Situ Scanning Electron Microscopy Research Articles

In situ SEM observation of electromigration phenomena in fully embedded copper interconnect structures

An experimental set-up is presented, that allows in situ scanning electron microscope (SEM) investigations of the progress of electromigration damage in fully embedded copper interconnect structures. A LEO Gemini 1550 SEM has been equipped with a heating stage and electrical connections for the experiment. The studied interconnect structures are usually used for reliability testing in electromigration ovens. These test structures are located within the scribelines of wafers. Therefore, they allow the characterization of the electromigration behaviour of products on the wafer. To enable the SEM observation, focused ion beam (FIB) was used to prepare cross-sections of the samples maintaining their electrical functionality. Thereby, a thin layer of passivation was left over in front of the interconnects to keep them fully embedded. The SEM images which were taken at an angle of 60° allow the observation of both the entire via/contact and the connecting lines. Multiple images were recorded during the degradation experiments. The resulting video sequences provide a good visualization of the formation, growth and motion of voids at the stressed interconnects. The dominant diffusion path has been identified.

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

Coalescence process of monodispersed Co cluster assemblies

Cluster-cluster coalescence process of monodispersed Co clusters with mean diameter d = 8.5 and 13 nm deposited a plasma-gas-condensation-type cluster beam deposition system was investigated by in situ electrical conductivity measurements and ex situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and analyzed by percolation concept. The electrical conductivity measurement and TEM observation indicated that, below temperature T≈ 100°C, the Co clusters in the assemblies maintain their original structure as deposited at room temperature, while that the inter-cluster coalescence takes place at T > 100°C, although the size distribution and the interface morphology of the clusters showed no marked change at substrate temperatures T s≤200°C.

Cellular dislocations: experiment and modeling

The concept of cellular dislocations (CDs) as topological defects in a regular grain array has previously been used as a convenient analogy with regular dislocations for modeling of deformation and grain growth processes on the scale of grain groups. This work provides experimental evidence supporting the existence of CDs. Results of observations of deformation relief at the surface of superplastically deformed samples under different strain states, distortion of marker lines due to grain group sliding, and in situ scanning electron microscope (SEM) deformation are provided. These data can be interpreted in terms of CDs. Finite element modeling (FEM) of sliding of grain groups indicates that stresses and strain are localized within a two grain size around CD.

Non-contact atomic force microscopy of an antiferromagnetic NiO(100) surface using a ferromagnetic tip

We have observed a cleaved NiO(100) surface by means of non-contact atomic force microscopy (NC-AFM) using a Fe-coated tip. The shape and magnetic properties of the Fe-coated tip are confirmed by ex situ scanning electron microscopy (SEM) and magnetic force microscopy (MFM) imaging. Also, in situ Δf–vmeasurement shows that the contact-potential difference (CPD) is different between non-coated Si and Fe-coated tips. The Fe-coated tip can produce atomically resolved NC-AFM images of a cleaved NiO(100) surface at room temperature.

Formation of a liquid film of AgNO3 on a silver surface

The behavior of a AgNO3/Ag2O/Ag “sandwich” upon heating in vacuum was studied by in situ X‐ray photoelectron spectroscopy (XPS) and ex situ scanning electron microscopy (SEM). The AgNO3/Ag2O/Ag “sandwich” was prepared by exposure of a silver foil to a NO : O2 mixture. The upper layer of the “sandwich” consists of AgNO3 crystals of a mean size between 0.1 and 0.4 μm. Heating at 550 K in vacuum results in melting of the AgNO3 crystals. A liquid film of AgNO3, readily wetting the silver, covers the surface. Cooling below the melting point of AgNO3 leads to the agglomeration of silver nitrate to long islands with a size reaching a few tens of micrometers (μm). The possible effects of AgNO3 liquid‐phase formation on surface processes are discussed.

Asymmetric behavior of monolayer holes after growth in GaAs molecular beam epitaxy revealed by in situ scanning electron microscopy

The behavior of monolayer holes on the GaAs (0 0 1) surface during post-growth annealing in molecular beam epitaxy (MBE) is examined in detail by in situ scanning electron microscopy (SEM). Submicron scale monolayer-deep holes are formed after small islands and holes are eliminated. Their growth and shrinkage are found to proceed asymmetrically: for example, they grow only into the right-hand side, and shrink only from the top. The mechanism is discussed in terms of environmental step structure. Ga adatoms migrate tenths of micron in several minutes. It was found that in the regrowth after annealing, three-dimensional islands are formed preferentially on the step edges.

Secondary electron imaging of nucleation and growth of semiconductors for nanostructure fabrication

In situ scanning electron microscopy (SEM) images of growing surfaces provide valuable insights into the control of growing structures. Surface morphology evolution due to 2D island nucleation and coalescence is clearly imaged in GaAs molecular beam epitaxy, and compared between (001) and (111)A surfaces. Nucleation sites of 3D islands are identified in solid phase epitaxy of Ge on Si(111). Nucleation sites can be controlled using atomic step networks, as demonstrated by the selective growth of GaAs wires and Au dots on Si(111).

