GB2382725A

GB2382725A - Transmission line structure

Info

Publication number: GB2382725A
Application number: GB0204322A
Authority: GB
Inventors: Joseph Alan Barnard
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-02-25
Filing date: 2002-02-25
Publication date: 2003-06-04
Also published as: GB0204322D0

Abstract

A transmission line structure for transmitting a signal over a substrate 2, including at least one longitudinal conducting strip 10 formed over the substrate and laterally encapsulated between underlying and overlying longitudinal low-loss dielectric layers 8,12 also formed over the substrate, the underlying and overlying longitudinal low-loss dielectric layers disposed between overlying and underlying electrical ground layers also formed over the substrate and in electrical contact with each other.

Description

TRANSMISSION LINE STRUCTURE The present invention relates to a transmission line structure for transmitting one or more signals over a substrate.

A number of devices involve the transmission of one or more signals over a substrate. Examples include high speed computers, high speed laser diode arrays and photodiode arrays, which typically include a parallel microwave signal bus.

It is an aim of the present invention to provide a transmission line structure for transmitting one or more signals over a substrate.

According to a first aspect of the present invention, there is provided a transmission line structure for transmitting a signal over a substrate, including at least one longitudinal conducting strip formed over the substrate and laterally encapsulated between underlying and overlying longitudinal low-loss dielectric layers also formed over the substrate, the underlying and overlying longitudinal low-loss dielectric layers disposed between overlying and underlying electrical ground layers also formed over the substrate and in electrical contact with each other.

According to another aspect of the present invention, there is provided a transmission line structure for parallelly transmitting a plurality of signals over a substrate, including: a common underlying ground layer formed over a substrate ; an array of laterally spaced parallel longitudinal underlying strips of a low-loss dielectric material formed over the underlying ground layer, each supporting at least one respective longitudinal conducting strip and a respective overlying longitudinal overlying strip of low-loss dielectric material so as to laterally encapsulate the at least one respective conducting strip between the respective pair of underlying and overlying low-loss dielectric strips; and a

common overlying ground layer formed over the longitudinal low-loss dielectric strips so as to make electrical contact with the underlying ground layer in the spaces between the underlying low-loss dielectric strips.

According to another aspect of the present invention, there is provided an optic system including an array of optoelectronic components and a transmission line structure as described above, wherein the at least one longitudinal conducting strip formed supported on each underlying low-loss dielectric strip is electrically connected to a respective optoelectronic component for transmitting signals in parallel to or from the optoelectronic components.

According to another aspect of the present invention, there is provided a method for transmitting a plurality of signals in parallel over a substrate, the method including the steps of : providing a transmission line structure including an array of laterally spaced longitudinal conducting strips, and introducing each signal into a respective one or more of the conducting strips associated therewith, wherein each one or more of the conducting strips respectively associated with each signal are laterally encapsulated between a respective one of a laterally spaced array of pairs of underlying and overlying longitudinal low-loss dielectric strips formed over the substrate; the low-loss dielectric strips disposed commonly between underlying and overlying continuous electrical ground layers also formed over the substrate and in electrical contact with each other via the spaces between each pair of underlying and overlying longitudinal low-loss dielectric strips.

Embodiments of the present invention are described hereunder, by way of nonlimiting example only, with reference to the accompanying drawings, in which :Figure 1 illustrates an optic system including a transmission line structure according to an embodiment of the present invention ;

Figure 2 illustrates in cross-section the transmission line structure employed in the device shown in Figure 1; Figure 3 illustrates the connection between the photodiodes and the transmission lines in the system shown in Figure 1.

