Diffusion Of Copper Atoms Research Articles

Evolution of phase, surface morphology and wettability of sputtered copper thin films on annealing in air: Formation of CuO/Cu2O/Cu nanocomposites

In this study, copper thin films have been prepared on glass substrates via DC magnetron sputtering with subsequent annealing in air up to 300 °C temperature and the evolution of phase, surface morphology and wettability of the samples have been studied in detail. X-ray diffraction, Raman spectroscopy studies and transmission electron microscopy images reveal formation of CuO/Cu2O/Cu nanocomposite for annealing at 300 °C. Atomic force microscopy and scanning electron microscopy images disclose that surface morphology is immensely influenced by annealing temperature. Upon annealing, the surface roughness varies non – monotonically due to interplay between Ostwald ripening, surface diffusion of copper atoms and oxidation. The values of roughness exponent, varying from 0.72 to 0.83, indicate change in local surface undulation on annealing. Fractal dimension and Minkowski functionals suggest that the sample annealed at 200 °C possesses the most irregular surface morphology and isolated holes are predominantly present in it. Such findings regarding surface morphology are important from tribological application point of view. Gradient value of the power spectral density function, varying from 3.24 to 3.71, suggests that the sample deposition and evolution of the sample on annealing are largely governed by surface diffusion and bulk diffusion. Static deionized water contact angle, varying non – monotonically with annealing temperature, appears to be strongly correlated with surface roughness. An equation is suggested to estimate the contact angle of copper thin films for a given roughness value in the bordering hydrophobic – hydrophilic domain.

Effect of the Diffusion of Copper Atoms in Polycrystalline CdTe Films Doped with Pb Atoms

The process of diffusion of labeled copper atoms in p-CdTe<Pb> coarse-block films with a columnar grain structure has been studied. The CdTe<Pb> film is a p-type semiconductor, where an increase in the Pb concentration in the composition of the CdTe films increases the resistivity ρ of the structure. When the Pb concentration in CdTe changes from 1018 to 5·1019 cm-3, the hole concentration decreases by more than 3 orders of magnitude at a constant operating level depth of EV + (0.4 ± 0.02) eV. This may indicate that the concentration of acceptor defects, which are formed in the films due to self-compensation upon doping with a PbCd donor, exceeds the number of the latter. Electrical measurements by the Hall method were carried out at a direct current and a temperature of 300 K. As a result, an increase in the temperature of films on a Mo-p-CdTe<Pb> substrate during annealing affects the electrical parameter of charge carrier mobility µ, it decreases significantly. X-ray diffraction analysis showed that on the diffraction patterns of samples of p-CdTe<Pb> films, all available reflections correspond to the CdTe phase and up to х = 0.08 do not contain reflections of impurity phases and have a cubic modification. Based on the results of the calculation, it was established that the low values of the diffusion coefficient of Cu atoms are due to the formation of associates of the A type , which are directly dependent on the concentration of atoms. Diffusion length Ln and lifetime τn of minority current carriers in large-block p-type cadmium telluride films, which can also be controlled by introducing lead atoms into cadmium telluride.

Microscopic Formation Mechanism and Physical Properties of Mo/Cu Composites with High Densification

The formation mechanism and physical properties of high-densification Mo/Cu composites are studied by analyzing materials' microstructure, atom diffusion near the phase interface and physical properties. In the liquid phase sintering, the atomic diffusion occurs at the interface of molybdenum and copper, mainly the diffusion of copper atoms into molybdenum phase. Copper atoms in the material diffuse into the molybdenum phase to form a micron sized Cu-Mo solid solution, and no compound phase is found in the material structure, which forms a good interface bonding effect and makes it have high densification. The average linear expansion coefficients, thermal conductivities, electrical conductivities and tensile strengths of high-densification Mo/Cu composites with different copper content are linearly correlated with copper content. Mo80Cu20 is organized as a connected molybdenum skeleton and a small amount of copper phase in the voids. The tensile fracture of Mo80Cu20 is mainly exhibited as brittle fracture of the sintering neck of the molybdenum phase. The copper phase in Mo70Cu30, Mo60Cu40 or Mo50Cu50 is in a connected state, with plasticity significantly increased. Under the action of tensile stress, the ductile fracture of copper phase and the brittle fracture of sintering neck of the molybdenum phase occur simultaneously in these materials.

