Process for making optical waveguides

A process for treating a waveguide structure which comprises a silicon substrate with an integrally formed rib waveguide is described. The waveguide has an end portion with a facet, the end portion overhanging the silicon substrate and having an oxide layer on its underside protruding from the facet of the waveguide. A nitride layer extends over the upper surface of the waveguide and the facet. The treatment process involves etching the oxide layer from the underside, growing a new oxide layer, etching the nitride layer and then depositing a new nitride layer.

FIELD OF INVENTION
 The present invention relates to a process for making optical waveguides.
 BACKGROUND OF THE INVENTION
 Optical fibre communication systems and optical fibre based instruments and
 devices often require the accurate alignment and reliable attachment of
 optical fibres with integrated optical devices such as waveguides
 integrated on a substrate. One important consideration in the design of
 such optical connections is that good alignment is achieved between the
 waveguide and the optical fibre. A typical structure of such an optical
 connection is that of a fibre set within a V-groove to a waveguide
 integrated on a silicon substrate. Such a structure based on silicon rib
 or ridge waveguides integrated on silicon insulator wafer is described in
 PCT GB96/01068. In order to achieve a good connection, the fibre should be
 brought to within a gap of 5.mu.m or less of the waveguide facet. Since
 the V-groove does not have an end face perpendicular to the base of the
 groove, but rather it is set at an angle towards the base, it is desirable
 to undercut the waveguide to form a waveguide structure overhanging the
 angled face like a "diving board". Such a concept can be found in
 PCT/GB96/01068. Whilst this particular concept is a desirable way of
 achieving the appropriate alignment features, processing tolerances can
 cause the overhanging structure to exhibit an unwanted "shelf" of buried
 oxide extending beyond the end facet of the waveguide after the V-groove
 has been formed. When a nitride layer is deposited over the end face
 containing the oxide shelf, a small nitride "shelf" can likewise be
 formed. If left, the shelves would distort the exit of optical waves from
 the waveguide. It is an aim of this invention to improve the surface
 quality of the waveguide facet to provide an improved optical connection
 to an optical fibre.
 SUMMARY OF THE INVENTION
 According to one aspect of the present invention there is provided a
 process for treating a waveguide structure comprising: a silicon substrate
 with an integrally formed rib waveguide, the waveguide comprising an end
 portion with a facet, the end portion overhanging the silicon substrate
 and having an oxide layer on its underside protruding from the facet of
 the waveguide, and a nitride layer extending over the upper surface of the
 waveguide and the facet, the process comprising:
 i) carrying out an oxide etch step to remove the oxide layer from the
 underside;
 ii) carrying out an oxide growth step to form a new oxide layer on exposed
 silicon on the underside, said new oxide layer terminating at the facet;
 iii) carrying out a nitride etch step to remove the nitride layer; and
 iv) depositing a new nitride layer extending over the upper surface and
 facet without protruding beyond the facet, such that silicon nitride is
 formed on the facet.
 According to another aspect of the invention there is provided a process
 for making a waveguide structure in a silicon-on-insulator wafer
 comprising a silicon substrate, a layer of oxide on top of the silicon
 substrate and a layer of epitaxial silicon on top of the layer of oxide,
 the process comprising: defining a rib waveguide in the epitaxial layer;
 etching through the epitaxial layer on either side of the rib waveguide at
 an end portion thereof to expose the buried oxide layer; depositing oxide
 and nitride layers successively on the rib waveguide; undercutting the end
 portion to form a V-groove in the silicon substrate for aligning an
 optical fibre to the waveguide structure, said undercutting step leaving
 an unwanted part of the buried oxide layer extending beyond an end facet
 of the rib waveguide; and treating the waveguide structure by a process as
 defined hereinabove.
 The etch steps and deposition steps (i) to (iv) above can be carried out as
 blanket processes, without the need for masking. Therefore, the process is
 quite simple to implement. The thickness of the new nitride layer can be
 controlled to control the optical properties of the silicon nitride formed
 on the facet.
 For a better understanding of the present invention and to show how the
 same may be carried into effect reference will now be made by way of
 example to the accompanying drawings.

