GB2372108A

GB2372108A - Alignment of optical component using V-grooves in an optical chip

Info

Publication number: GB2372108A
Application number: GB0103455A
Authority: GB
Inventors: Emma Jane Clarissa Dawnay; Mehdi Asghari
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-02-10
Filing date: 2001-02-10
Publication date: 2002-08-14
Also published as: WO2002065172A3; WO2002065172A2; GB0103455D0; AU2002229938A1

Abstract

A method of positioning an optical component (1) by:<BR> forming V-groove alignment marks (3a-3H) on a chip and positioning the component (1) in alignment with the marks (3A-3H). The chip can be a silicon-on-chip insulator. Also disclosed is a photodiode (1) which may be aligned with a V-groove (5) in which an optical fibre (6) is located, both types of V-grooves (3A-3H;5) being formed in the same lithographic step. The photodiode (1) may be located over a reflective facet (5A, figure 3) at the end of the V-groove (5) but does not overlap the end of the fibre (6).

Description

ALIGNMENT OF AN OPTICAL COMPONENT This invention relates to the alignment of an optical component on an optical chip and, in particular, to a coupling device for coupling an optical fibre with an optical component, where the optical component requires accurate alignment with the optical fibre.

Various ways of accurately positioning or aligning components are known, e. g. by butting the component up against a step or pedestal whose position is known, or visual alignment of the component with respect to alignment marks formed in a metal layer deposited on the chip.

A coupling device for coupling and aligning an optical fibre to a waveguide integrated on an optical chip is described in US5787214. A coupling device for coupling and aligning an integrated waveguide with an optical device such as a light source or light receiver is described in US6108472. It is also known to couple an optical fibre to an optical device mounted on an optical chip by positioning the fibre within a V-groove formed in the chip and mounting the optical device over the end of the fibre in the V-groove, the V-groove having an inclined, facet at the end thereof which reflects light emitted from an end face of the optical fibre towards the optical device. Metal coatings are provided on the chip for increasing the reflectivity of the facet and for providing electrical connection to the optical component. Marks are formed in the metal coating by means of the mask used to form the coating to assist in aligning the component with the V-groove. The positioning of these marks is subject to inaccuracies in the positioning of this mask.

The prior art thus uses a variety of techniques to ensure correct alignment of components of an optical device but these entail various disadvantages.

The present invention provides an alternative way of aligning an optical component on an optical chip which is particularly advantageous when used to align a component with a V-groove formed in the chip.

According to a first aspect of the invention, there is provided a method of positioning an optical component on an optical chip comprising the steps of: forming at least one alignment mark on the chip and positioning the component on the chip in alignment with the or each of said marks, wherein the or each of the alignment marks comprises an alignment V groove formed in the chip by a lithographic process.

According to a further aspect of the invention there is provided an optical component positioned on an optical chip, the component being aligned with alignment marks provided on the chip, wherein the alignment marks comprise alignment V-grooves formed in the chip. According to another aspect of the invention there is provided a coupling device for coupling an optical fibre with an optical component comprising: an optical chip having a V-groove in a surface thereof with a reflective facet at one end thereof; an optical fibre located in the V-groove; an optical component mounted on the optical chip over the reflective facet, whereby light leaving an end face of the optical fibre is reflected by the reflective facet towards the optical component, or vice versa, wherein the normal to the end face of the fibre is inclined to the optical axis of the fibre and the fibre is spaced from the reflective facet by a distance such that the optical component does not overlap the end face of the fibre.

Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a component mounted on an optical chip in accordance with the one embodiment of the invention; Figure 2 is a plan view similar to Figure 1 with the component omitted to reveal the features beneath the component; Figure 3 is a schematic side view of the arrangement shown in Figure 1; Figure 4 is a cross-sectional view taken along line A-A shown in Figure 1; Figure 5 is an enlarged side view of part of the device shown in Figure 3; and Figure 6 is a cross sectional view taken on line B-B in Figure 5.

Figure 1 shows an optical component 1, in this case a photodiode, mounted on an optical chip 2, e. g. formed of silicon. Alignment marks 3A-3H are formed on the surface of the chip 2 and the component 1 is aligned therewith. The alignment marks each comprise alignment V-grooves formed in the surface of the chip, hereafter referred to as"mini V-grooves".