Rapid and Full Densification of Carbonized Polymer Dot/Copper Composites by Low‐Temperature Sintering

To accomplish densification, pure copper (Cu) sintering, which is a solid phase sintering, necessitates extremely high pressure and temperature. The inclusion of an activator can speed up the diffusion of copper atoms, allowing the densification process to proceed more quickly. The carbonized polymer dot (CPD) via ball milling and spark plasma sintering to obtain a composite with full densification at low temperatures. Effects of sintering temperature on interface of CPD–Cu and the impact of CPD as an activator on pure Cu sintering densification are investigated in this study. Following are the results: Cu matrix composites can be densified at low temperatures by adding CPD, CPD/Cu composites have a relative density of 99.4%, and a hardness up to 130.94 HV while sintering temperature is just 550 °C.

Study of the damping behaviour in samples consisting of iron electro-deposited on copper in an ionic liquid

Copper-iron alloys were produced at room temperature by means of electrodeposition of iron on a copper substrate in an ionic liquid (1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate [Py 1,4 ]TfO). Samples with different electrodeposition times were studied using mechanical spectroscopy, scanning electron microscopy, light microscopy and magnetic loops techniques. Independent of the electrodeposition time the electrodeposition process leads to the promotion of a thin layer of iron onto the copper surface without iron diffusion into the substrate. The damping spectra for electrosposited samples in the as-electrodeposited state show the characteristic low and intermediate grain boundary damping peaks from copper as well as the solvent grain boundary damping peak from the electrodeposited iron. Thermal annealing at temperatures near 973 K leads to the appearance of Fe particles at the interface between the copper and iron (Cu + α-Fe phase) leading to a new damping peak at around 680 K whose driving force is the diffusion of copper atoms around the second phase particles. • Iron-copper alloys produced by electrodeposition in an ionic liquid at room temperature. • Grain boundaries relaxation for the copper and iron in bi-layered alloys. • Damping peak related to the diffusion of copper atoms around the (alpha-Fe) particles.

High-speed mass transfer in the W–Cu pseudo-alloy

Tungsten–copper alloys are heat-resistance materials for aerospace industry. Structure stability during processing and exploitation is controlled by the diffusion processes. In particular, the microstructure evolution is determined by the diffusion of copper atoms in tungsten matrix. Several experimental studies report different diffusivity of copper, which can be associated with the discrepancy of activation energy of diffusion depending on the structure of grain boundaries in tungsten. We performed density functional theory calculations of vacancy formation energies, segregation energies, and migration barriers for copper atoms in certain high-angle grain boundaries of special type using nudged elastic band method. Based on this information we calculated the corresponding diffusion coefficients. We found that the diffusivities in the studied grain boundaries exceed the extrapolated experimental data obtained at low temperatures at least by five times. The obtained data on diffusion path help to explain the depth, from which copper is removed during the annealing of the W–Cu alloy at 1500 K.

Time-Dependent Evolution Study of Ar/N2 Plasma-Activated Cu Surface for Enabling Two-Step Cu-Cu Direct Bonding in a Non-Vacuum Environment

In this paper, a two-step copper-copper direct bonding process in a non-vacuum environment is reported. Time-dependent evolution of argon/nitrogen plasma-activated copper surface is carefully studied. A multitude of surface characterizations are performed to investigate the evolution of the copper surface, with and without argon/nitrogen plasma treatment, when it is exposed to the cleanroom ambient for a period of time. The results reveal that a thin layer of copper nitride is formed upon argon/nitrogen plasma activation on copper surface. It is hypothesized that the nitride layer could dampen surface oxidation. This allows the surface to remain in an “activated” state for up to 6 h. Afterwards, the activated dies are physically bonded at room temperature in cleanroom ambient. Thereafter, the bonded dies are annealed at 300 °C for varying duration, which results in an improvement of the bond strength by a factor of 70∼140 times. A sample bonded after plasma activation and 2-h cleanroom ambient exposure demonstrates the largest shear strength (∼5 MPa). The degradation of copper nitride layer at elevated temperature could aid in maintaining a localized inert environment for the initial diffusion of copper atoms across the interface. This novel bonding technique would be useful for high-throughput three-dimensional wafer bonding and heterogeneous packaging in semiconductor manufacturing.

