Electroless Copper Metallization Research Articles

Advanced Ag nanoparticles for the catalyzation of glyoxylic acid oxidation in through-holes electroless copper metallization

Advanced Ag nanoparticles (Ag NPs) were prepared by wet chemical oxidation–reduction method, using mainly the tannic acid as reducing agent and carboxymethylcellulose sodium as stabilizer. The prepared Ag NPs uniformly disperse and are stable for more than one month without agglomeration. The studies of transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) absorption spectroscopy indicate that the Ag NPs are in homogeneous sphere with only 4.4 nm average size and narrow particle size distribution. Electrochemical measurements reveal that the Ag NPs behave excellent catalytic activity for electroless copper plating using glyoxylic acid as reducing agent. In situ fourier transform infrared (in situ FTIR) spectroscopic analysis combined with density functional theory (DFT) calculation illustrate that the molecular oxidation of glyoxylic acid catalyzed by Ag NPs is as the following routes: glyoxylic acid molecule first is adsorbed on Ag atoms with carboxyl oxygen terminal, then hydrolyzed to diol anionic intermediate, and last oxidized to oxalic acid. Time-resolved in situ FTIR spectroscopy further reveals the real-time reactions of electroless copper plating as follows: glyoxylic acid is continuously oxidized to oxalic acid and releases electrons at the active catalyzing spots of Ag NPs, and Cu(II) coordination ions are in situ reduced by the electrons. Based on the excellent catalytic activity, the advanced Ag NPs can replace the expensive Pd colloids catalyst and successfully apply in through-holes metallization of printed circuit board (PCB) by electroless copper plating.

Radio-Frequency Energy Harvesting Using Rapid 3D Plastronics Protoyping Approach: A Case Study

Harvesting of ambient radio-frequency energy is largely covered in the literature. The RF energy harvester is considered most of the time as a standalone board. There is an interest to add the RF harvesting function on an already-designed object. Polymer objects are considered here, manufactured through an additive process and the paper focuses on the rapid prototyping of the harvester using a plastronic approach. An array of four antennas is considered for circular polarization with high self-isolation. The RF circuit is obtained using an electroless copper metallization of the surface of a 3D substrate fabricated using stereolithography printing. The RF properties of the polymer resin are not optimal; thus, the interest of this work is to investigate the potential capabilities of such an implementation, particularly in terms of freedom of 3D design and ease of fabrication. The electromagnetic properties of the substrate are characterized over a band of 0.5–2.5 GHz applying the two-transmission-line method. A circular polarization antenna is experimented as a rapid prototyping vehicle and yields a gain of 1.26 dB. A lab-scale prototype of the rectifier and power management unit are experimented with discrete components. The cold start-up circuit accepts a minimum voltage of 180 mV. The main DC/DC converter operates under 1.4 V but is able to compensate losses for an input DC voltage as low as 100 mV (10 μW). The rectifier alone is capable of 3.5% efficiency at −30 dBm input RF power. The global system of circularly polarized antenna, rectifier, and voltage conversion features a global experimental efficiency of 14.7% at an input power of −13.5 dBm. The possible application of such results is discussed.

Multimaterial 3D Printing Technique for Electronic Circuitry Using Photopolymer and Selective Metallization

Herein, a 3D printing technique to enable the manufacturing of selectively plated polymer objects by photopolymer printing and copper metallization is presented. Metallized plastic has become popular in a number of industries due to its lightweight, flexibility of design, and cost‐effective fabrication. Common metallization techniques require multiple pretreatment steps, such as surface etching, sensitization/activation, and acceleration. The proposed method, by eliminating the sensitization step, allows selective metallization of 3D‐printed samples by direct plating. The method relies on the inclusion of silver seeds into a mixture of acrylate‐ and methacrylate‐based monomers and oligomers, which enables direct electroless copper metallization following 3D printing. Ag(I) ions act as catalytic sites for copper deposition. Copper films grown with embedded 2 and 4 wt% Ag(I) seeds reach thicknesses of 5.7 ± 1.2 and 7.7 ± 1.3 μm, respectively. Furthermore, printing with pristine and silver‐modified resins on the same sample allows selective plating: only the modified resin is plated leaving the pristine resin unaffected. A proof‐of‐concept temperature sensor is manufactured to demonstrate the performance of the printed interconnects.

