Lithium niobate crystal preparation method and lithium niobate device

Abstract

The invention provides a lithium niobate crystal preparation method and a lithium niobate device, and relates to the technical field of semiconductors, the method comprises the following steps: mixing and calcining Nb2O5 and Li2CO3 to obtain a lithium niobate polycrystal material; putting the lithium niobate polycrystal material into a crucible in a multi-layer temperature field heating furnace, and heating the lithium niobate polycrystal material to a molten state to obtain a crystal material melt; immersing the lithium niobate seed crystal into the crystal material melt through a seed crystal rod, and pulling the crystal through the seed crystal rod; in the initial lifting stage, controlling the seed rod and / or the crucible to rotate according to a first rotating speed; in the middle stage of pulling, the seed rod and / or the crucible are / is controlled to periodically rotate reversely according to a second rotating speed until the lithium niobate crystal is obtained, the direction of each reverse rotation is opposite to the direction of the last rotation, and the second rotating speed is smaller than the first rotating speed. And through dynamic rotation control, the convection uniformity of the crystal melt is improved, so that the defect density of the lithium niobate crystal is reduced, and the optical uniformity is improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and more particularly, to a method for preparing lithium niobate crystals and a lithium niobate device. Background Art

[0002] Lithium niobate (LiNbO3) is a key material in the fields of optical communication, electro-optic modulators, surface acoustic wave devices, etc., and is widely used due to its excellent piezoelectric, electro-optic, and acousto-optic properties. At present, the traditional Czochralski method is used to prepare lithium niobate crystals, but the lithium niobate crystals prepared by this method are prone to problems such as high dislocation density, poor optical uniformity, and low crystal processing yield.

[0003] Therefore, how to reduce the defect density of lithium niobate crystals and improve the optical uniformity is a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing lithium niobate crystals and a lithium niobate device to reduce the defect density of lithium niobate crystals and improve the optical uniformity.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] On the one hand, the present invention provides a method for preparing lithium niobate crystals, the method comprising:

[0007] Mixing Nb2O5 and Li2CO3 and calcining to obtain a lithium niobate polycrystalline material;

[0008] Placing the lithium niobate polycrystalline material in a crucible located in a multi-layer temperature field heating furnace, and heating the lithium niobate polycrystalline material to a molten state to obtain a crystal material melt;

[0009] After immersing a lithium niobate seed crystal into the crystal material melt through a seed crystal rod, the seed crystal rod pulls the crystal at a preset pulling rate;

[0010] In the initial stage of pulling, controlling the seed crystal rod and / or the crucible to rotate at a first rotation speed;

[0011] In the middle stage of pulling, controlling the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained; wherein, the direction of each reverse rotation is opposite to the direction of the previous rotation, and the second rotation speed is less than the first rotation speed.

[0012] Further, the step of mixing Nb2O5 and Li2CO3 and calcining to obtain a lithium niobate polycrystalline material includes:

[0013] After mixing the Nb2O5 and the Li2CO3 according to a preset molar ratio, doping 0.01 - 1.5 mol% MgO and 0.01 - 0.3 mol% ZnO, and uniformly mixing them, a calcination treatment is carried out to obtain the lithium niobate polycrystalline material.

[0014] Further, the molar ratio of the Nb2O5 to the Li2CO3 is 0.946:1, the molar percentage of the MgO is 1.2 mol%, and the molar percentage of the ZnO is 0.2 mol%.

[0015] Further, the method further includes:

[0016] In the initial stage of the pulling and the middle stage of the pulling, the axial temperature gradient at the crystal growth interface is controlled to be 10 - 30 °C / cm, and the radial temperature gradient is controlled to be 0 - 5 °C / cm by the multi-layer temperature field heating furnace.

[0017] Further, in the middle stage of the pulling, the step of controlling the seed rod and / or the crucible to perform periodic inversion at a second rotation speed until the lithium niobate crystal is obtained includes:

[0018] In the middle stage of the pulling, the seed rod and / or the crucible are controlled to be inverted once every 5 minutes at the second rotation speed until the lithium niobate crystal is obtained.

