A method for preparing a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide

Abstract

The application discloses a preparation method of high-quality ferroelectric domain structure in a ferroelectric thin film ridge waveguide, and comprises the following steps: according to a overlay mark, a waveguide shape profile is prepared on the surface of an x-cut lithium niobate / lithium tantalate thin film, then a waveguide structure is etched, electron beam glue is removed, and a lithium niobate / lithium tantalate thin film waveguide is obtained; a silicon dioxide cover layer is added on the surface of the waveguide; according to the overlay mark, a comb-shaped electrode shape profile is prepared on the surface of the x-cut lithium niobate / lithium tantalate thin film, then a layer of metal electrode is plated, photoresist is removed, and a polarized electrode structure is obtained; a positive electrode is connected to one side of the electrode, the other side of the electrode is grounded, then an electric field is applied, so that the domains between the electrodes are reversed, the electrode is removed, the end face of the waveguide is polished, and a lithium niobate / lithium tantalate thin film superlattice waveguide is obtained. Through the introduction of the cover layer, the problem of incomplete domain reversal caused by polarization after etching is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor integrated optoelectronic devices, in particular to a preparation method of high-quality ferroelectric domain structure in ferroelectric thin film ridge waveguide. BACKGROUND

[0002] The mainstream preparation process of lithium niobate thin film waveguide currently adopts the "polarization after etching" technology, but this process has obvious defects: after polarization, the sample is etched and cleaned, and due to the selective etching effect of the etching solution, wrinkles are generated on the surface of the waveguide with periodic domain inversion structure, which significantly increases the transmission loss of the optical waveguide.

[0003] Patent 2022116043891 discloses a preparation method of lithium niobate thin film superlattice waveguide and a comb-shaped electrode. The preparation method comprises: etching to obtain a lithium niobate thin film waveguide and a comb-shaped electrode structure; the comb fingers of the comb-shaped electrode are periodically arranged, the comb fingers are perpendicular to the z-axis of lithium niobate, the comb fingers of the two side electrodes are opposite, and the comb fingers of one side electrode are sharp, and the comb fingers of the other side electrode are flat; connect the positive electrode of the one side electrode with sharp comb fingers to the circuit, and connect the other side electrode to the ground, apply an electric field, and make the lithium niobate in the metal electrode coverage area domain flip; remove the electrode to obtain a waveguide region domain flip structure, and further obtain a lithium niobate thin film superlattice waveguide. This patent greatly reduces the waveguide loss caused by the selective etching of positive and negative polarity domains during the cleaning process in the polarization after etching waveguide process, but this process also has some problems, such as the domain structure formed after polarization presents a non-ideal triangular morphology or the domain flip is not complete, which causes a significant deviation between the theoretical prediction efficiency and the actual performance of the waveguide. SUMMARY

[0004] The present application aims to provide a preparation method of high-quality ferroelectric domain structure in ferroelectric thin film ridge waveguide with polarization after etching, which solves the problem of incomplete domain flip caused by polarization after etching.

[0005] Technical scheme: The preparation method of high-quality ferroelectric domain structure in ferroelectric thin film ridge waveguide according to the present application comprises the following steps:

[0006] (1) preparing a mask mark for mask alignment,

[0007] (2) preparing a waveguide shape profile on the x-cut lithium niobate / lithium tantalate thin film surface by electron beam lithography according to the mask mark;

[0008] (3) etching a waveguide structure on the x-cut lithium niobate / lithium tantalate thin film surface by IBE ion beam etching;

[0009] (4) removing the electron beam glue to obtain a lithium niobate / lithium tantalate thin film waveguide;

[0010] (5) Adding a cover layer on the surface of the lithium niobate / lithium tantalate thin film waveguide, the cover layer being silicon dioxide;

[0011] (6) According to the overlay mark, a comb-shaped electrode shape profile is prepared on the x-cut lithium niobate / lithium tantalate thin film surface by photolithography, wherein the comb fingers are perpendicular to the z-axis of the lithium niobate;

[0012] (7) A layer of metal electrode is plated on the photolithographed electrode shape profile;

[0013] (8) The photoresist is removed to obtain a polarized electrode structure;

[0014] (9) The positive electrode is connected to one side of the electrode, the other side of the electrode is grounded, and then an electric field is applied to make the domains between the electrodes flip;

[0015] (10) The electrode is removed by using dilute hydrochloric acid to obtain a waveguide domain flip structure;

[0016] (11) The waveguide end face is polished to obtain a lithium niobate / lithium tantalate thin film superlattice waveguide.

