A method for preparing a silicon nitride thin film on a substrate surface

Abstract

The application relates to the technical field of semiconductors, and provides a method for preparing a silicon nitride film on a substrate surface. The method comprises the following steps: (1) in a first cavity with a nitrogen atmosphere, a hot plate with a process temperature is provided, the substrate is preheated to a preheating temperature at a first distance; the material of the substrate comprises lithium tantalate or lithium niobate; (2) the substrate with the preheating temperature is placed on the surface of the hot plate, and the substrate is heated to the process temperature; (3) a silicon nitride film is deposited on the surface of the substrate with the process temperature; (4) the substrate with the deposited silicon nitride film is transferred to a second vacuum cavity for cooling. In the application, the silicon nitride film is directly deposited on the surface of the lithium tantalate or lithium niobate substrate in the complete condition of the lithium tantalate or lithium niobate substrate by combining the preheating and the slow cooling in the mode of heat radiation. The method provided by the application is simple in process, easy to realize, and has the industrial application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor, in particular to the preparation of silicon nitride film, and especially to a method for preparing silicon nitride film on the surface of a substrate. BACKGROUND

[0002] Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) are two important ferroelectric crystal materials, which are widely used in optoelectronics, surface acoustic wave (SAW) devices, integrated optics and nonlinear optics due to their excellent piezoelectric, electro-optic and acoustic properties. As substrate materials, they have a unique position in the semiconductor and optoelectronic industry.

[0003] However, due to the existence of spontaneous polarization domain structure inside lithium niobate and lithium tantalate, the movement of domain wall when heated may cause local stress concentration, and lithium niobate and lithium tantalate belong to the trigonal system, which is easy to cleave along the specific crystal surface, and the mechanical strength anisotropy is significant. In addition, lithium niobate and lithium tantalate have poor thermal conductivity, with LiNbO3 thermal conductivity of only ~5 W / (m·K) and LiTaO3 of only ~3 W / (m·K). Therefore, during the processing of lithium niobate and lithium tantalate substrates, thermal distribution is uneven, thermal gradient stress is generated, and thermal shock cracks are induced, resulting in lithium niobate and lithium tantalate substrate fragments. Therefore, lithium niobate and lithium tantalate substrates are usually bonded with silicon nitride layers.

[0004] CN117059631A discloses a hetero-optoelectronic fusion integrated chip based on wafer-to-wafer bonding, and the integration method includes preparing an erbium-doped lithium niobate wafer by ion doping process; obtaining a silicon nitride film on a silicon wafer by thin film deposition technology; then bonding the silicon nitride film and the erbium-doped lithium niobate wafer by wafer bonding process to prepare a silicon nitride-doped lithium niobate electro-optic modulator.

[0005] CN117572673A discloses a preparation method of an optical phase modulator, which includes bonding a lithium niobate single crystal sheet above a silicon nitride waveguide core through a bonding medium layer, and reducing the insertion loss by setting a slope structure on one side or both sides of the lithium niobate single crystal sheet in the length direction of the silicon nitride waveguide core.

[0006] CN118039727A discloses a preparation method of a lithium tantalate-silicon nitride photoelectric chip, comprising: providing a lithium tantalate-on-insulator material; the lithium tantalate-on-insulator material comprises a lithium tantalate thin film layer and a substrate layer distributed upward and downward; a silicon nitride thin film layer is prepared on the lithium tantalate thin film layer by wafer bonding; part of the silicon nitride thin film layer is removed to form a patterned silicon nitride thin film layer; a metal electrode is prepared on the lithium tantalate thin film layer to form a lithium tantalate-silicon nitride photoelectric chip; the metal electrode and the patterned silicon nitride thin film layer are positionally separated. The invention forms a lithium tantalate-silicon nitride heterostructure by wafer bonding, avoids high-temperature silicon nitride deposition process, and ensures the polarization integrity of lithium tantalate.

[0007] In the prior art, the combination of lithium niobate or lithium tantalate and a silicon nitride thin film layer is usually achieved by bonding, which is complicated and difficult to process.

