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Strain evolution in 0.45-μm-thick, 2-μm-wide, and 100-μm-long Cu conductor lines with a passivation layer has been investigated using synchrotron x-ray microdiffraction. A moderate electromigration-current density of 2.2 × 105 A/cm2 was used to minimize Joule heating in the Cu conductor lines. After 120 h of current flowing in the Cu lines at 270 °C, measurements show strain relaxation and homogenization occurring in the Cu lines with current flowing, but not in Cu conductor lines without current. Stronger interaction between electrons with Cu atoms in areas with higher strains was proposed to explain the observation.

We have observed abrupt, reversible resistance changes during electromigration (EM) testing of damascene Cu conductor lines. The tests were conducted at temperatures of 300°C or 350°C and with current densities from 0.6 × 106 A/cm2 to 1.6 × 106 A/cm2. In most cases, an incubation period with negligible resistance increase was followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. If the direction of current was reversed, the resistances of the Cu lines decreased, due to the refilling of voids by the back flowing atoms. With further EM, the resistance curves showed spikelike features, with a sudden resistance increase followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurred, and the line failed. We present resistance data, microstructural observations, and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating and melting of conductor line segments, and from voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases, line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to the voids.

Summary A study of the metallurgy and manufacturing techniques of a group of eight astrolabes (seven from Lahore, one attributed to India) using non-destructive methods has produced the earliest evidence for systematic use of high-zinc (α + β) brass. To produce this alloy, the brass industry supplying the Lahore instrument makers must have co-melted metallic copper and zinc. This brass-making technology was previously believed to have been developed on an industrial scale in the nineteenth century in Europe. This work hypothesizes that this technology was used in Lahore on an industrial scale as early as ad 1601. In addition, this work hypothesizes that the α + β brass alloy was used specifically for its ease in manufacturing the thin sheet brass required for astrolabe-component manufacture.

Luminescence variations in MBE-grown blue and red ZnCd(Mg)Se/InP QW LEDs have been studied by time-resolved and bias-dependent cathodoluminescence (CL) spectroscopy. CL from the red LED reversibly increased under forward bias and decreased under negative bias during several complete biasing cycles. CL from blue LEDs was bias dependent only during the first few cycles. The irreversible effects were QW CL intensity decreases after electron bombardment for the red LED and QW CL intensity enhancements seen as bright regions in CL images and dark regions in SE micrographs for blue LEDs. For the red LED, the bombarded regions were dark in CL images and had a dark dot and halo contrast in SE micrographs. The enhancements of CL intensity in the blue LEDs may result from buildup of internal fields due to persistent charge trapping in preexisting or electron bombardment created defects.

Synchrotron X rays were used to perform nondestructive transmission diffraction and fluorescence experiments on a group of 24 European and Islamic astrolabes dated between 1350–1720 A.D. in order to determine their compositions. A group of six astrolabes produced in Lahore between 1601–1662 A.D. were found to contain a mixed α+β brass microstructure, proving that the brass was produced by a comelting technique rather than the traditional cementation process. The results also show evidence of dezincification, attributed to heavy annealing of the brass during astrolabe manufacture. This effect was so severe that an accurate analysis of the bulk Zn composition could not be determined from the fluorescence results alone; transmission X-ray diffraction gives a more accurate measurement of the bulk Zn composition.

Synchrotron-based x-ray microbeam fluorescence and diffraction have been used for in situ measurements of Cu concentration and biaxial stress in a 200-μm-long, 10-μm-wide Al(0.25 at. % Cu) conductor line with 1.5-μm-thick SiO2 passivation during electromigration. Measurements over 48 h with T=300 °C and j=1.5×105 A/cm2 show that a stress gradient of 3 MPa/μm develops over the upstream 130 μm of line length where Cu concentration drops below 0.15 at. %, and a 10-μm-long void develops at the cathode end of the line, but little change in stress occurs over the downstream 70 μm of line length where Cu concentration remains above 0.15 at. %. These experimental results have been reproduced by a finite element model in which the downstream Cu transport is accompanied by a counter flow of Al in the upstream direction, and downstream Al motion is blocked where the local Cu concentration is above ∼0.15 at. %. Defect mediated coupling between Al and Cu diffusive flows, e.g., Cu–vacancy binding, is proposed as the ca...

