Method for bonding wafers stacked with high aspect ratio microstructures

Abstract

The invention provides a method for bonding wafers stacked with high aspect ratio microstructures, a first wafer and a second wafer are provided, the high aspect ratio microstructures are stacked on the surface of the first wafer or the second wafer, and the first wafer and the second wafer are cleaned and dried before being bonded. The cleaning and drying treatment comprises the following steps: pre-cleaning at normal temperature; performing infiltration treatment on the pre-cleaned first wafer and the pre-cleaned second wafer by using an IPA solution, and after the infiltration treatment is completed, putting the first wafer and the second wafer into a supercritical drying cavity, and introducing gas to perform supercritical drying treatment; the cleaned and dried first wafer and second wafer are pre-bonded, and according to the bonding method for the wafers stacked with the high aspect ratio microstructures, the cleaning efficiency is guaranteed, meanwhile, residues of water stains in the high aspect ratio microstructures are reduced, the cleaning effect is improved, and the yield of bonded products is guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of integrated circuit manufacturing technology, and particularly relates to a wafer bonding method for stacking microstructures with high aspect ratios. Background Technology

[0002] In microelectromechanical systems In this structure, the combination of high aspect ratio silicon etching technology and microelectronic processes enables the integration of a series of novel sensors and actuators into the chip. For example, MEMS microphones, speakers, pressure sensors, and accelerometers all require the construction of micromechanical cavities and backplanes with high aspect ratio microstructures to achieve the conversion of vibration and electrical signals. When stacking various microstructure devices using wafer bonding technology, deionized water or other cleaning liquids can easily remain in the microcavities and high aspect ratio structures. Due to the extremely small scale and large aspect ratio of the microstructures on the wafer, the surface tension and capillary action of the liquid are significant, making it difficult to completely remove them through conventional drying methods. The residual liquid will vaporize upon heating during subsequent bonding processes, forming localized high pressure, leading to voids at the interface, decreased bonding strength, or bonding interface failure, severely affecting device performance and yield.

[0003] In existing technologies, to address the aforementioned issues, the drying time is typically increased or combined with prolonged spin-drying after wafer cleaning to reduce the residue of deionized water in the wafer cavity structure. However, in these methods, extending the drying time between pre-cleaning and pre-bonding processes is inefficient due to the low efficiency of natural or hot air drying within the deep cavity structure. This not only fails to completely evaporate residual water but also significantly increases the risk of contamination on the bonding surface, impacting subsequent bonding processes. Furthermore, prolonged spin-drying can damage high aspect ratio microstructures on the wafer, leading to reduced wafer yield.

[0004] The aforementioned problems severely impact the yield of wafer bonding processes with stacked high aspect ratio microstructures, rendering many designs unfeasible. Therefore, it is necessary to develop a wafer bonding method for stacked high aspect ratio microstructures that, while ensuring cleaning efficiency, reduces water residue in the high aspect ratio structure, thereby guaranteeing the quality of subsequent wafer bonding. Summary of the Invention

[0005] This invention provides a wafer bonding method with stacked high aspect ratio microstructures, which reduces water residue in the high aspect ratio structure while ensuring cleaning efficiency, improves cleaning effect, and ensures the yield of the bonded product.

[0006] Other objects and advantages of the present invention can be further understood from the technical features disclosed herein.

[0007] To achieve one or more of the above objectives or other objectives, the present invention provides a wafer bonding method for stacked high aspect ratio microstructures, characterized in that a first wafer and a second wafer are provided, the surface of the first wafer or the second wafer being stacked with high aspect ratio microstructures; the first wafer and the second wafer are subjected to cleaning and drying treatments before bonding, the cleaning and drying treatments including: pre-cleaning at room temperature; immersing the pre-cleaned first wafer and the second wafer in an IPA solution; after the immersion treatment, placing the first wafer and the second wafer in a supercritical drying chamber and introducing gas for supercritical drying treatment; and pre-bonding the first wafer and the second wafer after cleaning and drying.

[0008] Before the first and second wafers are immersed in IPA solution, they are subjected to plasma treatment. Any one or a combination of gases, such as N2, argon, and O2, is used as the reaction gas. The power is 40W-100W and the flow rate of the reaction gas is 30sccm-50sccm. The wafer surface is treated for 15s-60s.

[0009] The IPA solution immersion treatment is a gradient immersion treatment, including: placing the first wafer and the second wafer in a cleaning machine containing a 30%-40% concentration IPA solution for a first-stage immersion treatment of 5 min-10 min; placing the first wafer and the second wafer in a cleaning machine containing a 70%-80% concentration IPA solution for a second-stage immersion treatment of 5 min-10 min; and placing the first wafer and the second wafer in a cleaning machine containing an IPA solution with a concentration of 95% or higher for a third-stage immersion treatment of 5 min-10 min.

