WO2003034115A2

WO2003034115A2 - Coupling a semiconducter laser to an optical waveguide

Info

Publication number: WO2003034115A2
Application number: PCT/GB2002/004692
Authority: WO
Inventors: John Hughes; Nicholas Parsons; Stephen Wood; Ian Flint; Paul Jerram
Original assignee: Bookham Technology Plc
Priority date: 2001-10-17
Filing date: 2002-10-17
Publication date: 2003-04-24
Also published as: GB0124916D0; GB2381082A8; GB2381082A; AU2002334184A1; WO2003034115A3

Abstract

A waveguide (10) is formed by using a single mask to define waveguide features and alignment features on the core layer (16). Etch stop is retained on the alignment features when the cladding layer (18) is added and a second mask defined which defines a mechanical stop and a waveguide facet aligned to the stop. The mask area is etched down to the substrate apart from the protected alignment features. Laser and other components can be attached. Lasers (65) may be attached by solder bumps (76) and have their own alignment feature spaced from the laser facet.

Description

LASER HYBRIDISATION

This invention relates to semiconductor lasers, and in particular to the hybridisation of semiconductor lasers onto a wafer which supports a waveguide structure.

There are many techniques known in the art for attaching lasers and other components to silicon or silica substrates using mechanical alignment features. Examples of these techniques are disclosed in US5,721,797 and WO01/06285. In US5,721,797, alignment features are created using a mask which is separate to that used to define the waveguide. This gives rise to alignment tolerances between the alignment features layer and the waveguide layer which are unsatisfactory. Furthermore, vertical height alignment is achieved by etching steps in the silicon and then growing the waveguide in the lower layer of silicon. As a result, errors in the etching of the silicon step and errors in the growth of the silica layer will both contribute to vertical alignment errors. This is highly unsatisfactory. ϊn WO01/06285, a single mask is used to create the core glass and the alignment features. This is advantageous as it reduces the possibility of alignment errors. However, the process disclosed requires the -alignment features, to be created in the top layer of the buffer glass. This approach is disadvantageous as it requires alignment features to be formed on top of a layer of etch stop. Etch stop . is not intended to have layers grown on it and the resultant^' features are not as well defined or as of high quality as would be preferred. Moreover, as the etch stop has to be removed, the chemicals used for the removal can attack and smooth the alignment features. WO01/06285 provides for tolerances along the optical axis by providing bumps on the laser which butt against slightly tapered rails on the waveguide . As the angle of the taper is low, tolerance in the etching of the waveguide or the growth of the alignment features on the laser will be magnified by the taper angle.

Furthermore, the process relies on precision cleaving of the laser facet and precision alignment of the gold bumps on the laser which contribute to alignment errors. The invention aims to overcome the disadvantages with the prior art arrangements and techniques outlined above. In its broadest form, the invention creates alignment features for the two critical axes in the core layer which are produced in the^' same operation as the waveguide. More specifically, there is provided a method of fabricating, a waveguide having a core layer deposited on a buffer layer arranged on a substrate, comprising forming a masking layer defining the waveguide and at least one component alignment feature, for aligning a component with the waveguide, on the core layer.

The invention also provides a waveguide comprising a substrate, a buffer layer arranged. on the substrate and a core layer deposited on the buffer layer, the core layer comprising waveguide features and at least one alignment feature for aligning a component with the waveguide.

Embodiments of the invention have the advantage that by forming the alignment features from the same masking layer and etch as the waveguide, the alignment features can be near perfectly aligned in two axes. Preferably, near perfect alignment in a third axis may be achieved by forming a further masking layer over protected alignment features, the mask overlapping the waveguide and defining a mechanical stop, and etching. This etch defines the waveguide facet which is therefore aligned with the mechanical stop. The original alignment features can then be revealed by removing the protection.

Preferably, the waveguide is a silica waveguide, the core layer is doped glass and the buffer is also glass.

A wide variety of components may be attached to a- waveguide embodying the invention.

The invention also provides a method of attaching a semiconductor laser to a waveguide, comprising fabricating the waveguide by the method according to the invention as defined above, forming an alignment feature on the laser and attaching the laser to the waveguide such that the alignment feature on the laser abuts the at least one alignment feature on the waveguide.

The invention also provides a semiconductor laser hybrid comprising a semiconductor laser attached to a waveguide according to the invention as defined above, the laser comprising an alignment feature abutting the at least one alignment feature^' on the waveguide.

The laser may include a mode expander or the waveguide end may include a lens. The laser may be attached using solder bumps .

Embodiments of this aspect of the invention have the advantage that the alignment of the laser and the waveguide may be so precise that the laser may be positioned and

.attached in place passively, with no optical feedback. As a result, many lasers may be fabricated onto a single silicon wafer providing a high volume low cost assembly technique. Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a waveguide on a substrate; Figure 2 is a flow chart showing steps in the process of embodying the invention;

Figure 3 shows schematically a waveguide section and alignment features;

Figure 4 shows a plan of the components of Figure 3 with a pattern defined on the top of the cladding glass; and Figure 5 shows side and end views of a laser showing an attachment feature and a facet.

