A method for reducing lithium tantalate bonded wafer bubble defects

Abstract

The application relates to the field of semiconductor materials, and specifically provides a method for reducing the air bubble defect of a lithium tantalate bonding wafer, steps of which are as follows: a) twice vacuumizing treatment is conducted on a bonding chamber, so that the target vacuum degree is less than 1.5e-3 mbar; b) a lithium tantalate wafer and a substrate wafer are respectively vacuum adsorbed on an upper loading platform and a lower loading platform of the bonding chamber, wherein the vacuum adsorption pressure of the upper loading platform and the lower loading platform is set according to the material of the substrate wafer; c) plasma is used to clean the lithium tantalate wafer and the substrate wafer; d) plasma is used to activate the surface of the substrate wafer; e) in the vacuum adsorption state of the lithium tantalate wafer and the substrate wafer, the two are aligned at the edges and are pressurized to be bonded, and the vacuum adsorption of the lithium tantalate wafer is released when the pressure reaches a certain value. Through the cooperation of the twice vacuumizing process, the plasma cleaning process and the vacuum adsorption combined with the free attachment bonding process, the air bubble defect problem in the bonding process is effectively solved, and the bonding quality is improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor materials, and more specifically to a method for reducing bubble defects in lithium tantalate bonding wafers. Background Technology

[0002] In recent years, the mobile communication industry has developed rapidly, and the development direction of filters has gradually shifted towards higher frequencies, composite materials, miniaturization, and integration. Traditional surface acoustic wave (SAW) filters suffer from problems such as low frequency and poor temperature stability. To solve these problems and meet the performance requirements of next-generation devices, heterogeneous wafer bonding technology is developing rapidly, which can achieve efficient and low-cost manufacturing and performance improvement through new structures.

[0003] Lithium tantalate (LiTaO3, LT) crystals possess excellent piezoelectric, acousto-optic, ferroelectric, and pyroelectric effects, making them a fundamental functional material in SAW devices, optical communications, lasers, and optoelectronics. They offer advantages such as a large piezoelectric coefficient, a small temperature coefficient of frequency, and a high electromechanical coupling coefficient. To improve the performance of SAW filters, bonding technology is needed to combine lithium tantalate wafers with wafers of other materials to meet the requirements of low insertion loss, high bandwidth, and good thermal stability. However, existing bonding processes are constrained by the bonding environment and the surface condition of the bonded wafers. Achieving truly suitable bonding conditions is difficult and time-consuming. When certain conditions are not met, defects such as bubbles are easily generated internally after bonding, leading to the loss of the entire bonded wafer. Therefore, an optimized process is urgently needed to provide a suitable environment for bonding and achieve appropriate surface conditions for the bonded wafers.

[0004] Chinese patent application CN118444430A discloses a composite thin film comprising a patterned composite substrate and its preparation method. This invention employs high-low temperature cyclic annealing during the composite thin film preparation process. Since the patterned layer in the composite thin film containing the patterned composite substrate comprises heterogeneous materials, high-low temperature cyclic annealing can eliminate stress changes caused by heating between the heterogeneous materials, thus avoiding bonding bubble problems due to differences in thermal expansion coefficients between the patterned layer and the dielectric layer. However, for materials with high thermal expansion coefficients, high-low temperature cyclic annealing may cause the wafer to expand and crack due to heat, resulting in losses. Furthermore, for room temperature bonding, high-low temperature cyclic annealing also introduces additional processes, increasing risks and uncertainties.

