JP2004226583A - Method for processing high dielectric material and optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for processing a high dielectric material, having simplified steps by proposing a mask material with a high mask selection ratio and etching to a depth of ≥2 μm, and an optical waveguide element. <P>SOLUTION: Silicon nitride is used as a mask on lithium niobate and gas with a large carbon/fluorine ratio is used to suppress the etching rate of silicon nitride, thereby realizing the etching to a depth of ≥10 μm. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for processing a high dielectric material using plasma and an optical waveguide device.
[0002]
[Prior art]
In recent years, lithium niobate having a perovskite structure, LiTaO ₃ , KTiOPO ₄ , CLBO, BBO, and the like are expected to be applied to various devices as functional materials having pyroelectric, electro-optical, nonlinear optical, and ferroelectric properties. Have been. As a part thereof, it is applied to various elements such as a piezo element, an infrared detection element, a nonvolatile memory element, and a second harmonic generation element. In recent years, as devices have become lighter and more compact, miniaturization and higher functionality of these devices have been demanded.
[0003]
As a conventional example, a second harmonic generation element (hereinafter, SHG) using a lithium niobate material will be described. Conventionally, embedded optical waveguides are generally used for optical waveguide devices. A method of manufacturing a conventional embedded optical waveguide will be described. FIG. 6 shows the shape of a conventional embedded optical waveguide. This is a method for forming a buried optical waveguide by proton exchange for exchanging proton ions (+ H) and heat treatment. In this method, since the proton exchange layer is diffused by heat treatment, an optical waveguide of the order of the number of widths + μm can be obtained.
[0004]
Furthermore, in order to realize an optical waveguide device that performs high-efficiency and fine confinement, ridge processing as shown in FIG. 2 may be performed (Patent Documents 1 and 2).
[0005]
An example of ridge processing of a conventional optical waveguide device will be described.
[0006]
First, a first conventional processing example will be described with reference to FIG. A metal film having a certain thickness is formed on a lithium niobate substrate, and a resist mask is formed on the metal film. The metal film 14 is processed using the resist 15 as a mask, the resist 15 is removed, and the ridge processing of the lithium niobate substrate 6 is performed using the processed metal film 14 as a mask (Non-Patent Document 1). Here, the metal mask is a heavy metal such as nickel or chromium (FIG. 7).
[0007]
Next, a description will be given of a second conventional processing example with reference to FIG. A metal film having a certain thickness is formed on a lithium niobate substrate, and a resist mask is formed on the metal film. The metal film 14 is processed using the resist 15 as a mask, and the resist is removed. Proton exchange and heat treatment are performed on the lithium niobate substrate 6 using the metal film as a mask to form a proton exchange layer 16. Further, ridge processing is performed by dry-etching the proton exchange layer 16. Since the proton exchange layer has a broken crystal structure, the etching rate is higher than that of single crystal lithium niobate. Here also, the metal mask is made of a heavy metal such as nickel and chromium.
[0008]
An etching method for the above two processes will be described.
[0009]
FIG. 1 shows a configuration diagram of a conventional etching apparatus. The inductive coupling coil 4 is installed on the upper part of the vacuum vessel 1 via the dielectric plate 5. The substrate 6 is placed on the electrode 7 in the vacuum vessel 1. While introducing the gas, the pressure is controlled to a predetermined value by the pressure controller 10 to generate plasma in the vacuum vessel 1, and the substrate 6 placed on the electrode 7 or the film on the substrate 6 is etched. Here, the film on the base 6 is a lithium niobate thin film or the substrate itself is lithium niobate. For plasma generation, high frequency power is applied to the inductive coupling coil 4 from a high frequency power supply to generate plasma. High-frequency power is applied to the lower electrode 7 to attract ions existing in the plasma in the vacuum chamber 1. The etching gas used is CF ₄ , CHF ₃ , and NF ₃ (Non-Patent Document 1).