[0019] Further, after the step of controlling the seed rod and / or the crucible to perform periodic inversion at a second rotation speed until the lithium niobate crystal is obtained in the middle stage of the pulling, the method further includes:

[0020] Cutting the lithium niobate crystal with a ultraviolet picosecond laser;

[0021] Chemically mechanically polishing the lithium niobate crystal with a polishing liquid mixed with SiO2 nanoparticles and oxalic acid;

[0022] Performing staged annealing on the lithium niobate crystal in an oxygen atmosphere.

[0023] Further, the step of performing staged annealing on the lithium niobate crystal in an oxygen atmosphere includes:

[0024] The temperature of the first annealing stage is 600 °C, and the holding time is 2 h;

[0025] The temperature of the second annealing stage is 950 °C, and the holding time is 5 h.

[0026] Further, the multi-layer temperature field heating furnace includes: a heat preservation cover, a heat preservation furnace, a heating element, a rotating rod, and a crucible; the crucible is disposed inside the heat preservation furnace, the heat preservation cover and the heat preservation furnace are detachably connected, a first through hole is formed at the top of the heat preservation cover, a second through hole is formed at the bottom of the heat preservation furnace, the rotating rod passes through the second through hole and is connected to the bottom of the crucible, and the heating element is disposed on the outer wall of the heat preservation furnace.

[0027] Further, the heat preservation furnace sequentially includes, from inside to outside: corundum, mullite cotton, and mullite; both the corundum and the mullite cotton are in a cylindrical bottomless structure, the mullite is in a cylindrical bottomed structure, the mullite cotton is disposed between the corundum and the mullite, and the heating element is disposed on the outer wall of the mullite.

[0028] On the other hand, the present invention also provides a lithium niobate device, which sequentially includes, from bottom to top: a lithium niobate crystal, an optical waveguide, an organic polymer layer, and an electrode, and the lithium niobate crystal is prepared by using the lithium niobate crystal preparation method according to any one of the foregoing embodiments.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention provides a method for preparing a lithium niobate crystal and a lithium niobate device. The method includes: mixing Nb2O5 and Li2CO3 and calcining to obtain a lithium niobate polycrystalline material; putting the lithium niobate polycrystalline material into a crucible located in a multi-layer temperature field heating furnace, and heating the lithium niobate polycrystalline material to a molten state to obtain a crystal material melt; immersing a lithium niobate seed crystal into the crystal material melt through a seed crystal rod, and then the seed crystal rod pulling the crystal at a preset pulling rate; in the initial stage of pulling, controlling the seed crystal rod and / or the crucible to rotate at a first rotation speed; in the middle stage of pulling, controlling the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained; wherein, the direction of each reverse rotation is opposite to the direction of the previous rotation, and the second rotation speed is less than the first rotation speed. By performing dynamic rotation control during the pulling process, the present invention improves the convection uniformity of the crystal material melt, thereby greatly reducing the defect density of the lithium niobate crystal and improving the optical uniformity.

[0031] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following specific preferred embodiments are given below in conjunction with the accompanying drawings for detailed description. Description of the Drawings

[0032] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and illustrated herein generally can be arranged and designed in a variety of different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0033] Figure 1 One of the flow diagrams of a method for preparing lithium niobate crystals provided by the present invention;

[0034] Figure 2 The structural diagram of a multi-layer temperature field heating furnace provided by the present invention;

[0035] Figure 3 The physical diagram of the lithium niobate crystal corresponding to Comparative Example 1;

[0036] Figure 4 The XRD diagram of the lithium niobate crystal corresponding to Comparative Example 1;

[0037] Figure 5 The physical diagram of the lithium niobate crystal corresponding to Comparative Example 2;

[0038] Figure 6 The XRD diagram of the lithium niobate crystal corresponding to Comparative Example 2;

[0039] Figure 7 The physical diagram of the lithium niobate crystal corresponding to the embodiment of the present invention;

[0040] Figure 8 The XRD diagram of the lithium niobate crystal corresponding to the embodiment of the present invention;

[0041] Figure 9 The comparative broken line diagram of the Li / Nb component deviation of Comparative Example 1, Comparative Example 2 and the embodiment of the present invention;

[0042] Figure 10 Another flow diagram of a method for preparing lithium niobate crystals provided by the present invention;

[0043] Figure 11 The side sectional view of a lithium niobate device provided by the present invention;

[0044] Figure 12 The top view of a lithium niobate device provided by the present invention.