[0005] Chinese Patent No. CN112635337A discloses a method for reducing bonding bubbles in ultra-thin stacked technology products. The invention provides a logic wafer with copper cracking defects generated by FSI WAT testing. A first oxide layer is formed on the surface of the logic wafer, completely covering the copper cracking defects. The thickness of the first oxide layer is between 25K and 35K. A planarization process is used to planarize the first oxide layer to between 5K and 9K. A second oxide layer is formed, again covering the copper cracking defects. The thickness of the second oxide layer is between 15K and 25K. A planarization process is used to planarize the second oxide layer to between 8K and 12K. The resulting logic wafer is then bonded to a pixel wafer. This method significantly improves the reduction of FSI copper pad cracking defects, resulting in substantial improvement in subsequent bonding bubbles. This technology uses an oxide coating to cover defects in bonding bubbles, which is used to improve bonding bubbles. However, this technology is mainly used in packaged products, which are actually small-sized devices packaged onto wafers. The method is not entirely applicable to the bonding of large-sized wafers. Moreover, improvement through coating requires additional cost investment and does not fundamentally solve this problem.

[0006] Therefore, there is a lack of efficient methods in the prior art that can effectively reduce the defects of bubbles in lithium tantalate bonded sheets. Summary of the Invention

[0007] This invention provides a method for reducing bubble defects in lithium tantalate bonded sheets, thereby overcoming the shortcomings of the prior art and solving the problem of bubble defects that may occur after bonding existing lithium tantalate bonded sheets.

[0008] The technical solution adopted by this invention to solve the problem is: A method for reducing bubble defects in lithium tantalate bonded sheets includes the following steps: a) After the bonding chamber of the bonding machine is evacuated for the first time and leak checked, it is filled with argon gas for flushing, and then evacuated and leak checked again. b) The substrate and the lithium tantalate wafer are transported from the transfer chamber to the bonding chamber respectively. The substrate is vacuum adsorbed on the download stage and the lithium tantalate wafer is vacuum adsorbed on the loading stage. c) Use an ion gun to perform plasma cleaning on the lithium tantalate wafer and the substrate; d) Use an ion gun to perform plasma surface activation on the substrate; e) With both the lithium tantalate wafer and the substrate under vacuum adsorption, align their edges and apply pressure bonding. When the pressure reaches a certain level, the lithium tantalate wafer is released from vacuum adsorption.

[0009] In step a) above, the first evacuation is carried out to below 3e-3 mbar, and leak detection begins. After 30 minutes, the leak rate standard is below 1e-2 mbar. The argon filling time is 20-30 minutes. The second evacuation is carried out to below 1.5e-3 mbar, and leak detection begins. After 30 minutes, the leak rate standard is below 4e-3 mbar.

[0010] In step b) above, the substrate is generally made of silicon, sapphire, etc., and the shape and size of the substrate are the same as those of the lithium tantalate wafer, with a size of 4 to 8 inches.

[0011] In step b) above, the vacuum adsorption pressure of the upper and lower stages is set according to the material of the substrate: if the substrate is a silicon wafer, the vacuum adsorption pressure of the upper stage is -31~-39 kPa, and the vacuum adsorption pressure of the lower stage is -40~-55 kPa; if the substrate is a sapphire or other wafer with a Mohs hardness of 8 or higher, the vacuum adsorption pressure of the upper stage is -41~-49 kPa, and the vacuum adsorption pressure of the lower stage is -50~-65 kPa.

[0012] In step c) above, the lithium tantalate wafer and the substrate are plasma cleaned using an ion gun. The inert gas flow rate is 500~600 sccm, the machine power is 50~300W, and the cleaning time is 1~2 min.

[0013] In step d) above, an ion gun is used to perform plasma surface activation on the substrate. The voltage is 1500±50kV, the current is 100±5mA, the initial argon flow rate is 35±2sccm, and the subsequent argon flow rate is 37±2sccm.

[0014] In step e) above, both the lithium tantalate wafer and the substrate are in a flattened state. After alignment, pressure is applied to the lithium tantalate wafer and the substrate to ensure close contact between them, eliminate air bubbles, and improve bonding quality. The bonding pressure is 1000~5000N, with an initial setting of 2000±500N. When the bonding pressure reaches 1550~1750N, the lithium tantalate wafer is released from vacuum adsorption.

