WO2021079403A1

WO2021079403A1 - Buried optical waveguide structure and method for producing same

Info

Publication number: WO2021079403A1
Application number: PCT/JP2019/041297
Authority: WO
Inventors: 信建小勝負; 藤原　裕士
Original assignee: 日本電信電話株式会社
Priority date: 2019-10-21
Filing date: 2019-10-21
Publication date: 2021-04-29

Abstract

A buried optical waveguide structure which has an Si wafer which functions as a substrate, a thermally oxidized Si core, and an upper cladding layer and a lower cladding layer which support the thermally oxidized Si core by directly sandwiching said core in the vertical direction, the buried optical waveguide structure being characterized in that the thermally oxidized Si core is separated from the Si wafer which functions as the substrate. Also, a method for producing the same.

Description

Embedded optical waveguide structure and its manufacturing method

The present invention relates to an optical waveguide structure, particularly a structure of an embedded optical waveguide that can be used in the visible light band at low cost and a method for producing the same. It relates to an optical waveguide for a planar optical circuit that can be used not only in the entire visible light band but also in the near-ultraviolet to near-infrared region. It can be used for ultra-broadband optical communication from light and visible light to the near-infrared light region.

(Explanation of current technology)
(Explanation of planar optical circuit PLC)
Until now, quartz-based planar optical circuits (PLCs) for optical communications have been widely used for optical communications in the near-infrared optical band, and they combine and demultiplex optical signals transmitted by optical fibers and have wavelengths. Optical switches that combine optical branching elements (optical splitters), AWG (Arrayed waveguide Grating) elements, and Machzenda interferometers that utilize the thermo-optical effect have been used to input and output signals each time. It was.

(PLC board for optical communication)
In conventional PLCs for optical communication, a silicon wafer (Si wafer) with an oxide film having an oxide film formed on the surface has been widely used as a substrate for production.

The reason is that Si wafers used in semiconductors are made of silicon with a purity of 99.999999999% (11N) or higher, and are used in CMOS (Complementary metal-oxide-semiconductor) processes, etc. Many are relatively inexpensive. It is also because there are few impurities and dust, the area is relatively large, the surface is smooth, and a silicon dioxide (SiO ₂ ) film can be formed by surface oxidation.

(Characteristics of Si wafer and SiO ₂ oxide film)
The Si wafer is naturally oxidized in the atmosphere and its surface is covered with a _{very thin SiO 2 film.} The adhesion between the Si of the substrate and the SiO _{2 film formed on the substrate is strong.} In particular, when oxidation is performed at a high temperature, a thick, dense and stable thermal oxide Si film (SiO ₂ film) can be formed. The melting point of Si is 1412 ° C., but _{the melting point of SiO 2} is 1732 ° C., and the thermally oxidized Si coating has very high heat resistance.

Not all metals and semiconductors have the property of easily forming and covering a dense oxide film with high adhesion. Therefore, in semiconductor devices using Si represented by the CMOS process, the thermally oxidized Si film is used. It has the characteristic of forming a very excellent insulating film, and can be said to be a very excellent material.

(Conventional method for manufacturing PLC for optical communication)
In the conventional PLC for optical communication, the manufacturing method as shown in FIG. 1 has been performed. First, as shown in FIG. 1A, thermal

oxide Si layers

2a and 2b are formed on both surfaces of the substrate 1 of the Si wafer, and one of them is used as the lower clad layer 2a. Next, as shown in FIG. 1B, Ge-doped SiO ₂ is laminated on the lower clad layer 2a to form the optical waveguide core layer 3. Finally, as shown in FIG. 1C, the upper clad layer 4 is sequentially formed, the optical waveguide core layer 3 is embedded, and the PLC of the embedded optical waveguide structure is formed.

At this time, since each layer is formed in sequence, the film formation temperature of each lower clad layer 2a, core layer 3, and upper clad layer 4 is very important. For example, when the film formation temperature of the upper clad layer 4 to be finally formed is higher than the melting point of the core layer 3, the structure of the core layer 3 produced at the time of film formation of the upper clad layer 4 is in the upper clad layer 4. The embedded optical waveguide structure disappears.

(Specific manufacturing method of PLC for optical communication)
In the conventional manufacturing method of FIG. 1, the thermal oxide Si film 2a has a _{lower refractive index than the Ge-doped SiO 2} which is the core layer 3, and therefore becomes a lower clad layer. On the surface of this thermal oxide Si film 2a, a flame deposition method (FHD: Flame Hydrolysis Deposition), a chemical vapor deposition method (CVD: Chemical Vapor Deiposition) such as thermal CVD (Thermal CVD) or plasma CVD (Plasma-enhanced CVD), etc. _{A Ge-doped SiO 2} core layer 3 in which the refractive index of the optical waveguide is adjusted is formed, and a ridge-shaped optical waveguide core 3 is formed by photolithography and plasma dry etching using a fluorine-based plasma such as _{CF 4.} To make. Boron-phospho-silicate-glass (BPSG) or the like, to which boron or phosphorus dopant having a lower melting point and lower refractive index than the core layer 3 is added to the surface by using an FHD method or a CVD method or the like. Was formed as the upper clad layer 4 to prepare an embedded optical waveguide.

At this time, by setting the relationship of the film forming temperature of each lower clad layer> the film forming temperature of the core layer> the film forming temperature of the upper clad layer, the other layers are not destroyed or disappeared when the core and the clad layer are formed. It is a process process like this.

(Possibility of PLC application in other fields)
On the other hand, in recent years, attention has been increasing in the fields of optical analysis, medical treatment, and display as new application fields of PLC.

