Enhancing the electrical properties of inkjet-printed silver ink by electrolyte sintering, photonic sintering, and electroless plating

Conductive inkjet printing is an emerging rapid manufacturing technology in
 the field of smart clothing and wearable electronics. The current challenge
 in conductive inkjet printing includes upgrading of electrical performance
 of printed inks to the equivalent level to traditional conductors such as
 bulk silver and copper, especially for high-performance electronic
 applications such as flexible antennas and circuits. Post-treatments are
 commonly employed to enhance the electrical conduction of inkjet-printed
 tracks. This research discusses the effects of electrolyte sintering,
 photonic sintering and electroless copper plating on the DC electrical
 resistance and resistivity of inkjet-printed silver nanoparticles. From
 experimental results and measurements, it was found that all the
 post-treatment methods effectively improved the electrical properties of
 printed silver ink, but in different ways. The lowest resistance of 4.5 ?
 (in 0.1 mm ? 10 mm) and thickest (4.5 ?m) conductor were achieved by
 electroless copper plating, whereas the lowest resistivity (7.5?10-8 ??m)
 and thinnest (1.0 ?m) conductor were obtained by photonic sintering.


Introduction
With growing interest in smart clothing and wearable electronics, conductive inkjet printing receives a great deal of attention as one of the most effective techniques to add electrical functionality to non-conductive substances. Conductive inkjet printing places droplets of conductive material, which are ejected from nozzles, into a desired pattern onto flat substrates. While the conventional printing techniques such as photolithography [1] and screen printing [2] are still favourably employed for their capabilities to produce a mechanically durable and stable conductive structure, the production process could become quicker, simpler and of higher resolution at lower manufacturing cost with the conductive inkjet technology [3]. Ready-to-use inkjet technology enables a rapid prototyping of electronically-integrated textile products and accelerates a smooth transition from laboratories to manufacturing sites. It allows circuit designers to immediately test a new design, quickly make necessary edits, and therefore, save on lead time and development cost.
One of the most significant concerns in conductive inkjet printing is the resulting electrical performance. When metal inks are inkjet-printed, they will usually form low-density structures with many pores, and thus the electric performance of printed tracks is inferior compared with their bulk counterparts [4]. In order to obtain electrical conductivity closer to the bulk metal, a sintering (curing) is required, which aims to coalesce contacting metal nanoparticles into a continuous solid in order to reduce the free surface area and increase the structural density of the printed paths ( Fig. 1) [5,6]. Thermal sintering is one of the most conventional methods, in which treating samples are exposed to relatively high temperature (~200 o C) [7]. Although thermal sintering is simple to perform, there are several drawbacks associated with this method, such as inevitable thermal damage to substrates, long treatment duration and energy inefficiency [8]. In order to ease these issues, quick room-temperature sintering methods have been investigated.
One such method is photonic sintering [9], which exposes printed inks to a highly energized photonic pulse to raise the ink temperature instantly and induce coalescence of neighboring metal particles. Another room-temperature method is chemical sintering with electrolytes [10,11]. In this method, printed metal nanoparticles are physically contacted with a solution containing either inorganic or organic salts. Although the exact underlying mechanisms appears not clear, it is believed that the zeta potentials at metal nanoparticle surfaces are largely reduced by electrolytes, leading to coalescence of neighboring metal particles [10,11].
An alternative approach to enhance the electrical properties of inkjet-printed metal nanoparticles is electroless plating, which deposits metallic nanoparticles from a plating solution onto target surfaces [12]. Thus, whereas the sintering techniques mentioned above aim to improve the electrical conductivity by reducing the porosity of printed materials, electroless plating adds an extra conductive layer on top of the existing conductive surface to boost the electrical conduction (Fig. 2). A reducing agent supplies the major driving force to reduce metal cations and sequential deposition [13]. Although these sintering and electroless plating techniques have extensively discussed in literature, one critical concern is to provide a guideline for engineers and scientists to determine the technique that is best-suited for applications of their interests. For instance, for microwave circuit applications at about several hundred gigahertzes, reducing the DC resistance by building a thicker conductor would not be substantially beneficial because the effective conductivity is capped by the skin effect. Instead, reducing the DC resistivity could be more promising [14][15][16]. On the other hand, for certain low frequency applications, the thick conductor with low resistance could bring appreciable benefits.
With this in mind, this research investigated the effects of electrolyte sintering, photonic sintering and electroless copper (Cu) plating on DC resistance and resistivity of inkjet-printed silver (Ag) nanoparticle track for flexible electronics applications to establish a guidepost to choose an appropriate post-treatment. In order to demonstrate the best practice of conductive printing, a commercially-available silver ink and a poly(ethylene terephthalate) (PET) film were used in the experiments. Statistical procedures were employed to validate the significance of the post-treatments, and electrical properties of the post-treated conductors were discussed.

