JP2009180898A - Method for forming polarization reversal structure and apparatus for forming polarization reversal structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a polarization reversal structure having a uniform duty in a thickness direction of a wafer. <P>SOLUTION: A method and an apparatus for forming a polarization reversal structure are disclosed, for forming a periodical polarization reversal structure in a wafer of a nonlinear optical crystal. The method and the apparatus for forming a polarization reversal structure includes a sequence of: a periodical pattern fabrication step of fabricating a periodical pattern from an insulating material, the pattern identical to the period of the polarization reversal structure, on one surface of a wafer of a nonlinear optical crystal subjected to a single domain process and to a mirror surface process; a counter face pattern fabricating step of fabricating a counter pattern from an insulating material opposing to the above periodical pattern, on the other surface of the wafer; and a polarization reversal structure forming step of applying a required DC voltage between both surfaces of the wafer so as to form a polarization reversal structure corresponding to the periodical pattern. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a domain-inverted structure forming method and a domain-inverted structure forming apparatus for forming a periodically domain-inverted structure on a nonlinear optical crystal wafer.

  A quasi-phase matching, which is a kind of second-order nonlinear effect generated in a nonlinear optical crystal, is used for second harmonic generation, sum frequency generation, difference frequency generation, and the like. FIG. 1 shows a configuration in which light having a sum frequency and a difference frequency is generated by a nonlinear optical crystal using quasi phase matching. In FIG. 1, 91 is a multiplexer, 92 is a nonlinear optical crystal, and 93 is a duplexer. The excitation light A and the excitation light B are combined by the multiplexer 91 and enter the nonlinear optical crystal 92. In the nonlinear optical crystal 92, the light is converted into light C having the sum frequency or the difference frequency of the excitation light A and the excitation light B, and is emitted from the nonlinear optical crystal 92 together with the excitation light A and the excitation light B. The light C having the sum frequency or the difference frequency is demultiplexed from the excitation light A and the excitation light B by the demultiplexer 93. When the frequency of the excitation light A and the frequency of the excitation light B are the same, or when only the excitation light A is incident, the frequency of the light C is twice that of the excitation light A due to the second harmonic generation. It becomes.

In a nonlinear optical crystal using quasi phase matching, it is necessary to form a polarization inversion structure in which the polarization direction is periodically inverted. The following methods are known as a method of forming a periodic domain-inverted structure (for example, refer nonpatent literature 1). A 500 μm thick MgO-containing LiNbO ₃ (hereinafter abbreviated as LN) crystal substrate which is a single-domain nonlinear optical crystal is used. A first electrode having a target pattern is formed on the + Z plane which is the surface of the substrate, and a uniform second electrode is formed on the −Z plane which is the back surface of the + Z plane. Thereafter, a positive voltage is applied to the first electrode and a negative voltage is applied to the second electrode. After the application of voltage is stopped, a domain-inverted structure corresponding to the target pattern is formed.

  The following method is also known as a method for forming a periodic domain-inverted structure (see, for example, Non-Patent Document 2). A 200 μm thick LN crystal substrate which is a single-domain nonlinear optical crystal is used. A first electrode having a target pattern is formed on the −Z plane of the substrate, and a uniform second electrode is formed on the + Z plane of the substrate. Thereafter, a negative voltage is applied to the first electrode and a positive voltage is applied to the second electrode. After the application of voltage is stopped, a domain-inverted structure corresponding to the target pattern is formed.

In order to form a conventional domain-inverted structure, a pattern corresponding to the period of the domain-inverted structure was produced only on the + Z plane or the −Z plane of the wafer of the nonlinear optical crystal. FIG. 2 is a diagram for explaining a process for producing a pattern that matches an inversion period with an insulator on one Z-plane of a conventional nonlinear optical crystal wafer. In FIG. 2, 95 is a wafer of nonlinear optical crystal, 96 is a uniform second electrode, 97 is a pattern matching the period of the domain-inverted structure, and 98 is a liquid electrode as the first electrode.
A single-domain nonlinear optical crystal wafer 95 is applied with a DC voltage between the liquid electrode 98 filled between the patterns 97 and the uniform second electrode 96, and the applied portion is polarized. Invert. However, since the cross section of the insulator constituting the pattern 97 has an edge, the electric field is concentrated on the edge portion, and the polarization inversion is started from the edge portion of the pattern 97. As shown in FIG. Will be disturbed.
M.M. Nakamura et. al. "Quasi-Phase-matched Optical Parametric Oscillator Using Periodically Poled MgO-Doped LiNbO3 Crystal", Jpn. J. et al. Appl. Phys. , Vol. 38, part 2, no11 App. L1234-1326, 1999 J. et al. Webjoen et. al. , “Quasi-phase-matched blue light generation in bulk lithium nanobate, electrically charged via periodic liquids”, Electronics Letters. 20, no. 11, p894-895, 1994

  In the conventional method of forming a domain-inverted structure, the domain-inverted structure formed on the surface of the wafer on which the first electrode having the pattern is formed is periodic, but the second electrode having no pattern is formed. The domain-inverted structure formed on the wafer surface is disordered. That is, there has been a problem that the inversion / non-inversion rate (hereinafter abbreviated as duty) of the domain-inverted structure is not uniform in the thickness direction of the wafer.