Investigation on electroless copper metallization on FDM built ABS parts

In this work, metallization of copper on acrylonitrile-butadiene-styrene (ABS) substrate has been fabricated by fused deposition modelling (FDM) using electroless process avoiding chemicals detrimental to environment. Four acidic baths containing five weight percentages of HF, H 2 SO 4 , HNO 3 and H 3 PO 4 varying copper sulphate (CuSO 4 ) at 10, 15 and 20 weight percentage in each bath has been used for metallization. The metallization process in each bath has continued for seventy two hours. Copper coating obtained on the substrate by the use of different baths has been evaluated for electrical resistance, thickness of copper coated layer, percentage of copper present on the coated surface and adhesion performance. Scanning electron microscopy (SEM) and energy dispersion X-ray spectroscopy (EDX) results have indicated that all the baths are quite capable of deposition of copper on ABS substrates. However, HF bath has exhibited superior coating performance as compared to other baths. The thickness of copper coated layer and percentage of copper present in the coated layer has been found to be highest by the use of HF bath with fifteen weight percentage of CuSO 4 .

Synergy of mercaptosilane monolayer embedding and extremely dilute cobalt alloying for metallization of copper without a conventional metallic barrier

TaN/Ta bilayer is the standard barrier for the metallization of copper interconnects of current integrated circuit devices. However, thickness reduction of the barrier becomes a major bottleneck for the successful metallization of the downsizing copper interconnects in sub-7-nm technology nodes. Hence, the feasibility of a mercaptosilane self-assembled monolayer (SAM) as a barrier, as well as a seed adsorber for electroless copper metallization, is investigated. The mercaptosilane SAM after proper functionalization in a RCA-1 aqueous solution has the capacity to adsorb 3-nm-sized nickel particles, subsequently catalyzing the plating of extremely dilute Cu(Co) films with only 0.13 at.% of Co. Using the adhesion strength (13.1 MPa) and threshold temperature for thermal failure (450 °C) of pristine Cu/SiO2 as a control, Cu/SAM/SiO2 (pristine state) yields markedly enhanced adhesion strength and threshold temperature of 39.4 MPa and 500 °C, respectively. Cu(Co)/SAM/Cu exhibits even a much higher adhesion of 53.8 MPa (pristine state) and 74.6 MPa (annealed state). Seeding of the nickel particles by the functionalized mercaptosilane monolayer is clarified from the viewpoints of solution chemistry and proton exchange mechanism, and why only a tiny amount of cobalt is incorporated onto copper matrix is explained. The synergetic effect of the mercaptosilane monolayer embedding and extreme dilution of cobalt alloying on enhancing the adhesion and thermal stability of copper films is discussed considering the context of previous studies.

Environmental friendly electroless copper metallization on FDM build ABS parts

In this paper, environment-friendly electroless metallization of copper (Cu) on acrylonitrile–butadiene–styrene (ABS) parts fabricated through fused deposition modelling route of rapid prototyping process has been proposed. The metallization process eliminates etching as well as the use of high cost palladium (Pd) for activation. For surface preparation, aluminium (Al)-enamel paste has been used. Five different acidic baths are used for electroless Cu deposition. Each bath contains 15 wt% copper sulfate (CuSO4) solution and 5 wt% of any one solution among hydrofluoric acid (HF), sulfuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid (HNO3) and acetic acid (CH3COOH). Metallization of Cu on ABS part has been made at different deposition time (2, 24, 48 and 72 h) in ambient condition. After successful deposition of Cu, the surfaces are characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. Adhesion assessment of samples in different baths has been made. It is observed that all baths are capable of formation of Cu crystals on ABS surface. However, superior deposition on surface in terms of electrical conductivity and uniformity in deposition surface is obtained in HF, H2SO4 and H3PO4 baths.

