Antenna in LTCC Technologies - IEEE Xplore

Antenna in LTCC Technologies: A Review and the Current State of the Art Ubaid Ullah, Mohd Fadzil Ain, Nor Muzlifah Mahyuddin, Mohamadariff Othman, Zainal Arifin Ahmad, Mohd Zaid Abdullah, and Arjuna Marzuki School of Electrical and Electronic Engineering, Universiti Sains Malaysia, Penang 14300, Malaysia E-mail: [email protected]

Abstract

This paper presents a chronological review of the research carried out on antennas in low-temperature cofired ceramics (LTCC) technology over the last ten years or so. Major breakthroughs in LTCC technologies and its shortcomings are highlighted. The current state of the art of LTCC-technology-based antennas is then evaluated. All realizable features of the LTCC-based antennas, which are compact and of light weight and offer high-speed functionality for portable electronic devices, are illustrated. Different techniques used by researchers for broadbanding, multiband designs, and fabrication of LTCC-based antennas are also presented. This paper ends with some recommendations and concluding remarks. Keywords: Antenna in package (AiP); antennas; integrated circuit (IC) packaging; IC technology; low-temperature cofired ceramics (LTCC) technology; microwave technology; millimeter-wave (mmWave) ICs; technology analysis

1. Introduction

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ow-temperature cofired ceramics (LTCC) have become an attractive technology for miniaturization of portable electronic devices over the last few decades. The modern-day portable electronic devices such as cellular phones, personal digital assistants, and personal computers used for data, voice, and video communications demand compactness of the circuit, light weight, and high-speed functionality. At millimeter-wavelength frequencies, high losses due to free space and atmospheric absorption of the radio waves limit communication to over a kilometer range [1]. Printed circuit board (PCB) and hybrid thick-film technology faces many problems in the extremely high frequency band, which severely affect the system performance and result in poor efficiency [2]. For certain military applications, few multilayer substrates such as polytetrafluoroethylene (PTFE) and liquid crystal polymers (LCPs) were taken into consideration. Although PTFE microwave substrates were able to meet the requirements of certain applications in wireless designs, such as low loss tangent (0.0018 at 10 GHz), good resistance to chemical processing, almost no absorption of water, and resistance to high temperature; however, for commercial applications, there

Digital Object Identifier 10.1109/MAP.2015.2414668 Date of publication: 17 April 2015

IEEE Antennas and Propagation Magazine, Vol. 57, No. 2, April 2015

were certain limitations associated with PTFE. The disadvantages of PTFE include high cost, the inert nature of PTFE, softness of the substrate, and a high thermal expansion coefficient [3]. In the same way, the LCP substrate showed some promise flexibly in the form of extreme nonreactive nature, inertness, extreme resistivity to fire, mechanical strength, and stiffness. Along with all these attractive features of LCP-based substrates, some shortcomings were also linked with the LCP, which limits its use in bulk production. Some major disadvantages of LCP in electrical applications are near-hermetic, poor thermal conductor, strong bases, and surface roughness. A detail comparative study on LTCC and PCB/PTFE can be found in [4], and a very good article on LCP was presented in [5]. On the other hand, LTCC technology circumvents many disadvantages and offers some reimbursement particularly at high frequencies. LTCC is a multilayer technology that has been used for packaging integrated circuits (ICs) and applied to actuators, sensors, and integrated microsystems with relatively low cost and high productivity [6–16]. LTCC-based circuits have an extensive range of applications in areas such as telecommunications, automotive aeronautics, radio frequency (RF) modules (Mobile phone, Bluetooth, Home RF, IEEE 802.11), microwave modules, optoelectronic modules, and medical, military, and sensors packaging [17–19]. Many researchers have investigated the feasibility of antenna in package (AiP) and system on package (SOP)

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using LTCC technology, and some major accomplishments are reported in the literature [20–29]. The purpose of this paper is to review the research carried out over the years on different aspects of LTCC-based antennas, their performance capabilities, design flexibility, and mass production feasibility. First, a brief historical review of LTCC technology is carried out, and major developments in LTCC technology in applications other than antenna in the last few decades are highlighted. This paper then looks at the recent progress made in the field of antennas using LTCC technology and current state of the art to give readers a sense of advantages of LTCC technology over the traditionally used techniques. It is highly anticipated that this paper will provide sufficient understanding to those who are not familiar with this technology of reducing a circuit footprint by forming a multilayer structure and 3-D integration of a passive device microstructure on a single chip. This paper will serve as a fundamental reference to all those who aim to do research in the field of LTCC-technology-based antennas, providing a detailed review of current developments made in the area. This paper will also address all achievable frequency ranges of already published work in LTCC-based antenna designs.

enhanced wiring density and short signal path electrical vias are used by applying smart design concepts. This type of packaging is known as multichip ICs (MCICs). It is further divided into three subgroups, namely, thick-film, LTCC, and high-temperature cofired ceramics (HTCC). Among these, LTCC turned out to be more promising and have many advantages over HTCC and thick-film technology. LTCC technology was invented to fulfill the requirements of high-performance, high-speed, and high-density MCM for realization of functional, reliable, and low-cost electronic devices. To give readers some idea of the versatility of LTCC technology, the processing characteristics of LTCC in comparison with HTCC and thick-film technology are summarized in Table 1. The history of LTCC technology actually dates back to 1980s when Hughes and DuPont first developed it for Table 1. Characteristics of LTCC, HTCC, and hybrid thick-film technology.

