JP2005157210A - Manufacturing method of silicon optical waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a silicon optical waveguide in which the optical propagation loss is suppressed in the state where a stacking fault is not caused. <P>SOLUTION: A silicon core 108 is formed by forming a silicon thin line 107 by etching a SOI layer 103 by reactive dry etching, and then etching the side faces of the silicon thin line 107 by about 20nm by wet etching with an etching solution which is a mixture of hydrogen peroxide aqueous solution and ammonium fluoride liquid, in the state in which the top face of the silicon thin line 107 is covered with a master pattern 106. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a silicon optical waveguide having a silicon core composed of fine wires made of silicon.

Recent progress in optoelectronic technology has been remarkable, and optical waveguides with an optical wavelength of 1.5 μm have been developed that allow silicon cores to have a cross-sectional dimension of 500 nm or less and a bending radius of several μm. . Such a silicon optical waveguide uses a silicon-based insulating film such as a silicon oxide film or a silicon oxynitride film as a cladding.
With this optical waveguide, a minute optical integrated circuit can be formed. Silicon is a semiconductor material that is also used in electronic devices, and development of optoelectronic devices that combine the optical waveguides and electronic devices is also being studied.

  By the way, the above-described silicon optical waveguide is often manufactured by, for example, using an SOI (Silicon On Insulator) substrate and finely processing an SOI layer. In this microfabrication, as is well known, the SOI layer is etched by reactive ion etching using an oxide film or the like as a mask pattern. However, there is a problem in that fine irregularities are formed on the side surface of the core formed by such etching, which increases propagation loss. That is, in the above silicon optical waveguide, the refractive index difference between the core and the clad is large, so if there are fine irregularities on the surface of the core, the light guided by the fine irregularities will be scattered and propagation loss will increase. The above-mentioned problem occurs.

As a technique for solving the above-described problem, a method of oxidizing the side surface of a silicon core (Non-Patent Document 1) has been studied.
The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
Daniel K Sparacin, Kazumi Wada, Lionel C. Kimerling, "Oxidation Kinetics of Waveguide Roughness Minimization in Silicon Microphotonics", Technical Digest of Integrated Photonics Research, pp.129-131, Optical Society of America Topical Meetings (June 16-18,2003 )

  However, the above-described processing method has a problem that oxidation-induced stacking faults (OSF) are easily introduced into the silicon core in the step of oxidizing the side surface of the silicon core (Non-patent Document: Semiconductor Silicon Crystal Engineering) Fumio Shimura, publisher: Maruzen Co., Ltd.). Since the cross-sectional dimension of the silicon core is as small as 1 μm or less, when a stacking fault is introduced as described above, the introduced stacking fault may easily penetrate the silicon core and form a crack in the silicon core. .

  The present invention has been made in order to solve the above-described problems, and it is possible to form a silicon optical waveguide in which light propagation loss is suppressed in a state where stacking faults and the like are not introduced. Objective.

  A method of manufacturing a silicon optical waveguide according to the present invention includes a step of preparing an SOI substrate having a silicon layer formed on a substrate via a buried oxide layer, and forming an insulating film made of silicon oxide on the silicon layer. A step of forming a resist pattern extending in a predetermined direction on the insulating film, a step of etching the insulating film using the resist pattern as a mask to form a mask pattern, and after removing the resist pattern, Etching a silicon layer by a dry etching method having vertical anisotropy to form a silicon fine wire as a mask, etching a predetermined amount of the exposed side surface of the silicon fine wire by wet etching, and forming a silicon core; Forming an upper cladding layer to cover the silicon core after removing the mask pattern; At least comprising, wet etching, it is large etching rate for the silicon than the etching rate for the silicon oxide, in which the etching rate is to perform an etchant which does not depend on the crystal orientation of the silicon.

According to this manufacturing method, a silicon core is formed on a buried oxide layer as a lower cladding layer, a lower surface is covered with a lower cladding layer, and an upper surface is covered with a mask pattern. A side surface of the silicon core formed and exposed to dry etching is etched by a predetermined amount by wet etching.
In the method for manufacturing the silicon optical waveguide, the etching solution may be composed of a mixed solution of an ammonium fluoride solution and a hydrogen peroxide solution.

  As described above, according to the present invention, the side surface of the silicon core exposed to dry etching is etched by a predetermined amount by wet etching, so that the side surface of the silicon core is smoothed, and stacking faults occur. An excellent effect is obtained in that a silicon optical waveguide in which light propagation loss is suppressed can be formed in a state in which no light enters.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 are process diagrams showing an example of a method for manufacturing an optical waveguide in the embodiment of the present invention. 1 and 2 are schematic cross-sectional views schematically showing cross sections in each step.

First, as shown in FIG. 1A, the insulating film 104 is formed on the SOI layer 103 constituting the SOI substrate 100. The SOI substrate 100 includes a substrate portion 101 made of single crystal silicon, a buried oxide layer 102 made of, for example, silicon oxide formed thereon, and an SOI layer made of single crystal silicon and having a film thickness of about 500 nm. 103. An insulating film 104 can be formed by depositing SiO ₂ on the SOI substrate 100 configured as described above by, for example, a CVD method. For example, the insulating film 104 may be formed to a thickness of about 50 nm.