The following description describes the fabrication of a structure in a waveguide and the assembly of a laser onto the waveguide. The structure of the waveguide is shown in

Figures 1, 3 and 4 and the process is shown in the flow chart of Figure 2. The waveguide 10 shown in Figure 1 is typically grown onto a silicon substrate 12. The waveguide comprises a buffer or under clad layer 14, a core glass layer 16 and cladding or over clad 18. The buffer layer 14 has a thickness of approximately 15 microns, the core layer 16 , a thickness of approximately 5 microns and the cladding 18, a thickness of approximately 15 microns.

The buffer, the core layer and the cladding are all typically made of silica glass. To confine the optical signal carried by the core layer 16, the core layer is doped, typically with germanium (Ge) to increase its refractive index with respect to the buffer and cladding layers 14, 18 by between approximately 0.3 to 1.3%. The buffer is first deposited on the silicon substrate (100 Fig. 2) . The core glass is then deposited over the whole of the buffer glass (102 Fig. 2) usually by plasma enhanced

Chemical Vapour Deposition (CVD) or by flame hydrolysis. The waveguide structure is formed in the core layer by defining the structure in a layer of etch stop, such as polysilicon, which is deposited on the core glass (104 Fig. 2) . The core glass is then dry etched away leaving the required waveguide features which were covered by etch stop. (106 Fig. 2) .

In embodiments of the present invention, the alignment features in the waveguide are created in the core glass for two critical axes. Thus, the alignment features are created in the same layer as the waveguide features and are produced in the same etch mask that defines the waveguide features. As a result, the alignment features are nearly perfectly aligned with the waveguide. Thus, at Figure 2, at step 104 both the waveguide structure and alignment features 'are defined in polysilicon etch stop.

Figure 3 shows a section of the waveguide 10 which includes a sacrificial end 20 which will be formed into a facet to couple to the laser. A pair of alignment features 30 are provided which are used to align the semiconductor laser when it is mounted. The alignment features 30 include a number of triangular projections 40 extending from one of the alignment features towards an opposite face of the other of the pair. These triangular projections abut a recessed edge of the laser and are intended to minimise the contact area with the laser to reduce stiction effects and remove the possibility of yield losses due to process problems. Other shaped projections could be used.

In a conventional fabrication process, the polysilicon etch stop would be removed at_. this stage. However, in the process embodying the invention, the polysilicon above the alignment feature is protected (108 Fig. 2) and then the polysilicon removed from the rest of the structure (110 Fig.. 2 ) . A cladding layer 18 (Fig. 1) is then grown onto the core ■ glass (112 Fig. 2) .

At this point, a pattern is defined on the top of the cladding glass which is aligned to the waveguide and the alignment features. This is shown at step 114 in Figure 2. .The alignment of this pattern, which is shown by the dashed line 50 in Figure 4 is not critical except that it must overlap the waveguide in area and must overlie the alignment features.

The pattern is used to define a second etch stop mask which is used to protect all the area outside the pattern area. (Step 116 Fig. 2). That is, all the area outside the

. dashed line 50 in Figure 3. All the silica within the pattern is then etched away in a second dry etch process, right down to the silicon substrate (118 Figure 2) . The polysilicon etch stop left on the area of the alignment features ensures that these features are untouched in the etching process. On completion of the etch, the polysilicon etch stop material .will be removed to leave an exposed reference surface 32 that is exactly coplanar with the top of the waveguide edge and having an exposed reference edge

34 perfectly aligned with the waveguide. (Step 120, fig. 2

) - During the etch, and due to the shape of the pattern

50, a pair of mechanical stops 60 are defined which fix the gap between the laser and the waveguide. It will be appreciated that the exact position of the waveguide facet

(90) will be formed by the second etch as the polysilicon on the waveguide is removed before the second etch. As the mechanical stops are defined in the same mask as defines the facet they will be perfectly aligned.

It will be appreciated that the process described produces mechanical features that provide for perfect alignment with the waveguide facet in all three axes. These features may be used to align and attach a laser or a number of other different components to the wafer such as fibre optic guides, filters, isolators and photodiodes. These are only examples and this list is not exclusive. The method described produces very accurate alignment to all three axes of the waveguide facet as the facet and alignment features are self aligned. This is achieved in two stages using two masks. The first producing alignment in two axes and the second providing alignment in the third.

Where the device to be attached to the waveguide structure is a laser, some limited processing of the laser is required. Referring to Figure 5, a feature 70 is created in the laser 65 at a position that is very well defined with respect to the laser stripe 72. This feature is preferably created using the same mask that is used to create the laser stripe, ensuring that the feature is aligned to the facet with a very high precision. The laser alignment feature may be created using a combination of dry and wet etch processes and must be sufficiently far away from the laser stripe 72 so as not to interfere with laser operation. The height of the laser alignment feature 70 can either be defined with a selective etch stop layer in the laser, or sufficient accuracy may be achieved by dry etching without a stop layer. It will be appreciated that the alignment is both a surface 74 and edge alignment 76.

