Additive Fabrication of Conductive Patterns by a Template Transfer

Research Article www.acsami.org

Additive Fabrication of Conductive Patterns by a Template Transfer Process Based on Benzotriazole Adsorption As a Separation Layer Yu Chang and Zhen-Guo Yang* Department of Materials Sciences, Fudan University, 220 Handan Road, Shanghai, China S Supporting Information *

ABSTRACT: The traditional subtractive process to fabricate conductive patterns is environmentally harmful, wasteful, and limited in line width. The additive process, including direct printing of conductive paste or ink, direct printing of catalytic ink, laser-induced forward transfer, etc., can solve these problems. However, the current additive process also faces many difficulties such as low electrical and adhesion properties, low pattern thickness, high cost, etc. Benzotriazole (BTA), as widely used corrosion inhibitor, can be adsorbed onto a copper surface. The electroplated copper film on BTA-adsorbed copper foil shows poor adhesion. On the basis of this phenomenon, a novel template transfer process to additively fabricate conductive patterns has been developed. A permeant antiadhesive mask is printed on carrier copper foil, and then, BTA is adsorbed onto the exposed area of the carrier foil, thus forming the template. The template is electroplated to grow conductive patterns in the exposed parts, and then can be adhered to the flexible substrate. The substrate is peeled off, with the transfer of the conductive patterns to the substrate, to form the designed conductive patterns on PET. By reimmersing the template into BTA solution, the template can be used again. The mechanism of BTA adsorption and the reason for the low peeling strength are researched using Raman spectra, XPS and electrochemical impedance spectroscopy. Copper patterns more than 20 μm in thickness can be prepared on PET, the resistivity of the prepared copper patterns is 2.01 μΩ cm, which is about the same as bulk copper, and the peeling strength of the pattern on PET is measured to be 6.97 N/cm. This template transfer process, with no waste, low pollution, high electrical and adhesion properties, and low cost, shows high potential in the large scale manufacturing of electronic devices, such as RFID circuitry, FPCs, etc. KEYWORDS: benzotriazole, additive process, conductive patterns, electroplating, peeling strength

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INTRODUCTION

wastes. Furthermore, the nonvertical etching at the mask margin will narrow the line, and the line width is limited.1−4 The additive process, which is free from etching, avoids the major problems of the subtractive process. Screen printing of conductive paste has been widely used for its simple procedure, no waste and low pollution, especially in the circuitry printing of sensors and photovoltaic cells.5,6 However, the conductivity of conductive paste is far below that of bulk metal. Non-noble metal conductive paste always faces the problem of oxidation while curing, thus further reducing the conductivity. Noble metal conductive paste is stable under heating, yet its cost is

Conductive patterns are the basic units of printed circuit boards (PCBs), flexible printed circuits (FPCs), radio frequency identification (RFID) tags, sensors, photovoltaic cells, etc. The conventional method to fabricate conductive patterns of PCBs, FPCs, and RFID tags, which is called the subtractive process, is thought to be wasteful, environmentally harmful and limited in line width. A photolithographed mask is prepared on a metal-clad substrate, and then, the exposed metal is etched, leaving the designed metal patterns. The etching step, on one hand causes waste of the metal, and on the other hand, it produces a large amount of pollutant with heavy metal ions. The photolithographic process, which is almost half of the production cost, is complicated and generates many liquid © XXXX American Chemical Society

Received: January 14, 2016 Accepted: May 12, 2016

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DOI: 10.1021/acsami.6b00499 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagrams of the template transfer process to additively fabricate conductive patterns on a flexible substrate.

