WO2023209819A1 - Method for inspecting periodical polarization inversion structure - Google Patents

Abstract

A method for inspecting a periodical polarization inversion structure according to the present disclosure comprises: a step for fixing a substrate to a stage via an electrostatic chuck (43); a step for selectively etching an inspection region of the periodical polarization inversion structure (44) near an opening of a capillary (45) by dry etching; and a step for determining nonuniformity or a defect of the periodical polarization inversion structure in the inspection region on the basis of recesses and projections formed, in a polarizing direction, through the dry etching.　This method can omit a complex process, and improves throughput in a case where the vicinity of a waveguide needs to be observed during a manufacturing process of an optical device including the periodical polarization inversion structure. Local inspection near the waveguide can be carried out, and thus the nonuniformity or the defect of the periodical structure can be discovered more accurately.

Description

Inspection method for periodically poled structure

The present invention relates to a method for inspecting a periodically poled structure of a dielectric material.

Optical elements that can generate and modulate coherent light in the ultraviolet, visible, near-infrared, and terahertz wavelength bands play an important role. Such optical elements can be applied in a wide variety of fields such as wavelength conversion of optical signals, optical modulation, optical measurement, and optical processing in optical communication systems. Among them, optical elements that utilize nonlinear optical effects have excellent characteristics in terms of nonlinear optical effects such as wavelength conversion and electro-optic effects.

Typical examples of optical materials having nonlinear optical effects and electro-optic effects are oxide-based compounds such as lithium niobate (LiNbO ₃ :LN) and lithium tantalate (LiTaO ₃ :LT). Taking advantage of the characteristic that LN and LT can spontaneously polarize at room temperature, PPLN (Periodically Poled Lithium Niobate) and PPLT (Periodically Poled Lithium Tantalate), which have a periodically polarized structure, are widely used. The periodically poled structure achieves high phase matching and high second-order nonlinear optical effects.

A wavelength conversion element is known as an optical device that uses the high nonlinearity of PPLN and PPLT. The wavelength conversion element uses mechanisms of second harmonic generation (SHG), differential frequency generation (DFG), and sum frequency generation (SFG). Wavelength conversion elements that utilize these mechanisms are useful for building large-capacity optical communication networks, including the development of light sources in new wavelength bands, optical routing and wavelength collision avoidance through batch wavelength conversion, and signal distortion compensation using phase conjugate light. Supports core technology.

In order to realize the above technology, it is essential to develop highly efficient nonlinear optical elements. Quasi-phase matching technology becomes important for increasing wavelength conversion efficiency (Non-Patent Document 1). The pseudo phase matching technique is a method of achieving pseudo phase matching in the light propagation direction by forming a structure in which the sign of nonlinear susceptibility is periodically inverted. Although quasi-phase matching will not be explained here, in order to periodically modulate the nonlinear optical coefficient in linear optical crystals, it is essential to fabricate with high precision a periodically poled structure that periodically reverses the crystal axis. It's technology.

Quasi-phase-matched second harmonic generation tuning and tolerances M. Fejer et al., 1992, IEEE Journal of Quantum Electronics

In order to exhibit a high second-order nonlinear optical effect, it is necessary to design a periodically poled structure with a theoretical value that provides the maximum efficiency, and to precisely manufacture it with that structure. In reality, it is very difficult to form a periodically poled structure as designed. In order to manufacture a chip including a periodically poled structure with sufficient performance, a step of inspecting the dimensional accuracy of the periodically poled structure and the presence or absence of defects is essential.

However, the conventional method for observing and inspecting periodically poled structures requires time and effort, and reduces the throughput of the manufacturing process when it is necessary to observe the vicinity of the waveguide. The present invention facilitates observation of periodic polarization structures near waveguides, improves throughput when it is necessary to inspect the vicinity of waveguides in the manufacturing process of optical devices including periodic polarization inversion structures, and improves the throughput of nonlinear optical elements. We provide an optical device inspection method that improves development efficiency.

One aspect of the present invention is a method for inspecting a periodically poled structure created on a substrate, comprising the steps of fixing the substrate to a stage via an electrostatic chuck, and dry etching to form a capillary. selectively etching the inspection region of the periodically poled structure in the vicinity of the opening, and etching the periodically poled structure in the test region based on the unevenness formed in the dry etching according to the polarization direction. determining non-uniformities or defects in the wafer.

Complicated work steps in testing nonlinear optical elements are omitted, and the periodic polarization structure near the waveguide can be easily observed.