[0010] The first and second wafers, after pre-cleaning, are dried by centrifugal drying at a speed not exceeding 700 r / min.

[0011] During the supercritical drying process, the first wafer and the second wafer are placed in a drying chamber filled with liquid CO2 or inert gas, the chamber temperature is maintained at 30℃-50℃, and the working pressure is 10MPa-20MPa; after the supercritical treatment is completed, the temperature is slowly reduced to room temperature and the pressure is reduced to standard atmospheric pressure.

[0012] Pre-cleaning at room temperature includes one or more of the following: DHF cleaning, SPM cleaning, SC1 cleaning, SC2 cleaning, and deionized water cleaning.

[0013] The thickness of the first wafer and the second wafer ranges from 400μm to 2200μm, and the diameter ranges from 100mm to 300mm.

[0014] The pre-bonding of the first wafer and the second wafer includes placing the surfaces of the first wafer and the second wafer opposite to each other, applying pressure to the first wafer and the second wafer in a vacuum environment or a protective atmosphere to achieve pre-bonding of the first wafer and the second wafer; and performing high-temperature annealing treatment on the pre-bonded wafer.

[0015] The first and second wafers were placed under a vacuum of 5 × 10⁻⁶. -4 Under conditions of mbar and 50°C, a pressure of 10000N-15000N is applied to the first and second wafers to achieve pre-bonding of the first and second wafers, with a pre-bonding time of 10min-20min; or the first and second wafers are placed in an environment with nitrogen gas flow rate of 20sccm, the temperature is raised to 40°C-50°C, and a pressure of 10000N-15000N is applied to the first and second wafers to achieve pre-bonding of the first and second wafers, with a pre-bonding time of 10min-20min.

[0016] The bonded wafers are annealed at a high temperature of 450℃-1100℃ for 1-2 hours.

[0017] Compared with the prior art, the beneficial effects of the present invention mainly include: 1. After wafer pre-cleaning, the present invention combines gradient IPA solution wetting treatment with supercritical gas drying for cleaning and drying wafers with stacked high aspect ratio microstructures before bonding. The gradient IPA solution wetting treatment can gradually replace the deionized water or other cleaning liquid in the microstructure. The gradient IPA solution wetting treatment assists supercritical gas drying to form a continuous phase transformation of "solvent transition - critical fluid replacement", reducing the gas-liquid interfacial tension during the drying process. Due to the gradient IPA solution wetting treatment, the microstructure can be completely wetted, achieving zero capillary stress deformation during the drying process, improving the drying effect, and protecting the morphology of the high aspect ratio microstructure.

[0018] 2. In this invention, a specific gas is used to perform plasma treatment on the wafer surface before the gradient wetting treatment of IPA solution and the supercritical gas drying, so as to loosen the wafer surface, enhance the flow and displacement efficiency of supercritical gas in the microstructure channels, and reduce the residue of IPA.

[0019] To make the above and other objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the wafer structure before and after bonding of the stacked high aspect ratio microstructures in this invention.

[0022] Figure 2 This is a flowchart of a wafer bonding method for stacking microstructures with high aspect ratios according to the present invention.

[0023] In the figure: 1. First wafer; 2. High aspect ratio microstructure; 3. Second wafer. Detailed Implementation

[0024] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings, which illustrate preferred embodiments of the application. However, this application may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be thorough and complete.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0026] It should be understood that when an element or layer is referred to as being "on," "adjacent to," or "connected to" other elements or layers, it may be directly on, adjacent to, connected to, or coupled to other elements or layers, or there may be intervening elements or layers. It should be understood that although the terms first, second, third, etc., may be used to describe various elements, components, regions, layers, doping types, and / or portions, these elements, components, regions, layers, doping types, and / or portions should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, doping type, or portion from another element, component, region, layer, doping type, or portion. Therefore, without departing from the teachings of this application, the first element, component, region, layer, doping type, or portion discussed below may be referred to as a second element, component, region, layer, or portion.

[0027] When used herein, the singular forms of “a,” “an,” and “the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that when the terms “comprise” and / or “comprising” are used in this specification, the presence of the stated feature, integer, step, operation, element, and / or part is established, but the presence or addition of one or more other features, integers, steps, operations, elements, parts, and / or groups is not excluded. Meanwhile, when used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0028] Embodiments of the invention are described herein with reference to cross-sectional views that serve as schematic diagrams of preferred embodiments (and intermediate structures) of this application, thus allowing for the anticipation of variations in the illustrated shapes due to, for example, manufacturing techniques and / or tolerances. Embodiments of this application should not be limited to the specific shapes of the regions shown herein, but rather include shape deviations due to, for example, manufacturing techniques. Therefore, the regions shown in the figures are substantially schematic, and their shapes do not represent the actual shapes of regions of the device, nor do they limit the scope of this application.

