High frequency signal transmission line structure having shielding conductor unit

A high frequency transmission line structure has a reference potential plane conductor layer, a plurality of strip line conductors, a dielectric material layer interposed between the reference potential plane conductor layer and the strip line conductors and a shielding conductor unit provided between adjacent two strip line conductors. The shielding conductor unit includes first and second slender conductor portions extending substantially in a direction parallel with a lengthwise direction of the strip line conductors and connected to be integral with each other at their first ends. The second ends of the first and second slender conductor portions being electrically connected with the reference potential plane conductor layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a wiring technique for various electronic 
components such as IC device packages, wiring circuit boards, and 
semiconductor chips, and in particular to a technique which is effective 
in application of transmission of high frequency signals. 
A multi-layer wiring circuit board aiming at reduction of crosstalk noises 
caused between signal conductors is disclosed in JP-A-63-155791 (laid-open 
on Jun. 28, 1988). In the above described multi-layer wiring circuit 
board, it is attempted to reduce crosstalk noises caused by signal 
propagation by forming ground conductors on both sides of a signal 
conductor sandwiched between upper and lower ground plane conductor 
layers, connecting electrically the above described ground conductors to 
the upper and lower ground plane conductor layers, and thus shielding each 
signal conductor with these ground conductors and ground plane conductor 
layers. 
SUMMARY OF THE INVENTION 
In the above described JP-A-63-155791, however, there is not described 
resonance caused between a signal conductor and a ground conductor by 
mutual inductance, which is formed between the signal conductor and the 
ground conductor, and self-inductance of the signal conductor and the 
ground conductor. As a result of a study made by the present inventor, it 
has been found that a high signal frequency ranging nearly from several 
GHz to several tens GHz degrades waveform or disabled transmission caused 
by the above described mutual inductance and/or self-inductance. 
An object of the present invention is to provide a high frequency signal 
transmission line structure suitable for transmission of high frequency 
signals. 
Another object of the present invention is to provide a packaged 
semiconductor device including the above described high frequency signal 
transmission line structure. 
Another object of the present invention is to provide a wiring circuit 
board including the above described high frequency signal transmission 
line structure. 
According to one aspect of the present invention, a high frequency signal 
transmission line structure has a reference potential plane conductor 
layer, a plurality of strip line conductors, a dielectric material layer 
interposed between the reference potential plane conductor layer and the 
strip line conductors and a shielding conductor unit provided between 
adjacent two strip line conductors. The shielding conductor unit includes 
first and second slender conductor portions extending substantially in a 
direction parallel with a lengthwise direction of the strip line 
conductors and connected to be integral with each other at their first 
ends. The second ends of the first and second slender conductor portions 
are electrically connected with the reference potential plane conductor 
layer. 
In the above described configuration, the shielding conductor unit 
interposed between adjacent strip line conductors includes a pair of 
slender conductor portions having first ends connected with each other and 
second ends respectively connected with the reference potential plane 
conductor layer. Therefore, the electromagnetic induction current induced 
in the shielding conductor unit by a signal current flowing through one 
signal conductor, i.e., through one strip line conductor has opposite 
senses in one conductor portion and the other conductor portion included 
in the shielding conductor unit. With respect to the direction of flow of 
the signal current, the electromagnetic induction current has a forward 
direction and a backward direction in one conductor portion and the other 
conductor portion, respectively. As a result, the self-inductance of the 
shielding conductor unit and the mutual inductance between the strip line 
conductors and the shielding conductor unit become small because 
respective inductance components cancel each other. Accordingly, the 
frequency f.sub.0 of resonance caused between a strip line conductor and 
the shielding conductor unit becomes large in accordance with the 
following expression. 
##EQU1## 
In this expression, L represents mutual inductance between the strip line 
conductor and the shielding conductor unit or self-inductance of each of 
them, and C represents capacitance between conductors. 
