Copper Diffusion Research Articles

Intrinsic Diffusivities Ratio Analysis in Double Multiphase Systems

Our investigations show that electrochemical corrosion of copper is faster than electrochemical corrosion of aluminium at temperatures below 100°C. Literature data analysis shows that the Al atoms diffuse faster than the Cu atoms at temperatures higher than 475°C, Al rich intermetallic compounds (IMCs) are formed faster in the Cu-Al system, and the Kirkendall plane shifts toward the Al side. Electrochemical corrosion occurs due to electric current and due to diffusion. An electronic devise working time, for example, depends on initial copper cover thickness on aluminium wire, connected to the electronic devise, temperature, and volume and dislocation pipe diffusion coefficients, so copper, iron, and aluminium electrochemical corrosion rates are investigated experimentally at room temperature and at temperature 100°C. Intrinsic diffusivities ratios of copper and aluminium at different temperatures and diffusion activation energies in the Cu-Al system are calculated by proposed here methods using literature experimental data. Dislocation pipe and volume diffusion activation energies of pure iron are calculated separately by earlier proposed method using literature experimental data. Aluminium dissolved into NaCl solution as the Al3+ ions at room temperature and at temperature 100°C, iron dissolved into NaCl solution as the Fe2+ (not Fe3+) ions at room temperature and at temperature 100°C, copper dissolved into NaCl solution as the Cu+ ions at room temperature and as the Cu+ and the Cu2+ ions at temperature 100°C. It is found experimentally that copper corrosion is higher than aluminium corrosion, and ratio of electrochemical corrosion rates, kCu/kAl>1, decreases with temperature increasing, although iron electrochemical corrosion rate does not depend on temperature below 100°C. It is obvious, because the melting point of iron is more higher than the melting point of copper or aluminium. It is calculated that the copper electrochemical corrosion rate is approximately equal to aluminium electrochemical corrosion at temperature about 300°C, so copper can dissolve into NaCl solution mostly as the Cu2+ ions at temperature about 300°C. The ratio of intrinsic diffusivities, DCu/DAl<1, increases with temperature increasing, and the intrinsic diffusivity of aluminium could be approximately equal to the intrinsic diffusivity of copper at temperature about 460oC. Intrinsic diffusivities ratios in the Cu-Zn system at temperature 400°C and in the Cu-Sn system at temperatures from 190°C to 250°C are analyzed theoretically using literature experimental data. Diffusion activation energies and pre-exponential coefficients for the Cu-Sn system are calculated combining literature experimental results.

Radiation-enhanced diffusion of copper in iron studied by three-dimensional atom probe

Radiation-enhanced diffusion (RED) of copper (Cu) in iron (Fe) is essential for understanding solute/impurity diffusion in nuclear materials, especially reactor pressure vessel steel, but has been rarely reported experimentally. In this study, we performed a high-precision investigation of RED using well-controlled electron irradiation and three-dimensional atom probe (3D-AP). Cu-Fe diffusion pairs were created using high-purity Fe and Cu as base materials, and irradiated by 2 MeV electron at a temperature of 773 – 893 K controlled to within ±3 K. Cu diffusion into the Fe matrix was observed at the atomic level using 3D-AP, and the diffusion coefficient was obtained directly using Fick's law. RED was clearly observed, and the ratio of diffusion under irradiation to thermal diffusion was increased as the irradiation temperature decreased. RED was quantitatively evaluated using the reaction kinetics model, and the model which consider only vacancies gave a good agreement. This gave experimental clarification that RED was dominated by irradiation-induced vacancies. In addition, the direct experimental results on the effect of irradiation on the solubility limits of Cu in Fe was obtained; solubility limits under irradiation were found to be lower than those under thermal aging.

Enhanced Stability of Tin Halide Perovskite Photovoltaics Using a Bathocuproine—Copper Top Electrode

AbstractUnencapsulated organo‐tin halide perovskite photovoltaic (PV) devices exhibiting record stability (for organo‐tin perovskite PV devices) when tested under continuous one sun solar illumination in ambient air and under electrical load are reported. This exceptional stability is made possible by the use of a bathocuproine | copper cathode in an inverted device architecture. A series of experiments designed to elucidate the underlying reasons for the high stability show that, compared to conventional silver electrodes, compact copper electrodes are far more resistant to corrosion by I2 gas (evolved when organo‐tin halide decomposes) and toward adverse morphological evolution and ingress of oxygen and water molecules through the top electrode into the device. The findings of these experiments show that copper should be the metal of choice for the reflective cathode in inverted tin perovskite PVs when the material interfacing the metal interacts strongly with it, enabling compact film formation and a stable interface toward copper diffusion into the adjacent charge transport layer.

