U.S. patent application number 10/034191 was filed with the patent office on 2002-07-04 for wiring substrate for high frequency applications.
Invention is credited to Maetani, Maraki.
Application Number | 20020084514 10/034191 |
Document ID | / |
Family ID | 18861893 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020084514 |
Kind Code |
A1 |
Maetani, Maraki |
July 4, 2002 |
Wiring substrate for high frequency applications
Abstract
A high frequency wiring substrate comprises surface transmission
line conductors formed on upper and lower surfaces of a dielectric
substrate and surrounded by respective surface ground conductors,
and a transmission line interconnect structure for transmitting a
high frequency signal between the surface transmission line
conductors via an interlayer transmission line conductor surrounded
by an interlayer ground conductor and connected at both ends
thereof to the respective ends of the surface transmission line
conductors by signal conducting through-hole conductors. The
surface ground conductors are connected together by two arrays of
top-to-bottom through-hole ground conductors, while the interlayer
ground conductor is connected to the surface ground conductors by
interlayer through-hole ground conductors.
Inventors: |
Maetani, Maraki;
(Soraku-gun, JP) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Family ID: |
18861893 |
Appl. No.: |
10/034191 |
Filed: |
December 26, 2001 |
Current U.S.
Class: |
257/662 ;
257/259; 257/664 |
Current CPC
Class: |
H01L 2924/3011 20130101;
H01L 2924/0002 20130101; H01L 23/66 20130101; H01L 2924/0002
20130101; H05K 1/0219 20130101; H01L 2223/6616 20130101; H01P 1/047
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/662 ;
257/664; 257/259 |
International
Class: |
H01L 029/80; H01L
031/112 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2000 |
JP |
P2000-396635 |
Claims
What is claimed is:
1. A wiring substrate for high frequency applications, comprising:
a pair of surface transmission line conductors formed in line with
each other on upper and lower surfaces of a dielectric substrate
composed of a stack of dielectric layers, each of the surface
transmission line conductors being surrounded by a surface ground
conductor; and a transmission line interconnect structure for
transmitting a high frequency signal between the pair of surface
transmission line conductors via an interlayer transmission line
conductor formed between the dielectric layers in such a manner as
to be in line with and parallel to the pair of surface transmission
lines, the interlayer transmission line conductor being surrounded
by an interlayer ground conductor and connected at both ends
thereof to the respective ends of the pair of surface transmission
line conductors by signal conducting through-hole conductors,
wherein in the transmission line interconnect structure, the
surface ground conductors on the upper and lower surfaces are
connected together via the interlayer ground conductor by
top-to-bottom through-hole ground conductors arranged in two
parallel arrays spaced a prescribed distance apart from each other
on both sides of the interlayer transmission line conductor, while
the interlayer ground conductor is connected to the surface ground
conductors on the upper and lower surfaces by interlayer
through-hole ground conductors arranged in an array at a prescribed
pitch along each of two sides extending perpendicularly to the
interlayer transmission line conductor; and the thickness of each
of the dielectric layers, representing the spacing between the
interlayer transmission line conductor and the respective surface
transmission line conductors, and the length of a straight line
section of the interlayer transmission line conductor between the
signal conducting through-hole conductors are each set not larger
than one quarter of a signal wavelength of the high frequency
signal, while through-hole ground conductors for connecting the
surface ground conductors on the upper and lower surfaces are
formed in a region where the interlayer ground conductor is not
formed between the interlayer transmission line conductor and the
top-to-bottom through-hole ground conductors arranged parallelly on
both sides of the interlayer transmission line conductor, and are
arranged at a pitch not larger than a prescribed pitch that has the
frequency of the high frequency signal as a cutoff frequency.
2. The wiring substrate for high frequency applications of claim 1,
wherein the distance between the two arrays of top-to-bottom
through-hole ground conductors is set about twice the spacing
between the pair of surface transmission line conductors.