Electromigration and diffusion of Cu have been investigated for polycrystalline Al(0.25 at. % Cu) conductor lines. In situ measurements of the evolution of Cu concentration profiles along 200 μm long, 10 μm wide conductor lines with 1.5 μm thick SiO2 passivation during electromigration have been obtained by synchrotron-based white x-ray microbeam fluorescence. The apparent effective charge ZCu* of Cu in Al(Cu) has been found to be −8.6±1.0. The evolution of Cu concentration profiles can be manipulated by controlling the direction and magnitude of the current flow at different temperatures. The effective grain boundary diffusivity DCueff has been determined by fitting the time dependent experimental Cu concentration profiles. The results show Arrhenius behavior of DCueff=D0 exp(−Q/kT) for T=275–325 °C with D0=10−(2.3±1.6) cm2/s and Q=0.76±0.19 eV.

Grain boundary segregation of Y in α -Al 2 O 3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α -Al 2 O 3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y 3 Al 5 O 12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α -Al 2 O 3 , and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y 2 O 3 . This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.

Dopants Y and Zr at 100 p.p.m. level (Y or Zr to Al atomic ratio) in ultra-high purity polycrystalline alumina have been found to be mainly segregated to the alumina grain boundaries. The atomic structural environments around the Y and Zr segregants have been investigated by Extended X-ray Absorption Fine Structure (EXAFS). On average, Y ions in α -Al 2 O 3 grain boundaries are coordinated by four oxygens, at a distance of 2.30 A, which corresponds nearly to the Y–O bond length in cubic Y 2 O 3 , and Zr ions are coordinated by five oxygens at a distance of 2.14 A, which is approximately the same as the average Zr–O bond length in monoclinic ZrO 2 . However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at 100 p.p.m. concentrations in α -Al 2 O 3 occupy grain boundary sites with well defined nearest neighbor cation–oxygen bond lengths similar to those in their parent oxides but with the next nearest neighbor cation–cation distances varying considerably from site to site. From EXAFS, the Y grain boundary saturation concentration is estimated to be 6.0 atoms/nm 2 , which is consistent with the estimate from STEM of 4.4 atoms/nm 2 . The differences in the Zr–O and Y–O nearest neighbor distances and coordination numbers in Al 2 O 3 grain boundaries are related to the Y–Al and Zr–Al size mismatches.

We have used cross sectional cathodoluminescence microscopy as a fast and nondestructive tool to characterize II–VI based green laser diodes. We find evidence for carrier mediated excitation of semiconductor layers that are not directly irradiated by the focused electron beam, from which diffusion lengths of lower mobility carriers (presumably holes) can be estimated. We find that N-doped (p-type) ZnMgSSe exhibits a very low (near) band edge luminescence efficiency. The diffusion length of minority carriers in p-type ZnMgSSe:N [(1–2)×1017 cm−3 net acceptor concentration] was found to be lower than for n-type ZnMgSSe:Cl with roughly equal dopant concentration. The diffusion length of minority carriers in n-type ZnMgSSe:Cl decreases from 0.21 μm for a doping level of (1–2)×1017 cm−3 to

Phosphor screens which convert x-ray images to visible light images are key components in two-dimensional charge coupled device (CCD) based detector systems used for x-ray diffraction. Some experimental and theoretical aspects of phosphor screen performance are described in this article. The efficiencies of x-ray-to-light conversion were measured using a CCD camera for transmission phosphor screens fabricated from two different phosphor powders, Y2O2S:Eu (P22R) and Gd2O2S:Tb (P43), for screen mass thicknesses of 3–50 mg/cm2 and for x-ray energies of 7–34 keV. A model was developed and evaluated for the dependence of the emitted light brightness on screen thickness and x-ray energy. Inputs to the model are x-ray absorption coefficients available from published compilations, and light attenuation versus thickness data, which were determined experimentally for the phosphors and found to be dominated by scattering rather than absorption. The angular distribution of emitted light, measured for one of the phosphor screens, was found to be nearly Lambertian. Broadening of image features in the x-ray-to-visible-light conversion by phosphors for 19.6 keV x-rays was found to increase approximately linearly with phosphor screen thicknesses in the range of 30–160 μm, but with a minimum width of 110 μm for P22R phosphor and 70 μm for P43 phosphor. In the range of 7–15 keV, maximum brightness was obtained for P43 phosphor screens of about 10 mg/cm2 mass thickness (60 μm). For P22R screens, the thickness for maximum brightness increased from about 8 mg/cm2 (50 μm) for 7 keV to more than 46 mg/cm2 (210 μm) for 15 keV. For 7 keV the maximum brightnesses for P22R and P43 phosphors were about the same. For 10 keV the maximum brightness for P43 phosphor was about 60% greater than the maximum brightness for P22R phosphor samples tested. For 15 keV the maximum brightness for P43 phosphor was again about 60% greater than that for the P22R samples tested. In the range of 20–34 keV, maximum brightness would occur for thicknesses greater than 46 mg/cm2 (210 μm) for P22R phosphor and greater than 40 mg/cm2 (160 μm) for P43 phosphor. Comparing the brightness for 90 μm thickness for the two phosphors, P43 was about 30% brighter for 20 keV, 20% brighter for 24 keV, and 10% brighter for both 29 and 34 keV.

