Abstract
This paper presents a silver-nanoparticle-based, screen-printed, high-performance, dual-band, bandstop filter (DBBSF) on a flexible polyethylene terephthalate (PET) substrate. Using screen-printing techniques to process a highly viscous silver printing ink, high-conductivity printed lines were implemented at a web transfer speed of 5 m/min. Characterized by X-ray diffraction (XRD), optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), the printed lines were shown to be characterized by smooth surfaces with a root mean square roughness of 7.986 nm; a significantly higher thickness (12.2 μm) than the skin depth; and a high conductivity of S/m. These excellent printed line characteristics enabled the implementation of a high-selectivity DBBSF using shunt-connected uniform impedance resonators (UIRs). Additionally, the inductive loading effect of T-shaped stubs on the UIRs, which were analyzed using S-parameters based on lumped parameter calculations, was used to improve the return losses of the geometrically optimized DBBSF. The measured minimum return loss and maximum insertion loss of 28.26 and 1.58 dB, respectively, at the central frequencies of 2.56 and 5.29 GHz of a protocol screen-printed DBBSF demonstrated the excellent performance of the material and its significant potential for use in future cost-effective, flexible WiMax and WLAN applications.
1. Introduction
Screen-printing, which is also widely known as thick-film printing, is a promising candidate for creating high demand, environmentally friendly, light-weight, and cost-effective electronic products at low cost, high throughput, and high reproducibility [1–3]. Due to its numerous advantages, such as its high processing speed, high throughput, and high reproducibility, as well as its capacity to process highly viscous printing ink, this technique has been used in a diverse range of applications, including printed energy sources, diodes, resistors, sensors, microwave antennas, and filters [4, 5]. Its capacity to work with several cheap substrates, such as paper, plastics, and glass, makes it suitable for creating light-weight, flexible, transparent, and disposable products [6, 7]. However, using these substrates dictates low working temperatures; to meet this requirement, conductive inks with metal micro-/nanoparticles have been widely researched and have been shown to be promising, because the increase in the ratio of surface area to volume with the reduction of the particles in volume significantly lowers the melting point of the particles compared with that of bulk materials [8]. As a consequence, several nanoparticles of noble metals, such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), or combinations of these metals, have been extensively investigated for the preparation of conductive pastes. However, Ag-particles-based ink is most widely used to manufacture conductive patterns in electronic devices due to its high stability in air and its high-conductivity [8, 9].
Currently, screen-printing-based implementations of radio-frequency, printed antennas and filters that are compact and highly selective are being investigated in many studies [10]. Various aspects such as the viscosity and surface tension of printing ink and the selection of an appropriate substrate and optimization of printing and sintering processes must be considered to successfully create a high-performance radio-frequency component. Kao et al. presented a silver-nanoparticle-colloidal-solution-based, inkjet-printed, radio-frequency bandpass filter on a liquid crystal polymer (LCP) substrate. Increasing the printed line conductivity and optimization of the line thickness with a uniform width were primarily considered to improve the performance of the filters. A hairpin bandpass filter [11], an interdigital coupled-line-based bandpass filter [12], and a third-order, dual-behavior resonator (DBR) bandpass filter [13] were inkjet-printed onto an LCP substrate. However, the large insertion losses indicated by the measurement results, which degraded the selective performance of the filters, were most likely caused by an insufficient conductivity of the printed lines. Therefore, increasing the conductivity of the printed lines to create a high-performance microwave filter is still an ongoing challenge.
In this study, we propose using Ag-nanoparticle-based screen-printing technique to create a high-performance, dual-band bandstop filter (DBBSF) on a flexible PET substrate. High-conductivity printed lines to obtain high return and low insertion losses that are indicative of excellent in-band and out-of-band selectivity were the primary objectives of this study. The Ag-nanoparticle-based ink used in this study was formulated to have a high viscosity that was suitable for screen-printing of high-conductivity, uniform filter lines with smooth surfaces. Screen-printed Ag line characterization, which was performed by optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), indicated that the printed lines exhibited uniform widths with a thickness (e.g., 12.2 μm) significantly larger than the skin depth () over nearly the entire operating frequency band; these analyses also showed that the lines had smooth surfaces with root mean square (RMS) value of roughness equal to 7.986 nm and a high-conductivity of 2 × 107 S/m. All of these excellent line characteristics allowed the creation of high-performance DBBSF with excellent selectivity and design flexibility based on stub-loaded, shunt-connected, uniform-impedance resonators (UIRs). A measured minimum return loss of 28.26 dB and a maximum insertion loss of 1.58 dB in the stop bands demonstrated the excellent selectivity of a protocol DBBSF operating at central frequencies of 2.56 and 5.29 GHz; and, therefore, the proposed process is a suitable candidate for use in low-cost WiMax and WLAN applications.