Teija Laine-Ma, etc., reported a transfer process to additively fabricate embedded copper patterns in a thermoplastic substrate for 3D circuitries.24 Electroplated copper film shows poor adhesion on a stainless steel surface for the passivation layer on the steel surface. Based on this mechanism, copper was electroplated on steel carrier foil with a plating resistant mask to form the designed patterns, followed by dissolution of the mask. The steel foil with copper patterns was hot pressed with a thermoplastic substrate, embedding the copper patterns into the substrate. By peeling off the steel foil, the copper pattern embedded in the substrate was thus prepared. This process enables additive preparation of non-noble conductive patterns with high electrical properties, high adhesion and low cost. However, it fails to solve the problem of multipreparation of the mask. Only some specific electroplating baths can be used for the passivation layer will be corroded by acid, chloride ions, and some other corrosives (an acidic copper plating bath is most often used in the manufacturing of PCBs). Benzotriazole (BTA) and its derivatives are effective corrosion inhibitors for copper and have been widely used to protect copper products from oxidation. It is known that a protective barrier layer consisting of Cu-BTA complexes is formed when copper is immersed in BTA solution. However, contradictory mechanisms and models of its action have been proposed. The structure of the Cu-BTA complex and the mechanism as to how BTA links onto the copper surface are not completely understood in spite of the application of various techniques.25−31 In addition, people have also found that the electroplated metal film on BTA adsorbed copper foil shows poor adhesion. However, the mechanism is also unknown.32,33 In this paper, a simple process based on pattern transfer and electroplating to additively fabricate conductive patterns on a flexible substrate is developed. The process, which can be called template transfer process, is divided into the following two steps: the fabrication of the template and the fabrication of the conductive patterns (see Figure 1). First, a copper-clad polyethylene terephthalate (PET) film is used as the carrier substrate, and then, a permanent antiadhesive mask is printed on the copper surface, exposing the designed patterns. Afterward, the carrier film is immersed into the BTA aqueous solution. BTA will react with the copper surface and selfassemble the separation layer with several layers of molecules on the exposed area. After rinsing and drying, the template is complete. The template is then electroplated under the coverage of the mask. Metal will grow on the separation layer

very high. The inkjet printing of nanoparticle conductive ink has been widely researched in recent years. Its potential applications cover the circuitry preparation of FPCs, RFID tags, sensors, organic transistors, organic light emitting diodes (OLEDs), etc.7−11 However, conductive ink is also troubled by its low conductivity, low stability and high cost. Moreover, the low production efficiency of inkjet printing limits its application in large scale manufacturing. Printing with catalytic ink followed by metallization by electroless plating is a promising additive method.12−16 The conductivity of the plated metal can reach the same level as that of bulk metal, no oxidation problem exists when non-noble metal is plated. The limitations of catalytic ink are its poor adhesion and low line thickness. The adhesion between the plated metal and the substrate is reduced significantly with increased plating time, even though many modifications to the ink and the substrate have been applied.17−20 To guarantee the adhesion, the plating time is decreased, leading to the low line thickness (less than 1 μm, commonly the copper thickness for PCBs is above12 μm in order to reduce the electrical and signal attenuation in circuitry), and thus the resistance of the pattern is large. The direct printing of conductive paste, conductive ink, and catalytic ink is much simpler and produces less pollution than the conventional subtractive process. However, in the situation that high conductivity, high line thickness, high adhesion, low cost, and large scale production are required, such as for PCBs, FPCs, RFID tags, etc., they cannot fulfill all of these requirements. Laser-induced forward transfer (LIFT) is a widely researched process to transfer metal, polymer, or biomacromolecules from a source film to the target substrate using a focused laser beam. It can be used to additively fabricate metal patterns without a lithography mask.21−23 A metal film is deposited on a transparent support as the metal donor, and the acceptor substrate is placed very close to the donor. The laser beam is focused on the designated area of the metal donor, and the metal will be made molten and transferred to the acceptor substrate. Submicrometer and even nanoscale metal points and patterns can be prepared using LIFT. However, as a point-topoint fabrication process, the production efficiency of LIFT is low. Moreover, a complicated and expensive laser device is needed in the LIFT process, and common polymer substrates cannot be used as an acceptor substrate because of the high process temperature. Therefore, LIFT is also not suitable for the circuitry fabrication of PCBs, FPCs, RFID tags, etc. B


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ACS Applied Materials & Interfaces and fill in the exposed part of the mask. Afterward, the flexible substrate with a pressure-sensitive adhesive (pressure-sensitive adhesive is adhesive that forms a bond when pressure is applied to marry the adhesive with the adherent, which is, mainly based on acrylic elastomer) covering is adhered to the electroplated template, with heating and pressure to remove the air out of the interface. The flexible substrate is then peeled off from the template. Meanwhile, the plated patterns can be easily transferred to the flexible substrate due to the existence of the separation layer. Finally, after being redipped in the BTA solution, the template can be electroplated and reused to fabricate conductive patterns As an etching-free additive method, the template transfer process possesses the advantages of no waste, low pollution, a line width only related to the accuracy of the mask, and a mask that can be used repeatedly, thus reducing the cost and pollution of mask preparation and removal. Compared with the other additive processes mentioned above, this process does not have the problem of oxidation, the conductivity of the fabricated patterns is about the same as that of bulk metal, the patterns can tightly link with the substrate with the assistance of the adhesive, the thickness of the line ranges from several micrometers to several hundred micrometers, the cost of the process is low because non-noble metals like copper and nickel can be plated, almost all types of substrates are available, and corrosive plating bath containing acid or chloride ions can be used. Combined with the continuous electroplating technology, the template transfer process can roll-to-roll manufacture conductive patterns with an extremely high production efficiency. The applications of the template transfer process can be found in the large-scale manufacturing of conductive patterns with a low cost, low pollution, and high performance, such as for FPCs and RFID tags.

