Research ArticleInkjet-Printed Interdigital Bandpass Filter with Wide StopbandUsing Multilayer Liquid Crystal Polymer Technique

Li-Chun Chang,1,2 Cheng-Lin Cho ,3 Sameer Kamrudin Bachani ,4

and Hsuan-Ling Kao 5,6

1Department of Materials Engineering, Ming Chi University of Technology, 84 Gongzhuan Rd., Taishan Dist.,New Taipei City 24301, Taiwan2Center for Thin Film Technologies and Applications, Ming Chi University of Technology, New Taipei City, Taiwan3Department of Engineering and System Science, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road,Hsin-Chu, Taiwan4Department of Mechanical Engineering, National Taiwan University of Science and Technology, No. 43, Keelung Rd., Sec. 4,Taipei 10607, Taiwan5Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan, Taiwan6Department of Dermatology, Chang Gung Memorial Hospital, Linkou Branches, Kwei-Shan Tao-Yuan, Taiwan

Correspondence should be addressed to Hsuan-Ling Kao; snoopy@mail.cgu.edu.tw

Received 10 February 2018; Revised 21 April 2018; Accepted 30 April 2018; Published 15 May 2018

Academic Editor: N. Nasimuddin

Copyright © 2018 Li-Chun Chang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article presents a two-layer inkjet-printed interdigital bandpass filter using lamination bonding process on liquid crystal polymer(LCP) substrates for radio frequency electronic applications. Various percentages of torque force were applied over a 4× 4 cm2 areawith a 942 kg fixed force in the lamination bonding process. The insertion loss and surface morphology of the inkjet-printed silverfilm were examined on various torque forces to develop the lamination bonding process. The lamination bonding was performedat 12% torque and 270°C. A three-dimensional bandpass filter was realized with a S21 of −2.2 dB at 11.5GHz with a 17%fractional bandwidth. A multilayer inkjet-printed bandpass filter was successfully developed to verify the design methodology andfabrication of inkjet-printing technology and lamination bonding technique for a three-dimensional integrated circuit package.

1. Introduction

Three-dimensional (3D) technologies [1–3] provide moredesigns and compact sizes for radio frequency (RF) and mil-limeter wave devices. Low-temperature cofired ceramic(LTCC) packaging method is useful for microwave applica-tions because of excellent physical and electrical properties[3–5]. However, the sintering temperature of LTCC shouldbe at the level of 850°C. Compared to printed circuit board,the cost of LTCC is expensive. Recently, liquid crystal poly-mers (LCPs) have become a favorable material for RF applica-tions because of an impressive loss tangent, lower thermalexpansion coefficient, and lower moisture absorption. Addi-tionally, the dielectric constant of LCP is 2.9, which is lowerthan that of LTCC [6, 7] and is favorable for multilayer lami-nation because it provides more tolerances for registration

error. Several authors have presented studies on the ultrawide-band bandpass filter using multilayer LCP technology at dif-ferent frequencies [6, 8]. Multilayer inductors and capacitorsusing LCP technology have been demonstrated for high-passfilter design [7, 9]. A low-loss integrated waveguide bandpassfilter using LCP substrate was published to provide an inex-pensive low-loss hermetic packaging solution [10]. No losseswere recorded over a frequency range of 25GHz and 15GHzfor coplanar waveguide (CPW), and microstrip via transitionswere examined, respectively [11]. Therefore, multilayer LCPtechnologies are suitable for RF applications.

Inkjet printing technology has gained popularity becauseof its cost-effective direct-write technology and being envi-ronmentally friendly. Several studies on the implementationand characterization of inkjet-printed silver film on an LCPsubstrate have been conducted [12, 13]. In our study, the

HindawiInternational Journal of Antennas and PropagationVolume 2018, Article ID 6161427, 7 pageshttps://doi.org/10.1155/2018/6161427

multilayer LCP technology was utilized at high torque forcesand a high temperature. The strength of the inkjet-printedsilver film at various torque forces was studied. Accordingto the strength results, a multilayer inkjet-printed interdigitalbandpass filter was realized to demonstrate a high perfor-mance and cost-effective integration circuit for electronicpackage applications. This paper is organized as follows.Section 2 describes the processes for fabricating the inkjet-printed silver film and the lamination bonding process formultilayer LCP substrates. Section 3.1 focuses on the inser-tion loss and surface morphology of inkjet-printed silverfilm under various torque forces. Section 3.2 describes thestructure and fabrication of the multilayer bandpass filter.In Section 4, we provide the conclusions.

