JP4372039B2 - Low propagation loss optical waveguide and manufacturing method thereof - Google Patents

Description

  The present invention relates to a low propagation loss optical waveguide having a small propagation loss used for an optical integrated circuit, a small optical sensor, and the like, and a manufacturing method thereof.

In recent years, development of an optical waveguide having a small bending radius has been progressing for use in a small optical integrated circuit and a small biochip / sensor. The bending radius of the optical waveguide has the greatest influence on the miniaturization of these optical devices.
In general, an optical waveguide is composed of a portion having a high refractive index (n) called a core and a portion having a low refractive index called a cladding, and the smaller the refractive index difference (Δn), the smaller the bending radius. It is known that it can be realized. This is because the larger the Δn, the stronger the light can be confined in the core portion, and the more difficult the light leaks to the outside even with a sharp bend.

  This type of optical waveguide is generally called an index guiding waveguide, and light propagates through an internal total reflection mechanism. A conceptual diagram of this type of optical waveguide is shown in FIG. In the figure, (a) is a channel-type waveguide, and (b) is a rib-type waveguide.

Apart from the refractive index waveguide type optical waveguide, many photonic crystal waveguide optical waveguides have been reported. This is because the optical propagation path is covered with a photonic crystal without using a total internal reflection mechanism like a normal optical waveguide, and a forbidden substance (Photonic
Bandgap) is designed to prevent light from leaking outside the propagation path. A conceptual diagram of this type of optical waveguide is shown in FIG. In the figure, (a) is a hole type photonic crystal waveguide, and (b) is a ball type photonic crystal waveguide. In both cases, a cross-sectional view (top) and a plan view (bottom) are shown in pairs.

Theoretically, in both the refractive index guided type and the photonic crystal type, not only can the bending radius be reduced to several μm, but also the propagation loss can be reduced as much as possible. Propagation loss is extremely high, reporting several tens of dB / cm or more. It has been reported that this is due to surface roughness of the side surface formed during processing of the optical waveguide (see Non-Patent Document 1).
Usually, the optical waveguide is manufactured by dry etching through an exposure process by a photoresist method. In this etching process, surface roughness occurs on the side wall of the optical waveguide. A typical waveguide manufacturing process is shown in FIG.

Usually, a refractive index waveguide type optical waveguide or a photonic crystal type optical waveguide is manufactured using an SOI (Silicon on Insulator) substrate. This is because a clad layer (in this case, SiO ₂ layer) is already formed under the silicon layer, and single crystal silicon can be used for the layer in which the waveguide is formed. Single crystal silicon is known to have less loss than amorphous or polysilicon because it is not absorbed or scattered by grain boundaries. Therefore, it can be said to be an ideal material for a refractive index waveguide type optical waveguide and a photonic crystal type optical waveguide.

However, even if steep bending can be realized, the total propagation loss is extremely high due to the rough surface of the interface, mainly the side wall of the optical waveguide.
In the case of a refractive index waveguide type optical waveguide, there is a report that this propagation loss is proportional to the third power of Δn (see Non-Patent Document 1). I can not say.
In particular, when SOI is used as the substrate, Δn = Si (n = 3.5) −SiO ₂ (n = 1.45) is 2.1, and Δn of silica-based optical waveguides actually used for AWG (Arrayed Waveguide Gratings) etc. Compared to <0.05, the propagation loss is also large. Similarly, in the case of a photonic crystal type optical waveguide, the actual propagation loss is extremely large, and in many cases it is not practical.

In order to reduce the roughness of the side wall, it is conceivable to improve the etching process. However, this requires enormous costs for the introduction of an etching apparatus and an exposure apparatus, and its effect is uncertain.
Further, in order to reduce the surface roughness of the side wall, there has been proposed a method of reducing the surface roughness by performing an oxidation process on the etched surface and forming an oxide film (see Non-Patent Document 2, Patent Documents 1 and 2). ). However, this method has a problem that the cross-sectional size of the optical waveguide changes due to the formation of the oxide film. In addition, surface roughness may remain on the side wall even after the oxidation treatment (see Non-Patent Document 3).