(Invited) Hybrid Bonding-Based Interconnects: A Status on the Last Robustness and Reliability Achievements

I. INTRODUCTION and BACKGROUND 3D integration technologies have emerged less than 10 years ago as viable solutions for meeting IC requirements such as higher performance, increased functionality, lower power consumption, and a smaller footprint. Some examples of 3D integration in products are CMOS Image Sensors (CIS), memories and interposers. Through-Silicon-Vias (TSVs) are integrated in most of these products, because they allow die-level assembly, and simplify some aspects of test and packaging. However, TSVs disadvantage is the silicon device keep out zones combined with a Cu pillar pitch typically limited to ~40 µm. For some applications, such as CMOS image sensors and memories, smaller pitches (< 10 µm) are required for the vertical die-to-die connections; direct hybrid bonding (HB) is the solution of interest. While the first HB products are put on the market [1] few detailed studies are published from the robustness/reliability point of view. This paper reviews the robustness/reliability achievements and include previously published data related to the HB module (Wafer-to-Wafer [W2W] or Die-to-Wafer [D2W]). II. RESULTS Compared to standard microelectronics processes, the main difference of 3D integration consists in the addition of a wafer-bonding step. Due to the bonding tool alignment accuracy, from 200 nm to 1 µm respectively for W2W and D2W, misalignment could lead to an electrical yield loss. W2W-based studies demonstrated 100 %-yield [2, 3] where D2W ones still show lower yield (>90 %) [4] mainly due to more recent developments.Silicon dioxide can be hydrophilic, this tends to highlight a potential risk of moisture sensitivity of the HB module. By means of uHAST, Beillard and Gambino did not evidence any issue [3, 5].Because different materials are assembled, thermomechanical stress issue could arise. ST/CEA-LETI works demonstrated [2, 6] no particular impact at 7.2 and 1.44 µm-interconnect pitches after thermal cycling tests. For larger pitches, Gambino and Gao reached a similar conclusion [3, 7].Electromigration (EM)-induced degradation is another potential reliability issue. As demonstrated by ST/CEA-LETI [8], for a pitch of 7.2 µm, HB integration is immune to EM whatever the electron flow. The failure always occurs in the BEoL layers. For a smaller pitch (3.24 µm), IMEC highlights a potential risk to the HB module itself [9].Cu diffusion in silicon and silicon dioxide is a well-known issue in microelectronics and is solved by using diffusion barriers (TaN/Ta, Ti/TiN...). Despite the lack of a diffusion barrier at the bonding interface in their integration, ST/CEA-LETI [10, 11] demonstrated by electrical and chemical characterization (TVS/BTS, ToF-SIMS/TEM-EELS) that no copper diffusion is nevertheless detectable. Gambino and Kagawa, with different integrations, reached the same conclusion [3, 12].To conclude, in the light of the published works, even if HB module could raise specific robustness/reliability issues, all the lights are green for mass production with micrometric pitches, which is consistent with the fact that this approach, at these pitches, is found in some products [1]. On the contrary, for sub-micrometric pitches some questions remain topical (IMD reliability, Cu diffusion, electromigration). Acknowledgment This work was supported by the French National Research Agency (ANR) under the ”Investissements d’avenir” programs: ANR 10-AIRT-0005 (IRT NANO-ELEC).

Electrochemical Investigation of Light-Induced Plating of Nickel on Si Solar Cells and Study of Nickel Silicide Formation

