r θ
s w
ground plane
Zi,bc
Energy reflected
Energy escapes
boundary sphere l=r
Zo,bc
ZL Zi,bc Zo,bc ZL
θbc l
(a)
r θ
s w
ground plane
Zi,bc
Energy reflected
Energy escapes
boundary sphere l=r
Zo,bc
ZL Zi,bc Zo,bc ZL
l (b)
Figure 11.3: (a) A finite biconical antenna with an input impedanceZi,bc, characteristic impedance Zo,bc and an equivalent load impedance ZL at the imaginary boundary sphere.
(b) Equivalent transmission line representation of a finite biconical antenna.
11.5 WR-03 Band CPW Bow-Tie Antenna in eWLB Package
Although antennas in eWLB package with good directivity have been demonstrated, wide bandwidth performance either in terms of input return loss or gain has still not been achieved.
A narrow bandwidth antenna poses a fundamental limitation on the overall radar system bandwidth and consequently its range resolution. As has been demonstrated analytically and by measurements throughout this work, the current SiGe HBT technologies with innovative design techniques at the circuit design level can support extensive bandwidths both at mm-wave and THz. Due to the geometry of the eWLB package, the designed antenna can be regarded as a planar wire antenna with a ground reflector and can not be strictly classified as a microstrip antenna. The natural direction of radiation, as shown in Fig. 11.1 is towards the package mold.
A part of the radiated power is directed towards the bottom where it gets reflected by the PCB ground metal. Published prior-art eWLB antennas have relied on dipoles, folded dipoles, differential patch, and rhombic structures [236]. Rhombic antennas, have specially been proven very effective for constructing highly directional arrays [239]. All of these antennas are narrow band resonant structures depending on λg/2 wavelength. Consequently, there is a need to design eWLB antennas which are wideband in nature and also could be used for single-ended as well as differential topologies.
11.5.1 Brief Analysis and Design of a Bow-Tie Antenna
There are extensive number of planar antenna structures and shapes which can provide broad-band radiation performance. However, the primary challenge arise in getting this performance in an eWLB package with a limited size, defined mold thickness, and quite stringent metal design rules. All these issues exacerbate at mm-wave and THz frequencies. One of the eWLB
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11. A WR-03 Radar Sensor with a Wideband Bow-Tie Antenna in eWLB Package
r
s w ground plane
θbc
want
lfeed
θbc= 45◦ r= 480µm want= 20µm s= 20µm w= 40µm lfeed= 300µm
Figure 11.4:Geometry of the proposed WR-03 band slot CPW bow-tie antenna. The design parameters of the antenna are annotated.
antenna structures investigated during this work was a coplanar waveguide (CPW) bow-tie2 antenna. This type of antenna was first introduced by Solimanet al.[240], which shows several advantages such as a very large bandwidth, good control of its input impedance, and ease of fabrication. The traditional bow-tie antenna has a dipole like radiation pattern. Slot antennas in general provide a wider bandwidth, better impedance match, and a possibility of obtaining bidirectional and unidirectional radiation patterns as compared to their microstrip counter parts. A slot antenna is mostly fed by a CPW line preferred because of its lower dispersion and more flexibility in impedance. A bow-tie antenna is based on a biconical antenna which in its infinite form behaves as a guide for spherical waves analogous to a uniform transmission line which guides a plane wave. This infinite antenna can be specified solely by the flare angles and has a characteristic impedance independent of the wavelength, as given by [241]
Zo,bc= ηo
π ln cotθbc
4 (11.3)
where, ηo is the free-space impedance andθbc is the flare angle from one edge to the other.
Equation (11.3) is known an Schelkunoff’s relations for the characteristic impedance of a biconical antenna. Following the analysis given by Schelkunoff for a finite biconical antenna, both TEM and higher-order modes may be present inside the cone (defined as a boundary sphere with radius r), but outside the cone only higher-modes can exist (see Fig. 11.3(b)).