We discuss the results of high resolution, low temperature X-ray diffraction measurements used to determine the lattice strain caused by electron emission from Sn- and Si-related DX centers in Ga1 − xAlxAs. DX centers are deep levels associated with donors in various III–V semiconductors, and they are responsible for persistent photoconductivity and other unusual electrical and optical properties of these materials. Although DX centers have been extensively studied for more than a decade, information is still needed about the atomic arrangements that comprise DX centers for various donor and host systems. Our lattice strain measurements show that there is lattice expansion attributable to local bonding changes that occur when the DX centers are emptied. This result is important in testing proposed structural models for the DX center, and it can be used to rule out a proposal that symmetric breathing-mode relaxation is responsible for a DX− state for Sn-doped Ga1 − xAlxAs. The theoretical proposal by Chadi (Phys. Rev. B, 46 (1992) 6777) requires an inward relaxation when a Sn-related DX center is emptied, but our experiments show an outward relaxation.

Modeling and simulation coupled with X-ray microbeam studies, where local elastic strains are measured, can be used to resolve some of the challenging questions regarding the physics of electromigration. This chapter provides an introduction to the prevalent modeling methodology and three key issues that are currently unresolved: (i) how does the diffusing mass arrange itself and the effect of diffusion path, (ii) what is the appropriate driving stress, and (iii) determination of effective charge number Z*. Analytic and finite element modeling formulations are presented and used with two X-ray microbeam data sets, exhibiting opposing trends, to demonstrate how these questions may be answered.

An X-ray microbeam study and a polycrystal finite element model of a 1010 m 2 section of a 1 m thick polycrystalline aluminum film on a silicon substrate are used to investigate the effect of microstructure on thermal stress variability. In the X-ray microbeam study, the grain orientations and deviatoric elastic strain field are measured at the subgrain level in the film during and after two thermal cycles. A finite element model of the observed grain structure is created and modeled with an elastoviscoplastic crystal constitutive model that incorporates film thickness and grain size effects as well as dislocation entanglement hardening. The experimental and simulation results are compared at both the film and subgrain scales. While the experiment and model agree fairly well at the film level, the experimental results show much greater elastic strain variability than the simulations. In considering the grain size effect, the experiment and model both predict a similar Hall‐Petch coefficient, which is consistent with literature data on free standing aluminum thin films. DOI: 10.1115/1.4002212

Extended X-ray absorption fine structure measurements were used to determine nearest neighbor coordination numbers and distances for As in amorphous Si prepared by 100 keV As ion-implantation of single crystal Si. In the limit of low As concentration, the average AsSi coordination number was 3.2±0.1 and the As-to-Si distance was 2.37±0.02 A , compared with 4 and 2.41±0.02 A , respectively, for As in crystalline Si. It is concluded that 20% of the As atoms are fourfold coordinated and 80% are threefold coordinated in the amorphous Si, and that the lack of electrical doping by the fourfold coordinated As results from electron trapping by dangling bonds or other structural defects.

