GB2257260A

GB2257260A - Optoelectronic assemblies.

Info

Publication number: GB2257260A
Application number: GB9113859A
Authority: GB
Inventors: Ian Richard Croston; Stephen Geoffrey Tyler
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-06-26
Filing date: 1991-06-26
Publication date: 1993-01-06
Anticipated expiration: 2011-06-26
Also published as: GB9113859D0; GB2257260B

Abstract

An optoelectronic assembly comprises an optoelectronic device 1 mounted on a motherboard 6 for ease of handling and to provide good optical and electrical connections thereto. The component bearing surface of the device is placed adjacent to the motherboard 6 which has an aperture 8 therethrough via which electrical connections 13 are taken. The reverse side of the motherboard 6 includes grooves in which optical fibres 14 and 15 are located and cooperating projecting and recessed regions to provide alignment. <IMAGE>

Description

Optoelectronic Assemblies This invention relates to optoelectronic assemblies and more particularly to the mounting of an optoelectronic device on a carrier.

Optoelectronic or integrated optical devices are known in which an interaction takes place between optical energy transmitted along a waveguide or waveguides in the device and an applied electrical field to give effects such as switching between adjacent waveguides. This may be achieved by using an electro-optic material such as, for example, lithium niobate in which titanium is diffused to form a region of high refractive index which acts as an optical waveguide.

Electrodes are arranged on the surface of the lithium niobate adjacent to the waveguide so that application of a potential difference between them produces an electrical field across the waveguide, altering its refractive index. Hence the passage of optical energy along the waveguide may be controlled.

In another branch of the technology, the substrate may be of a semiconductor material such as indium phosphide (InP) or galium arsenide (GaAs) in which a tertiary or quaternary of the material is used to define optical guiding regions.

This type of integrated optic device may be made more compact than the lithium niobate type and established techniques used in integrated semiconductor circuits may be used for its fabrication. However, the materials tend to be more brittle than lithium niobate.

The efficiency of optical waveguides formed in optoelectronic devices is generally quite good and only relatively insignificant amounts of optical energy are lost during its transmission along them. However, there tend to be losses where connections are made to and from the device by means of an optical fibre, due to the incomplete transfer of the optical energy from the optical fibre to the waveguide substrate.

In order to ruggedise the optical coupling efficiency to integrated optoelectronic devices, it has been proposed to mount such a device on a motherboard on which optical fibres for connection to the device are also mounted to permit more accurate alignment to be maintained. Use of a motherboard enables the integrated optic device to by more easily handled and packaged in a hermetically sealed container if desired.

One technique for mounting an integrated optoelectronic device on to a motherboard involves using solder bump technology. In this technique, the motherboard supports conductive tracks on which solder is deposited. The optoelectronic device is then presented to the motherboard with its electrode bearing face adjacent to the surface of the motherboard on which the solder bumps are deposited. The solder bumps are laid down in a pattern such that they are in register with the electrodes of the device when the device and motherboard are correctly aligned. The two substrates are held adjacent one another and the temperature increased so that the solder reflows. The device is brought into alignment with the motherboard because of the capillary action of the molten solder or by manipulation. When the temperature is reduced, the solder solidifies, bonding the device to the motherboard.Grooves in the motherboard accommodate optical fibres which are inserted along them and into contact with the optical waveguide regions of the device.

Although this technique has produced acceptable results where lithium niobate devices are used, it has been more difficult to achieve the accuracy required for optoelectronic devices of smaller dimensions, such as those borne on semiconductor substrates, in which much greater precision is required than can be repeatedly obtained using the solder bump method.

The present invention arose from an attempt to provide an improved optoelectronic assembly.

According to the invention, there is provided an optoelectronic assembly comprising an optoelectronic device mounted on a motherboard having an aperture therethrough, at least one electrical connection to a component of the device being made via the aperture.

As the electrical connection is made via the aperture, the precise alignment required when connections are made using solder bumps may be dispensed with, it only being necessary that the aperture permits easy connection to be made to electrical components on the device. The components may be active devices or may be electrodes or conductive tracks, for example. Thus by employing the invention, electrical alignment constraints are eased.

Preferably, the surface of the device adjacent to the motherboard bears optical and electronic tcomponents.

It is preferred that the surface of the motherboard adjacent to the device includes at least one groove therein in which a light guide is located. This enables connections to be made to optical waveguides carried by the device. The light guide may be any suitable material but preferably is an optical fibre as this may be readily inserted in position after the device has been mounted on the motherboard.

It is preferred that the surface of the device adjacent to the motherboard includes at least one projecting portion which interengages with a corresponding recess in the motherboard when the device is mounted thereon to provide alignment of the device and the motherboard. This is a particularly convenient configuration as the recess may be formed at the same time as the grooves which are required to accommodate light guides. Separate grooves may be provided for alignment and for carrying the light guides.

However, the projecting portion used for alignment may also include an optical waveguide of the device, in which case a groove may be used to both accommodate a light guide and to align the components.

In an alternative embodiment, the recess may be accommodated in the device and the projecting portion form part of the motherboard component or a combination of these two configurations could be used.

As these alignment projections and recesses can be provided independently of the necessary electrical connections to the device, design constraints on their configurations and positioning are much less than is the case when solder bumps are used.

It is preferred that the motherboard has only electrical components on one surface and only optical components on its other surface. This enables optical and electrical connections to be made independently of one another and therefore to be optimised without compromising the quality of the other connections.

