A low-temperature bonding method and bonding wafer

Abstract

This application discloses a low-temperature bonding method and a bonding wafer, belonging to the field of wafer bonding technology. The method includes the following steps: (1) forming a SiO2 layer by plasma-enhanced chemical vapor deposition on the non-bonding surfaces of the substrate wafer and / or piezoelectric wafer; (2) performing plasma activation treatment on the piezoelectric wafer and the substrate wafer; (3) cleaning and drying the activated piezoelectric wafer and the substrate wafer, and forming an oxide layer with a thickness of at least 1.5 nm on the surface of the cleaned and dried substrate wafer; (4) bonding the piezoelectric wafer and the substrate wafer obtained in step (3) at a temperature of 0.9 × 10⁻⁶ nm. 5 -1.2×10 5 Pre-bonding is performed at Pa and 50%-70% humidity; (4) the pre-bonded wafer is annealed to obtain the bonded wafer. This method can effectively improve the bonding strength and avoid defects such as voids, thereby improving the performance stability of the bonded wafer.

Description

Technical Field

[0001] This application relates to a low-temperature bonding method and a bonding wafer, belonging to the field of wafer bonding technology. Background Technology

[0002] Direct wafer bonding technology can combine wafer materials of different materials through strong covalent bonds formed by interatomic contact. Compared with traditional bonding methods, direct bonding technology can complete wafer bonding at low temperatures, overcoming the problems of mismatch in thermal expansion coefficients and lattice constants, and is an important technology for heterogeneous integration.

[0003] Currently, the mainstream direct bonding technologies mainly include two types: surface activated bonding, which uses Ar ion beams to activate the bonding surface of the wafer and then completes the direct bonding of the wafer at low temperature; and plasma activated bonding, which uses O2, N2, and Ar plasmas to activate the bonding surface of the wafer and then performs pre-bonding and annealing at low temperature.

[0004] Plasma-activated bonding is a hydrophilic bonding method, and its bonding strength is positively correlated with the number of hydroxyl groups on the bonding surface. Therefore, increasing the number of hydroxyl groups on the wafer surface can effectively improve the wafer bonding strength. However, traditional PAB bonding methods often require pre-bonding in a vacuum environment to ensure the cleanliness of the bonding surface. This process not only reduces bonding efficiency and increases costs due to the vacuum step, but more importantly, the continuous vacuum environment consumes hydroxyl groups on the wafer surface, leading to a reduction in the number of covalent chemical bonds formed at the bonding interface, thereby weakening the bonding strength.

[0005] If bonding could be performed directly in an atmospheric environment, the above problems would be solved. Existing technology CN110473778B provides a method for direct bonding of zirconium oxide and aluminum oxide using plasma activation. It activates the ZrO2 and Al2O3 wafers to be bonded using O2 / NH3 / H2O plasma, and then places them in an atmospheric environment with a humidity of 20~80% for bonding. This method is the first to achieve atmospheric bonding of two heterogeneous wafers.

[0006] The applicant found that when this method is extended to piezoelectric wafers made of lithium tantalum or lithium niobate, the above-mentioned plasma treatment and bonding under this humidity have two problems: first, there are a large number of particles on the wafer surface, making it difficult to ensure the cleanliness of the bonding surface, which leads to a decrease in wafer bonding strength; second, there are bubble defects caused by gas accumulation on the bonding surface. Therefore, it is difficult to prepare an ideal piezoelectric composite substrate under the above bonding conditions, and it is impossible to carry out large-scale production. Summary of the Invention

[0007] To address the aforementioned issues, a low-temperature bonding method is provided. This method involves forming a SiO2 layer at the piezoelectric wafer and substrate wafer through non-ionic plasma-enhanced chemical vapor deposition at the bonding interface, followed by plasma activation and cleaning. This enables atmospheric bonding and effectively prevents gas aggregation at the bonding interface, thus avoiding bubble defects. The synergistic effect of these measures significantly improves the strength and performance stability of the bonded wafer.

[0008] According to one aspect of this application, a low-temperature bonding method is provided, comprising the following steps:

[0009] (1) Plasma-enhanced chemical vapor deposition is performed on the unbonded surfaces of the piezoelectric wafer and / or the substrate wafer to form a SiO2 layer with a thickness of 200-600 nm;

[0010] (2) The piezoelectric wafer and the substrate wafer are subjected to plasma activation treatment using an activation gas, wherein the piezoelectric wafer is selected from lithium niobate or lithium tungstate, and the substrate wafer is selected from at least one of silicon, silicon oxide, silicon carbide, and sapphire;

[0011] (3) The activated piezoelectric wafer and the substrate wafer are cleaned and dried, and a first oxide layer of at least 1.5 nm is formed on the surface of the substrate wafer after cleaning and drying.