Fabrication of Copper Electrode on Flexible Substrate Through Ag+-Based Inkjet Printing and Rapid Electroless Metallization

Thin film electrodes on a flexible substrate have attracted significant interests from the community for developing flexible and wearable electronics. A prompted routine for inkjet printing conductive patterns on a flexible plastic substrate is developed in this study, in which silver ion-based catalytic ink is utilized as a seed layer for electroless copper metallization in room temperature. Through optimization of Ag <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ink composition and printing process parameters, the pattern definition and continuity of electroless plated Cu have been improved. The resistivity at the same order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> Ω · cm as bulk copper is achieved, and the adhesion has passed a variety of mechanical tests. Comprehensive characterizations of the printed samples at various processing stages are carried out and presented in this paper.

Electroless Plating of NiP and Cu on Polylactic Acid and Polyethylene Terephthalate Glycol-Modified for 3D Printed Flexible Substrates

Electroless nickel and copper metallization of 3D printed polymers like polylactic acid and polyethylene terephthalate glycol modified is presented. The plating process is tested on suitable samples, which reproduce the characteristic morphologies used in 3D printing of objects. An alkaline etching is used for both polymers in order to modify the surface properties and to enhance the adhesion and uniformity of the metallic coating. In the case of polylactic acid, a plasma treatment is applied as well to further improve adhesion of the metallic coating. For the activation of the surface, a tin free process involving an immersion in a palladium solution and subsequent reduction to form metallic nuclei is employed. Electrolytes are formulated and selected to operate in temperature ranges comparable with the glass transition temperatures of the polymers. Adherent and uniform layers of NiP (3–4% P wt) and Cu can be easily obtained for esthetic and functional applications, also on flexible substrates.

Ultrasound assisted dispersal of a copper nanopowder for electroless copper activation

This paper describes the ultrasound assisted dispersal of a low wt./vol.% copper nanopowder mixture and determines the optimum conditions for de-agglomeration. A commercially available powder was added to propan-2-ol and dispersed using a magnetic stirrer, a high frequency 850kHz ultrasonic cell, a standard 40kHz bath and a 20kHz ultrasonic probe. The particle size of the powder was characterized using dynamic light scattering (DLS). Z-Average diameters (mean cluster size based on the intensity of scattered light) and intensity, volume and number size distributions were monitored as a function of time and energy input. Low frequency ultrasound was found to be more effective than high frequency ultrasound at de-agglomerating the powder and dispersion with a 20kHz ultrasonic probe was found to be very effective at breaking apart large agglomerates containing weakly bound clusters of nanoparticles. In general, the breakage of nanoclusters was found to be a factor of ultrasonic intensity, the higher the intensity the greater the de-agglomeration and typically micron sized clusters were reduced to sub 100nm particles in less than 30min using optimum conditions. However, there came a point at which the forces generated by ultrasonic cavitation were either insufficient to overcome the cohesive bonds between smaller aggregates or at very high intensities decoupling between the tip and solution occurred. Absorption spectroscopy indicated a copper core structure with a thin oxide shell and the catalytic performance of this dispersion was demonstrated by drop coating onto substrates and subsequent electroless copper metallization. This relatively inexpensive catalytic suspension has the potential to replace precious metal based colloids used in electronics manufacturing.

Potential low‐cost palladium‐alternatives for activating electroless copper deposition

PurposeThe purpose of this paper is the investigation of the catalytic activities of selected metals in reductant oxidation.Design/methodology/approachElectrochemical measurements were carried out in order to obtain information about the catalytic activity of copper, silver, nickel, and palladium in the oxidation reactions of formaldehyde and glyoxylic acid.FindingsMetals with high‐catalytic activity for each reductant oxidation can be determined by using cyclic voltammetry. These metals are suitable for an improved activation process in electroless copper deposition. Electrochemical measurements show that palladium does not have the highest catalytic activity of all tested metals. Therefore, alternative and inexpensive metals are explored as catalysts in electroless copper plating reactions.Research limitations/implicationsAt the present time, formaldehyde is the established reducing agent in the electroless copper metallization of plated through‐holes. Because of its environmental impact, there is a need to replace formaldehyde. A change in reducing agent can lead to a change in the activation process, because there is no one metal that appears to be a good catalyst for the oxidation of all reducing agents that have been employed for electroless deposition. This paper suggests that further improvement in electroless copper plating processes may be possible with an alternative activation procedure that yields higher catalytic activity.Originality/valueBased on the results of experiments, the development and optimization of activation processes in electroless copper plating can be advanced.

