WO2004008203A1

WO2004008203A1 - Planar waveguide with tapered region

Info

Publication number: WO2004008203A1
Application number: PCT/AU2003/000843
Authority: WO
Inventors: Michael Bazylenko
Original assignee: Redfern Integrated Optics Pty Ltd
Priority date: 2002-07-11
Filing date: 2003-07-01
Publication date: 2004-01-22
Also published as: AU2003236568A1

Abstract

A method of photolithographically fabricating a photonic waveguide having a tapered region and an untapered region. The tapered region is formed by etching with a hard mask pattern (40) using a substantially isotropic etching process, such as wet etching, so that undercutting is achieved below the mask. The untapered region is formed by etching with a photoresist mask pattern using a directional etching process, such as ion beam milling. Vertical sidewalls are produced with well-controlled dimensions in the untapered region, and inwardly sloping sidewalls (60) converging to a sharply pointed tip are produced in the tapered region. The sharp tip can be used to reduce coupling losses when the waveguide is coupled to another waveguide of different refractive index.

Description

PLANAR WAVEGUIDE WITH TAPERED REGION

TECHNICAL FIELD

The present invention relates to photonic planar waveguides and to a method of fabricating such waveguides.

BACKGROUND OF THE INVENTION

There is a growing demand in the telecommunications industry for small-form- factor, low-cost, liigh-performance optical components such as modulators, multiplexers, demultiplexers, switches, lasers etc., for use in optical communications networks. One approach to reducing both the size and cost of optical components is to monolithically integrate multiple components on a common substrate, such as a silicon substrate. In order to integrate different types of optical components, it is can be necessary to couple together waveguides composed of different materials and having different refractive indices. Where the difference in refractive indices between such waveguide materials is large, optical power will be reflected or attenuated at the interface between the materials if they are simply butt-coupled together. In order to reduce coupling losses, it is necessary to provide a gradual transition in effective refractive index from one waveguide to another. This can be achieved by terminating one of the waveguides such that a mode travelling along the waveguide experiences a gradual change in effective refractive index, and ideally an adiabatic change in effective refractive index. Such structures are sometimes referred to as "mode-size converters" or simply "mode converters" since the size of a propagating optical mode is adjusted. A gradual change in effective refractive index may be implemented in a mode converter by creating a progressive change in material refractive index along the length of the waveguide. Alternatively, or additionally, there may be a gradual change in the cross-sectional dimensions of one of the waveguides. The latter technique is usually the most practical to implement in planar waveguides. For example, a planar waveguide may terminate in a tapered region in which sidewalls of the waveguide converge to a pointed tip. There are a number of fabrication constraints on fonning such a tapered waveguide structure having both tapered and untapered regions. Firstly, the sidewalls in the tapered region of the mode converter need to converge to a tip which is as sharp as possible. Secondly, untapered regions of the waveguide should be fabricated with sidewalls which are relatively smooth compared to the wavelength of photonic signals to be transmitted through the waveguide. Thirdly, the sidewalls of the untapered region should stand at a reproducible angle relative to the substrate- usually perpendicular to the substrate. Lastly, since the cross-sectional dimensions of a waveguide have an influence on the effective refractive index of the waveguide, the sidewalls of each untapered region need to be formed using a process which enables the waveguide width to be accurately controlled. Unfortunately, it has been found that when planar waveguides are formed by photolithographically-defmed etching, the etching processes suitable for forming a tapered waveguides with a sufficiently sharp tip are incapable of forming untapered waveguides with good sidewall roughness, verticality and dimension control. For example, whilst ion beam etching is capable of forming an untapered waveguide according to required specifications, this process is only capable of forming a tip with a radius of curvature of the order of 1 micron. Greater resolution at the tip is required in order to minimise optical losses. There is therefore a need for an improved method of forming planar waveguides having both tapered and untapered regions.

SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention there is provided a method of fabricating a photonic planar waveguide with a tapered region, the method comprising the steps of:

- depositing a layer of an optically-transmissive material on a substrate;

- forming a first waveguide structure by patterning a first mask on the layer and etching the layer using a first etching process which substantially avoids undercutting of the mask;

- forming a second waveguide structure by patterning a second mask on the layer of material and etching the layer using a second etching process which gives rise to undercutting below the mask, wherein the second waveguide structure is in optical communication with the first waveguide structure and has a cross-sectional area which tapers with progression along a propagation axis of the first waveguide structure.

