Thibaud Denneulin

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 2u2009×u200910−4 is obtained with a probe size approaching 1u2009nm in diameter.

Strain mapping of semiconductor specimens with nm-scale resolution in a transmission electron microscope

The last few years have seen a great deal of progress in the development of transmission electron microscopy based techniques for strain mapping. New techniques have appeared such as dark field electron holography and nanobeam diffraction and better known ones such as geometrical phase analysis have been improved by using aberration corrected ultra-stable modern electron microscopes. In this paper we apply dark field electron holography, the geometrical phase analysis of high angle annular dark field scanning transmission electron microscopy images, nanobeam diffraction and precession diffraction, all performed at the state-of-the-art to five different types of semiconductor samples. These include a simple calibration structure comprising 10-nm-thick SiGe layers to benchmark the techniques. A SiGe recessed source and drain device has been examined in order to test their capabilities on 2D structures. Devices that have been strained using a nitride stressor have been examined to test the sensitivity of the different techniques when applied to systems containing low values of deformation. To test the techniques on modern semiconductors, an electrically tested device grown on a SOI wafer has been examined. Finally a GaN/AlN superlattice was tested in order to assess the different methods of measuring deformation on specimens that do not have a perfect crystalline structure. The different deformation mapping techniques have been compared to one another and the strengths and weaknesses of each are discussed.

Strain mapping with nm-scale resolution for the silicon-on-insulator generation of semiconductor devices by advanced electron microscopy

Strain engineering in the conduction channel is a cost effective method of boosting the performance in state-of-the-art semiconductor devices. However, given the small dimensions of these devices, it is difficult to quantitatively measure the strain with the required spatial resolution. Three different transmission electron microscopy techniques, high-angle annular dark field scanning transmission electron microscopy, dark field electron holography, and nanobeam electron diffraction have been applied to measure the strain in simple bulk and SOI calibration specimens. These techniques are then applied to different gate length SiGe SOI pFET devices in order to measure the strain in the conduction channel. For these devices, improved spatial resolution is required, and strain maps with spatial resolutions as good as 1u2009nm have been achieved. Finally, we discuss the relative advantages and disadvantages of using these three different techniques when used for strain measurement.

Characterization and modeling of structural properties of SiGe/Si superlattices upon annealing

Stacked multichannel or nanowire CMOS transistors are foreseen as viable options in future technology nodes. Superior electric performances and a relative immunity to short channel effects have already been demonstrated in such devices. They rely on (i) the epitaxy of SiGe/Si superlattices, (ii) the anisotropic etching of the source and drain (S/D) blocks and the channels, and (iii) the high degree of selectivity that can be achieved when laterally etching the SiGe sacrificial layers. The voids left by the removal of SiGe are then conformally filled by HfO2/TiN/poly-Si gates, leading to the formation of multichannel devices. Doping elements can be included in situ in the SiGe layers during the epitaxial step in order to achieve a proper S/D doping after annealing. Precise knowledge of the diffusion behavior of all species is then crucial to understand and tailor final device performance. In this work, we investigated the properties of intrinsic or in situ doped (with B, C, or P) SiGe/Si superlattices upon...

The addition of strain in uniaxially strained transistors by both SiN contact etch stop layers and recessed SiGe sources and drains

SiN contact etch stop layers (CESL) and recessed SiGe sources/drains are two uniaxial strain techniques used to boost the charge carriers mobility in p-type metal oxide semiconductor field effect transistors (pMOSFETs). It has already been shown that the electrical performances of the devices can be increased by combining both of these techniques on the same transistor. However, there are few experimental investigations of their additivity from the strain point of view. Here, spatially resolved strain mapping was performed using dark-field electron holography (DFEH) on pMOSFETs transistors strained by SiN CESL and embedded SiGe sources/drains. The influence of both processes on the strain distribution has been investigated independently before the combination was tested. This study was first performed with non-silicided devices. The results indicated that in the channel region, the strain induced by the combination of both processes is equal to the sum of the individual components. Then, the same investigation was performed after Ni-silicidation of the devices. It was found that in spite of a slight reduction of the strain due to the silicidation, the strain additivity is approximately preserved. Finally, it was also shown that DFEH can be a useful technique to characterize the strain field around dislocations.