2. LTCC Technology: A Historical Review Microelectronics is a multi-billion industry in the world today, which offers consumers a wide range of indispensable features of new electronic products. All these revolutionary products are the result of a successful combination of an extensive range of modern technologies with the significant advancement in material science. Nevertheless, microelectronics packaging is the key technology to combine the aforementioned technologies and to determine the reliability, performance, and cost of the final product [30–32]. Techniques for the packaging of microsystems on a multichip module (MCM) have been started decades ago. MCM is a type of packaging in which the multiple-chip ICs and components are confined into a single layer. There are different technologies used for production of the MCM structure, which are classified on the basis of the substrate used, i.e., MCM-L (advanced PCB technology), MCM-D (thick-film deposition), and MCM-C (ceramic multilayer substrate). Among these substrates, the ceramic substrate showed the desired thermal, electrical, and mechanical characteristics compared with other MCM technologies. This hybrid single-layer packaging technique seems to be an easy solution, but it is not sufficient for modern-day packaging systems, as it requires high functional density, wiring, smaller size, and robustness [4, 33, 34]. The modern-day microelectronics packaging requires simultaneous packaging of active and passive components into a multilayer ceramic module [35]. All the components are integrated (buried) within and/or onto the surface layers, which facilitates the fabrication process of the monolithic structure with increased functionality, performance, and reliability. For efficient communication between different components,

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military applications [36, 37]. To date, DuPont has developed several materials for high-frequency applications up to 100 GHz and beyond, such as DuPont 9K7 and 951 green tape cofired material. Later on, the growth of the LTCC technology got accelerated when LTCC tape manufacturers co-operated with packaging companies in the late 1980s, and afterward, it was commercialized in automotive industries and avionics [17, 38]. LTCC-based MCIC made a rapid growth in the consumer electronic industries such as mobile phones (0.9–2 GHz), wireless local networks, Bluetooth (2.4 GHz), global positioning systems (GPS, 1.6 GHz), broadband access connection systems (5.8–40 GHz), etc. These are some of the examples to LTCC applications in the RF field [39–44]. Based on the demands of a particular application, many functional discrete electronic components can be wire bonded or soldered to the surface of an LTCC module, as illustrated in Figure 1. In this paper, the focus will be mostly on LTCC technology in the field of antennas. First, a general idea of the LTCC multilayer circuit is presented, so that the readers can get familiar with the basic processing steps; afterward, the developments in the area of antennas are highlighted. The LTCC is a multilayer ceramic technology in which green/soft tapes are autonomously processed. The processing of tapes includes printing conductors, embedding passive components, and forming vias, cavities, and the desired circuit pattern using conductive, dielectric and/or resistive pastes. The combination of different green sheet layers has to be laminated together and fired simultaneously. This technique not only saves time and money but also reduces circuit footprint. Another great advantage LTCC technology acquires is that, before firing, every single green sheet layer can be inspected and replaced in case of any inaccuracy or damaged circuit pattern. This helps the designer to fabricate the circuit with utmost accuracy and may prevent the need for manufacturing a whole new circuit [45]. Because of the fact that LTCC have a low firing temperature of about 900 C, it is thus possible to use good

Figure 1. Schematic view of an LTCC module illustrating multiple embedded components [39].


Figure 2. LTCC multilayer circuit processing steps [48]. conductive materials, such as silver and gold instead of molybdenum and tungsten (which have to be used in conjunction with the HTCC). Manufacturing an LTCC multilayer circuit is a multistep process [46, 47], as summarized in Figure 2 [48]. Usually, green sheets of low-temperature ceramics are shipped in rolled form. Before going to the blanking process, the tape has to be unrolled and cut using a razor or a laser or by punching it carefully. The size of the tape has to be kept larger than the actual size to account for preconditioning (if required). In the next step, the tape has to go through the blanking process in which a blanking die is used to make marks for orientation and holes for tooling. After blanking, the green sheet via holes has to be punched or drilled using low-power laser beams. Subsequently, via holes should be filled with conductive pastes using conventional thick-film screen printing or an extrusion via filler. Conductors that can be cofired with green sheets of LTCC have to be screen printed on the green sheets using conventional thick-film screen printing methods. For screen printing a standard emulsion type, thick-film screen is used. It merits mentioning here that there is a predefined standard for the gap between two conducting lines, width of the conducting line, via diameter, via pitch, isolation gap, metallization thickness, etc. LTCC developers, such as DuPont, VTT, and DT Corporation, have set guidelines [49–53] for the fabrication of LTCC circuits in which the value for the minimum gap between lines, width of the conducting strip, and all other design parameter values can be found. In the next step, the screen printed sheets are laminated with two

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possible methods. One is the uniaxial lamination method, and the other is the isostatic lamination method. Lamination is the last step before the circuit is exposed to fire. Cofiring of the circuit is performed at about 900 C, and some printing of additional circuit elements can be done on the top layer after cofiring is done. The last step is to validate the design by testing and characterizing the multilayered circuit.