2. Methods and Materials
2.1. Layout Design, Simulation, and Screen-Printing
The 3D layout of the proposed DBBSF, which is shown in Figure 1, consists of two shunt-connected resonators with impedance and characteristic electrical lengths of , , and . Using = = = , the input impedance () of the proposed shunt-connected UIRs can be expressed as [14] The solution of , which corresponds to the lower resonant frequency () and the upper resonant frequency () of the proposed shunt-connected UIRs, indicates that the frequency ratio () can be varied within a wide range and the required resonant frequencies can be obtained by varying and optimizing the length ratios of and , respectively. can be independently varied by shifting to a lower UIR position along the impedance line of the upper UIR [15, 16]. To verify the application of the proposed shunt-connected UIRs in the DBBSF, a protocol filter was designed on a PET substrate with a dielectric constant , a loss tangent = 0.0147, and a thickness = 250 μm. Figure 2 also shows the equivalent circuit of the proposed DBBSF. represents the combined resistive loss of the tuned circuits and that correspond to the lower and upper UIRs, which resonate at and , respectively; and represent the net inductive and capacitive effects of the T-shaped stub-loads, respectively. The designed filter was simulated and optimized using a full-wave EM simulator, Sonnet, and the optimized dimensional parameters of UIRs, which exhibited resonant frequencies of 2.9 and 5.65 GHz, as shown in the simulated results of Figure 3(a) are Ω, °, °, and °. To obtain DBBSF with improved return losses, the stub with the optimized dimensions of Ω, Ω, °, and ° was loaded onto the UIRs.
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The use of stub-loads resulted in DBBSF with central frequencies of 2.55 and 5.3 GHz, each of which was shifted downward by 0.35 GHz with respect to the resonant frequencies of the UIRs. To evaluate the effect of the T-shaped stub-load on the central frequencies and return loss, the net lumped element parameters for a propagation distance of 10 mm were calculated from the simulated -parameters for UIRs and DBBSF [17]. The calculated net lumped parameters, which are displayed in Figure 2, indicated that the use of stub-loads resulted in a significant increase in the net lumped inductance from 0.896 to 0.995 nH and 1.183 to 1.501 nH at the central frequencies of 2.9 and 5.65 GHz, respectively, as shown in Figure 2(b). Similarly, the net lumped capacitance increased from 0.055 to 0.224 pF and 0.034 to 0.061 pF at 2.9 and 5.65 GHz, respectively, as shown in Figure 2(c). Therefore, the increase in net lumped inductance and capacitance shifted the central frequencies () downward and noticeably increased the return losses. Additionally, Figure 2(d) shows that the net lumped resistance exhibited peak values, whose magnitude and frequency decreased and shifted downward, respectively, due to use of the stub-loads.
SM-S320 model Sun Mechanix screen printer system, as shown in Figure 3(a), with 250-mesh screen was used to print the Ag conductive lines onto a roll of polyethylene terephthalate (PET) film, which has a width of 200 mm and a thickness of 250 μm and was purchased from SKC Korea. The PET substrate is suitable for screen-printing due to its excellent flexibility and durability. Ag-nanoparticle-based conducting ink (PG-007 BB type, Paru Co., Korea) was used during printing. Because this ink contained 62 wt% of silver nanoparticles that were 20 to 200 nm in diameter, as shown in the SEM image displayed in Figure 2(b), the viscosity and surface tension of the silver inks required tuning through a simple ink formulation process. We first formulated the conductive inks to meet the wetting requirements on the PET substrate; then, we adjusted the viscosities by raising the silver-nanoparticle content to 75 wt%. The viscosity of the formulated ink was evaluated using a viscometer (SV-10 Vibro Viscometer, A&D Co., Japan). Additionally, the surface tension was measured using DCAT 21 (Dataphysics Co., Germany). Ethylene glycol (Aldrich) was used to formulate the Ag ink to achieve a viscosity of 15,000 cp and a surface tension of 47 mN/m. The ethylene glycol stabilized the metal nanoparticles using its ether group, which contains unshared electron pair, to allow the stable use of high concentrations of metal nanoparticles and to increase the viscosity of the solution. A higher viscosity in the printing ink is preferred to obtain a lower resistivity in the printed conductive films with optimized widths. The web transfer speed was set to 5 m/min under a roll pressure of 2.5 MPa. The printed silver layer was cured for 10 min by passing through an oven of 150°C without additional treatments. To make the back coating a ground conductive layer, we used same ink.
3. Results and Discussions
3.1. Characterization of the Printed Ag Lines
Ag-nanoparticle-based, screen-printed conductive films on PET substrates were characterized by XRD to study their crystallinity and structural characteristics. The XRD pattern, which is shown in Figure 4(a), shows a high intensity peak corresponding to the PET substrate at angle of 38.1°; this agrees with that reported in the literature [18, 19]. The presence of peaks at the values of 38.4°, 44.09°, and 64.36° corresponds to the , , and planes of silver, respectively. Thus, the XRD spectrum confirmed the crystalline structure of the silver nanoparticles. No peaks of other impurity crystalline phases were detected, and all peaks in the XRD pattern can be readily indexed to the face-centered cubic structure of silver as per available literature (JCPDS, file number 4-0783).
