Optical Module and a Mounting Structure Thereof

An optical module achieving optical coupling at a low cost and by a simple and convenient process is intended to be provided. For attaining the purpose, a transparent member sealing an optical device and an optical transmission channel are connected as an optical coupling structure. Specifically, optical coupling is achieved in an optical module having an optical device, a first substrate having the optical device mounted thereon, and a second substrate or a transparent resin provided over the first substrate so as to hermetically seal the optical device by connecting an optical transmission channel over the second substrate or the transparent resin at a portion in which light from the optical device is transmitted.

DETAILED DESCRIPTION

Preferred embodiments of the present invention are to be described with reference to the drawings. Substantially identical portions carry the same reference numerals, for which description is not repeated.

First Embodiment

First, the first embodiment of the present invention is to be described with reference toFIG. 1toFIG. 4B.

FIG. 1is a view for explaining a configuration of optical coupling in the first embodiment.FIG. 2is a cross sectional view for explaining an optical coupling process in the first embodiment.FIG. 3is a view for explaining welding between a sealing substrate (second substrate) and an optical transmission channel.

FIGS. 4A and 4Bare views for explaining an optical module in which the configuration of optical coupling is applied in the first embodiment. Persons skilled in the art can read easily that, inFIG. 4B, the upper surface and the lower surface of a substrate are not present at an identical cross section but are shown as developed cross sectional views. This is also identical in the following embodiments.

First, a light coupling configuration in the first embodiment is to be described with reference toFIG. 1. In the first embodiment, use of an optical module6put to wafer level packaging (WLP) by a first wafer substrate2wand a second wafer substrate3wis considered. The first substrate2is a Si wafer (heat expansion coefficient: 3.3 ppm/K) used most frequently as a substrate for semiconductor devices. A glass material can be used for the second substrate3and a transparent amorphous glass material (thermal expansion coefficient: 3.3 to 8.0 ppm/K) is used in this embodiment considering application to an optical device). Generally, borate type glass is used as the material for the second substrate3considering bonding with the Si wafer. This is because the thermal expansion coefficient of the borate type glass is approximate to that of Si and there is no problem of warp in the substrate caused by the difference of the thermal expansion coefficient. However, use of the amorphous glass material of excellent optical property for the second substrate3is preferred when importance is attached to the optical properties such as refractive index and transmittance.

A vertical cavity surface emitting laser (VCSEL) as an optical device1and a driver IC as a LSI1awhich is a driving element that drives the optical device1are incorporated into the optical module6put to the WLP. Optical signals outputted from the optical device1are transmitted through the second substrate3and emitted from the optical module6, and the optical module6serves as a sending optical module. The optical device1may also be a surface light receiving photodiode in which a trance impedance amplifier (TIA) is used as the LSI1aand the optical module6serves as a receiving optical module.

In this embodiment, a plastic optical fiber (POF) is welded as an optical transmission channel7on the second substrate3at a portion where optical signals from the optical device1are transmitted. Thus, the optical signals emitted from the optical device1are introduced into the core layer of the POF and propagated in the POL.

The first substrate2is not necessarily restricted to Si but may also be other semiconductor wafers formed of InP, GaAs, SiC, SiGe, GaN, etc. Naturally, the first substrate2is not restricted to the semiconductor material but it may comprise other materials such as a glass material, a ceramic material, and a metal material.

Also the second substrate3is not restricted to the glass material and may be formed of other materials such as a semiconductor material so long as the substrate allows a light at a wavelength emitted from or incident to the optical device1to transmit therethrough.

Further, also the optical transmission channel7is not restricted to the POF but may also be an organic optical waveguide.

Then, a specific optical coupling process in the first embodiment is to be described with reference toFIG. 2.