The tapered region may be tapered towards a pointed tip. The order of the steps of forming the first waveguide structure and forming the second waveguide structure may be reversed, depending on the type of waveguide. The first mask may be in the form of a waveguide pattern deposited on the layer of optically-transmissive material, and the first etching process may comprise a directional etching process which etches the layer such that the waveguide pattern is transferred to the layer. Examples of directional etching processes which may be used include ion beam milling, reactive ion etching, ion-assisted etching and ion beam reactive etching.

The second etching process may be arranged to produce sidewalls in the tapered region which are at least partially sloped relative to the substrate. The second mask may be in the form of a tapered pattern deposited on a portion of the layer of optically-transmissive material and the second etching process may comprise a substantially isotropic etching process. The second mask may comprise a hard mask pattern and the first mask may comprise a photoresist pattern. Alternatively, the second mask may comprise a hard mask pattern overlaid with a photoresist pattern, the hard mask being arranged to limit the extent of undercutting during the second etching process.

The waveguide produced by the method may have a channel geometry or a rib geometry. The step of forming the first waveguide structure may further comprise forming a rib structure protruding from a slab region, the rib structure comprising a substantially untapered region and a tapered region which converges with progression along the propagation axis to a first pointed tip. The step of formmg the second waveguide structure may comprise using the second etching process to define sidewalls of the slab region such which converge along the propagation axis to a second pointed tip. The first pointed tip of the rib structure may be spaced-apart along the propagation axis from the second pointed tip of the slab region.

In accordance with a second aspect of the present invention there is provided a photonic planar waveguide fabricated in accordance with the method of the first aspect of the invention.

In accordance with a third aspect of the present invention there is provided a method of fabricating a photonic planar waveguide with a tapered region, the method comprising the steps of: etching a trench in an optical buffer layer using a first photolithographically- defined etching process which substantially avoids undercutting sidewalls of the trench, the trench having the a shape chosen to be an inversion of a rib structure for a rib waveguide; - depositing a layer of an optically-transmissive material in and above the trench so as to form an inverted rib waveguide comprising a rib structure and a slab region above the rib structure; and

- forming a tapered region in the slab region waveguide by a second photolithographically-defined etching process which gives rise to undercutting below a mask, the tapered region having a cross-sectional area which is tapered with progression along a propagation axis of the waveguide.

In accordance with a fourth aspect of the present invention there is provided a photonic planar waveguide fabricated in accordance with the method of the third aspect of the invention.

In accordance with a fifth aspect of the present invention there is provided a photonic planar waveguide having a waveguide core formed on a generally-planar substrate, the core having a first portion in which its cross-sectional area tapers with progression along a propagation axis of the waveguide, wherein the edges of the first portion are defined by sloped sidewalls which extend non-perpendicularly away from the substrate and inwardly towards an upper surface of the first portion.

Embodiments of invention will now be described, by way of example only, with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

- Fig. 1 is cross-sectional view of a structure comprising a layer of waveguide material formed on a buffer layer, prior to being processed in accordance with the present invention.

- Fig. 2A is a plan view of the structure shown in Fig. 1 after a hard mask with a tapering region has been formed on the layer of waveguide material.

- Fig. 2B is a plan view of the structure shown in Fig. 1 after the waveguide has been subjected to an isotropic etching process.

- Fig. 3 is a view through section 3-3 of Fig. 2B.

- Fig. 4 is a view through section 4-4 of Fig. 2B. - Fig. 5 illustrates a subsequent step in the process illustrated in Figs. 1-4 wherein a strip of photoresist is formed over the tapering waveguide structure created during the isotropic etch.

- Fig. 6 is a view through section 6-6 of Fig. 5. - Fig. 7 is a view through section 6-6 of Fig. 5 after the tapering waveguide structure has been exposed to a directional etch.

- Fig. 8 is a perspective view of the waveguide structure shown in Fig.7 after a second channel waveguide has been formed thereon, thus forming a mode converter.

- Fig. 9 illustrates the masking and etching steps required to form the tapered waveguide shown in Fig. 5.

- Fig. 10 illustrates an early stage in the fabrication of a different mode converter arrangement in accordance with an embodiment of the invention, showing a narrow strip of photoresist which has been deposited on a layer of waveguide material.

