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Rapid and high-performance processing technology for large-area frequency selective surfaces
Journal of Materials Processing Tech. 264 (2019) 129–133
Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Rapid and high-performance processing technology for large-area frequency selective surfaces
T
⁎
Jun Aia, Huiqing Xianga, Qifeng Dua, Dongchu Chenb, Jianguo Liua, , Xiaoyan Zenga a
Functional Laboratory of Laser and Terahertz Technology, Wuhan National Laboratory for Optoelectronics(WNLO), Huazhong University of Science and Technology, Wuhan 430074, PR China b School of Materials Science and Energy Engineering, Foshan University, Guangdong 528000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser direct writing lithography Frequency selective surface Laser direct ablation Wet chemical etching
Frequency selective surfaces (FSSs) with periodic smart structures have widespread applications in many areas, especially in electromagnetic wave filters. In this paper, a rapid and high-performance processing technology for large-area FSSs was presented. This novel technology combined laser direct writing lithography (LDWL) with wet chemical etching (WCE) (i.e., LDWL + WCE). Basing on experiments and in the condition of optimal parameters (i.e., 40 mW laser power, 1000 mm/s scan speed, single scan, 20 μm hatch space, pattern-profile scan path, and ferric chloride aqueous solution as etchant) of LDWL + WCE, a large-area (200 × 200 mm2) aperture type FSS with 625 square-loop units was fabricated within 8 min on a FR-4 copper-clad plate (with 12 μmthickness copper layer). When the same FSS was prepared by commonly-used laser direct ablation (LDA) technology with itself optimal processing parameters, it needed about 62 min (nearly 8 times as much time as the former). Scanning electron microscopy (SEM) and surface probe profilometry analyses showed that the edges of the square-loop patterns fabricated by LDWL + WCE were steeper and smoother, and the FR-4 substrate had no damage. In an anechoic chamber, the electromagnetic wave filtering performances of the as-prepared FSSs were tested, and the results indicated that the FSS fabricated by LDWL + WCE had a bandpass response at 7.08 GHz with a transmittance of 97.75%. Its frequency response curve was in better agreement with the simulation result. The technology of LDWL + WCE also has the potential to rapidly fabricate large-area and/or non-planar FSSs with high performance.
1. Introduction Frequency selective surfaces (FSSs) with periodic smart structures are generally composed of conductive patches or aperture elements within a metallic screen (the screen was usually supported by a dielectric substrate) (Munk, 2005). They are designed and used to transmit, absorb, or reflect electromagnetic waves at certain frequencies. Thus, they have been widely used in such areas as radar cross section (RCS) reduction (Kim et al., 2008a,b), electromagnetic shielding (Rahman et al., 1995), microwave absorption (Wei et al., 2009), structural health monitoring (Sang-Dong et al., 2013), wireless communication (Yong et al., 2018), antenna design (Hong-Kyu et al., 2011), optical signal filtration (Lu et al., 2016), and so on. Accordingly, a great attention has been paid to the fabrication of FSSs. In the open literatures, commonly-used FSS fabrication technologies include standard contact/projection photolithography (Moallem and Sarabandi, 2012), inkjet printing (Cooper et al., 2012), screen
⁎
printing (Li et al., 2017), micro-pen dispensing (Marhefka et al., 2007), e-beam evaporation (Kim et al., 2014). For example, Kim et al. (2008a,b) fabricated stealth radome with FSS using standard photolithography. Batchelor et al. (2009) demonstrated inkjet printing as a facile digital fabrication tool for the manufacture of FSS. Liu and Kim (2016) manufactured a square loop FSS for wide-bandwidth microwave absorber by screen printing. Marhefka et al. (2007) fabricated an FSS antenna ground plane using micro-pen dispensing. Kim et al. (2014) proposed a composite FSS fabricated by an e-beam evaporator. However, the above processing technologies usually have their selfdrawbacks. For example, standard contact/projection photolithography usually requires a mask and then projects the mask pattern to a photoresist-coated substrate (Pease and Chou, 2008). This technology is able to achieve high line/space resolution, but it has less process flexibility, longer production cycle, more cost, and more difficulty for largearea and/or non-planar FSSs. Inkjet printing is a rapid and non-contact pattern-printing technology by directly depositing conductive droplets
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.jmatprotec.2018.09.009 Received 1 April 2018; Received in revised form 24 August 2018; Accepted 7 September 2018 Available online 09 September 2018 0924-0136/ © 2018 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 264 (2019) 129–133
J. Ai et al.
(e.g., silver ink) on substrates. Whereas, it is a more-defect and lessprecision patterning technology (Song and Nur, 2004). Screen printing is a stencil-based process, and it needs to pre-fabricate a stencil with designed patterns, and then squeezes the ink through open areas of the screen mesh onto the printing substrate (Novaković et al., 2016). Thus, this technology is lack of flexibility and wastes ink. Micro-pen dispensing is an additive mask-free patterning technology (Lewis and Gratson, 2004). But it has a critical demand for the substrate surface flatness, and is low-efficiency because of the low-speed movement of mechanical stages. E-beam evaporation needs a specially-designed mold and a vacuum chamber to make FSS patterns (Bishop, 2015). This technology is also more-defect and lower-efficiency. Over the past decade, laser direct ablation (LDA) technology has been developed rapidly. It utilizes a focused laser beam to directly ablate material layers and to produce patterns on substrates. Thus, this is a non-contact, maskless and flexible patterning technology (Hon et al., 2008), and it could be used to fabricate high-resolution patterns in small areas (Mizeikis et al., 2010). When LDA is used to fabricate large-area FSSs, in order to directly ablate the metal (e.g., copper or aluminum) layer on a substrate, it usually needs high laser power and very low scan speed (Li et al., 2018). This not only reduces the processing efficiency, but also inevitably damages the dielectric substrate frequently. In this paper, a novel fabrication technology for large-area FSS was proposed. It combined laser direct writing lithography (LDWL) with wet chemical etching (WCE) (i.e., LDWL + WCE). It utilized a focused laser beam to directly expose the photosensitive material (e.g., photoresist) coated on a substrate. After development and wet chemical etching, the patterns were transferred onto the substrate. Because of the very low exposure energy threshold of the photoresist, the demand laser power was very low. Meanwhile, the laser beam scan speed depending on a galvanometer scanner could be elevated greatly. Thus, this was a maskless, flexible, rapid, non-contact processing technology. As a comparison, LDA technology was also adopted.