Rapid fabrication of Cu/40-μm thick full Cu3Sn/Cu joints by applying pulsed high frequency electromagnetic field for high power electronics

With the application of wide bandgap semiconductors for high power electronics, the joints are inevitably faced with a trade-off between withstanding high temperature environments and remaining low bonding temperatures. In this study, a full Cu3Sn intermetallic compounds layer with thickness of 40 μm, which can survive at 679 °C, were obtained between Cu plates using pulsed High Frequency Electromagnetic Field (HFEF) within 10 min at ambient temperature. Output power and frequency of the HFEF was 6 kW and 530 kHz, respectively. In contrast to common Sn-based solder joints, the joints prepared through HFEF consisted predominately of Cu3Sn, because both the horizontal and vertical temperature gradients and the eddy currents within the joints strongly promoted the 3D diffusion of copper atoms. With increasing HFEF exposure time, the Cu3Sn changed from an irregular needle-like to smoothly scalloped or columnar morphology. The HFEF approach enabled Cu3Sn joints to achieve a superior shear strength (>56 MPa) and a greater stand-off height (>30 μm), which are useful to mitigate the coefficient of thermal expansion (CTE) mismatch problems; thereby yielding higher reliability. We envision that these full Cu3Sn joints prove useful in a wide range of applications in high-temperature and high-power electronics.

Fabrication and Characterization of CuO nanowires on V-shaped Microgroove surfaces

This study developed a facile and scalable thermal oxidation method to prepare CuO nanowires on the electro-discharge machining (EDM) processed V-shaped microgrooves. The formation feasibility, surface morphology, and wetting properties of nanowires on V-shaped microgrooves were explored by the variation of thermal oxidation temperatures from 300 to 600 °C, and oxidation times from 2 h to 8 h. Nanowires were found to successfully synthesized on the V-shaped microgrooves surfaces when annealing temperature was 400 °C or higher. The microvoids or microcavities on the EDM processed V-shaped microgrooves surfaces contributed to the stress grain boundary (GB) diffusion of copper atoms, and facilitated the growth of nanowires. The diameter of nanowires increased monotonically when the annealing temperatures increased, whereas it almost kept unchanged with increasing annealing times. The length of nanowires was influenced positively by both annealing temperature and annealing time. All the nanowire samples showed hydrophobic properties of water.

Reliable Nickel-Free Surface Finish Solution for High-Frequency, HDI PCB Applications

Abstract The evolution of Internet-enabled mobile devices has driven innovation in the manufacturing and design of technology capable of high-frequency electronic signal transfer. Among the primary factors affecting the integrity of high-frequency signals is the surface finish applied on printed circuit board copper pads—a need commonly met through the electroless nickel immersion gold process (ENIG). However, there are well-documented limitations of ENIG due to the presence of nickel, the properties of which result in an overall reduced performance in the high-frequency data transfer rate for ENIG-applied electronics, compared with bare copper. An innovation over traditional ENIG is a nickel-less approach involving a special nano-engineered barrier designed to coat copper contacts, finished with an outermost gold layer. In this study, assemblies involving this nickel-less novel surface finish have been subjected to extended thermal exposure, then intermetallics analyses, contact resistance comparison after every reflow cycle (up to six reflow cycles) to assess the prevention of copper atom diffusion into the gold layer, solder ball pull and shear tests to evaluate the aging and long-term reliability of solder joints, and insertion loss testing to gauge whether this surface finish can be used for high-frequency, high-density interconnect applications.

Two-Dimensional Observation of Copper Atoms After Forced Extinction of Vacuum Arcs by Laser-Induced Fluorescence

The distribution and dissipation of neutral atoms are crucial for understanding the dielectric recovery process after interrupting direct current (DC) vacuum arcs. This article aims to investigate the dissipation of copper atoms after a forced extinction of the vacuum arc experimentally, adopting the plane laser-induced fluorescence method. The change in the 2-D distribution of copper atoms with time is presented. The results show that the magnetic fields, the axial magnetic field (AMF), and the transverse magnetic field (TMF) have a limited effect on the initial density of copper atoms. For both the AMF and the TMF, the copper atom densities at current zero (CZ) vary in the range (6– $8) \times 10^{17}\,\,\text{m}^{-3}$ when 3-kA vacuum arcs are forced to 0 in 0.2 ms. Contrary to the TMF case, in the AMF case, the evaporation on the anode after the CZ results in long-existing atoms near it. Consequently, the atom density of the TMF decays faster than that of the AMF, which indicates that a vacuum interrupter with the TMF contacts has a better performance when interrupting a DC load. The difference between the two magnetic fields originates from the arc control patterns. Namely, the AMF tends to keep vacuum arcs in a stable mode, which is unfavorable for a DC interruption; on the contrary, the TMF drives the vacuum arcs to move at high velocities resulting in faster dissipation of copper atoms after the CZ. In addition, the composition proportion of CuCr contacts has a limited effect on the diffusion of copper atoms when a low-vacuum arc is interrupted.

