Abstract:
A thin-film multilayer structure on top of a substrate has three polymer layers having adapted refractive indices in which optical waveguides are formed. Signal conducting metal layers are located at these three thin-film layers. The metal is etched away at the waveguide cores so that ordinary optical waveguides of channel-type are obtained having a refractive index difference between the core and the cladding material. Holes are etched in the polymer layers in order to form electrical vias. Mirrors can be formed by laser ablating to provide connection of an optical waveguide to some component and to provide optical vias, in the case where another similar set of three polymer layers are applied on top of the layers shown. Hence, electrical and optical interconnections can be integrated in the multilayer structure using a minimum number of polymer layers and the optical waveguides can be constructed to have a low loss.

Description:
The present invention relates to an optoelectric multichip module and a method of fabricating it using basically polymer materials. 
     BACKGROUND 
     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 business offices, the so called local access network which is also called “Fibre To (In/From) the Home”, “Fibre To (In/From) the Customer (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 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 optical devices. 
     Only in a few cases attempts have been made to drastically reduce the costs when commercially manufacturing optoelectric and electrooptical modules. For example, the company Motorola has put a concept called “OPTOBUS” on the market. Some details thereof are disclosed in U.S. Pat. No. 5,659,648 for Knapp et al. In a multilayer structure based on a substrate made of polyimide optical signal layers are used as electrically isolating layers between electrically conducting metal layers. In FIGS. 1 and 2 the structures comprise waveguide cores  17 ,  18 , . . . and  45 ,  46  of a polymer material which at their sides have metal strips in a central layer, the metal strips forming part of the waveguide cladding. Layers under and on top of the central layer are made of polyimide and constitute an overcladding and an undercladding. In the structure shown in FIG. 3 whole metal layers  52 ,  60  are in addition placed between the central layer and the polymer cladding layers. No details are given in regard to positioning electrical signal conductors. 
     SUMMARY 
     It is an object of the invention to provide multilayer structures allowing both electrical and optical connections. 
     It is a further object of the invention to provide multilayer structures allowing electrical interconnections and optical connections having low losses. 
     It is a further object of the invention to provide a method of fabricating multilayer structures having electrical and optical connections which can be easily executed using a minimum number of processing steps. 
     A thin-film process is used for sequentially building a multilayer structure on top of a suitable substrate. In the multilayer structure at least on thin-film layer of a suitable polymer is used both as an electrically isolating layer separating signal conducting metal layers and as a layer of an optical waveguide. The materials of the thin-film structure are selected to be optically transparent to some suitable, selected light wavelength and have adapted refractive indices for this wavelength. Generally, signal conducting metal layers are located between and/or on top of and/or under the three thin-film layers forming the optical waveguide but the metal is etched away at the waveguide cores so that optical waveguides of the type having a refractive index difference between the core and the cladding material are obtained, i.e. the claddings are of a transparent optical material and are formed by portions of the bottom and top layers of the thin-film structure. Hence, electrical and optical interconnections can be integrated in the multilayer structure using a minimum number of polymer layers and the optical waveguides can be constructed having no metal layers for defining them or no metal layers in the direct vicinity of the waveguide cores. Various components can be mounted at the multilayer structure, such as lasers, photodiodes, electronic driver circuits for the optical devices, electronic logical and memory circuits. The components can e.g. be flip-chip mounted or wire bonded. For example a combined cable of ribbon type can be formed, accommodating both electrical conductors and optical waveguides. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which: 
     FIG. 1 is a plan view of a portion of a substrate covered by a multilayer structure forming electrical signal paths and optical waveguides, the portion in particular illustrating a transmitter module and a receiver module having connectors, 
     FIG. 2 is a plan view in a larger scale showing in particular optical waveguides and the mounting of a laser chip, 
     FIG. 3 is a cross-sectional view taken perpendicularly to optical waveguides showing the construction thereof, 
     FIG. 4 is a cross-sectional view showing the mounting of a surface-emitting laser chip and its connection to electrical signal paths and to an optical waveguide, 
     FIG. 5 is a perspective view illustrating the mounting of a connector device on substrate, 
     FIG. 6 is a cross-sectional view showing the mounting of a surface-emitting laser chip and its connection to electrical signal paths and to an optical waveguide, and 
     FIG. 7 is plan view similar to FIG. 1 illustrating an alternative embodiment of connectors for the transmitter and receiver modules. 
    
    
     DETAILED DESCRIPTION 
     The fabrication of the multichip module to be described is generally based on the use of materials as described 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, Norrköping, Sweden. There, photo-patternable polymer materials, ORMOCER®, are disclosed which are suitable for optoelectrical multichip modules in order to build optical waveguides. In particular the refractive indices of these materials can be varied for producing cores and claddings of optical waveguide structures. In addition the materials can be processed at relatively low temperatures of 120-180°. Also, the materials have good etching characteristics. The low processing temperatures result in that low-cost substrates not suited for high temperature processing can also be used, so that substrates such as standard materials used for circuit boards, FR4-epoxy, BT-laminates, silicon wafers, ceramics, glass, metal, thin foils of polymers and other materials can be used. 
