JP5362551B2 - Composite optical waveguide - Google Patents

Abstract

In a recessed section region on the side of one main surface of a silicon substrate (100) in a composite optical waveguide, a first region (141) and a second region (142), each of which contains silica as a main component, are arranged. A silicon thin line (300) composed of semiconductor is arranged to extend in proximity to the first region (141). In the recessed section region, the first region (141) relatively exists inside, and the second region (142) relatively exists outside. The refractive index of the first region (141) is higher than that of the second region (142), and the refractive index of the silicon thin line (300) is higher than that of the first region (141).

Description

  The present invention relates to an optical waveguide formed on a silicon substrate.

  Although an electronic circuit can be formed on a silicon single crystal substrate, if an optical integrated circuit can be further formed on the silicon single crystal substrate, the electronic circuit and the optical circuit can be monolithically integrated. it can. For this purpose, a silicon thin wire using a silicon-on-insulator (hereinafter referred to as “SOI”) substrate manufactured through a dielectric isolation layer of a silicon oxide film is used as an optical waveguide for near-infrared light. Research and development on laser oscillation and ultrafast optical modulators are very active. This is because it is considered that many advantages can be obtained compared to other compound semiconductor materials by using silicon, which is a semiconductor material most frequently used as an electronic integrated circuit, in an optical integrated circuit. .

For example, in Non-Patent Document 1, a silicon thin layer formed on a silicon single crystal substrate via a silicon oxide film layer (1 μm thick) is processed to have a cross-sectional area of 0.11 μm ² (= 0.48 μm × 0.22 μm). ) To produce a two-photon absorption and a few picoseconds by confining light at high density in the narrow-width silicon thin-film waveguide space made of single crystal silicon. It has been demonstrated that ultrafast optical modulation is possible. In this way, a great potential is expected for a device that processes an SOI substrate formed on a silicon substrate via a dielectric layer of a silicon oxide film and uses it for an optical waveguide.
JP 11-242125 A US Pat. No. 6,277,662 US Pat. No. 6,753,589 US Pat. No. 6,222,974 US Pat. No. 6,493,496 JP-A-57-41823 TK Liang et al., OpticsExpress 13, 7298-7303 (2005) Laser Focus World Japan 2005.12pp63-65 Laser Focus World Japan 2006.01pp52-54 K. Imai et al., IEEE Trans.Electron Devices ED-31 297-302 (1984) K. Imai, Solid State Electronics, 24, 159-164 (1981) K. Imai et al., J. Cryst. Growth, 63, 547-553 (1983) S. Nagata et al., Appl. Phys. Lett. 72, 2945-2947 (1998) S. Nagata et al., Appl. Phys. Lett. 82, 2559-2561 (2003)

However, it is an important practical issue that the optical coupling efficiency between the external optical circuit such as an optical fiber and the silicon thin wire waveguide is very small. The first reason optical coupling efficiency is small, (0.11 .mu.m 2 in the ^precedent) cross-sectional area of the silicon wire waveguide core diameter of itself is single-mode optical fiber (about 10 .mu.m, the area is about 78 .mu.m ²⁾ compared to the It is very small, and coupling with external light is made through the cross section of the silicon waveguide having a small cross section. The second reason is that the refractive index of silicon is as large as about 3.5, and there is a large difference from the refractive index of about 1.5 at the core portion of the silica optical fiber. Many methods have been studied to improve the small optical coupling efficiency (see, for example, Non-Patent Document 3).

  FIG. 22 is a diagram showing a basic configuration of a silicon fine wire optical waveguide manufactured on an SOI substrate reported in Non-Patent Documents 1 and 2. FIG. 4A shows a cross-sectional view of the optical waveguide. FIG. 4B shows the refractive index distribution along the line AA ′ in FIG. In FIG. 4B, the horizontal axis indicates the position in the thickness direction, and the vertical axis indicates the refractive index. In addition, the vertical axis in FIG. 4B indicates the downward direction in which the refractive index increases. In the configuration shown in this figure, a single-layer silica layer 170 is formed on one main surface of a silicon substrate 100 to form an SOI substrate, and a silicon fine wire 300 is formed on the silica layer 170. . The refractive index of the silicon fine wire 300 is larger than the refractive index of the silica layer 170. The oxide films 332 and 332 ′ shown in FIG. 5B are dense silica layers that are inevitably formed by oxidizing the surface of the silicon fine wire 300, and have a refractive index equal to the refractive index of the silica layer 170. Have Note that the reference numeral 350 shown in FIG. 3A includes the silicon thin wire 300 and the oxide films 332 and 332 'that are the silica layers around the silicon thin wire 300.

  Even if the silica layer 170 is a single layer, it has a function of dielectrically separating the silicon fine wire 300 from the silicon substrate 100. That is, when there is no need to guide light to the silica layer 170, the dielectric separation film between the silicon substrate 100 and the silicon fine wire 300 is formed by laminating a plurality of oxide films having different refractive indexes. There is no need for a structure having a waveguide function. When optical coupling between the silicon thin wire 300 having a very narrow cross-sectional area and an external light source is performed, the light that cannot enter the silicon thin wire 300 and flows into the silica layer 170 is diffused and propagated in the silica layer 170 to form a silicon substrate. There is a possibility of being absorbed by both 100 and silicon wire 300.

  However, since the area where the silicon substrate 100 is in contact with the silica layer 170 is overwhelmingly larger than the area where the silicon fine wire 300 is in contact with the silica layer 170, the light leaking into the silica layer 170 is substantially absorbed by the silicon fine wire 300. Instead, it is absorbed by the silicon substrate 100 side. Therefore, the optical coupling between the external light source and the silicon fine wire 300 is basically only through the very narrow cross section of the silicon fine wire 300.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a composite optical waveguide having excellent optical coupling efficiency with an external optical circuit.

The composite optical waveguide according to the present invention has a first region and a second region each containing silica as a main component in a recessed region on one main surface side of the silicon substrate, and extends close to the first region. A third region made of a semiconductor is provided, the first region relatively exists inside and the second region exists outside in the recessed region, and the refractive index of the first region is the refractive index of the second region. greater, the refractive index of the third region is formed rather greater than the refractive index of the first region, the first region is a core region becomes a waveguide second area is the cladding region, the third region, the first region The cross-sectional area is formed from a semiconductor having a relatively large refractive index compared to that of the waveguide. When external light is received by one end face of the waveguide formed by the first region and the second region, The light propagating in one region passes through the junction interface between the first region and the third region. Pair to the refractive index is larger cross-sectional area is optically coupled to the third region narrowing, the third region is a semiconductor wire waveguide.

In the composite optical waveguide according to the present invention, a fourth region is further provided between the first region and the third region, the refractive index of the fourth region is smaller than the refractive index of the first region, and the thickness of the fourth region is Is preferably smaller than the thickness of the second region. The fourth region is further provided between the first region and the third region, the refractive index of the fourth region is rather larger than the refractive index of the first region, than the refractive index of the fourth area is the refractive index of the third region It is also preferable that the thickness is set to be small and the thickness of the fourth region is smaller than the thickness of the second region.
It is also preferable that the first region and the second region are made of porous silica. A fourth region is further provided between the first region and the third region, the fourth region is formed by thermal oxidation of the semiconductor, the first region and the second region are made of porous silica, It is also preferable that the refractive index of the four regions is equal to or greater than the refractive index of the first region. Further, the third region may be crystalline, or the third region may be amorphous. Further, a fifth region mainly composed of silica is further provided between the first region and the second region, the refractive index of the fifth region is smaller than the refractive index of the first region, and the refractive index of the fifth region. Is preferably larger than the refractive index of the second region.

  In this composite optical waveguide, each of the first region, the second region, and the fifth region is obtained by selectively modifying a recessed region that was originally a part of the silicon substrate by anodic oxidation or the like. As a result of the modification, it may be a porous silicon region, a porous silica region, or a dense silica region. In addition, the refractive index of each of the first region, the second region, and the fifth region may be adjusted by adding an impurity, or the refractive index may be adjusted by adjusting the porosity or the porosity. May be.

  In this composite optical waveguide, the first region and the second region, each of which is mainly composed of silica and formed in the recessed region of the silicon substrate, function as an optical waveguide having the first region as a core and the second region as a cladding. obtain. The third region can also function as the core of the optical waveguide with the second region as the outer cladding and the first region as the inner cladding. The light incident on the first region is suppressed from being coupled to the silicon substrate by the barrier of the second region having a low refractive index, and can be strongly coupled to the third region having a high refractive index. The first region, the second region, and the fifth region can function as an optical waveguide having the first region as a core, the second region as an outer cladding, and the fifth region as an inner cladding. In this double clad waveguide structure, the second region, which is a low refractive index outer clad, functions as a barrier that prevents light from being coupled to the silicon substrate. Light that has entered a region having a wide cross-sectional area that includes the fifth region that is the inner cladding and the first region that is the core, has a large refractive index through a wide joint surface where the fifth region and the first region are joined. It is condensed in one area with high density. The area of the joint surface between the first region and the third region is small, but first, the light condensed at a high density by condensing the light at the first region is more than the third region having a high refractive index. It will be a strong bond.

