IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 7, JULY 2008 431
Artificial Dielectric Shields for
Integrated Transmission Lines
Yue Ma, Behzad Rejaei, and Yan Zhuang
Abstract—We present a novel shielding method for on-chip transmission lines built on conductive silicon substrates. The shield consists of an artificial dielectric with a very high in-plane dielectric constant, built from two patterned metal layers iso-lated by a very thin dielectric film. Inserted below an integrated coplanar transmission line, the artificial dielectric layer blocks the electric field of the line from entering the silicon substrate. Shielded coplanar waveguides fabricated on a conventional sil-icon wafer show a two- to three-fold loss reduction compared to unshielded lines at frequencies below 30 GHz.
Index Terms—Artificial dielectric layer (ADL), high dielectric, integrated transmission line, radio frequency (RF)/microwave sub-strate loss, shield.
I. INTRODUCTION
M
ONOLITHIC integration of high-quality microwave transmission lines still remains a challenge due to performance degradation caused by losses in the underlying silicon (Si) substrate, the main vehicle of the contemporary radio frequency integrated circuits (RFICs). Although substrate loss can be suppressed by using insulating substrates [1], micromachining [2], and substrate transfer techniques [3], such methods involve modification of the core device integration process, leading to significantly higher fabrication costs.Electric shields built from metal or polysilicon layers offer an IC-compatible alternative. Inserted beneath the device, the shield blocks the electric field from entering the conductive Si substrate, preventing the flow of unwanted currents [4]–[6]. Already coplanar waveguide (CPW) transmission lines with shields built from floating metal strips perpendicular to the line have been demonstrated to have very low substrate loss [6]. However, the inherently large parasitic capacitance between the CPW and the conductive shield substantially reduces the line characteristic impedance. Consequently, shielded lines with high characteristic impedance require narrow signal lines or large signal to ground spacing, which increase the line attenuation or device area. This is a crucial disadvantage in the implementation of resonant tanks, impedance matching networks and couplers, where a wide range of characteristic impedances (30–300 ) is required.
In this letter, we describe a new IC-compatible shielding method for integrated CPWs utilizing an artificial dielectric
Manuscript received February 4, 2008; revised February 12, 2008. This work was supported by “Stichting voor de Technische Wetenschappen (STW)”.
The authors are with the Laboratory of High Frequency Technology and Com-ponent (HiTeC), Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, Delft NL-2628CD, The Netherlands (e-mail: y.ma@tudelft.nl).
Digital Object Identifier 10.1109/LMWC.2008.924907
Fig. 1. (a) and (b): Geometrical layout of the ADL consisting of two thin, pat-terned metal layers isolated by a thin dielectric. The lattices of squares on the two metal layers are shifted with respect to each other so that each square on the top lattice partially faces four squares on the bottom lattice and vice versa. (c): Cross section of an integrated CPW line shielded by an ADL inserted be-tween the Si and oxide (thicknessh) layers.
layer (ADL) consisting of a conventional, thin dielectric film sandwiched between two patterned metal layers [7]. Due to its very high in-plane dielectric constant, the ADL prevents the electric field of the CPW from entering the Si substrate. Our experiments show the ADL shield to yield an up to three-fold reduction of the line attenuation below 30 GHz. Furthermore, the effect of the ADL on the characteristic impedance of the CPW is much smaller than that of conventional shields, allowing the realization of a broad range of characteristic impedances without the need for narrow signal lines or large signal-ground spacing. An additional benefit of the ADL, com-pared to conventional shielding structures, is the independence of its design of the individual CPW layout, simplifying the mask design and fabrication process.
II. ARTIFICIALDIELECTRIC AS ASHIELD
Fig. 1 shows the ADL built from two thin metal layers ver-tically separated by a thin dielectric film with the thickness and dielectric constant . Each metal layer is patterned to form a lattice of disconnected, but closely spaced squares. The lattices on the two metal layers are shifted with respect to each other so that each square on the top lattice partially faces four squares on the bottom lattice and vice versa [7], [8]. To reduce the flow of eddy currents inside individual patterns at high frequencies, their dimension is chosen in the m range.
To describe the dielectric properties of the ADL we use an effective medium approach which is justified at the frequencies
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432 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 7, JULY 2008
of interest in this work ( 30 GHz) where the electromagnetic wavelength exceeds by several orders of magnitude the dimen-sion and spacing of metallic squares (both in the m range). Using a method described in [7] it can then be shown that the ef-fective, in-plane dielectric constant of the ADL is
where is the overall ADL thickness and is the overlap area between metallic squares partially facing each other (Fig. 1). With being of the order of square m’s and ( ) not ex-ceeding several tens (hundreds) of nanometers, is typically several orders of magnitude larger than .
The effective dielectric constant of the ADL in the direc-tion normal to its plane can be easily estimated when the spacing between the patterns is much smaller than their size. For perpen-dicular electric fields, the ADL then practically behaves as two continuous metal layers separated by a dielectric film, resulting in . Since and do not differ by orders of mag-nitude, is much smaller than .
The shielding effect of the ADL on the electric field of a Si-based CPW line [Fig. 1(c)] can be demonstrated using the quasi-TEM approximation [9] where the (transverse) electric field is expressed as the gradient of an electrostatic potential on the line cross section. To simplify the analysis we assume an unbounded substrate and an infinitely thick Si layer. Solving the spectral domain Laplace equation, the Fourier trans-form (in the -direction) of inside the Si layer ( 0) can be expressed as
(1)
where is the potential on the (top) oxide
surface, is the oxide layer thickness [see Fig. 1(c)], and (2) (3) where is the dielectric constant of the oxide layer,
, , and
with the dielectric constant and the conductivity of Si. The (Fourier transformed) electric field inside Si is
.
