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(11) | EP 0 977 300 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Compact transmission lines |
| (57) Compact transmission lines 101, 102 have non-uniform conductive geometries including
tabs 111 and notches 112. The tabs 111 and notches 112 increase the capacitance of
the lines 101, 102 without increasing the inductance. This creates a delay in the
signal transmitted through the lines 101, 102. |
[0002] The known structure found in Figure 1(a) (cross-sectional view) and 1(b) (top view) can be used as a transmission line (or as a waveguide). It has one or more metal lines 101 carrying an AC current, or signal, and one or more metal lines 102 carrying the return signal with equal and opposite current. The current direction is indicated by the arrows 105 which are depicted as arrow heads and tails to indicate incoming and outgoing current direction in Figure 1a and lateral directions in Figure 1(b). Which lines we designate as the "signal" line or as the "return" line is arbitrary since the current direction switches every half-cycle. Because a typical transmission line is uniform along its length, a three-dimensional transmission line can normally be described as a two-dimensional geometry extruded in the length dimension. In that case the following relations hold and are independent of the two-dimensional cross-section shown in Figure 1(b):
For a lossless transmission line (which is approximately the case in most situations), the following relations also holds:
where C and L are the capacitance and inductance per length of the transmission line. The variable c is the speed of light in the medium and is the speed of a signal sent through the transmission line.
[0003] For metal lines on an integrated circuit primarily embedded in silicon dioxide, the dielectric constant ε = 4.0e0, and c= 1.50e10 cm/s. If the signal frequency is 5GHz then the wavelength is 3 cm. The above relationship shows that, if the capacitance can be increased without changing the inductance, the speed of the signal will be reduced. However, in the past, any change in capacitance was offset by a corresponding change in inductance such that both c and the wavelength are unchanged.
[0004] Consider the case of Fig. 1. If the lines move closer to each other, the capacitance between the lines carrying the signal and the return signal will be increased. However, by allowing the two current paths to be closer together, their contributions to the magnetic field (since they are in opposite directions) cancel more exactly which reduces the total magnetic field strength and therefore reduces the inductance per length of the transmission line. The inductance per length is reduced by the same factor that the capacitance per length is increased - as predicted assuming c is unchanged in Eq. 2.
[0005] The present invention includes a transmission line structure comprising a signal line comprising one or more parallel conducting paths capable of carrying an alternating current along its length, a return signal line comprising one or more conducting paths parallel to the signal line and capable of carrying an alternating current in the direction opposite to that of the signal line, an insulating medium in which the return and signal lines are embedded through which direct current cannot pass from the signal line to the return line, non-uniform conductive means coupled to the signal and return lines for increasing the effective capacitance between the signal and return lines without significantly reducing the overall inductance of the transmission line, and a semiconductor substrate and the insulating medium is disposed over the semiconductor substrate.
[0006] The invention also includes an integrated circuit comprising a transmission line structure comprising: a semiconductor substrate, a signal line over the substrate and comprising one or more parallel conducting paths capable of carrying an alternating current along its length, a return signal line over the substrate and comprising one or more conducting paths parallel to the signal line and capable of carrying an alternating current in the direction opposite to that of the signal line, an insulating medium in which the return and signal lines are embedded through which direct current cannot pass from the signal line to the return line, non-uniform conductive means coupled to the signal and return lines for increasing the effective capacitance between the signal and return lines without significantly reducing the overall inductance of the transmission line, in which the substrate comprises the insulating material.
[0007] Conveniently, by departing from the uniform-along-the-length geometry, C can be increased without decreasing L. The structure in Fig. 2 accomplishes this by the pattern of alternate tabs 111 and notches 112 on the two inside edges of the transmission line metal. The average distance for capacitive coupling is greatly decreased by the presence of the tabs, which raises C. Yet, because the notches 112 prevent current flow in the longitudinal direction, the current paths of the signal and return signal are forced to remain on the outside edges of the two conductors as if the tabs were never added. Since the current paths are essentially unchanged from the untabbed geometry, the magnetic fields are unchanged and so is L. With this structure Equation 1 does not apply any more although Equation 2 does but with a lower value of c, the signal speed. The wavelength, λ, of a signal in the transmission line is proportional to c.
