EP1412794A2

EP1412794A2 - Packaged optoelectronic device

Info

Publication number: EP1412794A2
Application number: EP02740933A
Authority: EP
Inventors: Luke Alcatel Optronics UK Limited O'CONNER
Original assignee: Alcatel Optronics UK Ltd
Current assignee: Kaiam Europe Ltd
Priority date: 2001-07-04
Filing date: 2002-07-03
Publication date: 2004-04-28
Also published as: WO2003005091A2; WO2003005091A3; GB0116298D0

Abstract

An optoelectronic device comprising: a substantially planar optical component having at least one electrical contact provided thereon; a housing containing the optical component; and a flexi-rigid printed circuit board (PCB) electrically connected to the electrical contact on the optical component and having at least a portion thereof disposed within the housing.

Description

PACKAGED OPTOELECTRONIC DEVICE

HELD OF THE INVENTION

The present invention relates to a package for an active optoelectronic device. More specifically, though not exclusively, it relates to a packaged planar waveguide type device having drive electronics associated therewith, the package thus requiring both optical and electrical inputs and outputs.

BACKGROUND ART Planar waveguide devices such as optical switches, attenuators and the like, for use in the telecommunications industry, present specific packaging problems. Where the planar waveguide component integrally includes one or more electrical components thereon, such as a resistive heater, for controlling the operation of the device, there is a requirement for a package which houses the optical component and allows both optical and electrical signals to be input to and output from the device. Making electrical connections from electronic drive circuitry to the electrical component(s) on the optical component presents specific difficulties. The final packaged device needs to be robust, and relatively insensitive to temperature variations of the environment in which the device will be used. Any electrical connection between the planar waveguide device and drive circuitry which may be either inside the package or external thereto, will be particularly sensitive to shock, vibration or thermal fluctuations.

WO 01/29591 proposes a package for an electrical/optical device in which a bridge component is disposed above an optical component. Electrical components are mounted on top of the bridge and are wire-bonded to electrical connections provided on an upper surface of the optical component. Alternatively, or additionally, plated through-hole connections extend through the bridge to allow conductive bonding of the components to electrical connections on the upper surface of the optical component. We believe the fabrication and assembly of such a structure would be relatively complex. A first described embodiment requires the discrete manufacture of small bridge structures which will have to be specially jigged in order to fit onto shoulders, where they have to be attached in some way - not an easy process to control. In a second embodiment the bridges are directly connected (by solder or conductive adhesive) to electrical connector pins mounted in the package housing - a very tricky operation for volume manufacture. This latter type of scheme puts demands on the package design itself : the base would have to be either specially made with a suitable shoulder or leads that accommodate such an electrical connection. In addition, the height of the leads from the package base will have to be custom designed for each desired thickness of optical die, in order to minimize the wire bond length. Hence, this is not a standardized solution. A further potential disadvantage is the wire-bonding. If the bridge is too thin excessive energy from the wire bond machine will be expended in bending the bridge itself- and a wire bond will not be formed. In the third described embodiment directly soldered connections are used and these also pose potential problems: controlling the heights involved is critical, and the scheme necessitates blind connections which have to be made between die / bridge and bridge / package base lead concurrently. This is not likely to be straightforward.

The present invention aims to avoid or minimize one or more of the foregoing disadvantages. It is another aim of the present invention to provide a packaged optoelectronic device which utilises standard materials and assembly techniques to provide electronic interconnection from an optical component to the environment external to the packaged device. It is a further aim to provide a package design which is flexible in the sense that it can be applied to different types of packages, for example hermetic, non- hermetic, or quasi-hermetic applications; high speed and/or high reliability and/or high temperature, etc applications.

SUMMARY OF THE INVENTION The present invention provides an optoelectronic device comprising: a substantially planar optical component having at least one electrical contact provided thereon; a housing containing the optical component; and flexi-rigid printed circuit board (PCB) means electrically connected to said at least one electrical contact on the optical component and having at least a portion thereof disposed within the housing. Flexi-rigid PCBs are commonly available in the electronics industry. Their basic structure comprises a set of thin electrically conductive (commonly copper) layers interspersed with a set of thin dielectric layers (forming the flexible part), a portion of which layered structure is bonded to a thicker backing material or "stiffener" normally of a high strength (forming the rigid part). The flexible part has a larger surface area than the rigid part, so that pre-defined portions are substantially unsupported. These portions can be easily shaped and bent to allow connections to be made out of the plane of the top surface of the supported portion. Because of the material stacking arrangement of its structure, the mechanical properties of the supported portion of the flexible part are normally nearly identical to the rigid part.