3. A wiring substrate for high frequency applications, comprising:
(a) a dielectric substrate composed of a stack of dielectric
layers; (b) a first surface transmission line conductor formed in a
straight line on one surface of the dielectric substrate; (c) a
second surface transmission line conductor formed in a straight
line on the other surface of the dielectric substrate; (d) a first
surface ground conductor formed to surround the first surface
transmission line conductor; (e) a second surface ground conductor
formed to surround the second surface transmission line conductor;
and (f) a transmission line interconnect structure for transmitting
a high frequency signal between the first surface transmission line
conductor and the second surface transmission line conductor, the
transmission line interconnect structure comprising: (f1) an
interlayer transmission line conductor formed in a straight line
between the dielectric layers; (f2) an interlayer ground conductor
formed between the dielectric layers in such a manner as to
surround the interlayer transmission line conductor; (f3) signal
conducting through-hole conductors for connecting the first surface
transmission line conductor to the interlayer transmission line
conductor, and for connecting the second surface transmission line
conductor to the interlayer transmission line conductor; (f4) first
through-hole ground conductors, arranged in two parallel arrays
spaced a prescribed distance apart from each other on both side of
the interlayer transmission line conductor, for connecting the
first surface ground conductor to the second surface ground
conductor via the interlayer ground conductor; (f5) second
through-hole ground conductors for connecting the first surface
ground conductor to the interlayer ground conductor, and for
connecting the second surface ground conductor to the interlayer
ground conductor, the second through-hole ground conductors being
arranged in an array at a prescribed pitch along each of two sides
extending perpendicularly to the interlayer transmission line
conductor; and (f6) third through-hole ground conductors for
connecting the first surface ground conductor to the second surface
ground conductor, the third through-hole ground conductors being
formed in a region where the interlayer ground conductor is not
formed between the interlayer transmission line conductor and the
first through-hole ground conductors arranged parallelly on both
sides of the interlayer transmission line conductor, wherein a
thickness of each of the dielectric layers, representing the
spacing between the interlayer transmission line conductor and the
first and second surface transmission line conductors, and a length
of a straight line section of the interlayer transmission line
conductor between the signal conducting through-hole conductors are
each set not larger than one quarter of a signal wavelength of the
high frequency signal, and the third through-hole ground conductors
are arranged at a pitch not larger than a prescribed pitch that has
the frequency of the high frequency signal as a cutoff frequency.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wiring substrate for high
frequency applications that has a transmission line structure for
transmitting therethrough a high frequency signal such as a
microwave signal, millimeter wave signal, or the like, and more
particularly to a wiring substrate for high frequency applications
that improves reflection characteristics for high frequency signals
at transmission line interconnects between the upper and lower
surfaces of a dielectric substrate.
[0003] 2. Description of the Related Art
[0004] In a high frequency wiring substrate that has a transmission
line structure for transmitting a high frequency signal
therethrough, interconnects between transmission line conductors
formed on the upper and lower surfaces of a dielectric substrate
have traditionally been achieved using through-hole conductors such
as plated via holes, as shown in the perspective view of FIG.
5.
[0005] In the perspective view of FIG. 5, the interior of the high
frequency wiring substrate is shown by dotted lines. As shown in
FIG. 5, surface transmission line conductors 2 and 3 as
transmission lines for high frequency signals are formed in line
with each other on the upper and lower surfaces of the dielectric
substrate 1 consisting of a single or multiple dielectric layers,
and the surface transmission line conductors 2 and 3 are connected
together at their opposing ends by a signal conducting through-hole
conductor 4 such as a plated via hole. On the upper and lower
surfaces of the dielectric substrate 1 are also formed surface
ground conductors 5 and 6 in such a manner as to surround the
respective surface transmission line conductors 2 and 3. The
surface transmission line conductors 2, 3 and the surface ground
conductors 5, 6 together constitute grounding coplanar transmission
lines on the upper and lower surfaces of the dielectric substrate
1, respectively. Further, the surface ground conductors 5 and 6 are
connected together by a large number of through-hole ground
conductors 7 formed in substantially parallel arrays on both sides
of the respective surface transmission line conductors 2 and 3.
This structure not only achieves stable grounding of the grounding
coplanar transmission lines formed on the upper and lower surfaces
of the dielectric substrate 1, but also serves as an impedance
matching grounding structure in the area surrounding the signal
conducting through-hole conductor 4.
[0006] However, in the high frequency wiring substrate of the prior
art shown in FIG. 5, as the frequency of the high frequency signal
to be transmitted increases, the length of the signal conducting
through-hole conductor 4 becomes closer to or nearly the same as
the guide wavelength at the signal frequency in the dielectric
substrate 1, resulting in the problem that the degree of impedance
mismatch increases, increasing the reflection of the high frequency
signal; there has also been the problem that since the thickness of
the dielectric layer also becomes close to the guide wavelength at
the signal frequency in the dielectric layer, unwanted mode
conversion to parallel plate mode or waveguide mode occurs within
the dielectric layer, degrading the transmission
characteristics.
[0007] In view of increasing frequency of high frequency signals to
be transmitted, it is desired to realize a transmission line
structure that can transmit high frequency signals between
transmission line conductors on the upper and lower surfaces of the
dielectric substrate without degrading the transmission
characteristics.
SUMMARY OF THE INVENTION
[0008] The present invention has been devised in view of the above
problems of the prior art, and an object of the invention is to
provide a wiring substrate for high frequency applications that has
a transmission line interconnect structure capable of efficiently
transmitting signals in high frequency regions including microwave
and millimeter wave regions, by improving the reflection
characteristics of high frequency signals at transmission line
interconnects between the upper and lower surfaces of a dielectric
substrate and suppressing the occurrence of conversion to an
unwanted mode within the dielectric substrate.
