JP2011043604A - Method of forming periodic polarization reversal structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a periodic polarization reversal structure so that the width of a polarization reversal region is equal to that of a non-polarization determining region and the polarization reversal period is uniform. <P>SOLUTION: A method of forming the periodic polarization reversal structure includes an insulating film forming step, an electrode contact step, and a polarization reversal structure forming step. In the insulating film forming step, an insulating film 20 (24) formed on a first main surface of the Z-surface of a ferroelectric crystal substrate is formed so that stripe-like openings 22 (26) are arranged periodically. The longitudinal direction of the stripe-like openings is matched with the direction parallel to the Y-axis direction of the crystal axis of the ferroelectric crystal substrate. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for forming a periodic domain-inverted structure on a parallel crystal plate and a single domain ferroelectric crystal substrate.

  A quasi-phase-matching wavelength converter that performs quasi-phase matching (QPM) using a periodically poled structure and performs wavelength conversion (hereinafter sometimes referred to as a “QPM-type wavelength converter”) has attracted attention. ing. This is because the QPM type wavelength conversion element enables wavelength conversion of wavelength-converted light having an arbitrary wavelength by setting the period of the periodically poled structure according to the wavelength of wavelength-converted light. With this excellent feature, the QPM type wavelength conversion element is being actively used not only in the field of optical fiber communication but also in the field of optical measurement.

  In addition, a ferroelectric crystal substrate having a periodically poled structure is used as the above-described wavelength conversion element, as well as an electro-optic polarizer (see, for example, Patent Document 1), a terahertz wave generator (See, for example, Patent Document 2) or optical modulators (see, for example, Patent Document 3) and have wide industrial applications.

LiNbO ₃ crystal or LiTaO ₃ crystal is actively used as the material for the QPM type wavelength conversion element, and the voltage application method is attracting attention as a promising formation method for forming the domain-inverted structure (for example, patent literature) 4).

  In the voltage application method, an electrode is placed in a region where polarization inversion occurs on the surface of the ferroelectric crystal substrate, that is, an electrode is placed in a cycle that satisfies the QPM condition, and high voltage is applied to this electrode to realize polarization inversion. Let However, there is a problem that it is difficult to form the polarization inversion region in a shape that accurately reflects the shape of the electrode pattern because a phenomenon occurs in which the polarization inversion occurs by protruding from the region directly under the electrode.

  In the wavelength conversion element, one of the conditions for realizing the greatest wavelength conversion efficiency is that the width of the domain-inverted region is equal to the width of the non-domain-inverted region. In other words, a common problem with the method of forming a periodically domain-inverted structure on a ferroelectric crystal substrate is that the domain-inverted period is uniform and the widths of the domain-inverted region and the non-domain-inverted region are equal. is there.

  In order to solve this problem, in the formation of the periodic domain-inverted structure by the voltage application method, the non-polarized domain is subjected to proton exchange treatment to degrade the ferroelectricity and suppress the domain inversion and further cover the electrode In this way, a technique has been proposed in which polarization inversion is prevented from spreading from a region covered with an electrode to a region not covered with an electrode by forming an insulating film on the surface of the ferroelectric crystal substrate (see, for example, Patent Documents). 5).

  Further, in forming a periodic domain-inverted structure by a voltage application method, a method of using an electrolyte solution as an electrode for voltage application means has also been proposed (see, for example, Patent Documents 6 to 8). In the following description, an electrode formed of an electrolyte solution may be referred to as a liquid electrode.

Japanese Patent Laid-Open No. 10-83001 JP 2005-77470 A JP-A-11-174390 Japanese Patent Laid-Open No. 08-220578 JP 2000-147484 A Japanese Patent No. 3999362 JP 2006-234939 A JP 2003-330053 A

  A method for suppressing the occurrence of polarization reversal by degrading ferroelectricity by subjecting a non-polarization inversion region to proton exchange treatment is a method that requires a proton exchange treatment step, is complicated, and is not suitable for mass production.

  In addition, even in the method of using an electrolyte solution as an electrode for the voltage application means, there is still room for improvement in forming the periodic domain-inverted structure so that the widths of the domain-inverted region and the non-domain-inverted region are equal. ing.