Therefore, it becomes possible to suppress resonance occurring between 
strip line conductors and the shielding conductor unit by shifting the 
above described resonance frequency to the outside of the frequency band 
of signal current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereafter be described in detail by referring to 
embodiments. In all diagrams for describing embodiments, components having 
the same functions are denoted by like numerals and will not be described 
repetitively. 
A packaged semiconductor device including a high frequency signal 
transmission line structure according to an embodiment of the present 
invention is shown in FIGS. 1 to 4. FIG. 1 is an oblique view of principal 
parts of the package and a semiconductor chip; FIG. 2 is a top view of the 
package; FIG. 3 is a sectional view seen along a line III--III shown in 
FIG. 2; and FIG. 4 is a top view showing the arrangement of conductors in 
the package. 
An IC package 1 in the present embodiment is a so-called ceramic package. 
The package main body includes a dielectric substrate 2 made from a 
dielectric material layer and an opening therein, a frame-shaped 
dielectric layer, i.e., a frame member 3, a cap plate 4, and a base plate 
5 functioning as a reference potential plane conductor layer for providing 
a reference potential (GND) plane. The above described dielectric 
substrate 2, frame-shaped dielectric layer 3, and cap plate 4 are made 
from ceramics such as alumina, mullite, or aluminum nitride (AlN). The 
base plate 5 is made from electric conductors such as Kovar, 42 alloy, or 
mixture of copper powder and tungsten powder. Within a cavity 6 surrounded 
by the dielectric substrate 2, the frame-shaped dielectric layer 3, the 
cap plate 4 and the base plate 5, a semiconductor chip 7 made from GaAs 
(gallium arsenide) having a logical integrated circuit performing 
switching operation at an ultrahigh speed, for example, is mounted. The 
semiconductor chip 7 is joined to the upper face of the base plate 5 by a 
soldering material 8 such as Au-Sn eutectic alloy. 
On the principal face of the dielectric substrate 2, a predetermined number 
of signal conductors, i.e., strip line conductors 9 are formed by means of 
thick film printing of metal having a high melting point such as W 
(tungsten). These conductors 9 have a characteristic impedance value 
(50.OMEGA., for example) equivalent to the impedance value of an external 
signal source for driving the logical integrated circuit included in the 
semiconductor chip 7. The strip line conductors 9 form so-called 
microstrip lines in conjunction with the dielectric substrate 2 having a 
predetermined dielectric constant value and the base plate, i.e., the 
reference potential plane conductor layer 5 joined to the bottom face of 
the dielectric substrate 2. First ends of the strip line conductors 9 are 
electrically connected with the semiconductor chip 7 via bonding wires 10 
made from conductors such as Au. External leads 11 made from conductors 
such as Kovar or 42 alloy are brazed to second ends of the conductors 9 by 
soldering. 
On the surface of the substrate 2 and between adjacent strip line 
conductors 9, a shielding conductor unit 12 extending substantially in a 
direction parallel with the lengthwise direction of the conductors is 
formed. One end of the shielding conductor unit is electrically connected 
with the base plate (reference potential plane) 5. That is to say, it is 
attempted in the packaged semiconductor device of the present embodiment 
to reduce crosstalk noise at the time of signal propagation by shielding 
respective strip line conductors 9 with the shielding conductor unit 12 
and the base plate (reference potential plane conductor layer) 5. The 
shielding conductor unit 12 may be made from the same conductive material 
and formed in the same production process as the strip line conductors, 
for example. As understood from the foregoing description, the frame 
member 3 is formed on the strip line conductors 9, the shielding conductor 
unit 12, and the substrate 2. The cap plate 4 is joined to the upper face 
of the frame member 3 via a soldering material 4' which is, for example, a 
conductive material. As evident from FIG. 3, the cap plate 4 covers a 
substantial part of the base plate 5 and contributes to the formation of a 
cavity. In order to enhance the above described crosstalk noise reduction 
effect, an alternative configuration may be adopted. In this alternative 
configuration, a second reference potential plane is provided above the 
layer 2 (and within or above the frame member 3, for example) and this 
reference potential plane is electrically connected with the above 
described shielding conductor unit. For example, the above described 
structure can be achieved by providing the layer of the soldering material 
4' with the reference potential (such as the ground potential). Or the cap 
plate 4 may be made of a conductive plate such as a metallic plate and 
provided with the reference potential. 
