220 IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007 Millimeter-Wave Bandpass Filters by Standard 0.18-µm CMOS Technology Sheng Sun, Member, IEEE, Jinglin Shi, Lei Zhu, Senior Member, IEEE, Subhash Chander Rustagi, Senior Member, IEEE, and Koen Mouthaan Abstract—Millimeter-wave (mm-wave) bandpass filters are presented using the standard 0.18-µm CMOS process. Without any postprocessing steps, thin film microstrip (TFMS) structure is properly constructed on the low-resistivity silicon substrate, aiming at reducing the substrate loss and crosstalk to a large extent. Using the broadside-coupled scheme, a tight coupling is achieved so as to make up a class of low-loss and broadband TFMS bandpass filters in the mm-wave range. To achieve a small size, one-stage and two-stage filters with sinuous-shaped resonators are designed and fabricated. A good agreement between the predicted and measured results has been observed up to 110 GHz. Index Terms—Bandpass filter, millimeter-wave (mm-wave), silicon substrate, thin film microstrip (TFMS) line. Fig. 1. Layout configuration of the proposed resonator on the standard 0.18-µm CMOS with six metal layers. I. I NTRODUCTION and 3.2-dB loss at 27 GHz [8] have been reported after the implantation of high-resistivity substrate transfer. On the other hand, thin film microstrip (TFMS) [9] has been demonstrated with several promising transmission characteristics over a wide frequency range (1–110 GHz), such as small linewidths, bend and via pad sizes, simple design procedure [9], [10], and only a single dominant propagating mode rather than two modes in conventional coplanar-waveguide structure [1]. In this letter, we present a novel class of mm-wave bandpass filters with reduced loss and miniaturized size using the sinuous-shaped TFMS line structure. By introducing a ground shield between a passive component and the silicon substrate [11], a current-return path is formed, thereby minimizing the electric field leaking into the silicon region. After optimization design, two two-stage TFMS filters are fabricated by a conventional 0.18-µm CMOS process without any postprocessing steps. The measured insertion losses are approximately 3.7 dB at 60 GHz and 2.7 dB at 65 GHz, respectively, which are considerably lower than those reported in [6]–[8]. M ICROWAVE passive bandpass filters are of great interest in the development of 60-GHz unlicensed millimeterwave (mm-wave) communication systems [1]. Due to the lowfabrication cost and matured process, the standard CMOS technology has always been preferable in industries. Much effort has been recently made to extend this technology to the exploration of CMOS-based passive components in the mm-wave range. Unfortunately, the low-resistivity silicon substrate in the standard CMOS always suffers from high-dielectric loss and brings out a dramatic crosstalk to the devices integrated on the silicon wafers. To break this limitation, many approaches have been tried to be proposed including ion implantation [2], micromachining technique [3], thick isolated interface layer [4], and so on. However, these nonstandard processes require additional processing steps or packaging considerations, thus tremendously increasing the process complexity and cost. By using the micromachining techniques, bandpass filters have been demonstrated for the mm-wave applications at 31 [3], 37, and 60 GHz [5]. To minimize an interaction with the low-resistivity silicon substrate, the use of a thick polyimide interface layer to build up a 30-GHz bandpass filter [6] has also been reported. Compared to the poor performance on the conventional lossy silicon substrate, 3.4-dB loss at 40 GHz [7] Manuscript received October 17, 2006; revised December 13, 2006. The review of this letter was arranged by Editor M. Ostling. S. Sun, J. Shi, and S. C. Rustagi are with the Institute of Microelectronics, Singapore 117685 (e-mail: sunsheng@ieee.org). L. Zhu is with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798. K. Mouthaan is with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119077. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2007.891305 II. P ROPOSED S INUOUS -S HAPED R ESONATOR W ITH TFMS Fig. 1 depicts the layout configuration of the proposed sinuous-shaped TFMS resonator in the standard 0.18-µm CMOS process technology, with six-metal aluminum interconnects on a low-resistivity (10 Ω · cm) silicon substrate. Typically, to achieve a higher inductive quality factor with thicker silicon-oxide layer [1], the top metal (M6 ) with a thickness of about 2 µm is used as signal line (thicker metal has lower series resistance), and the bottom layer (M1 ) with a thickness of about 0.5 µm is used as ground plane. As usual, the dielectric layer is thin with the total height (h) of about 6.7 µm in 0.18-µm CMOS and yields a large capacitive quality factor 0741-3106/$25.00 © 2007 IEEE SUN et al.: MILLIMETER-WAVE BANDPASS FILTERS BY STANDARD 0.18-µm CMOS TECHNOLOGY 221 Fig. 2. Frequency responses of the designed resonators at different layers M5 and M6 . around 30 (h = 4 µm) at mm-wave frequencies [1], which leads to a relative weak coupling degree between the coupled lines at the same layer. For this reason, the broadside-coupled structures with a large coupling degree are employed to form a class of mm-wave broadside-coupled bandpass filters with a wide bandwidth [12]. In the proposed mm-wave bandpass filter, the strip width of the line resonator is selected as 10 µm, while the straight length is fixed at 1030 µm, as shown in Fig. 1. The two coupling arms with 210 µm are located at the other layer to produce a strong coupling between the feed line and the resonator. To