Front-contact metallization plays an important role in the fabrication of a solar cell as it adds up to 40% cost to the total production cost.1 Silver (Ag) screen printing is the commercialized technology for the front contact metallization. On the other hand, silver screen printing causes higher Si wafer breakage and results in conversion loss due to high line resistivity, poor aspect ratio and large printing spot size. Moreover, the high price of silver also drives up the cost of a solar cell. Copper, more than a hundred times cheaper than silver, is an alternate option for the front contact as its conductivity is comparable to that of silver.2 However, copper has a tendency to diffuse through Si and create recombination sites, which decreases the cell performance. To avoid diffusion of copper in silicon, a metal film has to be deposited that acts as a barrier layer. Among various diffusion barrier materials, Ni is the most promising because of its low bulk resistivity that makes nickel silicide compatible with the semiconductor technology.In this work, a nickel seed layer is deposited on silicon samples with light-induced plating (LIP) to obtain the low-resistive NiSi (1:1) phase. In-situ X-ray diffraction (XRD) is carried out along with annealing to understand the phase changes associated with silicide formation (Figure 1). The obtained XRD patterns of nickel deposited by LIP were not comparable with that of EM (electroless metal deposition). Mott-Schottky analysis is carried out to investigate the interface properties of Ni deposited by LIP and electroless methods.3, 4 XPS (X-ray photoelectron spectroscopy) data confirms the interfacial oxide which hindered the nickel diffusion to form silicide. References M. Razykov; C.S. Ferekides; D. Morel; E. Stefanakos; H.S. Ullal; H.M. Upadhyaya., Sol. Energy, 85, 1580 (2011). Kamp; J. Bartsch; S. Nold; M. Retzlaff; M. Hörteis; S.W. Glunz., Energy Procedia, 8, 558 (2011). Priyadarshani, P. Leuaa, R. Maurya, A. Kottantharayil and M. Neergat, J. Phys. Chem. C, 124, 19990 (2020).B. Bera, A. Chakraborty, T. Kar, P. Leuaa and M. Neergat, J. Phys. Chem. C, 121, 20850−20856 (2017). Figure 1

Search for copper diffusion at hybrid bonding interface through chemical and electrical characterizations

Cu/SiO2 diffusion at hybrid bonding interface without diffusion barrier was investigated in order to validate the electrical insulation of interconnects. The Cu thermal diffusion was studied by ToF-SIMS analysis of the dielectrics stack facing bonding pads after a bonding annealing (400 °C, 2 h) and a diffusion annealing (400 °C, 14 h). No trace of copper was found above the limit of detection 1017 at·cm−3. Cu ions drift was followed by I-V measurements on specially designed comb-serpentine that maximize the electrical field at bonding interface. Their efficiency was confirmed since physical failure analysis located the dielectric breakdown damage between bonding pads. Breakdown voltages coupled to wafer-to-wafer misalignments enabled the extraction of the SiO2 breakdown strength: 3.4 MV·cm−1. This study proves that at room temperature, hybrid bonding interconnects remain electrically insulated despite thermal budgets involved by the bonding process.

Corrosion Behavior of Copper Bearing Steels and the Derived In-Situ Coating

Using a period immersion wet/dry cyclic corrosion test, in-situ copper-coated steels prepared by corroding copper-bearing steels were investigated in this study. The steel with a higher copper content (&gt;3%) has a higher initial corrosion rate due to its obvious two-phase microstructure. The corrosion rates of all copper bearing steels tend to be stable after a certain time of corrosion. A copper-rich layer is formed between the matrix and the rust layer, which is due to the diffusion of copper from the rust layer to the metal surface. The copper’s stability under this corrosion condition led to the formation of a thin copper-rich film, which was uncovered after removing the rust by choosing appropriate descaling reagents. The copper coating was generated from the matrix itself during the corrosion process at 25 °C, which provided a new approach for producing in-situ composite materials without any bonding defect. It is found that the corrosion rate, corrosion time, and copper content in steel all affect the formation of copper-rich layer. In addition to the noble copper surface, the electrochemical corrosion test results show that the corrosion resistance of copper-coated steel has been significantly improved.

Effect of substrate temperature on the crystalline phases of Cu2-xTe films grown by RF sputtering

The effect of the substrate temperature (Ts) on the growth of Cu2-xTe films using the RF sputtering technique has been studied through a structural assessment of the different trigonal (space group P3m1) phases encountered in the range 100–350 °C. A new stoichiometric Cu7Te4 phase (δ′) with Z = 1 is proposed, and it is present in all the temperatures studied. Low-angle peaks (2θ < 20°) of Glancing Incidence X-ray Diffraction (GIXD) patterns are consistent with the formation of commensurate phases (forms α′) through variations in the c parameter whose value can be as large as 25.47 Å. The base modulation vector is the c parameter of the unit-cell of phase δ′ that, in this work, resulted in a value of 3.61 Å. The origin of the observed commensurate phases has been ascribed to an occupancy-wave modulation. Whereas at high temperatures (Ts > 250 °C), the major phase is δ' (Cu7Te4 or Cu2-xTe with x = 0.25), the predominant phase, for Ts < 250 °C, is the commensurate superstructure 7C with higher [Cu]/[Te] ratio (x < 0.25). The transition between these two thermal regimes is stated at a temperature of about 250 °C. Maintaining a constant global composition, the devaluation of [Cu] in films, with the temperature above the transition range, is explained in terms of diffusion of copper downward the film and towards its edges.

