US-12620551-B2 - Tomographic atom probe with terahertz pulse generator

US12620551B2US 12620551 B2US12620551 B2US 12620551B2US-12620551-B2

Abstract

A tomographic atom probe includes an analysis chamber intended to analyze a sample of material in the form of a nanotip mounted on an anti-vibration support, the nanotip being brought to a temperature of between 0 kelvin and ambient temperature, the nanotip being biased at an adjustable voltage of between 1 kV and 15 kV, the analysis chamber comprising a position-sensitive and time of flight-sensitive ion detector. The atom probe comprises a generator for generating high-peak-intensity single-cycle ultrashort terahertz pulses, the analysis chamber comprising optical means for focusing the terahertz pulses, the focusing of the terahertz pulses causing the atoms of the nanotip to evaporate through the field effect without thermal effects. The terahertz pulses are generated by a femtosecond pulsed laser emitting very high-power ultrashort optical pulses at a high rate.

Inventors

Angela Vella
Jonathan HOUARD
Ammar Hideur
Laurent ARNOLDI
Lorenzo RIGUTTI

Assignees

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE ROUEN
UNIVERSITE DE ROUEN NORMANDIE

Dates

Publication Date: 20260505
Application Date: 20200723
Priority Date: 20190729

Claims (10)

1 . A tomographic atom probe comprising an analysis chamber operating at ambient temperature, said analysis chamber having a sample (E) of material in the form of a nanotip mounted on an anti-vibration support, said nanotip being brought to a temperature of between 0 kelvin and ambient temperature by a cryogenic device, said nanotip being biased at an adjustable positive voltage of between 1 kV and 15 kV by an electronic device, said analysis chamber comprising a position-sensitive and time of flight-sensitive ion detector, wherein said atom probe comprises a generator for generating high-peak-intensity single-cycle ultrashort terahertz pulses, the analysis chamber comprising optical means for focusing said terahertz pulses on the nanotip, the focusing of the terahertz pulses causing the atoms of the nanotip to evaporate through the field effect without thermal effects, wherein the terahertz pulse generator produces optical single-cycle pulses with an amplitude greater than 10 kV/cm.
2 . The tomographic atom probe as claimed in claim 1 , wherein the nanotip is brought to a temperature lower than 200 kelvins.
3 . The tomographic atom probe as claimed in claim 1 , wherein the terahertz pulse generator comprises: a femtosecond pulsed laser emitting ultrashort optical pulses in the near infrared spectral range, a non-linear crystal able to generate, through frequency doubling, optical radiation with a wavelength equal to half that of the optical pulses, a focusing optic for focusing said pulses and said optical radiation, the plasma formed at the focal point generating the terahertz pulses, an optical filter for absorbing the pulses and optical radiation and transmitting the terahertz waves.
4 . The tomographic atom probe as claimed in claim 3 , wherein the non-linear crystal is beta barium borate, known by the name “BBQ”, or lithium triborate, known by the name “LBO”, or potassium di deuterium phosphate, known by the name “KDP”, or potassium titanyl phosphate, known by the name “KTP”.
5 . The tomographic atom probe as claimed in claim 3 , wherein the optical filter is a silicon wafer.
6 . The tomographic atom probe as claimed in claim 3 , wherein the pulsed laser emits, at a wavelength of between 400 and 3000 nanometers, light pulses whose energy is between 0.1 and 4 mJ, the emission frequency being between 1 and 200 KHz.
7 . The tomographic atom probe as claimed in claim 1 , wherein the terahertz pulse generator comprises: a femtosecond pulsed laser emitting ultrashort optical pulses in the near infrared spectral range, a focusing optic for focusing said optical pulses, a non-linear crystal placed at the focal point of the focusing optic and able to generate the terahertz pulses through optical rectification or through frequency difference, an optical filter for absorbing the optical pulses and transmitting the terahertz waves.
8 . The tomographic atom probe as claimed in claim 1 , wherein the ion detector has a detection half-angle of between 1 and 90 degrees.
9 . The tomographic atom probe as claimed in claim 1 , wherein the ion detector is a time-of-flight mass spectrometer.