Figure 4 illustrates in plan view the termination of the transmission lines in Figure 3; Figures 5 (a) and 5 (b) illustrate examples of circuits to which the transmission line structure of the present invention has application; and Figure 6 illustrates in cross-section a transmission line structure according to another embodiment of the present invention; With reference to Figures 1 to 4, an example of a system for monitoring the power of the component channels of a division wavelength-multiplexed signal includes an optic chip 20 defining an AWG (array waveguide grating) type wavelength dispersive element. A wavelength-multiplexed signal is introduced from an optic fibre 30 into the array waveguide grating via an input waveguide 24 from which it is propagated through a first free propagation region 25 to an array 22 of waveguides of differing optical length. An array of output waveguides 27 extend from different points of the second free propagation region 28 at the output end of the waveguide array 22 so as to each selectively receive a portion of the dispersed signal corresponding to a respective channel. The optic chip may, for example, be a silicon-on-insulator chip with each of the waveguides defined as rib waveguides.

The output waveguides 27 terminate at the edge of the optic chip, and a photodiode array 26 is arranged to receive light from the output waveguides. Each photodiode of the array is electrically connected via a respective wire bond 36 to the end of a respective one of an array of transmission lines 34 provided on a separate substrate 2.

As shown in Figure 2, each transmission line of the array includes a longitudinal conducting metal strip 10 laterally encapsulated between respective pair of longitudinal underlying and overlying longitudinal low-loss dielectric strips 8,12, which are in turn commonly laterally encapsulated between underlying and overlying continuous metallic electrically conducting layers 6,14, with electrical contact established between the underlying and overlying ground layers where the two continuous ground layers contact in lateral spaces between the parallel underlying low-loss dielectric strips.

The low-loss dielectric strips may, for example, be made from dielectric materials used in the production of printed circuit boards. Suitable materials include polyimide, quartz and glass such as plasma deposited borosilicate glass which has a dielectric constant, 8r of 4.82. The low-loss dielectric strips preferably have a tangential loss ratio (TAND) of less than 0.03, more preferably less than 0.01.

In one embodiment, each low-loss dielectric strip has a thickness from about 3 to 8u. m, the metal ground layers and conducting strips have a thickness of about I l1n, and the lateral track width, W of the conducting strips is about 6 11m.

The configuration of the point of connection of the transmission lines to the wire bonds is illustrated in more detail in Figures 3 and 4. The termination of each transmission line includes a longitudinal extension 62 of the respective underlying low-loss dielectric strip 8 supporting a longitudinal extension 64 of the respective conducting strip 10.

As shown in Figure 4, the longitudinal extension 64 of the conducting strip 10 is wider than the conducting strip in the main body of the transmission line. The conducting strip is narrowed to compensate for the increased capacitance per unit length as the conducting strip is sandwiched between the underlying and overlying ground planes. The characteristic impedance (Zo== (L/C) )

should be kept constant (typically about 50Q), and since the capacitance per unit length increases as the conducting strip goes between the two ground planes, the conducting strip width needs to be decreased to increase the inductance and decrease the capacitance of the conducting strip.

As shown in Figure 6, the underlying ground layer 6 is configured such that longitudinal extensions 68 thereof extend to the lateral sides of each conducting strip extension in a substantially coplanar configuration therewith, but without underlying the underlying low-loss dielectric strip extensions. The width G, of the gap 66 between each ground plane extension 68 and the adjacent longitudinal strip extension 64 should be sufficiently small to rely on coplanar infringing capacitance.

The relatively high inductance of the bond wire can be compensated for in order to keep Zo constant by increasing the degree of capacitive coupling (grounding) of the conducting strip extension to the ground plane, or alternatively by adding additional capacitance at the point to which the bond wire is attached to form part of an artificial CLC (shunt capacitance-series inductance-shunt capacitance) transmission line.

Examples of circuits to which the present invention is applicable are shown in Figures 5 (a) and 5 (b).