  In order to solve the above problems, an object of the present invention is to form a domain-inverted structure having a uniform duty in the thickness direction of a wafer.

  In order to achieve the above object, the inventors have conducted intensive studies on a method of forming a periodically poled structure in a nonlinear optical crystal. As a result, both the + Z plane and the −Z plane of a mirror-processed nonlinear optical crystal wafer are obtained. In addition, a pattern that matches the period of the domain-inverted structure was made of an insulating material, and a domain-inverted structure was formed by applying a DC voltage to both sides of the pattern.

  Specifically, a method of forming a domain-inverted structure in which a periodically domain-inverted structure is formed on a non-linear optical crystal wafer, which is single-domained and mirror-processed on one surface of the non-linear optical crystal wafer. A periodic pattern manufacturing process for manufacturing a periodic pattern that matches the period of the domain-inverted structure with an insulator, and a facing pattern manufacturing process for manufacturing a facing pattern facing the periodic pattern with an insulator on the other surface of the wafer And a domain-inverted structure forming step of applying a required DC voltage between both surfaces of the wafer to form a domain-inverted structure corresponding to the periodic pattern.

  In order to produce a pattern that matches the period of the domain-inverted structure on both surfaces of the wafer with an insulator, the domain-inverted structure forming method according to the present invention forms a domain-inverted structure with a uniform duty in the thickness direction of the wafer. Can do.

  In the domain-inverted structure forming method of the present invention, the insulating material for producing the periodic pattern or the facing pattern is a photoresist material, and before the domain-inverted structure forming step, up to a temperature at which the photoresist material starts to melt. It is desirable to have a post-bake process for heating the wafer.

  Since the insulating material is a photoresist material, a periodic pattern or a facing pattern can be easily produced. In addition, since the cross section of the photoresist that has started to melt in the post-bake process becomes a semicircle, it is possible to prevent electric field concentration on a part of the wafer, and polarization inversion with a uniform duty in the wafer thickness direction. A structure can be formed.

  In the method for forming a domain-inverted structure according to the present invention, it is preferable that the wafer has a Z-cut crystal axis and has a thickness of 200 μm to 1 mm.

  If the wafer of nonlinear optical crystal is 200 μm or less, the wafer becomes too thin and difficult to handle, and the wafer may be damaged during a photo process such as exposure or development. On the other hand, if the wafer is thicker than 1 mm, the DC voltage required for polarization inversion becomes large, which is not practical.

In the method for forming a domain-inverted structure according to the present invention, the nonlinear optical crystal is preferably a LiNbO ₃ or LiTaO ₃ crystal, or a crystal containing at least one of MgO and ZnO as an additive.

  Since the polarization inversion structure forming method of the present invention uses a nonlinear optical crystal having a large second-order nonlinear effect, quasi phase matching becomes easy.

  The method for forming a domain-inverted structure according to the present invention is preferably performed in a heated environment.

  When the wafer of the nonlinear optical crystal is heated, the coercive electric field, which is the magnitude of the electric field required for the polarization inversion of the nonlinear optical crystal, is reduced. Therefore, it is sufficient to apply a low DC voltage even when the wafer is thick. Further, since the electric conductivity of the nonlinear optical crystal is increased, a uniform domain-inverted structure can be formed.

  In the domain-inverted structure forming method of the present invention, in the domain-inverted structure forming step, the periodic pattern forming surface of the wafer is immersed in the first liquid electrode, and the facing pattern forming surface is independent of the first liquid electrode. It is desirable to immerse in a second liquid electrode and apply a DC voltage between the first and second liquid electrodes.

  Since the liquid electrode uniformly contacts the area between the periodic patterns of the wafer and the area between the facing patterns, the liquid electrodes face each other when a DC voltage is applied between the first and second liquid electrodes. Since the path through which the applied DC electric field penetrates is limited between the areas to be applied and the electric field is linearly formed so as to have the shortest distance, a polarization inversion structure with a uniform duty in the thickness direction of the wafer should be formed. Can do.

  In the domain-inverted structure forming method of the present invention, in the domain-inverted structure forming step, a metal film is formed by vapor-depositing a metal on one surface of the periodic pattern and the facing pattern of the wafer, and the other surface It is preferable to immerse the substrate in a liquid electrode and apply a DC voltage between the metal film and the liquid electrode.