Conventionally, a relatively large analyzer using a spatial optical system has been used as an optical circuit for biological analysis such as a fluorescence microscope, an interference microscope, and a spectroscope, and as a light source in the display field. However, in response to recent demands for general-purpose analyzers, smaller size and lighter weight, and free optical alignment, expectations for highly integrated devices using PLCs are increasing. For example, RGB (Red Green Blue) combiner for retinal scanning display [Non-Patent Document 1], AWG for small spectroscopic sensor [Non-Patent Document 2], digital holographic microscope by two light beam interference [Non-Patent Document 3], etc. Is under consideration.

(Visible light of Ge-doped core, light damage by UV light)
However, in the conventional PLC device, in the Ge-doped SiO ₂ used for the optical waveguide core layer, the doped Ge (germanium) atom absorbs UV light (ultraviolet light) and short-wavelength visible light. , It is known that a bond defect called the color center of Ge occurs and the light loss in the core layer increases. [Non-Patent Document 4]
Therefore, the closer the light wavelength is to UV light and the higher the energy, and the higher the light intensity, the more likely it is that photodegradation will occur, which has been a factor in increasing the light loss of the PLC. ..

(Usefulness of optical waveguide in pure quartz core)
Therefore, in order to make the PLC resistant to high-intensity UV light and visible light, in the amorphous structure SiO ₂ glass, the number of bond defects is reduced as much as possible, and the impurities that can cause defects are also reduced as much as possible. Is desirable.

As shown in [Non-Patent Document 5], pure fused silica glass (Fused Silica glass) has very excellent light transmittance from about 200 nm to about 2500 nm, excluding the light absorption factor due to defects. There is.

Therefore, it is necessary to deposit as high-purity raw material as possible on the substrate, melt it to a temperature at which defects disappear, and then cool it to form a high-purity SiO ₂ glass film as a core layer.

(Problems with the substrate when manufacturing visible light PLC)
As described above, since _{the melting point of SiO 2} is 1732 ° C., in order to sufficiently melt the substrate, it is necessary that the substrate for producing the PLC is not deformed by the heating temperature at the time of melting. On the other hand, since the melting point of Si is 1412 ° C., when a Si wafer is used for a substrate, it is better to avoid a temperature process above the Si melting point.

(Difficulty in producing fused quartz core)
Therefore, it is very difficult to form a core layer of fused silica glass on the surface of a Si wafer.

Further, in the case of sequentially producing from the substrate (wafer) surface like the above-mentioned PLC for optical communication, it is necessary to use a material having higher heat resistance such as melting point than the core layer for the lower clad, which is also the point. , It becomes difficult to use the core layer of fused silica glass.

On the other hand, it is also conceivable to attach a separately produced fused silica glass on a substrate such as a Si wafer and then polish the entire surface to thin the fused silica glass as the core layer. However, in order to use it as a stable optical waveguide core, it is necessary to make the film thickness uniform with an accuracy of about submicron, and even with the current technology, the yield is high and there is no swell in the entire substrate. The method of grinding and polishing is very difficult.

(Purpose of Invention)
The present invention has been made in view of this situation, and an object of the present invention is to maintain compatibility with a CMOS process, that is, while using a Si wafer, from near-ultraviolet light to visible light, and further to near-infrared light. The purpose is to produce an optical waveguide having excellent light transmission in the entire range up to the light band (for example, 300 nm to 1700 nm).

(Visible light WG (optical waveguide) using thermally oxidized Si for the core layer and its manufacturing method)
As a result of diligent studies in view of the above problems, the present inventors have found that an optical waveguide structure using a thermal oxide film of a Si wafer as a core layer is possible, and reviewed the procedure for producing the optical waveguide and the optical waveguide structure. Further, by examining the clad material, the present invention has been completed.

Further, as described later, since it is possible to fabricate a PLC with the thermal oxide core layers of two PLCs adjacent to each other on the upper and lower sides, it is possible to fabricate an element that photobonds between the layers of the two thermal oxide cores. It also has features. Furthermore, by using a Si wafer, it is possible to fabricate a CMOS device on the surface of the Si wafer, which not only enables mass production at low cost, but also allows it to be used in the ultraviolet to near-infrared light region. It has a feature that an integrated device with a light receiving element such as a Si photodiode can also be manufactured.

An example of the configuration of the means for solving the above problem is as follows.

(Structure 1)
In the cross-sectional view of the substrate perpendicular to the light propagation direction of the optical waveguide,
A Si wafer as a substrate, a thermally oxidized Si core,
It has an upper clad layer and a lower clad layer that directly sandwich and support the thermally oxidized Si core vertically.
An embedded optical waveguide structure characterized in that the thermally oxidized Si core is located at a distance from the Si wafer serving as a substrate.

(Structure 2)
In the embedded optical waveguide structure described in Configuration 1,
An embedded optical waveguide structure further comprising a support substrate bonded to the upper surface of the upper clad layer.

(Structure 3)
A two-layer structure characterized in that the two embedded optical waveguide structures according to the configuration 1 have a two-layer structure in which two embedded optical waveguide structures are arranged back to back on the side of the upper clad layer with a spacer interposed therebetween and are aligned and joined. Embedded optical waveguide structure.

(Structure 4)
In the embedded optical waveguide structure according to the configuration 2,
The support substrate has a light-receiving element that photocouples to the optical waveguide, and has a composite type embedded optical waveguide structure that also has a light-receiving function as well as structural reinforcement.

(Structure 5)
A step of forming a thermal oxide Si layer on the upper surface of a Si wafer to be a substrate,
The step of etching the thermal oxide Si layer to form the thermal oxide Si core layer of the optical waveguide,
A step of forming an upper clad layer having a refractive index lower than that of the thermally oxidized Si core layer on the thermally oxidized Si core layer,
A step of partially etching and removing the Si wafer in a portion corresponding to the lower side of the thermal oxide Si core layer until the thermal oxide Si core layer is separated from the remaining portion of the Si wafer serving as a substrate.
A step of forming a lower clad layer having a low refractive index under the thermally oxidized Si core layer is provided.
An embedded optical waveguide structure characterized in that the thermally oxidized Si core is directly sandwiched and supported vertically by the upper clad layer and the lower clad layer to produce a structure located at a distance from the Si wafer as a substrate. Manufacturing method.