First, inFIG. 2A, the optical device1and the LSI1aare mounted over an electrode pattern21of a first wafer substrate2w. An Au—Sn vapor deposition solder is previously formed as a bonding member over the electrode pattern21. When a solder material such as a vapor deposition solder is used as the bonding material, metallization is formed over the electrode pattern21for ensuring solder wettability. The configuration of the metallized metal has a stacked plating structure comprising 2 to 5 μm Ni and 0.05 μm Au. Generally, when bonding is performed by a solder material, an inter-metallic compound is formed at the boundary between the solder material and Au after bonding. Since the inter-metallic compound is rigid and weak in the stress damping effect, it deteriorates the reliability of bonding against impact or the like. Further, if Au remains, the inter-metallic compound grows further by being left at high temperature subsequently to cause Kirkendall voids and bring about a worry of deterioration in the reliability and the hermetic seal. Therefore, it is preferred to reduce the thickness of Au plating as much as possible. In this embodiment, the thickness of Au plating is 0.05 μm.

For avoiding the complexity of the drawing, LSI1a, vapor deposited solder, and metallized metal are not illustrated inFIG. 2.

Then, inFIG. 2BandFIG. 2C, after wafer alignment of the first wafer substrate2wand the second wafer substrate3w, the first wafer substrate2wand the second wafer substrate3ware bonded by anodic bonding to hermetically seal the optical device1and the LSI1a. As described above, the mounting cost can be decreased, and the characteristics of the optical device1can be ensured to improve the reliability by hermetic seal in the state of the wafer.

The anodic bonding is to be described specifically. Generally, the anodic bonding is a technique of superposing a glass substrate to a Si wafer, pressing electrodes to the lower surface of the Si wafer and the upper surface of the glass substrate, and bonding them by applying a voltage to Si as an anode and to the glass as a cathode while heating the entire portion to about 400° C. Alkali ingredients such as Na contained in the glass tend to be diffused by heating. When the voltage is applied in this state on the anode of Si and the cathode of glass, such alkali ingredients are ionized and diffused. It is said that the cations of Na are attracted to the upper surface of the glass substrate, that is, to the cathode and a cation depleted layer is formed near the bonding boundary with the Si wafer. It is considered that the region is originally neutral in view of electric charges but positive electric charges are decreased by compulsory diffusion of the cations due to the voltage and the region is negatively charged relatively. The charging generates further intense electrostatic attraction with respect to the Si wafer, and firmly bonds the Si wafer and the glass substrate. At the same time, a firm bonding is formed at the boundary between Si and glass by oxidation of Si with oxygen.

An advantage obtained by the application of the anodic bonding to the sealing is that hermetic seal can be achieved at a low cost since glass is directly bonded to the Si substrate and, accordingly, no additional cost is caused.

In this embodiment, since Si is used as the first wafer substrate2w, anodic bonding is adopted as a method of bonding the first wafer substrate2wand the second wafer substrate3w, but this is not restrictive and they may be bonded also by means of a solder material, adhesive, etc.

Then, inFIG. 2D, the bonded wafers are cut into individual optical modules6in the form of wafer level packaging by wafer dicing using a dicing blade4. Solder bumps of Sn-3Ag-0.5Cu are formed as solder bumps5at the rear face of the optical module6for ensuring electric conduction as shown inFIG. 2E. Thus the individualized optical module6can be handled as a chip. The optical module6formed with the solder bumps5is bonded by way of the bumps5, for example, to an organic substrate formed with electric wirings.

Then, an optical transmission channel7is bonded to the second substrate3as shown inFIG. 2F. In this embodiment, a plastic fiber (POF) is used as the optical transmission channel7. For bonding the optical transmission channel7and the second substrate3, the optical transmission channel7is positioned to a portion of the second substrate3where optical signals from the optical device1transmit and then the clad layer71of the POF and the glass material of the second substrate3are welded by a laser light to fix the optical transmission channel7to the second substrate3as illustrated inFIG. 3. In this embodiment, the POF is used as the optical transmission channel7which provides the following advantage. Since the core layer72of the POF generally has a size of 125 μm φ which is larger than that of a single mode fiber, required positioning accuracy for the POF is moderated and the POF can be fixed easily by the laser welding, the alignment step can be simplified.

In this embodiment, laser welding is used as a method of fixing the optical transmission channel7to the second substrate3but this is not always accessory and a method such as adhesion may also be used so long as the optical transmission channel7is fixed to the second substrate3.