[0045] Icons: 10 - seed crystal rod; 20 - heat insulation cover; 30 - heat insulation furnace; 31 - corundum; 32 - mullite cotton; 33 - mullite; 40 - heating element; 50 - rotating rod; 60 - crucible; 70 - first through - hole; 80 - second through - hole; 100 - lithium niobate device; 110 - lithium niobate crystal; 120 - optical waveguide; 130 - organic polymer layer; 140 - electrode. Specific embodiments

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and illustrated in the accompanying drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0047] In the description of the present invention, it should be noted that relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

[0048] The following will describe in detail some embodiments of the present invention with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0049] As described in the background art, currently, the preparation method of lithium niobate crystals uses the traditional Czochralski method. However, the lithium niobate crystals prepared by this method are prone to problems such as high dislocation density, poor optical uniformity, and low crystal processing yield. Therefore, how to reduce the defect density of lithium niobate crystals and improve the optical uniformity is a technical problem that those skilled in the art urgently need to solve.

[0050] To solve the above - mentioned technical problems, please refer to Figure 1 , the embodiments of the present invention provide a method for preparing lithium niobate crystals, and the method includes the following steps:

[0051] Step S100: After mixing and calcining Nb2O5 and Li2CO3, a lithium niobate polycrystalline material is obtained.

[0052] Step S200: Put the lithium niobate polycrystalline material into a crucible located in a multi - layer temperature - field heating furnace, and heat the lithium niobate polycrystalline material to a molten state to obtain a crystal material melt.

[0053] Specifically, since the melting point of lithium niobate is generally 1250 °C, the lithium niobate polycrystalline material can be placed in a crucible and heated to 1300 °C for melting, and then held at this temperature for 6 hours to homogenize the melt, so as to obtain a crystal material melt.

[0054] Step S300: After immersing the lithium niobate seed crystal into the crystal material melt through a seed crystal rod, the seed crystal rod pulls up the crystal at a preset pulling rate.

[0055] In the embodiment of the present invention, after the seed crystal rod fixes the lithium niobate seed crystal and immerses it into the crystal material melt, it pulls up upward at a preset pulling rate (for example, 5 mm / h), so that the atoms or molecules in the melt gradually grow into a single crystal according to the crystal structure of the seed crystal.

[0056] Step S400: In the initial stage of pulling, control the seed crystal rod and / or the crucible to rotate at a first rotation speed.

[0057] Step S500: In the middle stage of pulling, control the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained.

[0058] Wherein, the direction of each reverse rotation is opposite to the direction of the previous rotation, and the second rotation speed is less than the first rotation speed.

[0059] Further, step S500 includes: in the middle stage of pulling, control the seed crystal rod and / or the crucible to reverse once every 5 minutes at the second rotation speed until a lithium niobate crystal is obtained.

[0060] It should be noted that the reverse rotation in the present invention can be forward rotation or reverse rotation. The specific rotation direction is determined by the previous rotation direction, as long as the direction of each reverse rotation is opposite to the direction of the previous rotation. Moreover, the present invention does not limit the specific values of the first rotation speed and the second rotation speed, as long as it is ensured that the first rotation speed is greater than the second rotation speed.

[0061] Optionally, the first rotation speed is 20 - 30 rpm, and the second rotation speed is 5 - 10 rpm. For example, in the embodiment of the present invention, the first rotation speed can be 25 rpm, and the second rotation speed can be 8 rpm.

[0062] Exemplarily, assume that in the initial stage of pulling (i.e., the seeding stage), the seed rod and / or the crucible rotate forward at a speed of 25 rpm. Then, in the middle stage of pulling (i.e., the shoulder release stage and the isodiameter stage), the rotation speed of the seed rod and / or the crucible is reduced to 8 rpm and rotates backward. After 5 minutes, the seed rod and / or the crucible maintain a rotation speed of 8 rpm and rotate forward. After another 5 minutes, the seed rod and / or the crucible maintain a rotation speed of 8 rpm and rotate backward again. The rotation of the seed rod and / or the crucible is controlled in the above-mentioned periodic reverse manner until the growth of the lithium niobate crystal is completed, and the total growth time is about 120 hours.