As shown in FIG. 1, the shielding conductor unit 12 has a hairpin-shaped or 
U-shaped pattern including a pair of slender conductor portions G.sub.1 
and G.sub.2 connected with each other at their first ends and connected 
with the reference potential plane conductor layer 5 at their second ends. 
When a signal current flows through a strip line conductor 9, therefore, 
an electromagnetic induction current flows through the shielding conductor 
unit on both sides of the strip line conductor. As shown in FIG. 5, the 
sense of the electromagnetic induction current in the first conductor 
portion (G.sub.1) is opposite to that in the second conductor portion 
(G.sub.2). With respect to the direction of the flow of the signal 
current, the electromagnetic induction current has a forward direction and 
a backward direction in the first conductor portion (G.sub.1) and the 
second conductor portion (G.sub.2), respectively. As a result, inductance 
of the first conductor portion (G.sub.1) and inductance of the second 
conductor portion (G.sub.2) cancel each other. Therefore, self-inductance 
of the shielding conductor unit 12 becomes smaller than the 
self-inductance of the case where the shielding conductor unit 12 is 
formed by a single rectilinear conductor. Mutual inductance between the 
strip line conductor 9 and the shielding conductor unit 12 also becomes 
smaller than that of the case where the shielding conductor unit 12 is 
formed by a single rectilinear conductor because respective inductance 
components cancel each other. 
The following table shows simulation values of self-inductance (L.sub.S) of 
the strip line conductor 9, self-inductance (L.sub.G) of the shielding 
conductor unit 12, and mutual inductance (M.sub.S-G) between the strip 
line conductor 9 and the shielding conductor unit 12 in case of a 
transmission line structure (the present embodiment) having the shielding 
conductor unit including a pair of conductor components G.sub.1 and 
G.sub.2 as described in the present embodiment between adjacent strip line 
conductors and a transmission line structure (prior art) having a single 
rectilinear conductor. It is assumed that the strip line conductors 9, the 
shielding conductor unit 12, and the reference potential plane conductor 
layer 5 have dimensions as shown in FIG. 6 (for the present embodiment) 
and FIG. 7 (for the prior art). 
TABLE 
______________________________________ 
PRESENT 
EMBODIMENT PRIOR ART 
______________________________________ 
L.sub.S 2.20 nH 2.22 nH 
L.sub.G 7.99 .times. 10.sup.-2 nH 
2.23 nH 
M.sub.S-G -5.92 .times. 10.sup.-3 nH 
-1.12 nH 
f.sub.0 
FOR L.sub.G 32.5 GHz 6.15 GHz 
FOR M.sub.S-G 
119.4 GHz 8.68 GHz 
______________________________________ 
As evident from this table, the self-inductance (L.sub.G) of the shielding 
conductor unit 12 could be reduced in the package of the present 
embodiment to one-twenty-eighth of that of the prior art and the mutual 
inductance (M.sub.S-G) between the strip line conductor 9 and the 
shielding conductor unit 12 could be reduced to one-hundred-eighty-ninth 
as compared with the prior art. The frequency (f.sub.0) of resonance 
caused between the strip line conductor 9 and the shielding conductor unit 
12 is expressed by 
##EQU2## 
where L represents self-inductance or mutual inductance of the conductor 
9, 12 or 12', and C represents capacitance between the conductors 9 and 12 
or 12'. As expressed by this equation, the resonance frequency f.sub.0 is 
in inverse proportion to the self-inductance (or mutual inductance). In 
packages according to the present embodiment, therefore, the above 
described resonance frequency f.sub.0 can be made larger than that of the 
prior art. By shifting the above described resonance frequency f.sub.0 to 
the outside of the frequency band (such as 30 GHz or less, for example) of 
the signal current flowing through the strip line conductor 9, therefore, 
resonance between the strip line conductor 9 and the shielding conductor 
unit 12 can be suppressed. In calculating f.sub.0 of the above described 
table, it was assumed that C was 0.3 pF. 