[0008] Therefore, there is an urgent need to provide a method for directly preparing a silicon nitride thin film on the surface of lithium niobate or lithium tantalate. SUMMARY

[0009] In view of the deficiencies in the prior art, the purpose of the present application is to provide a method for preparing a silicon nitride thin film on the surface of a substrate. The present application directly deposits a silicon nitride thin film on the surface of a substrate by preheating and slow cooling in a thermal radiation manner while maintaining the integrity of the lithium niobate or lithium tantalate substrate. The method provided by the present application is simple, easy to implement, and has industrial application prospects.

[0010] To achieve the purpose of the present application, the following technical solutions are adopted:

[0011] In a first aspect, the present application provides a method for preparing a silicon nitride thin film on the surface of a substrate, comprising:

[0012] (1) In a first chamber with a nitrogen atmosphere, a hot plate with a process temperature is provided, and the substrate is preheated to a preheating temperature at a first distance;

[0013] The material of the substrate includes lithium tantalate or lithium niobate;

[0014] (2) The substrate with the preheating temperature is placed on the surface of the hot plate, and the substrate is heated to a process temperature;

[0015] (3) A silicon nitride thin film is deposited on the surface of the substrate with the process temperature;

[0016] (4) The substrate with the deposited silicon nitride thin film is transferred to a second vacuum chamber for cooling.

[0017] In the present application, the distance between the substrate and the hot plate is achieved by adjusting the vertical position of the two in the cavity, wherein the substrate is supported by the probe when placed in the first cavity, located above the hot plate, and the distance between the hot plate and the substrate in the present application is actually the distance from the top of the hot plate to the bottom of the substrate.

[0018] The present application uses a hot plate with a process temperature to preheat the substrate spaced apart by a first distance, preheats the substrate by means of heat radiation with a slow temperature rise rate, which can prevent lithium niobate or lithium tantalate substrate from being broken due to too fast thermal expansion; and uses nitrogen as a medium for heating, which is more uniform. At the same time, after the deposition of the silicon nitride film is completed, the substrate with the deposited silicon nitride film is directly transferred to the second vacuum cavity for cooling, and in the vacuum environment, only heat radiation is relied on for cooling, so that the cooling process is gentle, and sudden cooling which can cause lithium niobate or lithium tantalate substrate to be broken is avoided.

[0019] Preferably, the first distance is 30 mil~80 mil.

[0020] Preferably, the process temperature is 10℃~30℃ higher than the preheating temperature.

[0021] Preferably, the process temperature is 150℃~260℃.

[0022] Preferably, the flow rate of the nitrogen gas is 800sccm~1200sccm.

[0023] Preferably, the substrate in step (1) is kept at the preheating temperature for 30s or more, preferably 30s~60s.

[0024] Preferably, the substrate in step (2) is kept at the process temperature for 45s or more, preferably 45s~120s.

[0025] Preferably, the method for depositing a silicon nitride film comprises:

[0026] At the process temperature, a precursor gas is introduced into the first cavity, and a silicon nitride film is deposited by plasma enhanced chemical vapor deposition; the precursor gas comprises silane, ammonia and nitrogen.

[0027] Preferably, after the precursor gas is introduced, it is stabilized for 45s~100s, and then plasma enhanced chemical vapor deposition is performed.

[0028] Preferably, the flow rate of the silane is 30sccm~50sccm.

[0029] Preferably, the flow rate of the ammonia is 30sccm~50sccm.

[0030] Preferably, the flow rate of the nitrogen gas is 1400sccm~1500sccm.

[0031] Preferably, the pressure in the first cavity during the deposition process is 1 torr ~ 1.5 torr.

[0032] Preferably, the power of the plasma enhanced chemical vapor deposition is 100 W ~ 200 W.

[0033] Preferably, the time of the plasma enhanced chemical vapor deposition is 30 s ~ 50 s.

[0034] Preferably, the method further comprises vacuumizing after the deposition of the silicon nitride film is completed.

[0035] Preferably, the temperature in the second vacuum cavity is 20℃ ~ 40℃.

[0036] Preferably, the time of cooling in the second vacuum cavity is 100 s ~ 200 s.