Figure 5 (a) shows a circuit for processing a signal generated by a photodiode 40 upon the incidence of light thereupon. The signal is transmitted to a transimpedance amplifier (TIA) 42 via a transmission line 34 of the type shown in Figure 2. The transmission line is connected to the TIA via the type of connection used for connecting the photodiode to the transmission line in the system shown in Figures 1 to 4. The differential parts of the signal output from the TIA are transmitted onwards for further processing via a limiting amplifier 46. A transmission line structure as illustrated in Figure 6 can be used for the differential line tracks 44,48 connecting the TIA to the limiting amplifier and

the limiting amplifier to the circuitry for further processing (not shown). The transmission line structure is the same as that illustrated in Figure 2, except that a pair of longitudinal conducting strips (one for each differential part of the signal) are laterally encapsulated between a common pair of underlying and overlying low-loss dielectric strips 8,12.

Figure 5 (b) shows an example of a circuit for driving a laser diode. A transmission line structure of the type shown in Figure 6 can be used for transmitting the differential signals to the laser diode driver and from the laser diode driver 52 to the laser diode 56.

The present invention has particular application, for example, where one or more signals are to be transmitted over a relative lossy substrate (such as silicon or other semiconductor substrates), where a plurality of microwave signals are to be routed in parallel round a board in a relatively compact manner (such as on a printed circuit board or a glass substrate), or where one or more signals need to be transmitted with shielding from external radiation (such as in sensitive radio and microwave receivers).

The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any definitions set out above. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS: I. A transmission line structure for transmitting a signal over a substrate, including at least one longitudinal conducting strip formed over the substrate and laterally encapsulated between underlying and overlying longitudinal low- loss dielectric layers also formed over the substrate, the underlying and overlying longitudinal low-loss dielectric layers disposed between overlying and underlying electrical ground layers also formed over the substrate and in electrical contact with each other.
2. A transmission line structure according to claim I or claim 2 wherein the low loss dielectric layers have a tangential loss ratio of less than 0.03.
3. A transmission line structure according to claim 3 wherein the low loss dielectric layers have a tangential loss ratio of less than 0. 01.
4. A transmission line structure according to claim 1 or claim 2 wherein the low-loss dielectric layers are selected from the group consisting of quartz, polyimide and glass layers.
5. A transmission line structure according to claim 5 wherein the low-loss dielectric layers are borosilicate glass layers.
6. An optic system including an optoelectronic component and a transmission line structure according to claim 1 electrically connected to the optoelectronic component for transmitting signals to or from the optoelectronic component.
7. A transmission line structure for parallelly transmitting a plurality of signals over a substrate, including: a common underlying ground layer formed over a substrate; an array of laterally spaced parallel longitudinal underlying

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strips of a low-loss dielectric material formed over the underlying ground layer, each supporting at least one respective longitudinal conducting strip and a respective overlying longitudinal overlying strip of low-loss dielectric material so as to laterally encapsulate the at least one respective conducting strip between the respective pair of underlying and overlying low-loss dielectric strips; and a common overlying ground layer formed over the longitudinal lowloss dielectric strips so as to make electrical contact with the underlying ground layer in the spaces between the underlying low-loss dielectric strips.
8. An optic system including an array of optoelectronic components and a transmission line structure according to claim 7, wherein the at least one longitudinal conducting strip formed supported on each underlying low-loss dielectric strip is electrically connected to a respective optoelectronic component for transmitting signals in parallel to or from the optoelectronic components.
9. A method for transmitting a plurality of signals in parallel over a substrate, the method including the steps of : providing a transmission line structure including an array of laterally spaced longitudinal conducting strips, and introducing each signal into a respective one or more of the conducting strips associated therewith, wherein each one or more of the conducting strips respectively associated with each signal are laterally encapsulated between a respective one of a laterally spaced array of pairs of underlying and overlying longitudinal low-loss dielectric strips formed over the substrate; the low-loss dielectric strips disposed commonly between underlying and overlying continuous electrical ground layers also formed over the substrate and in electrical contact with each other via the spaces between each pair of underlying and overlying longitudinal low-loss dielectric strips.

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10. A transmission line structure substantially as hereinbefore described with reference to Figure 2 or Figure 6 of the accompanying drawings.
11. An optic system substantially as hereinbefore described with reference to Figures I to 4 or Figure 2 or 6 with Figure 5 (a) or 5 (b).