10 . The tomographic atom probe as claimed in claim 1 , wherein the optical means for focusing said terahertz pulses are a spherical or parabolic mirror or a terahertz lens operating on-axis or a parabolic mirror operating off-axis.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a National Stage of International patent application PCT/EP2020/070775, filed on Jul. 23, 2020, which claims priority to foreign French patent application No. FR 1908599, filed on Jul. 29, 2019, the disclosures of which are incorporated by reference in their entirety. FIELD OF THE INVENTION The invention lies in the general technical field of analyzing the composition of materials on the atomic scale. It relates more particularly to analysis devices that operate by evaporating a sample of the material by way of an electrical or optical pulse. More precisely, the field of the invention is that of tomographic atom probes. BACKGROUND In a tomographic atom probe, a DC voltage is applied to the sample of material to be analyzed. The sample, cut in the form of a tip with a small radius, generally of between 10 and 200 nanometers, is placed in a vacuum chamber. The material may be a conductor, an insulator or a semiconductor. The intense electric field thus created is sufficient to tear the atoms from the surface, which atoms are ionized and projected onto a position and time detector. Superimposing electrical or light pulses on the DC voltage evaporates the surface atoms at precise times in the form of ions. Measuring the times of flight of the ions makes it possible to determine their chemical nature through time-of-flight mass spectroscopy. The data that are collected make it possible to represent the volume of the destroyed sample on the scale of the atomic lattice, as well as the chemical nature of each atom. The mass resolution of an electrical pulse-assisted atom probe depends on the duration of the pulse that controls the duration of the emission and the energy dispersion of the ions. The mass resolution of a femtosecond laser pulse-assisted atom probe depends on the duration for which ions are emitted following the absorption of a light pulse. It has been shown that this duration itself depends on the physical mechanism actually involved during evaporation. A distinction is thus drawn between a slow evaporation mechanism, the duration of which is greater than a few nanoseconds, induced by a thermal effect, and a fast evaporation mechanism, still induced by a thermal effect but in which, by virtue of the action of the intense electric field, the heating is more localized and the associated thermal dynamics are faster. This highly localized thermal mechanism produces a virtually instantaneous effect of tearing the elements from the sample. The first slow thermal mechanism is responsible for limiting the mass resolution of a laser pulse-assisted atom probe. Laser-assisted tomographic atom probes that are currently on the market comprise a laser emitting in the near-ultraviolet range, at a wavelength of 355 nanometers. In the case of metals, this wavelength makes it possible, for suitable geometries of the sample, to confine the absorption to the end of the sample. For semiconductor materials, having a gap less than 3 eV, the absorption of laser light in the near-UV is also confined to the end of the sample. This confinement of the absorption promotes the fast evaporation mechanism. However, in the case of insulators and semiconductors with a large gap, greater than 3 eV, the energy of the laser is weakly absorbed at the end of the tip and absorption maxima occur far from this end, giving rise to slow and delayed thermal evaporation that degrades the mass resolution of the instrument. In addition, the strong static field applied to the sample increases the absorption at the end of the tip. The energy of the laser thus has to be adjusted according to the value of the applied electric field. In practice, the analysis conditions are determined so as to promote the fast evaporation mechanism and to minimize the slow mechanism. However, these analysis conditions have a strong influence on the measurement of the composition of insulating and semiconductor materials. In particular, the measurement varies with the value of the applied static field. Strong fields thus promote fast emission, but may give rise to biases in the composition measurements. On the other hand, weaker fields may give more reliable composition analyses, but with poor mass resolution. Lastly, excessively high field or laser energy values lead to the destruction of the sample through electrostatic and thermal effects. It is therefore necessary to determine field and illumination conditions that are capable both of minimizing slow thermal evaporation and of giving composition values close to nominal values. These optimum values are difficult to determine. They depend on multiple parameters, such as the composition of the material under analysis or the geometry of the tip of the sample. However, the composition of the material under analysis is, as a principle, not exactly known. In the same way, the exact geometry of the sample is not fully understood, given the small dimensions involved.