Inkjet printing
Linear and planar prints were created in an exact dimension using Adobe Illustrator. The dimensions were 0.1 mm × 10 mm for lines and 20 mm × 20 mm for squares, respectively as shown in Figure 3. The linear print was placed in parallel to the direction of the ink nozzle movement for the best printing result. A commercially available piezoelectric inkjet printer (Epson America Inc.) was used for printing. A clean and empty ink cartridge was filled with the Ag ink and the pre-treated PET film was fed through the sheet feeder. "Best photo" and "matte paper" settings were chosen in the printer's control panel to achieve the best printing quality. The printed films were dried at room temperature for 24 hours and treated either by electrolyte sintering (Section 2.3), photonic sintering (Section 2.4) or electroless plating (Section 2.5).

Electrolyte sintering
An electrolyte sintering solution was prepared following [17]. It was, in short, a 0.3 mol/L sodium chloride (NaCl) solution prepared with deionized water. A clean and empty ink cartridge was filled with this solution, and the solution was inkjet-printed on the top of the Ag prints. The NaCl-sintered print was dried at room temperature for 24 hours.

Photonic sintering
Photonic sintering was achieved with a commercially available photonic sintering device (Xenon Corporation) (Fig. 4). This sintering device was composed of a linear flash lamp, its housing with a mechanical aperture, high voltage power controller, sintering chamber, and a blower. The flash lamp with a lower cut off wavelength of 370 nm was installed in the housing, and the distance between the sintering bed and the lamp was set to 25.48 mm. This flash lamp produces extremely intense, incoherent, full-spectrum visible light for very short durations for coalescence of silver nanoparticles. The size of the aperture was set to 8 mm to control the treatment intensity. The entire surface of Ag prints was sintered by a photonic pulse for 520 μs at 3.8 kV.

Electroless plating
An electroless copper plating solution was prepared referring to [12]. The plating bath contained 0.072 mol/L copper(II) sulfate pentahydrate, 0.16 mol/L ethylenediaminetetraacetic acid, 0.00014 mol/L potassium hexacyanoferrate(II) trihydrate and 22.5 mL/L formaldehyde, and the pH was adjusted to 12.8 by dissolving sodium hydroxide. Then, the Ag print was immersed in the plating bath for 30 minutes at room temperature to allow uniform formation of a Cu layer over the Ag layer (Fig. 2) and then gently rinsed with deionized water and dried at room temperature for 24 hours.

Measurements
Widths (W) and lengths (L) of the print samples were measured by a zoom microscope (Bausch & Lomb Inc.) and thickness was evaluated by a scanning electron microscope (SEM) (Phenom World) after cutting across the square prints for cross-sectional observations. DC electrical resistances of the linear prints (0.1 mm × 10 mm) were measured by a multimeter (Fluke Inc.) by the two-probe method (Fig. 1c) [18]. For each post-treatment method, the measurement was repeated for 20 specimens. Based on the measured dimensions and electrical resistance, the electrical resistivity of the conductive prints was calculated, where the printed geometries were assumed to be perfectly cuboidal as shown in Fig. 1c.