The lattice strain caused by electron emission from DX centers was determined by x-ray diffraction for Sn-doped and Si-doped Ga 1-x Al x As, with x Al =0.22-0.24. Measurements were made after cooling samples to 14 K in the dark, which filled DX centers and thereby produced low conductivity. The measurements were repeated after emptying the DX centers by ir illumination, giving ΔN e =(1.0-1.6)×10 18 cm -3 conduction electrons which persisted after illumination

Diffusion lengths of excited carriers in a CdxZn1−xSe multiple quantum well structure were determined for temperatures between room temperature and 8 K from cathodoluminescence measurements. The diffusion length was found to depend upon temperature and Cd concentration of the quantum well. For the highest Cd concentration (x=0.43), the diffusion length increased with temperature up to 225 K and then dropped at higher temperatures. Diffusion lengths were 0.21 μm at 8 K, 0.38 μm at 225 K, and 0.24 μm at room temperature. For the well with least Cd concentration (x=0.24), longer diffusion lengths were obtained. The nature of the diffusing carriers is also discussed.

Al conductor lines, with Ti-Al top and bottom layers and SiO2 passivation, were stressed with current density 1.5 × 106 A/cm2at 190°C, and the strains developed during electromigration were measured in-situ by white and monochromatic beam synchrotron x-ray microdiffraction. Grain-scale deviatoric strain measurements with 0.4µm beam size and perpendicular full strain measurements with 1.0µm beam size were made repeatedly during electromigration. A strong strain gradient developed along the upstream half of the conductor lines during electromigration, although no resistance changes, voids or extrusions were seen. Orientation maps showed near-bamboo grain structure. Results from an approximate analytic model, using the Eshelby inclusion theory, are consistent with the measurement results for late-stage electromigration-induced strains.

We report on the implementation of crystal ion slicing in lithium niobate (LiNbO3). Deep-ion implantation is used to create a buried sacrificial layer in single-crystal c-cut poled wafers of LiNbO3, inducing a large etch selectivity between the sacrificial layer and the rest of the sample. 9-μm-thick films of excellent quality are separated from the bulk and bonded to silicon and gallium arsenide substrates. These single-crystal films have the same room-temperature dielectric and pyroelectric characteristics, and ferroelectric transition temperature as single-crystal bulk. A stronger high-temperature pyroelectric response is found in the films.

X-ray microbeam techniques (spot size=3×20 μm2) have been applied to characterize the composition and structure of rare earth activated Gd(La, Sr)AlO3 phosphor thin films grown by combinatorial synthesis. Using x-ray fluorescence, x-ray diffraction and near-edge x-ray absorption spectroscopy, we have measured the chemical composition, crystallographic structure, and valence state of the rare earth activator atom Eu. These measurements represent the direct application of x-ray techniques to solid-state materials prepared by combinatorial synthesis and demonstrate the power of x-ray microbeam analysis to nondestructively characterize as-grown combinatorial libraries.

Spectral shifts associated with 〈100〉 dark line defects of degraded II-VI laser diodes based on ZnCdSe/ZnSSe/MgZnSSe separate confinement heterostructures have been studied by spatially resolved cathodoluminescence at room temperature. Dark line defects were induced by electron-beam bombardment. Peak shifts as large as 2 nm were observed towards the blue or the red depending on the local circumstances. Peak widths usually became narrower after degradation. Redshifts and blueshifts are explained in terms of strain relaxation and Cd out-diffusion associated locally with degradation, as well as the kinetic energy dependence of the degradation-related carrier capture cross section.

Electromigration-induced stress distributions in 200 μm long, 10 μm wide aluminum conductor lines in 1.5 μm SiO2 passivation layers have been investigated in real time using synchrotron-based white-beam x-ray microdiffraction. The results show that a steady-state linear stress gradient along the length of the line developed within the first few hours of electromigration and that the stress gradient could be manipulated by controlling the magnitude and the direction of the current flow. From the current density dependence of the steady-state stress gradient, the effective valence Z* was determined to be 1.6 at 260 °C. From the time dependence of the transient-state stress gradient, the effective grain boundary diffusion coefficient Deff was estimated to be 8.2×10−11 cm2/s at 260 °C using Korhonen’s stress evolution model [M. A. Korhonen, P. Bo/rgesen, K. N. Tu, and C.-Y. Li, J. Appl. Phys. 73, 3790 (1993)]. Both Z* and Deff values are in good agreement with the previously reported values.