2. Fabrication Process

Figure 1 illustrates the process flow of inkjet printingand lamination bonding for three-dimensional interdigitalbandpass filter. DGP-40LT-15C silver ink was used to printthe metal layers using a DMP-2800 inkjet printer. Two types

of Rogers LCP substrates including ULTRALM 3850 (corefilm) and 3908 bondply (bonding film) were adopted whichhave similar thermal expansion and dielectric constants[14]. The steps for fabricating the inkjet-printed three-dimensional interdigital bandpass filter are as follows:

(1) Plasma polymerization process was applied at 3W,5 s for surface treatment.

(2) Ten-pass silver ink was printed at 60°C onto thefront of the core.

(3) Three vias were drilled in the core film for subse-quent alignment.

(4) The LCP was flipped to layer 2 and treated byplasma polymerization.

(5) Layer 2 was aligned using the horizontal vias, andthen the intersected vertical via was the startingpoint for layer 2 printing.

(6) The backside of the silver film was printed.

Surface treatment

Plasma polymerization

Plasma polymerization

Drilling

Surface treatment Inkjet printing (back)

Sintering

Top view (final)

Alignment

Top view (middle) Lamination

Drilling Via by silver paste

Core film Core film

Core film Core film

Core film

Core film Core film

Core film

Heat substrate

Buffer layer

Buffer layerBonding film

Bonding film Bonding film

Inkjet printing (front)

Core film

Heat substrate

Figure 1: Inkjet printing and lamination bonding process flow.

2 International Journal of Antennas and Propagation

(7) The double-sided silver films were subjected to270°C for 1 h.

(8) A two-layer inkjet-printed silver film was achieved.

(9) The core film and a copper ground film were lami-nated using a bonding film at various torques and270°C, throughout which its properties were stud-ied. The sample was protected at the top and bottomby two buffer layers.

(10) Eight ground vias were drilled using a drillingmachine with a 0.2mm diameter needle.

(11) The eight vias were filled with a conductive paste toconnect the background plane.

(12) Fabrication of the inkjet-printed bandpass filter bylamination bonding was finished.

3. Results and Discussion

3.1. Various Lamination Bonding Percentages. The lamina-tion bonding process was applied at a constant pressure from25°C to 270°C, and the temperature was cooled down toroom temperature. Various percentages of torque force weretuned over a 4× 4 cm2 area with a 942 kg fixed force. Thesample was clearly open while the torque force was smallerthan 12%. An obvious crack was observed at 15% torqueforce. Therefore, the samples were measured at torque forceof 12 to 14%. Figure 2 shows the changes in the insertionlosses of the printed line (width, 100μm; length, 4000μm)and a probing pad on a 100μm LCP substrate during severaltorque force experiments. The inserted figure shows the S-parameters of the silver line, and the resonant frequency ofthe printed line was approximately 22GHz. The insertionloss at the resonant frequency was dependent on the torqueforce. The insertion loss at 12% (<0.68 dB at 22GHz) was

comparable to that of the 0% sample. The insertion lossesat the resonant frequency below the 12% printed line were0.22 and 0.09 dB when torque forces were 13% and 14%,respectively. However, the average degradations in the valueof insertion losses over a whole frequency range were −1and −1.9 dB at 13% and 14%, respectively. Figure 3 showsthe surface morphologies at various torque forces. The sur-face roughness of the silver film increased with torque force.No crack was observed at a torque force of 0% and 12%.At 13%, few cracks were found occasionally on the surfaceof the silver film. A lot of surface cracks were generated inthe microstructure images while bonding at 14%. The sur-face morphology was consistent with the S21 value. There-fore, 12% was the optimal torque force for the laminationbonding process.

