Low-temperature bonding method and bonded wafer

Abstract

The invention discloses a low-temperature bonding method and a bonded wafer, and belongs to the technical field of wafer bonding. The method comprises the following steps of: (1) performing plasma enhanced chemical vapor deposition on a non-bonding surface of a substrate wafer and / or a piezoelectric wafer to form a SiO2 layer; (2) performing plasma activation treatment on the piezoelectric wafer and the substrate wafer; (3) cleaning and blow-drying the activated piezoelectric wafer and the substrate wafer, and forming an oxide layer with the thickness of at least 1.5 nm on the surface of the cleaned and blow-dried substrate wafer; (4) laminating the piezoelectric wafer obtained in the step (3) with a substrate wafer, and carrying out pre-bonding under 0.9 * 10 < 5 >-1.2 * 10 < 5 > Pa and the humidity of 50%-70%; and (4) annealing the pre-bonded wafer to obtain a bonded wafer. According to the method, the bonding strength can be effectively improved, defects such as holes are avoided, and the performance stability of the bonded wafer is improved.

Description

TECHNICAL FIELD

[0001] The application relates to a low-temperature bonding method and a bonded wafer, and belongs to the technical field of wafer bonding. BACKGROUND

[0002] Wafer direct bonding technology can combine wafer materials of different materials through atomic contact to form strong covalent bonds. Compared with traditional bonding methods, direct bonding technology can complete wafer bonding at low temperatures, overcoming the problems of thermal expansion coefficient and lattice constant mismatch, and is an important technology for heterogeneous integration.

[0003] At present, the mainstream direct bonding technology mainly includes two kinds. One is surface activated bonding, which adopts Ar ion beam to activate the bonding surface of the wafer and then completes the direct bonding of the wafer at low temperature. The other is plasma activated bonding, which adopts O2, N2, Ar plasma to activate the bonding surface of the wafer and then pre-bonds at low temperature and anneals.

[0004] Plasma activated bonding belongs to hydrophilic bonding, and the bonding strength and the number of hydroxyl groups on the bonding surface are positively correlated, so increasing the number of hydroxyl groups on the wafer surface can effectively improve the wafer bonding strength. In order to ensure the cleanliness of the bonding surface, the traditional PAB bonding method often needs to be pre-bonded in a vacuum environment. This process not only reduces the bonding efficiency and increases the cost due to the vacuum step, but more importantly, the continuous vacuum environment consumes the hydroxyl groups on the wafer surface, resulting in a decrease in the number of covalent chemical bonds formed at the bonding interface, thereby weakening the bonding strength.

[0005] If it can be directly bonded in an atmospheric environment, the above problems will be solved. The prior art CN110473778B provides a method for directly bonding zirconia and aluminum oxide using plasma activation, which activates the ZrO2 and Al2O3 wafers to be bonded by using O2 / NH3 / H2O plasma, and then bonds them in an atmospheric environment with a humidity of 20-80%, which is the first time to realize the atmospheric bonding of two heterogeneous wafers.

[0006] The applicant found that when this method is extended to piezoelectric wafers such as lithium tantalate and lithium niobate wafer materials, using the above plasma treatment and bonding at the above humidity, firstly, there are many particles on the wafer surface, which makes it difficult to ensure the cleanliness of the bonding surface, thereby reducing the wafer bonding strength; secondly, there is a problem of air bubble defects caused by gas accumulation on the bonding surface, so it is difficult to prepare an ideal piezoelectric composite substrate under the above bonding conditions, and large-scale production cannot be carried out. SUMMARY

[0007] To solve the above problems, a low-temperature bonding method is provided, which can realize bonding under atmosphere by forming a SiO2 layer on the non-bonding surface of the piezoelectric wafer and the substrate wafer through plasma-enhanced chemical vapor deposition (PECVD) of the non-bonding interface, plasma activation and cleaning treatment, effectively preventing the accumulation of gas at the bonding interface to form bubble defects. The above measures work together to significantly improve the strength and performance stability of the bonded wafer.

