Focused Ion Beam Microscopy Research Articles

Microstructural Development and Possible Whiskering Behavior of Thin Sn Films Electrodeposited on Cu(Zn) Substrates

The aging behavior at room temperature of thin Sn films, electrodeposited on top of Cu(Zn) substrates containing 15 wt.% and 36 wt.% Zn, was investigated, using focused ion beam microscopy and x-ray diffraction analysis to evaluate the microstructural and (residual) stress development in the specimens. For comparison, parallel experiments and similar analyses were performed with Sn films electrodeposited on pure Cu substrates. Whereas Sn whiskering was observed for the Sn films deposited on the Cu substrates, such whiskering was not observed for the Sn films deposited on the Cu(Zn) substrates. It was found that alloying the Cu substrates with Zn strongly slows down the formation rate of the intermetallic compound Cu6Sn5 at the Sn/Cu(alloy) interface. The Sn films on the Cu(Zn) substrates remained whisker free for the entire time of investigation even though an overall compressive state of stress has developed after several weeks of aging. It was concluded that a homogeneous, compressive stress in the Sn film does not lead to whisker formation: the presence of negative stress gradients is essential for Sn whisker growth.

Micrometric rods grown by nanosecond pulsed laser deposition of boron carbide

Micrometric size rods have been fabricated via pulsed laser deposition in vacuum from boron carbide targets using nanosecond pulses of 1064 and 266nm and room temperature Si (100) substrates. Morphological, structural and chemical characterization of the microrods was made by applying scanning electron microscopy, focussed ion beam microscopy coupled to secondary ion mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Ablation at 1064nm favours the formation of microrods with high aspect ratio, sharp edges and pyramidal tips, typically 10μm long with a cross section of around 2μm×2μm. Differently, at 266nm the microrods are of smaller size and present a more globular aspect. The analyses of the microrods provide information about their crystalline nature and composition, based on a mixture which includes boron, boron oxide and boron carbide, and allows discussion of the wavelength dependent growth mechanisms involved.

Reliability Study of Silver, Copper and Gold Wire Bonding on IC Device

Copper wire bonding in IC packages is not always suitable for devices with active circuit under bonding pad because higher bonding power required for copper wire bonding may cause top aluminum metal splash and mechanically impact the circuit underneath. Silver wire is an alternative solution to this problem based on its physical properties and lower cost compared to gold wire. Ag88%Au8.5%Pd2.5%X1% and Ag95%Au1.5%Pd2.5%X1% alloyed silver wires are used in the study to compare with copper and gold wires of 99.99% in purity. As bonding power plays a dominating role in wire bonding, we focused on the effects of silver, copper and gold bonding wires with different bonding power on the top aluminum metal splash of power device by Optical Microscope(OM) and Scanning Electron Microscope(SEM). The ball shear strength of the bonding wires with different bonding power in samples without mold compound encapsulation was investigated before and after 24, 48, 96 and 192 hours of pressure cooker test (PCT). The intermetallic compound (IMC) formed between silver and aluminum was confirmed by focus ion beam (FIB) and transmission electron microscope (TEM). Although the top surface of the silicon device shows no significant difference after aluminum layer removal for all three wire types, the severity level of vertical deformation and side splash of aluminum layer due to copper wire bonding is much more than silver or gold wire using same amount of bonding power. Ball shear strength of non-encapsulated silver wire decreases dramatically after PCT aging compared with copper wire or gold wire and some samples show zero shear strength after PCT 96 hours and PCT 192 hours for silver wires doped with Pd/Au. Furthermore, larger bonding power induces higher ball shear strength. The major IMC compositions between silver and aluminum are Ag3Al and Ag2Al. A thermo dynamic model was built to explain why silver wire is prone to corrosion compared with copper wire by humidity although copper is easier to be ionized than silver. No electrical test was performed as the samples cannot be tested without package encapsulation and singulation. Furthermore, silver wire samples in SO8 package with mold compound encapsulation were subjected to highly accelerated stress test (HAST), PCT, temperature cycle test (TCT) after MSL1 preconditioning test as well as high temperature operation life test (HTOL) according to JEDEC procedures. The encapsulated samples using either Ag 88wt% or Ag95wt% alloys all passed MSL1 and PCT/HAST/TCT/HTOL. Drain to source on-resistance (Rdson) of the device including package parasitics was measured and it has no significant difference between silver wire and gold wire. The results from this study shows promising data using silver alloy wires but care should be taken to further understand the degradation of silver-aluminum interface under severe humidity condition. Using other metallization on silicon top surface such as NiAu or CuAu can significantly alleviate the interface problem related to AgAl.

