Dark-field Electron Holography Research Articles

Identification and Formation Mechanism of a New Interstitial Defect Found in Ion Implanted Silicon

For 40 years, extended defects are found in silicon after ion implantation and annealing. Widely studied for shallow implantation, thermal evolution of these defects follow the same path in the particular case of a Si layer transferred by Smart Cut™ technology.Being extrinsic or intrinsic precipitates, these defects are formed under supersaturated condition and grow by interchanging interstitials, or vacancies, during annealing. All along their growth, the precipitates rearrange them-selves to transform into a more stable structure.A transferred Si layer needs a combination of very high-temperature annealing in neutral and oxidizing atmospheres to thin and smooth the fractured surface. Despite these treatments, irregular topography has been detected on the extremely smooth surface. Studying them in detail, we have found a new type of interstitial-type crystalline defects, extremely stable, that we have named “tent-like” defects in reference to their shape. The presence of such structures was a surprise as it is generally believed that the high-temperature treatment following fracture was sufficient to dissolve all the precipitates initially present in the transferred Si layer.It is the goal of this work to report the characteristics, formation mechanism and origin of these structures. From that, we infer, and then experimentally confirm the process conditions promoting or hampering their formation.For this work, 300 nm-thick Si layers were transferred by using H+ and other species, wafer bonding and thermal initiation of the fracture. Afterwards, the transferred layers were subjected to different annealing steps, oxidizing or not, then annealed at temperatures up to 1200°C under Ar atmosphere.Figure 1a is a typical Scanning Electron Microscopy (SEM) image of one of these structures. Such images were obtained using a low voltage SEM with the ability to image mono-atomic steps separating flat (001) terraces all over the surface of a 300 mm wafer. Step bunching imaged on the right of the defect, as contour lines, reveals some “erosion” around the rectangular shape, at the origin of a shallow hole which was initially detected. In general, these structures are elongated along one of the two in-plane <110> directions (Fig. 1a). Their length increases during oxidizing annealing whereas their width stays almost constant. Since silicon oxidation results in the injection of Si interstitials into the underlying crystal, this growth suggests they are extrinsic defects i.e., contain excess Si atoms.Figure 1b is a HREM image of such a defect, cut in its body part, as shown in Fig. 1a. Two intersecting {111} stacking faults (SF) define a V-shape defect (Fig. 1b). Figure 1c shows the geometric phase distributions around the structure obtained by Dark Field Electron Holography (DFEH) for g= 004, g=111 and g=-1-11. From such images (no phase jump for g=004 and mirror symmetry between g=111 and g=-1-11), the Burgers vector of the dislocation, seen edge-on at the SFs intersection, is found to be 1/6[-1-10], i.e. that of a stair-rod Lomer-Cottrell dislocation. This finding suggests that the tent-like defect probably results from the dissociation of straight dislocation segments, each following a path such as 1/3<-1-11>=1/6<-1-10>+1/6<-1-12>.Based on these experimental data, we propose a scenario describing the formation of such structures. At the beginning an interstitial loop (Frank or perfect) was buried in the matrix. Such loops are known to form in ion implanted silicon annealed at high enough temperature. During oxidation, such a loop can grow up to, or may be cut by, the SiO2/Si interface. It then forms a half-loop. Based on energetic criteria, we show that such a half-loop tends to elongate and becomes segmented. During high-temperature annealing, the dislocation lines dissociate: a 2D defect transforms into a 3D “obtuse wedge elongated defect” bound by four {111} SFs and five segments of stair-rod Lomer-Cottrell dislocations. This dissociation is similar to the one describing the formation of tetrahedral defects from triangular Frank dislocation loops.In summary, we have found a new type of extended defects in Si, of interstitial character, which may appear as a result of the precipitation of silicon interstitials. This defect has not been identified before as the conditions for its formation are quite stringent. Beyond the process conditions under which they have been observed at first, the mechanisms which lead to their formation requires that a highly I-supersaturated region is initially located very close to a free surface (possibly an interface) where a half-dislocation loop can form and then dissociate, a condition rarely found in real cases with the notable exception of layers transferred by the Smart Cut™ process. Figure 1

Assessment of the strain depth sensitivity of Moiré sampling Scanning Transmission Electron Microscopy Geometrical Phase Analysis through a comparison with Dark-Field Electron Holography

