JP4866981B2 - Method for manufacturing quasi phase matching crystal and quasi phase matching crystal - Google Patents

Description

  The present invention relates to a method for manufacturing a quasi phase matching crystal and a quasi phase matching crystal.

Quasi-phase matching is realized by applying periodic stress to the paraelectric quartz (SiO ₂ ) near its α-β phase transition temperature to create a periodic twin structure and realize a periodically domain-inverted structure. Wavelength conversion optical elements have been proposed (S. Kurimura, R. Batchko, J. Mansell, R. Route, M. Fejer, and R. Byer: 1998 Spring Applied Physics Society Proceedings 28a-SG-18: Non-patent literature. 1). This utilizes Dauphine twin crystal, a method of fabricating a quasi-phase matched crystals by crystal by inverting the sign of the nonlinear optical constant d ₁₁ periodically.

In the case of quartz, the absorption edge has a wavelength of about 150 nm, and ultraviolet light absorption at a wavelength of 200 nm or less is achieved by conventional nonlinear optical elements (β-BaB ₂ O ₄ , CsLiB ₆ O _10, etc.) using birefringence phase matching, Compared to the case of using a nonlinear optical element (LiNbO ₃ , LiTaO _3, etc.) using a quasi-phase matching of a ferroelectric material, there is almost no difference. For this reason, light having a wavelength of about 193 nm equivalent to that of an ArF excimer laser can be generated with high efficiency by generating second optical harmonics, and a semiconductor exposure apparatus using the same has also been proposed (Japanese Patent Laid-Open No. 2002-122898). Publication: Patent Document 1). The crystal axis reversal period at this time is about 0.95 μm.

  As conventional quasi-phase-matched crystals, lithium niobate and lithium tantalate are well known, and research aimed at direct optical conversion in wavelength division multiplexing optical communication is active. However, in lithium niobate and lithium tantalate, photodamage due to the photorefractive effect has become a serious problem, and the use at high output is limited. On the other hand, the quartz crystal is not damaged by the photorefractive effect and can be used in a sufficiently stable state. The crystal axis inversion period at a wavelength of about 1.55 μm, which is often used for optical communication, is about 70 μm.

Furthermore, the crystal itself is a substance for which production technology has been established, and is easily available and can be kept at a low cost. The mechanical properties and chemical properties belong to the most excellent class of optical crystals. Further, there is no deliquescence that is noticeable in conventional nonlinear optical crystals using birefringence phase matching (for example, β-BaB ₂ O ₄ and CsLiB ₆ O ₁₀ ), which is very advantageous in terms of handling. . Furthermore, the d ₁₁ coefficient is about 0.3 pm / V, which is a little smaller than a general nonlinear optical crystal, and a sufficient conversion efficiency can be expected.

  Various methods are known as methods for producing an artificial twin structure in quartz, and a hot press method is currently proposed as a promising method. (S. Kurimura, I. Shoji, T. Taira, M. Fejer, Y. Usu, and H. Nakajima: 2000 Autumn Applied Physics Society Proceedings 3a-Q-1; Non-Patent Document 2). This method creates a periodic step structure on one surface of a quartz substrate, sandwiches the quartz substrate with a heater block from above and below, raises the temperature of the quartz substrate, and reaches a desired temperature. The pressure is applied. At this time, since the stress acts only on the portion corresponding to the convex portion of the step structure, the crystal axis component is inverted only in this portion. This crystal axis inversion portion grows to the inside of the crystal and propagates to the inside of the crystal, so that a periodic twin lattice having a large depth can be produced. That is, stress concentrates only on the convex portion, twins are generated therefrom, and gradually grows inward to produce a twin structure having a large aspect ratio.

  An example of such a quartz crystal substrate having a step structure formed on one surface will be described with reference to FIG. A step structure is formed on the surface of one side of the quartz substrate 1, and as a result, convex portions 2 having a rectangular parallelepiped shape are formed at predetermined intervals. This convex part 2 has a predetermined width in the left-right direction in the figure, and a plurality of convex parts 2 are formed in the left-right direction in the figure at the same interval as the width. The convex part 2 has a rectangular parallelepiped shape that is long in the depth direction of the figure. A direction parallel to the longitudinal direction is defined as an a-axis direction. Naturally, the a-axis is perpendicular to the normal line of the quartz substrate 1.

