Inkjet Printing RF Bandpass Filters on Liquid Crystal Polymer Substrates

Transcription

1 Inkjet Printing RF Bandpass Filters on Liquid Crystal Polymer Substrates Hsuan-ling Kao a*, Chia-Ming Kuo a, Cheng-Lin Cho b, Li-Chun Chang c a Dept. of Electronic Engineering, Chang Gung University, Tao-Yuan, Taiwan b Dept. of Engineering and System Science, Nat l Tsing Hua Univ., Hsinchu, Taiwan c Dept. of Materials Engineering, Ming Chi Univ. of Technology, New Taipei, Taiwan *snoopy@mail.cgu.edu.tw ABSTRACT This study investigates inkjet-printed radio frequency bandpass filters on liquid crystal polymer (LCP) substrates. Silver nanoparticle colloidal solution was used as the printing ink. A plasma polymerization technique was used to improve the LCP surface by optimizing the printing contact angle. Various numbers of passes were used to print the samples to study the changes in conductivity and uniformity. Samples were then sintered in an oven to remove excess solvent and material impurities. The silver-film conductivities of the conductive ink were approximately Siemens/m for 1-pL ink drops. The minimum insertion loss was 2.78 db and 2.87 db at 28.6 GHz and maximum return loss at 29.9 GHz was 27.7 db for bandpass filter. This study describes the optimization of the inkjet printing process and discusses the surface treatment study of the inkjet-printed seed layer and process integration are presented in detail. Keyword: Bandpass filter, Inkjet printing, Liquid crystal polymer (LCP), Silver. 1. Introduction Inkjet-printing technology has recently been adopted in RF (or microwave) applications, such as ultra-high-frequency antennas and filters on paper substrates [1]-[2]. Compared to traditional etching techniques, the design patterns of direct-write technology that does not require masks reduce material use and waste generated by wet processes. Inkjet-printing technology is generally faster and more economical than other additive manufacturing technologies. Inkjet-printing technology is an integrated and green chemical approach. Although paper substrate offers low-cost, fast, and simple inkjet printing, it is limited by high-frequency, absorption, and humidity concerns. Materials and integration techniques are becoming more and more critical for wireless applications. Liquid crystal polymer (LCP) is a favorable material for high frequency applications, with an impressive loss tangent (approximately over the entire RF range up to 110 GHz). Studies have used millimeter wave filters on LCP substrates for wide bandwidth design [3], multilayer circuit technology [4], and

2 integrated waveguides [5] due to its stable and desirable electrical characteristics for wide band, moisture absorption, and low thermal expansion coefficient. This material is also suitable for reel-to-reel processing and has good flexibility. Integrated the inkjet printing technology on LCP substrate required the uniformity of conductivity and thickness of the metal line for high frequency applications. Therefore, inkjet-printing conditions including surface treatment, additional thermal treatment, and multi-pass printing on the LCP substrate, should be established. This study observes and compares the microstructure and electrical properties of inkjet-printed Ag-film on an LCP substrate. A 29 GHz bandpass filter consists of two identical half-wavelength ( /2) microstrip lines using inkjet printing was developed to achieve superior performance. The bending effect is also observed. Inkjet printing technology on LCP substrate points enables low-cost and rapid manufacturing of RF and millimeter wave circuits. 2. Experimental Procedure The bandpass filter was produced on an LCP substrate using inkjet-printing technology. A LCP substrate is used the Rogers ULTRALAM 3850 [6]. An LCP substrate with constant relative permittivity of 3.2, a stable loss tangent below over the entire RF range up to 110 GHz and a substrate thickness of 100 m, are suitable for a high-frequency design with excellent performance. Before printing, it is necessary to treat the surface by plasma polymerization with appropriate conditions. Samples were held in a vacuum at a pressure of 30 mtorr. Adding oxygen to the chamber increased the working pressure to 70 mtorr. Surface treatment processes were conducted at room temperature. The plasma was sustained between parallel-plate electrodes by an RF generator operating at MHz at various plasma polymerization operation powers for 5 s. The filter was printed using a Dimatix DMP 2800 printer with a DMC cartridge. The cartridge contained 16 nozzles, 9 m in diameter and each nozzle generated 1-pL ink drops. The drops produced a 30- m spot when printed on a surface treated with an LCP substrate. Silver nanoparticle inks are usually used in the inkjet-printing process to ensure good metal conductivity. The ink (Advanced Nano Products Co., Ltd., DGP-40LT-15C) used in this study contained 30 to 35 wt% spherical silver nanoparticles, dispersed in triethylene glycol monoethyl ether. Silver nanoparticle ink was printed on a substrate heated to 60 C. Multi-pass printing is required to achieve uniform conductivity and appropriate conductor thickness. The sintering process is important because it removes excess solvent and material impurities from the deposition and increases bonding between the silver ink and substrate. The samples were sintered in an oven at various temperatures at atmospheric conditions. Sintering was conducted for 1 h and the temperature was

