Electroless Copper Deposition Research Articles

Pd nanocubes enclosed by {100} facets for activating electroless Cu deposition on liquid crystal polymer substrates with strong adhesion strength

In this study, we investigate the catalytic performance of palladium (Pd) nanocatalysts with different surface atomic structures for activating electroless copper (Cu) deposition on liquid crystal polymer (LCP) substrates. These nanocatalysts include Pd nanocubes featuring {100} facets of square atomic arrangement as their predominant surface structure and Pd spherical nanoparticles characterized by random atomic arrangements, synthesized using an aqueous method. Our electrochemical measurements reveal that the induction time for the growth of Cu films is significantly shorter when utilizing Pd nanocubes compared to Pd nanospheres. This shorter induction time signifies a quicker initiation of Cu growth, primarily due to the presence of Pd {100} facets. Furthermore, we also develop a surface modification approach involving a crosslinking reaction that utilizes the highly branched polyethylenimine and linear polyvinylpyrrolidone on the Pd nanocubes. This method not only facilitates the adsorption of Pd nanocubes onto the LCP substrates but also achieves a robust peel adhesion strength (630 gf/cm) of the resulting Cu films, exceeding the industrial adhesion goal of 500 gf/cm. Moreover, in-depth synchrotron X-ray photoelectron spectroscopy analysis provides valuable insights into the depth-dependent chemical changes within the Cu films and proves the removal of Cu oxides after the acid treatment.

Emerging Non-Noble-Metal Atomic Layer Deposited Copper as Seeds for Electroless Copper Deposition.

Copper metal catalyst seeds have recently triggered much research interest for the development of low-cost and high-performance metallic catalysts with industrial applications. Herein, we present metallic Cu catalyst seeds deposited by an atomic layer deposition method on polymer substrates. The atomic layer deposited Cu (ALD-Cu) can ideally substitute noble metals Ag, Au, and Pd to catalyze Cu electroless deposition. The optimized deposition temperature and growth cycles of an ALD-Cu catalyzed seed layer have been obtained to achieve a flexible printed circuit (FPC) with a high performance electroless plating deposited Cu (ELD-Cu) film. The ELD-Cu films on the ALD-Cu catalyst seeds grown display a uniform and dense deposition with a low resistivity of 1.74 μΩ·cm, even in the through via and trench of substates. Furthermore, the ALD-Cu-catalyzed ELD-Cu circuits and LED devices fabricated on treated PI also demonstrate excellent conductive and mechanical features. The remarkable conductive and mechanical characteristics of the ALD-Cu seed catalyzed ELD-Cu process demonstrate its tremendous potential in high-density integrated FPC applications.

Synthesis of Aptes-Capped Self-Adsorption Pd Nanoparticles and Application in Metallization on Glass Substrate with High Adhesion

Palladium is the most used catalysts in the electroless deposition process. The technology that use the amino silane compound (3-Aminopropyl) Triethoxysilane (APTES) as surface modifier is quite mature. Therefore, an innovative idea that one step palladium catalyst APTES-Pd with APTES as capping agent and surface modifier appeared. APTES-Pd is capable to adsorb on surface of glass substrate by forming Si-O covalent bond. Comparing to commercial palladium catalysts, the process is complex and need to work at high concentration, APTES-Pd has easier process and able to work at low concentration. The particle size of APTES-Pd is characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The configuration of APTES-Pd as well as the effect of modification on glass substrate is explained by X-ray photoelectron spectroscopy (XPS).Glass substrate is a good option as an interposer substrate. It has serval advantages, such as low k, high resistivity, high stability, low coefficient of thermal expansion (CTE) mismatch with copper material and good mechanical properties. However, because glass substrate (SiO2) is stable and surface is smooth, it’s hard to electroless deposit Cu layer with high adhesion on it. The adhesion that using commercial palladium catalysts Sn/Pd for electroless copper deposition is too low to pass tape test. Because interaction between Sn/Pd and substrate is electrostatic adsorption which is lower than covalent bond. Using APTES-Pd for electroless copper deposition is capable to improve the interaction between glass and Cu layer. APTES-Pd can pass tape test on glass substrate. But its adhesion is still too low to be measured by T-Peel. To enhance the adhesion of the metal films on glass substrates, we coat titanium dioxide (TiO2) as adhesion promoter. TiO2 can not only bond to glass substrate but increase surface roughness. After coating TiO2 film, the adhesion of metal film increases to 122 gf/cm.