  The c-axis direction of the crystal of the crystal is perpendicular to the a-axis, but in the normal direction of the quartz substrate 1 as described in Non-Patent Document 3 (Naoshi Kurimura, May 2000 issue of the Japan Society of Applied Physics). It is slightly tilted. That is, the quartz substrate 1 is cut so that the normal line and the c-axis have a slight angle. The larger the angle, the easier it is for twins to be formed, but in practice it is kept at around 10-20 degrees. As shown in the drawing, the light enters from the end face of the quartz substrate 1 and the polarization direction is the same as the a-axis direction. The light whose wavelength has been converted inside the quartz substrate 1 is emitted from the end surface opposite to the incident surface.

  Hereinafter, an example of a method for producing the convex portion 2 as shown in FIG. 4 will be described. First, a Cr film is formed on the quartz substrate 1 by a sputtering method so as to have a thickness of about 100 nm. A positive resist is applied onto the film, and a portion other than the portion that becomes the convex portion 2 is exposed and developed using a semiconductor exposure apparatus such as an i-line stepper. Next, the Cr film is removed using the remaining resist as a mask. Then, using the remaining resist film and Cr film as a mask, wet etching is performed with hydrofluoric acid to produce a step structure having a depth of about several μm. Thereby, the quartz substrate 1 having the step structure having the convex portions 2 as shown in FIG. 4 formed on the surface is completed. Note that the Cr film may or may not be peeled off before pressing.

  The smaller the angle between the c-axis and the quartz substrate normal, the better the wavelength conversion device, but the greater the stress required for twin formation. Therefore, the crystal is cut so that the angle between the c-axis and the crystal substrate normal is about several degrees to about 20 degrees.

  Although not a known technique, it is also conceivable to use dry etching instead of the above-described wet etching. In this case, for example, a Cr film is first formed on the quartz substrate 1 by a sputtering method so as to have a thickness of about 100 nm. A positive resist is applied onto the film, and a portion other than the portion that becomes the convex portion 2 is exposed and developed using a semiconductor exposure apparatus such as an i-line stepper. Then, the Cr film is removed using the remaining resist as a mask. Thereafter, using the remaining resist film and Cr film as a mask, dry etching such as RIE or ICP is performed to produce a step structure having a depth of about several μm. Finally, by removing the resist film and the Cr film, the crystal substrate 1 having the step structure having the convex portions 2 as shown in FIG. 4 formed on the surface is completed.

  In order to obtain a quartz substrate having a twin crystal structure, a quartz substrate 1 is sandwiched from above and below using a press device equipped with a cartridge heater, the temperature is raised to near the phase transition temperature, and the pressure is pressed when the desired temperature is reached. . Then, stress acts only on the convex portion 2 and the crystal axis component is inverted only in this portion. The crystal axis reversal portion grows to the inside of the crystal and propagates to the inside of the crystal, and a periodic twin lattice having a large depth can be produced. A high aspect ratio in the depth direction can be created by controlling the time change of pressing time and pressure.

In the above description, the step structure is formed on the surface of the quartz substrate 1, but it is also conceivable that the surface of the quartz is polished and a step process is formed on the pressing surface on the press side (not a known technique). When ceramic Si ₃ N ₄ or the like is used for the pressing surface on the press side, a periodic step structure can be produced using the lithography technique and dry etching processing.

JP 2002-122898 A 1998 Spring Applied Physics Society Proceedings 28a-SG-18 2000 Autumn Autumn Meeting of Applied Physics 3a-Q-1 The Journal of Applied Physics, May 2000

  By immersing the pressed quartz substrate with hydrofluoric acid for several minutes and using the difference in etching rate, the boundary where the crystal axes are reversed can be observed. FIG. 5 shows the quartz substrate 1 as viewed from the normal direction of the substrate. A rectangular convex portion 2 provided on the surface of the quartz substrate 1 is cut off by dry etching or the like. Therefore, when pressing on the plane of the press. Although the rectangular portion corresponding to the convex portion 2 should be inverted in crystal axis, the crystal axis inversion region 3 is actually shown by hatching, reflecting the three-fold symmetry around the c-axis of the crystal. It is often generated with a hexagonal pattern.