[0012] (4) The first oxide layer and the second oxide layer are bonded together at a depth of 0.8 × 10⁻⁶. 5 -1.2×10 5 Pre-bonding was carried out under 50%-70% humidity conditions;

[0013] (5) Anneal the prebonded wafer to obtain the bonded wafer.

[0014] In step (1) of this application, plasma-enhanced chemical vapor deposition is performed on the non-bonded surfaces of the piezoelectric wafer and / or the substrate wafer to form a SiO2 layer with a thickness of 200-600 nm. After the SiO2 layer is formed by plasma-enhanced chemical vapor deposition (PECVD), when it is cooled to room temperature, the shrinkage of the piezoelectric wafer and the substrate wafer is much greater than that of the SiO2 layer. This causes the SiO2 layer to generate a tensile stress on the back side of the piezoelectric wafer and the substrate wafer, thereby causing the front side of the piezoelectric wafer and the substrate wafer to bulge. This solves the problem that bonding under high pressure conditions is prone to residual gas forming bubble defects on the bonding surface. Therefore, this operation realizes convex bonding under atmospheric conditions, effectively preventing gas accumulation at the bonding interface to form bubble defects, which is convenient for bonding under the conditions of step (4), and significantly improves the strength and performance stability of the bonded wafer. Plasma-enhanced chemical vapor deposition (PECVD) to form SiO2 layers for convex bonding offers advantages over other methods of altering the warpage of piezoelectric wafers or substrate wafers, such as ion implantation, solution etching, and physical application of external forces. It is easier to implement, easier to operate, lower in cost, and does not cause lattice damage to the wafer.

[0015] Step (3) of the plasma-activated piezoelectric wafer and substrate wafer has the following effects: First, it can effectively remove organic matter and contaminants from the surface, which can improve the cleanliness of the bonding interface and the number of hydroxyl groups, thereby improving the bonding strength. Second, it can regulate the number and uniformity of hydroxyl groups and dangling bonds in the bonding interface, so as to suppress the generation of hydrogen during the bonding process while improving the bonding strength and bonding uniformity, thereby avoiding defects such as voids in the bonding interface and improving the quality of the bonded wafer. Third, an oxide layer is formed on the surface of both the piezoelectric wafer and the substrate wafer. The presence of this oxide layer can reduce the difference in the thermal expansion coefficient and lattice constant between the piezoelectric wafer and the substrate wafer, which not only increases the bonding strength, but also effectively suppresses acoustic energy leakage as an insulating layer, thereby improving the reliability of downstream devices.

[0016] The cleaning in step (3) is SC-1 cleaning. The first oxide layer formed on the bonding surface further enhances the plasma activation effect and can also serve as a buffer layer to further reduce the difference in thermal expansion coefficients between the piezoelectric wafer and the substrate wafer. At the same time, the first oxide layer helps to suppress hydrogen generation and provides an escape channel for hydrogen released during the annealing stage, thus doubly avoiding the formation of void defects.

[0017] Furthermore, this study found that increasing plasma activation power and time to improve bonding strength inevitably causes surface damage to the piezoelectric wafer and substrate wafer, resulting in increased surface roughness and defect density, which in turn reduces bonding strength. Therefore, the activation effect and oxide layer formation effect are currently in conflict with surface damage, and this conflict causes changes in bonding strength. The piezoelectric wafer and substrate wafer treated in steps (2) and (3) above can achieve a balance between surface damage and activation and cleaning effects, thus achieving optimal bonding strength.

[0018] Step (4) pre-bonding involves adding a fan filter unit (FFU) and an anti-static device to the existing bonding equipment. This can strictly control the cleanliness of the internal environment to within ISO 5 level, effectively preventing the wafer from being contaminated by particles in the atmospheric environment during handling and bonding, and improving the bonding strength and the quality of the bond.

[0019] Optionally, the thickness of the first oxide layer is at least 2.3 nm.

[0020] Preferably, the thickness of the first oxide layer is 1.5-15 nm. The thickness of the first oxide layer does not exceed 15 nm because the maximum oxide layer formed by hydrogen peroxide generally does not exceed this value. If the first oxide layer is too thin, the bonding effect cannot be significantly improved.