Figure 4 shows a cross-sectional view on line A-A of Figure 1 and this shows a section through mini V-grooves 3B and 3G.

As shown in the Figures, the mini V-grooves are small relative to the dimensions of the component 1. Typically, they have a width of 20 microns or less, and preferably 10 microns or less, and a length of 350 microns or less and preferably 50 microns or less.

A mini V-groove is preferably provided for alignment of at least two sides or edges of the component 1 and, preferably, these two sides are inclined to each other. In the example shown, mini V-grooves are provided for alignment of all four sides of the component 1, which, in this case, has a rectangular shape. In this example, two separate mini V-grooves are provided for alignment of each side of the component, one adjacent each end of the respective side. Thus, pairs of mini Vgrooves such as 3A and 3B are provided at right angles to each other and, together, can be used to align a corner of the component in the required position on the chip.

In another arrangement, just a single mini-V-groove may be provided, e. g. a mini V-groove of similar length to the component, against which one side of the component is aligned.

The component may be placed on the chip manually or using an automatic assembly machine and the alignment of the component relative to the mini Vgrooves judged by eye and/or optical sensing means on the assembly machine.

V-grooves can be accurately formed in the surface of the chip by well known lithographic processes. The etching process used follows crystallographic planes in the chip, the surface being etched preferably being formed of silicon, so the depth of the V-groove is accurately determined by the width and/or length of a window in a lithographic mask. Moreover, the position and alignment of each of the mini V-grooves can be determined by a single mask. They can therefore be accurately formed and accurately aligned with each other simply by fabrication of a single lithographic mask. Furthermore, the mini V-grooves can be accurately aligned with a further feature etched in the chip as the positions of both the mini Vgrooves and that of the further feature can be determined by a single mask.

Lithographic techniques for fabricating features such as V-grooves in a silicon layer, e. g. in a silicon-on-insulator chip, are well known, so will not be described further. it should also be noted that V-grooves may be etched so that the sides meet at a line at the base thereof as shown in Figure 4 or may only be etched to a depth which leaves a flat base between the sides thereof, e. g. to a depth sufficient to accommodate a fibre as shown in Figure 6.

V-grooves are formed on optical chips for a variety of other purposes as part of an optical device, e. g. for receiving and locating the end of an optical fibre on the chip. The mini V-grooves can thus be formed at the same time as such further Vgrooves using the same mask. When the optical component 1 is to be aligned with such a further V-groove, this ensures that the mini V-grooves are easily fabricated in the appropriate positions to ensure precise alignment of the component 1 with the further V-groove. This provides a significant advantage over the prior art as, in the prior art, alignment masks are formed by a separate process to such further V-grooves and complex, expensive processes and equipment are required to ensure that these different features are accurately positioned and aligned with each other.

The example of a component 1, such as a photodiode, mounted in alignment with a V-groove 5, which, in turn, receives and locates an optical fibre 6, is shown in the drawings.

The optical fibre 6 typically has a diameter of about 125 microns and, in the arrangement shown, is mounted in a V-groove 5 having a width in the range 180240 microns, and preferably around 210 microns (the width of the groove determines its depth). As shown in Figures 3 and 4, the fibre 6 projects from the V-groove 5 beyond the surface 2A of the chip 2.

As shown in Figures 2 and 3, the V-groove 5 terminates in an inclined end face 5A which forms a reflective facet. A metal coating 7, e. g. of aluminium or gold, is preferably provided on this end face 5A to increase its reflectivity. As will be discussed further below, the depth of the V-groove is selected relative to the end face 5A so as to optimise reflection from the face 5A to the component 1.

The component is mounted on the surface 2A of the chip 1 over the reflective end face 5A so light emitted from the end face 6A of the optical fibre 6 is reflected by the end face 5A towards the component 1, as shown schematically by dashed lines in Figure 3.

The component 1 needs to be accurately aligned in orthogonal directions parallel to the surface 2A of the chip to ensure that its input aperture (not shown) is accurately positioned to receive substantially all of the light reflected towards the component 1 by the end face 5A.