both x and y directions, and the both z stage could carry the galvanometer scanner and F-theta lens moving up and down in a range of ± 25 mm (larger movement ranges meant larger processing areas or volumes). These made the systems have the potential for larger-area, nonplanar and/or 3D fabrication. For LDWL + WCE, the processing procedures were shown in Fig. 2. First, the FCCP was coated with the above-mentioned photoresist layer. After baking on a hot plate under 100 ℃ for 1 min, using the above LDWL equipment system, it was directly written the pre-designed patterns on the photoresist. Next, it was developed in the developer solution. And then, it was implemented a wet-etching process in the abovementioned FeCl3 aqueous solution. Finally, the residual photoresist was washed away in acetone.
2. Experiment section
2.3. Measurements and characterization
2.1. Material
The microstructures of the as-prepared FSSs were observed using a sirion200 scanning electron microscope after the samples were sputtercoated with gold. The surface profiles of the substrate after WCE or LDA were measured by a KLA TencorP-16+ surface probe profiler. Electromagnetic wave characteristics were tested and evaluated in a microwave anechoic chamber using a free-space measurement setup (Fig. 3). Two waveguide horn antennas sweeping among a frequency range of from 2 GHz to18 GHz were connected to a signal source, and they were placed 1 m away from each side of the FSSs. The data simulation of electromagnetic wave characteristics was accomplished using a commercially available software named High Frequency Structure Simulator (HFSS).
Fig. 1. Schematic diagram of LDWL or LDA equipment system.
Commercially available FR-4 copper-clad plates (FCCP) were purchased from Shenzhen Haiqunli New Material Technology Co., Ltd (Shenzhen, China). The plates were cut into 200 mm × 200 mm in size (length × width), the copper layer was about 12 μm thick, and the FR-4 dielectric substrate was about 1 mm thick. Commercially available positive photoresist SUN-110 P and developer SUN-238D from Suntific Materials CO. Ltd (Weifang, China) were used in the experiment. The photoresist was a diazonaphthoquinone (DNQ) positive photoresist and the developer was mainly composed of tetramethylammonium hydroxide (TMAH). Self-prepared wet chemical etchant was 50 wt. % ferric chloride (FeCl3) aqueous solution.
3. Results and discussion 2.2. Experimental methods 3.1. Structure of FSSs Fig. 1 was the schematic diagram of LDWL or LDA equipment system. The both systems included a laser source, an optical system, and an x-y-z three-dimensional (3D) translation stage, respectively. For LDWL, the laser source was with a wavelength of 355 nm and a maximum output power of 0.5 W; For LDA, the laser source was with a wavelength of 1064 nm and a maximum output power of 20 W. The focused laser spot diameters at 1/e2 of their maximum intensity were approximately 10 μm and 20 μm, respectively. In either optical system, a beam expander was used to expand and collimate the laser beam and a high-speed galvanometer scanner was used to rapidly manipulate the laser beam, and the optical deflection angle range was between -20° and +20°; Besides, an F-theta lens was used to focus the laser beam on a plane. The both translation stages had movement ranges of 200 mm in
Generally speaking, FSSs are composed of the periodic structures containing such self-symmetric patterns as circular rings, square loops, dipole, etc. because of the good stability of these patterns (Panwar and Lee, 2017). Depending on corresponding demand, FSS can be designed and fabricated on a planar or curved surface, and the adopted materials can be pure conductors (e.g., metal) or composite materials consisting of conductive layers and dielectric layers. In our experiments, square loop was chosen as the periodic pattern unit of FSS, and FCCP was used as the fabrication material. Fig. 4 was the structure of a FSS. Its whole size was 200 mm × 200 mm, and it had 625 square loop periodic units. The geometry of each unit was: p = 8 mm, a = 7.05 mm, b = 6.45 mm and loop gap g = 300 μm. The 130
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Fig. 2. Schematic diagram of FSS fabricated by LDWL + WCE. (a) Coating with photoresist, (b) LDWL and developing, (c) WCE, and (d) Removing residual photoresist.
1000 mm/s, single scan, and hatch space of 20 μm. Fig. 5d showed the pattern processed by LDWL + WCE with the optimal laser parameters. It could be seen that the edge was smooth and straight, and the copper layer was completely removed. Similarly, for LDA, the orthogonal experiments for laser power (5–20 W), scan speed (100–2000 mm/s), scan times (1–4), and hatch space (10–40 μm) were also explored. The optimal laser parameters were laser power of 12 W, scan speed of 200 mm/s, 4 scan times, and hatch space of 25 μm. It indicated that LDA needed higher laser power, more scan times, and lower scan speed (compared with that of LDWL). Commonly, for both LDWL and LDA, there are two kinds of scan path to fabricate the square loop unit. One is raster scan (Fig. 6a), and the other is pattern-profile scan (Fig. 6b). Because of the frequent acceleration, uniform and deceleration of the laser beam during direct writing lines, when the raster scan was adopted, the square loop would produce jagged edges (Fig. 6c). However, when the pattern-profile scan was adopted, this would be alleviated, or even be eliminated. Here, smooth edge would be obtained, and the pattern quality was greatly improved (Fig. 6d). Eventually, under the optimal condition, the designed FSSs were fabricated within 8 min by LDWL + WCE. While it took about 62 min to fabricate the same patterns by LDA. The latter cost nearly 8 times as much time as the former. Obviously, LDWL + WCE are a higher-efficiency processing technology for large-area FSSs.