(Structure 6)
A method for producing an embedded optical waveguide structure according to the present invention, wherein the method for producing an embedded optical waveguide structure further includes a step of joining a support substrate on the upper clad layer.

(Structure 7)
Two embedded optical waveguide structures were produced by the method for producing the embedded optical waveguide structure of the configuration 5.
A step of arranging the two produced embedded optical waveguide structures back to back on the upper clad layer side with a spacer in between.
A method for producing an embedded optical waveguide structure having a two-layer structure, which further comprises a step of joining while aligning with a marker.

(Structure 8)
A method for manufacturing a composite embedded optical waveguide structure, which further comprises a step of manufacturing a light receiving element that photocouples to the optical waveguide on the support substrate in the method for manufacturing the embedded optical waveguide structure of the configuration 6.

According to the optical waveguide structure of the present invention described above and the method for producing the same, an embedded optical waveguide structure having excellent light transmission from the near infrared to the visible light band and the ultraviolet light band at low cost by a relatively simple method. Can be realized. By the method for producing an optical waveguide of the present invention, an optical waveguide element that combines and demultiplexes light over a wide band wavelength can be produced, and is used in various industrial fields such as medical treatment, bioanalysis, and display devices such as image projection. Is possible.

It is sectional drawing explaining the manufacturing method of the conventional PLC for optical communication. It is sectional drawing which shows the structure of the thermal oxide Si core embedded optical waveguide of Embodiment 1. FIG. It is a figure explaining the manufacturing process of the thermal oxide Si core embedded optical waveguide of Embodiment 1. It is sectional drawing which shows the structure of the thermal oxide Si core embedded optical waveguide of Embodiment 2. It is a figure explaining the manufacturing process of the thermal oxide Si core embedded optical waveguide of Embodiment 2. It is sectional drawing which shows the structure of the thermal oxide Si core embedded optical waveguide of Embodiment 3. It is a figure explaining the manufacturing process of the thermal oxide Si core embedded optical waveguide of Embodiment 3. It is sectional drawing which shows the structure of the thermal oxide Si core embedded optical waveguide of Embodiment 4. It is a figure explaining the manufacturing process of the thermal oxide Si core embedded optical waveguide of Embodiment 4.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Si wafer)
First, a method for producing a Si wafer, which is a prerequisite, will be described. _{Quartz sand (main component is SiO 2} ) is ubiquitous on the earth, but in order to efficiently obtain high-purity Si, high-quality silica sand with few impurities is used as the raw material. When silica sand is mixed with coke (C) in an electric furnace at 1800 ° C., oxygen is combined with carbon and degassed to obtain metallic silicon (Si) having a purity of about 98% in a molten state. The main impurities at this stage are Al (aluminum) and Fe (iron).

Next, when powdered metallic silicon is dissolved in hot hydrochloric acid (HCl), trichlorosilicon (HSICl ₃ ) is obtained. Trichlorosilicon is a liquid at room temperature and has a boiling point of 31.8 ° C, but it is continuously sent to a distillation column for purification. Next, liquid trichlorosilicon is vaporized and sent to a high-temperature reactor together with hydrogen gas, where it is decomposed to produce electronic-grade polysilicon (EGS) with a purity of about 99.99999999% (10N). obtain.

Subsequently, polycrystalline silicon is melted at a high temperature in an electric furnace and slowly cooled to precipitate crystals and to purify them. Mainly, silicon single crystal ingots with a purity of 99.999999999% (11N) or more are produced by two methods, the zone melting method (FZ: floating zone) and the Czochralski method (CZ: Czochralski). To do.

At this time, by forming, cutting, CMP (chemical-mechanical polishing), cleaning, etc., the single crystal ingot, the large swell over the entire surface of the wafer is ± 20 to 25 μm or less, and locally at the atomic level. A Si wafer that maintains smoothness can be obtained.

(Method for producing a thermal oxide Si film)
Further, a thermally oxidized Si film is formed on the surface of the Si wafer obtained above. There are three types of thermal oxidation of Si, dry oxidation, wet oxidation, and steam oxidation, depending on the type of gas used for oxidation, and the thermal oxidation temperature is in the temperature range of about 800 to 1100 ° C. Oxygen gas is used for dry oxidation, deionized steam is added to oxygen gas for wet oxidation, and only deionized steam is used for steam oxidation. _{In the hydrogen combustion steam oxidation method, steam (H 2} O) generated by natural combustion by flowing oxygen gas and hydrogen gas into the furnace is used.

In general, since the solubility of H ₂ _{O in the SiO 2} layer is high and the _{diffusion rate is high because the H 2} O molecules are small, wet oxidation proceeds faster than dry oxidation, but the formed SiO ₂ layer Denseness is higher in dry oxidation. Therefore, dry oxidation is usually used for forming a thick layer of 100 nm (0.1 μm) or less, and wet oxidation is used for forming a thicker oxide layer. In the present invention, since it is used as the core layer of the optical waveguide, a film thickness of about 10 μm or less is required, and a manufacturing method by wet oxidation is desirable.

At this time, the purity of the silicon itself to be oxidized can be controlled to be extremely high at 99.999999999% (11N) or more, and dopants such as phosphorus (P) and boron (B) are introduced to control the electrical resistance and semiconductor characteristics. If a high resistance silicon wafer is used, the influence of the dopant contained in the silicon substrate can be reduced. Furthermore, if the thermal oxidation environment is controlled in a clean room or the like, it is possible to limit the mixing of impurities into the thermal oxide film itself, so that the silicon oxide layer is caused by impurities, if not as much as pure fused silica glass. It is possible to reduce the light absorption derived from defects.

