PROCESS FOR THE HYBRID INTEGRATION OF AT LEAST ONE OPTO-ELECTRONIC COMPONENT AND A WAVEGUIDE, AND AN INTEGRATED ELECTRO-OPTICAL DEVICE

Abstract: The invention pertains to a process for the hybrid integration of at least one opto-electronic component (9) and a waveguide (14) provided on a substrate (13), in which process one or more opto-electronic components are provided on a connector piece (6) and the connector piece (6) is placed on the substrate (13) and moved up to the edge of the waveguide (14), with the shape of the connector piece (6) being wholly or largely complementary, and the connector piece (6) when it abuts against the edge of the waveguide (14) having only one degree of freedom in the plane of the substrate (13). The process provides an easy way of producing integrated electro-optical devices.

Description: PROCESS FOR THE HYBRID INTEGRATION OF AT LEAST ONE OPTO¬ ELECTRONIC COMPONENT AND A WAVEGUIDE, AND AN INTEGRATED ELECTRO-OPTICAL DEVICE The invention pertains to a process for the hybrid integration of at least one optoelectronic component and a waveguide provided on a substrate. Such a process is known from EP 0 617 303 A1. This patent specification describes how material from a planar waveguide built up of a substrate, a bottom deflection layer, a core layer, and a top deflection layer is removed. In the cavity thus created a semiconductor component obtained by a process known as epitaxial lift-off is arranged. Such a process can be used to incoφorate semiconductor components, such as LEDs, laser diodes, and VCSELs (Vertical Cavity Surface Emitting Laser Diodes), as well as detectors into an integrated structure containing polymers or glass in which light is transported and, optionally, modulated. These integrated structures have a number of important applications in the field of optical telecommunications (e.g., external modulation of light emitted by a laser diode, routing in interconnection networks, optical amplifiers, monitoring of signals, wavelength division multiplexing, etc.), high-speed interconnections in computers (optical backplane), optical sensors, etc., as well as being easier to use and handle than electro-optical structures composed of separate, unintegrated components. Other advantages of such structures include the possibility of incorporating a wide range of functionalities into a single electro-optical device and improved efficiency in coupling light into and out of waveguides. In EP O 617 303 A1 alignment of the semiconductor component and the waveguide structure in transversal direction (i.e., the direction perpendicular to the substrate of the waveguide, also known as the z- direction) is obtained by careful selection of the height of the stack on which the semiconductor component is provided, while alignment in lateral direction (i.e., the direction parallel to the edge of the waveguide) is obtained by not defining the waveguide channels until after the arrangement of the semiconductor component. Alignment in longitudinal direction (i.e., the direction perpendicular to the two other directions) is determined by the accuracy of the pick-and-place apparatus for the arrangement of the semiconductor component. There is need for a method by means of which optoelectronic components can be accurately aligned with waveguide structures in an easier way still. Preferably, the fullest possible use should be made in this alignment of conventional, hence reliable, techniques for the different process steps. The invention attains this objective by means of the process as described in the opening paragraph and providing one or more optoelectronic components on a connector piece, which connector piece is placed on the substrate and moved up to the edge of the waveguide, with the shape of the waveguide and the shape of the connector piece being wholly or largely complementary, and the connector piece when it abuts against the edge of the waveguide having only one degree of freedom in the plane of the substrate. When the connector piece is placed on the substrate, it has three degrees of freedom in the plane of the substrate (one rotation and two translations). Because of its complementary, clearing shape, the connector piece when abutting against the waveguide can be moved in just a single direction, i.e. straight away from the waveguide. Because of this complementary, clearing shape the connector piece can be moved easily up to the edge of the waveguide to take up its exact envisaged position. Such a shape permits passive (and thus inexpensive and rapid) alignment. Moreover, the connector piece makes for greater freedom in selecting the sequence of the various process steps required to obtain an integrated electrooptical device. For instance, the optoelectronic component (or components) can be provided on the connector piece before coupling to the waveguide takes place. This results in a ready-made module which is easy to move up against the waveguide and mount. Alternatively, the