In this study, the Moiré sampling Scanning Transmission Electron Microscopy Geometrical Phase Analysis (or STEM Moiré GPA) strain characterization method is compared to the well-established Dark-Field Electron Holography technique on a thin film stack grown by Molecular Beam Epitaxy. While experimental data obtained with the two techniques are, overall, in good qualitative agreement, small statistically relevant differences are locally observed between the two methods. The results obtained from both techniques are further confronted with Finite Element Method (FEM) mechanical simulations modeling the strain relaxation phenomena from a thin lamella. The FEM simulation highlights a non-uniform deformation field along the beam propagation direction with a higher deformation level near the surface of the lamella compared to the center of the same lamella. The center-surface strain differences obtained from modeling are consistent with the experimentally derived differences accounting for the fact that the SMG method is sensitive to the strain state of the surface of the lamella with a very narrow depth-of-field, and the DFEH technique is measuring the strain state of the center of the same lamella averaging over a large section of the thickness. The depth-of-field difference between both methods can be reasonably related to their respective contrast mechanisms (STEM vs Conventional Transmission Electron Microscopy). As the SMG method is using a convergent probe, the narrow depth-of-field might be used to sense the deformation field over different sections of the lamella using the defocus and potentially retrieve the three-dimensional strain field.

Highly spatially resolved mapping of the piezoelectric potentials in InGaN quantum well structures by off-axis electron holography

The internal fields in 2.2 nm thick InGaN quantum wells in a GaN LED structure have been investigated by using aberration-corrected off-axis electron holography with a spatial resolution of better than 1 nm. To improve the spatial resolution, different types of off-axis electron holography acquisitions have been used, including pi phase shifting and phase shifting holography. A series of electron holograms have been summed up to simultaneously improve the sensitivity in the measurements. A value of 20% of indium concentration in the quantum wells has been obtained by comparing the deformation measured by dark-field electron holography and geometrical phase analysis to finite element simulations. The electrostatic potential has then been measured by off-axis electron holography. The mean inner potential difference between the InGaN quantum wells and the GaN quantum barriers is high compared to the piezoelectric potential. Due to the improved spatial resolution, it is possible to compare the experimental results to simulations and remove the mean inner potential component to provide a quantitative measurement of the piezoelectric potential.

Dark-field electron holography as a recording of crystal diffraction in real space: a comparative study with high-resolution X-ray diffraction for strain analysis of MOSFETs

Diffraction-based techniques, with either electrons or photons, are commonly used in materials science to measure elastic strain in crystalline specimens. In this paper, the focus is on two advanced techniques capable of accessing strain information at the nanoscale: high-resolution X-ray diffraction (HRXRD) and the transmission electron microscopy technique of dark-field electron holography (DFEH). Both experimentally record an image formed by a diffracted beam: a map of the intensity in the vicinity of a Bragg reflection spot in the former, and an interference pattern in the latter. The theory that governs these experiments will be described in a unified framework. The role of the geometric phase, which encodes the displacement field of a set of atomic planes in the resulting diffracted beam, is emphasized. A detailed comparison of experimental results acquired at a synchrotron and with a state-of-the-art transmission electron microscope is presented for the same test structure: an array of dummy metal–oxide–semiconductor field-effect transistors (MOSFETs) from the 22 nm technology node. Both techniques give access to accurate strain information. Experiment, theory and modelling allow the illustration of the similarities and inherent differences between the HRXRD and DFEH techniques.

Compositional accuracy in atom probe tomography analyses performed on III-N light emitting diodes

Laser-assisted atom probe tomography (APT) and high-resolution dark-field electron holography (HR-DFEH) were performed to investigate the composition of a polar [0001] GaN/AlxGa1 − xN/InyGa1 − yN light emitting diode. In particular, the III-site fraction of both AlxGa1 − xN and InyGa1 − yN alloys was studied adopting a comparative approach. HR-DFEH allows mapping the projected strain with a subnanometer spatial resolution which is used for the calculation of the two-dimensional alloy composition distribution. APT provides three-dimensional alloys composition distribution with a nanometer spatial resolution. However, here we reveal that important inaccuracies affect local composition measurements. A Ga-poor composition is obtained in high DC-electric field regions. Moreover, such inaccuracies may be locally enhanced where the [0001] pole intersects the surface of the analyzed specimen, leading to a lower fraction of Ga measured. III-site fractions closer to the nominal values were measured at low field conditions. Ga loss is thought to be due to preferential DC field induced evaporation of Ga ions between laser pulses. This is explained in terms of formation of a metallic layer on the tip surface during APT analysis, where weak Ga-Ga bonds are formed, promoting the loss of Ga at high field conditions.