[0021] Optionally, the plasma-enhanced chemical vapor deposition temperature is 180-300℃, the radio frequency power (RF) is 70-200W, the vacuum degree is 100-200Pa, the heating and cooling rate is 3-5℃ / min, the ratio of silane:nitrogen:laughing gas is 36:300:700 (in sccm), and the time is 3-10min.

[0022] The parameters for plasma-enhanced chemical vapor deposition described above have the following advantages:

[0023] 1) By sacrificing film quality, the deposition rate of SiO2 of the same thickness can be increased by more than 50%.

[0024] 2) Control the process temperature within a relatively safe range (180-300℃) to prevent the piezoelectric wafer or substrate wafer from cracking due to excessive stress during cooling.

[0025] Deposition at 180-300℃ can ensure yield. The amount of silane, nitrogen and nitrous oxide used can accelerate the deposition rate. The processing time needs to be selected according to the wafer with different coefficients of thermal expansion, so as to control the size of the bow.

[0026] Optionally, before plasma-enhanced chemical vapor deposition in step (1), a 1-2 μm photoresist is spin-coated onto the bonding surface of the piezoelectric wafer and / or the substrate wafer and then heated and cured. After the SiO2 layer is formed by plasma-enhanced chemical vapor deposition in step (1), the photoresist is removed by immersion in acetone solvent and ultrasonic assistance, then rinsed with deionized water and dried with nitrogen before proceeding to step (2).

[0027] Before step (1), the photoresist is spin-coated to ensure that the photoresist layer is uniform, complete, and free of pinholes. This can protect the bonding surface, prevent plasma-enhanced chemical vapor deposition from affecting the subsequent processing of the bonding surface, and improve the bonding effect.

[0028] Optionally, after the activation treatment in step (2), the contact angle of the piezoelectric wafer surface is 2°-6°;

[0029] The contact angle of the substrate wafer surface is 2°-5°.

[0030] Optionally, the activation process in step (2) specifically involves:

[0031] Activation is performed using one or more mixed gases of N2, O2, and Ar. The vacuum degree of the plasma activation chamber is 10-60 Pa, the plasma gas flow rate is 50-200 sccm, the power of the plasma emitter at 400 kHz is 40-100 W, the power at 40 kHz is 10-50 W, and the process time is 15-90 s.

[0032] The above activation treatment can increase the number of hydroxyl groups and dangling bonds on the surface of the piezoelectric wafer and the substrate wafer. At the same time, the activation treatment can also effectively remove surface particles and organic matter from the piezoelectric wafer and the substrate wafer, reduce surface roughness, and lay the foundation for the subsequent step (2).

[0033] Plasma activation has limited effect on the wafer surface. If the processing time is too long, it will lead to increased wafer surface roughness and more defect sites due to overactivation. Therefore, the cleaning operation in step (3) can further control the number of hydroxyl groups and dangling bonds on the oxide layer surface on the basis of reducing surface damage to the piezoelectric wafer and the substrate wafer, so as to improve the bonding strength and avoid the generation of hydrogen gas during bonding, thereby reducing the probability of defects such as voids in the bonded wafer.

[0034] Optionally, the cleaning in step (3) is as follows: using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O=1:(5-10):(10-20), the piezoelectric wafer and the substrate wafer are immersed for 30 minutes at a temperature of 65-100℃.

[0035] Optionally, the contact angle of the piezoelectric wafer surface obtained in step (3) is <4°;

[0036] The contact angle of the substrate wafer surface is <4°.

[0037] Optionally, the average water droplet contact angle obtained in step (2) on the piezoelectric wafer surface and the substrate wafer surface is θ Then the bond strength σ = σ max • e -kθ

[0038] in σ Bond strength, in MPa;

[0039] θ The contact angle of the water droplet is expressed in degrees (°).

[0040] σ max The theoretical maximum bond strength is expressed in MPa.

[0041] k is the attenuation coefficient, with a value range of 0.05-0.5.

[0042] This application establishes a relationship between the number of hydroxyl groups on the surface of piezoelectric wafers and substrate wafers and the bonding strength. Based on this relationship, it can guide the preparation of bonded wafers using the method of this application. The approximate bonding strength can be calculated by testing the water droplet contact angle on the surface of piezoelectric wafers and substrate wafers. Some piezoelectric wafers and substrate wafers that obviously do not meet the bonding requirements can be eliminated or optimized, thereby improving production efficiency and reducing process losses. Furthermore, piezoelectric wafers and substrate wafers can be prepared in a targeted manner according to different bonding strength requirements, and the processing schemes of steps (2) and (3) can be adjusted more quickly according to the requirements.