  In the formation of the periodic domain-inverted structure by the voltage application method, domain-inverted regions are periodically provided at a plurality of locations. Electrodes are formed in these domain-inverted regions, and the electrodes formed in the domain-inverted regions are connected to each other so as to be equipotential when a voltage is applied. An electrode portion for realizing this connection (hereinafter also referred to as an equipotential electrode) and an electrode portion formed in the domain-inverted region are formed of a continuous metal film. That is, both electrode portions are formed by patterning a single metal electrode film by etching.

  For this reason, the portion covered with the equipotential electrode for realizing the connection is also reversed in polarity. Since the polarization inversion phenomenon starts from a portion where the electric field strength is large, the polarization inversion starts from a connection portion between the equipotential electrode where the electric field strength becomes the largest and the electrode portion formed in the polarization inversion region. From this fact, the inventor of this application is that the region where the polarization inversion occurs at the end of the electrode portion formed in the polarization inversion region is wide, and the ratio of the width of the polarization inversion region to the width of the non-polarization inversion region is in the longitudinal direction. It was found that it was not formed uniformly.

  Further, the inventor of this application applied the voltage to the ferroelectric crystal substrate to realize the domain inversion structure in which the domain inversion structure is formed, and the formation speed of the domain inversion region is larger in the Y axis direction than in the X axis direction. I found this experimentally.

  Therefore, the inventors of this application sought a method of forming a periodic polarization reversal structure by a voltage application method without requiring an equipotential electrode for connecting the electrode portion formed in the polarization reversal region, Using a method that does not require an equipotential electrode that causes non-uniform polarization inversion periods, and applying the property that the formation speed of the domain inversion region is larger in the Y-axis direction than in the X-axis direction, polarization inversion We have convinced that it is possible to form a periodic domain-inverted structure so that the period is uniform and the widths of the domain-inverted region and the non-domain-inverted region are equal.

  Therefore, an object of the present invention is to form a periodically domain-inverted structure so that the domain-inverted period is uniform and the widths of the domain-inverted region and the non-polarized region are equal without requiring proton exchange treatment. It is to provide a method.

  According to the gist of the present invention, the method for forming a periodic domain-inverted structure has the following features.

  The method for forming a periodically poled structure according to the present invention includes an insulating film forming step, an electrode contacting step, and a domain-inverted structure forming step.

  The insulating film forming step is a striped opening having a width equal to each other on the first main surface of the Z plane of a parallel-crystal single-domain ferroelectric crystal substrate cut by a plane orthogonal to the direction of spontaneous polarization. This is a step of forming an insulating film having portions periodically provided.

The Z plane refers to a plane orthogonal to the Z axis of the ferroelectric crystal substrate. In addition, if either the Z ^- plane, which is the starting point of the spontaneous polarization vector representing the spontaneous polarization of the ferroelectric crystal, or the Z ⁺ plane, which is the end point of the spontaneous polarization vector, may be used, or if any plane is indicated, simply Z Sometimes referred to as a surface.

  The electrode contact step is a step of bringing the first electrode into contact with the opening on the first main surface of the ferroelectric crystal substrate and bringing the second electrode into contact with the second main surface facing the first main surface.

  The domain-inverted structure forming step is a step of forming a periodic domain-inverted structure on the ferroelectric crystal substrate by applying a voltage between the first electrode and the second electrode.

  Then, in the insulating film formation step, the insulating film is formed by providing the opening so that the length direction of the opening formed in the stripe shape is parallel to the Y-axis direction of the ferroelectric crystal.

  In the method for forming a periodically poled structure according to a preferred embodiment of the present invention, an electrolyte solution may be used as the first electrode and the second electrode.

  In the method for forming a periodic domain-inverted structure according to another preferred embodiment of the present invention, a metal thin film electrode may be used as the first electrode and the second electrode.

  In the method of forming a periodically poled structure according to a preferred embodiment of the present invention, the insulating film is preferably formed as a photoresist mask, a silicon nitride film, or a silicon oxide film.

