GB2213957A

GB2213957A - Waveguide to opto-electronic transducer coupling

Info

Publication number: GB2213957A
Application number: GB8823873A
Authority: GB
Inventors: James Wilson Parker
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1987-12-15
Filing date: 1988-10-12
Publication date: 1989-08-23
Anticipated expiration: 2008-10-12
Also published as: GB8729253D0; GB8823873D0; GB2213957B

Abstract

Optical coupling between a ridge waveguide 20 and an optoelectronic transducer, e.g. a conventional surface sensitive photodetector, 22 is made by mounting the transducer (photosensitive face down) on a slotted silicon carrier 10 which is itself mounted with the ridge waveguide engaged in the slot. A crystallographically etched obliquely inclined surface at the root of the carrier slot reflects the light emerging from the ridge waveguide up to the transducer overhead. The place of the ridge waveguide may alternatively be taken by an injection laser diode. <IMAGE>

Description

WAVEGUIDE TO OPTO-ELECTRONIC TRANSDUCER COUPLING For a number of applications it is convenient to guide light by means of waveguides that extend close to associated substrates. (The term 'light' is used here and elsewhere in this specification to include not only that part of the electromagnetic spectrum which is in the visible range but also those parts adjacent to the visible range).

One form of waveguide that extends close to a substrate is a ridge waveguide. Such a waveguide takes the form of a substrate from which protrudes a ridge of material of one refractive index which envelopes a core of material of a higher refractive index. Typically the ridge may be no more than a few microns high and a few microns wide. Typically it is desirable to be able to couple light emerging from such a waveguide efficiently into a photodetector or into the waveguide from an optical source. Edge sensitive photodetectors exist which can simply be butted up against the end of a ridge waveguide so that the emergent light impinges upon the photo sensitive region, but such detectors are more complex, more costly and generally less readily available than the more conventional surface sensitive devices.To couple light efficiently intb a conventional surface sensitive device presents difficulties because the axis of the light emerging from the ridge waveguide is parallel to the plane of the substrate upon which the ridge waveguide is formed, typically not many microns away from that surface. In principle this problem is capable of being overcome by mounting the surface sensitive photodetector on edge in a pit formed in the substrate surface just beyond the end of the ridge waveguide, but such a technique is unattractive in view of the difficulties liable to be encountered in forming such a pit and in locating the detector in the pit, and in making electrical connection with the detector.

Another form of waveguide that extends close to a substrate comprises a double hetero-structure injection laser mounted on a heat sink. In a double hetero structure laser a waveguide is formed by having a layer of one material sandwiched between two layers of a different material having a lower refractive index.

Such an arrangement of layers provides a waveguiding effect in a direction normal to the plane of the layers. This waveguiding effect may be supplemented by incorporation into the laser a structure that provides a waveguiding effect in an orthogonal direction. For example a waveguiding effect in an orthogonal direction can be provided by a ridge of semiconductive material located above the planar layers of the hetero-structure and protruding into material of lower refractive index than that of which the ridge is made.

According to the present invention there is provided a waveguide to optoelectronic transducer coupling in which the waveguide extends parallel to one surface of a supporting substrate and the transducer is mounted on a carrier embracing one end of the waveguide, and wherein the carrier is provided with a crystallographically etched planar surface obliquely aligned with respect to the optical axis of the waveguide so as to reflect light emergent from the waveguide towards the active surface of the transducer.

There follows a description of ridge waveguide to photodetector couplings embodying the invention in preferred forms. The description refers to the accompanying drawings in which: Figure 1 depicts a perspective view of a coupling carrier, Figure 2 depicts the carrier in position around the end of a ridge waveguide, Figures 3a and 3b depict successive stages in the manufacture of the ridge waveguide of Figure 2, Figure 4 depicts a modified form of carrier, Figure 5 depicts the carrier of Figure 1 in position around an injection laser diode, and Figure 6 depicts the laser diode of Figure 5 in greater detail.