Application in Self-Adsorption Pd Nanoparticles with High Adhesion

Electroless plating (ELP) is widely used to metallize non-conductive substrates in printed circuit board industry. Due to the complexity of commercial palladium catalysts activation procedure and the high price of palladium, we synthesized a type of palladium catalyst with polyvinyl alcohol (PVA) as protector, formaldehyde (HCHO) as reducing agent. We used polyethylimine (PEI) to modify the surface of the substrate in advance.In this study, we developed a one-step catalyst by adding PEI into PVA-Pd. In addition to its stability, the catalyst has self-adsorption property. The process can be operated at low concentrations, which can effectively reduce the cost. Because of these characteristics, it can not only control the catalyst better than the commercial palladium catalyst but also simplify the process. The effect of self-adsorption was tested on Rogers RO3003 laminates. RO3003 high frequency laminates are ceramic-filled PTFE composites for use in printed circuit boards in commercial microwave and RF applications. The substrate has excellent characteristics such as stable dielectric constant over various temperatures and frequency and low coefficient of thermal expansion. Due to these properties, RO3003 has a great potential to be applied in 5G generation products. After activating, we used electroless copper plating to metallize the substrate.The study focused on the Cu/Pd ratio of the self-adsorption catalyst PEI+PVA-Pd to quantitatively analyze the adsorption capacity by ICP-OES, and the adhesion was analyzed by T-peel. The results indicates that the process is cost-effective, and the adhesion between substrate and copper foil is also higher. The copper layer by using self-adsorption catalyst will also have a better compactness and the surface will be pinhole-free. Therefore, this process can be successfully applied to electroless copper deposition with self-adsorption catalysts.

A facile and universally applicable additive strategy for fabrication of high-quality copper patterns based on a homogeneous Ag catalyst ink

Additive strategy has beenregardedas a promising approach for fabricating electronic copper patterns, since it has predominance over traditional lithography technology (subtractive strategy) in process simplicity, material economizing, and environmental protection. At present, the key challenge for additive strategy is how to precisely fabricate high-quality copper patterns adhering tightly to substrates, by facile and universally applicable process. Here a novel additive copper patterning process is explored by adopting a homogeneous Ag-based catalyst ink. The catalyst ink can be facilely patterned and tightly attached on various substrates by different patterning techniques, without any pre- and post-treatment procedures. In the catalyst ink formula, epoxy matrix contributes high adhesiveness and fixes ink form, valen Schiff base acts as a medium anchoring Ag catalyst, ethanol serves as both reductant and diluent, and polyethylene glycol improves hydrophility. The copper coating, forming on the catalyst layer by further electroless copper deposition process, exhibits compact structure and high electrical conductivity comparable to bulk copper. Moreover, the copper coating can be partially inserted into substrate body by porous surface design for substrates, thus achieving strong adhesion and high flexibility. This research contributes to the progress of next-generation copper patterning techniques.

Sugar‐Based Electroless Copper Deposition on Pectin‐Coated Alumina Microparticles

AbstractA novel approach for fabricating copper‐coated alumina microparticles utilizing a pectin‐based coating subsequently followed by electroless deposition is reported. The biopolymer pectin is modified with phosphate groups covalently binding to the particle surface, while the polymer itself promotes the formation of metal centers on the surface due to the high affinity to metal cations. The sugar‐based (galactose, xylose, and glucose) electroless plating process in combination with an organic‐based treatment ensures a homogeneous metal coating method. As a benefit, precious metal activation of alumina particles and hazardous reducing agents typically employed are not required, creating a low‐cost and sustainable process. Metal plated micro particles show a uniform and homogeneous coating rendering them as ideal additives for application in metal matrix composites. These ceramic metal composites can be used in a wide range of applications where high‐strength metal components are needed.

UV-induced graft polymerization of polyamide-6 for electroless copper deposition

A simple and chromium-free method for the metallization on engineering plastic polyamide-6 (PA6) by UV irradiation is proposed in this article. The surface graft polymers with specific adsorption activity are developed through one-step UV exposure operation, which captures Pd2+ in the activation process. The chemisorption of Pd2+ is further reduced into catalytic Pd0 to enable the substrate with catalytic nucleus for conventional copper electroless deposition (ELD). The grafting layer and the deposited copper coating are characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements, scanning electron microscopy, and X-ray diffraction. The results indicate that successful grafting occurs on PA6 after UV excitation. In addition, the as-deposited copper layer shows excellent conductive performance and high adhesion with the PA6 substrate. It’s expected that this emerging and feasible approach would have broad prospects for the metallization of plastic and offer significant value in the plating industry.

