JP2007206403A - Method of manufacturing optical crystal microstructure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To apply microfabrication of submicron order to a MiNbO<SB>3</SB>substrate. <P>SOLUTION: A crystal microstructure is equipped with an LiNbO<SB>3</SB>substrate 101, an Al<SB>2</SB>O<SB>3</SB>film 102, and an EB resist 103, where the EB resist 103 functions as an etching protection film during dry etching of the Al<SB>2</SB>O<SB>3</SB>film 102, and the Al<SB>2</SB>O<SB>3</SB>film 102, formed with a grooving pattern by etching, functions as an etching protective film during the etching of the LiNbO<SB>3</SB>substrate 102, and thereby a micropattern of a submicron side is formed in the LiNbO<SB>3</SB>substrate 101. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of manufacturing an optical delay element used for optical communication.

LiNbO ₃ (lithium niobate) has a large electro-optic constant, can form a low-loss photoconductive path in the communication wavelength band by Ti thermal diffusion or ion exchange, and LiNbO ₃ crystal has a diameter of 3 to 4 as an optical device material. Inch high-quality crystals can be obtained stably. Devices that use electro-optic effects such as optical modulators, optical gyros, optical switches, and electric field sensors, and acousto-optic effects are used. It has been used in many optical devices such as tunable filters.

In an optical fiber communication system, an optical modulator that functions to convert information into an optical signal and place the information on an optical fiber is important. An optical waveguide is formed on a LiNbO ₃ crystal having an electro-optic effect. The formed LiNbO ₃ modulator is used as a modulator for an optical system corresponding to a large-capacity main line.

FIG. 5 shows a schematic diagram of a LiNbO ₃ optical modulator.

The LiNbO ₃ optical modulator is a device that performs optical modulation by changing the refractive index using the Pockels effect, which is the primary electro-optic effect. Since the response speed of the electro-optic effect is very fast, a high-speed switching response is possible.

The optical modulator includes optical waveguides 502 and 503 having a Mach-Zehnder type structure formed on a LiNbO ₃ substrate 501 and modulation electrodes 504, 505, and 506, and light incident on the optical modulator is an electrode. Is emitted after being modulated in intensity. In addition, a buffer layer made of SiO ₂ or the like having a lower refractive index than that of the LiNbO ₃ substrate 501 is formed between the electrodes 504, 505, and 506 and the LiNbO ₃ substrate 501 in order to prevent light wave loss due to the electrode metal. Is done.

  The operation of the optical modulator will be briefly described.

  The incident light 507 is divided into two equal parts at the branch point 508 (510, 511), passes through the waveguides 502 and 503, and joins at the branch point 509.

  When voltages are applied in the directions of the electrodes 504 to 505 and 504 to 506, the incident light 57 is divided into two equal parts at the branch point 58, and electric fields in opposite directions are applied to the waveguides 502 and 503, respectively. Can be given a phase change of Δφ, and the waveguide 503 can be given a phase change of −Δφ.

  Here, Δφ is given by the number (1).

ただし、Δｎ＝ｎ₀ ³ｒ₃₃Ｅ_z／２は結晶のｚ軸方向に電界Ｅ_zを印加した際の屈折率変化、Ｌは電極長、λは入射光の波長、ｎ₀はＬｉＮｂＯ₃の屈折率、ｒ₃₃は電気光学定数（ポッケルス係数）を表す。

However, Δn = n ₀ ³ r ₃₃ E _z / 2 is a change in refractive index when an electric field E _z is applied in the z-axis direction of the crystal, L is an electrode length, λ is a wavelength of incident light, and n ₀ is LiNbO ₃ . refractive index, r ₃₃ represents a electro-optic constant (Pockels coefficient).

  Here, when a voltage is applied so that the phase difference between the two waveguides 502 and 503 is 2Δφ = π, the light passing through the waveguides 502 and 503 is recombined at the branch point 509 in an antiphase state, They cancel out due to interference and are not emitted from the branch point 509. This voltage is called a half-wave voltage Vπ, and the intensity can be modulated by moving the applied voltage V back and forth between 0 [V] and Vπ [V].

  When an optical modulator is actually mounted as a device, it is necessary to reduce the size of the modulator due to problems such as cost and mounting area.

  Here, in order to reduce the size of the optical modulator without reducing the optical modulation efficiency of the optical modulator, the light and electric field interact with the optical waveguides 502 and 503 formed in the optical modulator shown in FIG. The structure which increases the time to do was examined. Specifically, a structure for reducing the moving speed of light passing through the optical waveguides 502 and 503 was examined.