  Since the metal film is formed by vapor deposition, similarly to the liquid electrode, the metal film uniformly contacts the area between the periodic patterns of the wafer and the area between the facing patterns. For this reason, when a DC voltage is applied between the metal film and the liquid electrode, the path through which the applied DC electric field penetrates is limited between regions facing each other as described above, and the electric field is linearly set to the shortest distance. Since it is formed, a domain-inverted structure having a uniform duty in the thickness direction of the wafer can be formed.

  In the domain-inverted structure forming method of the present invention, in the domain-inverted structure forming step, a metal film is formed by vapor-depositing metal on both surfaces of the periodic pattern and facing pattern forming surface of the wafer, and a direct current is formed between the metal films on both surfaces. It is desirable to apply a voltage.

  Even when the metal film is formed on both surfaces of the wafer, the metal film uniformly contacts the area between the patterns on both surfaces as described above, so that when a DC voltage is applied between the metal films on both surfaces, the metal films face each other as described above. Since the path through which the applied DC electric field penetrates is limited between the areas to be applied and the electric field is linearly formed so as to have the shortest distance, a polarization inversion structure with a uniform duty in the thickness direction of the wafer should be formed. Can do.

  More specifically, the present invention relates to a polarization inversion structure forming apparatus for setting a non-linear optical crystal wafer that is made into a single domain and mirror-finished and forms a periodic polarization inversion structure on the wafer. A periodic pattern made of an insulator on one surface of the wafer in accordance with the period of the domain-inverted structure, and a facing pattern made of an insulator on the other surface of the wafer facing the periodic pattern; And holding application means for holding a wafer on which the periodic pattern and the facing pattern are fabricated and applying a DC voltage between the periodic pattern and facing pattern forming surfaces of the wafer in the holding state. Device.

  When a DC voltage is applied between the periodic pattern of the wafer held by the holding application means and the formation surface of the facing pattern, the area between the periodic patterns of the wafer and the area between the facing patterns face each other. Since the path through which the applied DC electric field passes is limited between the regions and the electric field is linearly formed so as to have the shortest distance, a domain-inverted structure having a uniform duty in the thickness direction of the wafer can be formed. it can.

  The domain-inverted structure forming apparatus of the present invention further includes a heating unit that uses a photoresist material as an insulator for forming the periodic pattern and the facing pattern on the wafer, and heats the wafer to a temperature at which the photoresist material starts to melt. It is desirable to have it.

  Since the insulating material is a photoresist material, a periodic pattern or a facing pattern can be easily produced. In addition, since the cross section of the photoresist that has started to melt in the post-bake process becomes a semicircle, it is possible to prevent electric field concentration on a part of the wafer, and polarization inversion with a uniform duty in the wafer thickness direction. A structure can be formed.

  In the domain-inverted structure forming apparatus according to the present invention, the holding application unit holds the wafer, the formation surface of the periodic pattern of the held wafer is immersed in the first liquid electrode, and the formation surface of the facing pattern is the surface A jig for housing the first and second liquid electrodes and a DC voltage applied to each of the first and second liquid electrodes in a state of being immersed in a second liquid electrode independent of the first liquid electrode. It is desirable that the first and second electrode rods are provided.

  When a DC voltage is applied between the first and second liquid electrodes via the first and second electrode rods, the region between the periodic patterns of the wafer and the region between the facing patterns are mutually connected. Since the path through which the applied DC electric field penetrates is limited between the opposed regions and the electric field is linearly formed so as to have the shortest distance, a polarization inversion structure with a uniform duty in the thickness direction of the wafer is formed. be able to.

  In the domain-inverted structure forming apparatus of the present invention, the wafer includes a metal film formed by metal vapor deposition on one surface of the periodic pattern and the facing pattern forming surface, and the holding application unit includes the wafer And a jig for housing the liquid electrode in a state where the formation surface and the relative surface of the metal film of the held wafer are immersed in the liquid electrode, and a first DC voltage application connected to the metal film. It is desirable to include an electrode rod and a second electrode rod for applying a DC voltage inserted into the liquid electrode.

  When a DC voltage is applied between the metal film and the liquid electrode via the first and second electrode rods, the path through which the applied DC electric field penetrates between the pattern non-formed regions facing each other is limited as described above. In addition, since the electric field is linearly formed so as to be the shortest distance, a domain-inverted structure having a uniform duty in the thickness direction of the wafer can be formed.

  In the domain-inverted structure forming apparatus according to the present invention, the wafer includes a metal film formed by metal deposition on both surfaces of the periodic pattern and the facing pattern, and the holding application unit is a jig for holding the wafer. And first and second electrode bars for applying a DC voltage connected to the metal films on both surfaces of the wafer held by the jig.

  When a DC voltage is applied between the metal films on both sides via the first and second electrode rods, the path through which the applied DC electric field penetrates is limited between the pattern non-formed regions of the wafer facing each other as described above. In addition, since the electric field is linearly formed so as to be the shortest distance, a domain-inverted structure having a uniform duty in the thickness direction of the wafer can be formed.

