High Resolution Secondary Electron Microscopy Research Articles

Local hydrogen accumulation after cold forming and heat treatment in punched advanced high strength steel sheets

Hydrogen embrittlement is one of the most crucial problems in the application of advanced high strength steel (AHSS) sheets for the automotive industry. Especially, the severe plastic deformation in punched edges makes the components susceptible to hydrogen assisted cracking (HAC). While small amounts of hydrogen are measured in the bulk material, hydrogen concentration increases in the micrometer-sized shear affected zone many times, along with severe plastic deformation. To contribute to the understanding of local microstructure and local stress states on the hydrogen accumulation in the shear affected zone, two industrial AHSS were investigated. Both steels had the same ultimate tensile strength of 1200 MPa, but different uniform elongations. High-pressure torsion (HPT) deformed samples were used to represent the material state in the shear affected zone. Thermal desorption spectroscopy (TDS), X-ray diffraction (XRD), magnetic retained austenite measurements and electron backscattering diffraction (EBSD) in a high resolution secondary electron microscopy (SEM) were applied among other techniques to gain more insights with microstructural resolution. The hydrogen analysis results of the HPT samples were correlated with the local hydrogen trapping capacity in punched edges. Finally, a simplified “two-zone” model considering the bulk and punched edge as separate zones was developed in order to estimate the local hydrogen concentration of punched AHSS sheets as a function of hydrogen bulk concentration. In a concluding remark it is shown, that the effect of the hydrostatic stress field as trap site is negligibly small compared to other traps.

Voiding Phenomena in Copper-Copper Bonded Structures: Role of Creep

For its good resistivity and reliability, copper remains the most widely used material for metal interconnections since the early 1990s [1]. Concerning the 3D-integration technological transition for next generation integrated circuits, copper still remains interesting as a bonding material. In this specific configuration, this metal allows wafer or die level assembly of active component strata with high bonding strength and it also ensures interlayer electrical conduction, reducing interconnection delays and increasing overall performances [2-3]. In the process temperature range from 300°C to 400°C, the voiding mechanism that drives the structure failure [4-5] shows similar characteristics to stress-induced voiding previously reported in basic encapsulated copper interconnections [6]. However, regardless of the bonding technique, void density, shape and location exhibit variations which cannot be explained by the mechanisms previously established. In this paper, we investigate several factors, including copper surfaces roughness, copper layer microstructure and bonding processing parameters to weight their impact on the voiding phenomenon. Better understanding of void formation has enabled the achievement of wafer assemblies exhibiting much higher reliability and bonding quality.200mm thermally oxidized silicon substrates are used in this study. 500nm-thick copper layer are then deposited on a 20nm-thick titanium nitride layer which is used to prevent copper diffusion into silicon substrate. Copper-copper bonding is studied through two different bonding techniques: direct bonding at room temperature under atmospheric pressure after surface activation by chemical and mechanical polishing (CMP activated bonding) and thermocompression bonding (TC bonding) using a combination of a thermal budget and compressive stress. In both cases, additional post-bonding annealing is accomplished under nitrogen atmosphere.In our bonded copper layers configuration, voiding phenomenon is supposed to have different origins:- Residues of the copper surface roughness which is brought into contact during bonding process and trap some volume- Presence of metal oxide on sample surfaces and its subsequent degradation by annealing- Nucleation and growth of cavities due to standard metallurgical creep mechanisms of copper layers during its thermomechanical loadingIn order to evaluate roughness impact on the voiding phenomenon, bonding was performed thanks the two different techniques on copper surfaces exhibiting identical surface topology properties (post CMP treatments) (Fig. 1a). In order to aggravate void creation, TC bonding was also performed with extreme parameters in terms of pressure values and copper surface roughness (Fig. 1b). Void densities and volumes were estimated by high resolution Secondary Electron Microscopy (SEM) cross-sections observations (Fig. 2). A special attention was paid to imaging conditions and analysis. Indeed we operated data collection over large sections of about 40µm. In the context of this study we were able to appreciate significant differences between bonded samples. More specifically we focused on volume variations and size distributions which enabled us for instance to highlight the role of roughness in voiding phenomenon.Other possible causes for void generation were also investigated using dedicated experiments:- Gold-gold TC bonding was performed as a bonding reference system involving oxide free metallic layers- Bonding of copper layers carefully deposited on copper bulk substrates to highlight the influence of thermomechanical stress (Fig. 3)In the light of these results, the impact of the different factors will be discussed and the main source of voiding due to creep deformation will be demonstrated. FIGURES CAPTIONS Fig 1 : 20 x 20 µm² AFM scans of copper surface. a- In CMP bonding configuration: RMS roughness is 0.2 nm. b- In TC bonding configuration: RMS roughness is 22 nm. Fig 2 : SEM cross section. a – On CMP activated bonding after a post bonding annealing at 400°C. b – On TC bonding performed at 400°C under a pressure of 2.9 MPa on high-roughness surfaces. Voids are arrowed. Fig 3 : SEM cross section of TC bonding involving copper layers deposited directly on copper bulk substrate. Voids are arrowed. ACKNOWLEDGEMENTS The authors would like to thanks the French Direction Générale de l’Armement (DGA), the French Direction Générale de la Compétitivité de l’Industrie et des Services (DGCIS) and SOITECSA for its financial support. REFERENCES [1] P. C. Andricacos, et al., IBM J. Res. Dev., vol. 42, no. 5, pp. 567–574, Sep. 1998.[2] N. Sillon, et al., pp. 1 –4., IEDM 2008. [3] R. Taibi, et al., pp. 219–225, ECTC 2010.[4] C. S. Tan, et al., Appl. Phys. Lett., vol. 87, no. 20, pp. 201909, 2005.[5] P. Gondcharton, et al., MRS Online Proc. Libr., vol. 1559, 2013, doi : 10.1557/opl.2013.718.[6] H. Okabayashi, Mater. Sci. Eng. R Rep., vol. 11, no. 5, pp. 191–241, 1993.

