OPTICAL CIRCUIT BOARD AND METHOD OF MANUFACTURING THE SAME

Abstract: An optical circuit substrate having optical fiber placed and fixed in the form of a circuit on one side or on both sides of a substrate can be prepared with good efficiency by providing thereto an optical interconnection portion which makes an area for optical connection smaller and registration between an optical element and optical fiber easy.

Description: The present invention relates to an optical circuit substrate comprising a substrate and optical fiber laid and fixed in the form of a circuit on the substrate. Optical fiber is employed in communication which use light having a very high frequency as the carrier wave, and has advanced as a communication medium which enables a high quality transmission with a small loss over a long distance. It has been proposed and developed in recent years to use the optical fiber, which has been developed thus for use in long distance communication, for transmission in a short distance communication or in an apparatus in place of electric wire, which permits transmission of only a relatively small amount of information. For example, JP-A-61-186908 discloses, in the field of communication systems or instrumentation systems which operate by utilizing inter-transformation between light and electrons, a system in which three or more layers of substrates each provided with a driving circuit comprising an optical active element, an optical passive element and an optical active element are provided and optical circuits are provided between the substrates. With regard to optical fiber, there is described in the disclosure that an optical branching-coupling device 60 mm in length has been provided by using two lines of optical fiber with a spacing of 2 mm on a UV resin substrate. JP-A-1-180505 discloses a method of forming an optical circuit wherein optical fiber is laid on a substrate through the medium of an adhesive layer. It is described therein that the optical fiber has been laid by using a laying tool on an acrylic adhesive layer formed on an ABS resin substrate. JP-A-4-311905 discloses a circuit board in which a flexible base material having optical fiber interposed therein is sticked to a rigid substrate such that the flexible base material rises over the substrate from the intermediate part of the substrate so that the tip of the optical fiber can be connected to a connector. Furthermore, as disclosed in JP-A-1-183605, there is known an optical circuit substrate which has on the end face of the substrate a condensing lens that receives aslant a light from the optical fiber. When, as disclosed in JP-A-61-186908, in communication systems or instrumentation systems which operate by utilizing inter-transformation between light and electrons, three or more layers of substrates each provided with a driving circuit comprising an optical active element, an optical passive element and an optical active element are provided, and optical circuits are provided between the substrates, there arise disadvantages in that the thickness of the resulting substrate is unfavorably large and moreover, when a fault occurs in the optical circuit, the entire part of the substrate including the electron circuit has to be replaced, leading to an economical disadvantage. In the above-mentioned method, furthermore, there is a large gap between the accuracy of alignment required for optical interconnection and the accuracy of actual electrical mounting of an optical element, etc., so that allowance for absorbing a considerable accuracy gap is necessary in the optical interconnection part. This leads to an increased size of the substrate and resultant disadvantage in practice. From such a viewpoint, it is economically more advantageous to provide an optical circuit alone independently, as disclosed in JP-A-1-180505 and JP-A-4-311905. However, when optical fiber is laid on an adhesive layer by means of a tool, as disclosed in JP-A-1-180505, the entire part of the optical fiber is fixed to the adhesive and, to take out signals from the optical circuit, it is necessary to peel off the necessary part or to take some means not to fix the necessary part, and the optical fiber which has not been fixed is apt to be broken. Further, even when, as disclosed in JP-A-4-311905, a flexible base material having optical fiber interposed therein is sticked to a rigid substrate such that the flexible base material is raised over the substrate from the intermediate part of the rigid substrate (to assume a form of so-called "pig tail") so that the tip of the optical fiber can be connected to a connector, there are problems in that the fiber which has not been fixed is similarly apt to be broken, that the optical fiber connected to the tip of the raised optical fiber needs an optical inter-connection part of increased size because the area of the member part is large