Low temperature bonding method for substrate
The fusion pre-bonding process combined with electrically driven bonding using a voltage with a pulse or AC component addresses the limitations of conventional methods by forming covalent bonds at lower temperatures, enhancing bonding strength and process control in substrate bonding.
Patent Information
- Application Number
- JP2025004038
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-01-11
- Filing Date
- 2025-01-10
- Publication Date
- 2025-07-24
AI Technical Summary
Existing bonding methods for substrates face challenges in achieving sufficient bonding strength and process control, particularly at moderate temperatures below 200°C, due to limited covalent bond formation and thermal stress issues, which are exacerbated by high-temperature annealing processes and surface roughness requirements.
A method combining a fusion pre-bonding process with an electrically driven bonding process using a voltage with a pulse or AC component to form covalent bonds between substrates, eliminating the need for high-temperature annealing and allowing for alignment correction.
This approach enables strong, homogeneous bonding at lower temperatures with reduced processing time and thermal stress, improving bonding strength and process control, while maintaining alignment precision.
Smart Images
Figure 2025109197000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method for low-temperature bonding of substrates.
Background Art
[0002] In recent years, the mounting density of semiconductor devices has been significantly increasing. Integrated circuit (IC) devices are becoming more powerful and at the same time are being miniaturized. Further increasing the mounting density of semiconductor devices and the metal wiring connecting them is an ever-growing challenge. To address the technical problems associated with the manufacture of such devices, special processing measures such as permanent or temporary substrate / wafer bonding, and substrate or wafer bonding techniques have been developed.
[0003] Furthermore, when processing devices with multiple stacked layers, it may be necessary to connect them by bonding two substrates.
[0004] Various bonding processes are known from the literature.
[0005] In common fusion processes, including the fusion pre-bonding process and the high-temperature annealing process, two substrates are chemically bonded without an additional intermediate layer therebetween. The bonding is based on intermolecular interactions including van der Waals forces, hydrogen bonds (formed in the fusion pre-bonding process), and strong covalent bonds between the substrates (formed in the high-temperature annealing process).
Summary of the Invention
Problems to be Solved by the Invention
[0006] The quality of the connection strongly depends on the surface roughness, the amount of silanol groups (which depends on the efficiency of plasma surface treatment), and the processing temperature. At a moderate bonding temperature below 200°C, only a limited number of silanol groups form covalent bonds at the bonding interface. To obtain sufficient bonding strength, a high-temperature annealing process (above 200°C) needs to be applied for a long time, for example, longer than 2 to 10 hours, which increases the number of covalent bonds.
[0007] However, due to the processing temperature, especially the high temperature in the annealing process, there are limitations in the application of the fusion process for bonding substrates with different coefficients of thermal expansion. When the bonded substrate stack is cooled, the internal stress may cause damage or breakage of the bonding interface and / or the substrate.
[0008] Not only the temperature, but also the quality of the fusion process strongly depends on the surface roughness of the two substrates. A smooth surface with an RMS (root mean square) roughness of less than 0.15 nm is required to obtain a sufficient contact interface for high-quality fusion. Therefore, for example, by chemical mechanical polishing (CMP), costly surface preparation is required before the bonding process.
[0009] Due to the above-mentioned drawbacks of the commonly applied bonding methods, there is a need for means to generally improve the bonding strength and process control and shorten the processing time.
Means for Solving the Problem
[0010] The object of the present invention is providing a first substrate and a second substrate, aligning the first substrate and the second substrate, pre-bonding the first substrate and the second substrate by a molten pre-bonding process, bonding the first substrate and the second substrate by applying a voltage between the first substrate and the second substrate, where the voltage includes a pulse component or an AC component, which is solved by a method for bonding substrates including the above.
[0011] This method is particularly suitable for bonding wafers, especially silicon wafers.