Application of the optical coupling structure in this embodiment to optical wirings is to be described with reference toFIG. 4. In this embodiment, an optical module6in the form of the WLP is mounted over an optical and electrical hybridization substrate9as in semiconductor devices in existent electronic equipment. Electric wirings91for transmitting electric signals and optical wirings92for transmitting optical signals are formed over the optical and electrical hybridization substrate. The optical module6and the optical and electrical hybridization substrate9ensure electric conduction between the optical module6and the optical and electrical hybridization substrate9by connection, for example, by means of a Pb-free solder. The bonding material for the optical module6and the optical and electrical hybridization substrate9is not restricted to the solder but other materials, for example, electroconductive adhesive may also be used so long as the electric conduction can be ensured.

The POF is welded to the optical module6mounted over the optical and electrical hybridization substrate9by laser welding as in the step illustrated inFIG. 2E. In the same manner, the top end of the POF on the side opposite to the side where it is welded with the second substrate3is fixed to the optical wirings92over the optical and electrical hybridization substrate9by laser welding.

Electric signals transmitted from the electric wirings91over the optical and electrical hybridization substrate9are transmitted to the LSI1aby way of through vias22formed in the first substrate. The LSI1agenerates signals corresponding to the transmitted electric signals which drive the optical device1and are converted into optical signals. The converted optical signals are guided by way of the second substrate3to the core layer72of the optical transmission channel7. The introduced optical signals are further propagated in the core layer72and the optical signals are transmitted in the optical wirings92over the optical and electrical hybridization substrate9.

As described above according to the optical coupling structure described for this embodiment, the optical module6in the form of the WLP can be handled as a chip and the optical module6can be handled in the same manner as that for the semiconductor device in existent electronic equipment. Further, since the required positioning accuracy is moderated and the optical transmission channel7can be fixed easily by laser welding by using the POF of a large core diameter as the optical transmission channel7, the aligning step can be simplified.

Second Embodiment

A second embodiment of the present invention is to be described with reference toFIG. 5. In this embodiment, a bonding method between the second substrate3and the optical transmission channel7in the first embodiment is changed but other structures and the processes are identical with those of the first embodiment.

In this embodiment, a laser light absorption resin10that absorbs a laser light at a wavelength utilized for laser welding is supplied previously to the second substrate3at a portion where the optical transmission channel7is bonded. Thus, the laser light upon irradiation is absorbed in the laser light absorption resin10upon laser welding and the laser light absorption resin generates heat. By the generation of heat, temperature of the laser welded portion increases and improvement in the bonding strength between the second substrate3and the optical transmission channel7can be expected.

The laser light absorption resin10supplied over the second substrate3is supplied preferably in an extremely thin state. When the laser light absorption resin is present between the second substrate3and the optical transmission channel7, optical signals emitted from the second substrate3are absorbed and scattered by the laser light absorption resin10and, as a result, the intensity of the optical signals propagated in the optical transmission channel7may possibly be lowered. Then, in this embodiment, the thickness of supplying the resin is made 10 μm or less by supplying the laser light absorption resin10to the second substrate3by spin coating.

Preferably, the refractive index of the laser light absorption resin is substantially equal with that of the second substrate3or the core layer72of the optical transmission channel7. Since the laser light absorption resin10is present between the second substrate3and the optical transmission channel7, when the difference of refractive index is present between the boundaries of the laser light absorption resin10, a Fresnel reflection loss due to the difference of the refractive index may be caused at the boundaries. In order to decrease the Fresnel reflection loss, the refractive index of the laser light absorption resin10is substantially made equal with the refractive index of the glass of the second substrate3.

As described above, according to the optical coupling structure described in this embodiment, since the temperature at the laser welded portion increases, bonding strength between the second substrate3and the optical transmission channel7can be improved.

Third Embodiment

A third embodiment of the present invention is to be described with reference toFIG. 6. Also in this embodiment, the bonding method between the second substrate3and the optical transmission channel7in the first embodiment is changed but other structures and the processes are identical with those of the first embodiment.

