Electron Backscattering Pattern Research Articles

Study on Subgrain Rotation Behavior at Different Interfaces of a Solder Joint During Thermal Shock

In order to investigate subgrain rotation behavior in the recrystallized region of lead-free solder joints, a ball grid array (BGA) specimen with a cross-sectioned edge row was thermally shocked. Electron backscattered diffraction (EBSD) was used to obtain the microstructure and orientations of Sn grains or subgrains in as-reflowed and thermally shocked conditions. Orientation imaging microscopy (OIM) showed that several subgrains were formed at the tilted twin grain boundaries, near the chip side and near the printed circuit board (PCB) side after 200 thermal shocks due to a highly mismatched coefficient of thermal expansion (CTE) of twin grains. Also, subgrains formed at the chip side and PCB side in the solder joint were selected to research the grain rotation behavior in lead-free solder joints. The analysis of subgrain rotation also indicated that the rotation behavior of subgrains was different between the chip side and PCB side. It was closely related with the large different crystal orientations between the chip side and PCB side. Furthermore, electron backscattered patterns (EBSPs) at several parts of the joint were not obtained after 300 thermal shocks due to the serious deformation caused by mismatched CTE during thermal shock. But 4 subgrains were selected and compared with that of the initial state and 200-thermal shock conditions. The results showed that the subgrains at the chip side were also rotated around the Sn [101] and [001] axes and the subgrains at the PCB side were also rotated around the Sn [100] axis, which indicated a continuous process of subgrain rotation.

Oxygen precipitates distributed around random grain boundaries in a cast-grown multicrystalline silicon crystal

We investigated oxygen precipitates and grain boundaries (GBs) in multicrystalline silicon for solar cells. We observed the distribution of GBs on both sides of the specimen by using an electron backscattering pattern to be able to compare it with the distribution of oxygen precipitates revealed by infrared absorption spectroscopy. We precisely examined the relationship between oxygen precipitates and various GBs, which are coincidence site lattice GBs () and random GBs, and found that the distribution of oxygen precipitates coincided with that of random GBs. Furthermore, we annealed the specimen in which the oxygen precipitates already existed around the random GBs, and found that the dispersion and coalescence of the precipitates took place.

Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling

Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous and interrupted thermal cycling on the number of cycles to failure. The interruptions given two of the samples were the result of periodic examinations using electron backscattered pattern mapping, leading to room temperature aging of 30 days–2.5 years after increments of about 100 cycles at several stages of the cycling history. The continuous thermal cycling resulted in solder joints with a much larger degree of recrystallization, whereas the interrupted thermal cycling tests led to much less recrystallization, which was more localized near the package side, and the crack was more localized near the interface and had less branching. The failure mode for both conditions was still the same, with cracks nucleating along the high angle grain boundaries formed during recrystallization. In conditions where there were few recrystallized grains, recovery led to formation of subgrains that strengthened the solder, and the higher strength led to a larger driving force for crack growth through the solder, leading to failure after less than half of the cycles in the continuous accelerated thermal cycling condition. This work shows that there is a critical point where sufficient strain energy accumulation will trigger recrystallization, but this point depends on the rate of strain accumulation in each cycle and various recovery processes, which further depends on local crystal orientations, stress state evolution, and specific activated slip and twinning systems.

Heel crack propagation mechanism of cold-rolled Cu/Al clad ribbon bonding in harsh environment

A cold-rolled Cu/Al clad ribbon was bonded on an electroless nickel immersion gold (ENIG)-finished Cu substrate by ultrasonic bonding. The bonding samples were subjected to harsh conditions such as thermal exposure at 200 °C and thermal shock at −40/250 °C. The microstructural evolution and textural transitions in the Cu/Al clad ribbons were analyzed using electron backscatter diffraction to understand the heel crack propagation mechanism. The heel cracks were initiated at the edge of the Al layer between the bonded and non-bonded zones and were propagated along the coarsened grain boundaries deep into the cladded Cu layer. The continuous dynamic recrystallization (CDRX) transition phenomena around the heel cracks were scrutinized by electron backscatter pattern analysis to reveal the different microstructural and textural evolutions as a function of the ultrasonic bonding process at the bonded part and of lift-up at the non-bonded part. The heel cracks were occurred by grain coarsening due to CDRX within the Al layer during the environmental tests. The region of coarsened grains by CDRX was extended with increasing test periods. Especially in the thermal shock tests at −40/250 °C, severe grain coarsening was found involving significant transformation of sub-grains into grains by rapid CDRX development. Thus, heel crack propagation accelerated more during thermal shock cycles than during high-temperature storage.

