Grain Boundary Diffusion Of Cu Research Articles

Diffusion of lithium in copper current collectors of lithium metal anodes in liquid cells

Conflicting views are found in the literature on the significance of lithium diffusion into copper current collectors of lithium metal anodes. Previously published in situ Neutron Depth Profiling (NDP) measurements in a liquid cell revealed reversible Li diffusion in the current collector during electrochemical cycling, resulting in concentrations of ∼ 1 at. %. Herein, a model for these experiments is presented, in which stress in the deposit is induced by Li diffusion accompanying plating and stripping. This in turn produces stress gradients in the current collector, which together with composition gradients drive reversible grain boundary diffusion in Cu during cycling. Quantitative agreement with the NDP results is obtained for a fit diffusivity comparable to previous in situ measurements, but much larger than values from ex situ measurements using Li–Cu bilayers. Creep due to grain boundary diffusion is found to relax stress in the copper foil, resulting in low levels of stress and deformation during cycling. The results clarify the significance of Li diffusion in Cu, and also help elucidate the origin of stress in the lithium anode during cycling, which may induce interfacial instability.

Investigation of bonding mechanism for low-temperature Cu[sbnd]Cu bonding with passivation layer

The bonding mechanism of low-temperature CuCu thermal compression bonding (TCB) with passivation layer has been investigated in this research. After the structures of Cu with Au passivation and Cu with Pd passivation were bonded at 180 °C, the analyses of transmission electron microscope (TEM) were performed for the bonded structures to explore the bonding behavior of passivated-Cu during different stages of the TCB process. The results indicate that grain boundary diffusion of Cu leads to formation of amorphous Cu at the bonding interface in the passivated-Cu structure. Furthermore, diffusion of Cu atoms and recrystallization of amorphous Cu into polycrystalline Cu can eliminate voids at the bonding interface during the TCB process. Comparing to conventional Cu direct bonding with high crystallinity Cu at the bonding interface, passivated-Cu bonding can be achieved by much lower energy, leading to high quality bonding results with low thermal budget.

An insight into the precipitate evolution and mechanical properties of a novel high-performance cast Al-Li-Cu-Mg-X alloy

High-performance cast Al-Li alloys are highly desirable for a wide range of weight-critical and stiffness-critical applications. This work was undertaken to investigate the microstructural evolution, coarsening kinetics of main precipitates and related mechanical characteristics of a newly-developed cast Al-Li-Cu-Mg-X alloy during aging at 150, 175, 200 and 225 °C. The as-aged alloys studied primarily comprised of δ′ (Al3Li), T1 (Al2CuLi), S′ (Al2CuMg) and core-shell Al3(Li, Sc, Zr) phases. δ′ phases exhibited the relatively lowest thermal stability. The increasing aging temperature led to the greatly enhanced coarsening rate of δ′. It was found that the coarsening of metastable δ′ was a diffusion-controlled process and a linear relationship of cube average particle radius (R̅3) against aging time t was revealed. The activation energy for the δ′ coarsening was estimated to be ~98.7 kJ/mol, which was somewhat lower than those in wrought Al-Li alloys. Besides, the activation energy for the growth of T1 plate was measured to be ~95.6 kJ/mol, which was close to that of grain boundary diffusion of Cu in α-Al (~105 kJ/mol), indicating that the growth of T1 should be governed by the diffusion rate of Cu toward T1 plates. With the successively increased aging temperature and prolonged aging time, the number density and volume fraction of δ′ particles were significantly reduced, primarily contributing to the significant strength reduction. The relatively best combination of strength and ductility was achieved in the studied alloy aged at 175 °C for 32 h.

Effect of internal stress on short-circuit diffusion in thin films and nanolaminates: Application to Cu/W nano-multilayers