  ADVANTAGE OF THE INVENTION According to this invention, the polarization inversion structure formation method and polarization inversion structure formation apparatus which can form the polarization inversion structure with a uniform duty in the thickness direction of the wafer of a nonlinear optical crystal can be provided.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  FIG. 3 is a diagram illustrating a process of forming a pattern that matches the polarization inversion period on one Z-plane of a nonlinear optical crystal wafer with an insulator. In FIG. 3, 21 is a wafer of nonlinear optical crystals, and 22 is a periodic pattern that matches the polarization inversion period. The period of the periodic pattern is, for example, 2 μm to 40 μm. First, a periodic pattern 22 corresponding to the polarization inversion period is formed with an insulator on one Z plane (+ Z plane) of a single-domain nonlinear optical crystal wafer 21 (periodic pattern manufacturing step). Further, a facing pattern (not shown) facing the periodic pattern 22 with an insulator is formed on the other opposing Z surface (-Z surface) (facing pattern manufacturing step).

As the nonlinear optical crystal, a crystal of LN or LiTaO ₃ (hereinafter abbreviated as LT) is desirable. Alternatively, it may be a crystal containing at least one of MgO and ZnO as an additive. Such a crystal has a large non-linear effect, and it becomes easy to generate light having a sum frequency and a difference frequency by using pseudo phase matching. In view of the magnitude of the nonlinear constant, it is desirable to use LN rather than LT. From the point of light damage resistance, it is desirable to use LN containing MgO or ZnO. From the viewpoint of polarization reversal when a DC voltage is applied, it is most desirable to use LN containing ZnO.

  The insulator material used for the pattern is preferably a photoresist material. A metal film may be vapor-deposited on the pattern formation surface made of a photoresist material. Examples of the photoresist material include S1818 (Rohm and Haas Japan Co., Ltd.). Examples of the material used for the metal film include gold, silver, platinum, copper, nickel, aluminum, chromium, and tantalum.

  Next, a method of applying a required DC voltage between both surfaces of a nonlinear optical crystal wafer will be described with reference to FIG. FIG. 4 is a cross-sectional view showing the configuration of the domain-inverted structure forming apparatus. In this figure, 21 is a wafer of nonlinear optical crystal, 22 is a periodic pattern matching the domain-inverted period, 23 is an O-ring, and 24 is from above. A jig having a donut-like shape, 25-1 is a first liquid electrode, 25-2 is a second liquid electrode, and 26-1, 26-2 are first electrodes connected to a DC power source (not shown). The rod and the second electrode rod, 27 is a hot plate, and 28 is a facing pattern facing the periodic pattern. The domain-inverted structure forming apparatus is configured to include all of the other components 22 to 28 excluding the wafer 21 to be domain-inverted structure formed. The O-ring 23, the jig 24 made of an insulator such as Teflon (registered trademark), the liquid electrodes 25-1 and 25-2, and the electrode rods 26-1 and 26-2 constitute a holding application unit.

  A mirror-processed Z-cut LN crystal substrate is used as the nonlinear optical crystal wafer 21. The wafer 21 has a size of 3 inches and a thickness of 300 μm. The thickness of the wafer 21 is desirably 200 μm or more and 1 mm or less. When the thickness is 200 μm or less, the wafer 21 becomes too thin and difficult to handle, and the wafer 21 may be damaged in a photo process such as exposure or development. On the contrary, when the thickness of the wafer 21 is 1 mm or more, the direct current voltage required for polarization inversion becomes large, which is not practical.

  An upward arrow displayed on the wafer 21 indicates the direction of polarization that is made into a single domain before a required DC voltage is applied. On the + Z plane of the wafer 21, a periodic pattern 22 that matches the period of the domain-inverted structure is formed with a photoresist as an insulator. The cross-sectional shape of the photoresist constituting the periodic pattern 22 is preferably approximately a semicircle.

  A facing pattern 28 facing the periodic pattern 22 is formed on the −Z surface of the wafer 21 with a photoresist as an insulator. The cross-sectional shape of the photoresist constituting the facing pattern 28 is preferably approximately a semicircle.

  Here, the periodic pattern 22 and the facing pattern 28 of the photoresist desirably have a substantially semicircular cross-sectional shape. FIG. 5 shows cross-sectional shapes of the nonlinear optical crystal wafer 21, the photoresist periodic pattern 22, and the facing pattern 28. The single-domain nonlinear optical crystal wafer 21 includes liquids in regions (referred to as upper regions) between the periodic patterns 22 and regions (referred to as lower regions) between the facing patterns 28. A DC voltage is applied from the electrode rods 26-1 and 26-2 via the electrodes 25-1 and 25-2. At the time of application, since the cross section of the periodic pattern 22 and the facing pattern 28 is substantially semicircular, the electric field does not concentrate on the edges of the periodic pattern 22 and the facing pattern 28 as in the prior art, and the upper region and the lower region Polarization reversal (indicated by a downward arrow) is started from the center line portion of the opposite portion. This facilitates duty control of the polarization inversion structure.