and that the length of the fiber which is not fixed increases because a certain extent of movement is necessary at the time of operation of connecting a connector. When a condensing lens which receives obliquely a light from optical fiber is provided at the end face of the substrate as disclosed in JP-A-1-183605, there arises a problem in that when the angle of inclination is decreased, the position of fixing the optical fiber has to be lowered, whereas when the angle is increased the height of the substrate has to be increased. An object of the present invention is to provide an optical circuit substrate which permits the connection of the optical circuit with good accuracy and without occupying much space and a process for preparing such an optical circuit substrate with good efficiency. The present invention provides an optical circuit according to claim 1. The present invention further provides a process for the preparation of an optical circuit according to claim 11. The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1, (a) and (b) are each a schematic sectional view for illustrating the principle of the present invention, and (c) is a plan view for illustrating one embodiment of the present invention. The optical circuit (referred to as "optical circuit substrate" in the following description) of the present invention is an optical circuit substrate having optical fiber placed and fixed in the form of a circuit on a surface of a substrate, wherein an end portion (or part) of the optical fiber is cut so as to have an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to the substrate surface, and said substrate having holes therethrough beneath the cut-section of the fiber, each hole having a metal film on the inner wall thereof. The angle which permits the end part of the optical fiber 1 to refract or reflect light most to the direction of 90 degrees relative to the surface of the substrate 11 is such an angle that, as shown in Fig. 1(a) or Fig. 1(b), not only the light 101 which has been transmitted in the optical fiber 1 and is parallel to the central axis of the optical fiber but also the light 102 which has been reflected by the internal periphery of the optical fiber are reflected or refracted at the cut-section 16, then introduced into a hole 13 provided beneath the cut-section 16 of the optical fiber 1 and having a metal film 14 provided on its internal wall, and thus can connect an optical circuit. The cut-section 16 is preferably cut at an angle of about 45 degrees to the length direction of the optical fiber 1. The hole 13 may also be filled with a light-transmittable resin, whereby the deposition of dust and dirt from surroundings can be prevented during use and hence the optical connection route can be secured more easily. In the hole 13, the light input and output part of a light receiving and transmitting element may be placed on a side not contacting with the optical fiber 1, and registered and connected to the hole 13 provided beneath the cut-section 16 of the optical fiber 1. In this case, since the element is attached perpendicularly to the substrate 11, the mounting area can be reduced and the increase of the thickness of the substrate 11 can be obviated. When the light-transmittable resin filled in the hole 13 is composed of resins different in refractive index between the resin of the internal wall side of the hole and that of the central side of the hole such that the refractive index of the resin used in the internal wall side of the hole is higher than that of the resin used in the central side of the hole, the loss in connection can be favorably reduced. Further, a lens may be provided in the hole. The substrate 11 used may be one which has a conductor circuit on the surface of an insulating substrate or of an inner layer circuit substrate insulation-treated with prepreg, or one which has a conductor circuit layer and an optical circuit layer separately, or one which comprises a combination of a conductor circuit substrate and an optical circuit substrate. It is also possible to use a resin film as the substrate material and to use an adhesive-coated resin film, an adhesive resin film or a photosensitive resin as a means for fixing the optical fiber 1. It is possible to use an optical circuit substrate wherein, in the plane shape thereof a cut or a projection is formed, the optical fiber 1 is laid continuously from the main portion of the optical circuit substrate to the portion surrounded by said cut or to the projection, and the cut-section 16 is provided for the portion surrounded by the cut or the projection. Such an optical substrate may be prepared, for example, by making a hole 13 which passes through a substrate 11, metallizing the inner wall of the hole, placing and fixing optical fiber 1 in the form of a circuit on a surface of the substrate 11, and then cutting an end part of the