3. LTCC Technology Applied to Antenna Researchers have started paying attention to antennas in LTCC technology, since the start of the new millennium due to the fast-growing demand for multifunctionality, high performance, and subminiaturization of the modern-day mobile terminal devices. In order to restrain increases in circuit size due to multifunctionality such as Bluetooth, GPS, and wireless local area networks (LANs), it is desirable to build various high-frequency functions and passive components into the substrate itself rather than mounting them on its surface [52]. In 1996, Sturzebecher presented a 20-GHz phase array module in LTCC, which can be considered the initial breakthrough of the LTCC technology as it helped in drawing researchers’ attention toward this versatile technology [53]. Afterward, in 2002, Zhang published a research paper on antenna in LTCC technology using a ceramic ball grid array (CBGA) integration technique [54]. Since then, many other researchers started investigating antenna in LTCC technology for modern-day single-chip transceivers and several other applications. Different approaches were adopted to improve the performance of the antenna in terms of bandwidth, gain, polarization, radiation efficiency, and miniaturization of the circuit. Zhang employed the CBGA technique for integrating antennas on a single-chip IC for wireless transceivers, as shown in Figure 3. It is comprised of three laminated LTCC layers with two buried layers and one top metallization layer in the structure. The top layer of the structure is utilized for the printed antenna, whereas the bottom layer acts as an antenna ground plane. Geometry and dimension of the patch antenna used by the author is shown in Figure 4. The center operating frequency of the antenna was reported to be 5.52 GHz with 4.65% impedance bandwidth and 65% radiation efficiency. After the successful

Figure 3. CBGA package antenna [54].

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Figure 4. Geometry of the first microstrip patch antenna integrated on an LTCC single-chip IC [54]. implementation of this novel idea of integration antenna to LTCC technology for miniaturization of the complex ICs, researchers started investigating the potential of this technology for modern wireless communication applications. To date, more or less 150 journals and magazines along with 400 or more conference papers are published in the period of less than 12 years. Antennas have been designed at different frequencies using the LTCC technology approach ranging from 1 to 270 GHz. In the following section, characteristics and applications of the frequencies utilized for LTCC antennas and the published work in each frequency band will be addressed.

4. LTCC Antenna in IEEE 802.15.3c Band Every wireless system designer wishes for replacing all the cables for high-speed data communication in an indoor environment. This was not possible due to insufficient dedicated unlicensed frequency spectrum. Therefore, the U.S. Federal Communication Commission allocated an unprecedented 7-GHz spectrum for license-free operation between 57 and 64 GHz [55]. The allocation of this spectrum made multigigabit RF links possible. In late 2009, IEEE-SA went on to standardize the 57–64-GHz band as IEEE 802.15.3c [56]. In comparison with other indoor wireless systems, 60-GHz systems have a very small wavelength; therefore, it has the potential of high directivity. 60 GHz is viewed as a complementary technology, working in tandem with other versions of Wi-Fi, and helps in maximizing the abilities of wireless devices. As today’s Wi-Fi has adequately allowed ethernet cables to be replaced, 60-GHz Wi-Fi thus additionally allows devices to replace all of the cables used for data and display with just a single additional radio. The 60-GHz band has the capability to stream high-definition videos


Table 2. Circularly polarized LTCC-based antennas for 60-GHz applications.

efficiently, and it still supports all other interfaces that consumer usually expects [57]. These distinctive characteristics of IEEE 802.15.3c opened a way for new integration option of single-chip transceivers using SOP and AiP methods. LTCC technology is effectively applied in this 60-GHz band, and many researchers published a vast majority of their work. The already-published work is categorized on the basis of the polarization of the antenna. A list of several circularly polarized antennas designed for 60-GHz applications using LTCC technology is provided in Table 2. Some major characteristics such as center frequency, type of antenna used, number of antenna elements, impedance bandwidth, axial ratio, gain, overall dimensions of the circuit, and references are listed in the table. Miniaturization of modern-day devices is one of the preliminary requirements; for that reason, the size of the complete circuit is mentioned in the table (when specified by the authors), so that it can be compared with other antennas reported in LTCC and other mainstream technologies. Polarization of the antenna plays a vital role in a millimeter-wave (mmWave) frequency band. Due to oxygen and water


absorption in the atmosphere, mmWave communication tends to be in short range (from 100 m to 5 km); hence, efficient transfer of electromagnetic energy is crucial. In circularly polarized antennas, the electric field of the wave only changes direction in a rotary manner, and the strength of the signal remains unchanged. For effective short-range communication, antennas with circular polarization and high gain can be more helpful. To date, the most prominent AiP in terms of gain with circular polarization is reported in [64]. Designing an integrated antenna for 60-GHz applications with circular polarization and high gain is a challenging task. In [64], a 16-element microstrip patch antenna array was integrated with a low-noise amplifier (LNA) on the same substrate using LTCC technology. The antenna was energized using a sequentially rotated feeding scheme [63] to achieve circular polarization. The explored 3-D view of the reported antenna is illustrated in Figure 5. As shown in the figure, the antenna is integrated with the LNA in five LTCC Ferro A6M ("s ¼ 5:9, tan ¼ 0:002) layers. The thickness of each LTCC

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("s ¼ 5:9, tan ¼ 0:002) LTCC substrate. The explored sketch of the AiP is given in Figure 7. There are four ceramic and four metallic layers in the circuit. The metallic layer on top of the substrate serves as a ground for the SOP