  Therefore, when the step period is short, for example, about several μm, the crystal axis reversal region 3 does not spread over the entire convex portion 2 of the step, but stops in a hexagonal shape as shown in the figure, and the step pattern ( Crystal axis reversal does not occur up to the end of the convex part 2). In such a case, in general, when a laser beam having a radius of several tens μm or more is incident, only a part of the laser beam is converted.

  On the other hand, the crystal axis reversal region of quartz is easily generated from the end of the step pattern (convex portion 2) where stress is concentrated. Therefore, when the step pattern has a long period, for example, several tens of μm, the crystal axis inversion region is generated only at the end of the step pattern, not at the center, and a desired twin periodic structure cannot be obtained. There was a problem.

  Furthermore, due to the concentration of extremely large stresses at the four corners of the convex portion 2 of the rectangular pattern, there may occur a phenomenon in which a crystal axis inversion region is generated in a portion deviated from the convex portion 2. When this occurs, particularly in the case of a periodic structure with a short step pattern, this region may stick to adjacent patterns, and the periodic structure may be broken.

  The present invention has been made in view of such circumstances, and a method of manufacturing a quasi-phase-matched quartz crystal that can easily form a crystal axis inversion region of a target shape, and a quasi-phase formed by an example of this method. It is an object to provide a matched crystal.

  The means for solving the problem is a method of manufacturing a quasi-phase-matched crystal substrate that periodically generates a crystal axis inversion region in which the crystal axis is inverted by a hot press method. The pressure surface corresponding to each is divided into a plurality in the surface direction of the pressure surface.

  In this means, the pressing surface corresponding to each of the crystal axis reversal regions is divided into a plurality in the plane direction. That is, a plurality of fine pattern convex portions (plural divisions) are gathered to form a convex portion consisting of one pattern when viewed macroscopically. A surface (concave portion) lower than the surface of the convex portion is formed at the boundary between fine patterns. Therefore, when the convex part of such a pattern is formed on the quartz substrate and pressed by the plane of the press, each convex part of the fine pattern is pressed by the plane of the press. Therefore, it is possible to prevent the desired twin-periodic structure from being obtained due to the fact that the stress does not spread as a whole or, conversely, concentrated stress is generated only at the corners of the crystal axis inversion region.

  No stress is applied to the recesses (surfaces lower than the surface of the projections) between the projections of the fine pattern, but by sufficiently narrowing the width of the recesses, the crystal axis inversion part generated in the projections is crystallized. When growing to the inside and propagating to the inside of the crystal, a crystal axis inversion portion is formed and connected to the inside of the crystal corresponding to the recess, and a crystal axis inversion portion having a desired shape can be formed.

  The surface of the quartz substrate is a polished surface, and a convex part (plural divisions) is provided on the surface (pressure surface) that presses the quartz substrate, thereby forming a crystal axis inversion part in the part of the quartz substrate that the convex part is in contact with. Also good. In this case, the surface that presses the crystal substrate is formed from a set (a plurality of divisions) of convex portions with fine patterns, so that the crystal axis inversion region does not spread over the whole, or conversely, the crystal axis By generating concentrated stress only at the corners of the inversion region, it is possible to prevent a desired twin periodic structure from being obtained.

  Another means for solving the above-mentioned problems is characterized in that the divided pieces are rectangular, square or hexagonal.

  In general, since the crystal axis inversion region is often rectangular when viewed in a plan view, the crystal axis inversion region can be obtained by making the shape of the convex portion (a plurality of divisions) of the finer pattern square or rectangular. Can be easily formed as a set of finer patterns. Further, if a finer pattern is a square or a rectangle, it is possible to obtain a feature that it is easy to process.

  As described above, when the quartz substrate is pressed, the crystal axis inversion region is often generated as a hexagonal pattern, reflecting the three-fold symmetry around the c-axis of the quartz crystal. Therefore, by setting the shape of the convex part (multiple divisions) of a finer pattern to a hexagonal shape, stress can be applied to the portion of the pattern that matches the generation of the crystal axis inversion region, and the crystal axis inversion is efficiently performed. Regions can be formed.