FIG. 2 is a schematic diagram of the structure of a PPLN element having a periodic polarization inversion structure. FIG. 3 is a diagram illustrating various non-uniformity and defect patterns of a periodically poled structure. FIG. 3 is a diagram showing a region where a periodically poled structure is observed in an inspection process on a wafer. FIG. 2 is a diagram comparing inspection flows of a conventional technique and a periodic polarization inversion structure of the present disclosure. FIG. 1 is a diagram showing the configuration of a periodic polarization structure inspection device according to the present disclosure. FIG. 3 is a diagram showing a state of dry etching near the surface of a period-reversed polarization structure. FIG. 3 is a diagram illustrating a pattern of a periodically reversed polarization structure obtained by dry etching. FIG. 3 is a diagram illustrating inspection by local dry etching along a waveguide.

The method for inspecting a periodically poled structure of the present disclosure fixes a sample using an electrostatic chuck and reveals a surface potential that depends on the polarization direction of the sample. By combining this method with dry etching, it is possible to conduct a local inspection near where the waveguide is to be fabricated. Compared to the conventional inspection method using wet etching, complicated steps such as masking can be omitted, making it easier to inspect and observe the periodic polarization inversion structure in the vicinity of the waveguide. Since it is possible to locally inspect the vicinity of the waveguide, it is possible to more accurately discover non-uniformities and defects in the periodic structure.

In the following explanation, first, the structure of the periodically poled structure and its manufacturing process will be briefly mentioned, and then non-uniformity and defects in the periodically poled structure will be described. Furthermore, a method for inspecting a periodically poled structure according to the present disclosure will be described in comparison with a conventional method for inspecting a periodically poled structure.

FIG. 1 is a schematic diagram of the structure of a PPLN element having a periodic polarization inversion structure. A ridge-type waveguide 13 having a periodic polarization inversion structure is formed in the PPLN element 10. As an example, the PPLN element 10 of FIG. 1 is constructed by directly bonding a first substrate 12 having a periodically polarized structure, which will be a core layer, and a second substrate 11, which will be a cladding layer, and forming a waveguide 13 by dry etching. It is obtained by processing it into the shape of. The ridge-type waveguide 13 is formed along the x direction, perpendicular to the boundary surface of each region of the periodically poled structure. The light 18 to be subjected to the nonlinear effect enters from one end of the ridge waveguide 13. In the method for inspecting a periodically poled structure of the present disclosure, there are no limitations on the structure of the optical waveguide that is produced in the device that is finally obtained. Whether it is a ridge type optical waveguide as shown in Figure 1 or a diffused type optical waveguide in which ions or titanium are diffused, there are laser-drawn optical waveguides that form a structure using the change in refractive index caused by laser irradiation. But it's okay.

Looking at the ridge waveguide 13 in the x direction, a region 1a polarized downward (-z) and a region 1b polarized upward (+z) in the z-axis direction perpendicular to the substrate surface are arranged at a constant period (pitch). ) are repeated alternately. A structure in which the polarization direction is periodically reversed between upward and downward in this manner is a periodic polarization inversion structure.

In a ferroelectric material represented by LN, a polarization inversion structure is formed using an electric field application method. Generally, a high voltage is applied to invert the polarization of a ferroelectric material, and in particular, in LN, a very high electric field of about 21 kV/mm is used. Voltage application is performed by generating a voltage pulse with an arbitrary waveform generator, amplifying it with a high-voltage amplifier, and then applying it to the sample via a high-voltage cable. A metal electrode or a liquid electrode is in contact with the surface of the z-cut LN substrate, and the electric field is spatially modulated by the pattern shape of the electrode to form a periodic polarization inversion structure. In such a method for manufacturing a domain-inverted structure, various non-uniformities, variations, and defects may occur in the periodic structure.

FIG. 2 is a diagram illustrating various defect patterns in a periodically poled structure. Both figures show a cross section (zx plane) perpendicular to the substrate surface (xy plane) of the periodically poled structure, and after the waveguide is fabricated, light propagates in the x-axis direction. FIG. 2(a) shows an ideal periodic polarization inversion structure in which there are no defects or non-uniformity, and downward polarized regions 1a and upward polarized regions 1b are regularly arranged at equal intervals. In (b), some areas 2-1 have a different pitch from other areas. In (c), the width 3a of the downward polarized region is different from the width 3b of the upward polarized region, resulting in non-uniformity of the structure. In (d), shape distortion in the z direction of each polarization region occurs in some regions 2-2. In (e), a defective region 3 in which polarization inversion is not formed occurs.

