Abstract:
The invention provides an optoelectronic assembly for coupling an optical conductor to a light emitting surface of an optoelectronic semiconductor device on a substrate. The optoelectronic assembly includes a multilayer having a cavity adapted to receive and electrically connect the optoelectronic semiconductor device to the multilayer substrate and a groove leading to the cavity and being adapted to receive and optically connect the optical conductor to the light emitting surface of said optoelectronic semiconductor device. The optoelectronic semiconductor device and the optical conductor are precisely positioned within the cavity and the groove, respectively, so that light emitted from the light emitting surface of the optoelectronic semiconductor device couples to an optical surface of the optical conductor.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
   This application claims the benefit of U.S. provisional application Ser. No. 60/388,437 filed on Jun. 13, 2002 and entitled INTEGRATED OPTOELECTRONIC ASSEMBLY WITH EMBEDDED OPTICAL AND ELECTRICAL COMPONENTS which is commonly assigned and the contents of which are expressly incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to an optoelectronic assembly, and more particularly, to an optoelectronic assembly with embedded optical and electrical components on a substrate. 
   BACKGROUND OF THE INVENTION 
   There are many methods for manufacturing optoelectronic assemblies that include integrated optical and electrical devices and components on a substrate. The optical and electrical devices and components may be among others optical fibers, lenses, mirrors, light emitting diodes, laser diodes, photodiodes, detectors, semiconductor devices, capacitors, inductors and resistors. 
   Several prior art methods utilise silicon “V-grooves” as fiber positioning elements. U.S. Pat. No. 4,767,174 makes use of the fact that certain crystalline orientations of silicon substrates can be preferentially etched to a high degree of accuracy. This is accomplished by a series of lithographic steps including resist coating and exposure, followed by liquid etching. This is an expensive method, which is not compatible with standard microelectronic infrastructures and processes. 
   Alternatively, the V-groove and optical fiber can be positioned relative to a waveguide by using additional positioning elements. These positioning elements also increase the complexity and cost of the method. Even when the V-groove technique is utilised only to couple two optical fibers to one another, as in U.S. Pat. No. 4,973,126, there are several additional positioning elements required. 
   U.S. Pat. No. 4,735,677 describes a method for providing guides for aligning optical fibers on the surface of a silicon substrate. In this method it is necessary to first grow a layer of glass on the silicon wafer by a soot process. This layer of glass is then lithographically patterned and etched, as by reactive ion etching (RIE), to form the positioning elements. After formation of these elements, an optical fiber can be inserted between them and fixing is accomplished with an adhesive or by melting the glass with a CO.sub.2 laser beam. This technique involves a great number of processing steps and is limited to substrates that are not damaged by high temperature processes or those that do not contain sensitive electronic devices that would be damaged by an RIE etch. 
   U.S. Pat. No. 4,750,799 teaches a hybrid optical integrated circuit having a high-silica glass optical waveguide formed on a silicon substrate, an optical fiber and an optical device coupled optically to the optical waveguide, and an optical fiber guide and an optical device guide on the substrate for aligning the optical fiber and the optical device at predetermining positions, respectively, relative to the optical waveguide. Islands carrying electrical conductors are disposed on the substrate, a first electrical conductor film is formed on the substrate, second electrical conductor films are formed on the top surfaces of the optical waveguide, the optical fiber guide, the optical device guide and the islands and are electrically isolated from the first electrical conductor film. 
   U.S. Pat. No. 4,796,975 teaches a method of aligning and attaching optical fibers to substrate optical waveguides. One or more slabs of preferentially etchable material and a waveguide substrate are placed adjacent to each other face down on a flat surface for aligning the tops of the slabs with the top of the waveguide. A backing plate is secured to the back surface to hold the entire assembly together. The preferentially etchable material is etched to form V-grooves in alignment with the light guiding region of the waveguide substrate. Optical fibers are secured within the groove and are optically aligned with the light guiding region. 
