Study on the Effects of Copper Oxide Growth on the Peel Strength of Copper/Polyimide

In this study, various copper oxides [CuO, Cu₂O, and Cu₂O + polybenzimidazole (PBI)] were studied as alternative adhesion layers. Specimens were aged under harsh conditions (300°C, 5% O₂ or humid condition), and then peel tests were conducted to investigate the reliability of Cu oxides/polyimide (PI). The peel strength of the bare Cu, CuO, and Cu₂O specimens dropped substantially, close to nil, due to void formation after 2 h of aging at 300°C. The degree of void formation near the Cu₂O/Cu interface showed a clear inverse relationship with the peel strength, suggesting that the formation of voids beneath the Cu₂O layer was directly responsible for the peel strength degradation. Void growth was controlled by Cu₂O layer growth, while voids originated from the difference between the diffusion rate of Cu atoms through the Cu₂O layer and Cu layers. Humidity tests did not lower the peel strength significantly in any of the specimens, none of which showed voids that were detrimental to the peel strength, in contrast to the results of the aging treatments at 300°C.     

Ultrasensitive Detection of VOCs Using a High‐Resolution CuO/Cu2O/Ag Nanopattern Sensor

The p‐type semiconducting copper oxides (CuO and Cu₂O) are promising materials for gas sensors, owing to their characteristic oxygen adsorption properties and low operation temperature. In this study, the sensing performance of a CuO‐based chemiresistor is significantly enhanced by incorporating Ag nanoparticles on high‐resolution p‐type CuO/Cu₂O nanopattern channels. The high‐resolution CuO/Cu₂O/Ag nanochannel is fabricated using a unique top‐down nanolithographic approach. The gas response (ΔR/R_a) of the CuO/Cu₂O/Ag gas sensor increases by a maximum factor of 7.3 for various volatile organic compounds compared with a pristine CuO/Cu₂O gas sensor. The sensors exhibit remarkable sensitivity (ΔR/R_a = 8.04) at 125 parts per billion (ppb) for acetone analytes. As far as it is known, this is the highest sensitivity achieved for p‐type metal oxide semiconductor (MOS)‐based gas sensors compared to previous studies. Furthermore, the outstanding gas responses observed in this study are superior to the most of n‐type MOS‐based gas sensors. The high sensitivity of the sensor is attributed to i) the high resolution (≈30 nm), high aspect ratio (≈12), and ultrasmall grain boundaries (≈10 nm) of the CuO/Cu₂O nanopatterns and ii) the electronic sensitization and chemical sensitization effects induced by incorporating Ag nanoparticles on the CuO/Cu₂O channels.     

Enhanced Wettability of Oxidized Copper with Lead-Free Solder by Ar-H<Subscript>2</Subscript> Plasmas for Flip-Chip Bumping

Enhanced solder wettability (SW) of oxidized-Cu (OC) with 96.5Sn-3Ag-0.5Cu lead-free solder (LFS) by Ar-H₂ plasmas was investigated. The SW of OC was significantly improved from 0% wetting of Cu oxidized in air at 260°C for 1 h to 100% wetting of OC modified by Ar-H₂ plasmas for 10 min. The SW of Cu was found to be highly dependent on the surface characteristics of Cu. By decreasing the total surface energy (TSE), decreasing the polar surface energy (PSE), and increasing the dispersive surface energy (DSE) on the surfaces of OC modified by Ar-H₂ plasmas, the SW with LFS improves. X-ray photoelectron spectroscopy (XPS) indicates that Ar-H₂ plasma treatment is used to remove the copper oxides CuO and Cu₂O from the OC surfaces. The ratio of the total amount of Cu₂O to CuO was found to be a good indication of how the copper oxides CuO and Cu₂O affect the PSE, DSE, and SW of Cu.     

