Compact Polyimide-Based Antennas for Flexible Displays - IEEE Xplore

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 2, FEBRUARY 2012

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Compact Polyimide-Based Antennas for Flexible Displays Haider R. Khaleel, Student Member, IEEE, Hussain M. Al-Rizzo, and Daniel G. Rucker

Abstract—In this paper, we present two compact ultra thin and flexible printed monopole antennas intended for integration with flexible displays, such as flexible organic light-emission displays (FOLEDs) and active matrix electro-phoretic displays (AM-EPDs). The proposed antennas are designed to provide Wireless local area network (WLAN) and Bluetooth connectivity for flexible displays. The first design is a dual band antenna operating at 2.45 GHz and 5.2 GHz while the second is a single band antenna operating at 2.4 GHz. Both antennas were printed on a Kapton polyimide-based substrate with dimensions (35 mm 25 mm) and (26.5 mm 25 mm) for the dual and single band respectively. Antenna properties, such as gain, far-field radiation patterns, scattering parameter 11 are provided. Moreover, the effect of folding/bending was performed experimentally on both designs to study its influence on the antennas performance. The proposed compact, thin and flexible designs along with antennas characteristics are perfectly suitable for integration into flexible displays for WLAN and Bluetooth connectivity. Index Terms—Active-matrix electro-phoretic display (AMEPD), bluetooth, flexible displays, flexible-organic light-emission display (FOLED), polyimide, printed monopole, WLAN.

I. INTRODUCTION

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HE past decade has witnessed escalating research activities focused on the development and optimization of flexible displays in response to the electronics market trends which reports an increased interest in portable, lightweight, thin, and flexible devices. Flexible displays are deemed as one of the most potential technologies to satisfy such technological demands as they can be applied to a wide spectrum of applications such as military, personal communication, and entertainment [1], [2]. Several appealing and technically progressive flexible display prototypes have been proposed [3]–[6]. Indeed, some of these designs have already been commercialized. The most commercially available flexible displays are the active matrix electrophoretic displays (AM-EPDs) which are expected to replace conventional publication media, such as books and papers, by utilizing flexible polymer substrates [7]. Besides, flexible organic light emission displays (FOLEDs) are becoming more popular due to their ability to offer high brightness, outstanding Manuscript received April 27, 2011; revised July 29, 2011; accepted August 01, 2011. Date of current version January 27, 2012. This work was supported in part by the National Science (NSF EPSCoR) under EPS-0701890 and under CNS-0619069. The authors are with the Department of Systems Engineering, George W. Donaghey College of Engineering and Information Technology, University of Arkansas, Little Rock, AR 72204 USA (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures are available online at http:// ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2011.2164235

viewing angle performance, low power consumption and fast response time. Another advantage which made FOLEDs extremely popular in today’s displays market is the ability to print such displays using printing techniques such as ink jet and flexography which provide low cost production and scalability [8]. On the other hand, today’s information oriented society demands electronic devices/gadgets to be equipped with wireless connectivity. Wireless Local Area Network (WLAN) is recognized as the most reliable and cost effective solution for wireless high speed data connectivity. The ongoing developments in WLAN technologies require the integration of IEEE 802.11 WLAN standards of the 2.4 GHz (2400–2484 MHz), and 5.2 GHz (5150–5350 MHz) bands into a single antenna unit [9]–[12]. For shorter distance radio communications, Bluetooth is the most widely used standard in today’s electronic devices. It utilizes the Industrial, Scientific, Medical (ISM) band operating in the 2.4–2.84 GHz frequency region. Obviously, the integration of a wireless connectivity with flexible displays triggers the need for ultra light/thin/flexible antennas. At the same time, these antennas should be robust, cost effective, and highly efficient with desirable radiation characteristics. Many design approaches of flexible and conformal antennas were proposed including: Electro-textile [13], paper-based [14], fluidic [15], and synthesized flexible substrates [16]. Needless to say, conventional microstrip antennas are not a practical solution for flexible displays due to their narrow bandwidth which is a function of the substrate’s thickness. In [14], a flexible inverted-F single band antenna printed on a 46 mm 30 mm paper-based substrate was proposed for integration into flexible displays for WLAN applications. However, paper based substrates are found to be not robust enough and introduce discontinuities when used in applications that require high levels of bending and rolling. Moreover, they have a high loss factor is around 0.07 at 2.45 GHz) which com(loss tangent promises the antenna’s efficiency [21]. In this paper, we present two compact ultra thin/flexible printed monopole antennas for WLAN and Bluetooth applications. The proposed antennas are based on a Kapton polyimide substrate which is known for its flexibility, robustness, and thermal endurance. The first design is a dual-band antenna operating at 2.45 and 5.2 GHz, while the second is a single band antenna which operates at 2.4 GHz. Furthermore, the proposed antennas are compact which is suitable for the targeted application and provide less susceptibility to performance degradation due to bending and folding effects. In Section II, we present the description for our proposed designs, principle of operation and fabrication process. In Section III, we discuss the radiation characteristics and performance of the antennas. In Section IV, the performance of

