Copper Films Research Articles

Study on the Damage Mechanism of an H62-Cu/7075-Al Tribo-Pair Under the Influences of Current Direction and Density

In the present study, we used 7075 Al-H62 Cu and H62 Cu-7075 Al pairs to study the effects of current density and direction on their tribological properties and on the damage caused by the current-carrying friction and wear. We found that, when the current density increased from 0 A/mm2 to 79.61 A/mm2, the coefficients of friction for both pairs decreased. Results obtained after wear indicate that the current direction influences the electromigration between the two tracks, leading to different kinds of damage on the worn surface. In the case of the 7075 Al-H62 Cu pair, damage mainly involved mechanical wear at low current densities. As the current density increased, electro-erosion damage gradually became more dominant. Under the action of a large electric arc, the material surface was severely eroded, and a dense oxide film formed on the material contact surface, ultimately leading to the failure of electrical conduction between the materials. In the case of the H62 Cu-7075 Al pair, damage mainly involved mechanical wear. A layer of copper film was found on the surface of the worn aluminum pin, which caused its mass to be greater than it was before wear.

Predicting the surface roughness of an electrodeposited copper film using a machine learning technique

Predicting the surface roughness of an electrodeposited copper film using a machine learning technique

Evaluating size effects on the thermal conductivity and electron-phonon scattering rates of copper thin films for experimental validation of Matthiessen’s rule

As metallic nanostructures shrink towards the size of the electronic mean free path, thermal conductivity decreases due to increased electronic scattering rates. Matthiessen’s rule is commonly applied to assess changes in electron scattering rates, although this rule has not been validated experimentally at typical operating temperatures for most of the electronic systems (e.g., near room temperature). In this study, we experimentally evaluate the validity of Matthiessen’s rule in determining the thermal conductivity of thin metal films by measuring the in-plane thermal conductivity and electronic scattering rates of copper (Cu) films with varying thicknesses (27 nm — 5 µm), microstructures, and grain boundary segregation. Comparing total electron scattering rates measured with infrared ellipsometry to infrared ultrafast pump-probe measurements, we find that the electron-phonon coupling factor is independent of film thickness, whereas the total electronic scattering rate increases with decreasing film thickness. Our findings provide experimental validation of Matthiessen’s rule for electron transport in thin metal films at room temperature and also introduce an approach to discern critical heat transfer processes in thin metal interconnects, which holds significance for the advancement of future CMOS technology.

Study on Oxygen Plasma-Based Copper Etching Process

Abstract A plasma-based, room-temperature copper etch process using the chlorine- or bromine-containing feed gas was reported. This simple process could potentially replace the chemical mechanical polishing method in preparing copper interconnects. However, the chlorine- and bromine-containing gases are corrosive and must be handled with expensive equipment following stringent safety procedures. In this paper, the oxygen plasma-based copper etch process is presented. The copper film was converted into a porous and polycrystalline copper oxide film which was subsequently dissolved in a dilute hydrochloric acid solution. The copper film was expanded when converted into an oxide film. The oxidation precursors, i.e., oxygen radicals and ions, were generated in the plasma phase and then transported through the oxide layer to the underneath copper film where the oxidation reaction proceeded. The oxide growth rate is affected by plasma parameters, such as pressure and power, and the kinetics of the oxidation reaction. This new oxygen plasma-based process is a simple solution for preparing copper interconnects for nano and microelectronic products.

Sol-Gel Derived Alumina Particles for the Reinforcement of Copper Films on Brass Substrates.

The aim of this study is to provide tailored alumina particles suitable for reinforcing the metal matrix film. The sol-gel method was chosen to prepare particles of submicron size and to control crystal structure by calcination. In this study, copper-based metal matrix composite (MMC) films are developed on brass substrates with different electrodeposition times and alumina concentrations. Scanning electron microscopy (FE-SEM) with energy-dispersive spectroscopy (EDS), TEM, and X-ray diffraction (XRD) were used to characterize the reinforcing phase. The MMC Cu-Al2O3 films were synthesized electrochemically using the co-electrodeposition method. Microstructural and topographical analyses of pure (alumina-free) Cu films and the Cu films with incorporated Al2O3 particles were performed using FE-SEM/EDS and AFM, respectively. Hardness and adhesion resistance were investigated using the Vickers microindentation test and evaluated by applying the Chen-Gao (C-G) mathematical model. The sessile drop method was used for measuring contact angles for water. The microhardness and adhesion of the MMC Cu-Al2O3 films are improved when Al2O3 is added. The concentration of alumina particles in the electrolyte correlates with an increase in absolute film hardness in the way that 1.0 wt.% of alumina in electrolytes results in a 9.96% increase compared to the pure copper film, and the improvement is maximal in the film obtained from electrolytes containing 3.0 wt.% alumina giving the film 2.128 GPa, a 134% hardness value of that of the pure copper film. The surface roughness of the MMC film increased from 2.8 to 6.9 times compared to the Cu film without particles. The decrease in the water contact angle of Cu films with incorporated alumina particles relative to the pure Cu films was from 84.94° to 58.78°.