We have carried out measurements of electromigration-induced strains in copper conductor lines using microbeam energy dispersive x-ray diffraction. Strains developed in random texture damascene Cu 2μm-wide, 0.16 μm-thick conductor lines with TaN liners in low-k dielectric during electromigration at 350°C are much smaller than electromigration-induced strains in (111) fiber texture Al-on-Si, 10μm-wide, SiO2 passivated conductor lines. The reasons for these differences in electromigration behavior may involve the different roles of grain boundary and interface diffusion paths and the different passivation structures and materials for the two types of samples.

X‐ray microbeam measurements of thermal and electromigration‐induced strains have been made at NSLS using white‐beam energy dispersive x‐ray diffraction, averaging over many grains, and at APS using white‐beam Laue x‐ray diffraction, from single grains. Grain‐by‐grain deviatoric strain measurements in Al films show wide variation in behavior for different grains in the films. Room temperature relaxation of residual strains was observed to occur at different rates for Al films with different bonding layers and substrates. X‐ray microbeam measurements of strain development during electromigration for Cu and Al conductor lines show that strain gradients do not develop in the copper lines under conditions similar to those for which large strain gradients have been seen for Al lines.

We have observed abrupt, reversible resistance changes during electromigration (EM) testing of 0.16μm thick, 100μm long damascene Cu conductor lines with TaN liners and W vias. Lines of 0.2μm, 0.8μm and 2μm widths were tested and compared. The tests were conducted at temperatures of 300C – 350C and with current densities of 0.6 × 106 A/cm2 to 1.6 × 106A/cm2. In most cases an incubation period with negligible resistance increase is followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. With further EM, the resistance curves show spike like features, with a sudden resistance increase, followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurs, and the line fails. We present resistance data, microstructural observations and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating melting of conductor line segments and voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to fill voids in the line.

A spatially resolved cathodoluminescence study of 〈100〉 dark line defects (DLDs) of degraded II–VI laser diodes based on a ZnCdSe/ZnMgSSe separate confinement heterostructure has been carried out at temperatures between room temperature and 8 K. Cathodoluminescence line scans were used to measure the change of contrasts between the DLDs and the adjacent material. The contrast decreased with decreasing temperature, which suggests that the nonradiative recombination processes associated with DLDs are thermally activated. Activation energies were found to be about 16 and 6 meV for temperatures above and below 200 K, respectively, which may reflect a transition between free carriers and bound excitons at this temperature.

Cathodoluminescence has been employed to investigate the luminescence and lateral transport properties of excited carriers at 8 K in GaAs-AlGaAs quantum well material and in submicron features fabricated in this material by magnetron reactive ion etching. A carrier diffusion length of 0.85±0.04 μm in quantum wells and a surface recombination velocity (5.4±0.8)×103 m/s at etched sidewalls were measured. Also, the effect of feature size on luminescence efficiency was examined and compared with model calculations using the measured values of diffusion length and surface recombination velocity.

Monochromatic CL imaging, CL spectra, WDS spectra, and EDS spectra and imaging demonstrate that electron beam bombardment of LEO-GaN causes decrease of near band edge cathodoluminescence intensity that cannot be attributed to absorption in a growing carbon contamination layer. An alternative explanation is needed, such as generation of defects, or charge injection and buildup of internal electric fields, caused by electron beam bombardment.

Thermally induced residual strains in polycrystalline Cu and Al films on single crystal Si and glass substrates, respectively, have been examined on a grain-by-grain basis by x-ray microbeam diffraction. The crystallographic orientation and the deviatoric strain tensor, εij*, are determined for each grain by white beam Laue diffraction. From grain orientation mapping and strain tensor measurements, information is obtained about the distributions of strains for similarly oriented grains, about strain variations within single grains, and about grain-to-grain correlations of strains. This type of information may be useful in developing and testing theories for intergrain effects in strain evolution in polycrystals.

We have studied luminescence and degradation of a ZnCdMgSe/ZnCdSe/ZnCdMgSe QW LED grown by MBE and lattice matched to its InP substrate. Results have been obtained from time-resolved and bias-dependent cathodoluminescence spectroscopy and from electroluminescence spectroscopy. Reversible and irreversible variations in luminescence intensity were observed during bias cycling and electron bombardment. Cathodoluminescence from the QW LED was modeled by calculating current generation and transport, bias-dependent energy levels, envelope wave functions, overlap integrals, and carrier escape rates. Results of these calculations agree qualitatively, but not quantitatively, with bias-dependent intensity and wavelength changes observed in the experiments.