[0008] According to one aspect of the present application, a low-temperature bonding method is provided, comprising the following steps: (1) Plasma-enhanced chemical vapor deposition (PECVD) is performed 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; (2) Plasma activation treatment is performed on the piezoelectric wafer and the substrate wafer using an activation gas, wherein the piezoelectric wafer is selected from lithium tantalate or lithium niobate, and the substrate wafer is selected from at least one of silicon, silicon oxide, silicon carbide, and sapphire; (3) The activated piezoelectric wafer and the substrate wafer are cleaned and dried, and a first oxide layer with a thickness of at least 1.5 nm is formed on the surface of the cleaned and dried substrate wafer; (4) The first oxide layer and the second oxide layer are bonded, and the bonding wafer is pre-bonded under a pressure of 0.8x10 5 -1.2x10 5 pa and 50%-70% humidity; (5) Annealing treatment is performed on the pre-bonded wafer to obtain a bonded wafer.

[0009] In step (1) of the present application, plasma-enhanced chemical vapor deposition (PECVD) is performed 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. After the PECVD deposition forms the SiO2 layer, the piezoelectric wafer and the substrate wafer shrink much more than the SiO2 layer when cooled to room temperature, resulting in a tensile stress on the back surface of the piezoelectric wafer and the substrate wafer, which causes the front surface of the piezoelectric wafer and the substrate wafer to protrude. This solves the problem of gas bubble defects caused by residual gas at the bonding surface under high gas pressure. Therefore, this operation realizes convex bonding under atmospheric environment, effectively prevents the accumulation of gas at the bonding interface to form bubble defects, facilitates bonding under the conditions of step (4), and significantly improves the strength and performance stability of the bonded wafer. The method of forming a SiO2 layer through PECVD to realize convex bonding has the advantages of easier implementation, easier operation, lower cost, and no lattice damage to the wafer for methods such as ion implantation, solution corrosion, and physical application of external force to change the warping of the piezoelectric wafer or the substrate wafer.

[0010] The step (3) operation on the piezoelectric wafer and the substrate wafer after the plasma activation treatment can effectively remove the surface organic matter and contamination, can improve the cleanliness and the number of hydroxyl groups of the bonding interface, and can improve the bonding strength; can control the number and distribution uniformity of the hydroxyl groups and the dangling bonds of the bonding interface, so as to inhibit the generation of hydrogen in the bonding process on the basis of improving the bonding strength and the bonding uniformity, thereby avoiding the defects such as the voids in the bonding interface, and improving the quality of the bonded wafer; and the surfaces of the piezoelectric wafer and the substrate wafer are formed with the oxide layer, the existence of the oxide layer can reduce the thermal expansion coefficient and the lattice constant difference of the piezoelectric wafer and the substrate wafer, not only can improve the bonding strength, but also can effectively inhibit the sound wave energy leakage as an insulation layer, and can improve the reliability of the downstream device.

[0011] The cleaning of the step (3) is SC-1 cleaning, the first oxide layer formed on the bonding surface can further improve the plasma activation effect, and can further reduce the thermal expansion coefficient difference of the piezoelectric wafer and the substrate wafer as a buffer layer, at the same time, the first oxide layer can help to inhibit the generation of hydrogen, and can provide an escape channel for the hydrogen released in the annealing stage, and can double avoid the formation of void defects.

[0012] In addition, the present application finds that in order to improve the bonding strength by increasing the plasma activation power and time, surface damage will inevitably occur to the piezoelectric wafer and the substrate wafer, which will cause the surface roughness to increase and the defect density to increase, and will also make the bonding strength decrease, therefore, the activation effect and the effect of forming the oxide layer are in an opposite relationship with the surface damage, and the bonding strength changes under the existence of the opposite relationship. The piezoelectric wafer and the substrate wafer treated by the above-mentioned steps (2) and (3) can balance the surface damage of the piezoelectric wafer and the substrate wafer and the activation effect and the cleaning effect, so as to obtain the optimal bonding strength.

[0013] The pre-bonding of the step (4) is to add a fan filter unit (FFU) and an electrostatic elimination device in the existing bonding equipment, the internal environment cleanliness can be strictly controlled within ISO 5 level, the wafer can be effectively prevented from being polluted by particles in the atmospheric environment during the handling and bonding process, and the bonding strength and the quality of the bonded body can be improved.