[0043] Optionally, the number of particles larger than 3 μm on the surface of the piezoelectric wafer obtained in step (3) is less than 10ea, and the number of particles larger than 3 μm on the surface of the substrate wafer is less than 10ea.

[0044] After step (2), the surface cleanliness of the piezoelectric wafer and the substrate wafer decreases. Vacuum bonding can improve their cleanliness and achieve effective bonding. However, under atmospheric bonding, this decrease in cleanliness will affect the bonding effect between the piezoelectric wafer and the substrate wafer, not only reducing the bonding strength but also causing Newton's rings to form on the bonding surface due to particles adhering to the wafer surface. Therefore, step (3) above can improve the surface cleanliness of the piezoelectric wafer and the substrate wafer, which is more conducive to their bonding under atmospheric conditions.

[0045] Optionally, the warpage of the piezoelectric wafer obtained in step (1) is Warp < 20 μm, 0 < Bow < 15 μm, and the warpage of the substrate wafer is Warp < 20 μm, 0 < Bow < 15 μm.

[0046] After the treatment in the above step (3), the crystallization quality of the first oxide layer and the second oxide layer formed on the surfaces of the piezoelectric wafer and the substrate wafer is improved, and the expansion coefficients and lattice constants of the first oxide layer and the second oxide layer are closer, so they are more easily matched during bonding to obtain a bonded wafer with better quality.

[0047] Optionally, after the pre-bonding in step (4), the chemical bonds at the bonding interface between the piezoelectric wafer and the substrate wafer include:

[0048] If the gas is O2, Ar, the chemical bonds at the bonding interface include Si-OH, Ta-OH, Nb-OH and a small amount of Si-O-Ta, Si-O-Nb; [[ID=​​​​​​​​​​​​​​​​​​​​The maximum difference in bond strength between regions is within 10%. Maximum difference = (maximum bond strength - minimum bond strength) / average bond strength × 100%.

[0056] Optionally, the warpage of the bonded wafer is Warp < 30, TTV < 5 μm, and 0. <Bow<20。

[0057] The beneficial effects of this application include, but are not limited to:

[0058] 1. The low-temperature bonding method for improving wafer bonding strength according to this application enables the bonding of lithium niobate or lithium tungstate piezoelectric wafers in an atmospheric environment, filling the gap that such piezoelectric wafers cannot be bonded in an atmospheric environment, and the bonding strength can be improved by more than 60% compared with vacuum bonding.

[0059] 2. According to the low-temperature bonding method for improving wafer bonding strength of this application, step (1) involves plasma activation treatment of the piezoelectric wafer and the substrate wafer, which can increase the number and distribution uniformity of hydroxyl groups and dangling bonds on the surface of both wafers, thereby increasing the number and distribution uniformity of covalent bonds formed at the interface after bonding, and thus improving bonding uniformity while increasing bonding strength.

[0060] 3. According to the low-temperature bonding method for improving wafer bonding strength of this application, step (2) can further control the number of hydroxyl groups on the surface of the piezoelectric wafer and the substrate wafer, improve the bonding strength and avoid defects such as voids. In addition, the first oxide layer formed in this step can reduce the difference in expansion coefficient and lattice constant between the piezoelectric wafer and the substrate wafer, and further improve the performance stability of the bonded wafer.

[0061] 4. According to the low-temperature bonding method for improving wafer bonding strength of this application, the bonding process can eliminate vacuum time, improve bonding efficiency, control costs, and avoid the decrease in the number of hydroxyl groups and dangling bonds on the surface of piezoelectric wafers and substrate wafers caused by vacuuming, thereby improving bonding strength.

[0062] 5. According to the bonding wafer of this application, the bonding strength and bonding uniformity are significantly improved, the stress at the bonding interface is reduced, and the warpage is also reduced, so as to improve the stability of the device prepared by the bonding wafer. Attached Figure Description

[0063] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0064] Figure 1 This is a comparison image of the surface profile of the piezoelectric wafer before and after plasma-enhanced chemical vapor deposition in Embodiment 3 of this application;

[0065] Figure 2 This is a schematic diagram of the structure of the piezoelectric wafer and the substrate wafer after plasma-enhanced chemical vapor deposition according to Embodiment 3 of this application;

[0066] Figure 3 This is a schematic diagram of the structure of the piezoelectric wafer and the substrate wafer after plasma activation in Example 3;

[0067] Figure 4 This is a schematic diagram of the structure of the piezoelectric wafer and the substrate wafer after cleaning, as described in Example 3.