Corrosion Behaviour of Cu/Carbon Steel Gradient Material

Research on improving the corrosion resistance of carbon steel has become a hot topic in the iron and steel field in recent years. Copper plating on the surface of carbon steel is considered an effective means to improve its corrosion resistance, but the copper-plated carbon steel material prepared by this method has the problems of poor abrasion resistance, easy delamination of copper layer and similar issues, which affect the service performance of the copper-plated carbon steel material. To solve this problem, a new type of material whose surface is copper and the copper element is gradually diffused into carbon steel was developed by a plating-diffusion method, which is defined as a copper-carbon steel gradient material. Carbon steel with a copper plated surface and the Cu-Fe/carbon steel gradient material with 80% Cu content on the surface were prepared by the same method. The cross-sectional microstructure and composition of different samples were analysed, and the corrosion behaviors of samples in 3.5% NaCl solution were studied by the linear polarization curve method and electrochemical impedance spectroscopy. The cross-sectional microstructure result shows that the diffusion of copper in carbon is mainly carried out along its grain boundary, and the diffusion of copper will inhibit the growth of grains during heat treatment. As shown in the results of corrosion behaviors, there is no pitting corrosion in the corrosion process of all samples, as well as the stable passive film. All samples showed active dissolution. Compared with carbon steel, the corrosion potential of the Cu/carbon steel gradient material becomes more positive from −600 mV to −362 mV, the corrosion current density decreases from 53.0 μA/cm2 to 30.6 μA/cm2 and the radius of electrochemical impedance spectroscopy enlarges while the corrosion resistance is improved, and the corrosion resistance is mainly obtained by its surface copper layer. The corrosion resistance of Cu-Fe/carbon steel gradient material is lower than that of Cu/carbon steel gradient material, while it is still better than carbon steel, and it shows a clear passivation trend during corrosion. Therefore, the copper/carbon steel gradient material can significantly improve the corrosion resistance of carbon steel. Even after the surface copper layer is destroyed, the gradient material can protect the matrix and improve the service life of the material.

Investigating the Effect of Electrical Discharge Process Input Parameters on Mechanical Properties of Aluminum Surface

One of the important parameters in electrical discharge machining is the presence of micro cracks on the workpiece surface (recast layer). Therefore, the aim of this study was to investigate the possibility of increasing the mechanical properties of aluminum surface by alloying elements (copper and nickel) diffusion to the recast layer and thus removing surface micro cracks. For this purpose, pulse on time and pulse current in with and without ultrasonic vibration have been considered as input parameters and the presence of surface micro cracks has been investigated using microscopic images. Also, the yield stress of the surface layer was calculated using the surface micro hardness. Based on the obtain results, surface without micro cracks has been created on the aluminum surface due to the diffusion of copper and nickel into the workpiece surface which increased aluminum surface yield strength from 90MPa to 280MPa without ultrasonic vibrations and to 310MPa while applying ultrasonic vibrations. In other words, ultrasonic vibrations cause an average of 20% increase in surface layer yield strength. In addition, according to the wear test, in the case of using ultrasonic vibration, improving the mechanical properties of the surface has caused thinner grooves on the aluminum surface.

Diffusion in Copper/Cobalt Systems under High Magnetic Fields.

Comprehensive research on a high magnetic field’s effect on diffusion is lacking; hence, this study investigates the effect of the magnetization of such a field on diffusion using a copper/cobalt diffusion couple in the diamagnetic/ferromagnetic states, respectively. The diffusion couple was formed using explosive welding to avoid diffusion during manufacturing. The diffusion couple annealed within a temperature range of 1165–1265 K under a 0–6-T high magnetic field. The angle between the diffusion and magnetic field directions was set as 0° and then 180°. The penetration profiles of cobalt volume diffusion in the copper and grain-boundary diffusion of copper in cobalt were constructed using an electron probe micro analyzer. The high magnetic field increased the volume diffusivity of cobalt in copper, but had no evident effect on the grain-boundary diffusivity of copper in cobalt, irrespective of the magnetic field direction. An Arrhenius plot of the cobalt volume diffusivity in copper demonstrated that the applied high magnetic field enhanced diffusion by changing the frequency factor rather than the activation energy; this can be attributed to the increased diffusion entropy caused by changing the vacancy concentration, which resulted from the introduction of magnetization under a high magnetic field.