Dynamical diffraction effects on the geometric phase of inhomogeneous strain fields

In specimens with an inhomogeneous displacement field in electron beam direction dynamical diffraction effects lead to complex non-linear properties of the diffracted electron wave. Consequently, the diffracted beam’s phase contains information about the inhomogeneous displacement field. These phases are experimentally and theoretically investigated under different excitation errors and specimen thicknesses as well as for different depths of the displacement field. An inclined InGaAs layer with a larger lattice constant than the surrounding GaAs matrix serves as controlled displacement field, which is inhomogeneous in electron beam direction with a continuously changing depth. The phase and amplitude of the diffracted beam are measured by dark-field electron holography. The measurements agree with calculations performed by numerical propagation of the electron wave using the Darwin-Howie-Whelan equations. A strong dependency on the excitation conditions is found showing that the interplay between dynamical effects and the strain field must be considered in the interpretation of the geometric phase.

Strain evolution of SiGe-on-insulator obtained by the Ge-condensation technique

Compressively strained SiGe-On-Insulator (SGOI) made by the Ge-condensation technique is used as a performance booster for ultrathin fully depleted silicon-on-insulator transistor technology. Here, we report on the evolution of the compressive strain in the SiGe film along the formation of local SGOI. For this, experimental maps of lattice strain with nanometer spatial resolution have been obtained by dark-field electron holography and compared to results from numerical models describing the mechanics of the structures. In particular, we report on unexpected strain evolutions when the top semiconductor layer is patterned to fabricate the shallow trench isolations that separate the Si nMOS from the SiGe pMOS areas. Dramatic and long-range relaxation of the compressive SiGe layers occurs, while no extended defects are formed in the crystal. The phenomenon involves relative horizontal displacements between the SiGe layer and the underlying Buried Oxide (BOX). We suggest that the Ge-enrichment of the layer close to this interface by the Ge-condensation technique modifies the SiGe/BOX interface and that strain relaxation results from the propagation of some interfacial defects from the edge to the center of the structure, driven by the shear stress at the interface.

(Invited) Impact of Some Processing Steps onto the Strain Distributions in FD-SOI CMOS Planar Devices: A Contribution of Dark-Field Electron Holography

For long time considered as detrimental, stress is now an integral feature of modern electronic devices. Indeed, correctly oriented, the resulting strains may significantly increase carrier mobility and thus boost device performance. In this context, Dark-Field Electron Holography (DFEH) was used to study the impact of some key steps of the manufacturing process of 22 nm fully depleted planar transistors (FDSOI). DFEH is an interferometry technique based on Transmission Electron Microscopy recently invented and still being developed at CEMES-CNRS, which allows us to map crystalline strains with nanometer resolution and an accuracy of 10-4 over micrometer fields of view. The comparison of these experimental maps with simulation results obtained from Finite Element Modelling (FEM) allows one to explore, then possibly identify, the mechanical phenomena involved during the processing steps of FDSOI transistors. In general, samples are fabricated using a Focused Ion Beam equipment and thinned down to 80-110 nm. Thus they suffer some elastic relaxation at, and close to, the two free surfaces. FEM is used to reproduce the observed strain in the same thin “measured sample”, taking into account the influence of thickness on the electron wave amplitudes and resulting phase contributions to the final hologram, then to retrieve the strain which was originally present in the bulk sample. Based on our (long) experience, these physically sound refinements never alter the measurements by more than 15%, most often by a mere 5%, for the planar devices we observe. In other words, the mappings directly obtained by DFEH reproduce very well the actual strain fields which reside in the transistor before thinning. Following the different steps leading to the co-integration of n and p-MOS, from the “Ge condensation technique” to locally form a thin highly compressively strained SiGe layer to the opening of trenches, we have evidenced abnormal large strain relaxations of these layers at the trench edges, extending well beyond elasticity predictions. Numerical approaches suggest some “weakened interface” exist between the SiGe layer and the BOX. It can be simulated introducing weak elastic spring forces to bind the interface, instead of the foreseen rigid connection. Later confirmed by Raman and electrical studies and despite various hypothesis have been investigated (H, Ge-O bonds…), this phenomenon remains unexplained. The figure shown above is a composite image of the strain along the channel of the foreseen n-MOS after the fabrication of the gate stack on the Si thin top layer. The stack was deposited “full sheet” and involves several different materials and energetic thermal treatments (oxidation, ALD, CVD, activation annealing, etc). In full sheet, the internal stresses cannot relax. Then, when the gates are fabricated by photolithography and etching, the symmetry breaking (edges) allows the various layers to react to these internal stresses and deform. This image shows that the channel is under tension (in plane) below the gate, with a maximum strain of about 0.8%. The gate tends to relax some internal compressive stress and deforms the underlying, originally unstrained Si channel. The same phenomenon is observed on the SiGe layers for p-MOS transistors, which are under compression after their fabrication by the Ge enrichment technique. The stress exerted by the gate is mostly due to presence of the TiN contact which, when back at room temperature after deposition, is highly compressively stressed (-2.35 GPa). This contact transfers its deformation through the gate dielectric putting the channel under tension. This effect is consolidated by the opposite effect of the SiN spacer (intrinsically tensile) which tends to compress further the Si film on both sides of the channel. Finally, we will follow two subsequent and important processing steps; the fabrication of the raised source and drain and of the contact silicide. Both steps affect the strain state of the channel of the devices (n- and p-MOS). The composition of the source and drain (Si, Ge, B, P, C…) can be craftily adjusted to manipulate further strain in the channel of the devices. Silicidation of the source and drain has almost no effect on the strain state of the channel as the raised source and drain can relax laterally without affecting the channel. Partial BOX creeping at high temperature and strain relaxation at free edges are the main ingredients needed to understand and simulate the strain behavior of the channel during these fabrication steps. In most cases, DFEH images not only provide more precise results than Nano Beam Diffraction (NBD) but additional important information such as the presence of defects, precipitates, heterogeneous compositions, etc., all locally but strongly impacting strain distributions. Figure 1