[0068] Figure 5 This is a schematic diagram of the structure of the piezoelectric wafer and the substrate wafer after bonding in Example 3;

[0069] Figure 6 The contact angle variation trend of the wafer after plasma activation under different humidity conditions is shown in Example 3.

[0070] Figure 7 This shows the trend of bonding strength variation of the wafers involved in Example 3 after bonding under different humidity conditions.

[0071] List of components and reference numerals:

[0072] 100, piezoelectric wafer; 110, first thin film layer; 120, first activation layer; 200, substrate wafer; 210, second thin film layer; 220, second activation layer; 230, first oxide layer. Detailed Implementation

[0073] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0074] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this application were all purchased commercially.

[0075] Unless otherwise specified, the methods used in the embodiments and comparative examples of this application are conventional methods in the prior art.

[0076] Example 1

[0077] This embodiment discloses a low-temperature bonding method for improving wafer bonding strength, comprising the following steps:

[0078] (1) Spin-coating 1μm photoresist onto the bonding surface of the piezoelectric wafer and heating to cure it, and performing plasma-enhanced chemical vapor deposition on the non-bonding surface of the piezoelectric wafer to form a SiO2 layer (first thin film layer) with a thickness of 600nm on the surface of the piezoelectric wafer; the plasma-enhanced chemical vapor deposition temperature is 300℃, RF (radio frequency power) is 70W, vacuum degree is 200Pa, heating and cooling rate is 5℃ / min, silane:nitrogen:laughing gas ratio is 36:300:700, unit is sccm, time is 10min; after immersion in acetone solvent and ultrasonic-assisted removal of photoresist, it is cleaned with deionized water and dried with nitrogen;

[0079] (2) Plasma activation treatment is performed on the bonding surface of the piezoelectric wafer and the substrate wafer. A first activation layer is formed on the piezoelectric wafer 1 and a second activation layer is formed on the substrate wafer. The piezoelectric wafer is selected from lithium niobate and the substrate wafer is selected from silicon. Ar is used for plasma activation. The vacuum degree of the plasma activation chamber is 60 Pa. The plasma gas flow rate is 50 sccm. The power of the dual-frequency plasma source is 100 W for the high frequency of 27.12 MHz and 50 W for the low frequency of 13.56 MHz. The process time is 15 s.

[0080] (3) The activated piezoelectric wafer and substrate wafer are cleaned and dried. After cleaning and drying, a first oxide layer with a thickness of 2.4 nm is formed on the surface of the substrate wafer 200. The cleaning is specifically carried out by using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O=1:5:10 and immersing the piezoelectric wafer and substrate wafer at a temperature of 65℃ for 30 min.

[0081] (4) The first oxide layer and the piezoelectric wafer are bonded together at a depth of 0.8 × 10⁻⁶. 5 Pre-bonding was carried out at Pa and 70% humidity;

[0082] (5) The prebonded wafer is heated at 0.5℃ / min and first held at five holding points of 80℃, 90℃, 100℃, 110℃ and 120℃ for 1h. Then it is heated to the annealing temperature of 150℃ and held for 10h. After annealing, it is cooled to room temperature at a cooling rate of 5℃ to obtain the bonded wafer.

[0083] Example 2

[0084] This embodiment discloses a low-temperature bonding method for improving wafer bonding strength, comprising the following steps:

[0085] (1) Spin-coating 2μm photoresist onto the bonding surface of the piezoelectric wafer and heating to cure it, and performing plasma-enhanced chemical vapor deposition on the non-bonding surface of the piezoelectric wafer to form a 200nm thick SiO2 layer (first thin film layer) on the surface of the piezoelectric wafer; the plasma-enhanced chemical vapor deposition temperature is 180℃, RF (radio frequency power) is 200W, vacuum degree is 100Pa, heating and cooling rate is 3℃ / min, silane:nitrogen:laughing gas ratio is 36:300:700, unit is sccm, time is 3min; after immersion in acetone solvent and ultrasonic-assisted removal of photoresist, it is cleaned with deionized water and dried with nitrogen;

[0086] (2) Plasma activation treatment is performed on the bonding surface of the piezoelectric wafer and the substrate wafer. A first activation layer is formed on the piezoelectric wafer 1 and a second activation layer is formed on the substrate wafer. The piezoelectric wafer 100 is selected from lithium tantalum and the substrate wafer 200 is selected from sapphire. O2 is used for plasma activation. The vacuum degree of the plasma activation chamber is 10 Pa. The plasma gas flow rate is 200 sccm. The power of the dual-frequency plasma source is 40 W for the high frequency of 27.12 MHz and 10 W for the low frequency of 13.56 MHz. The process time is 90 s.