[0014] Optionally, the thickness of the first oxide layer is at least 2.3 nm.

[0015] 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 of the hydrogen peroxide oxidation layer is generally not more than this value, if the first oxide layer is too thin, the bonding effect cannot be obviously improved.

[0016] Optionally, the temperature of the plasma enhanced chemical vapor deposition is 180-300℃, the radio frequency power (RF) is 70-200W, the vacuum degree is 100-200Pa, the rate of temperature rise and fall is 3-5℃ / min, the ratio of silane, nitrogen and laughing gas is 36:300:700 (unit: sccm), and the time is 3-10min.

[0017] The parameters of the above plasma enhanced chemical vapor deposition have the following advantages: 1) The deposition rate of SiO2 with the same thickness is increased by more than 50% by sacrificing the film quality.

[0018] 2) The process temperature is controlled in a relatively safe range (180-300℃), which prevents the piezoelectric wafer or substrate wafer from being broken due to excessive stress during the cooling process.

[0019] The deposition at 180-300℃ can ensure the yield, and the use amount of silane, nitrogen and laughing gas can speed up the deposition rate. The processing time needs to be selected according to the different thermal expansion coefficients of the wafers to control the size of bow.

[0020] Optionally, before the plasma enhanced chemical vapor deposition in step (1), the operation of spin coating 1-2μm of photoresist on the bonding surface of the piezoelectric wafer and / or substrate wafer and heating and curing is further included. After the plasma enhanced chemical vapor deposition in step (1) forms the SiO2 layer, the photoresist is removed by soaking in acetone solvent and ultrasonic assistance, and then ionized water is used for cleaning and nitrogen is used for drying, and then step (2) is performed.

[0021] The spin coating of photoresist before step (1) can protect the bonding surface, avoid the influence of plasma enhanced chemical vapor deposition on the subsequent processing of the bonding surface, and improve the bonding effect.

[0022] Optionally, after the activation treatment in 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°.

[0023] Optionally, the activation treatment operation in step (2) is specifically as follows: The gas used for activation is one or more of N2, O2 and Ar mixed gas, the vacuum degree of the plasma activation chamber is 10-60Pa, the flow rate of the plasma gas used is 50-200sccm, the power of the 400k high frequency plasma emitter is 40-100W, the power of the 40k low frequency plasma emitter is 10-50W, and the process time is 15-90s.

[0024] The activation treatment can increase the number of hydroxyl groups and dangling bonds on the surfaces of the piezoelectric wafer and the substrate wafer, and can effectively remove surface particles and organic matter of the piezoelectric wafer and the substrate wafer, and reduce the surface roughness, thereby laying a foundation for the subsequent step (2).

[0025] The activation effect of the plasma activation on the wafer surface is limited, and if the processing time is too long, the wafer surface roughness will increase and the defect points will increase due to over-activation. Therefore, the cleaning operation in step (3) can further control the number of hydroxyl groups and dangling bonds on the surface of the oxide layer on the basis of reducing the surface damage of the piezoelectric wafer and the substrate wafer, so as to improve the bonding strength and avoid the generation of hydrogen in the bonding process, thereby reducing the probability of defects such as cavities in the bonded wafer.

[0026] Optionally, the cleaning in step (3) is specifically: using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O = 1:(5-10):(10-20), and soaking the piezoelectric wafer and the substrate wafer at a temperature of 65-100℃ for 30min.

[0027] Optionally, 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°.

[0028] Optionally, the average water droplet contact angle of the surface of the piezoelectric wafer obtained in step (2) and the surface of the substrate wafer is 0°-4°. θ Therefore, the bonding strength is σ = σ max • e -kθ wherein σ is the bonding strength, and the unit is MPa; θ is the water droplet contact angle, and the unit is °; σ max is the theoretical maximum bonding strength, and the unit is MPa; k is the attenuation coefficient, and the value range is 0.05-0.5.