With reference to FIG. 4, the shielding conductor unit denoted by reference 
numeral 12e need not necessarily be provided. However, it may be provided 
to make adjustment so that a strip line conductor 9 adjacent to the 
shielding conductor unit 12e may have the same shape and characteristic 
impedance as other strip line conductors. 
As heretofore described, shielding conductor units each including a pair of 
slender conductor portions connected with each other at their first ends 
and connected to the reference potential at their second ends are disposed 
along strip line conductors in the packaged semiconductor device of the 
present embodiment. In such a packaged semiconductor device of the present 
embodiment, the following effects can be obtained. 
(1) Since resonance between the strip line conductor 9 and the shielding 
conductor unit 12 can be suppressed, signal waveform degradation and 
transmission disablement can be reduced. 
(2) Since the strip line conductor 9 is shielded with the shielding 
conductor units 12 and the reference potential plane conductor layer 5, 
crosstalk noise caused by signal propagation can be reduced. 
(3) As a result of the above described (1) and (2), a packaged 
semiconductor device suitable for transmission of high frequency signals 
ranging from several GHz to several tens GHz can be provided. 
Heretofore, the invention made by the present inventor has been concretely 
described by referring to the embodiment. However, it is a matter of 
course that the present invention is not limited to the above described 
embodiment, but various changes can be made without departing from the 
spirit of the present invention. 
Although the shielding conductor unit of the above described embodiment has 
a hairpin-shaped (U-shaped) pattern, it may have a looped pattern 22, for 
example, as shown in FIG. 8. 
The shielding conductor unit of the above described embodiment is disposed 
between a plurality of strip line conductors juxtaposed substantially on 
the plane (principal face of the circuit board) parallel with the surface 
of the reference potential plan conductor layer 5. As shown in FIG. 9A, 
however, the shielding conductor unit 32 may be disposed at a level (such 
as a level of an internal layer of the circuit board 2) which is different 
from that of the strip line conductors 9. 
FIG. 9B shows an embodiment in which the arrangement shown in FIG. 9A is 
applied to a multi-layer strip line structure. In FIG. 9B, the lower strip 
line conductor 39 is provided with a reference potential plane conductor 
layer 5 via the dielectric layer 2, while the upper strip line conductor 
39 is provided with a reference potential plane conductor layer 5' via the 
dielectric layer 2". The reference potential plane conductor layers 5 and 
5' are supplied with a reference potential, for example, the ground 
potential. The shielding conductor unit 32 is provided on the insulating 
layer 2 (i.e., in the insulating layer 2') is electrically connected, for 
example, with the reference potential plane conductor layer 5 so as to 
effect shielding between the lower and upper strip line conductors 39. 
As shown in FIG. 10, shielding conductor units 42 may be formed in two 
layers by making connections via through-holes 43 opened through the 
circuit board or the frame member. 
FIG. 11 shows an example of application of the arrangement shown in FIG. 9A 
to wiring board including a transmission line structure in which strip 
line conductors 39 are arranged to be laminated on the surface of the 
reference potential plane conductor layer 50 via an insulation layer 2'. 
That is to say, a shielding conductor unit 32 is disposed between two 
laminated strip line conductors adjacent in the direction of lamination. 
The layer 50 may be made of a Cu-W alloy, for example. 
Description has heretofore been given mainly by referring to the case where 
the present invention is applied to a signal transmission line in a 
package. However, the present invention is not limited to such a case. For 
example, the present invention can be applied to internal conductors of a 
semiconductor chip having a logical integrated circuit performing 
switching operation at an ultrahigh speed and a wiring structure of 
various electronic components such as a wiring circuit board for high 
frequency signal transmission. For example, it is possible to provide a 
high frequency signal transmission wiring circuit board by forming the 
structure shown in FIG. 1 or 9 on an insulative support plate.