  According to the method of forming a periodic domain-inverted structure of the present invention, the insulating film in which stripe-shaped openings having the same width are periodically provided is formed on the first main surface of the Z plane. And this opening part is not connected and formed. In addition, the first electrode is brought into contact with the stripe-shaped opening formed on the first main surface of the Z plane in the domain-inverted structure forming step, and this opening is formed at a portion that causes polarization inversion in the prior art. This corresponds to the parallel stripe portion of the comb-shaped electrode or ladder-type electrode.

  Therefore, according to the method for forming a periodic domain-inverted structure of the present invention, since there is no equipotential electrode for connecting the electrode parts formed in the domain-inverted region, the domain inversion is performed from the equipotential electrode. Will not spread. Therefore, it is possible to form the polarization inversion period uniformly and to make the widths of the polarization inversion region and the non-polarization inversion region equal.

  Further, the opening is provided so that the length direction of the opening formed in a stripe shape is parallel to the Y-axis direction of the ferroelectric crystal. This makes it possible to form the polarization inversion period uniformly even more strictly. This is because, as described above, the domain-inverted region has a characteristic of being formed earlier in the direction along the Y-axis than in the direction along the X-axis.

It is a figure where it uses for description of the formation process of the polarization inversion area | region by a comb-shaped electrode. (A) is a figure which shows a comb-shaped electrode pattern, (B) is a figure which shows the shape in the Z surface of the polarization inversion area | region formed by voltage application. It is a figure which shows a ladder type electrode pattern. It is a figure which shows the pattern of an insulating film. (A) is a diagram showing a case where the stripe-shaped opening is a rectangle, and (B) is a diagram showing a case where the stripe-shaped opening is a length extending over the entire Z plane of the ferroelectric crystal substrate. It is. FIG. 5 is a diagram for explaining an apparatus used in an electrode contact process when forming a periodically poled structure using an electrolyte solution for a first electrode and a second electrode, and is parallel to the Z axis of a ferroelectric crystal substrate. It is a schematic cutaway view cut and shown in any direction. FIG. 10 is a diagram for explaining a method of forming a periodic polarization inversion structure using a metal thin film electrode for the first electrode and the second electrode, and the Z axis of the ferroelectric crystal substrate in a state where the metal thin film electrode is formed. It is a schematic cut surface view cut and shown in a parallel direction.

  First, since there exists an equipotential electrode for connecting the electrode parts formed in the polarization inversion region, the situation where the polarization inversion starts from the end of the electrode part formed in the polarization inversion region is shown in FIG. This will be described with reference to FIG. An embodiment of the present invention will be described with reference to FIGS.

  Each of FIGS. 3 to 5 shows an example of the configuration according to the present invention, and schematically shows the cross-sectional shape and the arrangement relationship of each component to the extent that the present invention can be understood. However, the present invention is not limited to the illustrated example. Moreover, in each figure of FIGS. 1-3, although hatching is shown and shown to the component, these hatches do not represent a cross section, but in FIG.1 and FIG.2, it is an electrode part and in FIG. It only shows that it is an insulating film. In the following description, specific materials and conditions may be used. However, these materials and conditions are only one of preferred examples, and are not limited to these.

<Principle of formation of domain-inverted regions by voltage application method>
Referring to FIG. 1 (A), FIG. 1 (B), and FIG. 2, a typical shape of a metal electrode pattern that has been widely used for forming a domain-inverted region by a voltage application method is shown. The process of forming the domain-inverted region will be described. 1 (A) and 1 (B) are diagrams for explaining a process of forming a domain-inverted region by a comb-shaped electrode, and FIG. 1 (A) is a diagram illustrating a comb-shaped electrode pattern. B) is a diagram showing a shape on the Z plane of a domain-inverted region formed by voltage application. FIG. 2 is a diagram showing a ladder-type electrode pattern.

  In the comb-shaped electrode shown in FIG. 1 (A), the portion corresponding to the axis similar to the comb is the equipotential electrode 10C, and the parallel stripe portion 10D protruding perpendicularly to this axis is formed in the domain-inverted region. Corresponds to the electrode portion.