[0029] The application establishes the relationship between the number of hydroxyl groups on the surfaces of the piezoelectric wafer and the substrate wafer and the bonding strength, and based on the relationship, the bonding wafer prepared by the method of the application can be guided. The approximate bonding strength can be calculated by testing the water droplet contact angle of the surface of the piezoelectric wafer and the substrate wafer, and some piezoelectric wafers and substrate wafers that do not meet the bonding requirements can be rejected or optimized, thereby improving the production efficiency and reducing the process loss. In addition, piezoelectric wafers and substrate wafers can be prepared according to different bonding strength requirements, and the processing scheme of steps (2) and (3) can be adjusted according to the requirements.

[0030] Optionally, the number of particles greater than 3 μm on the surface of the piezoelectric wafer obtained in step (3) is less than 10 ea, and the number of particles greater than 3 μm on the surface of the substrate wafer is less than 10 ea.

[0031] After the piezoelectric wafer and the substrate wafer are treated in step (2), the cleanliness of the surfaces thereof is reduced, and the cleanliness can be improved by using vacuum bonding to achieve effective bonding. However, the reduced cleanliness will affect the bonding effect of the piezoelectric wafer and the substrate wafer in atmospheric bonding, which not only reduces the bonding strength, but also causes Newton's ring on the bonding surface due to the particles attached to the surface of the wafer. Therefore, the cleanliness of the surfaces of the piezoelectric wafer and the substrate wafer is improved after the treatment in step (3), which is more conducive to the bonding of the two wafers in the atmosphere.

[0032] Optionally, the warpage of the piezoelectric wafer obtained in step (1) is Warp < 20 μm, and 0 < Bow < 15 μm, and the warpage of the substrate wafer is Warp < 20 μm, and 0 < Bow < 15 μm.

[0033] After the treatment in step (3), the crystalline 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 coefficient and the lattice constant of the first oxide layer and the second oxide layer are closer, so that the bonding is more easily matched to obtain a better bonded wafer.

[0034] Optionally, after the pre-bonding in step (4), the chemical bonds at the bonding interface of the piezoelectric wafer and the substrate wafer include: If the gas is O2 or 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 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.

[0035] Optionally, the annealing treatment in step (5) is specifically as follows: The temperature is increased from room temperature to a preset annealing temperature of 110-150 °C at a temperature increasing rate of 0.5-5 °C / min, and then the temperature is decreased from the preset annealing temperature to room temperature at a temperature decreasing rate of 0.1-5 °C / min. During the temperature increasing process, 2-5 temperature holding points are set, each point is held for 1-10 h, and the temperature gradient between each temperature holding point is 10-40 °C.

[0036] The temperature uniformity of the wafer at different points in the above annealing process is maintained at ±1 °C.

[0037] The annealing heat treatment can make the hydrogen bond of the bonding interface contract into strong covalent bond, enhance the bonding force, and also can make the atoms rearrange, eliminate defects, release the interface stress, and improve the quality uniformity of the piezoelectric wafer and the substrate wafer. In addition, the gradient annealing, the appropriate temperature rising and falling rate, and the lower annealing temperature can also ensure the stability and reliability of the bonding, and avoid the wafer warping and cracking caused by the wafer thermal mismatch.

[0038] According to another aspect of the present application, the bonded wafer obtained by the low-temperature bonding method is provided.

[0039] Optionally, the bonded wafer has a maximum difference of bonding strength of any two 10 cm 2 regions within 10%, and the maximum difference of bonding strength = (maximum bonding strength - minimum bonding strength) / average bonding strength x 100%.

[0040] Optionally, the bonded wafer has a warpage of Warp < 30, a TTV < 5 μm, and a Bow < 20.

[0041] The beneficial effects of the present application include but are not limited to: 1. The low-temperature bonding method for improving the wafer bonding strength according to the present application can realize the bonding of lithium tantalate or lithium niobate as a piezoelectric wafer in an atmospheric environment, which makes up for the blank of the bonding of such piezoelectric wafer in an atmospheric environment, and the bonding strength can be improved by more than 60% compared with the vacuum bonding method.

[0042] 2. The low-temperature bonding method for improving the wafer bonding strength according to the present application, the step (1) of performing the plasma activation treatment on the piezoelectric wafer and the substrate wafer can improve the number and distribution uniformity of the surface hydroxyl and dangling bond of the two, so as to improve the number and distribution uniformity of the covalent bond formed at the interface after the bonding, and further improve the bonding strength and the bonding uniformity.