The stability, reactivity and functionality of modern nanostructured and nanoarchitectured materials, like nano-multilayers (NMLs) and nanocomposites (NCs), are generally ruled by fast short-circuit diffusion of atoms along internal interfaces, such as phase and grain boundaries (GBs), at relatively low temperatures. Residual stresses can have a significant effect on the kinetics of short-circuit diffusion in these nanomaterials, which has been largely neglected up to date. We present a combined experimental-modelling approach for deriving stress-free activation energies (and related diffusion coefficients) for the grain boundary diffusion of a given element across an inert barrier nanolayer, which can be utilized to perform fundamental investigations and model predictions of GB diffusion kinetics across inert barrier nanolayers in the presence of a varying stress field. The method was applied to investigate the diffusion of Cu along W grain boundaries in Cu/W nano-multilayers in the temperature range from 400 °C to 600 °C. Firstly, the GB diffusion kinetics as function of the temperature and stress state were determined from experiment by combining insitu heating AES analysis and exsitu XRD stress analysis. The experimental data were compared to model predictions from a modified Hwang–Balluffi model, which accounts for a varying stress level within the barrier during the annealing. The extracted stress-free activation energy for grain boundary diffusion of Cu in W (~0.29 eV) is substantially lower than the respective experimental value for a high compressive stress within the W barrier nanolayer. The corresponding stress-free Cu GB diffusion coefficient in W equals Db∗=3.2±0.6×10-15exp-0.29±0.02eV/kTcm2/s.

Grain boundary diffusion of 59Fe in high-purity copper

Grain boundary diffusion of iron in high purity polycrystalline copper is measured using the radiotracer technique and applying the 59Fe isotope. At lower temperatures, T<949 K, the measurements are performed under Harrison's C-type kinetic regime and the grain boundary diffusion coefficient of Fe in Cu, Dgb, is determined, Dgb=5.6⋅10−6×exp(−121kJmol−1/RT) m2/s. Unconventional penetration profiles are measured for Fe grain boundary diffusion in Cu at higher temperatures (≥ 949 K) under the intended B-type kinetic regime, in fact formal C-type profiles are systematically observed instead. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials [Ackland et al., Phil. Mag. A, 1997] discovered an unexpected response of Cu grain boundaries on a partial Fe coverage at temperatures above 900 K. A model of Fe penetration in polycrystalline Cu in such conditions is proposed, which explains the untypical shape of the penetration profiles. The combination of the B- and C-type grain boundary diffusion measurements predict a strong segregation of Fe in Cu with a low segregation enthalpy.

Effect of Atomic Complexes Formation in Grain Boundaries on Grain Boundary Diffusion

The peculiarities of grain boundary diffusion in Cu connected with the effect of atomic pairs formation in grain boundaries (GB) were studied using the molecular dynamics (MD) simulation. In present study Cu GB selfdiffusion was simulated with the use of semi-empirical potential. Besides, the ‘heterodiffusion’ simulation was performed with the artificially addеd energy of interaction (E) between identical atoms in arbitrary chosen pairs. To obtain reliable data on the mean square displacements (MSD) the simulation cell, consisted about three hundreds thousands atoms and two symmetrical GBs Σ5 (001)(012), was used. 70 pairs of identical Cu atoms in GBs, bonded into pairs, were chosen as initial state. Energy of interaction was varied between 0 and - 0.5eV/atomThe results obtained for selfdiffusion are in a good agreement with experimental results and other results of computer simulation. Two main effects for heterodiffusion are under discussion. The first is atomic exchange between GB zone and adjacent lattice zone, where the mobility of the atoms decreases significantly. As a result, the MSD decrease. Another effect is connected with attraction between the “marked” atoms, which leads to formation of relatively stable complexes and the MSD also decreases. The results obtained involve also dependence the number of the stable pairs on time and temperature and show the possibility of pairs to condense into ternary, quarterly and more numerous complexes.

Peculiarity of Grain Boundary Diffusion of Fe and Co in Cu

In this article new experimental evidences of anomalous grain boundary diffusion (GBD) of Fe and Co in Cu were describe. To demonstrate that the brief describing of results of grain boundary diffusion in Cu with following formulation of rules which can be established on the base of the analysis of the results is presented. Experimental results which are described here concern the attempt to change the effect negative segregation by microalloying by sulfur which did not change the situation and the diffusion through foil which allowed to demonstrate the absence of accelerated GB diffusion without specific sample preparation. It is shown that GBs do not give the additional effect to the flux of Fe and Co through the foil. The extended model of surface tension gradient as an additional driving force is described.

