pulses (superimposed on a high standing DC voltage) are used since the conception of the APT in the late 1960’s. This method only works 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, which attracted great interest from the semiconductor industry. In LAPT, the field evaporation is triggered by an ultrashort laser pulse. 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. It is not excluded that, to some extent, athermal effects play a role as well [Kelly et al., Cur. Opin. Solid State Mater. Sci. 18, 81 (2014)]. 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)].

Several challenges remain to be surmounted in order to unlock la-APT’s full potential, in which the exact nature of the tip-light interaction plays a key role. To tackle this topic, the candidate has the possibility to explore and extensive playground, including experimental work (tip fabrication, APT analysis,...), data-analysis (3D reconstruction and analysis,...) and possibly physical modelling (theoretical, simulation,...).

 

Type of project: Internship, Thesis

 

Duration: 6 months for internship (with imec allowance) / full academic year for master's thesis (without allowance)

Required degree: Master of Science

Required background: Physics, Nanoscience & Nanotechnology

Supervising scientist(s): For further information or for application, please contact: Claudia Fleischmann (Claudia.Fleischmann@imec.be)

Imec allowance will be provided.

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