  Another means for solving the above-mentioned problem is characterized in that the crystal axis reversal region is substantially rectangular and the longitudinal end of the substantially rectangular shape is aligned with the twin boundary of the quartz substrate.

  In this means, since the end of the substantially rectangular crystal axis reversal region is shaped to follow the twin boundary of the crystal, stress is applied to the pattern portion in accordance with the generation of the crystal axis reversal region. And the crystal axis reversal region can be prevented from spreading outside the pressing pattern. In this way, the pattern shape is close to a hexagonal shape, but many corners can be formed compared to the pattern shape that is rectangular, so stress concentration can be reduced. Can do.

  Another means for solving the above problem is a quasi-phase-matched quartz crystal substrate obtained by the above-described manufacturing method, which has a crystal axis inversion region generated by pressurization by a plurality of pressurization surfaces. It is characterized by.

  In this means, since the crystal axis inversion region can be formed by using the above method, the crystal axis inversion region does not spread over the entire area, or conversely, concentrated stress is applied only to the corner of the crystal axis inversion region. It can be prevented that a desired twin periodic structure is not obtained due to the occurrence.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram of a quasi-phase matched quartz crystal substrate, which is a first example of an embodiment of the present invention, viewed from the normal direction of the substrate, and corresponds to FIG. 5 of the conventional example. The overall structure of the quasi-phase matched quartz crystal substrate is not different from the conventional one shown in FIG.

  In FIG. 1, a rectangular convex portion 2 is formed on one surface of the quartz substrate 1, and this convex portion 2 is formed as an aggregate of finer rectangular convex portions 4. This is different from the conventional one shown in FIG. That is, when viewed microscopically, a concave portion 5 lower than the surface of the convex portion 4 is formed between the convex portions 4, but the width of the concave portion 5 is narrow and viewed macroscopically. In some cases, a large number of convex portions 4 are gathered to form one convex portion 2.

  The convex part 2 is formed over the part which wants to reverse the crystal axis of quartz. Such a quartz substrate 1 is sandwiched by a heater block from above and below, and when the temperature of the quartz substrate is increased to reach a desired temperature, it is pressed by a press. Then, stress acts only on the portion corresponding to the convex portion 4, and the crystal axis component is inverted only in this portion. This crystal axis inversion portion grows to the inside of the crystal and propagates to the inside of the crystal, so that a periodic twin lattice having a large depth can be produced.

  When the crystal axis reversal portion propagates inside the crystal (the portion below the convex portion 2 of the quartz substrate 1), the propagation is also made to the concave portion 5, and the concave portion 5 between the convex portions 4 is formed inside the crystal. A crystal axis reversal part is formed in a buried and connected form. Finally, an inversion portion is formed in the entire desired region, and it grows toward the opposite surface of the quartz crystal substrate 1 along the c-axis direction. That is, a substantially rectangular portion 6 as shown by a broken line in FIG. 1 is a crystal axis inversion region. In other words, the width of the concave portion 5 needs to be such that when the pressing is performed, the crystal axis inversion region is connected inside the crystal due to the propagation of the crystal axis inversion portion. Actually, it is preferably at least several μm or less.

  Further, the width of the crystal axis reversal portion protruding from the convex portion 4 can be changed by controlling the temperature at the time of pressing and the time pattern of the applied load. In this way, by changing the width of the concave portion 5 or controlling the temperature at the time of pressing and the time pattern of the applied load, the portion of the concave portion 5 is filled and the crystal axis inversion portion is connected. Further, by changing the aspect ratio of the convex portion 4, the crystal axis inversion portion may be easily connected.

  In this means, the portion that receives pressure by the press is divided into fine convex portions 4, so that each convex portion 4 receives the pressure from the press evenly. Therefore, the crystal axis inversion region does not spread over the entire convex part consisting of periodic or arbitrary patterns, or conversely, concentrated stress is generated only at the corners of the convex part, so that a desired twin periodic structure is obtained. Can be prevented from being obtained.

  In this example, the fine convex portion 4 is formed in a rectangular shape. Thus, by forming the fine convex portion 4 in a rectangular or square shape, the rectangular convex portion 2 as a whole can be easily configured.