Improving Copper Nucleation on Liner Materials

The damascene process for fabrication of interconnects requires a copper seed conductive layer on top of a liner material, which acts as a copper diffusion barrier. Copper metal is electrochemically deposited from solutions containing additives onto the conductive seed layer, for void-free bottom up fill. However, as interconnect dimensions continue to decrease, copper damascene faces significant challenges1. As such, alternative metallization approaches are under development. These include electrodeposition of copper directly onto liner materials such as ruthenium2,3 or cobalt4-6. Direct plate onto liner materials pose new challenges that must be overcome prior to implementation. For example, ruthenium and cobalt are easily oxidized upon exposure to air7. This native oxide film inhibits nucleation, resulting in heterogeneous growth of copper islands8. The literature shows that oxide removal improves nucleation at ruthenium and that copper can successfully be deposited onto cobalt using additives6. While there are some insights on how to achieve nucleation of copper onto resistive substrates, this must be improved prior to commercialization. We present methods for pretreatment of ruthenium in order to reduce the native oxide, allowing for more uniform deposition of the copper films (as shown by Fig. 1). Upon addition of our additives, early film coalescence and uniform deposition of copper is achieved, resulting in void-free bottom up filling. These findings will be presented and discussed.Fig. 1. Nucleation of copper onto (a) as received compared to (b) pretreated ruthenium substrate plated from a typical copper electrolyte containing no additives.

Copper diffusion rates and hopping pathways in superionic Cu2Se

The ultra-low thermal conductivity of Cu2Se is well established, but so far there is no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron scattering. Reasonable agreement was found between these approaches, consistent with a diffusion coefficient of 3.1 ± 1.3× 10−5 cm2.s−1 at 675 K and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetrahedral and interstitial octahedral sites. This process forms dynamic Frenkel defects. Despite the latter processes, there is no major loss of the phonon mode intensity in the superionic state, and there is no strong correlation between the phonon spectra and the increased diffusion rates. Instead, intrinsic anharmonic phonon interactions appear to dictate the thermal conductivity above and below the superionic transition, and there is only subtle mode broadening associated with the monoclinic-cubic structural transition point, with the phonon density-of-states remaining almost constant at higher temperatures.

High-temperature behaviour of HVOF (Co,Ni)O coated Cu-Ni-Fe anodes

Cu-Ni-Fe alloys are promising inert anodes for green Al production but their corrosion resistance must be improved. Here, protective (Co,Ni)O coatings are deposited by high velocity oxygen fuel (HVOF) process on Cu-Ni-Fe alloys. The influence of the coating and substrate compositions on their behavior is studied at 1000 °C under argon and air. On Cu-rich alloy, CoxNi1−xO coatings with higher nickel content slow down oxygen diffusion to the substrate, as well as copper diffusion to the sample surface. On Ni-rich alloy, the formation of a NiFe2O4 scale is observed, whose thickness decreases as the Ni content of the CoxNi1−xO coatings increases.

Atomic diffusivities in amorphous and liquid Cu-Zr: Kirkendall effects and dependence on packing density

A novel method for measurement of atomic interdiffusivity is applied to amorphous Cu-Zr close to its glass-transition temperature Tg. Sputter-deposited multilayers are examined in cross-section by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Mapping the evolution of composition profiles gives the interdiffusivity, which is orders of magnitude higher than if coupled to the viscosity expected near Tg. Kirkendall drift of interlayer interfaces in both amorphous and supercooled liquid states (i.e. below and above Tg), and associated voiding in the liquid, show that the diffusivity of copper greatly exceeds that of zirconium. Amorphous Cu-Zr is known to show maxima in atomic packing density at sharply defined compositions. The comparison of the two compositions in the present work provides the first direct evidence that denser packing is associated with lower atomic interdiffusivity. The lower interdiffusivity is governed by a lower diffusivity of copper, and reflects a lessened degree of decoupling of the copper (fast) and zirconium (slow) diffusivities in an efficiently packed glass. The new insights help to understand issues ranging from glass-forming ability to the controlled generation of nanovoided structures.

Thermal and oxidation stability of TixW1−x diffusion barriers investigated by soft and hard x-ray photoelectron spectroscopy

The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO2, WO3, and a WO3 oxide coordinated with a titanium environment. This combinatorial characterization approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.

