Polycrystalline Copper Films Research Articles

Electrodeposition of copper on niobium for cryocooler application

The electrodeposition of copper onto niobium using commercial acidic and alkaline electrolytes was tested. The continuous dense polycrystalline copper films were successfully obtained in aqueous alkaline-type bath containing copper sulphate, sodium hydroxide and sodium gluconate. The effect of benzotriazole and sodium lauryl sulphate additives on the morphology and crystal structure of the deposited copper was investigated by optical and scanning electron microscopy, and X-ray diffraction. No copper oxides were found in the grown films. Copper films had moderate adhesion properties that would be insufficient for cryocooler application. We are currently exploring different compositions of electrolyte baths for obtaining the coatings on niobium with improved adhesion.

Grain boundary diffusion of Si in polycrystalline copper film

Grain boundary (GB) diffusion of Si in polycrystalline Cu film was studied in the temperature range of 403–453 K, in the C-type kinetic regime. The amorphous Si layer (80 nm) and polycrystalline Cu layer (40 nm) were successively deposited by magnetron sputtering at room temperature onto a Si (111) wafer. Appearance of Si atoms on the copper surface due to GB diffusion through the copper layer was detected by low energy ion scattering spectroscopy with high sensitivity. The depth distribution of Si in Cu grain boundaries was revealed by secondary neutral mass spectrometry. Surface morphology of Cu films was investigated by scanning tunneling microscopy. Identification of Si chemical bonds on the surface layer was made by X-ray photoelectron spectroscopy. GB diffusion coefficients were estimated by the relation used for calculation of diffusant distribution from a constant source in assumption that the diffusion path equals to the film thickness at the moment of appearing Si atoms on the Cu surface. At 453 K we estimated the surface segregation factor and detected formation of Cu–O–Si atomic bonds on the Cu film surface.

Quantitative analysis of void initiation in thermo-mechanical fatigue of polycrystalline copper films

The early stages of thermo-mechanical fatigue in 20-μm-thick copper films are studied using accelerated stress tests on poly‑silicon heater test chips. During loading, the samples are heated up from a base temperature of 100 °C to a maximum temperature of 400 °C within 200 μs with a waiting time of 1 s between subsequent pulses. With applied heating rates up to 106 K/s, followed by studies of microstructural damage after cycle numbers between 0 and 1000, the formation of voids at grain boundaries and triple points is revealed. With image processing on scanning electron microscopy images of stressed samples, void formation and growth as a function of cycle number is quantified. The results from metallography are compared to 3D reconstructions obtained by FIB tomography.

Infrared Analysis of Interfacial Phenomena during Electrochemical Reduction of CO2 over Polycrystalline Copper Electrodes

Using attenuated total reflection (ATR) infrared spectroscopy and ∼10 nm thick, sputtered Cu-films on single bounce Si-ATR-crystals, we have analyzed the electrochemical conversion of CO2 in 0.1 M NaOH/D2O solutions. By using cyclic voltammetry, transitions in the composition of dissolved and surface-adsorbed species could be identified. At a highly negative potential [more negative than −1.2 V (vs RHE)], the formation of OD– and D2 is dominant, resulting in a relatively high concentration of dissolved carbonate, with a maximum IR intensity at ∼1410 cm–1. When the potential is less negative than −1.2 V, spectroscopically resolved interconversion of carbonate (CO32–) to bicarbonate (D)CO3– is evident, explained by a decrease in the local pH. Furthermore, adsorbed carbonate can now be distinguished from dissolved carbonate due to the strongly potential-dependent peak position of adsorbed carbonate ranging from ∼1510 to 1570 cm–1. In the potential range of −1.2 to −0.5 V (vs RHE), using D2O, the recently proposed CO2-dimer-radical-anion was observed, adsorbed on the polycrystalline copper film. We also assign a previously unresolved band at ∼1610 cm–1 to this species. The dimer disproportionates to adsorbed CO and CO32–, the latter being converted to bicarbonate by proton addition. Adsorbed CO is sensitive to a Stark shift, that is, a shift as a function of applied potential. Eventually, CO disappears, and the infrared signature of (dissolved) formate at ∼1590 cm–1 appears at ∼ −0.5 V. We discuss the spectra and chemistry in detail, based on the reference spectra of carbonate, bicarbonate, and formate and using 13CO2 to substantiate the formation of the dimer intermediate. The results are discussed and compared to recent literature on infrared analysis of electrochemical reduction of CO2.