Statistical analysis
Statistical analysis was performed using the R Studio ® (R Studio Inc.). The twosample t-tests were conducted to examine whether there was a significant difference in the electrical resistance or resistivity before and after each post-treatment. The 99 % confidence level was chosen to determine statistical significance. Fig. 5 shows the top-view and side-on images of the Ag prints. For both linear and square prints, Ag ink was successfully deposited on the PET film. All the printed edges were sharp, and the measured dimensions of the silver print were as intended (0.1 mm by 10 mm). The print thickness was 1.3 μm. For the sample size of 20, the average electrical resistance of the Ag prints was 46.8 (±1.4) Ω and the average electrical resistivity was 6.1×10 -7 (±0.2×10 -7 ) Ω·m. Although the ink contained highly pure silver nanoparticles suspended in a completely volatile aqueous solution, due to the porous structure of the printed layer, the electrical resistivity was about 37 times higher than that of bulk silver. Fig. 6 shows the top-view and side-on images of the Ag prints sintered by the electrolyte solution. NaCl-sintering process did not critically change the dimensions of the Ag print. The measured electrical resistance of the NaCl-sintered print was 37.9 (±2.2) Ω, and the electrical resistivity was 4.9×10 -7 (±0.3×10 -7 ) Ω·m (Table I). By comparing the electrical properties before and after the NaCl sintering using the t-test, it was demonstrated that there was significant improvement (p < 0.01) in electrical propertieson average, NaCl sintering lowered both DC electrical resistance (R) and resistivity (ρ) by ~19 % from those of the Ag print (Table I).   Fig. 7 shows the top-view and cross-sectional images of the photonic-sintered prints. It is seen that photonic sintering caused a substantial reduction (by 23 %) in thickness while maintaining the same width and length. This evidences that the deposited Ag nanoparticles were coalesced by photonic sintering, and the air trapped between the Ag nanoparticles were effectively removed. From the electrical measurements, it was found that the mean electrical resistance of the photonic-sintered prints was 7.5 (±0.2) Ω and the calculated electrical resistivity was 7.5×10 -8 (±0.2×10 -8 ) Ω·m. From the statistical analysis (Table 2), it was confirmed that the electrical properties were significantly improved by the photonic sintering (p <0.01). Fig. 8 shows the top-view and side-on images of the Cu-plated prints. Although the linear prints retained the same width and length as the raw Ag prints, the copper-plated prints suffered from numerous cracks and holes as shown in Fig. 8b and 8d. These cracks were due to the pre-treated layer on the PET film partially dissolved in the plating solution. The detailed information on the pre-treated substance is protected by proprietorial rights of the manufacturer and therefore is not known. Incompatibility with certain pre-treated substances would give considerable limitations in the use of electroless plating along with conductive inkjet printing.  plated print became 4.5 μm, which was 3.2 μm thicker than that of the raw Ag prints and this was due to deposition of copper over the Ag prints by following the mechanism given in Fig.  2.

Cu-plated print
Although the resulting print quality was inferior with numerous cracks, the average electrical resistance and resistivity of the Cu-plated print were 4.5 (±1.3) Ω and 2.0×10 -7 (±0.6×10 -7 ) Ω·m, respectively and these were substantially lower (p < 0.01) than those of the raw Ag print according to the t-test (Table III). However, it should be addressed that while the electrical resistance was lowered by 90%, the resistivity was decreased merely by 67 %. The major reason for this would be the inferior electrical properties (i.e., ρ = 6.1×10 -7 Ω·m) of the silver layer underneath the newly established Cu-layer that served as a limiting factor.

Conclusions
An experimental study was presented on the effects of three post-treatments on the DC electrical resistance and resistivity of inkjet-printed Ag print. The Ag nanoparticles were successfully deposited onto the PET substrate by inkjet printing. The prints were treated either by electrolyte sintering, photonic sintering or electroless plating to improve the electrical resistance and resistivity. A series of experimental data demonstrated that there was statistically significant improvement in the electrical properties for all these three posttreatments.
Electroless plating gave the smallest electrical resistance of 4.5 Ω, but there was a compatibility challenge associated with this technique. Unlike the electrolyte and photonic sintering methods with which the prints retained good printing quality, numerous cracks were formed on the printed track after electroless plating. This was because the pre-treated material on the substrate dissolved in the aqueous solution during the electroless plating process. In addition, the improvement of the electrical resistivity was limited because the relatively highly resistive layer of silver remained underneath the copper layer.
On the other hand, photonic-sintered prints exhibited reasonable electrical resistance (7.5 Ω) and lowest resistivity (7.5×10 -8 Ω·m). It was more than 20 % of the electrical conductivity of the bulk silver and could be more-suited for microwave applications such as antennas and transmission lines [15,16].