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Fig. 2. Proposed flexible printed single and dual band monopole antennas.

Fig. 1. Polyimide-based printed monopole antenna to be integrated with flexible displays.

the antennas under bending effect is discussed. Comparative study based on the electrical properties and robustness of the proposed antennas and other types of flexible antennas are introduced in Section V. Finally, conclusions are given in Section VI. II. ANTENNA DESIGN AND FABRICATION In order to comply with the WLAN and Bluetooth technologies requirements, low profile, compact and high performance antennas with excellent radiation characteristics are required. For both technologies, the printed monopole antennas have received much interest over other antenna types due to their wide impedance bandwidth, low profile, ease of fabrication, and omni directional radiation pattern which is highly preferred in WLAN and Bluetooth systems. To comply with flexible technologies, integrated components need to be highly flexible and robust; they also have to exhibit high tolerance levels in terms of bending repeatability and thermal endurance. Polyimide Kapton was chosen as antennas substrate since it exhibits a good balance of physical, chemical, and electrical properties with a low loss factor over a wide frequency range. Furthermore, Kapton polyimide offers a very low profile (50.8 m) yet very robust with a tensile strength of 165 MPa at 73 F, a dielectric strength of 3500–7000 V/mil, and a temperature rating of 65 C to 150 C [20]. The radiating elements were printed on a 50.8 m flexible Kapton polyimide substrate with a dielectric constant of 3.4 and a loss tangent of 0.002. A conductive ink based on sliver nano particles is deposited over the substrate by a Dimatix DMP 2831 printer followed by a thermal annealing at 100 C for 9 hours. It is worth mentioning that three layers of ink were deposited on the substrate to achieve a robust and continuous radiating element. A partial flexible copper ground plane was placed on the opposite side of the Kapton substrate (the dark lower area in Fig. 2). Both antennas were fed by SMA connecters for measurement purposes. A. Dual Band Printed Monopole The proposed dual band antenna is a modified version of a design reported in [19]. Our modified version achieved a reduction of more than 50% in size while maintaining the antenna’s

Fig. 3. Geometry and dimensions of the proposed dual band printed monopole antenna (shaded area represents the ground plane on the opposite side).

TABLE I DUAL BAND ANTENNA DIMENSIONS IN MILLIMETER

efficiency and almost the same impedance bandwidth. As can be seen in Fig. 3, the proposed design comprises two T-shaped branches monopole, both operated as quarter wavelength structures. They are fed by a 2 mm wide 50 microstrip line. The two branches and the microstrip line are printed on the same side of a 35 mm 25 mm Kapton polyimide substrate. On the other side of the substrate, a 12 mm 25 mm flexible copper ground plane is placed below the microstrip line. The widths and lengths of the branches are the main controlling parameters that determine the resonance frequency of the antenna. The dominant mode (2.45 GHz) is controlled by the upper monopole branch of the antenna while the lower branch controls the second band (5.2 GHz). The parameters were optimized to achieve a dual resonance at 2.45 and 5.2 GHz which is suitable for the intended WLAN application. It should also be noted that the dimensions of the ground plane can affect the resonant frequency and bandwidth of the two modes [19]. Dimensions of the dual band antenna are depicted in Table I.