The explanation is as follows: as TEMs waves reach the boundary of the sphere, part of the energy get reflected as a TEM wave. However, since this reflection is not uniform, some of the waves are reflected in the form of higher order modes. At the center, most of the energy escapes into free space, while at the cones most TEM waves are reflected back [242]. Thus, a finite biconical antenna, can be considered as a transmission line of characteristic impedance Zo,bc terminated in the load impedanceZL, as shown in Fig. 11.3(b), signifying that its input impedance is now both on flare angle and length of the cone. Furthermore, it has been shown that terminal impedance of a bow-tie antenna with wider flare angles (lower characteristic
2also known as a butterfly antenna
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11.5 WR-03 Band CPW Bow-Tie Antenna in eWLB Package
Backside metal
Mold PCB
Integrated TRX chip Slot antenna
Ground metal
Figure 11.5: Perspective view of the TX and RX CPW bow-tie antenna in eWLB package. The slot antenna is designed in RDL 1 and the backside metallization is manufactured using RDL 2. The embedded TRX chip can be seen in the right-hand side figure.
Figure 11.6: Simulated realized gain of the bow-tie AiP at 240 GHz. The antenna gain does not include the loss of the transition from the MMIC pads to the RDL layer.
impedance) is a weaker function of cone length as compared to thinner antennas. This leads to the conclusion that wider bow-tie antennas have a characteristic impedance more suitable for broadband applications.
11.5.2 Simulation and Optimization in CST Microwave Studio®
Geometry of the proposed CPW-fed bow-tie antenna is shown in Fig. 11.4. Based on the geometry there are four main design parameters: radiusr and flare angleθbc of the bow-tie, and spacingsand widthw of the CPW line. The CPW line is designed to be 50-Ω compatible with the output impedance of the TX and RX chain. An alternate to the curved bow-tie is a triangular bow-tie, however simulations show an overall better performance with the curved form. The bow-tie AiP was 3D EM simulated in CST Microwave Studio®. The perspective view of the eWLB package with a TX and an RX bow-tie antenna, embedded Si-block for the MMIC with the exact fabricated dimensions, and the backside metal at RDL 2 is presented
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11. A WR-03 Radar Sensor with a Wideband Bow-Tie Antenna in eWLB Package
2000 220 240 260 280 300
2 4 6 8 10
Frequency (GHz)
Gain(dBi)
GainRe.ection coe/cient
200 220 240 260 280 300-35
-30 -25 -20 -15 -10 -5
200 220 240 260 280 300-35
-30 -25 -20 -15 -10 -5
Re.ectioncoe,.(dB)
Figure 11.7:Simulated realized gain and input reflection coefficient of the bow-tie AiP as a function of frequency.
-90 -60 -30 0 30 60 90
-10 -5 0 5 10
3(deg.)
Gain(dBi)
230 GHz 240 GHz 250 GHz
(a)
-90 -60 -30 0 30 60 90
-10 -5 0 5 10
3(deg.)
Gain(dBi)
230 GHz 240 GHz 250 GHz
(b)
Figure 11.8:Simulated radiation pattern of the bow-tie AiP. (a) H-plane. (b) E-plane.
in Fig. 11.5. The higher dielectric constant of the Si substrate as compared to the mold (11.7 compared to around 3 of the mold) is a major reason for beam deformity. The size and the aspect ratio of the open window at the backside metal aids in optimizing the beam pattern and the gain bandwidth of the antenna. The antenna is designed to provide good gain performance from 220 GHz and above. The optimized dimensions of the antenna are presented in Fig. 11.4.
It should be noted that the minimum spacing allowed in the RDL layer is 20µm which restricts the design flow.
The simulated realized gain of the bow-tie AiP at 240 GHz is shown in Fig. 11.6. The point of reference for all the simulations presented here is from the CPW feed line as shown in Fig. 11.4 and do not include the loss of the transition from the MMIC RF pads to the RDL layer and the loss of the additional CPW line to the edge of the IC. The loss of the transition can not be much optimized and it is mostly governed by the package design rules. This transition loss is estimated to be around 2 dB above 220 GHz. It should also be noted that losses incurring
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