optical fiber 1 at the place of the hole 13 provided previously so as to make the resulting end part have an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to the surface of the substrate 11. The optical substrate may also be prepared by making a hole 13 which passes through a substrate 11, metallizing the inner wall of the hole and then, while placing optical fiber 1 in the form of a circuit on the surface of the substrate, cutting an end part of the optical fiber 1 so as to fix the resulting cut-section 16 just above the hole 13 provided previously and to have an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to the surface of the substrate 11. Further, the optical substrate may be prepared by placing and fixing optical fiber 1 in the form of a circuit on a surface of a substrate 11, cutting the optical fiber 1 at the place of the hole 13 provided previously so as to have an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to the surface of the substrate 11, then providing a hole 13 beneath the cut-section 16 of the optical fiber 1 from the back side of the substrate 11, and metallizing the inner wall of the hole. Still further, the optical substrate may be prepared by cutting an end part of optical fiber 1, while placing and fixing the optical fiber 1 in the form of a circuit on a surface of a substrate 11, so as to fix the cut-section 16 thus formed just above a hole 13 to be provided later and to have an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to the surface of the substrate 11, then providing a hole 13 beneath the cut-section 16 of the optical fiber 1 from the back side of the substrate 11, and metallizing the inner wall of the hole. The laying of optical fiber 1 may be conducted by pressure bonding or heat-pressure bonding of the optical fiber 1, while feeding the optical fiber 1 onto a sticky film or adhesive film provided on the substrate 11, at fixing positions varied by means of an NC apparatus. The cut-section 16 of the optical fiber 1 may be nearly specularly finished by treating it with abrasives, hydrofluoric acid, borofluoric acid, solvents, etc., whereby the loss can be favorably suppressed. The form of the signal light transmittable resin surface may be controlled according to the extent of filling of the resin in the hole; a convex lens form may be obtained by filling the hole with the resin in an amount more than the inside volume of the hole and a concave lens form may be obtained by filling the hole with the resin in an amount less than the inside volume of the hole. It is also possible to adhere a lens to the opening of the hole 13 which is not in contact with the optical fiber 1, after filling a signal light transmittable resin in the hole. The hole 13 for optical connection may be provided by using a laser, or the hole 13 may be provided by using a photosensitive resin sheet and subjecting it to light exposure and development. It is also possible, by using two kinds of photosensitive resins, to fill the hole 13 with a light-transmittable resin having a low refractive index, then bore a hole of a diameter smaller than that of the original hole and fill therein a light-transmittable resin having a high refractive index. The optical fiber 1 used in the present invention is preferably quartz fiber because of its excellent resistance to laser beams. However, plastics fiber or metal film-coated plastics fiber may also be used so long as they are satisfactorily resistant to laser beams. Optical fibers 1 coated with a resin or the like may also be used so long as the resin or the like can be removed with laser beams. The placing and fixing of optical fiber 1 in the form of a circuit on the substrate 11 may be achieved by laminating an adhesive sheet having stickiness on the substrate 11, then pressure-bonding the fiber thereon by heat and/or an ultrasonic jig, and forming the circuit by such means as an X-Y table. Further, if necessary, a protective fixing layer may be provided on the fixed fiber by using a prepreg, an adhesive resin film or a photosensitive resin. The hole 13 provided for the optical fiber 1 from the back side of the substrate 1 in the present invention may be formed by using an infrared laser or eximer laser. To reduce adverse thermal influence, laser beams with a short pulse and a high energy density per unit time are suitably used. The hole 13 can also be made by using plasma, but the use of a mask is necessary. The resin which can be bored therethrough by using laser beams may be such materials conventionally used for laminated sheets as epoxy resin, polyimide resin and the like. Organic or inorganic fillers may also be used in the resin in a range not harmful to laser boring. The method used for filling may be pressing, casting, printing, etc. The method used for metallizing the internal wall of the hole 13 may be any desired one but it is economically advantageous