A METHOD OF FABRICATING MIRRORS IN POLYMER WAVEGUIDES

Abstract: The position of an oblique mirror structure inside a polymer waveguide (1) in relation to an optical device is accurately defined by using an integrated mask of metal which is defined using photolithography, on top of the waveguide structure. When irradiating the surface of the waveguide with laser light, it is only let through where no metal is left, so that the recess (19) formed is defined by edges of strips (15) of the metal layer. In the same metal layer also electrical contact pads (13) are produced used for self-aligning flip-chip mounting of devices such as lasers and photodiodes. Another metal layer (21) can be applied inside the waveguide structure at a place under the waveguide core so that it is hit by and stops the laser light. The energy of the laser will atomize the metal and the dispersed metal will then be deposited on the walls of the recess (19).

Description: A METHOD OF FABRICATING MIRRORS IN POLYMER WAVEGUIDES TECHNICAL FIELD The present invention relates to a method of fabricating mirrors in polymer waveguides. 5 BACKGROUND OF THE INVENTION AND STATE OF THE ART Telecommunication systems using light propagating in different waveguides expand more and more today. There is a large interest in extending the optical networks even up to private homes and local business estates, the so called local access network which is also called "Fibre To (In/From) the Home", "Fibre To (In/From) the Customer ιo (Business)", etc. Also, there is a large interest in extending the use of optical networks in LANs, i.e. local area networks, used for interconnecting computers in a business estate and furthermore for communication inside computer equipment and for communication between computers and peripheral devices such as printers etc. In order to achieve this expansion, the costs of the components of the optical networks of course have to be is reduced as much as possible. Very important costs are related to producing the optical transmitter and receiver modules including lasers, LEDs, etc. and other active or passive devices. In optical transmitter and receiver modules and other optical products which have integrated waveguides for light there is a need for mirrors which can reflect light or 20 generally make a abrupt change of the direction of light propagated in the optical waveguides such as to deflect the light out of a waveguide to some receiver. Mirrors which can generally be formed by end surfaces of optical waveguides can be produced using different methods. Mirrors in the waveguide plane achieving a reflection at a surface to air has been disclosed by Honeywell, for example for creating sharp 90° bends 25 in a waveguide, and mirrors for deflecting light out of a waveguide plane have been disclosed by Dupont and IBM, see for example Lawrence A. Hornack, editor "Polymers for lightwave in integrated optics", Marcel Decker K.K., New York, 1992, Chapter 9 by B. Booth, Dupont. In the company Ericsson, the department MIRC has in cooperation with the Institute for Optical Research disclosed mirrors in waveguides for deflecting out 3o of a waveguide plane produced by means of an UV-excimer laser, see Gunnar Bostrδm, "Waveguide grating couplers", graduate report at the Royal Institute of Technology, Stockholm, Trita. Phys. 2138, Sep. 15, 1994. In U.S. patent No. 5,327,415 a method of producing a reflector for a laser in a semiconductor material is described, see Figs. 2a - 2c. A metal mask having portions 28, 35 34 defines the opening of the reflector at the surface of the material. SUMMARY OF THE INVENTION It is an object of the invention to provide a method providing an accurate positioning of an optoelectrical device or chip to be correctly coupled to a channel-type waveguide formed in the surface layer of a substrate. It is a further object of the invention to provide a method for positioning an optoelectrical device in relation to a reflector formed in the surface layer of a substrate. It is a further object of the invention to provide a method for making a reflector in an optical waveguide allowing a simultaneous metallization of reflecting surfaces. Thus the position of an oblique mirror structure inside a polymer waveguide is accurately defined by using an integrated mask of metal, which is defined using photolithography, on top of the waveguide structure. The metal mask only lets the laser light through where material is to be removed, i.e. ablated, in order to manufacture one or several mirrors. The need for a high accuracy of the position in space of the laser radiation beam can be reduced since in the lithographic mask which defines the metal mask intended for producing the reflector also a pattern can be included defining the contact pads intended for flip-chip mounting of lasers and photodiodes. Self-aligning flip- chip mounting comprising soldering isles allows a very accurate positioning of the optocomponents which will then have a correct location in relation to the mirror obtained. The edge inside the integrated metal mask, which defines the position of the reflecting surface can be a straight line or be curved, for example having a parabolic profile, in order that more light which diverges from e.g. a laser diode will be collected in the waveguide structure or that light from the waveguide will be better focused on for example the input surface of a photodiode. Another metal layer can be applied directly under the undercladding or even at a suitable place under the waveguide core in order to use this for metallizing the mirror surface. Then the metallizing of the surfaces which will form the mirrors is made in the same process and directly after the cavity being made, when the radiation beam reaches this metal surface. The laser beam will then also affect this inner metal layer but will at a substantially lower velocity than the polymer material. Metal will then be removed in a very finely divided form having a high velocity of the particles therein. They will then deposit on available surfaces, and thus on the mirror surface owing to that it is located very close to the surface of the inner metal region. The surface of the inner metallization should advantageously be diffusely reflecting in order to mi_r___mize ablation due to reflections of the laser beam. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of non-limiting embodiments with reference to accompanying drawings in which: Fig. 1 is partial sectional view of an optical waveguide structure in which a reflector is formed, Fig. 2 is partial sectional view of the optical waveguide structure of Fig. 1, on which a surface-emitting laser is mounted, Figs. 3 and 4 are top views of the multilayer structure forming the optical waveguide in two embodiments of a mirror-defining edge, and Fig. 5 is a partial sectional view similar to that of Fig. 1 of optical waveguide structures arranged on top of each other. DESCRIPTION OF PREFERRED EMBODIMENTS In Fig. 1 a fragmentary cross-sectional view of a waveguiding structure 1 applied on a surface of a substrate 3 is shown. The substrate 3 can be made of a suitable plastics material. The light waveguiding structure 1 comprises three layers, an undercladding 5, a waveguide core layer 7 and an overcladding 9. All the waveguiding layers are assumed to be made of suitable polymer materials. The polymer materials can be selected as suggested in M. Robertsson, A. Dabek, G. Gustafsson, O.-J. Hagel, M. Popall, "New Patternable Dielectric and Optical Materials for MCM-L/D- and o/e-MCM-packaging" , First IEEE Int. Symp. on Polymeric Electronics packaging, Oct. 26-30, 1997, Norrkδping, Sweden, and the optical waveguide 10 can be formed as is disclosed in the simultaneously filed Swedish patent application "Optoelectric multichip module". On top of the overcladding 9 a metal layer 11 is applied which has been patterned to an accurately defined configuration, including soldering islands or pads 13 for connecting an optoelectric or electrooptic component, not shown, and electrical conductors, not shown, such as between the pads 13 and current supply or control circuits, not shown. In addition to these areas the metal layer 11 includes a frame like structure 15 having an elongated opening defining the location, where a reflector of 45° is to be formed. The metal layer 11 and its different areas 13, 15 are also visible in the top view in Fig. 3. By directing a powerful laser beam 17 from a laser 18 in the desired angle to the surface of the structure 1 to hit the frame 15 and in particular the opening therein, an oblique recess or trench 19 can be formed. The laser beam 17 is located in relation to the long inner edges 16, 16', see also Fig. 3, of the frame 17 so that it hits at least one of these edges along which it can move in order to form the recess 19. The laser 18 producing the beam 17 can e.g. be an UV-excimer laser. At the border surface between the substrate 3 and the undercladding 5 a metal layer may have been applied and patterned. Then a portion 21 can be left at the bottom of the recess 19 in order to stop the laser beam, so that it cannot penetrate into the substrate 3. For suitable refractive indices of the materials in the layers forming the optical waveguide 10 and in particular in the waveguide core layer 7 the upper one of the two parallel surfaces of the recess 19, which surfaces both have an angle of 45° in relation to the surface of the structure, hence also to the different layers and to the substrate surface, light propagating in the waveguide structure 10 can be reflected at the boarder surface to air out of the waveguide 10 to hit for example a light detector, not shown. The same reflecting surface can be used for injecting light in the waveguide if for example a light generating unit such as a surface emitting laser 23 is placed topside down above the reflector recess 19, as is illustrated in Fig. 2. The component mounted on the surface, such as the surface emitting laser 23, is preferably aligned to or positioned in an accurately defined position using surface-tension forces of solder in solder balls or dots 25 applied at accurately defined positions to the bottom side of the component. These balls or dots 25 are when melted and when the component is placed at an approximate position adhering to and wetting the soldering 5 pads 13, moving the component to the intended position in relation to the frame-like structure 15 and to the recess 19 and its reflecting surfaces. In that way the positioning of the component can be easily made, not requiring complicated aligning operations. In order to make the reflecting surface less sensitive to dirt and pollution and in order to allow a refilling of the cavity or recess 19, the surface of the cavity recess 19 o can be metallized. The cavity 19 can then be refilled by a suitable material, for example a polymer cured by means of light, the polymer having a normal refractive index. For a suitable material in the inner metal region 21 the metal can be evaporated when hit by the laser beam 17. The evaporated metal can then be deposited on adjacent surfaces, i.e. the surfaces inside the cavity 19 and also thus the surface which is to be 5 used as a mirror for the hght propagating in the waveguide structure. The surface of the metallization has advantageously a diffuse reflection of the light of the laser beam in order to miiiimize ablation due to the reflected beam. As is visible in Fig. 3 the frame-like structure 15 for defining the opening of the cavity 19 