The Role of Silver in Mitigation of Whisker Formation on Thin Tin Films

The mitigating effect of alloying Sn thin films with Ag on the formation of Sn whiskers was investigated by time-resolved investigations employing x-ray diffraction for phase and stress analyses and focused ion beam microscopy for morphological characterization of the surfaces and cross-sections of the specimens. The investigated Sn-6 wt.%Ag thin films were prepared by galvanic co-deposition. The results are compared with those obtained from investigation of pure Sn films and discussed with regard to current whisker-growth models. The simultaneous deposition of Sn and Ag leads to a fine-grained microstructure consisting of columnar and equiaxed grains, i.e. an imperfect columnar Sn film microstructure. Isolated Ag3Sn grains are present at the Sn grain boundaries in the as-deposited films. Pronounced grain growth was observed during aging at room temperature, which provides a global stress relaxation mechanism that prevents Sn whisker growth.

Microstructure-hardness-fretting wear resistance correlation in ultrafine grained Cu–TiB2–Pb composites

The retention of the desired combination of mechanical/tribological properties in ultrafine grained materials presents important challenges in the field of bulk metallic composites. In order to address this aspect, the present work demonstrates how one can achieve a good combination of hardness and wear resistance in Cu–Pb–TiB2 composites, consolidated by spark plasma sintering at low temperatures (<500°C). Transmission electron microscope (TEM) studies reveal ultrafine grains of Cu (100–400nm) with coarser TiB2 particles (1–2μm) along with fine scale Pb dispersoid at triple junctions or at the grain boundaries of Cu. Importantly, a high hardness of around 2.2GPa and relative density of close to 90% relative density (ρtheo) have been achieved for Cu–15wt% TiB2–10wt% Pb composite. Such property combination has never been reported for any Cu-based nanocomposite, by conventional processing route. In reference to the tribological performance, fretting wear tests were conducted on the sintered nanocomposites and a good combination of steady state COF (0.6–0.7) and wear rate (10–4mm3/Nm) were measured. An inverse relationship between wear rate and hardness was recorded and this commensurates well with Archard’s relationship of abrasive wear. The formation of a wear–resistant delaminated tribolayer consisting of TiB2 particles and ultrafine oxide debris, (Cu, Fe, Ti)xOy as confirmed from subsurface imaging using focused ion beam microscopy has been identified as the key factors for the low wear rate of these composites.

The sliding wear of TiC and Ti(C,N) cermets prepared with a stoichiometric Ni3Al binder

TiC and Ti(C,N) based cermets offer several improved characteristics relative to conventional WC-based “hardmetals”, such as lower mass and improved oxidation resistance, which are combined with high toughness, hardness and wear resistance. In the present work the tribological behaviour of TiC and Ti(C,N) cermets has been evaluated under reciprocating sliding conditions. The cermets were produced using an in-situ, reaction sintering procedure to form the stoichiometric Ni3Al binder, with the binder contents varied from 20 to 40vol%. Wear tests were conducted using a ball-on-flat geometry, with a WC–6wt% Co sphere as the counter-face material, for loads from 20 to 60N. The wear response was characterised using a combination of optical profilometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, and focused ion beam microscopy. Initially, two-body abrasive wear was observed to occur, which transitions to three-body abrasion through generation of debris from the cermet and counter-face materials. Ultimately, this wear debris is incorporated into a thin tribolayer within the wear track, which indicates a further transition to an adhesive wear mechanism. It was found that specific wear rates of the cermets increased with both applied load. The highest wear resistance was found for intermediate Ni3Al contents.