Analysis of Tempering Effects on LDS-MID and PCB Substrates for HF Applications

Mechatronic Integrated Devices or Molded Interconnect Devices (MID) are three-dimensional (3D) circuit carriers. They are mainly fabricated by laser direct structuring (LDS) and subsequent electroless copper plating of an injection molded 3D substrate. Such LDS-MID are used in many applications today, especially antennas. However, in high frequency (HF) systems in 5G and radar applications, the demand on 3D circuit carriers and antennas increases. Electroless copper, widely used in MID, has significantly lower electrical conductivity compared to pure copper. Its lower conductivity increases electrical loss, especially at higher frequencies, where signal budget is critical. Heat treatment of electroless copper deposits can improve their conductivity and adhesion to the 3D substrates. This paper investigates the effects induced by tempering processes on the metallization of LDS-MID substrates. As a reference, HF Printed Circuit Boards (PCB) substrates are also considered. Adhesion strength and conductivity measurements, as well as permittivity and loss angle measurements up to 1 GHz, were carried out before and after tempering processes. The main influencing factors on the tempering results were found to be tempering temperature, atmosphere, and time. Process parameters like the heating rate or applied surface finishes had only a minor impact on the results. It was found that tempering LDS-MID substrates can improve the copper adhesion and lower their electrical resistance significantly, especially for plastics with a high melting temperature. Both improvements could improve the reliability of LDS-MID, especially in high frequency applications. Firstly, because increased copper adhesion can prevent delamination and, secondly, because the lowered electrical resistance indicates, in accordance with the available literature, a more ductile copper metallization and thus a lower risk of microcracks.

Time-resolved monitoring of electroless copper deposition on woven cellulose fabrics

This study investigates the performance of electroless copper deposition on woven cellulose fabrics. Silver seeds were printed in a straight line of defined dimensions on the fabrics, which were then subjected to electroless copper deposition. Varying the deposition time allowed to study the evolution of the layer formation on the coated fabrics and their conductivity. With increasing deposition time, an initial rapid reduction of electrical resistance could be observed, which was followed by a plateau after 120 min. The initially high strain sensitivity of 105 found after 30 min deposition decreased significantly as the deposition time increased. Percolation efficiencies were calculated from empirically measured and theoretically calculated resistance values. Microscopic imaging showed that feature of the substrate construction, e.g. yarn crossings, exert a strong influence on overall conductive performance and percolation efficiency of the coated fabrics.

Highly Ordered Thermoplastic Polyurethane/Aramid Nanofiber Conductive Foams Modulated by Kevlar Polyanion for Piezoresistive Sensing and Electromagnetic Interference Shielding

HighlightsKevlar polyanionic chains induced the formation of highly ordered porous thermoplastic polyurethane foams with adjustable pore sizes.A tiny amount of Ti3C2Tx MXene was performed as reducing agent for the electroless deposition of copper nanoparticles.The conductive foam reinforced by aramid nanofibers exhibited excellent piezoresistive sensing and electromagnetic interference shielding performance.

The role of citrate in heterogeneous silver metal catalyst formation: A mechanistic consideration

The formation of silver particles as initial catalyst for the electroless copper deposition is an essential part of the formation of conductive structures in flexible electronics. In this work, we have investigated the basic mechanism using the citrate anion for the fixation of silver species and later metal catalyst formation directly on a textile substrates surface. During the particle formation process and the subsequent electroless copper deposition, silver changes its oxidation state several times. While the decomposition of bulk silver citrate leads to silver metal, the distribution on the substrate surface leads to silver oxide due to the oxidation under air. Finally, the reducing conditions in the copper deposition step, i.e. alkaline formaldehyde, convert any silver-ion to silver metal. In parallel, the thermal decomposition of silver citrate also forms CO2 and hexane-2,5-dione. The latter is the result of an auto oxidation reaction similar to Kolbe electrolysis, i.e. symmetrical radical recombination of two acetonyl radicals remaining from the decarboxylated citrate scaffold.

Fluidic-chemical characteristics of electroless copper deposition of ordered mass-fabricated pillars in a microchannel for chip packaging applications

The trend of chip-packaging in the semiconductor industry is pointing towards 3D integrated circuits and scaling down to attain devices with enhanced efficiency and performance. To overcome the reassimilating challenges, we propose the usage of an electroless plated deposition layer to connect pillar bumps and create high-density interconnections. The substrates and pillar arrangements encompass our microchannel with the height of 50 µm or 25 µm where in between the electrolyte is forced with a superficial velocity of 0.3 to 300 mm s−1 in our experiments. The results are compared to a numerical multi-physics model which simulates idealized plating conditions without any disruptions to characterize and quantify the different flow regimes and undesired side-effects. In the low-velocity regime, hydrogen bubbles cannot be dislodged and remain in the microchannel which is accompanied with the formation of extraneous deposition on the resist surface. The mid-velocity regime shows a good match to the simulation model and is considered as optimal plating condition. The chemical reaction slows down for high velocities.