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room temperature for 2 min and 5 wt % aquaria solution of H2SO4 at 50 °C for 5 min to remove the organic contamination and oxide on the surface. After rising and drying, the antiadhesive paste was screen printed onto the copper surface and cured at 80 °C for 1 h to form the mask. The copper foil with the mask was then immersed into 0.5 wt % BTA water solution at 50 °C, and the separation layer was thus formed on the exposed area on the copper surface. Finally, the template is complete after rising and drying. Preparation of Conductive Patterns on the Flexible Substrate. The template is electroplated in the electroplating bath containing CuSO4·5H2O (250 g/L) and H2SO4 (150 g/L) with a temperature of 45 °C and a current density of 15 A/dm2. Copper conductive patterns were deposited in the exposed area at a rate of approximately 3.73 μm/min. After rinsing and drying, the electrodeposited template is adhered to the flexible PET substrate with pressure-sensitive adhesive and then treated at 80 °C in a plate hotpresser to expel air at the interface. Afterward, the flexible PET substrate was peeled off from one side of the template at an angle of approximately 90°. The conductive patterns were transferred to the PET substrate, thus, a flexible PET substrate with copper conductive patterns adhered to the substrate was produced. After peeling, the template after peeling was reimmersed in a 0.5 wt % BTA aquaria solution at 50 °C for 5 min to rebuild the separation layer, and then the template can be reused to prepare conductive patterns. Characterization. Electrochemical impedance spectroscopy (EIS, CHI 660d) was operated in the frequency range of 0.01 Hz to 0.1 M Hz with an excitation signal amplitude of 50 mV at 25 °C. Measurements were obtained between the large area plate Pt electrode and the copper foil adsorbed with a separation layer. The exposed area of the copper foil is approximately 1 cm2, and the electrolyte is a 5 wt % water solution of H2SO4. The Raman spectra were obtained by a laser confocal micro-Raman spectrometer (Horiba Jobin Yvon XploRA). X-ray photoelectron spectroscopy (XPS) was carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). The surface profile was obtained on a Veeco Dektak 150 profilometer. The resistance of the patterns was measured by a four-point probe (Qingfeng, SB100/A). A 3D stereoscopic microscope (Hirox, KH-7700) was used for optical observations.

EXPERIMENTAL SECTION

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Materials. PET films (90 μm in thickness, Toray ZV10), pressuresensitive adhesive (467MP, 3M company, 60 μm in thickness, acrylic based adhesive), BTA (AR, Sinopharm), CuSO 4·5H2O (AR, Sinopharm), H2SO4 (AR, Sinopharm), copper foil (50 μm in thickness, 0.031 μm in surface roughness, Sinopharm), polyvinyl butyral (M.W. 170 000−250 000, Butvar B-72), ethylene glycol butyl ether (AR, Sinopharm), terpineol (AR, Sinopharm), hydroxylterminated poly(dimethylsiloxane) (3500 cSt, Aldrich), 3-mercaptopropyltriethoxysilane (95%, Shanghai Aladdin), hydrophobic nanosilica (7−40 nm, Shanghai Aladdin), silicon dioxide powder (2−5 μm in grain size, Shanghai Aladdin), and ethyl acetate (AR, Sinopharm). Preparation of the Antiadditive Paste. The antiadhesive mask is prepared by the screen printing of the antiadditive paste. Polyvinyl butyral (3 g), hydroxyl terminated poly(dimethylsiloxane) (3 g), hydrophobic nanosilica (0.45 g), and silicon dioxide powder (2.4 g) were mixed and stirred with ethylene glycol butyl ether (10 g) and terpilenol (5 g) at 50 °C for 2 h to obtain the uniform paste. 3mercaptopropyltriethoxysilane (0.2 g) was added before printing. Hydroxy-terminated poly(dimethylsiloxane) plays the main role in the antiadhesive performance because of its low surface free energy. Polyvinyl butyral is added to improve the adhesion with copper, while hydrophobic nanosilica is used to increase the compatibility between different polymers. Silicon dioxide powder as the pigment can improve the rheological property of the paste, making the paste more suitable for screen printing. 3-Mercaptopropyltriethoxysilane on one hand acts as the curing agent of hydroxyl terminated poly(dimethylsiloxane) and on the other hand increases the adhesion of the paste with copper via Cu−S bonds. Preparation of the Template. Copper foil was adhered to the PET film to form the copper-clad PET film, with one side of the copper foil exposed. Then, the foil was immersed into ethyl acetate at