  FIG. 6 shows an outline of the waveguide structure provided in the optical waveguides 502 and 503 of FIG. 5 that operates on the light of the 1.55 μm band examined.

With respect to the longitudinal direction of the optical waveguide 602 formed on the LiNbO ₃ substrate 601, holes by grooves are provided in a direction perpendicular to the longitudinal direction of the optical waveguide 602, and an uneven pattern 603 is formed. Here, as the design of the concavo-convex pattern 603, the concave width: 180 nm and the convex width: 180 nm. Further, a portion (herein referred to as a line defect portion 64) obtained by adding two convex structures is provided. The width of the line defect portion 604 is 360 nm. The optical waveguide 602 is provided with concavo-convex patterns 605 and 606 having 200 periods with the line defect portion 64 as the center of line symmetry. Here, in the waveguide structure of FIG. 6, the periodic structure of the concavo-convex pattern 603 is omitted except for a part (indicated by a dotted line in FIG. 6).

  FIG. 7 shows the waveguide characteristics related to the TM mode of the optical waveguide 602 introduced with the optical delay structure shown in FIG.

For example, when a groove depth of 300 nm is given to the structure of the concavo-convex pattern 603 on the LiNbO ₃ substrate 601 shown in FIGS. 7 to 6, the 1.5 μm band is affected by the periodic structure of the concavo-convex pattern 603. It is apparent that the group speed ratio v / c can be made 0.2 or less when the speed v of light passing through the optical waveguide is divided by the speed of light c in vacuum. That is, if the periodic structure of the uneven pattern 603 having unevenness with a depth of 300 nm or more can be manufactured, the speed of light passing through the fine periodic structure with unevenness with a depth of 300 nm or more can be reduced to 1/5 or less. It can be seen that a modulation efficiency of 5 times is obtained.

  That is, since Δφ in the equation (1) can be increased by five times, the electrode length L in the equation (1) is the difference between the light and the electric field in the optical waveguides 502 and 503 formed in the optical modulator shown in FIG. The interaction length (electrode length) can be reduced to 1/5, and the optical modulator can be miniaturized.

  As a conventional manufacturing method for producing the optical delay element structure, there is a processing technique having a high aspect ratio by dry etching. For example, Patent Document 1 describes a method for manufacturing a semiconductor device by dry etching.

  A method for manufacturing the semiconductor device disclosed in Patent Document 1 will be described below.

  As shown in FIG. 8A, a positive resist film 2-1 is spin-coated on the HgCdTe substrate 1 to a thickness of about 1 μm, and dried and solidified at a temperature of 100 ° C.

  Next, as shown in FIG. 8B, a metal film 3 made of aluminum is deposited to a thickness of about 0.5 μm or deposited by means such as sputtering. Next, as shown in FIG. 8C, a resist film 2-2 is applied on the metal film 3 to a thickness of about 1 μm, and a predetermined pattern having a width on the order of μm is formed by an exposure and development process. The metal film 3 is etched into a predetermined pattern using the patterned resist film 2-2 as a mask.

  Next, as shown in FIG. 8D, the lower resist film 2-1 is removed by dry development by a reactive ion etching method using oxygen plasma. In the step of removing the lower resist film 2-1, the uppermost resist film 2-2 is also removed at the same time.

  In this step, as shown in FIG. 9A, the surface of the metal film 3 is exposed. Therefore, as shown by an arrow A, the surface of the metal film 3 is oxidized by irradiating oxygen plasma. Since the metal film 3 is an aluminum film, a metal oxide film 4 made of an alumina film having a thickness of about 0.05 μm is formed by oxidation.

Next, as shown in FIG. 9B, CF ₄ gas gas plasma that can etch the HgCdTe substrate 1 is irradiated as shown by an arrow B, and the groove 5 is formed by etching the HgCdTe substrate 1 by a reactive ion etching method. Form.

  In this case, the etching selectivity of the HgCdTe substrate 1 and the metal oxide film 4 made of alumina is 1:50, and the etching selectivity of the HgCdTe substrate 1 and the metal film 3 made of aluminum is 1: 5. In the case of the present embodiment, the high aspect ratio groove 5 having a depth of about 5 μm can be formed in the HgCdTe substrate 1 by one etching and one etching mask formation.