Assessment of photovoltaic junction position using combined focused ion beam and electron beam‐induced current analysis of close space sublimation deposited CdTe solar cells

ABSTRACTA major limitation of the cross‐section electron beam‐induced current method—the use of roughly fractured surfaces to provide the cross‐section—has been overcome with the use of focused ion beam microscope sample preparation. Using this method, it was possible to undertake a study of the relation between junction position and the corresponding external quantum efficiency (EQE) curves. For the case of cadmium telluride (CdTe) cells, it was demonstrated that the EQE curve shape that indicates a buried CdTe homojunction only arises if the junction is buried by more than 0.9 µm from the heterointerface. This highlights the limitations of interpreting EQE curve shape to determine junction position. It was also shown that extended postgrowth annealing degrades the cadmium sulfide by Kirkendall voiding, and this leads to efficiency loss. © 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley &amp; Sons, Ltd.

Study of Morphological Defects on Dual-Band HgCdTe on CdZnTe

HgCdTe dual-band epitaxial layers on lattice-matched CdZnTe substrates often have morphological defects. These defects, unlike normal void and microvoid defects, do not contain a polycrystalline core and, therefore, do not offer a good contrast for observation using optical and electron microscopes. This paper reports a way of identifying these defects by using a Nomarski optical microscopy image overlay on focused ion beam microscopy images for preparation of thin cross-sectional foils of these defects. Transmission electron microscopy was used to study the defect cross-sections to identify the origin and evolution of the morphological defects and their effect on the epitaxial layer. This paper reports cross-sectional analysis of four morphological defects of different shape and size.

DESIGN AND PERFORMANCE OF TWO ORTHOGONAL EXTRACTION TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETERS FOR FOCUSED ION BEAM INSTRUMENTS

The design and performance of two orthogonal extraction time-of-flight mass spectrometers are reported that were adapted to existing focused ion beam microscopes for secondary ion mass spectrometry. The performances of these designs were compared to that of a prototype previously described by our group. The differences include newly designed transfer ion optics and in the use of a larger microscope chamber. The two new prototypes allow a mass resolving power of either 600 Th/Th (compact design) or 3000 Th/Th (high resolution design) while simultaneously achieving a lateral spatial resolution of less than 50 nm. The spectrometers and their performance (effective ion yield, mass resolving power, lateral, and depth resolution) are described and compared. Additionally, example applications are presented with multivariate statistical methods to visualize the data sets. Both time-of-flight mass analyzers use orthogonal extraction which avoids the need to pulse the primary ion beam, and the of use monoisotopic gallium to preserve the mass resolution. The goal of the design was a cost-effective accessory to augment typical focused ion beam-scanning electron microscopy applications as an alternative to the cost of a dedicated secondary ion mass spectrometer. The modified instrument allows excellent non destructive imaging and easy sample access, and benefits from the presence of complementary non destructive analytical and imaging techniques that exploit the presence of an electron microscope.