RESULTS AND DISCUSSIONS Adsorption of BTA onto the Copper Surface. BTA and its derivatives have been widely used as corrosion inhibitor for copper, because BTA can react with copper to form an insoluble water and oxygen-proof polymer on the surface. However, the reaction between the BTA and copper surface is not totally understood. The widely accepted viewpoint when BTA is adsorbed in aquaria solution is that several molecular layers of cuprous oxide are generated when copper is directly exposed in air. BTA has three N atoms (the N bonded with H designated as N1, the N next to N1 designated as N2, and the N opposite N1 designated as N3, shown in Figure 2a), and can react with cuprous oxide on the copper surface to produce a Cu(I)-BTA complex via a Cu−N1 bond, as shown in Figure 2b. After the surface of the copper is covered by one BTA molecular layer, the growth of the Cu(I)-BTA film is controlled by the transport of cuprous ions through the film itself. The cuprous ions from the copper surface transfer through the Cu(I)-BTA film, react with free BTA to form Cu(I)-BTA complexes and linked with existing Cu(I)-BTA via Cu−N3 coordination bond (as shown in Figure 2c). With continuous reactions at the end of the Cu(I)-BTA chain, the Cu(I)-BTA linear polymer is thus generated.34−42 The structure of the Cu(I)-BTA film on the copper surface is shown in Figure 3. The thickness of the separation layer is calculated through EIS data. The EIS spectra of the copper foil with different BTA adsorption times, the equivalent circuits and the fitted curves C


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layer of copper foil (including oxide and Cu(I)-BTA layers). Correspondingly, R1 represents the resistive behavior of the oxide layer, and C1 represents the capacitive behavior at the copper/oxide interface. Q2 represents the capacitive behavior of the electrical double layer at the copper/electrolyte interface, and R2 is the charge transfer resistance of the double layer.31,32,34 Q3 and R3 represent the capacitive and resistive behavior of the electrical double layer at the Pt/electrolyte interface. However, C1 and (Q3R3) can be ignored because their impendences are extremely low compared with the other units. Therefore, the equivalent circuits can be simplified to Re(Q1(R1(Q2R2)). The fitted curve is operated from 0.1 MHz to 1 Hz in order to avoid the lower frequency spectra, which represent the diffusion impendence of the ion. This will not affect the numerical calculation of the units in the equivalent circuits. The capacitance of the separation layer, which is located between the electric double layers at copper/electrolyte interface, is calculated from Q2 according to the equation C = (R12 − n2Q2)1/n2. The thickness of the film can then be calculated using the equation d = ε0εS/C, where ε0 is vacuum dielectric constant of 8.85 × 10−14 F/cm and S is the surface area of 1 cm2. The dielectric constant of the separation layer ε is 5.3, and is an average value of cuprous oxide (7.6) and the organic film (3).31,34,41 The surface area of the sample may be larger than 1 cm2 for the surface of the carrier copper foil is not totally flat, resulting in a small deviation of the thickness calculation. Because this systematic error exists in all the samples, the calculated results using the carrier copper foil with the same surface structure are comparable. The fitting results of the EIS spectra with different immersion time, including all of the units of the equivalent circuits, are listed in Table S1. Figure 4b indicates the relationship between the immersion time and the BTA thickness based on EIS data. During the first 10 s, the thickness of the separation layer increases rapidly to approximately 0.7 nm. This stage is the fast reaction between BTA and cuprous oxide to form the initial molecular layers of

Figure 2. (a) BTA molecular structure, (b) the equation between BTA with cuprous oxide, and (c) the formation of coordination bonds between Cu(I)-BTA.

are shown in Figure 4a. This experimental model, in which BTA adsorbed onto copper foil acts as the working electrode, a large-area Pt plate acts as the counter electrode, and dilute sulfuric acid acts as the electrolyte, is equivalent to the circuit of Re(Q1(R1C1(Q2R2)) (Q3R3) at higher frequency. Q is a constant phase element (CPE) representing the nonideal capacitance. Its impendence is calculated by Q = (Cjωn)−1. For n = 1, Q becomes an ideal capacitor, and for n = 0, Q becomes a simple resistor. Re is the impendence of the electrolyte between two electrodes. Q1 represents the capacitive behavior of the epitaxial