[0063] Similarly, assume that in the initial stage of pulling, the seed rod and / or the crucible rotate backward at a speed of 25 rpm. Then, in the middle stage of pulling, the rotation speed of the seed rod and / or the crucible is reduced to 8 rpm and rotates forward. The subsequent reverse direction is opposite to the previous rotation direction, which will not be elaborated here.

[0064] Based on the above design, compared with the current constant rotation method, the present invention controls the dynamic rotation during the pulling process, improves the convective uniformity of the crystal material melt, thereby greatly reducing the defect density of the lithium niobate crystal and improving the optical uniformity.

[0065] In order to further improve the optical uniformity of the lithium niobate crystal, as an optional implementation manner, a method for preparing a lithium niobate crystal provided by the present invention further includes:

[0066] In the initial stage of pulling and the middle stage of pulling, the axial temperature gradient at the crystal growth interface is controlled to be 10 - 30 °C / cm, and the radial temperature gradient is 0 - 5 °C / cm by a multi-layer temperature field heating furnace.

[0067] It should be noted that the axial temperature gradient refers to the temperature change along the crystal growth direction (from the melt to the crystal). The radial temperature gradient refers to the temperature change perpendicular to the crystal growth direction (i.e., at different positions on the same crystal growth interface).

[0068] Optionally, in the embodiment of the present invention, the axial temperature gradient at the crystal growth interface is 25 °C / cm, and the radial gradient is 3 °C / cm.

[0069] By reducing the axial temperature gradient and the radial gradient at the crystal growth interface, the Li / Nb component supercooling is inhibited, the internal stress concentration of the crystal is avoided, thereby reducing the dislocation density of the lithium niobate crystal and improving the optical uniformity.

[0070] In the embodiment of the present invention, the structure of the multi-layer temperature field heating furnace is as Figure 2 shown. The multi-layer temperature field heating furnace includes: a heat preservation cover 20, a heat preservation furnace 30, a heating element 40, a rotating rod 50, and a crucible 60.

[0071] Among them, the crucible 60 is arranged inside the heat preservation furnace 30, and the heat preservation cover 20 and the heat preservation furnace 30 are detachably connected. A first through hole 70 is opened at the top of the heat preservation cover 20, and a second through hole 80 is opened at the bottom of the heat preservation furnace 30. The rotating rod 50 passes through the second through hole 80 and is connected to the bottom of the crucible 60, and the heating element 40 is arranged on the outer wall of the heat preservation furnace 30.

[0072] The seed crystal rod 10 passes through the first through hole 70 to immerse the lithium niobate seed crystal into the crystal material melt in the crucible 60. During the pulling process, the pulling up rate, rotation speed and rotation direction of the lithium niobate seed crystal are controlled through the seed crystal rod 10; the crucible 60 is driven to rotate through the rotating rod 50, so as to control the rotation speed and rotation direction of the crystal material melt; the heat preservation furnace 30 is heated through the heating element 40 to accurately control the axial temperature gradient and radial temperature gradient at the crystal growth interface, inhibit the Li / Nb component supercooling, and avoid the internal stress concentration of the crystal. By adopting the cooperative control method of dynamic rotation and temperature field optimization, the dislocation density of the lithium niobate crystal is greatly reduced, and the optical uniformity is improved.

[0073] Furthermore, the heat preservation furnace 30 sequentially includes from inside to outside: corundum 31, mullite cotton 32 and mullite 33. Among them, both the corundum 31 and the mullite cotton 32 are cylindrical structures without bottoms, and the mullite 33 is a cylindrical structure with a bottom. The mullite cotton 32 is arranged between the corundum 31 and the mullite 33, and the heating element 40 is arranged on the outer wall of the mullite 33.

[0074] It should be noted that the specific type of the heating element 40 is not limited in the present invention. For example, the heating element 40 can be a resistor or an induction coil.