[0043] 3. The low-temperature bonding method for improving the wafer bonding strength according to the present application, the step (2) operation can further control the number of surface hydroxyl of the piezoelectric wafer and the substrate wafer, improve the bonding strength, and avoid the defects such as voids, and in addition, the first oxide layer formed in this step can reduce the difference of the expansion coefficient and the lattice constant of the piezoelectric wafer and the substrate wafer, and further improve the performance stability of the bonded wafer.

[0044] 4. The low-temperature bonding method for improving the wafer bonding strength according to the present application, the bonding process can save the time of vacuum, improve the bonding efficiency, control the cost, and avoid the decrease of the number of surface hydroxyl and dangling bond of the piezoelectric wafer and the substrate wafer caused by the vacuum, so as to improve the bonding strength.

[0045] 5. The bonded wafer according to the present application has significantly improved bonding strength and bonding uniformity, reduced stress at the bonding interface, and reduced wafer bow, thereby improving the stability of the device prepared from the bonded wafer. BRIEF DESCRIPTION OF DRAWINGS

[0046] The accompanying drawings, which are included to provide a further understanding of the present application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and together with the description serve to explain the present application. In the drawings: Figure 1 A comparison of the surface profile of a piezoelectric wafer before and after plasma enhanced chemical vapor deposition according to Embodiment 3 of the present application; Figure 2 A structural schematic of a piezoelectric wafer and a substrate wafer after plasma enhanced chemical vapor deposition according to Embodiment 3 of the present application; Figure 3 A structural schematic of a piezoelectric wafer and a substrate wafer after plasma activation according to Embodiment 3 of the present application; Figure 4 A structural schematic of a piezoelectric wafer and a substrate wafer after cleaning according to Embodiment 3 of the present application; Figure 5 A structural schematic of a piezoelectric wafer and a substrate wafer after bonding according to Embodiment 3 of the present application; Figure 6 A trend of the contact angle of a wafer after plasma activation at different humidity according to Embodiment 3 of the present application; Figure 7 A trend of the bonding strength of a wafer after bonding at different humidity according to Embodiment 3 of the present application.

[0047] List of components and reference numerals: 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 DESCRIPTION

[0048] The present application will be described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0049] Unless otherwise specified, the raw materials in the examples and comparative examples of the present application are purchased through commercial channels.

[0050] Unless otherwise specified, the methods used in the examples and comparative examples of the present application are conventional methods in the prior art.

[0051] Example 1 This embodiment discloses a low-temperature bonding method for improving the bonding strength of a wafer, comprising the following steps: (1) spin 1 pm photoresist on the bonding surface of the piezoelectric wafer and heat to cure, and perform plasma enhanced chemical vapor deposition on the non-bonding surface of the piezoelectric wafer to form a 600 nm-thick SiO2 layer (first thin film layer) on the surface of the piezoelectric wafer; the temperature of the plasma enhanced chemical vapor deposition is 300°C, the RF (radio frequency power) is 70 W, the vacuum degree is 200 Pa, the heating and cooling rate is 5°C / min, the ratio of silane, nitrogen and laughing gas is 36:300:700, in units of sccm, and the time is 10 min; after removing the photoresist by acetone solvent immersion and ultrasonic assistance, the wafer is cleaned with ionized water and dried with nitrogen; (2) perform plasma activation treatment on the bonding surface of the piezoelectric wafer and the substrate wafer, to form a first activation layer on the piezoelectric wafer 1 and a second activation layer 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 used is 50 sccm, the power of the double-frequency plasma source 27.12 MHz high frequency is 100 W, the power of the 13.56 MHz low frequency is 50 W, and the process time is 15 s; (3) clean and dry the activated piezoelectric wafer and substrate wafer, and a 2.4 nm-thick first oxide layer is formed on the surface of the cleaned and dried substrate wafer 200, the cleaning is as follows: using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O = 1:5:10, immersing the piezoelectric wafer and the substrate wafer at a temperature of 65°C for 30 min; (4) bond the first oxide layer and the piezoelectric wafer, and pre-bond at 0.8 x 10 5 Pa and 70% humidity; (5) heat the pre-bonded wafer at 0.5°C / min, first at 80°C, 90°C, 100°C, 110°C, 120°C for 1 h respectively, then to the annealing temperature of 150°C for 10 h, and then cool to room temperature at a rate of 5°C, to obtain a bonded wafer.