[0008] In an integrated circuit, there are practical reasons for reducing both. Delay lines are used to increase the flight time of a signal, and this would reduce the necessary length of a delay line. It is also desirable sometimes to shift the phase of a signal by a given fraction of a wavelength and the reduced wavelength of a signal in this structure reduces the length of the transmission line necessary to achieve this shift.
[0009] The invention will now be described, by way of example, with reference to the accompanying drawings in which:
FIG. 1(a) is a cross-sectional view of prior art.
FIG. 1(b) is a top view of FIG. 1(a).
FIG. 2(a) is a cross-sectional view of an embodiment of the present invention.
FIG. 2(b) is a top view of FIG. 2(a).
FIG. 3(a) is a cross-sectional view of an alternative embodiment of the present invention.
FIG. 3(b) is a top view of FIG. 3(a).
FIG. 4(a) is a cross-sectional view of a second alternative embodiment of the present invention.
FIG. 4(b) is a top view of FIG. 4(a).
FIG. 5(a) is a cross-sectional view of a third alternative embodiment of the present invention.
FIG. 5(b) is a top view of FIG. 5(a).
FIG. 6 is a top view of a fourth alternative embodiment of the present invention.
[0010] Figures 2(a), 2(b) illustrate an embodiment of this invention. AC current 105 is carried by one or more signal lines 101 and one or more return lines 102 which carry current in the opposite direction. The signal and return lines in general are composed of any conducting material such as metal while the surrounding medium 103 is normally composed of an insulating material. Tabs 111, also composed of a conducting material, are connected to the signal and return lines, the presence of which breaks the uniformity of the transmission line along its length. The average distance for capacitive coupling is greatly decreased by the presence of the tabs, which raises C. Yet, because the notches 112 prevent current flow in the longitudinal direction, the current paths 105 of the signal and return signal are forced to remain on the outside edges of the two conductors as if the tabs were never added. Since the current paths are essentially unchanged from the untabbed geometry, the magnetic fields are unchanged and so is L. The effect on the velocity and the wavelength, λ, of a signal in the transmission line is that they are both reduced by a factor equal to the square root of the ratio, r, that C was increased by. In an integrated circuit, there are practical reasons for reducing both. Delay lines are reduced to increase the flight time of a signal, and this would reduce the necessary length of a delay line. It is also desirable sometimes to shift the phase of a signal by a given fraction of a wavelength and the reduced wavelength of a signal in this structure reduces the length of line necessary to achieve this shift. On an integrated circuit (IC) ratios of r = 300 are achievable with present processes using standard technology. This would decrease the velocity by a factor of (1/r)1/2 = 0.058 to 8.7 x 108 cm/sec and the wavelength at 5GHz to 0.17 cm.
[0011] Figures 3(a) and 3(b) illustrate a variation of this invention more applicable to transmission lines on an IC where metal and insulator are patterned onto the chip in successive layers. This structure is similar to that illustrated in Figures 1(a) and 1(b) except that an additional layer of metal 122 is used. The main feature of this structure is that the increased capacitive coupling is achieved by the use of this second layer 122 which is patterned into a series of strips 123, 124 not necessarily rectangular in shape. The strips are generally oriented transverse to the lines 101, 102 with gaps 106 between each so that current is not able to flow in the strips in the same direction as the lines 101, 102. A preferred embodiment includes vias 121 which connect each of the strips to either the signal line(s) or return line(s) of the transmission line. This helps to maximize the capacitance per length, C, of the transmission line which is about equal to that between the fat part 124 of each of the strips and the signal (or return signal) line above. For small gaps 131 between the two metal layers, the capacitance will be high. To maximize the capacitance, most of the area of line 102 should have a part of a strip 124 below it. Other than this criterion, the exact shape of the strips is rather arbitrary. A rectangular shape with a uniform width equal to that of the fat part 124 of the strips would probably work just as well in this case. Figures 4(a), 4(b) illustrate a structure that contains features from both of the structures of Figures 2 and 3. Tabs 111 connected to signal line 101 fill in most of the area between the signal and return lines in first level of metal. Strips 123 in a second layer of metal 122 extend beneath both the tabs and the signal line and are attached to the return line with conducting vias 121. This structure thus utilizes both the area of the signal line and the area between the signal and return line for capacitive coupling between the two metal layers and thus between the signal and return lines.