Reversed scan direction reduces electron beam damage in EBSD maps

The deleterious effects of electron beam damage on high-resolution electron backscatter diffraction (EBSD) maps of undeformed quartz are significantly reduced by scanning in the direction opposite to that dictated by widely used EBSD acquisition software. Higher quality electron backscatter patterns are produced when the electron beam moves progressively down the sample (the apparent 'up' direction in the resulting maps) for all step sizes where beam damage affects EBSD map quality (≤ ∼0.4 μm in this study). The relative improvement associated with downward scanning increases as step size is reduced. A comparison of high-resolution maps made in experimentally deformed quartz demonstrates that downward scanning reduces by a factor of ∼2 the lower limit in step size relative to maps scanned in the conventional direction. The electron beam damages quartz at its point of entry, forming ∼0.1-μm diameter bumps visible in Scanning electron microscope (SEM) images. Downward scanning produces better results because it minimizes the flux of electrons through these loci of damaged crystal.

An alternative way to orient the parent phase in the cubic/orthorhombic martensitic transformation of titanium shape memory alloys

Titanium-based shape memory alloys undergo a cubic to orthorhombic martensitic transformation (β to α'' martensite). A new easy way operating as a reconstruction of the parent microstructure is used from electron backscattered diffraction. It is shown that the electron backscattered patterns of the martensitic microstructure can be directly indexed as the parent phase in order to obtain the correct orientation of the high temperature microstructure. This method is however specific to the cubic/orthorhombic martensitic transformation in titanium shape memory alloys.

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.

Strength improving mechanism of laser arc hybrid welding of wrought AA 2219 aluminium alloy using AlMg5 wire

Laser arc hybrid welding of wrought AA 2219 aluminium alloy was studied. The cross-weld tensile strength of the joint using AlMg5 wire was 20% higher than that using AlCu7 wire. The reasons for strength improvement were analysed by microstructure observations via scanning electron microscopy, transmission electron microscopy and electron backscattering patterns. The results showed the strength improvement of the joint using AlMg5 wire depended on two reasons. One is the formation of large amount of fine S–Al2CuMg precipitates. The other is the preformed pinning effect of the precipitates on the dislocations. Finally, the formation of the fine S–Al2CuMg precipitates of the joint using AlMg5 wire was discussed by thermodynamic calculation, and the formation of the preformed dislocation pining effect was discussed by the collision theory of adjacent dendrites during solidification.

Tensor Evaluation of Stress Relaxation Profile in Strained SiGe Nanostructures on Si Substrate

A strained SiGe layer will be used in next-generation transistors to improve device performance along with device scaling. However, the stress relaxation of SiGe layer may be inevitable in nanodevices, because the SiGe layer is processed into nanostructure. In this study, we evaluated the stress relaxation profiles in mesa-shaped strained SiGe layers on Si substrate by electron back scattering pattern (EBSP), super-resolution Raman spectroscopy (SRRS) measurements, and finite element method (FEM) simulation. As a result, the stress relaxation profiles with high spatial resolution were obtained by each measurement. The range of σxx stress relaxation is mostly 100 nm from edge. The drastically σxx stress relaxation was over than approximately 25 percent in that range. Thus, the stress relaxation is inevitable in nanostructure with less than 200 nm scale. Moreover, there is a good correlation between the results of EBSP, SRRS measurements, and FEM simulation. The spatial resolution of EBSP and SRRS measurements were estimated less than 100 nm. Thus, super-resolution algorithm improved the spatial resolution of Raman spectroscopy from approximately 400 nm to sub-100 nm. It is prospective to evaluate the precise stress relaxation profile the sub-100 nm order structures by EBSP and SRRS measurements, respectively. Moreover, the complement of FEM simulation is important to verify the results of EBSP and SRRS. We believe that EBSP, SRRS measurements, and FEM simulation will be indispensable to for evaluating stress in future MOSFETs with high spatial resolution, which will surely surpass the present ones in complexity.