Fig. 3. Setup for testing electromagnetic wave characteristics of FSSs.
processing error of loop gap size should be within ± 5% (i.e., ± (300 × 5%) = ± 15 (μm)). In addition, the electromagnetic wave characteristics of the FSS would be better when the edges of the square loops were smoother and steeper, and the FR-4 dielectric layer substrate was less damage. 3.2. Fabrication of FSSs by LDWL + WCE and LDA
3.3. Profile and micromorphology of as-prepared FSSs During the fabrication, laser processing parameters mainly included laser power, scan speed, scan times, hatch space and scan path. For LDWL, preliminary experiments showed that laser power, scan speed, hatch space and scan times had great impacts on the photoresist exposure, overlap along scan direction, overlap perpendicular to scan direction, and processing efficiency. Thus, when inappropriate parameters were used, poor patterns would be obtained, like rough and unsmooth edges (Fig. 5a), or jagged edges (Fig. 5b), or uncompleted removal of copper layer (Fig. 5c). All would affect the precision and performance of the as-prepared FSSs. Basing on the preliminary experiments, a 4 × 4 orthogonal experiment for laser power (20–50 mW), scan speed (500–2000 mm/s), scan times (1–4), and hatch space (10–40 μm) was implemented. The obtained optimal parameters were laser power of 40 mW, scan speed of
Fig. 7a and b were the photographs of the FSSs fabricated by LDWL + WCE and LDA, respectively. It seemed to have no obvious difference from macroscopic scale. The average widths of the square loop gaps were about 292 μm and 287 μm, respectively, which had a 2.67% and 4.33% deviation from the designed value (i.e., 300 μm). Both were within the above-mentioned error. However, from microscopic scale, there existed big differences. In Fig. 7(c)–(e), it could be seen that the square loops fabricated by LDWL + WCE had smooth, straight and steep edges, and the FR-4 substrates had no damage. This was because the copper layer was removed by wet chemical etching. The etchant only reacted with the naked copper layer and it had no damage to the FR-4 substrates or the photoresist.
Fig. 4. Structure of FSS (left: whole FSS; right: periodic square loop unit). 131
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Fig. 5. Different square loop profiles fabricated by LDWL + WCE with different laser parameters. (a) Rough edges due to overlarge laser power, (b) Jagged edges due to overlarge scan speed, (c) Uncompleted removal of copper layer due to overlarge hatch space, and (d) perfect profile by optimal laser parameter.
Fig. 6. Two scan paths ((a) raster scan or (b) pattern-profile scan) of square loop fabrication by LDWL or LDA, and micrographs of square loop edges fabricated by LDWL + WCE or LDA using raster scan (c) or pattern-profile scan (d).
Fig. 7. As-prepared FSSs (a (by LDWL + WCE) and b (by LDA)), (c) profiles of the aperture gaps of square loops, and magnified images of the aperture gaps of square loops by LDWL + WCE (d and e) and by LDA (f and g).
While the square loop edges fabricated by LDA had large heat affected zones (HAZ) beside the grooves (Fig. 7(f)). Moreover, the edges had lots of burrs and remelting residues, and the FR-4 substrate was damaged severely (Fig. 7(g)). In addition, from Fig. 7(c), it could be seen that the grooves fabricated by LDA were rougher than that by LDWL + WCE. This was because the copper was removed by laser heat melting and evaporating. The high laser power generated a large amount of heat, and leaded to large HAZs and remelting residues on both sides of the grooves. Besides, the FR-4 substrate was easily damaged when the copper layer was removed because of large amounts of heat and heat conduction. Therefore, from microscopic scale, the FSSs fabricated by LDWL + WCE had better square-loop profile and micromorphology than that by LDA. 3.4. Electromagnetic wave characteristics of as-prepared FSSs In the frequency range of from 2 GHz to 18 GHz, the electromagnetic wave transmission coefficients of the as-prepared FSSs were shown in Fig. 8. It could be seen that the FSSs were good bandpass filters and had special frequency selections. For example, the FSS processed by LDWL + WCE had transmission response at 7.08 GHz, and the transmission coefficient (S) at the passband peak was about
Fig. 8. Electromagnetic wave transmission response curves of simulation and experiment results of FSSs.
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-0.099 dB (the solid line in Fig. 8). According to the equation S = 10 ∙ lg (T), the calculated transmittance T was equal to 97.75%. The -3 dB bandwidth was about 3.04 GHz (5.48–8.52 GHz). Obviously, these experimental results were in good agreement with the simulation results (the red and dashed line in Fig. 8), especially in low frequency range (i.e. < 12 GHz). On the other hand, the FSS fabricated by LDA had transmission response at 7.44 GHz. The transmission coefficient (S) at the passband peak was about -1.73 dB, and the corresponding transmittance (T) was 67.14%. The -3 dB bandwidth was only 1.76 GHz (6.52–8.28 GHz) (the blue and dashed line in Fig. 8). Whereas, these data had a big deviation from the simulation results (the red and dashed line in Fig. 8) in the both low and high frequency ranges. Depending on the analysis to the electromagnetic wave characteristics measurement setup, it was found that the both deviations in high frequency range were because the setup itself was not very stable in this range. From the above experiment and simulation results, it showed that the FSS processed by LDWL + WCE had much better electromagnetic wave characteristics than that of the one by LDA. This was directly related to the smoother and steeper edges of the square-loop and the undamaged substrate. Further experiments showed that the FSSs with periodic structure units of different shapes and sizes had different electromagnetic wave characteristics. Thus, LDWL + WCE are a general and high-performance processing technology for the FSSs basing on the composite materials consisting of conductive layers and dielectric layers. In addition, it can be expected that this technology of LDWL + WCE is also able to fabricate non-planar or curved FSSs with larger size when the LDWL equipment system adopts larger-range translation stage. This will be published elsewhere in the near future.