  Next, a method for creating a substantially semicircular cross-sectional pattern will be described. After a suitable amount of a normal photoresist material is dropped on the surface of the nonlinear optical crystal wafer 21, a photoresist film is formed by a spin coater. A film having a thickness of about 1.8 μm can be formed at a speed of 3000 rpm. The photoresist film is pre-baked at 90 ° C., exposed and developed using a contact aligner, and a periodic convex pattern is produced. At this stage, the photoresist pattern is edged and the cross-sectional shape is square. The developed nonlinear optical crystal wafer 21 is placed in an oven as a heating means and post-baked. In the post-bake, the nonlinear optical crystal wafer 21 is heated to a temperature (150 ° C. to 250 ° C.) at which the photoresist material starts to melt (post-bake step). For example, in the material of the previous S1818, the temperature range is 160 ° C. to 200 ° C. At such a temperature, the edge portion of the pattern is softened, and the cross-sectional shapes of the periodic pattern 22 and the facing pattern 28 are substantially semicircular as shown in FIG.

  In FIG. 4, the wafer 21 is sandwiched from above and below by a doughnut-shaped jig 24 that is made of acrylic or polycarbonate through an O-ring 23. As a result, the upper surface of the wafer 21 is brought into close contact with the doughnut-shaped hollow portion of the upper jig 24 to form a waterproof recess, and the circular recess of the lower jig 24 is sealed in a lid shape on the lower surface of the wafer 21. As a result, a cavity is formed. The liquid electrodes 25-1 and 25-2 are filled in these recesses and cavities, whereby the upper and lower surfaces of the wafer 21 are in close contact with the liquid electrodes 25-1 and 25-2. Examples of the liquid electrodes 25-1 and 25-2 include lithium chloride aqueous solutions. Further, mercury may be used as the liquid electrodes 25-1 and 25-2.

  Also, the electrode rods 26-1 and 26-2 connected to the DC power source are immersed in the liquid electrodes 25-1 and 25-2. An anode electrode rod 26-1 is disposed on the liquid electrode 25-1 in contact with the + Z plane, and a cathode electrode rod 26-2 is disposed on the liquid electrode 25-2 in contact with the -Z plane. Either the anode electrode rod 26-1 or the cathode electrode rod 26-2 may be grounded. The lower jig 24 is disposed on the hot plate 27, and the hot plate 27 heats the wafer 21 that is the substrate of the LN crystal together with the jig 24.

  The periodic pattern 22 and the facing pattern 28 of the photoresist prepared so as to coincide with the period of the domain-inverted structure function as an insulating film, and direct current is applied to the LN crystal wafer 21 from the liquid electrodes 25-1 and 25-2. A voltage is applied (a process of forming a domain-inverted structure).

  A periodic pattern 22 and a facing pattern 28 are formed on both the + Z plane and the −Z plane of the wafer 21. Therefore, as shown by the downward arrow in FIG. 5, the path through which the applied DC electric field penetrates is restricted, and the electric field is formed linearly so as to have the shortest distance. The duty of the domain-inverted structure is uniform in the vertical direction.

  In addition, it is desirable that the polarization inversion structure forming step of applying a DC voltage is performed in a state where the nonlinear optical crystal wafer 21 is heated. When the heating is performed, the coercive electric field, which is the magnitude of the electric field required for reversing the polarization of the nonlinear optical crystal, is reduced. Therefore, it is sufficient to apply a low DC voltage even when the wafer 21 is thick. Furthermore, since the electric conductivity of the nonlinear optical crystal is increased, a uniform domain-inverted structure can be formed. The heating temperature is preferably 50 ° C. or higher and 150 ° C. or lower, and more preferably 90 ° C. or higher and 100 ° C. or lower. When the temperature is 150 ° C. or higher, the water in the liquid electrode evaporates.

Example 1
In this example, an LN crystal containing ZnO was used as the nonlinear optical crystal, and a 300-μm thick 3-inch substrate was used as the wafer 21 to form a domain-inverted structure as shown in FIG.

  As described above, first, the periodic pattern 22 and the facing pattern 28 were respectively made of a photoresist material on both the + Z plane and the −Z plane. The prepared LN crystal wafer 21 is set on a jig 24, a lithium chloride aqueous solution is injected as each of the liquid electrodes 25-1 and 25-2, and the wafer 21 is heated to 80 ° C. by a hot plate 27, and then a direct current is applied. A DC voltage was applied from the power source via the electrode rods 26-1 and 26-2. At this time, when a voltage of 0.8 kV (electric field strength of 2.7 kV / mm) was applied from a DC power source for 300 msec, an inversion charge amount of 2 mC flowed, and a desired periodic domain-inverted structure could be formed.