[0037] Compared with the prior art, the present application has the following beneficial effects:

[0038] The present application directly deposits a silicon nitride film on the surface of a substrate in the case of a complete lithium niobate or lithium tantalate substrate by preheating and slow cooling in a thermal radiation manner. The method provided by the present application is simple in process and easy to implement, and has an industrial application prospect. DETAILED DESCRIPTION

[0039] The technical solutions of the present application will be further described by specific embodiments. Those skilled in the art should understand that the embodiments are only used to understand the present application and should not be regarded as specific limitations of the present application.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion.

[0041] In the description of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0042] In one specific embodiment, the present application provides a method for preparing a silicon nitride film on the surface of a substrate, the method comprising:

[0043] (1) in the first cavity of nitrogen atmosphere, provide the hot disc with process temperature, interval first distance, preheat the substrate to preheat temperature;

[0044] The material of the substrate includes lithium tantalate or lithium niobate;

[0045] (2) the substrate with the preheat temperature is placed on the surface of the hot disc, and the substrate is heated to the process temperature;

[0046] (3) depositing a silicon nitride film on the surface of the substrate with the process temperature;

[0047] (4) the substrate with the deposited silicon nitride film is transferred to the second vacuum cavity for cooling.

[0048] The present application uses the hot disc with process temperature to preheat the substrate with interval first distance, preheats the substrate by the way of heat radiation with slow heating rate, which can prevent the lithium niobate or lithium tantalate substrate from being broken due to too fast thermal expansion; and the heating is more uniform with nitrogen as medium. At the same time, after the deposition of the silicon nitride film is completed, the substrate with the deposited silicon nitride film is directly transferred to the second vacuum cavity for cooling, which only relies on heat radiation for cooling in the vacuum environment, so that the cooling process is smooth, and the lithium niobate or lithium tantalate substrate is prevented from being broken due to sudden cooling.

[0049] In the present application, the distance between the hot disc and the lithium niobate or lithium tantalate affects the heating rate of the substrate and the final preheat temperature, and the appropriate distance can ensure preheating with slow heating rate, and can also ensure that the substrate is preheated to a sufficient preheat temperature, so that the substrate is not broken due to too large temperature difference when placed on the surface of the hot disc.

[0050] In some embodiments, the first distance is 30 mil-80 mil, for example, it can be 30 mil, 40 mil, 50 mil, 60 mil, 70 mil or 80 mil, including but not limited to the listed values, and other values not listed in the value range are also applicable.

[0051] In the present application, the process temperature and the preheat temperature should not differ too much, so as to prevent the preheated substrate from being broken due to sudden heating when placed on the surface of the hot disc, and at the same time, it should not be too small, otherwise it will lead to too long heating time, causing resource waste and being not conducive to cost saving.

[0052] In some embodiments, the process temperature is 10-30℃ higher than the preheat temperature, for example, it can be 10℃, 15℃, 20℃, 25℃ or 23℃, including but not limited to the listed values, and other values not listed in the value range are also applicable.

[0053] In some embodiments, the process temperature is 150°C to 260°C, such as 150°C, 175°C, 200°C, 225°C, 250°C, or 260°C, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable.

[0054] In some embodiments, the flow rate of the nitrogen gas is 800 sccm to 1200 sccm, such as 800 sccm, 900 sccm, 1000 sccm, 1100 sccm, or 1200 sccm, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable.

[0055] In some embodiments, the substrate is maintained at the preheating temperature for 30 seconds or more, preferably 30 seconds to 60 seconds, such as 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, 55 seconds, or 60 seconds, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable.

[0056] In some embodiments, the substrate is maintained at the process temperature for 45 seconds or more, preferably 45 seconds to 120 seconds, such as 45 seconds, 50 seconds, 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds, 110 seconds, or 120 seconds, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable.

[0057] In some embodiments, the method for depositing a silicon nitride thin film comprises:

[0058] At the process temperature, a precursor gas is introduced into the first cavity, and a silicon nitride thin film is deposited by plasma enhanced chemical vapor deposition; the precursor gas comprises silane, ammonia, and nitrogen.

[0059] In some embodiments, after the precursor gas is introduced, it is stabilized for 45 seconds to 100 seconds, such as 45 seconds, 50 seconds, 55 seconds, 60 seconds, 65 seconds, 70 seconds, 75 seconds, 80 seconds, 85 seconds, 90 seconds, 95 seconds, or 100 seconds, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable, and then plasma enhanced chemical vapor deposition is performed.