to use the electroless plating method employed in metallizing the through-hole of a printed wiring board. The metal used may be copper, but metals which are not susceptible to oxidation and are glossy, for example nickel, are more preferred. For cutting the optical fiber in the present invention, a rotary knife, e.g., a diamond cutter or a dicing saw which has a thickness of 250 µm and in which the knife edge has an angle which permits refraction or reflection of light most in the direction of 90 degrees relative to one side of the rotary plane of the rotary knife may be used to form an intended section. It is also possible to cut the fiber by using a similar rotary knife having a knife edge angle of 90 degrees, at an inclination of 45 degrees relative to the substrate surface. By filling a light-transmittable resin in the hole 13 of the substrate formed by laser beams in the present invention, the input and output of light signals from and to the optical fiber 1 can be improved. When the part of the light-transmittable resin exposed on the substrate surface is made to take the form of a lens, the signal transmission efficiency can be improved. It is also favorable to adhere a minute lens-formed article to the part of the light-transmittable resin exposed on the substrate surface. Such a signal light transmittable resin is preferably a transparent resin, such as acrylic resin, but any resins may be used so long as they are light-transmittable, examples of which include epoxy resins, particularly acrylic-modified epoxy resins and fluorine-containing epoxy resins, and polyimide resins. It is further preferable for improving the signal transmission efficiency of the hole part that, after filling a signal light transmittable resin in the hole 13 formed, a hole having a smaller diameter is formed again from the back side of the substrate 11 and then a resin having a slightly higher refractive index than the initially filled resin is filled therein. It is further preferable for improving the signal transmission efficiency to connect, with a light transmittable resin, the optical element for receiving and transmitting light signals positioned on the back side of the substrate 11 with the part of light-transmittable resin exposed on the back side of the substrate, or the lens-formed article on the substrate surface or, directly, the cut-part of the optical fiber. It is favorable and preferable for suppressing the leakage of light at the cut-section 16 to form a low refractive index material layer at the cut-section 16 of the optical fiber 1. For forming a U-shaped cut 15 as shown in Fig. 1(c) on the substrate 11 at the signal take-out site of the optical fiber 1, a cutter knife, router, or the like may be used. According to the present invention, since no "pig tail" is used, the risk of the optical fiber 1 being broken is low and, since a light take-out hole 13 is formed by using laser beams, light is taken out easily. Further, since a method of cutting a fixed fiber is used for forming the reflecting surface of the optical fiber 1 which serves for taking out light, the preparation is easy. According to the above-mentioned structures, an optical fiber wiring substrate for optical interconnection which requires only a small area for optical connection is obtained. Moreover, by forming the whole of the optical fiber wiring substrate with a flexible material, the alignment of the entrance and exit part of the fiber with an optical element can be easily achieved by utilizing the deformation of the substrate 11. The U-shaped cut 15 formed in the substrate 11 similarly makes the alignment be achieved more easily and, at the same time, makes it possible to take out only the part of optical connection between the element and the fiber, and thus can enhance the reliability of connection. The present invention is described in more detail below with reference to the following Examples. Example 1 Polyimide film of 70 µm in thickness was used as a substrate. An acrylic rubber-based adhesive film was sticked to one side of the substrate and a multi-mode quartz optical fiber 125 µm in diameter was laid thereon by using a pressure-bonding jig utilizing ultrasonic vibration and an X-Y table of NC drive. An amide-epoxy photosensitive resin which retains flexibility after cure was coated in a thickness of 100 µm on the surface having the fiber laid thereon obtained above, then defoamed under vacuum and cured by ultraviolet light. The above-mentioned operation of forming a photosensitive resin layer was repeated twice to form a protective fixing layer for the fiber. Then, the signal take-out region of the optical fiber was cut with a dicing saw from the surface till the bottom of the optical fiber to obtain a cut-section of the optical fiber having an angle of 45 degrees. On the surface thereof was laminated a film for plating resist, HW-440 (a trade