Applying a Statistical Model to the Observed Texture Evolution of Fatigued Metal Films

Reliably modeling fatigue induced degradation of metal films requires a consistent mathematical description of the physically relevant damage driving forces. The processes, which trigger deformation, damage and eventually failure, have to be included and combined in a physically meaningful way to obtain a valid model. Therefore, it is essential to consider the material response to applied stress, as the microstructural changes are responsible for the initialization of the degradation process. In this work, the material response of a polycrystalline copper film under repetitively applied thermo-mechanical stress is investigated by means of a test structure. The changes in the microstructure are studied by means of electron backscatter diffraction data, where the focus is laid on the grain boundary character distribution - a distribution of relative lengths of grain boundaries and corresponding misorientations. It is investigated whether the data can be described by a Fokker-Planck model that accounts for the system’s energy. Parameter estimation is performed on basis of a Maximum Likelihood approach, which is supplemented by an adjoint method to efficiently compute the gradient of the loss function with respect to the parameters. Additionally, uncertainty quantification of the estimates is required, which is done by computing the credible sets of the parameters. To this end, the results of local approximations, Markov Chain Monte Carlo simulations and profile likelihoods are compared.

Large-area grain-boundary-free copper films for plasmonics

Ultrasmooth single-crystalline metallic thin films provide several key advantages for the fabrication of well-defined and high-resolution plasmonic nanostructures, particularly complex integrated nanocircuits. For this purpose, copper is generally regarded as a poor plasmonic material compared to gold and silver because of its notorious oxidation issues when subjected to air exposure. Here, we report on the use of large-area grain-boundary-free copper films grown epitaxially on sapphire substrates in combination with focused ion beam milling to pattern plasmonic nanostructures with superior quality. The copper surfaces prepared using a single-crystalline copper sputtering target exhibit a very low roughness without any grain boundaries for varying film thicknesses and a strong resistance to oxidation, overcoming the bottleneck in conventional copper film fabrication. Surface plasmon resonance measurements show that improved dielectric constants with higher conductivity and long-term stability can be achieved using the single-crystalline copper films. Plasmonic nanohole arrays patterned from these high-quality films are found to display a stronger field enhancement compared to those made from polycrystalline copper films, thus resulting in an enhanced extraordinary optical transmission performance. This study suggests that our fabrication method is ideally suited for applications in copper-based plasmonic and nanophotonic devices as well as integrated nanocircuits on a large scale.

Impact of Granularity on the Oxidation Kinetics of Copper

Polycrystalline copper films with mean grain sizes varying by three orders of magnitude are prepared to correlate morphology and oxidation kinetics of metals. The films are oxidized to cuprous oxide at well‐defined pressure and temperature conditions, monitoring the progress of the reaction by in situ optical transmission spectroscopy. The oxidation kinetics is retrieved by fitting the optical data to a three‐layer Cu2O/Cu2O–Cu/Cu model with the central layer accounting for the inhomogeneity of the oxidation front due to grain boundaries. The analysis reveals the highest oxidation rates for Cu films with finest granularity, demonstrating the decisive role of grain boundaries for Cu mass transport during oxidation. Moreover, reaction rates are found to differ by one order of magnitude for oxidation along the grain boundaries and into the crystalline Cu grains. In fact, slow oxidation into the Cu crystallites is responsible for an incomplete metal‐oxide conversion in the case of coarse‐granular films. The experiments demonstrate the direct interplay between morphology and oxidation kinetics for metals.

