Today, the integration of an antenna with an electronic circuit consists in most cases of positioning the antenna-component on the surface of a Printed-Circuit-Board (PCB)   , or directly etching the antenna over this PCB. Matching circuits and Radio-Frequency (RF) feed lines are generally still needed for proper operation which is time consuming during the design stages and occupies some space on the PCB . The first solution we propose in this paper is a compact three-dimensional antenna, taking advantage of the multilayer properties of a Low-Temperature-Co-fired-Ceramics (LTCC) substrate. The goal is to achieve a 2.4 GHz Antenna in Package (AiP) and to directly integrate this antenna and the electronic circuit that is the RF components and the Integrated Circuits (ICs) in a single System in Package (SiP). Existing solutions of integrated antennas in SiP are generally working at higher frequencies  . Therefore, no interface between the antenna and these components is anymore required. We also demonstrate the possibility to apply this technique with a FR4 multilayer substrate.
The antenna is designed to be integrated with an existing 2.4 GHz SiP module having the following dimensions: 8 x 8 x 1.4 mm3. The complete module is built using a 0.5 mm thick LTCC substrate having a relative permittivity ?r of 7.8 and a tan? of 0.005. This module incorporates built-in passive components (filters, baluns). Active components and ICs are mounted on the substrate and over-molded to form a single SiP module.
The basic radiating structure is an Inverted-F Antenna (IFA). To take advantage of the seven-layers LTCC substrate, the solution consists in meandering in three dimensions through several layers the main radiating arm of the IFA as a kind of an helical antenna. The overall size of the antenna does not exceed 2 x 8 x 0.5 mm3 (Fig. 1) including a 0.2 mm spacing between the antenna and the edges of the substrate (required to comply with manufacturing rules). The two metallization layers (top and bottom) used for the main arm of the antenna are connected together by vertical vias having a 100 μm diameter. The feed and the ground arm of the antenna are directly connected to the output of the front-end module on the top layer. No additional matching circuit is therefore necessary. This kind of 3D-LTCC concept allows the fabrication of the antenna and the module in one shot during the same industrial process.
Fig.1: Example of an integrated multilayer antenna in a 2.4 GHz LTCC Quad Flat No-leads (QFN) SiP module.
After a complete investigation of different antenna-shapes within the help of the Ansoft HFSS electromagnetic package tool , a first batch of prototypes including ten different selected structures has been manufactured and tested. One of them is presented in Fig. 2.
Fig.2: Picture of a LTCC AiP module sample (without components and over-molding). Some parts of the main meandered arm of the integrated antenna are seen in white color.
Fig.3: AiP module prototype (containing only the antenna) mounted on a FR4 mobile phone test board.
The AiP module containing only the antenna is mounted on a FR4 mobile phone PCB (40 x 80 x 1 mm3) where a 2 x 8 mm2 clearance is made in this PCB just below the antenna. A grounded co-planar feeding line ended by a SMA connector is used to feed the AiP (Fig. 3). Simulation and measurement of one prototype are shown in Fig. 4.
Fig.4: Simulated and measured |S11| of a 2 x 8 x 0.5 mm3 LTCC AiP.
With a relative -6 dB impedance bandwidth of 3.4 % (83 MHz centered at 2.47 GHz) and a minimum measured radiation efficiency of 25 % on this band, simulations and measurements are in a good agreement (less than 1 % shift). The radiation efficiency only takes into account the losses of the antenna and not the mismatch losses. We further transmitted a WiFi-modulated signal with a dedicated source and measured its reception on a spectrum analyzer with the AiP prototype sample being mounted on a mobile phone PCB (fig. 5). A Bluetooth test also shown that a good signal level was efficiently received with our AiP module, then compared to a 6 cm reference monopole case.
Fig.5: Comparison of the received WiFi signal level between our AiP prototype mounted on a mobile phone PCB and a reference monopole.
During the tests, some frequency offsets were observed, due to manufacturing tolerances. Even if the ISM band was correctly covered for applications like Bluetooth and WiFi, our antenna design was not able to cover a relative bandwidth of 6 % that is suggested by customers in order to anticipate for any frequency offset effects that could appear in different conditions of use or different sizes of the ground plane where the AiP is positionned.
III.IMPROVEMENTS OF THE STRUCTURE
Further investigations on the antenna have been carried out in order to operate in a wider bandwidth. New topologies (like dimensions, shapes, number of layers used and ground connections) have been studied and optimized by electromagnetic simulations on a large range of ground planes corresponding to various applications (mobile phone, USB dongle). Each of the twenty new structures designed has a different three-dimensional shape and uses several layers .
