IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 5, MAY 2012
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Compact High-Gain mmWave Antenna for TSV-Based System-in-Package Application Sanming Hu, Yong-Zhong Xiong, Senior Member, IEEE, Lei Wang, Rui Li, Member, IEEE, Jinglin Shi, and Teck-Guan Lim, Member, IEEE
Abstract— This paper presents a cavity-backed slot (CBS) antenna for millimeter-wave applications. The cavity of the antenna is fully filled by polymer material. This filling makes the fabrication of a silicon CBS antenna feasible, reduces the cavity size by 76.8%, and also maintains the inherent high-gain and wide bandwidth. In addition, a through-silicon via-based architecture is proposed to integrate the 135-GHz CBS antenna with active circuits for a complete system-in-package. Results show that the proposed structure not only reduces the footprint size but also suppresses the electromagnetic interference. Index Terms— Cavity-backed slot (CBS) antenna, electromagnetic interference (EMI), millimeter-wave (mmWave), system-in-package (SiP), through-silicon via (TSV).
gain, wide impedance bandwidth, and unidirectional radiation pattern. However, the air-filled metal cavity of this antenna results in not only bulky size but also incompatibility with active-integrated circuits. In this paper, a silicon-based CBS antenna with a polymerfilled cavity [11] rather than an air-filled one is designed to solve the abovementioned issues. To integrate this silicon antenna with active circuits to form a silicon-based mmWave system-in-package (SiP) with reduced footprint and low electromagnetic interference (EMI), a through-silicon via (TSV)-based architecture is proposed and investigated. II. A NTENNA D ESIGN AND FABRICATION A. Antenna Design
I. I NTRODUCTION
S
ILICON-BASED millimeter-wave (mmWave) systems are greatly driven by the applications including short-range ultra-high-speed communications, remote sensing, biomedical imaging, and automotive radars [1], [2]. Many countries or regions have reserved specified frequency bands for these applications. For example, Singapore has selected the frequency band of 116–144 GHz as the unlicensed industrial, scientific, and medical (ISM) band [3]. In a silicon-based mmWave system, an antenna, which is one of the most important components, is challenging to be realized with high-performance [1], [4]–[6]. First, an antenna occupies large chip area which results in highfabrication cost. Second, due to the limited output power of the silicon-based transmitter and the increased free-space propagation loss at mmWave frequencies, high-gain antennas are required. However, most on-chip mmWave antennas have gain of −10 dBi or lower [5], [7]. Third, antennas with wide bandwidth and flat gain are necessary for many applications. Nevertheless, most reported on-chip antennas are narrowband. In [8]–[10], a metal cavity and a wide slot are investigated to form a cavity-backed slot (CBS) antenna to achieve high Manuscript received April 21, 2011, revised November 22, 2011; accepted February 12, 2012. Date of publication April 3, 2012; date of current version May 3, 2012. This work was supported in part by the A*STAR SERC under Grant 082 141 0040. Recommended for publication by Associate Editor M. Cases upon evaluation of reviewers’ comments. The authors are with the Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), 117685, Singapore (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2012.2188293
The explored 3-D view and 2-D cross-sectional view of the CBS antenna are presented in Fig. 1(a) and (b), respectively. Polymer and benzocyclobutene (BCB) are used in this antenna design. To obtain the mmWave characteristics of these dielectric materials which are very important for an antenna design, some ring resonators, and transmission lines are fabricated using the same process for de-embedding and extraction. At 135 GHz, the extracted electrical characteristics of polymer are εr = 2.65 and tanδ = 0.01, and that of BCB are εr = 2.8 and tanδ = 0.01, respectively. As illustrated in Fig. 1(a), the footprint of the whole antenna is 1.6 × 1.2 mm2 (W0 × L 0 ). Three metal layers (M1, M2, and M3) are efficiently used in this antenna structure. On the M3 layer, a wide slot of 1.3 × 0.3 mm2 (W3 × L 3 ) rather than a narrow slot is employed to expand the frequency bandwidth of the CBS antenna. The wide slot is then excited by an open-ended stub connecting with a coplanar waveguide (CPW) feeding line. The CPW structure makes the on-wafer measurement easy and also reduces the surface wave effect. The input impedance of a center-fed CBS antenna is very large. Therefore, the presented antenna is offset-fed to match the 50- CPW line. Different from the reported CBS antennas [8]–[10], an additional metal layer (M2) with an opening of 1.3 × 0.4 mm2 (W2 × L 2 ) is adopted to slightly narrow the antenna beamwidth and enhance the boresight antenna gain. M1 layer avoids the electromagnetic energy being trapped into the silicon substrate which has low resistivity (10 •cm). In this case, the high-loss silicon substrate is, for structural purpose, to support the antenna and will not reduce the antenna performance. The M1 layer is also used to shape a polymerfilled metal cavity. The cavity depth (h = 150 μm) is set by
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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 5, MAY 2012
Fig. 3.
