Width determination of SiO2-films in Si-based devices using low-loss EFTEM: image contrast as a function of sample thickness

Energy filtering transmission electron microscopy (EFTEM) has become one of the most efficient tools for specimen characterization at nanometer length scales. EFTEM imaging is most often carried out in the core-loss region but image intensity becomes more and more a limiting factor with decreasing feature size. Alternatively, it is possible to record EFTEM images in the low-loss region, where intensities are essentially higher and where in many cases the images contain material specific contrasts. In this paper we investigate the influence of the important parameters on the material contrast between silicon and silicon dioxide, e.g. specimen thickness, specimen orientation, energy-loss and energy selecting slit width. We show that sample thickness plays an important role and present two methods to calculate material contrast as a function of energy-loss and sample thicknesses. The first method uses spectra taken from both materials at different sample thickness by electron energy-loss spectroscopy, the second calculates contrast directly from a series of energy filtered images. From the results we determine the ideal acquisition parameters for the Si/SiO(2) system and demonstrate imaging at sufficient resolution below 2nm with a test sample of thin SiO(2) layers on Si.     

Size and density control of silicon oxide nanowires by rapid thermal annealing and their growth mechanism

In this work, we demonstrate a fast approach to grow SiO₂ nanowires by rapid thermal annealing (RTA). The material characteristics of SiO₂ nanowires are investigated by field emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field (HAADF) imaging, electron energy loss spectroscopy (EELS), and energy-filtered TEM (EFTEM). The HAADF images show that the wire tip is predominantly composed of Pt with brighter contrast, while the elemental mappings in EFTEM and EELS spectra reveal that the wire consists of Si and O elements. The SiO₂ nanowires are amorphous with featureless contrast in HRTEM images after RTA at 900°C. Furthermore, the nanowire length and diameter are found to be dependent on the initial Pt film thickness. It is suggested that a high SiO₂ growth rate of >1 μm/min can be achieved by RTA, showing a promising way to enable large-area fabrication of nanowires.     

Transmission electron microscopy and X-ray diffraction study of microstructural evolution in magnetoresistive Cu–Fe–Ni ribbons

The evolution of the microstructure of a granular Cu₈₀Fe₁₀Ni₁₀ (at%) melt-spun ribbon is studied by transmission electron microscopy (TEM), energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction. This system is interesting as large giant magnetoresistance (GMR) values have been measured for this composition. We have shown the presence of two face-centred cubic phases, an (Fe,Ni)-rich phase and a Cu-rich phase. The lattice parameters of these two phases are close and no diffraction or elastic contrast is involved in displaying the two phases in TEM bright-field mode. With EFTEM imaging, we have shown the presence of a fine-scale (Fe,Ni)-rich precipitation inside the Cu-rich fcc matrix. The precipitates are 2–4 nm in the as-spun state and 4–6 nm after annealing for 2 h at 400°C. The lattice parameter of the Cu-rich phase in the as-spun sample is 0.3608 nm and 0.3610 nm for the (Fe,Ni)-rich phase. After a 24-h annealing treatment at 600°C, the mean diameter of the particle is 20 nm and the lattice parameter of the (Fe,Ni)-rich phase has decreased to 0.3600 nm, while that of the Cu-rich phase has increased to 0.3613 nm, which is consistent with a segregation of Fe and Ni in the precipitates. The composition and volume fraction of the two phases measured for this annealed sample are in good agreement with the Thermocalc® predictions.     

Measurement of γ′ precipitates in a nickel-based superalloy using energy-filtered transmission electron microscopy coupled with automated segmenting techniques

Precipitates of the ordered L1₂ γ′ phase (dispersed in the face-centered cubic or FCC γ matrix) were imaged in Rene 88 DT, a commercial multicomponent Ni-based superalloy, using energy-filtered transmission electron microscopy (EFTEM). Imaging was performed using the Cr, Co, Ni, Ti and Al elemental L-absorption edges in the energy loss spectrum. Manual and automated segmentation procedures were utilized for identification of precipitate boundaries and measurement of precipitate sizes. The automated region growing technique for precipitate identification in images was determined to measure accurately precipitate diameters. In addition, the region growing technique provided a repeatable method for optimizing segmentation techniques for varying EFTEM conditions.     

New techniques in electron energy-loss spectroscopy and energy-filtered imaging

This article is a survey of hardware and software advances that promise to increase the power and sensitivity of electron energy-loss spectroscopy (EELS) and energy-filtered imaging (EFTEM) in a transmission electron microscope. Recent developments include electron-gun monochromators, lens-aberration correctors, and software for spectral sharpening, spectral processing and interpretation of fine structure. Future improvements could include the deployment of new electron sources. The expected enhancements in energy and spatial resolution are compared with fundamental limitations that arise from the natural widths of spectral peaks, the delocalization of inelastic scattering and the problem of electron-irradiation damage.     