Adhesion strength of leadframe/EMC interfaces

Cu-based leadframe sheets were oxidized in alkaline solutions to produce brown and/or black oxide on the surfaces, and molded with epoxy molding compound (EMC). The adhesion strength of leadframe/EMC interface was measured using sandwiched double-cantilever beam (SDCB) specimens and pull-out specimens. Results showed that the adhesion strength of leadframe/EMC interface was inherently very poor but could be increased drastically with the nucleation of acicular CuO precipitates. The presence of smooth-faceted Cu₂O on the surface of the leadframe gave close to zero fracture toughness (G_C) and suitable pull strength (PS). A direct correlation between G_C and PS showed that PS can be a measure of G_C only in a limited range.     

Effects of molarity on structural,optical, morphological and CO2 gas sensing properties of nanostructured copper oxide films deposited by spray pyrolysis

In this paper, we have exposed the effects of molarity on structural, optical, morphological and gas sensing properties of copper oxide films deposited by pneumatic spray pyrolysis method. The molar concentration was varied from 0.05 to 0.3 M. X-ray diffractograms showed the formation of a single phase CuO for films prepared with 0.05 and 0.1 M concentrations. A secondary phase Cu₂O was obtained for 0.2 and 0.3 M concentrations. Optical measurements showed that 0.05 M concentration provides a film with the best transparency in the visible and near infrared regions. The thickness values were between 2 and 110 µm. Moreover, the contact angle measurements have shown that all the deposited films are hydrophobic with angles between 103° and 121°. The morphological properties were investigated using SEM and AFM. According to SEM and AFM micrographs, 0.05 M is the concentration that leads to porous structure. The gas sensing measurements confirm that this porous surface structure is the most sensitive to different CO₂ concentrations.     

Effects of post-annealing by the rapid thermal process on the characteristics of MOCVD-Cu/TiN/Si structures

Effects of rapid thermal annealing on the characteristics of Cu films deposited from the (hfac)Cu(VTMS) precursor and on the barrier properties of TiN layers were studied. By the post-annealing, the electrical characteristics of Cu/TiN and the microstructures of Cu films were significantly changed. The properties of Cu films were more sensitive to the annealing temperature than the annealing time. Sheet resistances were decreased in 400–450°C ranges, and abrupt increases were observed above 750°C. It was also found that the copper films showed pronounced grain growth with the (111) preferred orientation. The grain growth and condensation of copper were observed below 500°C without formation of the CuO and Cu₂O phase resulting in surface degradation. Above 500°C, the oxide compound of copper was partially formed on the surface and the inter-reaction on the Cu-TiN interface was started. The inter-reaction of Cu-Ti and Cu-Si interface vigorously occurred and the surface roughness was continuously deteriorated above 650°C. It revealed that the optimum annealing conditions for MOCVD-Cu/PVD-TiN structures to enhance the electrical characteristics without degradation of TiN barriers were in the range of 400°C.     

Fabrication of Source/Drain Electrodes for a-Si:H Thin-Film Transistors Using a Single Cu Alloy Target

A Cu alloy/Cu alloy oxide bilayer structure was formed on an n ⁺-a-Si:H substrate using a single Cu alloy target. It was employed for the source/drain electrodes in the fabrication of a-Si:H thin-film transistors with good electrical performance, high thermal stability, and good adhesion. Transmission electron microscopy and electron energy-loss spectroscopy analyses revealed that the initial sputtering of the Cu alloy in O₂/Ar allowed for preferential oxidation of Si and the formation of a SiO _x /Cu-supersaturated a-Si:H bilayer at the copper oxide–a-Si:H interface. This bilayer turned into an SiO _x /Cu₃Si bilayer after annealing at 300°C. It provided a stable contact structure with low contact resistance.     

Nanoscale Cu<Subscript>2</Subscript>O films: Radio-frequency magnetron sputtering and structural and optical studies

Nanoscale copper (I) oxide layers are formed by magnetron-assisted sputtering onto glassy and silicon substrates in an oxygen-free environment at room temperature, and the structural and optical properties of the layers are studied. It is shown that copper oxide formed on a silicon substrate exhibits a lower degree of disorder than that formed on a glassy substrate, which is supported by the observation of a higher intensity and a smaller half-width of reflections in the diffraction pattern. The highest intensity of reflections in the diffraction pattern is observed for Cu₂O films grown on silicon at a magnetron power of 150 W. The absorption and transmittance spectra of these Cu₂O films are in agreement with the well-known spectra of bulk crystals. In the Raman spectra of the films, phonons inherent in the crystal lattice of cubic Cu₂O crystals are identified.     