- Fig. 11 is a view through section 11-11 of Fig. 10. - Fig. 12 is view through section 11-11 of Fig. 10 after the layer of waveguide material has been subjected to a directional etching process so as to form a rib waveguide.

- Fig. 13 is a plan view of the rib waveguide shown in Fig. 12 after a hard mask with a tapering shape has been patterned thereon. - Fig. 14 is a view through section 14-14 of Fig. 13.

- Fig. 15 is a plan view of the rib waveguide of Fig. 13 after being subjected to an isotropic etch, thus forming a tapered terminating region in the rib.

- Fig. 16 is a view through section 16-16 of Fig. 15.

- Fig. 17 shows the rib structure of Fig. 16 after a channel waveguide structure has been formed upon it.

- Fig. 18 is a plan view of another embodiment of a mode converter in an early stage of fabrication in accordance with the invention. In this embodiment a rib waveguide structure is formed with two tapering regions, the first of which (shown here) is formed using a directional etching process, while the second tapering region (shown in Fig. 19) is formed with an isotropic etch. - Fig. 19 shows a plan view of the stmcture shown in Fig. 18 after a slab region has been patterned and etched with an isotropic etching process.

- Fig. 20 is a view through section 20-20 of Fig. 19.

- Fig. 21 shows the tapered rib waveguide of Fig. 19 after a channel waveguide has been formed directly upon it so as to optically couple the tapered waveguide to the channel waveguide.

- Fig. 22 is a view through section 22-22 of Fig. 21.

- Fig. 23 shows a cross-sectional view of an alternative mode converter arrangement in which an inverted rib structure is formed on a buffer layer and coupled to a channel waveguide above it.

- Fig. 24 is a view through section 22-22 of Fig. 23.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. For the purposes of this specification it is to be clearly understood that the word

"comprising" means "including but not limited to", and that the word "comprises" has a corresponding meaning.

DETAILED DESCRIPTION OF THE DRAWINGS

In order to better illustrate the concepts of the invention, a number of embodiments will now be described. Each embodiment provides a method of fabricating a photonic planar waveguide having an untapered region which terminates in a tapered region . converging to a pointed tip. Such a structure can be used to form a mode converter for reducing optical coupling losses in the transition from one waveguide to another waveguide of different refractive index. An embodiment of the invention will initially be described with reference to Figs. 1 to 8 which illustrate the formation of a channel waveguide which terminates in a tapering region. Referring to Fig. 1, the method is applied to a stmcture 10 comprising a waveguide layer 20 composed of an optically-transmissive material deposited on a substrate comprising an optical buffer layer 30 formed on a wafer of silicon (not shown). The optical buffer layer is also optically transparent and has a lower refractive index than the waveguide layer 20 so as to significantly reduce optical absorption in the silicon wafer. In this embodiment, the buffer layer is composed of silica and the waveguide layer 20 is composed of aluminium oxide.

Referring to Fig. 2A, a hard mask 40 is then formed on the waveguide layer 20. In this case the hard mask is composed of a thin film of silica. As is known in the art, a hard mask is formed by the following steps:

1. depositing a layer of hard mask material on the waveguide layer 20;

2. patterning a photoresist mask on the layer of hard mask material;

3. etching the hard mask material using an etching process which substantially avoids etching the waveguide layer20, so as to transfer the photoresist pattern into the hard mask; and

4. stripping off the photoresist mask, leaving the patterned hard mask.

Other hard mask materials such as chromium or aluminium can be used instead, depending on the material in the waveguide layer. In the embodiment shown Fig. 2A, the hard mask 40 has a terminating region 50 which tapers in width towards a sharp tip 55. The waveguide layer 20 and hard mask 40 are then subjected to a substantially isotropic etching process which etches away the waveguide material in preference to the hard mask so as to causes undercutting below the mask 40. Fig. 2B shows a plan view of the same hard mask 40 after the waveguide layer has been, subjected to an isotropic etch. The outline of the waveguide sidewalls 60 below the hard mask 40 are shown in dashed lines. The edges of the hard mask 40 overhang the waveguide sidewalls because the waveguide material is etched in preference to the hard mask. The advantage of using an isotropic etchant is that the resultant tip 65 of the waveguide is sharper than the tip 55 of the hard mask. Two cross-sections 3-3 and 4-4 through the terminating region 50 are shown in Figs. 3 and 4, respectively, after the isotropic etch has been completed. It can be seen that the isotropic etch produces sidewalls 60 in the terminating region 50 which extend non-perpendicularly from the buffer layer 30 such that they are sloped towards each other. In other words, the width of the tapered region is tapered across the thickness of the waveguide as well as along the propagation axis.