name, mfd. by Hitachi Chemical Co., Ltd.). Thereafter, an infrared laser beam was irradiated from the back side of the substrate to make a hole 0.3 mm in diameter until it reached the optical fiber. Then, a plating catalyst was attached to the inner wall of the hole and to the surface of the plating resist, and the plating resist was peeled off. The substrate having the hole made therethrough was dipped in a nickel plating solution, Blue Shumer (a trade name, mfd. by Japan Kanizen Co., Ltd.), at 85°C for 20 minutes to obtain a nickel plating 5 µm in thickness. Then, a U-shaped cut was formed on the substrate with a cutter knife so as to surround the cut fiber, whereby a flexible optical fiber wiring substrate was obtained. Example 2 The surface copper foil of a glass cloth epoxy resin double-sided copper-plated laminate 0.25 mm in thickness, MCL-E-679 (a trade name, mfd. by Hitachi Chemical Co., Ltd.), was removed by etching using a known subtractive process. Then, a hole 5 mm in diameter was provided beforehand in the substrate region corresponding to the signal take-out region of optical fiber, and epoxy resin was filled in the hole by casting and then cured, to obtain a substrate. Then, acrylic rubber-based adhesive film was sticked onto one side of the substrate, and multi-mode quartz optical fiber 125 µm in diameter was laid thereon by using a pressure-bonding jig which utilizes ultrasonic vibration and an X-Y table of NC drive. An epoxy prepreg was laid on the top of the surface having the fiber laid thereon and heat-cured by means of cushion pressing, to obtain a protective fixing layer for the fiber. Then the signal take-out region of the optical fiber was cut with a dicing saw from the surface to the bottom of the optical fiber to obtain a cut-section of the optical fiber having an angle of 45 degrees. On the surface thereof was laminated a film for plating resist, HW-440 (a trade name, mfd. by Hitachi Chemical Co., Ltd.). Thereafter, an infrared laser beam was irradiated from the back side of the substrate to make a hole 0.3 mm in diameter until it reached the optical fiber. Then, a plating catalyst was attached to the inner wall of the hole and to the plating resist surface, and the plating resist was peeled off. The substrate having the hole made therethrough was dipped in a nickel plating solution, Blue Shumer (a trade name, mfd. by Japan Kanizen Co., Ltd.), at 85°C for 20 minutes to obtain a nickel plating of 5 µm in thickness. Further, a fluorine-containing type epoxy resin was filled in the hole thus nickel-plated to swell above the substrate surface to such an extent as to become roundish owing to surface tension, and was cured by irradiation of ultraviolet light to obtain an optical fiber wiring substrate. Example 3 A flexible optical fiber wiring substrate was obtained wherein a light receiving and transmitting element was provided to the opening of the hole of the flexible optical fiber wiring substrate prepared in Example 1 which opening was not in contact with the optical fiber, such that the light input-output site of the element might be in alignment and the opening. Example 4 A flexible optical fiber wiring substrate was obtained in the same manner as in Example 1 except that the cut-section of the optical fiber was polished by blasting alumina powder, an abrasive, then polished by buffing and washed with a diluted solvent to obtain specular finishing. Example 5 An optical fiber wiring substrate was obtained in the same manner as in Example 2 except that, after the signal light transmittable resin had been filled in the hole, a convex lens was adhered to the opening part of the hole which was not in contact with the optical fiber. Example 6 An optical fiber wiring substrate was obtained in the same manner as in Example 2 except that a photosensitive resin sheet was used as the substrate and the hole was formed by light exposure and development. Example 7 An optical fiber wiring substrate was obtained in the same manner as in Example 2 except that two kinds of light-transmittable resins were used, the diameter of the first hole was changed to 1.0 mm, the hole was filled with a light-transmittable resin having a low refractive index, the resin was cured by heating, then a hole 0.3 mm in diameter was made through the center of the original hole, and a light-transmittable resin having a high refractive index was filled therein. As set forth above, according to the present invention, there can be prepared with good efficiency an optical circuit substrate that has an optical interconnection part which requires only a small area for optical connection and permits easy alignment between the optical element and the optical fiber and an optical circuit substrate which permits optical interconnection with a through-hole. These optical circuit substrates can be used effectively in communication instruments and the like.