The flexi-rigid PCB means may conveniently be a single flexi-rigid PCB (hereinafter referred to as the "flexi") formed and arranged to carry electrical signals between the optical component and electronic circuitry connected to the optoelectronic device.

Preferably, the flexi is directly connected to an electrical connector means of the device, for example an array of pin connectors extending through a wall of the device housing, via which externally generated electrical signals may be applied to the device. Alternatively the flexi may itself extend through an aperture provided in the housing, for connection to an external electrical signal source.

Preferably, the electrical connection of the electrical contact(s) on the substantially planar optical component (hereinafter referred to as the "die") to the flexi may conveniently be achieved by flip-chip bonding or wire-bonding of the optical component to the flexi. From the point of view of electrical connection, the benefit of the PCB means being flexi-rigid is that standard and high quality electrical connections can be easily made from the die to the supported portion of the flexi, and that the electrical connections can be carried by the unsupported portion of the flexi to any part of the package for attachment to, for example, pins or other connectors. This removes the requirement for the package pins / connectors to be specially positioned - for example in the plane of the die. In addition, the flexi structure is a standard structure amongst PCB manufacturers and relatively easy to fabricate using standard techniques. The fabrication methods do not alter significantly with the use of different rigid materials. These can be selected for different applications including: thermal coefficient of Expansion (TCE) matching to other materials used in the device (e.g. silicon); mechanical rigidity; and enhanced thermal conductivity.

The substantially planar optical component preferably may have at least one electrical contact on an upper surface thereof, which contact is electrically connected to an upper surface of the flexi. The flexi can be in substantially side-by-side relationship with the die, or the die can be mounted onto the top surface of the flexi, for example via a pedestal on which the die is mounted.

In a preferred embodiment the flexi includes an aperture provided therein through which the die is connected to a base of the housing via pedestal means. However, in certain circumstances, the die may instead be connected directly to the housing base. For example, a step may be machined in the housing base so as to effectively form a pedestal upon which the flexi can be mounted above the level of a remaining portion of the housing base. Wire bond connections may conveniently be made between one or more electrical connections on an upper surface of the die to electrical connections on an upper surface of the flexi. The thickness of the flexi can be easily chosen - and specified to PCB manufacturers - to minimize wire bond length.

A significant advantage of the die being raised up off the base of the housing (by provision in the design of either a pedestal or machining a step in the package base) is that this enables optical input/outputs of the die to be aligned with mounted input/output optical fibres for attachment to the die, for example input/output fibres mounted in a fibre V- groove array (FVA) device having a thickness greater than the thickness of the planar optical component, while still allowing the die and attached FVAs to be contained within the housing. Also in this manner the input/output fibres can be arranged to extend generally parallel to the planar optical device, between the optical device and input/output apertures provided therefore in the housing, at desired positions on the housing, without requiring bending of the fibres. Bending of the fibres can cause signal loss or distortion, and can lead to breakage of the fibres during assembly of the packaged device. Alternatively, the relative dimensions can be adjusted to make provision for an 'S' bend in the fibre for stress relief during package base movement when subject to temperature excursions. Provision of the pedestal means also has the advantage that it provides heat sinking to dissipate heat from the die to the housing, and thus to the exterior environment of the package.

A flexible portion of the flexi-rigid PCB preferably comprises a substantially planar member incorporating at least one layer of polymer material and having conductive pathways provided thereon, for example printed circuit means deposited on the surface of the planar member. The polymer material preferably has a low dielectric constant, for example in the range of 2.0 - 5.0. This enables electrical signals to be carried in the flexi at high speeds. By choosing a material having an appropriate dielectric constant and characteristic impedance, we envisage that in principle electrical signals up to the gigahertz (GHz) range can be carried in the flexi.