[0075] As an optional implementation manner, the dimensions of each component in the multi-layer temperature field heating furnace are as follows:

[0076] The diameter of the crucible 60 is 150 - 300 mm, the height is 50 - 100 mm, and the thickness is 2 - 5 mm. The inner diameter of the corundum 31 is 160 - 320 mm, the height is 50 - 200 mm, and the thickness is 1 - 10 mm. The height of the mullite cotton 32 is 50 - 200 mm, the thickness is 5 - 30 mm, and the mullite cotton 32 closely adheres to the outer wall of the corundum 31. The inner diameter of the mullite 33 is 180 - 360 mm, the height is 100 - 500 mm, and the wall thickness is 10 - 100 mm. The heat preservation cover 20 is a zircon brick with an inner diameter of 10 - 80 mm, an outer diameter of 190 - 360 mm, and a thickness of 30 mm. The heating element 40 completely wraps the outer wall of the mullite 33.

[0077] In addition, in order to improve the optical damage threshold of the lithium niobate crystal, in an optional implementation manner, step S100 includes:

[0078] After mixing high-purity Nb2O5 and Li2CO3 according to a preset molar ratio, doping 0.01-1.5 mol% MgO and 0.01-0.3 mol% ZnO, and mixing evenly, calcination treatment is carried out to obtain lithium niobate polycrystalline material.

[0079] Optionally, the molar ratio of Nb2O5 to Li2CO3 is 0.946:1. The molar percentage of MgO is 1.2 mol%, and the molar percentage of ZnO is 0.2 mol%.

[0080] The anti-photorefractive performance of the crystal is enhanced by double-element doping (MgO+ZnO), the anti-light damage threshold of the crystal is improved, and the requirements of high-precision optical devices are met.

[0081] To better reflect the advantages of the lithium niobate crystal preparation method provided by the present invention, please refer to Figures 3 to 9 and the following two comparative examples will be compared and analyzed with the examples of the present invention.

[0082] The steps for preparing lithium niobate crystals in Comparative Example 1 (traditional Czochralski method) are as follows:

[0083] (1) Mix high-purity Nb2O5 and Li2CO3 in a molar ratio of 0.946:1 without doping;

[0084] (2) Use a single-layer temperature field heating furnace with an axial temperature gradient of 80 °C / cm and a radial temperature gradient of 15 °C / cm;

[0085] (3) The seed crystal rotation rate is constant at 20 rpm.

[0086] Result: As Figure 3 shown, the diameter of the lithium niobate crystal prepared in Comparative Example 1 is 80 mm, obvious growth stripes appear in the isodiametric section, and the anti-light damage threshold < 100 MW / cm 2 . The XRD pattern of Comparative Example 1 is as Figure 4 shown, and XRD detection shows that the Li / Nb component deviation (i.e., the difference between the edge and the center) reaches 2.1%, and the dislocation density is 1×10 4 cm -2 .

[0087] Defect analysis: In Comparative Example 1, the lack of doping with other elements results in a low anti-light damage threshold of the crystal, serious shoulder broadening, many crystal defects, and serious cracking phenomena. In addition, the constant rotation leads to unstable melt convection, and the large temperature field gradient causes constitutional supercooling, resulting in stress concentration inside the crystal and a high dislocation density.

[0088] The steps for preparing lithium niobate crystals in Comparative Example 2 (existing magnesium-doped crystal process) are as follows:

[0089] (1) Mix high-purity Nb2O5 and Li2CO3 at a molar ratio of 0.946:1, and incorporate 1.0 mol% MgO;

[0090] (2) Use a single-layer temperature field heating furnace with an axial temperature gradient of 80 °C / cm and a radial temperature gradient of 15 °C / cm;

[0091] (3) The seed crystal rotation rate is kept constant at 15 rpm.

[0092] Results: As Figure 5 and Figure 6 shown, the diameter of the lithium niobate crystal prepared in Comparative Example 2 is 80 mm. The growth striations are reduced, but there are still local compositional fluctuations (Li / Nb compositional deviation is 1.2%), the dislocation density is 5×10 3 cm -2 , the optical damage resistance threshold is 300 MW / cm 2 , which does not meet the requirements of high-power devices, and the crystal shows slight cracking after annealing, indicating insufficient stress release.