In Situ Ramp Anneal X-ray Diffraction Study of Atomic Layer Deposited Ultrathin TaN and Ta1-xAlxNy Films for Cu Diffusion Barrier Applications

Ultrathin TaN and Ta1-xAlxNy films with x = 0.21 to 0.88 were deposited by atomic layer deposition (ALD) and evaluated for Cu diffusion barrier effectiveness compared to physical vapor deposition (PVD) grown TaN. Cu diffusion barrier effectiveness was investigated using in-situ ramp anneal synchrotron X-ray diffraction (XRD) on Cu/1.8 nm barrier/Si stacks. A Kissinger-like analysis was used to assess the kinetics of Cu3Si formation and determine the effective activation energy (Ea) for Cu silicidation. Compared to the stack with a PVD TaN barrier, the stacks with the ALD films exhibited a higher crystallization temperature (Tc) for Cu silicidation. The Ea values of Cu3Si formation for stacks with the ALD films were close to the reported value for grain boundary diffusion of Cu whereas the Ea of Cu3Si formation for the stack with PVD TaN is closer to the reported value for lattice diffusion. For 3 nm films, grazing incidence in-plane XRD showed evidence of nanocrystallites in an amorphous matrix with broad peaks corresponding to high density cubic phase for the ALD grown films and lower density hexagonal phase for the PVD grown film further elucidating the difference in initial failure mechanisms due to differences in barrier crystallinity and associated phase.

Atomic Layer Deposition of Ultrathin TaN and Ternary Ta1-XAlXNy Films for Cu Diffusion Barrier Applications in Advanced Interconnects

The continuous scaling of Cu interconnects for sub-10nm technology nodes calls for development of ultrathin and conformal Cu diffusion barriers. Due to its ability to conformally deposit films with nanoscale thickness control, atomic layer deposition (ALD) is an attractive method for depositing refractory metal nitride diffusion barrier layers, as opposed to a line-of-sight technique such as physical vapor deposition (PVD).[1] In this regard we have previously demonstrated extendability of Cu fill (with Ru seed layer) below 25 nm linewidth (200 nm height) when PVD TaN is replaced with ALD TaN.[2] In 40 nm Cu dual damascene structures with 3 nm barrier layers, ALD TaN has also been shown to reduce via resistance by ~28% compared to PVD TaN in spite of 20x lower blanket resistivity for PVD TaN.[3] In order to further improve ALD grown TaN and minimize Cu-diffusion through these ultrathin barrier films, it is desirable to avoid forming polycrystalline films with inherent grain boundary diffusion pathways. Accordingly, alloying refractory transition metal nitrides with a third element (as for example Al) may offer a promising method for maintaining metastable amorphous films by interrupting the typical polycrystalline phase formation.[4,5] In this study, ultrathin TaN and ternary Ta1-xAlxNy films (denoted TaAlN) were deposited by ALD using metallorganic precursors and were evaluated for their effectiveness in Cu diffusion barrier applications compared to a PVD grown TaN control. TaN was deposited using tert-butylimido tris(ethylmethylamido)tantalum (TBTEMT, Ta(NCMe3)(NEtMe)3) and NH3 at 350 °C with a growth per cycle (GPC) of ~ 0.7 Å/cycle. ALD TaAlN ternary alloy films were deposited by addition of trimethylaluminum/NH3 cycles to the TBTEMT/NH3cycles using Ta:Al cycle ratios in the range 16:1 to 2:1, resulting in Al/(Al+Ta)% in the range of 21% to 88%. The effectiveness of the barrier layers against Cu-diffusion in advanced interconnects were investigated using in-situ ramp anneal synchrotron x-ray diffraction (XRD) [6,7] to study the effect of annealing on Cu diffusion with the presence of these ultrathin (~1.8 nm) barrier layers interposed between 50 nm PVD grown Cu and the Si substrate. The barrier failure temperature (Tc) was determined based on the onset of Cu3Si XRD peaks during the in-situ ramp anneal. The kinetics of Cu3Si formation was assessed by XRD using multiple ramp anneal rates and performing a Kissinger-like analysis [6] to determine the effective activation energy (Ea) for silicidation in presence of the Cu diffusion barrier layers. Compared to the film stack with PVD TaN barrier, the stacks with ALD TaN and TaAlN (Ta:Al = 8:1) exhibit a slightly higher Tc for Cu silicidation. The effective activation energy of Cu3Si formation for stacks with ALD TaN (Ea ~ 1.5 eV) and TaAlN (Ea ~ 1.3 eV) are close to the reported value for grain boundary diffusion of Cu (1.3 eV) [8], whereas the Cu3Si effective activation energy for the stack with PVD TaN (Ea ~ 2.3 eV) is closer to the reported value for lattice diffusion (2.7 eV).[8] Although the temperature of Cu3Si crystallization and corresponding barrier failure temperature is higher for these ALD barrier films compared to the PVD barrier TaN film, the failure mechanism for the ALD barriers is most likely dominated by grain boundary diffusion whereas the initial failure mechanism for the PVD barrier may be dominated by lattice diffusion.