3 increased at approximately 5 C/min. The conductivities of the printed metal lines were measured using the four-probe method. The surface morphologies of the films were examined by field emission scanning electron microscopy (FESEM, JSM7000F, JEOL, Japan) under a 3-kV accelerating voltage. The cross sectional images of the films were evaluated by using a scanning electron microscope (SEM, S3400N, Hitachi, Japan) under a 15-kV accelerating voltage. The cross sectional SEM samples were prepared by cold mounting. Bandpass filter characteristic was measured using an HP E8364C network analyzer for S-parameter measurements. 3. Results and Discussion Capillary forces and surface wetting determine how the ink drop spreads on the substrate. Studies have used surface treatments with Argon and Oxygen plasma [7], UV-ozone [8] or wet bath treatments [9] to increase substrate surface energy [10]. In this study, the plasma polymerization surface treatment on an LCP substrate enhances the surface energy and renders it hydrophilic, producing the correct printing conditions. Plasma treatment enhances the substrate surface energy. The contact angle monitors the surface tensions of the corresponding interfaces that intersect at the three-phase contact line. An optimal contact angle is required for uniform distribution of silver nanoparticles. Fig. 1 shows the Ag ink droplet contact angle and single-pass printing silver thickness measurement at different plasma polymerization operation powers for 5 sec. Without any plasma treatment, the contact angle was 111 which means the LCP substrate is too hydrophobic. A saturation operating power of 10 W produced a contact angle of 51. Increasing the plasma operation power decreased the contact angle. Optical microscopy photographs of a 1 1 mm 2 Ag pattern show a larger contact angle, which produces discontinuous silver dots because the substrate was too hydrophobic. By contrast, with a smaller contact angle, the ink spreads and the pattern edge is distorted. The surface treatment was applied at 3 W for 5 s to produce an optimal contact angle of 79. This setting was used in subsequent experiments. (a) (b) (c) (d) Fig. 1. Water contact angle and silver film OM versus (a) 0 W, (b) 3 W, (c) 5 W, and (d) 10W operation power of plasma polymerization for surface treatment. Substrate temperature was used to control the droplet spreading when it touched the LCP substrate. In this study, the maximal substrate temperature of the printer was

4 Conductivity (x10 7 S/m) Silver Thickness ( m) 60 C; therefore, ink drying at the edge of a pattern occurred simultaneously, providing sufficient edge-drying for each pass. A post-bake process accelerated the sintering rate of the solvent in the ink to improve the conductivity of the silver-film. Fig. 2 shows the conductivities and thicknesses at various sintering temperatures for 10-pass printing. The temperature was limited by 315 C melting point of the LCP substrate. The conductivity depends on the sintering temperature and is saturated at 270 C, but the silver thicknesses are almost identical Note that the lower sheet resistance is required for gigahertz frequency performance. Therefore, a sintering temperature of 270 C was used during fabrication to sufficiently cure the nanoparticle ink on the LCP substrate Substrate Temp:60 o C 10 layers Sintering Temperature ( o C) Fig. 2. Conductivity and silver thickness of the Ag film with various sintering temperatures for 10-pass printing. Multi-pass printing is required to obtain better uniformity and higher conductivity. The substrate temperature and the duration between each pass are the key parameters of multi-pass printing. The substrate temperature controls the spreading of droplets and the sintering of ink when it contacts the substrate. The ink dried at the edge of the pattern because of an adequate temperature and 1-pL ink volume. Fig. 3 shows the surface morphologies and thicknesses of the printed results with 5, 7, 10, and 15 passes. The images show little microstructure evolution. Multi-pass printing improved the uniformity of Ag film. More printing passes created smoother surface morphology. After 10-pass printing, the surface smoothness was saturated. Fig. 4 shows the conductivity and silver thickness of Ag film printed with 5 to 15 passes. Silver thickness and conductivity were proportional to the number of printing passes as the surface morphologies in Fig. 3. The bulk silver conductivity for 10-pass printing was approximately Siemens/m.

5 Conductivity (x10 7 S/m) Silver Thickness ( m) Substrate Temp:60 o C Curing Temp:270 o C No. of passes printed Fig. 3. Conductivity and silver thickness of the Ag line from 5 to 20 passes printing. 5-layer 1.15 m 7-layer 2.42 m 10-layer 3.37 m 15-layer 4.77 m Fig. 4. Surface morphology and thickness of Ag film printed with 5, 7, 10 and 15 of passes.