[0029] Example 1 Example 1 provides a wafer bonding method with stacked high aspect ratio microstructures, including: providing a first wafer and a second wafer, the surface of which is stacked with high aspect ratio microstructures; cleaning and drying the first and second wafers before bonding, the cleaning and drying including: pre-cleaning at room temperature; wetting the pre-cleaned first and second wafers with IPA solution; after the wetting treatment, placing the first and second wafers into a supercritical drying chamber and introducing gas for supercritical drying treatment; and bonding the first and second wafers after cleaning and drying.

[0030] This invention combines IPA solution immersion treatment with supercritical gas drying for cleaning and drying wafers with stacked high aspect ratio microstructures before bonding. IPA displacement can remove deionized water or other cleaning solutions from the microstructure. IPA displacement assists supercritical gas drying to form a continuous phase transformation of "solvent transition - critical fluid displacement", which reduces the gas-liquid interfacial tension during drying, improves the drying effect, and reduces the liquid residue in the high aspect ratio microstructure.

[0031] The wafer bonding method for stacked high aspect ratio microstructures of the present invention will be explained in detail below with reference to the accompanying drawings.

[0032] See Figure 1The wafers used for bonding in this invention include a first wafer 1 and a second wafer 3. The first wafer 1 has a high aspect ratio microstructure 2 stacked on it. The high aspect ratio microstructure 2 includes deep holes or deep cavities. The deep holes or deep cavities are prone to leaving water stains during wafer cleaning. When the second wafer 3 is bonded to the first wafer 1, the water stains remaining in the high aspect ratio microstructure 2 on the first wafer 1 cannot be completely removed. During bonding heating, the water vapor evaporates and forms high pressure on the bonding surface, affecting the bonding yield.

[0033] Example 1 proposes a wafer bonding method for stacking high aspect ratio microstructures to address the above problems, such as... Figure 2 As shown, it includes the following steps: Step 1: Pre-cleaning of the first wafer 1 and the second wafer 3: Pre-cleaning of wafers at room temperature includes methods such as DHF cleaning (using hydrofluoric acid solution diluted with high-purity deionized water to clean silicon wafers, typically with a dilution ratio between 100:1 and 200:1, the specific dilution ratio determined according to process requirements), SPM cleaning (using a mixed solution of concentrated sulfuric acid and hydrogen peroxide as the cleaning fluid, typically with a volume ratio of concentrated sulfuric acid to hydrogen peroxide of 4:1 to 3:1; SPM cleaning has strong oxidation and stripping capabilities, effectively removing organic contaminants, photoresist, and some metallic impurities from the wafer surface), and SC1 cleaning (Standard Clean). 1 is the RCA standard cleaning process, which is usually composed of ammonia, hydrogen peroxide, and deionized water. The mixing ratio of the above solutions is usually between 1:1:5 and 1:2:7. SC1 cleaning is used to remove particulate contamination, organic acids, and some metal impurities from the surface of silicon wafers. SC2 cleaning (composed of hydrochloric acid, hydrogen peroxide, and deionized water, with a mixing volume ratio of usually 1:1:50, SC2 cleaning can be used to remove deionized metals) and one or more combinations of deionized water cleaning.

[0034] In actual cleaning, different cleaning methods and corresponding cleaning processes can be selected according to the type of residual impurities on the silicon wafer.

[0035] In other embodiments, after pre-cleaning at room temperature, a centrifugal drying process is also included. Specifically, the wafer is dried by centrifugal drying, with the speed of the centrifuge not exceeding 700 r / min. Centrifugal drying of the wafer not only ensures the removal of most water stains on the wafer but also reduces damage to high aspect ratio microstructures on the wafer.

[0036] Step 2: IPA (isopropanol) solution immersion treatment of wafer 1 and wafer 3: The first wafer 1 and the second wafer 3 are subjected to IPA solution immersion treatment to replace the deionized water or other cleaning liquid remaining on the wafers.