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Fig. 4. Geometry and dimensions of the proposed single band printed monopole (the shaded area represents the ground plane on the opposite side).

TABLE II SINGLE BAND ANTENNA DIMENSIONS IN MILLIMETERS

Fig. 5. Measured and simulated return loss for the proposed dual band printed monopole at 2.5 and 5.2 GHz.

A. Dual Band Printed Monopole

B. Single Band Printed Monopole The proposed antenna is suitable for Bluetooth connectivity utilizing the ISM 2.4 GHz band. As shown in Fig. 4, the antenna design consists of a U-shaped monopole. This type of winding reduces the structure size without significant degradation of the efficiency or disturbance to the radiation pattern. The separation between the arms is chosen as 6 mm which achieves the least return loss. It should be noted that smaller separations lead to increased capacitive coupling between the arms which results in an increased impedance mismatch. The U-shaped monopole is fed by a 1.5 mm wide 50 microstrip line. Both the monopole and the microstrip line are printed on the same side of a 26.5 mm 25 mm Kapton polyimide substrate. On the other side of the substrate, a 12.5 mm 25 mm flexible copper ground plane is adhered below the microstrip line. The electrical length of the U-shaped monopole in addition to the ground plane size controls the resonance frequency of the antenna. The antenna’s dimensions are depicted in Table II. III. SIMULATIONS AND MEASUREMENTS Design and analysis of the proposed printed monopole antennas have been carried out using the full wave simulation software CST Microwave Studio which is based on the Finite Integration Technique (FIT) [18]. The antennas S-parameters were obtained using an Agilent PNA-X series N5242A Vector Network Analyzer (VNA) with (10 MHz–26.5 GHz) frequency range. The farfield radiation patterns of the principal planes (E and H) were measured in the University of Arkansas at Little Rock’s anechoic chamber. The Antenna Under Test (AUT) was placed on an ETS Lindgren 2090 positioner and aligned to a horizontally polarized horn antenna.

The simulated and measured return loss versus frequency for the dual band antenna is presented in Fig. 5. The simulated return loss for the antenna is 23 dB at 2.5 GHz, with a 10 dB bandwidth of 290 MHz. The measured return loss is 18 dB at 2.48 GHz with a 10 dB bandwidth of 305 MHz. For the second band, the simulated return loss is 19 dB at 5.2 GHz with a 10 dB bandwidth of 280 MHz while the measured return loss is 17 at 5.35 GHz with a 480 MHz 10 dB bandwidth. Inevitable fabrication discrepancies led to a slight shift in the resonance frequency as observed in Fig. 5 when compared to results obtained from numerical simulations. In particular, it is observed that the dominant mode is slightly shifted to the lower side (about 1.7%), while the second mode is shifted in the opposite direction by 2.8%. This observation inspired further simulations to investigate this trend in the shift of the resonance frequencies. mainly conOur investigation revealed that the parameter trols the resonance frequency of the first mode of the dual band antenna, while the second mode is controlled mainly by and partially by . For example, decreasing by 0.5 mm increases the resonance frequency by 27 MHz. (about 1%). On by 0.5 mm increases the second the other hand, decreasing band resonance frequency by 85 MHz (1.6%), while decreasing by the same amount increases the resonance frequency by 22 MHz only (0.04%). Thus, a small amount of conductive ink residue during prototyping might have independently altered the two resonance frequencies, i.e. a shift to a higher or lower frequency. Obviously, the second (higher) mode is more affected since the effective wavelength is smaller at 5.2 GHz. controls the The length of the partial ground plane impedance matching. A minor disposition of the ground plane due to the adhesion process would affect the return loss and consequently the 10 dB impedance bandwidth. As seen in Fig. 5, the measured bandwidths of the prototype antennas constructed from silver nano particle are obviously larger than those obtained from CST microwave studio due to the assumption of perfectly conducting patch and ground plane.

