GB2215124A

GB2215124A - Integrated circuit underpasses

Info

Publication number: GB2215124A
Application number: GB8803506A
Authority: GB
Inventors: George Hedley Storm Rokos; Robert William Hunt
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-02-16
Filing date: 1988-02-16
Publication date: 1989-09-13
Also published as: GB8803506D0

Abstract

In an integrated circuit transistor array an elongate underpass 11 is provided between parts of a conductive line 13. The underpass may have two or more elongate parallel contacts 14 which can be separated on the underpass as required. This allows for customer configuration of contact positions. <IMAGE>

Description

IMPROVEMENTS IN INTEGRATED CIRCUITS.

This invention relates to integrated circuits and in particular to the provision of underpasses in such circuits.

Transistor array devices, for example bipolar analogue arrays, are conventionally customised by the application of one or more layers of configurable metal whereby circuit elements are coupled in a particular way to provide a desired circuit function. Usually only a single layer of metal is available and in such cases it is normal for the chip to contain a number of relatively low impedance semiconductor underpasses to facilitate metal routing. Such underpasses are relatively small and have their silicon to metal contact windows in a fixed position as the contact mask is not configurable by the customer. This can provide serious constraints on the layout of the metal routing.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a transistor array adapted to provide a predetermined circuit function by the application thereto of a conductor pattern, said array including a plurality of similar cells of transistors, routing paths between said cells for a conductor pattern whereby selected transistors are interconnected, and one or more underpasses whereby cross-overs of said pattern are provided, wherein each said underpass is elongate in configuration whereby contact windows providing access to the underpass may be disposed at positions determined by the function of the underpass.

The elongate underpass allows the use of a customer configurable mask for the positioning of the contact windows. This allows each underpass to be tailored to the local topological requirements thus improving design flexibility.

Embodiments of the invention will now be described with reference to the accompanying drawings in which Figures 1 to 4 illustrate various configurationa of an elongate underpass.

Referring to Figure 1 of the drawings, this depicts an elongate underpass 11 which provides a cross-over between a power bus 12 and a signal line 13.

The underpass 11 is provided by a diffusion or implantation in a semiconductor substrate and is covered by an insulating layer (not shown). To accomodate the relatively wide power bus 12 the contacts 14 whereby the signal line 13 is connected to the underpass are displaced towards the end of the underpass.

In Figure 2 the underpass 11 provides a cross-over between a pair of signal lines 21 and 22. In this structure the contact windows 23 are disposed near the centre of the underpass thus minimising thne resistance of the underpassing signal line 22.

Figure 3 illustrates the use of the underpass 11 on a multijunction arrangement e.g. for use in encoders, decoders and signal buses. In this structure further contacts 31, 32 are provided between the contacts 33 to which signal line 34 is connected. These further contacts provide a connection between signal line 34 and signal lines 35 and 36. The underpass also provides a cross-over between signal line 34 and further signal lines 37.

Figure 4 shows an alternative cross-over between a power bus 41 and a signal line 42. In this arrangement elongate contacts 43 are disposed longitudinally on the underpass 11 and provide connection to the power bus 41. As the contacts 41 are close together and extend for most of the underpass length, a very low resistance path is provided.

The underpass arrangement described above are of particular application in bipolar analogue arrays. It will however be understood that their use is not limited to such arrays and that underpasses of this type may be used in other integrated circuit constructions.

Claims

CLAIMS.

1. A transistor array adapted to provide a predetermined circuit function by the application thereto of a conductor pattern, said array including a plurality of similar cells of transistors, routing paths between said cells for a conductor pattern whereby selected transistors are interconnected, and one or more underpasses whereby cross-overs of said pattern are provided, wherein each said underpass is elongate in configuration whereby contact windows providing access to the underpass may be disposed at positions determined by the function of the underpass.

2. A transistor array as calimed in Claim 1, wherein at least one of said underpasses is provided with elongate parallel contacts adapted to provide connection to a power bus.

3. A transistor array as claimed in claim 1, wherein at least one of said underpasses is provided with more than two contacts whereby to provide a circuit node.

4. A transistor array substantially as described herein with reference to and as shown in any one or more of Figures 1 to 4 of the accompanying drawing.