Abstract:
An optical printed circuit board which can transfer optical signal and electric signals simultaneously, and a method of fabricating the optical printed circuit board. An optical printed circuit board which includes an upper cladding layer, a core layer positioned in the upper cladding layer that has a first reflecting surface and a second reflecting surface at both ends to guide optical signals, a lower cladding layer of which one side is in contact with the upper cladding layer and which has a circuit pattern and light connecting bumps on the other side corresponding to the first reflecting surface and the second reflecting surface, may provide the advantage of high optical connection efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2006-0076807 filed with the Korean Intellectual Property Office on Aug. 14, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an optical printed circuit board which can transfer optical signal and electric signals simultaneously, and a method of fabricating the optical printed circuit board. 
         [0004]    2. Description of the Related Art 
         [0005]    Optical connection technology is widely used in data transmission between systems separated several to several hundreds of meters, data bus transmission between back planes in systems separated several tens of centimeters to several meters, connection between boards separated several tens of centimeters, and data links between chips separated several to several tens of centimeters, etc. 
         [0006]    Various connecting methods are used, such as using free space connection, optical fiber ribbons, and polymer optical waveguides, etc. In order to apply optical connection to boards having a relatively short transmission distance, the connecting method should be improved to be applicable to present printed circuit board technology. 
         [0007]    An optical printed circuit board according to prior art is disclosed in Korean patent publication No. 10-2005-0072736. The optical printed circuit board has dielectric layers formed on both sides of the optical waveguide respectively and a reflecting surface that refracts the incident light vertically. Also, transmission chips and receipt chips are mounted at the circuit pattern formed in the printed circuit board. On the chips, VCSEL&#39;s (Vertical-Cavity Surface-Emitting Lasers) or photodiodes are protruded downward, so that light is irradiated or received in the reflecting surface. The irradiated light is refracted at the reflecting surface to be transmitted through the optical waveguide. 
         [0008]    The optical waveguide according to the prior art has a gap between the photodiode or VCSEL and dielectric layer, so that optical connection efficiency is decreased due to the diffusing property of light irradiated or refracted on the VCSEL or the photodiode. Also, the surface of the dielectric layer is given roughness in order to form a metal layer easily, but such a roughened surface leads to light loss. 
       SUMMARY 
       [0009]    An aspect of the present invention aims to provide an optical printed circuit board having a high optical connection efficiency and a method of fabricating the optical printed circuit board. 
         [0010]    One aspect of the invention provides an optical printed circuit board which includes an upper cladding layer, a core layer positioned in the upper cladding layer that has a first reflecting surface and a second reflecting surface at both ends to guide optical signals, a lower cladding layer of which one side is in contact with the upper cladding layer and which has a circuit pattern and light connecting bumps on the other side corresponding to the first reflecting surface and the second reflecting surface. 
         [0011]    An optical printed circuit board according to embodiments of the invention may include one or more of the following features. For example, the light connecting bump can be shaped as a protrusion projected to a predetermined distance from the other side of the lower cladding layer. The light connecting bump can be formed with the lower cladding layer as a single body. 
         [0012]    One end of the light connecting bump can have a curved surface having a predetermined radius of curvature, and the first reflecting surface and the second reflecting surface can have reflecting angles of 45° and can face each other. The upper cladding layer and the core layer can have an air layer which exposes the first reflecting surface and the second surface to the exterior, where the air layer can be sealed by a protective film. 
         [0013]    One aspect of the invention provides a fabricating method for an optical printed circuit board, which includes forming a hole in a metal foil and forming a light connecting bump by stacking a lower cladding layer to fill the hole, stacking a core layer on the lower cladding layer and forming a first reflecting surface and a second reflecting surface corresponding to the hole respectively, and stacking an upper cladding layer on the lower cladding layer and forming a circuit pattern on the metal foil. 
         [0014]    Another aspect of the invention provides a fabricating method for an optical printed circuit board, which includes forming a hole in a metal foil and forming a light connecting bump by stacking a lower cladding layer to fill the hole, stacking a core layer and an upper layer in sequence and forming a circuit pattern on the metal foil, and forming a first reflecting surface and a second reflecting surface corresponding to the hole by forming an air layer in the core layer and the upper cladding layer. 