[0015] In step e) above, during pressure bonding, the pressure rate switching point is when the distance between the substrate and the lithium tantalate wafer is 0.5~0.8mm, where the fast pressure rate is 20±5mm / s, the slow pressure rate is 0.15±0.05mm / s, and the pressing time is 40±10s.

[0016] In step e) above, the lithium tantalate wafer and the substrate are edge aligned, wherein the coarse alignment rotation angle is -5° to 5°, the visual recognition diameter range is 80 to 120 μm, the judgment point error range is 2 μm, and the compression ratio is 2 to 4.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] (1) The secondary evacuation technology effectively solves the problem of residual gas adsorption layer on the surface of the cavity: a single evacuation can only remove the free gas in the cavity. By a secondary evacuation and rinsing with inert gas between the two evacuations, the residual gas on the inner wall of the cavity can be efficiently replaced, shortening the time to evacuate to the target vacuum level and avoiding the failure of a single evacuation to meet the target. A single evacuation is difficult to achieve a vacuum level target of less than 1.5e-3mbar, which can easily lead to impurity particles remaining in the cavity and increase the risk of bubble generation.

[0019] (2) Before the wafer surface is activated, the lithium tantalate wafer and the substrate are plasma cleaned with an ion gun. This effectively avoids poor bonding caused by surface dirt and improves the bonding strength between the two, further reducing the possibility of poor bonding.

[0020] (3) Both the lithium tantalate wafer and the substrate are in a vacuum adsorption state in the early stage of bonding, which can effectively avoid the bubble problem caused by micro-warping of the wafer; when the bonding pressure reaches a certain value, the vacuum adsorption of the lithium tantalate wafer is released, allowing it to adhere freely during the bonding process, further promoting the tight bonding of the two. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention.

[0022] Figure 2 This is a micrograph of the bonded sheet from Example 1.

[0023] Figure 3 This is a micrograph of poorly bonded bubbles, as shown in Comparative Example 1. Detailed Implementation

[0024] The present invention will be further described below with reference to embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0025] Example 1:

[0026] The flowchart of this embodiment is as follows: Figure 1 As shown, a method for reducing bubble defects in lithium tantalate bonded sheets is provided, comprising the following steps: a) The bonding chamber of the bonding machine was first evacuated to 2.46e-3 mbar, and leak detection was started. After 30 minutes, the leakage rate was 6.12e-3 mbar. Argon gas was then introduced for 30 minutes, and the chamber was evacuated again to 1.38e-3 mbar. Leak detection was started again. After 30 minutes, the leakage rate was 3.9e-3 mbar. b) The substrate and the 6-inch lithium tantalate wafer are transported from the transfer chamber to the bonding chamber respectively. The silicon wafer is used as the substrate and is vacuum adsorbed on the download stage. The vacuum adsorption pressure of the download stage is -50kPa. The lithium tantalate wafer is vacuum adsorbed on the loading stage. The vacuum adsorption pressure of the loading stage is -35kPa. c) Use an ion gun to perform plasma cleaning on the lithium tantalate wafer and the substrate. The inert gas flow rate is 550 sccm, the power is 55W, and the cleaning time is 2 min. d) Use an ion gun to perform plasma surface activation on the substrate at a voltage of 1500kV, a current of 100mA, an initial argon flow rate of 35sccm, and a subsequent argon flow rate of 37sccm. e) First, keep both the lithium tantalate wafer and the substrate in a vacuum adsorption and flattened state, and align them by edge alignment. The coarse alignment rotation angle is 2°, the visual recognition diameter range is 110μm, the judgment point error range is 2μm, and the compression rate is 3. Then, place the aligned wafer into the bonding cavity and pressurize to ensure tight contact between the bonding wafers and eliminate air bubbles. The bonding cavity pressure is set to 2000N, and the pressurization speed switching point is when the distance between the substrate and the lithium tantalate wafer is 0.5mm. The rapid pressurization speed is 20mm / s, the slow pressurization speed is 0.15mm / s, and the slow pressurization speed is maintained during the pressurization process. When the bonding pressure reaches 1550N, the lithium tantalate wafer is released from vacuum adsorption, and the pressing time is 40s.

