Copper Ink Research Articles

Anomalous Temperature Effect on the Deposition Morphology in Reactive Inkjet Printing

Reactive inkjet (RIJ) printing represents an important approach for printed electronics. Different from the traditional inkjet printing of colloidal inks with nanosized metal particles, RIJ prints the solution of metal organic compound, that is, metal‐organic decomposition (MOD) ink, and reduces it to the pure metal during the post‐treatment to form conductive electrodes. Successful RIJ printing of MOD inks has been demonstrated; however, the fundamental understanding of droplet deposition involved in RIJ printing is lacking. Herein, the effect of substrate temperature and nozzle temperature during inkjet printing of copper MOD ink on the deposition morphology is investigated. It exhibits distinct behaviors in the deposition morphology at elevated temperatures when compared with the ones at room temperature. The different deposition morphology of the copper ink at various temperatures is attributed to the generation of a localized seeding layer which attracts the copper ions to deposit. In addition, the copper MOD ink printing is compared with a silver MOD ink. The different deposition morphology between the copper ink and silver ink is due to the solvent system. Understanding the deposition mechanism for reactive inks provides valuable information and guidance in the fabrication of printed functional devices using particle‐free reactive inks.

Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease

This paper describes the process of producing chemosensors based on hybrid nanostructures obtained from Al2O3, as well as ZnO ceramic nanotubes and the following conducting polymers: poly(3-hexylthiophene), polyaniline emeraldine-base (PANI-EB), and poly(3, 4-ethylenedioxythiophene)-polystyrene sulfonate. The process for creating ceramic nanotubes involves three steps: creating polymer fiber nets using poly(methyl methacrylate), depositing ceramic films onto the nanofiber nets using magnetron deposition, and heating the nanotubes to 600 °C to burn off the polymer support completely. The technology for obtaining hybrid nanostructures from ceramic nanotubes and conducting polymers is drop-casting. AFM analysis emphasized a higher roughness, mainly in the case of PANI-EB, for both nanotube types, with a much larger grain size dimension of over 5 μm. The values of the parameter Rku were close or slightly above 3, indicating, in all cases, the formation of layers predominantly characterized by peaks and not by depressions, with a Gaussian distribution. An ink-jet printer was used to generate chemiresistors from ceramic nanotubes and PANI-EB structures, and the metallization was made with commercial copper ink for printed electronics. Calibration curves were experimentally generated for both sensing structures across a wider range of NH3 concentrations in air, reaching up to 5 ppm. A 0.5 ppm detection limit was established. The curve for the ZnO:PANI-EB structure presented high linearity and lower resistance values. The sensor could be used in medical diagnosis for the analysis of breath ammonia and biomarkers for predicting CKD in stages higher than 1. The threshold value of 1 ppm represents a feasible value for the presented sensor, which can be defined as a simple, low-value and robust device for individual use, beneficial at the patient level.

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.

Chemiresistors Based on Hybrid Nanostructures Obtained from Graphene and Conducting Polymers with Potential Use in Breath Methane Detection Associated with Irritable Bowel Syndrome.

This paper describes the process of producing chemiresistors based on hybrid nanostructures obtained from graphene and conducting polymers. The technology of graphene presumed the following: dispersion and support stabilization based on the chemical vapor deposition technique; transfer of the graphene to the substrate by spin-coating of polymethyl methacrylate; and thermal treatment and electrochemical delamination. For the process at T = 950 °C, a better settlement of the grains was noticed, with the formation of layers predominantly characterized by peaks and not by depressions. The technology for obtaining hybrid nanostructures from graphene and conducting polymers was drop-casting, with solutions of Poly(3-hexylthiophene (P3HT) and Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). In the case of F8T2, compared to P3HT, a 10 times larger dimension of grain size and about 7 times larger distances between the peak clusters were noticed. To generate chemiresistors from graphene-polymer structures, an ink-jet printer was used, and the metallization was made with commercial copper ink for printed electronics, leading to a structure of a resistor with an active surface of about 1 cm2. Experimental calibration curves were plotted for both sensing structures, for a domain of CH4 of up to 1000 ppm concentration in air. A linearity of the curve for the low concentration of CH4 was noticed for the graphene structure with F8T2, presenting a sensitivity of about 6 times higher compared with the graphene structure with P3HT, which makes the sensing structure of graphene with F8T2 more feasible and reliable for the medical application of irritable bowel syndrome evaluation.