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Fig. 7. Measured and simulated return loss for the proposed single band printed monopole at 2.45 GHz.

Fig. 6. Measured E-plane and H-plane radiation patterns for: (a) dual band printed monopole at 2.45 GHz (b) dual band printed monopole at 5.2 GHz.

Fig. 8. Measured E-plane and H-plane far-field radiation patterns for the single band printed monopole at 2.4 GHz.

B. Single Band Printed Monopole The electrical conductivity of the silver nano particle based ink has been measured using the traditional four-probe method and was found to be 8.9 10 S/m. When this value is used in the second run of simulations, the bandwidths fairly match with the measured ones. This property decreases the quality factor and consequently increases the impedance bandwidth. However, both simulated and measured bandwidths sufficiently cover the required frequency range for the intended WLAN application. It should be noted that the difference in bandwidth between measured and simulated results is mainly attributed to the above-mentioned reason rather than to the observed shift in resonance frequencies. E-plane and H-plane far-field radiation patterns are shown in Fig. 6. It can be seen that the total radiation power is nearly omni-directional at both bands. The antenna achieved a gain of 0.6 dBi at 2.4 GHz and 0.4 dBi at 5.2 GHz which are typical values for such antennas size and resonant frequency.

The simulated and measured return loss versus frequency for the proposed single band antenna is presented in Fig. 7. The simulated return loss for the antenna is 30 dB at 2.4 GHz, with a 10 dB bandwidth of 140 MHz. The measured return loss is 29 dB at 2.36 GHz with a 10 dB bandwidth of 270 MHz. The shift in resonance frequency can be attributed to the same fabrication discrepancies discussed in the dual band antenna section. However, the amount of shift is only 1.6% which lies within the acceptable error range encountered during simulation, fabrication and measurement. On the other hand, the increase in the operational impedance bandwidth is also attributed to the conductivity finiteness of the conductive ink used in the printing process. The SMA connector size has a very slight effect on the impedance bandwidth discrepancy as well. E-plane and H-plane far-field radiation patterns in the polar form are shown in Fig. 8. It can be inferred from the graph that the total radiation power is nearly omni-directional. The antenna achieved a gain of 0.4 dBi at 2.4 GHz.

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TABLE III COMPARATIVE STUDY OF DIFFERENT TYPES OF FLEXIBLE ANTENNAS

IV. COMPARATIVE STUDY The antennas proposed in this paper were compared to different types of flexible antennas reported in [13]–[16]. Given the applications envisioned in this paper, the comparative study is focused on compactness (size and thickness), electrical properties and robustness. Robustness encompasses the major mechanical properties related to flexible/conformal electronic devices such as tensile strength, flexural strength, deformability, and thermal stability. Fabrication complexity criterion is also considered in this comparative study. Table III depicts these characteristics of the antennas under study. As shown in Table III, the proposed antennas offer relatively smaller sizes, highly robust and flexible designs. Furthermore, these antennas are printable and provide low cost and roll to roll production. V. PERFORMANCE UNDER BENDING EFFECT Since the antennas are expected to be bent, rolled, and folded when integrated within a flexible display, three qualitative tests need to be conducted for operative validation, as noted below. • Resonance frequency and return loss will need to be evaluated under bending conditions since they are prone to shift/decrease due to impedance mismatch and a change in the effective electrical length of the radiating elements. • Repeated testing of the antennas under bending, folding, and twisting are required to monitor the deposited conductive ink for any deformations, discontinuities or cracks. • The substrate will need to be tested for rolling, folding and twisting to make sure no wrinkles or permanent folds are introduced which might compromise the antennas performance. As stated before, polyimide Kapton substrate was chosen in this application mainly due to its physical robustness and high flexibility, furthermore, the fabricated prototypes demonstrated an excellent performance as they were tested repeatedly against bending, twisting, and rolling effects. To test the performance of the antennas under different bending extents, AUTs were rolled on foam cylinders with difmm and the second is mm) to ferent radii (the first is emulate different extents of bending while they were connected to the network analyzer. The following results were observed: • dual band antenna was more sensitive to the effect of bending than the single band antenna. The shift was

Fig. 9. Flexibility test setup (AUT is conformed over a cylindrical foam with different radii to reflect different extents of folding/bending).

