Armand Béché

Improved precision in strain measurement using nanobeam electron diffraction

Improvements in transmission electron microscopy have transformed nanobeam electron diffraction into a simple and powerful technique to measure strain. A Si0.69Ge0.31 layer, grown onto a Si substrate has been used to evaluate the precision and accuracy of the technique. Diffraction patterns have been acquired along a ⟨110⟩ zone axis using a FEI-Titan microscope and have been analyzed using dedicated software. A strain precision of 6×10−4 using a probe size of 2.7 nm with a convergence angle of 0.5 mrad has been reached. The bidimensional distortion tensor in the plane perpendicular to the electron beam has been obtained.

Atomic electric fields revealed by a quantum mechanical approach to electron picodiffraction.

By focusing electrons on probes with a diameter of 50 pm, aberration-corrected scanning transmission electron microscopy (STEM) is currently crossing the border to probing subatomic details. A major challenge is the measurement of atomic electric fields using differential phase contrast (DPC) microscopy, traditionally exploiting the concept of a field-induced shift of diffraction patterns. Here we present a simplified quantum theoretical interpretation of DPC. This enables us to calculate the momentum transferred to the STEM probe from diffracted intensities recorded on a pixel array instead of conventional segmented bright-field detectors. The methodical development yielding atomic electric field, charge and electron density is performed using simulations for binary GaN as an ideal model system. We then present a detailed experimental study of SrTiO3 yielding atomic electric fields, validated by comprehensive simulations. With this interpretation and upgraded instrumentation, STEM is capable of quantifying atomic electric fields and high-contrast imaging of light atoms.

Strain measurement at the nanoscale: Comparison between convergent beam electron diffraction, nano-beam electron diffraction, high resolution imaging and dark field electron holography.

Convergent beam electron diffraction (CBED), nano-beam electron diffraction (NBED or NBD), high resolution imaging (HRTEM and HRSTEM) and dark field electron holography (DFEH or HoloDark) are five TEM based techniques able to quantitatively measure strain at the nanometer scale. In order to demonstrate the advantages and disadvantages of each technique, two samples composed of epitaxial silicon-germanium layers embedded in a silicon matrix have been investigated. The five techniques are then compared in terms of strain precision and accuracy, spatial resolution, field of view, mapping abilities and ease of performance and analysis.

Magnetic monopole field exposed by electrons

Magnetic monopoles continue to be elusive. However, an experiment now shows that the interaction of an electron beam with the tip of a nanoscopically thin magnetic needle—a close approximation to a magnetic monopole field—generates an electron vortex state, as expected for a true magnetic monopole field.

Improved strain precision with high spatial resolution using nanobeam precession electron diffraction

NanoBeam Electron Diffraction is a simple and efficient technique to measure strain in nanostructures. Here, we show that improved results can be obtained by precessing the electron beam while maintaining a few nanometer probe size, i.e., by doing Nanobeam Precession Electron Diffraction (N-PED). The precession of the beam makes the diffraction spots more uniform and numerous, making N-PED more robust and precise. In N-PED, smaller probe size and better precision are achieved by having diffraction disks instead of diffraction dots. Precision in the strain measurement better than 2 × 10−4 is obtained with a probe size approaching 1 nm in diameter.

Dark field electron holography for quantitative strain measurements with nanometer-scale spatial resolution

Strain measurements on strained SiGe specimens have been performed using dark field electron holography. By combining the excellent stability of state-of-the-art electron microscopes with careful specimen preparation we have been able to acquire two-dimensional strain maps of the layers with a spatial resolution of 5 nm, a background noise of 2×10−4, an interference width of 1500 nm, and a field of view of more than 500×500 nm2. We also show that the strain measurements are quantitative to within experimental error.

Exploiting lens aberrations to create electron-vortex beams.

A model for a new electron-vortex beam production method is proposed and experimentally demonstrated. The technique calls on the controlled manipulation of the degrees of freedom of the lens aberrations to achieve a helical phase front. These degrees of freedom are accessible by using the corrector lenses of a transmission electron microscope. The vortex beam is produced through a particular alignment of these lenses into a specifically designed astigmatic state and applying an annular aperture in the condenser plane. Experimental results are found to be in good agreement with simulations.

A new way of producing electron vortex probes for STEM

A spiral holographic aperture is used in the condensor plane of a scanning transmission electron microscope to produce a focussed electron vortex probe carrying a topological charge of either � 1, 0 or þ 1. The spiral aperture design has a major advantage over the previously used forked aperture in that the three beams with topological charge m ¼� 1, 0, and 1 are not side by side in the specimen plane, but rather on top of each other, focussed at different heights. This allows us to have only one selected beam in focus on the sample while the others contribute only to a background signal. In this paper we describe the working principle as well as first experimental results demonstrating atomic resolution HAADF STEM images obtained with electron vortex probes. These results pave the way for atomic resolution magnetic information when combined with electron energy loss spectroscopy.

Strain evolution during the silicidation of nanometer-scale SiGe semiconductor devices studied by dark field electron holography

SiGe is routinely used to induce strain in modern semiconductors in order to improve the mobility of the carriers in the channel. Due to the absence of a technique that can accurately measure the strain in these devices with nanometer-scale resolution it has been difficult to assess the effects of processing such as silicidation on the compressive strain in the conduction channel. Here we show that by using dark field electron holography, the strain evolution at various stages of the device processing can be observed, showing that the silicidation process does in fact significantly reduce the strain in the conduction channel.

Strain mapping for the semiconductor industry by dark-field electron holography and nanobeam electron diffraction with nm resolution

There is a requirement of the semiconductor industry to measure strain in semiconductor devices with nm-scale resolution. Here we show that dark-field electron holography and nanobeam electron diffraction (NBED) are both complementary techniques that can be used to determine the strain in these devices. We show two-dimensional strain maps acquired by dark holography and line profiles that have been acquired by NBED of recessed SiGe sources and drains with a variety of different gate lengths and Ge concentrations. We have also used dark-field electron holography to measure the evolution in strain during the silicidation process, showing that this can reduce the applied uniaxial compressive strain in the conduction channel by up to a factor of 3.