   U.S. Pat. No. 5,359,687 teaches an optical coupling device having a substrate with a surface region at a pre-determined position on the surface of the substrate for placing an optical waveguide. The substrate further includes a channel on its surface for optically aligning and coupling an optical fiber to an optical waveguide positioned at the predetermined position. The longitudinal axis of the channel is in alignment with the predetermined position such that on placement of the optical fiber in the channel and placement of the optical waveguide in the position the light carrying core of the fiber and the waveguide are substantially in optical alignment. 
   U.S. Pat. No. 6,266,472 describes a process of splicing optical fibers having of a substrate and at least one optical fiber gripper on the substrate. The gripper includes adjacent parallel, polymeric strips each having a base attached to a surface of the substrate, a top surface in a plane parallel to the substrate, and side walls, which form a groove between adjacent strips. The invention provides strips of polymeric splice elements, also known as elastomeric polymer grippers, to provide a splice means for optical fibers. Once these grippers are deposited on a substrate, optical fibers can be snapped between them. The splicing elements allow for accurate lateral and longitudinal alignment and improved collinearity of spliced optical fibers, achieving low coupling loss. 
   U.S. Pat. No. 6,371,655 provides a molded plastic housing that incorporates a device and other optical elements including an optical fiber and lenses. The device may be a transmitter. The entire housing assembly may be connected to another similar assembly housing a receiver. The transmitter package may be mounted side-by side in another plastic housing. 
   U.S. Pat. No. 6,285,808 and U.S. Pat. No. 6,324,328 provide a circuit carrier having an optical polymer layer embedded between layers of insulating or conducting materials. The circuit carrier structure including the optical layer is formed by conventional laminating processes. 
   U.S. Pat. No. 6,376,268 provides an optoelectronic assembly having an insulating substrate with a planar surface and a metal layer bonded to the planar surface. The metal layer is patterned and adapted to receive active and passive optical components. Structures formed in the metal layer (i.e., steps) are utilized as fiduciaries for positioning of the optical components. Furthermore there is an optical fiber placed into a groove and attached to the substrate. 
   U.S. Pat. No. 5,875,205 and U.S. Pat. No. 6,271,049 describe an optoelectronic assembly having active optical components (i.e., a laser chip) and passive optical components (i.e., a lens) integrated onto a substrate. 
   None of these prior art methods teach how to accurately position and align optical fibers and other optical and electrical devices on a substrate. There is a need for an assembly and a method for accurately positioning and aligning of optical components such as fibers, optical waveguides, and semiconductor devices on a substrate. The invention further needs to compactly integrate optical and electrical passive and active structures within the same coupling platform. The invention further needs to provide means for optimising the assembly of a complete optoelectronic component, and not only one of its constituents. The invention also needs to make use of standard processes and materials currently used in the microelectronics and micro-electromechanical systems industries, allowing for large-scale manufacturing at reduced cost, while still improving the accuracy and repeatability of the assembly. 
   SUMMARY OF THE INVENTION 
   The invention provides an assembly and a method for accurate placing and aligning active and passive devices and components on a substrate by precise patterning of an optical polymer layer formed on the substrate and adapted to receive the mentioned devices. 
   In general, in one aspect, the invention provides an optoelectronic assembly for coupling an optical conductor to a light emitting surface of an optoelectronic semiconductor device. The optoelectronic assembly includes a multilayer substrate having a cavity adapted to receive and electrically connect the optoelectronic semiconductor device to the multilayer substrate and a groove leading to the cavity and being adapted to receive and optically connect the optical conductor to the light emitting surface of the optoelectronic semiconductor device. The optoelectronic semiconductor device and the optical conductor are precisely positioned within the cavity and the groove, respectively, so that light emitted from the light emitting surface of the optoelectronic semiconductor device couples to an optical surface of the optical conductor. 