K.K.Lee et al., Appl. Phys. Lett., 77, pp. 1617-1619 (2000) K.K.Lee et al., Opt.Lett., 26 (23), pp.1888-1890 (2001) L. Pavesi, Silicon Photonics, Springer, 2004 Special Publication 2004-510181 PCT / US2001 / 041632

  An object of the present invention is to provide a low propagation loss optical waveguide that can be easily manufactured at low cost, has a small bending radius, and has a small propagation loss, and a method for manufacturing the same.

The method for producing a low propagation loss optical waveguide according to the present invention comprises forming an optical waveguide in a single crystal layer using a substrate having a single crystal layer of Si , and performing a heat treatment in an argon atmosphere at a temperature of 500 to 1350 ° C. The present invention is characterized in that a minute surface roughness existing on the side surface of the optical waveguide is improved. Note that an SOI (Silicon on Insulator) is preferably used for the substrate. Moreover, it is preferable to remove the natural oxide film on the surface of the single crystal by immersing in an aqueous solution containing fluorine ions before the heat treatment.
The top clad layer may be formed after heat treatment if necessary, and may be formed using a CVD (Chemical Vapor Deposition) method typified by plasma CVD or a PVD (Physical Vapor Deposition) method such as a sputtering method. Can do. The optical waveguide is a refractive index guided optical waveguide type or a photonic crystal type.

The low propagation loss optical waveguide of the present invention is obtained by forming an optical waveguide in a single crystal layer using a substrate having a single crystal layer of Si , and performing heat treatment in an argon atmosphere at a temperature of 500 to 1350 ° C. It is characterized by improving minute surface roughness existing on the side surface of the waveguide. Note that the top clad layer may be appropriately provided according to the application if necessary after the surface roughness is improved by heat treatment.

  According to the present invention, the minute surface roughness existing on the side surface of the optical waveguide is improved, and a low propagation loss optical waveguide with a small propagation loss is obtained. By using this, a small optical device can be manufactured.

In the present invention, a general method as shown in FIG. 3 is used until an optical waveguide is formed in a single crystal layer on a substrate. At the stage where the optical waveguide is formed, surface roughness (micro roughness) that causes propagation loss exists on the side wall of the optical waveguide. In other words, the surface roughness has a large surface area compared to a flat and smooth case.
The present inventor considers that the surface roughness of the side wall is reduced if the surface area is reduced, and as a result of intensive studies, it has been found that it is effective to smooth the rough rough surface. The present invention was achieved by heat treatment in an argon atmosphere at a temperature of 500 to 1350 ° C. after the formation of the waveguide.

  The present invention is characterized in that heat treatment is performed to reduce the surface roughness of the side wall. Specifically, high-temperature annealing is performed in an argon atmosphere in a short time. As a result, silicon is rearranged in order to reduce the surface energy of the sidewalls and tries to take the minimum surface area, so that the microroughness is reduced and the surface roughness becomes smooth, thereby propagating the optical waveguide. Loss is also reduced.

The reason why the heat treatment is performed in an argon atmosphere is that when an active gas such as oxygen is used, a film such as an oxide film is formed on the silicon surface, and the side walls are fixed by the film. This is because it does not occur sufficiently.
Heat treatment in an argon atmosphere has an excellent feature that it can improve the minute surface roughness of the sidewall considered to be on the order of nm while minimizing the deformation of the cross-sectional shape of the waveguide on the order of μm or sub-μm. .

The reason for not using hydrogen, hydrogen and argon or other inert gas in the heat treatment atmosphere is that hydrogen promotes rearrangement of silicon atoms and minimizes the surface area, but at the same time silicon In addition to etching the surface and changing the cross-sectional size of the waveguide, hydrogen gas is explosive and difficult to handle. Furthermore, when hydrogen gas is used, rearrangement of silicon atoms on the surface proceeds excessively, and the cross-sectional shape of the waveguide may be destroyed.
Further, in the nitrogen atmosphere, the rearrangement of silicon atoms hardly proceeds and the propagation loss is not improved.