In this embodiment, a transparent resin11is previously supplied to a second substrate3at a portion where an optical transmission channel7is to be bonded. For the transparent resin11, a resin having a refractive index higher than that of a core layer72of an optical transmission channel7is used. Upon laser welding, the glass of a second substrate3and the optical transmission channel7are melted by laser light irradiation, in which the transparent resin11is diffused into the optical transmission channel7. Thus, a diffusion layer12of the transparent resin11is formed in the optical transmission channel7. A distribution of refractive index is formed in the diffusion layer12such that the refractive index on the side of the boundary with the second substrate3is higher than the refractive index on the side remote from the boundary. When such a distribution of refractive index is present in the core layer72of the optical transmission channel7, the same effect as a Grin (Graded Index) lens is formed in the diffusion layer12. In other words, optical signals emitted from the second substrate3are converged in the diffusion layer12, and as a result, improvement in the intensity of the optical signals propagating in the optical transmission channel7can be expected.

Also in this embodiment, the thickness of the supplied resin is defined as 10 μm or less by supplying the transparent resin11to the second substrate3by spin coating. The method of supplying the transparent resin11is not restricted to the spin coating so long as an amount capable of forming the diffusion layer12can be supplied.

As described above, according to the optical coupling structure described in this embodiment, distribution of the refractive index is caused in the optical transmission channel7, and this provides the Grin lens effect, and can improve the intensity of the optical signals propagating in the optical transmission channel7.

Fourth Embodiment

A third embodiment of the present invention is to be described with reference toFIG. 7. Also in this embodiment, the bonding method between the second substrate3and the optical transmission channel7in the first embodiment is changed but other structures and the processes are identical with those of the first embodiment.

In this embodiment, a glass material having a refractive index higher than the refractive index of the core layer72of the optical transmission channel7is used for the second substrate3. Thus, upon laser welding, the glass material of the second substrate3and the optical transmission channel7are melted by the laser light irradiation in the same manner as in the third embodiment. In this case, the glass material of the second substrate3diffuses into the optical transmission channel7. Thus, a diffusion layer12of the glass material of the second substrate3is formed in the optical transmission channel7. As a result, a distribution of refractive index is formed in the diffusion layer12such that the refractive index on the side of the boundary at the second substrate3is higher and the Grin lens effect is generated as in the third embodiment.

As described above, also in this embodiment, distribution of refractive index is caused in the optical transmission channel7, and this provides the Grin lens effect, and the intensity of the optical signals propagated in the optical transmission channel7can be improved.

Fifth Embodiment

A fifth embodiment of the present invention is to be described with reference toFIG. 8toFIG. 10.

FIG. 8is a view for explaining a configuration of optical coupling in the fifth embodiment.FIG. 9is a cross sectional view for explaining an optical coupling process in the fifth embodiment.FIG. 10is a view for explaining an optical module in which the configuration of the optical coupling in the fifth embodiment is applied.

First, the optical coupling configuration in the fifth embodiment is to be described with reference toFIG. 8.

The fifth embodiment is identical with the first embodiment in that a first wafer substrate2, an optical device1, a LSI1a, an optical transmission channel7, and solder bumps5are provided and detailed description for the structures and the processes thereof are omitted. The fifth embodiment is different from the first embodiment in that the second wafer substrate is not provided and a transparent resin13is provided at the periphery of the optical device1. The transparent resin13covers the optical device1and is connected with the optical transmission channel7. Optical signals emitted from the optical device1are guided by way of the transparent resin13to the core layer of the optical transmission channel13and propagated therein.

Next, a specific optical coupling process in the fifth embodiment is to be described with reference toFIG. 9.

First, in the same process as in the first embodiment, an optical device1and a LSI1aare disposed over a first wafer2and solder bumps5are disposed therebelow as illustrated inFIG. 9A. Different from the first embodiment, this embodiment has no second wafer and lacks in the process of bonding the second wafer and the first wafer2.

Then, an optical transmission channel7is bonded to the optical device1as illustrated inFIG. 9B. In this embodiment, a plastic fiber (POF) is used as the optical transmission channel7. First, a transparent resin13is coated and cured over the optical device1. In this step, the transparent resin13covers the entire optical device1and hermetically seals the optical device1. Then, for the optical transmission channel7and the optical device1, the POF is positioned at a portion where the optical signals from the optical device1transmit therethrough, and then the transparent resin13and the POF are welded by a laser light.