[0052] Example 2 The present embodiment discloses a low-temperature bonding method for improving wafer bonding strength, comprising the following steps: (1) spin 2 pm of photoresist on the bonding surface of the piezoelectric wafer and heat and cure, and perform plasma-enhanced chemical vapor deposition on the non-bonding surface of the piezoelectric wafer to form a 200 nm-thick SiO2 layer (first thin film layer) on the surface of the piezoelectric wafer; the temperature of the plasma-enhanced chemical vapor deposition is 180°C, the RF (radio frequency power) is 200 W, the vacuum degree is 100 Pa, the heating and cooling rate is 3°C / min, the ratio of silane, nitrogen and laughing gas is 36:300:700, in units of sccm, the time is 3 min; after removing the photoresist by acetone solvent immersion and ultrasonic assistance, ionized water is used for cleaning and nitrogen is used for drying; (2) perform plasma activation treatment on the bonding surface of the piezoelectric wafer and the substrate wafer, form a first activation layer on the piezoelectric wafer 1 and a second activation layer on the substrate wafer, the piezoelectric wafer 100 is selected from lithium tantalate, 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 used is 200 sccm, the power of the double-frequency plasma source 27.12 MHz high frequency is 40 W, the power of the 13.56 MHz low frequency is 10 W, and the process time is 90 s; (3) clean and dry the activated piezoelectric wafer and substrate wafer, and a 2.3 nm-thick first oxide layer is formed on the surface of the cleaned and dried substrate wafer 200, the cleaning is as follows: a cleaning solution with a volume ratio of NH4OH:H2O2:H2O = 1:10:20 is used, the piezoelectric wafer and the substrate wafer are soaked at a temperature of 100°C for 30 min; (4) bond the first oxide layer and the piezoelectric wafer, and pre-bonding is performed at 1.2 x 10 5 Pa and 50% humidity; (5) increase the temperature of the pre-bonding wafer at a rate of 5°C / min, and keep it at two holding points of 80°C and 100°C for 10 h respectively, then increase it to a preset annealing temperature of 110°C and keep it for 30 h, after annealing, decrease it to room temperature at a rate of 0.1°C, and obtain a bonded wafer.

[0053] Example 3 Reference Figures 2-5 The embodiment discloses a low-temperature bonding method for improving the bonding strength of wafers, comprising the following steps: (1) Spin 1 μm photoresist on the bonding surface of the piezoelectric wafer 100 and the substrate wafer 200 and heat to cure, and perform plasma enhanced chemical vapor deposition on the non-bonding surface 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 is 200℃, the RF (radio frequency power) is 150 W, the vacuum degree is 150 Pa, the heating and cooling rate is 4℃ / min, the ratio of silane, nitrogen and laughing gas is 36:300:700, in units of sccm, the time is 5 min; after removing the photoresist by acetone solvent immersion and ultrasonic assistance, the wafer is cleaned with ionized water and dried with nitrogen; (2) Perform plasma activation treatment on the bonding surface of the piezoelectric wafer 100 and the substrate wafer 200 to form a first activation layer 120 on the piezoelectric wafer 100 and a second activation layer 220 on the substrate wafer 200, the piezoelectric wafer 100 is selected from lithium niobate, and the substrate wafer 200 is selected from silicon carbide; use N2 for plasma activation, the vacuum degree of the plasma activation chamber is 40 Pa, the plasma gas flow used is 150 sccm, the power of the double-frequency plasma source 27.12 MHz high frequency is 80 W, the power of the 13.56 MHz low frequency is 40 W, and the process time is 60 s; (3) After the activated piezoelectric wafer 100 and the substrate wafer 200 are cleaned with SC-1 and dried, a 2.6 nm thick first oxide layer 230 is formed on the surface of the second activation layer 220 of the cleaned and dried substrate wafer 200, and the cleaning is as follows: using a cleaning solution with a volume ratio of NH4OH:H2O2:H2O=1:8:15, immersing the piezoelectric wafer and the substrate wafer at a temperature of 90℃ for 30 min; (4) Bond the first oxide layer 230 and the first activation layer 120 of the piezoelectric wafer 100, and pre-bond at 1.0×10 5 Pa and 60% humidity; (5) Increase the pre-bonded wafer at a rate of 0.5℃ / min, and keep it at 80℃, 100℃ and 120℃ for 6h respectively, and then increase it to a preset annealing temperature of 140℃ and keep it for 20h, and then decrease it to room temperature at a rate of 0.5℃ / min, to obtain a bonded wafer.