Next, as shown in FIG. 1B, a line-shaped resist pattern 105 extending in a predetermined direction is formed on the insulating film 104. The resist pattern 105 may be formed by a known lithography technique.
Next, the lower insulating film 104 is selectively removed by etching using the resist pattern 105 as a mask, so that a mask pattern 106 is formed as shown in FIG. For example, the mask pattern 106 may be formed by using a dry etching method having vertical anisotropy such as reactive ion etching.

  Next, after removing the resist pattern 105, the lower SOI layer 103 is selectively removed by etching using the mask pattern 106 as a mask, so that a silicon fine wire 107 is formed as shown in FIG. . Also in this etching, for example, a dry etching method having vertical anisotropy such as reactive ion etching may be used. By such dry etching, the silicon fine wire 107 can be formed in a state in which the cross section is substantially rectangular. The thin silicon wire 107 is a portion that becomes a core of an optical waveguide constituting the optical integrated circuit.

  Next, with the mask pattern 106 covering the upper surface of the silicon fine wire 107, the etching rate for silicon is higher than the etching rate for silicon oxide, and the etching rate is wet with an etching solution that does not depend on the crystal plane orientation of silicon. By etching, a predetermined amount of the side surface of the silicon fine wire 107 is etched. As a result, as shown in FIG. 1E, the silicon core 108 having a smoothed side surface is formed.

  For example, the side surface of the silicon fine wire 107 is etched by about 20 nm by wet etching using an etching solution in which hydrogen peroxide solution (30 wt%) and ammonium fluoride solution (ammonium fluoride aqueous solution: 40 wt%) are mixed. Is formed. The etching solution may be a mixture of the hydrogen peroxide solution 3 and the ammonium fluoride solution 2. Although silicon oxide is also etched by this etching solution, the silicon oxide etching amount is about 2 nm in film thickness, which is almost negligible.

  As shown in FIG. 3, the ratio of the etching rate of silicon with respect to silicon oxide can be increased by increasing the ratio of the hydrogen peroxide solution in the etching solution. By increasing the etching rate ratio, it is possible to etch only the side surface of the silicon fine wire 107 while the upper surface and the lower surface are protected by the mask pattern 106 and the buried oxide layer 102.

In the wet etching, the etching rate acts isotropically without depending on the crystal plane orientation of silicon, so that fine irregularities on the side surface formed by dry etching are smoothed.
Therefore, the side surface of the core 108 is formed in a smoothed state without fine irregularities.

  Finally, after removing the mask pattern 106, as shown in FIG. 2F, an upper clad layer 109 is formed on the buried oxide layer 102, and the core 108 is buried by the upper clad layer 109. . The upper clad layer 109 may be made of, for example, silicon oxide or silicon oxynitride. By forming the upper clad layer 109, an optical waveguide composed of the core 108 whose upper and lower sides are covered with the clad is formed.

  As described above, according to the present embodiment, the fine irregularities on the side surface of the silicon fine wire 107 formed by dry etching are smoothed by wet etching. As a result, the side surface of the core 108 is smoothed, and the propagation loss due to fine irregularities on the core surface is suppressed in the optical waveguide formed by the core 108.

It is process drawing which shows the example of the manufacturing method of the optical waveguide in embodiment of this invention It is process drawing which shows the example of the manufacturing method of the optical waveguide in embodiment of this invention It is a characteristic view which shows the correlation with the ratio of each component in an etching liquid, and the etching rate ratio of the silicon | silicone with respect to a silicon oxide.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... SOI substrate, 101 ... Substrate part, 102 ... Embedded oxide layer, 103 ... SOI layer, 104 ... Insulating film, 105 ... Resist pattern, 106 ... Mask pattern, 107 ... Silicon fine wire, 108 ... Silicon core, 109 ... Upper clad layer.

Claims (2)

Preparing an SOI substrate in which a silicon layer is formed on a substrate via a buried oxide layer, and forming an insulating film made of silicon oxide on the silicon layer;
Forming a resist pattern extending in a predetermined direction on the insulating film;
Etching the insulating film using the resist pattern as a mask to form a mask pattern;
After removing the resist pattern, using the mask pattern as a mask, etching the silicon layer by a dry etching method having vertical anisotropy to form a silicon fine wire;
Etching a predetermined amount of the exposed side surface of the silicon fine wire by wet etching to form a silicon core;
After removing the mask pattern, forming an upper clad layer so as to cover the silicon core,
The method for producing a silicon optical waveguide is characterized in that the wet etching is performed with an etching solution in which the etching rate for silicon is larger than the etching rate for silicon oxide and the etching rate does not depend on the crystal plane orientation of silicon. In the manufacturing method of the silicon optical waveguide according to claim 1,
The method for producing a silicon optical waveguide, wherein the etching solution is composed of a mixed solution of an ammonium fluoride solution and a hydrogen peroxide solution.

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