3.2. Multilayer Inkjet-Printed Bandpass Filter. Figure 4illustrates the interdigital bandpass filter comprising twoLCP layers and three metal layers. The bandpass filtercomprises two feeding lines on layer 1, two quarter-wavelength resonators on layer 2, and a ground on layer3 for the transmission lines on the other layers. The feed-ing lines provided a broadside coupling to the resonatorson layer 2. At the other end of the feeding lines, theground vias were connected to generate three transmissionzeros (TZs) because of the inductive source-load coupling[15]. Two resonators were meandered on layer 2 andcoupled with each other using an interdigital capacitor.The bandpass filter was simulated using electromagneticsimulation software. The proposed filter was designed withthe following parameters: a permittivity (εr) of 2.9 and aloss tangent (tanδ) of 0.0025 for LCP substrate and a con-ductivity of 1 × 107 S/m and a thickness of 3.4μm for silverfilm. Figure 5 shows the simulated transmission character-istic of the layout for various spaces of inductive source-load coupling (S1). No transmission zero was producedwithout inductive source-load coupling, and the strengthof broadside coupling was weak. With inductive source-load coupling (S1 = 0.3mm), two transmission zeros (TZ1and TZ2) near the passband were produced because ofthe cross coupling between feeding lines. Additionally,transmission zero (TZ3) at stopband is generated whileS1 ≦ 0.2mm because the two transmission paths for the sig-nals (source-load path and source-resonator 1-resonator 2-load path) have the same magnitudes but are out of phase.The transmission zeros can be tuned by S1 because thestrength of the source-load coupling is changed. This alsoverifies that the three transmission zeros were generated bythe inductive source-load coupling. Figure 6 shows the fre-quency responses of various finger numbers (nr) for interdi-gital capacitor. The interdigital capacitor shortened thelength of the coupling region, and the resonant frequencywas tuned by adjusting the capacitance. The coupling ratio(k) between two resonators affected the insertion loss.Figure 7 shows the extracted coupling ratio at f0 against L13and S2. From the resonant frequency and the required k,the size of the interdigital capacitor was determined.Figure 8 shows the three TZ positions relative to the variouscouple length of two feeding lines (L7). The TZ3 can be tuned

−4.0

−3.5

−3.0

−2.5

−2.0

−1.5

−1.0

−0.5

0.0

|S21

| (dB

)

Frequency (GHz)

0%12%

13%14%

0 5 10 15 20 25 30

0 5 10 15 20 25 30 35 40−30−25−20−15−10

−50

S11S-

para

met

ers (

dB)

Frequency (GHz)0%12%

13%14%

S21

Figure 2: Insertion loss and S-parameters (inserted figure) of silverlines at various torque forces.

3International Journal of Antennas and Propagation

0% 12%

13% 14%

Figure 3: Surface morphology of silver film at various torque forces.

100 𝜇m

150 𝜇mLayer 2

Layer 1

Layer 3

(a)

L2

W2

L3

L4L5

L6

L7

L8 W4

W3

W5

L1 W1

S 1

(b)

L13

S 2

W6

S 3W7

L11

L10

L9

W8

L12

Capacitor

nr

(c)

Figure 4: Structure and photograph of proposed bandpass filter.

4 International Journal of Antennas and Propagation

by adjusting the coupling length between two feeding lines onlayer 1 to improve the skirt selectivity and to enhancestopband while the TZ1 and TZ2 remain fixed. The TZ3was saturated at 28.7GHz, while L7 was 0.95mm. Consid-ering a wide stopband and compact size, we selected L7 as1.15mm. Based on the required responses, the parametersare determined as follows: L1 = 0 7, L2 = 0 3, L3 = 1 18, L4 =0 3, L5 = 1 05, L6 = 0 79, L7 = 1 15, L8 = 0 8, L9 = 0 7, L10 =0 88, L11 = 1, L12 = 0 45, L13 = 1, W1 = 0 07, W2 = 0 3, W3 =0 2, W4 = 0 4, W5 = 0 3, W6 = 0 12, W7 = 0 3, W8 = 0 6,S1 = 0 1, S2 = 0 15, S3 = 0 2 mm , and nr = 4. These proto-types occupied an LCP area of 5.28 × 3.32 × 0.25mm3