Fatigue damage-healing model based on plasticity change for copper film by laser repair

A healing method for fatigue damage was investigated by laser shock peening and laser irradiation with excimer laser for polycrystalline copper film. It is found that the sum of the consumed fatigue ductility and the total fatigue ductility for damaged specimen after both laser treatments is equal to the total fatigue ductility for undamaged specimen after both laser treatments. According to the relationship of plasticity, two variables are defined. One is healing variable and the other one is enhancement variable. Based on the relationship of damage variable, healing variable and enhancement variable, a residual fatigue life prediction method is proposed. The results show that the predicted lives by the proposed method agree well with the experimental results.

Oxidation of polycrystalline copper films – Pressure and temperature dependence

Oxidation of polycrystalline Cu films is investigated in a wide range of temperatures and O2 pressures. The composition of the resulting oxide is derived from transmission and absorption spectra, analyzed with the transfer-matrix and a modified Tauc method, as well as from photoelectron spectroscopy. No Cu oxidation occurs at oxygen pressures below 1 mbar, indicative for a pressure-dependent barrier. At higher pressures, either cuprous or cupric oxide forms at oxidation temperatures below 460 K and above 500 K, respectively, while both phases coexist at intermediate temperatures. Oxide films of unknown stoichiometry, possibly Cu4O3, are revealed in the pressure range between 1 and 10 mbar. Based on these results, a formation diagram for copper oxides is developed and compared to respective phase diagrams in the literature.

Alkanethiol self-assembled monolayer on copper polycrystalline thin films: Influence on resistivity

Thin polycrystalline copper films of low surface roughness and low electrical resistivity were deposited by physical vapor deposition onto mica at room temperature and in thicknesses ranging from 10 to 80 nm. The crystalline orientation of the films was mainly in the [111] direction, with a surface roughness of under 8.0 nm for thicker films. The copper films were coated with dodecanethiol (DDT), CH3(CH2)10HS by direct immersion into a millimolar solution of the molecules in ethanol for 20 h. Self-Assembled monolayers (SAMs) formation was observed based on XPS studies. The results showed that DDT-SAMs were an effective barrier against the oxidation of the metallic surface, and the resistivity increased between 15% and 70% over the bulk value, depending on the film thickness.

Effect of lead on the thermal dispersion of continuous polycrystalline copper films

It has been found that the kinetics of heating has an essential effect on copper films dispersion temperature. Annealing of nonequilibrium defects slightly slow down the diffusion processes in comparison with unannealed samples and it naturally increases the temperature of de-wetting of continuous films. It has been shown that the formation of liquid lead in continuous Cu-Pb polycrystalline films accelerates their thermal dispersion compared to the copper films of the same thickness. Minimum concentration of lead that can stimulate de-wetting of polycrystalline copper films of 50 nm thickness, is 3 wt %. Structural studies using electron and X-ray diffraction have shown that Cu and Cu-Pb films annealing and dispersing do not result in new phases and crystalline modifications. Only a slight increase in the solubility of the given system in comparison with bulk samples was found. The activation energy of de-wetting processes in Cu and Cu-Pb films, (which is equal to 1.6 and 0.2 eV respectively) has been determined by means of electron microscopic studies of kinetics of through pores growth. This process may be considered as the liquid phase mass transfer not the diffusive one due to the low activation energy of de-wetting for the films containing lead.

Reflectometry-Ellipsometry Reveals Thickness, Growth Rate, and Phase Composition in Oxidation of Copper.

