JP3549374B2 - Manufacturing method of polarization diffraction element - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a polarization diffraction element used in the field of utilizing the polarization of light, such as optical recording and optical information processing, whose diffraction characteristics differ depending on the incident polarization.
[0002]
[Prior art]
For an expensive and large polarizing element using a prism or the like, a diffraction grating type polarizing diffraction element using an anisotropic crystal which is advantageous for miniaturization and cost reduction has been proposed as disclosed in JP-A-6-27322. ing. This element has the function of exchanging protons on the surface of lithium niobate crystal and utilizing the anisotropy of the change in the refractive index to transmit most of a certain polarized light component, while diffracting most of the polarized light in a direction perpendicular to that. are doing.
The diffraction characteristics of a polarization diffraction element having an ion exchange region include a refractive index change (Δn) due to proton exchange and its exchange depth (d), and a groove depth when a groove is used as a phase adjustment method as shown in FIG. In order for one polarized light of a certain wavelength (λ) to be completely diffracted, the optical path difference between the ion-exchange region and the non-exchange region should be a half integral multiple of the wavelength. Where n is the refractive index of the crystal substrate, 1 is the refractive index of air, and m is an integer with respect to
(Equation 1)
Δnd− (n−1) g = (m + ／) λ
Therefore, the diffraction condition depends on the wavelength of the light used. Usually, when protons are exchanged into the crystal, the change in the refractive index for extraordinary light is about △ ne = 0.13 and the change in the refractive index for ordinary light is about −no = −0.04, and the wavelength normally used is 500 nm. With respect to the above light, the depth of the ion exchange region required to completely diffract at least one polarization component is 1 μm or more.
[0003]
Although a large refractive index change can be obtained in the polarization diffraction element subjected to the proton exchange, this element has a problem that the crystallinity of the ion exchange region is deteriorated. For example, in the document "A. Campari et al.," Strain in and surface damage induced by proton exchange in Y-cut LiNbO3 ", J. Appl. Phys., Vol. 58, No. 12, p. It is described that when ion exchange of a depth of 0.2 μm (2,000 angstroms) or more is performed on a lithium niobate crystal Y-cut substrate, cracks or the like occur on the crystal surface. It has been pointed out that when the particle size becomes large, many fine cracks are generated on the crystal surface, and the reproducibility of proton exchange deteriorates and the scattering loss of transmitted light increases.
To solve this problem, it has been proposed to thermally diffuse a metal in advance into a region where proton exchange is performed (Japanese Patent Laid-Open No. 2-12105). This method requires as long as 7 to 8 hours at 1050 ° C. in order to perform sufficient heat diffusion, and it takes almost a whole day when the time for raising and lowering the temperature is included, resulting in extremely low production efficiency. Was.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to perform a pretreatment for removing the entire surface of the crystal substrate and further distortion inside the crystal substrate before performing the ion exchange without performing a method based on thermal diffusion of a metal having a low production efficiency. An object of the present invention is to suppress the generation of cracks caused by the above, and to manufacture a polarization diffraction element with good productivity and reproducibility. As a result, it is possible to obtain a low-loss cracking-free polarization diffraction element.
[0005]
[Means for Solving the Problems]
In general, an optical crystal such as lithium niobate has an affected layer caused by polishing of the surface and a variation due to a processing strain, a change in a proton concentration and a composition in the crystal, and the like. It is said that these have a great effect on device characteristics when using the neighborhood. This is the same in the case of the polarization diffraction element. Since the proton exchange deforms the crystal lattice, it is inevitable that the distortion inherent in the crystal itself greatly affects the crack generation.
The present invention is characterized in that, in order to prevent cracks caused by proton exchange, the crystal substrate is subjected to a pre-treatment before the proton exchange to remove the crystal surface and internal strains. It is also characterized by the fact that crystal distortion can be relaxed and the affected layer can be removed by etching.
[0006]
In the present invention, as the anisotropic crystal, lithium niobate, lithium tantalate and a solid melt thereof are used. When these are used, proton exchange is generally used as ion exchange, and proton exchange is performed by immersing the crystal substrate in benzoic acid or pyrophosphoric acid that has been heated and melted. For example, as shown in FIG. 1 or 2, a proton exchange region 2 is formed by proton-exchanging a lithium niobate substrate 1 at regular intervals. When benzoic acid is used, a phase adjusting film 4 made of a dielectric film such as niobium oxide is applied as shown in FIG. 2 as a phase adjusting means for increasing the polarization extinction ratio, or when pyrophosphoric acid is used. As shown in FIG. 1, a method of performing phase adjustment by forming a phase adjustment groove 3 by selective etching of a proton exchange region with hydrofluoric acid is used.
The crystal substrate has a uniaxial anisotropy, and a crystal substrate whose optical axis is included in the plane of the substrate surface is usually used in order to function as a polarizing element. It is named cut or Y-cut. The present invention can be applied to both X-cut and Y-cut crystal substrates. Crystal quality is classified into expensive high-quality type used for ordinary optical applications and inexpensive low-grade type with poor crystal uniformity according to the presence or absence of sub-grains and the content of impurities in the substrate. The method enables proton exchange with good reproducibility regardless of the quality of the crystal, so that a high-quality ion exchange region can be formed even by using such an inexpensive crystal substrate. It becomes possible to produce a polarization diffraction element.
[0007]
