JP4893431B2 - Method for manufacturing polarization reversal element - Google Patents

Description

  The present invention relates to a method for manufacturing an optical functional element, an electronic device, etc., in which a polarization inversion structure is applied to a nonlinear optical crystal substrate made of a ferroelectric material.

When an electric field is applied to the nonlinear optical crystal substrate using an electron beam or a shape-processed electrode, a domain-inverted region can be formed at an arbitrary location, and an element having a memory function or a light modulation function can be manufactured.
For example, a method for forming a periodically poled structure as disclosed in Patent Document 1 has been known. In this method, with respect to a crystal substrate of lithium niobate (LiNbO ₃ ) cut in the Z plane, a comb-shaped first electrode is formed on the + C plane of the crystal substrate, and a planar second electrode is formed on the −C plane. Providing a predetermined polarization inversion voltage between the first electrode and the second electrode to grow a domain inversion region in the C-axis direction immediately below the first electrode to form a periodic domain inversion structure. there were. However, this manufacturing method has a problem in that the voltage application to the crystal substrate cannot be performed with good reproducibility because discharge occurs at the end of the comb electrode pattern to which the voltage is applied. Therefore, in the periodic domain-inverted structure created by this manufacturing method, the domain-inverted region that can be directly under the comb-shaped electrode pattern and the non-domain-inverted region of the part that was not electrode-reversed without the electrode pattern are not the same region width. It was uneven.

Further, as disclosed in Patent Document 2, there is a method of forming an electrode by forming a metal film after forming an insulating film pattern on a non-polarized surface. By using this method, there is no need to make the electrode into a comb shape, and there is an advantage that the discharge phenomenon, which is a disadvantage of the method of Patent Document 1 described above, can be prevented. However, in this polarization inversion method using an insulating film, an electric field leaks through the insulating film, so that the difference in electric field strength between the polarization inversion region and the non-polarization inversion region cannot be increased, and any reproducibility is arbitrary. It was difficult to form a pattern inversion structure. For example, when a 0.5 μm thick SiO ₂ insulating layer is formed on a 0.5 mm thick lithium niobate substrate and then created by this polarization reversal method, the difference in electric field strength between the non-polarization reversal region and the polarization reversal region There is a problem that even if the holding temperature of the substrate and the applied voltage slightly change, polarization inversion occurs in the non-polarization inversion region, and a uniform periodic polarization inversion structure cannot be formed.

  Further, as disclosed in Patent Document 3, a film having slit-like holes is placed on the upper surface of the nonlinear optical crystal substrate, and the conductive liquid is brought into contact with the substrate through the slit-like holes. There was a method of applying a polarization reversal voltage using the portion as an electrode. By using this method, a metal electrode pattern having a comb-teeth structure is not used, and an air layer can be used as an insulating layer. Therefore, a discharge phenomenon and electric field leakage can be suppressed, and a domain-inverted region can be used even when it is not necessary. Was able to be prevented. However, in this method, since the contact width between the conductive liquid and the substrate depends on the pressure applied to the conductive liquid, it is difficult to control the electrode width, and the non-uniformity in the width of the domain-inverted region has not been improved.

  Also, as disclosed in Non-Patent Document 1, a method has been proposed in which a mold having an electrode pattern shape is pressed against a substrate to form an electrode. In this method, the air layer becomes an insulating layer, and dielectric breakdown and electric field leakage are suppressed. However, there is a problem that the contact state between the electrode pattern of the mold and the nonlinear optical crystal substrate is not stable, and it is difficult to uniformly manufacture a domain-inverted region having a certain width.

JP-A-4-19719 (pages 4-5, FIG. 1) Japanese Patent Laid-Open No. 9-230403 (pages 3 to 4, FIG. 1) Japanese Patent Laid-Open No. 10-48680 (page 3, FIG. 2) M.M. Sato, P.M. G. R. Smith and D.C. C. Hanna, “Contact electrode method for fabricating bulk periodically polled LiNbO3”, Electronics Letters, Vol. 34, pp. 660-661, 1998.

  The present invention has been made to solve the above-described problems caused by the discharge phenomenon and the decrease in electrode width accuracy, and the manufacture of a domain-inverted device that realizes a uniform and high-accuracy periodic domain-inverted structure with a high production yield. It aims to provide a method.