[0093] Defect analysis: Although doping only MgO in Comparative Example 2 can improve the photorefractive property of the crystal and the shoulder broadening problem of the crystal can be improved, the shoulder release stage is relatively long, resulting in a large waste in the later processing and application. In addition, Comparative Example 2 also uses the method of constant rotation and a large temperature field gradient, resulting in unstable melt convection, Li / Nb compositional supercooling, stress residue, and low crystal processing yield.

[0094] The steps for preparing a lithium niobate crystal in the embodiment of the present invention (using dynamic rotation control + optimized design of temperature field gradient + dual-element doping) are as follows:

[0095] (1) Mix high-purity Nb2O5 and Li2CO3 at a molar ratio of 0.946:1, and dope 1.2 mol% MgO and 0.2 mol% ZnO;

[0096] (2) A multi-layer temperature field heating furnace with an axial temperature gradient of 25 °C / cm and a radial temperature gradient of 3 °C / cm;

[0097] (3) Dynamically adjust the rotation rate and direction: At the initial stage of pulling, control the rotation speed of the seed crystal rod and / or the crucible to be 25 rpm. At the middle stage of pulling, control the rotation speed of the seed crystal rod and / or the crucible to drop to 8 rpm, and reverse it once every 5 minutes until a lithium niobate crystal is obtained, and the total growth time is 120 hours.

[0098] Results: As Figure 7 and Figure 8As shown, the lithium niobate crystal prepared in the embodiment of the present invention has a diameter of 80 mm, and there are no visible growth stripes in the isodiametric section. The Li / Nb component deviation is 0.3% (XRD consistency between the edge and the center), and the dislocation density <10 2 cm -2 (measured by the etch pit method), and the optical damage resistance threshold > 500 MW / cm 2 (tested with a 532 nm laser). There is no cracking phenomenon after annealing, indicating that the crystal stress distribution is uniform.

[0099] The lithium niobate crystal prepared in the embodiment of the present invention not only excellently suppresses the shoulder problem, but also has a low crystal defect density and is not easy to crack. Moreover, the growth length of the crystal is increased, greatly improving the utilization rate of the crystal.

[0100] Please refer to the following Figure 9 , Figure 9 which is a comparative line graph of the Li / Nb component deviation of Comparative Example 1, Comparative Example 2 and the embodiment of the present invention. The smaller the slope of the broken line, the better the component uniformity of the prepared lithium niobate crystal. It can be seen that the Li / Nb component deviation corresponding to Comparative Example 1 is +2.1%, the Li / Nb component deviation corresponding to Comparative Example 2 is +1.2%, and the Li / Nb component deviation corresponding to the embodiment of the present invention is +0.3%.

[0101] In summary, the respective data corresponding to Comparative Example 1, Comparative Example 2 and the embodiment of the present invention are shown in Table 1.

[0102] Table 1 Comparative data table

[0103]

[0104] It can be seen that compared with Comparative Example 1 and Comparative Example 2, the embodiment of the present invention significantly reduces the crystal dislocation density and Li / Nb component deviation by adopting the collaborative control method of dynamic rotation and temperature field optimization, and the component uniformity is improved. The optical damage resistance threshold of the crystal is improved through the synergistic effect of dual-element doping, meeting the requirements of high-precision optical devices.

[0105] As an optional implementation manner, after obtaining the lithium niobate crystal through the above steps S100 to S500, it is also necessary to perform processing on it to prepare for the subsequent preparation of lithium niobate devices.

[0106] Specifically, please refer to Figure 10 , after step S500 of controlling the seed rod and / or crucible to perform periodic reversal at the second rotation speed until the lithium niobate crystal is obtained, the method for preparing a lithium niobate crystal provided by the present invention further includes the following steps:

[0107] Step S600: Cut the lithium niobate crystal with an ultraviolet picosecond laser.

[0108] In an embodiment of the present invention, a lithium niobate crystal is cut along the Z-axis into wafers with a thickness of 100 μm, and an ultraviolet picosecond laser (wavelength 355 nm, pulse width < 10 ps, energy density 2 J / cm 2 , scanning speed 200 mm / s) is used for stealth cutting to form a modified layer inside the wafer, and the cutting depth error is < ±5 μm.