[0060] In some embodiments, the flow rate of the silane is 30 sccm to 50 sccm, such as 30 sccm, 35 sccm, 40 sccm, 45 sccm, or 50 sccm, including but not limited to the listed numerical values, and other numerical values within the range of values are also applicable.

[0061] In some embodiments, the flow rate of the ammonia gas is 30 sccm ~ 50 sccm, for example, it can be 30 sccm, 35 sccm, 40 sccm, 45 sccm or 50 sccm, including but not limited to the listed values, other values not listed in the value range are also applicable.

[0062] In some embodiments, the flow rate of the nitrogen gas is 1400 sccm ~ 1500 sccm, for example, it can be 1400 sccm, 1420 sccm, 1440 sccm, 1460 sccm, 1480 sccm or 1500 sccm, including but not limited to the listed values, other values not listed in the value range are also applicable.

[0063] In some embodiments, the pressure in the first cavity during the deposition process is 1 torr ~ 1.5 torr, for example, it can be 1 torr, 1.1 torr, 1.2 torr, 1.3 torr, 1.4 torr or 1.5 torr, including but not limited to the listed values, other values not listed in the value range are also applicable.

[0064] In the present application, during the plasma enhanced chemical vapor deposition process, appropriate power is beneficial to maintain the integrity of the lithium niobate or lithium titanate substrate, and if the power is too large, the risk of fragmentation will increase.

[0065] In some embodiments, the power of the plasma enhanced chemical vapor deposition is 100 W ~ 200 W, for example, it can be 100 W, 120 W, 140 W, 160 W, 180 W or 200 W, including but not limited to the listed values, other values not listed in the value range are also applicable.

[0066] In some embodiments, the time of the plasma enhanced chemical vapor deposition is 30 s ~ 50 s, for example, it can be 30 s, 35 s, 40 s, 45 s or 50 s, including but not limited to the listed values, other values not listed in the value range are also applicable.

[0067] In some embodiments, the radio frequency system used in the plasma enhanced chemical vapor deposition is AENI, and the frequency is 13.56 MHz.

[0068] In some embodiments, the method further comprises vacuumizing after the deposition of the silicon nitride film is completed.

[0069] In the present application, in a vacuum environment, the lithium niobate or lithium titanate substrate on which the silicon nitride film is deposited is subjected to a slow cooling process by means of thermal radiation, thereby avoiding the risk of fragmentation caused by rapid cooling.

[0070] In some embodiments, the temperature within the second vacuum cavity is between 20 °C and 40 °C, for example, it can be 20 °C, 25 °C, 30 °C, 35 °C or 40 °C, including but not limited to the listed values, and other values within the range of values are also applicable.

[0071] Preferably, the time for cooling in the second vacuum cavity is between 100 s and 200 s, for example, it can be 100 s, 120 s, 140 s, 160 s, 180 s or 200 s, including but not limited to the listed values, and other values within the range of values are also applicable.

[0072] Example 1

[0073] The present example provides a method for preparing a silicon nitride film on a surface of a lithium niobate substrate, the method comprising:

[0074] (1) nitrogen gas with a flow rate of 1000 sccm is introduced into a first vacuum cavity, a hot plate is heated to 180 °C, and a lithium niobate substrate is preheated to 180 °C at an interval of 50 mil for 45 s;

[0075] (2) the hot plate is heated to 200 °C, and the preheated lithium niobate substrate is placed on the surface of the hot plate, and the lithium niobate substrate is heated to 200 °C and maintained for 90 s;

[0076] (3) silane with a flow rate of 40 sccm, ammonia with a flow rate of 40 sccm and nitrogen with a flow rate of 1450 sccm are introduced, so that the pressure in the first cavity is 1.2 torr, and the pressure is stabilized for 90 s; an AENI radio frequency system is used, the frequency is adjusted to 13.56 MHz, plasma enhanced chemical vapor deposition is carried out, the power for deposition is controlled to 150 W, and deposition is carried out for 40 s; after the deposition is completed, the pressure in the first cavity is pumped to vacuum;

[0077] (4) the lithium niobate substrate with the deposited silicon nitride film is transferred to a second vacuum cavity with a temperature of 30 °C, and naturally cooled for 150 s, and the preparation is completed.

