5. DIFFERENTIAL ANTENNA IN EWLB PACKAGE
5.2 Folded dipole AiP with eWLB package
5.2.1 eWLB structure
The term eWLB stands for embedded wafer level ball grid array. It is a fan-out wafer-level package technology which was first introduced in 2006 [71,72]. Figure5.7shows the comparison between standard wafer-level packages (WLP) and fan-out wafer level packages (fan-out WLP).
In standard WLP, the ball grid arrays (BGAs) are directly under the MMICs. The I/Os number is limited by the MMICs’ size and pitch size. In eWLB, the package size is largely increased by introducing the redistribution layer (RDL). RDL is the addition of metal and dielectric layer onto the surface of wafer. RDL reroutes I/Os from MMICs to a large area of BGAs. Therefore, the interconnection gap between MMICs and PCBs is largely reduced. Secondly, RDL supports the connection from MMICs to BGAs instead of the bonding wire and laminate substrate. It benefits the implementation in mmW applications.
Figure 5.7: Comparison of standard WLP (top) and fan-out WLP (bottom). c 2006 IEEE [71].
Figure5.8shows the process flow for the eWLB manufacturing. The starting point is a front-end processed wafer. When the wafer is singulated, the chips typically stick on an adhesive carrier foil with a dicing frame (see Figure5.8(a)). The singulated chips are picked and placed onto a carrier with large spacing. Standard “Pick and Place” equipment is used to place the dice onto a metal carrier. In the next step, the dice on the metal carrier is molded (see Figure 5.8(b)). The encapsulation process is the core process in the embedded die technology. On the obtained “Reconfigured Wafer”, the redistribution layer is deposited (Figure 5.8(c)), and afterwards, the balls for the second level are interconnected (Figure 5.8(d)). More details are referred to [71,72].
The RDL has a much higher resolution compared with transmission lines on PCBs. For instance, the 100 Ohm coupled transmission line on PCB is about W/S/W = 240 µm/200 µm/240 µm, while in RDL, it is W/S/W = 35 µm/20 µm/35 µm. It brings a new concept for building the antenna in this layer. The following sections of this chapter give a couple of design examples for the differential feed AiP in eWLB packaging. The impact of MMICs and package size on antenna performance is also analyzed. Different design proposals for ultrawide 97
5. DIFFERENTIAL ANTENNA IN EWLB PACKAGE
Figure 5.8: Schematic process flow for a fan-out wafer level package. c2006 IEEE [71].
bandwidth, high-gain antenna, etc., are discussed in the following sections.
5.2.2 Folded dipole AiP design
The cross section of eWLB AiP is shown in Figure5.9, together with MMICs. The antenna was built in the RDL layer. The ground on PCB beneath the antenna area behaves as a reflector for the AiP. The maximum gain of radiation pattern is about the broadside direction of the antenna. The distance between RDL and ground plane is defined by the height of BGAs, which has a typical value of 180µm.
In RDL, the coupled transmission line has very compact size with W/S/W = 35 µm/20 µm/35 µm. From the discussion in Chapter 3, it is required to have a minimal distance for the electric separation of a single patch DMPA. The spacing between the line – 20 µm – is
Figure 5.9: Cross section of AiP with MMICs in eWLB package. The antenna is built in RDL layer. c2012 IEEE [164].
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much smaller than the required separation. It is necessary to develop other types of antenna for eWLB AiP.
The first AiP in eWLB packaging designed in this work is a folded dipole antenna. The reason for dipole-like antenna is that dipole antenna is suitable for tight coupled transmission line. It matches the coupled transmission line in RDL. The classic dipole is a half wavelength open-end transmission line. The typical antenna impedance is 73 Ohm without ground plane below the antenna. In eWLB package, the ground-to-antenna distance is 180 µm, which is about 0.05λ0 at 79 GHz. Taking this effect into consideration, the dipole antenna impedance is around 20 to 30 Ohm [165]. It is far from the 100 Ohm port impedance of MMICs. It requires a bulky matching network for impedance matching. Therefore, a folded dipole antenna is proposed for a compact solution. The principle of the folded dipole is shown in Figure5.10.