A.Simulations and RF Performance
We optimized new shapes offering a better bandwidth and enhanced radiation efficiency. Fig. 6 shows the simulated return loss of a 3.5 x 8 x 0.5 mm3 (14 mm3) antenna enclosed in a 8 x 8 x 0.5 mm3 module. A 9 % relative bandwidth of 220 MHz centered at 2.43 GHz is achieved with a minimum radiated efficiency of 50 % over this bandwidth. It is especially revealed that this new structure is robust versus different variations of the size of the ground plane of a USB or a mobile phone device (the whole ISM band is -6 dB covered in both cases).
Fig.6: Simulated |S11| of a 3.5 x 8 x 0.5 mm3 multilayer antenna for two different size of the ground plane.
A parameter study has been performed in order to integrate these new antennas to a variety of LTCC SiP configurations. Several parameters were investigated like the antenna geometry, the antenna connections and the size of the ground plane. Furthermore, various sizes of the overall module (modifying the available volume for the antenna structure and the components) have been tested. The best performance we obtained in terms of bandwidth and efficiency is reported in Fig. 7 and 8 for each antenna-volume.
Fig.7: Relative bandwidth vs. antenna volume for two different size of the ground plane.
Fig.8: Radiated efficiency vs. antenna volume for two different size of the ground plane.
Those results confirm that the antenna-volume is an important parameter for both the maximization of the impedance bandwidth and the radiation efficiency : a trade-off can be found between the overall size of the module (antenna + components) and the Radio-Frequency performance of the system.
B.Tests of the Prototypes
A second batch of prototypes including more than twenty different shapes has been manufactured. Measurements confirm the performance improvements versus the first batch of prototypes in terms of return loss bandwidth (up to 150 % wider) and radiation efficiency (up to 100 % enhancement).
Fig.9: Simulated and measured |S11| of a 2 x 8 x 0.5 mm3 multilayer antenna placed over a USB dongle PCB.
Fig. 9 shows the good agreement found between simulation and measurement of a 2 x 8 x 0.5 mm3 (8 mm3) antenna-module soldered on a USB dongle PCB. With a -6 dB relative bandwidth of 5 % (125 MHz centered at 2.46 GHz), this structure offers enough margin to cover the whole ISM band and handle detuning. The same AiP module mounted on a mobile phone PCB suffer only from a small frequency shift but achieves a larger bandwidth (6.5 % centered at 2.42 GHz).
With our improved integrated antennas, the goal of covering the whole ISM band for both USB dongle and mobile phone ground planes is fulfilled. Furthermore, with a 50 % minimum radiation efficiency over the impedance bandwidth, these small antennas have the same performance as larger structures used in actual systems (typically 3 x 10 x 1 mm3 for 2.4 GHz ISM band applications ).
Fig.10: Measured radiation patterns (Etotal) of one prototype placed on two different size of ground plane (dongle USB and mobile phone).
Fig. 10 shows the measured radiation patterns in an anechoic chamber of a prototype mounted on two different ground planes. Both vertical and horizontal polarizations arerepresented. With a maximum gain of 0.51 dBi and a quasi-omnidirectional pattern, these results are in good agreement with the simulations. We can observe a slight deformation of the radiation pattern and a better maximum gain when the size of the ground plane increases.
Nevertheless, the omnidirectional behavior of the AiP module is a key factor for this kind of applications (WLAN, Bluetooth, …) where the communicating device has to properly work in any position and should receive any kind of polarization in a multi-path environment.
IV.FR4 SUBSTRATES DESIGNS
Similar antenna topologies have been optimized for multilayered laminate FR4 SiPs. Because of a lower dielectric permittivity (between 3.5 and 5), the antenna structure is larger if we want to achieve the same level of performance. A prototype of a full module including the antenna and the RF components  is presented in Fig. 11 and 12 and achieves the same functionalities as a reference design which is four times larger.
Fig.11: Picture of a 6 x 8 mm2 antenna-module in a 12 x 8 mm2 laminate over-molded QFN SiP.
Fig.12: Picture of an AiP module on its USB dongle test board.
Satisfactory performance (10 % relative bandwidth at -6 dB centered at 2.45 GHz and at least 50 % radiation efficiency over the bandwidth and omnidirectional radiation patterns) validates the suitability of this kind of three-dimensional multilayered integrated antennas in low-cost modules.
In this paper, a technique to integrate miniature antennas (about 2 x 8 x 0.5 mm3) in complete Systems in Package was presented. It consists in creating 3D shapes by taking advantage of multilayer substrates like LTCC or laminate FR4.
This Antenna in Package (AiP) concept is particularly suitable for 2.4 GHz applications because of the good performance we could achieve (at least 6 % relative impedance bandwidth and 50 % radiation efficiency and quasi-omnidirectional radiation patterns).
This method allows an industrial production of full systems in packages, reduces costs and time to market for Bluetooth and WiFi wireless applications.
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