[0010]
After forming the ridge, silicon oxide having a low refractive index is deposited on the surface of lithium niobate to confine light within the ridge.
[0011]
[Patent Document 1]
JP 07-159637 A [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-147584 [Non-Patent Document 1]
Etching Characteristics of Lithium Niobate Crystal by Fluorine Gas Plasma RIE, Electronics A, Vol. 120, No. 2, 2000
[Problems to be solved by the invention]
However, in each of the conventional examples, many steps are performed using a metal mask, and a long etching time is required because of a low etching rate. In addition, metals such as nickel and chromium have low reactivity and are hardly etched, and it is difficult to process them on the order of microns on a mask. The reason for using a metal mask is that the resist mask cannot be processed to a desired depth because the etching rate of the resist mask is faster than the etching of lithium niobate, that is, the selectivity to resist is low.
[0013]
Even if the etching can be performed, there is a problem that the number of steps increases because the mask is removed in the next step and silicon oxide having a low refractive index is deposited on the ridge surface.
[0014]
SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and proposes a mask material having a high selectivity to a mask, enables etching to a depth of 2 μm or more, and simplifies the process. It is an object of the present invention to provide a processing method and an optical waveguide element.
[0015]
[Means for Solving the Problems]
In the method for processing a high dielectric material according to the first invention of the present application, the material formed on the substrate is a high dielectric material or the substrate itself is a high dielectric material, and the organic material is used as a first layer on the high dielectric material. A first step of laminating a mask material that does not exist, a second step of applying a resist as a second layer thin film on the first layer, a third step of patterning the resist, and a first layer. Plasma processing for etching the high dielectric material using the patterned first layer as a mask, including a fourth step of etching silicon nitride to a high dielectric material and a fifth step of removing a second layer of resist. The method is characterized in that the first layer mask material is silicon nitride.
[0016]
Further, in the method for processing a high dielectric material according to the second invention of the present application, the material formed on the substrate is a high dielectric material or the substrate itself is a high dielectric material, and the organic material is formed as a first layer on the high dielectric material. A first step of laminating a mask material that is not a material, a second step of applying a resist as a second layer thin film on the first layer, a third step of patterning the resist, and a first layer A fourth step of etching silicon nitride to a high dielectric material, and a plasma processing method of etching the high dielectric material using the patterned first layer and a resist as a mask. It is characterized by being.
[0017]
Further, in the optical waveguide device according to the third invention of the present application, the material formed on the substrate is a high dielectric material or the substrate itself is a high dielectric material, and the silicon nitride is used as a mask to dry-etch the high dielectric material to form a pattern. The light is transmitted to the silicon nitride side of the high dielectric material formed with.
[0018]
At this time, in the optical waveguide device according to the third invention of the present application, it is preferable that the high dielectric material or the high dielectric substrate is made of a material having a relative dielectric constant of 20 or more.
[0019]
Further, the high dielectric material or the high dielectric substrate formed on the substrate is preferably a material containing at least one of lithium niobate, LiTaO ₃ , KTiOPO ₄ , CLBO, and BBO.
[0020]
In the method for processing a high dielectric material according to the first and second aspects of the present invention, the means for generating plasma in the vacuum vessel is an inductively coupled plasma source, an electron cycloton resonance plasma source, a helicon plasma source, or a VHF plasma source. It is suitable.
[0021]
Further, in the method of processing a high dielectric material according to the first invention of the present application, the gas introduced into the vacuum vessel is CHF ₃ , CF ₄ , C ₄ F ₈ , CH ₂ F ₂ , C ₂ F ₆ , C ₃ F ₅ , or a gas or a mixed gas containing at least one of C ₅ F ₈ .
