Florent Houdellier

Nanoscale holographic interferometry for strain measurements in electronic devices

Strained silicon is now an integral feature of the latest generation of transistors and electronic devices because of the associated enhancement in carrier mobility. Strain is also expected to have an important role in future devices based on nanowires and in optoelectronic components. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions. Developing methods of strain measurement at the nanoscale has therefore been an important objective in recent years but has proved elusive in practice: none of the existing techniques combines the necessary spatial resolution, precision and field of view. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometre scale, whereas transmission electron microscopy allows strain measurement at the nanometre scale but only over small sample areas. Here we present a technique capable of bridging this gap and measuring strain to high precision, with nanometre spatial resolution and for micrometre fields of view. Our method combines the advantages of moiré techniques with the flexibility of off-axis electron holography and is also applicable to relatively thick samples, thus reducing the influence of thin-film relaxation effects.

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe L2,3 edge.

Optimal aperture sizes and positions for EMCD experiments

The signal-to-noise ratio (SNR) in energy-loss magnetic chiral dichroism (EMCD)--the equivalent of X-ray magnetic circular dichroism (XMCD) in the electron microscope--is optimized with respect to the detector shape, size and position. We show that an important increase in SNR over previous experiments can be obtained when taking much larger detector sizes. We determine the ideal shape of the detector but also show that round apertures are a good compromise if placed in their optimal position. We develop the theory for a simple analytical description of the EMCD experiment and then apply it to dynamical multibeam Bloch wave calculations and to an experimental data set. In all cases it is shown that a significant and welcome improvement of the SNR is possible.

Experimental and physics-based modeling assessment of strain induced mobility enhancement in FinFETs

This study combines direct measurements of channel strain, electrical mobility measurements and a rigorous modeling approach to provide insight about the strain induced mobility enhancement in FinFETs and guidelines for the device optimization. Good agreement between simulated and measured mobility is obtained using strain components measured directly at device level by a novel technique. A large vertical compressive strain is observed in FinFETs and the simulations show that this helps recover the electron mobility disadvantage of the (110) FinFETs lateral interfaces w.r.t. (100) interfaces, with no degradation of the hole mobility. The model is then used to systematically explore the impact of the fin-width, fin-height and fin-length stress components on n- and p-FinFETs mobility and to identify optimal stress configurations.

Energy-loss magnetic chiral dichroism (EMCD): Magnetic chiral dichroism in the electron microscope

A new technique called energy-loss magnetic chiral dichroism (EMCD) has recently been developed [P. Schattschneider, et al. Nature 441, 486 (2006)] to measure magnetic circular dichroism in the transmission electron microscope (TEM) with a spatial resolution of 10 nm. This novel technique is the TEM counterpart of x-ray magnetic circular dichroism, which is widely used for the characterization of magnetic materials with synchrotron radiation. In this paper we describe several experimental methods that can be used to measure the EMCD signal [P. Schattschneider, et al. Nature 441, 486 (2006); C. Hebert, et al. Ultramicroscopy 108(3), 277 (2008); B. Warot-Fonrose, et al. Ultramicroscopy 108(5), 393 (2008); L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); P. van Aken, et al. Microsc. Microanal. 13(3), 426 (2007)] and give a review of the recent improvements of this new investigation tool. The dependence of the EMCD on several experimental conditions (such as thickness, relative orientation of beam and sample, collection and convergence angle) is investigated in the transition metals iron, cobalt, and nickel. Different scattering geometries are illustrated; their advantages and disadvantages are detailed, together with current limitations. The next realistic perspectives of this technique consist of measuring atomic specific magnetic moments, using suitable spin and orbital sum rules, [L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); J. Rusz, et al. Phys. Rev. B 76, 060408 (2007)] with a resolution down to 2 to 3 nm.

Development of TEM and SEM high brightness electron guns using cold-field emission from a carbon nanotip

A newly developed carbon cone nanotip (CCnT) has been used as field emission cathode both in low voltage SEM (30 kV) electron source and high voltage TEM (200 kV) electron source. The results clearly show, for both technologies, an unprecedented stability of the emission and the probe current with almost no decay during 1h, as well as a very small noise (rms less than 0.5%) compared to standard sources which use tungsten tips as emitting cathode. In addition, quantitative electric field mapping around the FE tip have been performed using in situ electron holography experiments during the emission of the new tip. These results show the advantage of the very high aspect ratio of the new CCnT which induces a strong enhancement of the electric field at the apex of the tip, leading to very small extraction voltage (some hundred of volts) for which the field emission will start. The combination of these experiments with emission current measurements has also allowed to extract an exit work function value of 4.8 eV.