Chemical “kick start” for the autocatalytic formaldehyde‐free electroless copper plating process

PurposeThe purpose of this paper is to examine the use of additives in formaldehyde‐free copper‐plating solutions with low reducing agent (RA) concentration to improve the start reaction of electroless copper deposition and to enable a copper‐plating process which is more environmentally friendly.Design/methodology/approachDifferent additives were investigated and their influence on the plating reaction and deposition rate was elucidated using several deposition trials.FindingsOn palladium‐activated base material, the additives reacted with the palladium and generated additional electrons in the initial phase of the deposition. Thus, the adequate supply of electrons from two sources (RA and additive) permits the deposition of a homogeneous and compact copper layer.Research limitations/implicationsAt the present time, formaldehyde is the established RA in the electroless copper metallization process used with plated through‐holes. Because of its environmental impact, there is a need to replace formaldehyde. In this investigation, the more environmentally friendly glyoxylic acid is used as an autocatalytic RA. However, glyoxylic acid is more expensive and causes undesirable side reactions. In order to keep process costs under control, the concentration of glyoxylic acid in the copper bath should be reduced without affecting the quality of the copper deposits.Originality/valueAdditives can compensate for the lower RA concentration, and thus the lack of essential electrons for the copper deposition.

Atmospheric-Pressure Plasma Amination of Polymer Surfaces

Using dielectric barrier discharges (DBDs) in suitable gas atmospheres, appreciable densities of amino groups can be generated on polymer surfaces. After the introduction and a few remarks on analytical methods for the determination of functional groups densities, this paper presents a short summary of recent studies on the mechanism of the polymer surface amination from nitrogen and nitrogen/hydrogen mixtures, and possible relevant precursor species. Combination of chemical derivatization with quantitative FT-IR spectroscopy was employed for the determination of primary amino groups densities introduced on polyolefin surfaces in DBD afterglows in N2 and N2 + H2 mixtures. Owing to the possibility to generate atmospheric-pressure plasmas in sub-mm3 volumes, DBD plasmas can be used to modify polymer surfaces area selectively: a new process termed 'plasma printing' can be applied for the achievement of micropatterned surface modifications, such as hydrophilization/hydrophobization or chemical functionalization. Direct-patterning polymer surface modification processes are of interest for biochemical/biomedical applications as well as for polymer electronics. Two examples are presented in more detail: • the area-selective plasma amination of carbon-filled polypropylene minidiscs to manufacture microarrays with peptide libraries utilizing parallel combinatorial chemical synthesis, and •the continuous treatment of polymer foils by means of reel-to-reel patterned plasma amination for the subsequent electroless copper metallization, leading to a fast and highly efficient process for the manufacture of structured metallizations for flexible printed circuits or RFID antennas.

Control of Topographical Selectivity in Palladium-Activated Electroless Copper Metallization

From our study on copper electroless deposition, we have successfully controlled selective copper growing only in vias and trenches of TiN/SiO 2 /Si and Ta/SiO 2 /Si substrates by adapting the accelerating and inhibiting effects of additives and modulating the condition of palladium activation. 2,2'-Dipyridyl and polyethylene glycol play a significant role of inhibitor and accelerator, which contributed to the selective growing during the electroless deposition process. Furthermore, we found that the postcleaning conditions of deionized water rinsing and N 2 blowing after Pd activation also strongly affects the topographical selectivity of copper growing in our electroless deposition.