Growth of ZnSe on GaAs(1 1 0) surfaces by molecular beam epitaxy

Single crystal films of ZnSe have been grown on nonpolar GaAs(1 1 0) surfaces by molecular beam epitaxy (MBE). The epitaxial films have been characterized by in situ reflection high-energy electron diffraction (RHEED), ex situ scanning electron microscopy (SEM) and X-ray diffraction. RHEED and SEM images of the surfaces reveal the formation of facets which are aligned along the [0 0 1] direction. The formation of facets indicates that ZnSe growth on the (1 1 0) surface proceeds 6°–13° off the vicinal (1 1 0) surface. Facet-free ZnSe surface has been successfully grown on a GaAs(1 1 0) 6°-off the substrate.

Tensile damage in a symmetric [ [formula omitted]] 2s silicon-carbide fiber-reinforced titanium-matrix composite

The results of a detailed study of the effects of composite microstructure on the micromechanisms of tensile damage in a symmetric [ 0 90 ] 2s Ti-15Al-3Cr-3Al-3Sn (Ti-15-3)/SiC (SCS-6) composite are presented. Matrix micro-structure is controlled by heat treatment, which is used to produce metastable β or Widmänstatten α + β microstructures. The sequence of damage initiation and evolution at room temperature is identified using ex situ scanning electron microscopy (SEM) during incremental monotonic loading to failure. Damage mechanisms are also studied using non-destructive acoustic emission (AE) techniques, and matrix hardening is characterized using matrix micro-hardness measurements within individual plies. Idealized and actual microstructure-based finite-element models are used to rationalize the observed tendency of damage to propagate from the outer plies to the inner plies. The paper highlights the importance of simplified micromechanical models in the development of a fundamental understanding of the effects of composite architecture on damage initiation phenomena.

The effect of consolidation temperature on microstructure and mechanical properties in powder metallurgy-processed 2XXX aluminum alloy composites reinforced with SiC particulates

The effects of consolidation temperature on the development of microstructure and resulting mechanical properties of 2XXX aluminum composites were studied in an effort to fabricate composites with enhanced properties. Type 2009 and 2124 aluminum composites reinforced with 15 pct SiC particulates were produced at four different consolidation temperatures, i.e., 560 °C, 580 °C, 600 °C, and 620 °C, followed by extrusion at 450 °C. The 2124 Al-SiC p composites consolidated at 560 °C showed the most homogeneous and the finest microstructures with the best mechanical properties, which were even better than the whisker-reinforced counterparts. All the results of the tensile tests, hardness tests, in situ scanning electron microscope (SEM) observations of the fracture process, and the apparent fracture toughness indicated that the prominent mechanical property improvement observed in the 2124 Al-SiC p was associated largely with the reduction of volume fraction of the detrimental coarse and brittle manganese-containing particles, as well as grain refinement. The detrimental manganese-containing particles that were routinely observed in the 2124 Al-SiC composites were very effectively refined by the reduction of consolidation temperature, and they rather contributed to the overall mechanical properties of the composites through Orowan-type strengthening and grain growth inhibition.

In situ observation of MEE GaAs growth using scanning electron microscopy

We have used in situ scanning electron microscopy (SEM) for real-time observation of migration enhanced epitaxy (MEE) processes. The surface morphology developed during MEE growth is either monolayer islands or monolayer holes depending on the amount of Ga supply per growth cycle. When island coverage is low, the islands disappear immediately after growth terminates, while the holes remain longer. Resulting surface roughness is much smaller than in molecular beam epitaxy even at a low substrate temperature. The present observations directly confirm enhancement of surface atom migration in MEE.

Principle and application of the constant strain rate tensile test with in situ scanning electron microscopy for evaluation of the adhesion of films

Based on the theory of mechanics of materials, a constant strain rate tensile test with in situ scanning electron microscopy (SEM) for measuring the adhesion of PVD films was developed, and the adhesion of a film to a substrate was determined in terms of the shear strain energy of the film during the elongation of the film/substrate combination, i.e. the adhesion depends on the elongation ΔL of the film/substrate combination. The adhesion of three different TiN films prepared by different processes was evaluated by the constant strain rate tensile test with in situ SEM and the conventional scratch test. The results of the two tests show that (1) when the films are thin the adhesion values are in good agreement, but for the thick films with increasing thickness the adhesion decreases in the tensile test while the adhesion first increases and then decreases in the scratch test and (2) for three different TiN films of the same thickness, the identical adhesion order was obtained in the two tests.