The invention may be applied to assemblies in which the optoelectronic device comprises a lithium niobate substrate but is particularly advantageous for use with devices having semiconductor substrates formed for example from a compound of group III and V materials. A particularly suitable material for the motherboard's substrate is silicon.

One way in which the invention may be performed is now described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an integrated optoelectronic device; Figure 2 is an end view of the device shown in Figure 1; Figure 3 schematically illustrates a motherboard on which the integrated optic device of Figure 1 is to be mounted; Figure 4 shows the opposite face of the motherboard to that shown in Figure 3; Figure 5 shows a step in the method of mounting the device of Figure 1 on the motherboard; Figure 6 is an end view of the optoelectronic device positioned on the motherboard; Figure 7 illustrates the assembly of the motherboard and optoelectronic device; and Figure 8 schematically illustrastestpart of another assembly in accordance with the invention.

With reference to Figure 1, an optoelectronic device 1 has a substrate of a III-V semiconductor material, which in this particular embodiment is indium phosphide (InP). The device is a switch and includes two optical waveguides 2 and 3, the waveguides being defined by ridges 2a and 3a of InP laid down on a layer of InGaAsP deposited on the indium phosphide substrate 1. Electrodes 4 are located adjacent the waveguide regions 2 and 3 such that when a suitable electric field is applied across them, the refractive index of the waveguides is modified to change the optical coupling between the waveguides 2 and 3. The integrated optic device 1 also includes elongate silicon dioxide ridges 5 on its upper surface as shown in Figure 1.The ridges 5 are used to assist in alignment of the device when it is mounted on a motherboard and typically are an order of magnitude larger than the waveguide defining ridges 2a and 3a.

The device 1 illustrated in Figures 1 and 2 is to be mounted on a motherboard for ease of handling and also to enable accurate alignment to be maintained between optical fibres and the waveguides 2 and 3. The motherboard 6 is shown in Figure 3 and comprises a silicon substrate 7 having a central rectangular aperture 8 therethrough which may be produced by etching or laser ablation. The substrate 7 has a plurality of grooves 9 etched in its upper surface, each groove having a V or U shaped profile. Some of the grooves 9 accommodate optical fibres in the finished assembly and others are used in aligning the device 1 and motherboard 6.

The upper face of the silicon substrate 7 may be termed the optical plane 10 as it bears only optical connection means and does not carry any electrical components.

Figure 4 illustrates the reverse side of the motherboard 6 showing the electrical plane 11 on which is laid down conducting pads and tracks 12.

The optoelectronic device 1 is mounted on the motherboard 6 by placing its optical waveguide bearing surface against the optical plane 10 of the motherboard 6 as illustrated in Figure 5. As shown in Figure 6, the alignment ridges 5 interengage with some of the V-shaped grooves 9 to provide accurate alignment between the channels in which the optical fibres are located and the waveguides 2 and 3 of the optoelectronic device 1. Once the device 1 has been brought into registration with the motherboard 6, it is fixed in position. The electrical connections are then made between the bond pads 12 on the underside ofvthetmotherboard 6 and the electrodes 4 of the optoelectronic device, shown in Figure 7. In this case, the connections are made via wire bonds 13. Optical fibres 14 and 15 are then inserted in the grooves 9 collinear with the waveguide regions so that the inner cores of the optical fibres line up with the waveguiding regions of the device 1.

In another embodiment of the invention, shown in Figure 8, the waveguide defining part 16 of an optoelectronic device 17 is combined with a projecting portion 18 used for alignment in cooperation with a groove 19 etched in the facing surface of a motherboard 20. The height a of the portion 18 is an order of magnitude greater than the height b of the ridge 16. In this embodiment, the substrate 21 of the device 17 is of InP and has a layer 22 of InGaAsP and a layer of InP 23 to define the waveguide.

Claims

1. An optoelectronic assembly comprising an optoelectronic device mounted on a motherboard having an aperture therethrough, at least one electrical connection to a component of the device being made via the aperture.

2. An assembly as claimed in claim 1 wherein the surface of the device adjacent to the motherboard bears optical and electronic components.

3. An assembly as claimed in claim 1 or 2 wherein the surface of the motherboard adjacent to the device includes at least one groove therein in which a light guide is located.

4. An assembly as claimed in claim 3 wherein the groove has a V-shaped profile.

5. An assembly as claimed in claim 3 or 4 wherein the light guide is an optical fibre.

6. An assembly as claimed in any preceding claim wherein the surface of the motherboard facing away from the device bears electrically conductive material to which said electrical connection is made.

7. An assembly as claimed in any preceding claim wherein the surface of the device adjacent to the motherboard includes at least one projecting portion which inter-engages with a corresponding recess in the motherboard when the device is mounted thereon to provide alignment of the device and the motherboard.

8. An assembly as claimed in claim 7 wherein the projection comprises an elongate ridge and the recess is a groove in the surface of the mother board.

9. An assembly as claimed in claim 8 wherein the groove which engages with the ridge has the same profile as a groove in the motherboard in which an optical fibre is located.

10. An assembly as claimed in any of claims 7 to 9 wherein the projecting portion includes an optical waveguide region.

11. An assembly as claimed in any preceding claim wherein the motherboard bears only electrical components on one surface and only optical components on its other surface.

12. An assembly as claimed in any preceding claim wherein the optoelectronic device has a semiconductor substrate.

13. An assembly as claimed in any preceding claim wherein the substrate of the motherboard is of silicon.

14. An optoelectronic assembly substantially as illustrated in and described with reference to Figures 1 to 7 or Figure 8 of the accompanying drawings.