This paper reviews results obtained by x-ray microbeam energy dispersive Laue diffraction for electromigration-induced stresses in passivated pure Al conductor lines. Combined x-ray microdiffraction and microfluorescence experiments are described which provide new information on relationships between local Cu concentration and Al diffusivity during electromigration in Al(0.25 at.% Cu) conductor lines.

Effects of electrical bias on the cathodoluminescence (CL) have been investigated for a blue II-VI quantum well (QW) light emitting diode structure of ZnCdMgSe, lattice-matched to InP. In CL wavelength scans, the observed effects include largely reversible changes in QW CL intensity and wavelength and changes in cladding CL intensity. In CL time-based scans, the QW CL intensity showed both immediate and long term changes with bias. Irreversible, degradationrelated decreases in QW CL intensity were also observed. Effects of bias on CL were modeled by calculating the rates of carrier production by electron bombardment and the resulting electron and hole currents with different applied bias fields. These model calculations do not explain many of the experimental observations, because the model does not include effects of bias on carrier escape and redistribution in the QW and effects of bias on generation and transport of atomic scale defects.

Near-band-edge cathodoluminescence emission for some nominally undoped GaN samples decreases with time during electron bombardment in a scanning electron microscope. The rate of decrease depends on the incident beam voltage and current and on the size of the area being scanned, which is determined by the magnification used for the scanning electron microscope. Faster intensity decreases occur with lower beam voltages (10kV versus 20kV), higher beam currents (600nA or 200nA versus 35nA) and smaller scanned areas (120K magnification versus 15K magnification). For larger scanned areas, the maximum luminescence intensity occurs 10's or 100's of seconds after the start of electron bombardment, and the intensity then decreases with further bombardment. In some cases the size of the degraded region is larger than the bombarded area. These observations suggest that the degradation is caused by electromigration of impurities or other charged defects within the GaN resulting from local charging by electron bombardment.

We have used x-ray microbeam fluorescence and diffraction for in-situ measurements of electromigration-induced Cu diffusion and stress evolution in passivated, polycrystalline 10[.proportional]m-wide, 200[.proportional]m-long Al(0.25 at.% Cu) conductor lines. Cu migration is in the direction of the electron flow and is determined by the direction and magnitude of the current and by the temperature during electromigration. The effective charge and diffusivity of Cu in Al(Cu) have been obtained from analysis of the Cu concentration profiles. The evolution of electromigration-induced strains normal to the sample surface has been monitored by x-ray microbeam diffraction. A linear strain profile developed after about 9 hrs of electromigration with 1.5[.dotmath]105 A/cm2 at 300°C, corresponding to 3MPa/[.proportional]m equi-biaxial stress. From the Cu profile measured at the same time, the critical Cu concentration for significantly slowing down Al grain boundary diffusion is estimated to be ∼0.15 at. %. These data also confirm that downstream Cu transport is accompanied by a counter flow of Al in the upstream direction.

We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings.

Dopants Y and Zr at 100 ppm levels in high purity, micron grain-size polycrystalline alumina are mainly segregated to the alumina grain boundaries and strongly reduce high temperature creep. Information about this segregation has come from high resolution STEM composition mapping experiments. Information about local structural surroundings of the dopant atoms has come from EXAFS experiments, and information about local bonding of the dopant atoms has come from XANES experiments. Structural models for dopant grain boundary segregation provide a context for these experimental results and for effects of dopant incorporation on grain boundary mediated transport. Recent experimental and theoretical results are discussed in this paper.

We describe a method for determining the local strain state of passivated aluminum metal lines from single grains within 2.6 µm × 7.0 µm × 0.75 µm sized regions along the line. X-ray microbeam diffraction is used to obtain localized measurements of thermal and electromigration-induced strain during 37 hours of electromigration in a passivated 2.6 µm-wide, 300 µm-long pure Al conductor line at a current density of 4.2×105 A/cm2 and temperature of 270°C. Diffraction from single grains is used to measure both the in-plane and normal components of strain and their evolution during electromigration at several positions along the line.

The interplay between stress and electromigration has been recognized since I. A. Blech et al. used x-ray topography in 1976 to demonstrate that stress gradients developed during electromigration. Availability of high brightness synchrotron x-ray sources, high stability energy dispersive detectors, high resolution area detectors, and pinholes, capillaries and other optical elements for forming x-ray microbeams, has made possible more quantitative, real time measurements of strains and composition changes which develop in polycrystalline metal conductor lines during electromigration. This paper describes advances made in this area, implications of results which have been obtained, and prospects for further progress.

