JP2006018243A - Method for manufacturing wavelength converting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a wavelength converting element having a large light wavelength conversion efficiency and low transmission loss of light, with high accuracy so as to control a periodically polarization-inversion structure for stably manufacturing a wavelength converting element having a periodical polling inversion structure in an optical waveguide. <P>SOLUTION: In a step of forming an interdigital electrode 25 comprising periodical strip electrodes 23 and a common electrode 24 on a first surface 22 of a ferroelectric substrate in a single domain, having the first surface 22 and a second surface 27 facing each other, the pattern of the interdigital electrode 25 is formed and is further subjected to overetching to produce level difference on the surface of the ferroelectric substrate 21 and an insulating thin film 26 is formed on the first surface 22. Then an electric field is applied between the interdigital electrode 25 and a counter electrode 28 to form a periodically polarization-inverted region 29. Thus, a SHG (second harmonic generation) device with high conversion efficiency can be realized. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing an optical wavelength conversion element such as an optical second harmonic generation waveguide element in which an optical waveguide using a ferroelectric material having a nonlinear electro-optic effect is integrally formed.

  In recent years, a light emitting device such as a laser diode is used as a laser beam generating means for an optical recording medium such as a DVD. However, as a small and short wavelength coherent light source for the demand for higher density, an optical second harmonic wave is used. A laser beam using a generation element (hereinafter, an optical second harmonic generation element is abbreviated as an SHG element) has attracted attention.

  This SHG element is made of an inorganic oxide such as lithium niobate or lithium tantalate, and has a large nonlinear optical constant. As a technique for amplifying the output of the light that comes out by artificially matching the wave numbers of the input wave and the output wave of light incident on the SHG element, a method using a periodically poled structure has been proposed. .

  As a method for producing this periodically poled structure, (1) a method of forming a titanium (Ti) film for each period in a part of the optical waveguide and diffusing it by high-temperature heat treatment, (2) a proton exchange treatment, A method of rapid heating and quenching, (3) a method of using corona discharge, (4) a method of applying a high voltage to a comb-shaped electrode, and the like are known.

  Among them, the high voltage application method (4) is most promising because it has few steps in the process and can be applied to a bulk material.

  However, when a high voltage is applied to a fine pattern such as a comb-shaped electrode by this method, the domain-inverted region grows from the tip of the electrode. It has not been possible to form a domain-inverted structure having a domain-inverted region that has a sufficiently long linear domain-inverted region that is parallel to the entire region of the inversion region length. For this reason, an SHG element having high efficiency of the generated SHG wave has not been obtained particularly for generation of ultraviolet or blue light having a short wavelength.

As prior art document information related to the invention of this application, for example, Patent Document 1 is known.
JP-A-4-335620

  When an SHG element is formed by providing a periodic domain-inverted region in a waveguide, ideally, the duty ratio is desirably 0.5 at any position in the waveguide. However, when a periodic domain-inverted structure is produced in the optical waveguide portion by applying a high voltage using a comb-shaped electrode, a uniform electric field is usually applied to the desired region to form the domain-inverted region. However, at that time, a domain-inverted region is formed due to the influence of the leakage electric field up to a region where there is no electrode other than the desired region. This is because the domain-inverted region grows also in the region protruding from the comb-shaped electrode. Since the growth has not been suppressed so far, it is difficult to control the domain-inverted structure formed. Only devices with a domain-inverted boundary showing a linear growth pattern were obtained.

  As a result, in the cross section perpendicular to the first surface and parallel to the waveguide direction, the polarization reversal boundary in the waveguide thickness direction becomes a curve whose center is swollen. In an element having such an inversion boundary, the length of polarization inversion in the waveguide direction is different between the vicinity of the center and the portion near the end (that is, the duty ratio is different). It has a problem that it is difficult to produce an SHG element that is large and small in value and has high SHG conversion efficiency.

  This invention is made | formed in view of the said subject, The place made into the objective is to provide the manufacturing method of an optical wavelength conversion element with large conversion efficiency.

