Copper Thin Films Research Articles

Functional organic adlayers for controlling the adhesion strength of electrolessly deposited copper on super-engineering plastics

Effective metallization of super-engineering plastics such as liquid crystal polymers (LCP) and polyphenylene sulfides (PPS) via electroless deposition of copper thin films is an important step in the manufacturing of a wide range of devices. Copper deposition typically requires a pre-treatment step involving chemical and/or mechanical roughening to ensure high adhesion at the polymer/Cu interface; however such treatments are detrimental to patterns and topographic features of the polymer that possess µm resolution and/or high aspect ratio. Herein, we demonstrate that it is possible to regulate and improve adhesion of electrolessly deposited copper thin films at LCP and PPS materials via multilayer functionalization with aryldiazonium cations of a p-aminobenzoic acid precursor. We first demonstrate that aryldiazonium grafting conditions can be optimized to overcome the chemical inertness of LCP/PPS without resorting to harsh oxidative or mechanical treatments. We then show that the density of Ar-COOH groups can be regulated by modifying the number of functionalization cycles. X-ray photoelectron spectroscopy studies shows that this has a clear impact on the composition of the catalytic nanoparticle layer required for copper deposition. Adhesion strength studies demonstrate that such changes translate into improved adhesion strengths without any adverse effects on surface roughness. Control experiments with alternative chemical moieties suggest that specific chemical interactions between catalytic seeds and grafted carboxylate groups play an important role in the observed improved adhesion.

Effects of grain size, dislocation density, and texture type on the etching behavior of Cu target in magnetron sputtering process

Abstract Copper thin film is commonly applied in the microelectronics and battery industries, and environmentally friendly magnetron sputtering process is one of the main coating processes. The effects of grain size, dislocation density, and texture type of Cu target on sputtering efficiency were researched in this work. High-purity copper cast ingots were rolled into plates. Pieces were then cut from different surfaces of the plates and assembled into the targets named as Target 1, Target 2, and Target 3 for magnetron sputtering. To investigate the etching behavior of the three targets, EBSD technology was utilized for analysis, and the micro-morphologies of the etching grooves were observed using SEM. After the sputtering process, the weight loss rate and etching depth of the various targets were measured to compare the respective sputtering efficiency. Target 1 exhibited the highest sputtering weight loss rate at any time period compared to Target 2 and Target 3, and reached its maximum depth after every 15 minutes of sputtering. In the first 45 minutes, the etching depth of Target 2 was slightly greater than that of Target 3. While the etching depth of Target 3 increased significantly in the last 15 minutes compared to that of Target 2. The most significant factor influencing the sputtering efficiency of the target material was grain size, with finer grains leading to higher sputtering efficiency. The dislocation density had a greater impact in the first 30 minutes, but the texture type became more important as the sputtering process continued.

Plasmonic properties of silicon nitride encapsulated copper

We investigated the optical properties (i.e. plasmonic loss) of thin-film copper that was deposited and processed in a 300mm CMOS pilot line. The optical properties at 1550 nm (QSPP,Cu = 3921 ± 350) were evaluated by measuring the propagation loss of dielectric loaded plasmonic waveguides on samples with high and low root mean square surface roughness (Rq) with and without a silicon nitride diffusion barrier and at room temperature and cryogenic temperatures. In our fabrication result, the average grain size is 2.08±0.05μm, which is much larger than the mean free path of free electrons in copper, so the surface roughness becomes the main cause of the waveguide propagation loss from the fabrication constrains. Further, we show that copper can be encapsulated by a 15 nm silicon nitride diffusion and oxidation barrier without degrading the optical properties.

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.

Size Effects of Copper(I) Oxide Nanospheres on Their Morphology on Copper Thin Films under Near-Infrared Femtosecond Laser Irradiation.