[0022]
Furthermore, in the method for processing a high dielectric material according to the first invention of the present application, it is preferable that the ratio of fluorine to carbon of the gas introduced into the vacuum vessel is 20% or more.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0024]
FIG. 1 shows a configuration diagram of the etching apparatus used in the experiment. The inductive coupling coil 4 is installed on the upper part of the vacuum vessel 1 via the dielectric plate 5. The substrate 6 is placed on the electrode 7 in the vacuum vessel 1. While introducing the gas, the pressure is controlled to a predetermined value by the pressure controller 10 to generate plasma in the vacuum vessel 1, and the substrate 6 placed on the electrode 7 or the film on the substrate 6 is etched. Here, the film on the substrate 6 is a lithium niobate thin film, but the substrate itself may be lithium niobate. The object to be etched on the substrate 6 may be a material containing at least one of lithium niobate, LiTaO ₃ , KTiOPO ₄ , CLBO, and BBO. These substances have a relative dielectric constant of 20 or more.
[0025]
For plasma generation, high frequency power is applied to the inductive coupling coil 4 from a high frequency power supply to generate plasma. High-frequency power is applied to the lower electrode 7 to attract ions existing in the plasma in the vacuum chamber 1.
[0026]
A silicon nitride film 17 is grown on the lithium niobate 6 on the substrate 6 as shown in FIG. A resist mask 15 is formed thereon, and a pattern 15 is formed by a stepper. Thereafter, silicon nitride is etched using the dry etching apparatus until the silicon nitride reaches lithium niobate. Next, after the resist is removed by ashing, the lithium niobate is etched using the etching apparatus. At this time, a fluorine-based gas is used as an etching gas, but the carbon / fluorine ratio of the fluorine-based gas may be 30% or more. Here, an example in which C ₄ F ₈ is used as an etching gas will be described. FIG. 4 shows the carbon / fluorine dependence of the silicon nitride selectivity. According to this figure, the composition of the etching gas is at least one of C ₄ F ₈ , CH ₂ F ₂ , C ₂ F ₆ , C ₃ F ₅ , and C ₅ F ₈ so that the carbon / fluorine ratio becomes 30% or more. It is understood that a gas or a mixed gas may be included. The etching rate of silicon nitride is slower than that of lithium niobate.
[0027]
This etching will be described. In the etching of lithium niobate, oxygen atoms are extracted by carbon atoms in the gas, evaporation of niobium fluoride, and sputtering of lithium fluoride are simultaneously performed. Therefore, even if a gas having a high carbon ratio is used, the etching rate does not decrease. On the other hand, silicon nitride is composed of nitrogen atoms and silicon atoms, and these atoms react with fluorine in the gas and silicon, but do not react with nitrogen atoms. Therefore, when the carbon ratio is increased, the reaction gas decreases, and an etching stop occurs, so that it functions as a mask.
[0028]
The degree of vacuum for etching the lithium niobate is preferably a high vacuum of 1.0 Pa or more. At a high vacuum of 1.0 Pa or less, a fluorine compound in which lithium niobate and fluorine have reacted tends to evaporate and react more than the reaction between the resist and fluorine, so that the etching rate of lithium niobate increases (FIG. 5). In order to maintain the plasma at a high vacuum of 1.0 Pa or less, an example using inductively coupled plasma has been described here. However, an electron cyclotron resonance plasma source, a helicon plasma source, or a VHF plasma source may be used as the plasma source. It is valid. In this example, the resist was removed by ashing, but the lithium niobate may be etched with the resist remaining.
[0029]
Normally, a silicon nitride mask is formed on lithium niobate, but some intermediate film may be present between the lithium niobate and the silicon nitride layer. At this time, the intermediate layer is removed immediately after the etching of the silicon nitride or immediately before the etching of the lithium niobate.
[0030]
When the lithium niobate ridge is formed, the silicon nitride film used as a mask remains. This silicon nitride may be removed after etching lithium niobate to a predetermined depth.
[0031]
FIG. 2 shows an example in which an optical waveguide is manufactured after forming a ridge of lithium niobate and leaving silicon nitride. Although the ridge surface is usually covered with a silicon oxide film, a silicon nitride mask is used for the device as it is.