Ni Grain Boundary Diffusion in Cu-Co Alloys

The influence of Co as an alloying element on grain boundary diffusion (GBD) in Cu attracts particular interest due to anomalous GBD of Co in Cu. Ni as a neutral to Co and Cu element was chosen for GBD study. The triple products of Ni GBD in Cu and Cu-Co alloys (with concentration up to 2.9 wt. %) were determined in temperature range 500 – 700 °C by X-ray microprobe analysis. It was shown, that in spite of some scattering the triple product does not depend on Co concentration at all temperatures of experiments. From the obtained results it follows that Co does not change the GB structure.

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25°C to 725°C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500°C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500°C. Under uniaxial tension at temperatures higher than 650°C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.

Barrier layer and annealing temperature dependent microstructure evolution of nanocrystalline Cu films

The effects of barrier layers and annealing temperature on texture variation, grain growth and void forming of nanocrystalline Cu films were investigated by X-ray diffraction, transmission electron microscope and scanning electron microscope (SEM). The variation in texture and grain size of Cu films with annealing temperature is different for Cu/Ti and Cu/Ta. The activation energies of grain growth of Cu films on Ti and Ta, respectively, are 19.7 and 23.4 kJ mol −1, which are much closer to that of grain boundary diffusion of Cu. The average diameter of about 400 nm for surface voids of Cu/Ti is larger than that of Cu/Ta structure. Furthermore, both the electrical resistivity measurement and SEM observation imply that Cu/Ti rather than Cu/Ta structure tend to fail easier as annealing temperature exceed 400 °C.

Residual Stress Effect on Self-Annealing of Electroplated Copper

The remarkable feature of electroplated film was grain growth at room temperature and this phenomenon was named `self-annealing'. The driving force of self-annealing was expected as highly increased defect and surface energy, but the exact mechanism was unknown. In this work, residual stress effect on self-annealing was investigated. During self-annealing, increased temperature accelerated grain growth, and it is concluded that self-annealing is caused by Cu grain boundary diffusion. From the stress measurement by X-ray difractometer (XRD), it was the shown that the average stress release was 500 MPa during self-annealing. The average strain energy is lower than the grain boundary energy, but locally high stress originated from trapped poly ethylene glycol (PEG) whose molecular size is large at the grain boundary, can accelerate grain boundary diffusion of Cu. Therefore, stress is major driving force of self-annealing, and it plays a role in decomposing PEG molecules and providing sufficient driving force for grain boundary diffusion to Cu atoms near PEG.

Activation energy determination for recrystallization in electroplated-copper films using differential scanning calorimetry

The apparent activation energy for recrystallization during self-annealing of 1.5-µm-thick electroplated copper films was determined using constant-heating-rate scans in a differential scanning calorimeter (DSC). The apparent-activation energy was measured to be 0.62 eV/atom. Ex-situ microscopy studies showed that self-annealing in electroplated Cu films is characterized by site-saturated nucleation and diffusion-limited, two-dimensional grain growth. For a transformation with these characteristics, the apparent activation energy measured using a differential scanning calorimeter corresponds to the activation energy for grain-boundary motion. The measured activation energy is reasonably close to values reported for grain boundary motion from in-situ microscopy studies. The value is also close to the activation energy for grain boundary diffusion in Cu. This work demonstrates the feasibility of using differential scanning calorimetry (DSC) as a relatively straightforward method to study the kinetics of the self-annealing process.

Grain Boundary Diffusion and Linear and Non-Linear Segregation of Ag in Cu

Ag grain boundary (GB) diffusion was measured in the Cu-0.2at%Ag alloy in a wide temperature range from 473 to 970 K. The direct measurements of Ag GB diffusivity D alloy gb under conditions of the Harrison C regime revealed that D alloy gb is almost identical to D pure gb determined earlier for Ag diffusion in high-purity Cu (Divinski, Lohmann, and Herzig, 2001). The penetration profiles determined in the Harrison B regime showed a complex, multi-stage shape. This diffusion behavior can be rationalized assuming that besides GBs significantly covered by segregated Ag atoms, some fraction of GBs remains almost free from Ag atoms in the studied temperature interval. The total amount of “pure” GBs drastically decreases with decreasing temperature. This hypothesis was proven by measurements of Ag GB diffusion in Cu near Σ5 bicrystals, which allowed us to analyze in detail the non-linear segregation of Ag in Cu GBs.