This paper reviews the measurements of local strain distribution in individual 10 μm-wide 200 μm-long passivated Al conductor lines using white-beam x-ray microdiffraction. Local strain changes were examined in real-time when the Al lines were subjected to temperature changes as well as to electrical current stressing. Results of thermal strain measurements show that the stress in the Al lines can be treated as equibiaxial, and that the passivation does not hinder the overall expansion of the conductor line along the film normal. Results of electromigration strain measurements show relief of the initial biaxial compressive stress near the cathode end of the conductor line, and increase of compressive stress near the anode end. A linear stress gradient was observed when steady-state electromigration was reached. Steady-state measurements at various current densities below the threshold value, jth=1.6×105 A/cm2, yield an effective valence Z*=1.6 for Al electromigration, and transient-state measurements as a...

We have used x-ray microbeam fluorescence and diffraction for in-situ measurements of Cu concentration and strain during 36 hours of electromigration with current density 5×105 A/cm2 at temperature 310°C for a passivated 10μm-wide, 200μm-long Al(0.5 at.% Cu) conductor line. These measurements indicate that Cu is quickly depleted at the cathode end of the line and is piled up at the anode end, where the Cu concentration first increases, then decreases, and finally remains constant. A void begins growing at the cathode end of the line about two hours after the start of current flow, with a nearly constant void growth rate of 1 μm/hr for the remainder of the test. The evolution of stresses within the line during electromigration involves changes by 100's of MPa and is more complex than seen previously for electromigration under similar conditions for pure Al conductor lines.

Complex faceted features of micrometer sizes and with intense luminescence rise 200-300 nm above the surface of a GaN thin film grown by molecular beam epitaxy on (0001) sapphire. Cathodoluminescence measurements at room temperature and at 8K were used to investigate the luminescence properties of these microfeatures in comparison with those of the background GaN material. The morphology of the micro-features was studied by scanning electron microscopy and by atomic force microscopy.GaN and related compounds have been considered as promising materials for light emitting devices in the short wavelength visible and UV spectral regions because of their direct, wide band gaps and high luminescence efficiency. Remarkable, rapid success has been achieved in developing devices based on III-V nitrides, which allows these material systems to rival other systems under development [1]. Blue-green LEDs fabricated from InGaN/AlGaN double-heterostructure layers on sapphire substrates by Nichia Chemical Industries in Japan are commercially available, despite the presence of high extended defect densities in the 1010 /cm2 range [2]. In December 1995, an important milestone in the development of nitride lasers was reached; Nakamura et al. in Nichia demonstrated the first nitride-based laser diode which operated at 417 nm under pulsed conditions at room temperature [3].The morphology and spatial distribution of luminescence in GaN thin films have been investigated by several research groups [4], [5], [6], although most efforts have been directed to developing appropriate growth conditions for improved film quality. From observations by scanning electron microscopy (SEM) and atomic force microscopy (AFM), Trager-Cowan et al. described an MBE-grown GaN film which contained an assembly of oriented hexagonal crystallites rising above a background of polycrystalline or amorphous material [4]. All the crystallites, oriented in a similar fashion, had roughly the same sizes, 1 or 2 μm across a hexagonal face and about 1 μm high. These crystallites were much brighter than the surrounding background material in panchromatic low-temperature cathodoluminescence (CL) images. Trager-Cowan et al. concluded that the crystallites were of better quality than the background material. They also observed a green emission band, attributed to impurities, which became weaker for higher electron beam voltages, generating luminescence from deeper in the film. From this observation, they concluded that higher quality material is located closer to the film's outer surface. Spatial variation of the luminescence efficiency from MOCVD-grown films has also been observed by Ponce et al[5]. in their room-temperature CL microscopy studies. Their results showed significant nonuniformities in both the band-edge and yellow band emissions. Although they reported no faceted island structures, one of their samples had “marked surface features” and consisted of hexagonal crystals 10 to 50 μm in diameter. The crystals gave strong band-to-band (364 nm) luminescence.In this paper, the morphology of a GaN film grown on (0001) sapphire by MBE is characterized by SEM and AFM, and CL measurements are used to investigate the luminescence properties of the film at room temperature and at 8K. Some regions of this film are similar to the one described by Trager-Cowan et al. [4] in having micron sized, brightly luminescent islands. The hexagonal facet angles and bright luminescence of islands in our film also resemble the larger scale, more regularly shaped hexagonal crystals of Ponce et al. [5]. These regions lie on the boundary of excess gallium growth conditions. Our results differ from those of Trager-Cowan in terms of the island morphologies and the spectral character of luminescence from the islands and from the background material. Possible causes of the large variations in luminescence efficiency are discussed.