Figure 3. Structure of the Cu(I)-BTA film on the copper surface. The first several molecular layers of copper are oxidized to cuprous oxide. D


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Figure 4. (a) EIS spectra, the fitted curves and the equivalent circuits of the carrier copper foil with different BTA immersion times, (b) the thickness of the separation layer on copper foil with different BTA immersion times, (c) the EIS spectra, the fitted curves and the equivalent circuits of the carrier copper foil and peeled copper film with different peeling times; “CC” means carrier copper foil, “PC” means peeled copper film, and “CC RI” means the carrier copper foil reimmersed in BTA solution for 5 min, and (d) the thicknesses of the separation layers on carrier copper foil and peeled copper film with different peeling times.

peeled for a second and third time. The Cu(I)-BTA layer thickness on the carrier foil is reduced to 0.20 and 0.16 nm after the second and third peelings, respectively, but the Cu(I)-BTA thicknesses on the peeled copper films are just 0.10 and 0.07 nm. That is because the first BTA layer is connected with the carrier copper surface via strong Cu−N covalent bonds, which cannot be easily broken. After the first peeling, some part of the carrier copper foil is covered by just one layer of BTA molecules. Thus, during the next peeling, these parts will remain on the carrier foil, and only the Cu(I)-BTA not directly bonded to the carrier copper foil can be separated, which explains the unequal separation of the Cu(I)-BTA layer at second or third peeling. It is difficult to peel the plated copper more than three times, because the Cu(I)-BTA layer is too thin to completely cover the carrier copper surface after third peeling. The carrier copper foil is reimmersed into 0.5 wt % BTA solution to recover the Cu(I)-BTA layer, and the layer grows to 0.82 nm after 5 min. After peeling, the Cu(I)-BTA layer on the carrier copper surface becomes thin and cannot prevent the oxidation of copper. When the carrier copper is reimmersed into BTA solution, oxygen from the BTA solution can penetrate through the Cu(I)-BTA film to the copper surface and oxidize copper to cuprous ions. Meanwhile, the cuprous ions can also penetrate through the Cu(I)-BTA film and react with free BTA molecules to thicken the layer, until the Cu(I)-BTA layer is thick enough and the transport rates of cuprous ion and oxygen are significantly reduced. The rebuilding process of the Cu(I)-BTA separation layer on the copper surface guarantees repeated usage of the template. Spectrum Analysis. Raman analysis was used to detect the existence of Cu(I)-BTA on the surfaces of the carrier copper foil and peeled copper film, as shown in Figure 6. The broad

Cu(I)-BTA. With the thickening of the Cu(I)-BTA, the transport rate of the cuprous ions decreases greatly. Therefore, after 10 s, the layer grows slowly and remains stable at approximately 0.8 nm. Considering the molecular sizes of BTA (approximately 0.5 nm in width, 0.6 nm in length and 0.2 nm in height, calculated by bond length) and the cuprous ion (approximately 0.15 nm in diameter), there may be just two or three layers of Cu(I)-BTA after immersion for 5 min.37,40 Function of BTA while Peeling. To detect the role of Cu(I)-BTA during peeling, the thicknesses of the separation layers on the template and the peeled copper film were measured, as shown in Figure 4c, d. The fitting results of EIS data are listed in Table S2. The initial thickness of the Cu(I)BTA film on the carrier copper surface after 5 min of immersion was calculated to be 0.79 nm. After electroplating and peeling, the thickness of the Cu(I)-BTA on the carrier copper foil is 0.39 nm, which is about half of the initial thickness. A separation layer of approximately 0.44 nm was found on the peeled copper film, indicating the Cu(I)-BTA layer is divided equally during peeling. This result confirms that the separation happens in the Cu(I)-BTA complexation chain. The Cu(I)-BTA chain is connected via the weak coordination bond between the cuprous ion and N3 from another Cu(I)BTA molecule. Thus, in the peeling step, the Cu(I)-BTA chain breaks from the coordination bonds randomly; therefore, the separation layer will separate into half on the surfaces of both the carrier foil and the plated film, as shown in Figure 5a. The EIS curves and the Cu(I)-BTA layer thicknesses of the samples under the same growth conditions are also measured, and they are shown in Figure S1 and Table S3. Rebuilding of the Cu(I)-BTA Layer after Peeling. After the first peeling, the carrier copper foil is re-electroplated and E