Figure 5. High-gain LTCC antenna integrated with LNA [64]. layer is shown in the figure, and the overall size of the package is 13 20 1:4 mm3 . There are five metallic layers in the circuit. Metallic layers are referred as M layers. Layers M1 and M2 provide metallization for antenna array and for antenna ground with aperture and grounded coplanar waveguides (GCPWs), respectively. Layers M3 and M4 are used to provide metallization for feeding lines and for the GCPW ground plane, respectively. Finally, M5, which is the bottom most layer of the circuit and provides metallization for the system ground plane. The gain of the antenna is reported to be 35 dBi with impedance bandwidth more than 8 GHz. Several research articles on antennas for the 60-GHz band in LTCC technology, with circular polarization, are reviewed in Table 2, and few selected ones are illustrated in Figure 6. From the values provided in the table, a comparison in terms of size of the package, impedance bandwidth, polarization, and axial ratio for each design can be easily made. More work on LTCC-based antennas in the 60-GHz band with linear polarization is provided in Table 3. Few selected antennas with a single element and array elements are addressed here. A single-element linearly polarized slot antenna with the highest gain is reported in [68]. The author of the paper reported to have followed the technique of thin cavity-down CBGA [86, 87] package in LTCC technology for implementation of the design. A substrate-modulated slot antenna is integrated in Ferro A6

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Figure 6. (a) 3-D view of a helical antenna array in an LTCC substrate for 60-GHz applications reported in [58]. (b) U-slot patch antenna array for 60-GHz applications using an LTCC substrate reported in [59]. (c) Grid array antenna for 60-GHz applications by NTU Singapore taken from [65].


Table 3. Linearly polarized LTCC-based antenna for 60-GHz applications.

and antenna guard rings. The second and third buried metallic layers provide metallization for antenna and signal trace lines in the package. The bottom layer of the circuit serves as a ground plane for antenna and solders ball pads. A perfect electromagnetic environment is created within the immediate vicinity of the slot antenna, so as to make the antenna less sensitive to the carried radio die, permittivity of substrate, and buried metallic structures. The slot antenna is designed so that the slot acts as a main radiator, a guard ring is used as director, a ground plane as reflector and fences of vias that shorts the outer metal edge of


the slot radiator to the reflector, so as to confine the electromagnetic fields. To have control over surface waves due to high dielectric constant, air holes are modulated underneath a slot antenna. With this approach, the impedance bandwidth achieved is reported to be 6 GHz from 59 to 65 GHz for a single-element antenna with a peak gain value of 11 dBi. It is believed that this method can be implemented in the array form to further increase the performance of the antenna in terms of gain and bandwidth.Table 3 lists many single-element and array element designs with linear polarization for the 60-GHz band, and few of them are illustrated in Figure 8.

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modulation technique used in the original version was extended. With this alteration in modulation techniques, a maximum data rate of 11 Mb/s in the 2.4-GHz band was realized. 802.11g was standardized in 2003 with the third modulation standard for WLANs. It uses the same 2.4-GHz band as 802.11b with a maximum data rate of 54 Mb/s. 802.11n was another amendment that improved the previous 802.11 standard by introducing multiple-input–multipleoutput antennas. 802.11n operates at 2.4 GHz and at 5 GHz with a maximum net data rate from 54 to 600 Mb/s [89].

Figure 7. Substrate-material-modulated slot AiP for 60-GHz applications reported in [68].

5. LTCC Antenna in IEEE 802.11a/b/g/n Band In the previous section, antennas designed in LTCC for the 60-GHz band are addressed. Here, we will review the work that has been done on antenna in LTCC technology for 2.4 and 5 GHz. Before going to the main topic of this paper, a brief overview of the IEEE 802.11a/b/g/n band is provided. The aim is to give some idea to the readers about the basic features of IEEE802.11, the subsequent amendments that were made, and its role in wireless communication. Over a decade ago, IEEE 802.11 wireless communication protocols have been introduced; since then, a rapid progress in all fronts has been witnessed. IEEE 802.11 is a standard Internet protocol for wireless LANs (WLANs), which features a set of media access control (MAC) and physical layer (PHY). The original version of the standard was released by the IEEE LAN/MAN standard committee (IEEE802.11) in 1997 [88]. Since the introduction of the base version, successive amendments were made and subsequently revoked after each new standard is released. IEEE 802.11 was set to specify two net bit rates of 1 or 2 Mb/s with a forward error correction code. IEEE 802.11a was the first amendment that used the same data link layer protocol and frame format as standardized in the original version. The operating frequency of this band was set to 5 GHz, and the maximum net data rate is 54 Mb/s with an error correction code. With this amendment, the maximum throughput of 20 Mb/s is realized. 802.11b was another amendment to the legacy 802.11. This was also based on the same data media access method as defined in the original standard; only the