  In this composite optical waveguide, the depth and width (for example, several tens of μm) that define the cross-sectional area of the first region can be made sufficiently larger than the core diameter of the single mode optical fiber. Moreover, since the material of the first region is basically silica, the refractive index of the first region is approximately the same as the refractive index of the silica optical fiber. Therefore, this composite optical waveguide can be easily optically coupled with an optical fiber as an external optical circuit.

  In this composite optical waveguide, the silica waveguide formed by the first region and the second region mainly composed of silica also forms a two-dimensional light confinement structure. As described above, the silica region is a multilayer, and a third region as a semiconductor thin wire waveguide is formed in the vicinity of the first region as the core portion of the silica waveguide having a two-dimensional optical confinement structure. Has been.

  The third region (semiconductor waveguide portion) is joined close to the first region (core portion) of the silica waveguide, and the refractive index of the third region (semiconductor waveguide portion) is the first region (silica core). Part), the optical coupling between the two is performed through the joint interface between the two. That is, optical coupling is performed through the entire area of the interface where both are bonded.

  When coupling externally introduced light to a semiconductor thin-line optical waveguide having a very narrow cross-sectional area, external light is once received by a silica waveguide having a sufficiently large cross-sectional area and a refractive index similar to that of silica fiber. , A new method for coupling light through the interface of the semiconductor waveguide close to the core of the silica waveguide has been proposed, and the effective optical coupling efficiency of external light to the semiconductor waveguide has been greatly increased and the coupling has jumped. It became easy.

It is a figure which shows the structure of the composite optical waveguide which concerns on this embodiment. It is a graph which shows the relationship between the refractive index of the high refractive index impurity doped in the silica, using the porosity of silica as a parameter. It is a figure which shows the other example of the refractive index distribution of the composite optical waveguide which concerns on this embodiment. It is a graph which shows the relationship between a porosity and a refractive index supposing two components of a silica and a void | hole. 3 is a chart summarizing oxidation characteristics of porous silicon and silicon fine wires. It is process drawing explaining the 1st method of manufacturing the compound-light waveguide concerning this embodiment. It is a figure which shows typically the optical characteristic of the composite optical waveguide manufactured by the 1st method. It is process drawing explaining the 2nd method of manufacturing the compound-light waveguide concerning this embodiment. It is process drawing explaining the 3rd method of manufacturing the compound-light waveguide based on this embodiment. It is process drawing explaining the 4th method of manufacturing the compound-light waveguide concerning this embodiment. It is a figure which shows the structure and the mask for anodization of the 1st modification of the compound-light waveguide based on this embodiment. It is a figure which shows the structure and the mask for anodization of the 2nd modification of the compound-light waveguide based on this embodiment. It is a figure which shows the structure of the 3rd modification of the compound-light waveguide based on this embodiment. It is a figure which shows the structure of the 4th modification of the composite optical waveguide which concerns on this embodiment. It is process drawing explaining the manufacturing method of an experiment example. It is a graph which shows the wavelength dependence of the extinction coefficient of light when light propagates the inside of silica or silicon. It is process drawing explaining the manufacturing method of the composite optical waveguide which uses aluminum nitride (AlN) as a semiconductor fine wire. It is process drawing explaining the manufacturing method of the composite optical waveguide which uses amorphous silicon (a-Si) as a semiconductor fine wire. It is a figure which shows the structure of the composite optical waveguide which concerns on other embodiment. It is process drawing explaining the 1st method of manufacturing the composite optical waveguide which concerns on other embodiment. It is a figure which shows the example of the refractive index distribution of the composite optical waveguide which concerns on other embodiment. It is a figure which shows the basic composition of the prior art example of the silicon | silicone thin wire | line optical waveguide produced on the SOI substrate.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Silicon substrate, 110 ... Carbon thin film, 112 ... Opening, 120 ... 5th area | region (porous silicon area | region), 121 ... 1st area | region (porous silicon area | region), 122 ... 2nd area | region (porous silicon area | region) 125, first region doped with metal impurities, 130, fifth region (dense silica region), 131, first region (dense silica region), 132, second region (dense silica region), 140, fifth. Region (porous silica region), 141 ... first region (porous silica region), 142 ... second region (porous silica region), 300 ... silicon fine wire (third region), 332 ... oxide film (fourth region) ), 350... Silicon thin wire waveguide, 500... N-type aluminum nitride region (third region), 600... Amorphous semiconductor region (third region).

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same or similar elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a diagram showing a configuration of a composite optical waveguide according to the present embodiment. FIG. 4A shows a plan view of the composite optical waveguide. FIG. 2B shows a cross-sectional view of the composite optical waveguide. FIG. 4C shows the refractive index distribution along the line AA ′ in FIG. In FIG. 3C, the horizontal axis indicates the position in the thickness direction, and the vertical axis indicates the refractive index. Also, in the vertical axis in FIG. 3C, the direction in which the refractive index increases is the downward direction. This is because when light propagates through a medium having a different refractive index, the light has a property along a medium having a large refractive index.

  In the composite optical waveguide shown in this figure, a first region 131 and a second region 132 each containing silica as a main component are formed in a recessed region on one main surface side of the silicon substrate 100. In the recessed region, the first region 131 that is the core region is present on the inner side, and the second region 132 that is the cladding region is present on the outer side. The refractive index of the first region 131 is larger than the refractive index of the second region 132. Therefore, the first region 131 and the second region 132, each of which is mainly formed of silica and formed in the recessed region of the silicon substrate 100, function as an optical waveguide having the first region 131 as a core and the second region 132 as a cladding. Can do.

  In addition, the composite optical waveguide according to the present embodiment is provided with a silicon fine wire 300 as a third region made of a semiconductor extending in the longitudinal direction in the vicinity of the first region 131 so as to be capable of optical coupling. The refractive index of the silicon fine wire 300 is larger than the refractive index of the first region 131. This silicon thin wire 300 can also function as the core of the optical waveguide.

  However, the silicon fine wire waveguide 350 has a single crystal silicon fine wire 300 and an oxide film 332 as a thin fourth region covering the outer peripheral portion thereof. A thin oxide film 333 is formed between the single crystal region of the silicon substrate 100 and the second region 132. The oxide films 332 and 333 inevitably cause the surface of the silicon fine wire 300 and the surface of the silicon substrate 100 to be oxidized by an oxidizing atmosphere when the porous region is oxidized to form the first region 131 and the second region 132. It is a dense silica layer that is formed.

  FIG. 4C shows the distribution of the refractive index in the thickness direction at a wavelength of 1.5 μm. Here, assuming that the main application of the composite optical waveguide according to the present embodiment will be the propagation and control of light in the optical communication wavelength band, a handbook of Optical Properties of Solids, Edited by Edward D. (Palik, Academic Press), and the refractive index at a wavelength of 1.5 μm was used.

  The silicon substrate 100 and the silicon fine wire 300 have a very large refractive index of 3.48. Of the oxide film formed by oxidation of the surface of the silicon fine wire 300, an oxide film 332 is formed on the side in contact with the first region 131, and an oxide film 332 ′ is formed on the surface side. An oxide film 333 is also formed at the interface between the silicon substrate 100 and the second region 132. These oxide films 332, 332 ′, and 333 are formed by thermally oxidizing the silicon substrate 100 and the silicon fine wire 300, and are basically dense oxide films with a refractive index of 1.444 at a wavelength of 1.5 μm. It is understood. Each of the first region 131 and the second region 132 is also densified silica, and an impurity for increasing the refractive index is added to the first region 131. The core region R1 is composed of the first region 131. The dense silica WG region R2 includes a first region 131, a second region 132, and an oxide film 333. The silica region R3 includes a dense silica WG region R2 and an oxide film 332.

  As shown in FIGS. 4A and 4B, a second region 132 that is a cladding region having a low refractive index exists on the substrate side of the first region 131 that is the core region of the silica waveguide. Light incident on the first region 131 having a relatively high refractive index is two-dimensionally confined by the barrier of the second region 132 having a lower refractive index. In addition, a barrier of the oxide film 332 exists also between the first region 131 and the silicon thin wire 300.

  However, if the second region 132 is sufficiently thick as will be described later, light leakage from the first region 131 to the silicon substrate 100 is prevented. On the other hand, when the thickness of the oxide film 332 that is a thermal oxide film is thin, light can leak through the barrier of the oxide film 332 and flow into the thin silicon wire 300. In this manner, optical coupling can be maintained between the first region 131 and the silicon fine wire 300 while preventing optical coupling between the light incident on the first region 131 and the silicon substrate 100.