The oxidation of copper is a complicated process. Copper oxide develops two stable phases at room temperature and standard pressure (RTSP): cuprous oxide (Cu2O) and cupric oxide (CuO). Both phases have different optical and electrical characteristics that make them interesting for applications such as solar cells or resistive switching devices. For a given application, it is necessary to selectively control oxide thickness and cupric/cuprous oxide phase volume fraction. The thickness and composition of a copper oxide film growing on the surface of copper widely depend on the characteristics of as-deposited copper. In this Research Article, two samples, copper films prepared by two different deposition techniques, electron-beam evaporation and sputtering, were studied. As the core part of the study, the formation of the oxidized copper was analyzed routinely over a period of 253 days using spectroscopic polarized reflectometry-spectroscopic ellipsometry (RE). An effective medium approximation (EMA) model was used to fit the RE data. The RE measurements were complemented and validated by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). Our results show that the two samples oxidized under identical laboratory ambient conditions (RTSP, 87% average relative humidity) developed unique oxide films following an inverse-logarithmic growth rate with thickness and composition different from each other over time. Discussion is focused on the ability of RE to simultaneously extract thickness (i.e., growth rate) and composition of copper oxide films and on plausible physical mechanisms responsible for unique oxidation habits observed in the two copper samples. It appears that extended surface characteristics (i.e., surface roughness and grain boundaries) and preferential crystalline orientation of as-deposited polycrystalline copper films control the growth kinetics of the copper oxide film. Analysis based on a noncontact and nondestructive measurement, such as RE, to extract key material parameters is beneficial for conveniently understanding the oxidation process that would ultimately enable copper oxide-based devices at manufacturing scales.

Thermal and electrical conductivity of approximately 100-nm permalloy, Ni, Co, Al, and Cu films and examination of the Wiedemann-Franz Law

We present measurements of thermal and electrical conductivity of polycrystalline permalloy (Ni-Fe), aluminum, copper, cobalt, and nickel thin films with thickness $l200$ nm. A micromachined silicon-nitride membrane thermal-isolation platform allows measurements of both transport properties on a single film and an accurate probe of the Wiedemann--Franz (WF) law expected to relate the two. Through careful elimination of possible effects of surface scattering of phonons in the supporting membrane, we find excellent agreement with WF in a thin Ni-Fe film over nearly the entire temperature range from 77 to 325 K. All other materials studied here deviate somewhat from the WF prediction of electronic thermal conductivity with a Lorenz number, $L$, suppressed from the free-electron value by $10%\phantom{\rule{4.pt}{0ex}}\text{to}\phantom{\rule{4.pt}{0ex}}20%$. For Al and Cu we compare the results to predictions of the theoretical expression for the Lorenz number as a function of $T$. This comparison indicates two different types of deviation from expected behavior. In the Cu film, a higher than expected $L$ at lower $T$ indicates an additional thermal conduction mechanism, while at higher $T$ lower than expected values suggests an additional inelastic scattering mechanism for electrons. We suggest the additional low-$T\phantom{\rule{4pt}{0ex}}L$ indicates a phonon contribution to thermal conductivity and consider increased electron-phonon scattering at grain boundaries or surfaces to explain the high-$T$ reduction in $L$.

Experimental study of electron-phonon coupling and electron internal thermalization in epitaxially grown ultrathin copper films

Electron interaction in nanoscale metal structures is of major interest in understanding and designing nanoscale electronic devices. In this study, electron interaction in epitaxially grown 20-nm-thick copper film on Si(100) was studied using femtosecond pump-probe technique with tunable probe photon energy near the $d\text{-band}$ to Fermi-level transition. It was found that probe-photon-energy dependence on the transient reflectivity signal can be divided into four regimes, and the borders of these regimes depend on the pump power. Based on a phenomenological model, the excited electrons were separated into nonthermalized and thermalized electrons, and it was seen that the probe-photon-energy dependence of the signal can be explained by the difference of the ratios of nonthermalized to thermalized electron components. Furthermore, it was found that each of these two components can be selectively excited by properly choosing the probe photon energy. Each electron-phonon (el-ph) coupling and electron internal thermalization time were investigated individually without interference from each other's processes. We show that el-ph coupling factor is larger in epitaxial copper film than the previously reported value in polycrystalline-copper film. In addition, the electron internal thermalization time is smaller than the reported value for the polycrystalline gold and silver films.

Equivalent damage-healing approach to residual fatigue life prediction for copper film by laser repair

A healing method for fatigue damage was investigated by laser shock peening and laser irradiation with excimer laser for polycrystalline copper film. It is found that the sum of the consumed fatigue ductility and the total fatigue ductility for the damaged specimen after both laser treatments is equal to the total fatigue ductility for the undamaged specimen after both laser treatments. The evolution of the damage healing can be replaced by that for the undamaged specimen after laser treatment. Based on the equivalent fatigue damage evolution, a residual fatigue life prediction method is proposed during the damage-healing process for the copper specimen. The predicted lives by the proposed method agree well with the experimental results.