[0037] To ensure better displacement results, a gradient IPA solution impregnation treatment can be used, specifically including: Step 2-1: Place the first wafer 1 and the second wafer 3 in a cleaning machine containing a 30%-40% concentration IPA solution for the first stage of immersion treatment for 5 min-10 min; Step 2-2: Place the first wafer 1 and the second wafer 3 into a cleaning machine containing a 70%-80% concentration IPA solution for a second-stage immersion treatment for 5-10 minutes; Steps 2-3: Place the first wafer 1 and the second wafer 3 into a cleaning machine containing IPA solution with a concentration of 95% or higher for a third-stage immersion treatment for 5-10 minutes.

[0038] The above-mentioned IPA gradient wetting treatment can better achieve the wetting of the structure on the wafer, better replace the deionized water or other cleaning solution remaining on the wafer, facilitate zero capillary stress deformation in the subsequent supercritical drying process, improve the drying effect, and at the same time protect the high aspect ratio microstructure morphology.

[0039] In other embodiments, plasma treatment of the first wafer 1 and the second wafer 3 is further performed prior to the IPA solution immersion treatment: After pre-cleaning, the first wafer 1 and the second wafer 3 undergo plasma treatment using any one or a combination of gases selected from N2, argon, and O2 as the reactant gas. The power is 40W-100W, and the reactant gas flow rate is 30sccm-50sccm. The wafer surface treatment lasts for 15s-60s. Plasma treatment of the surfaces of the first wafer 1 and the second wafer 3 can loosen the surface layer, enhancing the flowability and displacement efficiency of the supercritical gas in the microstructure channels during subsequent supercritical drying, thereby reducing the residue of IPA solution on the wafers.

[0040] Step 3: Supercritical drying treatment of the first wafer 1 and the second wafer 3: Place the first wafer 1 and the second wafer 3 in a drying chamber filled with CO2 (liquid) or inert gas; after the supercritical drying process begins, the temperature of the working environment of the drying chamber is raised to 30℃-50℃, and the chamber pressure is maintained at 10MPa-20MPa. After 30 minutes of supercritical treatment, the temperature and pressure are reduced. Specifically, 30 minutes after the end of the supercritical treatment, the temperature is slowly reduced to room temperature (e.g., 25℃), and the pressure is slowly reduced to standard atmospheric pressure (e.g., 0.1MPa).

[0041] This invention combines IPA solution gradient displacement with supercritical drying for cleaning and drying of wafers with stacked high aspect ratio microstructures before bonding. The IPA solution gradient wetting gradually replaces the deionized water or other cleaning solutions within the microstructure. The IPA solution gradient wetting assists supercritical gas drying in forming a continuous phase transformation of "solvent transition - critical fluid displacement," reducing the gas-liquid interfacial tension during drying. Simultaneously, plasma treatment of the surfaces of the first wafer 1 and the second wafer 3 can loosen the surfaces of the first wafer 1 and the second wafer 3, enhancing the flow and displacement efficiency of supercritical gas in the microstructure channels, reducing IPA solution residue, and improving the drying effect.

[0042] Step 4: Pre-bonding of the first wafer 1 and the second wafer 3: The surfaces of the first wafer 1 and the second wafer 3 are placed opposite each other, wherein the thickness of the first wafer 1 and the second wafer 3 ranges from 400μm to 2200μm and the diameter ranges from 100mm to 300mm. Pressure is applied to the first wafer 1 and the second wafer 3 in a vacuum environment or under a protective atmosphere to achieve pre-bonding of the first wafer 1 and the second wafer 3.

[0043] During pre-bonding in a vacuum environment, the first wafer 1 and the second wafer 3 are placed in a vacuum of 5×10⁻⁶. -4 At an environment of mbar and 50°C, a pressure of 10000N-15000N is applied to the first wafer 1 and the second wafer 3 to achieve pre-bonding of the first wafer 1 and the second wafer 3, and the pre-bonding time is maintained at 10min-20min.

[0044] Alternatively, in a protective atmosphere, the first wafer 1 and the second wafer 3 are placed in an environment with nitrogen gas flowing through them at a flow rate of 20 sccm, and the temperature is raised to 40°C-50°C.

[0045] A pressure of 10000N-15000N is applied to the first wafer 1 and the second wafer 3 to achieve pre-bonding of the first wafer 1 and the second wafer 3, and the pre-bonding time is maintained at 10min-20min.

[0046] Step 5: Annealing of the pre-bonded wafer: After pre-bonding, the wafers were annealed at 450℃-1100℃ for 1-2 hours. Following annealing, the bonding effect was assessed using CSAM (Chemical Sequencing and Amplification). CSAM utilizes the reflection or transmission of ultrasonic waves at different media interfaces for non-destructive testing, enabling the detection of defects in the bonding layer and thus evaluating the impact of the IPA gradient replacement treatment and supercritical gas drying treatment of this invention on the wafer bonding effect.