Inelastic electron scattering observation using energy filtered transmission electron microscopy for silicon -- germanium nanostructures imaging

This paper presents a new technique using energy filtered TEM (EFTEM) for inelastic electron scattering contrast imaging of Germanium distribution in Si-SiGe nanostructures. Comparing electron energy loss spectra (EELS) obtained in both SiGe and Si single crystals, we found a spectrum area strongly sensitive to the presence of Ge in the range [50-100 eV]. In this energy loss window, EELS spectrum shows a smooth steeply shaped background strongly depending on Ge concentration. Germanium mapping inside SiGe can thus be performed through imaging of the EELS background slope variation, obtained by processing the ratio of two energy filtered TEM images, respectively, acquired at 90 and 60 eV. This technique gives contrasted images strongly similar to those obtained using STEM Z-contrast, but presenting some advantages: elastic interaction (diffraction) is eliminated, and contrast is insensitive to polycrystalline grains orientation or specimen thickness. Moreover, since the extracted signal is a spectral signature (inelastic energy loss) we demonstrate that it can be used for observation and quantification of Ge concentration depth profile of SiGe buried layers.     

Quantitative TEM analysis of Al/Cu multilayer systems prepared by pulsed laser deposition

Thin films composed of alternating Al/Cu/Al layers were deposited on a (111) Si substrate using pulsed laser deposition (PLD). The thicknesses of the film and the individual layers, and the detailed internal structure within the layers were characterized by means of transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-filtered TEM (EFTEM). Each Al or Cu layer consists of a single layer of nano-sized grains of different orientations. EFTEM results revealed a layer of oxide about 2 nm thick on the surface of the Si substrate, which is considered to be the reason for the formation of the first layer of nano-sized Al grains. The results demonstrate that the PLD technique is a powerful tool to produce nano-scale multilayered metal films with controllable thickness and grain sizes.     

Electron irradiation-enhanced core/shell organization of Al(Cr,Fe, Mn)Si dispersoids in Al–Mg–Si alloys

The age hardening 6061-T6 aluminium alloy has been chosen as structural material for the core vessel of the material testing Jules Horowitz nuclear reactor. The alloy contains incoherent Al(Cr, Fe, Mn)Si dispersoids whose characterization by energy-filtered transmission electron microscopy (EFTEM) analysis shows a core/shell organization tendency where the core is (Mn, Fe) rich, and the shell is Cr rich. The present work studies the stability of this organization under irradiation. TEM characterization on the same particles, before and after 1 MeV electron irradiation, reveals that the core/shell organization is enhanced after irradiation. It is proposed that the high level of point defects, created by irradiation, ensures a radiation-enhanced diffusion process favourable to the unmixing forces between (Fe, Mn) and Cr. Shell formation may result in the low-energy interface segregation of Cr atoms within the (Fe, Mn) system combined with the unmixing of Cr, Fe and Mn components.     

Elastic strain determination in semiconductor epitaxial layers by HREM

A new technique that is independent of image contrast and robust to the presence of experimental noise is presented to analyze strains from high resolution electron microscopy (HREM) lattice images. This approach involves the analysis of the cumulative sum of deviations (CUSUM) in lattice-fringe spacings from a target value. The effects of surface roughness at an interface, and surface relaxation due to transmission electron microscope (TEM) sample preparation are discussed. The CUSUM method was applied to two simulated and two experimental HREM images of semiconductor strained layer structures in [ ] zone axis projection. The CUSUM technique was able to accurately reproduce the strain profiles from the simulated and experimental images in all cases studied except for the component of the strain in the slip direction (e_xx) of an edge dislocation in a simulated image. In this case, the strain field near the core appeared hemispherical rather than lobed as expected.     

HRTEM image contrast of short range order in Ni4Mo

The high resolution transmission electron microscope (HRTEM) imaging of short range order (SRO) in Ni4Mo was investigated by means of multi-slice image simulations. The HRTEM images of Ni4Mo exhibit locally bright dot patterns corresponding to the [001] projections of the N2M2-type (chalcopyrite-like) structure. The multi-slice simulations revealed that the N2M2 patterns are rationalized as the projection patterns of the SRO structure which consists of subunit cell clusters of D1a, D022 and Pt2Mo structures. The N2M2-type image contrast appears when both the fundamental fcc lattice reflections and the 1 1/2 0 diffuse scattering of SRO contribute enough to imaging. This suggests that a good coincidence in intensity distribution between the Fourier power spectra of HRTEM images and the electron diffraction patterns is one of the conditions for the image contrast of SRO to be interpreted in terms of the projection contrast.     