Once the isotropic etch has been completed, the hard mask is removed and a layer of photoresist is deposited over the waveguide layer 20, exposed through a shadow mask and developed to form another etching mask in the shape of an untapered rectangular strip 70 as shown in Figs. 5 and 6. The strip 70 of photoresist has a constant width w along its length for defining a channel waveguide of width w in the waveguide layer 20. The strip 70 is positioned such that it overlays part of the tapered terminating region 50 and is longitudinally aligned with a central axis 90 of the terminating region. The stmcture shown in Figs. 5 is then subjected to a directional etching process so as to transfer the plan-view profile of the photoresist pattern 70 into the waveguide layer 20, forming a channel waveguide structure 100. Figures 6 and 7 are respective views through section 6-6 of Fig. 5 before and after, respectively, the directional etching step. The untapered region of the channel waveguide structure 100 has opposed sidewalls 110 which are substantially parallel to each other and oriented substantially perpendicular to the buffer layer 30. The resultant etched structure 110 is shown in perspective in Fig. 8 after the layer of photoresist 70 has been removed. It can be seen that the terminating region 50 has been left intact during the directional etch and an untapered region 130 with vertical and parallel sidewalls 120 has been formed.

The inventor has recognised that when transferring a sharp feature from a mask into a layer, a tip can be form with greater sharpness than that of the mask feature if the layer is subjected to an etching process which is isotropic and preferentially etches the layer. Further, it has been recognised that the tip resolution achievable with an isotropic etching process cannot be realised with a directional etching process. The inventor believes that the improved tip resolution resulting from isotropic etching is largely a result of two effects which work in synergy: (a) the mask undercutting which takes place during isotropic etching tends to produce inwardly-sloping sidewalls; and (b) in a tapered structure, inwardly-sloping sidewalls tend to narrow the width of the tapered region and thus sharpen the tip. A further advantage of the sloping sidewalls is that they converge to produce a sloped leading edge 140. As can be seen in Fig. 8, the resultant tapered region 50 is thus tapered in both width and to some extent thickness due to the slope of the leading edge 140.

It is believed that the additional tapering in the thickness dimension has the effect of reducing optical losses in a mode converter. It should be noted that isotropic etching tends to form rougher sidewalls than directional etching. Although sidewall roughness is generally undesirable in a waveguide (due to optical scattering losses caused by sidewall roughness), the length of the terminating region in which isotropic etching is used is typically sufficiently short that any optical scattering losses arising from sidewall roughness in the terminating region will be negligible. Referring again to Fig. 8, the tapered waveguide 110 can then be coupled to a second waveguide, in this case a channel waveguide 160 (shown in dashed lines) composed of a material having a different refractive index from the tapered waveguide 110, thus completing the mode converter. The second waveguide 160 is formed by depositmg a layer of material over the tapered waveguide 110 and using a photolithographically-defined directional etching process to form a waveguide with channel geometry.

The term "isotropic etching" is used here to mean an etching process which etches a material in substantially all directions, but not necessarily at the same rate in all directions. The expression "directional etching" is used here to indicate that the etching proceeds substantially in one direction only. In this embodiment, the etching only proceeds in a direction towards the substrate at an angle of substantially 90°. Thus, there is no undercutting of the photoresist pattern 70, resulting in sidewalls 120 which are substantially perpendicular to the buffer layer 30. Any etching process capable of providing directional etching without undercutting a mask may be used. Examples include ion beam milling, reactive ion etching, ion-assisted etching and ion beam reactive etching. It will be understood that the photoresist pattern 70 (shown in Figs. 6 and 7) needs to have a thickness sufficient to prevent it being etched away before the etching of the waveguide layer 20 is completed.