[0012] By pre-bonding by a well-known fusion process, the substrates are reversibly bonded to each other. Thereby, the relative positions of each other can be controlled and corrected as needed. Furthermore, regarding the subsequent bonding step, the handling of the substrates becomes easy without impairing the alignment.
[0013] In the bonding step, the substrates are strongly bonded together by applying a voltage to both substrates, and the voltage includes a pulse component or an AC component. Thus, the bonding step can be interpreted as an electrically driven bonding process.
[0014] In particular, after the bonding step, the substrates are permanently bonded to each other and / or inseparably connected.
[0015] The bonding step can be carried out, in particular, by applying a so-called impulse current bonding (ICB) process, which is applicable to a number of different substrate materials.
[0016] Without wishing to be limited to theory, the application of a voltage having a pulse component or an AC component is thought to define the movement of substances such as atoms and / or ions at the substrate interface.
[0017] Furthermore, due to the formation of a chemical and / or charge gradient in the interface region, covalent and / or ionic bonds may occur between the substrates.
[0018] The frequency of the pulse voltage component or the alternating voltage component may be higher than the dielectric relaxation frequency of the substrate material used. In particular, the frequency may be between 0.1 kHz and 100 kHz.
[0019] Preferably, the voltage also has a DC component with an AC component or a pulse component superimposed thereon. The direction in which the DC component is applied defines the direction of material movement.
[0020] In the method according to the invention, the voltage application with an AC component or a pulse component replaces the annealing process of the conventional fusion process. Thus, the two substrates can be firmly connected without being exposed to high temperatures.
[0021] In other words, the main idea is to combine two different bonding techniques based on different physical and / or chemical functional principles, namely the melting pre-bonding process of the conventional fusion process and the electrically driven bonding process according to the present invention, in particular the ICB process.
[0022] This makes it possible to combine and use the advantages of both bonding processes in one method. These are, in particular, the low processing temperature, the high processing speed, the bonding strength that is strongly and homogeneously distributed over the entire bonding interface, which is especially an advantage of the electrically driven bonding process, the possibility of pre-bonding, which is especially an advantage of the melting pre-bonding process, and the possibility of correcting the alignment of the substrates if necessary.
[0023] Specifically, in the conventional fusion bonding, the annealing step has a great influence on the final bonding strength and often requires a processing time of several hours. The approach of combining the melting pre-bonding with the electrically driven bonding process at a substantially lower temperature and processing time according to the present invention still results in covalent bonds being formed at the bonding interface and provides a substantially higher bonding strength compared to the conventional fusion bonding where the annealing step is carried out at the same temperature.
[0024] Due to the different nature of the electrically driven bonding process, the resulting bond is formed by a chemical reaction between the entire surface areas of the bonding interface. In contrast, in the case of conventional fusion bonding, the chemical reaction at the bonding interface occurs only at the positions of the silanol groups formed during surface activation, and the number thereof is of course limited (depending on the activation conditions). However, the electrically driven bonding process according to the present invention enables a true hermetic bond.
[0025] During pre-bonding, some of the Si-OH silanol groups at the interface between the substrates begin to form hydrogen bonds by (atomic) contact between the substrates and change to covalent bonds during the electrically driven bonding process.
[0026] For example, the covalent bond can be a Si-O-Si covalent bond.
[0027] The amount of covalent bonds formed during the electro-driven bonding process, particularly Si-O-Si covalent bonds, and thus the bond strength, is considered to increase with the processing temperature and processing time.
[0028] In particular, such a connection can be established between two silicon wafers having chemisorbed water and / or silanol groups on the surface.
[0029] However, it is also possible to connect substrates made of materials other than silicon with Si-O-Si bonds. In particular, silanol groups can be introduced onto the substrate surface intended for bonding by a silylation step prior to the pre-bonding step.
[0030] In a preferred embodiment, the pre-bonding step may be carried out at a temperature below 200 °C, particularly at room temperature. It has been found that even at such low processing temperatures, sufficient bond strength for substrate handling in subsequent processing steps is obtained. This reduces the thermal stress on the substrate and / or the internal stress within the bonded substrate stack.