[0087] (3) The activated piezoelectric wafer and substrate wafer are cleaned and dried. After cleaning and drying, a first oxide layer with a thickness of 2.3 nm is formed on the surface of the substrate wafer 200. The cleaning is specifically carried out by using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O=1:10:20 and immersing the piezoelectric wafer and substrate wafer at a temperature of 100℃ for 30 min.

[0088] (4) The first oxide layer and the piezoelectric wafer are bonded together at a depth of 1.2 × 10⁻⁶. 5 Pre-bonding was performed at 50% humidity and at pa;

[0089] (5) The prebonded wafer is heated at 5℃ / min and held at two holding points of 80℃ and 100℃ for 10h respectively. Then it is heated to the preset annealing temperature of 110℃ and held for 30h. After annealing, it is cooled to room temperature at a cooling rate of 0.1℃ to obtain the bonded wafer.

[0090] Example 3

[0091] refer to Figure 2-5 This embodiment discloses a low-temperature bonding method for improving wafer bonding strength, comprising the following steps:

[0092] (1) A 1 μm photoresist was spin-coated onto the bonding surfaces of the piezoelectric wafer 100 and the substrate wafer 200 and cured by heating. Plasma-enhanced chemical vapor deposition was performed on the non-bonding surfaces of the piezoelectric wafer 100 and the substrate wafer 200 to form a first thin film layer 110 and a second thin film layer 210 on the piezoelectric wafer 100 and the substrate wafer 200, respectively. Both the first thin film layer 110 and the second thin film layer 210 are SiO2 layers with a thickness of 400 nm. The temperature of the plasma-enhanced chemical vapor deposition was 200 °C, the RF (radio frequency power) was 150 W, the vacuum degree was 150 Pa, the heating and cooling rate was 4 °C / min, the ratio of silane: nitrogen: nitrous oxide was 36:300:700 (in sccm), and the time was 5 min. The photoresist was removed by immersion in acetone solvent and ultrasonic assisted removal, followed by cleaning with deionized water and drying with nitrogen.

[0093] (2) Plasma activation treatment is performed on the bonding surface of piezoelectric wafer 100 and substrate wafer 200. A first activation layer 120 is formed on piezoelectric wafer 100 and a second activation layer 220 is formed on substrate wafer 200. Piezoelectric wafer 100 is selected from lithium niobate and substrate wafer 200 is selected from silicon carbide. N2 is used for plasma activation. The vacuum degree of the plasma activation chamber is 40 Pa. The plasma gas flow rate is 150 sccm. The power of the dual-frequency plasma source is 80 W for the high frequency of 27.12 MHz and 40 W for the low frequency of 13.56 MHz. The process time is 60 s.

[0094] (3) The activated piezoelectric wafer 100 and substrate wafer 200 are cleaned with SC-1 and dried. After cleaning and drying, a first oxide layer 230 with a thickness of 2.6 nm is formed on the surface of the second activation layer 220 of the substrate wafer 200. The cleaning is specifically performed by using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O=1:8:15 and immersing the piezoelectric wafer and substrate wafer at a temperature of 90℃ for 30 min.

[0095] (4) The first oxide layer 230 and the first activation layer 120 of the piezoelectric wafer 100 are bonded together at a depth of 1.0 × 10⁻⁶ mm. 5 Pre-bonding was performed at 60% humidity and at pa;

[0096] (5) The prebonded wafer is heated at 0.5℃ / min and held at three holding points of 80℃, 100℃ and 120℃ for 6 hours. Then it is heated to the preset annealing temperature of 140℃ and held for 20 hours. After annealing, it is cooled to room temperature at a cooling rate of 0.5℃ / min to obtain the bonded wafer.

[0097] Example 4

[0098] The difference between this embodiment and embodiment 3 is that the plasma gas flow rate for plasma activation in step (2) is 250 sccm.

[0099] Example 5

[0100] The difference between this embodiment and embodiment 3 is that in step (2), the power of the plasma emitter at the 27.12MHz high frequency is 120W and the power at the 13.56MHz low frequency is 60W.