The thicknesses of the masks and type of type etchants will depend on the materials being etched. Referring to Fig. 9, the above embodiment uses the following process:

(a) A layer of silica 40A is deposited on the aluminium oxide waveguide layer 20 with a thickness of 30 to 150 nm (Fig. 9A) and a photoresist mask 155 is patterned on the silica ;

(b) The layer of silica 40 A is etched with 7:1 buffered oxide etch solution at ~22°C (Fig. 9B) thus forming a silica hard mask 40;

(c) The photoresist mask 155 is removed (Fig. 9C);

(d) The aluminium oxide waveguide layer 20 is etched in hot phosphoric acid at ~85°C until the waveguide material is etched down to the buffer layer 30, leaving an aluminium oxide waveguide with sloped sidewalls 60 (Fig. 9D); (e) The silica hard mask 40 is etched away using reactive ion etching (Fig. 9E).

In the above process, there is no photoresist present during the isotropic etching stage. Alternatively, the combination of a hard mask overlaid with a wider layer of photoresist can also be used during the isotropic etching. Such an embodiment can be used to increase the amount of undercutting in a controlled manner. Undercutting tends to be greater below a layer of photoresist due to the tendency of the etchant to penetrate along the waveguide-photoresist interface, but is reduced substantially once it reaches the interface between the waveguide and hard mask. Thus the hard mask can be used to limit the extent of undercutting which takes place.

Although the waveguide material in the above embodiment is aluminium oxide, the method is equally applicable to many other waveguide materials including, but not limited to, other optically-transmissive metal oxides, silica glasses or phosphide glasses. One or more dopants can be incorporated within the waveguide. For example, dopants such as erbium or alternatively erbium and ytterbium can be included in waveguides to form optical amplifiers. Also, refractive-index-modifying dopants such as germanium can be included. The invention can be used to form a mode converter between an amplifying waveguide structure and a passive waveguide, such as a silica waveguide. A second embodiment of the invention will now be described with reference to

Figs. 9 - 16. This embodiment differs from the embodiment shown in Figs. 1 - 8 in that the waveguide formed by the method has a rib geometry rather than a channel geometry.

Referring initially to Figs. 10 and 11, the method is applied to a structure 200 comprising a waveguide layer of an optically transmissive material 210 formed on an optical buffer layer 220 which is in turn formed on a wafer of silicon (not shown). As usual, the optical buffer layer 220 is optically transparent and has a lower refractive index than the waveguide layer 210. An elongate rectangular strip of photoresist 230 is first formed directly on the surface of the waveguide layer 210 by exposing and developing a layer of photoresist in the usual manner. The strip of photoresist 230 has a constant width >₂ which is the width of a rib to be formed in the layer 210.

Referring to Fig. 12, the waveguide layer 210 is then subjected to a directional etching process so as to transfer the plan-view profile of the strip 230 into the waveguide layer, thereby forming a rib or ridge structure 240 protruding from a slab region 250. The resultant rib-geometry waveguide is designed to guide light within and below the rib structure 240. The height h of the rib structure 240 above the slab region 250 is controlled by controlling the duration of the directional etching. In the next step, shown in Figs. 13 and 14, a hard mask 260 is deposited over the rib waveguide and patterned so as to form a tapering region 270 which terminates in a pointed tip 280. The hard mask 260 is arranged symmetrically either side of a longitudinal axis 290 of the rib structure 240 such that the tip 280 is disposed directly above the axis 290. The rib stracture 240 and slab region 250 are then subjected to a substantially isotropic etching process and the mask 260 is removed, resulting in the structure shown in Figs. 15 and 16. It can be seen that the isotropic etch has produced inwardly-sloping sidewalls 290 which converge towards a sharply-pointed tip 300 in the centre of the rib structure 240. Finally, referring to Fig. 17, the completed rib waveguide 310 is coupled to a channel waveguide 320 by depositing another layer of material over the rib waveguide and etching sidewalls

330 into the layer using photolithographically-defined etching.