The femtosecond laser direct writing of metals has gained significant attention for micro/nanostructuring. Copper (I) oxide nanospheres (NSs), a promising material for multi-photon metallization, can be reduced to copper (Cu) and sintered through near-infrared femtosecond laser pulse irradiation. In this study, we investigated the size effect of copper (I) oxide nanospheres on their morphology when coated on Cu thin films and irradiated by near-infrared femtosecond laser pulses. Three Cu2O NS inks were prepared, consisting of small (φ100 nm), large (φ200 nm), and a mixture of φ100 nm and φ200 nm NSs. A unique phenomenon was observed at low laser pulse energy: both sizes of NSs bonded as single layers when the mixed NSs were used. At higher pulse energies, the small NSs melted readily compared to the large NSs. In comparisons between the large and mixed NSs, some large NSs remained intact, suggesting that the morphology of the NSs can be controlled by varying the concentration of different-sized NSs. Considering the simulation results indicating that the electromagnetic fields between large and small NSs are nearly identical, this differential morphology is likely attributed to the differences in the heat capacity of the NSs.

Investigation of the impact of environmental conditions on the mechanical properties of thin-film copper wafers

Thin-film copper offers excellent film texture for multilevel interconnections in integrated circuit fabrication due to its superior resistance to electromigration and high electrical conductivity. To perform a chemical mechanical planarization process during semiconductor fabrication of copper, it is necessary to have a thorough understanding of the nanomechanical properties of thin-film copper. In this study, thin-film copper and reacted passivation layers on silicon substrate wafers are investigated for their nanomechanical properties under various environmental conditions. The results of this study indicate that thin-film copper passivation layers have different properties in deionized (DI) water and polishing slurry environments compared to thin-film copper exposed to ambient air. Interestingly, variations in temperature within wet environments do not significantly affect the properties of thin-film copper wafers; but changes in properties are largely driven by chemical processes. The insights gained from this study emphasize the significance of considering both the passivation layers and wet environments in semiconductor fabrication processes, which contributes to the advancement of copper-based interconnect materials and optimization of the chemical mechanical planarization process in semiconductor manufacturing.

Effect of Thickness and Sidewall Slope of Photoresist Mask on Etch Profile of Copper Interconnect

We performed high density plasma reactive ion etching of copper thin films using organic gas mixture to define fine patterns less than 10 μm for the application of display devices. However, since redeposition in dry etching of copper thin films frequently occurs due to extremely low reactivity, the etch variables which affect the formation of redeposition were investigated. In this study, the photoresist (PR) masks were employed to etch copper thin films. The effects of the thickness of PR and the sidewall slope of PR on etch profile and redeposition were examined.The thickness of PR mask were in the range of 3 μm ~ 1 μm and the sidewall slopes of the PR mask were in the range of 90° ~ 45°. The etch profile and etch mechanism of dry etching using different masks were evaluated using field emission scanning electron microscopy and X-ray photoelectron spectroscopy. Finally, an etching method to reduce and/or avoid the redeposition in dry etching of copper thin films will be proposed.Acknowledgement : This research was supported by the MOTIE(Ministry of Trade, Industry & Energy (20019504) and KSRC(Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.

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.

Correlation between Crystallite Characteristics and the Properties of Copper Thin Film Deposited by Magnetron Sputtering: Bias Voltage Effect

This work investigates the properties of copper thin films deposited by magnetron sputtering. The substrate is biased by a negative voltage (Vs), which controls the energy ions bombardment during the deposition of the thin films. In order to focus solely on the ions energy contribution, the power supply was fixed and the working pressure was selected at 5 Pa. This ensures energetic sputtered particles completely thermalized, by a sufficient number of collisions with the Argon gas. X-ray diffraction analysis revealed that substrate voltage Vs affects essentially the structure and size of the formed crystallites. The preferred orientation (111) and the larger crystallite size (30 nm) were achieved at Vs = - 60 V. The Cu (111)/(200) peak intensity ratio is maximal (12.55) at - 60 V, corresponding to the lowest resistivity value (6.33 mW.cm). Optimum corrosion resistance of the deposited thin film was achieved at -60 V. At high crystallite sizes, nanoindentation analysis showed a thin film that is more elastic (133 GPa) and less hard (1.96 GPa).