[0054] Example 4 The difference between this example and Example 3 is that the plasma gas flow of the plasma activation in step (2) is 250 sccm.

[0055] Example 5 The difference between this embodiment and embodiment 3 is that the power of the plasma emitter in step (2) is 120 W at a high frequency of 27.12 MHz and 60 W at a low frequency of 13.56 MHz.

[0056] Example 6 The difference between this embodiment and embodiment 3 is that the pre-bond wafer is heated to the annealing temperature of 140°C at a rate of 0.5°C / min in step (5), and then cooled to room temperature at a rate of 0.5°C / min, to obtain the bonded wafer.

[0057] Example 7 The difference between this embodiment and embodiment 3 is that the pre-bond wafer is annealed at a rate of 10°C / min in step (5).

[0058] Example 8 The difference between this embodiment and embodiment 3 is that the pre-bond wafer is cooled to room temperature at a rate of 10°C after the annealing is completed, to obtain the bonded wafer.

[0059] Comparative Example 1 The difference between this comparative example and embodiment 3 is that step (1) is not performed on the piezoelectric wafer and the substrate wafer.

[0060] Comparative Example 2 The difference between this comparative example and embodiment 3 is that in step (1), the bonding surface of the piezoelectric wafer and the substrate wafer is spin-coated with 1 μm of photoresist and heated and cured, and the non-bonding surface of the piezoelectric wafer is subjected to plasma-enhanced chemical vapor deposition to form a SiO2 layer with a thickness of 800 nm; the temperature for the plasma-enhanced chemical vapor deposition is 200°C, the RF (radio frequency power) is 150 W, the vacuum degree is 150 Pa, the rate of heating and cooling is 4°C / min, the ratio of silane:nitrogen:nitrous oxide is 36:300:700 in units of seem, and the time is 10 min; after the photoresist is removed using acetone solvent immersion and ultrasonic assistance, ionized water is used for cleaning and nitrogen is used for drying.

[0061] Comparative Example 3 The difference between this comparative example and embodiment 3 is that step (3) is not performed on the piezoelectric wafer and the substrate wafer.

[0062] Comparative Example 4 The difference between this comparative example and embodiment 3 is that the humidity in step (4) is 40%.

[0063] Comparative Example 5 The difference between this comparative example and embodiment 3 is that the humidity in step (4) is 80%.

[0064] Test Example 1 The surface contact angle, surface root mean square roughness of the piezoelectric wafer and the substrate wafer prepared in step (1) and step (3) of the above examples and comparative examples were tested, and the test results are shown in Table 1. In Table 1, " / " represents the same data as that of Example 3.

[0065] Table 1

[0066] Figure 1 The surface profile comparison chart of the piezoelectric wafer of Example 3 before and after plasma enhanced chemical vapor deposition is shown in Figure 1 Fig. (a) is the surface profile chart before plasma enhanced chemical vapor deposition, Figure 1 Fig. (b) is the surface profile chart after plasma enhanced chemical vapor deposition, from Figure 1 As can be seen from Fig., the plasma enhanced chemical vapor deposition on the piezoelectric wafer can produce a tensile stress on the back surface of the wafer, so that the front surface of the wafer is raised, which is convenient for subsequent bonding.

[0067] Test Example 2 Using the method of the above examples and comparative examples, 50 bonded wafers were prepared for each example or comparative example, the pass rate and the average defect number were counted, and the bonding strength, Warp, Bow and TTV of the finally obtained bonded wafer were calculated, and the test results are shown in Table 2. In Table 2, the bonding strength of any two 10 cm 2 The maximum difference of the bonding strength of the bonded wafer after 3 mm edge removal = (maximum bonding strength - minimum bonding strength) / average bonding strength x 100%.