In the figures like reference numerals indicate like parts.
 DESCRIPTION OF THE PREFERRED EMBODIMENT
 The rib waveguide described herein is based on a silicon-on-insulator chip.
 A process for forming this type of chip is described in a paper entitled
 "Reduced defect density in silicon-on-insulator structures formed by
 oxygen implantation in two steps" by J. Morgail et al, Applied Physics
 Letters, 54, page 526, 1989. This describes a process for making
 silicon-on-insulator wafer. The silicon layer of such a wafer is then
 increased, for example by epitaxial growth, to make it suitable for
 forming the basis of the integrated waveguide structure described herein.
 FIG. 1 shows a view of an end portion of an optical waveguide formed on
 such a chip. The chip comprises a layer of silicon 1 which is separated
 from the silicon substrate 4 by a layer of silicon oxide 3. The rib
 waveguide 2 is formed in the silicon layer 1. FIG. 1 also shows an oxide
 cladding 7 formed on the top of the rib waveguide 2. Further details of
 this form of waveguide are given in a paper entitled "Low loss single mode
 optical waveguides with large cross-section in silicon-on-insulator" by J.
 Schmidtchen et al in Electronic Letters, 27, page 1486, 1991 and in PCT
 Patent Specification No. WO95/08787.
 This form of waveguide provides a single mode, low loss (typically less
 than 0.2 dB/cm for the wavelength range 1.2 to 1.6 microns) waveguide
 typically having dimensions in the order of 3 to 5 microns which can be
 coupled to optical fibres and which is compatible with other integrated
 components.
 FIG. 2 is a side view of the final form of the integrated structure for
 coupling of the waveguide to an optical fibre 6. The buried oxide layer 3
 has a lower refractive index than the rib waveguide 2 so that optical
 waves are confined within the rib waveguide. A layer of thermal oxide 7 is
 formed over the waveguide to provide waveguide cladding. An etched back
 slope 5A is formed during the etching of a V-groove in the substrate 4 to
 create an overhanging "diving board" structure 50 for connection to the
 optical fibre 6.
 A process for making such a structure will now be described, starting from
 the basic silicon-on insulator wafer.
 The first stage is to deposit a layer of thermal silicon dioxide 7 which
 will provide a cladding layer on the epitaxial silicon 1 to a thickness of
 about 7000 .ANG.. Then the wafer is patterned in resist and an etch into
 the thermal oxide 7 is performed to form an initial ridge structure. This
 exposes the epitaxial silicon on either side of the initial ridge
 structure. The resist is removed, and then the epitaxial silicon is etched
 1.45 .mu.m to form the ridge waveguide 2. A cross-section through the
 ridge at this stage is shown in FIG. 1A.
 The next step is to fully form the end facet 52 of the waveguide. The wafer
 is patterned in resist, avoiding the partially formed facet in order to
 maintain alignment. The exposed epitaxial silicon is then over etched in
 the area that will later contain the V-groove to reveal the buried oxide
 layer 3. The waveguide facet is now fully formed and has a "T-junction"
 shape at the facet when viewed from above as shown in FIG. 1.
 FIG. 2 further shows an optical fibre 6 having a core 6A. It can be seen
 from the figure that due to the angle of the etched back slope 5A it would
 be difficult to place the optical fibre close enough to the waveguide
 facet 52 if the waveguide were not overhanging the back slope. Therefore,
 in order to produce the arrangement of FIG. 2, the next stages of the
 process include the formation of the V-groove and undercutting of the
 waveguide at its end portion, thus allowing a fibre to be closely aligned
 with the waveguide.
 The resist remaining from the previous step is removed and a layer of wet
 silicon dioxide approximately 0.35 .mu.m thick is thermally grown to cover
 the entire wafer. The facet is then patterned in resist and a wet overetch
 is performed, which means the facet is now free of oxide, while the rest
 of the ridge structure is still covered. A suitable etchant is hydrogen
 fluoride.
 The resist is removed and a thin layer of dry oxide (200 .ANG.) is grown to
 protect the facet from a subsequent wet nitride etch. Such an etch could
 potentially cause light phosphorous doping in the silicon.
 The next step is to deposit a 500 .ANG. layer of LPCVD silicon nitride over
 the rib structure and exposed buried oxide. Because this layer is
 deposited in a furnace, reaction with the atmosphere in the furnace causes
 a nitox layer (40 .ANG. thick) to be formed on top of the nitride. A
 nitride removal layer is patterned in resist and the nitox is wet etched.
 The resist is then stripped and the nitride is wet etched from the area
 around the ridge which will form the V-groove area in the next stages. In
 this way the nitox is the etch mask and the nitride removal defines where
 the oxide will be removed in the following stage.
 The following step is the start of formation of the V-groove. The V-groove
 area is patterned in resist and the buried oxide is wet etched 0.8 .mu.m
 in the exposed areas as defined by the nitox mask down to the substrate 4.
 The final step is to remove the resist and etch into the silicon substrate
 4 to define the V-groove. This is a fast crystallographic etch which
 undercuts the ridge waveguide leaving its end portion as a "diving board"
 overhanging the V-groove. The actual ridge is 1.45 .mu.m high and the
 height of the remainder of the diving board is 2.8 .mu.m. The structure at
 this stage is shown in FIG. 3 in side section.
 As shown in FIG. 3, there is an unwanted "shelf" growing beyond the end
 facet 52 of the waveguide. This consists of an oxide part 11 which is a
 residue of the buried oxide layer and a nitride part 12. Both were left
 during the etching process because it is not possible to precisely align
 an etch mask with the facet. The rib waveguide 2 is in the epitaxial
 silicon 1 and is totally encased by oxide and nitride. The oxide on the
 underside of the waveguide 2 is indicated by reference numeral 3 because
 it is part of the buried oxide layer and the layer of nitride over the
 upper surface and facet of the waveguide is indicated by 10. Furthermore
 there is a small amount of unwanted oxide 14 remaining on the facet 52
 which was left after growing the 200 .ANG. layer of dry oxide.
 The remaining figures show the removal of the unwanted shelves and
 subsequent production of an efficient waveguide structure which has
 protective layers on upper and lower surfaces and an anti-reflective
 silicon nitride coating on the end facet.
 Firstly the oxide layer 3 and extra part 11 is removed using a wet etch.
 This results in the structure as depicted in FIG. 4. Then a protective
 oxide layer 13 is re-grown on the exposed silicon on the underside of the
 waveguide and around the V-groove. The purpose of this layer, as for any
 oxide layer deposited around the waveguide, is to smooth the oxide/silicon
 interface and to prevent any dust particles from affecting the effective
 refractive index of the waveguide. At this stage the nitride part 12 still
 remains. A nitride etch removes this part 12 and the nitride 10 on the
 upper surface leaving the structure shown in FIG. 6. The facet 52 is now
 exposed apart from a small thickness of unwanted oxide remaining on it.
 Then, the facet is dipped in etchant for a short period of time to remove
 this oxide, leaving a pure silicon facet 52 as shown in FIG. 7. All the
 etches involved between the stages shown in FIGS. 3 to 8 are "blanket"
 etches which is cost-effective because no patterning is required.
 The final treatment is to re-deposit a nitride layer 15 around the entire
 waveguide and V-groove area. On the facet 52, this forms a layer of
 silicon nitride 17 which acts as an anti-reflective coating to ensure that
 there is a low-loss fibre-waveguide interference.