Layer Thickness Effect on Lifetime of Copper Oxide Passivated Plasma Etched Copper Line

Previously, authors reported that copper oxide could be a good passivation material for the copper line because it prevented copper diffusion to adjacent layer at the high temperature. In this paper, the copper oxide thickness effect on the electromigration failure mechanism is studied. The copper oxide passivation layer was formed by exposing the copper line to the oxygen plasma in a parallel-plate plasma etcher. The copper oxide passivation layer has a smooth surface and a crystalline structure. The copper line with a very thin copper oxide passivation layer has an electromigration lifetime similar to that of the unpassivated copper line. However, the lifetime deteriorates with the increase of the copper oxide thickness due to the enlargement of the mechanical and thermal stress mismatches between the bulk copper film and the passivation layer. Therefore, the thickness of the copper oxide is critical to the effectiveness of its passivation function.

From Electrical to Physical-Chemical Characterization of the Cu/SiO₂ Hybrid-Bonding Interface—A Cu₂O-Layer as a Cu Diffusion Barrier?

This letter discusses copper (Cu) diffusion in the framework of the development of future 3D stacked devices by means of hybrid bonding-based interconnects. Indeed, at the bonding interface, due to the bonding tool accuracy, an inherent misalignment exists between top and bottom tiers leading to a direct contact between copper and neighboring dielectrics. Our experimental studies do not evidence any copper diffusion for the analyzed hybrid bonding-based test structures. Indeed, BTS/TVS (Bias Temperature Stress/Triangular Voltage Sweep) electrical characterization does not evidence any mobile ions diffusion. Our Electron Energy Loss Spectroscopy (EELS) analysis reveals the presence of a thin layer (~3 nm) of cuprous oxide (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) at the copper/dielectric interface that most probably acts as a diffusion barrier to copper.

Investigation of abnormal forming process current caused by copper diffusion in Cu/GeSO/TiN resistance random access memory

Understanding its resistance switching (RS) properties is beneficial to the development of resistance random access memory (RRAM). In this work, a RRAM device with a Cu/GeSO/TiN structure (from top to bottom) is investigated. Before forming, the device can operate with a small ratio (<10) of high resistance state (HRS) to low resistance state (LRS). The distribution of this ratio is dependent on device cell size, and is a result of oxygen accumulation. By applying a positive bias during forming process, an abnormal relatively flat current is observed. These observations are confirmed by testing devices of different cell sizes. Bias stresses and capacitance-voltage (C–V) measurements are also used to confirm the changes in film characteristics. In addition, material analyses of transmission electron microscopy (TEM) images are also applied to examine the diffusion of Cu ions that affect the forming process of this Cu/GeSO/TiN device.

Void-dynamics in nano-wires and the role of microstructure investigated via a multi-scale physics-based model

With scaling of nano-interconnect linewidth toward a 3 nm technology node, electromigration in copper nano-interconnects is becoming a major limitation. In this context, the increase of texture polycrystallinity plays a major role and necessitates better understanding of the impact of void dynamics and its interaction with texture at operating conditions. To this end, comprehensive, yet efficient, physics-based numerical models are warranted given that electromigration experiments at low operating temperatures are not feasible. Albeit, development of such models has been a challenge since it involves large length-scale discrepancy between features, i.e., from wire dimensions (hundreds of micrometers) down to grains and voids (tens of nano-meters and below) leading to substantial computational cost. To this end, in this study an efficient multi-scale physics-based electromigration modeling approach is demonstrated, where an experimentally calibrated Korhonen-type 1D model solves electromigration at the global scale (entire interconnect), and a 2D local model simulates void dynamics considering the impact of electron wind, void surface energy, and stress gradients. The role of copper texture, i.e., grain boundaries, grain orientation and anisotropic properties is thoroughly investigated. We demonstrate that by tailoring the copper grain structure, void behavior and thus resistance evolution of nanowires can be effectively controlled. Compared to wider interconnects, in scaled nanowires, surface energy is found to play a dominant role on void morphology compared to electron wind. Strong anisotropy of diffusivity of copper's FCC lattice fosters faceted morphologies and emergence of slit morphology during void transition through the polycrystalline texture. Bamboo segments are shown to effectively pin the voids migrating toward the cathode in cases with cobalt cap. In addition, voids migrating from low diffusivity to high diffusivity regions adopt a longitudinally elongated morphology, which can be detrimental in a down-stream operation mode.