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The IEEE’s defined standard for different frequency bands plays a significant role in wireless communication networks. Modern-day wireless communication is entirely based on these standards. LTCC technology has been extensively applied in different frequency bands for single-chip RF transceivers and numerous other applications. The concept of integrating antennas in LTCC technology is introduced by Zhang, as stated in the previous section. The operating frequency of the antenna integrated for the first time in LTCC technology was reported to be 5.52 GHz [54]. Several antennas designed in LTCC technology for IEEE 802.11a/b/g/n bands are listed in Table 4. Some of the listed designs will be briefly discussed in the following section to highlight their performance characteristics. Few of the listed antennas have shown multiband characteristics; some are designed for wideband operation using different band widening techniques. Moreover, different types of antennas are integrated in the LTCC substrate for AiPs and single-chip transceivers. Zhang et al. in [90] reported a microstrip line antenna integrated in LTCC technology for modern single-chip transceivers. The scalability and trimability of an antenna for AiPs operating at different frequencies is a figure of merit as scalability shortens the design cycles, and trimability helps improve production yield. Few microstrip and inverted F antennas are reported for AiP with these characteristics, but due to the large metallic surface area of the aforementioned antennas, they are potentially vulnerable to cause mechanical problems in the prototyping of the AiP on a large scale. In [90], Zhang et al. presented a theoretical design of the microstrip line antenna with good mechanical and electrical characteristics for AiP applications. Figure 9 illustrates two different views on the reported antenna. The microstrip line antenna is designed using a silver conductor on the top surface of the LTCC substrate having a ground plane for antenna on the bottom surface. The permittivity of the substrate used is "s ¼ 7:8 with tan ¼ 0:002. The values of permittivity and loss tangent mentioned in this paper were given by VTT Finland. The datasheet value of permittivity for the substrate is 7.8, which is same as stated in the paper, but the loss tangent differs from 0.006 at 3 GHz to 0.014 at 40 GHz for DuPont 951. The microstrip line antenna printed on the top surface of the substrate is excited with a metallic via through a circular aperture in the ground plane. The operating frequency of the antenna can be fine tuned by changing the length and width of each arm of the microstrip line antenna. With the single-current-path configuration of


Figure 8. (a) 3-D view and side view of the L-probe patch array antenna for 60-GHz applications [71]. (b) Integration of the linearly polarized Yagi antenna to LTCC package for 60-GHz applications [76]. (c) Simulated model of a 24-element linearly polarized antenna in LTCC for 60-GHz applications. (i) Patch antenna 3-D. (ii) Top view. (iii) Simulated AiP model [77]. (d) A 16-element array antenna cavity-backed slot antenna for 60-GHz applications [73]. (e) Example of a linearly polarized microstrip grid array antenna for 60-GHz applications. (i) Explored view. (ii) Microstrip grid. (iii) Feeding network [72].


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Table 4. LTCC-based antenna for IEEE 802.11a/b/g/n band.

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Figure 9. Top and bottom views of the microstrip line antenna reported in [90]. the antenna shown in Figure 9, the impedance bandwidth of 50 MHz is achieved from 5.755 to 5.805 GHz. The gain of the antenna with this configuration is 4.7 dBi. To broaden the impedance bandwidth of the antenna, the technique of merging two close resonances is used. To generate another resonance close to the original resonance of the antenna, an extra current path is created in the antenna, as shown in Figure 10. By adjusting the different parameters of the microstrip line, a second resonance is created and merged with the original resonance. Meshing of the ground plane of the antenna is done primarily from the mechanical viewpoint. It can also help in improving the impedance bandwidth of the antenna. By creating the second current patch in the structure, the impedance of the antenna has been improved to 80 MHz from 5.795 GHz to 5.875 GHz with 0.1-dBi improvement in gain. The peak gain reported is 4.8 dBi for the two-current-path structure. The microstrip line antenna is integrated to AiP using the CBGA technique. Three LTCC layers are used to integrate the antenna with two buried metallic layers for signal traces and ground plane. The top metallic layer is used for the microstrip line antenna in the package. After integration of the line antenna with a single-chip transceiver, a bandwidth enhancement technique was developed for the AiP.

Figure 11. Configuration of the antenna reported in [92]. (a) Lower patch. (b) Upper patch. (c) Top view. (d) Side view. In [92], a multiband circularly polarized hexagonal microstrip antenna is reported. The antenna is made to operate in a multiband manner by using a stacked-patch configuration, which also helped in keeping the antenna more compact. The configuration of the antenna is illustrated in Figure 11. The polarization of the antenna is controlled by adjusting the size of the slit inserted in the patch and by truncated corners of the patch. The antenna is printed on an LTCC Ferro A6 substrate having dielectric constant 5.9. The lower patch acts as the ground plane for the patch on the top layer. The lower patch of the antenna resonates at 3.5 GHz with 120-MHz impedance bandwidth from 3.44 to 3.56 GHz. The upper patch of the antenna resonates at 5.2 GHz having 250-MHz impedance bandwidth, which is higher than the lower frequency band. The antenna was made to propagate circularly polarized field in both upper and lower bands by adjusting the length of the inserted slit and truncating corners of the patch. A 24-MHz axial ratio (3 dB) bandwidth of the antenna is reported for the lower band with a 3.5-GHz center frequency, whereas for the upper band, the reported axial ratio bandwidth is 53 MHz with respect to a 5.18-GHz center frequency. Several other antennas are summarized in Table 4, and few selected ones are illustrated in Figure 12.

6. Selected LTCC Antennas in Different Frequency Bands Figure 10. Top view of the two-current-path microstrip line antenna with meshed ground plane [90].