  A comb-shaped electrode pattern 10 shown in FIG. 1 (A) is obtained by patterning a metal thin film formed on the Z surface of a ferroelectric crystal substrate by photolithography. If polarization reversal occurs ideally, polarization reversal occurs only immediately below the comb-shaped electrode pattern 10 on the Z plane of the ferroelectric crystal substrate. However, as shown in FIG. 1B, the domain-inverted region 12 extends to a region other than the region immediately below the comb-shaped electrode pattern 10. The polarization inversion starts from the corner portion 14C of the electrode boundary line 14 where the electric field strength formed by the applied voltage is strong and gradually spreads.

  In FIG. 1B, an electrode boundary line 14 is superimposed on the domain-inverted region 12. It can be seen that the domain-inverted region 12 extends also to the inner side of the electrode boundary line 14 (indicated by a wavy line in FIG. 1B). In particular, since the electrode boundary line 14 has a large spread from the corner portion 14C, the boundary between the domain-inverted region 12 and the non-domain-inverted region has a substantially lens shape.

  Therefore, if the polarization inversion is performed by the comb electrode pattern 10 shown in FIG. 1 (A), the width of the polarization inversion region 12 is at the end (right side of FIG. 1 (B)) as shown in FIG. 1 (B). The shape becomes wider and gradually narrows as it moves to the left. That is, it can be seen that the domain-inverted period of the domain-inverted region 12 cannot be formed uniformly.

  Similarly, even when the domain-inverted region is formed by the ladder-type metal electrode 16 shown in FIG. 2, the equipotential electrode 16S for connecting the ladder portions formed on the left and right sides of the ladder-type metal electrode 16 is used. In the portion close to, the polarization inversion starts, the width of the polarization inversion region is widened on both sides of the ladder portion, and the boundary between the polarization inversion region and the non-polarization inversion region has a substantially lens shape. Therefore, even if the domain-inverted region is formed by the ladder-type metal electrode 16, the domain-inverted period and width of the domain-inverted region cannot be formed uniformly in the longitudinal direction.

<Method for Forming Periodically Polarized Inversion Structure of Embodiment of the Invention>
With reference to FIGS. 3A and 3B, an insulating film used in the method for forming a periodic domain-inverted structure according to an embodiment of the present invention and a process for forming the insulating film will be described. 3 (A) and 3 (B) show the Z plane of a parallel plate and single domain ferroelectric crystal substrate (not shown) cut by a plane perpendicular to the direction of spontaneous polarization. FIG. 3 is a diagram showing a pattern of an insulating film formed on a first main surface. FIG. 3 (A) is a diagram showing a case where the stripe-shaped opening is rectangular, and FIG. 3 (B) is a length in which the stripe-shaped opening extends over the entire Z surface of the ferroelectric crystal substrate. It is a figure which shows a case.

  As shown in FIG. 3A, stripe-like openings 22 are periodically formed in the insulating film 20. The stripe-shaped opening 22 is formed such that the longitudinal direction thereof coincides with the direction parallel to the Y-axis direction of the crystal axis of the ferroelectric crystal substrate 28. Further, as shown in FIG. 3B, stripe-shaped openings 26 are periodically formed in the insulating film 24, and the openings 26 reach from end to end of the Z plane of the ferroelectric crystal substrate 28. You may form in length.

  As shown in FIG. 3 (A), when the stripe-shaped opening 22 is formed in the insulating film 20 and the polarization inversion is performed by applying a voltage, the polarization inversion starts from the corner portions on both the left and right sides of the opening 22. However, polarization inversion in the Y-axis direction ends before the domain-inverted region spreads in the X-axis direction, and then the polarization inversion in the X-axis direction proceeds uniformly, so the width of the domain-inverted region is formed uniformly. Is done.

  The width of the stripe-shaped opening 22 is adjusted by adjusting the timing of voltage application when the polarization inversion region finishes polarization inversion in the Y-axis direction and the polarization inversion in the X-axis direction progresses uniformly. Is possible. In other words, the width of the opening 22 in the X-axis direction is set to be narrower than the width of the domain-inverted region in the X-axis direction, and the width of the domain-inverted region in the X-axis direction is designed by adjusting the voltage application time. By determining the value, it becomes possible to form a desired periodic domain-inverted structure. As for the width in the X-axis direction of the opening 26 in FIG. 3B, a desired periodic domain-inverted structure can be formed by adjusting the voltage application time in the same manner as the opening 22.