Further, since the Si wafer is a high-quality wafer substrate that is used in a very large amount in a CMOS process or the like, it is relatively inexpensive in terms of price. Therefore, in order to fabricate a wide-band optical waveguide from the ultraviolet band to the near-infrared light region at low cost, it is superior to using a thermally oxidized Si film as a core layer rather than pure fused silica glass.

On the other hand, the amorphous SiO ₂ film can be produced not only by thermal oxidation but also by CVD (chemical vapor deposition) method, sputtering method, FHD (flame deposition) method, etc., but reduction and control of contamination of impurities As a material for a wide-band optical waveguide core from the ultraviolet band to the near-infrared light region, it is more unsuitable than the thermally oxidized Si film because it is difficult to absorb light due to impurities.

Furthermore, since the thermally oxidized Si film is produced by surface oxidation of a high-purity Si single crystal, although slight point defects (bonding defects) and oxygen deficiency defects occur, it is controlled with high accuracy in a low impurity environment. Since it undergoes an oxidation process at about 800 to 1100 ° C, the defect density is close to that of fused silica glass, and it has excellent transparency from the near-ultraviolet light region of about 200 nm to the near-infrared light band of about 2500 nm. A glass film is obtained.

In the present invention, such thermally oxidized Si is used as a core, and by selecting the structure and manufacturing procedure of the embedded optical waveguide and the clad material shown below, it is inexpensive and has a wide band from the ultraviolet light band to the near infrared light band. We have found a usable optical waveguide.

(First Embodiment)
(Basic structure of thermal oxide Si core optical waveguide)
FIG. 2 is a cross-sectional view showing the structure of the thermal oxide Si core embedded optical waveguide according to the first embodiment of the present invention. In the following cross-sectional view, unless otherwise specified, the cross-sectional view of the substrate perpendicular to the optical propagation direction of the optical waveguide is shown, including other embodiments.

In the first embodiment of FIG. 2, the optical waveguide has an embedded optical waveguide structure of a thermally oxidized Si core. A part of the Si wafer 21 to be a substrate is hollowed out, and the thermal oxide Si core 23 to be the core of the optical waveguide is directly sandwiched and supported by the upper clad 24 and the lower clad 22, and is positioned at a distance from the Si wafer 21. The structure is such that the thermally oxidized Si core 23 is surrounded by a clad and floats.

That is, in the cross-sectional view of the substrate in the cross section perpendicular to the optical waveguide in FIG. 2, the upper clad layer 24 and the lower clad layer 22 that directly sandwich and support the thermally oxidized Si core 23 and the thermally oxidized Si core 23 vertically. It has an embedded optical waveguide structure in which the thermally oxidized Si core 23 is located at a distance from the Si wafer 21 as a substrate. The thermally oxidized Si layer 20 initially formed on the back surface of the Si wafer 21 may remain between the Si wafer 21 and the lower clad 22, and the Si wafer 21 is separated from the thermally oxidized Si core 23. At this position, it may be sandwiched between the upper clad 24 and the lower clad 22.

Such an embedded optical waveguide structure cannot be produced by the conventional method for producing an optical waveguide for optical communication as shown in FIG. Hereinafter, the method for producing the first embodiment of the present invention will be described in detail with reference to FIG.

(Basic method for manufacturing a thermal oxide Si core optical waveguide)
First, as shown in FIG. 3A, thermal oxide Si layers 30a and 30b are prepared on both the front and back surfaces of the Si wafer 31 to prepare a Si wafer. The thermal oxide Si layer 30a on the upper surface side (front surface side) of the Si wafer 31 is formed with a sufficient film thickness required as a core of an optical waveguide.

Alignment markers 37 required for the process process are formed on the thermal oxide Si layers 30a and 30b on both the front and back surfaces. This is done by a photolithography process using a photoresist, covering areas other than the alignment marker position with a resist film, and by a reactive dry etching method using a fluorine-based plasma such as _{CF 4, thermal oxide Si layers 30a, b and Si wafers.} It can be produced by digging into 31.

Next, as shown in FIG. 3B, a photoresist 38 is formed on the surface-side thermal oxide Si layer 30a by a photolithography step in a pattern of an optical waveguide core. Then, the thermally oxidized Si layer 30a is removed by etching by a reactive dry etching method using a fluorine-based gas such as _{CF 4, and as shown in FIG. 3C, the thermally oxidized Si core layer 33 of the ridge-shaped optical waveguide} Is produced on the surface of the Si wafer 31.

If the film thickness of the thermally oxidized Si core layer 33, which is the optical waveguide core, is on the order of 100 μm, a very long time is required even for steam oxidation, which is not a realistic process process. Therefore, the film thickness of the thermally oxidized Si core layer 33 is several tens of μm at the thickest. Further, assuming that the propagating signal light of the optical waveguide is taken out by an optical fiber or the like, the film thickness is preferably 10 μm or less.

Next, as shown in FIG. 3D, the upper clad layer 34 is laminated on the surface of the thermal oxide Si core layer 33.

(Description of clad material)
Since the material of the clad layer is an optical waveguide having an embedded structure, it is necessary to have a lower refractive index than the thermally oxidized Si core layer 33 in the ultraviolet light band to the near infrared light band. Further, in the present invention, it is also necessary to structurally support the thermal oxide Si core layer 33, so that it is desirable that the mechanical strength is high.