were grateful to the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for SEM testing, and the assistance from engineer Haili Zhang in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for profile measurements. References Batchelor, J.C., Parker, E.A., Miller, J.A., Sanchez-Romaguera, V., Yeates, S.G., 2009. Inkjet printing of frequency selective surfaces. Electron. Lett. 45, 7–8. Bishop, C.A., 2015. Chapter 15 - electron beam (E-beam) evaporation. In: Bishop, C.A. (Ed.), Vacuum Deposition Onto Webs, Films and Foils, Third edtion. William Andrew Publishing, Boston, pp. 289–299. Cooper, J.R., Kim, S., Tentzeris, M.M., 2012. A novel polarization-independent, freespace, microwave beam splitter utilizing an inkjet-printed, 2-D array frequency selective surface. IEEE Antennas Wirel. Propag. Lett. 11, 686–688. Hon, K., Li, L., Hutchings, I., 2008. Direct writing technology—advances and developments. CIRP Ann.-Manuf. Technol. 57, 601–620. Hong-Kyu, J., Won-Jun, L., Chun-Gon, K., 2011. Design and fabrication of a microstrip patch antenna with a low radar cross section in the X-band. Smart Mater. Struct. 20, 015007. Kim, P.C., Lee, D.G., Seo, I.S., Kim, G.H., 2008a. Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces. Compos. Sci. Technol. 68, 2163–2170. Kim, P.C., Lee, D.G., Seo, I.S., Kim, G.H., 2008b. Nanocomposite stealth radomes with frequency selective surfaces. Compos. Struct. 86, 299–305. Kim, J.Y., Kim, D.S., Woo, D.W., Shin, D.S., Park, W.S., Hwang, W., 2014. Electromagnetic wave characteristics of second-order composite frequency-selective surfaces fabricated by e-beam evaporator. Electron. Lett. 50, 189–190. Lewis, J.A., Gratson, G.M., 2004. Direct writing in three dimensions. Mater. Today 7, 32–39. Li, W., Chen, M., Zeng, Z., Jin, H., Pei, Y., Zhang, Z., 2017. Broadband composite radar absorbing structures with resistive frequency selective surface: optimal design, manufacturing and characterization. Compos. Sci. Technol. 145, 10–14. Li, J.S., Li, Y., Zhang, L., 2018. Terahertz bandpass filter based on frequency selective surface. IEEE Photonics Technol. Lett. 30, 238–241. Liu, T., Kim, S.-S., 2016. Design of wide-bandwidth electromagnetic wave absorbers using the inductance and capacitance of a square loop-frequency selective surface calculated from an equivalent circuit model. Opt. Commun. 359, 372–377. Lu, Z., Liu, Y., Wang, H., Zhang, Y., Tan, J., 2016. Optically transparent frequency selective surface based on nested ring metallic mesh. Opt. Express 24, 26109–26118. Marhefka, R.J., Young, J.D., Towle, J.P., 2007. Design, fabrication and measurement of an FSS antenna ground plane. Jensen, M. (Ed.), IEEE Antennas and Propagation Society International Symposium 3972–3975. Mizeikis, V., Juodkazis, S., Sun, K., Misawa, H., 2010. Fabrication of frequency-selective surface structures by femtosecond laser ablation of gold films. J. Micro/Nanoeng. 5, 115–120. Moallem, M., Sarabandi, K., 2012. Miniaturized-element frequency selective surfaces for millimeter-wave to terahertz applications. IEEE Trans. Terahertz Sci. Technol. 2, 333–339. Munk, B.A., 2005. Canada. Frequency Selective Surfaces: Theory and Design. John Wiley & Sons, pp. 1–4. Novaković, D., Kašiković, N., Vladić, G., Pál, M., 2016. 15 - screen printing. In: Izdebska, J., Thomas, S. (Eds.), Printing on Polymers. William Andrew Publishing, Chadds Ford, USA, pp. 247–261. Panwar, R., Lee, J.R., 2017. Progress in frequency selective surface-based smart electromagnetic structures: a critical review. Aerosp. Sci. Technol. 66, 216–234. Pease, R.F., Chou, S.Y., 2008. Lithography and other patterning techniques for future electronics. Proc. IEEE 96, 248–270. Rahman, H., Dowling, J., Saha, P.K., 1995. Application of frequency sensitive surfaces in electromagnetic shielding. J. Mater. Process. Technol. 54, 21–28. Sang-Dong, J., Byung-Woo, K., Jaehwan, K., 2013. Frequency selective surface based passive wireless sensor for structural health monitoring. Smart Mater. Struct. 22, 025002. Song, J.H., Nur, H.M., 2004. Defects and prevention in ceramic components fabricated by inkjet printing. J. Mater. Process. Technol. 155–156, 1286–1292. Wei, X., Hai-Feng, C., Zeng-Yong, C., Yong-Jiang, Z., Hai-Tao, L., Zhao-Hui, C., 2009. Effect of FSS on microwave absorbing properties of hollow-porous carbon fiber composites. Mater. Des. 30, 1201–1204. Yong, W.Y., et al., 2018. Flexible convoluted ring shaped FSS for X-Band screening application. IEEE Access 6, 11657–11665.