(0 32 λg × 0 2 λg × 0 015 λg) including probing pads.Figures 9(a) and 9(b) show the photographs of the band-

pass filter on layer 1 and layer 2, respectively. With 20μmdrop spacing and 30μm droplet size, the resolution ofinkjet-printed film resolution was 50μm. The registrationerror is approximately 50μm because the diameter of align-ment is 100μm. The registration error was approximately

50μm because the diameter of alignment was 100μm.Figure 9(c) compares the simulated and measured S-parameters of the proposed bandpass filter. The measuredmaximal S21 of 2.24 dB at 11.4GHz and the S11 was largerthan the 12dB in the passband. A 3dB passband was cen-tered at 11.5GHz with a fractional bandwidth of 17%. Thethree transmission zeros located at 8.5, 17.5, and 26.5GHzprovided high skirt selectivity and enhanced rejection levels.The stopband rejection is more than 24 dB up to 31GHz.Note that the transmission zero at 20GHz is due to the highresistance of drilled vias. Drilling process causes cracks on thesurface of silver films resulting in high resistance of vias(inserted in Figure 9(c)). The high resistance vias inducedleakages of the signal and produced a redundant transmis-sion zero in the stopband. It was difficult to control theformation of cracks by drilling vias. Moreover, the slightdiscrepancy between the measured and simulated resultsis mainly from the resolution of the inkjet-printed filmsand the registration error of alignment process. The

Frequency (GHz)

0−5−10−15−20−25−30−35−40−45−50−55

4 6 8

w/o inductive S-L couplingS1 = 0.3 mm

S1 = 0.2 mmS1 = 0.1 mm

10 12 14 16 18 20 22 24 26 28

|S21

| (dB

)

Figure 5: Simulated frequency responses without inductivesource-load coupling and with inductive source-load coupling atvarious S1.

4 6

nr = 2 nr = 8nr = 10nr = 4

nr = 6

8 10 12 14 16 18 20

0−5−10−15−20−25−30−35−40−45

Frequency (GHz)

|S21

| (dB

)

Figure 6: Simulated frequency responses for the finger number ofinterdigital capacitor.

S2 (mm)

0.300.280.260.240.220.200.180.160.140.120.100.08

L13 = 0.8 mmL13 = 1.0 mmL13 = 1.1 mm

k

0.10 0.15 0.20

Figure 7: Coupling ratio with the value of S2 and L13.

L7

323028262422201816141210

86

0.7

Zero

pos

ition

s (G

Hz)

TZ1TZ2TZ3

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Figure 8: Three transmission zeros versus L7.

5International Journal of Antennas and Propagation

overall agreement between the simulated results and mea-sured data was observed.

4. Conclusions

Inkjet-printed interdigital bandpass filter using multilayertechnique on LCP substrates was successfully developedbased on the insertion loss and surface morphology. Thethree-dimensional interdigital bandpass filter achieved aminimum insertion loss of 2.2 dB with a 17% bandwidth ata centered frequency of 11.5GHz. Combining inkjet printingand multilayer LCP lamination bonding processes providesinexpensive and compact electronic components for elec-tronic package applications.

Data Availability

The data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was partially supported by the Ministry of Scienceand Technology Taiwan (no. 106-2221-E-182-062) and theChang Gung Memorial Hospital at Linkou (BMRP957).

References

[1] C.-F. Chen, T.-Y. Huang, T.-M. Shen, and R.-B. Wu, “Design ofminiaturized filtering power dividers for system-in-a-package,”IEEE Transactions on Components, Packaging andManufactur-ing Technology, vol. 3, no. 10, pp. 1663–1672, 2013.

[2] J. C. Riou, E. Bailly, C. Bunel, L. Lenoir, and M. Pommier,“3D TSV system in package (SiP) for aerospace applications,”in 2013 European Microelectronics Packaging Conference(EMPC), pp. 1–7, Grenoble, France, September 2013.