The method of manufacturing the polarization inversion element of the present invention is to form an electrode having a contact portion for contacting a substrate on a substrate of nonlinear optical crystal, a method of making a polarization inversion element by applying a voltage to the electrodes, a predetermined A step of forming a sacrificial layer of a photosensitive resin material provided at intervals of photolithography , and a substrate between the sacrificial layer and the sacrificial layer so that an electrode contact portion is formed between the sacrificial layer Forming an electrode material and forming an electrode made of the electrode material; removing the sacrificial layer after forming the electrode material; and applying a voltage to the electrode to invert the polarization after removing the sacrificial layer. It is characterized by having.

According to the method of manufacturing a polarization inverting element according to the present invention, a sacrificial layer of a photosensitive resin material provided at a predetermined interval is formed by photolithography, and an electrode material is formed between the sacrificial layers to form an electrode. By forming a contact portion that contacts the substrate, the contact portion between the electrode and the substrate can be controlled with the dimensional accuracy of photolithography, so that a polarization inversion element that realizes a uniform and highly accurate domain-inverted structure with a high manufacturing yield A method can be provided.

Embodiment 1 FIG.
A method of manufacturing a polarization inversion element for periodically forming a polarization inversion structure in a nonlinear optical crystal by polarization inversion will be described. FIGS. 1-7 is the figure which showed the manufacturing method of the polarization inversion element which concerns on this Embodiment 1 in order of a process. Each figure is a perspective view of the polarization inverting element in the process, and also shows the element end face. In order to avoid complicated explanation, a lithium niobate substrate is simply referred to as a “substrate”, and an embodiment of the present invention will be described in detail with reference to the drawings.
In addition, what attached | subjected the same code | symbol is the same or it corresponds, This is common in the whole text of a specification. Further, the description of the constituent elements appearing in the whole specification is merely an example and is not limited to these descriptions. Further, although the manufacturing method described below will be described by taking a lithium niobate substrate as an example, any non-linear optical crystal that undergoes polarization inversion by applying an electric field can be used without being limited to lithium niobate.

  Referring to FIG. 1, first, a metal film to be the second electrode 102 is formed on the back surface of a substrate 101 having a thickness of 0.5 mm by a sputtering method. For example, Cr can be used as the metal film. Here, the surface of the substrate 101 to which the second electrode 102 attached in FIG. 1 is attached is the −C surface of the lithium niobate substrate. Note that the surface opposite to this surface is the + C surface.

  Next, referring to FIG. 2, a photosensitive material (for example, a novolac positive photoresist) having a thickness of 2.0 μm is applied to the surface of the substrate 101, and a line-and-space pattern is formed using photolithography. A sacrificial layer 106 is formed. Note that the surface on which the sacrificial layer 106 is formed is the + C surface of the lithium niobate substrate.

  Further, as shown in FIG. 3, the first electrode 104 is formed by sputtering a metal film having a thickness of 0.5 μm. For example, Cr can be used as the metal film. Since the metal film is laminated on both the portion where the sacrificial layer 106 is present and the portion where the substrate 101 is exposed, the first electrode 104 is in direct contact with the + C plane of the substrate 101 where the sacrificial layer 106 is not present. A portion of the sacrificial layer 106 is formed so as to cover it.

  Next, as shown in FIG. 4, an etching hole 105 for patterning the metal film to remove the sacrificial layer 106 is formed. Then, the substrate 101, the first electrode 104, and the second electrode 102 are immersed in a chemical solution (for example, acetone) that does not damage, and the sacrifice layer 106 is dissolved and removed through the etching hole 105. As a result, a tunnel-like structure 110 as shown in FIG. 5 is completed. Since the hollow portion of the tunnel is the portion where the sacrificial layer 106 was originally, it becomes an air layer after removal. Therefore, the surface of the substrate 101 in contact with the air layer becomes a region where the crystal is not reversed in polarity (non-polarized inversion surface 107) in the subsequent polarization inversion step.

  Here, in the step of removing the sacrificial layer 106, organic residues can be reduced by using a dry process such as plasma ashing. The material used for the metal film of the first electrode and the second electrode may be a base metal such as Ni, Al, Cu or Ti in addition to Cr, or a noble metal such as Au or Pt, or an etching solution for patterning. If there is, it can be used.