Simulated 3-D radiation pattern of the CBS antenna at 135 GHz.
Fig. 4.
Microphotograph of the CBS antenna.
Fig. 1. (a) Explored view of the CBS antenna. The optimal values are (unit: millimeter) W0 = 1.6, L 0 = 1.2, W1 = 1.4, L 1 = 0.8, W2 = 1.3, L 2 = 0.4, W3 = 1.3, and L 3 = 0.3. (b) Cross-sectional view of the antenna design. 8 7
Gain (dBi)
6 5 4 3 2
BCB (tan δ =0) + polymer (tan δ =0) + PEC BCB (tan δ =0) + polymer (tan δ =0) + Cu BCB (tan δ =0.01) + polymer (tan δ =0) + Cu BCB (tan δ =0.01) + polymer (tan δ =0.01) + Cu
1 0 120
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Frequency (GHz)
Fig. 2.
Antenna gain variations with material properties.
the fabrication process, and the cavity cross section is chose as 1.4 × 0.8 mm2 (W1 × L 1 ) to obtain optimal antenna gain and bandwidth. Theoretically, the cavity size follows a cubic wavelength (λ3 ) dependence. There is √ 3 c/ εr 3 ∝ εr−1.5 (1) λ = f where c is the speed of light in free space, εr is the relative permittivity of the material filled in the cavity, and f is the operation frequency. Therefore, the size of a polymer- (εr = 2.65) filled cavity is 76.8% (i.e., 1− εr−1.5 ) smaller than that of a conventional air (εr = 1) cavity when the operation frequency keeps the same. In addition to the antenna size reduction, the antenna gain variation caused by polymer is also investigated using the 3-D full-wave simulator Ansoft high-frequency structure simulator.
As shown in Fig. 2, the highest antenna gain can be obtained if: 1) the dielectric materials, i.e., polymer and BCB, are lossless (tanδ = 0); and 2) the metal is set as perfect electric conductor (PEC, conductivity σ = ∞). Compared with the above ideal case, the antenna gain is reduced by ∼0.7 and ∼0.3 dB due to the losses of copper (Cu, σ = 5.8 × 107 S/m) and BCB (tanδ = 0.01), respectively. On the other hand, the antenna gain is dropped by ∼0.2 dB, if tanδ of polymer changes from 0 (lossless) up to 0.01. These results indicate that, among these three materials, i.e., polymer, BCB, and copper, polymer is the smallest factor resulting in the reduction of antenna gain (∼0.2 dB), whereas copper is the biggest one (∼0.7 dB). Based on the extracted dielectric characteristics of the materials, the simulated 3-D radiation pattern at 135 GHz is illustrated in Fig. 3. The shape is in good agreement with that of the CBS antennas in [8] and [9]. B. Antenna Fabrication The optimized antenna is then fabricated by an in-house developed Si-BCB process. A deep cavity with 150-μm depth is first etched in a low-resistivity silicon wafer using Borsch Etch process. After that, a 1-μm layer of SiO2 and a 1000-Å thin film of titanium are deposited on the silicon substrate to
HU et al.: COMPACT HIGH-GAIN mmWAVE ANTENNA
Fig. 5.