[0078] Example 2

[0079] The present example provides a method for preparing a silicon nitride film on a surface of a lithium niobate substrate, the method comprising:

[0080] (1) nitrogen gas with a flow rate of 800 sccm is introduced into a first vacuum cavity, a hot plate is heated to 140 °C, and a lithium niobate substrate is preheated to 140 °C at an interval of 30 mil for 30 s;

[0081] (2) the hot plate is heated to 150 °C, and the preheated lithium niobate substrate is placed on the surface of the hot plate, and the lithium niobate substrate is heated to 150 °C and maintained for 45 s;

[0082] (3) introduce silane with a flow rate of 30 seem, ammonia with a flow rate of 30 seem and nitrogen with a flow rate of 1400 seem to make the pressure in the first cavity 1 torr, stabilize for 45 s; use AENI radio frequency system, adjust the frequency to 13.56 MHz, perform plasma enhanced chemical vapor deposition, control the power of deposition to be 100 W, and deposit for 30 s; after the deposition is completed, the pressure in the first cavity is pumped to vacuum;

[0083] (4) transfer the lithium niobate substrate on which the silicon nitride film is deposited into the second vacuum cavity with a temperature of 20℃, naturally cool for 100 s, and end the preparation.

[0084] Example 3

[0085] The embodiment provides a method for preparing a silicon nitride film on a lithium tantalate substrate surface, and the method comprises the following steps:

[0086] (1) introduce nitrogen with a flow rate of 1200 seem into the first cavity which is pumped to vacuum, heat the hot plate to 230℃, and preheat the lithium tantalate substrate to 230℃ at an interval of 80 mil for 60 s;

[0087] (2) heat the hot plate to 260℃, place the preheated lithium tantalate substrate on the surface of the hot plate, heat the lithium tantalate substrate to 260℃, and keep for 120 s;

[0088] (3) introduce silane with a flow rate of 50 seem, ammonia with a flow rate of 50 seem and nitrogen with a flow rate of 1500 seem to make the pressure in the first cavity 1.5 torr, stabilize for 100 s; use AENI radio frequency system, adjust the frequency to 13.56 MHz, perform plasma enhanced chemical vapor deposition, control the power of deposition to be 100 W-200 W, and deposit for 30 s-50 s; after the deposition is completed, the pressure in the first cavity is pumped to vacuum;

[0089] (4) transfer the lithium tantalate substrate on which the silicon nitride film is deposited into the second vacuum cavity with a temperature of 40℃, naturally cool for 200 s, and end the preparation.

[0090] Example 4

[0091] The embodiment provides a method for preparing a silicon nitride film on a lithium tantalate substrate surface, and the method comprises the following steps:

[0092] Example 5

[0093] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that in step (1) the hot plate is heated to 195°C and the lithium niobate substrate is heated to 195°C, i.e. only 5°C below the process temperature.

[0094] Example 6

[0095] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that in step (1) the lithium niobate substrate is preheated to 180°C only for 20 mils.

[0096] Example 7

[0097] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that after the lithium niobate substrate is preheated to 180°C in step (1), it is heated to 200°C only for 20s.

[0098] Example 8

[0099] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that after the lithium niobate substrate is heated to 200°C in step (2), plasma enhanced chemical vapor deposition is performed only for 30s.

[0100] Example 9

[0101] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that after the precursor gas is introduced in step (3), plasma enhanced chemical vapor deposition is performed only for 30s.

[0102] Example 10

[0103] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that the power of plasma enhanced chemical vapor deposition in step (3) is 75W.

[0104] Example 11

[0105] The present example provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that the power of plasma enhanced chemical vapor deposition in step (3) is 225W.

[0106] Comparative Example 1

[0107] The comparative example 2 provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that step (1) is not performed.

[0108] During the preparation of the comparative example 2, the lithium niobate substrate is broken.

[0109] Comparative example 2

[0110] The comparative example 2 provides a method for preparing a silicon nitride film on the surface of a lithium tantalate substrate, which is the same as example 1 except that step (1) is not performed.

[0111] During the preparation of the comparative example 2, the lithium niobate substrate is broken.