[0101] Example 6

[0102] The difference between this embodiment and embodiment 3 is that in step (5), the pre-bonded wafer is heated to the annealing temperature of 140°C at a rate of 0.5°C / min, held at that temperature for 20 hours, and then cooled to room temperature at a rate of 0.5°C / min to obtain the bonded wafer.

[0103] Example 7

[0104] The difference between this embodiment and embodiment 3 is that in step (5), the pre-bonded wafer is annealed at a heating rate of 10°C / min.

[0105] Example 8

[0106] The difference between this embodiment and embodiment 3 is that in step (5), after annealing, the temperature is reduced to room temperature at a rate of 10°C to obtain a bonded wafer.

[0107] Comparative Example 1

[0108] The difference between this comparative example and Example 3 is that step (1) was not performed on the piezoelectric wafer and the substrate wafer.

[0109] Comparative Example 2

[0110] The difference between this comparative example and Example 3 is that, in step (1): a 1 μm photoresist is spin-coated onto the bonding surface of the piezoelectric wafer and the substrate wafer and heated to cure; plasma-enhanced chemical vapor deposition is performed on the non-bonded surface of the piezoelectric wafer to form a SiO2 layer with a thickness of 800 nm; the plasma-enhanced chemical vapor deposition temperature is 200 °C, the RF (radio frequency power) is 150 W, the vacuum degree is 150 Pa, the heating and cooling rate is 4 °C / min, the ratio of silane: nitrogen: nitrous oxide is 36:300:700 (in sccm), and the time is 10 min; after immersion in acetone solvent and ultrasonic-assisted removal of the photoresist, the wafer is cleaned with deionized water and dried with nitrogen.

[0111] Comparative Example 3

[0112] The difference between this comparative example and Example 3 is that step (3) was not performed on the piezoelectric wafer and the substrate wafer.

[0113] Comparative Example 4

[0114] The difference between this comparative example and Example 3 is that the humidity in step (4) is 40%.

[0115] Comparative Example 5

[0116] The difference between this comparative example and Example 3 is that the humidity in step (4) is 80%.

[0117] Test Example 1

[0118] The surface contact angle and root mean square roughness of the piezoelectric wafers and substrate wafers prepared in steps (1) and (3) of the above embodiments and comparative examples were tested. The test results are shown in Table 1. In Table 1, " / " represents the same data as in Example 3.

[0119] Table 1

[0120]

[0121] Figure 1 These are comparison images of the piezoelectric wafer in Example 3 before and after plasma-enhanced chemical vapor deposition. Figure 1 Image (a) shows the surface profile before plasma-enhanced chemical vapor deposition. Figure 1 Image (b) shows the surface profile after plasma-enhanced chemical vapor deposition. Figure 1 As can be seen, plasma-enhanced chemical vapor deposition on piezoelectric wafers can generate a tensile stress on the back side of the wafer, thereby causing the front side of the wafer to bulge, which facilitates subsequent bonding.

[0122] Test Example 2

[0123] Using the methods described in the above embodiments and comparative examples, 50 bonded wafers were prepared for each embodiment or comparative example. The pass rate and average number of defects were statistically analyzed, and the bond strength, warp, bow, and TTV of the final bonded wafers were calculated. The test results are shown in Table 2. Table 2 shows any two 10cm sections after the bonded wafers have been trimmed by 3mm. 2 The maximum difference in bond strength between regions = (maximum bond strength - minimum bond strength) / average bond strength × 100%.

[0124] In Table 2, the pass rate is calculated as (number of qualified wafers / 50) × 100%.

[0125] The average defect rate in Table 2 is calculated as the total number of Newton's rings and cracks in 50 bonded wafers / 50. For example, in Example 2, the number of Newton's rings in 50 bonded wafers is 2, the number of cracks is 4, and the total number of Newton's rings and cracks is 6. Therefore, the average defect rate in Example 2 is 6 / 50 = 0.12.

[0126] Table 2

[0127]

[0128] In Comparative Example 2 of Table 2, the warpage of the piezoelectric wafer and the substrate wafer is too large, making bonding impossible. Therefore, the parameters in Table 2 are difficult to obtain.