[0009] The present invention provides a wiring substrate for high
frequency applications, comprising: a pair of surface transmission
line conductors formed in line with each other on upper and lower
surfaces of a dielectric substrate composed of a stack of
dielectric layers, each of the surface transmission line conductors
being surrounded by a surface ground conductor; and a transmission
line interconnect structure for transmitting a high frequency
signal between the pair of surface transmission line conductors via
an interlayer transmission line conductor formed between the
dielectric layers in such a manner as to be in line with and
parallel to the pair of surface transmission lines, the interlayer
transmission line conductor being surrounded by an interlayer
ground conductor and connected at both ends thereof to the
respective ends of the pair of surface transmission line conductors
by signal conducting through-hole conductors, wherein in the
transmission line interconnect structure, the surface ground
conductors on the upper and lower surfaces are connected together
via the interlayer ground conductor by top-to-bottom through-hole
ground conductors arranged in two parallel arrays spaced a
prescribed distance apart from each other on both sides of the
interlayer transmission line conductor, while the interlayer ground
conductor is connected to the surface ground conductors on the
upper and lower surfaces by interlayer through-hole ground
conductors arranged in an array at a prescribed pitch a long each
of two sides extending perpendicularly to the interlayer
transmission line conductor; and the thickness of each of the
dielectric layers, representing the spacing between the interlayer
transmission line conductor and the respective surface transmission
line conductors, and the length of a straight line section of the
interlayer transmission line conductor between the signal
conducting through-hole conductors are each set not larger than one
quarter of a signal wavelength of the high frequency signal, while
through-hole ground conductors for connecting the surface ground
conductors on the upper and lower surfaces are formed in a region
where the interlayer ground conductor is not formed between the
interlayer transmission line conductor and the top-to-bottom
through-hole ground conductors arranged parallelly on both sides of
the interlayer transmission line conductor, and are arranged at a
pitch not larger than a prescribed pitch that has the frequency of
the high frequency signal as a cutoff frequency.
[0010] Furthermore, the invention also provides a wiring substrate
for high frequency applications, comprising:
[0011] (a) a dielectric substrate composed of a stack of dielectric
layers;
[0012] (b) a first surface transmission line conductor formed in a
straight line on one surface of the dielectric substrate;
[0013] (c) a second surface transmission line conductor formed in a
straight line on the other surface of the dielectric substrate;
[0014] (d) a first surface ground conductor formed to surround the
first surface transmission line conductor;
[0015] (e) a second surface ground conductor formed to surround the
second surface transmission line conductor; and
[0016] (f) a transmission line interconnect structure for
transmitting a high frequency signal between the first surface
transmission line conductor and the second surface transmission
line conductor, the transmission line interconnect structure
comprising:
[0017] (f1) an interlayer transmission line conductor formed in a
straight line between the dielectric layers;
[0018] (f2) an interlayer ground conductor formed between the
dielectric layers in such a manner as to surround the interlayer
transmission line conductor;
[0019] (f3) signal conducting through-hole conductors for
connecting the first surface transmission line conductor to the
interlayer transmission line conductor, and for connecting the
second surface transmission line conductor to the interlayer
transmission line conductor;
[0020] (f4) first through-hole ground conductors, arranged in two
parallel arrays spaced a prescribed distance apart from each other
on both side of the interlayer transmission line conductor, for
connecting the first surface ground conductor to the second surface
ground conductor via the interlayer ground conductor;
[0021] (f5) second through-hole ground conductors for connecting
the first surface ground conductor to the interlayer ground
conductor, and for connecting the second surface ground conductor
to the interlayer ground conductor, the second through-hole ground
conductors being arranged in an array at a prescribed pitch along
each of two sides extending perpendicularly to the interlayer
transmission line conductor; and
[0022] (f6) third through-hole ground conductors for connecting the
first surface ground conductor to the second surface ground
conductor, the third through-hole ground conductors being formed in
a region where the interlayer ground conductor is not formed
between the interlayer transmission line conductor and the first
through-hole ground conductors arranged parallelly on both sides of
the interlayer transmission line conductor,
[0023] wherein a thickness of each of the dielectric layers,
representing the spacing between the interlayer transmission line
conductor and the first and second surface transmission line
conductors, and a length of a straight line section of the
interlayer transmission line conductor between the signal
conducting through-hole conductors are each set not larger than one
quarter of a signal wavelength of the high frequency signal,
and
[0024] the third through-hole ground conductors are arranged at a
pitch not larger than a prescribed pitch that has the frequency of
the high frequency signal as a cutoff frequency.
