Method of forming optical waveguide

Parallel-aligned core layers are formed by patterning a core sheet laminated on a base plate, and a clad/core bonded body is formed by laminating a cladding sheet. The base plate is peeled from one surface of the clad/core bonded body and a dicing tape is pasted on the other surface of the clad/core bonded body. An inclined surface is formed by bevel-cutting both end portions of the core layers. Clad/core bonded pieces are formed by straight-cutting the cladding sheet between core layers and on an outside of outermost core layers. A mask is disposed on the clad/core bonded pieces, and then a metal film is formed on the inclined surface. The clad/core bonded pieces are separated individually by peeling the pieces from the dicing tape after the mask is removed. The clad/core bonded piece is brought into contact with the liquid adhesive coated on a circuit substrate and aligned thereon. Then, the liquid adhesive is cured.

This application is based on and claims priority from Japanese Patent Application No. 2007-230646, filed on Sep. 5, 2007, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates to a method of forming an optical waveguide on a circuit substrate.

2. Related Art

In various electronic circuits, with enhancement of a signal carrying speed (increase of a higher frequency), optical/electrical hybrid circuits a part of which is replaced with optical interconnection using the optical waveguide in place of the conductive wirings made of copper has been developed.

InFIGS. 1A and 1B, a typical example of an optical waveguide mounted on a circuit substrate is shown. As shown inFIG. 1A, both surfaces of a core layer12serving as a light traveling path of an optical waveguide10are covered with a lower cladding layer14aand an upper cladding layer14b, and the optical waveguide10has a light polarizing plate16on an inclined surface of the end portion. As shown inFIG. 1B, the optical waveguide10is mounted on a circuit substrate20by an adhesive18. For example, the circuit substrate20is constructed by forming an insulating layer26and a wiring layer28as a build-up layer on both surfaces of a double-sided copper-clad core substrate up to a predetermined number of layers. According to the double-sided copper-clad core substrate, a copper foil24is pasted onto both surfaces of a core material22. Then, a solder land30and a solder resist layer32used for the external connection are formed on the upper surface. The optical waveguide10is adhered onto the uppermost insulating layer26by the adhesive18, for example.

As disclosed in JP-A-2000-199827, for example, in the known method of forming the optical waveguide, a triple-layered structure consisting of lower cladding layer/core layer/upper cladding layer is formed by three steps of laminating and curing a lower cladding sheet, laminating and patterning a core sheet, and laminating and curing an upper cladding sheet, and then this triple-layered structure is bonded to the circuit substrate with the adhesive.

According to the above method, much takt time and cost are required for the above three steps, and also the upper/lower cladding layers formed by laminating the cladding sheet have a certain thickness respectively. Therefore, the above method has such a disadvantage that this method is unsuitable for the slimming down of the circuit.

Meanwhile, as described in JP-A-2004-341454, an upper cladding layer and a core layer are sequentially laminated on a metal layer for wiring layer formation, then a cover film for protection is pasted on the core layer, and then a V-groove is formed on the core layer by applying the cutting process. The unnecessary core layer of the optical waveguide in one side of the V-groove is melted and removed while the core layer on the other side of the V-groove remains as the core layer of the optical waveguide, so that a laminated product is formed. This laminated product has a double-layered structure such that the upper cladding layer and the core layer are laminated on the metal layer for forming the wiring layer. Then, the core layer side the laminated product is adhered onto the circuit substrate via the adhesive. Accordingly, the optical waveguide consisting of upper cladding layer (laminated layer)/core layer (laminated layer)/lower cladding layer (adhesive layer) is formed on the circuit substrate. The electric circuit may be provided on the upper surface of the circuit substrate to which the laminated product is adhered.

However, the method described in JP-A-2004-341454 has the following disadvantages.

Namely, the substrate on which the upper cladding layer and the core layer are formed always requires the metal layer for forming the wiring layer, and thus the laminated product is restricted to such a structure that a predetermined wiring layer is provided directly on the upper cladding layer of the completed optical waveguide. Therefore, the laminated product must be designed integrally with the circuits on the circuit substrate. This leads to lack of versatility. Alternately, although not described in JP-A-2004-341454, even though the laminated product is used after the metal layer is removed, the wet process is required to melt/remove the metal layer and thus steps become complicated.