  The method of manufacturing an optical wavelength conversion device according to the present invention includes a periodic strip electrode and a common electrode on a first surface of a monodomain ferroelectric substrate having a first surface and a second surface facing each other. A step of forming a comb electrode comprising: a step of forming a counter electrode on the second surface; a step of forming an insulator thin film on the first surface; and an electric field between the comb electrode and the counter electrode A method of manufacturing an optical wavelength conversion device comprising a step of forming a periodic domain-inverted region by applying a step, and a step of removing the insulator thin film and the comb-shaped electrode, wherein the pattern of the comb-shaped electrode is formed After forming a comb-shaped electrode pattern using dry etching and performing over-etching, a step is provided on the surface of the ferroelectric substrate, and the boundary of the domain-inverted regions formed on the ferroelectric substrate Over almost the entire region The parallel straight, but with a reduced position dependency of an inversion forming region of the duty ratio which is the ratio of the width and the period of the domain inversion regions.

  An optical wavelength conversion element having a domain-inverted structure in an optical waveguide according to the present invention has a sufficiently large domain-inverted region boundary in a straight line parallel to the entire length of the domain-inverted region when used as a short wavelength SHG device. As a result, an excellent SHG element with little position dependency in the inversion formation region of the duty ratio can be manufactured regardless of variations in the waveguide formation position with respect to the inversion region, and the conversion efficiency of the SHG element can be improved. Can do.

(Embodiment)
Hereinafter, the manufacturing method of the optical wavelength conversion element in embodiment of this invention is demonstrated, referring drawings.

  FIG. 1 is a configuration diagram of an SHG element according to the present invention, in which a ferroelectric substrate 21 having a domain-inverted region 29 and a support substrate 13 are bonded together by a main surface having a direct bonding region 14 and a non-direct bonding region 15. In this configuration, the optical waveguide 16 is formed on the bonded substrate. A manufacturing method of the optical wavelength conversion element configured as described above will be described in detail. FIG. 2 is a diagram for explaining a main part of the manufacturing method according to the embodiment of the present invention.

First, a lithium niobate substrate doped with 5 ° Y-cut MgO polarized in the Z direction is used as the ferroelectric substrate 21, and the first surface 22 of the ferroelectric substrate 21 has a thickness of about 150 nm. An aluminum film is formed, a resist pattern is formed thereon by photolithography, and then a comb-shaped electrode 25 including a strip electrode 23 and a common electrode 24 is formed by dry etching. For dry etching, a mixed gas of Cl ₂ and BCl ₃ is used to perform further over-etching even after the comb-shaped electrode pattern is formed, and parts other than the electrode are shaved by about 10 nm. Overetching is under the same conditions as aluminum etching may be performed only by extending the time, increasing the proportion of BCl ₃ to increase the proportion of ionic etching, or may be used gases such as Ar.

  Next, after removing the resist, an insulator thin film 26 made of silicon oxide is formed on the first surface 22 with a thickness of about 100 nm. By doing in this way, the stress state of the portion where the insulator thin film 26 on the surface of the first surface 22 is adhered can be made a compressive stress state of about 150 MPa.

  With the insulator thin film 26 attached to the first surface 22, a pulse voltage of about 5 kV is applied between the common electrode 24 and the counter electrode 28 provided on the second surface 27 to spontaneous polarization of the MgO: LN substrate. Application is made in insulating oil so that the positive side becomes positive potential and the negative side becomes negative potential, and a domain-inverted region 29 is grown in the Z-axis direction from the vicinity of the tip of the strip electrode 23. Note that the step of providing the counter electrode 28 on the second surface may be performed at any stage before the pulse voltage is applied.

  Thereafter, after the insulator thin film 26 and the comb-shaped electrode 25 are removed, the first surface 22 side and the support substrate 16 are joined, the second surface 27 side is polished to a predetermined thickness, and then the ridge is further removed. By processing, an optical wavelength conversion element having periodic polarization inversion in the optical waveguide as shown in FIG. 1 can be obtained.