[0025] According to the high frequency wiring substrate of the
present invention, since the surface transmission line conductors
on the upper and lower surfaces of the dielectric substrate are
connected together using the above-described transmission line
interconnect structure, the signal conducting through-hole
conductors provided at both ends of the interlayer transmission
line conductor can each be held shorter than the guide wavelength
in the dielectric substrate. Further, since the interlayer ground
conductor, which serves as a ground plane in the high frequency
transmission line structure having the upper (lower) surface
transmission line conductor as the signal conductor, is connected
by the interlayer through-hole ground conductors to the lower
(upper) surface ground conductor which serves as a ground plane in
the high frequency transmission line structure having the
interlayer transmission line conductor as the signal conductor,
ground discontinuities where the structure changes are eliminated,
and therefore, the increase in reflections due to the signal
conducting through-hole conductors and the interlayer transmission
line conductor connecting between the upper and lower surface
transmission line conductors can be held low. Generally, between
discontinued points along a transmission line, there can exist a
standing wave such that the spacing between the discontinued points
becomes equal to an integral multiple of a half wavelength;
however, in the present invention, since the straight line section
of the interlayer transmission line conductor between the signal
conducting through-hole conductors is set not longer than one
quarter of the guide wavelength of the high frequency signal, the
wavelength of the standing wave occurring between the discontinued
points at the signal conducting through-hole conductors connected
to both ends of the interlayer transmission line conductor can be
shifted to a frequency region higher than the signal frequency.
Further, in a rectangular waveguide structure, the cutoff frequency
increases as the length of the longer side of the cross section
decreases; in view of this, the through-hole ground conductors for
connecting the upper and lower surface ground conductors, which are
formed in the region where the interlayer ground conductor is not
formed between the interlayer transmission line conductor and the
top-to-bottom through-hole ground conductors arranged parallelly on
both sides of the interlayer transmission line conductor, are
arranged at a pitch not larger than the prescribed pitch that has
the frequency of the high frequency signal as the cutoff frequency.
As a result, the cutoff frequency of the dielectric waveguide
transmission line structure can also be shifted to a frequency
region higher than the signal frequency, the dielectric waveguide
transmission line structure here comprising: the top-to-bottom
through-hole conductors arranged in two parallel arrays spaced a
prescribed distance apart from each other on both sides of the
interlayer transmission line conductor and extending along the
signal transmission direction thereof in such a manner as to
surround the interlayer transmission line conductor; the upper and
lower surface ground conductors; the interlayer transmission line
conductor; and the through-hole ground conductors for connecting
the upper and lower surface ground conductors, which are formed in
the region where the interlayer ground conductor is not formed
between the interlayer transmission line conductor and the
top-to-bottom through-hole ground conductors arranged parallelly on
both sides of the interlayer transmission line conductor. This
improves the reflection characteristics of high frequency signals
at the transmission line interconnects between the upper and lower
surfaces of the dielectric substrate and suppresses the occurrence
of conversion to an unwanted mode within the dielectric substrate;
as a result, the high frequency wiring substrate thus constructed
has the transmission line interconnect structure capable of
efficiently transmitting signals in high frequency regions
including microwave and millimeter wave regions.
[0026] As described above, according to the high frequency
substrate of the present invention, which comprises a pair of
surface transmission line conductors formed in line with each other
on the upper and lower surfaces of a dielectric substrate, each
surface transmission line conductor being surrounded by a surface
ground conductor, and a transmission line interconnect structure
for transmitting a high frequency signal between the surface
transmission line conductors via an interlayer transmission line
conductor formed between the dielectric layers in such a manner as
to be in line with and parallel to the surface transmission lines,
the interlayer transmission line conductor being surrounded by an
interlayer ground conductor and connected at both ends thereof to
the respective ends of the surface transmission line conductors by
signal conducting through-hole conductors, the transmission line
interconnect structure is characterized in that: the surface ground
conductors on the upper and lower surfaces are connected together
via the interlayer ground conductor by top-to-bottom through-hole
ground conductors arranged in two parallel arrays spaced a
prescribed distance apart from each other on both sides of the
interlayer transmission line conductor, while the interlayer ground
conductor is connected to the surface ground conductors on the
upper and lower surfaces by interlayer through-hole ground
conductors arranged in an array at a prescribed pitch along each of
two sides extending perpendicularly to the interlayer transmission
line conductor; and the thickness of each of the dielectric layers,
representing the spacing between the interlayer transmission line
conductor and the respective surface transmission line conductors,
and the length of the straight line section of the interlayer
transmission line conductor between the signal conducting
through-hole conductors are each set not larger than one quarter of
the signal wavelength of the high frequency signal, while
through-hole ground conductors for connecting the surface ground
conductors on the upper and lower surfaces are formed in a region
where the interlayer ground conductor is not formed between the
interlayer transmission line conductor and the top-to-bottom
through-hole ground conductors arranged parallelly on both sides of
the interlayer transmission line conductor, and are arranged at a
pitch not larger than a prescribed pitch that has the frequency of
the high frequency signal as the cutoff frequency. With the
structure, the increase in the inductance component associated with
the signal conducting through-hole conductors and the interlayer
transmission line conductor connecting between the upper and lower
surface transmission line conductors can be held low. Furthermore,
the wavelength of the standing wave occurring between the
discontinued points at the signal conducting through-hole
conductors connected to both ends of the interlayer transmission
line conductor can be shifted to a frequency region higher than the
signal frequency. At the same time, the cutoff frequency of the
dielectric waveguide transmission line structure formed in such a
manner as to surround the interlayer transmission line conductor
can also be shifted to a frequency region higher than the signal
frequency. This improves the reflection characteristics of high
frequency signals at the transmission line interconnects between
the upper and lower surfaces of the dielectric substrate and
suppresses the occurrence of conversion to an unwanted mode within
the dielectric substrate; as a result, the high frequency wiring
substrate thus constructed has the transmission line interconnect
structure capable of efficiently transmitting signals in high
frequency regions including microwave and millimeter wave
regions.