Specifically, magnesium fluoride (MgF ₂ ), calcium fluoride (CaF ₂ ), hafnium fluoride (HfF ₄ ), aluminum fluoride (AlF ₃ ), lanthanum fluoride (LaF ₃ ), yttrium fluoride (YF). ₃ ), Fluorine compounds such as zirconium fluoride (ZrF ₄ ), low refractive index amorphous SiO ₂ _{such as boron-doped SiO 2} glass and fluorine-doped SiO ₂ glass can be used.

Examples of the polymer compound include polytetrafluoroethylene (PTFE), perfluoroalkoxy alkane (PFA), perfluoroethylene-propene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), and polyvinylidene fluoride (PVDF). , EFEP and other fluororesins, amorphous fluoropolymers, (partially) fluorinated epoxy resins, (partially) fluorinated acrylate resins, (partially) fluorinated silicone resins, and mixtures of the above resins are also used. You can also.

Further, quartz glass in which these fluorine compounds are made into fine particles and dispersed, an organic compound, a mixture or a melt of _{a fluorine compound or a low refractive index oxide and amorphous quartz (SiO 2) can also be used.}

Further, the upper clad layer 34 in FIG. 3D may have a multilayer structure of two or more layers instead of a one-layer structure. For example, a material having a low refractive index such that the propagating light is sufficiently confined in the thermally oxidized Si core 33 is first laminated so as to cover the surface of the core layer 33 with a thickness that does not allow light to seep out, and then laminated on the material. A material having an excellent mechanical strength even if the refractive index is close to that of the thermally oxidized SiO _{2 may be laminated as the second layer.}

Next, as shown in FIG. 3 (e), a photoresist 38 is formed on the back surface side (lower surface side in the drawing) of the Si wafer 31, and a pattern wider than that of the core 33 is formed at a position corresponding to the back surface of the optical waveguide core 33. Then, as shown in FIG. 3 (f), the thermally oxidized Si layer 30b at the position corresponding to the back surface of the optical waveguide core 33 is etched and removed by a reactive dry etching method using a fluorine-based gas such as _{CF 4.}

After that, as shown in FIG. 3 (g), a mixed solution of hydrofluoric acid, nitrate, and acetic acid showing isotropic etching properties, potassium hydroxide (KOH) showing anisotropic etching, and tetramethylammonium hydroxide (TMAH). ), Wet etching method using alkaline aqueous solution such as ethylenediamine / pyrocater (EDP), reactive dry etching method using fluorine-based gas such as _{CF 4,} and Bosch process using _{SF 6} and C ₄ F _{8 gas, etc.} The lower layer of the wafer 31 is also further etched and removed.

The Si wafer layer 31 in the lower portion of the optical waveguide core layer 33 is partially etched and removed until the thermally oxidized Si core layer 33 is separated from the remaining portion of the Si wafer 31 as a substrate. At this time, since it is necessary to etch the Si layer to such an extent that the propagating light in the waveguide core 33 does not seep into the remaining Si wafer layer 31, at least the distance between the core 33 and the remaining portion of the Si layer 31 is set. It should be 10 μm or more.

Finally, as shown in FIG. 3H, the lower clad layer 32 is laminated under the optical waveguide core layer 33 from the back surface side as in the upper clad 34, and the embedded optical waveguide structure by the thermal oxide Si core of the first embodiment is formed. Can be produced.

(Second Embodiment)
(Thermal Oxide Si Core Optical Waveguide with Support Substrate Joined)
FIG. 4 is a cross-sectional view showing the structure of the thermal oxide Si core optical waveguide according to the second embodiment of the present invention.

_{Since the SiO 2} thermal oxide film on the surface of the Si wafer has excellent adhesion, it does not peel off much even if it is processed into a ridge shape on the surface of the Si wafer as shown in FIG. 3C. On the other hand, the coefficients of thermal expansion of _{Si and SiO 2} ^{are about 3.6 × 10 -6} (1 / K) and about 0.65 × 10 ^-6 (1 / K), respectively, which are quite different, so that they are thermally oxidized at 800 to 1100 ° C. Residual stress is generated at the interface _{between Si and SiO 2} due to the temperature difference between the temperature and room temperature.

If the mechanical strength of the clad material is large enough to overcome this residual stress, the optical waveguide core will not be displaced or the optical waveguide structure will not be destroyed by the residual stress. However, if the low refractive index characteristics of the clad material are not due to the atomic refraction of the constituent elements but simply due to the low density of the material, the mechanical strength of the clad material is low and there is a risk of stress failure. Increases sex. The structure of the second embodiment of the present invention has been found to solve such a problem.

In the thermal-oxidized Si core-embedded optical waveguide structure of the second embodiment shown in FIG. 4, the portion corresponding to that of the second embodiment has the same structure, and the description thereof will be omitted. In the second embodiment, the support substrate 49 is bonded to the upper surface of the upper clad layer 44, and the support substrate 49 _{prevents the positional deviation of the thermal oxide film SiO 2} core and the structural destruction of the light confinement structure.

Hereinafter, the method for manufacturing the optical waveguide of the second embodiment will be described in detail with reference to FIG.

(Method of manufacturing a thermal oxide Si core optical waveguide with a support substrate bonded)
In the method of manufacturing the optical waveguide of the second embodiment, the steps of FIGS. 5 (a) to 5 (c) are the same as the steps of the method of manufacturing the first embodiment of FIGS. 3 (a) to 3 (c).

In the manufacturing method of the second embodiment, as shown in FIG. 5D, the upper clad layer 44 is laminated on the thermally oxidized Si core 43, and then the support substrate 49 is bonded on the upper clad layer 44. As a method of joining the support substrate 49, when the upper clad layer 44 is a thermoplastic clad material, a method of softening by heating and directly joining can also be used, but an adhesive / adhesive material is applied to the upper surface of the upper clad layer 44. It is also possible to join by.