Tensor Evaluation of Anisotropic Stress Relaxation in Mesa-Shaped SiGe Layer on Si Substrate by Electron Back-Scattering Pattern Measurement: Comparison between Raman Measurement and Finite Element Method Simulation

A strained SiGe layer will be used in next-generation transistors to improve device performance along with device scaling. However, the stress relaxation of the SiGe layer may be inevitable in nanodevices, because the SiGe layer is processed into a nanostructure. In this study, we evaluated the anisotropic stress relaxation in mesa-shaped strained SiGe layers on a Si substrate by electron backscattering pattern (EBSP) measurement. Moreover, we compared the results of EBSP measurement with those of anisotropic Raman measurement and finite element method (FEM) simulation. As a result, the anisotropic stress relaxation obtained by Raman spectroscopy was confirmed by EBSP measurement. Additionally, we obtained a good correlation between the results of EBSP measurement and FEM simulation. The σxx and σyy stresses were markedly relaxed and the σzz and σxz stresses were concentrated at the SiGe layer edges. These stresses were mostly relaxed in the distance range from the SiGe layer edges to 200 nm. Therefore, in a SiGe nanostructure with a scale of less than 200 nm, stress relaxation is inevitable. The results of EBSP and Raman measurements, and FEM simulation show a common tendency. We believe that EBSP measurement is useful for the evaluation of stress tensors and is complementary to Raman measurement.

Effect of Cold-Rolling Rate on Texture in Ti-Mo-Al-Zr Shape Memory Alloy

Effect of rolling rate on texure of Ti-Mo-Al-Zr shape memory alloy was investigated using X-ray diffraction pole figure measurement and electron backscattering pattern (EBSP) analysis to optimize the thermomechanical processing. Ti-Mo-Al-Zr alloy is a candidate Ni-free shape memory alloy to replace Ti-Ni alloy in medical applications. The alloy was single phase of β before rolling and then α''-martensite was induced during the cold-rolling. Unlike the recrystallization texture in Ti-Nb alloy system, {110}ββ recrystallization texture of β-phase was developed by the solution-treatment especially in the specimens with the reduction rate higher than 90%. This texture has never been observed in Ti-Nb based alloys and can exhibit a larger recovery strain in both tension and compression.

Boundary Migration during Recrystallization of Heavily Deformed Pure Nickel

The detailed microstructure in front of recrystallization boundaries and their migration during annealing were traced using ex-situ electron backscatter pattern maps of one and the same surface area taken after annealing. It is observed that many protrusions/detrusions form on the recrystallizing boundaries. During annealing, the recrystallization boundary segments migrate in a stop-go type of fashion, while protrusions and detrusions alternately form and disappear. The correlation between the protrusions/detrusions and the stop-go type of migration are briefly discussed.

Ti-4Fe-7Al 合金の焼戻しに伴う特異現象

Unprecedented phenomena were discovered by tempering the Ti-4Fe-7Al alloy quenched from the β (bcc) field. The alloy became very hard when it was tempered at 450℃ for several minutes, and severely rugged surface was generated. The inverse shape recovery phenomenon was also discovered when a quenched specimen that had been bent at room temperature was heated. The tempered microstructure showed almost β grains and some of the usual martensitic acicular structure areas. However, electron back scattering pattern (EBSP) measurements showed that the β-like grain was not the bcc structure but was the hcp or orthorhombic structure. X-ray diffraction (XRD) measurements clarified that an orthorhombic α″ structure (a=0.2995 nm, b=0.4913 nm, c=0.4659 nm) was formed from the β phase by tempering. Moreover, this α″ structure was confirmed to be a type of martensitic transformation because no concentration distribution was detected in scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis of the microstructure. It was suggested that the essential Ms point of the alloy should be higher than room temperature; however, the martensite transformation could not operate by fast quenching. The newly-discovered α″ martensite is formed without atomic diffusion by heating. When the β grain transforms into the single α″-variant, a very huge lattice strain is generated, resulting in the severely rugged surface or the inverse shape recovery phenomenon.