[3] J.-H. Lee, G. DeJean, S. Sarkar et al., “Highly integratedmillimeter-wave passive components using 3-D LTCCsystem-on-package (SOP) technology,” IEEE Transactions onMicrowave Theory and Techniques, vol. 53, no. 6, pp. 2220–2229, 2005.

[4] Y. Li, L. Li, X. Dai, C. Zhu, F. Huo, and G. Dong, “Compactshorted stacked-patch antenna integrated with chip-packagebased on LTCC technology,” International Journal of Anten-nas and Propagation, vol. 2014, Article ID 235847, 11 pages,2014.

[5] L. Li, Y. Zhang, J. Wang, W. Zhao, S. Liu, and R. Xu, “Band-width and gain enhancement of patch antenna with stackedparasitic strips based on LTCC technology,” InternationalJournal of Antennas and Propagation, vol. 2014, Article ID461423, 5 pages, 2014.

[6] Z.-C. Hao and J. Hong, “UWB bandpass filter with switchablenotching band using multilayer LCP technology,” in 2010European Microwave Conference (EuMC), pp. 17–20, Paris,France, September 2010.

[7] E. Arabi and A. Shamin, “High Q, miniaturized LCP-basedpassive components, and filter design for SoP applications,”in 2011 41st European Microwave Conference (EuMC),pp. 111–114, Manchester, UK, October 2011.

[8] S. Qian and J. Hong, “Miniature quasi-lumped-elementwideband bandpass filter at 0.5–2-GHz band using multi-layer liquid crystal polymer technology,” IEEE Transactionson Microwave Theory and Techniques, vol. 60, no. 9,pp. 2799–2807, 2012.

[9] S. Qian, Z.-C. Hao, J. Hong, J. P. Parry, and D. P. Hand,“Design and fabrication of a miniature highpass filter usingmultilayer LCP technology,” in 2011 41st EuropeanMicrowaveConference (EuMC), pp. 187–190, Manchester, UK, October2011.

[10] K. S. Yang, S. Pinel, I. K. Kim, and J. Laskar, “Low-lossintegrated-waveguide passive circuits using liquid-crystal poly-mer system-on-package (SOP) technology for millimeter-waveapplications,” IEEE Transactions on Microwave Theory andTechniques, vol. 54, no. 12, pp. 4572–4579, 2006.

[11] D. J. Chung, S. K. Bhattacharya, and J. Papapolymerou,“Low loss multilayer transitions using via technology on LCPfrom DC to 40 GHz,” in 2009 59th Electronic Componentsand Technology Conference, pp. 2026–2029, San Diego, CA,USA, May 2009.

[12] H.-L. Kao, C.-S. Yeh, X. Y. Zhang et al., “Inkjet printed series-fed two-dipole antenna comprising a balun filter on liquidcrystal polymer substrate,” IEEE Transactions on Components,Packaging and Manufacturing Technology, vol. 4, no. 7,pp. 1228–1236, 2014.

[13] H.-L. Kao, C.-L. Cho, and L.-C. Chang, “Inkjet-printed inter-digital coupled line filter on liquid crystal polymer substrate,”IEEE Electron Device Letters, vol. 34, no. 12, pp. 1584–1586,2013.

(a) (b)

Frequency (GHz)

|S11

| & |S

21| (

dB)

0

0

Line: simulatedSymbol: measured

Drilling via

S11

S21

−5−10−15−20−25−30−35−40−45−50−55−60

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

(c)

Figure 9: Photographs of the bandpass filter on (a) layer 1 and (b)layer 2. (c) S-parameters of the interdigital bandpass filter.

6 International Journal of Antennas and Propagation

[14] ULTRALAM 3850, Rogers Corporation, Rogers, CT, USA,2012, http://www.rogerscorp.com/documents/730/index.aspx.

[15] X. Y. Zhang, X. Dai, H.-L. Kao, B.-H. Wei, Z. Y. Cai, andQ. Xue, “Compact LTCC bandpass filter with wide stopbandusing discriminating coupling,” IEEE Transactions on Compo-nents, Packaging and Manufacturing Technology, vol. 4, no. 4,pp. 656–663, 2014.

7International Journal of Antennas and Propagation

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