Claims: An optical circuit comprising an optical fiber (1) placed and fixed in the form of a circuit on a surface of a substrate (11), wherein an end portion (16) of the optical fiber is cut so as to have an angle which permits refraction or reflection of light at the cut end face such that most of the light is directed towards the substrate at 90 degrees relative to the substrate surface, and said substrate having a hole (13) therethrough beneath the cut-section of the fiber, said hole having a metal film (14) on the inner wall thereof. The optical circuit according to claim 1, wherein the hole provided beneath the cut-section of the optical fiber is filled with a light-transmittable resin. The optical circuit according to claim 1 or 2, wherein the substrate has on a side not contacting with the optical fiber a light receiving and transmitting element containing a light input and output part which is registered and connected to the hole beneath the cut-section of the optical fiber. The optical circuit according to any one of claims 1 to 3, wherein the light-transmittable resin filled in the hole is composed of resins different in refractive index between the internal wall side and the central side of the hole, wherein the refractive index of the resin of the central side is higher than that of the resin of the internal wall side. The optical circuit according to any one of claims 1 to 4, wherein the hole has a lens therein. The optical circuit according to any one of claims 1 to 5, wherein the substrate has a conductor circuit on a surface of an insulating substrate or an internal layer circuit substrate insulation-treated with a prepreg. The optical circuit according to any one of claims 1 to 5, wherein the substrate has a conductor circuit layer and an optical circuit layer separately. The optical circuit according to claim 7, wherein the substrate comprises a combination of a conductor circuit substrate and an optical circuit substrate. The optical circuit according to any one of claims 1 to 8, wherein the substrate material is a resin film and the means for fixing the optical fiber is the use of an adhesive-coated resin film, adhesive resin film or photosensitive resin. The optical circuit according to claim 9, wherein, in the plane shape of the substrate, a cut (15) or a projection is formed, the optical fiber is laid continuously from the remaining portion of the substrate to the portion surrounded by the said cut or to the projection, and the cut-section of the fiber is provided at the portion surrounded by the cut or at the projection. A process for the preparation of an optical circuit, which comprises the steps of: (a) making a hole (13) which passes through a substrate (11), (b) metallizing the inner wall of the hole, (c) placing and fixing an optical fiber (1) in the form of a circuit on a surface of the substrate, and (d) cutting an end portion (16) of the optical fiber at the place of the hole so as to have an angle which permits refraction or reflection of light at the cut end face such that most of the light is directed towards the substrate at 90 degrees relative to the surface of the substrate. The process according to claim 11, wherein the steps (c) and (d) are conducted first, followed by the steps (a) and (b) so as to form the hole beneath the cut-section of the optical fiber from the back side of the substrate. The process according to claim 11 or 12, wherein the placing and fixing of the optical fiber in the step (c) is conducted simultaneously with step (d) so as to fix the optical fiber just above the place of the hole. The process according to claim 12, wherein the hole is provided by boring a hole having a larger diameter than the final one, filling the hole with a resin capable of being bored with a laser beam, heat curing the resin and using a laser beam for providing the hole. The process according to any one of claims 11 to 14, wherein the optical fiber is laid on a sticky film or an adhesive film provided on the substrate while feeding the optical fiber and subjecting to pressure bonding or heat-pressure bonding at fixing positions varied by means of an NC wiring machine. The process according to any one of claims 11 to 15, wherein the cut-section of the optical fiber is specularly finished by treating it with an abrasive, hydrofluoric acid, borofluoric acid, or a solvent. The process according to any one of claims 11 to 16, wherein the hole is provided by boring a hole, metallizing the inner wall thereof, and filling the hole with a signal light transmittable resin. The process according to claim 17, wherein the form of the surface of the signal light transmittable resin is controlled by the extent of filling of the resin. The process according to claim 18, wherein after filling the hole with the signal light transmittable resin, a lens is adhered to the opening of the hole not contacting with the optical fiber. The process according to any one of claims 17 to 19, wherein, after filling the hole with the signal light transmittable resin, a light receiving and transmitting element is provided on the opening of the hole not contacting with the optical fiber by registering a light input and output site thereof with the hole. The process according to any one of claims 11 to 20 , wherein the hole is provided by using a laser. The process according to any one of claims 11 to 20, wherein the substrate is obtained from a photosensitive resin sheet, and the hole is provided by light exposure and subsequent development. The process for the preparation of an optical circuit substrate according to any one of claims 17 to 22 , wherein the hole is filled with two kinds of light-transmittable resins by filling first with a light-transmittable resin having a lower refractive index, boring a hole having a smaller diameter than the original hole, and filling the hole with a light-transmittable resin having a higher refractive index.

Inventors: TSURU YOSHIYUKI, TSURU, YOSHIYUKI
Applicants: HITACHI CHEMICAL CO LTD, HITACHI CHEMICAL COMPANY, LTD.
Application year: 1998
Main classification: G02B6
IPC codes: G02B6, H01L27