  In another embodiment of the present invention, porous silica can be used for the first region (core region) and the second region (cladding region) instead of the densified silica. This will be described with reference to FIG. FIG. 2 is a graph showing the relationship between the refractive index and the high-refractive index impurity concentration doped in silica, using the porosity of silica as a parameter. In the figure, the refractive index of dense silica is 1.444, the refractive index of a single high-refractive index impurity is 2.0, and the refractive index of pores in porous silica is 1.0. It is.

Here, the additive rule, silica which are the components, a simple refractive index of the components of the high refractive index impurities and pores and n _i, the concentration of each component as c _i, the wavelength of the observation light In comparison, when each element is evenly mixed with very fine dimensions (several nanometer size in the case of porous silica used in the present embodiment), the total refractive index n is approximated by an approximate expression “n = Σn _i. c _i / Σc _i ”.

  In FIG. 2, the refractive index is calculated using the porosity of porous silica as a parameter and the impurity concentration as a variable. The refractive index of dense silica (point C) in which both the porosity and the impurity concentration are zero is 1.444. On the other hand, the refractive index of porous silica (point A) having a porosity of 15% and an impurity concentration of zero is 1.377. Further, the refractive index of porous silica (point B) having a porosity of 15% and a high refractive index impurity concentration of 8 mol% is calculated to be 1.422.

  FIG. 3 shows an example in which a relationship equivalent to the refractive index configuration in FIG. FIG. 3 is a diagram illustrating another example of the refractive index distribution of the composite optical waveguide according to the present embodiment. Between the first region 141, which is a core region composed of doped porous silica, and the silicon substrate 100, there is a second region 142, which is a cladding region composed of non-doped porous silica. The second region 142 prevents light incident on the first region 141 from leaking into the silicon substrate 100.

  On the other hand, an oxide film 332 as the fourth region is also formed around the silicon thin wire 300 as the third region according to the oxidation conditions. The oxide film 332 is basically a dense film and has a refractive index of 1.444. The refractive index of the oxide film 332 has a larger value than the refractive index of the first region 141. Therefore, even though the oxide film 332 exists between the first region 141 and the silicon thin wire 300, what is the barrier that the light incident on the first region 141 is coupled to the silicon thin wire 300 having a very high refractive index? Don't be. Thus, the light incident on the first region 141 is directly and strongly coupled to the silicon fine wire 300. As shown in FIG. 3, the OPSWG region R4 made of porous silica (Oxidized Porous Silicon (OPS)) includes a first region 141 and a second region 142. The silica region R5 includes an OPSWG region R4 and oxide films 332 and 333.

  Here, the relationship between FIG. 1C and FIG. 3 will be described. The refractive index magnitude relationship of the porous silica waveguide shown in FIG. 3 is created depending on whether or not the porous silica is doped with a high refractive index impurity. Each of the first region 141 and the second region 142 in FIG. 3 is shown by the relationship between the point B and the point A in FIG. When such a porous silica material is further oxidized under steam oxidation conditions of, for example, 1100 ° C. or higher, fluidity appears in the silica, voids are eliminated, and the porous silica is densified. When the porous silica is densified, point A moves to point C and point B moves to point D. The state in which the porous silica is densified in this way corresponds to FIG.

  Further, in the present embodiment, a refractive index difference is formed between the core region and the cladding region of silica by controlling the porosity of the porous silica without using a high refractive index impurity. The same refractive index relationship can be obtained. This will be described with reference to FIG. FIG. 4 is a graph showing the relationship between the porosity and the refractive index, assuming two components of silica and pores. Also here, the refractive index was calculated by the additive rule as described above.

  For example, when the porosity of porous silica is 15%, the refractive index is 1.377, which is sufficiently smaller than the refractive index 1.444 of a dense thermal oxide film. If the porosity of the porous silica constituting the second region 142 in FIG. 3 is set to 15%, for example, and the porosity of the porous silica constituting the first region 141 is set to be smaller than 15%, the first region The refractive index of 141 is larger than the refractive index of the second region 142 and smaller than the refractive index of the dense silica oxide film 332 formed on the surface of the silicon fine wire 300. In FIG. 4, a point P1 indicates a cladding, a point P2 indicates a thermal oxide film, and a range R6 indicates a range in which a core can be formed.

  Consider a composite optical waveguide in which the refractive index distribution as described above is built. The refractive index 1.444 of the dense oxide film 332 present on the surface of the silicon fine wire 300 is larger than the refractive index of the first region 141. Therefore, the oxide film 332, which is a dense silica layer, cannot serve as a barrier in which light incident on the first region 141 of the silica waveguide is directly coupled to the silicon fine wire 300 having a very high refractive index. On the other hand, since the refractive index of the second region 142 is smaller than the refractive index of the first region 141, the light confined in the first region 141 is sufficiently prevented from flowing into the silicon substrate 100.

  Here, an outline of the similarities and differences between the oxidation characteristics of porous silicon and silicon thin wires will be described. FIG. 5 is a chart summarizing the oxidation characteristics of porous silicon and silicon fine wires. In this figure, the outline of how the above-described porous silicon and the silicon fine wire produced in close contact with the porous silicon are oxidized by strengthening the oxidizing conditions of the atmosphere is shown in a general manner. .

  Porous silicon is produced by anodic oxidation in which a current is passed in a hydrofluoric acid solution with a silicon substrate as an anode. The properties of the porous silicon thus produced vary greatly depending on various parameters of anodization. Porosity and pore size are the main factors that separate the properties of porous silicon. Here, the porous silicon is present between a solid portion which is a thin pillar of silicon (hereinafter referred to as “silicon thin pillar”) having a crystallinity of several nanometers in diameter and between these silicon thin pillars. It is defined as the entire region including pores having approximately the same diameter. The porosity of the porous silicon is defined as the volume ratio of the pores in the unit volume of porous silicon.

  In the present embodiment, the inside of the porous silicon region manufactured from the p-type substrate used as the silicon substrate 100 has a diameter of several nanometers, but is from the surface of the porous silicon region to the bottom of the porous silicon region. It is composed of pores penetrating to the contact surface with the substrate, and thin silicon pillars (silicon thin pillars) holding a nano-sized single crystal atomic arrangement almost equal to the pores.

  The technologies described in Non-Patent Documents 7 and 8 utilize a fact that the interaction between nano-sized pores and metal organic molecules greatly depends on the size of the pores. Fabricate an optical waveguide with a core region with a large refractive index and a small cladding region by making use of the ability to selectively dope metal organic molecules into a specific porous silicon layer. To do.

When oxidizing porous silicon, oxygen in the oxidizing atmosphere is supplied deeply into the porous silicon region through the pores. As a result, oxidation proceeds from the entire surface of the silicon column. Nano-sized silicon thin pillars are oxidized from the entire surface of the thin pillars in an oxidizing atmosphere. In an oxidizing atmosphere of about 300 ° C., the monolayer on the outermost surface of the thin column is oxidized to become SiO ₂ .

When the oxidation temperature rises further, the oxidation proceeds from the entire surface of the silicon fine column to the inside of the fine column. When the temperature reaches about 800 ° C., the oxidation reaches the center of the silicon fine column, and the silicon atoms in the silicon fine column are All combine with oxygen to form SiO ₂ . The temperature at which the entire thin column is oxidized (total oxidation temperature) depends on the thickness of the thin column, and the total oxidation temperature increases as the diameter of the thin column increases. Even if all the silicon atoms in the porous silicon region are oxidized, the structure is porous, and this state is called Oxidized Porous Silicon (OPS).

When silicon is oxidized to SiO ₂ , the volume increases approximately 2.2 times. When porous silicon is oxidized to silica, the volume of the solid portion as SiO ₂ increases, and the solid fills part of the volume existing as pores in the porous silicon state. However, since there are still pores, it is said to be porous silica. Therefore, the ratio of the volume occupied by the pores in the porous silica formed by oxidation of the porous silicon (referred to as “porosity” in this specification) is smaller than the porosity in the porous silicon state. . However, since the porosity of the porous silica depends on the porosity of the porous silicon, when producing a multilayer porous silicon for an optical waveguide, by controlling the porosity and thickness of each layer, The porosity and thickness of the porous silica as the optical waveguide material can be adjusted.

  In order to densify the porous silica, the temperature may be heated to about 1100 ° C. or higher under oxidizing conditions including water vapor. It has already been demonstrated that the silica film densified in this way has properties similar to those of a silicon thermal oxide film. Thus, in the porous silicon, when the oxidation temperature is increased from 800 ° C., the porous silica maintains the porous state up to about 1100 ° C. When the oxidation temperature is about 1100 ° C. or higher and water vapor is added to the oxygen stream, fluidity appears in the porous silica, and the porous silica becomes dense silica.

  On the other hand, the silicon fine wire 300 as the third region is a thin silicon wire having a small cross-sectional area used as the core of the optical waveguide. The size of the silicon wire 300 varies depending on the usage, but one side is 0. It is about several μm to several μm. Nevertheless, the micron-sized silicon fine wire is about two to three orders of magnitude larger than the nano-size of the porous silicon thin column described above.

A silicon oxide film SiO ₂ usually used in a device or the like has a submicron thickness. It is considered that the oxidation of micron-sized silicon thin pillars is treated in the same way as the oxidation of silicon single crystal. When the temperature is about 300 ° C., the monoatomic layer on the outermost surface of the silicon fine wire is oxidized. When the temperature reaches about 800 ° C., the film grows to a film thickness (up to several nm) according to the oxidation conditions. When the temperature is further raised, detailed data is accumulated as the basis of the silicon integrated circuit technology, and the oxide film thickness increases as the oxidizing atmosphere becomes stronger. This oxide film is basically a dense oxide film.

  The fine silicon wires remaining on the porous silicon are oxidized not only from the main surface surface side of the silicon substrate but also from the porous silicon side supporting the fine silicon wires, and the oxide films 332 and 332 ′ shown in FIGS. Is formed. The oxide film 332 ′ may be removed in a later process or the like, but when the oxide film 332 is removed, the silicon fine wire 300 is also lifted off. In the present embodiment, the fact that the silicon fine wire 300 is close to the first region 131 or the first region 141 that is the core region of the silica waveguide recognizes the interposition of the oxide film 332 in which the silicon fine wire material is oxidized. is there.

  The basic configuration of the embodiment of the composite optical waveguide according to the present invention has been described above. However, the present invention can be variously modified. Further, the composite optical waveguide according to the present embodiment can be manufactured by various methods. Hereinafter, a method for manufacturing the composite optical waveguide according to the present embodiment (or the modification) will be described, and the configuration of the modification will also be described.

  A first method for manufacturing the composite optical waveguide according to the present embodiment will be described. FIG. 6 is a process diagram illustrating a first method for manufacturing the composite optical waveguide according to the present embodiment.

  In the first manufacturing method, first, a carbon thin film 110 having a thickness of about 100 nm is formed on one main surface of the p-type silicon substrate 100, and a resist having a thickness of about 1.5 μm is applied thereon. Then, a resist pattern is formed by exposure using a photomask, a desired opening 112 is formed in the carbon thin film 110 by reactive ion etching (RIE) mainly composed of oxygen, and then the resist is removed. (Figure (a)). The carbon thin film 110 in which the opening 112 is formed is used as a mask layer used in selective anodic oxidation.

  At this time, the etching rate with respect to oxygen plasma is higher in the resist than the carbon thin film 110, but since the carbon thin film 110 is thin, the carbon thin film 110 can be patterned without any problem. As for the relationship between the width w of the opening 112 and the width d of the carbon thin film 113 between the two openings 112, a predetermined combination dimension is used.

  Subsequently, a porous silicon region is selectively formed. That is, p-type silicon obtained by patterning the carbon thin film 110 using a pulse current formation method in which the formation current is increased in proportion to the area of the interface between the porous silicon and the silicon crystal as the porous silicon region grows. Porous silicon having a depth of about 15 μm is formed by using a substrate 100 as an anode and an opposing platinum electrode as a cathode, holding a first predetermined concentration of hydrofluoric acid solution between the two electrodes, and anodizing (forming) the silicon substrate 100. The first region 121 is formed. Next, the hydrofluoric acid solution concentration is changed to the second predetermined concentration, and the second region 122 of porous silicon having a thickness of about 5 μm is formed using the second interface current density (FIG. 5B). In this step, if each of the hydrofluoric acid concentration, the formation current density, and the formation depth is controlled to predetermined values, the silicon thin film 113 between the two openings 112 is formed at a substantially central portion by a silicon single crystal. The thin silicon wire 300 can be left.

  Subsequently, metal organic compound molecules (Ti-MO) of titanium are selectively doped. That is, the silicon substrate 100 on which the first region 121 and the second region 122 are formed by the above process is immersed in a Ti-MO-containing solution, and the first region 121 is selectively doped with Ti-MO molecules. As a result, the first region 121 becomes a region 125 in which the organometallic molecules of titanium are selectively doped, but the second region 122 can be controlled with almost no titanium doping (FIG. 5C).

  Subsequently, the silicon substrate 100 is oxidized. That is, the silicon substrate 100 selectively doped with titanium by the above process is heat-treated in an oxygen stream at a temperature of 300 ° C. for 1 hour, thereby removing the organic component of Ti-MO from the porous silicon. At the same time, the porous silicon structure is stabilized by forming an oxide layer of a monoatomic layer on the surface of the silicon fine column inside the porous silicon. Next, the thin silicon pillars in each of the region 125 and the second region 122 are completely oxidized to the inside of the thin pillars by an oxidation treatment at a temperature of 800 ° C. By this step, the porous silicon region is oxidized to become porous silica. That is, the porous silicon region 125 becomes a porous silica first region 141 (core region), and the porous silicon second region 122 becomes a porous silica second region 142 (cladding region) (FIG. d)).

  This state is a state of a composite optical waveguide using the porous silica waveguide shown in FIG. By this oxidation step, the dense oxide film 332 described with reference to FIG. 5 having a thickness corresponding to the oxidation condition is formed on all surfaces of the silicon fine wire 300. In this specification, as shown in FIGS. 1 and 3, a region where the silicon fine wire 300 and the oxide film 332 around the fine wire are combined is shown as a silicon fine wire waveguide 350. Further, although the oxide film 333 shown in FIG. 3 is also formed at the interface between the second regions 132 and 142 and the silicon substrate 100, this oxide film is not shown in the present embodiment.

  After that, oxidation is performed for 1 hour at a temperature of 1200 ° C. in an oxygen stream added with water vapor to densify the porous silica, the first region 131 having a relatively large refractive index, and the refractive index is relatively small. A second region 132 is formed. By this oxidation process, the dense oxide film 332 described with reference to FIG. 1 having a thickness corresponding to stronger oxidation conditions is formed on all surfaces of the silicon fine wire 300.

  A composite optical waveguide was actually manufactured by the first method described so far, and the optical characteristics of the composite optical waveguide were examined. FIG. 7 is a diagram schematically showing the optical characteristics of the composite optical waveguide manufactured by the first method. FIG. 4A shows a plan view of the composite optical waveguide. FIG. 2B shows a cross-sectional view of the composite optical waveguide.

  FIG. 6C schematically shows an image observed with an optical microscope using both epi-illumination and transmission illumination on a composite optical waveguide cut to a length of 15 mm. The second region 132 appears dark, which indicates that the transmitted illumination light is not transmitted through this portion. Of the first region 131, the region 131 ′ remote from the silicon fine wire waveguide 350 looks bright, which indicates that transmitted illumination light is transmitted through this portion. On the other hand, in the first region 131, the peripheral region 150 of the silicon fine wire waveguide 350 was observed dark because the transmitted light intensity decreased.

  FIG. 4D shows a near field pattern when helium neon laser light having a wavelength of 633 nm is transmitted through the composite optical waveguide having a length of 15 mm. Each of the silicon substrate 100 and the second region 132 did not transmit the laser beam and was observed dark. Of the first region 131, the region 131 ′ far from the silicon wire waveguide 350 was observed brightly. On the other hand, the peripheral region 150 ′ of the silicon thin wire waveguide 350 in the first region 131 was observed dark although it was in the first region 131. The region observed dark in the first region 131 was similarly observed both in the case of white visible light transmission illumination of a microscope and in the observation of helium neon laser light.

  FIG. 5 (e) shows a first example in which the above-described composite optical waveguide having a length of 15 mm is etched with buffered hydrofluoric acid to remove the oxide film on the surface of the silicon substrate 100 and to form a dense silica waveguide. The surface of each of the region 131 and the second region 132 is etched, and a sample from which the silicon thin wire waveguide 350 is removed by lift-off is produced. The result of microscopic observation using both epi-illumination and transmission illumination is shown for this sample. The peripheral areas 150 and 150 ′ that appeared dark in FIGS. 3C and 3D disappeared, and the entire first area 131 was observed brightly.

  The above observation results are interpreted as follows. The white light of the microscope's visible transmission illumination and the helium neon laser light having a wavelength of 633 nm have a photon energy larger than the band gap energy of 1.1 eV of single crystal silicon. While such short-wavelength light propagates through a long waveguide having an optical path of 15 mm, the light leaks through the relatively thin oxide film 332 and is coupled to the silicon fine wire 300 and flows into the silicon fine wire 300 and is absorbed. Show. That is, as shown in FIG. 1C, if the thickness of the oxide film 332 is relatively small compared to the thickness of the second region 132, light having a larger photon energy than the bandgap energy of silicon can be obtained. It is interpreted that this indicates that the barrier was leaked and combined with the silicon fine wire 300 to absorb it. According to a technical book, the thickness of a dense thermal oxide film formed on the silicon surface by treatment at a temperature of 1200 ° C. for 1 hour with steam humidification is estimated to be about 0.8 μm. In the case of this example, the thickness of the oxide film 332 formed on the surface of the silicon fine wire is considered to be approximately the same.

  Next, a second method for manufacturing the composite optical waveguide according to this embodiment will be described. FIG. 8 is a process diagram illustrating a second method of manufacturing the composite optical waveguide according to the present embodiment.

  In the second manufacturing method, first, an n − type silicon layer 210 having a thickness of about 1 μm is epitaxially grown on one main surface of the p − type silicon substrate 100. Next, two parallel openings 212 having a width of 30 μm are formed in the n− type silicon layer 210 by photolithography, and a thin and narrow n− type silicon thin line region having a width of about 4 μm is provided between the two openings 212. 210 'is formed ((a) of the figure).

  Subsequently, a carbon thin film 110 having a thickness of about 100 nm is stacked on the silicon substrate 100 subjected to the above-described treatment, and openings 212 having a width of about 4 μm are formed on both sides of the thin line region 210 ′ by photolithography. This carbon thin film 110 is used as a mask layer used in selective anodic oxidation (FIG. 5B).

  Subsequently, similarly to the first manufacturing method, a first region 121 and a second region 122 of porous silicon having a two-layer structure are formed by anodic oxidation, and titanium organometallic compound molecules are selectively transferred to the first region 121. Doping is performed ((c) in the figure). What is important here is that holes are required as valence electrons when porous silicon is formed by anodic oxidation. Many holes exist as valence electrons in the p-type region, but no holes exist in the n-type region. Due to this characteristic, the p-type region is selectively made into porous silicon, but the n-type region retains its characteristic without being anodized. Due to this selectivity, the thin line region 210 ′ remains as a silicon crystal.

  Then, the silicon substrate 100 having been subjected to the above treatment is treated at a temperature of 300 ° C. for 1 hour in an oxygen stream, and then subjected to an oxidation treatment at a temperature of 900 ° C. for 1 hour, thereby oxidizing the porous silicon. It is converted to a porous silica ((d) in the figure). In this way, the first region 141 of doped porous silica and the second region 142 of undoped porous silica are formed, and the independent silicon fine wire waveguide 350 is formed into the first region of doped porous silica. It can be manufactured in the vicinity of the region 141.

  In this manufacturing method, if the remaining n − type silicon layer 210 remains or contacts the surface of the first region 141 or 142 of the silica waveguide, the light in the silica waveguide is converted into the n − type silicon layer 210. Inconvenience of direct coupling with However, in this manufacturing method, the n − type silicon layer 210 excluding the silicon fine wire waveguide 350 functioning as an optical waveguide is prevented from remaining on or contacting the surface of the first regions 141 and 142 of the silica waveguide. can do. The carbon thin film 110 functioning as a mask layer for selective anodic oxidation is spontaneously extinguished by combustion due to an oxidation treatment at a temperature of 900 ° C. Also by this second manufacturing method, it is possible to manufacture a composite optical waveguide having a refractive index configuration as illustrated in FIG.

  Next, a third method for manufacturing the composite optical waveguide according to the present embodiment will be described. FIG. 9 is a process diagram illustrating a third method of manufacturing the composite optical waveguide according to the present embodiment.

  In the third manufacturing method, first, an n − type silicon layer 210 having a thickness of about 1 μm is epitaxially grown on one main surface of the p − type silicon substrate 100. Next, by ion implantation, a partial region 220 of the n− type silicon layer 210 is selectively converted into a p− type, and an n− that is elongated between the two p− type regions 220 in a direction perpendicular to the paper surface. The fine wire region 210 'of the mold is left ((a) in the figure).

  Subsequently, a carbon thin film 110 having a thickness of about 100 nm is laminated on the main surface of the silicon substrate 100, and then openings 112 having a width of about 4 μm are formed on both sides of the thin wire region 210 ′ by photolithography. Then, this carbon thin film 110 is used as a mask layer used in selective anodic oxidation ((b) in the figure).

  Subsequently, similarly to the first manufacturing method, a first region 121 and a second region 122 of porous silicon having a two-layer structure are formed by anodic oxidation, and titanium organometallic compound molecules are selectively transferred to the first region 121. Doping is performed ((c) in the figure). What is important here is that the width 228 of the two p-type regions 220 is set larger than the width 227 of the second region 122 made of porous silica. By doing so, the region 220 converted to the p-type is porous siliconized in a self-aligning manner with the progress of anodization. As a result, the n − -type silicon layer 210 does not remain on the first region 121 and the second region 122 of porous silicon.

  Then, the silicon substrate 100 having been subjected to the above treatment is treated at a temperature of 300 ° C. for 1 hour in an oxygen stream, and then oxidized at a temperature of 900 ° C. for 1 hour to oxidize porous silicon. Then, it is converted into porous silica ((d) in the figure). In this way, a first region 141 of doped porous silica and a second region 142 of undoped porous silica are formed, and a first region of porous silica doped with an independent silicon wire waveguide 350 is formed. It can be made close to 141.

  Further, the n − -type silicon layer 210 which is an epi region excluding the silicon fine wire waveguide 350 functioning as an optical waveguide can be prevented from covering or contacting the surface of the first regions 141 and 142 of the silica waveguide. . In this manufacturing method, if the n− type silicon layer 210 covers or contacts the surfaces of the first regions 141 and 142 of the silica waveguide, the light in the silica waveguide is directly coupled to the n− type silicon layer 210. To cause inconvenience. Therefore, in this manufacturing method, it is desirable to set the width of the p-type region 220 so that the remaining p-type region 220 remains slightly on both sides of the second region 142 of the second porous silica. . The carbon thin film 110 functioning as a mask layer for selective anodic oxidation is spontaneously extinguished by combustion due to an oxidation treatment at a temperature of 900 ° C.

  Next, a fourth method for manufacturing the composite optical waveguide according to the present embodiment will be described. FIG. 10 is a process diagram illustrating a fourth method for manufacturing the composite optical waveguide according to the present embodiment.

  In the fourth manufacturing method, first, a resist mask is formed on one main surface of the p-type silicon substrate 100, and a part of the region 221 on the main surface of the substrate 100 is n-typed by proton ion implantation. (FIG. 2A). Thereafter, in the same manner as in the manufacturing method described above, a carbon thin film 110 having a thickness of about 100 nm is laminated on the main surface of the silicon substrate 100, and then an opening 112 having a width of about 4 μm is formed on the carbon thin film 110 by n − The carbon thin film 110 is formed on both sides of the mold region 221 and used as a mask layer for selective anodic oxidation (FIG. 5B).

  Subsequently, similarly to the first manufacturing method, a first region 121 and a second region 122 of porous silicon having a two-layer structure are formed by anodic oxidation, and titanium organometallic compound molecules are selectively transferred to the first region 121. Doping is performed ((c) in the figure). By treating the silicon substrate 100 that has undergone this treatment in an oxidizing atmosphere at a temperature of 900 ° C., the porous silicon is converted into porous silica. In this way, the second region 142 and the first region 141 of porous silica can be formed, and a composite optical waveguide having the silicon fine wire waveguide 350 close to the first region 141 can be formed (FIG. d)).

  As described above, the region 221 that has been n-typed by ion implantation is left as a crystal, and the silicon fine wire waveguide 350 can be formed on the first region 141. Note that the n-type region by proton ion implantation returns to the p-type again after a heat treatment at a temperature of 800 ° C. when oxidizing the porous silicon. If it is desirable to change the silicon wire waveguide 350 to n-type, a group V donor impurity may be ion-implanted.

  Next, the configuration of the first modification of the composite optical waveguide according to the present embodiment will be described, and a method for manufacturing the composite optical waveguide will be described. The composite optical waveguide manufacturing method of the first modification is substantially the same as the first manufacturing method described above, but differs in the shape of the carbon thin film 110 used as a mask layer during selective anodic oxidation. Therefore, the composite optical waveguide of the first modification has a configuration corresponding to this mask shape.

  FIG. 11 is a diagram showing a configuration of a first modification of the composite optical waveguide according to this embodiment and an anodizing mask. FIG. 5A is a plan view showing a pattern of the carbon thin film 110 used as a mask layer during selective anodic oxidation during the manufacturing process of the composite optical waveguide. In FIG. 6A, a carbon thin film 110 formed as a mask layer on the main surface of the p-type silicon substrate 100 is shown as a white region, and an opening 112 provided in the carbon thin film 110 (that is, The region where the p-type silicon substrate 100 is exposed) is shown as a cross-hatched region. As shown in this figure, one opening 112 is provided with a constant width and extending in a predetermined direction. On the other hand, the other opening 112 has a portion whose width gradually increases along the predetermined direction.