(Invited) Impact of Some Processing Steps onto the Strain Distributions in FD-SOI CMOS Planar Devices: A Contribution of Dark-Field Electron Holography

Clever strategies have been set up and are today routinely used to generate strains of desired directions and amplitudes in the channels of CMOS transistors. Unfortunately, many of the processing steps which follow this strain engineering have a strong impact on the final strain state of these channels, sometimes even not foreseen. Here, we report on the use of Dark-Field Electron Holography to image strain in the channels of FD-SOI CMOS devices all along their fabrication route, from the fabrication of the co-integrated Si-SiGe layers to contact formation. We show that, in general, good background knowledge of the different materials characteristics and of the physics of the process, as well notions of structural mechanics, are enough to understand then eventually model the impact of these processing steps. However, our results evidence a particular mechanical weakness of the SiGe/BOX interface following the Ge condensation process which seriously challenges strain manipulation in the pMOS channel.

Lattice contraction due to boron doping in silicon

In modern electronic devices, strain is used to increase carrier mobility. It is thus mandatory to know precisely the effect of doping on the lattice parameter of silicon. However, there are many experimental biases which prevent one from measuring this effect with high accuracy. For this reason, we have designed and fabricated a step-like structure consisting of five 50 nm-thick Si layers of increasing substitutional boron concentrations. Then, we have used Dark Field Electron Holography, a Transmission Electron Microscopy based technique, to measure the strain in these pseudomorphic and defect-free layers. Using Finite Element Modelling, we show that the observed out-of-plane strains can be reproduced by assuming that the expansion coefficient of boron is about − 6.5 × 10–24 cm3. This value is slightly larger (in absolute value) than those previously reported. It is otherwise about 20% larger than predicted from the size mismatch between B and Si atoms.

Off-axis electron holography combining summation of hologram series with double-exposure phase-shifting: Theory and application

In this paper we discuss developments for Lorentz mode or “medium resolution” off-axis electron holography such that it is now routinely possible obtain very high sensitivity phase maps with high spatial resolution whilst maintaining a large field of view. Modifications of the usual Fourier reconstruction procedure have been used to combine series of holograms for sensitivity improvement with a phase-shifting method for doubling the spatial resolution. In the frame of these developments, specific attention is given to the phase standard deviation description and its interaction with the spatial resolution as well as the processing of reference holograms. An experimental study based on Dark-Field Electron Holography (DFEH), using a SiGe/Si multilayer epitaxy sample is compared with theory. The method’s efficiency of removing the autocorrelation term during hologram reconstruction is discussed. Software has been written in DigitalMicrograph that can be used to routinely perform these tasks. To illustrate the real improvements made using these methods we show that a strain measurement sensitivity of ± 0.025 % can be achieved with a spatial resolution of 2 nm and ± 0.13 % with a spatial resolution of 1 nm whilst maintaining a useful field of view of 300 nm. In the frame of these measurements a model of strain noise for DFEH has also been developed.