Parameter maps of 1H residual dipolar couplings in tendon under mechanical load

Proton multipolar spin states associated with dipolar encoded longitudinal magnetization (DELM) and double-quantum (DQ) coherences of bound water are investigated for bovine and sheep Achilles tendon under mechanical load. DELM decay curves and DQ buildup and decay curves reveal changes of the 1H residual dipolar couplings for tendon at rest and under local compression forces. The multipolar spin states are used to design dipolar contrast filters for NMR 1H images of heterogeneous tendon. Heterogeneities in tendon samples were artificially generated by local compression parallel and perpendicular to the tendon plug axis. Quotient images obtained from DQ-filtered images by matched and mismatched excitation/reconversion periods are encoded only by the residual dipolar couplings. Semi-quantitative parameter maps of the residual dipolar couplings of bound water were obtained from these quotient images using a reference elastomer sample. This method can be used to quantify NMR imaging of injured ordered tissues.     

Interface characterization and indium content of indium-rich quantum dots in InGaN/GaN multiple quantum wells

We report on the interface characterization of InGaN/GaN multiple quantum wells with indium aggregation grown by metalorganic chemical vapor deposition. The interface related microstructure was analyzed by high-resolution transmission electron microscopy, high-resolution X-ray diffraction and high angle annular dark field. Luminescence measurements were carried out by micro-photoluminescence measurement. In addition, quantitative determination of the indium concentration inside the ultra-small dots was attempted. We demonstrate that the quantum dots are coherent and the interfaces remain sharp. The In content inside ∼2 nm InGaN dots is about 65% determined by spectrum imaging in energy-filtered transmission electron microscopy combined with multiple linear least squares fitting, which is slighter higher than the value obtained either from HRTEM or theoretical calculations. This discrepancy is briefly discussed but demands further studies for complete understanding.     

Atypical X-linked adrenoleukodystrophy: new MRI observations with FLAIR,magnetization transfer contrast,diffusion MRI,and proton spectroscopy

An 11-year-old boy with an atypical form of X-linked adrenoleukodystrophy is reported predominantly involving the frontal lobes, and later spreading to temporal lobes. Magnetization transfer contrast T1-weighted, and FLAIR images without intravenous paramagnetic contrast medium, clearly identified the leading edge of a central necrotic zone, as well as a splenial lesion. Zonal differences were identified on ADC maps of an echo-planar diffusion MRI sequence that the ADC value of central zone (2.06 x 10(-3) mm(2)/sec) was higher than that of the peripheral zone (1.67 x 10(-3) mm(2)/sec). On proton spectroscopy, besides changes in the peaks of NAA, choline, and myoinositol, prominent peaks between 0.9 and 1.4 ppm were shown belonging to macromolecules, probably to very long chain fatty acids, a diagnostic feature of adrenoleukodystrophy. In addition, a distinct and prominent glycine peak was observed at 3.50 ppm, reflecting excitotoxic brain damage.     

Comparison of laser Doppler and laser speckle contrast imaging using a concurrent processing system

Full field laser Doppler imaging (LDI) and single exposure laser speckle contrast imaging (LSCI) are directly compared using a novel instrument which can concurrently image blood flow using both LDI and LSCI signal processing. Incorporating a commercial CMOS camera chip and a field programmable gate array (FPGA) the flow images of LDI and the contrast maps of LSCI are simultaneously processed by utilizing the same detected optical signals. The comparison was carried out by imaging a rotating diffuser. LDI has a linear response to the velocity. In contrast, LSCI is exposure time dependent and does not provide a linear response in the presence of static speckle. It is also demonstrated that the relationship between LDI and LSCI can be related through a power law which depends on the exposure time of LSCI.     

X射线菲涅耳波带板成像和投影式相衬成像的数值模拟比较

通过解析分析和数值模拟,比较了钛K线（4.5 keV）与铜K线（8.0 keV）等X射线源背光透视物体情况下,菲涅耳波带板直接成像与投影式相衬成像对被透视物体的空间分辨能力。结果表明,波带板成像可实现优于1 m的高空间分辨能力,而且使用较大尺度背光源更有利于成像。对于高透射或弱吸收的透视物体,波带板难以成像,可采用投影式相衬成像实现m级空间分辨。计入了以前文献没有考虑到的更高阶影响后,解析给出了点光源照射下相衬像的强度分布与对比度。模拟了微焦点X射线源照射存在厚度起伏的薄膜靶以及密度空间调制靶的相衬成像,点光源情况下模拟结果和解析结果相符。讨论了光源大小、成像距离等参数对相衬成像对比度和空间分辨能力的影响,结果表明,通过减小光源尺度和调节物体到探测面的距离,空间分辨能力可优化到1～4 m。     