Where a pedestal means is provided via which the die is connected to the housing base, the material of the pedestal means may advantageously be expansion matched to the die and/or to the material of the housing base. For example, the die may include a substrate having planar waveguides formed thereon, and the pedestal means may be made of the same material as the die substrate, for example silicon. This has the advantage that changes in temperature will not cause stress at the join between the pedestal and the die. Any stress on the die may cause malfunction or degradation of performance of the device. Preferably, the rigid portion of the flexi-rigid PCB and the package base are constructed from similarly expansion matched material, whereby excessive stress on the die and wire bonds is substantially avoided during temperature excursions.

In an alternative possible embodiment, as mentioned above, the die may be mounted on pedestal means which is mounted on an upper surface of the flexi and the flexi is attached to the base of the package housing. In this case, the stiffener material of the flexi is preferably made of a material having a similar coefficient of thermal expansion (within 4.5 ppm / C) to the pedestal means. With suitable choice of stiffener thickness, this scheme ensures an amount of stress isolation between die and housing in the case where the latter has a substantially larger TCE than the former. An example would be when the die and housing are fabricated from silicon and aluminum respectively. In a further possible embodiment the die is flip chip bonded directly to the upper surface of the flexi. This is a solution suited to high speed electronic interconnect. The flexible portion of the flexi-rigid PCB may advantageously contain a compatible low dielectric constant dielectric material such as PTFE.

Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a plan view of a packaged optoelectronic device, according to one embodiment of the invention, in which the top cover of the package housing has been removed to reveal the contents of the housing;

Fig.2 is a perspective view of the packaged device; Fig.3 is a perspective view of a flex-rigid PCB used in the device of Fig.1 ;

Fig.4 is a schematic perspective view of an example optical component of known type;

Fig.5 is a schematic cross-sectional view of a portion of the device of Fig.l, taken along the line A-A' of Fig.l;

Fig.6 is a perspective view of a packaged device according to an alternative embodiment of the invention;

Fig.7 is a schematic cross-sectional view of modified version of the embodiment of Fig.1, also taken along the line A- A' in Fig.l;

Fig.8 is a schematic cross-sectional view of a portion of a modified version of the device of

Fig.l, also taken along the line A- A' in Fig.l; and Fig.9 is a schematic cross-sectional view of a portion of a further modified version of the device of Fig.l, also taken along the line A- A' in Fig.l.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Like reference numerals will be used throughout to refer to like parts in the different described embodiments.

Fig.l shows an optoelectronic device 1 comprising a planar waveguide component or "die" 3 contained in a housing 5. A top cover of the housing has been removed in Fig.1 to reveal the contents of the housing. The planar waveguide component, hereinafter referred to as the "die", comprises a silicon substrate having provided thereon waveguide cores (hereinafter referred to as the "waveguides") embedded in cladding material. One or more electrical heating elements are deposited on the upper surface of the die, for heating predetermined portions of the waveguides in order to control operation of the device. Fig.4 shows an example of this type of planar waveguide component, in the form of an optical switch based on a Mach-Zender Interferometer (MZI) arrangement. This component comprises a silicon substrate 48 having a layer 49 of silicon oxide deposited thereon, and two waveguides 50,52 provided thereon on the oxide layer 49, the waveguide being covered by cladding 56. A resistive heater 54 is positioned on the cladding 56, above one of the waveguides 50. By applying current to the heater 54 from a power source 55 (external to the packaged device), an input signal supplied to an input end 57 of one of the waveguides can be switched from one of the output ends 58, 59 of the two waveguides to the other. However, it will be appreciated that the package design now described could be used for any planar waveguide device or other substantially planar optical component which receives at least one input optical signal which enters the package and outputs at least one optical signal which exits from the package, and which also requires input/output electronic connections on one or more sides of the package for electrical signals to go to/from the device inside the package.

The packaged device 1 has at least one optical input fibre 6 and at least one optical output fibre (in the embodiment of Fig.l, there are two optical output fibres 7,8) which protrude from first and second opposing sides 16,17 of the housing 5. A rubber boot 18,19 of conventional type surrounds the input and output fibres respectively, adjacent the external faces of the housing sides, in order to minimize bending of the fibres where they protrude from the housing. A respective FVA 32,33 is attached to the optical input and output edge of the die 3, these FVAs mounting the input and output fibres 6,7,8 which carry optical signals into/out of the die 3.