Recent advances in focused ion beam technology and applications

Abstract

Temperature Dependence of Defect Evolution and Distribution in Thermally Cycled Cu-TSVs

Through-silicon vias (TSV) are copper (Cu) interconnects used in three dimensional integrated circuits (3D-IC) and other packaging to allow inter-chip communication. A thermo-mechanical reliability concern for Cu-TSVs, are the formation of voids, which are indicative of the stresses undergone by the structures. Voids can degrade signal propagation by acting as scattering centers and if large enough, can prevent the passage of current [1]. Simulations of fabrication conditions such as back-end-of-the-line (BEOL) addition, have been conducted by isothermal heating or minimal cycling [2-4]; however, few studies have focused on in-use conditions where chips undergo many cycles at lower temperatures, simulating environmental changes and joule heating from use. This study replicates these in-use conditions to evaluate defect evolution and distribution in thermally cycled Cu-TSVs. Silicon (Si) chips containing blind Cu-TSVs were cycled 2000 times from room temperature to maximum cycling temperatures of 100 °C, 150 °C, or 200 °C. Residual voiding from fabrication was considered by examining two sample types (shown in Fig. 1), specimens that contained Cu-TSVs with seams and large bottom voids and those that exhibited only microvoids. The S11 parameter was measured at 500 cycle increments and determined signal loss trended with maximum cycling temperature and initial void area, but not with number of cycles. To further investigate void development, the TSVs were cross-sectioned for internal evaluation using a combination of grinding, polishing, and ion beam milling. Voids were then studied using focus ion beam (FIB) and scanning electron microscope (SEM) images to record void dimensions and locations within the Cu of the TSVs. Void area was observed to increase with increasing maximum cycling temperature from 100 °C to 200 °C, however, this increase is a result of void nucleation as opposed to pre-existing void growth. While rapid cycling (~5 min/cycle) under in-use conditions for these Cu-TSVs did not allow diffusion and growth of voids, maximum cycling temperatures provided sufficient driving force for stresses to form new voids in the Cu.Fig. 1: SEM micrographs of the TSV cross-sections for the two as-received sample types: (a) containing seams and large bottom voids and (b) solely containing microvoids.

Sectioning of Individual Hematite Pseudocubes with Focused Ion Beam Enables Quantitative Structural Characterization at Nanometer Length Scales

A dual-beam focused ion beam microscope equipped with a nanomanipulator was used to fabricate slices from within individual hematite (α-Fe2O3) pseudocubes with selected orientations with respect to the original pseudocubes. Transmission electron microanalysis through selected area electron diffraction enabled assignment of each thin section to a particular zone of the hematite lattice. While the pseudocubes are composed of numerous crystallites, 25-50 nm in size, they are not simply polycrystalline particles. Electron diffraction of thin sections showed that while the pseudocubic hematite particles are composed of numerous coherent domains, the individual thin sections display a net crystallographic orientation to the underlying hematite lattice. Quantitative analysis of the lattice misorientation between coherent domains was calculated from the azimuthal spread of electron diffraction peaks and is consistent with a structure that contains small-angle grain boundaries. Based upon this analysis, we conclude that the pseudocubic hematite particles are mosaic crystals, composed of highly oriented coherent domains.

Anodic polarization of nanocrystalline titanium

TiO2 nanotubes are extensively investigated because of their unique properties and wide range of applications, e.g., in biomedicine. They are used as coatings on titanium implant materials accelerating osteoblast (bone cell) adhesion and improving osteointegration. Owing to its high mechanical properties, nanocrystalline titanium is likely to replace the widely used titanium alloys, which contains harmful ions such as V and Al. The performance properties of nanocrystalline titanium can be modified by subjecting it to various surface treatments tailored to the demands of a given application. The aim of this study is to determine whether the grain refinement of the titanium substrate has an influence on the formation of TiO2 nanotubes. The TiO2 nanotubes were fabricated by anodic polarization of micro- and nanotitanium at a constant voltage of 10, 15, and 20 V for 2 h in an electrolyte containing fluoride ions. The nanocrystalline bulk titanium (grade 2) with grain size of about 90 nm and high density of dislocations was obtained using hydrostatic extrusion. Commercially available coarse-grained titanium with grain size of 20 μm was used as a reference sample. The microstructure of the fabricated nanotubular layers was revealed using scanning electron microscopy and focus ion beam microscopy. Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to determine the chemical composition of the fabricated layers. The results indicate that grain refinement influences the morphology of TiO2 nanotubes while their chemistry remains unchanged.