         [0015]    A method of fabricating an optical printed circuit board according to certain embodiments of the invention may include one or more of the following features. For example, one end of the light connecting bump can have a curved surface having a predetermined radius of curvature. The first reflecting surface and the second reflecting surface can have reflecting angles of 45° and can face each other. The air layer can be sealed by a protective film. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of a metal foil on which a circuit pattern is formed according to one embodiment of present invention. 
           [0017]      FIG. 2  is a cross-sectional view in which holes are formed in the metal foil in  FIG. 1 . 
           [0018]      FIG. 3  is a cross-sectional view in which a lower cladding layer is stacked on to fill the holes of the metal foil in  FIG. 2 . 
           [0019]      FIG. 4  is a cross-sectional view in which a core layer stacked on the lower cladding layer in  FIG. 3 . 
           [0020]      FIG. 5  is a cross-sectional view in which a first reflecting surface and a second reflecting surface are formed by processing the ends of the core layer in  FIG. 4 . 
           [0021]      FIG. 6  is a cross-sectional view in which an upper cladding layer is stacked on the core layer in  FIG. 5 . 
           [0022]      FIG. 7  is a cross-sectional view in which a circuit pattern is formed by processing the metal foil in  FIG. 6 . 
           [0023]      FIG. 8  is a cross-sectional view of a metal foil having holes, a lower cladding layer, and an upper cladding layer stacked in sequence according to another embodiment of the present invention. 
           [0024]      FIG. 9  is a cross-sectional view in which a circuit pattern is formed in the metal foil in  FIG. 8 . 
           [0025]      FIG. 10  is a cross-section view in which an air layer is sealed by a protective film in  FIG. 9 . 
           [0026]      FIG. 11  is a cross-sectional view in which a transmission chip and receipt chip are mounted on the optical printed circuit board according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments of the invention will be described below in more detail with reference to the accompanying drawings. In the description with reference to the accompanying drawings, those components are rendered the same reference number that are the same or are in correspondence regardless of the figure number, and redundant explanations are omitted. 
         [0028]    As illustrated in  FIG. 1 , a metal foil  11  is provided on which a circuit pattern (not illustrated) is formed by a subsequent process. The metal foil  11  may have a thickness of several to several tens of micrometers and may be made of copper or aluminum, etc. One side of the metal  11  may be given roughness to increase adhesive strength between the metal foil  11  and a lower cladding layer, and the lower cladding layer may be stacked on the roughened metal foil  11 . If the metal foil  11  is a copper sheet, brown black oxide treatment may be performed to increase the roughness. 
         [0029]      FIG. 2  is a cross-sectional view in which a pair of holes  13  are formed in the metal foil  11  illustrated in  FIG. 1 . 
         [0030]    Referring to  FIG. 2 , the holes  13  are perforated to a predetermined distance. Because the positions of the holes  13  correspond to the positions of light connecting bumps formed by a subsequent process, the holes  13  may advantageously be processed with precision. The holes  13  may be formed using a laser drill or a mechanic drill, etc. The holes  13  can have various shapes such as circles or ellipses, etc., as long as the holes  13  do not obstruct the path of the light reflected on the first or second reflecting surface. 
         [0031]      FIG. 3  is a cross-sectional view in which a lower cladding layer  15  is formed on a surface of the metal foil  11  of  FIG. 2 . 
         [0032]    Referring to the  FIG. 3 , the lower cladding layer  15  may be formed by applying a cladding using a laminating method, rolling method, screen printing, or spraying method, and then drying and hardening at a predetermined temperature. 
         [0033]    The rolling method is to use rollers to spread a cladding, supplied by pumping from a container filled with a liquid photo-resist polymer, on the metal foil  11  fed by a feeding means. The lamination method is of forming a cladding layer by coating a cladding, supplied from a cylinder roll wound with cladding material, onto the metal foil  11  fed by a feeding means. In addition, the screen printing method is of applying a cladding on the metal foil  13 , which forms the lower cladding layer  15 , with a screen printing plate. The spraying method is of spraying the cladding on the metal foil  13  with a spray. 
         [0034]    Here, the cladding may be filled in the hole  13  formed in the metal foil  11  and hardened, so that light connecting bumps  17  may be formed. The lower cladding layer  15  may be formed to have a lower refractive index than that of the core layer  19 . Accordingly, the light energy irradiated on the core layer  19  may not be emitted to the exterior of the printed circuit board. 