Next, the metal film 3 made of aluminum remaining on the HgCdTe substrate 1 can be easily removed by removing it by a so-called lift-off method when the lower resist film 2-1 is removed with a resist film remover of an organic solvent. A groove 5 as shown in FIG. 9C can be formed in the HgCdTe substrate 1.
JP-A-5-275327 JP-A-60-191255 JP-A-61-188502

  However, in the conventional method for manufacturing a semiconductor device, as a mask for etching formed on the HgCdTe substrate 1, there is a 1 μm resist film 2-1 in the lowermost layer, and a metal film aluminum 3 is formed thereon. Since a semiconductor device manufactured by a conventional method of manufacturing a semiconductor device is used as an optical device, there is a problem that if the aluminum film 3 is left directly formed on the HgCdTe substrate 1, the aluminum film 3 absorbs the guided light. . Therefore, the process of removing the aluminum 3 will be needed at the end.

  Therefore, the conventional method for manufacturing a semiconductor device requires a step of lifting off and removing the resist film 2-1 immediately above the HgCdTe substrate 1.

In order to solve this problem, for the processing of LiNbO ₃ substrate by ICP dry etching, EB resist and Al ₂ O ₃ having a refractive index smaller than that of LiNbO ₃ substrate are used as a mask to transfer a fine pattern of EB resist. LiNbO ₃ performed accurately in the substrate, and LiNbO ₃ Al ₂ O ₃ is a lower refractive index than the LiNbO ₃ substrate after processing of the substrate is a method for producing an optical crystal microstructure which enables the use as a buffer layer.

In order to solve the above-described problems, the optical crystal microstructure manufacturing method of the present invention has an Al ₂ O ₃ film with a thickness of 30 nm to 90 nm so as to have a shape dimension to be processed on a LiNbO ₃ substrate. Then, an EB resist is formed thereon with a film thickness of 150 nm to 500 nm, and then a mask pattern of EB resist is selectively formed by development and exposure, and then ICP in a mixed gas atmosphere of Ar and CF ₄ The Al ₂ O ₃ film corresponding to the portion where the mask pattern of the EB resist is not formed is dug by dry etching, and then the Al ₂ O _{3 is} etched by ICP dry etching in an Ar gas atmosphere. An optical crystal microstructure manufacturing method is characterized in that the LiNbO ₃ substrate is dug in a portion corresponding to a dug portion of the film.

Further, Al ₂ O ₃ film formed on said LiNbO ₃ substrate is after etching the LiNbO ₃ substrate by ICP dry etching, a method of manufacturing an optical crystal microstructure characterized by not removed.

As described above, the optical crystal microstructure manufacturing method of the present invention has a structure in which an Al ₂ O ₃ film is formed on a LiNbO ₃ substrate and an EB resist is further formed thereon, whereby the EB resist is made of Al _2. O ₃ film serves as an etching protective film during dry etching, the etched Al ₂ O ₃ pattern is formed by film works as an etching protection film when the LiNbO ₃ substrate etching, and the LiNbO ₃ Al ₂ O ₃ than the substrate Since the film has a small refractive index, there is an effect that it is not necessary to remove the film from the LiNbO ₃ substrate.

Hereinafter, a method for manufacturing the optical crystal microstructure in the LiNbO ₃ substrate.

In the present invention, an EB resist and an Al ₂ O ₃ film are used as an etching protective film, and the LiNbO ₃ substrate is etched.

Because LiNbO ₃ the Al ₂ O ₃ film formed on the substrate have a dry etching selection ratio with Ar gas to the LiNbO ₃ substrate is 10 or more, by using the EB resist and the Al ₂ O ₃ film as an etching protection film, LiNbO It is possible to produce a fine groove shape with a high aspect ratio for _three substrates.

Furthermore, since the Al ₂ O ₃ film formed on LiNbO ₃ substrate is a refractive index less than 2.2 of the LiNbO ₃ substrate (1.7), it is not necessary to remove the LiNbO ₃ substrate after etching It is possible to provide a function as a buffer layer sandwiched between the LiNbO ₃ substrate and the electrode for applying an electric field to the LiNbO ₃ substrate.

The group velocity ratio characteristics of light in the 1.55 μm band depending on the etching depth of the concave and convex groove pattern 603 provided in the longitudinal direction of the optical waveguide 602 formed on the LiNbO ₃ substrate 601 shown in FIG. 6 (shown in FIG. 7) ), By giving an etching depth of the groove pattern 603 of 200 nm or more, the group velocity ratio characteristic of light in the 1.55 μm band is 0.35 or less. Further, the etching depth of the groove pattern 603 up to 500 nm is given from the 1.55 μm band light group velocity ratio characteristics (shown in FIG. 7) depending on the etching depth of the concave and convex groove pattern 603. A group velocity ratio characteristic of light in the 55 μm band is 0.01.