  In this example, the period of the domain-inverted structure was formed using a 6.3 μm mask, but it was confirmed that the domain-inverted structure could be formed with an arbitrary period of 2 μm or more. When the wavelength conversion element is cut out in a strip shape in a direction orthogonal to the direction of the period from the produced LN crystal wafer 21, both end faces of the cut out wavelength conversion element are polished, and excitation light having a wavelength of 1.06 μm is incident. It was possible to generate 532 nm green SHG light.

(Example 2)
In this example, a domain-inverted structure was formed by using a LN crystal containing ZnO and using a 300 μm-thick 3 inch substrate as the wafer 21 in the same manner as in Example 1.

  The periodic pattern and the facing pattern were produced in the same manner as in Example 1, and the periodic pattern 22 and the facing pattern 28 having a period of 4.5 μm were produced using a photoresist material. In this example, the polarization inversion structure was formed by grounding the electrode rod 26-1 connected to the + Z plane and applying a negative voltage via the electrode rod 26-2 connected to the -Z plane. An LN crystal wafer 21 is set on a jig 24, and a voltage of -1.35 kV is applied from a DC power source for 300 msec in a state where the wafer 21 is heated to a temperature of 80 ° C. by a hot plate 27, and a desired cycle is obtained. A domain-inverted structure could be formed. When the wavelength conversion element is cut out from the wafer 21 in a strip shape, etched in a dilute hydrofluoric acid solution, and the internal polarization inversion structure is observed, the polarization is uniform in the thickness direction of the wafer from the −Z plane to the + Z plane. An inversion structure was formed, and there was no difference in the duty of the polarization inversion structure between the + Z plane and the −Z plane.

(Example 3)
In this example, a domain-inverted structure was formed using an LN crystal containing ZnO as the nonlinear optical crystal and a 300-inch-thick 3-inch substrate as the nonlinear optical crystal in the same manner as in Example 1.

  In the present example, as shown in FIG. 6, after the desired periodic pattern 22 and the facing pattern 28 are formed with a photoresist material on the + Z plane and the −Z plane of the wafer 21 by the same method as in the first embodiment. Gold was deposited only on the surface on the pattern 22 side to form a metal film 29. This wafer 21 is set on a jig 32 made of an insulator such as Teflon (registered trademark), and a liquid electrode 25 is injected into the cavity on the −Z plane side in the same manner as described above. The anode electrode rod 26-1 was connected to the metal film 29. Further, a hot plate 27 is disposed under the lower jig 24 as in the configuration of FIG. In the present apparatus configuration, the O-ring 23 on the + Z plane side can be omitted.

  In this configuration, a DC voltage was applied from the DC power source through the electrode rods 26-1 and 26-2 while the wafer 21 was heated to 80 ° C. by the hot plate 27. At this time, when a voltage of 0.8 kV (electric field strength of 2.7 kV / mm) was applied from a DC power source for 300 msec, an inversion charge amount of 2 mC flowed, and a desired periodic domain-inverted structure could be formed.

  Even when an LN crystal containing MgO or an LT crystal is used as the wafer 21 in addition to the above LN crystal, a uniform domain-inverted structure is produced by applying a DC voltage corresponding to the coercive electric field of each wafer. We were able to. Wavelength conversion elements are cut into strips from each wafer, etched in dilute hydrofluoric acid solution, and the internal domain inversion structure is observed. Uniform polarization in the wafer thickness direction from -Z plane to + Z plane An inversion structure was formed, and no difference was seen in the duty between the + Z plane and the −Z plane.

Example 4
In this example, an LN crystal containing ZnO was used as the nonlinear optical crystal, and a domain-inverted structure was formed using a 3 inch substrate having a thickness of 300 μm as the wafer 21.

  In this embodiment, a pattern having a period with a so-called chirp with a varying period was produced. When having a chirp compared to a non-chirp, a wide matching temperature range can be obtained. A periodic pattern 22 having a desired chirp in the left-right direction is made of a photoresist material on the + Z plane, and a facing pattern 28 having the same period is made on the −Z plane, and then a DC voltage is applied in the same manner as in the first embodiment. did.

  When the produced LN crystal wafer 21 was etched in a hydrofluoric acid solution, a portion where the domain-inverted structure was formed and a portion where it was not formed appeared in a striped pattern. This is because the same photomask that exposed the + Z plane was used when exposing and developing the −Z plane, so that the direction of the chirp was reversed left and right, and the periodic pattern 22 produced on the + Z plane and the −Z plane and This is because a place where the facing pattern 28 does not match in the Z-axis direction (thickness direction) appeared. As described above, in the present invention, it is important to produce the periodic pattern 22 and the facing pattern 28 that coincide with a desired periodic structure so as to face each other in the + Z plane and the −Z plane.