The polarization diffraction element of the present invention is made by the following steps:
1) First, a single crystal such as lithium niobate is prepared.
2) Cut a substrate from the single crystal to create a wafer.
3) Polish the wafer. It is considered that processing distortion occurs in this step.
4) Pre-treat the wafer to remove distortion.
5) Using a photolithography technique, the wafer is immersed in benzoic acid or the like to form a proton exchange region in a lattice.
6) The phase of the polarization diffraction element is adjusted by applying a dielectric film or by selective etching.
Usually, between the step 3) and the step 5), pickling of the crystal surface is not performed. However, the present invention is characterized in that the pretreatment is performed between the steps 3) and 5) to remove distortions on the crystal surface and inside immediately before proton exchange. When the pretreatment according to the present invention is performed, defects such as cracks hardly occur even when proton exchange is performed thereafter, and it can be seen that there is no fear of a fluctuation in the composition of the crystal.
[0008]
As a heat treatment method performed before proton exchange, it is preferable to perform heating at 300 ° C. or more for 1 hour or more in order to remove crystal surface and internal strains. The conditions are determined within a range where quality deterioration does not occur. This heat treatment is performed with the Curie temperature as the upper limit, and is about 1200 ° C. in the case of lithium niobate crystals. Further, for example, in order to avoid the disadvantage that lithium niobate is diffused out, the heat treatment is performed at about 900 ° C. or less. The heat treatment atmosphere is appropriately determined depending on the crystal quality used, such as in the air, in dry oxygen, or in humid air. In addition, since the heat treatment does not use a chemical, there is also an advantage that there is no possibility of a composition change due to elution of components due to the chemical.
On the other hand, as an etching method, there are a wet method using an acid or alkali solution and a dry method such as reactive ion etching (RIE), and any method can be used as long as it is suitable for the purpose of removing crystal surface and internal strain. . Examples of the acid include hydrofluoric acid, nitric acid, phosphoric acid, organic acids, and mixtures thereof. Examples of the alkali include sodium hydroxide and potassium hydroxide. Examples of the dry method include argon and chlorine-based gas. Can be used.
[0009]
【Example】
Example 1
As an example of a method for manufacturing the polarization diffraction element of the present invention (the structure of which is shown in FIG. 1), the strain on the crystal surface was relaxed by heat treatment. First, the X-cut lithium niobate substrate 1 is annealed at 700 ° C. for 1 hour, and then a lattice-like tantalum mask pattern at a constant interval (50 μm) is formed on the surface as a mask for ion exchange by ordinary photolithographic techniques. . After immersion in pyrophosphoric acid at 230 ° C. for 2 hours to form a proton exchange region 2 having a depth of 2.4 μm, the surface layer of the proton exchange region was etched away with hydrofluoric acid to form a phase adjusting groove 3 and at the same time tantalum. The mask was melted to produce a desired device. In addition, a sample that was not heat-treated before proton exchange was used as a comparative sample. As shown in FIG. 4, in the comparative sample, many fine cracks were generated in the direction perpendicular to the lattice. On the other hand, the state of the element surface after proton exchange in this example was as shown in FIG. It can be seen that it has been greatly reduced. Further, when the transmittance at λ = 780 nm (the sum of ± first-order light and zero-order light divided by the incident light amount) is measured after applying an antireflection film to the polarization diffraction element, no processing is performed. The transmittance was 40% or less, whereas the transmittance of this embodiment was greatly improved to 80% or more due to a decrease in scattering loss accompanying the decrease in cracks.
Example 2
The same crystal substrate as in Example 1 was immersed in 50% hydrofluoric acid for 20 minutes to etch the crystal surface, and then subjected to proton exchange with pyrophosphoric acid at 230 ° C. for 2 hours in the same manner as in Example 1. Also in this case, the surface of this polarization diffraction element was almost the same as that shown in FIG. 3, and the number of fine cracks was significantly reduced as compared with the case where no such treatment was performed. It can be seen that the distortion inside and on the surface of the crystal was sufficiently removed by such an etching treatment.
[0010]
【The invention's effect】
In the present invention, since the distortion inside the crystal and the surface during the proton exchange are sufficiently removed, the occurrence of cracks after the proton exchange is suppressed, and a low-loss polarization diffraction element can be manufactured with good reproducibility. In addition, proton exchange with good reproducibility becomes possible regardless of the quality of the crystal, so that a high-quality ion exchange region can be formed even by using an inexpensive crystal substrate having poor crystal uniformity. An inexpensive polarization diffraction element can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic view of a polarization diffraction element.
FIG. 2 is a schematic view of another example of the polarization diffraction element.
FIG. 3 is a surface photograph of a polarization diffraction element according to the present invention.
FIG. 4 is a photograph of the surface of a polarization diffraction element without pretreatment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Lithium niobate substrate 2 Proton exchange area 3 Phase adjustment groove 4 Phase adjustment film

Claims (3)

The method of manufacturing a polarization diffraction element in the anisotropic crystal surface provided with a lattice-like ion exchange region, before the ion exchange, the entire surface of the front Symbol anisotropic crystal surface, in the anisotropic crystal A method for manufacturing a polarization diffraction element, comprising performing a pretreatment for removing distortion. The method for manufacturing a polarization diffraction element according to claim 1, wherein the pretreatment is a heat treatment. The method for manufacturing a polarization diffraction element according to claim 1, wherein the pretreatment is an etching treatment.

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