Characterization of GLAD-grown TiCu thin films for thermo-resistive sensing applications

Titanium-copper thin films were prepared by Glancing Angle Deposition (GLAD) to assess their suitability for temperature sensors, by measuring the temperature resistance coefficient (TCR). The films were deposited with zigzag and spiral architectures, while the substrate holder was maintained at a fixed angle of α = 20º relative to the incident flux of the sputtered particles. The films were produced through DC co-deposition magnetron sputtering, using two targets of pure Ti and Cu. A wide range of compositions was achieved by varying the current on the Cu target from 6 mA up to 20 mA. The obtained architectures were stabilized through in-vacuum annealing treatments to minimize the hysteresis effects of the temperature on the electrical resistance of the films. The sheet resistance showed a direct correlation with the formation/precipitation of the Ti-Cu intermetallic phases in the film. The measured Temperature Coefficient of Resistance (TCR) values ranged from −1.08×10−3 to - 5.1×10−3 °C−1, closely resembling the absolute value of platinum's TCR (3.93×10−3 °C−1). Moreover, the elimination of hysteresis from the TCR plot and consistent results obtained during multiple cycles of heating and cooling highlight the potential of titanium copper thin films as promising alternatives for temperature sensors.

Improved mechanical and fire-retardant properties of fiber-reinforced composites manufactured via modified resins and metallic thin films

Fiber-reinforced polymeric composites have been extensively used in different industrial applications because of their excellent mechanical and other properties but have lower tolerance levels for fire and lightning damage. The thermal, mechanical, and electrical conductivity of these composites can be substantially increased using some thin metallic films for higher fire resistance. The objective of this study was to develop fire-retardant fiber-reinforced composites using modified resins and metallic copper (Cu) thin films and test and characterize the mechanical and thermal properties of these prepared composites. Standard hand wet layup process was used to manufacture composite panels, and then the flame retardant and other physical and chemical properties were determined before and after resin modification and surface metal film coatings. These modified resins and the conductive metallic films of the composite provided superior flame retardancy and higher mechanical strength. The prepared composite panels made from modified epoxy via 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO) inclusion and with metallic surface coatings passed the UL-94 vertical flame testing with a V-0 rating. This composite achieved an average flexural strength of 344.2 MPa, a mean tensile strength of 400.82 MPa, and a shear strength of 6.54 MPa for single lap shear joint studies. Fractography results also show better bonding of the matrix and fiber with no significant damage. This study may open new opportunities in various composite industries.

Correlation between morphology and hardness of electrolytically produced copper thin films

Correlation between morphology and hardness of electrolytically produced copper thin films

64‐4: ECR Plasma Source for Copper Thin Film Dry Etching

Dry etching process of thin copper films using high electron temperature ECR (electron cyclotron resonance) plasma source is developed. With ECR source and RIE (reactive ion etching) mode, etching rate of 175 nm/min is achieved. Dry etching is performed under high electron temperature plasma source with low temperature substrate and employing a reactive ion etching mode. To compensate the large area etching uniformity, scanning low temperature susceptor is adopted. Rectangular‐type microwave slot antenna (ReSLAN) is used to generate ECR plasma.

Thickness dependence of the room-temperature ethanol sensor properties of Cu2O polycrystalline films