Local deformation gradients in epitaxial Pb(Zr0.2Ti0.8)O3 layers investigated by transmission electron microscopy

Lead zirconate titanate samples are used for their piezoelectric and ferroelectric properties in various types of micro-devices. Epitaxial layers of tetragonal perovskites have a tendency to relax by forming ferroelastic domains. The accommodation of the a/c/a/c polydomain structure on a flat substrate leads to nanoscale deformation gradients which locally influence the polarization by flexoelectric effect. Here, we investigated the deformation fields in epitaxial layers of Pb(Zr0.2Ti0.8)O3 grown on SrTiO3 substrates using transmission electron microscopy (TEM). We found that the deformation gradients depend on the domain walls inclination ( or to the substrate interface) of the successive domains and we describe three different a/c/a domain configurations: one configuration with parallel a-domains and two configurations with perpendicular a-domains (V-shaped and hat--shaped). In the parallel configuration, the c-domains contain horizontal and vertical gradients of out-of-plane deformation. In the V-shaped and hat--shaped configurations, the c-domains exhibit a bending deformation field with vertical gradients of in-plane deformation. Each of these configurations is expected to have a different influence on the polarization and so the local properties of the film. The deformation gradients were measured using dark-field electron holography, a TEM technique, which offers a good sensitivity (0.1%) and a large field-of-view (hundreds of nanometers). The measurements are compared with finite element simulations.

Impact of Some Processing Steps onto the Strain Distributions in FD-SOI CMOS Planar Devices: A Contribution of Dark-Field Electron Holography

Clever strategies have been set up and are today routinely used to generate strains of desired directions and amplitudes in the channels of CMOS transistors. Unfortunately, many of the processing steps which follow this strain engineering have a strong impact on the final strain state of these channels, sometimes even not foreseen. Here, we report on the use of Dark-Field Electron Holography to image strain in the channels of FD-SOI CMOS devices all along their fabrication route, from the fabrication of the co-integrated Si-SiGe layers to contact formation. We show that, in general, good background knowledge of the different materials characteristics and of the physics of the process, as well notions of structural mechanics, are enough to understand then eventually model the impact of these processing steps. However, our results evidence a particular mechanical weakness of the SiGe/BOX interface following the Ge condensation process which seriously challenges strain manipulation in the pMOS channel.

2D strain mapping using scanning transmission electron microscopy Moiré interferometry and geometrical phase analysis

A strain characterization technique based on Moiré interferometry in a scanning transmission electron microscope (STEM) and geometrical phase analysis (GPA) method is demonstrated. The deformation field is first captured in a single STEM Moiré hologram composed of multiple sets of periodic fringes (Moiré patterns) generated from the interference between the periodic scanning grating, fixing the positions of the electron probe on the sample, and the crystal structure. Applying basic principles from sampling theory, the Moiré patterns arrangement is then simulated using a STEM electron micrograph reference to convert the experimental STEM Moiré hologram into information related to the crystal lattice periodicities. The GPA method is finally applied to extract the 2D relative strain and rotation fields. The STEM Moiré interferometry enables the local information to be de-magnified to a large length scale, comparable to what can be achieved in dark-field electron holography. The STEM Moiré GPA method thus extends the conventional high-resolution STEM GPA capabilities by providing comparable quantitative 2D strain mapping with a larger field of view (up to a few microns).

Morphology of GaAs crystals heterogeneously integrated on nominal (001) Si by epitaxial lateral overgrowth on tunnel oxide via Ge nano-seeding

In this study, GaAs crystals were grown by chemical beam epitaxy on nominal (001) Si substrate over Ge nano-seeds placed within nano-holes opened through a 0.6nm silica layer. GaAs crystal morphology and atomic organization at the interface between Ge and GaAs were studied by using complementary Scanning Transmission Electron Microscopy, energy dispersive X-ray spectrometry and dark-field electron holography. Fourfold symmetry GaAs crystals were obtained and found to be completely relaxed and twin free. Thus, the use of Ge nano-seeds to initiate the growth of GaAs results in the suppression of twins previously observed for direct GaAs growth on nominal (001) Si. Nevertheless, anti-phase domains were detected. A simple atomistic model is proposed which explains how Anti-Phase Boundaries develop at the junctions between Ge {113} facets, and {113} and {111} facets for As-stabilized GaAs growth.