Characterization of Coatings by SEM Based Microdiffraction

The behaviour of a coating is directly related to its microstructure as well as the crystal structure and composition of different phases that are formed in the deposition process. A comprehensive study of the microstructure and local phase assembly in coatings would help in arriving at structure-property correlations that can help understand the coating behaviour. SEM-based diffraction techniques provide a simple method for obtaining local crystallographic information without the need for complex synchrotron sources. In this study, we present a method for characterization of coatings using SEM-based microdiffraction which involves the combined use of the EDS and EBSD capabilities, citing a Ti-Al-Cr-N multilayer coating as an example. The different layers in the coating were observed and the electron beam focused in each region to first obtain Energy Dispersive Spectra and electron backscatter patterns (EBSPs). The elemental constituents were identified from EDS maps and used to shortlist the possible phases present. The diffraction pattern for each possible phase was then calculated and the EBSPs of the observed and calculated patterns were compared for the closest match. The identified phase was then used as an input to set up EBSD scans across the coating. A qualitative picture of the compositional variation in multilayer coatings was obtained that could help in arriving at the exact stoichiometry of the different layers. Hence, SEM-based microdiffraction allows identification of local crystallographic phases and composition, permitting detailed microstructural studies that would find special application in the study of coatings.

Measurement of residual elastic strain and lattice rotations with high resolution electron backscatter diffraction

A set of dynamically simulated electron backscatter patterns (EBSPs) for α-Ti crystals progressively rotated by 1° steps were analysed using cross-correlation to determine image shifts from which strains and rotations were calculated. At larger rotations the cross-correlation fails in certain regions of the EBSP where large shifts are generated. These incorrect shifts prevent standard least square error procedures from obtaining a valid solution for the strain and rotation, where the applied rotation exceeds ∼ 8°. Using a robust iterative fitting routine reliable strains and rotations can be obtained for applied rotations of up to and including ∼ 11° even though some image shifts are measured incorrectly. Finally, high resolution electron backscatter diffraction has been used to analyse the residual elastic strain, lattice rotations and density of stored geometrically necessary dislocations in a sample of copper deformed to 10% total strain. The robust iterative fitting analysis allows reliable analysis of a larger proportion of the map, the difference being most obviously beneficial in regions where significant lattice rotations have been generated.

An Open-Source Engine for the Processing of Electron Backscatter Patterns: EBSD-Image

An open source software package dedicated to processing stored electron backscatter patterns is presented. The package gives users full control over the type and order of operations that are performed on electron backscatter diffraction (EBSD) patterns as well as the results obtained. The current version of EBSD-Image (www.ebsd-image.org) offers a flexible and structured interface to calculate various quality metrics over large datasets. It includes unique features such as practical file formats for storing diffraction patterns and analysis results, stitching of mappings with automatic reorganization of their diffraction patterns, and routines for processing data on a distributed computer grid. Implementations of the algorithms used in the software are described and benchmarked using simulated diffraction patterns. Using those simulated EBSD patterns, the detection of Kikuchi bands in EBSD-Image was found to be comparable to commercially available EBSD systems. In addition, 24 quality metrics were evaluated based on the ability to assess the level of deformation in two samples (copper and iron) deformed using 220 grit SiC grinding paper. Fourteen metrics were able to properly measure the deformation gradient of the samples.