This study investigates the fabrication process of copper thin films via thermal evaporation, with precise control over film thickness achieved through Z-position adjustment. Analysis of the as-fabricated copper films reveals a discernible relationship between grain size (〈D〉) and Z-position, characterized by a phenomenological equation 〈D〉XRDn(Z)=〈D〉0n1+32rZ2+158rZ4 , which is further supported by a growth exponent (n) of 0.41 obtained from the analysis. This value aligns well with findings in the literature concerning the growth of copper films, thus underlining the validity and reliability of our experimental outcomes. The resulting crystallites, ranging in size from 20 to 26 nm, exhibit a resistivity within the range of 3.3–4.6 μΩ · cm. Upon thermal annealing at 200 °C, cuprite Cu2O thin films are produced, demonstrating crystallite sizes ranging from ∼9 to ∼24 nm with increasing film thickness. The observed monotonic reduction in Cu2O crystallites relative to film thickness is attributed to a recrystallization process, indicating amorphization when oxygen atoms are introduced, followed by the nucleation and growth of newly formed copper oxide phase. Changes in the optical bandgap of the Cu2O films, ranging from 2.31 to 2.07 eV, are attributed mainly to the quantum confinement effect, particularly important in Cu2O with size close than the Bohr exciton diameter (5 nm) of the Cu2O. Additionally, correlations between refractive index and extinction coefficient with film thickness are observed, notably a linear relationship between refractive index and charge carrier density. Electrical measurements confirm the presence of a p-type semiconductor with carrier concentrations of ∼1014 cm−3, showing a slight decrease with film thickness. This phenomenon is likely attributed to escalating film roughness, which introduces supplementary scattering mechanisms for charge carriers, leading to a resistivity increase, especially as the roughness approaches or surpasses the mean free path of charge carriers (8.61 nm). Moreover, ab-initio calculations on the Cu2O crystalline phase to investigate the impact of hydrostatic strain on its electronic and optical properties was conducted. We believe that our findings provide crucial insights that support the elucidation of the experimental results. Notably, thinner cuprite films exhibit heightened sensitivity to ethanol gas at room temperature, indicating potential for highly responsive gas sensors, particularly for ethanol breath testing, with significant implications for portable device applications.

Experimental study of taper shape bistable beam for improved vibration energy harvesting

Unwanted vibrations cause discomfort and affect the accuracy of the machinery. Vibrations of low frequency (<30 Hz) are usually inutile. Various researchers focus on harvesting and enhancing energy output through low-frequency vibrations. This paper focuses on improving energy harvesting from low-frequency vibration sources through taper shape (TAP) bistable beam fabricated using thin Piezoelectric Polyvinylidene fluoride (PVDF) film sandwiched between thin copper and aluminium films on both sides. Voltage power output through a PVDF film depends on the film’s strain distribution; hence, in this study, five different beam designs are studied for strain distribution through the ANSYS workbench. Further, an experimental harmonic analysis study for voltage output is performed on rectangular beam (RECT) and TAP beam for both straight and bistable configuration using three different harmonic input conditions, which concluded that the voltage output for the TAP beam is much more than that of the RECT beam, with a much more significant voltage output increase in bistable conditions for all three harmonic input conditions.

Influence of annealing temperature on nano crystalline description for CuZnS thin films

Copper Zinc Sulphide (Cu0.5Zn0.5S) alloy and thin films were fabricated in a vacuum. Nano crystallized (CZS) film with thick 450±20 nm was deposit at substrates glasses using thermal evaporation technique below ~ 2 × 10− 5 mbar vacuum to investigated the films structural, morphological and optical properties depended on annealing temperatures ( as-deposited, 423, 523 and 623) K for one hour. The influences annealed temperature on structurally besides morphologically characteristics on these films were investigated using XRD and AFM respectively. XRD confirms the formation a mixed hexagonal phase of CuS-ZnS in (102) direction with polycrystalline in nature having very fine crystallites size varying from (5.5-13.09) nm. AFM analysis shows the uniform distribution of closely packed grains, grain size for that film diverge on ranges as of (52.37 to 89.25) nm after annealed. The optical properties of all films prepared had been examined for the wavelength range 400 - 1000 nm using UV-Vis-NIR spectrometer. The band gaps of (Cu0.5Zn0.5S) films are obtained in the range of 2.4 to 1.9 eV, which makes it a suitable absorber as well as buffer/window layer for solar cell applications.