The Impact of Substrate Temperature on the Adhesion Strength of Electroplated Copper on an Al-Doped ZnO/Si System.

This research, which involved a comprehensive methodology, including depositing electroplated copper on a copper seed layer and Al-doped ZnO (AZO) thin films on textured silicon substrates using DC magnetron sputtering with varying substrate heating, has yielded significant findings. The study thoroughly investigated the effects of substrate temperature (Ts) on copper adhesion strength and morphology using the peel force test and electron microscopy. The peel force test was conducted at angles of 90°, 135°, and 180°. The average adhesion strength was about 0.2 N/mm for the samples without substrate heating. For the samples with substrate heating at 100 °C, the average peeling force of the electroplated copper film was about 1 N/mm. The average peeling force increased to 1.5 N/mm as the substrate heating temperature increased to 200 °C. The surface roughness increases as the annealing temperature of the Cu/AZO/Si sample increases. These findings not only provide a reliable and robust method for applying AZO transparent conductive films onto silicon solar cells but also underscore its potential to significantly enhance the efficiency and durability of solar cells significantly, thereby instilling confidence in the field of solar cell technology.

Na2SO4-assisted shift in the nucleation mechanism of copper-electrocrystallization from an acidic bath

In this study, the effect of sodium sulphate (Na2SO4) on the nucleation mechanism of copper films during electrodeposition was investigated. Voltammetric and chronoamperometric studies were employed to analyze the kinetics of electrochemical nucleation. Phase analysis of the nucleated copper was performed via X-ray diffraction, while morphological and topographical studies were conducted using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to further validate the electrochemical findings. The deposits obtained in the presence of Na2SO4 were finer and more closely packed compared to those formed without Na2SO4. Additionally, the roughness of the deposits varied between the two conditions, with films formed in the presence of Na2SO4 being slightly smoother. The results indicate that the addition of Na2SO4 changed the nucleation mechanism from charge transfer controlled to mass transfer controlled.

An additively manufactured dual circularly polarized open‐ended waveguide antenna using outer ridges with wideband characteristic

AbstractThis paper presents a novel additively manufactured wideband dual circularly polarized (CP) open‐ended waveguide (OEW) antenna, which adopts a pair of wedge‐shaped outer ridges acting as a circular polarizer. Study found that compared to the con‐ ventional designs that employ internal polarization converting structures, such outer ridges are capable of achieving wider axial ratio bandwidth (ARBW) and better reflection coefficients. On each ridge, two rectangular slots are designed to suppress the higher order mode caused by the increased waveguide size, and further expanding the ARBW. The proposed antenna is fed by two differential ports. By switching the feeding ports, the CP states can be altered to either right‐handed CP (RHCP) or left‐handed CP (LHCP). In terms of configuration, the proposed antenna has a simple one‐piece structure and is fabricated by using dielec‐ tric 3‐D printing technology. To form the waveguide walls, its partial surfaces are coated by copper films through electroplat‐ ing. The proposed antenna features easy fabrication and light weight. Experimental results show that the proposed antenna has an impedance bandwidth of larger than 47.6% (8–13 GHz) with reflection coefficients lower than −15 dB, while the 3‐dB ARBW is 37.7% from 8.4 to 12.3 GHz.

Improving the interfacial bonding strength of a silane anticorrosion film on copper surfaces using small-molecule corrosion inhibitors

The weak binding ability of silane to copper surfaces restricts its use in copper anticorrosion. In this study, the P–OH groups in etidronic acid (HEDP) were utilized to establish chemical bonds with copper and to react with 3-mercaptopropyltrimethoxysilane (PropS-SH), serving as a bridging mechanism to enhance the anchoring of silane onto copper surfaces. Moreover, the presence of HEDP promoted the condensation of silane, leading to the formation of a denser and thicker film with outstanding corrosion resistance. SEM results indicated that the thickness of the PropS-SH/HEDP film was about 125 nm, and defects at the copper–film interface were repaired. Electrochemical tests demonstrated that the PropS-SH/HEDP film exhibited remarkable corrosion resistance, achieving an anticorrosion efficiency of 99.95 %. Immersion testing showed that the PropS-SH/HEDP film was capable of withstanding immersion in a 3.5 wt% NaCl solution for 10 days, displaying a durability 5 times greater than that of the standalone silane film.