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peeling is confirmed. XPS is also used to verify the existence of BTA and is shown in Figure S2. Measurement of the Peeling Strength. The peeling strengths of the plated copper film from the carrier foil at different BTA immersion times were measured according to ASTM D3330 (adhesive tape is adhered to the plated film and peeled at 90°) as shown in Figure 7a, whereas the concrete data are listed in Table S4. The initial 10 s is the period of construction of first several Cu(I)-BTA layers, the peeling strength is very large, and some part of the plated film remains on the carrier foil. For the situation that monolayer BTA is adsorbed onto the foil, inevitably, there are many voids among BTA molecules, exposing the copper surface underneath.42 As the molecular size of cupric ions and copper atoms is much smaller than that of BTA, during electroplating, cupric ions can pass through the voids and be deposited on the surface. As a result, the plated copper film is linked with the carrier foil via these connections at the voids, thus the peeling strength is very large (shown in Figure 5b). With the growth of the Cu(I)-BTA film, these voids can be covered, avoiding the connections during electrodeposition. Therefore, the peeling strengths of the plated copper films reduces gradually and remains stable at approxianately 0.07 N/cm after 2 min. The relationship between the peeling strength and peeling time is shown in Figure 7b, whereas the concrete data are listed in Table S5. The peeling strength of the plated copper film from the carrier foil after BTA immersion for 5 min is 0.074 N/ cm. Copper is then replated on the carrier foil and peeled for second and third times, with the peeling strengths increasing to 0.15 N/cm and 1.08 N/cm, respectively. With the peeling of the plated film, the separation layer on the carrier foil becomes thinner, and some voids among BTA molecules are exposed, leading to the increase in the peeling strength. After being peeled three times, the carrier foil is then reimmersed into BTA solution to rebuild the separation layer. With the thickening of the Cu(I)-BTA layer, the peeling strength decreases to 0.084 N/cm, which is about the same as the first peeling strength. The peeling strength of the plated copper film from the carrier copper foil is also influenced by the surface roughness of the carrier foil, which is shown in the Figure S3. The peeling strength between the antiadhesive mask and the adhesive is measured to be 1.96 N/cm. The carrier foil with the mask can be easily peeled, with consideration of the prevention of mistaken peeling. The peeling strength of the conductive patterns on PET was measured to be 6.97 N/cm, which means it can meet the requirements of the FPCs and RFID tags (the industry standard is 0.7 kg/cm, 6.86 N/cm). The peeling strength of the copper patterns is related to the surface morphology of the plated copper and the properties of the adhesive, and the peeling strength can be greatly promoted by improvement in the electroplating bath, post-treatment of the copper patterns and modification of the adhesive. Electrical Measurements. The electroplating of copper is a widely used technology in the PCB industry to thicken the conductive patterns and manufacture conductive holes. The resistivity of the electroplated copper can reach the same level as that of bulk copper, and is affected by the electroplating bath formula, current density, temperature, etc. In this paper, a simple electroplating bath without special additives is used for the research. The resistivity of the electroplated copper is calculated to be 2.01 μΩ cm at 20 °C with the plating conditions described in the Experimental Section, and is approximately 16.9% larger than the resistivity of bulk copper

Figure 5. (a) Electroplated copper film is peeled off from the carrier copper foil with a multilayer Cu(I)-BTA covering. (b) Plated copper film is connected with the carrier copper foil covered by monolayer Cu(I)-BTA via the voids between BTA molecules.

Figure 6. Raman spectra of the surfaces of the carrier copper foil and peeled copper film.

bands from 350 to 650 cm−1 are assigned to the symmetric and asymmetric stretching vibrations of cuprous oxide. Characteristic Raman bands of BTA can be found at 787 cm−1 (carrier foil) and 789 cm−1 (peeled film) for the ring deformation mode; 1048 cm−1 (carrier foil) and 1046 cm−1 (peeled film) for the ring breathing mode; 1206 cm−1 (carrier foil) and 1208 cm−1 (peeled film) for the asymmetric stretching vibrations of (N−N−N); and 1384 cm−1 (carrier foil) and 1391 cm−1 (peeled film) for the ring stretching mode.25,42 Through Raman analysis, the division of the separation layer during F


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Figure 7. (a) Peeling strengths of the plated films from the carrier foil at different BTA immersion times, and (b) the peeling strengths of the plated films from the carrier foil with different peeling times.

Figure 8. (a) Cross-section of conductive patterns on a PET substrate, (b) the bottom surface of the conductive patterns, (c) the RFID tag circuit fabricated by the electroplating transfer process with the template, and (d) the RFID tag shows high flexibility.