has in the simplest case two long parallel sides of metal material opaque to the o light of the strong laser beam 17. Also end portions to close the frame can be arranged at the ends of these long, defining side portions. Only the position of that side-edge 16, 16' of a long parallel side of the frame 15 is important where the reflecting surface is to be formed. Thus, in the general case only one strip of masking metal material is required for positioning the reflecting side of the reflective recess accurately, the strip corresponding 5 to a long side of the frame-like structure 15. The laser beam has then to be moved along the intended edge of the strip to hit the surface of the polymer structure at this edge, the laser staying at each position for a sufficiently long period until polymer material down to the substrate 3 has been ablated. The inner edge of a long side of the frame-structure 15 can also be curved, as is 0 visible at 16" in Fig. 3 in order to form a reflecting surface for example of a paraboloidic shape. Hence a focusing of the hght propagating in the waveguide 10 which is to hit a photodetector element, not shown, can for example be obtained. Multiple optical waveguide structures formed by e.g. a polymer structure 1' of the kind illustrated in Figs. 1 and 2 located on top of an already formed bottom polymer 5 structure 1 can also be obtained, as is illustrated in Fig. 5. Then contact pads 13 are only arranged in the top-most polymer structure 1'. Reflecting surfaces for light waves propagating in the waveguides of each individual structure are produced as above. Then first the under-most polymer structure including suitably arranged optical waveguides is produced, then mirrors in this structure and thereupon a new polymer structure on top of the already formed structure. When applying the first polymer layer in this new structure, the recesses 19 for the mirrors can be filled with the polymer material. Thereupon the reflecting recesses for the waveguides of the new structure are produced, etc.

Claims: CLAIMS 1. A method of connecting a hght generating or receiving electrooptical or optoelectrical device to an optical waveguide formed in a surface layer of a substrate, comprising the steps of applying a metal layer on the surface, patterning the metal layer to form an edge of a region, the edge defining the location of a reflecting surface to be formed, irradiating the surface at the edge to remove material in the surface layer so that a recess is formed having a light reflecting sidewall starting at the edge and extending into the surface layer, characterized in that in patterning the metal layer it is patterned to form metal areas positioning the device in an accurately defined position in relation to the edge and thus to the reflecting sidewall. 2. A method according to claim 1, characterized in that in patterning the metal layer the edge is given such a shape, that when irradiating the region, the sidewall of the recess obtains a correspondingly curved shape, which allows focusing of light propagating along the waveguide or which allows light to be injected in the waveguide to be shaped to a parallel beam. 3. A method according to any of claims 1 - 2, characterized in that before irradiating the surface an interior metal layer is arranged inside the surface layer in a position below or inside the optical waveguide, so that when irradiating the surface with a hght beam the hght beam after removing material in the optical waveguide will hit the interior metal layer, whereby the material of the interior metal layer will be evaporated, sputtered or atomized and be deposited on the sidewall. 4. A method of making a reflector in an optical waveguide formed in a surface layer of a substrate, comprising the steps of applying a metal layer to a surface of the surface layer, patterning the metal to form a metal edge and a region having no metal at the edge, irradiating the region at the edge by a beam to remove material in the surface layer so that a recess is formed having a hght reflecting sidewall, the recess and sidewall extending into the surface layer, characterized in that in patterning the metal layer the edge is given such a curved shape, that when irradiating the region the sidewall of the recess will have a corresponding curved shape allowing focusing of hght propagating in the waveguide or allowing that hght to be injected in the waveguide is made to a parallel beam. 5. A method according to claim 4, characterized in that before irradiating the surface a metal layer is arranged inside the surface layer in a position below or inside the optical waveguide, so that when irradiating the surface with a hght beam the light beam after removing material in the optical waveguide will hit the interior metal layer, whereby the material of the interior metal layer will be evaporated, sputtered or atomized and be deposited on the sidewall. 6. A method of making a reflector in an optical waveguide formed in a surface layer of a substrate, comprising the step of irradiating the surface by a hght beam to remove material in the surface layer so that a recess is formed having a hght reflecting sidewall extending into the surface layer, characterized by the further steps of placing, before irradiating the surface, a metal layer inside the surface layer at a position below or inside the optical waveguide, the position being selected so that in irradiating the surface the beam will, after removing material in the optical waveguide, hit the metal layer, whereby the material of the metal layer will be evaporated, atomized or finely dispersed and be deposited on the sidewall.

Inventors: ROBERTSSON MATS, ROBERTSSON, MATS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: G02B6
IPC codes: G02B6