optoelectronic component can be mounted on the connector piece after it has been moved up against the edge of the waveguide. The opto-electronic component preferably is mounted on the connector piece using the so-called flip-chip technique (familiar to the skilled person), since this technique permits accurate positioning of the chip vis-a-vis the connector piece. Examples of the flip-chip technique are solder bump flip- chip and Au-Au thermocompression. When this technique is employed, the connector piece will have to be provided with a reflecting surface to enable optical signals emitted by the waveguide to be coupled into the opto- electronic component or components (when the opto-electronic component is a detector), or vice versa (when the opto-electronic component is a source). When one of the aforementioned techniques is used to mount the opto- electronic component on the connector piece after the connector piece has been placed up against the edge of the waveguide, it is preferred to select a material having good thermal conduction for the connector piece. In that case, very local heating to make a connection between the opto-electronic component and the connector piece can be rapidly dispersed over the entire connector piece and, if so desired, transferred to the substrate (which will then function as a heat sink). This prevents the waveguide material being affected by the heat. For that matter, when materials having good thermal conduction are employed, any heat which may be generated by the opto-electronic component will be rapidly dissipated. The waveguide may take the form of a planar, preferably polymeric, waveguide; alternatively, it may comprise a row of parallel optical fibres (fibre ribbon) arranged, e.g., in grooves in the substrate. Planar waveguides generally consist of one or more layers of polymeric material provided on a substrate. The waveguide may be a complete waveguide, in which case it will commonly comprise a bottom deflection layer, a core layer (in which the waveguide channels are defined), and a top deflection layer, but the structure may equally well be incomplete and be made up, say, of just a bottom deflection layer and a core layer. The polymeric material can be provided on a substrate in the form of, say, a polymer solution, preferably by means of spincoating, followed by evaporation of the solvent. Depending on its nature the polymer can also be moulded, injection moulded, or cast using other processing techniques known as such. Suitable substrates include silicon wafers or synthetic laminates, e.g., on the basis of a reinforced or unreinforced epoxy resin. Suitable substrates are known to the skilled person. The substrate is not essential for carrying out the process according to the present invention. Preference is given to the planar waveguide (over fibre ribbons) because its edge (at least at the point where it is complementary to the connector piece) can easily be made complementary to the connector piece by the removal of waveguide material. A further advantage of planar waveguides over optical fibres consists in that as regards the direction in which they extend, the waveguide channels in a planar waveguide are not dependent on the substrate on which the waveguide has been provided. Optical fibres, by contrast, are usually fixed in V-grooves, and the direction of such V- grooves, which is almost invariably obtained by wet-chemical etching, is dictated by the crystal lattice of the substrate (generally silicon). The waveguide material (at least where it is complementary to the connector piece) may be removed by means of any suitable etching technique, e.g., those known from the manufacture of integrated circuits (ICs). Techniques that come to mind in this case are wet-chemical etching techniques, e.g., with use being made of organic solvents or strong bases. However, preference is given to photolithographic etching techniques, such as sputter etching (non-reactive plasma etching), laser ablation, reactive ion etching (RIE) or reactive plasma etching. Such techniques are known to the skilled person and require no further elucidation here. Alternatively, etching can be performed mechanically, e.g., by grinding, cutting, drilling, or through bombardment with sanding particles such as alumina, silica, and, more particularly, pumice. The skilled person is expected to be able to select an appropriate etchant for the polymer in question without undue experimentation. It is of particular relevance that the polymeric material be so removed by etching as to give a smooth surface (facet). Furthermore, the surface subjected to etching should not exhibit any foreign substances or roughnesses. To remove the desired portion of the polymer when using non-mechanical etching techniques, a mask is applied to cover those parts which should remain free from attack by the etchant. These masks, the chief prerequisite of which is that they be resistant to the action of