Effect of novel non-inhibitor slurry on chemical mechanical polishing properties of copper interconnect copper film based on experiments and theoretical calculations

In order to simplify the component of copper slurry during chemical mechanical polishing (CMP) and improve the surface corrosion inhibition efficiency, anionic surfactant dodecylbenzene sulfonic acid (DBSA) and nonionic surfactant decyl glucoside (DG) as mixed surfactants were used instead of inhibitors, and the corrosion inhibition properties were thoroughly studied by experiments and theoretical calculations. Polishing experiments and surface morphology tests show that the mixed surfactants can effectively enhance the surface quality after CMP compared with traditional inhibitor TAZ. Electrochemical experiments show that DG and DBSA can form a dense passivation film on the Cu surface to prevent corrosion. The inhibition efficiency of mixed surfactants is as high as 95 %. The theoretical calculations indicate that DBSA and DG form a stable long-chain structure, which leads to a higher adsorption energy, a denser passivation film, and stronger corrosion inhibition, in agreement with the experimental findings. These studies confirmed the stronger inhibitory effect of mixed surfactant on Cu film CMP and provided a new reference for the design of simplified slurry without inhibitor components.

Effects of organic compounds rich in phosphate functional groups as multifunctional inhibitors on copper film chemical-mechanical polishing properties: Combined experiment and theoretical calculation

For copper (Cu) interconnection chemical mechanical polishing (CMP) process, the Cu film slurry follows the principles of high efficiency, environmentally friendly and low cost. In this study, hydroxyethylene diphosphonic acid (HEDP), amino-trimethylene phosphonic acid (ATMP) and ethylenediamine etramethylenephosphonic acid (EDTMP) were used as green inhibitors to study the effects of organic compounds rich in two, three and four phosphate functional groups on the corrosion inhibition properties of Cu films. The results indicated that the Cu film material removal rate (MRR) of HEDP reaches 5879 A/min and the surface roughness (Sq) is 0.85 nm, superior to the other two inhibitors. Through mechanical action analysis, electrochemical experiment, UV-Visible, X-ray photoelectron spectroscopy (XPS) test analysis and theoretical calculation, it was observed that three inhibitors could not only form stable complexes with Cu, which have the effect of corrosion inhibition, but also play a lubricating role of surfactant, and simplify the composition of slurry, and the inhibition efficiency order of them is HEDP > ATMP > EDTMP.

Effect of Superfilling and Leveling Inhibitors in Hybrid Additives on the Microstructure Evolution of Electroplated Copper Interconnect Films during Self-Annealing

Abstract As the feature size shrinks, the main challenge for copper interconnect electroplating is to minimize interconnect resistance while ensuring the void-free feature filling, which is realized by the interaction of additives during electroplating. Among additives, hybrids are considered suitable for damascene filling because they combine type-I (superfilling) and type-II (leveling) inhibitors. However, it is generally believed that the more inhibitory an additive is, the more impurities it introduces into the films, thereby hindering grain growth during self-annealing, which paradoxically limits the conductivity of copper interconnects. To balance the inhibition of hybrid additives with their ability to introduce impurities, the effect of hybrid additives on the microstructure evolution of the films during self-annealing is explored in terms of texture, grain size, and impurity density, which are considered to determine the conductivity of copper films. We find that the {111} texture increases as the percentage of type-I inhibitors increases, whose relative texture coefficient reaches 97% after self-annealing due to the adsorption of hybrid additives on non-<111>-oriented crystal planes. Additionally, the impurity content and grain size increase as type-I inhibition increases, and so does the film conductivity after self-annealing, due to the possible promotion from impurities to the grain growth.