     In FIG. 1 a plan view of a portion of a substrate coated with various layers is shown, having optical and electric components mounted thereon. After manufacturing the substrate and having components mounted thereon the substrate is intended to be divide in square modules  1 , the division lines being indicated at  3  and marks for cutting the substrate being shown at  5 . At  7  a surface-emitting laser chip is shown, comprising five individual laser units. The laser units emit light into corresponding five optical waveguides  9  extending in parallel to each other and perpendicularly to a division line  3  and beyond this division line into a neighboring module  1 . On the module on which the laser chip  7  is mounted an electronic driver circuit chip  11  is located and furthermore electric contact pads  13  are placed in the margin region of the considered module  1  for wired connection of chip capacitors, not shown. On the neighboring module, into which the optical waveguides  9  extend, a photodetector chip  15  is located receiving light from the five parallel optical waveguides  9 . Three electronic driver chips  17  are also located on this module and also electric contact pads are provided. The optical and electronic chips  7 ,  15 ,  11 ,  17  can be electrically connected to the substrate by for example the ball grid array method, as is illustrated by the contact pads  18  drawn in dashed lines. 
     In the partial elevational view of FIG. 4, the different layers which form the optical waveguides  9  and are located on top of the very substrate  19  are shown, and also electrically conducting layers and interconnections between layers at different levels. A substrate  19  of standard multilayer type is used having metal layers  21 ,  23  on its bottom and top surfaces respectively and two interior metal layers  25 ,  27 . These metal layers can all be patterned to form suitable electrical conductors paths between various parts. Also, vias can be formed interconnecting the metal layers  21 - 27  of the substrate at suitable places in the conventional way. On top of the upper metal layer  23  of the substrate  19  is a first patterned thin metal layer  29  applied forming vias  30  for contacting the top interior thick metal layer  27  in the substrate  19 . On top of the thin metal layer  29  a first polymer layer  31  is applied and on top of this layer in turn a sequence of a second thin, electrically conducting metal layer  33  for signal transmission, a second polymer layer  35 , a third electrically conducting, thin metal layer  37  for signal transmission, a third polymer layer  39 , a fourth electrically conducting, thin metal layer  41  for signal transmission is located. The driver circuit  11  is here as an alternative shown to be connected by wires  18  to contact areas of the third metal layer  37 . 
     The polymer layers  31 ,  35 ,  39  are made of the polymer ORMOCER as described in the article cited above. The polymer layers  31 ,  35 ,  39  have adapted refraction indices in order to be capable of forming an undercladding, a waveguide core and an overcladding of optical waveguides as will be described hereinafter. Typical thicknesses are 5-20 μm, e.g. 10 μm, for the first or bottom and the third or top polymer layers  31 ,  39  and 5-70 μm, e.g. 20 μm, for the second or intermediate polymer layer  35 . These polymer layers can all be patterned but they will only comprise a very small total area of holes or cutouts, primarily only via holes for allowing electrical interconnections between different levels. The second polymer layer  35  is in addition patterned to allow that cladding portions are formed at the sides of the waveguide cores formed in this layer, as will be described hereinafter. The top polymer layers  35 ,  39  can be patterned to comprise long parallel grooves  43  for allowing alignment of optical connectors  44  intended to be connected to one or two modules  1 , see FIG.  1 . Also, the top polymer layer  39  can have cut-outs to allow electrical contacting of the third electrical signal layer  37  from the top side of the structure with contact pads  13  provided in that metal layer. 
     The electrical signal layers  29 ,  33 ,  37 ,  41  are all very thin and can have a thickness of typically 3 μm to be compared to the thickness of the substrate metal layers  21 ,  23 ,  25 ,  27  which can be of the order of 200 μm. The three inner layers  29 ,  33 ,  37  in the top multilayer structure can be made of aluminum by sputtering. The top layer  41  is made of a sequence of layers comprising undermost a sputtered layer of aluminium, thereon a very thin titanium layer and a thin copper layer and on top a thicker nickel-layer coated with a thin gold layer. They are all patterned to form conductor paths and possibly electric contact pads and to fill via holes in the underlying polymer layer for contacting the electrically conducing layer located next thereunder. Rather little metal material may be left in each electrical signal layer and in particular there is no metal material at the bottom and top surfaces of the third polymer layer  39  at the areas in which it forms optical waveguide cores in order not to interfere with the propagation of light in the waveguides and allow a straight extension and a uniform cross-section of the waveguide cores. 