Equations (1)–(3) show that, for any non-uniform field ( 0), a high value of reduces and, therefore, the electric field and power dissipated inside the conductive Si. In particular, since the lateral variation of the field over distances comparable to the ADL thickness ( 1 m) is negligible in typical CPW configurations, it suffices to consider the limit , where
, . Evidently,
increasing leads to an increase in and, therefore, a re-duction of . (Although is given below the CPW conduc-tors, a rigorous treatment of this problem requires its self-con-sistent computation inside the CPW slots. But the argument pre-sented is sufficient to show the shielding effect.)
Note that the magnetostatic equations of the quasi-TEM ap-proach [9] are not influenced by the dielectric properties of the medium. Therefore, as long as the quasi-TEM approximation holds (i.e., at not too high frequencies), the effect of the ADL on the magnetic field of the CPW is expected to be negligible.
Fig. 2. Attenuation versus frequency of a CPW line (w = 30 m, s = 30 m) shielded by ADL’s with 32 3, 5 2 5, and 7 2 7 m patterns. Results for an unshielded line are shown for comparison. Dashed lines show the HFSS sim-ulation results with the ADL modeled as an anisotropic dielectric. Taking into account a 0.3 m reduction of the ADL pattern size due to lithography is-sues, a value of" 700, 4200, and 10 000 was calculated for the three shields, respectively, and used in the HFSS simulations." 75 in all cases. A Si re-sistivity of 7cm was assumed in the simulations.
III. EXPERIMENTS
CPW transmission lines with different signal line width ( ) and signal to ground spacing ( ) were fabricated using a 3 m thick aluminum (Al) layer on a 5–10 cm Si wafer, isolated by a 9 m thick silicon oxide (SiO ) layer. The ADL shields were built using two 100 nm thick Al layers isolated by a 30 nm thick PVD-sputtered aluminum oxide film with a rela-tive dielectric constant of 10. For each CPW line, three different ADL structures were built using 3 3, 5 5, and 7 7 m metallic patterns, all laterally separated by 1 wide gaps. The propagation constant ( ) and characteristic impedance ( ) of the CPW lines (of length ) were found from
(4) (5) where and denote the impedance of the open- and short terminated lines, respectively, extracted from -parameter data obtained using a HP8510C vector network analyzer.
Fig. 2 shows the attenuation constant
of a shielded CPW ( 30 m, 30 m) in comparison with that of an identical, but unshielded line. The effectiveness of shielding clearly increases with the ADL pattern size. At frequencies below 30 GHz, the ADL with the largest patterns (7 7 m ) yields a nearly three fold reduction of compared to the unshielded device. HFSS simulations based on the effec-tive description of the ADL yield lower values of compared to the experimental data (Fig. 2). Especially at high frequencies, this can be attributed to the inability of the dielectric model to account for the eddy current loss inside the individual patterns of the ADL. Nevertheless, the overall agreement is reasonable below 20 GHz.
Fig. 3 shows the attenuation and characteristic impedance of different CPW’s shielded using the ADL with 7 7 m patterns. Unlike the conventional lines, the characteristic impedance of the shielded CPW’s changes little as a function
MA et al.: ARTIFICIAL DIELECTRIC SHIELDS 433
Fig. 3. Attenuation (a) and real part of the characteristic impedance (b) of shielded (solid markers) and unshielded (hollow markers) CPW lines with the signal line widthw and signal to ground spacing s.
of frequency and has a very small imaginary component (not shown here). This is due to the ADL masking the influence
of the complex permittivity of Si whose
imaginary part is large and strongly frequency-dependent. Note that, due to its high in-plane dielectric constant, the ADL lowers the characteristic impedance of the CPW lines (up to ) due to an increase in the line capacitance per unit length. Nevertheless, this effect is much smaller than that of conventional metallic shields [6]. Therefore, ADL-based CPW lines with a wide range of characteristic impedances can be real-ized without the need for extremely narrow signal lines or large signal-ground spacing.
IV. CONCLUSION
We experimentally demonstrated a novel method for reducing the substrate loss of Si-based integrated CPW transmission lines. The method exploits the shielding effect of an ADL with a high in-plane dielectric constant. The ADL comprises two
patterned metal layers separated by a thin conventional dielec-tric, and can be readily built in a standard IC process. It was found that the ADL shield significantly lowers the attenuation, but only moderately changes the characteristic impedance of the transmission lines. Consequently, CPW lines equipped with an ADL shield can be used to implement low-loss passive (sub-) circuits where a variety of characteristic impedances is required.
The shielding effectiveness of the ADL can be further in-creased by increasing its in-plane dielectric constant . This can be achieved by increasing the pattern size, provided that the flow of parasitic eddy currents inside individual patterns can be effectively suppressed. can also be increased by reducing the thickness of the dielectric layer, or using a dielectric with a high intrinsic dielectric constant. Finally, in view of the many metal-lization layers available in modern IC processes, one can stack several ADL’s on top of each other to increase the shielding effect.
It is worth mentioning that, unlike conventional shielding structures [4]–[6], the geometrical patterning of an ADL does not depend on the layout of individual CPW lines (e.g., their direction). Therefore, a predefined uniform ADL shield can be built underneath the whole passive (sub-) circuit, which simplifies the mask design and fabrication process.
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