In this embodiment, while a UV-curable resin is preferred as the transparent resin13, a thermosetting resin may also be used.

The method of fixing the optical transmission channel7is not restricted to the laser welding. The operation time can be decreased remarkably, for example, by using a UV-curable resin, positioning the optical transmission channel7and the optical device1after coating and before curing the UV-curable transparent resin13, curing the transparent resin13by UV light irradiation, and adhering to fix the optical transmission channel7by the transparent resin13.

Also in this embodiment, the POF is used as the optical transmission channel7. This is advantageous since the diameter of the core layer72of the POF is generally large as 125 μm φ which is larger than that of the single mode fiber and, accordingly, required accuracy for positioning the POF is moderated and the bondability with the transparent resin13is satisfactory.

In this embodiment, while the UV curable resin is used as the method of fixing the optical transmission channel7, this is not always necessary but other transparent resin such as a thermosetting resin may also be used so long as the optical transmission channel7is fixed. Further, when the optical transmission channel is connected by laser welding, the second embodiment of the invention may also be combined such that configuration of the laser light absorption resin may be disposed between the transparent resin13and the optical transmission channel7and welded by laser light. Further, the transparent resin13may be diffused into the optical transmission channel7upon laser welding as in the third embodiment. While coating and curing of the transparent resin13and the connection of the optical transmission channel7are performed in the state where the wafer is cut into individual chips, the processing up to the coating and curing of the transparent resin13and, further, connection of the optical transmission channel7may be performed also in the state of the wafer before it is cut into individual chips.

Then, a configuration of applying the optical coupling structure of this embodiment to optical wirings is to be described with reference toFIGS. 10A and 10B.FIG. 10Ais a perspective view of an optical module mounting structure according to this embodiment andFIG. 10Bis a cross sectional view thereof. Description for the configurations of this embodiment in common with those of the first embodiment illustrated inFIG. 4is to be omitted. In this embodiment, an individualized optical module6is mounted over the optical and electrical hybridization substrate9as in semiconductors device in existent electronic equipment. As illustrated inFIG. 9B, the POF is welded by a transparent resin13to the optical module6mounted over the optical and electrical hybridization substrate9. In the same manner, the top end of the POF on the side opposite to the side adhered with the optical device1is fixed to the optical wirings92over the optical and electrical hybridization substrate9.

Electric signals transmitted from electric wirings91over the optical and electrical hybridization substrate9are transmitted by way of through vias22formed in the first wafer substrate to the LSI1a. The LSI1agenerates signals corresponding to the transmitted electric signals, which drive the optical device1and are converted into optical signals. The converted optical signals are guided by way of the transparent resin13to the core layer72of the optical transmission channel7. Further, the guided optical signals are propagated in the core layer72and the optical signals are transmitted in the optical wirings92of the optical and electrical hybridization substrate9.

As described above according to the optical coupling structure described for this embodiment, the optical module6in the form of the WLP can be handled as a chip and the optical module6can be handled in the same manner as that for the semiconductor device in the existent electronic equipment. Further, since the required positioning accuracy is moderated by using the POF of a large core diameter as the optical transmission channel7and the optical transmission channel7can be fixed easily by the transparent resin, the alignment step can be simplified.

Sixth Embodiment

A sixth embodiment according to the present invention is to be described with reference toFIG. 11. Also in this embodiment, an optical transmission channel7is bonded by a transparent resin14in the same manner as in the fifth embodiment and structures and processes are identical with those of the fifth embodiment.

In this embodiment, after cutting the wafer into individual optical modules6by dicing, the entire optical module including not only the optical device1but also the LSI1ais hermetically sealed by a transparent resin13. This can prevent intrusion of obstacles in the optical channel of the optical module6. Further, moisture proofness can also be improved.

Alternatively, optical devices1and LSIs1aof a plurality of optical modules may be sealed by a lump of the transparent resin in the state of a wafer before cutting into individual chips and they may be cut into individual chips together with transparent resin by dicing.

As described above, in this embodiment, reliability can be improved by resin encapsulation of the optical device.