渐变折射率Ta2O5/SiO2薄膜多脉冲激光损伤特性

Ta2O5/SiO2 gradient-index optical thin films for 1064 nm have been prepared with Ta2O5 and SiO2 by ion beam sputtering (IBS). The optical property and laser-induced damage threshold (LIDT) of the coatings were investigated, respectively. Sidelobe ripples beside the stopband are suppressed by the gradient-index design, meanwhile the design achieves above 99.8% reflectivity at 1064 nm. The multi-shot LIDT decreases as the number of shots increases, but after 20 pulses, it remains almost unchanged all the way to 100 pulses. Typical damage micrographs demonstrate a significant dependence of the initiated damage on the diameter of nodular defect. Scanning electron microscope (SEM) and focused ion beam microscope (FIB) pictures of the nodular defects show that, ablation pits caused by the initiated nodular defect expanded to farther nodular defects under multi-shot pulses. Damage accumulation effects may be attributed to the ejection of farther nodular defects.

Correlation of Microstructure and Transport Properties of Multilayered Graphene Spin Valves on SiO2/Si

Multi-layered graphene based spin valves were deposited on SiO2/Si substrates and the Co ferromagnetic and non-magnetic electrodes were patterned using electron-beam lithography. Aberration-corrected (scanning) transmission electron microscopy imaging and energy dispersive X-ray spectroscopy were employed to study the interfacial structure of the device components from thin cross-sectional lamellae obtained by focused ion beam microscopy. The robustness of the spin injection with a constant spin polarization under positive and negative bias voltage was attributed to the interface smoothness which leads to suppression of spin scattering at the interface between the ferromagnet and the multi-layered graphene.

Insight into the core–shell structures of Cu–In–S microspheres

In this study we report about the inner and outer structure of CuInS2 microspheres which might be used e.g. in pastes for simple, low-cost solar cell preparation, as well as in electrodes for light-driven water splitting. The microspheres are synthesized via a mild, template-free solvothermal synthesis route and characterised by electron and focused ion beam microscopy, X-ray diffraction, inductively coupled plasma atomic emission and energy dispersive X-ray spectroscopy. The investigations of cross sections prepared by focused ion beam showed that the spheres consist of compact cores and flaky surface structures. Depending on the reaction time, the core possesses a stoichiometric or Cu-rich chemical composition surrounded by an In-rich shell. The flaky surface always comprises a stoichiometric composition in tetragonal chalcopyrite crystal structure, whereas the other areas additionally show minor contributions of CuS, and CuInS2 in hexagonal wurtzite structure. The presence of different phases can be beneficial for future applications since they offer different absorption behaviour in the visible range.

Microstructure–Wear Resistance Correlation and Wear Mechanisms of Spark Plasma Sintered Cu-Pb Nanocomposites

The dispersion of a softer phase in a metallic matrix reduces the coefficient of friction (COF), often at the expense of an increased wear rate at the tribological contact. To address this issue, unlubricated fretting wear tests were performed on spark plasma sintered Cu-Pb nanocomposites against bearing steel. The sintering temperature and the Pb content as well as the fretting parameters were judiciously selected and varied to investigate the role of microstructure (grain size, second-phase content) on the wear resistance properties of Cu-Pb nanocomposites. A combination of the lowest wear rate (~1.5 × 10−6 mm3/Nm) and a modest COF (~0.4) was achieved for Cu-15 wt pct Pb nanocomposites. The lower wear rate of Cu-Pb nanocomposites with respect to unreinforced Cu is attributed to high hardness (~2 to 3.5 GPa) of the matrix, Cu2O/Fe2O3-rich oxide layer formation at tribological interface, and exuding of softer Pb particles. The wear properties are discussed in reference to the characteristics of transfer layer on worn surface as well as subsurface damage probed using focused ion beam microscopy. Interestingly, the flash temperature has been found to have insignificant effect on the observed oxidative wear, and alternative mechanisms are proposed. Importantly, the wear resistance properties of the nanocomposites reveal a weak Hall–Petch-like relationship with grain size of nanocrystalline Cu.