Since the impedance of the folded dipole antenna is four times that of a dipole antenna (see Equation5.1[166]), it is feasible to build AiP without any matching network.
Zf olded−dipole= 4×Zdipole (5.1)
The initial size of the folded dipole is calculated from half wavelength in the mold material.
Figure 5.11 shows the top view of the antenna as well as the simulation results. The final optimized length of the folded dipole is 1.24 mm. The simulation results show that the folded dipole antenna AiP has 6 GHz bandwidth for 10 dB return loss. It covers the full bandwidth of automotive radar applications (76 GHz to 81 GHz).
5.2.3 Manufacturing and measurement
The designed AiP – folded dipole (FD) was manufactured and tested. Figure 5.12 shows the manufactured AiP as well as the test board.
The folded dipole AiP was manufactured together with MMICs. Figure 5.12(a) shows the bottom view of the manufactured folded dipole type AiP. Antenna is integrated with an
×18 frequency multiplier in a 6 mm by 6 mm eWLB packaging [167]. The MMIC includes a
Figure 5.10: Folded dipole and equivalent regular dipole.
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frequency multiplier [167], an amplifier and a balun. It generates the RF signal (76.5 GHz) by multiplying the frequency of the input signal of LOin (4.25 GHz) by a factor of 18. The MMIC has a differential topology and RF output interface. Figure 5.12(b)shows the test board with mounted AiP. LOin and LOout signals are aligned on the right and left side of the board with SMA connectors. The power supply signals is aligned on the bottom side of the boards.
The performance of AiP was measured with an active setup, which means antenna was measured together with MMIC. Figure 5.13shows the measurement configuration and photo of the setup in the antenna chamber. The antenna under test is set as Tx antenna. The input LO signal of the DUT was generated by an Agilent E8257D signal source. The RF signal was transmitted by the AiP and received by an E-band standard horn antenna, which was placed at a distance d = 1.8 m in front of the transmitter. The received signal was measured by a spectrum analyzer (Rohde & Schwarz FSQ40) combined with a harmonic mixer (Rohde &
Schwarz FSZ90). The equivalent isotropic radiated power (EIRP) of the antenna is calculated
Figure 5.11: Simulated S11 of folded dipole AiP in eWLB packaging, LF D = 1.24 mm, line width = 0.035 mm, line spacing = 0.02 mm. c2012 IEEE [164].
(a) Photo of manufactured folded dipole AiP (bot-tom view).
(b) Photo of test board for folded dipole AiP (top view)
Figure 5.12: Photo of folded dipole AiP (bottom view) (a) and test board (top view) (b). c 2012 IEEE [164].
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from the measurement as
PEIRP(dB) =Pr−Gr+LF S (5.2)
wherePr is the received power in the spectrum analyzer and Gr is the gain of the receive antenna. In this measurement setup, Gr is 20 dBi. LF S is the propagation loss which can be calculated from free space path loss equation:
LF S(dB) = 20log10(4πd
λ ) (5.3)
The gain of AiP (Gt) can be further calculated as follows:
Gt=PEIRP−Pout (5.4)
where Pout is the measured output power of the MMICs.
The calculated gain of folded dipole AiP is about 7 dBi over a wide frequency range.
(a) AiP measurement configuration (b) Photo of measurement setup of folded dipole AiP in chamber.
Figure 5.13: Radiation pattern measurement of AiP configuration (a) and photo of chamber (b). c2012 IEEE [164].
The measured radiation patterns at 76.5 GHz are shown in Figure5.14, together with the simulated results for comparison. From the measurement of the radiation pattern, a notch for beam pattern in H-plane has been found. This is mainly because of the package size and the MMICs’ back metallization. More detailed discussions are shown in Section5.5.1.
The first design of folded dipole AiP proves the concept for the eWLB AiP. From the next section, different designs for bandwidth and radiation pattern improvement will be shown step by step.
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(a) (b)
Figure 5.14: Measurement and simulated radiation pattern of folded dipole AiP at 76.5 GHz:
(a) E-plane and (b) H-plane. c2012 IEEE [164].