Optical and structural characterization of AlInN layers for optoelectronic applications

Al 1 − x In x N layers with an indium content between x=10.5% and x=24% were grown by metal-organic vapor-phase epitaxy and characterized concerning their optical, structural and morphological properties with regard to the realization of optoelectronic devices. The indium content and the strain of these layers were measured by high resolution x-ray diffraction. Ellipsometric measurements were used to determine the optical constants [refractive index n(λ) and extinction coefficient κ(λ)] in dependence of wavelength and indium content. The values determined for the electronic bandgaps are in good agreement with theoretical predictions and previous publications on this topic but are more focused on AlInN layers which are pseudomorphically grown on GaN. A bowing parameter of b=10.3±0.1 was determined for fully strained layers with an indium content between 13% and 24%. In order to investigate the suitability of these layers for use in distributed Bragg reflectors, the surface morphology is characterized with respect to the indium content. Furthermore, the influence of an annealing step which often is necessary during device growth, was studied. The influence of this annealing step on the roughness was analyzed by atomic force microscopy, while structural features are monitored by high resolution secondary electron microscopy images. Based on these results distributed Bragg reflectors for the green spectral region with up to 40 pairs and a peak reflectivity of 97% have been realized. Transmission electron microscopic analysis of the layer interfaces are in good agreement with the atomic force and secondary electron microscopy images of the single layer surfaces.

Ultrastructure in frozen/etched saline solutions: on the internal cleansing of ice.

Seawater, with its 3.5% salt content, freezes into hexagonal ice (Ih) that encloses concentrated brine within its matrix. When unsubmerged sea ice reaches a certain height and temperature, the brine drains downward through narrow channels. This mechanism was now modeled by frozen 2-3.5% saline as investigated by cryo-etch high-resolution secondary electron microscopy. Thus, saline was either plunge-frozen in liquid ethane at -183 degrees C or else high-pressure frozen to -105 degrees C in 5-6 ms. Ice from a freshly exposed surface was then subjected to a high-vacuum sublimation ("etching"), a procedure that removes pure bulk ice in preference to ice from frozen hydrated salt. After chromium-coating the etched surface with a 2-nm film, the sample was examined by cryo-HRSEM. Granular icy "fences" were seen surrounding empty areas where amorphous ice had originally resided. Since the fences, about 1-2 mum high, survived the etching, it is likely that they consist of frozen brine. The presence of such fences suggests that, during freezing, saline can purge itself of salt with remarkable speed (5-6 ms). Alternatively, channels (perhaps routed around submicroscopic crystallites of cubic ice (Ic) embedded in the amorphous ice at -105 degrees C) can guide the migration of salt to the periphery of ice patches. Macromolecules fail to form fences because they diffuse too slowly or because they are too large to pass through the channels.

Microscopic evaluation of spatial variations in material and charge transport properties of CdZnTe radiation detectors

The material uniformity and charge transport properties in CdZnTe radiation detectors are investigated by various non-destructive room temperature techniques. High-resolution secondary electron microscopy shows evidence of Te-enriched regions near the metal contacts. Photoluminescence (PL) mapping is used to image material defects and variations in Zn composition. Close to the electrodes PL spectra show strong defect-activated emission which is not observed in the material bulk. Ion-beam-induced-charge (IBIC) microscopy using a 2 μm resolution focussed proton beam is used to investigate the spatial variation in charge transport across detectors. Localized regions of poor charge collection efficiency seen in the IBIC images correlate with material defects observed in PL such as tellurium precipitates. The effects of these material defects in terms of the spectroscopic performance of the detectors to X-ray and γ-ray irradiation is discussed.