An uneven groove pattern having an etching depth of 200 nm or more and 500 nm or less of the uneven groove pattern 603 provided on the LiNbO ₃ substrate 601 in which the group velocity ratio characteristic of the light is 0.35 to 0.01 is provided. LiNbO ₃ illustrating embodiment examples of processing in the substrate specifically below.

  The dimensions, dimensional ratios, and positional relationships in the drawings used for explanation are not necessarily accurate. Hereinafter, preferred embodiments of a method for producing an optical crystal microstructure according to the present invention will be described with reference to the drawings.

An embodiment of the present invention is shown in FIG. In FIG. 1, 101 is a LiNbO ₃ substrate, 102 is an Al ₂ O ₃ film, and 103 is an EB resist film.

As shown in FIG. 1A, an Al ₂ O ₃ film 102 having a film thickness of 30 nm was formed on a LiNbO ₃ substrate 101 by sputtering or vapor deposition. Further, an EB resist film 103 was applied thereon with a thickness of 200 nm by spin coating. After application, pre-baking was performed at 190 ° C. for 25 minutes.

  Thereafter, as shown in FIG. 2, an exposure pattern 201 (shaded portion) of 30 μm × 0.18 μm was produced by electron beam exposure of the EB resist 103. As the arrangement of the exposure pattern 201, the shape of the exposure part 201 and the non-exposure part 202 is a periodic structure of about 30 μm × 0.36 μm.

  Further, the exposure pattern 201 of FIG. 2 was removed by performing a development process, and a mask pattern of the EB resist 103 having a width of 0.18 μm was formed as shown in FIG. Thereafter, post-baking was performed at 120 ° C. for 25 minutes to solidify the EB resist 3.

Subsequently, the Al ₂ O ₃ film 102 was etched by ICP dry etching on the non-masked portion of the EB resist 103 formed by the development process.

The dry etching conditions of ICP dry etching were RF Bias power: 100 W, the gas atmosphere was a mixed gas system of Ar and CF ₄ , and the total pressure in the chamber was 8 mTorr. Here, with respect to pre-Ar gas ratio of CF ₄ for the mixed flow rate of _{CF 4 (CF 4 / (Ar} + CF 4)), the etching rate selectivity of EB resist film 103 and the Al ₂ O ₃ film 102 in FIG. 3 I checked as there was.

Than 3, the selection ratio between the Al ₂ O ₃ film 102 and the EB resist film 103 in the Ar and regions gas ratio of CF ₄ is 12.5% ～25.1% for mixed flow rate of CF ₄ (Al ₂ O ₃ It was confirmed that 0.2 or more was obtained by dividing the etching rate of the film 102 by the etching rate of the EB resist film 103.

When the gas ratio of CF _{4 to} the mixed flow rate is 12.5% to 25.1% under etching conditions, the EB resist film 103 has a maximum thickness of about 150 nm in order to etch the 30 nm thick Al ₂ O ₃ film 102. It can be seen that it is etched.

As a result, a fine structure can be formed in the Al ₂ O ₃ film 102 as shown in FIG.

This time, describes dry etching of the Al ₂ O ₃ film 2 for the case the gas ratio of CF ₄ to the total flow rate of Ar and CF ₄ is 19%.

If for the mixed flow rate of the gas ratio of 19% CF _4, because the selection ratio between the Al ₂ O ₃ film 102 and the EB resist film 103 is 0.27, performs grooving by 30nm etching to the Al ₂ O ₃ film 102 At this time, the EB resist 103 is etched by about 110 nm. Therefore, after etching of the 30 nm Al ₂ O ₃ film 102, the EB resist 103 remains about 90 nm.

Next, ICP dry etching was performed on the LiNbO ₃ substrate 101 using the Al ₂ O ₃ film 102 onto which the pattern of the EB resist 103 was transferred by the etching process described above as a mask.

As conditions for ICP dry etching, the RF bias power is set to 100 W, the total pressure in the chamber is set to 8 mTorr, the gas atmosphere during ICP dry etching of the LiNbO ₃ substrate 101 is a mixed gas system of Ar and CF ₄ , and Ar and CF gas ratio of CF ₄ for the mixed flow rate of _{4 (CF 4 / (Ar +} CF 4)) with respect to, Al ₂ O ₃ film 102 and LiNbO ₃ etch rate selectivity of the substrate 101 (LiNbO ₃ the etching rate of the substrate 101 Al ₂ (Divided by the etching rate of the O ₃ film 102).