(1.72 μΩ cm at 20 °C). This may result from the micro void and crackle in the plated copper film; and can be improved by adding particular additives to the electroplating bath or using a commercial electroplating bath. The plating rate is measured to be 3.73 μm/min, and this value is mainly influenced by the plating current density. The plated copper patterns may be oxidized in the atmosphere, thus the conductivity will decrease. Surface coating, surface plating with antioxidation metal, organic solderability preservatives, etc., can be used to prevent the oxidation of copper, and these methods are well researched and have been widely used in industry. The resistance stability of the conductive patterns after cyclical bending is shown in Figure S4. Microstructure Analysis. The cross-section of the conductive patterns on the PET substrate (Figure 8a)

demonstrates the structure of the substrate-adhesive-copper. The coarse bottom surface (the surface at the copper/adhesive interface, that is also the surface against the electroplating bath during electroplating) of the patterns enhances the adhesion by increasing the mechanical anchor. The magnified image of the bottom surface of the conductive patterns shown in Figure 8b exhibits a morphology of small knobs and the surface roughness Ra is measured to be 1.56 μm. The surface morphology of the bottom surface can be modified by changing the electroplating bath formula to meet the needs of different applications. The sample of the RFID tag circuitry (20 μm in pattern thickness) fabricated by the template transfer process is shown in Figure 8c, d with the template. The properties of the RFID circuitry, more images about the microstructure and more sample images are shown in Table S6 and Figures S5 and S6. G



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CONCLUSION

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ASSOCIATED CONTENT

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REFERENCES

(1) Teng, K. F.; Vest, R. W. Liquid Ink Jet Printing with MOD Inks for Hybrid Microcircuits. Components, Hybrids, and Manufacturing Technology. IEEE Trans. Compon., Hybrids, Manuf. Technol. 1987, 10, 545−549. (2) Lee, Y.; Choi, J.; Lee, K. J.; Stott, N. E.; Kim, D. H. Large-scale Synthesis of Copper Nanoparticles by Chemically Controlled Reduction for Applications of Inkjet-printed Electronics. Nanotechnology 2008, 19, 415604. (3) Perelaer, J.; Smith, P. J.; Mager, D.; Soltman, D.; Volkman, S. K.; Subramanian, V.; Korvink, J. G.; Schubert, U. S. Printed Electronics: the Challenges Involved in Printing Devices, Interconnects, and Contacts Based on Inorganic Materials. J. Mater. Chem. 2010, 20, 8446−8453. (4) Lim, J. H.; Shim, J. H.; Choi, J. H.; Joo, J. H.; Park, K.; Jeon, H.; Moon, M. R.; Jung, D. G.; Kim, H. S.; Lee, H. J. Solution-Processed Ingazno-Based Thin Film Transistors for Printed Electronics Applications. Appl. Phys. Lett. 2009, 95, 012108. (5) Sanghavi, B. J.; Mobin, S. M.; Mathur, P.; Lahiri, G. K.; Srivastava, A. K. Biomimetic Sensor for Certain Catecholamines Employing Copper (II) Complex and Silver Nanoparticle Modified Glassy Carbon Paste Electrode. Biosens. Bioelectron. 2013, 39, 124−132. (6) Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-To-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36−49. (7) Ko, S. H.; Pan, H.; Grigoropoulos, C. P.; Luscombe, C. K.; Frechet, J. M. J.; Poulikakos, D. All-Inkjet-Printed Flexible Electronics Fabrication on a Polymer Substrate by Low-Temperature HighResolution Selective Laser Sintering of Metal Nanoparticles. Nanotechnology 2007, 18, 345202. (8) Kang, J. S.; Kim, H. S.; Ryu, J. G.; Hahn, H. T.; Jang, S. H.; Joung, J. W. Inkjet Printed Electronics Using Copper Nanoparticle Ink. J. Mater. Sci.: Mater. Electron. 2010, 21, 1213−1220. (9) Reinhold, I.; Hendriks, C. E.; Eckardt, R.; Kranenburg, J. M.; Perelaer, J.; Baumann, R. R.; Schubert, U. S. Argon Plasma Sintering of Inkjet Printed Silver Tracks on Polymer Substrates. J. Mater. Chem. 2009, 19, 3384−3388. (10) Jeong, S.; Woo, K.; Kim, D. J.; Lim, S. K.; Kim, J. S.; Shin, H. J.; Xia, Y. N.; Moon, J. Controlling the Thickness of the Surface Oxide Layer on Cu Nanoparticles for the Fabrication of Conductive Structures by Ink-Jet Printing. Adv. Funct. Mater. 2008, 18, 679. (11) Dearden, A. L.; Smith, P. J.; Shin, D. Y.; Reis, N.; Derby, B.; O'Brien, P. A Low Curing Temperature Silver Ink for Use in Ink - Jet Printing and Subsequent Production of Conductive Tracks. Macromol. Rapid Commun. 2005, 26, 315−318. (12) Tseng, C. C.; Chang, C. P.; Sung, Y.; Chen, Y. C.; Ger, M. D. A Novel Method to Produce Pd Nanoparticle Ink for Ink-Jet Printing Technology. Colloids Surf., A 2009, 339, 206−210. (13) Chang, Y.; Tao, Y.; Zhang, Q.; Yang, Z. G. Selective Adsorption of Catalyst and Copper Plating for Additive Fabrication of Conductive Patterns and Through-Holes. Electrochim. Acta 2015, 158, 7−12. (14) Chen, W. D.; Lin, Y. H.; Chang, C. P.; Liu, Y. M.; Ger, M. D. Fabrication of High-Resolution Conductive Line via Inkjet Printing of Nano-Palladium Catalyst onto PET Substrate. Surf. Coat. Technol. 2011, 205, 4750−4756. (15) Horiuchi, S.; Nakao, Y. Platinum Colloid Catalyzed Etchingless Gold Electroless Plating with Strong Adhesion to Polymers. Surf. Coat. Technol. 2010, 204, 3811−3817. (16) Lu, Y. X. Improvement of Copper Plating Adhesion on Silane Modified PET Film by Ultrasonic - Assisted Electroless Deposition. Appl. Surf. Sci. 2010, 256, 3554−3558. (17) Cheng, K.; Yang, M. H.; Chiu, W. W. W.; Huang, C. Y.; Chang, J.; Ying, T. F.; Yang, Y. Ink - Jet Printing, Self - Assembled Polyelectrolytes, and Electroless Plating: Low Cost Fabrication of Circuits on a Flexible Substrate at Room Temperature. Macromol. Rapid Commun. 2005, 26, 247−264. (18) Garcia, A.; Polesel-Maris, J.; Viel, P.; Palacin, S.; Berthelot, T. Localized Ligand Induced Electroless Plating (LIEP) Process for the