Strain-induced indium clustering in non-polar a-plane InGaN quantum wells

In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.

An Application of High-Resolution Dual-Lens Dark-Field Electron Holography in Strain Analysis for Nanometer Semiconductor Device in Wafer-foundries

An Application of High-Resolution Dual-Lens Dark-Field Electron Holography in Strain Analysis for Nanometer Semiconductor Device in Wafer-foundries

Strain measurement in ferromagnetic crystals using dark-field electron holography

This study proposes a method to separate the geometric phase shift due to lattice strain from the undesired phase information, resulting from magnetic fields that are superposed in the dark-field electron holography (DFEH) observations. Choosing a distinct wave vector for the Bragg reflection reversed the sense of the geometric phase shift, while the sense of the magnetic information remained unchanged. In the case of an Nd-Fe-B permanent magnet, once the unwanted signal was removed by data processing, the residual phase image revealed a strain map. Even though the applications of DFEH have thus far been limited to non-magnetic systems, the method proposed in this work is also applicable to strain measurements in various ferromagnetic systems.

Strain mapping in a nanoscale-triangular SiGe pattern by dark-field electron holography with medium magnification mode.

Recent years have seen a great deal of progress in the development of transmission electron microscopy-based techniques for strain measurement. Dark-field electron holography (DFEH) is a new technique offering configuration of the off-axis principle. Using this technique with medium magnification (Holo-M), we carried out strain measurements in nanoscale-triangular SiGe/(001) Si with (004), (2-20) and (-111) diffraction spots. The reconstruction of holograms and interpretation of strain maps in term of strain precision were discussed and the strain distributions in the SiGe/(001) Si patterns were visualized. Based on linear anisotropic elastic theory for strain simulation, the simulated results obtained by the finite element method compared with the experimental results acquired byDFEH. The strain values were found to be 0.9-1.0%, 1.1-1.2% and 1.0-1.1%, for the (004), (2-20) and (-111) diffracted beams, respectively, and the strain precisions were determined to be ~2.1×10-3, 3.2×10-3 and 9.1×10-3 for the corresponding diffraction spots. As a result, DFEH is highlighted as a powerful technique for strain measurement, offering high-strain precision, high-spatial resolution and a large field of view.

(Invited) Transistor Strain Measurement Using Electron Beam Techniques

Strain and the related stress in nanoelectronic devices play a critical role in semiconductor technologies. In general, strain arises from the lattice mismatch between dissimilar materials and thus strain is critical for the design and growth of epitaxial heterostructures in micro- and nano-electronics. In transistors, strained silicon channels provide a major boost to the performance of metal-oxide-semiconductor field-effect transistors (MOSFETs), e.g., the use of epitaxial strain of Si and Ge alloys in PMOS [1]. In the near term, the transistor technology is pushing towards the sub 10 nm technology node. At this length scale, resolving strain in the devices requires a resolution of sub nanometer with not only high resolution, also high accuracy and reliability. The use of novel architectures and alternative materials in the transistor technology also demands metrology capable of handling these structural and material changes. High resolution strain measurement can be performed using high energy electrons either in reciprocal space using convergent beam electron diffraction (CBED), scanning electron nanodiffraction (SEND), nanobeam electron diffraction (NBED) [2], or scanning precession electron diffraction (SPED) or in real space through direct imaging using high resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) [3] or dark-field electron holography (DFEH)[4]. The resolution and strain measurement sensitivities range from Å to several nm and 10-3 to 10-4respectively. Recent developments in aberration corrected STEM especially have improved the spatial resolution to sub- Å and the precision of measuring atomic column positions to picometers [3]. This talk will review high resolution and high sensitivity electron beam techniques for strain characterization in nanodevices and device materials. Examples include planar and a finfet transistors and III-V heterostructures. For finfet transistors, we demonstrate that NBED provides an effective approach for strain mapping (See figure below) and some of challenges will also be highlighted. In III-V heterostructures, we will describe atomic resolution Z-contrast imaging based methods for high resolution interfacial strain analysis and its applications for the investigation of interfacial intermixing and atomic layer-by-layer strain profiles in InAs/GaSb and InAs/InAsSb superlattices, targeted for middle- and long-wavelength IR detection.