[0027] In the invention it is preferable that the distance between
the two arrays of top-to-bottom through-hole ground conductors is
set about twice the spacing between the pair of surface
transmission line conductors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Other and further objects, features, and advantages of the
invention will be more explicit from the following detailed
description taken with reference to the drawings wherein:
[0029] FIG. 1 is a perspective view showing one embodiment of a
high frequency wiring substrate according to the present
invention;
[0030] FIG. 2 is an exploded perspective view showing the one
embodiment of a high frequency wiring substrate according to the
present invention;
[0031] FIG. 3 is a cross sectional view showing the one embodiment
of a high frequency wiring substrate according to the present
invention;
[0032] FIG. 4 is a diagram showing the reflection coefficient
versus frequency characteristics of the high frequency wiring
boards of the embodiment and a comparative example; and
[0033] FIG. 5 is a perspective view showing one example of a prior
art high frequency wiring board.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Now referring to the drawings, preferred embodiments of the
invention are described below.
[0035] FIG. 1 is a perspective view showing one embodiment of a
high frequency wiring substrate according to the present invention.
FIG. 2 is an exploded perspective view of the high frequency wiring
substrate of FIG. 1, and FIG. 3 is a cross sectional view taken
along a direction parallel to the transmission line conductors
formed on the high frequency wiring substrate of FIG. 1. In the
perspective view of FIG. 1, the interior of the high frequency
wiring substrate is shown by dotted lines.
[0036] In the above figures, the high frequency wiring substrate
comprises a dielectric substrate 11, surface transmission line
conductors 12 and 13 as first and second surface transmission line
conductors, an interlayer transmission line conductor 14, surface
ground conductors 15 and 16 as first and second surface ground
conductors, and a transmission line interconnect structure. The
dielectric substrate 11 is composed of a stack of dielectric layers
11a, 11b (in this embodiment, two layers) In other words, the
dielectric layers 11a and 11b together constitute the dielectric
substrate 11. The surface transmission line conductors 12 and 13
are formed in line with each other, one on one surface, i.e., the
upper surface, of the dielectric substrate 11 and the other on the
other surface, i.e., the lower surface. The interlayer transmission
line conductor 14 is formed between the dielectric layers 11a and
11b in such a manner to be in line with and parallel to the surface
transmission line conductors 12 and 13. The surface ground
conductors 15 and 16 are formed in such a manner as to surround the
respective surface transmission line conductors 12 and 13 on the
upper and lower surfaces of the dielectric substrate 11.
[0037] The transmission line interconnect structure comprises an
interlayer ground conductor 17, signal conducting through-hole
conductors 18 and 19, top-to-bottom through-hole ground conductors
20 as first through-hole ground conductors, interlayer through-hole
ground conductors 21 and 22 as second through-hole ground
conductors, and through-hole ground conductors 24 as third
through-hole ground conductors. The interlayer ground conductor 17
is formed between the dielectric layers 11a and 11b in such a
manner as to surround the interlayer transmission line conductor
14. Therefore, a microstrip transmission line is formed by the
surface transmission line conductor 12 and interlayer ground
conductor 17 with the dielectric layer 11a as its base. Likewise, a
microstrip transmission line is formed by the surface transmission
line conductor 13 and interlayer ground conductor 17 with the
dielectric layer 11b as its base. A strip transmission line
(triplate transmission line) is formed by the interlayer line
conductor 14 and the surface ground conductors 15 and 16 with the
dielectric layers 11a and 11b as its base. The signal conducting
through-hole conductors 18 and 19, the top-to-bottom through-hole
ground conductors 20, the interlayer through-hole ground conductors
21 and 22, and the through-hole ground conductors 24 will be
described later.
[0038] In the example shown in FIGS. 1 to 3, the surface ground
conductors 15 and 16 extend parallelly on both sides of the
respective surface transmission line conductors 12 and 13 in such a
manner as to surround the respective transmission line conductors,
but this does not necessarily mean a coplanar transmission line
structure. That is, if the gap between the surface transmission
line conductor 12, 13 and the surface ground conductor 15, 16 is
essentially wider than the transmission line width, the structure
can be regarded as a microstrip transmission line. FIG. 1 shows an
example of the structure that can be regarded as a microstrip
transmission line.