  When the heat treatment is performed at a temperature lower than 500 ° C., a sufficient smooth surface cannot be obtained. When the heat treatment is performed at a temperature higher than 1350 ° C., the shape of the optical waveguide changes. The time required for smoothing varies depending on the heat treatment temperature, but if it is too short, the variation in quality (smoothing) will increase. Is preferable.

  The present invention can use various existing heat treatment facilities by using argon which is an inert gas. A sufficient effect can be expected at low cost by adjusting the temperature, time, etc., depending on the type of existing equipment (diffusion furnace characterized by normal batch processing, Rapid Thermal Annealer: RTA, etc.).

In addition, after forming the optical waveguide, prior to the above heat treatment, it is preferable to remove the natural oxide film on the surface of the single crystal by immersing it in a fluorine ion-containing aqueous solution, thereby eliminating the heat treatment time for evaporating the oxide film. In addition, it is possible to obtain effects such as eliminating the need for considering new surface roughness due to in-plane unevenness of the oxide film evaporation rate.
Examples of the fluorine ion-containing aqueous solution include aqueous solutions of HF, NH ₄ F, and the like.

Optical waveguide may be either a refractive index waveguide-type optical waveguide type or photonic crystal type.

  An 8-inch SOI wafer was used as the substrate. The thickness of the buried oxide film layer to be the cladding layer is 3 μm, a single crystal silicon layer having a thickness of 0.2 μm is provided, a photoresist layer is formed thereon, and then the width of the single crystal layer is 0.5 μm by exposure / dry etching. A refractive index waveguide type optical waveguide shown in FIG. 4 having a length of 1 to 5 mm was produced. 4A is a cross-sectional view, and FIG. 4B is a plan view.

  Subsequently, heating was performed at 1000 ° C. for 1 minute in an argon atmosphere using a lamp heating furnace (RTA). The rate of temperature increase up to 1000 ° C. was 50 ° C./sec., And the temperature decrease from 1000 ° C. was 33 ° C./sec. Prior to this heating, in order to remove the natural oxide film on the silicon surface as a pretreatment, the wafer was immersed in 2% hydrogen fluoride solution for 30 seconds, then washed with pure water and then dried. After the heat treatment, a top cladding layer having a thickness of 4 μm was formed by plasma enhanced chemical vapor deposition (PECVD). In addition, in comparison with the argon atmosphere, in addition to the conventional heat treatment, it was also performed in a nitrogen gas and hydrogen gas atmosphere.

Both end surfaces of the obtained waveguide were polished, and propagation loss was measured by the Fabry-Perot resonance method and the cut-back method. The wavelength used for the measurement was 1.55 μm, and the TE mode was selected as the incident light. The results are shown in FIG.
It was observed that the loss was reduced in any atmosphere gas as compared to the case without heat treatment. In the case of a nitrogen atmosphere, the loss is larger than in other cases, but this is considered to be due to insufficient rearrangement of the silicon surface. In the case of hydrogen gas, the loss is larger than that of argon, but this is thought to be due to the collapse of the cross-sectional shape of the waveguide.

A refractive index waveguide type optical waveguide is formed in the same manner as in Example 1 except that a diffusion furnace is used and heating is performed at an atmospheric temperature of 850 ° C. for 1 hour, and a natural oxide film is removed, and argon, nitrogen and hydrogen are formed. After performing the heat treatment for each of the gases, a top clad layer was formed. Both end faces of the obtained waveguide were polished, and propagation loss was measured by Fabry-Perot resonance method and cutback method. The results are shown in FIG.
The same results as in Example 1 were obtained, and the argon atmosphere was extremely effective in reducing the propagation loss.