Referring now to Figs. 18-22, a further embodiment of the method of forming a tapered rib waveguide for use in a mode converter will be described. Turning initially to Fig. 18, the method begins by forming a tapered rib structure 400 which projects slightly above a slab region 410. The slab region is formed upon a buffer layer 420 which can be seen in Fig. 20. The rib structure 400 is formed in a similar manner to that of the stracture 240 illustrated in Fig. 12, that is, a layer of photoresist is deposited, exposed, developed and finally etched using a directional etching process such as ion beam milling. The rib structure 400 shown in Fig. 18 differs from the rib structure 240 shown in Fig. 12 in that the rib structure 400 terminates in a region which tapers in width towards a tip 440. Since the tapering region 430 is formed with a directional etching technique, the tip 440 is relatively rounded compared with the tip that can be produced with isotropic etching. Referring now to Figs. 19 and 20, the next step in the process involves defining sidewalls 450 in the slab region 410 using a tapering hard mask (not shown) and an isotropic etching process. The resultant sidewalls 450 are inwardly-sloping as a result of undercutting below the hard mask and have a tapered region 460 which converges in width towards a pointed tip 470. Since the tip 470 of the slab region 410 is formed in an isotropic etching process, the tip 470 is much sharper than the tip 440 formed at the end of the rib structure 400. It can be seen that the tip 470 of the slab region 410 is aligned along the same longitudinal axis as the rib structure 400 and that the tip 470 is separated from the tip 440 of the rib structure by a distance z. When a photonic signal propagates along the rib structure 400 it will first encounter the tapered region 430 of the rib, where the mode size will gradually decrease until the mode is confined within the slab region 410 alone. Thereafter, confinement of the signal in the slab will again gradually decrease as it propagates along the tapered region 460 within the slab towards the tip 470, until it completely couples into an overlying or underlying waveguide. It is not essential that the tip 440 at the end of the rib structure is formed with a sha ly-defined apex as this transition is not between materials of different refractive index. However, a photonic signal propagating through the second tapering region 460 of the slab region 410 will experience a transition into a material of different refractive index so it is important that the slab region 410 terminates in a tip 470 which is as sharp as possible.

Finally, referring to Figs. 21 and 22, a channel waveguide 480 is formed directly upon the rib waveguide so as to encapsulate the rib structure 400 and much of the second tapering region 460. It will be understood that the principle of reciprocity means that the mode converter may be used to couple light either from the channel waveguide 480 to the rib waveguide or vice versa.

A further embodiment of a waveguide coupling arrangement is illustrated in Figs. 23 and 24. In this embodiment, an inverted rib waveguide 500 is formed directly beneath a channel waveguide 510. The rib waveguide 500 is identical to the rib waveguide shown in

Fig. 21 except that the rib structure 520 in this embodiment projects below the slab region 530 into a buffer layer 540 rather than above the slab region into the channel waveguide 510. The rib structure is formed by etching away an inversion of the shape of a rib in the buffer layer 540 using a directional etching process and then filling the channel 550 with waveguide material. Sidewalls 560 of the slab region 530 are again formed with an isotropic etching process.

Although not illustrated, the invention includes within its scope embodiments in which a rib waveguide is formed directly on top of a channel waveguide rather than between the channel waveguide and buffer layer. The above embodiments are described in terms of coupling aluminium-oxide-based waveguides to silica-based waveguides, but the present invention is not limited to such materials. In fact, the tapered stracture described above may be used to couple waveguides which are composed of the same or similar materials but have the different refractive indices. Further, although the method of the invention can be used to form mode converters, the invention is not limited to forming such devices. Rather, the method can be used wherever it is necessary to form a photonic waveguide with both an untapered region and a tapered region which converges to a sharp tip. It should be understood that the ratio of the length of each tapered region to the width of each untapered waveguide can be much greater than that shown in the drawings. The ratio has been reduced in the drawings in order to clearly illustrating the concepts of the invention. In practice, tapered regions with a length-to-width ratio in excess of 100 can be achieved in order to create a very gradual mode size conversion.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiment without departing from the spirit or scope of the invention as broadly described. The present embodiments are therefore to be considered in all respects illustrative and not restrictive.

Claims

1. A method of fabricating a photonic planar waveguide with a tapered region, the method comprising the steps of:

- depositing a layer of an optically-transmissive material on a substrate; - forming a first waveguide structure by patterning a first mask on the layer and etching the layer using a first etching process which substantially avoids undercutting of the mask;

- forming a second waveguide stracture by patterning a second mask on the layer of material and etching the layer using a second etching process which gives rise to undercutting below the mask, wherein the second waveguide stracture is in optical communication with the first waveguide structure and has a cross-sectional area which tapers with progression along a propagation axis of the first waveguide stracture.