Enhancement of fracture strength of cutted plate glass by the application of SiO2 sol-gel coatings

The influence of SiO2 sol-gel coatings on the bending strength of soda lime plate glass samples has been investigated by 3 point bending and double ring bending strength measurements. The samples were prepared by manual glass cutting and were coated with two different types of SiO2 coating: A TEOS coating (thickness of about 100 nm) was obtained from sols synthesized by acid catalysis of tetraethoxysilane and a MTKS coating (thickness of about 1 μm) was obtained from a methyl modified nanoparticulate SiO2 sol. The coatings were applied by dipping and were densified at 500°C for 1 h. The fracture strength σ0 of edge coated plate glass (60×25×3.8 mm), determined by 3 point bending test (according to EN 843-1), was increased after coating with TEOS from σ0=24 MPa to 40 MPa (67% increase). The fracture strength increased to σ0=56 MPa (133% increase) on applying MTKS or a double coating of TEOS followed by MTKS. The evaluation of these measurements using Weibull statistical analysis resulted in m values of 4.1 and 4.2 for the TEOS and MTKS coating and 3.5 for double coating (uncoated glasses m=5.7), respectively. These results combined with high resolution secondary electron microscopy investigations led to the conclusion that the strengthening effect of the two different sol-gel systems is due to a partial flaw healing and clamping of the crack sides.

Stripe-shaped facetted morphology and domain structure of epitaxial CeO 2(110) layers on Si(100) substrates

The microscopic structure of epitaxially grown CeO 2(110) layers on Si(100) substrates is investigated using high-resolution electron microscopic techniques. Surface morphology observations using high-resolution secondary electron microscopy indicate that the layer has a texture structure with stripes elongated in two directions perpendicular to each other. From cross-sectional high-resolution transmission electron microscopy (HRTEM) observations, it is found for the first time that the surface corresponding to these stripes has a triangular shape, which indicates that the surface consists of (111) facets. Plan view HRTEM observations verify that there are two kinds of domains, and that the crystallographic in-plane directions of the two domains are exactly perpendicular to each other. Stripes located in parallel to each other belong to a single-crystal domain. Grain boundaries are seen where domains having different in-plane direction get in touch together. Reflection high-energy electron diffraction with 〈110〉 azimuth shows split streaks, resulting in a mesh pattern, which directly verifies that the surface consists of a combination of (111) and (11 1 ) facets.

The origins of high-resolution secondary-electron microscopy

The production or generation of SE by focused, fast electron beams is a multistage process which includes excitation of target electrons by the energetic incident beam, subsequent decay yielding hot SE, transport to the surface, and transmission over the surface potential barrier. The ultimate spatial resolution of a SE image formed by scanning a well focussed probe across a surface is limited by the excitation events' spatial derealization through the transverse momentum transferred to the specimen.The accepted model for SE production is not well characterized since it is extremely difficult to separate experimentally the generation, transport and transmission processes during a given SE creation event. Here, we examine the SE generation pathway by correlating SE of a given energy produced by an initial inelastic excitation using time coincidence detection. This technique can be used, for example, to isolate the role of plasmon decay in the SE generation process. The experiments were performed in a Vacuum Generators HB501-S UHV scanning transmission electron microscope (STEM), operating at base pressures of 5 × 10−11 torr.

Imaging small particles with secondary electrons

High resolution secondary electron microscopy (HRSEM) in dedicated scanning transmission electron microscopy (STEM) instruments has proved very useful for characterizing supported catalysts. In order to understand the contrast mechanisms of HRSEM images of small particles, clean samples and UHV environments are needed since the emission of secondary electrons (SEs) can be significantly influenced by contamination on the particle surface. In this paper we report results on imaging small metal particles by HRSEM in a UHV STEM instrument. In addition, we show that with computer assistance digitized HRSEM images of small metal particles can be used to estimate the average inelastic mean free path and the mean escape depth of the collected SEs.The Vacuum Generators UHV STEM HB-501S, codenamed MIDAS (a Microscope for Imaging, Diffraction and Analysis of Surfaces), was used for these experiments. Detailed descriptions of the MIDAS system have been published previously. The collection efficiency of SEs approaches 100% through the use of magnetic ‘parallelizers’ situated inside the objective pole pieces.