         [0035]    In this manner, the light connecting bump  17  may be formed as a single body with the lower cladding layer  15  and may protrude downwards. A circuit pattern may be formed in the vicinity of the light connecting bump  17 , as in  FIG. 7 . The circuit pattern may decrease the gap between the photodiode or VCSEL of the receipt chip or the transmission chip mounted on the circuit pattern and the lower cladding layer  15  to increase optical connection efficiency. The ends of the light connecting bumps  17  can have predetermined radii of curvature. That is, after the spreading, printing, drying, and hardening of the cladding, one end of the light connecting bump  17  may be processed as a curved surface  18  having a predetermined shape. The curved surface  18  may serve as a lens to focus the light from the light emitting diode. 
         [0036]      FIG. 4  is a cross-sectional view in which a core layer  19  is stacked on the lower cladding layer  15 . 
         [0037]    The core layer  19  may be composed of a polymer material, which has better workability and lower cost than inorganic material, as well as an inorganic material, such as quartz glass or multi-ingredient glass, which has a wide transmission band. The polymer material can include polymethyl methacrylate (PMMA) or polystyrene. The core layer  19  may be formed by applying and drying a liquid polymer, using a laminating method, rolling method, or squeeze printing method, etc. The core layer  19  may be formed to have a higher refractive index than those of the lower cladding layer  15  and the upper cladding layer  29  described later, so that the optical signals undergo total reflection in the lower cladding layer  15  and the upper cladding layer  29  to be guided through the core layer  19 . 
         [0038]      FIG. 5  is a cross-sectional view in which both ends of the core layer  19  are processed to form a first reflecting surface  21  and a second reflecting surface  23 . 
         [0039]    Referring to  FIG. 5 , the first reflecting surface  21  and the second reflecting surface  23  may be formed in correspondence to the positions of the light connecting bumps  17 . That is, the positions of the first refracting surface  21  and the second refracting surface  23  may be adjusted, so that the light incident on the core layer  19  through the light connecting bump  17  and the light guided by the core layer  19  may be reflected on the first reflecting surface  21  and the second reflecting surface  23 . Because the first reflecting surface  21  and the second reflecting surface  23  may be formed to have 45° reflection angles with respect to the paths of light, the light transmitted through the light connecting bumps  17  may be refracted twice by the first reflecting surface  21  and the second reflecting surface  23 . The first reflecting surface  21  and the second reflecting surface  23  may be formed by a method of grooving with a diamond blade. Also, metal coating may be formed on the first reflecting surface  21  and the second reflecting surface  23  to increase reflection efficiency. 
         [0040]      FIG. 6  is a cross-sectional view with an upper cladding layer  29  stacked on. 
         [0041]    Referring to  FIG. 6 , the upper cladding layer  29  may be stacked on the core layer  19  to accommodate the core layer  19 . The upper cladding layer  29  may be formed by applying and drying a liquid cladding, using the lamination method, rolling method, or squeeze method, etc. Also, the upper cladding layer  29  may be formed to have a lower refractive index than that of the core layer  19 , through which the optical signals pass, and may block the optical signals irradiated into the core layer  19  from being emitted to the exterior. After the upper cladding layer  29  is formed, as presented in  FIG. 7 , a circuit pattern  27  may be formed in the metal foil  11 . A transmission chip and a receipt chip may be mounted on the circuit pattern  27  by subsequent processes. 
         [0042]    Because the light coming from the photo diode or VCSEL of the transmission chip and the receipt chip have a property of diffusion, the bigger the gap between the lower cladding layer  15  and the photo diode or VCSEL, the more the light loss, resulting in decreased light connecting efficiency. However, the light connecting bumps  17  according to this embodiment of the present invention may decrease the gap between the lower cladding layer  15  and photo diode or VCSEL, so that optical connection efficiency can be increased. 
         [0043]    The conventional optical printed circuit board has a roughened surface formed on the metal foil  11  to facilitate attachment between the lower cladding layer  15  and the metal foil  11 , where the light coming from the photo diode or VCSEL is reflected diffusedly on the roughened surface to result in light loss. However, in an optical printed circuit board according to an embodiment of the present invention, the light connecting bumps  17  may receive the light to decrease the light loss. 