[0109] Step S700: Chemically mechanically polish the lithium niobate crystal with a mixed polishing solution of SiO2 nanoparticles and oxalic acid.

[0110] In an embodiment of the present invention, after chemically mechanically polishing (Chemical Mechanical Polishing, CMP) the lithium niobate crystal with a mixed polishing solution of SiO2 nanoparticles and oxalic acid (pH = 4 - 5), the surface roughness Ra = 0.38 nm.

[0111] Step S800: Anneal the lithium niobate crystal in stages in an oxygen atmosphere.

[0112] Among them, the temperature in the first annealing stage is 600 °C, and the holding time is 2 h. The temperature in the second annealing stage is 950 °C, and the holding time is 5 h, thereby effectively eliminating residual stress and lattice distortion.

[0113] The present invention combines laser stealth cutting, chemical mechanical polishing, and staged annealing to achieve an ultra-thin wafer (thickness of 100 μm), and reduces the cutting loss and surface roughness (cutting loss < 5%, surface roughness is 0.38 nm), improving the quality.

[0114] Furthermore, please refer to Figure 11 and Figure 12 , an embodiment of the present invention further provides a lithium niobate device 100, which sequentially includes from bottom to top: a lithium niobate crystal 110, an optical waveguide 120, an organic polymer layer 130, and an electrode 140. Among them, the lithium niobate crystal 110 is prepared by the lithium niobate crystal preparation method described in any one of the foregoing embodiments.

[0115] In addition, the optical waveguide 120 is prepared by ultraviolet lithography technology, and the organic polymer layer 130 (for example, photosensitive material) and the electrode 140 are prepared by one or more of spin coating, spraying, evaporation coating, and pasting.

[0116] In summary, the embodiments of the present invention provide a method for preparing lithium niobate crystals and lithium niobate devices. The method includes: mixing Nb2O5 and Li2CO3 and calcining them to obtain lithium niobate polycrystalline material; placing the lithium niobate polycrystalline material in a crucible located in a multi-layer temperature field heating furnace, and heating the lithium niobate polycrystalline material to a molten state to obtain a crystal material melt; immersing a lithium niobate seed crystal into the crystal material melt through a seed crystal rod, and then pulling the crystal at a preset pulling rate by the seed crystal rod; in the initial stage of pulling, controlling the seed crystal rod and / or the crucible to rotate at a first rotation speed; in the middle stage of pulling, controlling the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained; where the direction of each reverse rotation is opposite to the direction of the previous rotation, and the second rotation speed is less than the first rotation speed. Compared with the current constant rotation method, the present invention improves the convection uniformity of the crystal material melt by performing dynamic rotation control during the pulling process, thereby greatly reducing the defect density of the lithium niobate crystal and improving the optical uniformity.

[0117] In addition, compared with the current high-temperature field gradient design (axial temperature gradient is 80 °C / cm, radial temperature gradient is 15 °C / cm), the present invention adopts a low-temperature field gradient design (axial temperature gradient is 10 - 30 °C / cm, radial temperature gradient is 0 - 5 °C / cm), which can suppress the supercooling of Li / Nb components, avoid stress concentration inside the crystal, further reduce the defect density of the lithium niobate crystal, and improve the crystal quality. By doping with two elements (MgO + ZnO), the photorefractive performance of the crystal is enhanced, and the optical damage threshold of the crystal is improved (test value > 500 MW / cm 2 , which is better than 300 MW / cm of the traditional magnesium-doped crystal 2 ). By adopting methods such as laser stealth cutting, chemical mechanical polishing, and staged annealing, an ultra-thin wafer with low loss and high surface quality (thickness < 500 μm) can be prepared.

[0118] The lithium niobate devices (such as surface acoustic wave devices, electro-optic modulators) prepared by using the method for preparing lithium niobate crystals provided by the present invention have low waveguide transmission loss and high electro-optic modulation efficiency.