  Separately from this, a photomask used for + Z plane exposure and a photomask with left-right reversal symmetry are prepared, and in the same manner as in Example 1, the + Z plane is a photomask with chirp, and the -Z plane is left and right. When a pattern was produced using a photomask having inversion chirp and the LN crystal wafer 21 to which a DC voltage was applied was etched in the same manner as in Example 1, a desired domain-inverted structure was uniformly produced. I was able to confirm.

  As seen in the present embodiment, in the present invention, it is desirable that the periodic pattern 22 and the facing pattern 28 formed on the + Z plane and the −Z plane match when viewed from the Z-axis direction. In the case of using a photomask having a certain period, it is desirable to separately prepare a backside photomask having left-right inversion symmetry.

(Example 5)
In the present embodiment, the periodic pattern 22 and the facing pattern 28 are formed of a photoresist material on the + Z plane and the −Z plane of the wafer 21 using the same method as in the third embodiment, and as shown in FIG. A metal film 29 was formed by depositing gold only on the pattern surface of the Z plane. The wafer 21 is sandwiched and set by jigs 24 and 30 through an O-ring 23 from above and below. In the case of the configuration of this embodiment, since the liquid electrode 25 is used only on the + Z plane side, the O-ring 23 only needs to be on the + Z plane side. The wafer 21 is sandwiched from above and below by the jig 30 and the upper jig 24. Further, a + voltage applying electrode rod 26-1 was inserted into the liquid electrode 25 on the + Z plane side, and the grounded electrode rod 26-2 was connected to the metal film 29 on the −Z plane side. A hot plate 27 is disposed under the lower jig 30.

  In this configuration, with the hot plate 27 heated to 80 ° C., a positive voltage was applied to the electrode rod 26-1 connected to the + Z plane. At this time, when a voltage of 0.8 kV (electric field strength of 2.7 kV / mm) was applied from a DC power source for 300 msec, an inversion charge amount of 2 mC flowed, and a desired periodic domain-inverted structure could be formed.

When the wavelength conversion element was cut out from the wafer 21 in a strip shape, etched in a dilute hydrofluoric acid solution, and the internal domain inversion structure was observed, it was confirmed that domain inversion domains were connected in the vicinity of the -Z plane.
(Example 6)
In the present embodiment, the periodic pattern 22 and the facing pattern 28 are formed of a photoresist material on the + Z plane and the −Z plane of the wafer 21 by using the same method as in the embodiment 6, and then, as shown in FIG. Gold was vapor-deposited on the pattern surface of both the surface and the −Z surface to form a metal film 29. The wafer 21 is placed and set on a jig 30, a + voltage application electrode rod 26-1 is connected to the + Z plane metal film 29, and an electrode rod 26 − grounded to the −Z plane metal film 29 is connected. 2 connected. A hot plate 27 is disposed under the lower jig 30.

  In this configuration, with the hot plate 27 heated to 80 ° C., a positive voltage was applied to the electrode rod 26-1 connected to the + Z plane. At this time, when a voltage of 0.8 kV (electric field strength of 2.7 kV / mm) was applied from a DC power source for 300 msec, an inversion charge amount of 2 mC flowed, and a desired periodic domain-inverted structure could be formed.

  When the wavelength conversion element was cut out from the wafer 21 in a strip shape, etched in a dilute hydrofluoric acid solution, and the internal domain inversion structure was observed, it was confirmed that domain inversion domains were connected in the vicinity of the -Z plane.

  The domain-inverted structure forming method and the domain-inverted structure forming apparatus of the present invention can be applied to the manufacture of wavelength conversion elements and the like.

It is a block diagram which generates the light of a sum frequency and a difference frequency with a nonlinear optical crystal using a quasi phase matching. It is a figure explaining the process of producing the pattern which corresponds to a polarization inversion period with the insulator on one Z surface of the wafer of the conventional nonlinear optical crystal. It is a figure explaining the process of producing the pattern which corresponds to a polarization inversion period with an insulator in one Z surface of the wafer of a nonlinear optical crystal. It is sectional drawing which shows the structure of the polarization inversion structure forming apparatus in case a liquid electrode is used for a wafer upper and lower surface. It is a cross-sectional shape figure of the periodic pattern and the facing pattern of the wafer and photoresist of the nonlinear optical crystal of this embodiment. It is sectional drawing which shows the structure of the polarization inversion structure formation apparatus in the case of using a metal film for a wafer upper surface, and a liquid electrode for a lower surface. It is sectional drawing which shows the structure of the polarization inversion structure formation apparatus in case a liquid electrode is used for a wafer upper surface, and a metal film is used for a lower surface. It is sectional drawing which shows the structure of the polarization inversion structure formation apparatus in case a metal film is used for a wafer upper and lower surface.