Electroplated Ru and RuCo films as a copper diffusion barrier

Thin, stacked films of Cu (1 μm)/RuxCo1-x(100 nm) were deposited on textured NiSiy/Si by electroplating. The Cu/RuxCo1-x/NiSiy/Si structure was then annealed at temperatures of 300–800 °C to evaluate the thermal stability. The structures of Cu/RuxCo1-x/NiSiy/Si characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometer (EDS), and powder X-ray diffraction (XRD). The results demonstrated that the Cu/RuxCo1-x/NiSiy/Si stack could be maintained up to 400 °C. For the Cu/RuxCo1-x/NiSiy/Si stack with x = 0.9–0.6 sample, the diffusion of Cu atoms into the silicon substrate was effectively suppressed. However, the copper atom diffusion into silicon can be found at Cu/Ru/NiSiy/Si sample at a temperature of 400 °C. The pure ruthenium thin film as a diffusion barrier to copper is limited. The RuCo barrier is ascertained to be a more robust barrier for copper diffusion than the Ru barrier. It can be attributed to the addition of Co in Ru thin film, which reduces the copper atoms to penetrate the silicon substrate.

Simulation of the Interaction of Graphene with a Copper Surface Using a Modified Morse Potential

A new carbon–copper interaction potential is proposed to simulate the moire structure of graphene on the copper surface. It is shown that the resulting moire structure is in qualitative agreement with scanning tunneling microscopy images. The thickness of the moire structure and the binding energy of graphene with the surface agree within the error with the existing experimental data. The proposed potential can also be used to simulate the diffusion of copper atoms over the graphene surface. The diffusion of an atom and a copper dimer in a wide temperature range is studied. It is found that the contribution of the vibrational free energy of copper atoms should be taken into account when simulating diffusion.

A diffusion model and growth kinetics of interfacial intermetallic compounds in Sn-0.3Ag-0.7Cu and Sn-0.3Ag-0.7Cu-0.5CeO2 solder joints

The growth kinetics of the intermetallic compounds (IMCs) between the low-silver composite solders (Sn–0.3Ag–0.7Cu-xCeO2) and copper substrates during reflow soldering process has been studied in this work. The thickness and grain size of the interfacial IMCs increase with an increase of reflow time. The addition of CeO2 nanoparticles can effectively suppress the growth of the interfacial IMCs. The interfacial reaction between composite solders and copper substrates is a dynamic response to the comprehensive effects of IMC growth at the interface, diffusion rate of copper atoms, and changes in composition at the solder/copper interface. A diffusion-controlled kinetics model is put forward to investigate the comprehensive effects. The modeling results show that, the diffusion rate of copper atoms at the solder/copper interface is related to the interfacial IMC grain size, IMC thickness and reaction time, and the addition of CeO2 nanoparticles can lead to a relatively lower diffusion rate of Cu atoms at the solder/copper interface. The change characteristic of the copper atom diffusion introduced in this model is supported by a good agreement between experimental and theoretical results. The mechanism of CeO2 nano-additive on the diffusion behavior of Cu atoms at the interface and interfacial IMC growth is explained by using the theory of adsorption and heterogeneous nucleation.

Suppressing Grain Growth on Cu Foil Using Graphene

The effect of graphene coating on the growth of grains on bulk copper film was studied. When methane gas is catalytically decomposed on the surface of copper, and a carbon–copper solid solution is formed at high temperature, precipitated carbon on the copper surface forms graphene during rapid cooling through strong sp2 covalent bonding. The graphene layer can prevent the growth of grains by suppressing the diffusion of copper atoms on the surface, even after continuous heat treatment at high temperatures. The actual size of the copper grains was analyzed in terms of repetitive high-temperature heat treatment processes, and the grain growth process was simulated by using thermodynamic data, such as surface migration energy and the binding energy between copper and carbon. In general, transition metals can induce graphene growth on surfaces because they easily form carbon solid solutions at high temperatures. It is expected that the process of graphene growth will be able to suppress grain growth in transition metals used at high temperatures and could be applied to materials that are prone to thermal fatigue issues such as creep.