The coupler of the present invention relies upon the use of a plane reflecting surface to direct the light emerging from the ridge waveguide into a direction inclined at a relatively large angle to the plane from which the ridge waveguide protrudes. In order to provide in a relatively cheap manner a reflector of suitable size, flatness and orientation, reliance has been placed upon the fact that under suitable circumstances crystallographic etching will produce etch planes of specific crystal plane orientation. Thus for instance it is well known that a single crystal slice of silicon extending in the (100) plane can readily be etched so as to produce wells with (111 '3 plane side walls that are inclined at approximately 54.7 to the plane of the slice.To produce a specific angle of inclination all that is necessary is to choose an appropriate plane for the cutting of the original slice and the orientation of photolithographic mask which defines the well.

Referring now to Figure 1 there is depicted a carrier 10 made of silicon. This is cut from a (100) oriented slice and is initially cuboid in shape before the etching of a slot in such a way as to leave plane side walls lla, llb and llc.

In Figure 2 this carrier is shown in position around the end of a ridge waveguide 20 that protrudes from one face of a substrate 21. The ridge waveguide comprises a strip of optical core material 20a encased in an optical material 20b of lower refractive index than that of the optical core 20a. Light propagating along the ridge waveguide 29 to emerge from the end -inserted into the slot in the carrier 10 strikes the end wall llb and is reflected upwardly to strike the active (photosensitive) surface of a photodetector 22 (shown in broken outline) mounted upon the carrier photosensitive surface face down. If desired, the end wall llb may be metallised in order to provide improved reflectivity.

Typically the ridge waveguide 20 of Figure 2 is made by the process now to be described with reference to Figures 3a and 3b. First the surface of the substrate 21, which is made of silicon, is provided with a silica buffer layer 30 and then this is covered with a further layer 31 of higher refractive index glass, typically silica doped for instance with germania or titania. These layers are each typically about 10 microns thick. In the next processing step the core 20a of the ridge is delineated by masking and reactive ion etching the silica layers. After this etching the core 20a formed by the residual material of layer 31 is supported upon an underlying strip 32 of lower refractive index cladding glass formed by the residual material of layer 30, but the other three sides of the strip of core glass remain exposed. These exposed sides are covered by the deposit of a further layer 33 of silica, and finally further masking and etching is used to remove unwanted material of layer 33 and leave the ridge waveguide structure as depicted in Figure 2 comprising a strip of core glass 20a surrounded on all four sides by lower refractive index cladding glass 20b. This second etching stage is only required in the region of the carrier.

Having regard to the small size of the core of a typical ridge waveguide in relation to the area of the photosensitive surface of a photodiode, the width of the slot in the carrier can readily be determined with sufficient tolerance to provide adequate lateral alignment of the photodetector with respect to the ridge. The end of the ridge waveguide within the carrier slot should preferably have a smooth plane surface whose normal is approximately aligned with the waveguide axis, but, again because of the relative sizes of guide and diode, precise alignment is not required.

Generally it is convenient to advance the carrier 10 over the end of the ridge waveguide until the end of the guide becomes butted against the foot of the end face llb of the carrier. However, if the angle of this end facet is such that the reflected light clips the'top of the ridge, rather than rely upon an arbitrary spacing of the components, it may be preferred to provide this end face of the carrier with a stepped structure as depicted in Figure 4. This provides a reflecting facet 41b rearwardly displaced with respect to two abutment shoulders 42.

If the photodetector is of the type provided with solder bumps for terminal connection on its front surface, electrical connection with these may be provided by metallisation tracks and pads (not shown) deposited on the carrier at the same time as the reflecting end facet llb is metallised. Alternatively, wire bonding between the metallised pads on the detector and metallisation provided on the carrier or the substrate may be used to provide electrical connection.