The support substrate 49 in FIG. 5D is preferably made of a material having mechanical strength, and further, stress generated due to a difference in thermal expansion coefficient from that of a Si wafer due to a temperature change during the manufacturing process. It is desirable that the material has a coefficient of thermal expansion similar to that of a Si wafer. Further, the support substrate 49 is made of a low-refractive material or the film thickness of the upper clad layer 44 is sufficient so that the light propagating through the thermal oxide Si core 43 does not photobond with others. It needs to be thick, and if possible, the film thickness of the upper clad layer 44 is preferably 10 μm or more.

The manufacturing steps of FIGS. 5 (e) to 5 (g) following the second embodiment are the same as those of FIGS. 3 (e) to 3 (h) of the first embodiment.

(Third Embodiment)
(Two-layer optical waveguide with thermal oxide Si core)
FIG. 6 is a cross-sectional view showing the structure of a two-layer optical waveguide using a thermally oxidized Si core according to a third embodiment of the present invention.

In the present invention, since a Si wafer that is smooth and has small waviness on the entire surface is used as the substrate material, the thermal oxide Si layer that is the core also has the same smoothness. Therefore, in the third embodiment, by introducing a spacer that maintains the distance between the two core layers, it is possible to realize an embedded optical waveguide structure having two layers of optical waveguide cores relatively easily.

As shown in FIG. 6, in the two-layer embedded optical waveguide structure by the thermal oxide Si core of the third embodiment, two embedded optical waveguides of the thermal oxide Si core of the first embodiment of FIG. A two-layered embedded optical waveguide structure is formed by laminating and joining back to back on the layer side.

In the two-layer optical waveguide of FIG. 6, the two thermal

oxide Si cores

63a and 63b are included in the clad layer 64 formed by joining the upper clad layers of the original two optical waveguides with the spacer 69 interposed therebetween. The clad layer 64 is sandwiched between the

Si wafers

61a and 61b of the original two optical waveguides, and further sandwiched between the

clad layers

62a and 62b which are the lower clad layers of the two original optical waveguides.

The spacer 69 may also be configured by joining two portions formed in the upper clad layer of the original two optical waveguides. The thermal oxide Si layers 60a and 60b are derived from the thermal oxide Si layers on the back surfaces of the original two optical

waveguide Si wafers

61a and 61b.

(Production method of Embodiment 3)
Hereinafter, the method for producing the third embodiment of the present invention will be described in detail with reference to FIG. 7.
The manufacturing method of the third embodiment of FIG. 7 is roughly a method for manufacturing a two-layer optical waveguide in which two thermally oxidized Si-core optical waveguides are vertically bonded to each other.

In the production method of the third embodiment, first, the steps of FIGS. 7 (a) and 7 (b) correspond to the steps of the production method of the first embodiment, FIGS. 3 (a) and 3 (b).

Next, in the step of FIG. 7C, after the ridge-shaped thermally oxidized Si core layer 63b is produced, a plurality (for example, two) spacers 69b designed to have a desired two-layer core spacing are produced. As a method for producing the spacer, another structure of thermally oxidized Si processed into a ridge shape can be used, but as shown in FIG. 7C, after the spacer material is deposited, patterning by a photolithography step or It can also be processed by a dry etching method using plasma in vacuum. It can also be produced by spin-coating a pre-curing solution of a UV-curable resin that is cured by a UV reaction with ultraviolet light and directly exposing it to UV light.

Then, in the step of FIG. 7 (d), the two structures of FIG. 7 (c) were inverted with each other on the core side (the surface on the upper clad layer side in which the spacer was created) back to back, and the clad layer 64 was formed. A two-layer optical waveguide structure of a thermally oxidized Si core having a desired interval can be formed by sandwiching the materials and joining them while aligning them with markers.

At this time, if it is possible to have a thermoplastic clad material that becomes a solution such as perfluoroalkanes, the upper and lower Sis in FIG. 7 (d) can be obtained without preparing the spacer 69 in FIG. 7 (c). By aligning and fixing the wafers and pouring the thermoplastic clad material, it is possible to form a two-layer optical waveguide structure of a thermally oxidized Si core having a desired interval as shown in FIG. 7 (d).

At this time, the alignment markers 67 on the front and back surfaces produced in FIG. 7A make it possible to align the relative positions of the two wafers with high accuracy.

The following steps of FIGS. 7 (e) to 7 (g) can be produced by basically repeating the same method as the steps of FIGS. 3 (e) to 3 (h) of the first embodiment on both the front and back surfaces.

(Fourth Embodiment)
The fourth embodiment of the present invention is roughly a composite type thermal oxide Si core embedding in which a Si light receiving element (PD) is manufactured on the upper support substrate of the second embodiment and has a light receiving element function as well as structural reinforcement. The structure and manufacturing method of an optical waveguide are disclosed.

8 (a) to 8 (c) are cross-sectional views illustrating the structure of the thermal oxide Si core optical waveguide according to the fourth embodiment of the present invention. FIG. 8A is a cross-sectional view of the substrate perpendicular to the light propagation direction of the optical waveguide similar to the others, but FIGS. 8B and 8C are cross sections including the propagating light along the light propagation direction of the optical waveguide. It is a cross-sectional view of the substrate of.

In the fourth embodiment of FIG. 8A, the upper support substrate 49 in the second embodiment (FIG. 4) is replaced with the N-type Si substrate 101, and the implant region of the P-type Si is contained in the N-type Si substrate 101. 102 is formed to form a Si photodiode (PD). Propagation light can be received by the compositely integrated PD, and an electric signal can be detected from the PD electrode 105. The insulating layer 104 may be appropriately formed for electrical separation from the PD.