miniaturize the overall size of on-wafer area, the mm-wave bandpass filters with the sinuous-shaped resonators at different layers (M5 for 60 GHz and M6 for 65 GHz) are designed. Fig. 2(a) and (b) shows the measured and simulated frequency responses of the designed resonator using the SONNET EM Suite. The unloaded Q factors of resonators are about 15.3 for M5 and 25.3 for M6 , respectively. The minimum insertion losses in the passbands are found to be 2.1 dB at 60 GHz and 1.7 dB at 65 GHz, respectively. III. R ESULTS OF T WO -S TAGE F ILTERS To further sharpen and deepen the concerned outer stopbands, the two-stage bandpass filters are designed and implemented. Fig. 3(a) illustrates the top-view photograph of the fabricated two-stage mm-wave bandpass filters. Two sinuousshaped resonators are formed at M5 or M6 , and they are cascaded via a weakly coupled section. The predicted and measured frequency responses of insertion and return losses for these two filters with the resonators installed at M5 and M6 are shown in Fig. 3(b) and (c), respectively. Over the wide frequency range from 1.0 to 110.0 GHz, the predicted and measured results are found in reasonable agreement with each other. The filter specifications are shown in Table I. More than 50% of the measured bandwidth is achieved due to the enlarged coupling degree of a broadside-coupled structure. The minimum insertion losses are found to be 3.9 and 2.7 dB, respectively. Similar to the one-stage case, we can again Fig. 3. Photograph, predicted, and measured results of the proposed two-stage mm-wave bandpass filters. (a) Photograph. (b) Insertion and return losses for the 60-GHz filter with a resonator layer at M5 . (c) Insertion and return losses for the 65-GHz filter with a resonator layer at M6 . TABLE I SPECIFICATIONS OF THE DESIGNED BANDPASS FILTERS observe here that the insertion loss gets a gradual reduction as the resonator is installed in the upper layer. In this case, the field distribution is dispersed in the air space, while the current density on the strip conductor of such a resonator is reduced, thereby simultaneously reducing the substrate loss and conductor loss. 222 IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007 IV. C ONCLUSION In this letter, a class of novel mm-wave bandpass filters with a broadside-coupled line are proposed, designed, and implemented by the standard 0.18-µm CMOS technology. By forming a sinuous-shaped resonator at the top layer of the six metal layers and partially introducing the broadside-coupled arms in the input and output ports, a good mm-wave passband with two transmission poles is achieved, as illustrated in simulation and verified in the experiment. For the applications in 0.13 µm or beyond technology, more stacked dielectric layers are always preferable to reduce parasitic capacitance, and thicker metal of signal line can be equivalently realized by connecting the two top-metal layers through a via. ACKNOWLEDGMENT The authors would like to thank the support of the Institute of Microelectronics, Singapore. This work was carried out for the A*STAR Project No. 0 421 140 045. R EFERENCES [1] C. H. Doan, S. Emami, A. M. Niknejad, and R. W. Brodersen, “Millimeter-wave CMOS design,” IEEE J. Solid-State Circuits, vol. 40, no. 1, pp. 144–155, Jan. 2005. [2] A. Chin, K. Lee, B. C. Lin, and S. Horong, “Picosecond photoresponse of carriers in Si ion-implanted Si,” Appl. Phys. Lett., vol. 69, no. 5, pp. 653–655, Jul. 1996. [3] T. M. Weller, K. J. Herrick, and L. P. B. Katehi, “Quasi-static design technique for mm-wave micromachined filters with lumped elements and series stubs,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 6, pp. 931– 938, Jun. 1997. [4] C.-M. Nam and Y.-S. Kwon, “Coplanar waveguides on silicon substrate with thick oxidized porous silicon (OPS) layer,” IEEE Microw. Wireless Compon. Lett., vol. 8, no. 11, pp. 369–371, Nov. 1998. [5] P. Blondy, A. R. Brown, D. Cros, and G. M. Rebeiz, “Low-loss micromachined filters for millimeter-wave communication systems,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 12, pp. 2283–2288, Dec. 1998. [6] J. Papapolymerou and G. E. Ponchak, “Microwave filters on a low resistivity Si substrate with a polyimide interface layer for wireless circuits,” in Proc. IEEE RFIC Symp. Dig., Jun. 2001, pp. 125–128. [7] K. T. Chan, C. Y. Chen, A. Chin, J. C. Hsieh, J. Liu, T. S. Duh, and W. J. Lin, “40-GHz coplanar waveguide bandpass filters on silicon substrate,” IEEE Microw. Wireless Compon. Lett., vol. 12, no. 11, pp. 429– 431, Nov. 2002. [8] D. S. Yu, C. F. Cheng, K. T. Chan, A. Chin, S. P. Mcalister, C. Zhu, M. F. Li, and D. L. Kwong, “Narrow-band bandpass filters on silicon substrate at 30 GHz,” in Proc. IEEE MTT-S Int. Microw. Symp. Dig., 2004, pp. 1467–1470. [9] G. E. Ponchak and A. N. Downey, “Characterization of thin film microstrip lines on polyimide,” IEEE Trans. Compon., Packag., Manuf. Technol. B, vol. 21, no. 2, pp. 171–176, May 1998. [10] K. Nishikawa, K. Kamogawa, K. Inoue, K. Onodera, T. Tokumitsu, M. Tanaka, I. Toyoda, and M. Hirano, “Miniaturized millimeter-wave masterslice 3-D MMIC amplifer and mixer,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 9, pp. 1856–1862, Sep. 1999. [11] T. S. D. Cheung and J. R. Long, “Shielded passive devices for siliconbased monolithic microwave and millimeter-wave integrated circuits,” IEEE J. Solid-State Circuits, vol. 41, no. 5, pp. 1183–1200, May 2006. [12] M. Tran and C. Nguyen, “Modified broadside-coupled microstrip lines suitable for MIC and MMIC applications and a new class of broadsidecoupled band-pass filters,” IEEE Trans. Microw. Theory Tech., vol. 41, no. 8, pp. 1336–1342, Aug. 1993.