[0047] The foregoing has provided a detailed description of a wafer bonding method for stacked microstructures with high aspect ratios, as provided by this invention. Specific examples have been used to illustrate the structure and working principle of this invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of this invention. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims of this invention.

Claims

1. A method for wafer bonding of a stack of high aspect ratio microstructures, characterized in that, The first wafer and the second wafer are provided, and high aspect ratio microstructures are stacked on the surfaces of the first wafer or the second wafer. Before the first wafer and the second wafer are bonded, cleaning and drying treatment is performed, and the cleaning and drying treatment comprises the following steps: Pre-cleaning is performed at room temperature; After the pre-cleaning, the first wafer and the second wafer are subjected to IPA solution immersion treatment, and after the immersion treatment is completed, the first wafer and the second wafer are placed in a supercritical drying cavity and subjected to supercritical drying treatment by introducing gas; After the cleaning and drying are completed, the first wafer and the second wafer are pre-bonded.

2. The method of claim 1, wherein the high aspect ratio microstructure is a microstructure having an aspect ratio of 10 or more. Before the first wafer and the second wafer are subjected to the IPA solution immersion treatment, plasma treatment is performed, and any one of N2, argon, and O2 or a combination of multiple gases is used as a reaction gas, the power is 40 W-100 W, the reaction gas flow is 30 sccm-50 sccm, and the wafer surface is treated for 15 s-60 s.

3. The method of claim 1, wherein the high aspect ratio microstructure is a nanostructure. The IPA solution immersion treatment is gradient immersion treatment, which comprises the following steps: The first wafer and the second wafer are placed in a cleaning machine containing 30%-40% concentration IPA solution for first-stage immersion treatment for 5 min-10 min; The first wafer and the second wafer are placed in a cleaning machine containing 70%-80% concentration IPA solution for second-stage immersion treatment for 5 min-10 min; The first wafer and the second wafer are placed in a cleaning machine containing 95% or more concentration IPA solution for third-stage immersion treatment for 5 min-10 min.

4. The method of claim 1, wherein the high aspect ratio microstructure is a nanostructure. The pre-cleaned first wafer and the second wafer are subjected to spin-drying by a centrifugal spin-drying method, and the rotation speed is not higher than 700 r / min.

5. The method of claim 1, wherein the high aspect ratio microstructure is a nanostructure. During the supercritical drying treatment, the first wafer and the second wafer are placed in a drying cavity filled with liquid CO2 or inert gas, the cavity temperature is kept at 30℃-50℃, and the working pressure is 10 MPa-20 MPa; After the supercritical treatment is completed, the temperature is slowly reduced to room temperature, and the pressure is reduced to standard atmospheric pressure.

6. The method of claim 1, wherein the high aspect ratio microstructure is a microstructure having an aspect ratio of 10 or more. The pre-cleaning at room temperature comprises one or more of the following: DHF cleaning, SPM cleaning, SC1 cleaning, SC2 cleaning, and deionized water cleaning.

7. The method of claim 1, wherein the high aspect ratio microstructure is a nanostructure. The thickness of the first wafer and the second wafer ranges from 400 μm to 2200 μm, and the diameter ranges from 100 mm to 300 mm.

8. The method of claim 1, wherein the high aspect ratio microstructure is a microstructure having an aspect ratio of at least 10:

1. The pre-bonding of the first wafer and the second wafer comprises the following steps: The first wafer and the second wafer are placed in a vacuum environment or a protective atmosphere, and pressure is applied to the first wafer and the second wafer to achieve the pre-bonding of the first wafer and the second wafer; 9. The method of claim 8, wherein the high aspect ratio microstructure is a microstructure having an aspect ratio of 10 or more. The first wafer and the second wafer are placed in a vacuum degree of 5x10 -4 mbar, temperature 50℃, and a pressure of 10000N-15000N is applied to the first wafer and the second wafer to realize pre-bonding of the first wafer and the second wafer, and the pre-bonding time is 10min-20min. The pre-bonded wafer is subjected to high-temperature annealing treatment.

10. The method of claim 8, wherein the high aspect ratio microstructure is a microstructure having an aspect ratio of 10 or more. Or the first wafer and the second wafer are placed in an environment where nitrogen gas with a flow rate of 20 sccm is introduced, the temperature is raised to 40℃-50℃, a pressure of 10000 N-15000 N is applied to the first wafer and the second wafer to achieve the pre-bonding of the first wafer and the second wafer, and the pre-bonding time is 10 min-20 min. The bonded wafer is annealed at a high temperature of 450℃-1100℃ for 1 h-2 h.

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