The presence of non-uniformity and defects in the periodically poled structure shown in Figure 2 is due to non-uniformity of the period of inversion, and structures with different quasi-phase matching conditions are distributed in the light propagation direction (x direction). means that This means that when light of the wavelength intended for operation is input to the nonlinear optical element, there are structural parts where the quasi-phase matching condition is not completely satisfied, and the effective conversion efficiency is reduced.

In order to maximize the wavelength conversion efficiency through quasi-phase matching in wavelength conversion elements, it is essential to have the technology to fabricate a periodically polarized structure as designed and to evaluate and inspect that structure with precision. Further, since it is difficult to form the periodically poled structure uniformly within the plane of the substrate (wafer), it is also difficult to accurately predict the distribution of non-uniformity and defects 3a and 3b. Therefore, it is preferable to accurately evaluate and inspect the periodically poled structure on a substrate in which the structure is formed as close as possible to a region where a waveguide will be formed in a subsequent process.

FIG. 3 is a diagram showing a region where a periodically poled structure is observed in the inspection process. FIG. 3A shows a state in which a region 12 having a periodic polarization inversion structure is formed on, for example, an LN wafer 14. FIG. 3 shows the state before the waveguide is fabricated, and shows along the x direction a plurality of regions 13-1 to 13-5 where the waveguide is to be fabricated. FIG. 3 is a schematic diagram for explaining the region, and it should be noted that the period of the periodically poled structure and the width and spacing of the waveguides are not accurate. For example, in the actual manufacturing process of a PPLN element, it is possible to manufacture approximately several hundred waveguides within a 3-inch substrate.

As described above, when a large number of waveguides are fabricated in a periodically poled structure on a wafer, it is preferable to evaluate and inspect the periodically poled structure as close to the waveguide as possible. However, in the evaluation and inspection of the periodically poled structure in the prior art, since destructive testing is involved as will be described later, it was possible to evaluate only the end region 15 of the region 12 of the periodically poled structure on the wafer. Originally, as shown in the enlarged view of the periodically poled structure in FIG. It is preferable to make the judgment easier. Therefore, it is desirable to be able to perform evaluation in the inspection area 15 as close as possible to the area 13-1 where the waveguide is to be fabricated.

FIG. 4 is a diagram showing the flow of a method for inspecting a periodically poled structure according to the prior art and the present disclosure. FIG. 4A is a flow diagram illustrating an example of a conventional inspection method. FIG. 4B is a flowchart showing an example of a method for inspecting a periodically poled structure according to the present disclosure, and the details will be described later. Although FIG. 4 is shown as an inspection method in the manufacturing process of an optical element such as a wavelength conversion element, it should be noted that it also has aspects of an observation method and an evaluation method for a periodic polarization inversion structure. The flowchart 20 in FIG. 4(a) starts at step 21, in which a periodically poled structure is fabricated on, for example, an LN substrate, as in the wafer state shown in FIG. 3(a).

In order to observe the periodically poled structure of the LN crystal, first, the end region of FIG. A step 22 of masking the area where the other waveguides are to be fabricated except for the area 15 is required. This is to prevent the waveguide from being damaged by wet etching in the next step. Next, in a wet etching step 23 using hydrofluoric acid and hydrofluoric nitric acid, the end region 15 is etched. In the periodically poled structure, there is a difference in etching rate between a region polarized in the z-direction of the crystal axis and a region polarized in the -z-axis direction. Non-uniformity and the presence of defects can be visualized by the step difference in the etched surface in the thickness direction of the substrate caused by the difference in etching rate.

After the wet etching step 23 is completed, a masking removal step 24 is performed to observe the periodic polarization inversion structure. In the observation step 25, if non-uniformity or defects in the periodic structure as shown in FIG. 2 are discovered, they are reflected in the next step, the waveguide structure fabrication step 26, according to certain criteria. For example, if more non-uniformities or defects are found than a certain standard, the wafer can be discarded.

In the inspection flow shown in FIG. 4(a), the wet etching step 23 is essential, so it is very difficult to observe the polarization inversion structure only in the vicinity of the crystal plane where the waveguide is actually created on the wafer. During wet etching, only the area to be inspected is immersed in a solution such as hydrofluoric acid and hydrofluoronitric acid. In order to selectively etch only the vicinity of the crystal plane where the waveguide is to be formed, a step 22 of masking the area outside the region to be wet etched and a step 24 of removing the masking are essential, as shown in FIG. 4(a). These essential steps associated with wet etching take up a large amount of time in the periodic polarization inversion structure inspection process, and the work is complicated, resulting in cost and time.