   Implementations of this aspect of the invention may include one or more of the following features. The multilayer substrate may further include first and second sets of patterned layers. The first set of patterned layers may include at least one metal layer, at least one dielectric layer, and a plurality of passive electrical components embedded within the patterned layers and connected to each other and to the metal layer thereby forming an electrical network. The second set of patterned layers may be formed upon the first set of patterned layers, and it may include at least one polymeric layer. The polymeric layer may include the cavity and the groove. The cavity may include a first opening formed in a cavity wall. The optoelectronic semiconductor device is oriented and positioned within the cavity so that light emitted from the light emitting surface exits the cavity via the first opening. The groove may include a second opening formed in a groove wall. The second opening is aligned with the first opening, thereby allowing light exiting the cavity through the first opening to couple to an optical surface of the optical conductor aligned with the second opening. The optoelectronic semiconductor device is precisely positioned within the cavity in X and Y directions by placing two adjacent walls of the device in contact with two adjacent walls of the cavity. The optoelectronic semiconductor device may further be precisely positioned within the cavity in a Z direction by placing a bottom surface of the device upon a bottom surface of said cavity. The optoelectronic semiconductor device may further be precisely positioned within the cavity in a Z direction at a predetermined height by placing a bottom surface of the device upon a first spacer positioned upon a bottom surface of the cavity and having the predetermined height. The cavity wall having the first opening may be spaced apart the groove wall having the second opening by a predetermined distance by placing a second spacer between the cavity wall and the groove wall. The second spacer has a length equal to the predetermined distance. The optoelectronic semiconductor device may be a laser diode, a light emitting diode, a photodiode or a light detector. The optical conductor may be an optical fiber, an optical waveguide, a lens, a mirror, a grating, a diffraction element, or combinations thereof. The first set of patterned layers may have a height in the range between 10 micrometers and 25 micrometers. The polymeric layer may a height in the range between 30 micrometers and 150 micrometers. The optoelectronic semiconductor device may be flip chip mounted or wire bonded within the cavity. The cavity may include at least one metal contact and the metal contact may be in contact with the metal layer and with the optoelectronic semiconductor device thereby electrically connecting the optoelectronic semiconductor device to the metal layer. The first and second spacer may be made of a polymer. 
   In general, in another aspect, the invention features a method of coupling an optical conductor to a light emitting surface of an optoelectronic semiconductor device. The method includes the following steps. First, providing a multilayer substrate having first and second sets of patterned layers. The first set of patterned layers includes at least one metal layer and at least one dielectric layer and the second set of patterned layers includes at least one polymeric layer. Next, forming a cavity within the at least one polymeric layer. The cavity is adapted to receive and electrically connect the optoelectronic semiconductor device to the metal layer. Next, forming a groove within the polymeric layer. The groove leads to the cavity and is adapted to receive and optically connect the optical conductor to the light emitting surface of the optoelectronic semiconductor device. Finally, precisely positioning the optoelectronic semiconductor device and the optical conductor within the cavity and the groove, respectively, so that light emitted from the light emitting surface of the optoelectronic semiconductor device couples to an optical surface of the optical conductor. 
   Implementations of this aspect of the invention may include one or more of the following features. The method may further include forming a first opening in a cavity wall and positioning the optoelectronic semiconductor device within the cavity so that light emitted from the light emitting surface exits the cavity via the first opening. The method may further include forming a second opening in a groove wall, wherein the second opening is aligned with the first opening, thereby allowing light exiting the cavity through the first opening to couple to an optical surface of the optical conductor aligned with the second opening. The positioning of the optoelectronic semiconductor device within the cavity in X and Y directions includes placing two adjacent walls of the device in contact with two adjacent walls of the cavity. The precise positioning of the optoelectronic semiconductor device within the cavity in a Z direction includes placing a bottom surface of the device upon a bottom surface of the cavity. The precise positioning of the optoelectronic semiconductor device within the cavity in a Z direction at a predetermined height may also include placing a bottom surface of the device upon a first spacer formed upon a bottom surface of the cavity and having the predetermined height. The method may further include placing apart the cavity wall that has the first opening from the groove wall that has the second opening by a predetermined distance by forming a second spacer between the cavity wall and the groove wall. The second spacer has a length equal to the predetermined distance. 