Here, we would like to address some more antennas designed in LTCC technology for different frequency bands. In Table 5, some selected antennas in LTCC technology

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Figure 12. (a) Expanded view of the stacked-patch antenna reported in [95]. (b) Slotted-patch chip antenna in LTCC technology [96]. (c) Ultrawideband slot antenna in LTCC technology with dimensions in millimeters. (i) Top view. (ii) Bottom view [100]. (d) Ultrawideband modified patch antenna in LTCC technology reported in [104]. (e) Two-layered monopole antenna in LTCC technology reported in [104]. (i) Top view. (ii) Side view.

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Table 5. Selected LTCC-based antenna.

operating at different frequencies are summarized. Few of the listed antennas will be briefly discussed, to highlight their prominent features such as configuration of the circuit, impedance bandwidth, axial ratio, gain, and overall size. To date, 270 GHz is the highest known frequency reported for which an LTCC integrated antenna is designed, fabricated, and characterized [109]. In that paper, a Fresnel zone plate


(FZP) lens antenna integrated in an LTCC substrate to operate at 270 GHz with high gain is proposed. The gain of the antenna is improved by using a back cavity, which was formed by via holes sidewall and the ground plane. A compact feed mechanism is introduced in the paper to achieve a broadband impedance bandwidth. The measured peak gain of the antenna is reported to be 20.8 dBi at 270 GHz. With

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this configuration, the cavity-backed FZP lens antenna achieved a broadband 3-dB gain bandwidth of 9.1 GHz (266.2–275.3 GHz). The structure of the proposed FZP lens antenna is shown in Figure 13. The antenna is integrated in LTCC Ferro A6M tapes having dielectric constant "s ¼ 5:9 and tan ¼ 0:002. The cross-sectional view of the antenna is given in Figure 13(a). The antenna consisted of 20 ceramic layers and three metallic layers. A metallic sidewall is created on the edge of all 20 layers by using metallic vias. Figure 13(b) shows the top view of the FZP and sidewalls on the edge. The vias for sidewall is formed from a large diameter to ease the fabrication of the circuit. The sidewall on the circumference of the circle suppresses radiation from the edge of the antenna, and therefore, energy propagation in the unwanted direction is controlled. A new feeding transition is introduced and successfully implemented to achieve broadband impedance bandwidth. Characterization of the fabricated antenna was done to validate the design concept of the FZP lens antenna at 270 GHz for high-gain applications.

Figure 13. FZP lens antenna integrated in LTCC. (a) Side view. (b) Top view [109].

Figure 14. Side view of a dielectric loaded SIW slot antenna array in LTCC [110].

In [110], a 140-GHz dielectric loaded substrate integrated waveguide (SIW) slot antenna array is reported. The antenna element is integrated in an LTCC substrate with feeding network and dielectric loadings. The side view of the antenna array is shown in Figure 14. The antenna is integrated in a total of 11 LTCC Ferro A6M substrate layers and six metal layers. The array is comprised of antenna elements, a feeding network with an eight-way power divider, E-plane couplers, fourelement subarrays, and dielectric loading. To simplify the structure of the 8 8 element array, a high-order TE20 mode is used, as with higher order modes, the internal sidewalls are removed, and the size of the waveguide is enlarged. A pair of radiating slots is loaded to the structure by a double-width open-ended via-fence structure, and high-order TE20 mode is excited in it. A 3-D schematic view and top view of the proposed slot antenna array is shown in Figure 15. The maximum boresight gain value of 21.3 dBi is achieved at 140.6 GHz with impedance bandwidth from 129. 2 to 146 GHz. Utilizing higher order mode in antenna design at the upper mmWave band can simplify the structure, can enhance electrical robustness, and reduces the fabrication cost of the circuit.

Figure 15. (a) 3-D schematic view and (b) top view of the slot antenna array reported in [110].

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Further in the section, a 79-GHz planar antenna array using a ceramic-filled cavity resonator in LTCC technology reported in [111] is briefly discussed. Figure 16(a) illustrates the overall structure and geometry of the proposed antenna.

The antenna is fabricated in nine TDK LTCC layers having dielectric constant "s ¼ 7:28 and tan ¼ 0:006. The radiating element of the antenna is designed by a rectangular ceramic-filled cavity resonator formed by a via-fence and four ceramic layers. These radiating elements are energized by a transverse slot in the upper metallization of the laminated waveguide. Elements were designed in such a way that they radiate energy in the same manner as an open-ended integrated waveguide with the dominant propagating mode being TE10 . The radiating elements with the feed structure are shown in Figure 16b. Part (i) of the structure shows cavity resonator antenna elements fed by a slot, part (ii) illustrates the inclined slot radiator element, and part (iii) shows the equivalent circuit model of the feeding configuration. The structure and function of a laminated waveguide is discussed in detail in [112]. An eight-element array was designed with which was fed in center with the element impedances combining at the center. The antenna elements in the array were designed with identical shape, and mutual coupling between them is optimized. With this configuration, the antenna yielded an impedance bandwidth of 5 GHz from 76 to 81 GHz. It has been reported that the antenna showed a stable radiation pattern over the entire impedance bandwidth with a peak gain value of 13.2 dBi achieved at 79 GHz. The maximum directivity of the antenna reported is 15.1 dBi with 65% overall efficiency of the antenna. To date, the lowest frequency antenna designed in LTCC technology is reported in [127]. Lin et al. presented a technique to design a multiband antenna for mobile communication applications. The antenna was designed to operate in a multiple-frequency GSM band of 900/1800 MHz, which is the lowest known frequency for which LTCC technology is implemented. A printed helical monopole antenna is designed in two LTCC layers for dual-band operation. A 3-D schematic view and cross-sectional view of the antenna is shown in Figure 17. The cross-sectional view of the antenna shows that the antenna is printed and placed in the middle layers. Two-layer monopoles are created using a metallic strip on two different layers, and via holes are used to connect the layers. LTCC tapes with dielectric constant and loss tangent value "s ¼ 7:8 and tan ¼ 0:0047, respectively, are used to integrate the monopole helix antenna. It is worth mentioning here that the tangent loss values given in the paper are different from the values given in the datasheet for DuPont 951.