[0039] Further, since the distances in the signal transmission
direction from the surface transmission line conductor 12 to the
surface ground conductor 15, from the surface transmission line
conductor 13 to the surface ground conductors 16 and from the
interlayer transmission line conductor 14 to the interlayer ground
conductor 17 correspond to distances of portions of their
associated ground planes from which portions respective ground
conductors are removed, in the microstrip transmission lines
(surface transmission line conductor 12+interlayer ground conductor
17 and surface transmission line conductor 13+interlayer ground
conductor 17) and the strip transmission line (interlayer
transmission line conductor 14+surface ground conductors 15 and
16), respectively, it is desirable that the distances be made as
small as the manufacturing process can allow.
[0040] The signal conducting through-hole conductors 18 and 19
connect the end of the surface transmission line conductor 12 to
one end of the interlayer transmission line conductor 14 and the
end of the surface transmission line conductor 13 to the other end
of the interlayer transmission line conductor 14.
[0041] Here, to prevent positional displacements of the signal
conducting through-hole conductors 18 and 19 due to layer
registration errors, land portions 14a and 14b, substantially
circular in shape, are formed at both ends of the interlayer
transmission line 14. The diameter of the land portions 14a and 14b
is usually not equal to the width of the straight line section of
the interlayer transmission line conductor 14; considering this,
the entire length of the interlayer transmission line conductor 14
minus the length equal to the diameters of the land portions 14a
and 14b is defined as the length of the straight line section of
the interlayer transmission line conductor 14. The length of the
straight line section is set not greater than one quarter of the
signal wavelength of the high frequency signal, and the thickness
of each of the dielectric layers 11a and 11b between the interlayer
transmission line conductor 14 and the upper and lower surface
transmission line conductors 12 and 13, that is, the length of each
of the signal conducting through-hole conductors 18 and 19
connecting between the interlayer transmission line conductor 14
and the respective surface transmission line conductors 12 and 13,
is also set not greater than one quarter of the signal wavelength
of the high frequency signal. With this structure, the length of
each of the signal conducting through-hole conductors 18 and 19
provided at the respective ends of the interlayer transmission line
conductor 14 can be held shorter than the guide wavelength in the
dielectric substrate 11. Further, since the interlayer ground
conductor 17, which corresponds to the ground plane in the high
frequency transmission line structure having the upper (lower)
transmission line conductor 12 (13) as the signal conductor, is
connected by the interlayer through-hole ground conductors 21 (22)
to the lower (upper) surface ground conductor 13 (12), which
corresponds to the ground plane in the high frequency transmission
line structure having the interlayer transmission line conductor 14
as the signal conductor, ground discontinuities where the high
frequency transmission line structure changes are eliminated. This
serves to alleviate the effects caused by discontinuities in the
characteristic impedance of the high frequency transmission line at
transmission line interconnects constructed of the microstrip
transmission line/strip transmission line/microstrip transmission
line structure.
[0042] The top-to-bottom through-hole ground conductors 20 are
arranged in two parallel arrays spaced a prescribed distance apart
from each other on both sides of the interlayer transmission line
conductor 14, and connect the upper and lower surface ground
conductors 15 and 16 via the interlayer ground conductor 17. The
interlayer through-hole ground conductors 21 and 22 are arranged in
an array at a prescribed pitch along each of two sides extending
perpendicularly to the interlayer transmission line conductor 14,
and connect the upper surface ground conductor 15 to the interlayer
ground conductor 17 and the lower surface ground conductor 16 to
the interlayer ground conductor 17.
[0043] More specifically, the interlayer through-hole ground
conductors 21 are formed passing through the dielectric layer 11b
in its thickness direction and arranged at a prescribed pitch in an
array on one side extending perpendicularly to the interlayer
transmission line conductor 14. On the other hand, the interlayer
through-hole ground conductors 22 are formed passing through the
dielectric layer 11a in its thickness direction and arranged at a
prescribed pitch in an array on the other side extending
perpendicularly to the interlayer transmission line conductor
14.
[0044] The top-to-bottom through-hole ground conductors 20 and the
interlayer through-hole ground conductors 21 and 22 should in their
respective arrays be formed as closely spaced and as many as the
manufacturing process can allow, because the high frequency
grounding at the respective portions can then be reinforced. For
example, the through-hole conductors in each top-to-bottom
through-hole ground conductor array 20 and in each interlayer
through-hole ground conductor array 21, 22 should be arranged as
closely spaced as possible at a pitch smaller than one half of the
signal wavelength of the high frequency signal. This pitch need not
necessarily be fixed, but may be made variable as long as the pitch
is held smaller than one half of the signal wavelength of the high
frequency signal. Further, the distance, that is, the spacing,
between the two top-to-bottom through-hole ground conductor arrays
20 should preferably be set about twice the spacing between the
upper and lower surface transmission line conductors 12 and 13. In
that case, the top-to-bottom through-hole ground conductors 20 and
the surface ground conductors 15 and 16 together form a dielectric
waveguide transmission line structure surrounding the interlayer
transmission line conductor 14, and the structure effectively acts
as an electromagnetic shield.