X-ray microdiffraction was applied to study the thermal and electromigration strains in 10 μm-wide Al conductor lines with 10 μm spatial resolution. X-rays were collimated either by pinholes or by tapered glass capillaries to form x-ray microbeams. Measurements were made in a symmetric-reflection geometry so that the strains normal to the sample surface could be examined at different positions along the conductor lines. Results of thermal strain measurements show that the SiO2 passivation plays an important role in limiting relaxation of in-plane compressive thermal stresses in the Al lines, but that the passivation is not effective in confining the overall thermal expansion of the Al line along the film normal. Electromigration strain measurements show that a linear stress gradient developed within the first hour of electromigration. The magnitude of the stress gradient changed little until fast stress relaxations occurred near the anode end of the line. Possible mechanisms are discussed in light of these observations.

We describe how x-ray microbeam diffraction is being used to measure strain with micronscale spatial resolution. Micron-scale x-ray beams can be obtained using tapered glass capillaries. With the high brightness and broad energy spectrum of synchrotron radiation and the energy dispersive capabilities of commercially available, liquid nitrogen cooled x-ray detectors, spatially resolved strains in a sample can be determined along different directions without having to rotate the sample, in contrast with more conventional methods using monochromatic x-ray diffraction. This is a major advantage in achieving micron-scale spatial resolution. Strain sensitivities on the order of 2×10−4 have been achieved in such measurements. White beam x-ray microdiffraction has been applied for the first time in real-time studies of thermal and electromigration related strain distributions in passivated Al-on-Si conductor lines. Results of measurements on a single 10μm-wide line are described.

Design of reliable thin film metallization is a complex problem that requires information about local parameters such as composition, microstructure, residual and loading stresses, etc. On the other hand, such information has been traditionally obtained using techniques that have sampling volumes that are appreciably larger than the BEOL dimensions currently in use. In this article, the x-ray residual stress analysis technique and its extensions to micro-domain measurements are reviewed. These concepts are then applied to strain data from in-situ x-ray measurements, obtained in real-time from Al metallization lines subjected to thermal stressing and electromigration.

We present the results of an investigation of the dependence of the photoluminescence (PL) spectra on preparation conditions, the resulting microstructure, and post-anodization treatment of porous silicon carbide films which were formed from both p and n-type 6H-SiC substrates. Porous samples were prepared by anodic dissolution under different galvanostatic conditions, resulting in different porosities and crystallite sizes. Selected-area electron diffraction patterns taken on similarly prepared porous silicon carbide (PSC) samples confirmed that the films were monocrystalline. Transmission electron microscopy of as-anodized films revealed an isotropie porous network; a dependence of porosity and nanocrystallite size on porous layer formation current density was established. Some PSC samples were passivated using a short, thermal oxidation treatment. The effects of porosity and crystallite size, and of oxide passivation in these PSC films, on PL spectra and intensity were studied using a 365 nm Kr-ion laser as excitation. Under certain conditions, the spectrally integrated PL intensity of a passivated film is more than 450x that for the original bulk SiC substrate. PL spectra are presented, and possible mechanisms are discussed.

Cathodoluminescence (CL) spectroscopy and microscopy were used to study the luminescent properties of a variety of GaN films, both Si-doped and unintentionally-doped, grown on sapphire substrates. A narrow and intense near band-edge emission was found in the CL spectrum of each film examined, and deep-level emission was also observed for some of the films. The luminescence efficiency of near band-edge emission increased with a faster rate than that of deep-level emission when the pumping current was increased. Spatial nonuniformities of luminescence were observed in monochromatic CL microscopy, and microstructures were observed in scanning electron microscopy. No correlations between luminescence features and microstructural features were seen. Degradation of near band-edge luminescence was observed, accompanied by growth of deep-level emission.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.