[0017] Preferably, the thickness of the cover layer is the thickness of the etched waveguide ± 100 nm. The thickness of the cover layer should be as close as possible to the height of the waveguide, otherwise it may cause the polarization to be unable to completely reverse.

[0018] Preferably, the preparation method of the overlay mark comprises the following steps:

[0019] (1) An overlay mark photoresist pattern is prepared on the sample by ultraviolet photolithography;

[0020] (2) A layer of 30-300 nm thick metal that is not easy to remove is plated on the photolithographed structure;

[0021] (3) The photoresist is removed to leave a metal mark for overlay, and an overlay mark is obtained.

[0022] Preferably, in step (4), the method for removing the electron beam resist is to use RCA solution; further preferably, the RCA solution formula is ammonia water: hydrogen peroxide: water = 1:2:7 (volume ratio).

[0023] Preferably, in step (5), the method for adding a cover layer on the sample surface is plasma enhanced chemical vapor deposition (PECVD) or electron beam evaporation.

[0024] Preferably, in step (6), the photolithography can be electron beam lithography or ultraviolet lithography or a combination of both.

[0025] Preferably, in step (6), the width of the comb-shaped electrode is 1 / 2 to 1 / 4 of the period length.

[0026] Preferably, the comb-shaped electrode is 20-100 um long, the period length is 5-50 um, the electrode width is 1 / 4-1 / 2 of the period length, and the positive and negative electrode spacing is 10-20 um.

[0027] Preferably, in step (7), the metal electrode is aluminum, nickel, chromium or titanium.

[0028] Preferably, in step (8), the method for removing the photoresist is etching removal using an acetone or N-methyl pyrrolidone (NMP) solution.

[0029] Preferably, in step (9), the applied electric field is 100-1400 V, determined according to the period size and the coercive field.

[0030] Invention mechanism: The present application covers the surface of the lithium niobate thin film waveguide with a layer of silicon dioxide, which on the one hand raises the electrode position, and the domain grows from the vicinity of the positive electrode, and on the other hand increases the electric field size on the upper surface of the waveguide. This makes the waveguide surface more prone to domain, so that the waveguide region domain inversion penetrates from top to bottom, making the inversion more thorough.

[0031] Advantages: Compared with the prior art, the present application has the following significant advantages: (1) process sequence optimization: placing the polarization treatment after waveguide formation greatly improves the compatibility and flexibility of the process flow; (2) comprehensive performance improvement: the introduction of the cover layer not only solves the problem of domain structure morphology distortion, but also ensures that the polarization region is completely reversed, ultimately obtaining a low-loss, high-uniformity superlattice waveguide structure; (3) precise quality control: through the measured data of the waveguide parameters in the previous process, the polarization period design can be accurately controlled, effectively avoiding experimental errors. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The flowchart of the preparation method of the present application is shown in the figure;

[0033] Figure 2 The waveguide structure cross-section schematic diagram of Example 1 of the present application is shown in the figure;

[0034] Figure 3 The confocal characterization diagram of the lithium niobate superlattice prepared in Example 1 and Example 2 of the present application is shown in the figure;

[0035] Figure 4 The PFM diagram and electron microscope diagram of the lithium niobate superlattice prepared in Comparative Example 1 are shown in the figure. DETAILED DESCRIPTION

[0036] The technical solutions of the present application are further described below in conjunction with the examples.

[0037] Example 1

[0038] The application discloses a preparation method of high-quality ferroelectric domain structure in a ferroelectric thin film ridge waveguide.