The photonic crystal type optical waveguide shown in FIG. 7 was produced using an 8-inch SOI wafer as a substrate. The thickness of the SOI buried oxide layer was 0.5 μm and the thickness of the single crystal silicon layer was 0.3 μm to form an optical waveguide. The sizes of the constituent elements are as shown in FIG. 7, wherein (a) is a sectional view and (b) is a plan view.
Before the heat treatment, 30 HF buffer solution (Buffered HF: HF / NH ₃ solution) is used to remove the native oxide film on the silicon surface and remove the oxide film layer below the single crystal silicon layer. The wafer was immersed for 1 minute, then washed with pure water and then dried. Next, heat treatment was performed by heating at 830 ° C. for 1 hour in each atmosphere of argon, nitrogen, and hydrogen using a diffusion furnace.

Both end faces of the obtained waveguide were polished, and propagation loss was measured by Fabry-Perot resonance method and cutback method. The wavelength used for the measurement was 1.55 μm, and the TE mode was selected as the incident light. The results are shown in FIG.
The loss in the nitrogen atmosphere was almost the same as in Examples 1 and 2, but was significantly worsened in the hydrogen atmosphere. This is probably because the cross-sectional shape of the photonic structure was changed by hydrogen and the light confinement effect was weakened, resulting in an increase in loss. The loss in the argon atmosphere is extremely small, which is thought to be due to the preservation of the photonic structure itself while effectively suppressing minute roughness on the side surface.

  Contributes to miniaturization of optical devices.

It is sectional drawing which shows the outline of a refractive index waveguide type | mold optical waveguide, (a) is a channel type | mold waveguide, (b) is a rib type | mold waveguide. In the figure, (a) is a Hall-type photonic crystal waveguide, (b) is an outline of a pole-type photonic crystal waveguide, the top is a cross-sectional view, and the bottom is a plan view. It is sectional drawing which shows the outline of the manufacturing process of an optical waveguide. The outline of the refractive index waveguide type optical waveguide obtained in Examples 1 and 2 is shown, (a) is a sectional view and (b) is a top view. 3 is a graph showing measurement results of Example 1. 6 is a graph showing measurement results of Example 2. The outline of the photonic crystal type | mold optical waveguide obtained in Example 3 is shown, (a) is sectional drawing, (b) is a top view. 10 is a graph showing measurement results of Example 3.

Claims (10)

Using a substrate having a single crystal layer of Si, an optical waveguide is formed on the single crystal layer, and heat treatment is performed in an argon atmosphere at a temperature of 500 to 1350 ° C., thereby causing minute surface roughness present on the side surface of the optical waveguide. A method for producing a low propagation loss optical waveguide, characterized by The method for producing a low propagation loss optical waveguide according to claim 1, wherein the substrate is SOI .   The method for producing a low propagation loss optical waveguide according to claim 1 or 2, wherein the natural oxide film on the surface of the single crystal is removed by immersion in an aqueous solution containing fluorine ions before the heat treatment.   The method for producing a low propagation loss optical waveguide according to claim 1, wherein a top clad layer is formed after the heat treatment.   The method for producing a low propagation loss optical waveguide according to claim 4, wherein the top clad layer is formed by a CVD method or a PVD method.   The method for producing a low propagation loss optical waveguide according to claim 1, wherein the optical waveguide is a refractive index guided optical waveguide type or a photonic crystal type. Using a substrate having a single crystal layer of Si, an optical waveguide is formed on the single crystal layer, and heat treatment is performed in an argon atmosphere at a temperature of 500 to 1350 ° C., thereby causing minute surface roughness present on the side surface of the optical waveguide. A low propagation loss optical waveguide characterized by improving the above. The low propagation loss optical waveguide according to claim 7, wherein the substrate is SOI .   The low propagation loss optical waveguide according to claim 7 or 8, wherein a top clad layer is formed after the heat treatment.   The low propagation loss optical waveguide according to claim 7, wherein the optical waveguide is a refractive index guided optical waveguide type or a photonic crystal type.

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