The position of the component 1 in the direction normal to the surface 2A is determined by its location on the surface 2A, although a thin metal layer 8 may be provided on the surface 2A to provide electrical contact with the component 1.

Figure 2 shows a plan view of the arrangement shown in Figures 1 and 3 but with the component 1 omitted so as to show the metal coatings 7 and 8 and the end face 5A.

A shown in Figures 1 and 3, the end face 6A of the optical fibre is spaced from the end face 5A by a sufficient distance to permit the component 1 to be mounted on surface 2A of the chip over the facet 5A. Preferably, the end face 6A of the fibre is spaced from an edge of the component by a distance D of at least 10 microns.

This arrangement provides a further advantage as the normal to the end face 6A is inclined to the optical axis of the fibre 6, e. g. by an angle of up to 10 degrees, in order to reduce back reflections from the end face 6A. In order to provide a low loss coupling, it is important to locate the fibre 6 within the V-groove 5 in the appropriate angular orientation about its axis so that light emitted from the end face 6A is not refracted sideways away from the end face 5A. The component 1 is usually mounted on the chip first, aligned with the mini Vgrooves described above, and the fibre 6 subsequently positioned in the V-groove 5. If the end face 6A of the fibre 6 lay underneath the component 1, it would be hidden so it would be difficult to determine whether or not the end face 6A was in the correct angular orientation. However, in the arrangement shown in the Figures, the fibre 6 is positioned so its end face 6A is visible from above the chip so it is easier to ensure that it is in the appropriate angular position before the fibre 6 is secured in place.

Mini V-grooves 9 and 9B (or just one of these) may also be provided on opposite sides of the V-groove 5 with which the end face 6A of the fibre is aligned to determine the described spacing Z between the end face 6A of the fibre and the end face 5A of the V-groove (see Figure 1).

The above arrangement is made possible due to the fact that the optical fibre 6 is a single mode fibre so has a relatively narrow core 6A, typically 2 to 15 microns in diameter, so light emitted from the core 6A diverges less than for a multi-mode fibre. Light emitted from the core 6A diverges to some extent and, to keep losses to a minimum, it is important to ensure that substantially all the light falls upon the end face 5A and is then reflected into the input window 1A of the component 1. There are several factors which affect this as illustrated in Figures 5 and 6. These factors include the following : P: The half width of the component from its overhanging edge to the centre of its input window 1A.

Z: The spacing between the end face 5A and the end face 6A.

E: the depth of the centre of the fibre core 6A beneath the surface of the chip L: the distance the centre of the component's input window 1A overhangs the end of the V-groove.

W: the width of the V-groove at the surface of the chip.

8 : the angle of the end face 6A to a plane normal to the optical axis of the fibre 6.

These factors are related in a number of ways so that to maintain low losses a change in one parameter requires changes to be made to the other parameters.

For instance: E should increase as P increases, Z should increase as P increases, L should increase as P increases, and W should increase as P increases.

Although losses generally increase as Z increases with an appropriate selection of the other parameters, these losses can be kept to an acceptable level.

As shown in the Figures, the V-groove 5 and fibre 6 are arranged so that the fibre core 6A lies beneath the surface of the chip in alignment with the end face 5A of the V-groove. However, the fibre is not buried within the chip but projects above the surface of the chip. Accordingly, the end face 5A of the fibre 6 is positioned adjacent or spaced from the edge of the component 1 as it cannot be slid beneath the component 1. Taking the above factors into account, the arrangement can be designed such that the variation in the position at which the light beam from the optical fibre 6 falls on the input window 1A of the component 1 is relatively small as the angular position of the fibre (and hence the angled end face 6A thereof) about the fibre axis is varied. By appropriate design, it can be arranged so that said position varies by less than 5 microns in any direction if the angled end face is positioned with 10 degrees of its optimum angular position about the fibre axis.

The above arrangement thus provides alignment V-grooves accurately positioned relative to a V-groove for receiving an optical fibre or to some other alignment feature. Optical components aligned with these respective features are thus accurately aligned with each other. In the example given, an optical component such as a photodiode is accurately aligned with an optical fibre by aligning the photodiode with alignment V-grooves and locating the optical fibre in a further Vgroove. The arrangement described also enables the rotational alignment of the fibre to be accurately determined.