Preferably, the laser includes a mode expander to better match the laser mode shape to the waveguide. If an expanded mode laser is not used, there will be significant losses unless the waveguide is lensed. The laser is attached to the waveguide using solder bumps 76 under the laser stripe in area 74 of Figure 3 between the pair of alignment features 30. The solder bumps 76 pull the laser down onto the top of the reference surface and conduct heat away from the laser into the silicon substrate. The very high thermal conductivity of silicon makes it a preferred substrate material when the process described is used to mount a laser.

The laser is mounted, flipped so that the active layer is down onto the waveguide and soldered into position. During soldering the laser is pushed against the mechanical stops 60 and the shaped projections 40. Alternatively, the solder bumps could be offset slightly causing them to pull the laser up to the stops. It will be appreciated that the embodiment described has the advantage of providing a very high precision alignment of the alignment features to the waveguide facet. Alignment along two axes is provided by creating the alignment features in the same masks as the waveguides and the facet. Alignment along the third axes is achieved by forming mechanical stop during the same process which creates the facet.

_.When used with a laser, the laser output can be aligned with such precision to the waveguide structure that the laser may be positioned and attached entirely passively, that is without any feedback. The process enables many lasers to be fabricated onto a single silicon wafer thus providing a low cost, high volume, assembly technique.

Many modifications to the embodiments described are possible and will occur to those skilled in the art without departing from the scope of the invention which is defined in the following claims. For example, the waveguide has been described as a silica waveguide, but other waveguides may be used, for example silicon or polymer waveguides.

Claims

1. A method of fabricating a waveguide having a core layer deposited on ^• a buffer layer arranged on a substrate, comprising forming a masking layer defining the waveguide and at least one component alignment feature, for aligning a component with the waveguide, on the core layer.

2. A method according to claim 1, comprising selectively etching the masking layer to remove a portion of the masking layer defining the waveguide but retaining a portion of the masking layer defining the of at least one alignment feature.

3. A method according to claim 2, comprising forming a cladding layer onto the core layer, defining a further masking layer over the at least one alignment feature, the further masking layer overlapping the waveguide, and etching the area defined by the further masking layer but not including the retained first masking layer portion to the substrate to define a waveguide facet.

4. A method according to claim 3, wherein the further^' masking layer defines the waveguide facet and a mechanical stop referenced to the waveguide facet to define a gap between the component and the waveguide facet.

5. A method according to claim 3 or 4, comprising removing the further masking layer and the retained masking layer portion.

6. A method according to any of claims 1 to 5, wherein the waveguide is a silica waveguide having a doped glass core layer and a glass buffer layer.

7. A method of attaching a semiconductor laser to a waveguide, comprising fabricating the waveguide according to the method of any of claims 1 to 6, forming an alignment feature on the laser and attaching the laser to the waveguide such that the alignment feature on the laser abuts the alignment feature of the waveguide .

8. A method according to claim 7, wherein the laser includes a mode expander.

9. A method according to claim 7 or 8, wherein the laser is attached by solder bumps arranged on the waveguide under the laser stripe.

10. A method according to claim 9, wherein the solder bumps are slightly offset from the axis of the laser .

11. A method according to any of claims 6 to 9 wherein the alignment feature on the laser is formed using the same mask used to form the laser edge stripe, and the alignment feature comprises a step having a height defined by etch stop or etch time.

12. A waveguide, comprising a substrate, a buffer layer arranged on the substrate and a core layer deposited on the buffer layer,, the core layer comprising waveguide features and at least one alignment feature for aligning a component with the waveguide .

13. A waveguide according to claim 12, wherein the waveguide features and the at least one alignment feature are created using a single mask.

14. A waveguide according to claim 12 or 13, comprising a cladding layer.

15. A waveguide according to claim 14, wherein the cladding layer includes at least one mechanical stop aligned to a waveguide facet.

16. A semiconductor laser hybrid comprising a semiconductor laser attached to a waveguide according to any of claims 11 to 14, the laser comprising an alignment feature abutting the at least one alignment feature on the waveguide.

17. A semiconductor laser hybrid according to claim 16, wherein the alignment feature on the laser is spaced from the laser facet sufficiently to avoid interference with operation of the laser.

18. A semiconductor laser hybrid according to claims 16 or 17, wherein the laser includes a mode expander.

19. A semiconductor laser hybrid according to any of claims 16 to 18, wherein the substrate is silicon.

20. A semiconductor laser hybrid according to any of claims 16 to 19, wherein the laser is attached to the waveguide by solder bumps.

21. A semiconductor laser hybrid according to any of claims 16 to 20, wherein the alignment feature on the laser is formed using the same mask as used to form the laser stripe, and the alignment feature comprises a step having a height defined by etch stop or etch time.

22. A device according to any of claims 12 to 21, wherein the waveguide is a silica waveguide, the core layer is a doped glass core and the buffer layer is a glass layer.

23. A method of fabricating a waveguide substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings .

24. A method of attaching a semiconductor laser to a waveguide substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings.

25. A waveguide substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings.

26. A hybrid semiconductor laser substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings .