JP3886958B2 - Manufacturing method of two-dimensional photonic crystal with thin wire waveguide - Google Patents

Description

  The present invention relates to a two-dimensional photonic crystal used in a multiplexer / demultiplexer or the like in the field of wavelength division multiplex communication or the like, and in particular, a two-dimensional with a thin line waveguide connected with a thin line waveguide for introducing or introducing light outside the crystal. The present invention relates to a method for producing a photonic crystal.

  In recent years, photonic crystals, which are optical functional materials having a periodic refractive index distribution, have attracted attention. The photonic crystal has a feature that a band structure is formed with respect to the energy of light and electromagnetic waves due to its periodic refractive index distribution, and an energy region (photonic band gap) where propagation of light and electromagnetic waves is impossible is formed. Have. Note that “light” used in the present specification and claims includes electromagnetic waves other than light.

  By introducing an appropriate defect in the photonic crystal, an energy level (defect level) is formed in the photonic band gap. Accordingly, only light having a wavelength corresponding to the energy of the defect level in the wavelength (frequency) range corresponding to the energy in the photonic band gap can exist at the position of the defect. A waveguide is formed by providing this defect in a line shape, and an optical resonator is formed by providing the defect in a dot shape. The wavelength (resonance wavelength) of light that resonates in this point defect depends on its shape and refractive index.

  Various optical devices are being studied using this resonator and waveguide. For example, by arranging this resonator in the vicinity of the waveguide, among the light of various wavelengths propagating in the waveguide, light having a wavelength that matches the resonance wavelength of the resonator is externally transmitted from the waveguide through the resonator. In addition to functioning as a demultiplexer for taking out light, the demultiplexer also functions as a multiplexer that introduces light having the resonance wavelength of the resonator into the waveguide from the outside via the resonator. For example, in the field of optical communication, such a multiplexer / demultiplexer is used for wavelength division multiplexing communication in which light of a plurality of wavelengths is propagated through a single fiber and a separate signal is placed on the light of each wavelength. it can.

  Photonic crystals include one-dimensional crystals, two-dimensional crystals, and three-dimensional crystals. Among these, two-dimensional photonic crystals have an advantage that they are relatively easy to manufacture. As an example, Patent Document 1 discloses a two-dimensional photonic crystal in which substances having a refractive index lower than that of a material are periodically arranged on a plate material (slab) having a high refractive index. A two-dimensional photonic crystal and an optical multiplexer / demultiplexer in which a waveguide having defects in a shape and a point defect that disturbs the periodic arrangement are provided adjacent to the waveguide are described. Note that the waveguide formed in the two-dimensional photonic crystal in this way is referred to as an “in-crystal waveguide” in the present application.

Japanese Patent Laid-Open No. 2001-272555 ([0019] to [0032], FIG. 1)

  In the two-dimensional photonic crystal, the low refractive index region periodically disposed in the slab having a high refractive index is air (that is, a hole), so that the difference in refractive index can be increased. And the most common in terms of manufacturing ease.

  In the two-dimensional photonic crystal described in Patent Document 1, the slab is in contact with air both above and below. As described above, since the refractive index difference between the slab and air is large, most of the light propagating in the in-crystal waveguide is confined in the slab by total reflection. Obtainable.

  In such a two-dimensional photonic crystal, in order to introduce light from the outside into the waveguide or to derive light from the waveguide to the outside, the inventors of the present application used a thin-line waveguide in the two-dimensional photonic crystal. We are studying connected two-dimensional photonic crystals with fine wire waveguides. An example is shown in FIG. The two-dimensional photonic crystal 10 is formed by periodically arranging holes 12 in the slab 11, and the in-crystal waveguide 13 is formed by deleting the holes 12 by one row. The thin wire waveguide 14 is connected on the extension of the intracrystalline waveguide. By forming the fine wire waveguide from the same material as the slab, the two-dimensional photonic crystal and the fine wire waveguide can be integrated.

  The inventors of the present application calculated a relationship (waveguide mode) between the frequency and the wave number of light guided through the intra-crystal waveguide and the thin wire waveguide. As a result, when all the surfaces of the thin wire waveguide are in contact with air as shown in FIG. 1, it has been clarified that there are two types of waveguide modes in the thin wire waveguide. That is, both modes are guided simultaneously in one waveguide, resulting in a multimode in which light of two wave numbers can exist for one frequency. Since light in both modes has different propagation speeds, the presence of such a multimode may be an obstacle in optical communication.

  On the other hand, as shown in FIG. 2, when a clad member 15 made of a material having a refractive index lower than the refractive index of the fine wire waveguide and higher than the refractive index of air is provided on one side surface of the fine wire waveguide 14, As shown in FIG. 3, the waveguide mode of the thin wire waveguide is a single mode in which only one wave number of light can exist for one frequency, and the above problem does not occur.

  As described above, in the two-dimensional photonic crystal with a thin wire waveguide, the two-dimensional photonic crystal is in contact with the air on both the upper and lower sides of the slab (thus, not in contact with the clad member), whereas the thin wire waveguide is It is desirable to contact the clad member.

Patent Document 1 describes a method of manufacturing a two-dimensional photonic crystal that is in contact with air both above and below the slab (however, no thin wire waveguide is provided). In this method, a substrate having a layer made of InGaAsP or SiO ₂ (hereinafter referred to as “clad layer”) under a layer made of InP or Si (hereinafter referred to as “slab layer”) is used. First, a two-dimensional photonic crystal is formed by periodically forming holes that penetrate the slab layer. At that time, a point-like defect and an intra-crystal waveguide are formed by appropriately setting the size and arrangement of the holes. Next, an etching agent is introduced from the formed holes to etch the cladding layer under the holes. At this time, by etching for a certain time or longer, the cladding layer between the vacancies is also etched, so that one cavity is formed in the vacancy-provided region, that is, the entire lower part of the two-dimensional photonic crystal. To do. In the two-dimensional photonic crystal manufactured in this way, a bridge-like structure is formed in which a bridge formed by a slab of the two-dimensional photonic crystal is formed on the cavity. This is called “air bridge structure” and this cavity is called “air bridge cavity”.

  When the two-dimensional photonic crystal and the fine wire waveguide are integrally formed, when the two-dimensional photonic crystal with the fine wire waveguide is manufactured, the pattern of the two-dimensional photonic crystal and the fine wire waveguide is formed together on the slab layer, It is natural to create it at once by etching or the like. However, when such a manufacturing method is adopted, when an air bridge cavity is subsequently formed in the lower part of the two-dimensional photonic crystal, the etching agent enters the cladding layer from the periphery of the thin wire waveguide, and the two-dimensional photonic crystal Not only the lower part but also the part directly under the thin wire waveguide is etched. Therefore, the thin wire waveguide does not contact the clad layer, and multimode propagation occurs as described above. Further, there is a risk that the thin wire waveguide may be broken due to insufficient strength. In order to prevent this, even if the periphery of the thin wire waveguide is masked, it is difficult to completely prevent the etching agent from entering through the gap between the thin wire waveguide and the mask, and the part immediately below the thin wire waveguide is also etched. End up.

  The problem to be solved by the present invention is that the slab of the two-dimensional photonic crystal is in contact with the air both above and below, while a clad member is provided below the thin wire waveguide connected to the two-dimensional photonic crystal. Another object of the present invention is to provide a method for producing a two-dimensional photonic crystal with a thin wire waveguide.

A method for producing a two-dimensional photonic crystal with a thin-line waveguide according to the present invention, which has been made to solve the above-mentioned problems, comprises a two-dimensional photonic crystal having a waveguide from a plate material in which a slab layer and a clad layer are laminated. And a method of manufacturing a two-dimensional photonic crystal with a thin wire waveguide comprising a thin wire waveguide connected to the waveguide,
a) a two-dimensional photonic crystal formation step of periodically providing holes in a predetermined range of the slab layer and forming an intracrystalline waveguide in the predetermined range ;
b) an air bridge cavity forming step of etching a lower cladding layer in the predetermined range to form a cavity in the cladding layer by introducing an etching agent through the holes ;
c) From the outer edge of the predetermined range toward the outside of the predetermined range, a slab layer with a predetermined width is left on the extension of the in-crystal waveguide, and a slab layer around the slab layer is removed to form a thin-line waveguide wire waveguide forming step of,
It is characterized by performing in order .

Embodiments and effects of the invention

In the method for producing a two-dimensional photonic crystal with a thin wire waveguide of the present invention, a plate material formed by laminating a slab layer and a clad layer is used as a material. This plate material includes those in which another layer is laminated after the slab layer and the clad layer. For example, a commercially available SOI (Silicon On Insulator) substrate in which a SiO ₂ thin film is formed on a thick Si film and a Si thin film is formed thereon can be used. In the case of an SOI substrate, the Si thin film becomes a slab layer, and the SiO ₂ thin film becomes a cladding layer. The thick Si film under the SiO ₂ thin film serves as a substrate that supports the slab layer and the clad layer.

  First, holes are periodically provided in a predetermined range of the slab layer. As a result, a periodic distribution of the refractive index is formed, so that a two-dimensional photonic crystal is formed in the range where the holes are provided. At the same time, an in-crystal waveguide is formed. The intracrystalline waveguide is generally formed by linear disturbance of the periodic arrangement of holes, but is typically formed by linearly providing a region where holes are not formed. It should be noted that even if the width is narrowed by increasing the arrangement of the holes on both sides, or conversely expanded, it can sufficiently function as an in-crystal waveguide. Furthermore, it is of course possible to form an in-crystal waveguide by arranging holes having a diameter different from that of the surrounding holes in a line or by embedding a different substance. The holes are formed by, for example, photolithography, electron beam lithography, or dry etching used for manufacturing various semiconductor devices. A point-like defect is formed in the two-dimensional photonic crystal as necessary. In this step, the thin wire waveguide is not formed.

Next, an etching agent is introduced into the cladding layer through the holes. The etching agent enters the cladding layer from the holes, and etches the lower part of the holes and the surrounding cladding layer. As a result, an air bridge cavity is formed in the area where the holes are provided, that is, in the lower part of the two-dimensional photonic crystal. At this time, by adjusting the etching time, the air bridge cavity region can be made substantially coincident with the two-dimensional photonic crystal region. Here, the area of the air bridge cavity does not need to completely coincide with the area of the two-dimensional photonic crystal, and may be slightly larger or smaller than that. An existing etchant or etching gas can be used as the etchant. For example, when the cladding layer is made of SiO _{2, an} aqueous hydrogen fluoride solution can be used.

  Next, a thin-line waveguide is formed on the extension of the in-crystal waveguide from the predetermined range, that is, from the outer edge to the outer side of the two-dimensional photonic crystal. That is, the slab layer is removed while leaving the slab layer by the width of the thin wire waveguide. Since the clad layer remains outside the air bridge cavity, the lower part of the thin-line waveguide formed in this way is in contact with the clad layer. The removal of the slab layer can be performed using photolithography, electron beam lithography, dry etching, or the like, similar to the case of forming the holes. Here, in order to properly arrange the thin-line waveguide to be formed on the extension of the previously formed intracrystalline waveguide, it is preferable to use a superposition function of a commercially available lithographic apparatus. This overlay function is widely used in the field of semiconductor manufacturing. When further patterns are formed in accordance with the patterns formed in the previous process, the positional deviation of both patterns is below a predetermined size. It can be suppressed to.

  When removing the slab layer in the thin wire waveguide forming process, it is not necessary to remove all the slab layer except for the portion of the thin wire waveguide, only removing the slab layer so as to form grooves on both sides of the thin wire waveguide. But you can. For example, when removing a slab layer by electron beam lithography, it is easier to remove only the groove portion in this way than to remove the slab layer over a wide range.

The width of the groove is preferably set to one period or more of the hole arrangement for the following reason. In the present application, “a period of hole arrangement” means a distance between adjacent holes.
FIG. 4 shows the electric field distribution of light propagating through the thin wire waveguide in the width direction of the waveguide. The horizontal axis represents the position in the width direction of the thin waveguide, and the range of reference numeral 41 in the drawing corresponds to the region of the thin waveguide. The vertical axis represents the electric field amplitude, and the larger the absolute value of the vertical axis, the larger the electric field. Although the electric field exists also outside the fine wire waveguide 41, the electric field becomes almost zero at a position 42 that is separated from the end of the fine wire waveguide by one period a of the hole arrangement. Therefore, if the width of the groove is equal to or longer than one period of the hole arrangement, light hardly leaks into the slab layer on the side of the thin wire waveguide.

  In a two-dimensional photonic crystal, light propagates while leaching from the intra-crystal waveguide to a certain range in the width direction of the waveguide (however, the light does not spread outside the range). This causes no loss of light in the waveguide within the crystal). The range of this oozing is about 5 periods of hole arrangement. If the two-dimensional photonic crystal is directly connected to the slab layer left outside the grooves on both sides of the thin wire waveguide, the light oozed out of the waveguide inside the crystal enters the remaining slab layer. As a result, light is lost. Therefore, in the fine wire waveguide forming step, it is desirable to further remove the slab layer at the outer edge of the two-dimensional photonic crystal from the fine wire waveguide for five or more periods of the hole arrangement. As a result, it is possible to prevent the light that has leached from the intra-crystal waveguide from entering the slab layer left outside the groove and losing it.

  According to the present invention, the following effects can be obtained. In the present invention, after forming a two-dimensional photonic crystal and forming an air bridge cavity below the two-dimensional photonic crystal, a thin-line waveguide is formed from the outer edge of the two-dimensional photonic crystal toward the outside. It is possible to ensure that the cladding layer is in contact with. For this reason, in the thin wire waveguide manufactured by this method, the waveguide mode becomes a single mode, and problems due to the multimode do not occur. On the other hand, since an air bridge cavity is formed in the lower part of the two-dimensional photonic crystal, the direction of light in the two-dimensional photonic crystal is perpendicular to the slab due to the refractive index difference between the air and the slab existing above and below it. The loss of light can be minimized.

  An embodiment of a method for producing a two-dimensional photonic crystal with a thin wire waveguide according to the present invention will be described with reference to FIGS. In FIG. 5, a perspective view of each process is shown on the left side, and a cross-sectional view taken along AA ′ or BB ′ shown there is shown on the right side.

An SOI substrate 21 composed of three layers of a slab layer 211 made of Si, a clad layer 212 made of SiO ₂ and a substrate layer 213 made of Si is prepared. A commercially available SOI substrate 21 can be used. The thickness of each layer of the SOI substrate 21 used in this example is 0.25 μm for the slab layer 211, 1.5 μm for the cladding layer 212, and 725 μm for the substrate layer 213. First, a resist 28 is applied to the surface of the slab layer 211, and the resist 28 is removed by an electron beam in a pattern in which holes are periodically arranged ((a)). In this embodiment, the radius of the holes is 0.12 μm, the holes are arranged in a triangular lattice, and the lattice constant is 0.42 μm. At this time, no hole is formed at a position to be the intracrystalline waveguide. The width of the in-crystal waveguide is 0.48 μm in this embodiment. The intracrystalline waveguide can also be formed by making the shape of the holes different from others or by shifting the positions of the holes. Next, holes 22 are formed in the slab layer 211 by dry etching using an etching gas (SF ₆ gas). At this time, the portion of the in-crystal waveguide 23 (and, if a point defect is provided, the point defect) is not etched ((b)). Thereafter, the resist 28 is removed. A two-dimensional photonic crystal is formed in the slab layer 211 by the steps so far (two-dimensional photonic crystal formation step). However, at this stage, the cladding layer 212 is in contact with the lower part of the two-dimensional photonic crystal.

Next, the SOI substrate 21 is immersed in a 5% concentration hydrogen fluoride aqueous solution for 1 minute and 30 seconds. The aqueous hydrogen fluoride solution that has entered through the holes 22 etches only the SiO _{2 in the} cladding layer 212 without affecting the Si in the slab layer 211 and the substrate layer 213. As a result, the cladding layer 212 is etched to a certain distance from each hole 22, and the etched region is connected between the holes. Thus, the air bridge cavity 24 is formed in the lower part of the two-dimensional photonic crystal ((c)). In this embodiment, no hole is provided in a region having a certain width at the side end of the slab layer 211. As a result, the cladding layer 212 remains without being etched by a certain width in the lower portion 27 at the side end. The remaining portion of the cladding layer 212 plays a role of supporting the two-dimensional photonic crystal.

Next, a resist 29 is applied on the slab layer 211, and the resist 29 on both sides of the position where the thin wire waveguide is formed is removed by an electron beam by a predetermined width. At that time, by using the registration (superposition) function of the electron beam drawing apparatus, the formed (remaining) thin wire waveguide is correctly aligned with the extension of the intra-crystal waveguide 23. The width of the groove is one period of the hole arrangement. In this embodiment, simultaneously with the formation of this groove, a groove in a direction substantially perpendicular to the thin wire waveguide is formed in the resist 29 at the outer edge of the two-dimensional photonic crystal ((d)). The length of the groove on the outer edge of the crystal is longer than 5 periods of the hole arrangement, and is 6 lines (5.2 periods) of the hole array, and the width is the width of the groove on both sides of the thin wire waveguide. Similarly, it is one cycle of hole arrangement. Next, two grooves 26 are formed in the slab layer 211 by dry etching using SF ₆ gas. Thereafter, when the resist 29 is removed, the slab layer 211 left between the two grooves 26 becomes the thin-line waveguide 25 ((e)).

  As described above, the two-dimensional photonic crystal with the fine wire waveguide having the air bridge cavity 24 under the two-dimensional photonic crystal and the clad layer 212 under the fine wire waveguide 25 is completed. In the intracrystalline waveguide 23 of the two-dimensional photonic crystal with a thin wire waveguide, an air bridge cavity 24 is formed in the lower portion thereof, so that light does not leak in the direction of the substrate layer 213. Further, when the lower portion of the thin wire waveguide 25 is in contact with the cladding layer 212, the waveguide mode of the thin wire waveguide 25 becomes a single mode. In addition, since the width of the grooves 26 on both sides of the thin wire waveguide 25 is one period of the hole arrangement, the light in the thin wire waveguide 25 does not leak into the slab layer 211 on the side of the waveguide 25. Further, since the grooves 26 are formed on the outer edge of the crystal by six rows of hole holes (for 5.2 periods) in a direction substantially perpendicular to the thin-line waveguide 25, the grooves 26 are immersed in the width direction from the intra-crystal waveguide 23. The emitted light does not leak into the slab layer 211 on the side of the thin wire waveguide 25.

  In the case of providing a point defect, it is only necessary not to form a hole at the target position in the step shown in FIG. Alternatively, the shape of the holes at that position may be different from others, or the holes may be arranged at positions that disturb the periodic arrangement. FIG. 6A-1 shows an example in which three holes are not formed at the position 61, and FIG. 6A-2 shows an example in which the diameter of the hole formed at the position 62 is larger than the other holes. FIG. By performing the other steps in the same manner as described above, the point-like defect 63 ((b-1)) lacking three holes or the point-like defect 64 ((( A two-dimensional photonic crystal with a thin wire waveguide provided with b-2)) is obtained. Note that these point defects are described in detail in Patent Document 1 and Japanese Patent Application Laid-Open No. 2003-279764.

FIG. 7 shows a scanning electron microscope (SEM) photograph of the two-dimensional photonic crystal with a thin wire waveguide manufactured according to this example. (a) is a photograph of a cross section of the two-dimensional photonic crystal portion (corresponding to the cross section AA ′ in FIG. 5) obliquely from above, and (b) is an enlarged photograph thereof. Holes 32 are periodically formed in the slab layer 311, and an in-crystal waveguide 33 in which the holes 32 are missing for one row is formed. An air bridge cavity 34 is formed below the two-dimensional photonic crystal. The reason why the clad layer 312 is etched obliquely is that the upper part of the clad layer 312 near the slab layer 311 is etched farther from the holes 32 and the crystallinity of SiO ₂ . (c) is a photograph of the cross section of the thin wire waveguide portion (corresponding to the cross section BB ′ in FIG. 5) obliquely from above, and (d) is a photograph of the cross section taken obliquely from the side. By forming the two grooves 36, the thin wire waveguide 35 is formed between them. Further, as shown in (c), the lower portion of the thin wire waveguide 35 is in contact with the cladding layer 312. Thereby, the waveguide mode of the thin wire waveguide 35 becomes a single mode.

The figure which shows an example of the two-dimensional photonic crystal with a thin wire | line waveguide. The figure which shows an example of the two-dimensional photonic crystal with a thin wire | line waveguide which provided the clad member. The graph which shows the waveguide mode of the thin wire | line waveguide and the intracrystal waveguide which provided the clad member. The graph which shows the spread of the light to the width direction of a thin wire | line waveguide. The perspective view and sectional drawing which show one Example of the manufacturing method of the two-dimensional photonic crystal with a thin wire | line waveguide concerning this invention. The perspective view which shows the example which provides a point defect in the manufacturing method of the two-dimensional photonic crystal with a thin wire | line waveguide which concerns on this invention. The scanning electron micrograph of the two-dimensional photonic crystal with a thin wire | line waveguide manufactured by the present Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Slab 12, 22, 23 ... Hole 13, 23, 33 ... In-crystal waveguide 14, 25, 35 ... Thin wire waveguide 15 ... Cladding member 21 ... SOI substrate 211, 311 ... Slab layer 212, 312 ... Cladding layer 213: Substrate layers 24, 34 ... Air bridge cavities 26, 36 ... Grooves 28, 29 ... Resist

Claims (7)

In a method for manufacturing a two-dimensional photonic crystal with a thin wire waveguide comprising a two-dimensional photonic crystal having a waveguide and a thin wire waveguide connected to the waveguide, from a plate material formed by laminating a slab layer and a clad layer ,
a) a two-dimensional photonic crystal formation step of periodically providing holes in a predetermined range of the slab layer and forming an intracrystalline waveguide in the predetermined range ;
b) an air bridge cavity forming step of etching a lower cladding layer in the predetermined range to form a cavity in the cladding layer by introducing an etching agent through the holes ;
c) From the outer edge of the predetermined range toward the outside of the predetermined range, a slab layer with a predetermined width is left on the extension of the in-crystal waveguide, and a slab layer around the slab layer is removed to form a thin-line waveguide wire waveguide forming step of,
A method for producing a two-dimensional photonic crystal with a thin wire waveguide, which is performed in the following order .   2. The method for producing a two-dimensional photonic crystal with a thin wire waveguide according to claim 1, wherein the in-crystal waveguide is a linear region in which the holes are not formed.   The thin wire waveguide forming process according to claim 1 or 2, wherein, in the thin wire waveguide forming step, the slab layer is removed by a width of one period or more of the hole arrangement in a groove shape on both sides of the thin wire waveguide. A method for producing a two-dimensional photonic crystal.   4. The fine wire waveguide forming step, further comprising removing a slab layer at an outer edge of the two-dimensional photonic crystal from the fine wire waveguide for five or more periods of hole arrangement. The manufacturing method of the two-dimensional photonic crystal with a thin wire | line waveguide of description. The made slab layer from Si plate material, a manufacturing method of the wire waveguide with the two-dimensional photonic crystal according to claim 1, the cladding layer, characterized in that the made of SiO _2.   The method for producing a two-dimensional photonic crystal with a thin wire waveguide according to claim 5, wherein the plate material is an SOI substrate. The method for producing a two-dimensional photonic crystal with a thin wire waveguide according to claim 5 or 6, wherein the etching agent is an aqueous hydrogen fluoride solution.