Low-power laser manufacturing of copper tracks on 3D printed geometry using liquid polyimide coating.

Silver nanoparticle photoreduction synthesis by direct laser writing is a process that enables copper micro-track production on very specific polymers. However, some important 3D printing polymers, such as acrylonitrile butadiene styrene (ABS) and acrylates, do not accept this treatment on their surface. This work presents an approach to produce copper microcircuitry on 3D substrates from these materials by using direct laser writing at low power (32 mW CW diode laser). We show that by coating a thin layer of polyimide (PI) on a 3D-printed geometry, followed by a sequence of chemical treatments and low-power laser-induced photoreduction, copper tracks can be produced using silver as catalyst. The surface chemistry of the layer through the different stages of the process is monitored by FTIR and X-ray photoelectron spectroscopy. The copper tracks are selectively grown on the laser-patterned areas by electroless copper deposition, with conductivity (1.2 ± 0.7) × 107 S m-1 and a width as small as 28 μm. The patterns can be written on 3D structures and even inside cavities. The technique is demonstrated by integrating different circuits, including a LED circuit on 3D printed photopolymer acrylate and a perovskite solar cell on an ABS 3D curved geometry.

Synthesis of carbon-based Ag-Pd bimetallic nanocomposite and the application in electroless copper deposition

In this paper, carbon dots (C-dots) based AgPd/C nanocomposite (AgPd/C NCs) was synthesized and firstly applied as the catalyst in the electroless copper deposition. The hydroxyl-rich C-dots synthesized by one-pot electrochemical carbonization method can reduce Ag+ into Ag nanoparticles to form Ag/carbon nanocomposites (Ag/C NCs), which were then utilized as the template to synthesize AgPd/C NCs that have palladium-rich surface and silver-rich core. The results show that AgPd/C NCs can act as the activator and be used in electroless copper deposition of printed circuit boards (PCBs) with the backlight coefficient of 9.5 with the palladium concentration as low as 7 ppm. The speed in the deposition of copper and the stability of the catalyst are better than those of the traditional Sn/Pd colloidal activator at the same concentration, which promises the AgPd/C NCs good substitutes in electroless copper deposition in the future.

Microstructural and physicochemical origins of electroless copper deposition on graphite enhanced by acid pretreatment

Acid treatment is the most widely used surface modification method for enhancing the electroless metal deposition (EMD) on carbon reinforcement materials (CRMs) for metal matrix nanocomposites. However, specific microstructural and physicochemical origins of the enhanced EMD on carbon surfaces by acid treatments have been rarely studied. Here, we investigated the effects of the nitric acid treatment on graphite, a prototypical combination of acid treatment and CRM, on the fidelity of Cu EMD and their structural and chemical origins. Complementary materials characterizations and density functional theory calculations revealed the acid-induced formation of broken C–C/CC graphitic bonds and resulting surface micropores on graphite; this enabled a uniform dispersion of catalytic Sn/Pd nanoparticles during pre-EMD sensitization/activation processes via spontaneous binding of Sn and Pd ions and, consequently, a much more uniform Cu layer EMD compared to the untreated graphite. We proposed a general mechanism illustrating how the acid-induced microstructural and chemical modifications of carbon surface affected the spatial uniformity of catalytic metal reduction during EMD and, finally, the quality of deposited metal layer. The results clearly reveal the origins of the enhanced EMD on carbon materials by acid treatments, providing guidelines for optimizing EMD on general CRMs for high-performance metal matrix nanocomposites.

Design of Multiphase Metal Balls via Maskless Localized Anodization Based on Bipolar Electrochemistry

AbstractNanoporous metal‐oxide films formed on some metals and their alloys during anodization are crucial for both basic research and commercial applications. They are used as important materials for various types of device fabrication due to their distinctive solid‐state shapes, such as nanopores and nanotubes. However, preparing nanoporous films using anodization, which is a surface treatment for plate‐like specimens, requires a direct electrical connection to the workpiece. Developing new materials independent of the specimen's shape and size will vary if the need for direct energization is eliminated. Here, bipolar electrochemistry is employed to fabricate two‐phase (insulator–conductor) and three‐phase (insulator–conductor–insulator) aluminum balls by controlling the redox reaction region in an electric field. The aluminum balls are site‐selectively anodized employing a wireless and maskless operation. The multiphase aluminum balls’ versatility is demonstrated in the site‐selective electroless deposition of copper on their exposed regions. Additionally, it is demonstrated that various types of materials, including aluminum and titanium, can be oxidized simultaneously in the same cell.