the etchant, are known, int. al., from IC technology. Such a mask may be preformed and made up, e.g., of metal or synthetic material; alternatively, it can be made by applying a photosensitive resin (photoresist) and subsequently exposing and developing said resin in accordance with the desired pattem. When a planar waveguide is used, the waveguide channels can be provided, int. al., by removing portions of the flat waveguide, e.g., with the aid of wet-chemical or dry etching techniques, and filling the thus formed cavities with a material having a lower index of refraction (thus forming a channel of core layer material enclosed on all sides by deflection layer material). Alternatively, it is possible to use photosensitive material which can be developed after irradiation; for instance, a negative photoresist, that is to say, material which is resistant to a particular solvent (developer) after being irradiated. The developer in that case may be used to remove non- irradiated material. In the case of a positive photoresist, on the other hand, it is the irradiated portion which is removed by the developer. According to this invention, use may also be made of a core material in which a waveguide pattern can be provided without any material being removed by etching. For instance, there are core layer materials which are chemically converted into materials with a different index of refraction under the influence of heat, light or UV irradiation. In the case of an increase in the index of refraction the treated material can be used as core material. This may take the form of carrying out the treatment using a mask, with the holes in the mask being identical with the desired waveguide pattern. If, on the other hand, a reduction of the index of refraction is involved, the treated material will be suited for use as cladding material. The treatment in question in that case may be carried out using a mask of which the closed portions are identical with the desired waveguide pattem. Use may be made of a planar waveguide of which the core layer comprises a polymer bleachable under the influence of irradiation. This is a particular type of light- or UV-sensitive core layer material. Probably because of a chemical rearrangement reaction, irradiation, preferably generally using blue light, lowers the index of refraction of such a material without essentially affecting the remaining physical and mechanical properties. Preferably, the flat waveguide is provided with a mask covering the desired pattern of channels, so that the surrounding core layer material can have its index of refraction lowered ("be bleached") by means of irradiation. Thus, as desired, waveguide channels are formed which are enclosed on all sides by material having a lower index of refraction (the bottom and top deflection layers and the surrounding bleached core layer material). Such bleachable polymers have been described in EP 358476. In principle, the channels can be defined either before or after the connector piece is contacted with the waveguide. In actual practice, however, it is easiest to define the channels before the connector piece is contacted with the waveguide. The end faces of each of the waveguide channels (in planar waveguides as well as in optical fibres) preferably are at an angle to said waveguide channel's optical axis. This greatly reduces the back reflection of signals being coupled in or out. Such back reflections can have a very harmful effect on the signal, e.g., through ending up in the "laser cavity" of a laser diode again. It was found that back reflection is reduced particularly strongly when the angle is greater than 8 degrees. When a planar waveguide is employed, these oblique angles can be photolithographically defined together with the shape of the edge of the waveguide which is to connect to the connector piece. In that case a single mask will suffice for both purposes. One very efficient way of shaping the connector piece such as to render it suitable for use in the process according to the invention is by etching a rectangular hole in it. When the connector piece is made of a single crystal, a hole with three bevelled edges will be formed. In embodiments where the opto-electronic component(s) is (are) provided on the connector piece, one of these edges may serve as the aforementioned reflecting surface if so desired. An additional advantage of the thus obtained reflecting surface is that it extends the entire height of the connector piece. This means that there is no need for transversal or z-direction alignment. Several connector pieces can be made simultaneously from a single wafer by etching square or rectangular holes in the wafer and then cleaving it. This will be eludidated in greater detail in the example. The invention further pertains to an integrated electro-optical device which can be obtained by the above-described process among others. Preferably, the opto-electronic component (or components) is (are) mounted on top of the connector piece, while the connector piece comprises a mirror capable of coupling optical signals from the opto- electronic component or components into the waveguide and vice versa. Because of the use of the mirror the invention is not restricted to components detecting or emitting laterally, but also allows for the use of detectors (and sources) which are surface-detecting (emitting). When the angle of the mirror to the substrate of the waveguide is smaller than 40 degrees or greater than 50 degrees, back reflections on the end face of the waveguide channels or on the surface of the opto-electronic component can be reduced or avoided. In addition, the invention pertains to a module comprising a connector piece suitable for use in the invention with at least one opto-electronic component mounted thereon and to the connector piece itself. Preferably, the connector piece is on its underside provided with one or more grooves (this embodiment will be discussed in detail in the Example below). Also, it should be noted that EP 420 029 A1 discloses a device for reflecting and focusing light emitted by a laser chip. The light is coupled into a silicon body and reflected from an angled surface in this body in the direction of a focusing device. No mention is made of the silicon body and the laser chip being aligned. Furthermore, EP 607 524 describes a device comprising a silicon body provided with a V-groove in which is placed an optical fibre. Light emitted by the optical fibre is coupled into the silicon body and reflected from an angled surface in this body in the direction of a receiver element. This document fails to describe opto-electronic components integrating with waveguides provided on a substrate. For that matter, according to the invention preference is given to embodiments in which the light is reflected onto, not coupled into, the connector piece. First of all, the coupling in as such results in additional losses and reflections. Secondly, the connector piece material (say, silicon) is always transparent in a restricted wavelength range only. The invention will be further illustrated below with reference to an embodiment shown in the figures. Needless to say, the invention is not restricted to said example. Fig. 1 shows a top view of a silicon wafer in which several square holes have been etched. Fig. 2 shows a side view of a cross-section of an integrated electro-optical device according to the invention. Fig. 3 shows a top view of the integration of a connector piece on which a detector array has been mounted with a planar waveguide. Fig. 4 shows a view in perspective of an embodiment according to the invention. Fig. 5 also shows a view in perspective of an embodiment according to the invention. Example A double-side polished Si (silicon of the crystal type indicated as (100)) wafer 1 is coated on both sides with a layer of SiNx (250 nm thick) by means of PECVD (Plasma Enhanced Chemical Vapour Deposition). On one side of the wafer 1 a metal (Au) pattem is provided (electric paths 7; Fig. 3). This metal pattern is thickened by means of electrolytic deposition, after which a second layer of SiNx is coated over said metal pattem. Next, the SiNx is removed (by RIE) in conformity with holes 2 and lines 4 and 5. The wafer 1 is then immersed in a hot KOH-IPA solution, resulting in the anisotropic removal by etching of the exposed silicon. Once the silicon has been removed to a sufficient degree (it will have disappeared completely from sites 2), the wafer 1 is taken out of the bath and rinsed thoroughly. At the points where the electric paths 7 have to be contacted with other components, the SiNx is removed by RIE. The central side wall 3 (which because of the nature of the Si (100) crystal is at angle of 54.7 degrees to the bottom face of the wafer 1) is coated with a layer of Au to optimise its reflective properties (Au being a good reflector for IR light). Next, using the solder bump (8) flip-chip technique known to the skilled person, a detector array 9 provided with eight detectors is mounted on the connector piece 6, and each of the detectors 10 is electrically connected to one of the paths 7. The other ends of the paths 7 can be electrically connected to other electric components via Tape Automated Bonding. Finally, the wafer 1 in which the connector pieces 6 were realised is broken along lines 4 and 5 to form small individual connector pieces. Simultaneously with the manufacture of the connector piece 6 a number of waveguide channels 14 are realised in a known manner in a planar waveguide structure comprising two deflection layers 11 and a core layer 12 provided on a substrate 13. Next, the RIE process is used to remove a portion of the planar waveguide, with a portion complementary to the shape of the connector piece 6 being provided on the edge of the waveguide. The end faces of each of the waveguide channels 14 are defined such as to be at an angle of 10 degrees to