Figure 16. (a) Geometry of the 79-GHz planar antenna array reported in [111]. (b) (i) Cavity resonator antenna fed by a slot, (ii) standard inclined slot radiator element, and (iii) equivalent circuit of a slot-fed antenna [111].


The first monopole of the antenna operates in the higher frequency (1800 MHz) band. The printed helical antenna is tuned to lower frequency (900 MHz) band. To enhance the impedance bandwidth of the antenna, three different lengths of the monopole were printed and integrated with the helix. The antenna chip in LTCC was mounted on an FR4 substrate without a ground plane underneath the antenna circuit for measurement purposes. With this arrangement, the antenna yielded just enough bandwidth to fulfill the requirement for mobile communication applications. The measured directivity of the antenna is reported to be 2 dBi in both lower and higher frequency bands. The gain of the antenna is −4 dBi for lower frequency band and −6 dBi for higher frequency band.

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Figure 17. Design of a dual-band 900/1800 antenna. (a) Monopole-Helix in LTCC chip. (b) Chip antenna mounted on FR4 substrate. (c) Cross-sectional view of the reported antenna [127].

7. LTCC Application in Electronically Steerable Antenna Here, an imperative feature of LTCC technology in electronically steerable antennas is presented. Steerable antennas are the antennas that can change or steer its beam to the desired direction at a particular angle. This beamforming technique can be used for transmission and reception. The rapid growth of broadband services has created an attractive market for several applications such as mobile satellite terminals as well as maritime, aeronautical, and land applications. These terminals provide on-the-move multimedia satellite broadcasting services for several bands of frequency (L-band, Ku-band, and K/Ka-band), and they all require cost-effective, agile, and steerable antenna front-ends [129, 130]. The requirement for more complex circuitry and functionality of the RF module for the broadband services stipulated for the development of new integration techniques. LTCC technology was effectively applied in integrated beam steerable antennas by different researchers [131–133]. For instance, a highly integrated RF module in which the antenna elements, an RF circuitry calibration network, and a cooling system were integrated to a multilayer LTCC chip is considered [134]. Figure 18 shows the single-element schematic top view of the square microstrip antenna with an integrated 90 -hybrid. A complex high-level integration

Figure 18. Schematic top view of the square microstrip antenna with an integrated 90°-hybrid [134].

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technique is used to integrate the entire communication system to single-chip LTTC layers. Figure 19 shows the top and bottom views of LTCC tile that consists of the antenna elements, hybrid ring feeds for each element, and an integrated calibration network. The 3-D drawing shows the antenna element in red. Green and blue are the distribution network and the cooling system, respectively, integrated in the chip. The novel design proposed in the paper for a transmission front-end antenna is fabricated in one 15-layer LTCC tile. The antenna front-end consists of 8 8 antenna elements with separate hybrid ring feed for each element. To give insight into the LTCC structure, both the top and bottom views are partly shown. A comprehensive discussion over advanced high-level integration of integrated antennas with high polarization purity, which provides polarization multiplexing, is given in the paper. Furthermore, some novel techniques for beam-steering features of the antenna were also addressed.

8. Summary and Conclusion This paper is written with the endeavor to provide the readers an idea of the research that has been carried out in LTCC technology in the field of antennas. The historical review of the technology is summarized, and the current state of the art is also stated in this paper. Almost all of the major breakthroughs in LTCC technology in the field of antennas are highlighted and briefly addressed. From the numerous publications addressed in this paper, it is proven that LTCCbased antennas can be designed to best suit modern-day miniaturized communications devices operating almost anywhere in the frequency spectrum. Plenty of work has been reported in the IEEE 802.15.3c license-free band compared with the rest of the frequency spectrum. LTCC technology can be further investigated for successful implementation in different bands of frequency. After reviewing the literature on LTCCbased planar antennas, we concluded that almost 70% of the antennas integrated in LTCC were microstrip-patch-type antennas; therefore, other kinds of printed antenna can be investigated to study their potential as future LTCC antennas. It was also observed that almost all researchers designed their antenna on the same LTCC tape with the same dielectric constant and tangent loss; further study can be done on LTCC


Figure 19. Different views of the advanced electronically steerable antenna. (a) Top view. (b) Bottom view [134]. material to introduce more LTCC tapes with low-, medium-, and high-dielectric constants to best suit the requirements of modern-day fast-growing demands. The performance of the reported antenna can be improved by using different permittivity materials. In several cases, fabrication tolerance severely degrades the performance of the actual antenna due to high sensitivity of LTCC circuits, which poses a serious challenge for the advancement of LTCC technology. Hence, to resolve these issues, new, easy, and reliable methods of fabrication of the antennas should be explored. Addressing the aforementioned problems will make the LTCC-based antenna a better choice for future wireless communication applications and an even more viable option compared with other traditionally used integration technologies.