The etching rate of the LiNbO ₃ substrate 101 obtained by the above conditions, the etching rate of the Al ₂ O ₃ film 102, the etching rate selectivity of the Al ₂ O ₃ film 102 and the LiNbO ₃ substrate 101 shown in FIG. As shown in FIG. 4, when ICP dry etching was performed in a gas atmosphere containing only Ar gas, the etching rate selectivity between the LiNbO ₃ substrate 101 and the Al ₂ O ₃ film 102 was 14 and a very high etching rate selectivity was obtained. .

For this reason, when dry etching is performed under the conditions of RF Bias power: 100 W and the total pressure in the chamber of 8 mTorr, the Al ₂ O ₃ film 102 having a thickness of 30 nm is etched by 22 nm, which is shown in FIG. Thus, a groove processing depth of about 330 nm was obtained on the surface of the LiNbO ₃ substrate 101.

At this time, from the calculation result obtained from the mode coupling theory in FIG. 7, by forming a groove processing depth of 330 nm on the surface of the LiNbO ₃ substrate 101, a group velocity ratio of 0.15 is obtained, and the optical delay element is It can be confirmed that it is obtained.

Further, although the Al ₂ O ₃ film 102 remains on the LiNbO ₃ substrate 101 after etching by 8 nm, it can be used as a buffer layer for the LiNbO ₃ substrate 101 when applied to an optical modulator, so that the Al ₂ O ₃ film 102 can be used. No removal is necessary.

In the present invention, a method for manufacturing a group velocity ratio of 0.15, which is a function as an optical delay element in the 1.5 μm band, is shown. The structural parameters such as the groove depth for forming the LiNbO ₃ substrate 101 by dry etching, the film thickness of the Al ₂ O ₃ film 102, and the film thickness of the EB resist 103 can be changed as appropriate.

  By the method for producing an optical crystal microstructure of the present invention, a microstructure such as an optical delay element can be produced on the optical crystal.

Sectional drawing in each dry etching process in the embodiment of the present invention The fine pattern figure formed in the EB resist in embodiment of this invention Graph showing the etch rate selectivity between the EB resist in the etching rate and the Al ₂ O ₃ film etching rate and the EB resist and the Al ₂ O ₃ film of Graph showing the Al ₂ O ₃ film etch rate selectivity between the etching rate and the etching rate of the LiNbO ₃ substrate and the Al ₂ O ₃ film and the LiNbO ₃ substrate It shows a schematic of a LiNbO ₃ optical modulator The figure which shows the outline of the structure of the optical delay element examined by this invention The graph which shows the delay characteristic by the mode coupling theory of the optical delay element examined by this invention The figure which shows the manufacturing method of the conventional semiconductor device The figure which shows the manufacturing method of the conventional semiconductor device

Explanation of symbols

101 LiNbO ₃ substrate 102 Al ₂ O ₃ film 103 EB resist 201 Exposed portion 202 Non-exposed portion 501 LiNbO ₃ substrate 502 Waveguide 503 Waveguide 504 Electrode 505 Electrode 506 Electrode 507 Incident light 508 Branch point 509 Branch point 510 Branch light 511 Branch light Light 601 LiNbO ₃ substrate 602 Optical waveguide 603 Uneven pattern 604 Line defect 605 Uneven pattern of 200 periods 606 Uneven pattern of 200 periods 1 HgCdTe substrate 2-1 Resist film 2-2 Resist film 3 Metal film 4 Metal oxide film 5 Groove A Oxygen plasma B CF ₄ gas

Claims (2)

An EB resist having a thickness of 150 nm to 500 nm;
Al ₂ O ₃ having a film thickness of 30 nm or more and 90 nm or less,
A method for producing an optical crystal microstructure formed on a LiNbO ₃ substrate, comprising:
The fine pattern formed on the EB resist is transferred to Al ₂ O ₃ by ICP dry etching using a CF ₄ / Ar gas system with a CF ₄ flow ratio of 12.5% to 25.1%,
The fine pattern formed on the Al ₂ O ₃ is transferred onto the LiNbO ₃ substrate by ICP dry etching using Ar gas,
A method for producing an optical crystal microstructure, wherein a microstructure is formed on a LiNbO ₃ substrate. _{2. The} method of manufacturing an optical crystal microstructure according to claim 1, wherein Al ₂ O ₃ on the LiNbO ₃ substrate is not removed after the fine pattern is transferred to the LiNbO ₃ substrate.

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