2. A method in accordance with claim 1, wherein the second waveguide structure is tapered towards a pointed tip.

3. A method in accordance with claim 1, wherein the first mask is in the form of a waveguide pattern deposited on the layer of optically-transmissive material, and the first etching process comprises a directional etching process which etches the layer such that the waveguide pattern is transferred to the layer.

4. A method in accordance with claim 1, wherein the first mask comprises a photoresist pattern.

5. A method in accordance with claim 1, wherein the second etching process is arranged to produce sidewalls in the second waveguide structure which are at least partially sloped relative to the substrate.

6. A method in accordance with claim 1, wherein the second mask is in the form of a tapered pattern deposited on a portion of the layer of optically-transmissive material and the second etching process comprises a substantially isotropic etching process.

7. A method in accordance with claim 1, wherein the second mask comprises a hard mask pattern.

8. A method in accordance with claim 1, wherein the second mask comprises a hard mask pattern overlaid with a photoresist pattern, the hard mask being arranged to limit the extent of undercutting during the second etching process.

9. A method in accordance with claim 1, wherein the second mask comprises a hard mask having a tapered pattern, and the second etching process comprises a substantially isotropic etch in which the hard mask is undercut to an extent which produces sidewalls in the second waveguide stracture that are sloped relative to the substrate.

10. A method in accordance with claim 1, wherein the first etching process is carried out such that sidewalls are produced in the waveguide which are oriented at substantially 90° to the substrate.

11. A method in accordance with claim 1, wherein the first waveguide structure comprises a waveguide with a channel geometry.

12. A method in accordance with claim 1, wherein the first waveguide stracture comprises a rib stracture formed on a slab region.

13. A method in accordance with claim 1, wherein: the step of forming the first waveguide stracture further comprises forming a rib stracture protruding from a slab region, the rib structure comprising a substantially un-tapered region and a tapered region which converges along the propagation axis to a first pointed tip; and the step of forming the second waveguide stracture comprises using the second etcliing process to define sidewalls of the slab region such which converge along the propagation axis to a second pointed tip; wherein the first pointed tip of the rib structure is spaced-apart along the propagation axis from the second pointed tip of the slab region.

14. A method in accordance with claim 1, wherein: the step of forming the first waveguide stracture further comprises forming a rib structure protruding from a slab region, the rib structure being initially substantially un-tapered along the propagation axis; and the step of forming the second waveguide structure comprises using the second etching process to define sidewalls of the slab region and a portion of the rib, the sidewalls converging to a pointed tip.

15. A photonic planar waveguide fabricated in accordance with claim 1.

16. A method of fabricating a photonic planar waveguide with a tapered region, the method comprising the steps of:

- etching a trench in an optical buffer layer using a first photolithographically- defined etching process which substantially avoids undercutting sidewalls of the trench, the trench having the a shape chosen to be an inversion of a rib structure for a rib waveguide;

- depositing a layer of an optically-transmissive material in and above the trench so as to form an inverted rib waveguide comprising a rib structure and a slab region above the rib stracture;

- forming a tapered region in the slab region waveguide by a second photolithographically-defmed etching process which gives rise to undercutting below a mask, the tapered region having a cross-sectional area which is tapered along a propagation axis of the waveguide.

17. A photonic planar waveguide fabricated in accordance with claim 16.

18. A photonic planar waveguide having a waveguide core formed on a generally- planar substrate, the core having a first portion in which its cross-sectional area tapers with progression along a propagation axis of the waveguide, wherein the edges of the first portion are defined by sloped sidewalls which extend non- peφendicularly away from the substrate and inwardly towards an upper surface of the first portion.

19. A photonic planar waveguide in accordance with claim 18 wherein the sidewalls are formed in a generally-isotropic etching process.

20. A photonic planar waveguide in accordance with claim 18 wherein the first portion terminates in a pointed tip.

21. A photonic planar waveguide in accordance with claim 18 wherein the substrate comprises an optical buffer layer formed on a wafer.

22. A photonic planar waveguide stracture in which a first waveguide is optically coupled to a second waveguide, the first and second waveguides having different refractive indices, wherein the second waveguide is in accordance with the waveguide claimed in claim 18 and the first portion is in contact with and parallel to a coupling region of the first waveguide.