Inside the housing 5, the die 3 is mounted on a pedestal 12 which is made of the same material to that of the die substrate (in this case, silicon). The pedestal sits on a base 11 of the housing base 11. Figs 5 and 7 (see below) are representative of structures best suited to applications where the base is manufactured from a material with a coefficient of thermal expansion (TCE) similar (within 4.5 ppm /°C) to that of the die (substrate) material, where: TCE = (change in linear dimension/original linear dimension) per unit increase of temperature. As both the die substrate and the pedestal are made of silicon, the die and the pedestal are expansion matched i.e. have substantially the same coefficient of thermal expansion. The die 3 is wire bonded 35 to the top flexible layer 26 of a flexi-rigid printed circuit board (PCB) 20 (the "flexi") which carries electrical signals between the die 3 and electrical input/output pins 25 which protrude through a third side 13 of the housing 5 i.e. so that the heating elements 15 are wire bonded 35 to electrical contacts 29 on the upper surface 45 of the flexi 20. The flexi also includes a lower layer 30 made of a substantially rigid material (hereinafter referred to as the "stiffener") which is bonded to a portion of the flexible layer 26 so as to support this portion, making it substantially rigid. An aperture 22 is provided in the flexi 20 through which aperture the pedestal 12 connects the die 3 to the housing base 11. The heights of the silicon pedestal 12 and the flexi are chosen so that the die 3 is disposed in approximately the same plane as the top surface of the flexi. The upper surface 4 of the die 3 may be slightly above the top surface of the flexi, as shown in Fig.5. This package design is suitable for hermetic applications if the materials chosen for the structure are non-outgassing and generally compatible, and as long as a suitable (i.e. hermetic) seal is made between the pin hole connectors 25 and the housing 5, and also between the optical inputs and outputs 6, 7, 8 and the housing.

The flexi is electrically connected to the pin connectors 25 directly by means of complementary pin hole receptor apertures 35 provided therefor on the flexi. In an alternative possible embodiment, illustrated in Fig.6, the flexi itself has a free end 23 which passes through an aperture 40 provided therefore in the side 13 of the housing, so as to extend from the packaged device for direct connection to external electrical drive circuitry. This design is suitable for non-hermetic package applications where sub-system designers may wish to have this type of design flexibility.

A significant advantage of using the flexi to carry the electrical signals from the die 3 to the exterior of the package is that it is a commercially available component which can be manufactured according to existing standardized procedures currently used in the electronics industry. It enables robust electrical connections between the die and the pin connectors 25 to be simply achieved, without long and fragile/sensitive bonding wires being necessary. The flexi can be made of specific materials, e.g special polymers, to achieve desired performance features. For example, flexi-rigid polymer PCBs for high speed electrical connectivity are commercially available. Using such a board would enable high speed connection to be achieved between the optical device and the exterior of the package. Making a PCB out of a polymer material with low dielectric constant e.g. in the region of 2.0 to 5.0, will generally speed up connectivity times. A suitable such high speed polymer is PTFE.

In another possible embodiment, shown in side view in Fig.7, the housing base 11 is made of Copper / Tungsten having a TCE of approximately 4-5ppm/°C, and the stiffener 30 is made of Cu/In/Cu stack material i.e. a layer of Invar sandwiched between two layers of copper, the thicknesses being in the ratio of 20:60:20 for the Cu/In/Cu layers. This stiffener is thermally well matched to the Cu/W housing base 11. This allows the possibility of placing a temperature sensing element 42 on top of the flexi 20, close to the die 3, to monitor the temperature of the die 3 via the thermally conductive path (indicated by the arrows T) therebetween provided by the flexi' s flexible layer 26 and stiffener 30, the housing base 11 and pedestal 12. Additionally, if desired the die 3 can be physically connected via thermally conductive adhesive 39 to the stiffener 30 as indicated schematically in Fig.7, to provide additional lateral heat spreading from the die.

Fig.8 illustrates another possible embodiment in which the die 3 is still mounted on a pedestal 12, but the pedestal is this time mounted directly onto the upper surface 45 of the flexi 20. The flexi is attached directly to the base 11 of the package housing. The electrical contacts on the die 3 are wire bonded 44 to the upper surface 45 of the flexi 20. The rigid material of the stiffener 30 of the flexi is made of a material having a similar coefficient of thermal expansion (within 4.5 ppm / C) to the pedestal 12. The pedestal is also expansion matched to the die substrate. The main application of such a design is to provide stress isolation between the package base and the pedestal (and hence the die) when these members have significantly different thermal expansion coefficients. For instance, when the die (substrate) is silicon and the package base is aluminium.