4. Conclusion A rapid and high-performance processing technology for large-area FSSs was presented. This novel technology combined LDWL with WCE, and had the potential to fabricate non-planar or curved FSSs. Basing on experiments, the optimal processing parameters of LDWL + WCE were laser power of 40 mW, scan speed of 1000 mm/s, single scan, hatch space of 20 μm, pattern-profile scan, and ferric chloride aqueous solution as etchant. Under this optimal condition, the square-loop FSS with 625 units was fabricated on a 200 × 200 mm2 FCCP. The total processing time was within 8 min. In comparison to LDA, this technology improved the processing efficiency nearly 7 times. Moreover, the FSS fabricated by LDWL + WCE was with higher quality and higher performance. The electromagnetic wave frequency response of this FSS was at 7.08 GHz, and the corresponding transmittance reached up to 97.75%, which showed a better capability of bandpass filters. Its experiment and simulation results were in much better agreement. The technology of LDWL + WCE is a general processing technology for the FSSs basing on the composite materials consisting of conductive layers and dielectric layers. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 51775209). Also, the authors
133
Contents lists available at ScienceDirect
Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Rapid and high-performance processing technology for large-area frequency selective surfaces
T
⁎
Jun Aia, Huiqing Xianga, Qifeng Dua, Dongchu Chenb, Jianguo Liua, , Xiaoyan Zenga a
Functional Laboratory of Laser and Terahertz Technology, Wuhan National Laboratory for Optoelectronics(WNLO), Huazhong University of Science and Technology, Wuhan 430074, PR China b School of Materials Science and Energy Engineering, Foshan University, Guangdong 528000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser direct writing lithography Frequency selective surface Laser direct ablation Wet chemical etching
Frequency selective surfaces (FSSs) with periodic smart structures have widespread applications in many areas, especially in electromagnetic wave filters. In this paper, a rapid and high-performance processing technology for large-area FSSs was presented. This novel technology combined laser direct writing lithography (LDWL) with wet chemical etching (WCE) (i.e., LDWL + WCE). Basing on experiments and in the condition of optimal parameters (i.e., 40 mW laser power, 1000 mm/s scan speed, single scan, 20 μm hatch space, pattern-profile scan path, and ferric chloride aqueous solution as etchant) of LDWL + WCE, a large-area (200 × 200 mm2) aperture type FSS with 625 square-loop units was fabricated within 8 min on a FR-4 copper-clad plate (with 12 μmthickness copper layer). When the same FSS was prepared by commonly-used laser direct ablation (LDA) technology with itself optimal processing parameters, it needed about 62 min (nearly 8 times as much time as the former). Scanning electron microscopy (SEM) and surface probe profilometry analyses showed that the edges of the square-loop patterns fabricated by LDWL + WCE were steeper and smoother, and the FR-4 substrate had no damage. In an anechoic chamber, the electromagnetic wave filtering performances of the as-prepared FSSs were tested, and the results indicated that the FSS fabricated by LDWL + WCE had a bandpass response at 7.08 GHz with a transmittance of 97.75%. Its frequency response curve was in better agreement with the simulation result. The technology of LDWL + WCE also has the potential to rapidly fabricate large-area and/or non-planar FSSs with high performance.
1. Introduction Frequency selective surfaces (FSSs) with periodic smart structures are generally composed of conductive patches or aperture elements within a metallic screen (the screen was usually supported by a dielectric substrate) (Munk, 2005). They are designed and used to transmit, absorb, or reflect electromagnetic waves at certain frequencies. Thus, they have been widely used in such areas as radar cross section (RCS) reduction (Kim et al., 2008a,b), electromagnetic shielding (Rahman et al., 1995), microwave absorption (Wei et al., 2009), structural health monitoring (Sang-Dong et al., 2013), wireless communication (Yong et al., 2018), antenna design (Hong-Kyu et al., 2011), optical signal filtration (Lu et al., 2016), and so on. Accordingly, a great attention has been paid to the fabrication of FSSs. In the open literatures, commonly-used FSS fabrication technologies include standard contact/projection photolithography (Moallem and Sarabandi, 2012), inkjet printing (Cooper et al., 2012), screen
⁎
printing (Li et al., 2017), micro-pen dispensing (Marhefka et al., 2007), e-beam evaporation (Kim et al., 2014). For example, Kim et al. (2008a,b) fabricated stealth radome with FSS using standard photolithography. Batchelor et al. (2009) demonstrated inkjet printing as a facile digital fabrication tool for the manufacture of FSS. Liu and Kim (2016) manufactured a square loop FSS for wide-bandwidth microwave absorber by screen printing. Marhefka et al. (2007) fabricated an FSS antenna ground plane using micro-pen dispensing. Kim et al. (2014) proposed a composite FSS fabricated by an e-beam evaporator. However, the above processing technologies usually have their selfdrawbacks. For example, standard contact/projection photolithography usually requires a mask and then projects the mask pattern to a photoresist-coated substrate (Pease and Chou, 2008). This technology is able to achieve high line/space resolution, but it has less process flexibility, longer production cycle, more cost, and more difficulty for largearea and/or non-planar FSSs. Inkjet printing is a rapid and non-contact pattern-printing technology by directly depositing conductive droplets
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.jmatprotec.2018.09.009 Received 1 April 2018; Received in revised form 24 August 2018; Accepted 7 September 2018 Available online 09 September 2018 0924-0136/ © 2018 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 264 (2019) 129–133
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(e.g., silver ink) on substrates. Whereas, it is a more-defect and lessprecision patterning technology (Song and Nur, 2004). Screen printing is a stencil-based process, and it needs to pre-fabricate a stencil with designed patterns, and then squeezes the ink through open areas of the screen mesh onto the printing substrate (Novaković et al., 2016). Thus, this technology is lack of flexibility and wastes ink. Micro-pen dispensing is an additive mask-free patterning technology (Lewis and Gratson, 2004). But it has a critical demand for the substrate surface flatness, and is low-efficiency because of the low-speed movement of mechanical stages. E-beam evaporation needs a specially-designed mold and a vacuum chamber to make FSS patterns (Bishop, 2015). This technology is also more-defect and lower-efficiency. Over the past decade, laser direct ablation (LDA) technology has been developed rapidly. It utilizes a focused laser beam to directly ablate material layers and to produce patterns on substrates. Thus, this is a non-contact, maskless and flexible patterning technology (Hon et al., 2008), and it could be used to fabricate high-resolution patterns in small areas (Mizeikis et al., 2010). When LDA is used to fabricate large-area FSSs, in order to directly ablate the metal (e.g., copper or aluminum) layer on a substrate, it usually needs high laser power and very low scan speed (Li et al., 2018). This not only reduces the processing efficiency, but also inevitably damages the dielectric substrate frequently. In this paper, a novel fabrication technology for large-area FSS was proposed. It combined laser direct writing lithography (LDWL) with wet chemical etching (WCE) (i.e., LDWL + WCE). It utilized a focused laser beam to directly expose the photosensitive material (e.g., photoresist) coated on a substrate. After development and wet chemical etching, the patterns were transferred onto the substrate. Because of the very low exposure energy threshold of the photoresist, the demand laser power was very low. Meanwhile, the laser beam scan speed depending on a galvanometer scanner could be elevated greatly. Thus, this was a maskless, flexible, rapid, non-contact processing technology. As a comparison, LDA technology was also adopted.