A unified damage-healing model for laser shock repair of copper film

A healing method for fatigue damage was studied by laser shock peening (LSP) with excimer laser for polycrystalline copper film. It is found that the reduction of plastic strain range due to laser shock peening could be responsible in the improvement of residual fatigue lives for the damaged specimens by LSP, and the degree of reduction in plastic strain range for the damaged specimens by LSP is more evident when the stress range is larger. According to the variation of the fatigue ductility of the specimen before and after LSP, a unified damage-healing model is proposed during the damage-healing process for copper specimens. The predicted lives by the proposed method agree well with the experimental results.

Residual life prediction for healing fatigue damaged copper film by laser shock peening

ABSTRACTA healing method for fatigue damage was studied by laser shock peening (LSP) with excimer laser for polycrystalline copper film. It is found that work hardening due to LSP could be responsible for the improvement of residual fatigue lives for the damaged and undamaged specimens by LSP, and the hardening degree for the damaged specimen by LSP is obviously higher than that for the undamaged specimen by LSP. In this paper, two basic mechanisms were identified. One is the dissipated energy enhancement mechanism, which improves the fatigue life caused by laser shock stress, and the other is the healing mechanism, which leads to a further improvement. Based on the two mechanisms, a residual fatigue life prediction method is proposed by the view of energy consumption before and after LSP. The predicted lives by the proposed method agree well with the experimental results.

Healing fatigue damage by laser shock peening for copper film

A repair method for fatigue damage was studied by laser shock peening (LSP) with excimer laser for polycrystalline copper film. Two basic effects were defined. One is the enhanced effect, which improves the fatigue life for the non-damaged specimen by LSP, and the other is the healing effect, which has a further improvement for the damaged specimens. The results showed that, in most cases, LSP has a pure healing effect in addition to an enhanced effect for the damaged specimens. The energy analysis revealed the work hardening due to LSP is the main reason for the improvement of residual fatigue lives for the damaged specimens.

Mechanism of Healing Fatigue Damage by Pulsed Laser Surface Irradiation for Polycrystalline Copper Film

The mechanism of healing fatigue damage was investigated by pulsed laser surface irradiation for polycrystalline copper film, X-ray Diffraction(XRD), optical microscopic and scanning electron microscopy(SEM). The results indicate that surface melting and grain refinement were found in the melted surface layer. Tensile residual stresses were introduced in the resolidified surface layer due to laser surface melting which produced deleterious effect on fatigue property. Remelting and grain refinement in the surface layer is mainly responsible for healing fatigue damage and fatigue life improvement. The fatigue damage could be healed completely or partly by laser surface irradiation under the proper parameters of laser treatment for the specimens.

Strain in electroless copper films monitored by X-ray diffraction during and after deposition and its dependence on bath chemistry

X-ray diffraction based strain measurements have been carried out during the deposition and subsequent relaxation of electroless (autocatalytic) polycrystalline copper films. Thin polymer substrates were mounted on the surface of an electrolyte-filled plating cell, and the X-rays traversed the substrate to scatter of the growing Cu layer. The plating cell was rotated back and forth by up to 70° in order to find the strain of Cu crystallites within and perpendicular to the plane of the film ( sin 2 ψ method). Three types of plating solutions were investigated. A Ni-free solution C leads to compressive strain during steady-state film growth, followed by an exponential relaxation of the film to a residual tensile strain. Electrolytes A and B contain Ni ions, and the resulting Cu(Ni) films have nearly constant strain with small counteracting strain variations during and after film growth. Cyanide-stabilized solution A yields films with a slight compressive strain, while solution B, stabilized by an aromatic nitrogen compound, yields films with tensile strain. Different and reproducible evolution patterns observed for these three electrolyte types establishes in situ X-ray diffraction strain monitoring as a method to evaluate chemical formulations for electroless deposition.