Also, a bottom portion of the V-groove on the core layer formed by the cutting process must match up with a boundary between the upper cladding layer and the core layer. Thus, processing accuracy is required in forming the V-groove.

SUMMARY OF TH INVENTION

Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above.

It is an aspect of the present invention to provide a method of forming an optical waveguide, capable of reducing a takt time and a cost required for formation of a triple-layered structure, slimming down a circuit structure, and enhancing versatility without need of complicated processes.

According to one or more aspects of the present invention, there is provided a method of forming an optical waveguide on a circuit substrate. The method includes:

(a) laminating a core sheet on a base plate;

(b) patterning the core sheet, thereby forming a plurality of core layers each aligned in parallel on the base plate;

(c) laminating a cladding sheet on the base plate such that the core layers are embedded in the cladding sheet except one surface of each of the core layers contacting the base plate, thereby forming a clad/core bonded body;

(d) peeling the base plate from the clad/core bonded body;

(e) pasting a dicing tape on one surface of the clad/core bonded body opposite to said one surface of each of the core layers exposed from the cladding sheet;

(f) bevel-cutting the core layers and the cladding sheet in a width direction of the core layers on both side portions of the core layers, thereby forming an inclined surface in the clad/core bonded body;

(g) straight-cutting the cladding sheet between the core layers and on an outside of outermost core layers of the core layers along a length direction of the core layers, thereby collectively forming a plurality of clad/core bonded pieces;

(h) disposing a mask on the clad/core bonded pieces such that the inclined surface of the clad/core bonded pieces is exposed from the mask;

(i) forming a metal film on the inclined surface, wherein the metal mask serves as a reflecting film;

(k) peeling the dicing tape from the clad/core bonded pieces, thereby separating the clad/core bonded pieces individually;

(l) coating a liquid adhesive having a composition that is able to constitute a cladding layer of the optical waveguide, on an area of the circuit substrate on which the optical waveguide is to be arranged;

(m) bringing at least one of the clad/core bonded pieces into contact with the liquid adhesive such that said one surface of the core layer exposed from the cladding sheet contacts the liquid adhesive;

(n) aligning the at least one of the clad/core bonded pieces on the area of the circuit substrate on which the optical waveguide is to be arranged; and

(o) curing the liquid adhesive, thereby forming the optical waveguide on the circuit substrate, wherein the optical waveguide comprises: a lower cladding layer formed of the cured adhesive; the core layer of the clad/core bonded piece; and a upper cladding layer formed of the cladding sheet.

The clad/core bonded pieces having a double-layered structure are formed on the dicing tape. Therefore, takt time and cost required for forming the triple-layered structure can be reduced, and high versatility can be assured without imposing restrictions on design, which are indispensable in forming the double-layered structure on the metal layer used for forming the wiring layer, and without needing the complicated wet process, which is applied to remove the metal layer. Also, processing accuracy is not required since the bevel cutting applied in forming the inclined surface can be applied to reach the dicing tape, and the liquid adhesive adhered onto the circuit substrate can be employed as the lower cladding layer. Therefore, the slimming down can be attained more easily than the formation of the optical waveguide by laminating the sheet, and a positional accuracy of the clad/core bonded piece can be enhanced.

Furthermore, the core layer contains a plurality of sub-core layers, and the same material as the cladding layer is filled between the sub-core layers. Therefore, a plurality of optical waveguides can be formed collectively by a single clad/core bonded piece.

Other aspects and advantages of the present invention will be apparent from the following description, the drawings, and the claims.

Exemplary embodiments of the present invention will be described with reference to the drawings, hereinafter.

Embodiments in which an optical waveguide is formed according to the present invention will be described with reference toFIG. 2toFIG. 6hereinafter.

As shown in (a) a plan view, (b) a cross sectional view, and (c) a side view inFIG. 2A, a core sheet42A is laminated on a base plate40. As the base plate40, acrylic, polycarbonate, PET plate may be employed, and the base plate40having a high flatness is advantageous. As the core sheet42A, photo curable acrylic resin, epoxy resin, polyimide resin, fluororesin, electron curable resin may be used. As the resin whose solubility relative to the solvent is enhanced by the irradiation of an activation energy ray, a naphthoquinone resin with a photodecomposition property, or the like can be used. Out of them, the resin having high transparency and high thermal resistance is advantageous.

As shown inFIG. 2A, the core sheet42A is patterned by the exposure/development. Thus, a plurality of core layers42aligned in parallel are formed.