Insight into the Types of Alkanolamines on the Properties of Copper(II) Formate-Based Conductive Ink.

Conductive inks are one of the most important functional materials for printed flexible electronic devices, and their properties determine the methods of subsequent patterning and metallization. In comparison with copper nanoparticle or nanowire inks, copper particle-free inks employing copper(II) formate (Cuf) as a precursor have attracted the interest of researchers due to their flexibility in preparation, excellent stability, and lower conversion temperature. Alkanolamines can provide Cuf with excellent solubility in alcohols while being less toxic and having a certain reducibility, making them preferable ligands in comparison with aliphatic amines and pyridine. However, there have been few studies on the effects of the alkanolamine types on the performance of Cuf inks. Also, the decomposition mechanism of copper-alkanolamine complex inks is not clear. In this work, different kinds of alkanolamines were chosen as ligands to formulate Cuf inks to address the mentioned issues. The influences of amine types on the stability, wettability, thermal decomposition behavior, and electrical performance of the formulated Cuf particle-free inks were investigated in detail. The results show that the utilization of alkanolamines could provide Cuf with excellent solubility in alcohols, resulting in an ink with good stability and favorable wetting properties. The thermal decomposition temperature and electrical performance of the formulated copper ink are largely dependent on the amine used. When amines with a longer carbon chain and more branches were utilized to prepare the ink, a decreased decomposition temperature was observed on the derived inks because of the steric hindrance effect. Copper films with good morphology and conductivity could be obtained at low temperatures by selecting the appropriate alkanolamine. Copper particle-free conductive ink from 2-amino-2-methyl-1-propanol demonstrated better morphology and electrical performance (16.09 μΩ·cm) and was successfully used for conductive circuits by direct-writing.

Solderability of Additively Manufactured Copper Surfaces

As additive manufacturing technology for copper rapidly evolves, the likelihood for downstream joining like soldering and brazing exists. A solderability study was performed to assess process fundamentals over a variety of AM approaches. The solderability of 7 different copper (Cu) surface types were evaluated using the wetting balance technique: 4 Cu coatings (CP-007 and CP-008 conductive copper pastes, IMC-4118 conductive laser sinter paste, IMC-2501 copper ink), 2 laser processed Cu surfaces, and 1 bulk additively manufactured (AM) Cu coupon type. Laser processed surfaces were subjected to either circular or linear laser raster patterns. AM coupons were fabricated via a powder-bed laser process. In-situ force measurements were recorded while coupons were dipped into and removed from a 63Sn-37Pb (SnPb) or 96.4Sn-3.0Ag-0.6Cu (SAC305) solder bath, allowing contact angles and surface energies to be calculated. Meniscus height values were also measured. Surfaces were characterized before and after dipping via optical and electron microscopy. Results conclusively indicated that although all samples were intended to represent pure Cu, wettability was found to be strongly dependent on surface type with several samples exhibiting no wetting. Although the CP-007 paste performed the best of any non-traditional surface, all nontraditional surfaces saw signifi cant wettability improvements with aggressive cleaning via HCl etching.