AES analysis of silicon nitride formation by 10 keV N + and N 2+ ion implantation

Reactions of N + and N 2 + ions with the Si(100) surface as a function of ion dose and angle of incidence by 10 keV N + and N 2 + bombardment have been studied by in situ Auger electron spectroscopy (AES) analysis. The AES measurements show that a continued exposure of the ion beam to a saturation dose of > 5 × 10 16 N 2 +/ cm 2 leads to a surface stoichiometry close to that of Si 3N 4. Complete nitridation, i.e. the formation of Si 3N 4, could be achieved at incident angles between 0 ° and 30 ° to the surface normal. At incident angles exceeding 30 °, the degree of nitridation decreases. Above 60 ° incident angle, no nitride was found in the near-surface region. A simplified model has been proposed to explain the observed angular dependence. The samples with higher ion doses were also examined by ex situ scanning electron microscopy (SEM). No blister or bubble was found on the surface suggesting that under saturation conditions the self-sputtering of captured nitrogen atoms near the surface by further incoming projectiles is the major contributor to nitrogen removal in our experiment. No pronounced difference between N + and N 2 + bombardment was observed in this experiment.

Molecular dynamics simulation of void electromigration under a high‐density electric current stress in an aluminum interconnection

AbstractThree‐dimensional molecular dynamics simulation of behaviors of a void in an Al interconnection under high current stress has been accomplished employing empirical two‐body potential and the Huntington‐Grone ballistic model for an electron wind force. Stability of a void under a uniform strain field was first investigated; it was shown that a void was stable under a tensile strain, while it was unstable under a compressive strain. Then, a void movement into cathode direction was clearly demonstrated in the single‐crystalline Al interconnection at the high current density of 1 × 1010 A/cm2. The activation energy of the drift velocity of the void was 0.76 eV. Next, behaviors of a void in [111] bicrystal grains with a periodic boundary condition which resembled a bamboo structure interconnection was investigated. A void was stabilized at the grain boundary at low current density conditions; however, it could go across the grain boundary and move further to the cathode direction when the current density was high enough. It was shown that when a void comes close to the grain boundary, it suddenly annihilates into vacancies and rapid movement of vacancies into the grain boundary occur. A void was formed again at the grain boundary. This model will explain the void annihilation phenomena, reported by the in situ scanning electron microscope (SEM) observations. After stopping the current, a backflow of a void was observed as a consequence of the strain energy relaxation when the current density was high. These obtained results explained well the experimental observations qualitatively; furthermore, new predictions were obtained with quantitative analysis based on total potential energy analysis.

Analysis of the Mechanical Failure in a Multilayered Thin Film System Tested by Microtensile Loading

AbstractMultilayered thin films were deposited on pure Al substrate test bars for in situ scanning electron microscope (SEM) microtensile deformation. The films consisted of a TiN layer covered by either SiO2 or W. Deformation of the samples showed that the TiN film dominated cracking events and caused cracking in the W to occur at lower critical stress values than previously reported. Crack deflections along the TiN-W interface were also seen. W film strain was measured by X-ray diffraction before and after testing to look at the effect of cracking and deformation on stress in this film.

Direct observation of the fracture of CAS-glass/SiC composites

The fracture behaviour of a CAS-glass/SiC-fibre-reinforced composite was observed by dynamic in situ scanning electron microscopy (SEM). In a companion paper [1], tests on common delamination geometries are described and the basis of micromechanics models is critically evaluated. Flexure geometries and also the unnotched tensile response of ceramicmatrix composites (CMCs) have received considerable attention, both theoretically and experimentally. The effect of through-thickness notches on the tensile fracture of CMCs has however been relatively neglected. Previous work on polymer-matrix composites demonstrates the strong influence of subcritical damage on the fracture behaviour. In this paper we examine failure of notched CAS-glass/SiC composites in tension, under static- and fatigue-loading conditions, using a combination of in situ and conventional test methods. The subcritical damage which forms is compared with that in polymer-matrix composites, and the consequences for prediction of the notched strength are discussed.

Hydride embrittlement in ZIRCALOY-4 plate: Part I. Influence of microstructure on the hydride embrittlement in ZIRCALOY-4 at 20 °C and 350 °C

The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350 °C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, andβ-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably.In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20 °C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350 °C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect.

Fracture toughness analysis of heat-affected zones in high-strength low-alloy steel welds

This study is concerned with a correlation of fracture toughness with microstructural factors in heat-affected zones (HAZs) of a normalized high-strength low-alloy (HSLA) steel. In order to explain weld joint performance, tensile and plane strain fracture toughness tests were conducted for the simulated coarse-grained HAZ microstructures. The micromechanisms of fracture processes involved in void and microcrack formation are identified byin situ scanning electron microscopy (SEM) fracture observations and void initiation study. The fracture toughness results are also interpreted using simple fracture initiation models founded on the basic assumption that a crack initiates at a certain critical strain or stress developed over some microstructurally significant distance. The calculated KIc values are found to scale roughly with the spacing of the stringer-type martensite islands associated with voids, confirming that martensite islands play an important role in reducing the toughness of the coarse-grained HAZs. These findings suggest that the formation of martensite islands should be prevented by controlling the chemical compositions and by using the proper welding conditions to enhance fracture toughness of the welded joints of the HSLA steel.

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K1 = 1 MPa√m/s and a dynamic rate of ·K1 = 2 × 106 MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.