As shown inFIG. 3A, a cladding sheet44A is laminated on the base plate40on which the plurality of core layers42are aligned in parallel. Accordingly, the plurality of core layers42except contact surfaces to the base plate40are embedded in the cladding sheet44A. That is, the cladding sheet44A and the core layers42are pasted on the base plate40to constitute an integrally-bonded body (a clad/core bonded body42/44A). As the cladding sheet44A, a thermosetting resin such as epoxy resin, polyimide resin, unsaturated polyester resin, epoxy aclyrate resin may be used, in addition to the photo curable acrylic resin like the core sheet42A. Also, a flame retardant or an ultraviolet absorbent based on addition-type or reaction-type halogen, phosphorus, silicon, or the like may be contained in this resin to give a flame retardancy or to absorb an activation energy ray.

The base plate40is peeled from the clad/core bonded body42/44A. Thus, the surfaces of the core layers contacting the base plate40are exposed from the cladding sheet44A of the clad/core bonded body42/44A. The cladding sheet44A side of the clad/core bonded body42/44A is pasted onto a dicing tape46. This state is shown inFIG. 3B.

Then, as shown inFIG. 4A, inclined surfaces48are formed by bevel-cutting (V) the core layers42and the cladding sheet44A provided to fill the core layers42, in the width direction of the core layers42(the lateral direction onFIG. 4A) on both side portions of the plurality of core layers42on the dicing tape46. A cutting depth of the bevel cutting (V) is set adequately to such an extent that the cutting depth enters slightly into the dicing tape46to cut completely the core layers42and the cladding sheet44A. As an advantage of the present invention, a high processing accuracy is not required in executing the bevel cutting.

Then, as shown inFIG. 4B, the cladding sheet44A is subjected to straight-cut (L) between the core layers42and on the outside of the outermost core layer42on the dicing tape46along a length direction of the plurality of core layers42. Thus, a plurality of clad/core bonded pieces42/44embedded in the cladding layer44are collectively formed in a state that one surfaces of the core layers42are exposed.

Then, as shown inFIG. 5A, a mask50is arranged over the dicing tape46such that the cladding layers44and the core layers42of the plurality of clad/core bonded pieces42/44are covered but the inclined surfaces48are exposed. As the mask50, for example, a metal mask made of nickel may be used.

Then, as shown inFIG. 5B, a metal film52made of gold is formed on the dicing tape46by the sputter via the mask50. Accordingly, the metal film52as a reflecting film is formed on the inclined surfaces48of the clad/core bonded pieces42/44exposed from the mask50.

As shown inFIG. 6B, a liquid adhesive54having a composition that can construct the cladding layer of the optical waveguide is coated on optical waveguide aligning portions of a circuit substrate60. The circuit substrate60has connection pads68, which are exposed from openings66in a solder resist layer64, on the uppermost layer of a multilayer wiring structure62. In the illustrated example, the liquid adhesive54is coated on exposed surfaces of the multilayer wiring structure62, which are exposed from the large opening70in the solder resist layer64.

As the liquid adhesive54, any liquid adhesive can be used if a refractive index after the adhesive is cured becomes lower than the core layers42. It is advantageous that the liquid adhesive54should have the same composition as the cladding layer44.

Then, as shown inFIG. 6C, the exposed surfaces of the core layers42of the clad/core bonded pieces42/44are brought into contact with the liquid adhesive54and then are aligned with the liquid adhesive54. Then, the liquid adhesive54is cured. Accordingly, an optical waveguide72consists of the lower cladding layer54made of the cured adhesive, the core layer42of the clad/core bonded piece42/44, and the upper cladding layer44formed of the cladding layer of the clad/core bonded piece42/44, and has the reflecting film52on both sides respectively, and the optical waveguide72is formed on the circuit substrate60.

As one advantage of the present invention, as described above, the clad/core bonded pieces42/44can be aligned such that these pieces are brought into contact with the liquid adhesive54. As a result, alignment accuracy can be higher than that in the conventional method in which the optical waveguide is formed by press-bonding these pieces to the cladding sheet.