the optical axes of said channels. The waveguide channels 14 run through the planar waveguide at a slight angle. Because of said angle of 10 degrees the optical signal is very slightly deflected on egress. This can be counterbalanced by having the waveguide channels themselves run at an angle. Then the module (connector piece 6 provided with array 9) is provide on the substrate 13 of the waveguide, fitted smoothly against the waveguide, and fastened with glue. Fig. 5 shows shows a special embodiment of the device according to the invention in which a connector piece 15 is provided on its underside with grooves 16 (e.g., through sawing or etching) for letting through the waveguide channels 17. The waveguide channels 17 have branches 18, which open into cavities 19 in the waveguide structure matching the sections of the underside of the connector piece 15 between the grooves 16. In addition, the waveguide structure has comparatively large cavities 20, into which fits the remaining part of the underside of the connector piece 15. The surface area of the cavities (19 and 20) preferably is slightly larger than the surface area of the underside of the connector piece 15, enabling easy and rapid insertion of this connector piece. Part of a signal travelling through one of the waveguide channels 17 will reach the connector piece 15 via the corresponding branch 18 and be reflected to an opto-electronic component (not shown). In this way the connector piece according to the invention can be used to monitor the status of one or more waveguides, and so to rapidly find and localise breakdowns. The value of the optical component or optical network of which this component is part will be greatly enhanced as a result. For, such a control mechanism is highly desirable for high-grade components and networks (and probably for all networks in the future).

Claims: Claims 1. A process for the hybrid integration of at least one optoelectronic component and a waveguide provided on a substrate, characterised in that one or more optoelectronic components are provided on a connector piece and the connector piece is placed on the substrate and moved up to the edge of the waveguide, with the shape of the waveguide and the shape of the connector piece being wholly or largely complementary, and the connector piece when it abuts against the edge of the waveguide having only one degree of freedom in the plane of the substrate. 2. A process according to claim 1 , characterised in that the waveguide is a planar waveguide. 3. A process according to claim 2, characterised in that the edge of the waveguide, at least where it is complementary to the connector piece, is photolithographically defined. 4. A process according to claim 2 or 3 in which the waveguide is provided with one or more waveguide channels, characterised in that the end face of each of these channels is defined such as to be at an angle to optical axes of said channels. 5. A process according to claim 4, characterised in that the angle is greater than 8 degrees. 6. A process according to any one of the preceding claims, characterised in that on the side where the connector piece abuts against the waveguide a hole has been etched. A process according to any one of the preceding claims, characterised in that the connector piece ts made from a single crystal A process according to any one of the preceding claims, characterised in that on the underside of the connector piece at least one groove is provided An integrated electro-optical device obtainable by means of a process according to any one of the preceding claims An integrated electro-optical device according to claim 9, characterised in that the optoelectronic component is mounted on top of the connector piece and the connector piece comprises a mirror capable of coupling optical signals from the opto-electronic component or components into the waveguide and vice versa An integrated electro-optical device according to claim 10, characteπsed in that the angle of the mirror to the substrate on which the waveguide is provided is smaller than 40 degrees or greater than 50 degrees A module comprising a connector piece suitable for use in the process according to any one of claims 1-8, with at least one optoelectronic component mounted thereon A connector piece suitable for use in the process according to any one of claims 1-8 14. A connector piece according to claim 3, characterised in that on its underside the connector piece is provided with one or more grooves.

Inventors: DE DOBBELAERE PETER MARTIN CYR, VAN DAELE PETER PAUL, VAN DER LINDEN JOHAN EDUARD, DE DOBBELAERE, PETER, MARTIN, CYRIEL, VAN DAELE, PETER, PAUL, VAN DER LINDEN, JOHAN, EDUARD
Applicants: AKZO NOBEL NV, DAELE PETER PAUL VAN, DOBBELAERE PETER MARTIN CYRIEL, LINDEN JOHAN EDUARD V D, AKZO NOBEL N.V., DE DOBBELAERE, PETER, MARTIN, CYRIEL, VAN DAELE, PETER, PAUL, VAN DER LINDEN, JOHAN, EDUARD
Application year: 1997
Main classification: G02B6
IPC codes: G02B6