         [0044]      FIG. 8  to  FIG. 10  are cross-sectional views representing a method of fabricating an optical printed circuit board according to another embodiment of present invention. In this embodiment, the first reflecting surface  21  and the second reflecting surface  23  may be formed by forming an air layer  31  with the core layer  19  and the upper cladding layer  29  stacked, instead of forming the first reflecting surface  21  and the second reflecting surface  23  before forming the upper cladding layer  29 . Accordingly, the method of fabricating an optical printed circuit board according to this embodiment of the claimed invention has the same process described referring to  FIG. 1  to  FIG. 4 , up until stacking the core layer  19  on the lower cladding layer  15 . 
         [0045]    Referring to  FIG. 8 , after the core layer  19  is stacked in the lower cladding layer  15 , the upper cladding layer  29  may be stacked on the core layer  19  instead of processing the first and second reflecting surfaces. The upper cladding layer  29  may be formed by applying cladding which is to be dried and hardened, using the aforementioned lamination method, rolling method, screen printing method, or spray method, etc. 
         [0046]    As illustrated in  FIG. 9 , a circuit pattern  27  may be formed on the metal foil  11 . After a dry film is laminated on the metal foil  11  by a predetermined temperature and pressure, a working film may be laminated on the dry film, and irradiation of a predetermined intensity may be applied on the working film to form a pattern image. After a developing solution is used to remove parts of the dry film, the parts not covered by the dry film may be removed by way of an etching process. Finally, by removing the dry film with NaOH or KOH, etc., the circuit pattern  27  may be formed. 
         [0047]      FIG. 10  is a cross-section view in which a first reflecting surface and a second reflecting surface are formed by the forming of an air layer in  FIG. 9 . 
         [0048]    Referring to  FIG. 10 , using a dicing machine set to 45 degrees, parts of the upper cladding layer  29  and the core layer  19  may be removed to form an air layer  31  having a “V” shape cross-section at both ends. Because of the air layer  31  thus structured, a first reflecting surface  21  and a second reflecting surface  23  may be formed which have reflecting surfaces of 45 degrees. Furthermore, by forming a metal coating with gold or silver on the first reflecting surface  21  and the second reflecting surface  23  to form a reflecting mirror, the air layer  31  may be sealed from the exterior by the protective film  33 . 
         [0049]      FIG. 11  is a cross-sectional view in which a receipt chip  35  and transmission chip  37  are mounted on an optical printed circuit board  10  according to an embodiment of the present invention. 
         [0050]    Referring to  FIG. 11 , the receipt chip  35  and transmission chip  37  may be mounted on the circuit pattern  27  by way of solder balls. The receipt chip  35  may be connected to the photo diode  36  to amplify electrical signals. The photo diode  36  may convert the received optical signals to electrical signals. The transmission chip  37  may be connected to a VCSEL (Vertical-Cavity Surface-Emitting Laser)  38  to generate optical signals, where the VCSEL  38  is an active element that converts electrical signals to optical signals. In addition, a heat sink (not illustrated) can be disposed above the receipt chip  35  and the transmission chip  37  to emit the heat generated from the chips. 
         [0051]    Regarding the operation of the optical printed circuit board  10  with reference to  FIG. 11 , an optical signal generated at the VCSEL  38  may be outputted to the light connecting bump  17  by the transmission chip  37 . The optical signal entered through the light connecting bump  17  may be transmitted through the lower cladding layer  15  and reflected on the second reflecting surface  23 , and the optical signal may undergo total reflection along the core layer  19  to be entered into the first reflecting surface  21 . The optical signal may be reflected vertically on the first reflecting  21  to penetrate the lower cladding layer  15  and enter into the photo diode  36  through the light connecting bump  17 . Here, the receipt chip  35  may convert the received optical signal to an electrical signal. 
         [0052]    As described above, the gap between the VCSEL  38  and the light connecting bump  17  may be deceased, because the light connecting bump  17  may be protruded upward towards the VCSEL  38 . Accordingly, the optical connection loss can be decreased by the light connecting bump  17 . In addition, the gap between the photo diode  36  and the light connecting bump  17  may be decreased, because the light connecting bump  17  may be protruded upward towards the photo diode  36 . Therefore, light from the light connecting bump  17  does not diffuse greatly, so that light loss can be decreased. 
         [0053]    While the present invention has been described with reference to particular embodiments, it is to be appreciated that various changes and modifications may be made by those skilled in the art without departing from the spirit and scope of the present invention, as defined by the appended claims and their equivalents.