The inspection process shown in FIG. 4(a) involves inspecting the periodic polarization inversion structure near the waveguide before dicing and cutting out the optical circuit including the nonlinear optical element fabricated on the wafer as a chip. The case where this is implemented is shown. After dicing the wafer into chips, each chip may be inspected for a periodic polarization inversion structure similar to that shown in FIG. 4(a).

The method for inspecting a periodically poled structure of the present disclosure greatly simplifies the process shown in FIG. 4(a) in the prior art, and more easily visualizes the periodically poled structure near the waveguide. When it is necessary to observe the vicinity of a waveguide, it is also possible to improve the throughput of the manufacturing process of optical elements such as wavelength conversion elements.

[Target nonlinear optical materials]
The nonlinear optical material that can be used in the method of inspecting a periodically poled structure of the present disclosure may be any material as long as it is transparent at a light wavelength of 400 to 2000 nm. Further, the nonlinear optical material may have a second-order nonlinear optical effect or a third-order or higher nonlinear optical effect as long as it has a nonlinear optical effect. Examples include, in addition to the above-mentioned LN and LT, beta-barium bolite (BBO), potassium titanyl phosphate (KTiOPO ₄ , KTP), and the like. The nonlinear optical material in the inspection method of the present disclosure is a material that can form a periodically poled structure to increase the nonlinear optical effect.

[Inspection device for periodically polarized structure]
FIG. 5 is a diagram showing the configuration of an inspection device that plays a part in the inspection method for a periodically inverted polarization structure of the present disclosure. The inspection device 40 is a dry etching device in which a sample 44 to be inspected is placed in a vacuum chamber 41 . The chamber 41 is evacuated to a vacuum state by a vacuum pump 50, and etching gas 47 is introduced into the chamber from a cylinder 46. As the etching gas, argon or CF-based gas is mainly used, and the gas is used depending on the material to be etched. The sample 44 is fixed on a three-dimensionally movable stage 42 via an electrostatic chuck 43. Above the sample 44 is an insulating capillary 45 connected to a vacuum pump 50. The etching gas flows into the capillary due to the pressure difference between the chamber internal pressure (V2) and the capillary internal pressure (V1), and is exhausted by a vacuum pump.

A high-frequency coil 48 is installed around the capillary outside the chamber, and by applying a high voltage to the high-frequency coil 48 from a high-frequency power source 49, plasma is generated from the capillary into the chamber. A part of the plasma is also generated on the entrance side (sample side) of the capillary 45, and the diameter of the plasma can be narrowed down to about several mm, for example. The diameter of this plasma depends on the inner diameter of the capillary and can be controlled by the shape and material conditions of the capillary. Further, it should be noted that the inspection device 40 in FIG. 5 is a schematic diagram, and the length of the capillary, the relative position and distance to the high-frequency coil 48 and the sample 44 are different from the actual one.

In the inspection apparatus 40 configured as shown in FIG. 5, the plasma locally generated on the surface of the sample 44 makes it possible to perform local dry etching at any desired position on the periodically poled structure. The inspection device 40 shown in FIG. 5 uses an electrostatic chuck on the sample 44 to generate a surface potential on the sample that depends on the polarization direction of the sample, thereby realizing an etching rate that depends on the polarization direction. do. The sample 44 in FIG. 5 may be a wafer before waveguide fabrication, or a cut chip.

FIG. 6 is a diagram showing the state of dry etching near the surface of the periodically reversed polarization structure. The cross section (xz plane) of the sample 44 fixed by the electrostatic chuck 43 is seen, and (a) in FIG. 6 is before the start of dry etching, and (b) is during dry etching, and the flow of the etching gas is schematically shown. It shows. Before the start of dry etching in FIG. 6A, the vertical position of the capillary 45 on the z-axis is away from the substrate surface, and the plasma is also away from the substrate surface. As shown in FIG. 6B, when the capillary 45 descends to the vicinity of the substrate and the plasma limited to a certain area reaches the surface, local dry etching begins. Etching gas is constantly supplied from the outside to the inside of the capillary 45 due to the pressure difference (V ₁ , V ₂ ) between the inside and outside of the capillary 45 .