  In order to confirm the effect of the present invention, a pulse voltage is applied as shown in FIG. 2 to form the domain-inverted region 29, and then at a position where the optical waveguide 16 is formed in a later step, parallel to the waveguide direction. The shape of the domain-inverted region 29 was confirmed by cutting and polishing the surface perpendicular to the first surface 22 (CC in FIG. 2) and performing etching. FIG. 3A is a cross-sectional view of the domain-inverted region 29 formed according to the embodiment of the present invention, and the shape of the boundary of the domain-inverted region 29 is a straight line that is horizontally straight under the first surface 22. The domain-inverted region boundary in the direction perpendicular to the first surface 22 is also close to a straight line. This is considered to be because the growth of the domain-inverted region 29 is regulated by the step of the first surface 22 and the insulator thin film 26 and grows linearly. On the other hand, FIG. 3B is formed by a conventional method and has a shape in which the boundary of the domain-inverted region 29 is rounded. Note that the horizontal direction in these figures is the waveguide direction. 3A and 3B, if the polarization period is P and the width of the polarization inversion region 29 near the center is W, W / P is a duty ratio. If the width of the polarization determination region 29 in the near part is W1 and W2, in FIG. 3B according to the conventional method, W2 / P and W / P are different, but according to the embodiment of the present invention. In FIG. 3A, since W1 / P and W / P are almost the same, the duty ratio is constant for light passing through any position, so that the conversion efficiency as an SHG element is improved. Can do.

  By inputting about 50 mW light of the SHG element obtained as described above, an SHG output of about 3.5 mW is obtained, and for an output of about 2.8 mW produced by a conventional method, The conversion efficiency was improved by about 20%.

  Here, the reason why the effect of the present invention can be obtained will be described. The inventors have confirmed that when the surface of the ferroelectric material is kept in a compressive stress state, the polarization inversion becomes difficult to grow. By forming the insulator thin film 26 around the strip electrode 23, the ferroelectric surface other than the strip electrode 23 is brought into a compressive stress state, and the portions other than the comb-shaped electrode 25 are shaved by over-etching. Since the step is provided on the surface of the ferroelectric substrate 21 in the portion with and without the portion, the domain-inverted region 29 grown from directly under the strip electrode 23 is prevented from spreading to the periphery by this step, so that it is linear. Easy to grow. Thereby, the linearity in the substrate thickness direction at the domain-inverted region boundary can be improved, and the conversion efficiency can be improved.

  When the relationship between the effect of the present invention and the step on the surface of the ferroelectric substrate 21 was examined, when the step was smaller than 5 nm, the effect was reduced and the linearity in the substrate thickness direction at the domain-inverted region boundary was deteriorated. . On the other hand, when the thickness is larger than 100 nm, the linearity is not deteriorated, but the growth of the domain-inverted regions 29 is hindered, which is not preferable. Further, after the domain-inverted region 29 is grown, the first surface 22 is polished and bonded directly to the support substrate 13. However, if the step of the surface of the ferroelectric substrate 21 is larger than 100 nm, it takes time to polish. Or the substrate tends to warp. As described above, the step on the surface of the ferroelectric substrate 21 is preferably formed between 5 nm and 100 nm.

  Further, when the strength of the compressive stress was also examined, when the pressure was less than 100 MPa, the effect was reduced, and the linearity of the domain-inverted region boundary in the substrate thickness direction was deteriorated. On the other hand, when the pressure is higher than 200 MPa, the shape of the domain-inverted region 29 itself is different from the other invertible regions, and is deformed so that only the vicinity of the edge becomes the inverted region, and a desired inverted shape cannot be obtained. As described above, it is preferable that the compressive stress is formed between 100 MPa and 200 MPa.

The compressive stress is intended to make it difficult to grow the polarization inversion, and it is desirable that the compressive stress is not in a compressive stress state immediately below the strip electrode 23 on which the polarization inversion is grown. FIG. 4 is a diagram for explaining another embodiment of the present invention. After forming an insulator thin film 26 made of a silicon oxide thin film on the entire first surface 22, a resist pattern is formed, and a part of silicon oxide on the strip electrode 23 is removed using a mixed gas of CF ₄ and O _2. This is more preferable because the stress on the surface of the ferroelectric substrate 21 immediately below the strip electrode 23 can be relaxed. In an SHG element that normally emits blue light, the polarization inversion period is about 4.6 μm. Since the width of the strip electrode 23 is about half that of the strip electrode 23, even if the overlay accuracy of the resist pattern is taken into consideration, it can be sufficiently handled by a normal photolithography technique.