Geometric linewidth and the impact of thermal processing on the stress regimes induced by electroless copper metallization for Si integrated circuit interconnect technology

Mechanical strains and stresses are a major concern in the development of copper-based on-chip metallization. Synchrotron x-ray topography (SXRT), micro-Raman spectroscopy, finite element modeling (FEM), and atomic force microscopy (AFM) have been used to examine the strain fields imposed by electroless Cu metallization on the underlying Si. As expected, we have observed enhanced strain regions close to the metal line edges. These strain fields tend to zero at annealing temperatures approaching 200°C, and thereafter the magnitudes of the strain fields at 300°C and 400°C are much higher, implying a return to a higher strain regime. Although the strain transition point is slightly different from the SXRT result, the FEM results confirm the existence of a zero-strain transition point as a function of thermal anneal. We have also examined the generated stress in Si as a function of Cu linewidth L. We have found that the stress σXX due to the electroless copper metallization is empirically related to the Cu linewidth in terms of an exponential distribution. For Cu linewidths less than 20μm, the stress magnitudes increased with decreasing Cu linewidth due to the thermal stress in the absence of self-annealing, whereas the stress decreased with increasing linewidths in the range of 60–100μm due to a relief of the thermal stress possibly via the self-annealing effect. This self-annealing phenomenon was observed using AFM. It is observed that the stresses in the Si shifted to a compressive state after annealing at 400°C.

Comparison of processes for selective electroless copper plating on 96% alumina photo-selectively activated using chlorinated and non-chlorinated photochemical coatings: photoactivity and reliability studies

Photo-activation processes using a wide variety of photochemical coatings as activation precursors have been reported in recent years. This work presents a comparison of a series of chlorinated and non-chlorinated photochemicals in a photo-activation process for electroless copper metallization on alumina ceramic substrates. A purpose-built KrCl * excimer lamp contact mask aligner was used for the photo-selective activation step. UV spectral studies show that two chlorinated photochemicals are less susceptible to photolysis in a 222 nm excimer lamp contact mask aligner than the non-chlorinated photochemical. Evidence has been found that the photolysis process is temperature assisted and requires elevation of the substrate to temperatures in the range 60–80 °C for decomposition to take place on a time scale of the order of tens of minutes. Adhesion, resistivity and sawability test results for the electroless copper metallizations on 96% alumina activated are presented.

Application Of Electroless Copper Plating Technology For Thin Film Laminated Multilayer Wiring Board

AbstractA thin film laminated high density multilayer wiring board was developed using electroless copper plating technology. The basic steps in the fabrication process are: laminating the high heat resistant polymer film, forming via holes by plasma etching and filling them by electroless copper metallization. The 25 μm diameter via holes can be completely filled with copper using the selective electroless plating method. This paper describes the fabrication process, focusing particularly on electroless copper plating.

Laser assisted metallization of polyphenylquinoxaline

Electroless copper metallization of polyphenylquinoxaline (PPQ) was carried out using a palladium acetyl acetonate (Pd(acac) 2) precursor. A continuos wave Ar ion laser was used to decomposed Pd(acac) 2 organometallic layer deposited onto the PPQ. The samples were then processed in a copper electroless plating solution of conventional composition. The sample surfaces were analysed by X-ray photoelectron spectroscopy before any treatment, after laser irradiation and at the very first stages of copper growth. An estimate of the temperature reached by the polymer under the laser beam was obtained by analysing the interference pattern reflected by the surface. Breakdown fields of the polymer dielectric were found in the region of 1 MV cm −1 and are weakly affected by the electrochemical processes.

Selective electroless copper metallization of palladium silicide on silicon substrates

A scheme for selective electroless copper patterning of Si wafers has been developed with palladium silicide as the catalytic layer initiating copper deposition. Thermal conversion of a palladium layer to silicides on a SiO2 patterned silicon substrate, followed by an acid etching of the unreacted palladium on the SiO2 surfaces, leaves only the silicided regions at the base of the windows for electroless copper deposition. Excellent via-filling down to 0.5-μm dimensions and an aspect ratio of 6 has been demonstrated. The thin copper deposited on the Pd2Si has a resistivity of ∼2.0 μΩ cm. Contactless photocarrier decay measurements indicate virtually no degradation of Si lifetimes by these processing steps.