Light enhanced direct Cu bonding for advanced electronic assembly

An innovative and efficient surface modification pre-treatment that enhances Cu–Cu direct bonding through electromagnetic irradiation, including pulsed Xenon flash and near infrared rays is proposed. Without vacuum, short but critical electromagnetic radiation exposure on faying faces prior to bonding can significantly improve the joint strength up to 50% or even more. Copper atom diffusion acceleration by increased compressive residual surface stresses resulting from sudden heating/cooling accounts for the joint reinforcement. A close relationship between the increase in joint strength and the change in surface physics properties due to electromagnetic irradiations can be found.

Analysis of the Structure and Thermal Stability of Cu@Si Nanoparticles

In this research core-shell Cu@Si nanoparticles were obtained through evaporation of elemental precursors by a high-powered electron beam. The structures of the particles were investigated in order to elucidate their mechanisms of formation. The thermal stability of the particles was studied with the help of molecular dynamics calculations. The parameters of the thermal stability of the composite nanoparticles Cu@Si with different size were determined. It was concluded that with the temperature increasing the diffusion of copper atoms on the surface begins, leading to a reversal of the structure and the formation of particles having a particle type Si@Cu.

Computer Modeling of the Formation Process of Core-Shell Nanoparticles Cu@Si

The process of nanoparticle Cu@Si formation by the molecular dynamic method using MEAM-potentials was studied. Modeling the droplet behavior demonstrates that a core-shell structure with a copper core and a silicon shell can be formed if the drop is in the liquid state, until the material is finally redistributed. The parameters of thermal stability of Cu@Si composite nanoparticles of different sizes have been determined. It is concluded that as the temperature increases, the diffusion of copper atoms to the surface begins, which leads to a change in the structure and the formation of particles with a core of the Cu@Si type.

Effect of the Crystallinity on the Electromigration Resistance of Electroplated Copper Thin-Film Interconnections

Dominant factors of electromigration (EM) resistance of electroplated copper thin-film interconnections were investigated from the viewpoint of temperature and crystallinity of the interconnection. The EM test under the constant current density of 7 mA/cm2 was performed to observe the degradation such as accumulation of copper atoms and voids. Formation of voids and the accumulation occurred along grain boundaries during the EM test, and finally the interconnection was fractured at the not cathode side but at the center part of the interconnection. From the monitoring of temperature of the interconnection by using thermography during the EM test, this abnormal fracture was caused by large Joule heating of itself under high current density. In order to investigate the effect of grain boundaries on the degradation by EM, the crystallinity of grain boundaries in the interconnection was evaluated by using image quality (IQ) value obtained from electron backscatter diffraction (EBSD) analysis. The crystallinity of grain boundaries before the EM test had wide distribution, and the grain boundaries damaged under the EM loading mainly were random grain boundaries with low crystallinity. Thus, high density of Joule heating and high-speed diffusion of copper atoms along low crystallinity grain boundaries accelerated the EM degradation of the interconnection. The change of Joule heating density and activation energy for the EM damage were evaluated by using the interconnection annealed at 400 °C for 3 h. The annealing of the interconnection increased not only average grain size but also crystallinity of grains and grain boundaries drastically. The average IQ value of the interconnection was increased from 4100 to 6200 by the annealing. The improvement of the crystallinity decreased the maximum temperature of the interconnection during the EM test and increased the activation energy from 0.72 eV to 1.07 eV. The estimated lifetime of interconnections is increased about 100 times by these changes. Since the atomic diffusion is accelerated by not only the current density but also temperature and low crystallinity grain boundaries, the lifetime of the interconnections under EM loading is a strong function of their crystallinity. Therefore, it is necessary to evaluate and control the crystallinity of interconnections quantitatively using IQ value to assure their long-term reliability.

Enhanced Cu-to-Cu direct bonding by controlling surface physical properties

Cu-to-Cu direct bonding is one of the key technologies for three-dimensional (3D) chip stacking. This research proposes a new concept to enhance Cu-to-Cu direct bonding through the control of surface physical properties. A linear relationship between bonding strength and the value of the bonding face (H: subsurface hardness, R: surface roughness) was found. Low vacuum air plasma and thermal annealing were adopted to adjust the surface physical conditions. Instead of surface activation, an acceleration in copper atom diffusion due to plasma-induced compressive stress accounts for the improvement in bonding strength.