[0027] After bonding using the method of Example 1, the possibility of air bubbles being generated at the edge and random parts inside the bonded sheet is small, and the probability of air bubble defects in the bonded sheet is about 5%.

[0028] The lithium tantalate bonded sheet prepared in this embodiment was inspected and found to be free of scratches and surface defects. See the micrograph for details. Figure 2 This meets the requirements for subsequent device processing.

[0029] Example 2:

[0030] This embodiment provides a method for reducing air bubble defects in lithium tantalate bonded sheets, including the following steps: a) The bonding chamber of the bonding machine was first evacuated to 2.53e-3 mbar, and leak detection was started. After 30 minutes, the leakage rate was 5.28e-3 mbar. Argon gas was then introduced for 30 minutes, and the chamber was evacuated again to 1.59e-3 mbar. Leak detection was started again. After 30 minutes, the leakage rate was 3.33e-3 mbar. b) The substrate and the 4-inch lithium tantalate wafer are transported from the transfer chamber to the bonding chamber respectively. The sapphire wafer is used as the substrate and is vacuum adsorbed on the download stage with an adsorption pressure of -55 kPa. The lithium tantalate wafer is vacuum adsorbed on the loading stage with an adsorption pressure of -41 kPa. c) Use an ion gun to perform plasma cleaning on the lithium tantalate wafer and the substrate. The inert gas flow rate is 550 sccm, the power is 55W, and the cleaning time is 2 min. d) Use an ion gun to activate the surface of the substrate at a voltage of 1500kV, a current of 100mA, an initial argon flow rate of 35sccm, and a subsequent argon flow rate of 37sccm. e) First, keep both the lithium tantalate wafer and the substrate in a vacuum adsorption and flattened state, and align them by edge alignment. The coarse alignment rotation angle is 2°, the visual recognition diameter range is 110μm, the judgment point error range is 2μm, and the compression rate is 3. Then, place the aligned wafer into the bonding cavity and pressurize it to ensure tight contact between the bonding wafers and eliminate air bubbles. The bonding cavity pressure is set to 2300N, and the pressurization speed switching point is when the distance between the substrate and the lithium tantalate wafer is 0.5mm. The rapid pressurization speed is 20mm / s, the slow pressurization speed is 0.15mm / s, and the slow pressurization speed is maintained during the pressurization process. When the bonding pressure reaches 1750N, the lithium tantalate wafer is released from vacuum adsorption, and the pressing time is 40s.

[0031] After bonding using the method of Example 2, the possibility of air bubbles being generated at the edge and random parts inside the bonded sheet is small, and the probability of air bubble defects in the bonded sheet is about 4%.

[0032] The lithium tantalate bonded wafer prepared in this embodiment was inspected and found to be free of scratches and surface defects, meeting the requirements for subsequent device processing.

[0033] Comparative Example 1:

[0034] The steps for this comparative example are as follows: a) Evacuate the bonding chamber of the bonding machine to 2.33e-3 mbar and start leak detection. After 30 minutes, the leak rate is 7.23e-3 mbar. b) The substrate and the 6-inch lithium tantalate wafer are transported from the transfer chamber to the bonding chamber respectively. The silicon wafer is used as the substrate and vacuum adsorbed on the download stage. The vacuum adsorption pressure of the download stage is -50kPa. The lithium tantalate wafer is placed on the bonding fixture. c) Use an ion gun to activate the surface of the substrate at a voltage of 1500kV, a current of 100mA, an initial argon flow rate of 35sccm, and a subsequent argon flow rate of 37sccm. d) With the substrate in an unadsorbed state, perform edge alignment and pressure bonding on both. Align the lithium tantalate wafer with the substrate edge by rotating the coarse alignment angle by 2°, visual recognition diameter range of 110μm, judgment point error range of 2μm, and compression ratio of 3. Place the aligned wafer into the bonding cavity and apply pressure to ensure tight contact between the bonded wafers. The bonding pressure is set to 2000N and the pressing time is 40s.