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SEM image of a cross section of a printed Ag line with a width 613 μm on PET substrate with a thickness of 250 μm is shown in Figure 4(c). A magnified SEM image of a cross section of a printed Ag line, which is depicted in Figure 4(d), shows the thickness results: the middle of the printed line was 12.2 μm thick; however, the line thickness marginally decreased to 11.6 μm near the edge. This marginal change of the line thickness from center to the edge of the line is expected due to the sintering process of the conductive phase [20]. Based on a four-point measurement technique [21], the conductivity () of the printed silver ink after sintering was found to be 2 × 107 S/m. Compared with pure silver, the measured of the printed line was lower; however, was noticeably higher than previously reported values [22–24] and sufficiently high for practical microwave filter applications. The skin depth (), which is shown in Figure 4(b) for the entire operating frequency band of the proposed DBBSF, was calculated using the following equation [21]:where indicates the permeability of the printed line and equals 1 and is the angular frequency in rad/sec. The results indicated that was significantly lower than the measured thickness of the printed line at higher frequencies (e.g., from 2 to 8 GHz). For lower frequencies (e.g., below 0.5 GHz), the printed line thickness was marginally higher than . Figure 5(a) shows optical microscopy images of the printed lines representing the stub-loads of the proposed DBBSF. The images show that the printed lines were continuous with no voids. However, the center of the lines appeared thicker than the edges; these results agree with the thickness results found by the SEM images and can be caused by a quick viscosity recovery that prevented the ink from completely flattening during drying [23, 24]. Additionally, the thickness results for the stub-load show a width of 456 μm, which is 4 μm less than the optimized theoretical width indicated in the layout design. This difference in width may be attributed to the tape shrinkage and network compactness after densification by sintering. A decreasing height slope was observed near both edges of the line at a width of 35 μm. The resulting printed pattern was profiled using atomic force microscopy (AFM), and the corresponding images are displayed in Figure 5(b). The surface roughness results indicate a smooth surface with roughness peaks of 18.64 nm to 44.79 nm. The surfaces were shown to have a root mean square (RMS) surface roughness of 7.986 nm.
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3.2. Measurement Results of the Printed DBBSF
The screen-printed DBBSF, whose photographs in both normal and bent position are shown in Figure 6, was connected with port connectors using Ag paste to perform electrical measurements. The central frequencies of the fabricated DBBSF that were tested and characterized using Agilent 8510C vector network analyzer (VNA) were 2.55 and 5.3 GHz, each of which was shifted upward by 10 MHz with respect to the simulation results. This shift in frequency may be attributed to the slight difference between fabricated conductive line dimensions compared with the theoretical dimensions; the dielectric loss of the PET substrate; and the dispersion loss at the bends of the UIRs. The return and insertion losses of the first and second stop band were measured to be 27.4 and 28.9 dB and 1.17 and 1.58 dB, respectively. The measured maximum insertion loss and minimum return loss of 0.68 and 14.5 dB, respectively, for all of the pass bands between the stop bands indicated the excellent out-of-band selectivity of the fabricated DBBSF. The 3-dB fractional bandwidths of the first and second stop bands were measured to be 1.8 and 3.09 GHz, respectively.
Table 1, which displays the comparison of the performance of the proposed Ag-nanoparticle-based screen-printed bandstop filter with recently reported several inkjet-printed bandpass filters, indicates that our printed filter exhibits significantly lower insertion losses because of its geometrical structure and relatively higher conductivity printed Ag lines. Additionally, the present DBBSF exhibits higher number of operation bands with large return losses. Moreover, the proposed DBBSF has a compact size of 92.8 mm2.
4. Conclusion
In this study, we created an environmentally friendly, high-performance DBBSF based on shunt-connected, T-shaped stub-loaded UIRs. The proposed DBBSF was screen-printed on a low weight, cost-effective PET substrate via screen-printing. Highly conductive ( = 2 × 107 S/m) lines with favorable characteristics such as uniformity and smooth surfaces were successfully printed using highly viscous Ag conductive paste. The agreement of the simulated and measured results of DBBSF with a compact size of 10 × 9.28 mm2, measured central frequencies of 2.56 and 5.29 GHz, and excellent in-band and out-of-band selectivity indicated that silver-nanoparticle-based screen-printing can be used to successfully fabricate high-performance microwave filters.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning (no. 2011-0030079) and a grant support from the Korean government (MEST) no. 2012R1A1A2004366. This work was also supported by a Research Grant of Kwangwoon University in 2015. The authors also appreciate the financial support provided by the Ministry of Education of Korea through the Basic Science Research Program at Sunchon National University and by the Global Leading Technology Program of the Office of Strategic R&D Planning (OSP) through the Ministry of Commerce, Industry and Energy, Republic of Korea (10042537, Printed Electronics Total Solution Development).