both x and y directions, and the both z stage could carry the galvanometer scanner and F-theta lens moving up and down in a range of ± 25 mm (larger movement ranges meant larger processing areas or volumes). These made the systems have the potential for larger-area, nonplanar and/or 3D fabrication. For LDWL + WCE, the processing procedures were shown in Fig. 2. First, the FCCP was coated with the above-mentioned photoresist layer. After baking on a hot plate under 100 ℃ for 1 min, using the above LDWL equipment system, it was directly written the pre-designed patterns on the photoresist. Next, it was developed in the developer solution. And then, it was implemented a wet-etching process in the abovementioned FeCl3 aqueous solution. Finally, the residual photoresist was washed away in acetone.
2. Experiment section
2.3. Measurements and characterization
2.1. Material
The microstructures of the as-prepared FSSs were observed using a sirion200 scanning electron microscope after the samples were sputtercoated with gold. The surface profiles of the substrate after WCE or LDA were measured by a KLA TencorP-16+ surface probe profiler. Electromagnetic wave characteristics were tested and evaluated in a microwave anechoic chamber using a free-space measurement setup (Fig. 3). Two waveguide horn antennas sweeping among a frequency range of from 2 GHz to18 GHz were connected to a signal source, and they were placed 1 m away from each side of the FSSs. The data simulation of electromagnetic wave characteristics was accomplished using a commercially available software named High Frequency Structure Simulator (HFSS).
Fig. 1. Schematic diagram of LDWL or LDA equipment system.
Commercially available FR-4 copper-clad plates (FCCP) were purchased from Shenzhen Haiqunli New Material Technology Co., Ltd (Shenzhen, China). The plates were cut into 200 mm × 200 mm in size (length × width), the copper layer was about 12 μm thick, and the FR-4 dielectric substrate was about 1 mm thick. Commercially available positive photoresist SUN-110 P and developer SUN-238D from Suntific Materials CO. Ltd (Weifang, China) were used in the experiment. The photoresist was a diazonaphthoquinone (DNQ) positive photoresist and the developer was mainly composed of tetramethylammonium hydroxide (TMAH). Self-prepared wet chemical etchant was 50 wt. % ferric chloride (FeCl3) aqueous solution.
3. Results and discussion 2.2. Experimental methods 3.1. Structure of FSSs Fig. 1 was the schematic diagram of LDWL or LDA equipment system. The both systems included a laser source, an optical system, and an x-y-z three-dimensional (3D) translation stage, respectively. For LDWL, the laser source was with a wavelength of 355 nm and a maximum output power of 0.5 W; For LDA, the laser source was with a wavelength of 1064 nm and a maximum output power of 20 W. The focused laser spot diameters at 1/e2 of their maximum intensity were approximately 10 μm and 20 μm, respectively. In either optical system, a beam expander was used to expand and collimate the laser beam and a high-speed galvanometer scanner was used to rapidly manipulate the laser beam, and the optical deflection angle range was between -20° and +20°; Besides, an F-theta lens was used to focus the laser beam on a plane. The both translation stages had movement ranges of 200 mm in
Generally speaking, FSSs are composed of the periodic structures containing such self-symmetric patterns as circular rings, square loops, dipole, etc. because of the good stability of these patterns (Panwar and Lee, 2017). Depending on corresponding demand, FSS can be designed and fabricated on a planar or curved surface, and the adopted materials can be pure conductors (e.g., metal) or composite materials consisting of conductive layers and dielectric layers. In our experiments, square loop was chosen as the periodic pattern unit of FSS, and FCCP was used as the fabrication material. Fig. 4 was the structure of a FSS. Its whole size was 200 mm × 200 mm, and it had 625 square loop periodic units. The geometry of each unit was: p = 8 mm, a = 7.05 mm, b = 6.45 mm and loop gap g = 300 μm. The 130
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Fig. 2. Schematic diagram of FSS fabricated by LDWL + WCE. (a) Coating with photoresist, (b) LDWL and developing, (c) WCE, and (d) Removing residual photoresist.
1000 mm/s, single scan, and hatch space of 20 μm. Fig. 5d showed the pattern processed by LDWL + WCE with the optimal laser parameters. It could be seen that the edge was smooth and straight, and the copper layer was completely removed. Similarly, for LDA, the orthogonal experiments for laser power (5–20 W), scan speed (100–2000 mm/s), scan times (1–4), and hatch space (10–40 μm) were also explored. The optimal laser parameters were laser power of 12 W, scan speed of 200 mm/s, 4 scan times, and hatch space of 25 μm. It indicated that LDA needed higher laser power, more scan times, and lower scan speed (compared with that of LDWL). Commonly, for both LDWL and LDA, there are two kinds of scan path to fabricate the square loop unit. One is raster scan (Fig. 6a), and the other is pattern-profile scan (Fig. 6b). Because of the frequent acceleration, uniform and deceleration of the laser beam during direct writing lines, when the raster scan was adopted, the square loop would produce jagged edges (Fig. 6c). However, when the pattern-profile scan was adopted, this would be alleviated, or even be eliminated. Here, smooth edge would be obtained, and the pattern quality was greatly improved (Fig. 6d). Eventually, under the optimal condition, the designed FSSs were fabricated within 8 min by LDWL + WCE. While it took about 62 min to fabricate the same patterns by LDA. The latter cost nearly 8 times as much time as the former. Obviously, LDWL + WCE are a higher-efficiency processing technology for large-area FSSs.