  Whether the opening is formed in FIG. 3 (A) or FIG. 3 (B) belongs to the matter of design of an element having a periodically poled structure.

  As the insulating film 20 or 24, any of a photoresist mask, a silicon nitride film, and a silicon oxide film can be used. The silicon nitride film or the silicon oxide film can be formed by a known method such as a sputtering method or a vacuum evaporation method. The opening 22 or 26 can be formed by a known photolithography and etching process.

  Next, an electrode contact process in which the first electrode is brought into contact with the first main surface of the ferroelectric crystal substrate and the second electrode is brought into contact with the second main surface facing the first main surface will be described.

  First, a method of using an electrolyte solution that is a liquid electrode for the first electrode and the second electrode will be described with reference to FIG. FIG. 4 is a diagram for explaining an apparatus used in an electrode contact process when forming a periodically poled structure using an electrolyte solution for the first electrode and the second electrode. It is a schematic cutaway view shown cut in a direction parallel to the Z axis.

The Z ⁺ plane of the ferroelectric crystal substrate 30 is set as the first main surface, and the Z ⁻ plane is set as the second main surface. On the first main surface, an insulating film 32 having a stripe-shaped opening is formed.

  The electrolyte solution 50 is configured to be in contact with the second main surface of the ferroelectric crystal substrate 30 as the second electrode. The electrolyte solution 50 is filled in the cylindrical first container 40, and is in contact with the second main surface with the O-ring 42 interposed therebetween so that the electrolyte solution 50 does not leak.

  On the other hand, the electrolyte solution 52 is configured to be in contact with the first main surface of the ferroelectric crystal substrate 30 as the first electrode. The electrolyte solution 52 is filled in a cylindrical second container 46, and is in contact with the first main surface with the O-ring 44 interposed therebetween so that the electrolyte solution 52 does not leak.

  A first electrode rod 36 is inserted into the electrolyte solution 52, and a second electrode rod 38 is inserted into the electrolyte solution 50. The second electrode rod 38 is inserted from the bottom of the first container 40, and is treated with an adhesive or the like so that the electrolyte solution 50 does not leak from the insertion portion of the second electrode rod 38.

As described above, the electrolyte solution 50 filled in the first container 40 and the electrolyte solution 52 filled in the second container 46 are respectively the Z ^- plane and the second main surface of the ferroelectric crystal substrate 30. 1 It is configured to be in contact with the Z ⁺ surface, which is the main surface. This completes the electrode contact process.

  When the electrode contact process is completed, the first electrode rod 36 is subsequently connected to the positive pole of the voltage source 34, and the second electrode rod 38 is connected to the negative pole of the voltage source 34 to apply a voltage, thereby reversing the polarization. A structure forming step is performed.

Here, a lithium niobate crystal (LiNbO ₃ ) substrate was used as the ferroelectric crystal substrate 30. The voltage source 34 supplied a voltage signal in the form of a pulse having a voltage of 6 kV / mm and a pulse width of several hundred milliseconds. When the domain-inverted structure forming step was performed in this manner, it was confirmed that the domain-inverted region and the non-domain-inverted region had the same width and the domain-inverted period was formed uniformly. The pulse width of the voltage signal supplied from the voltage source 34 corresponds to the voltage application time.

  When the domain-inverted structure forming process is completed, the electrolyte solutions 50 and 52 are removed by washing with water, and the insulating film 32 is removed by well-known chemical etching to complete the formation of the periodically domain-inverted structure on the ferroelectric crystal substrate 30. To do.

  As the electrolyte solutions 50 and 52, an aqueous solution of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl) or the like as an electrolyte can be appropriately used.

  Next, a method of using metal thin film electrodes for the first electrode and the second electrode will be described with reference to FIG. FIG. 5 is a diagram for explaining a method of forming a periodic domain-inverted structure using a metal thin film electrode for the first electrode and the second electrode, and a ferroelectric crystal substrate in a state where the metal thin film electrode is formed. FIG. 3 is a schematic cross-sectional view cut along a direction parallel to the Z axis.