6 Fig. 5 shows a schematic drawing and an image of the bandpass filter. The bandpass filter consists of two identical half-wavelength ( /2) microstrip lines and two tapped I/O lines at one side of each /2 microstrip line. The bandpass filter was applied on an LCP substrate with a dielectric constant r =3.2, a loss tangent =0.0025, and thickness h=100 m. The physical dimensions of the 29 GHz bandpass filter were D 1 =0.6, D 2 =0.2, D 3 =0.63 D 4 =1.6, D 5 =0.37, D 6 =0.07, D 7 =0.2, and D 8 =0.05 (millimeters). The bandpass filters were printed on the LCP according to these parameters. These prototypes occupied an LPC area, including probing pads, of mm 2. For the bandpass filter, the minimum insertion loss at 28.5 GHz was 2.78 db and maximum return loss at 29.9 GHz was 27.7 db, respectively. Its central frequency was 29 GHz with 14% relative bandwidth. A close agreement between the measured and simulated S-parameters was obtained, as shown in Fig. 6. The results show that inkjet printing on LCP is an efficient option for mm-wave filter implementation. The 29 GHz bandpass filter was bent on vehicles with radii of 7.5 mm and 5 mm in the middle of the structure. The measurement setup for bending of the inkjet printed microstrip line was also shown in Fig. 6. We used bending vehicles with radii (r) of 7.5 mm and 5 mm to achieve bending (mechanical strain). The bending effects of insertion loss and return loss of the microstrip line are also presented. Compared to the flat filter, the insertion losses were increased to 2.8 db at 28.6 GHz and 2.87 db at 28.6 GHz for bending vehicles with curvature radii of 7.5 mm and 5 mm, respectively. Compared to the flat filters, the resonance frequencies were increased by 200 MHz and 600 MHz for bending radii of 7.5 mm and 5 mm, respectively. The observed frequency shifts corresponded to length variations of μm (1.01 ) and μm (3.034 ) for bending radii of 7.5 mm and 5 mm, respectively. The reason for frequency shifts and insertion losses is a mechanical deformation of the metallization [11]. It is possible to apply large bending (mechanical strain) to the inkjet printed line on a flexible LCP substrate without cracking. Fig. 5. Schematic and image of printed line bandpass filter.

7 Magnitude (db) S 21 S 11 S 11 S 21 Flat Bend'n r=7.5 mm Bend'n r=5 mm simulated Symbol:Measured Line:Simulated Frequency (GHz) r Fig. 6. S-parameters of the inkjet printed bandpass filter before and after bending. 4. Conclusion This study uses the inkjet-printing process to produce an interdigital coupled line bandpass filter on LCP substrates. The morphology and electrical properties of inkjet-printed silver film significantly affects insertion loss. Optimization of the inkjet-printing process and surface treatment of LCP substrates can create very low-cost and high-performance integrated circuits for RF and millimeter-wave applications. REFERENCES [1] Shaker, G., Nasri, F., Reynolds, T., Nikolaou, S., and Tenzeris, M., Inkjet printing of dual band conformal antenna for use in wifi frequency bands, IEEE Radio and Wireless Symp.Dig., Jan. 2010, [2] Marroncelli, M., Trinchero, D., and Tentzeris, M. M., Paper-based, inkjet-printed, text-meandered UHF resonant antennas for RFID applications, IEEE General Assembly and Scientific Symp., Aug [3] Zhang, X., Kuylenstierna, D., Liu, J., Cae, P., Andersson, C., Morris, J., and Zirath, H., A compact V-band planar wideband bandpass filter based on Liquid Crystal Polymer substrates, 2 nd Electronics System integration Technology Conference, Sept. 2008, [4] Lee, J. H., Sarkar, S., Pinel, S., Papapolymerou, J., Laskar, J., and Tentzeris, M. M., 3D-SOP Millimeter-Wave Functions For High Data Rate Wireless Systems Using LTCC and LCP Technologies, Electronic Components and Technology Conference, May 2005, [5] Karpour, A., and Wenham, S.R., The importance of surface roughness in the adhesion of electroless-plated metal in inkjet printed grooves, IEEE Trans. on

8 Microwave Theory and Techniques, 54 (12), Dec. 2006, [6] ULTRALAM 3850, Rogers Corporation, [7] B. K. Lok, and X. Hu, Transient Contact Angle of Evaporating Inkjet Droplet on Trans-parent Polymer Substrate, 12th Electronics Packaging Technology Conference, 2010, [8] Lee, S., Shin, K. Y., Hwang, J. Y., Kang, K., and Lee, G. B., Modification of Surface Properties for Industrial Ink-jet Printing, International Conference on Smart Manufacturing Application, 2008, [9] Lok, B. K., Liang, Y. N., Gian, P. W., Xuechuan, S., and Lu, A.C. W., Process Integration of Inkjet Printing and Electroless Plating for LTCC Substrates, IEEE Electronics Packaging Technology Conference, 2007, [10] Shaker, G., Tentzeris, M., and Safavi-Naeini, S., IEEE Antennas and Propagation Society International Symposium, 1, 2010 [11] Geise, A., and Jacob, A. F., Investigations of transmission lines and resonant structures on flexed liquid crystal polymer (LCP) substrates up to 67 GHz, 39 th European Microwave Conference, Sept. 2009,