In-situ fabrication of CuO/ZnO heterojunctions at room temperature for a self-powered UV sensor

This work presents a novel, low-cost, wet chemical fabrication technique to develop a self-powered photonic sensor. Thin films of zinc and copper were DC-sputtered on an ITO-coated glass slide, followed by an in-situ one-step oxidation of copper and zinc to form CuO (copper oxide) and ZnO (zinc oxide) heterojunctions at room temperature. The surface morphologies and chemical compositions characterized by a scanning electron microscope (SEM), X-ray diffraction (XRD), and an X-ray photoelectron spectroscopy (XPS) analysis confirmed the formation of heterojunctions between p-type CuO nanowires and n-type ZnO nanoparticles. The material's light absorbance was measured using ultraviolet-visible (UV–vis) spectroscopy. The fabricated device works in a photovoltaic mode; as photons strike the substrate, the built-in potential separates excitons, resulting in an electrical current without an external bias. The current-voltage characteristics of the device studied using a 365 nm centered laser radiation revealed an ideal p-n junction behavior. The sensor demonstrated stable and reproducible responsivity and photosensitivity of 0.108 A/W and 114, respectively, under a periodic UV radiation condition.

Conformal Coating Testing in Various Test Environments

Conformal coatings have traditionally been tested by determining the mean time to failure of conformally coated hardware exposed to corrosive test environments. This test approach has serious shortcomings: The test temperatures are most often too high. At these high temperatures, the conformal coating properties may be quite different from those at the application temperatures. In addition, the times to failure are unacceptably long extending into many months. Overcoming these shortcomings is an iNEMI championed test that involves exposing conformally coated thin films of copper and silver to sulfur vapors at 40-50 oC in flowers of sulfur (FoS) chamber and using the corrosion rates of the coated metal thin films as a measure of the corrosion protection capabilities of the conformal coatings. The test temperatures are similar to the application temperatures, the test durations are no more than a week and can be conducted under various temperature and humidity conditions. The purpose of this paper was to determine if testing in the industry-standard mixed-flowing gas corrosion chamber would give similar results as those using the FoS chamber. Acrylic, fluorinated acrylate, and atomic layer deposition conformal coatings were tested in three environments: (a) flowers of sulfur (FoS), (b) mixed-flowing gas (MFG), and (c) iodine vapor. The performance of the coatings tested in the FoS and the MFG corrosion chambers were quantitatively similar. The iodine vapor test results were in qualitative agreement with the FoS and MFG test results. In addition, we present early results pointing to the utility of terahertz-frequency imaging as a technique for measuring conformal-coating thickness nondestructively.

Influences of parameters of film on acoustical absorption performance of copper film-copper fiber composite structure

Fiber porous materials excel at sound absorption, but their effectiveness suffers at thin thicknesses. This study proposed a novel copper film-copper fiber porous material (FFM) structure to overcome this limitation. We designed a composite material with improved sound absorption, by coating a thin copper film onto an existing copper fiber matrix. This innovative approach enables effective sound absorption when the sample thickness is small. Our analysis revealed that FFM structures are better than the individual copper film and fiber porous material regarding sound absorption, particularly near their resonance frequency. Notably, a 2 mm-thick FFM structure boasts an average sound absorption coefficient of 0.35, a remarkable fourfold increase compared to the individual copper fiber matrix (0.08). The further investigation explores the influence of copper film thickness and bond area percentage on acoustic performance. We successfully validate our simulation model against experimental data, enabling us to calculate energy consumption ratios for various design parameters. This provides research ideas for optimizing FFM structures for specific noise reduction applications.

Decomposition and dewetting of super-saturated Cu-15 at. % Co solid solution film

Nano-structured copper thin films present numerous applications for miniaturization of electronic devices. Their long term stability is key for reliable functionality. This work explores the thermal stability and solid state dewetting of a Cu-Co solid solution thin film and compares it to pure Cu. As according to the phase diagram Co is practically immiscible in Cu at room temperature, a metastable solid solution of Cu-15 at. % Co was deposited as thin film on a sapphire wafer. The microstructural evolution of the Co super-saturated film at temperatures ranging from 673 to 1073 K was evaluated using X-ray diffraction and high resolution microscopy techniques such as scanning electron and transmission electron microscopy. Interestingly, there is a competition between grain growth and phase separation. Co precipitates at grain boundaries acting as pinning sites preventing rapid grain growth during heat treatment at intermediate annealing temperature. However, grain growth occurs very quickly at higher temperatures, once the elemental phases of Cu and Co are formed. The competition between grain growth and phase separation and their consequence for dewetting are discussed.