In FIG. 8B, the PD receives the propagating light indicated by the thick dotted arrow from the PD optical waveguide 103 that photocouples to the thermal oxidation core 93, and in the structure of FIG. 8C, the thermal oxidation core is received. A mirror structure 106 that bounces the propagating light is provided at the end of the 93, and the implant region 102 of the P-type Si directly receives the propagating light.

As in the third embodiment (FIGS. 6 and 7), the upper and lower Si wafers (101, 91) can be joined while being aligned by the alignment marker, and the optical wave guide and the light receiving element are compositely integrated. It is an embodiment.

According to the fourth embodiment, not only a wide band planar optical waveguide circuit of an ultraviolet light band to a near infrared light band can be realized, but also the planar optical waveguide circuit is structurally reinforced as in the second embodiment. Si light receiving elements with a sensitivity of about 190 to about 1100 nm can also be integrated.

At this time, the Si photodiode (PD) may be any of a pn junction type photodiode, a pin structure photodiode, and an avalanche amplification photodiode (avalanche photodiode), and an InP / InGaAs-based crystal is separately formed on the Si wafer surface. The compound semiconductor of the above may be sliced and joined to prepare a PD.

8 (a) and 8 (b) are schematic cross-sectional views perpendicular to and parallel to the propagating light of the optical waveguide core in the form in which the optical waveguide 103 for light reception is formed on the surface of the light receiving element. It is sectional drawing parallel to the propagating light of an optical waveguide core when the mirror structure 106 which bounces the propagating light is made on the optical waveguide core part. The dotted arrow in the optical waveguide core shows the propagation path image of the propagating light. In addition to the optical waveguide and the reflection mirror, there are various methods for guiding the propagating light of the optical waveguide core to the light receiving element, such as diffracted light by grating and radiation from the end of the optical waveguide.

(Production method of embodiment 4)
Hereinafter, the method for producing the fourth embodiment of the present invention will be described in detail with reference to FIG.

First, the steps of FIGS. 9 (a) to 9 (c) of the manufacturing method of the fourth embodiment can be manufactured by the same steps as those of FIGS. 3 (a) to 3 (c) of the first embodiment.

Next, in the step of FIG. 9D, the insulating layer 104 and the electrode wiring are separately formed on the front and back surfaces of the PN junction photodiode formed by forming the P-type layer 102 on the surface of the N-type Si wafer 101 by ion implantation. A wafer 101 on which the 105 is formed is prepared. In the case of the structure of FIG. 8B, the optical waveguide core layer 103 for PD is prepared on the surface of the P-type layer 102 with, for example, a UV curable resin. In the case of the structure of FIG. 8C, a mirror structure 106 that bounces propagating light is provided at the end of the thermal oxidation core 93.

After that, as in FIG. 7D of the third embodiment, the Si wafers are joined to each other using the material of the clad layer 94 while aligning the upper and lower Si wafers (101, 91).

Finally, the steps of FIGS. 9 (e) to 9 (g) can be produced by basically repeating the same steps as those of FIGS. 3 (e) to 3 (h) of the first embodiment.

(Production example and characteristic evaluation of the embodiment)
Hereinafter, production examples of the embodiments of the present invention and evaluation of their characteristics will be specifically described, but the present invention is not limited to these production examples.

(Production Example 1 of Embodiments 1 and 3)
(Si wafer production)
First, thermal oxidation of a thickness of 3 μm on the surface of a high-purity 4 inch bare Si wafer with a substrate thickness of 500 μm and a plane orientation (100) having an electrical resistance value of 1 kΩ or more, using a vertical thermal oxidation furnace of a hydrogen combustion steam oxidation method. A Si film (SiO ₂ film) was produced.

(Making alignment marker)
Next, a photolithography step, a parallel plate type reactive ion etching apparatus using a CF ₄ plasma, the alignment marker on the Si wafer surface was more prepared, the same on the back surface by photolithography process using a double-sided mask aligner The alignment marker was dry-etched to prepare the marker.

(Preparation of thermal oxide Si core layer)
Next, with reference to the alignment marker of the Si wafer surface, a thermally oxidized Si core the core width 3μm ridged, using a parallel plate reactive ion etching apparatus using the same photolithographic and CF ₄ plasma Made.

(Preparation of upper clad layer)
Next, the upper clad layer was prepared by a flame deposition method (FHD: Flame Hydrolysis Deposition). _{A glass raw material having a composition of SiO 2-} B ₂ O ₃ is deposited on a substrate by burning a weight ratio of SiCl ₄ to BCl ₄ of 1: 1 in hydrogen and oxygen, and heated to 1100 ° C. or higher to thermally oxidize SiO2. An upper clad layer _{of SiO 2-} B ₂ O ₃ having a lower refractive index than the film, being transparent, and having a thickness of about 50 μm was prepared.

(Back side processing)
Next, using the alignment marker on the back surface as a reference, the same pattern as the optical waveguide having a width of about 100 μm was produced on the back surface by a photolithography process. First, the thermal oxide Si layer on the back surface was dry-etched by a parallel plate type reactive ion etching apparatus using _{CF 4 plasma to expose the Si surface on the back surface.} Next, using a dry etching device by the Bosch process that alternately performs the etching step (SF ₆ plasma) and the passion step (C ₄ F ₈ plasma), the Si wafer is deeply dug to about 450 μm, and the remaining about 50 μm is used. Wet etching was performed using an aqueous solution of potassium hydroxide (KOH) to completely remove Si at the bottom of the optical waveguide core.

(Preparation of lower clad layer)
Subsequently, in order to prevent the inflow of the lower clad layer into the upper clad layer by the flame deposition method at the time of film formation, _{a magnetron sputtering apparatus using a mixed plasma of Ar and O 2} was used to open the opening on the back surface with a SiO of about 500 nm. _{A glass film of 2-} B2O ₃ was covered, and then a lower clad film of the same condition of the upper clad was deposited, and a lower clad layer was prepared by heating at a temperature lower than that of the upper clad at about 1000 ° C.