Fig. 3. Setup for testing electromagnetic wave characteristics of FSSs.
processing error of loop gap size should be within ± 5% (i.e., ± (300 × 5%) = ± 15 (μm)). In addition, the electromagnetic wave characteristics of the FSS would be better when the edges of the square loops were smoother and steeper, and the FR-4 dielectric layer substrate was less damage. 3.2. Fabrication of FSSs by LDWL + WCE and LDA
3.3. Profile and micromorphology of as-prepared FSSs During the fabrication, laser processing parameters mainly included laser power, scan speed, scan times, hatch space and scan path. For LDWL, preliminary experiments showed that laser power, scan speed, hatch space and scan times had great impacts on the photoresist exposure, overlap along scan direction, overlap perpendicular to scan direction, and processing efficiency. Thus, when inappropriate parameters were used, poor patterns would be obtained, like rough and unsmooth edges (Fig. 5a), or jagged edges (Fig. 5b), or uncompleted removal of copper layer (Fig. 5c). All would affect the precision and performance of the as-prepared FSSs. Basing on the preliminary experiments, a 4 × 4 orthogonal experiment for laser power (20–50 mW), scan speed (500–2000 mm/s), scan times (1–4), and hatch space (10–40 μm) was implemented. The obtained optimal parameters were laser power of 40 mW, scan speed of
Fig. 7a and b were the photographs of the FSSs fabricated by LDWL + WCE and LDA, respectively. It seemed to have no obvious difference from macroscopic scale. The average widths of the square loop gaps were about 292 μm and 287 μm, respectively, which had a 2.67% and 4.33% deviation from the designed value (i.e., 300 μm). Both were within the above-mentioned error. However, from microscopic scale, there existed big differences. In Fig. 7(c)–(e), it could be seen that the square loops fabricated by LDWL + WCE had smooth, straight and steep edges, and the FR-4 substrates had no damage. This was because the copper layer was removed by wet chemical etching. The etchant only reacted with the naked copper layer and it had no damage to the FR-4 substrates or the photoresist.
Fig. 4. Structure of FSS (left: whole FSS; right: periodic square loop unit). 131
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Fig. 5. Different square loop profiles fabricated by LDWL + WCE with different laser parameters. (a) Rough edges due to overlarge laser power, (b) Jagged edges due to overlarge scan speed, (c) Uncompleted removal of copper layer due to overlarge hatch space, and (d) perfect profile by optimal laser parameter.
Fig. 6. Two scan paths ((a) raster scan or (b) pattern-profile scan) of square loop fabrication by LDWL or LDA, and micrographs of square loop edges fabricated by LDWL + WCE or LDA using raster scan (c) or pattern-profile scan (d).
Fig. 7. As-prepared FSSs (a (by LDWL + WCE) and b (by LDA)), (c) profiles of the aperture gaps of square loops, and magnified images of the aperture gaps of square loops by LDWL + WCE (d and e) and by LDA (f and g).
While the square loop edges fabricated by LDA had large heat affected zones (HAZ) beside the grooves (Fig. 7(f)). Moreover, the edges had lots of burrs and remelting residues, and the FR-4 substrate was damaged severely (Fig. 7(g)). In addition, from Fig. 7(c), it could be seen that the grooves fabricated by LDA were rougher than that by LDWL + WCE. This was because the copper was removed by laser heat melting and evaporating. The high laser power generated a large amount of heat, and leaded to large HAZs and remelting residues on both sides of the grooves. Besides, the FR-4 substrate was easily damaged when the copper layer was removed because of large amounts of heat and heat conduction. Therefore, from microscopic scale, the FSSs fabricated by LDWL + WCE had better square-loop profile and micromorphology than that by LDA. 3.4. Electromagnetic wave characteristics of as-prepared FSSs In the frequency range of from 2 GHz to 18 GHz, the electromagnetic wave transmission coefficients of the as-prepared FSSs were shown in Fig. 8. It could be seen that the FSSs were good bandpass filters and had special frequency selections. For example, the FSS processed by LDWL + WCE had transmission response at 7.08 GHz, and the transmission coefficient (S) at the passband peak was about
Fig. 8. Electromagnetic wave transmission response curves of simulation and experiment results of FSSs.
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-0.099 dB (the solid line in Fig. 8). According to the equation S = 10 ∙ lg (T), the calculated transmittance T was equal to 97.75%. The -3 dB bandwidth was about 3.04 GHz (5.48–8.52 GHz). Obviously, these experimental results were in good agreement with the simulation results (the red and dashed line in Fig. 8), especially in low frequency range (i.e. < 12 GHz). On the other hand, the FSS fabricated by LDA had transmission response at 7.44 GHz. The transmission coefficient (S) at the passband peak was about -1.73 dB, and the corresponding transmittance (T) was 67.14%. The -3 dB bandwidth was only 1.76 GHz (6.52–8.28 GHz) (the blue and dashed line in Fig. 8). Whereas, these data had a big deviation from the simulation results (the red and dashed line in Fig. 8) in the both low and high frequency ranges. Depending on the analysis to the electromagnetic wave characteristics measurement setup, it was found that the both deviations in high frequency range were because the setup itself was not very stable in this range. From the above experiment and simulation results, it showed that the FSS processed by LDWL + WCE had much better electromagnetic wave characteristics than that of the one by LDA. This was directly related to the smoother and steeper edges of the square-loop and the undamaged substrate. Further experiments showed that the FSSs with periodic structure units of different shapes and sizes had different electromagnetic wave characteristics. Thus, LDWL + WCE are a general and high-performance processing technology for the FSSs basing on the composite materials consisting of conductive layers and dielectric layers. In addition, it can be expected that this technology of LDWL + WCE is also able to fabricate non-planar or curved FSSs with larger size when the LDWL equipment system adopts larger-range translation stage. This will be published elsewhere in the near future.