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One-port S-parameter measurement setup.
0
Fig. 7. Antenna gain measurement setup using the gain-transfer method. (a) Commercial horn antenna is used as a Tx antenna for reference. (b) CBS antenna is used as a Tx antenna under test.
-10
| S11 | (dB)
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Frequency (GHz)
Fig. 6.
Measured and simulated return loss curves of the CBS antenna.
enhance the isolation and adhesion. Then, a 1-μm Cu (M1) is built up through electroplating process. Next, the polymer (DuPont PerMX 3000) is filled into the cavity by vacuum lamination of permanent dry film photoresist, followed by lithography to remove the unwanted area. The cavity wafer with polymer is hard cure at 200 °C for two hours. Subsequently, the 10-μm photosensitive BCB 4026-26 from Dow Chemical is spin coated on M1 and photolithographically patterned. The wafer is then baked at 250 °C for one hour. Same process is repeated for the second metal (M2) and dielectric layers. After another baking process, the last metal layer (M3) is electroplated. Microphotograph of the fabricated CBS antenna is demonstrated in Fig. 4. This process is TSV-compatible and suitable for other passive designs with low loss. III. M EASUREMENT R ESULTS AND D ISCUSSION A. Impedance Bandwidth As illustrated in Fig. 5, one-port S parameters of the fabricated antenna are on-wafer measured using a vector network analyzer and its D-band extender. Fig. 6 shows that the simulated and measured 10-dB return loss bandwidths
are 116–141 GHz, and about 110–147 GHz, respectively. The results also show that, similar with the CBS antennas studied in [8] and [9], there are two adjacent resonant frequencies which broaden the antenna bandwidth. The simulated and measured second resonant frequencies, which are 136 and 137 GHz, are close to each other. Nevertheless, the measured value of the first resonant frequency is lower than the simulated one. One possible reason is the fabrication variation of the polymer-filled cavity. The down-shifting of the first resonance frequency results in the measured 10-dB bandwidth being wider than the simulated one.
B. Antenna Gain The gain-transfer method [12] is used to measure the antenna gain. As shown in Fig. 7(a), two identical commercial horn antennas with gain (G horn ) of 20 dBi are adopted as transmitting and receiving antennas, respectively. Signal is generated by a commercial mmWave source, and radiated by the horn antenna. Subsequently, the mmWave signal is received by the other horn antenna, and then measured by a frequency spectrum analyzer with a sub-harmonic mixer. To reduce the measurement errors, averaging factor, resolution bandwidth, and frequency span of the spectrum analyzer are set as 32, 1, and 500 kHz, respectively. The measured power is noted as Phorn . After that, as shown in Fig. 7(b), the commercial horn antenna for transmitting is replaced by the CBS antenna under test, whereas other equipments and their settings are identical as that in Fig. 7(a). In this case, the received power measured using the same frequency spectrum analyzer is recorded as PAUT . Therefore, we can get the measured antenna gain (G AUT) of the CBS antenna as G AUT = G horn − Phorn + PAUT + L probe
(2)
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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 2, NO. 5, MAY 2012
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8 Measured Gain Simulated Gain
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Efficiency (%)
95
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85
Fig. 9.
Proposed TSV-based SiP architecture.
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Frequency (GHz)
Fig. 8.
Antenna gain and efficiency of the CBS antenna.
where L probe = 1.65 dB is the insertion loss of the groundsignal-ground (GSG) probe. The measured antenna gain is shown in Fig. 8. From 123 to 142 GHz, the measured results are within 1.5-dB variation and have