  In FIG. 2, a comb-shaped electrode 25 is provided on the first surface 22 and a counter electrode 28 is provided in the extending direction of the strip electrode 23 on the second surface 27, and a pulse voltage is applied therebetween to form polarization reversal. However, the present invention is not limited to this. As shown in FIG. 5, the comb electrode 25 and the floating electrode 53 are provided on the first surface 22 in the extending direction, and the comb electrode 25 and the floating surface on the second surface 27 are provided. The counter electrode 28 may be provided so as to face both the electrodes 53, and a pulse voltage may be applied between the comb electrode 25 and the counter electrode 28. Electric charges are induced in the floating electrode 53 by the voltage applied to the counter electrode 28, whereby an electric field is generated between the comb electrode 25 and the floating electrode 53, and an electric field generated between the counter electrode 28 and the comb electrode 25. And an electric field in an oblique direction, whereby the domain-inverted region 29 can be grown. In this way, the method of FIG. 2 requires a distance between the comb electrode 25 and the counter electrode 28, but this distance can be shortened and the area for forming the domain-inverted region 29 can be reduced. can do. Even in this case, the floating electrode 53 can be formed at the same time as the comb-shaped electrode 25 by photolithography, so that it is not necessary to add a process.

  In the method of manufacturing an optical wavelength conversion element according to the present invention, the boundary of a domain-inverted region formed on a ferroelectric substrate is made to be a straight line that is substantially parallel to the entire region, and a duty that is a ratio of the width of the domain-inverted region and its period The position dependency in the ratio inversion formation region is reduced, and the efficiency of the optical wavelength conversion element can be increased.

Configuration diagram of SHG element according to the present invention The figure for demonstrating the principal part of the manufacturing method of one Embodiment of this invention. The figure for comparing the polarization inversion area | region shape of this invention and a prior art example The figure for demonstrating another embodiment of this invention The figure for demonstrating the principal part of the manufacturing method of another embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 13 Support substrate 14 Direct bonding area | region 15 Non-direct bonding area | region 16 Optical waveguide 21 Ferroelectric board | substrate 22 1st surface 23 Strip electrode 24 Common electrode 25 Comb-shaped electrode 26 Insulator thin film 27 2nd surface 28 Counter electrode 29 Polarization inversion Region 53 Floating electrode

Claims (6)

Forming a comb-shaped electrode composed of a periodic strip electrode and a common electrode on the first surface of a monodomain ferroelectric substrate having a first surface and a second surface facing each other; Forming a counter electrode on the second surface; forming an insulator thin film on the first surface; and applying an electric field between the comb electrode and the counter electrode to periodically polarize A method of manufacturing an optical wavelength conversion device, comprising: a step of forming an inversion region; and a step of removing the insulator thin film and the comb electrode, wherein dry etching is performed in the step of forming a pattern of the comb electrode. A method for manufacturing an optical wavelength conversion element, wherein a step is formed on the surface of a ferroelectric substrate by performing over-etching after forming the pattern of the comb-shaped electrode. The method for producing an optical wavelength conversion element according to claim 2, wherein the step on the surface of the ferroelectric substrate is formed between 5 nm and 100 nm. A part of the ferroelectric substrate where the insulator thin film is formed is in a state of compressive stress, and an electric field is applied between the comb electrode and the counter electrode to form a periodically poled region. Item 2. A method for producing an optical wavelength conversion element according to Item 1. The method for producing an optical wavelength conversion element according to claim 3, wherein the compressive stress is in a state of 100 to 200 MPa. 2. The method of manufacturing an optical wavelength conversion element according to claim 1, wherein in the step of forming the comb-shaped electrode, a floating electrode is simultaneously formed on the first surface in the extending direction of the strip electrode. 2. The optical wavelength according to claim 1, wherein in the step of forming the insulator thin film on the first surface, after forming the insulator thin film on the entire first surface, at least a part of the insulator thin film on the strip electrode is removed. A method for manufacturing a conversion element.

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