Turning attention now to Figure 5 there is in this figure depicted an arrangement in which light emitted from the near face of an injection laser 50 mounted on a heat sink 51, for instance a ridge structure injection laser, is reflected by the reflecting facet 52 of a silicon carrier 53 of similar basic construction as the carrier 10 of Figures 1 or 4 on to the active (photosensitive) surface of a monitor photo diode 54. The form of a specific ridge structure laser 50 given by way of example is depicted in greater detail in Figure 6. This comprises an n±type indium phosphide (InP) substrate 61 upon which are grown a succession of layers 62 to 66 by liquid or vapour phase epitaxy. The first epitaxial layer to be grown, layer 62, is an n-type InP buffer layer, typically between 2 and 5 um in thickness.Its growth is succeeded by the growth of layer 63 which is the active layer of the device. The active layer is thinner, typically being in the range from 0.08 um to 0.50 um in thickness, and is made of p-type or n-type InGaAsP. Its composition is chosen having regard to the wavelength of emission required from the device. Layer 64 is a p-type anti-meltback/guide layer, also of quaternary InGaAsP, but of a composition corresponding to a shorter emission wavelength than that of the active layer. The thickness of layer 64 lies typically in the range 0.1 to 0.3 um.

The remaining epitaxial layers of the structure, layers 65 and 66, are respectively a p-type cladding layer of InP which is typically about 1.5 um thick, and a p-type contact layer of InGaAsP which is typically about 0.2 um thick and may conveniently have the same composition as that of layer 64. Wet chemical etching is employed to etch two channels through the contact and cladding layers 66 and 65 so as to define an intervening ridge 67 that is typically between 3 and 5 um in width. These channels are etched through an oxide mask (not shown) after photolithography.

Layer 69 of silica to a depth of about 0.3 um is deposited over the surface to provide electrical insulation and then a window registering with the ridge is opened in this silica. Next the substrate 61 is thinned from about 300 um to about 100 um before the deposition of an evaporated TiPtAu p-type contact metallisation layer 70 on the top surface and an evaporated AuSnAu n-type contact metallisation layer 71 on the bottom surface. These metallisation layers are then alloyed in to the semiconductor material. In the case of layer 70 this alloying-in occurs only on the ridge 67 because elsewhere the silica insulation 69 acts as a mask.

Referring once again to Figure 5, the laser chip 50 is mounted epitaxial layers face up or face down, as desired, upon the heat sink 51 which provides one electrical connection for the chip while the other is provided by a wire bonding flying lead 55.

Claims

CLAIMS:

1. A waveguide to optoelectronic transducer coupling in which the waveguide extends parallel to one surface of a supporting substrate and the transducer is mounted on a carrier embracing one end of the waveguide, and wherein the carrier is provided with a crystallographically etched planar surface obliquely aligned with respect to the optical axis of the waveguide so as to reflect light emergent from the waveguide towards the active surface of the transducer.

2. A coupling as claimed in claim 1 wherein the crystallographically etched planar surface is provided with a reflection enhancing coating.

3. A coupling as claimed in claim 1 or 2 wherein the carrier is located with respect to the waveguide in part by an abutment on the carrier abuting a portion of the end face of the waveguide.

4. A coupling as claimed in claim 1, 2, or 3 wherein the carrier is located with respect to the waveguide in part by a slot formed in the carrier the width of which is dimensioned to receive the waveguide.

5. A coupling as claimed in any preceding claim wherein the carrier is made of silicon.

6. A coupling as claimed in any preceding claim wherein the waveguide is a ridge waveguide.

7. A coupling as claimed in any claim of claims 1 to 5 wherein the waveguide is a waveguide forming a constituent part of an injection laser diode.

8. A coupling as claimed in claim 7 wherein the laser diode is a ridge structure laser diode.

9. A ridge waveguide to photodetector coupling substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

10. A laser diode to monitor photodiode coupling substantially as hereinbefore described with reference to Figure 6 or Figures 5 and 6 of the accompanying drawings.