[0039] (1) preparing an overlay mark for overlay

[0040] a. preparing an overlay mark photoresist pattern on a sample by using ultraviolet photoetching;

[0041] b. continuously plating a 30nm / 70nm thick chromium / gold layer on the photoetched structure;

[0042] c. removing the photoresist by using NMP, leaving a metal mark for overlay, and obtaining an overlay mark;

[0043] (2) preparing a waveguide shape profile on the surface of an x-cut lithium niobate thin film by using the overlay mark prepared in step (1) and adopting electron beam photoetching;

[0044] (3) etching the waveguide shape profile photoetched in step (2) by using IBE ion beam etching, and obtaining a waveguide structure with an etching depth of 300nm;

[0045] (4) removing the photoresist by using NMP, and removing etching residual substances by using RCA solution (ammonia water: hydrogen peroxide: water volume ratio 1:2:7), and only leaving the waveguide structure;

[0046] (5) evaporating a 200nm silicon dioxide layer on the surface of the sample by using electron beam evaporation;

[0047] (6) preparing a comb-shaped electrode shape profile on the surface of the x-cut lithium niobate thin film by using photoetching according to the overlay mark, wherein the comb fingers are perpendicular to the z-axis of the lithium niobate, the comb-shaped electrode is 50um long, the period length is 30um, the electrode width is 15um, and the positive and negative electrode spacing is 16um;

[0048] (7) evaporating a 100nm metal electrode nickel layer on the photoetched electrode structure by using electron beam evaporation;

[0049] (8) removing the photoresist by using NMP solution, and leaving the electrode partial structure;

[0050] (9) connecting the electrode positive electrode to the upper electrode, connecting the lower electrode to the ground, and then applying an electric field of about 1200V, so that the domains between the electrodes are completely flipped and connected, and the polarization of the whole waveguide region is completed;

[0051] (10) removing the electrode by using dilute hydrochloric acid, and obtaining a mode conversion frequency doubling waveguide structure;

[0052] (11) polishing the waveguide end face, and obtaining a lithium niobate thin film superlattice waveguide.

[0053] Figure 2For the waveguide structure cross section of the embodiment, a thin film waveguide surface is covered with a layer of silicon dioxide, and electrodes are added on both sides of the waveguide.

[0054] Embodiment 2

[0055] The preparation method of the high-quality ferroelectric domain structure in the ferroelectric thin film ridge waveguide of the application comprises the following steps:

[0056] (1) Preparation of a set of set marks

[0057] a. A set of set marks photoresist patterns are prepared on the sample using ultraviolet lithography;

[0058] b. A 30nm / 70nm thick chromium / gold layer is continuously plated on the lithographed structure, which is not easy to remove;

[0059] c. The photoresist is removed using acetone, leaving a metal mark for set marking, and a set of set marks is obtained;

[0060] (2) Using the set of set marks prepared in step (1), the waveguide shape profile is prepared on the surface of the x-cut lithium niobate thin film using electron beam lithography;

[0061] (3) IBE ion beam etching is used to etch the waveguide shape profile lithographed in step (2), with an etching depth of 300nm, to obtain a waveguide structure;

[0062] (4) RCA solution (ammonia: hydrogen peroxide: water volume ratio 1:2:7) is used to remove the electron beam glue, leaving only the waveguide structure;

[0063] (5) A 200nm thick silicon dioxide layer is deposited on the surface of the sample using electron beam evaporation;

[0064] (6) According to the set of set marks, a comb-shaped electrode shape profile is prepared on the surface of the x-cut lithium niobate thin film using lithography, wherein the comb fingers are perpendicular to the z-axis of the lithium niobate, the comb-shaped electrode is 50um long, the period length is 30um, the electrode width is 15um, and the positive and negative electrode spacing is 16um;

[0065] (7) A 100nm thick metal electrode nickel is deposited on the lithographed electrode structure by electron beam evaporation;

[0066] (8) NMP solution is used to remove the photoresist, leaving the electrode structure;

[0067] (9) The positive electrode is connected to the upper electrode, and the lower electrode is connected to the ground, and then an electric field of about 1060V is applied, so that the domains between the electrodes are completely reversed and connected, and the polarization of the entire waveguide region is completed;

[0068] (10) The electrodes are removed using dilute hydrochloric acid to obtain a mode conversion frequency doubling waveguide structure;

[0069] (11) Polish the waveguide end face to obtain a lithium niobate thin film superlattice waveguide.

[0070] Comparative Example 1

[0071] Based on Example 1, step (5) is omitted, no silica covering layer is applied, and all other conditions remain unchanged.