[0112] Performance test:

[0113] The thickness and uniformity of the silicon nitride film prepared in all the above examples were measured by an ellipsometer, and the test results are shown in Table 1.

[0114] Table 1

[0115]

[0116] According to the test results in Table 1, the present application effectively prevents the lithium niobate or lithium tantalate substrate from being broken due to rapid thermal expansion by preheating the substrate by means of thermal radiation with a slow heating rate using a hot plate with a process temperature at a first distance, and the heating is more uniform using nitrogen as a medium. At the same time, after the deposition of the silicon nitride film is completed, the substrate on which the silicon nitride film is deposited is directly transferred to a second vacuum cavity for cooling, and in a vacuum environment, the cooling process is gentle only by relying on thermal radiation, which avoids sudden cooling leading to the lithium niobate or lithium tantalate substrate being broken.

[0117] According to the test results of example 1 and examples 4 and 5, a suitable temperature difference between the preheating temperature and the process temperature is more conducive to improving the uniformity of the substrate temperature, and thus improving the uniformity of the subsequent deposition of the silicon nitride film.

[0118] According to the test results of example 1 and example 6, if the distance between the hot plate and the substrate during the preheating process is too small, the heating rate of the substrate is too fast, which leads to uneven heating of the wafer, local thickness difference, and poor uniformity.

[0119] According to the test results of example 1 and examples 7 and 8, if the holding time after preheating is only 20s, or the holding time after reaching the process temperature is only 30s, the holding time at the target temperature is too short, which will lead to uneven temperature distribution inside the substrate, resulting in a decrease in the uniformity of the prepared silicon nitride film.

[0120] According to the test results of Example 1 and Example 9, if the precursor gas is unstable after being introduced, the stability of the silicon nitride film is poor.

[0121] According to the test results of Example 1 and Example 10 and Example 11, if the power of the plasma enhanced chemical vapor deposition is too small or too large, it is not conducive to the improvement of the uniformity of the silicon nitride film.

[0122] The above merely provides a specific implementation of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily conceived by those skilled in the art, and all such changes and replacements fall within the protection scope and disclosure scope of the present application.

Claims

1. A method for preparing a silicon nitride thin film on a substrate surface, characterized in that, The method includes: (1) In the first chamber under a nitrogen atmosphere, a hot plate with a process temperature is provided, spaced 30mil~80mil apart, to preheat the substrate to the preheating temperature; The substrate is made of lithium tantalate or lithium niobate. (2) Place the substrate with the preheating temperature on the surface of the hot plate and heat the substrate to the process temperature; the process temperature is 10°C to 30°C higher than the preheating temperature; the process temperature is 150°C to 260°C. (3) Deposit a silicon nitride thin film on the substrate surface at the process temperature described above; (4) The substrate with the silicon nitride film deposited is transferred to the second vacuum chamber for cooling; the temperature in the second vacuum chamber is 20℃~40℃; In step (1), the substrate is held at the preheating temperature for more than 30 seconds; In step (2), the substrate is held at the process temperature for more than 45 seconds; The method for depositing silicon nitride thin films in step (3) includes: At the process temperature, a precursor gas is introduced into the first cavity, and a silicon nitride thin film is deposited by plasma-enhanced chemical vapor deposition. The precursor gas includes silane, ammonia, and nitrogen; The power of the plasma-enhanced chemical vapor deposition is 100W~200W.

2. The method as described in claim 1, characterized in that, The nitrogen flow rate in step (1) is 800 sccm to 1200 sccm.

3. The method as described in claim 1, characterized in that, The flow rate of the silane in step (3) is 30 sccm to 50 sccm; And / or, the flow rate of ammonia in step (3) is 30 sccm to 50 sccm; And / or, the flow rate of nitrogen in step (3) is 1400 sccm~1500 sccm; And / or, during the deposition process described in step (3), the pressure in the first cavity is 1 torr to 1.5 torr.

4. The method as described in claim 1, characterized in that, The plasma-enhanced chemical vapor deposition time is 30s~50s.

5. The method as described in claim 1, characterized in that, The method in step (3) further includes evacuating a vacuum after the silicon nitride thin film deposition is completed.

6. The method as described in claim 1, characterized in that, The cooling time in the second vacuum chamber is 100s~200s.

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