As-deposited and dewetted Cu layers on plasma treated glass: Adhesion study and its effect on biological response

Improving the adhesion of nanosized copper films to a glass substrate is vital for their application in electronics and medicine, as it enhances their overall reliability. For this purpose, we employed Ar plasma etching (240 s) and magnetron sputtering to create copper layers on a glass substrate. Furthermore, we investigated the effect of subsequent solid state dewetting (at 300 °C) of Cu nanolayers on the interface stability. Increasing the sputtering time resulted in elevated copper concentration, UV-Vis absorption, conductivity, and surface roughness. The as-deposited and dewetted samples exhibited very good wettability with water contact angles below 60°. Importantly, plasma treatment improved the adhesion of the Cu layers to the glass. Subsequent dewetting accelerated surface diffusion and the oxidation of Cu atoms, causing structural and morphological changes. The presence of CuO after dewetting caused loss of the surface plasmon resonance (SPR) band in the UV-Vis spectrum and a decrease in sample conductivity due to the transformation of the copper layer from a metal to an oxide. Biological testing revealed a more pronounced bactericidal effect for the as-deposited Cu layer against E. coli and S. epidermidis on contrary to dewetted samples. The similar cytotoxic trend was observed for human dermal fibroblasts and hepatocytes. Nonetheless, biological testing confirmed better cell adhesion on dewetted Cu layers compared to the as-deposited ones. Therefore, our copper nanostructured samples could find application as antibacterial coatings of biomedical devices.

Low temperature Cu-Cu direct bonding in air ambient by ultrafast surface grain growth.

Fine-grain copper (Cu) films (grain size: 100.36 nm) with a near-atomic-scale surface (0.39 nm) were electroplated. Without advanced post-surface treatment, Cu-Cu direct bonding can be achieved with present-day fine-grain Cu films at 130℃ in air ambient with a minimum pressure of 1 MPa. The instantaneous growth rate on the first day is 164.29 nm d-1. Also, the average growth rate (∆R/∆t) is evaluated by the present experimental results: (i) 218.185 nm d-1 for the first-day period and (ii) 105.58 nm d-1 during the first 14-day period. Ultrafast grain growth and near-atomic-scale surface facilitate grain boundary motion across the bonding interface, which is the key to achieve Cu-Cu direct bonding at 130℃ in air ambient.

Influence of crystallite size and impurity density on the grain structure evolution of electroplated copper films during thermal and laser annealing

In this paper, the changes in microstructure and conductivity of the films before and after thermal and laser annealing are observed, which corresponds to various combinations of crystallite sizes and impurity densities. The results show that the impurities promote grain boundary migration at larger initial crystallite sizes, which contradicts previous reports. This is because the rate of grain growth is determined by the driving force and resistance to grain growth, the former provided by the energy of grain boundaries and the impurities outside the nanograin boundaries, and the latter by the impurities enriched within the boundaries. Additionally, laser annealing is more effective than thermal annealing in stimulating impurity diffusion, resulting in a reduced influence of impurities on grain growth.

Effect of thiol adsorption on the electrical resistance of copper ultrathin films

The impact of thiol adsorption on thin copper films, covered with a copper oxide layer, was investigated using electrical resistance measurements at various stages: during film growth, aging, exposure to air, and immersion in thiol solutions. Thin copper films (20 nm) were thermally evaporated, with variations in substrate temperature (RT, 330 and 390 K). Films deposited at 330 K exhibited the smallest percolation thickness and aging rates due to their compact morphology, showcasing lower surface roughness and correlation length. Exposure to air led to the formation of a Cu2O layer on the film surface. Subsequent immersion in a dodecanethiol solution in ethanol resulted in a resistance increase, ranging from 0.1 % to 0.4 %. This change was dependent on the substrate temperature, with the largest difference observed at 330 K. This observation suggests that samples grown at this temperature exhibited the highest electron-surface scattering. Moreover, by depositing a chromium surfactant layer, the impact of this scattering mechanism was amplified, leading to a resistance increase of up to 1.2 %. Mayadas-Shatzkes theory provided a good description of these resistance changes. The negatively charged S-head of the adsorbed thiols alters the electric field experienced by conduction electrons in the Cu2O/Cu interface, modifying the electron-surface scattering.