Flashlight Sintering Characteristics of the Inkjet-Printed Nanosized Copper Ink on an Auxiliary Heated Paper Substrate

Flashlight Sintering Characteristics of the Inkjet-Printed Nanosized Copper Ink on an Auxiliary Heated Paper Substrate

Inkjet printing electroless copper plating on aluminum nitride substrate

Aluminum nitride (AlN) as a ceramic substrate with high thermal conductivity and mechanic strength is highly desired for high-powder density electrode devices. However, precisely metalating of AlN in an environmental friendly way is still challenging. Here, we develop a inkjet printing electroless plating technology for fabricating cupper electrode on AlN substrate. The copper ink prepared by CuSO4·5 H2O is firstly inkjet printed on AlN substrate followed reduced by a dimethylamine borane solution to form a copper seed. The AlN substrate with the copper seed layer was immersed into the formaldehyde-free environment-friendly electroless plating solution to deposit the copper layer. The prepared copper electrode demonstrated a smooth surface with robust tangential peel strength reaches 302.67 N·cm−1 and excellent conductivity to 8.70 μΩ cm. Furthermore, AlN substrate after 8 h electroless copper plating has excellent heat dissipation performance. Therefore, the inkjet printing electroless plating technology provide a promising route for scalable and precise metalating the ceramic substrates.

The effect of conductive ink alternation on the sustainability and functioning of printed electronics

The demand for printed electronics (PE) is expected to increase for improved functionality and autonomy of products in the context of the Internet of Things, especially for sensing and communication. With this trend, the environmental performance of novel technologies is of growing importance, and researchers are exploring ways to make them more environmentally friendly using bio-based substrates and additive printing methods. Conductive inks have the highest environmental impact in the life cycle of PE, and there are two options to decrease the impact: increase recycling rates or replace commonly used silver inks with less impactful materials such as copper or graphite. With the current mixed waste management system, a higher recycling rate for silver is not achievable. Copper and graphite have a lower environmental impact than silver, however, the electrical conductivity of copper and graphite inks is lower than silver ink. This article introduces a double-parameter comparison to simultaneously assess the environmental and electrical conductivity of PE using three inks of silver, copper, and graphite inks. Then based on the technical feasibility, this study proposes suitable applications for each substrate and ink combination.

Unraveling the Mechanism of Plasma‐Induced Curing of Particle‐Free Complex Inks for Manufacturing of High‐Performance Flexible Copper Films and Patterns

AbstractAlthough cost‐effective copper inks show great potential in printed flexible electronics, the poor stability and high sintering temperatures have hindered their practical application seriously. Herein, the mechanism of plasma‐induced curing of particle‐free complex copper inks is investigated systematically to realize the low‐temperature fabrication of high‐performance copper films and patterns for advanced flexible electronics. The relationships among the ink formulations, plasma characteristics, microstructures, and the properties of the printed copper patterns are identified; the mechanisms of plasma‐induced curing of copper complex inks are summarized. The fabricated copper films exhibit high conductivity (4.1 µΩ cm), excellent flexibility (bending at a radius of 2 mm), and environmental stability because of the dense microstructures and robust interface adhesion, comparable to that of electroless‐plated films. Moreover, intricate copper antenna patterns on flexible substrates can be obtained rapidly using direct writing and gravure printing methods. These results provide a feasible low‐temperature route to fabricate high‐performance copper films and patterns, promoting the practical application of low‐cost copper inks in printed flexible electronics.

Printing conformal and flexible copper networks for multimodal pressure and flow sensing.

Flexible multimodal sensors with ultrasensitive detection capabilities are an indispensable component of wearable electronics and are highly sought-after involving a wide range of signal monitoring such as artificial skin and soft robotics. Here we report a flexible and wireless multimodal sensor using low-temperature additive manufacturing of copper nanoplates on elastic polyurethane substrates for temperature, pressure, and flow monitoring. The positive temperature coefficient and piezoresistive performance of the copper nanoplate network translates to a reliable temperature, steady-state and dynamic pressure/flow sensing for detecting pressures as small as 0.64 Pa with a response time of 130 ms, as well as velocity detection ranging from 2.5-6.8 m s-1. Additionally, by incorporating a printed antenna, it enables a self-powered, battery-free system, offering a wireless readout of printed multimodal sensors with superior real-time sensing performance in conjunction with wearable flexibility.