[0129] Using the preparation method of Example 3, the humidity in step (4) was changed, and bonding was performed at humidity levels of 20-80%. The contact angle and bonding strength of the bonded wafers after plasma activation were tested under different humidity levels. The results of the contact angle changes after plasma activation under different humidity levels are shown in […]. Figure 6 ;

[0130] The results of the bonding strength variation trend after plasma activation under different humidity conditions are shown in the figure. Figure 7 .from Figure 6 , Figure 7 It can be seen that the contact angle gradually decreases with the increase of humidity, and reaches a plateau at 60% humidity. As the contact angle decreases, the bonding strength also gradually increases, reaching its highest value at 60% humidity. However, when the humidity is greater than 60%, although the contact angle does not change much, the bonding strength decreases. This is because too many water molecules remain on the bonding surface, resulting in insufficient interfacial chemical reaction during annealing.

[0131] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A low temperature bonding method characterized by, The method comprises the following steps: (1) performing plasma enhanced chemical vapor deposition on the non-bonding surface of the piezoelectric wafer and / or the substrate wafer to form a SiO2 layer with a thickness of 200-600 nm; The temperature of the plasma enhanced chemical vapor deposition is 180-300 ℃, the radio frequency power is 70-200 W, the vacuum degree is 100-200 Pa, the heating and cooling rate is 3-5 ℃ / min, and the silane: The ratio of the nitrogen gas to the laughing gas is 36:300:700, the unit is sccm, and the time is 3-10 min; (2) performing plasma activation treatment on the bonding surface of the piezoelectric wafer and the substrate wafer, wherein the piezoelectric wafer is selected from at least one of lithium tantalate or lithium niobate, and the substrate wafer is selected from at least one of silicon, silicon carbide or sapphire; (3) cleaning and blowing dry the activated piezoelectric wafer and the substrate wafer, and forming a first oxide layer with a thickness of at least 1.5 nm on the surface of the cleaned and blown dry substrate wafer; (4) The first oxide layer and the piezoelectric wafer are attached at 0.8 x 10 5 -1.2 x 10 5 under 50% - 70% humidity; (5) performing annealing treatment on the pre-bonding wafer to obtain a bonding wafer; The specific operation of the annealing treatment is as follows: heating from room temperature to a preset annealing temperature of 110-150 ℃ at a heating rate of 0.5-5 ℃ / min, maintaining for 10-30 h, and then cooling from the preset annealing temperature to room temperature at a cooling rate of 0.1-5 ℃ / min; 2-5 holding points are set during the heating process, each holding point is maintained for 1-10 h, and the temperature gradient between each holding point is 10-40 ℃; The average contact angle of the piezoelectric wafer surface and the substrate wafer surface obtained in step (3) is θ The bonding strength is wherein σ is the bond strength in MPa; θ contact angle, in °; σ max Maximum bond strength for the theoretical maximum, in MPa; k is an attenuation coefficient, and the value range is 0.05-0.

5.

2. The method of claim 1, wherein, After the activation treatment of step (2), the contact angle of the surface of the piezoelectric wafer is 2°-6°; The contact angle of the surface of the substrate wafer is 2°-5°.

3. The method of claim 1, wherein, The activation treatment operation of step (2) is specifically as follows: The activation is performed using one or more mixed gases of N2, O2 and Ar, the vacuum degree of the plasma activation chamber is 10-60 Pa, the flow rate of the used plasma gas is 50-200 sccm, a double-frequency plasma source is adopted, the power of the 27.12 MHz high frequency is 40-100 W, the power of the 13.56 MHz low frequency is 10-50 W, and the process time is 15-90 s.

4. The method of claim 1, wherein, The contact angle of the surface of the piezoelectric wafer obtained in step (3) is less than 4°; The contact angle of the surface of the substrate wafer is less than 4°.

5. The method of claim 1, wherein, The Warp of the piezoelectric wafer obtained in step (1) is less than 20 μm, 0 < Bow < 15 μm, and the Warp of the substrate wafer is less than 20 μm, 0 < Bow < 15 μm.

6. The method of claim 1, wherein, After the pre-bonding of step (4), the chemical bonds at the bonding interface of the piezoelectric wafer and the substrate wafer include: If the plasma activation gas is O2 and Ar, the chemical bonds at the bonding interface include Si-OH, Ta-OH, Nb-OH and a small amount of Si-O-Ta and Si-O-Nb; If the plasma activation gas is N2, the chemical bonds at the bonding interface include Si-OH, Ta-OH, Nb-OH and a small amount of Si-O-Ta and Si-O-Nb.

7. The bonding wafer obtained by the low-temperature bonding method according to any one of claims 1-6.

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