Thin film resonant metasurface absorbers using patch-based arrays on liquid crystal polymer substrates for centimeter-wave applications

This paper reports the design and development of thin-film resonant absorbers for narrowband and multiband operation in the frequency regions centered at 10 GHz. The structure of the resonant metasurface absorber (RMA) is based on a liquid crystal polymer (LCP) thin-film spacer with a copper patch array on the front surface and un-patterned copper film on the back surface of the LCP film. The design and simulation works were carried out using full-wave analysis of the RMA characteristics. The copper-based periodic patch array acts as a metasurface. The perfect RMA for a given LCP film thickness can be obtained through impedance optimization by adjustment of the dimensions of the lattice periods. The electric and magnetic field distributions were studied. The resonant film absorber based on a 100 μm thick LCP film has an electrical thickness of λ/300 at 10 GHz. The experimental work was conducted using a narrowband RMA prototype consisting of 11×11 cells. The measured result of the resonant absorption is at 10.1 GHz, which is in close agreement with the design frequency of 10 GHz. For multiband functionality, double- and quad-band film resonant absorbers have been designed based on a coplanar supercell utilizing the superposition of the resonance effect. The LCP film-based absorbers have the potential to be used in EM shielding and sensing applications in centimeter-wave applications.

Numerical and experimental investigation of 2-thiazoline-2-thiol and nitrotetrazolium blue chloride compound as levelers for through-hole copper electroplating

Leveler is essential in the electroplating process of the through-hole (TH), the adsorption characteristics of which are strongly influenced by the aspect ratios of the THs. However, only a single kind of leveler usually appears in the most frequently performed THs’ filling electrolyte, which is highly possible to limit the electrolyte's capability to fill THs and improve the copper film properties. In this paper, two levelers, 2-Thiazoline-2-Thiol (T1) and Nitrotetrazolium Blue chloride (NBT), with significant molecular weight differences, were combined to fill THs with multiple ARs. The interactions among the leveler compounds and other additives become the primary factor controlling the THs electroplating process, which was systematically investigated through a combination of flow field numerical simulation and electrochemical measurements. T1 presents a weaker inhibition effect and is more sensitive to convection than NBT, which also exhibits a convection-enhanced inhibition effect with other additives. The convection of the electrolyte in the THs center continuously decreases with the ARs, guiding to the increasing electroplating rate at the THs center, high throwing power (TP) of 141%, and flawless filling of the THs with ARs of 4.75:1∼1.9:1 in 2.5 h. The copper film on the surface maintains extremely high flatness and low thickness.

Room temperature compressed air-stable conductive copper films for flexible electronics

The state-of-the-art technology of fabricating printed copper electronics is focussed largely on thermal sintering restricting transition towards heat sensitive flexible substrates. Herein we report a pioneering technology which eliminates the need for conventional sintering. Biopolymer-stabilised copper particles are prepared such that they can be compressed at room temperature to generate air-stable films with very low resistivities (2.05 – 2.33 × 10−8 Ω m at 20 °C). A linear positive correlation of resistivity with temperature verifies excellent metallic character and electron microscopy confirms the formation of films with low porosity (< 4.6%). An aqueous ink formulation is used to fabricate conductive patterns on filter paper, first using a fountain/dip pen and then printing to deposit more defined patterns (R < 2 Ω). The remarkable conductivity and stability of the films, coupled with the sustainability of the approach could precipitate a paradigm-shift in the use of copper inks for printable electronics.

The Design and Experimentation of a Differential Grain Moisture Detection Device for a Combined Harvester.

To conveniently implement the online detection of grain moisture in combined harvesters and the address the influence of the no-load measurement baseline, thereby enhancing detection accuracy and measurement continuity, this study developed a differential grain moisture detection device. For its convenient installation and integration on combined harvesters, a single-pole plate measurement element with a 1.6 mm thick epoxy resin coated with a 2-ounce copper film was designed, and a grain moisture detection device was constructed based on the STM32F103 microprocessor (STMicroelectronics International NV, Geneva, Switzerland). To enhance the device's interference resistance, a differential amplification measurement circuit integrated with high-frequency excitation was designed using a reference capacitance. To improve the resolution of the measurement circuit, Malab simulations were conducted at different excitation frequencies, ultimately selecting 30 kHz as the system's excitation signal frequency. To validate the effectiveness of the measurement circuit, validity tests were performed on the constructed sensor, which showed that the sensor's measurement voltage could effectively distinguish the moisture levels in grains, with a determination coefficient (R²) reaching 0.9978. To address the errors in moisture measurement caused by changes in grain temperature, an interaction experiment of the effect of moisture content and temperature on the measurement voltage was conducted using an integrated temperature sensor, resulting in the construction of a moisture content calculation model. Both the indoor static detection and field testing of the moisture detection device were conducted, indicating that the maximum average error in static measurements was 0.3%, with a maximum relative error of 0.47%, and the average relative error in field tests was ≤0.4%.