Next, the polarization of the substrate 101 manufactured through the above steps is reversed. Specifically, a voltage is applied between the first electrode 104 and the second electrode 102 on the substrate 101 to invert the polarization of the substrate 101. FIG. 6 is an explanatory diagram of such a process, and the vicinity of the portion in contact with the first electrode undergoes polarization inversion by application of voltage to become the polarization inversion region 103, and the portion not in contact with the first electrode shows the non-polarization inversion region 109. It becomes. Therefore, by appropriately adjusting the pattern dimensions of the line-and-space sacrificial layer 106, a periodically poled structure in which the domain-inverted region and the non-domain-inverted region are repeated at a desired cycle can be manufactured.
Here, the polarization inversion occurs when a polarization inversion electric field unique to the ferroelectric material is applied, and the intensity of the polarization inversion electric field of lithium niobate is about 20 kV / mm at room temperature. Therefore, in the example of the 0.5 mm-thick lithium niobate substrate described in this embodiment, the polarization inversion voltage is preferably about 10 kV at room temperature.

  Finally, when the first electrode 104 and the second electrode 102 are removed, a periodic polarization reversal element having excellent uniformity as shown in FIG. 7 can be manufactured. For example, when the electrode material is Cr, the electrode can be removed using an acid solvent.

  In the present embodiment, the electrode pattern for polarization inversion is formed by using a photolithography technique used in the semiconductor industry, so that a desired electrode pattern can be formed with high accuracy and uniformity. . Further, since the electrode pattern is formed by a technique used in the semiconductor industry, such as sputtering, the adhesiveness with the substrate is excellent, and the contact state with the substrate does not become a problem. Therefore, it is possible to control the width and shape of the part where the polarization is reversed precisely and uniformly over the entire substrate.

  Further, in the process of this embodiment, the pattern of the sacrificial layer is once formed and then removed, so that a portion where the electrode pattern is in contact with the substrate and a portion where the electrode pattern is not in contact are separately formed. Therefore, the electrode has a tunnel-like structure, and the portion where the sacrificial layer is provided becomes an air layer. By this manufacturing method, an air layer having a low dielectric constant can be used as the insulating layer, and the thickness of the insulating layer can be easily set to a desired thickness. That is, since the electric field applied to the non-polarization inversion region 109 is relaxed by the air layer, the difference in electric field strength between the polarization inversion region 108 and the non-polarization inversion region 109 can be easily increased. In the case of the present embodiment, an electric field of 20 kV / mm is applied to the domain-inverted region 108, but only an electric field of 15 Kv / mm is applied to the non-domain-inverted region 109. As a result, even if the holding temperature and applied voltage fluctuate by several percent from the set values, an electric field that causes polarization inversion is not applied to the non-polarization inversion region 109. Therefore, unlike the conventional case, inconvenient polarization inversion in the non-polarization inversion region does not occur, and furthermore, the contact area between the substrate and the electrode can be controlled with photolithography accuracy, so that a periodic polarization inversion structure can be formed with high accuracy and uniformity. Can do.

  In addition, by adjusting the shape of the sacrificial layer and the arrangement period of the sacrificial layer, a periodically poled structure in which the domain-inverted region and the non-domain-inverted region are repeated at a desired cycle can be manufactured. Therefore, a structure for obtaining a desired nonlinear optical characteristic can be easily created.

  Furthermore, in the process of the present embodiment, the etching hole is formed by patterning the metal film in order to remove the sacrificial layer. By providing this hole, the sacrificial layer can be easily removed, so that the process time and the process cost can be reduced.

In addition, when the sacrificial layer pattern has an elongated shape, an etching hole can be provided by opening a metal film near both ends of the sacrificial layer pattern. In this case, the periodic domain-inverted structure can be formed with higher accuracy and uniformity than in the case where the etching hole is provided near the center of the pattern of the sacrificial layer.
That is, in order to reverse the polarization of the nonlinear optical crystal, an electric field is applied to the crystal substrate via the electrode. Since this electrode shape affects the electric field, it is preferable that the electrode shape does not have a discontinuous portion.
Therefore, an etching hole that may affect the electric field is arranged as close to both ends of the sacrificial layer pattern as possible, and the polarization inversion process is performed. Since the central portion of the structure thus formed is a portion that is not affected by the disturbed electric field, it becomes a highly accurate and uniform periodic polarization inversion structure. Specifically, a domain-inverted structure is once created, and a crystal plate in a central portion that is not affected by a disturbed electric field is cut out and used.