[0035] After bonding using the method in Comparative Example 1, there is a high probability of air bubbles forming at the edges and random areas inside the bonded wafer. The probability of air bubble defects in the bonded wafer is approximately 20%. For details of the bonding air bubble defects produced in this comparative example, please refer to [link to relevant documentation]. Figure 3 .

[0036] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for reducing air bubble defects in lithium tantalate bonded sheets, characterized in that, Includes the following steps: a) The bonding chamber is evacuated twice to achieve a target vacuum level of less than 1.5e-3 mbar; b) Provide lithium tantalate wafers and substrates, which are vacuum adsorbed onto the loading stage and unloading stage of the bonding chamber, respectively, wherein the vacuum adsorption pressure of the loading and unloading stages is set according to the material of the substrate. c) Use plasma to clean the lithium tantalate wafer and substrate; d) Surface activation of the substrate using plasma; e) With both the lithium tantalate wafer and the substrate under vacuum adsorption, align their edges and apply pressure bonding. When the pressure reaches a certain level, release the vacuum adsorption of the lithium tantalate wafer.

2. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1, characterized in that, In step a), the initial evacuation is carried out to below 3e-3 mbar and leaks are detected. The leak rate standard after 30 minutes is below 1e-2 mbar. Then, argon gas is introduced for 20-30 minutes to clean the bonding chamber, followed by a second evacuation and leak detection. The leak rate standard after 30 minutes is below 4e-3mbar.

3. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1, characterized in that, In step b), the substrate includes a silicon wafer and sapphire, and the shape and size of the substrate are consistent with those of the lithium tantalate wafer, which is 4 to 8 inches.

4. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1 or 3, characterized in that, In step b), when the substrate is a silicon wafer, the vacuum adsorption pressure of the upper stage is -31~-39 kPa, and the vacuum adsorption pressure of the lower stage is -40~-55 kPa.

5. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1 or 3, characterized in that, In step b), when the substrate is sapphire, the vacuum adsorption pressure of the upper stage is -41~-49 kPa, and the vacuum adsorption pressure of the lower stage is -50~-65 kPa.

6. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1, characterized in that, In step c), plasma cleaning is performed using an ion gun with an inert gas flow rate of 500-600 sccm, a machine power of 50-300W, and a cleaning time of 1-2 minutes.

7. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1, characterized in that, In step d), surface activation is performed using an ion gun with a voltage of 1500±50kV, a current of 100±5mA, an initial argon flow rate of 35±2sccm, and a subsequent argon flow rate of 37±2sccm.

8. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 1, characterized in that, In step e), both the lithium tantalate wafer and the substrate are in a flattened state. The pressure for pressure bonding is 1000~5000N, the initial pressure is 2000±500N, and when the pressure reaches 1550~1750N, the vacuum adsorption of the lithium tantalate wafer is released.

9. The method for reducing bubble defects in lithium tantalate bonded sheets as described in claim 8, characterized in that, In step e), the pressure rate switching point is when the distance between the lithium tantalate wafer and the substrate is 0.5~0.8mm, where the rapid pressure rate is 20±5mm / s, the slow pressure rate is 0.15±0.05mm / s, and the pressing time is 40±10s.

10. The method for reducing bubble defects in lithium tantalate bonded sheets as described in any one of claims 1 and 8-9, characterized in that, In step e), the coarse alignment rotation angle is -5° to 5°, the visual recognition diameter range is 80 to 120 μm, the judgment point error range is 2 μm, and the compression rate is 2 to 4.

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