   Among the advantages of this invention may be one or more of the following. The invention provides an assembly and a method for aligning, coupling and fixing elements allowing accurate lateral and longitudinal alignment upon a planar substrate. The invention offers the advantages of reduced thickness of the assembly, higher placement accuracy, easier automation and large-scale manufacturability with reduced cost. The positioning and fixing of the elements on the substrate are manufactured with standard microelectronic processing techniques, which offer a higher accuracy and repeatability in large-scale production than conventional approaches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side, cross-sectional view of the integrated optoelectronic assembly showing embedded electrical components, the thick polymeric material, the optical fiber, and a flip chip mounted semiconductor die connected with solder bumps to the underlying metal pads; 
       FIG. 2  is a front, cross-sectional view of the optoelectronic assembly of  FIG. 1 ; 
       FIG. 3  is a top view of the optoelectronic assembly of  FIG. 1  showing the positioning of the optical fiber within the polymeric groove; 
       FIG. 4  is a top view of the optoelectronic assembly of  FIG. 1 , showing the optical fiber and the die positioned and fixed in their final configuration; 
       FIG. 5  is a side view of another embodiment of the optoelectronic assembly; and 
       FIG. 6  is a top view of another embodiment of the optoelectronic assembly. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the optoelectronic assembly  90  includes a carrier substrate  100 , a dielectric layer  104  with embedded patterned metal layers  102 ,  103 , and a polymeric layer  106 . Dielectric layer  104  is positioned on top of the carrier substrate  100  and the polymeric layer  106  is positioned on top of the dielectric layer  104 . The thickness of the carrier substrate  100  and the dielectric layer  104  is in the range of 10 to 20 micrometers. The thickness of the polymeric layer  106  varies between 30 and 150 micrometers. The assembly  90  further includes a semiconductor die  110 , connected to the patterned metal layers  103  via metal bonding pads  130 . In a preferred embodiment the semiconductor die  110  is flipchip mounted into a cavity  140  formed within the polymeric layer  106 . The semiconductor die  110  may be an edge emitting laser, photodiode or other semiconductor optical devices. The polymeric layer is patterned to include a groove  150 , stoppers  108  (shown in FIG.  3 ), cavity  140 , and spacers  107 . Groove  150  is adapted to receive a fiber  120  and stoppers  108  are used for positioning of the fiber  120  so that it is aligned with the light emitting edge of the semiconductor die  110 . The walls of the cavity  140  are used for x-y positioning of the semiconductor die  110  and the spacer  107  is used for z-positioning of the semiconductor die. 
   Referring to  FIGS. 3 and 4 , the semiconductor die  110  is placed upside down into the cavity  140  and onto the spacers  107 . For the x-y positioning of the semiconductor die  110 , two adjacent and vertical to each other sides of the semiconductor die  10   a ,  10   b  are placed in contact with the cavity walls  140 A and  140 B. This positioning of the die leaves spaces  141 ,  142  between the other two semiconductor die sides  110   c ,  110   d  and the cavity walls  140 C,  140 D, respectively. The spacing  141 ,  142 , between the die  110  and the cavity walls  140 C,  140 D accommodate any thermal expansions that may occur during the operation of the device and allow the use of materials with different thermal expansion coefficients. The z-positioning of the semiconductor die  110  is determined by the thickness of the polymeric spacers  107 . The final relative positioning of the fiber  120  and the die  110  are therefore defined by design, using microelectronic processes, which exhibit a much greater accuracy and reproducibility than a mechanical assembly. The accuracy of the placement in the present assembly is of the order of micrometers. 