Off-Axial Aberration Correction using a B-COR for Lorentz and HREM Modes

A dedicated Hitachi HF3300C microscope, “I2TEM” , has recently been installed in CEMES. This microscope has been specially designed to carry on electron interferometry and in - situ TEM experiments. I2TEM is a 300 kV cold FEG microscope fitted with a multibrism set - up, two stages capability, a GIF quantum ER, a 4k X 4k camera and a Cs - corrector “B - COR” from CEOS. The first sta ge location within the objective pole piece allows performing classical HREM experiments while the second location is in a field free region above the objective lens and below the third condenser lens and allows carrying TEM imaging or electron holography in Lorentz mode. Contrary to non - dedicated microscope, I2TEM allows, in Lorentz mode, using apertures in the focal plane of the objective lens (i.e. used as “ Lorentz lens ” ) to select diffracted beams. In addition, the B - COR can be adjusted to correct for t he objective lens aberrations when excited in HREM or in Lorentz modes at voltages of 60kV, 80kV, 200kV and 300kV. This unique multipolar optical system (also called “ Aplanator ” ) has been specially designed to correct not only for the Cs, the axial coma (B 2), the three - fold astigmatism (A2), but also to compensate for the radial and azimuthal off - axial coma [1]. These off - axis corrections are achieved thanks to two additional pair of short hexapoles located in between two image planes inside the corrector. These planes are located between three long hexapoles used to compensate the axial aberrations (Cs, B2, A2, . . . ) like in the standard C - COR (Fig. 1). As conventional Cs - correctors allow for correcting most of the important first and second order aberration s confined close to the optic axis in HREM images of few ten of nanometers wide, the Aplanator compensates for aberrations in much larger field of view images (the number of equally resolved point regarding the standard π/4 limit is indeed considerably hig her). It is therefore of huge interest for large field of view HREM images recorded with a 4k X 4k camera and for low magnification images or holograms obtained in Lorentz mode. We will present recent results showing the capacity of the B - COR to correct f or the axial and off - axial aberrations of the 11 mm pole piece gap of the I2TEM objective lens and achieve 80pm spatial resolution in HREM mode (Fig. 2a). Results will also be presented showing the capacity of the Aplanator to correct for the objective len s aberration when used in Lorentz mode where 0.5 nm spatial resolution has been achieved (Fig. 2b).

On the influence of elastic strain on the accommodation of carbon atoms into substitutional sites in strained Si:C layers grown on Si substrates

Measurements of strain and composition are reported in tensile strained 10- and 30-nm-thick Si:C layers grown by chemical vapor deposition on a Si (001) substrate. Total carbon concentration varies from 0.62% to 1.97%. Strain measurements were realized by high-resolution x-ray diffraction, convergent-beam electron diffraction, and geometric phase analysis of high-resolution transmission electron microscopy cross-sectional images. Raman spectroscopy was used for the deduction of the substitutional concentration. We demonstrate that in addition to the growth conditions, strain accumulating during deposition, thus depending on a layer thickness, has an influence on the final substitutional carbon composition within a strained Si:C layer.

Study of nanocrystalline BiMnO3-PbTiO3: synthesis, structural elucidation, and magnetic characterization of the whole solid solution.

In the last ten years, the study and the search for new multiferroic materials have been a major challenge due to their potential applications in electronic technology. In this way, bismuth-containing perovskites (BiMO(3)), and particularly those in which the metal M position is occupied by a magnetically active cation, have been extensively investigated as possible multiferroic materials. From the point of view of synthesis, only a few of the possible bismuth-containing perovskites can be prepared by conventional methods but at high pressures. Herein, the preparation of one of these potential multiferroic systems, the solid solution xBiMnO(3)-(1-x)PbTiO(3) by mechanosynthesis is reported. Note that this synthetic method allows the oxides with high x values, and more particularly the BiMnO(3) phase, to be obtained as nanocrystalline phases, in a single step and at room temperature without the application of external pressure. These results confirm that, in the case of Bi perovskites, mechanosynthesis is a good alternative to high-pressure synthesis. These materials have been studied from the point of view of their structural characteristics by precession electron diffraction and magnetic property measurements.

Differential phase-contrast dark-field electron holography for strain mapping.

Strain mapping is an active area of research in transmission electron microscopy. Here we introduce a dark-field electron holographic technique that shares several aspects in common with both off-axis and in-line holography. Two incident and convergent plane waves are produced in front of the specimen thanks to an electrostatic biprism in the condenser system of a transmission electron microscope. The interference of electron beams diffracted by the illuminated crystal is then recorded in a defocused plane. The differential phase recovered from the hologram is directly proportional to the strain in the sample. The strain can be quantified if the separation of the images due to the defocus is precisely determined. The present technique has the advantage that the derivative of the phase is measured directly which allows us to avoid numerical differentiation. The distribution of the noise in the reconstructed strain maps is isotropic and more homogeneous. This technique was used to investigate different samples: a Si/SiGe superlattice, transistors with SiGe source/drain and epitaxial PZT thin films.