Interdiffusion, stress, and microstructure evolution during annealing in Co/Cu/Co trilayers

In order to find interdiffusion and microstructural changes on sputtered 90 nm Co/180 nm Cu/90 nm Co trilayers during annealing, the evolution of the stress and electrical resistance was studied in situ during temperature ramps and isothermal annealing as well as concentration-depth profiles, the grain morphology, and the phase and texture formation were investigated after heat treatment at various temperatures. Up to 450 °C, no distinct lattice interdiffusion was observed. Grain-boundary diffusion of Cu through the Co top layer to the surface as well as a distinct growth of Cu grains into the Co top layer start to occur at about 450 °C. The conclusions corroborate findings on Cu/Co nanoscale multilayers, which show a deterioration of the giant magnetoresistance effect at high temperatures due to similar microscopic mechanisms.

SIMS DEPTH PROFILING ANALYSIS OF Cu/Ta/SiO2 INTERFACIAL DIFFUSION AT DIFFERENT ANNEALING TEMPERATURE

The interfacial reactions of Cu (60 nm )/ Ta (35 nm )/ SiO 2/ Si and Cu (500 nm )/ Ta (35 nm )/ SiO 2/ Si have been investigated in the 350°C ~ 700°C temperature range. We have found that the thickness of Cu top layer is one of the key factors to influence Ta barrier performance in the Cu/barrier/SiO 2/ Si system. SIMS depth profile spectra were obtained by using O 2+ primary ion beam with 12.5keV high voltage and a 5.0keV extraction field of the secondary ions. In the thinner (60nm) Cu system, Si has not penetrated the whole 35nm Ta layer after annealing at 600°C for 30 min under high vacuum. This indicated Ta still functions as a diffusion barrier between Cu and Si at 600°C, though Ta 5 Si 3 formed at 500°C as shown by XRD results. As a good diffusion barrier material for Cu metallization, 35nm Ta layer prevent Cu and Si inter diffusion at 650°C for at least 30 min. In the thicker Cu (500nm) system, top Cu layer changes its structural from small grain to big grain during annealing. This could suppress the grain boundary diffusion of Cu to Ta and SiO 2. 35nm Ta layer still functions as diffusion barrier between Cu and SiO 2 at the annealing temperature of 700°C.

Determination of grain-boundary diffusion of Ag in nanocrystalline Cu by the Hwang–Balluffi method

From the Ag and Cu Auger signals measured on the surface of nanocrystalline Cu/Ag thin bilayers, the temperature dependence of the parameter ω′ for Ag grain-boundary diffusion in Cu (ω′=δkbDb/δsks, where δ and δs are the width of the grain boundary and the segregated layer and kb and ks are the grain-boundary and surface segregation factors, respectively) have been determined by the Hwang–Balluffi method in the C-kinetics regime over the temperature range of 393–428 K. These values have been compared with triple products, P=δkbDb, determined in the temperature range 584–804 K by Bernardini et al. [Phil. Mag. A 73, 237 (1996)] using radio tracer technique in the B-kinetics regime and the temperature dependence of the surface segregation factor has been extracted. The surface segregation energy (34±16 kJ/mol) agrees well with other data published in the literature.

Grain Boundary Motion during Ag and Cu Grain Boundary Diffusion in Cu Polycrystals

Grain boundary (GB) motion in high-purity Cu material (5N8 and 5N Cu) is investigated using the results of radiotracer GB diffusion measurements with tracers exhibiting fundamental differences in the solute-matrix atom interactions. The results on GB solute diffusion of Ag (revealing a miscibility gap in the Ag-Cu phase diagram) and Au (forming intermetallic compounds with Cu) in Cu and on Cu self-diffusion are analyzed. The initial parts of the Ag and Cu penetration profiles turned out to be substantially curved. The profile curvature is explained via the effect of GB motion during 110m Ag and 64Cu GB penetration. The activation enthalpies of GB motion in these two independent measurements occurred to be very close, 95 and 103 kJ/mol, respectively. Moreover, these values turn out to be close, but still somewhat larger than the activation enthalpy of Cu GB self-diffusion in Cu material of the same very high purity, Q Cu gb = 72 kJ/mol. Although tracer diffusion measurements of Au GB diffusion in Cu yielded only limited information on GB motion, the absolute values of GB velocities are consistent with those calculated from the Ag and Cu GB diffusion data.

Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is Db≈6×10−11exp(−0.29/(kBT )) cm2/s.