  Using the carbon thin film 110 having such an opening 112 formed, selective anodic oxidation is performed in the same manner as in the first manufacturing method described above. Also thereafter, the respective steps of impurity selective doping, pre-oxidation, oxidation and densification treatment are performed in the same manner as in the first manufacturing method. FIG. 2B is a plan view of the composite optical waveguide manufactured as described above, and FIG. 2C is a cross-sectional view of the composite optical waveguide.

  In the composite optical waveguide manufactured as described above, the silicon fine wire waveguide 350 is provided with a constant width and extending in a predetermined direction. On the other hand, the first region 131 has a portion whose width gradually increases along the predetermined direction. That is, the vicinity of the silicon fine wire waveguide 350 can be used as the main waveguide 135, and the waveguide portion away from the main waveguide 135 can be used as the side waveguide 136. For example, in the vicinity of the main waveguide 135, signal light to be propagated through the silicon thin wire waveguide 350 is input, and in the side waveguide 136, excitation light for amplifying the signal light and control light for controlling the signal light are separately input. It becomes possible to do.

  Next, the configuration of the second modification of the composite optical waveguide according to the present embodiment will be described, and a method for manufacturing the composite optical waveguide will be described. The method of manufacturing the composite optical waveguide of the second modification is substantially the same as the first manufacturing method described above, but differs in the shape of the carbon thin film 110 used as a mask layer during selective anodic oxidation. Therefore, the composite optical waveguide of the second modification has a configuration corresponding to this mask shape.

  FIG. 12 is a diagram showing a configuration of a second modification of the composite optical waveguide according to the present embodiment and an anodic oxidation mask. FIG. 5A is a plan view showing a pattern of the carbon thin film 110 used as a mask layer during selective anodic oxidation during the manufacturing process of the composite optical waveguide. In FIG. 6A, a carbon thin film 110 formed as a mask layer on the main surface of the p-type silicon substrate 100 is shown as a white region, and an opening 112 provided in the carbon thin film 110 (that is, The region where the p-type silicon substrate 100 is exposed) is shown as a cross-hatched region. As shown in this figure, one opening 112 is provided with a constant width and extending in a predetermined direction. On the other hand, the other opening 112 has a portion provided in a predetermined width and extending in a predetermined direction, and a portion provided extending in an oblique direction from the middle. .

  Using the carbon thin film 110 having such an opening 112 formed, selective anodic oxidation is performed in the same manner as in the first manufacturing method described above. Also thereafter, the respective steps of impurity selective doping, pre-oxidation, oxidation and densification treatment are performed in the same manner as in the first manufacturing method. FIG. 2B is a plan view of the composite optical waveguide manufactured as described above, and FIG. 2C is a cross-sectional view of the composite optical waveguide.

  In the composite optical waveguide manufactured as described above, the silicon fine wire waveguide 350 is provided with a constant width and extending in a predetermined direction. On the other hand, the first region 131 has a portion provided in a predetermined width and extending in a predetermined direction, and a portion provided extending in an oblique direction from the middle thereof. That is, the waveguide portion along the extending direction of the silicon thin wire waveguide 350 can be the main waveguide 135, and the waveguide portion branched from the main waveguide 135 can be the branch waveguide 137. For example, signal light L1 can be input / output to the main waveguide 135, and light L2 such as excitation light and control light can be input to the branch waveguide 137 in a form separated from the main waveguide.

  Next, the configuration of the third modification of the composite optical waveguide according to the present embodiment will be described. The composite optical waveguide manufacturing method of the third modification is substantially the same as the first manufacturing method described above, but differs in the shape of the carbon thin film 110 used as a mask layer during selective anodic oxidation. Therefore, the composite optical waveguide of the third modification has a configuration corresponding to this mask shape. The composite optical waveguide of the third modified example is also different in terms of the pattern of the silicon fine wire waveguide 350.

  FIG. 13 is a diagram illustrating a configuration of a third modification of the composite optical waveguide according to the present embodiment. FIG. 4A is a plan view of the composite optical waveguide, and FIG. 4B is a cross-sectional view taken along the line BB ′ shown in the plan view. As shown in this figure, the silicon thin wire waveguide 350 indicated by a thick black line extends in a spiral shape on the first region 141 surrounded by the second region 142, and both ends thereof are on the end surface of the silicon substrate 100. Has reached. In the composite optical waveguide having such a configuration, a silicon fine line pattern is formed by an n-type silicon layer 210 formed on a p-type silicon substrate 100, and then a selective anodic oxidation mask pattern is appropriately formed. If the process is advanced to, it can be manufactured.

  In the composite optical waveguide having such a configuration, the input terminal and the output terminal of the signal light L1 can be separated from each other, and the light L2 such as control light and excitation light can be input from another part. The input light L2 such as control light and excitation light is confined two-dimensionally in the first region 141 having a large refractive index. While propagating through the first region 141, these lights are strongly coupled to the silicon wire waveguide 350 as shown in FIG.

  Next, the configuration of the fourth modification of the composite optical waveguide according to the present embodiment will be described. The composite optical waveguide of the fourth modified example has a configuration substantially similar to that of the third modified example described above, but differs in the shape of each of the first region 141 (core region) and the second region 142 (cladding region). To do.

  FIG. 14 is a diagram illustrating a configuration of a fourth modification of the composite optical waveguide according to the present embodiment. FIG. 4A is a plan view of the composite optical waveguide, and FIG. 4B is a cross-sectional view along CC ′ shown in the plan view. As shown in this figure, the silicon thin wire waveguide 350 indicated by a thick black line extends in a spiral shape on the first region 141 surrounded by the second region 142, and both ends thereof are on the end surface of the silicon substrate 100. Has reached. The first region 141 is divided into a central portion where the spiral portion of the silicon fine wire waveguide 350 is formed and an introduction portion where portions connecting to both end faces of the silicon fine wire waveguide 350 are formed. The silicon substrate 100 is separated. That is, a region 400 surrounded by a dotted line on the right side in the drawing is constituted by the silicon substrate 100 and the second region 142 of silica having a low refractive index. There are similar components on the left side of the figure.

  In the composite optical waveguide having such a configuration, the input terminal and the output terminal of the signal light L1 can be separated from each other, and the light L2 such as control light and excitation light can be input from another part. In addition, the input / output terminal of the signal light L1 and the input end of the light L2 such as control light and excitation light can be separated by the region 400. The region 400 has a structure in which a part of the silicon substrate 100 having a very high refractive index reaches the main surface, and the surface is surrounded by a second region 142 having a low refractive index. Such a structure functions as a strong light shield. When light propagating through the first region 141 exceeds the barrier of the second region 142, it is strongly absorbed by the silicon substrate 100 having an extremely high refractive index. It is expected to function as a light earth.

  Next, an experimental example manufacturing method to be compared with the composite optical waveguide manufacturing method according to the present embodiment will be described. FIG. 15 is a process diagram illustrating the manufacturing method of the experimental example. The manufacturing method of the experimental example shown in this figure is different from the second manufacturing method shown in FIG. 8 in that the carbon thin film 110 as a mask layer for selective anodic oxidation is not used.

  In the manufacturing method of the experimental example, first, an n-type silicon layer 210 is deposited on one main surface of the p-type silicon substrate 100 to form two openings 212 extending in directions parallel to each other and independently. A thin silicon wire region 210 'is formed (FIG. 2A). After that, a porous silicon first region 121 and a second region 122 are formed by anodic oxidation without using a carbon film as a mask layer for selective anodic oxidation (FIG. 5B).

  It should be noted that in the case of this experimental example in which the carbon thin film 110 which is a mask layer for selective anodic oxidation is not used, an n − type silicon layer is formed on the surface of the first region 121 and the second region 122 of porous silicon. 210 remains. When the device is completed after the subsequent heat treatment step or the like, it is preferable that the region 215 in FIG. If the silicon region corresponding to the region 215 in FIG. 5C remains, the light confined in the first region 141 of the silica waveguide is not only the original silicon wire waveguide 350. The region 215 is also strongly coupled. Therefore, it is preferable that the region 215 is completely removed from at least the upper portion of the first region 141.

  Compared with the manufacturing method of this experimental example, in the manufacturing method of the composite optical waveguide according to this embodiment, the carbon thin film 110 is used as a mask layer for selective anodic oxidation, so that the region is formed above the first region 141. Since 215 is removed, the light confined in the first region 141 of the silica waveguide can be strongly coupled only with the silicon wire waveguide 350.

  So far, the case where silicon is used as the material of the third region (semiconductor wire) of the composite optical waveguide according to the present embodiment has been described. However, in the present invention, the material suitable for the semiconductor fine wire is not limited to silicon. The usefulness of other semiconductor materials will be described below.

FIG. 16 is a graph showing the wavelength dependence of the extinction coefficient of light when light propagates inside silica or silicon. In the figure, of the wavelength dependence of the extinction coefficient due to interband absorption, that of silica is indicated by a solid line 370 and that of silicon is indicated by a dotted line 372. The extinction coefficient of silica due to infrared absorption is indicated by a solid line 371. As shown in this figure, the wavelength range of light (SiO ₂ transparent region R7) in which the silica material is transparent is a wide range from a far ultraviolet region of about 0.16 μm to a near infrared region of about 4 μm. On the other hand, the wavelength range in which silicon is transparent is a wavelength range longer than 1.1 μm having a photon energy smaller than the band gap energy of silicon.

  When a combination of silica and silicon is used as the material of the composite optical waveguide according to the present embodiment, the wavelength range in which the composite optical waveguide can propagate is a wavelength range in which both silica and silicon are transparent (both materials transparent region R8). ). That is, the wavelength range is about 1.1 μm to about 4 μm, and is limited to a relatively narrow wavelength range in the infrared region.

  In this embodiment, as described below, the wavelength range that can be used in the composite optical waveguide is changed from the visible region to the ultraviolet region according to the characteristics of the semiconductor material to be combined by changing the semiconductor material of the third region as necessary. It can be expanded to the area.

A semiconductor material having a large band gap energy (for example, aluminum nitride (AlN), gallium nitride (GaN), mixed crystal of Al _x Ga _1-x N, silicon carbide (SiC), etc.) is formed on a silicon substrate. Epitaxial growth is possible. Further, selective epitaxial growth is possible, and the single crystal epitaxial region is left only in a desired portion on the silicon substrate, and the film of other undesired portions can be removed. In addition, these materials can control the conductivity by doping, and the epitaxial film can be made n-type. Furthermore, these materials are chemically very stable and can withstand an anodizing step of forming porous silicon using high-concentration hydrofluoric acid. Moreover, it is excellent in resistance to an oxidizing atmosphere, and the surface layer is only oxidized in the process of oxidizing porous silicon to silica.

  FIG. 17 is a process diagram for explaining a method of manufacturing a composite optical waveguide using aluminum nitride (AlN) as a semiconductor fine wire. First, an n-type aluminum nitride region 500 is formed at a desired position on the p-type silicon substrate 100 (FIG. 1A). Subsequently, a carbon thin film 110 having an opening 112 is formed as a mask layer used in selective anodic oxidation ((b) in the figure). Subsequently, the first region 121 and the second region 122 of two layers of porous silicon are formed in the silicon substrate 100 as in the second manufacturing method described above ((c) in the figure). Then, by oxidizing the silicon substrate 100, the porous silicon first region 121 becomes the silica first region 131 (core region), and the porous silicon second region 122 becomes the silica second region 132. As a (cladding region), a silica waveguide composed of the first region 131 and the second region 132 is formed, and a composite optical waveguide having an n-type aluminum nitride region 500 close to the silica waveguide can be formed. ((D) in the figure).

  Aluminum nitride has a large band gap energy (Eg) of 5.1 eV and is a direct transition semiconductor. Therefore, it is transparent to light having a wavelength longer than that of light having a wavelength of 0.24 μm (ultraviolet region) corresponding to Eg. is there. Further, the refractive index is as large as about 2.1 or more even in the near-infrared region, and has excellent optical characteristics. Accordingly, the refractive index 2.1 is not as large as the silicon refractive index of 3.48 described above, but the wavelength of light that can be handled is applicable up to the ultraviolet region, and a composite optical waveguide having a large refractive index region can be formed. .

  FIG. 18 is a process diagram for explaining a method of manufacturing a composite optical waveguide using amorphous silicon (a-Si), which is a typical material of an amorphous semiconductor, as a semiconductor fine wire. First, a carbon thin film 110 having an opening 112 is formed by photoetching as a mask layer used for selective anodic oxidation on a p-type silicon substrate 100 (FIG. 1A). Subsequently, the first region 121 and the second region 122 of porous silicon are formed by anodic oxidation (FIG. 5B). Thereafter, the first region 121 is doped with a high refractive index impurity material, and porous silicon is oxidized and densified to form a silica waveguide having a dense first region 131 and a second region 132 of silica. (FIG. (C)). An amorphous silicon (a-Si) layer 650 is deposited on the silicon substrate 100 on which the silica waveguide is formed in this way (FIG. 4D), and is etched to a desired shape to be amorphous. A composite optical waveguide having a quality semiconductor region 600 can be formed ((e) in the figure).

  In the process shown in FIG. 18, amorphous silicon (a-Si) which is a semiconductor can be deposited after the porous silicon is oxidized. In other words, it is possible to deposit an amorphous semiconductor after performing a high-temperature heat treatment step required to oxidize porous silicon. Therefore, as amorphous semiconductor materials, hydrogenated amorphous silicon (a-Si: H), hydrogenated diamond-like carbon, etc., materials that compensate dangling bonds of amorphous semiconductors with hydrogen, in other words, For example, a semiconductor material whose properties deteriorate after a high-temperature heat treatment step can be used.

  As described above, the composite optical waveguide according to the present embodiment has a sufficiently large cross-sectional area when coupling light introduced from the outside to the third region (semiconductor wire waveguide) having a very narrow cross-sectional area and First, external light is temporarily received in the first region having the same refractive index as that of the silica optical fiber, and light is coupled through the interface where the first region and the third region (semiconductor wire waveguide) are joined. The effective optical coupling efficiency to the region (semiconductor wire waveguide) is greatly increased, and the coupling is greatly facilitated.

  Further, in the composite optical waveguide according to the present embodiment, densified silica or porous silica can be selectively used as a constituent material of each of the first region and the second region depending on the application. When porous silica is used as a constituent material for each of the first region and the second region, there is an advantage that the refractive index can be adjusted by the porosity of the silica. When silicon is used as the third region (thin wire waveguide), a porous material having a lower refractive index than the dense silica formed around the third region (thin silicon wire) in the heat treatment step is used for the first region. The fourth region (thermal oxide film) formed around the third region (silicon thin wire) does not become a barrier when the light in the silica waveguide is coupled to the third region (silicon thin wire waveguide).

  Further, the signal light to be processed in the third region (semiconductor thin wire waveguide) and the input portion of the excitation light and control light for amplifying the signal light can be divided and optically coupled. A silicon thin wire waveguide can be formed without using an expensive SOI substrate.

  Further, as the material of the third region (semiconductor fine wire), not only silicon but also many other main materials can be used. In particular, when a crystalline semiconductor that can be epitaxially grown in advance on a silicon substrate is used, a third region (semiconductor fine wire) can be formed in the vicinity of the first region. Furthermore, when a material such as amorphous silicon or amorphous diamond-like carbon is used, the third region (semiconductor fine wire) can be formed in the step after forming the first region and the second region, The degree of freedom of the process also increases.

  Furthermore, the degree of freedom in selecting the material of the third region (semiconductor thin wire waveguide) is greatly expanded, and the wavelength width of light that can be transmitted through the composite optical waveguide is greatly expanded. When silicon is used in the third region, it is limited to the near-infrared wavelength region having a photon energy equal to or lower than the band gap energy of silicon, but there is a possibility that the device application can be expanded to the visible and ultraviolet wavelength range. is there.

  FIG. 19 is a diagram illustrating a configuration of a composite optical waveguide according to another embodiment. FIG. 4A shows a plan view of the composite optical waveguide. FIG. 2B shows a cross-sectional view of the composite optical waveguide. The composite optical waveguide shown in FIG. 19 includes a fifth region 130 mainly composed of silica in addition to the configuration of the composite optical waveguide shown in FIG. The fifth region 130 is provided between the first region 131 and the second region 132. The refractive index of the fifth region 130 is smaller than the refractive index of the first region 131 and larger than the refractive index of the second region 132. This composite optical waveguide can function as an optical waveguide having the first region 131 as a core, the second region 132 as an outer cladding, and the fifth region 130 as an inner cladding. The light incident on the region having a wide cross-sectional area including the fifth region 130 and the first region 131 is suppressed from being coupled to the silicon substrate 100 by the barrier of the second region 132 having a low refractive index. The light incident on the combined region of the fifth region 130 and the first region 131 is first condensed with high density onto the first region 131 having a relatively large refractive index and a small cross-sectional area through a wide joint interface between the two regions. The By concentrating the light in this way, it can be strongly coupled to the silicon fine wire 300 as the third region of high refractive index having a smaller junction interface.

  FIG. 20 is a process diagram illustrating a first method for manufacturing a composite optical waveguide according to another embodiment. In the first manufacturing method, first, similarly to the step shown in FIG. 6A, a carbon thin film 110 having an opening 112 formed is formed on a silicon substrate 100 (FIG. 20A).