were grateful to the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for SEM testing, and the assistance from engineer Haili Zhang in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for profile measurements. References Batchelor, J.C., Parker, E.A., Miller, J.A., Sanchez-Romaguera, V., Yeates, S.G., 2009. Inkjet printing of frequency selective surfaces. Electron. Lett. 45, 7–8. Bishop, C.A., 2015. Chapter 15 - electron beam (E-beam) evaporation. In: Bishop, C.A. (Ed.), Vacuum Deposition Onto Webs, Films and Foils, Third edtion. William Andrew Publishing, Boston, pp. 289–299. Cooper, J.R., Kim, S., Tentzeris, M.M., 2012. A novel polarization-independent, freespace, microwave beam splitter utilizing an inkjet-printed, 2-D array frequency selective surface. IEEE Antennas Wirel. Propag. Lett. 11, 686–688. Hon, K., Li, L., Hutchings, I., 2008. Direct writing technology—advances and developments. CIRP Ann.-Manuf. Technol. 57, 601–620. Hong-Kyu, J., Won-Jun, L., Chun-Gon, K., 2011. Design and fabrication of a microstrip patch antenna with a low radar cross section in the X-band. Smart Mater. Struct. 20, 015007. Kim, P.C., Lee, D.G., Seo, I.S., Kim, G.H., 2008a. Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces. Compos. Sci. Technol. 68, 2163–2170. Kim, P.C., Lee, D.G., Seo, I.S., Kim, G.H., 2008b. Nanocomposite stealth radomes with frequency selective surfaces. Compos. Struct. 86, 299–305. Kim, J.Y., Kim, D.S., Woo, D.W., Shin, D.S., Park, W.S., Hwang, W., 2014. Electromagnetic wave characteristics of second-order composite frequency-selective surfaces fabricated by e-beam evaporator. Electron. Lett. 50, 189–190. Lewis, J.A., Gratson, G.M., 2004. Direct writing in three dimensions. Mater. Today 7, 32–39. Li, W., Chen, M., Zeng, Z., Jin, H., Pei, Y., Zhang, Z., 2017. Broadband composite radar absorbing structures with resistive frequency selective surface: optimal design, manufacturing and characterization. Compos. Sci. Technol. 145, 10–14. Li, J.S., Li, Y., Zhang, L., 2018. Terahertz bandpass filter based on frequency selective surface. IEEE Photonics Technol. Lett. 30, 238–241. Liu, T., Kim, S.-S., 2016. Design of wide-bandwidth electromagnetic wave absorbers using the inductance and capacitance of a square loop-frequency selective surface calculated from an equivalent circuit model. Opt. Commun. 359, 372–377. Lu, Z., Liu, Y., Wang, H., Zhang, Y., Tan, J., 2016. Optically transparent frequency selective surface based on nested ring metallic mesh. Opt. Express 24, 26109–26118. Marhefka, R.J., Young, J.D., Towle, J.P., 2007. Design, fabrication and measurement of an FSS antenna ground plane. Jensen, M. (Ed.), IEEE Antennas and Propagation Society International Symposium 3972–3975. Mizeikis, V., Juodkazis, S., Sun, K., Misawa, H., 2010. Fabrication of frequency-selective surface structures by femtosecond laser ablation of gold films. J. Micro/Nanoeng. 5, 115–120. Moallem, M., Sarabandi, K., 2012. Miniaturized-element frequency selective surfaces for millimeter-wave to terahertz applications. IEEE Trans. Terahertz Sci. Technol. 2, 333–339. Munk, B.A., 2005. Canada. Frequency Selective Surfaces: Theory and Design. John Wiley & Sons, pp. 1–4. Novaković, D., Kašiković, N., Vladić, G., Pál, M., 2016. 15 - screen printing. In: Izdebska, J., Thomas, S. (Eds.), Printing on Polymers. William Andrew Publishing, Chadds Ford, USA, pp. 247–261. Panwar, R., Lee, J.R., 2017. Progress in frequency selective surface-based smart electromagnetic structures: a critical review. Aerosp. Sci. Technol. 66, 216–234. Pease, R.F., Chou, S.Y., 2008. Lithography and other patterning techniques for future electronics. Proc. IEEE 96, 248–270. Rahman, H., Dowling, J., Saha, P.K., 1995. Application of frequency sensitive surfaces in electromagnetic shielding. J. Mater. Process. Technol. 54, 21–28. Sang-Dong, J., Byung-Woo, K., Jaehwan, K., 2013. Frequency selective surface based passive wireless sensor for structural health monitoring. Smart Mater. Struct. 22, 025002. Song, J.H., Nur, H.M., 2004. Defects and prevention in ceramic components fabricated by inkjet printing. J. Mater. Process. Technol. 155–156, 1286–1292. Wei, X., Hai-Feng, C., Zeng-Yong, C., Yong-Jiang, Z., Hai-Tao, L., Zhao-Hui, C., 2009. Effect of FSS on microwave absorbing properties of hollow-porous carbon fiber composites. Mater. Des. 30, 1201–1204. Yong, W.Y., et al., 2018. Flexible convoluted ring shaped FSS for X-Band screening application. IEEE Access 6, 11657–11665.
4. Conclusion A rapid and high-performance processing technology for large-area FSSs was presented. This novel technology combined LDWL with WCE, and had the potential to fabricate non-planar or curved FSSs. Basing on experiments, the optimal processing parameters of LDWL + WCE were laser power of 40 mW, scan speed of 1000 mm/s, single scan, hatch space of 20 μm, pattern-profile scan, and ferric chloride aqueous solution as etchant. Under this optimal condition, the square-loop FSS with 625 units was fabricated on a 200 × 200 mm2 FCCP. The total processing time was within 8 min. In comparison to LDA, this technology improved the processing efficiency nearly 7 times. Moreover, the FSS fabricated by LDWL + WCE was with higher quality and higher performance. The electromagnetic wave frequency response of this FSS was at 7.08 GHz, and the corresponding transmittance reached up to 97.75%, which showed a better capability of bandpass filters. Its experiment and simulation results were in much better agreement. The technology of LDWL + WCE is a general processing technology for the FSSs basing on the composite materials consisting of conductive layers and dielectric layers. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 51775209). Also, the authors
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