  In this embodiment, as described above, there is no reduction in manufacturing yield due to a discharge phenomenon, electric field leakage, or a decrease in electrode width accuracy that has occurred in the prior art, and a highly uniform periodic polarization inversion structure is high. It is possible to provide a method for manufacturing a polarization inversion element that is realized with a manufacturing yield.

Embodiment 2. FIG.
In the first embodiment described above, an example in which a photosensitive material such as a novolac positive photoresist is used for the sacrificial layer has been described, but in this embodiment, a case where a metal is used for the sacrificial layer will be described.

  When a photoresist is used as a sacrificial layer, there is a drawback that the photoresist changes in quality when it becomes high temperature in the subsequent process and cannot be easily removed. Therefore, it was decided to avoid such a problem by using a metal for the sacrificial layer.

  The manufacturing method of the polarization inverting element of this embodiment is different from the process of the first embodiment except that the sacrificial layer is made of metal and the etchant when removing the sacrificial layer in the subsequent process is made of metal. Since it is the same process procedure as Embodiment 1, description of a process is abbreviate | omitted in order to avoid the complexity of description. Since the metal sacrificial layer needs to be removed by etching, it is necessary to select a metal resistant to the etchant for the first electrode and the second electrode.

  For example, Al is used for the sacrificial layer, and Cr is used for the first electrode and the second electrode. Moreover, hot phosphoric acid is used for the etchant. A periodic domain-inverted structure can be created through the same process procedure as in the first embodiment. If an etchant is selected as appropriate, base metals such as Ni, Cu, Cr or Ti can be used in addition to Al.

Further, depending on the metal species used for the first electrode and the second electrode, an inorganic insulating material such as SiO ₂ or Si ₃ N ₄ , Si, or poly-Si can be used for the sacrificial layer. For example, when Cr or Ta is selected as the electrode material, the sacrificial layer of SiO ₂ can be removed by using buffered hydrofluoric acid (a mixed solution of ammonium fluoride and hydrofluoric acid) as an etchant.

  In the present embodiment, in addition to the effects described in the first embodiment, a high-temperature process can be adopted, so that there is an advantage that the degree of freedom of the manufacturing method is further improved. In addition, the adverse effects caused by residues resulting from the use of organic materials such as photoresist for the sacrificial layer are also due to the use of metals or inorganic materials such as silicon, polysilicon, silicon oxide, and silicon nitride for the sacrificial layer. Can be solved.

It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention. It is the perspective view which showed the manufacturing process of Embodiment 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Nonlinear optical crystal substrate, 102 2nd electrode, 103 Polarization inversion area | region, 104 1st electrode, 105 Etching hole, 106 Sacrificial layer, 107 Non-polarization inversion surface, 108 Polarization inversion surface, 109 Non-polarization inversion area | region, 110 Tunnel Structure, 110 power supply

Claims (4)

  In a method of manufacturing a polarization reversal element by forming an electrode having a contact portion in contact with the substrate on the substrate of the nonlinear optical crystal and applying a voltage to the electrode, the polarization inversion element is provided at a predetermined interval on the substrate. Forming a sacrificial layer of photosensitive resin material by photolithography, and an electrode material on the substrate between the sacrificial layer and the sacrificial layer so that a contact portion of the electrode is formed between the sacrificial layer Forming the electrode made of the electrode material, removing the sacrificial layer after forming the electrode material, and applying a voltage to the electrode after removing the sacrificial layer to reverse polarization And a method for manufacturing a polarization inversion element.   2. The method of manufacturing a polarization inversion element according to claim 1, wherein sacrificial layers having a predetermined shape are periodically arranged on the substrate of the nonlinear optical crystal. 2. The method of manufacturing a polarization inversion element according to claim 1, wherein a part of an end portion of the electrode material formed on the sacrificial layer is opened, and the sacrificial layer is removed through the opening.   The method of manufacturing a polarization inverting element according to claim 3, wherein after the polarization inversion, an end portion of the substrate including a portion facing the opening is removed.

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