The optical waveguide structure similar to that of the first embodiment was manufactured by the manufacturing process process as described above.

(Chip processing and optical characterization)
Then, the chip was cut with a dicing saw, and the SC (Supercontinuum) light source and the light receiving element were provided to evaluate the propagation loss of the obtained optical waveguide chip. As a result, the light propagation loss at each light wavelength of 450 nm (blue), 520 nm (green), and 630 nm (red) was 0.3 dB / cm or less.

From the above, the feasibility of

Embodiments

1 and 3 was confirmed.

(Production Example 2 of Embodiments 2 and 4)
(Si wafer production)
_{First, by the same method as in Production Example 1, two} high-purity 4 inch Si wafers having a thickness of 3 μm, a thermally oxidized Si film (SiO 2 film), a substrate thickness of 500 μm having alignment markers on the front and back surfaces, and a plane orientation (100). Was produced. Next, on one Si wafer, an optical waveguide core having a ridge shape and a core width of 3 μm was produced in the same manner as in Production Example 1.

(Preparation of upper clad layer)
Next, the AB agent of fluorosilicone resin FER-7061 manufactured by Shin-Etsu Silicone Co., Ltd. was mixed, applied to the core surface by a spin coating method, and sandwiched with another uncore-processed Si wafer to form a sandwich structure at 150 ° C. × It was heat-cured in 3 hours.

(Back side processing)
Next, as in Production Example 1, the same pattern as the optical waveguide having a width of about 100 μm was produced on the back surface by a photolithography step with reference to the alignment marker on the back surface of the Si wafer having a core. First, the thermal oxide Si layer on the back surface was dry-etched by a parallel plate type reactive ion etching apparatus using _{CF 4 plasma to expose the Si surface on the back surface.} Next, using a dry etching device by the Bosch process that alternately performs the etching step (SF ₆ plasma) and the passion step (C ₄ F ₈ plasma), the Si wafer is deeply dug to about 450 μm, and the remaining about 50 μm is used. Wet etching was performed using an aqueous solution of potassium hydroxide (KOH) to completely remove Si at the bottom of the optical waveguide core.

(Preparation of lower clad layer)
Subsequently, the exposed opening on the lower surface of the core was filled with an AB agent of fluorosilicone resin FER-7061 manufactured by Shin-Etsu Silicone Co., Ltd., and this time, it was cured in a room temperature environment.

(Chip processing and optical characterization)
Then, the chip was cut with a dicing saw in the same manner as in Production Example 1, and the SC (SuperContinuum) light source and the light receiving element were provided to evaluate the propagation loss of the obtained optical waveguide chip. As a result, the light propagation loss at each light wavelength of 450 nm (blue), 520 nm (green), and 630 nm (red) was 0.3 dB / cm or less.

From the above, the feasibility of Embodiments 2 and 4 was confirmed.

As described above, the present invention realizes an embedded optical waveguide having excellent light transmission from the near infrared to the visible light band and the ultraviolet light band at low cost by a relatively simple method. According to the manufacturing method of the present invention, an optical waveguide element that combines and demultiplexes light over a wide band wavelength can be manufactured, and can be used in various industrial fields such as medical treatment, bioanalysis, and display devices such as image projection. Its industrial utility value is extremely high.

Claims

In the cross-sectional view of the substrate perpendicular to the light propagation direction of the optical waveguide,
A Si wafer as a substrate, a thermally oxidized Si core,
It has an upper clad layer and a lower clad layer that directly sandwich and support the thermally oxidized Si core vertically.
An embedded optical waveguide structure characterized in that the thermally oxidized Si core is located at a distance from the Si wafer serving as a substrate.
In the embedded optical waveguide structure according to claim 1,
An embedded optical waveguide structure further comprising a support substrate bonded to the upper surface of the upper clad layer.
The two-layer structure according to claim 1, wherein the two embedded optical waveguide structures are arranged back to back on the side of the upper clad layer with a spacer interposed therebetween and are aligned and joined. Embedded optical waveguide structure.
In the embedded optical waveguide structure according to claim 2.
The support substrate has a light-receiving element that photocouples to the optical waveguide, and has a composite type embedded optical waveguide structure that also has a light-receiving function as well as structural reinforcement.
A step of forming a thermal oxide Si layer on the upper surface of a Si wafer to be a substrate,
The step of etching the thermal oxide Si layer to form the thermal oxide Si core layer of the optical waveguide,
A step of forming an upper clad layer having a refractive index lower than that of the thermally oxidized Si core layer on the thermally oxidized Si core layer,
A step of partially etching and removing the Si wafer in a portion corresponding to the lower side of the thermal oxide Si core layer until the thermal oxide Si core layer is separated from the remaining portion of the Si wafer serving as a substrate.
A step of forming a lower clad layer having a low refractive index under the thermally oxidized Si core layer is provided.
An embedded optical waveguide structure characterized in that the thermally oxidized Si core is directly sandwiched and supported vertically by the upper clad layer and the lower clad layer to produce a structure located at a distance from the Si wafer as a substrate. Manufacturing method.
The method for producing an embedded optical waveguide structure according to claim 5, further comprising a step of joining a support substrate on the upper clad layer.
Two embedded optical waveguide structures were produced by the method for producing an embedded optical waveguide structure according to claim 5.
A step of arranging the two produced embedded optical waveguide structures back to back on the upper clad layer side with a spacer in between.
A method for producing an embedded optical waveguide structure having a two-layer structure, which further comprises a step of joining while aligning with a marker.
The method for manufacturing an embedded optical waveguide structure according to claim 6, further comprising a step of manufacturing a light receiving element that photocouples to the optical waveguide on the support substrate.