Wave transmission lines and networks – Long lines – Strip type
Reexamination Certificate
2000-03-23
2004-01-06
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Long lines
Strip type
C333S161000, C333S246000
Reexamination Certificate
active
06674347
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multi-layer substrate used in a millimeter wave band and a microwave band, and more specifically to a multi-layer substrate configured to suppress an unwanted propagation mode.
In the prior art, a composite microwave integrated circuit having a number of semiconductor devices mounted on a multi-layer substrate is known, which was disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-09-167825. In the circuit disclosed in this patent publication, a plurality of semiconductor devices are accommodated in a cavity formed in a dielectric substrate, so as to avoid influence of an external electromagnetic field and to ensure an airtight condition. Each of the semiconductor devices are connected to microstrip lines.
In addition, T. Hirose et al, “A FLIP-CHIP MMIC DESIGN WITH CPW TECHNOLOGY IN THE W-BAND”, IEEE MTT-S, International Microwave Symposium Digest, pp.525-528, 1998 (the content of which is incorporated by reference in its entirety into this application) discloses a high frequency circuit including a coplanar waveguide formed on a single-layer substrate as an interconnection line. In a coplanar waveguide line, a signal line and a ground line are formed in the same plane, and the coplanar waveguide line is suitable to a flip chip bonding which can make connection with a low inductance and good repeatability.
When the coplanar waveguide line is used, however, a leakage of a signal wave in an unwanted propagation mode within the substrate has become a problem. This problem of leakage is described in detail in H. Shigesawa et al, “CONDUCTOR-BACKED SLOT LINE AND COPLANAR WAVEGUIDE: DANGERS AND FULL-WAVE ANALYSES”, IEEE MTT-S, International Microwave Symposium Digest, pp.199-202, 1988 (the content of which is incorporated by reference in its entirety into this application). The leakage mode includes a surface wave mode occurring when a ground conductor is formed on only one surface of a dielectric plate (as the ground conductor constituting the coplanar waveguide line) and a parallel-plate mode occurring when a ground conductor is formed on each of opposite surfaces of the dielectric plate (as the ground conductor constituting the coplanar waveguide line, and a ground conductor formed on a back surface of the dielectric plate). The leakage in these modes increases a transmission loss in the coplanar waveguide line, and causes interference between adjacent lines in the substrate, between adjacent devices in the substrate, and/or between adjacent line and device in the substrate.
Under the above mentioned circumstances, various approaches for suppressing the unwanted modes in the case that the coplanar waveguide line is formed in a single-layer substrate or in a multi-layer substrate, have been proposed.
A method for suppressing the surface wave mode is described in, for example, Misao HANEISHI, “Modern Planar Antenna”, Kabushiki Kaisha Sogou Gijutsu Center, Page 63. In this literature, it was designed so that the following equation (1) holds:
t<c
/{4
f
·(∈
r
−1)
e,fra 1/2
} (1)
where t is a thickness of a dielectric substrate
c is velocity of light
f is an operating frequency
∈
r
is a dielectric constant of a dielectric substrate material
Alternatively, a method for suppressing the parallel-plate mode is described in, for example, N. K. Das, “Methods of Suppression or
1
0
Avoidance of Parallel-Plate Power Leakage from Conductor-Backed Transmission Lines”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 44, NO. 2, pp. 169-181, FEBRUARY 1996 (the content of which is incorporated by reference in its entirety into this application). Specifically, there are a method for interconnecting, by means of a via contact, between a ground conductor of a coplanar waveguide line and a back surface ground electrode which are formed on opposite surfaces of a dielectric substrate, respectively, and another method for forming a coplanar waveguide line on a double-layer substrate consisting of two layers having different dielectric constants.
However, a problem occurring when the coplanar waveguide line is formed on an internal layer within a multi-layer substrate, cannot be dissolved with only the above mentioned methods.
Here, referring to
FIG. 18A
, there is shown a diagrammatic plan view of a prior art multi-layer substrate in which the coplanar waveguide line is formed.
FIG. 18B
is a sectional view taken along the line H—H in FIG.
18
A. The prior art multi-layer substrate is generally designated with the reference number
101
, and includes a signal line
107
is selectively formed on a surface of a first dielectric layer
103
, and a pair of ground conductor layers
108
formed on opposite sides of the signal line
107
, separately from the signal line
107
, the signal line
107
and the ground conductor layers
108
being formed from the same conductor layer
104
. Thus, a coplanar waveguide line
102
is formed. Furthermore, a second dielectric layer
105
is formed on the coplanar waveguide line
102
to cover the signal line
107
and the ground conductor layers
108
.
In order to suppress the surface wave mode in the multi-layer substrate of this structure, the thickness of each of the first and second dielectric layers
103
and
105
must be smaller than the value of “t” obtained from the equation (1). Therefore, in order to use a high signal frequency, it is necessary to make the thickness of the dielectric layers as thin as possible. Accordingly, a problem occurs in which the number of layers cannot be increased because of the limited thickness of the dielectric layer, even if it is desired to increase the number of layers in the multi-layer substrate. On the other hand, since the thickness of the overall multi-layer substrate becomes thin, it becomes short of a mechanical strength.
Furthermore, various problems have been encountered in a prior art multi-layer high frequency circuit substrate intended to suppress the parallel-plate mode. Referring to
FIG. 19A
, there is shown a diagrammatic plan view of a prior art multi-layer substrate intended to suppress the parallel-plate mode.
FIG. 19B
is a sectional view taken along the line I—I in FIG.
19
A.
The prior art multi-layer substrate intended to suppress the parallel-plate mode, is designated with the reference number
111
, and includes a signal line
117
is selectively formed on a surface of a first dielectric layer
113
, and a pair of ground conductor layers
118
formed on opposite sides of the signal line
117
, separately from the signal line
117
, the signal line
117
and the ground conductor layers
118
being formed from the same conductor layer
114
. Thus, a coplanar waveguide line
112
is formed. Furthermore, a second dielectric layer
115
is formed on the coplanar waveguide line
112
to cover the signal line
117
and the ground conductor layers
118
. Ground conductor layers
119
are formed on a back surface of the first dielectric layer
113
and a front surface of the second dielectric layer
115
, respectively. Furthermore, via contacts
121
are formed to penetrate through each of the first and second dielectric layers
113
and
115
, in order to interconnect between the ground conductors
118
and the ground conductor layers
119
. Incidentally, the via contacts
121
are located along an extending direction of the coplanar waveguide line at intervals which is at least not longer than a half of a signal wavelength.
With this arrangement, a structure equivalent to a section of a hollow-pipe waveguide is formed by the via contact
121
and the ground conductor layers
119
. In this waveguide structure, however, since it is assumed that a medium having a dielectric constant higher than that of air is used, a propagation in an unwanted waveguide mode occurs at a frequency lower than that at which a waveguide mode occurs in the hollow-pipe waveguide. Considering that for example an alumina substrate having the dielectric constant of 10 is used, an interval of the via
Ito Masaharu
Maruhashi Kenichi
Ohata Keiichi
Choate Hall & Stewart
Glenn Kimberly E
NEC Corporation
Pascal Robert
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