In particular, a height of a top surface (upper surface of the upper cladding layer) of the optical waveguide aligned on the circuit substrate requires a high dimensional accuracy. This is because its positioning to the light emitting element/the light receiving element that are essential to the configuration of the optical wiring is important. As a factor of deciding a height of the optical waveguide, a variation in thickness of the materials of the cladding sheet cannot be excluded in the conventional method using the press-bonding. In contrast, according to the method of the present invention, a height of the top surface can be adjusted in the layer of the liquid adhesive. Therefore, a variation of thickness of the materials can be absorbed.

As described above, according to the present invention, the clad/core bonded pieces having a double-layered structure can be fabricated by simple processing steps that need no complicated process such as the wet process, etc., and can be adhered onto the circuit board by the liquid adhesive. Therefore, a take time and a cost can be reduced as compared with the conventional method in which the triple-layered structure is fabricated by laminating the sheet material. Also, the restrictions of design in the conventional method in which the optical waveguide is formed on the metal layer for forming the wiring layer are not imposed, and a high versatility can be ensured.

FIG. 7shows an optical wiring100that is constructed by forming the optical waveguide72on the circuit substrate60likeFIG. 6Caccording to the present invention. Light receiving/emitting portions76,76are provided on a pair of connection pads68,68positioned near both ends of the optical waveguide72via bumps74,74respectively. The light emitting element such as Vertical Cavity Surface Emitting Laser (VCSEL) and the light receiving element such as the photo diode are built in the light receiving/emitting portions76,76respectively, and the light receiving/emitting portions76,76transmit/receive the light signal. Namely, the light receiving/emitting portions76,76are connected optically by a light signal transmission T passing through the core layer42of the optical waveguide72, thereby constituting the optical wiring100. An optical/electrical hybrid circuit is constructed by the optical wiring100and the electric wirings on the circuit board60.

FIG. 8shows the optical/electrical hybrid circuit in which a pair of optical interconnection110,110, which are constructed by forming the optical waveguides72,72on two circuit boards60,60shown inFIG. 6Caccording to the present invention respectively, are remotely connected via an optical fiber80. In this case, this configuration is partially different from a configuration shown inFIG. 6C. Namely, the light receiving/emitting portion76similar to the Embodiment 2 is provided on the connection pads68positioned near one end of the optical waveguide72on the circuit substrate60respectively via the bumps74. Unlike the Embodiment 2, an optical connector78is provided to the other end of the optical waveguide72to come into contact with the upper surface of the upper cladding layer44.

The optical interconnection110,110on two circuit substrates60,60are connected to both ends of the optical fiber80by two optical connectors78,78of the optical waveguides72,72respectively. Optical signal transmissions ΣT are constructed by an optical signal transmission T passing through the optical waveguide72of one optical wiring110, an optical signal transmission Tx passing through the optical fiber80, and an optical signal transmission T passing through the optical waveguide72of the other optical wiring110. The optical signal transmissions ΣT are formed by an optical wiring200in which one optical wiring110, the optical fiber80, and the other optical wiring110are integrally composed mechanically and optically. The optical/electrical hybrid circuit is constructed by the optical wiring200and respective electric wirings of two circuit substrates60,60.

FIG. 9shows a mode in which a plurality of optical waveguides are formed collectively by using one clad/core bonded piece according to the present invention. Namely, the clad/core bonded piece42/44shown inFIG. 6Acontains one core layer42in one piece. In contrast, a clad/core bonded piece42′/44inFIG. 9contains four sub-core layers42′ in one piece. The respective sub-core layer42′ is put between the cladding layers44on both sides, and four optical waveguides each consisting of the cladding layer44/the sub-core layer42′/the cladding layer44are constructed. The same reference symbols are affixed to the portions corresponding toFIG. 6A.

Four sub-core layers42′ are obtained by patterning the core layers42in the above step2(seeFIG. 2B) such that four sub-core layers42′ are formed in the area of the core sheet42A constituting one core layer42. Following manufacturing steps are similar to those in the case where one core layer42is contained per piece.

The clad/core bonded pieces42′/44formed as shown inFIGS. 9A to 9Care bonded to the circuit substrate60by the liquid adhesive54similarly to the above step10and the step11(seeFIGS. 6B and 6C). Thus, four optical waveguides72are formed collectively. The sub-core layers42′ of individual optical waveguides72function as the individual light signal lines mutually.

According to the present invention, there is provided a method of forming the optical waveguide that can reduce takt time and cost required for forming a triple-layered structure, slimming down a circuit structure, and enhancing a versatility without the need of complicated processes.

While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.