[0045] In the high frequency wiring substrate of the invention, it
is important that the through-hole ground conductors 24 connecting
between the upper surface ground conductor 15 and the lower surface
ground conductor 16 be arranged at a pitch not larger than the
spacing that has the frequency of the high frequency signal as the
cutoff frequency, in a region 23 where the interlayer ground
conductor 17 is not formed between the top-to-bottom through-hole
ground conductor arrays 20 arranged parallelly on both sides of the
interlayer transmission line conductor 14.
[0046] The pitch at which the through-hole ground conductors 24 are
arranged is not larger than the spacing that has the frequency of
the high frequency signal as the cutoff frequency. For example,
when two dielectric layers each having a thickness of 0.2 mm and a
dielectric constant of 8.8 is stacked one on top of the other, then
the structure constructed from such a stack can be regarded as a
dielectric waveguide whose spacing in the vertical direction is 0.4
mm, and when the spacing in the horizontal direction relative to
the direction of transmission is 0.9 mm, 0.8 mm, and 0.7 mm, then
the cutoff frequency is about 56.2 GHz, 63.2 GHz, and 72.2 GHz,
respectively. When designing the structure with the signal
frequency upper limit at 66 GHz, the through-hole ground conductors
24 are arranged at a pitch of 0.7 mm. This pitch should be made as
small as possible in order to suppress the waveguide mode. However,
if the pitch is made too small, it will affect the impedance of the
high frequency transmission line structure having the interlayer
transmission line conductor 14 as the signal conductor; therefore,
the through-hole ground conductors 24 should be arranged so that
their lands 25 (to be described later) are each spaced from the
interlayer transmission line conductor 14 by a distance not smaller
than the thickness of one dielectric layer (in the above example,
0.2 mm). Further, the through-hole ground conductors 24 to be
formed at positions parallel to the interlayer transmission line
conductor 14 should be arranged at positions substantially
coinciding with the midpoint of the straight line section of the
interlayer transmission line conductor 14 so that the waveguide
mode suppression effect can be provided equally at both ends of the
interlayer transmission line conductor 14.
[0047] Further, the through-hole ground conductors 24 and the two
arrays of top-to-bottom through-hole ground conductor 20 need not
necessarily be separated by an equal distance from the respective
sides of the interlayer transmission line conductor 14, and one may
be arranged closer to the interlayer transmission line conductor 14
than the other is, provided that the arrangement does not affect
other requirements, for example, the impedance of the high
frequency transmission line structure having the interlayer
transmission line conductor 14 as the signal conductor; however,
usually they should be arranged symmetrically on both sides of the
interlayer transmission line conductor 14.
[0048] Moreover, more than one through-hole ground conductor 24 may
be arranged on each side of the interlayer transmission line
conductor 14; in that case, the through-hole ground conductors 24
should preferably be arranged avoiding the positions on both sides
of the signal conducting through-hole conductors 18 and 19 so as
not to affect the impedance design at the microstrip transmission
line/strip transmission line conversion point.
[0049] The lands 25 substantially circular in shape are formed in
the region where the interlayer ground conductor 17 is not formed.
The lands 25 are formed between the upper dielectric layer 11a and
the lower dielectric layer 11b to prevent positional displacements
of the through-hole ground conductors 24 due to layer registration
errors.
[0050] According to the high frequency wiring substrate of the
invention, by arranging the through-hole ground conductors 24 as
describe above, a dielectric waveguide transmission line structure
having a thickness equal to the combined thickness of the
dielectric layers 11a and 11b is formed for the interlayer
transmission line conductor 14 which serves as an interconnect
between the upper surface transmission line conductor 12 and the
lower surface transmission line conductor 13. The cutoff frequency,
that is, the lower limit of the frequency that can be propagated in
the waveguide mode through the interior of the waveguide structure,
is defined by the width of the waveguide. When the through-hole
ground conductors 24 are arranged on both sides of the interlayer
transmission line conductor 14 at a pitch not larger than the
spacing that has as the cutoff frequency the frequency of the high
frequency signal transmitted through the transmission line
interconnect structure, that is, at a pitch having a higher cutoff
frequency, the cutoff frequency for the waveguide mode, an unwanted
mode, can be made higher without affecting the transmission in the
TEM mode, i.e., the transmission mode in the high frequency
transmission line structure having the interlayer transmission line
conductor 14 as the signal conductor. This makes it possible to
suppress the occurrence of the waveguide mode in the strip
transmission line section consisting of the interlayer transmission
line conductor 14 and surface ground conductors 15 and 16 and
having as its base the dielectric substrate 11 formed from the
dielectric layers 11a and 11b. As a result, it becomes possible to
suppress the occurrence of mode conversion from the TEM mode, the
desired mode, to the waveguide mode, an unwanted mode, within the
dielectric substrate 11. This achieves efficient transmission in
the TEM mode which is the desired mode.
[0051] [Embodiments]
[0052] Next, the high frequency wiring substrate of the invention
will be described by way of example.