Evaluation of Strained Silicon by Electron Back Scattering Pattern Compared with Raman Measurement and Edge Force Model Calculation

Global and local strained-Si samples, namely strained-Si on insulator (SSOI) wafer and a Si substrate with a patterned SiN film were each evaluated by electron backscattering pattern (EBSP). In the EBSP measurements for SSOI, biaxial tensile stresses (biaxial tensile strains and compressive strain perpendicular to the surface) were obtained, whose values were consistent with those obtained by UV-Raman spectroscopy. One-dimensional stress distributions in the Si substrate with the patterned SiN film were obtained by EBSP, UV-Raman spectroscopy with a deconvolution method, and edge force model calculation. The results were well consistent with each other. EBSP allows us to measure stress and strain in the patterned SiN sample with 150-nm wide space. Furthermore, anisotropic biaxial stress including shear stress was also obtained by EBSP.

Evaluation of Strained-Silicon by Electron Backscattering Pattern Measurement: Comparison Study with UV-Raman Measurement and Edge Force Model Calculation

We demonstrate the results of strain (stress) evaluation obtained from electron backscattering pattern (EBSP) measurement for samples of a strained Si-on-insulator (SSOI) and a Si substrate with a patterned SiN film. Two-dimensional stress distributions were obtained in 40×40 µm2 areas of the SSOI. The biaxial stress state was also obtained in the SSOI. Furthermore, clear cross-hatch contrast was observed, especially in the distribution of shear stress Sxy, in contrast to with the other distributions of normal stress Sxx and Syy. One- and two-dimensional stress distributions in the Si substrate with the patterned SiN film were also obtained from EBSP measurement. Moreover, the results were compared with those of UV-Raman measurement and edge force model calculation, and were found to have a good correlation with each other. EBSP measurement was used to measure the complicated biaxial stress including the shear stress in a sample with a 150-nm-wide space pattern. We can conclude that EBSP measurement is a useful method for precisely measuring stress with high spatial resolution.

Evaluation of Strained-Silicon by Electron Backscattering Pattern Measurement: Comparison Study with UV-Raman Measurement and Edge Force Model Calculation

We demonstrate the results of strain (stress) evaluation obtained from electron backscattering pattern (EBSP) measurement for samples of a strained Si-on-insulator (SSOI) and a Si substrate with a patterned SiN film. Two-dimensional stress distributions were obtained in 40×40 µm2 areas of the SSOI. The biaxial stress state was also obtained in the SSOI. Furthermore, clear cross-hatch contrast was observed, especially in the distribution of shear stress Sxy, in contrast to with the other distributions of normal stress Sxx and Syy. One- and two-dimensional stress distributions in the Si substrate with the patterned SiN film were also obtained from EBSP measurement. Moreover, the results were compared with those of UV-Raman measurement and edge force model calculation, and were found to have a good correlation with each other. EBSP measurement was used to measure the complicated biaxial stress including the shear stress in a sample with a 150-nm-wide space pattern. We can conclude that EBSP measurement is a useful method for precisely measuring stress with high spatial resolution.

Microstructural evolution of fine-grained layers through the firn column at Summit, Greenland

AbstractWe present a microstructural characterization of fine-grained layers from the top 90 m of firn from Summit, Greenland, performed using a combination of scanning electron microscopy techniques including secondary electron imaging, energy-dispersive spectroscopy and electron backscattered patterns, and X-ray microcomputed tomography. The impurities in the firn, both soluble impurities and dust particles, were found largely in the grain interiors. Both c- and a-axis pole figures do not show strong evidence of a preferred orientation of the grains even at the bottom of the firn column. The firn structure became increasingly anisotropic with vertical alignment in the top 3 m, probably due to vapor transport associated with dry-snow metamorphism. The anisotropy decreases below this level until at ∼50 m the average firn structure is close to isotropic. In the near surface, the level of anisotropy is weaker than at Hercules Dome, Antarctica, confirming that differences in accumulation rates and temperatures leave enduring evidence in the structure of the firn. The fraction of closed-off pores is relatively low until ∼65 m; below that it rises through the end of our sampling at 90 m. Our microstructure measurements on the microscale are consistent with in situ firn-air sampling measurements on a decimeter scale, both indicating the existence of the lock-in zone starting near 69 m depth, and pore close-off at 81 m at this site.