  FIG. 2 is a diagram of a quasi-phase matched quartz crystal substrate, which is a second example of the embodiment of the present invention, viewed from the normal direction of the substrate, and corresponds to FIG. In this example, the convex portion 2 corresponding to the portion where the crystal axis reversal portion is to be formed is formed from a hexagonal minute convex portion 4, and a narrow concave portion 5 is formed between the minute convex portions 4. . That is, in this example, the convex part 4 in the first example is configured in a rectangular shape, but there is no difference from the first example except that it is configured in a hexagonal shape.

  When the hot press method is applied to the quartz substrate 1 formed in this way, a portion 6 indicated by a broken line in FIG. 2 becomes a crystal axis inversion region inside the crystal. As described above, the example shown in FIG. 2 basically has the same function and effect as the example shown in FIG. 1, but the fine protrusion 4 has a hexagonal shape. The quartz crystal substrate 1 can be pressed in a form that matches the three-fold symmetry.

  FIG. 3 is a diagram of a quasi-phase matched quartz crystal substrate as a third example of the embodiment of the present invention, viewed from the normal direction of the substrate, and corresponds to FIG. In this embodiment, the convex part 2 is not divided into fine patterns, but by making the four corners of the convex part 2 hexagonal, the ends of the convex part 2 follow the twin boundaries of the crystal. It is shaped. Thereby, the quartz substrate 1 can be pressed in a form that matches the three-fold symmetry around the c-axis of the quartz. Therefore, it becomes difficult to form the crystal axis reversal region in a form protruding from the convex portion 2. In addition, as compared with the case where the convex portion 2 is rectangular, the number of corner portions increases, resulting in relaxation of stress concentration. The convex portions 2 in FIG. 3 may be formed as a set of fine convex portions in the same manner as in FIGS.

  In any of the above embodiments, a convex portion having a step structure is provided on one surface of the quartz crystal substrate 1. If the surface of the quartz substrate 1 is a polished surface and the stepped structure as shown in FIGS. 1 to 3 is provided on the pressing surface of the press without performing this process, the same effect can be obtained. There will be no need for.

  As described above, according to the present invention, a method of manufacturing a quasi phase matching crystal that can easily form a crystal axis reversal region having a target shape, and a quasi phase matching crystal formed by an example of this method Can be provided.

It is the figure which looked at the board | substrate of the quasi phase matching quartz crystal which is the 1st example of embodiment of this invention from the board | substrate normal line direction. It is the figure which looked at the board | substrate of the quasi phase matching quartz crystal which is the 2nd example of embodiment of invention from the board | substrate normal line direction. It is the figure which looked at the board | substrate of the quasi phase matching quartz crystal which is the 3rd example of embodiment of this invention from the board | substrate normal line direction. It is a figure which shows the example of the quartz substrate in which the level | step difference structure was formed on the surface of the single side | surface used for the conventional hot press method. It is the figure which looked at the quartz crystal substrate shown in FIG. 4 from the substrate normal line direction, and is a figure which shows the shape of the formed crystal axis inversion area | region.

Explanation of symbols

1: Quartz substrate 2: Convex part 3: Crystal axis reversal region 4: Convex part (fine convex part)
5: Recess 6: Crystal axis reversal part (region)

Claims (3)

A method of manufacturing a quasi-phase-matched quartz substrate that periodically generates a crystal axis reversal region in which a crystal axis is reversed by a hot press method,
The pressing surface corresponding to each of the crystal axis reversal regions is divided into a plurality of directions in the surface direction of the pressing surface ,
Each of the plurality of divisions in the surface direction of the pressure surface is a convex portion having a shape selected from the group consisting of a rectangle, a square, and a hexagon .   2. The method according to claim 1, wherein the crystal axis reversal region is substantially rectangular, and a longitudinal end of the substantially rectangular shape is aligned with a twin boundary of the quartz crystal substrate. . A quasi-phase matched quartz crystal substrate obtained by the production method according to any one of claims 1 or 2, and characterized in that it has a crystal axis inversion region generated by pressurization by pressure surface divided into a number A quasi-phase matched crystal substrate.