Explanation of symbols

21: Wafer 22: Periodic pattern 23: O-rings 24, 30, 32: Jigs 25, 25-1, 25-2, 98: Liquid electrodes 26, 26-1, 26-2: Electrode rod 27: Hot plate 28: Face-to-face pattern
29: metal film 91: multiplexer 92: nonlinear optical crystal 93: duplexer 95: wafer of nonlinear optical crystal 96: second electrode 97: pattern

Claims (13)

A domain-inverted structure forming method for forming a periodic domain-inverted structure on a wafer of nonlinear optical crystals,
A periodic pattern manufacturing step of manufacturing a periodic pattern that matches the period of the domain-inverted structure with an insulator on one surface of a single-domain and mirror-processed nonlinear optical crystal wafer;
On the other surface of the wafer, a facing pattern manufacturing step of manufacturing a facing pattern facing the periodic pattern with an insulator; and
A domain inversion structure forming step of applying a required DC voltage between both surfaces of the wafer to form a domain inversion structure corresponding to the periodic pattern;
A method for forming a domain-inverted structure comprising:   The insulator material for producing the periodic pattern or the facing pattern is a photoresist material, and the post-baking step of heating the wafer to a temperature at which the photoresist material starts to melt before the domain-inverted structure forming step is included. The method of forming a domain-inverted structure according to claim 1.   3. The method of forming a domain-inverted structure according to claim 1, wherein the wafer has a crystal axis that is Z-cut and has a thickness of 200 μm to 1 mm. The nonlinear optical crystal is crystal of LiNbO ₃ or LiTaO _3, or polarization inversion according to any one of claims 1 to 3, characterized in that these are crystals containing at least one MgO and ZnO as additives Structure formation method.   The method for forming a domain-inverted structure according to any one of claims 1 to 4, wherein the domain-inverted structure forming step is performed in a heated environment.   In the domain-inverted structure forming step, the periodic pattern forming surface of the wafer is immersed in a first liquid electrode, the facing pattern forming surface is immersed in a second liquid electrode independent of the first liquid electrode, 6. The method of forming a domain-inverted structure according to claim 1, wherein a DC voltage is applied between the first and second liquid electrodes.   In the domain-inverted structure forming step, a metal film is formed by vapor-depositing a metal on one of the periodic pattern and facing pattern forming surfaces of the wafer, and the other surface is immersed in a liquid electrode. 6. The method of forming a domain-inverted structure according to claim 1, wherein a DC voltage is applied between the liquid electrodes.   In the domain-inverted structure forming step, a metal film is formed by vapor-depositing metal on both surfaces of a periodic pattern and a facing pattern forming surface of the wafer, and a DC voltage is applied between the metal films on both surfaces. The method of forming a domain-inverted structure according to any one of claims 1 to 5. A domain-inverted structure forming apparatus for setting a single-domain and mirror-processed nonlinear optical crystal wafer and forming a periodic domain-inverted structure on the wafer,
On one surface of the wafer, a periodic pattern made of an insulator in accordance with the period of the domain-inverted structure;
A facing pattern made of an insulator facing the periodic pattern on the other surface of the wafer;
Holding application means for holding the wafer on which the periodic pattern and the facing pattern are produced, and applying a DC voltage between the formation surfaces of the periodic pattern and the facing pattern of the held wafer;
A device for forming a domain-inverted structure comprising:   10. A heating means for heating the wafer to a temperature at which the photoresist material begins to melt using a photoresist material as an insulator for forming the periodic pattern and the facing pattern on the wafer. The domain-inverted structure forming apparatus according to 1.   The holding application unit holds the wafer, the periodic pattern forming surface of the held wafer is immersed in the first liquid electrode, and the facing pattern forming surface is independent of the first liquid electrode. A jig for housing the first and second liquid electrodes in a state of being immersed in the liquid electrode, and first and second electrode bars for applying DC voltage inserted into the first and second liquid electrodes, respectively. The polarization inversion structure forming apparatus according to claim 9 or 10, characterized by comprising:   The wafer includes a metal film formed by metal vapor deposition on one surface of the periodic pattern and the facing pattern, and the holding application unit holds the wafer, and the wafer in the holding state With the metal film forming surface and the relative surface immersed in the liquid electrode, a jig for housing the liquid electrode, a first electrode rod for DC voltage application connected to the metal film, and the liquid electrode are inserted. The polarization inversion structure forming apparatus according to claim 9, further comprising: a second electrode rod for applying a DC voltage.   The wafer includes a metal film formed by metal vapor deposition on both surfaces of the periodic pattern and the facing pattern, and the holding application unit includes a jig for holding the wafer, and a wafer held by the jig. The polarization inversion structure forming apparatus according to claim 9 or 10, further comprising first and second electrode bars for applying a DC voltage connected to the metal films on both sides.

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