Evaluation of particle size distributions of precipitates in a 9% chromium steel using energy filtered transmission electron microscopy

Energy filtered transmission electron microscopy (EFTEM) has been used to evaluate particle size distributions of different phases in the creep resistant 9% chromium steel P92, aged at 600 or 650°C for up to 26,000 h. Precipitates of type VN, M₂₃C₆ and Laves phase were visualised by forming so-called jump ratio images of the elements vanadium, chromium and tungsten. Digital image analysis was used to measure the size of the precipitates at different ageing times. As a complement to EFTEM, atom probe field ion microscopy (APFIM) has been used to monitor the composition of the ferritic matrix. This indirectly gives information about precipitated volume fraction of different phases. In the case of VN precipitates the volume fraction was also determined from EFTEM images and the result was in good agreement with the data obtained with APFIM. The high-temperature stability of the secondary phases present in steel P92 and their relation to creep properties are discussed. To further study the applicability of EFTEM, steel P122 (with a composition similar to steel P92, but with an addition of Cu) was investigated and a Cu-rich phase was found.     

Novel contrast mechanisms in photoelectron microscopy

Two previously undiscussed contrast mechanisms in Auger and photoelectron microscopy, namely photoelectron diffraction contrast and molecular orbital orientation contrast, are presented. The former contrast mechanism is based on the phenomenon of photoelectron diffraction and forward focusing of Auger and photoelectrons, the latter is based on near-edge absorption fine structure (NEXAFS) spectroscopy and stems from the dependence of the NEXAFS resonance intensity on the orientation of the electric field vector of the incoming linearly polarized light with respect to the molecular orbital associated with this resonance. The applicability of both contrast mechanisms was demonstrated using a nickel polycrystal and a monolayer of benzoic acid (BA) on this polycrystal as test systems. Within the photoelectron diffraction approach well-resolved images of the individual microcrystallites on the surface of Ni polycrystal were obtained by using the photoelectrons from both the localized core level (Ni 2p_3/2) and the free-electron-like valence band. Within the molecular orbital orientation approach the well-resolved images of the azimuthally aligned BA molecules on the surface of the (110) microcrystallites incorporated into a Ni polycrystal were acquired. In these experiments the photon energy was tuned to the excitation energy of the π* orbital of the phenyl ring, which is a constituent of a BA molecule, and the C1s → π* excitation was monitored by the carbon KLL Auger electrons. The distinction between photoelectron diffraction and molecular orbital orientation contrast mechanisms within an imaging experiment is discussed.     

A New Method for the Measurement of Thickness in Single Crystals

A new method for thickness determination of single-crystal thin samples at exact zone axis orientation, based on pattern recognition in convergent beam electron diffraction (CBED), is presented. The method is especially well suited to materials with a large unit cell in zone axis directions where the reciprocal lattice is uniformly dense with diffraction points. The new method is based on comparison of a measured CBED zone axis pattern with a set of calculated ones. Its accuracy was estimated to be around 10% in the 5–100 nm thickness range as checked for garnets at the [111] zone orientation.     

Study of changes in composition and EELS ionization edges upon Ni4Ti3 precipitation in a NiTi alloy

Concentration gradients surrounding Ni4Ti3 precipitates grown by appropriate annealing in a Ni51Ti49 B2 austenite matrix are investigated by electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM). Concentration gradients of approximately 1.0-2.0 at.% in Ni within the surrounding B2 matrix can be detected by both EELS and EFTEM, revealing a Ni depleted zone in the matrix. Besides the concentration gradients, the EELS integrated cross-section of the Ni L(2,3) edges for the Ni-depleted region increased slightly, when compared with a matrix region away from the precipitate and not depleted in Ni.     

Investigation of the oxide shell forming on epsilon-Co nanocrystals

This paper reports on the TEM characterization of the surface oxide layer forming on Co nanocrystals (NCs) prepared using a standard method [Puntes, V.F., Krishnan, K.M., Alivisatos, P., 2001. Synthesis, self-assembly, and magnetic behavior of a two-dimensional superlattice of single-crystal epsilon-Co nanoparticles. Appl. Phys. Lett. 78 (15), 2187-2189]. Complementary transmission electron microscopy (TEM)-related techniques presented direct evidence of a 1.5-3nm CoO shell forming on varepsilon-Co NCs. The crystalline structure of the Co NCs was confirmed by selected area diffraction study while the nature of the shell was probed by energy-loss near-edge spectroscopy and energy-filtered TEM. Based on these results, we comment on the detection of nanoscale feature with energy-filtered imaging.