Hyperthyroïdie clinique induite par l’amiodarone dans les suites d’un bypass gastrique : à propos d’un cas

In this study we report about the inner and outer structure of CuInS2 microspheres which might be used e.g. in pastes for simple, low-cost solar cell preparation, as well as in electrodes for light-driven water splitting. The microspheres are synthesized via a mild, template-free solvothermal synthesis route and characterised by electron and focused ion beam microscopy, X-ray diffraction, inductively coupled plasma atomic emission and energy dispersive X-ray spectroscopy. The investigations of cross sections prepared by focused ion beam showed that the spheres consist of compact cores and flaky surface structures. Depending on the reaction time, the core possesses a stoichiometric or Cu-rich chemical composition surrounded by an In-rich shell. The flaky surface always comprises a stoichiometric composition in tetragonal chalcopyrite crystal structure, whereas the other areas additionally show minor contributions of CuS, and CuInS2 in hexagonal wurtzite structure. The presence of different phases can be beneficial for future applications since they offer different absorption behaviour in the visible range.

Overview of strength, crack propagation and fracture of nuclear reactor moderator graphite

Nuclear reactor moderator graphite is an aggregate of needle coke filler particles within a matrix of fine coke flour particles mixed with pitch binder. Following extrusion in green condition, impregnation with liquid pitch binder and graphitisation, a polygranular aggregate with orthotropic properties is produced. Its mechanical properties under several different loading conditions and associated cracking behaviour were examined to establish crack initiation and propagation behaviour. Both virgin and radiolytically oxidised material were examined using optical and electron optical microscopy, focused ion beam microscope and digital image correlation. The appearance of force vs. displacement curves varied with type of loading. Mostly linear elastic traces occurred in uniaxial tensile and flexural tests. Large departures from linear elastic behaviour were observed in standard uniaxial and diametral compression testing. Digital image correlation has shown that the initiation of cracking involves formation of a process zone which grows to a critical size of approximately 3–5mm before a macro-crack is initiated. Cracks straddle a torturous path which zigzags between the filler particles through the matrix consistent with crack propagation along the filler matrix interface. This paper provides an overview of strength, crack propagation and fracture of nuclear reactor moderator graphite. It reviews the physical processes and mathematical approaches that have been adopted to describe the behaviour of brittle materials and then considers if they apply to reactor core graphites.

Imaging after Local Tumor Therapies: Kidney and Liver

Nuclear reactor moderator graphite is an aggregate of needle coke filler particles within a matrix of fine coke flour particles mixed with pitch binder. Following extrusion in green condition, impregnation with liquid pitch binder and graphitisation, a polygranular aggregate with orthotropic properties is produced. Its mechanical properties under several different loading conditions and associated cracking behaviour were examined to establish crack initiation and propagation behaviour. Both virgin and radiolytically oxidised material were examined using optical and electron optical microscopy, focused ion beam microscope and digital image correlation. The appearance of force vs. displacement curves varied with type of loading. Mostly linear elastic traces occurred in uniaxial tensile and flexural tests. Large departures from linear elastic behaviour were observed in standard uniaxial and diametral compression testing. Digital image correlation has shown that the initiation of cracking involves formation of a process zone which grows to a critical size of approximately 3–5 mm before a macro-crack is initiated. Cracks straddle a torturous path which zigzags between the filler particles through the matrix consistent with crack propagation along the filler matrix interface. This paper provides an overview of strength, crack propagation and fracture of nuclear reactor moderator graphite. It reviews the physical processes and mathematical approaches that have been adopted to describe the behaviour of brittle materials and then considers if they apply to reactor core graphites.