[0053] First, the dielectric substrate 11 was fabricated by
stacking two dielectric layers 11a and 11b each made of alumina
ceramics having a dielectric constant of 8.8 and a thickness of 0.2
mm. Microstrip transmission lines (each with a strip line width of
0.22 mm) as the surface transmission line conductors 12 and 13 were
formed in line with each other on the upper and lower surfaces,
respectively. Each microstrip transmission line was formed at its
end with a land portion 12a or 13a (with a diameter of 0.15 mm) for
connecting thereto a signal conducting via (with a diameter 0.1 mm)
formed as the signal conducting through-hole conductor 18 or 19.
The signal conducting vias connected to the respective land
portions 12a and 13a were formed passing through the respective
dielectric layers 11a and 11b, and connected to the interlayer
transmission line conductor 14, a strip transmission line (with a
strip line width of 0.1 mm), formed between the dielectric layers
11a and 11b and having signal conducting via connecting land
portions 14a and 14b (each with a diameter of 0.15 mm) at both ends
thereof. The signal conducting vias were spaced apart from each
other by 0.5 mm in the signal propagation direction.
[0054] The surface ground conductors 15 and 16 and the interlayer
ground conductor 17 were formed in such a manner as to surround the
surface transmission line conductors 12 and 13 formed on the upper
and lower surfaces of the dielectric substrate 11 and the
interlayer transmission line conductor 14 formed between the
layers, respectively, each ground conductor being separated from
the end of its associated transmission line conductor by a distance
of 0.15 mm in the signal propagation direction. The surface ground
conductors 15 and 16 were connected to the interlayer ground
conductor 17 by ground vias, i.e., the through-hole ground
conductors 20, 21, and 22 formed at a pitch of 0.25 mm. Further,
ground vias as the through-hole ground conductors 24 were formed at
a pitch of 0.25 mm parallelly on both sides of the interlayer
transmission line conductor 14 to connect the upper and lower
surface ground conductors 12 and 13, the ground vias being located
at a distance of 0.6 mm from the centerline of the layer
transmission line conductor 14. The high frequency wiring substrate
A of the invention was fabricated with the above configuration.
[0055] As a comparative example, using the same material and layer
structure as described above, grounding coplanar transmission lines
(each with a strip line width of 0.22 mm and a stripline to ground
conductor gap of 0.14 mm) were formed as the upper and lower
surface transmission line conductors; the surface transmission line
conductors were connected by signal conducting vias, i.e, the
signal conducting through-hole conductors formed through the
respective dielectric layers, and the upper and lower surface
ground conductors were connected by ground vias, i.e., the
top-to-bottom through-hole ground conductors formed through the
respective dielectric layers, to fabricate the high frequency
wiring substrate B of the prior art configuration.
[0056] Next, S parameters, associated with the high frequency
transmission from the surface transmission line conductor formed on
the upper surface of the multilayer substrate to the lower surface
transmission line conductor via the signal conducting vias and the
interlayer transmission line conductor, were measured on the high
frequency wiring substrates A and B by using a network analyzer,
and S11 as the reflection coefficient of each substrate was
obtained. The reflection coefficient versus frequency
characteristics are shown in FIG. 4.
[0057] FIG. 4 is a diagram showing the reflection coefficient
versus frequency characteristics of the respective high frequency
wiring substrates. The frequency (unit: GHz) is plotted along the
horizontal axis, and the reflection coefficient S11 (unit: dB)
along the vertical axis. Of the characteristic curves, the solid
curve represents the results of the measurements made on the high
frequency wiring substrate A of the present embodiment, and the
dashed line shows the results of the high frequency wiring
substrate B of the comparative example. As shown in FIG. 4, the
rate of increase of the reflection coefficient S11 with increasing
frequency is held lower in the high frequency wiring substrate A of
the embodiment than in the high frequency wiring substrate B of the
comparative example, achieving good reflection characteristics and
good high frequency transmission characteristics with the
reflection coefficient S11 held low in the high frequency
regions.
[0058] As a result, in the high frequency wiring substrate of the
invention, the increase in the inductance component associated with
the signal conducting through-hole conductors 18 and 19 connecting
the surface transmission line conductors 12 and 13 to the
interlayer transmission line conductor 14 can be held low.
Furthermore, the wavelength of the standing wave occurring between
the discontinued points at the signal conducting through-hole
conductors 18 and 19 connected to both ends of the interlayer
transmission line conductor 14 can be shifted to a frequency region
higher than the frequency of the high frequency signal. Moreover,
the cutoff frequency in the dielectric waveguide transmission line
structure, formed by the two top-to-bottom through-hole ground
conductor arrays 20 arranged parallelly on both sides of the
interlayer transmission line conductor 14 and the surface ground
conductors 12 and 13 connected by the ground conductor arrays 20,
can also be shifted to a frequency region higher than the frequency
of the high frequency signal. It has therefore been confirmed that
the high frequency wiring substrate of the invention achieves the
transmission line interconnect structure having good high frequency
transmission characteristics.
[0059] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description and all changes which come within the meaning
and the range of equivalency of the claims are therefore intended
to be embraced therein.
* * * * *