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.

Practical aspects of strain measurement in thin SiGe layers by (004) dark-field electron holography in Lorentz mode.

Dark-field electron holography (DFEH) is a powerful transmission electron microscopy technique for mapping strain with nanometer resolution and high precision. However the technique can be difficult to set up if some practical steps are not respected. In this article, several measurements were performed on thin Si(1-x)Gex layers using (004) DFEH in Lorentz mode. Different practical aspects are discussed such as sample preparation, reconstruction of the holograms and interpretation of the strain maps in terms of sensitivity and accuracy. It was shown that the measurements are not significantly dependent on the preparation tool. Good results can be obtained using both FIB and mechanical polishing. Usually the most important aspect is a precise control of the thickness of the sample. A problem when reconstructing (004) dark-field holograms is the relatively high phase gradient that characterises the strained regions. It can be difficult to perform reconstructions with high sensitivity in both strained and unstrained regions. Here we introduce simple methods to minimise the noise in the different regions using a specific mask shape in Fourier space or by combining several reconstructions. As a test, DFEH was applied to the characterization of eight Si(1-x)Gex samples with different Ge concentrations. The sensitivity of the strain measured in the layers varies between 0.08% and 0.03% for spatial resolutions of 3.5-7 nm. The results were also compared to finite element mechanical simulations. A good accuracy of ±0.1% between experiment and simulation was obtained for strains up to 1.5% and ±0.25% for strains up to 2.5%.

Advanced GeSn/SiGeSn Group IV Heterostructure Lasers

Abstract Growth and characterization of advanced group IV semiconductor materials with CMOS‐compatible applications are demonstrated, both in photonics. The investigated GeSn/SiGeSn heterostructures combine direct bandgap GeSn active layers with indirect gap ternary SiGeSn claddings, a design proven its worth already decades ago in the III–V material system. Different types of double heterostructures and multi‐quantum wells (MQWs) are epitaxially grown with varying well thicknesses and barriers. The retaining high material quality of those complex structures is probed by advanced characterization methods, such as atom probe tomography and dark‐field electron holography to extract composition parameters and strain, used further for band structure calculations. Special emphasis is put on the impact of carrier confinement and quantization effects, evaluated by photoluminescence and validated by theoretical calculations. As shown, particularly MQW heterostructures promise the highest potential for efficient next generation complementary metal‐oxide‐semiconductor (CMOS)‐compatible group IV lasers.

HRTEM Studies of Stress Assisted Sintered BaLa4Ti4O15.

The role of interfaces on the grain growth of polycrystals during sintering has been recently recalled into attention and important theoretical work has been developed, supported by experimental results on ceramic oxide materials [1,2]. It was shown that by controlling the nature of the interfaces one can tune the grain growth during sintering and, therefore, the subsequent control of related properties is envisaged. One way to control the interface nature is by changing the sintering conditions and there are many studies proving the importance of the sintering temperature, holding time, heating rate and atmosphere, but very few about the effect of an external pressure applied during the consolidation process [3, 4]. nIn this work we are presenting a HRTEM study of BaLa4Ti4O15 (BLT) ceramics sintered under compressive stresses applied by Hot Pressing (HP) and Hot Isostatic Pressing (HIP) and the comparison with a free sintered sample. This study clearly reveals that the external pressure has a say on the nature of the grain boundaries in BLT.

Electron microscopy by specimen design: application to strain measurements

A bewildering number of techniques have been developed for transmission electron microscopy (TEM), involving the use of ever more complex combinations of lens configurations, apertures and detector geometries. In parallel, the developments in the field of ion beam instruments have modernized sample preparation and enabled the preparation of various types of materials. However, the desired final specimen geometry is always almost the same: a thin foil of uniform thickness. Here we will show that judicious design of specimen geometry can make all the difference and that experiments can be carried out on the most basic electron microscope and in the usual imaging modes. We propose two sample preparation methods that allow the formation of controlled moiré patterns for general monocrystalline structures in cross-section and at specific sites. We developed moiré image treatment algorithms using an absolute correction of projection lens distortions of a TEM that allows strain measurements and mapping with a nanometer resolution and 10−4 precision. Imaging and diffraction techniques in other fields may in turn benefit from this technique in perspective.