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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
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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.
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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.
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