10–25 MHz and 10–30 MHz to higher frequencies for the first and second bands, respectively; • single band antenna experienced a shift of 5–20 MHz to higher frequencies. However, the impedance bandwidths of the proposed antennas are relatively large, which could overcome the minor shift caused by the bending effect. Fig. 9 shows the flexibility test setup for the dual band antenna rolled on a foam cylinder with an 8 mm radius. VI. CONCLUSION Two ultra flexible/thin printed monopole antennas were presented in this paper. The first design is a dual band antenna operates at 2.45 and 5.2 GHz which is suitable for WLAN applications, while the second is a single band antenna which operates at 2.4 GHz for Bluetooth connectivity. Both antennas were printed on a 50.8 m Kapton polyimide substrate. Flexible, compact, thin and robust designs along with good antenna radiation characteristics suggest that the proposed antennas are suitable to be integrated into flexible displays for WLAN and Bluetooth connectivity. Moreover, the proposed antennas expressed a very low susceptibility to performance degradation and resonance frequency shift due to bending/folding effects.

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Haider R. Khaleel (S’09) received the B.Sc. degree in control and systems engineering from the University of Technology Baghdad, Iraq, in 2003, the M.Sc. degree in electrical and computer engineering from New York Institute of Technology (highest hons), New York, in 2006, and is currently working towards the Ph.D. degree in systems engineering at the University of Arkansas at Little Rock. He has been a Graduate Research and Teaching Assistant in the Applied Science Department at the University of Arkansas at Little Rock since August 2008. His main research interests include flexible and wearable antennas; Metamaterial based antennas, implantable antennas, electromagnetic bandgap (EBG) structures, artificial magnetic conductors (AMC), and antennas for multiple input multiple output (MIMO) systems.

Hussain M. Al-Rizzo received his B.Sc. in Electronics and Communications (1979) (High Honors), Postgraduate Diploma in Electronics and Communications (1981) (High Honors) and M.Sc. in Microwave Communication Systems (1983) from the University of Mosul, Mosul, Iraq. From May 1983 to October 1987 he was working with the Electromagnetic Wave Propagation Department, Space and Astronomy Research Center, Scientific Research Council Baghdad, Iraq. On December, 1987, he joined the Radiating Systems Research Laboratory, Electrical and Computer Engineering Department, University of New Brunswick, Fredericton, NB, Canada where he obtained his Ph.D. (1992) in Computational Electromagnetics, Wireless Communications, and the Global Positioning System. For his various academic achievements he won the nomination by the University of New Brunswick as the best doctoral graduate in science and engineering. Since 2000, he joined the Systems Engineering Department, University Arkansas at Little Rock where he is currently a Professor of Systems Engineering. He has published over 40 peer-reviewed journal papers, 70 conference presentations, and several patents. His research areas include implantable antennas and wireless systems, smart antennas, WLAN deployment and load balancing, electromagnetic wave scattering by complex objects, design, modeling and testing of high-power microwave applicators, design and analysis of microstrip antennas for mobile radio systems, precipitation effects on terrestrial and satellite frequency re-use communication systems, and field operation of NAVSTAR GPS receivers.

Daniel G. Rucker received the B.S. degree in systems engineering from the University of Arkansas at Little Rock, in 2007. Currently, he is pursuing a Ph.D. degree in applied science at University of Arkansas at Little Rock (UALR). He was accepted by the National Science Foundation for a Research Experience for Undergraduates program in the summer of 2006 at the Arecibo Observatory in Puerto Rico, where he worked on digital electronics and radar control. Following this work, he shifted his undergraduate research to microstrip antennas for biomedical devices at the University of Arkansas at Little Rock (UALR). His current research areas are microstrip antennas, low power wireless sensor systems, and systems engineering design. In the area of microstrip antennas, his work is focused on implantable and wearable microstrip antennas for biomedical applications.