     In the enlarged view in FIG. 2 the top metal layer and the top polymer are shown and in particular the area under and at the laser chip  7 . The waveguides  9  are here seen to comprise waveguide cores  51  formed of strips of the second polymer layer  35 . At the sides of the cores  51  in this layer grooves  53  have been formed which then have been filled with material from the next polymer layer, the third or top polymer layer  39 , see also the cross-sectional view of FIG.  3 . The grooves  53  can have a width corresponding to the thickness of the overcladding and undercladding layers, i.e. having a width of e.g. 10 μm. The grooves  53  and thus the waveguides  9  extend under the laser chip  7 , and there, in each waveguide, a mirror  55  is formed by a deep, oblique recess produced by laser ablating from the top of the layer assembly, see also FIG.  4 . At least one edge of the mirror recesses  55  at the surface of the layer assembly is limited by strips  57  of metal of the top metal layer  41 , the appropriate side of these metal strips  57  defining the position of the respective mirror recess  55 . Contact pads  59  for electrically contacting the laser chip  7  and for aligning it by the use of surface tension forces when soldering the laser chip are also formed by the top metal layer  41 . The contact and aligning pads  59  and the mirror defining strips  57  are thus formed by the same metal layer and using the same mask step for patterning this metal layer. 
     In the lower portion of FIG. 1, and in particular in FIG. 5, a connector structure  44  is visible. It is intended to form connectors of a kind as described in Swedish Patent Application No. 9504549-8. The connector structure  44  has the shape of an elongated rectangular plate from which, at the long sides thereof, ribs  61  project downwards. The ribs are symmetrically located at the long sides and end at some distance from the short sides of the rectangular connector body. The ribs  61  are intended to approximately position the connector structure by placing them in cut-out grooves  63  passing all through he polymer layers and the substrate  19 . The grooves are machined, for example, after applying all the metal and polymer layers, but before mounting components. For a fine positioning or alignment, the connector structure  44  also has low alignment ribs, not visible, placed on the underside of the connector structure plate portion between the high ribs  61 . The low alignment ribs cooperate with the grooves  43  in the top polymer layers in the structure as described above. 
     The connector structure  44  bridges two neighbouring modules  1 . It is located above waveguides  9  extending between the modules and is intended to form aligning connectors for the waveguides. The connector structure  44  is mounted at the same time as other components are mounted on the coated substrate  19  and may e.g. be attached to the surface of the substrate by glue pads  67 , see FIG.  1 . In the embodiment illustrated in FIGS. 1 and 5 end portions of the plate-shaped body of the connector structure extend over the laser chip  7  and the photodetector chip  15  to form a protection thereof. After mounting all components and connector structures the substrate  19  is split into individual modules  1 , by e.g. sawing, along the lines  3  as defined by the marks  5 . After such a sawing operation the waveguides  9  are also cut off to have ends at the module borders. The ends of the waveguides will then be located in the same perpendicular or vertical plane as the cut-off ends of the connector structure  44 , which by the splitting operation is divided into two connectors, one on each module  1 . 
     Attaching the connector structures  44  before splitting the substrate into modules can lower the manufacturing costs, both by attaching basically two connectors in one operation and by having the end surfaces of the individual connectors located in the same cut-off plane as the end surfaces of the waveguides, which facilitates polishing the end surfaces which is necessary in order to form well-aligned waveguide connections having a low attenuation. 
     In the surface structure allowing waveguides to be formed of course also edge emitting lasers can be used. This is illustrated in the sectional view of FIG.  6 . Here an edge-emitting laser unit  7 ′ is mounted in a recess  71  made in the two top polymer layers  35  and  39 . The laser unit comprises a plurality of individual lasers issuing light into respective waveguides  9 . The laser unit  7 ′ can be protected by an additional polymer layer  73 . This layer  73  can also penetrate between the surface of the laser unit  7 ′ and the opposite end surfaces of the waveguides  9 . It is then advantageously given a suitable refractive index in order to match the refractive indices of the laser units and waveguides in order to lower the attenuation of light coupled from the laser units into the waveguides  9  and to reduce back reflection of light into the laser units. 
     An alternative embodiment of the connector structure is illustrated in FIG.  7 . The connector structure  44 ′ is there illustrated to cover the main portions of two adjacent multilayer modules and thus all the components of each module. The cut-out grooves  63 ′ are located at two opposite edges of each module and the parallel low ribs  43 ′ extend next to the grooves. The low ribs can be designed as a continuous rib parallel to three edges of each module for sealing each module at these three edges in addition to their main function of accurately positioning the connector structure  44 ′ and thereby the individual connectors formed when splitting the substrate. 
     While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fail within a true spirit and scope of the invention.