1,8‐Octanedithiol as an Effective Intermediate Layer for Deposition of Cu Electrodes via Inkjet Printing and Laser Sintering on III–V Triple‐Junction Solar Cells

This research study combines surface modification techniques with back‐end‐of‐line (BEOL) methods for cost‐effective, scalable front contact electrode deposition on III–V solar cells. Copper nanoparticle grids are deposited by inkjet printing on surface‐modified III–V solar cells. The deposition of a self‐assembled monolayer (such as 1,8‐octanedithiol) as an intermediate layer is a proven method for surface modification to improve the wettability of the substrate surface and the adhesion of the printed copper nanoparticle structures on the substrate to perform inkjet printing of coherent and narrow electrode structures. Then, the printed copper ink is converted to a conductive copper grid by a picosecond pulsed laser with optimized settings and an additional galvanic plating step is required for the thickening of inkjet‐printed and laser‐sintered seeding layer for solar cell applications. As a result, an ohmic copper contact on III–V layer with low contact resistivity (5 mΩ cm2) is realized successfully. The processed solar cell shows a functioning behavior with 20% conversion efficiency.

3D-printable, lightweight, and electrically conductive metal inks based on evaporable emulsion templates jammed with natural rheology modifiers

Conductive metal inks with 3D-printable rheological properties have gained considerable attention, owing to their potential for manufacturing 3D electronics. Typically, such inks are formulated with high volume fractions of metal particles to achieve both rheological and electrical percolation. However, this leads to a high product cost and weight, making this approach potentially undesirable for practical application. In this study, naturally occurring ingredients, i.e., bee pollen microparticles (BPMPs) and citric acids (CAs), are used to produce a jammed hexane-in-aqueous suspension-type emulsion with controllable viscoelasticity as a template for conductive metal particles. Correspondingly, it is possible to develop 3D-printable, lightweight, and conductive inks. The BPMPs and CAs, as rheology modifiers, facilitate the 3D printability of the ink. After drying, the ink forms 3D networks without macroscopic discontinuities. Hexanes co-dispersed with BPMPs and CAs in the aqueous continuous phase improve the ink rheological processability and create internal macropores within the 3D-printed structure upon evaporation under ambient conditions, decreasing the product density. A conductive copper ink based on the emulsion template shows excellent 3D printability and electrical percolation at low metal loadings (<10 vol%); moreover, the printed ink with the optimized formulation has a remarkably low density (<2 g ∙ cm−3).

Low-temperature calcination of convenient micro-sized copper ink with surface activation and synchronous protection by in-situ chemisorbed cupric formate

Low-temperature calcination of convenient micro-sized copper ink with surface activation and synchronous protection by in-situ chemisorbed cupric formate

Improvement of discharge system in cool plasma sintering method for copper fine traces

As a wiring material for printed electronics (PE), copper is strongly needed instead of silver. For a sintering method of copper traces, we have developed the method using oxygen pump and atmospheric pressure plasma, and demonstrated the ink composed of nanoparticles can be sintered to bulk-like structure. In this study, we have improved the sintering system to make it effective especially for copper inks made of submicron particles. We have tried to increase the thickness of the sintered layer and decrease the resistivity by changing the power supply system of plasma discharge and increasing the plasma density. The improved system has successfully demonstrated that a copper trace at least 4 μm thick can be sintered and that the resistivity decreases to approximately 3.3 μΩ·cm. These results can lead to huge breakthroughs in PE based on copper.