[0032]
【The invention's effect】
The process can be reduced by leaving the silicon nitride film on the ridge. By the presence of the silicon nitride film having a low refractive index on the ridge, light is collected immediately below the silicon nitride, and an optical waveguide with low loss can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a dry etching apparatus used in the present invention. FIG. 2 is a diagram showing a ridge shape of lithium niobate processed in the present invention. FIG. 3 is a sample structure diagram used in the present invention. FIG. 5 is a diagram showing the gas dependence of the selectivity ratio of silicon nitride to silicon in the embodiment of the present invention. FIG. 5 is a diagram showing the pressure dependence of the etching rate of lithium niobate in the vacuum vessel in the embodiment of the present invention. FIG. 7 is a diagram showing a conventional light confinement region. FIG. 7 is a sample structure diagram using a conventional metal mask. FIG. 8 is a diagram showing an etching shape 1 of lithium niobate etched using a conventional metal mask. FIG. 9 is a diagram showing an etched shape 2 of lithium niobate etched using a conventional metal mask.
DESCRIPTION OF SYMBOLS 1 Vacuum container 2 High frequency power supply for inductive coupling coil application 3 Matching box for inductive coupling coil 4 Inductive coupling coil 5 Dielectric plate (quartz plate)
6. Substrate (lithium niobate)
7 Lower electrode 8 High frequency power supply for lower electrode 9 High frequency power supply for lower electrode application 10 Pressure controller

Claims (8)

A first step in which a material to be formed on the substrate is a high dielectric material or the substrate itself is a high dielectric material, and a mask material which is not an organic material is laminated as a first layer on the high dielectric material; A second step of applying a resist thereon as a thin film of a second layer, a third step of patterning the resist, and a fourth step of etching the first layer of silicon nitride to a high dielectric material; In a plasma processing method for etching a high-dielectric material using the patterned first layer as a mask, the method includes a fifth step of removing the resist of the second layer, wherein the mask material of the first layer is silicon nitride. Characteristic high dielectric material processing method. A first step in which a material to be formed on the substrate is a high dielectric material or the substrate itself is a high dielectric material, and a mask material which is not an organic material is laminated as a first layer on the high dielectric material; A second step of applying a resist thereon as a thin film of a second layer, a third step of patterning the resist, and a fourth step of etching the first layer of silicon nitride to a high dielectric material; A plasma processing method for etching a high dielectric material using a patterned first layer and a resist as a mask, wherein the mask material of the first layer is silicon nitride. The material to be formed on the substrate is a high-dielectric material or the substrate itself is a high-dielectric material, and the high-dielectric material is dry-etched using silicon nitride as a mask, and light is emitted to the silicon nitride side of the patterned high-dielectric material. An optical waveguide element characterized by passing through. 4. The optical waveguide device according to claim 3, wherein the high dielectric material or the high dielectric substrate is a material having a relative dielectric constant of 20 or more. High dielectric material or high dielectric substrate of lithium niobate formed on the _{substrate, LiTaO 3, KTiOPO 4, CLBO} , optical waveguide device according to claim 3, characterized in that the material containing at least one of BBO . 3. The method for processing a high dielectric material according to claim 1, wherein the means for generating plasma in the vacuum vessel is an inductively coupled plasma source, an electron cyclotron resonance plasma source, a helicon plasma source, or a VHF plasma source. . The gas introduced into the vacuum vessel is a gas or a mixed gas containing at least one of CHF ₃ , CF ₄ , C ₄ F ₈ , CH ₂ F ₂ , C ₂ F ₆ , C ₃ F ₅ , and C ₅ F _8. 2. The method for processing a high dielectric material according to claim 1, wherein: 2. The method for processing a high dielectric material according to claim 1, wherein the ratio of fluorine to carbon in the gas introduced into the vacuum vessel is 20% or more.

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