Figs.9 illustrates a further possible embodiment in which the die 3 is flip-chip bonded to the flexi 20. In this manner, electrical contacts provided on a lower surface 69 of the die 3 are directly connected to electrical contacts provided on the upper surface 45 of the flexi via, for example, solder beads 68 or other small amounts of electrically conductive material. The die 3 is in inverted orientation, as compared with the other described embodiment, such that the heaters (or other electrical contacts) are now on the lower surface 69 of the die. In this embodiment the rigid portion of the flexi (i.e. the stiffener 30) should be constructed from a material expansion matched (as closely as possible) to the die (substrate) material.

Further modifications and variations are possible within the scope of the invention. For example, instead of a single flexi-rigid PCB, two or more flexi-rigid PCBs may be arranged inside the housing 5, one or more of these being connected to the optical component. One or more of these flexis could be interconnected.

Claims

1. An optoelectronic device comprising: a substantially planar optical component having at least one electrical contact provided thereon; a housing containing the optical component; and flexi-rigid printed circuit board (PCB) means electrically connected to said at least one electrical contact on the optical component and having at least a portion thereof disposed within the housing.

2. A device according to claim 1, wherein the flexi-rigid PCB means is a single flexi-rigid PCB.

3. A device according to claim 2, wherein the substantially planar optical component has at least one said electrical contact on an upper surface thereof, which contact is wire-bonded to an upper surface of the flexi-rigid PCB.

4. A device according to claim 2, wherein the optical component is flip-chip bonded to the flexi-rigid PCB.

5. A device according to any of claims 2 to 4, wherein the flexi-rigid PCB is formed and arranged to carry electrical signals between the optical component and external electronic circuitry connected to the optoelectronic device.

6. A device according to claim 5, wherein the flexi-rigid PCB is directly connected to an electrical connector means of the device extending through the device housing, via which externally generated electrical signals may be applied to the device.

7. A device according to claim 6, wherein flexi-rigid PCB is directly connected to electrical connection pins extending through at least one wall of the housing.

8. A device according to claim 7, wherein a flexible portion of the flexi-rigid PCB is bent out of the plane of the top surface of a substantially rigid portion of the PCB to allow the connection of the PCB to the connector pins to be made.

9. A device according to claim 5, wherein the flexi-rigid PCB extends through an aperture provided in the housing, for connection to an external electrical signal source.

10. A device according to any preceding claim, wherein the flexi-rigid PCB means is disposed in substantially side-by-side relationship with the optical component.

11. A device according to any preceding claim wherein the optical component is mounted on pedestal means.

12. A device according to claim 11, wherein the pedestal means has a base which is attached to an inner surface of the housing.

13. A device according to claim 12, wherein the flexi-rigid PCB means has an aperture provided therein through which the die is connected to the housing base via the pedestal means.

14. A device according to any of claims 11 to 13, wherein the optical component comprises a substrate having planar waveguides formed thereon, and the pedestal means is made of the same material as the substrate.

15. A device according to claim 12 or claim 13, wherein the pedestal means is made of a material having a thermal coefficient of expansion which is approximately equal to the thermal coefficient of expansion of the housing base.

16. A device according to claim 11, wherein the pedestal means is mounted on the flexi- rigid PCB means.

17. A device according to claim 16, wherein the flexi-rigid PCB means comprises a flexible member which is mounted on a substantially rigid member, and the substantially rigid member is made of a material having a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the pedestal means.

18. A device according to claim 16 or claim 17, wherein the substantially rigid member is made of a material having a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the housing base.

19. A device according to any preceding claim, wherein the flexible PCB means comprises at least one flexi-rigid PCB having a structure comprising a polymer material having a dielectric constant in the range of 2.0 to 5.0.

20. An optoelectronic device substantially as described herein and with reference to Figs.l, 3 and 5.

21. An optoelectronic device substantially as described herein and with reference to Figs.l, 3 and 8.

22. An optoelectronic device substantially as described herein and with reference to Figs.l, 3 and 9.