Self-Organizing, Environmentally Stable, and Low-Cost Copper–Nickel Complex Inks for Printed Flexible Electronics

Cost-effective copper conductive inks are considered as the most promising alternative to expensive silver conductive inks for use in printed electronics. However, the low stability and high sintering temperature of copper inks hinder their practical application. Herein, we develop rapidly customizable and stable copper-nickel complex inks that can be transformed in situ into uniform copper@nickel core-shell nanostructures by a self-organized process during low-temperature annealing and immediately sintered under photon irradiation to form copper-nickel alloy patterns on flexible substrates. The complex inks are synthesized within 15 min via a simple mixing process and are particle-free, air-stable, and compatible with large-area screen printing. The manufactured patterns exhibit a high conductivity of 19-67 μΩ·cm, with the value depending on the nickel content, and can maintain high oxidation resistance at 180 °C even when the nickel content is as low as 6 wt %. In addition, the printed copper-nickel alloy patterns exhibit high flexibility as a consequence of the local softening and mechanical anchoring effect between the metal pattern and the flexible substrate, showing strong potential in the additive manufacturing of highly reliable flexible electronics, such as flexible radio-frequency identification (RFID) tags and various wearable sensors.

Preparing Copper Nanoparticles and Flexible Copper Conductive Sheets.

Nanotechnology is used in a wide range of fields, including medicine, cosmetics, and new material development, and is one of the most popular technologies in the field of flexible electronic products. For the present work, the chemical reduction method with environmentally friendly reducing agents was used to synthesize copper nanoparticles (CuNPs) with good dispersibility. The CuNPs were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and ultraviolet–visible spectrophotometry (UV–vis). After the CuNPs were formed, the solvent, polymers, and additives were added to form copper ink. Finally, the prepared copper inks were applied to flexible polyethylene terephthalate (PET) substrate under low sintering temperature and the effects of sintering time and different concentrations of sintering agent on resistivity were investigated. The results show that the copper nanoparticles synthesized by secondary reduction were smaller, more uniform, and better dispersed than those formed by primary reduction. Ethylene glycol has reducing effects under high temperatures; therefore, the CuNPs formed using the mixed solvent were small and well dispersed. The copper ink was applied on the PET substrate, treated with a formic acid aqueous solution, and sintered at 130 °C for 60 min, and its resistivity was about 1.67 × 10−3 Ω cm. The proposed synthesizing method is expected to have potential applications in the flexible electronic products field.

Copper inks for printed electronics: a review.

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

Ohmic contact formation for inkjet-printed nanoparticle copper inks on highly doped GaAs

GaAs compound-based electronics attracted significant interest due to unique properties of GaAs like high electron mobility, high saturated electron velocity and low sensitivity to heat. However, GaAs compound-based electronics demand a significant decrease in their manufacturing costs to be a good competitor in the commercial markets. In this context, copper-based nanoparticle (NP) inks represent one of the most cost-effective metal inks as a proper candidate to be deposited as contact grids on GaAs. In addition, Inkjet-printing, as a low-cost back-end of the line process, is a flexible manufacturing method to deposit copper NP ink on GaAs. These printed copper NP structures need to be uncapped and fused via a sintering method in order to become conductive and form an ohmic contact with low contact resistivity. The main challenge for uncapping a copper-based NP ink is its rapid oxidation potential. Laser sintering, as a fast uncapping method for NPs, reduces the oxidation of uncapped copper. The critical point to combine these two well-known industrial methods of inkjet printing and laser sintering is to adjust the printing features and laser sintering power in a way that as much copper as possible is uncapped resulting in minimum contact resistivity and high conductivity. In this research, copper ink contact grids were deposited on n-doped GaAs by inkjet-printing. The printed copper ink was converted to a copper grid via applying the optimized settings of a picosecond laser. As a result, an ohmic copper on GaAs contact with a low contact resistivity (8 mΩ cm2) was realized successfully.

Additive manufacturing of copper vertical interconnect accesses by laser processing

This paper introduces a new manufacturing process for vertical interconnect accesses (VIA). In contrast to industrially established VIA metallization technologies, the presented approach takes place without any chemical plating by combining copper ink and epoxy insulator coating with CO 2 laser processing for VIA drilling and copper ink sintering. The minimum VIA resistances are less than 50 mΩ, fitting the theoretically calculated value. A laboratory application scenario testing a 10 × 10 contact pad array with a pitch of 800 µm successfully demonstrates routing across five printed metallization layers, including 128 blind and 112 buried VIA.