[0072] Comparative Example 2

[0073] Performance testing

[0074] Figure 3 The confocal characterization of the lithium niobate superlattices prepared in Examples 1 and 2 is shown in Figure (3a). As can be seen from Figure (3b), in Example 1, the color of the polarized region is the same as that of the unpolarized region, indicating that the intensity is comparable and the domain inversion in the polarized region is complete. As can be seen from Figure (3b), in Example 2, the color of the periodically polarized region is the same as that of the unpolarized region, indicating that the intensity is comparable, the domain walls are thinner, and the domain inversion in the periodically polarized region is complete.

[0075] Figure 4 The PFM diagram of the lithium niobate superlattice prepared in Comparative Example 1 is shown below. Figure 4 a and 4b) and electron micrographs ( Figure 4 (c and 4d). From Figures (4a, 4b), it can be seen that there are no domain signals in the waveguide surface region; from Figures (4c, 4d), due to selective etching, a significant difference in positive and negative z can be seen in the waveguide cross-section, but not in the upper part of the waveguide, indicating that the domains do not extend to the upper part. Because Comparative Example 1 does not have a silicon dioxide capping layer, the flipping is incomplete.

Claims

1. A method for fabricating high quality ferroelectric domain structures in ferroelectric thin film ridge waveguides, comprising: The method comprises the following steps: ​ (1) preparing an overlay mark for overlaying, (2) preparing a waveguide shape profile on the surface of the x-cut lithium niobate / tantalum film according to the overlay mark by using electron beam lithography, (3) etching the waveguide structure on the surface of the x-cut lithium niobate / tantalum film by using IBE ion beam etching, (4) removing the electron beam glue to obtain the lithium niobate / tantalum film waveguide, (5) adding a cover layer on the surface of the lithium niobate / tantalum film waveguide, and the cover layer is silicon dioxide, (6) preparing a comb-shaped electrode shape profile on the surface of the x-cut lithium niobate / tantalum film according to the overlay mark by using photolithography, wherein the comb fingers are perpendicular to the z-axis of the lithium niobate, (7) plating a layer of metal electrode on the photolithographic electrode shape profile, (8) removing the photoresist to obtain the polarized electrode structure, (9) connecting the positive electrode on one side of the electrode and grounding the other side of the electrode, and then applying an electric field to make the domains between the electrodes flip, (10) removing the electrode by using dilute hydrochloric acid to obtain the waveguide domain flip structure, (11) polishing the waveguide end face to obtain the lithium niobate / tantalum film superlattice waveguide.

2. The method of claim 1, wherein the ferroelectric thin film ridge waveguide high quality ferroelectric domain structure is prepared by the steps of: depositing a ferroelectric thin film on a substrate; forming a first electrode on the ferroelectric thin film; forming a second electrode on the ferroelectric thin film; and forming a ridge on the ferroelectric thin film. In step (5), the thickness of the cover layer is the thickness of the etched waveguide ± 100 nm.

3. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, The preparation method of the overlay mark comprises the following steps: (1) preparing an overlay mark photoresist pattern on a sample by using ultraviolet photolithography, (2) plating a layer of 30-300 nm thick metal which is not easy to remove on the photolithographic structure, (3) removing the photoresist to leave the metal mark for overlaying to obtain the overlay mark.

4. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, In step (4), the method for removing the electron beam glue is to remove it by using RCA solution corrosion.

5. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, In step (5), the method for adding a cover layer on the surface of the sample is plasma enhanced chemical vapor deposition or electron beam evaporation.

6. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, In step (6), the width of the comb-shaped electrode is 1 / 2 to 1 / 4 of the period length.

7. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 6, characterized in that, The comb-shaped electrode is 20-100 um long, the period length is 5-50 um, the electrode width is 1 / 4 to 1 / 2 of the period length, and the positive and negative electrode spacing is 10-20 um.

8. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, In step (7), the metal electrode is aluminum, nickel, chromium or titanium.

9. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1 or 3, characterized in that, The method for removing the photoresist is to remove it by using acetone or N-methyl pyrrolidone solution corrosion.

10. The method for fabricating a high-quality ferroelectric domain structure in a ferroelectric thin-film ridge waveguide according to claim 1, characterized in that, In step (9), the applied electric field is 100 V-1400 V.

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