An additive process to fabricate conductive patterns on a flexible substrate based on electroplating and pattern transfer is developed in this paper. The antiadhesive mask is printed on copper-clad PET, with BTA adsorbed on the exposed area to form the template. The template is electroplated to grow conductive patterns in the exposed parts and then adhered to the PET substrate with a pressure-sensitive adhesive covering. By peeling the PET substrate off from the template, the conductive patterns can be transferred to the substrate with very a low peeling strength of 0.074 N/cm, thus forming the designed conductive patterns on a PET substrate with 2.01 μΩ cm in resistivity and 6.97 N/cm in peeling strength. The template can be used again by redipping in BTA solution. The mechanism as to why the electroplated copper film shows a low peeling strength on the carrier copper foil with adsorbed BTA is also researched. BTA can react with cuprous oxide on the copper surface and form Cu(I)-BTA complexes. The Cu(I)BTA complexes are connected to form the linear polymer separation layer by weak coordination bonds between cuprous ions and the N atom of each Cu(I)-BTA complex. When peeling the plated copper film from the template, the Cu(I)BTA layer at the interface will be divided due to the weak coordination bonds, resulting in a low peeling strength. The Cu(I)-BTA layer on the template after peeling can be rebuilt by repeated immersion in BTA solution, making the template recyclable. The applications of this template transfer process can be found in the large-scale manufacturing of conductive patterns with low waste, low pollution, high performance, and low cost, such as FPCs, RFID tags, etc.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00499. Fitting results of EIS spectra recorded, EIS curves of the carrier foil under the same BTA adsorption condition, XPS spectra, peeling strengths of the plated film from the carrier foil with different BTA immersion time and peeling times, relationship between the peeling strength of the plated copper film with the surface roughness of the carrier copper foil, properties of the RFID tag circuitry fabricated by the template transfer process, resistance stability of the conductive patterns after cyclical bending, surface morphologies of the carrier copper foil and the electroplated copper film, and the copper line approximately 150 μm in width (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-21-65642523. Fax: +86-21-65103056. Author Contributions

The manuscript was written through contributions all of the authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. H


Research Article

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Additive Fabrication of Conductive Patterns by a Template Transfer

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