A review of recent advances in transparent p-type Cu2O-based thin film transistors

One of the crucial challenges that face the wide-spread implementation of flexible and transparent electronics is the lack of high performance p-type semiconductor material. Cu₂O in thin-film form is a potentially attractive material for such applications because of its native p-type semi-conductivity, transparency, abundant availability, non-toxic nature, and low production cost. This review summarizes recent research on using copper oxide Cu₂O thin films to produce p-type transparent thin-film transistors (TFTs) and complementary metal–oxide–semiconductor (CMOS) devices. After a short introduction about the main advantages of Cu₂O semiconductor material, different methods for depositing and growing Cu₂O thin films are discussed. The hi-tech development, along with the associated obstacles, of the Cu₂O-based thin-film transistors is reviewed, with special emphasis on those made of sputtered Cu₂O films. Finally, the bilayer scheme as one of the most exciting and promising technique for both TFTs and CMOS devices will be considered.     

Formation of Cu<Subscript>2</Subscript>O and ZnO Crystal Layers by Magnetron Assisted Sputtering and Their Optical Characterization

Copper (I) oxide and zinc oxide films are formed on silicon and glassy quartz substrates by magnetron assisted sputtering. The thickness of the films is tens and hundreds of nanometers. The films are grown at different substrate temperatures and different oxygen pressures in the working chamber. The film samples are studied by the X-ray diffraction technique, scanning electron microscopy, and optical methods. It is established that an increase in the substrate temperature yields a change in the surface morphology of copper (I) oxide films towards the formation of well-pronounced crystallites. The reflectance and Raman spectra suggest that the quality of such films is close to that of bulk Cu₂O crystals produced by the oxidation of copper. As concerns ZnO films, an increase in the substrate temperature and an increase in the partial oxygen pressure make it possible to produce films, for which a sharp exciton structure is observed in the reflectance spectra and the emission of excitons bound at donors is observed in the luminescence spectra.     

Properties of Mn-doped Cu2O semiconducting thin films grown by pulsed-laser deposition

Semiconducting oxides offer the potential for exploring and understanding spin-based functionality in a semiconducting host material. Theoretical predictions suggest that carrier-mediated ferromagnetism should be favored for p-type material. Cu₂O is a p-type, direct wide bandgap oxide semiconductor that may hold interest in exploring spin behavior. In this paper, the properties of Mn-doped Cu₂O are described. Activities focused on understanding Mn incorporation during thin-film synthesis, as well as magnetic characterization. The epitaxial films were grown by pulsed-laser deposition. X-ray diffraction was used to determine film crystallinity and to address the formation of secondary phases. SQUID magnetometry was employed to characterize the magnetic properties. Ferromagnetism is observed in selected Mn-doped Cu₂O films, but appears to be associated with Mn₃O₄ secondary phases. In phase-pure Mn-doped Cu₂O films, no evidence for ferromagnetism is observed.     

LaNiO3 and Cu3Ge contacts to YBa2Cu3O7-x films

Metallization of high-T_c superconductors using low resistivity metal oxides and Cu-Ge alloys has been investigated on high quality pulsed laser deposited epitaxial YBa₂Cu₃O_7-x (YBCO) films. Epitaxial LaNiO₃ (LNO) thin films have been grown on YBCO films at 700°C using pulsed laser deposition. The specific resistivity of LNO was measured to be 50 μΩ-cm at 300K which decreases to 19 μΩ-cm at 100K indicating good metallicity of the LNO films. The contact resistance of LNO-YBCO thin film interface was found to be reasonably low (of the order of 10^-4Ω-cm² at 77K) which suggests that the interface formed between the two films is quite clean and LNO can emerge as a promising metal electrode-material to YBCO films. A preliminary investigation related to the compatibility of Cu₃Ge alloy as a contact metallization material to YBCO films is discussed. The usage of other oxide based low resistivity materials such as SrRuO₃ (SRO) and SrVO₃ (SVO) for metallization of high-T_c YBCO superconductor films is also discussed.     