  Subsequently, a porous silicon region is selectively formed. That is, p-type silicon obtained by patterning the carbon thin film 110 using a pulse current formation method in which the formation current is increased in proportion to the area of the interface between the porous silicon and the silicon crystal as the porous silicon region grows. Porous silicon having a depth of about 5 μm is formed by using the substrate 100 as an anode and the opposing platinum electrode as a cathode, holding a first predetermined concentration of hydrofluoric acid solution between the two electrodes, and anodizing (forming) the silicon substrate 100. The first region 121 is formed. At this time, the current density uses a predetermined value suitable for forming the first region 121. Next, a fifth region 120 of porous silicon having a depth of about 10 μm is formed. The current density used at this time is different from the current density of the first region 121 and has a predetermined value suitable for forming the fifth region 120. Next, the hydrofluoric acid solution concentration is changed to the second predetermined concentration, and the second region 122 of porous silicon having a thickness of about 5 μm is formed using the second interface current density (FIG. 5B). In this step, if each of the hydrofluoric acid concentration, the formation current density, and the formation depth is controlled to predetermined values, the first region 121, the fifth region 120, and the second region 122 made of porous silicon are formed. A silicon thin wire 300 made of a silicon single crystal can be left in the substantially central portion of the carbon thin film 113 between the two openings 112.

  Subsequently, the silicon substrate 100 is oxidized. That is, the silicon substrate 100 on which the first region 121, the fifth region 120, and the second region 122 made of porous silicon are formed by the above process is subjected to a heat treatment at a temperature of 300 ° C. for 1 hour in an oxygen stream. Then, an oxide layer of a monoatomic layer is formed on the surface of the silicon fine pillar inside the porous silicon to stabilize the porous silicon structure. Next, the silicon thin pillars in each of the first region 121, the fifth region 120, and the second region 122 are oxidized to the inside of the thin pillars by an oxidation treatment at a temperature of 800 ° C. By this step, the porous silicon region is oxidized to become porous silica. That is, the first region 121 made of porous silicon becomes the first region 141 made of porous silica (the core region of the so-called double clad waveguide), and the fifth region 120 made of porous silicon becomes the fifth region made of porous silica. The region 140 (inner cladding region) is formed, and the second region 122 made of porous silicon becomes the second region 142 (outer cladding region) made of porous silica (FIG. 5C).

  FIG. 21 is a diagram illustrating an example of a refractive index distribution of a composite optical waveguide according to another embodiment. This figure shows the refractive index distribution along the line AA ′ in FIG. 21 is compared with FIG. 3, it can be seen that in the composite optical waveguide of FIG. 21, a fifth region 140 having a smaller refractive index than that of the first region 141 is added. By adopting such a configuration, light incident on the first region 141 and the fifth region 140 having a wide cross-sectional area surrounded by the second region 142 is traveling in the waveguide while the first region 141 and the fifth region 140 Through a wide bonding interface with the region 140, the light is condensed with high density on the first region 141 having a large refractive index and a small cross-sectional area. The light condensed at a high density is coupled to the silicon fine wire 300 through the junction interface between the first region 141 and the silicon fine wire 300. The OPSWG region R9 includes a first region 141, a fifth region 140, and a second region 142. The silica region R10 includes an OPSWG region R9 and oxide films 332 and 333.

  Subsequently, the refractive index relationship of the three regions (the first region 141, the fifth region 140, and the second region 142) of the porous silica in the composite optical waveguide manufactured by the above process will be described in more detail with reference to FIG. explain. As the relationship of the porosity after oxidation at 800 ° C., the porosity of the first region 141 is the smallest, the porosity of the fifth region 140 is next small, and the porosity of the second region 142 is the largest. To do. In this way, as for the refractive index relationship between the three regions, the refractive index of the first region 141 is the largest, the fifth region 140 is the next largest, and the refractive index of the second region 142 is the smallest. Thus, the composite optical waveguide has a so-called double clad waveguide configuration in which the first region 141 is the core, the fifth region 140 is the inner cladding, and the second region 142 is the outer cladding. With the above configuration, the silicon fine wire waveguide 350 is intimately joined to the first region 141 of the double clad waveguide made of porous silica.

  As described above, in the composite optical waveguide, the coupling of light between the silica waveguide portion and the silicon fine wire is stronger and more efficient. The propagation characteristics of the waveguide having the silicon fine wire waveguide 350 shown in FIG. 20C and the propagation characteristics of the waveguide not having the silicon fine wire waveguide 350 are both visible using a sample waveguide having a length of 2 mm. The amount of transmitted light was compared. One end of a sample waveguide having a length of 2 mm was illuminated, and light transmitted through the other end was observed. In the waveguide that does not have the silicon thin wire waveguide 350, strong light propagated in the first region 141 and brightly shined, but the fifth region 140 and the second region 142 were dark, and the core region of the double clad waveguide The light condensing function to (first region 141) was confirmed. Further, the amount of light transmitted through the first region 141 in the waveguide having the silicon fine wire waveguide 350 was as small as several tenths of the amount of light transmitted through the first region 141 in the waveguide not having the silicon fine wire waveguide 350. . This decrease in the amount of visible light transmitted can be interpreted as being absorbed by the silicon fine wire 300 having a large refractive index through the bonding interface between the first region 141 and the silicon fine wire 300.

  The waveguide length of the sample illustrated in FIG. 7 is 15 mm, and the waveguide length of the sample illustrated in FIG. 20C is 2 mm. The refractive index configuration of the sample illustrated in FIG. 7 corresponds to FIG. 1C, and the refractive index configuration of the sample illustrated in FIG. 20C corresponds to FIG. The main differences between FIG. 1C and FIG. 21 will be compared. In FIG. 1 (c), the refractive index of the first region 131 composed of the doped dense silica is larger than the refractive index of the oxide film 332, which is unavoidably formed around the fine silicon wire 300, and is non-doped dense silica. . Therefore, the oxide film 332 having a refractive index smaller than that of the first region 131 acts as a barrier when the light in the first region 131 is combined with the silicon thin wire 300. When the thickness of the oxide film 332 is sufficiently thin, the light in the first region 131 passes through the oxide film 332 and leaks and can be combined with the silicon fine wire 300. In other words, it can be said that the thickness of the oxide film 332 regulates the magnitude of light coupling. On the other hand, in FIG. 21, the refractive index of the oxide film 332 inevitably formed around the silicon thin wire 300 is larger than the refractive indexes of the first region 141 and the fifth region 140. For this reason, the oxide film 332 does not become an obstacle that the light in the core region is combined with the silicon thin wire 300. The same can be said in the comparison between FIG. 1C and FIG.

  The difference between FIG. 3 and FIG. 21 is that, in FIG. 21, a double clad waveguide structure is used to concentrate light at a high density in the first region 141, and the light density at the junction interface between the core region and the silicon thin wire 300 is changed. As a result, the strength of optical coupling between the core region and the silicon fine wire 300 is increased.

  In addition, you may form the 5th area | region 120,130,140 in all the above-mentioned embodiments, modifications, etc.

Claims (8)

Each having a first region and a second region mainly composed of silica in a recessed region on one main surface side of the silicon substrate;
A third region comprising a semiconductor extending proximate to the first region is provided;
In the recessed area, the first area is present on the inner side and the second area is present on the outer side,
The refractive index of the first region is greater than the refractive index of the second region;
The refractive index of the third region is formed rather greater than the refractive index of the first region,
The first region becomes a core region, and the second region becomes a waveguide that becomes a cladding region,
The third region is formed of a semiconductor having a relatively high refractive index as compared with the first region, and has a narrow cross-sectional area.
When external light is received by one end face of the waveguide formed by the first region and the second region, the light propagating through the first region is relatively transmitted through the junction interface between the first region and the third region. A composite optical waveguide characterized in that it is optically coupled to the third region having a large refractive index and a small cross-sectional area, and the third region becomes a semiconductor thin wire waveguide. A fourth region is further provided between the first region and the third region;
The refractive index of the fourth region is smaller than the refractive index of the first region;
The composite optical waveguide according to claim 1, wherein a thickness of the fourth region is smaller than a thickness of the second region. A fourth region is further provided between the first region and the third region;
The refractive index of the fourth region is rather larger than the refractive index of the first region,
The refractive index of the fourth region is set smaller than the refractive index of the third region;
The composite optical waveguide according to claim 1, wherein a thickness of the fourth region is smaller than a thickness of the second region .   The composite optical waveguide according to claim 1, wherein the first region and the second region are made of porous silica. A fourth region is further provided between the first region and the third region;
The fourth region is formed by thermal oxidation of a semiconductor;
The first region and the second region are made of porous silica;
The composite optical waveguide according to claim 1, wherein a refractive index of the fourth region is equal to or greater than a refractive index of the first region.   The composite optical waveguide according to claim 1, wherein the third region is crystalline.   The composite optical waveguide according to claim 1, wherein the third region is amorphous. A fifth region mainly composed of silica is further provided between the first region and the second region,
The refractive index of the fifth region is smaller than the refractive index of the first region;
The composite optical waveguide according to claim 1, wherein a refractive index of the fifth region is larger than a refractive index of the second region.

Images