Claims

CLAIMS 1. A method of positioning an optical component on an optical chip comprising the steps of: forming at least one alignment mark on the chip and positioning the component on the chip in alignment with the or each of said marks, wherein the or each of the alignment marks comprises an alignment V groove formed in the chip by a lithographic process.
2. A method as claimed in claim 1 in which the or each of the alignment V grooves has small length and/or depth dimensions relative to the dimensions of the component.
3. A method as claimed in claim 1 or 2 in which the or each of the alignment V-grooves has a width of 20 microns or less and preferably 10 microns or less.
4. A method as claimed in claim 1,2 or 3 in which the or each of the alignment V-grooves has a length of 350 microns or less and preferably 50 microns or less.
5. A method as claimed in any preceding claim in which at least two sides or edges of the component are each aligned with a respective alignment V groove.
6. A method as claimed in claim 5 in which said at least two sides or edges are inclined to each other.
7. A method as claimed in any preceding claim in which a further feature is etched in the chip, the alignment V-groove (s) being for aligning said component relative to this feature.
8. A method as claimed in claim 7 in which said feature is a further V-groove for receiving and locating the end of an optical fibre on the chip.
9. A method as claimed in claim 7 or 8 in which the positions of the alignment V-groove (s) and the further feature are defined by the same lithographic mask.
10. A method as claimed in claim 9 in which the optical component is mounted in alignment with said further V-groove, the alignment V-grooves being used as alignment marks with which the component is aligned to ensure alignment with said further V-groove.
11. An optical component positioned on an optical chip, the component being aligned with alignment marks provided on the chip, wherein the alignment marks comprise alignment V-grooves formed in the chip.
12. An optical component as claimed in claim 11 in which the alignment V grooves have small length and/or depth dimensions relative to the dimensions of the component.
13. An optical component as claimed in claim 11 or 12 in which the alignment V-grooves have a width of 20 microns or less and preferably 10 microns or less.
14. An optical component as claimed in claim 11, 12 or 13 in which the alignment V-grooves have a length of 350 microns or less and preferably 50 microns or less.
15. An optical component as claimed in any of claims 11 to 14, in which a further V-groove is provided on the chip for receiving and locating the end of an optical fibre, the further V-groove terminating in a reflective face, wherein the component is mounted on the chip over the reflective face so that light is reflected by the reflective face towards the component, or vice versa, the component being aligned with said alignment V-grooves which are positioned to ensure the component is aligned with the reflective face.
16. An optical component as claimed in claim 15 in which the component comprises a photodiode.
17. An optical component as claimed in claim 15 or 16 in which an optical fibre is located within the further V-groove, the further V-groove being of a size such that the fibre projects from the V-groove beyond the surface of the chip, the end face of the fibre being spaced from the reflective face by a sufficient distance to allow the component to be mounted on the surface of the chip over the reflective face.
18. An optical component as claimed in any of claims 11 to 17 in which the part of the chip in which the alignment V-grooves are formed is of silicon.
19. An optical component as claimed in claim 18 in which the chip is a silicon on-insulator chip.
20. A method of positioning an optical component on an optical chip substantially as hereinbefore described with reference to the accompanying drawings.
21. An optical component positioned on an optical chip substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.
22. A coupling device for coupling an optical fibre with an optical component comprising: an optical chip having a V-groove in a surface thereof with a reflective facet at one end thereof; an optical fibre located in the V-groove; an optical component mounted on the optical chip over the reflective facet, whereby light leaving an end face of the optical fibre is reflected by the reflective facet towards the optical component, or vice versa, wherein the normal to the end face of the fibre is inclined to the opticai axis of the fibre and the fibre is spaced from the reflective facet by a distance such that the optical component does not overlap the end face of the fibre.
23. A coupling device as claimed in claim 22 in which the V-groove is of a size such that the fibre located therein stands proud of the surface of the optical chip.
24. A coupling device for coupling an optical fibre with an optical component substantially as hereinbefore described with reference to the accompanying drawings.