   The carrier substrate  100  has a high quality surface finishing on at least its top side. In the preferred embodiment depicted in  FIG. 1 , two subsequent layers of metals  102 ,  103  are deposited and patterned, alternated with two layers of high planarity, low moisture absorption, low dielectric constant and low loss dielectric material  104 . If only one set of metal-dielectric layers is used, only interconnection traces and bond pads can be manufactured. If two sets of metal-dielectric layers are used, traces, pads, inductors and low-value capacitors can be integrated. If more than two sets of metal-dielectric layers are deposited, resistors and high value capacitors can be included. By alternating several layers of dielectric material with different refractive indices, optical waveguides can be integrated as well. The overall thickness of this first set of layers ranges typically between 10 micrometers and 25 micrometers. This first set of layers is sufficiently planar to allow the further deposition and patterning of a thicker set of layers of polymeric material  106 , ranging from about 30 micrometers to about 150 micrometers. This polymeric material can be deposited and patterned in one single step, or in a series of steps, in case different heights are required. As depicted in  FIG. 1 , a first height of the polymeric material  106 , referenced as polymer spacer  107 , is used to accurately define the vertical positioning of the semiconductor die  110 . A second height of the polymeric material  106  is used to create sufficiently high walls  150   a ,  150   b  (shown in  FIG. 3 ) forming groove  150  used to guide the optical fiber  120  into its final position. The relative vertical positioning of the optical fiber  120  and the semiconductor die  110  is such that the fiber core  122  is accurately aligned with the edge of the active surface  112  of the die  110  (shown in FIG.  2 ). This embodiment is suitable for aligning optical fibers with edge emitting lasers or edge receiving photodiodes or other semiconductor optical devices. 
   Referring again to  FIG. 3 , a third wall  150   c  of the groove  150 , transverses the two polymeric walls  150   a ,  150   b , and is patterned so that it exhibits a through slot  152  in the middle and two side portions  108 . Side portions  108  act as fiber stoppers, precisely defining the final position of the fiber. The middle slot  152  allows the light to couple to and from the optical fiber. The groove  150  has a rectangular cross-section that provides sufficient space for absorbing the excess of the glue used for fixing the fiber in place. By manufacturing thicker fiber stoppers  108 , tapered fibers can be accurately positioned as well. The cavity  140  is shown with 4 sides  140 A,  140 B,  140 C and  140 D. Within the cavity, two die spacers  107  are shown. 
     FIG. 4  shows the same top view with all elements in their final positions. The fiber  120  is placed against stoppers  108  and the semiconductor die  110  is placed upside down into the cavity  140  and onto the spacers  107 , and is mounted in such a way that it is pressed against the cavity sides  140 A and  140 B, leaving some space  141 ,  142  between the other two die edges and cavity sides  140 C and  140 D, respectively. The cavity  140  is therefore not symmetrically placed relative to the fiber guide axis. In this way, cavity sides  140 A and  140 B are used to accurately position the semiconductor die into its final, predefined position. 
   Referring to  FIG. 5 , another preferred embodiment includes an optical fiber  120  whose tip  92  has been cleaved to allow for a total internal reflection of the light beam, according to industry accepted techniques. The light beam is subject to a 90° angle reflection and travels through the fiber cladding, through the optical quality glue, through the top part of the dielectric material  104  and hits the tip of the embedded optical waveguide  105 , which causes the light to be reflected forward and then again upward at the end of the waveguide, until the beam hits the light sensitive portion  114  on the active surface of the semiconductor device  110 . The inverse path is applicable in case of a transmitter device. 
   Referring to  FIG. 6 , in another embodiment the polymer layer  106  is configured to form a chamfer  98  for receiving the fiber  120 . Furthermore, additional optical elements such as a lens  96  may be integrated in the assembly. The lens  96  focuses the light signal from the optical fiber cable  122  to the active surface  112  of the device  110  and the reverse. The polymer layer  106  is configured to receive the lens  96 . Similarly, a mirror (not shown) may be integrated in  FIG. 5  at the edge  92  for deflecting the light toward the waveguide  105 . 
   Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.