Low‐Temperature Processable High‐Performance Electrochemically Deposited p‐Type Cuprous Oxides Achieved by Incorporating a Small Amount of Antimony

The development of an electrochemically robust method for the low‐temperature deposition of cuprous oxide (Cu₂O) thin films with reliable and conductive p‐type characteristics could yield breakthroughs in earth abundant and ecofriendly all oxide‐based photoelectronic devices. The incorporation of the group‐V element antimony (Sb) in the solution‐based electrodeposition process has been investigated. A small amount of Sb (1.2 at%) in the Cu₂O resulted in rapid nucleation and coalescence at the initial stage of electrochemical reaction, and finally made the surface morphology smooth in 2D. The growth behavior changed due to Sb addition and produced a strong diffraction intensity, single‐domain‐like diffraction patterns, and low angle tilt boundaries in the Cu₂O:Sb film, implying extremely improved crystallinity. As a result, these films exhibited extraordinary optical transmittance and band‐to‐band photoluminescence emission as well as higher electrical conductivity. The Cu/Cu₂O:Sb Schottky diode showed good rectifying characteristics and more sensible photoresponsibility.     

Photovoltaic response of Cu2O/In2S3 hetero-structure grown on Cu substrate

A thin layer of p-type Cu₂O was grown over flexible 30 μm thick copper substrates. Using Injection Chemical Vapor Deposition technique, n-type In₂S₃ thin films were grown over the Cu₂O layer. A p–n junction was thus realized. The Cu₂O/In₂S₃ hetero-structure showed photovoltaic behavior. A solar cell with the structure Cu/Cu₂O/In₂S₃/Ag could be fabricated. An acidic texturization sequence was developed which increased the photo-sensitivity of the In₂S₃ window layer. The Cu/Cu₂O/In₂S₃/Ag hetero-structure with the textured window layer had an open circuit voltage of 377 mV, short circuit current density of 0.118 mA/cm² and fill factor of 33.34%. It was found that the efficiency of the solar cell depended upon the photo-sensitivity of the In₂S₃ window layer. The work demonstrates the use of copper substrate for thin film solar cell fabrication.     

Structural and Passivative Behaviors of Cu(In) Thin Film

This investigation prepares a low-resistivity and self-passivated Cu(In) thin film. The dissociation behaviors of dilute Cu-alloy thin films, containing 1.5–5at.%In, were prepared on glass substrates by a cosputter deposition, and were subsequently annealed in the temperature range of 200–600 °C for 10–30 min. Thus, self-passivated Cu thin films in the form In₂O₃/Cu/SiO₂ were obtained by annealing Cu(In) alloy films at an elevated temperature. Structural analysis indicated that only strong copper diffraction peaks were detected from the as-deposited film, and an In₂O₃ phase was formed on the surface of the film by annealing the film at an elevated temperature under oxygen ambient. The formation of In₂O₃/Cu/SiO₂ improved the resistivity, adhesion to SiO₂, and passivative capability of the studied film. A dramatic reduction in the resistivity of the film occurred at 500 °C, and was considered to be associated with preferential indium segregation during annealing, yielding a low resistivity below 2.92 μΩcm. The results of this study can be potentially exploited in the application of thin-film transistor–liquid crystal display gate electrodes and copper metallization in integrated circuits.     

Fabrication and Microstructures of Sequentially Electroplated Sn-Rich Au-Sn Alloy Solders