Flexible Copper Metal Circuits via Desktop Laser Printed Masks

AbstractA novel process is demonstrated that produces patterns of electrically conductive copper on a flexible polyimide film substrate using standard desktop laser printed toner and near room temperature aqueous chemistry. The laser toner acts as a mask to selectively block the ion exchange self‐metallization (IESM) reduction reaction that forms nanoscale silver or palladium coatings at the polyimide surface. The silver or palladium IESM coating is then used as a catalyst for electroless deposition of copper. Under appropriate conditions, the copper is deposited selectively on top of the catalyst layer. The resulting copper layer has a measured sheet resistance as low as 0.3 Ohms/sq. Electrical isolation is measured between copper traces spaced as close as 300 microns, and high conductivity is measured along traces with widths as low as 200 microns. The minimum pattern size appears to be limited primarily by the resolution of the laser toner pattern, as the IESM metal layer is observed to follow the contours of individual toner particles. The process avoids the use of high temperature, vacuum, and organic solvents and is thus suitable for very low cost prototyping or distributed manufacturing of simple electronic devices.

Fabrication of metallic patterns on ordinary polymer substrates by laser direct activation and electroless plating

In recent years, the fabrication of metallic patterns on polymer substrates has been given increasing attention because of its wide and important applications in many fields. This paper reported a selective metallization technology on ordinary polymer substrates basing on laser direct activation of copper hydroxyl phosphate [Cu2(OH)PO4] and electroless deposition of copper. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the interaction mechanism between nanosecond-pulsed fiber laser (1064 nm wavelength) and Cu2(OH)PO4. It was found that after laser modification and activation by appropriate parameters, the special surface microstructure (e.g., porous sponge-like) appeared and explained the good adhesion strength (the highest grade of 5B) between the copper layer and substrate. At the same time, Cu+ was generated from the Cu2(OH)PO4, adsorbed on the microstructure, and acted as catalytic active centers to realize the selective copper deposition in the laser-activated zone. In addition, the copper layer possessed good selectivity (< 3% error of line space and width) and electrical conductivity (10−6 Ω·cm volume electrical resistivity). By comparison with laser direct structuring (LDS) technology and with palladium chloride as laser sensitive material, this was a more universal and less-cost technical pathway to fabricate planar and/or three dimensional (3D) metallic patterns on more kinds of ordinary polymer substrates.

Complexation-mediated surface modification of polyamide-66 textile to enhance electroless copper deposition

A key element for the development of electronic textiles is the formation of conductive tracks and circuits on fibres and textile structures. For achieving a reliable conductive structure during the entire usage lifetime, the modification of the substrate is often indispensable to improve the adhesion between the textile substrate the conductive structure.In this work, we have further developed the complexation mediated surface treatment technique using CaCl2/EtOH/H2O solution (CEW) for the modification of PA66 fibre and fabric which were subsequently coated with copper by currentless deposition. As a result of the treatment, a porous shell layer was formed at the fibre surface. Apart from the topographical change of the fibre/fabric surface, there was no further significant influence on other polymer parameters. Compared to untreated and state-of-the-art plasma modified samples, CEW modification resulted in the highest abrasion resistance as a result of the better migration of metal ions into the thin porous shell structure, allowing stronger anchoring of the metal layer to the substrate. After 10 000 abrasion cycles, 90% copper still remained on CEW modified coated fabric, exhibiting very low resistance. Thus, CEW complexation mediated modification technique is proven to be simple to implement, but efficient to enhance the polymer/metal interface in copper coated PA66 fibre.

A facile method to fabricate lightweight copper coated polyimide film current collectors for lithium-ion batteries

Large inactive component mass ratio and internal resistance are two critical challenges in further realizing high energy and power density lithium-ion batteries. Herein, lightweight copper coated polyimide (PI@Cu) current collectors with a high electrical conductivity (2.25 × 105 S cm−1) and low area density (1.99 mg cm−2) are fabricated via a facile electroless copper deposition method. Compared with the commercial Cu foil current collector, lithium titanate anodes equipped with PI@Cu current collectors decrease the inactive component mass ratio and improve the gravimetric capacity based on the whole mass of the electrode. Meanwhile, the rough surface morphology of deposited Cu layers reduces the electrode layer/current collector interfacial resistance, resulting in a lower internal resistance and better rate capability. PI@Cu composite films are promising alternatives to the traditional copper foil current collectors for high energy density and power density lithium-ion batteries.