[0012] Figures 5(a), 5(b) show a structure where the signal line 101 and return line 102 are on separate metal layers. Each line has a series of tabs 111, 123 on its own level that extend toward the opposite line and may overlap it as shown in the figures.
[0013] Figure 6 is a variation on Figure 2(b) in which the tabs 111 are interleaved to achieve a higher capacitance.
[0014] The invention is embodied in the structure of an IC where the surrounding medium 103 is composed of typical semiconductor processing materials such as silicon, silicon dioxide, silicon nitride, plastic, air, etc. The invention is also embodied in a multilayer PCB (Printed Circuit Board) where the surrounding medium is that of the nonmetallic parts of the PCB. The signal and return lines are on one side of an insulating layer and transverse, spaced apart metal lines are on the opposite side. Another embodiment of the invention, and the simplest, is in a cable. A particular example of this is a coaxial cable with conductive tabs reaching inward from the outer conductor and/or tabs reaching outward from the inner conductor.
[0015] For the IC embodiment, nichrome is a preferred conductive layer 122 because it is a trimmable metal. One may use the invention by trimming the nichrome strips to interrupt the current path from the tabs to the signal or return line(s). As more strips are cut, the wavelength and speed of the line gradually increase. This can be done after the rest of the processing and while the line is being measured so that very exact delays or phase shifts can be achieved.
[0016] Compact transmission lines 101, 102 have non-uniform conductive geometries including tabs 111 and notches 112. The tabs 111 and notches 112 increase the capacitance of the lines 101, 102 without increasing the inductance. This creates a delay in the signal transmitted through the lines 101, 102.
1. A transmission line structure comprising a signal line comprising one or more parallel
conducting paths capable of carrying an alternating current along its length, a return
signal line comprising one or more conducting paths parallel to the signal line and
capable of carrying an alternating current in the direction opposite to that of the
signal line, an insulating medium in which the return and signal lines are embedded
through which direct current cannot pass from the signal line to the return line,
non-uniform conductive means coupled to the signal and return lines for increasing
the effective capacitance between the signal and return lines without significantly
reducing the overall inductance of the transmission line, and a semiconductor substrate
and the insulating medium is disposed over the semiconductor substrate.
2. A transmission line as claimed in claim 1 including a multilayer printed circuit board
with alternate layers of metal lines and insulating material, an insulating layer
has on one side a first layer comprising signal and return lines and on its other
side a second layer comprising spaced apart metal lines oriented transverse with respect
to the signal and return lines to increase capacitance between the signal and return
lines, including a cable structure including signal and return lines separated from
each other by insulating material, at least one set of conductive tabs extending from
one of the lines toward the other line, said tabs oriented with respect to each other
to increase the capacitance between the signal and return lines.
3. A transmission line as claimed in claim 1 wherein the nonuniform conductive means
comprises a pattern of conductive strips, said conductive strips oriented transverse
to the signal and return lines to increase capacitance between the signal and return
lines, said conductive strips separated by gaps to prevent inductively induced current
in said conductive strips, in which the insulating medium is a printed circuit board.
4. An integrated circuit comprising a transmission line structure comprising: a semiconductor
substrate, a signal line over the substrate and comprising one or more parallel conducting
paths capable of carrying an alternating current along its length, a return signal
line over the substrate and comprising one or more conducting paths parallel to the
signal line and capable of carrying an alternating current in the direction opposite
to that of the signal line, an insulating medium in which the return and signal lines
are embedded through which direct current cannot pass from the signal line to the
return line, non-uniform conductive means coupled to the signal and return lines for
increasing the effective capacitance between the signal and return lines without significantly
reducing the overall inductance of the transmission line, in which the substrate comprises
the insulating material.
5. An integrated circuit as claimed in claim 4 wherein the insulating material is an
insulating layer over the substrate.
6. A transmission line as claimed in claim 5 wherein the nonuniform conductive means
comprises a pattern of conductive strips, said conductive strips oriented transverse
to the signal and return lines to increase capacitance between the signal and return
lines, said conductive strips separated by gaps to prevent inductively induced current
in said conductive strips.
7. A transmission line as claimed in claim 6, including further comprising vias extending
from the conductive strips to either the signal line or the return line, said vias
filled with conductive material for electrically connecting the strips to the respective
signal or return lines.