Three Sn-rich, Au-Sn alloy solders with eutectic, hypoeutectic, and hypereutectic Sn compositions were fabricated by sequential electroplating of Au and Sn and then the dual-layer films were reflowed at 250°C. The microstructures and phase compositions of the deposited Au/Sn dual-layer film and the reflowed Sn-rich Au-Sn alloys were studied. Microhardness values of the different phases or phase zones for the reflowed alloys were also tested. Finally, two Si wafers were bonded together with the eutectic Sn-rich Au-Sn alloy solder. For as deposited Au/Sn dual-layer films, reaction between Au and Sn occurs at room temperature leading to the formation of Au₅Sn, AuSn, and AuSn₂ at the Au/Sn interface. After reflowing at 250°C, two phases remain, Sn and AuSn₄, with the morphology and phase distribution depending on the original solder composition. In the Sn-rich, eutectic Au-Sn alloy, AuSn₄ particles are distributed uniformly in the Sn matrix. In the Sn-rich hypoeutectic/hypereutectic Au-Sn alloys, the proeutectic phase, AuSn₄ (Vickers hardness, Hv 125) or Sn (Hv 14.2), is larger in size and is surrounded by the eutectic zone (Sn + AuSn₄) (Hv 16.1). In all cases, the TiW adhesion and barrier layer remains intact during annealing. After reflowing at 250°C under a pressure of 13 kPa, two Si wafers are joined by the Sn-rich eutectic Au-Sn alloy solder, without crack or void formation at the Si wafer/solder interface or within the solder.     

Investigation of the blue–green emission and UV photosensitivity of Cu-doped ZnO films

Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (100) substrates using radio-frequency reactive magnetron sputtering. The structure and optical properties of the films were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and fluorescence spectroscopy. XRD and SEM results revealed that ZnO:Cu film had a better preferential orientation along the c-axis compared with pure ZnO film. The chemical state of copper and oxygen in ZnO:Cu films was investigated by XPS. The results suggest that the Cu ion has a mixed univalent and bivalent state. The integrated Cu²⁺/Cu⁺ intensity ratio increased with the O₂ partial pressure. Photoluminescence measurements at room temperature revealed a double peak in the blue regions and a green emission peak. The close relationship between the valence state of Cu ions and the blue–green emission is discussed in detail. A higher photocurrent was observed for ZnO:Cu films under UV illumination. UV photodetectors based on ZnO:Cu films have high sensitivity and fast response and recovery times. Under periodic UV illumination at 380 nm the ZnO:Cu films showed stable photocurrent growth and decay, so the films are potential candidate materials for UV photodetectors.     

Solution flow rate influence on properties of copper oxide thin films deposited by ultrasonic spray pyrolysis

The present work is an investigation of the solution flow rate influence on copper oxide (CuO) thin film properties deposited by ultrasonic spray pyrolysis. A set of CuO thin films were deposited, with various solution flow rates, on glass substrate at 300 °C. The precursor solution is formed with copper salt dissolution in distilled water with 0.05 molarity. The solution flow rate was ranged from 10 to 30 ml/h. Films composition and structure were characterized by means of XRD (X Rays diffraction) and Raman scattering. The optical properties were studied using UV–visible spectroscopy. The electrical conductivity, carrier mobility and concentration were determined by Hall Effect measurements. The obtained results indicate that flow rate is a key parameter controlling CuO films growth mechanism and their physical properties. The prepared films are mainly composed with a CuO monophase, the crystallite size is reduced with increasing the flow rate. A ZnO/CuO heterojunction structure has been realized and its rectifying behavior is tested.     

<Emphasis Type="Italic">In Situ</Emphasis> Study of Reduction Process of CuO Paste and Its Effect on Bondability of Cu-to-Cu Joints

A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu₂O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.     

Dissolution and electrochemical impedance spectroscopy studies of thin copper oxide films on copper in semi-aqueous fluoride solutions

The selective dissolution of thin copper oxide films grown on copper in semi-aqueous formulations containing dimethyl sulfoxide (DMSO), ammonium fluoride (NH₄F) and water was studied. Optimization of the formulations was carried out by systematic evaluation of the effect of solvent content and pH on the removal rates of copper oxide films and selectivity towards copper and carbon doped oxide (CDO) low k dielectric film. Copper oxide removal rate of ∼180 Å/min with a selectivity of ∼130:1 towards copper and ∼20:1 selectivity towards CDO was obtained in a formulation containing 29% DMSO, 1% NH₄F and 70% H₂O at pH 4. Electrochemical impedance spectroscopy studies were performed on this system and the data were analyzed to characterize the copper oxide/electrolyte interface with the ultimate objective of developing an end point detection technique for copper oxide removal.