[0068] The pass rate in Table 2 = (the number of qualified wafers / 50) x 100%; The average defect rate in Table 2 = the total number of Newton's rings and cracks in 50 bonded wafers / 50, for example, the number of Newton's rings in 50 bonded wafers in Example 2 is 2, the number of cracks is 4, the total number of Newton's rings and cracks is 6, then the average defect rate of Example 2 = 6 / 50 = 0.12.

[0069] Table 2

[0070] The warpage of the piezoelectric wafer and the substrate wafer of Comparative Example 2 in Table 2 is too large, which leads to the failure of bonding, so the parameters in Table 2 are difficult to obtain.

[0071] Using the preparation method of Example 3, the humidity of step (4) was changed, and bonding was carried out under the humidity of 20-80%, the contact angle change trend and the bonding strength change trend of the bonded wafer after plasma activation under different humidity were tested, and the results of the contact angle change trend after plasma activation under different humidity are shown in Figure 6 ; The results of the bonding strength trend under different humidity after plasma activation are shown in Table 1. Figure 7 As can be seen from Table 1, Figure 6 , Figure 7 the contact angle gradually decreases with the increase of humidity, and reaches a plateau at 60% humidity. With the decrease of the contact angle, the bonding strength gradually increases, and reaches the highest at 60% humidity. However, when the humidity is greater than 60%, although the contact angle changes little, the bonding strength decreases, because too much water molecules on the bonding surface lead to insufficient interface chemical reaction during annealing.

[0072] The above merely illustrates the embodiments of the present application, and the protection scope of the present application is not limited to these specific embodiments, but determined by the claims of the present application. Any modification, equivalent replacement, improvement, etc. within the technical thought and principle of the present application shall be included in the protection scope of the present 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 a piezoelectric wafer and / or a substrate wafer to form a SiO2 layer with a thickness of 200-600 nm; (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, and sapphire; (3) cleaning and drying the activated piezoelectric wafer and the substrate wafer, and forming a first oxide layer on the surface of the cleaned and dried substrate wafer with a thickness of at least 1.5 nm; (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-bonded wafer to obtain a bonded wafer.

2. The method of claim 1, wherein, After the activation treatment in 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 in step (2) is performed as follows: The activation is performed using a mixed gas of one or more of N2, O2, and Ar, the vacuum degree of the plasma activation chamber is 10-60 Pa, the flow rate of the plasma gas used is 50-200 sccm, a double-frequency plasma source is used, 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 <4°; The contact angle of the surface of the substrate wafer is <4°.

5. The method of claim 1, wherein, The average contact angle of the piezoelectric wafer surface and the substrate wafer surface obtained in step (3) is θ The bonding strength is σ = σ max • e -kθ wherein σ is the bond strength in MPa; θ contact angle, in °; σ max Theoretical maximum bond strength, in MPa; k is an attenuation coefficient, and the value range is 0.05-0.

5.

6. The method of claim 1, wherein, In step (1), the temperature for 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 ratio of silane: nitrogen: laughing gas is 36:300:700, in units of sccm, and the time is 3-10 min.

7. The method of claim 1, wherein, In step (1), the warp of the piezoelectric wafer is <20 μm, 0 < Bow < 15 μm, and the warp of the substrate wafer is <20 μm, 0 < Bow < 15 μm.

8. The method of claim 1, wherein, After pre-bonding in step (4), the chemical bonds at the bonding interface of the piezoelectric wafer and the substrate wafer include: If the 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 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.

9. The method of claim 1, wherein, The annealing treatment in step (5) is performed as follows: The temperature is increased from room temperature to a preset annealing temperature of 110-150℃ at a rate of 0.5-5℃ / min, and then the temperature is decreased from the preset annealing temperature to room temperature at a rate of 0.1-5℃ / min, the process is maintained for 10-30 h, 2-5 temperature holding points are set during the heating process, each point is maintained for 1-10 h, and the temperature gradient between each temperature holding point is 10-40℃.

10. The bonded wafer obtained by the low-temperature bonding method according to any one of claims 1-9.

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