The Z ⁺ plane of the ferroelectric crystal substrate 60 is set as the first main surface, and the Z ⁻ plane is set as the second main surface. On the first main surface, an insulating film 62 having a stripe-shaped opening is formed.

  A metal thin film electrode 72 is formed in contact with the first main surface of the ferroelectric crystal substrate 60 as the first electrode. A metal thin film electrode 70 is formed in contact with the second main surface of the ferroelectric crystal substrate 60 as the second electrode.

  As described above, when the formation of the first and second electrodes is completed, that is, when the electrode contact process is completed, the metal thin film electrode 72 as the first electrode is subsequently connected to the positive electrode of the voltage source 64 to be the second electrode. By connecting the metal thin film electrode 70 to the negative pole of the voltage source 64 and applying a voltage, the domain-inverted structure forming step is performed.

Here again, a lithium niobate crystal (LiNbO ₃ ) substrate was used as the ferroelectric crystal substrate 60, as in the case where a voltage was applied by the liquid electrode using the electrolyte solution described above. A voltage signal having a pulse voltage of 6 kV / mm and a pulse width of several hundred milliseconds was supplied from the voltage source 64. When the domain-inverted structure forming step was performed in this manner, it was confirmed that the domain-inverted region and the non-domain-inverted region had the same width and the domain-inverted period was formed uniformly.

  When the domain-inverted structure forming step is completed, the metal thin film electrode 72 as the first electrode, the metal thin film electrode 70 as the second electrode, and the insulating film 62 are removed by a well-known chemical etching, thereby forming the ferroelectric crystal substrate 60. The formation of the periodically poled structure is completed.

As the ferroelectric crystal substrate 30 or 60, lithium tantalate (LiTaO ₃ ), potassium titanyl phosphate (KTP: KTiOPO ₄ ), potassium niobate (KNbO ₃ ), etc. can be used as appropriate in addition to the above LiNbO _3. It is. In addition, in order to make it difficult for light damage to occur, the refractive index changes depending on the electric field of light, a strong element doped with elements such as magnesium (Mg), zinc (Zn), scandium (Sc), indium (In), etc. It is preferable to use a dielectric crystal substrate.

10: Comb electrode pattern
10C, 16S: equipotential electrode
10D: Striped part
12: Polarization inversion region
14: Electrode boundary line
16: Ladder type metal electrode
20, 24: Insulating film
22, 26: Opening
28: Ferroelectric crystal substrate
30, 60: Ferroelectric crystal substrate
32, 62: Insulating film
34, 64: Voltage source
36: First electrode rod
38: Second electrode rod
40: First container
42, 44: O-ring
46: Second container
50, 52: Electrolyte solution
70, 72: Metal thin film electrode

Claims (6)

Periodic stripe-shaped openings of equal width are provided on the first principal surface of the Z-plane of a parallel-plate, single-domain ferroelectric crystal substrate cut by a plane perpendicular to the direction of spontaneous polarization. An insulating film forming step for forming the insulating film;
An electrode contact step of bringing a first electrode into contact with the opening of the first main surface of the ferroelectric crystal substrate and contacting a second electrode with a second main surface opposite to the first main surface;
Including a domain-inverted structure forming step of applying a voltage between the first electrode and the second electrode to form a periodic domain-inverted structure in the ferroelectric crystal substrate,
In the insulating film formation step, the insulating film is formed by providing the opening so that the length direction of the opening formed in the stripe shape is parallel to the Y-axis direction of the ferroelectric crystal substrate. A method for forming a periodic domain-inverted structure, wherein:   2. The method for forming a periodic domain-inverted structure according to claim 1, wherein an electrolyte solution is used as the first electrode and the second electrode.   2. The method for forming a periodically poled structure according to claim 1, wherein a metal thin film electrode is used as the first electrode and the second electrode.   4. The method for forming a periodically poled structure according to claim 1, wherein the insulating film is formed as a photoresist mask.   4. The method for forming a periodic domain-inverted structure according to any one of claims 1 to 3, wherein the insulating film is formed as a silicon nitride film.   4. The method for forming a periodic domain-inverted structure according to claim 1, wherein the insulating film is formed as a silicon oxide film.

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