[0119] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0120] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any aspect, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A method for preparing lithium niobate crystal, characterized in that The method includes: After mixing and calcining Nb2O5 and Li2CO3, a lithium niobate polycrystalline material is obtained; Put the lithium niobate polycrystalline material into a crucible located in a multi-layer temperature field heating furnace, and heat the lithium niobate polycrystalline material to a molten state to obtain a crystal material melt; After immersing a lithium niobate seed crystal into the crystal material melt through a seed crystal rod, the seed crystal rod pulls the crystal at a preset pulling rate; In the initial stage of pulling, control the seed crystal rod and / or the crucible to rotate at a first rotation speed; In the middle stage of pulling, control the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained; wherein, the direction of each reverse rotation is opposite to the direction of the previous rotation, and the second rotation speed is less than the first rotation speed; 2. The method for preparing a lithium niobate crystal according to claim 1, wherein The step of obtaining a lithium niobate polycrystalline material after mixing and calcining Nb2O5 and Li2CO3 includes: Mix the Nb2O5 and the Li2CO3 according to a preset molar ratio, dope 0.01 - 1.5 mol% MgO and 0.01 - 0.3 mol% ZnO, mix evenly and then perform calcination treatment to obtain the lithium niobate polycrystalline material.

3. The method for preparing lithium niobate crystal according to claim 2, wherein The molar ratio of the Nb2O5 to the Li2CO3 is 0.946:1, the molar percentage of the MgO is 1.2 mol%, and the molar percentage of the ZnO is 0.2 mol%.

4. The method for preparing a lithium niobate crystal according to claim 1, wherein, The method further includes: In the initial stage of pulling and the middle stage of pulling, control the axial temperature gradient at the crystal growth interface to be 10 - 30 °C / cm and the radial temperature gradient to be 0 - 5 °C / cm through the multi-layer temperature field heating furnace.

5. The method for preparing lithium niobate crystal according to claim 1, wherein, The step of controlling the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained in the middle stage of pulling includes: In the middle stage of pulling, control the seed crystal rod and / or the crucible to reverse once every 5 minutes at the second rotation speed until the lithium niobate crystal is obtained.

6. The method for preparing lithium niobate crystal according to claim 1, wherein After the step of controlling the seed crystal rod and / or the crucible to perform periodic reverse rotation at a second rotation speed until a lithium niobate crystal is obtained in the middle stage of pulling, the method further includes: Cut the lithium niobate crystal with an ultraviolet picosecond laser; Chemically mechanically polish the lithium niobate crystal with a polishing liquid mixed with SiO2 nanoparticles and oxalic acid; Perform staged annealing on the lithium niobate crystal in an oxygen atmosphere.

7. The method for preparing lithium niobate crystal according to claim 6, wherein The step of performing staged annealing on the lithium niobate crystal in an oxygen atmosphere includes: The temperature in the first annealing stage is 600 °C, and the holding time is 2 h; The temperature in the second annealing stage is 950 °C, and the holding time is 5 h.

8. The method for preparing a lithium niobate crystal according to claim 1, characterized in that, The multi-layer temperature field heating furnace includes: a heat preservation cover, a heat preservation furnace, a heating element, a rotating rod, and a crucible; the crucible is arranged in the heat preservation furnace, the heat preservation cover and the heat preservation furnace are detachably connected, a first through hole is opened at the top of the heat preservation cover, a second through hole is opened at the bottom of the heat preservation furnace, the rotating rod passes through the second through hole and is connected to the bottom of the crucible, and the heating element is arranged on the outer wall of the heat preservation furnace.

9. The method for preparing a lithium niobate crystal according to claim 8, characterized in that, The heat preservation furnace sequentially includes, from the inside to the outside: corundum, mullite cotton, and mullite; both the corundum and the mullite cotton are cylindrical structures without a bottom, the mullite is a cylindrical structure with a bottom, the mullite cotton is disposed between the corundum and the mullite, and the heating element is disposed on the outer wall of the mullite.

10. A lithium niobate device, characterized in that, The lithium niobate device sequentially includes, from the bottom to the top: a lithium niobate crystal, an optical waveguide, an organic polymer layer, and an electrode, and the lithium niobate crystal is prepared by using the lithium niobate crystal preparation method according to any one of claims 1-9.

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