FIG. 6(c) is a diagram illustrating the potential near the surface of the periodically reversed polarization structure during dry etching. In a periodically reversed polarization structure, regions where upward polarization in the z-axis direction (upward arrow) occurs and regions where downward polarization in the z-axis direction (downward arrow) occur are formed alternately along the x-axis. . At this time, positively charged regions and negatively charged regions alternately appear on the outermost surface of the substrate. Therefore, the potential on the substrate surface changes periodically in the x-axis direction. Since the reactivity between the molecules of the etching gases 47a and 47b on the outermost surface of the substrate and the nonlinear optical material whose potential distribution varies, different etching rates are produced depending on the position on the x-axis.

FIG. 7 is a diagram illustrating the pattern of a periodically reversed polarization structure obtained by dry etching. As shown in FIG. 7(a), a region 12 having a periodically reversed polarization structure is formed on the wafer 14. The waveguide will be formed along the x-axis direction. When the reaction with the etching gas progresses at the etching rate dependent on the polarization direction as explained in FIG. 6, an uneven pattern depending on the polarization direction is locally generated on the surface. If dry etching can be performed continuously along the x-axis direction, an uneven pattern with a height at a periodic pitch of the polarization structure will appear in the x-axis direction, as shown in FIG. 7(b). By observing this concavo-convex pattern depending on the polarization direction using various measuring instruments such as an optical microscope or a step measurement system, it is possible to locally visualize the periodic polarization inversion structure.

[Procedure of inspection method for periodically poled structure]
FIG. 8 is a diagram illustrating inspection of a periodically reversed polarization structure by local dry etching along a waveguide. A stage 42 that holds a sample in the inspection apparatus 40 shown in FIG. 5 can move at least in the horizontal direction (x direction). With respect to a capillary 45 fixed to the apparatus 40, local dry etching can be performed at any position on the substrate surface by a movable stage 42 that determines the position of the sample 44 in the horizontal plane. For example, in FIG. 8, by moving (sweeping, scanning) the stage 42 in the x-axis direction parallel to the light propagation direction 18, the generally circular etching region 16 is swept, making it possible to perform linear etching. By making the area 17 to be processed into a line parallel to the area 13-1 where the waveguide of the wavelength conversion element is planned to be fabricated, the periodic polarization inversion structure can be visualized along the propagation direction of the light of the wavelength conversion element. . Although it depends on which places in the wafer are to be etched, the time required for local etching using the inspection apparatus 40 of FIG. 5 is about one hour at most.

Referring again to FIG. 4(b), a flowchart 30 of the method of testing a periodically poled structure of the present disclosure is shown in contrast to the flowchart of the prior art testing method of FIG. 4(a). As is clear from the flow diagram 30, the method for inspecting a periodically poled structure of the present disclosure does not require a masking step before the local dry etching 32 or a masking removal step. Therefore, compared to the conventional inspection flow 20 using wet etching, complicated work steps are omitted, and throughput can be improved when it is necessary to observe the vicinity of the waveguide. The work time and cost for observing and inspecting the periodically poled structure are significantly reduced, and wavelength conversion elements can be evaluated and manufactured efficiently.

In addition, in the inspection method shown in FIG. 4(b), the location and number of areas to be locally etched can be changed depending on the required quality level, and only good areas on the wafer can be used in the next process. It is also possible to provide such judgment information.

Easily observe periodic polarization structures near waveguides, improve throughput when it is necessary to inspect the vicinity of waveguides in the manufacturing process of optical devices that include periodic polarization inversion structures, and improve efficiency in the development of nonlinear optical elements. Realize.

The present invention can be used for manufacturing and developing optical devices.

Claims (3)

A method for inspecting a periodically poled structure created on a substrate, the method comprising:
fixing the substrate to a stage via an electrostatic chuck;
selectively etching the inspection region of the periodically poled structure near the opening of the capillary by dry etching;
and determining non-uniformity or defects in the periodically poled structure in the inspection area based on the unevenness formed in the dry etching that corresponds to the polarization direction. The stage is movable in a horizontal direction with respect to the opening of the capillary,
2. The method according to claim 1, further comprising: sweeping the inspection area on the substrate in parallel to a region where a waveguide is to be formed. The material of the substrate is a dielectric material such as lithium niobate (LiNbO ₃ :LN), lithium tantalate (LiTaO ₃ :LT), beta barium bolite (BBO), potassium titanyl phosphate (KTiOPO ₄ , KTP). The method according to claim 1 or 2.

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