9. Acknowledgment Authors would like to acknowledge Global Fellowship Scheme of Universiti Sains Malaysia and research grant number 1001/PELECT/854004 RUT.

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Ubaid Ullah received the B.S. degree in electrical engineering from CECOS University of IT and Emerging Sciences, Peshawar, Pakistan in 2010 and the M.S. degree in electronic engineering from Universiti Sains Malaysia, Penang, Malaysia, in 2012, where he is currently working toward the Ph.D. degree. He has published several articles in ISIindexed journals and some well-reputed international conferences. His current research interests include dielectric resonator antennas (DRAs), wideband DRAs, microwave circuits, low-temperature-cofired-ceramics-based antenna in package, applied electromagnetics, and small antennas.

Dr. Mahyuddin is currently involved in the IEEE Communications Society. Subsequently, she is also a member of the Institution of Engineering and Technology and a Professional Member of the Association for Computing Machinery. She is also registered under the Board of Engineers Malaysia.

Mohamadariff Othman received the B.S. degree in electronic engineering from Universiti Multimedia, Malaysia in 2006 and the M.S. degree in radio frequency and microwave from Universiti Sains Malaysia, Penang, Malaysia, in 2009, where he is currently working toward the Ph.D. degree in dielectric resonator antennas (DRAs). His current research interests include solid dielectric and thick-film fabrication, dielectric characterization, wideband DRAs, and patch antennas.

Zainal Arifn Ahmad received the B.S. degree in materials engineering from Universiti Sains Malaysia, Penang, Malaysia; the M.S. degree from the University of Manchester Institute of Science and Technology, Manchester, U.K.; and the Ph.D. degree from the University of Sheffield, Sheffield, U.K. He is currently a Senior Professor with the School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. His current research interests include ZTA ceramic for cutting insert, low-temperature-cofired-ceramics-based circuits, metal–ceramic joining, crystal glaze ceramic, TCP bioceramic, and dielectric ceramic for antennas.

Mohd Fadzil Ain received the B.S. degree in electronic engineering from Universiti Teknologi Malaysia, Johor Bahru, Malaysia, in 1997; the M.S. degree in radio frequency and microwave from Universiti Sains Malaysia (USM), Penang, Malaysia, in 1999; and the Ph.D. degree in radio frequency and microwave from the University of Birmingham, Birmingham, U.K., in 2003. In 2003, he joined USM. He is actively involved in technical consultancy with several companies in repairing microwave equipment. His current research interests include wireless circuit design, low-temperature-cofired-ceramics-based antenna in package, rain propagation, microwave links, and dielectric resonator antennas.

Mohd Zaid Abdullah received the B.App.Sc. degree in electronics from Universiti Sains Malaysia (USM), Penang, Malaysia, in 1986 and the M.Sc. degree in instrument design and application and the Ph.D. degree in electrical impedance tomography from the University of Manchester Institute of Science and Technology, Manchester, U.K., in 1989 and 1993, respectively. He was a Test Engineer with Hitachi Semiconductor. In 1993, he joined USM. From 2001 to 2006, he was an Associate Professor and the Deputy Dean with the School of Electrical and Electronic Engineering, USM. In 2006, he was promoted to Full Professor and is currently the Dean. He has published numerous research articles in international journals and conference proceedings. His research interests include microwave tomography, digital image processing, computer vision, and ultrawideband sensing. Dr. Abdullah was awarded The Senior Moulton Medal for the best article published by the Institute of Chemical Engineering in 2002.

Nor Muzlifah Mahyuddin received the B. Eng. degree in electrical telecommunications from Universiti Teknologi Malaysia, Johor Bahru, Malaysia, in 2005; the M.Sc. degree in electronics system design from Universiti Sains Malaysia, Penang, Malaysia, in 2006; and the Ph.D. degree in microelectronics system design from Newcastle University, Newcastle upon Tyne, U.K., in 2011. She was an intern with Agilent Technologies, Penang. Since March 2012, she has been a Lecturer with Universiti Sains Malaysia. She has produced several papers on lowswing signaling schemes. Her current research interests include radio frequency and microwave engineering, reliability, and signal integrity. The topic of interests include the modeling design of split-ring resonators in high-performance application, the impact of variability on the design of microstrip-based circuits, and the power integrity in high-performance circuits.

Arjuna Marzuki received the B.Eng. degree (Hons) in electronics engineering (Com) from the University of Sheffield, Sheffield, U.K., in 1997; the M.Sc. degree from Universiti Sains Malaysia, Penang, Malaysia, in 2004; and the Ph.D. degree in microelectronics engineering from Universiti Malaysia Perlis, Arau, Malaysia, in 2010. From 1999 to 2006, he was an Integrated Circuit Design Engineer with Hewlett-Packard/Agilent/Avago and IC Microsystem in the United States and Malaysia. He is currently a Lecturer with the School of Electrical and Electronic Engineering, Universiti Sains Malaysia. Dr. Marzuki is a registered Professional Engineer with the Society of Professional Engineers UK. He attained Chartered Engineer (Engineering Council, UK) status through the Institution of Engineering and Technology. He was a recipient of 30 IETE J C Bose Memorial Award for the year 2010.

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