[0031] Preferably, in the step of bonding the first substrate and the second substrate, a voltage is applied to the first substrate and / or the second substrate so as to define the movement of substances, particularly ions or vacancies, perpendicular to the contact interface between the first substrate and the second substrate.
[0032] This can be achieved by placing the substrate between two electrodes connected to a power source. Alternatively, in the case of a substrate that is at least partially conductive and / or semiconductive, the substrate itself can function as an electrode.
[0033] By the movement of substances perpendicular to the contact interface, interdiffusion of substances can occur between the substrates and / or the formation of covalent and / or ionic bonds at the substrate interface can be defined. Therefore, a very strong and durable substrate connection can be achieved.
[0034] In one example of this method, the step of bonding the first substrate and the second substrate is performed at a temperature below 250°C, particularly between 100°C and 200°C. It has been found that in this temperature range, a good compromise can be obtained between the thermal stress on the substrate and the bonding strength. Since heating promotes mass transfer and / or bond formation, it is considered that the bonding strength at a given bonding time is improved and / or the bonding time required to achieve the desired bonding strength is shortened.
[0035] The method according to the present invention may also include any additional optional steps, particularly steps of cleaning and / or planarizing the first substrate and / or the second substrate, and / or steps of treating the first substrate and / or the second substrate with a reactive plasma for surface activation, particularly according to conventional plasma-based low-temperature fusion bonding.
[0036] These processing steps improve the surface quality and / or reactivity, and thus improve the substrate bonding quality, particularly the bonding strength and / or homogeneity.
[0037] It is also possible to hydrate the first substrate and / or the second substrate before the pre-bonding step in order to increase the number of silanol groups on the surface where bonding is intended. Furthermore, this optional step particularly improves the bonding strength and / or homogeneity of the pre-bonded substrate stack.
[0038] In another preferred embodiment, the method includes a step of observing the alignment of the first substrate and the second substrate after the pre-bonding step and before the bonding step. If it is found as a result of the observation that there is a misalignment between the first substrate and the second substrate, at least the alignment step and the pre-bonding step are repeated, particularly after the substrates are separated again.
[0039] This additional step ensures that only well-aligned substrates are irreversibly bonded. The amount of waste products is reduced, and thus the processing yield is improved.
[0040] In other variations of this method, in the step of bonding the first substrate and the second substrate, a voltage is applied between the first substrate and the second substrate for less than 20 minutes, particularly less than 10 minutes. It has been found that sufficient bonding strength can be obtained even with such a short bonding time.
[0041] Further advantages and features will become apparent from the following description and the accompanying drawings showing non-limiting embodiments of the invention.
Brief Description of the Drawings
[0042]
Figure 1
Embodiments for Carrying Out the Invention
[0043] In the first step a) of this method, a first substrate 10 and a second substrate 12 are provided. The substrates 10, 12 shown are both silicon wafers.
[0044] Of course, the described embodiments are merely illustrative. In particular, other types of substrates and / or materials, such as SiC, glass, sapphire, etc. can also be bonded using the method according to the present invention.
[0045] In the second step b) of this method, both substrates 10, 12 are cleaned and treated with plasma to activate the surface. This step increases the surface energy of the substrates 10, 12, thus improving the formation of chemical bonds during pre-bonding and / or bonding.
[0046] In the third step c) of this method, the substrates 10, 12 are hydrated. This can be achieved by exposing the substrates 10, 12 to a hydrating liquid or gas. In particular, the substrates 10, 12 can be sprayed or rinsed with distilled water and / or deionized water. This step ensures that a sufficient amount of Si-OH groups are formed on the substrate surface, for example by chemisorption of water molecules.
[0047] In the fourth step d) of this method, the substrates 10, 12 are aligned with each other. A robot using a camera system that recognizes the wafer contour and / or positioning marks can be applied for high-precision alignment. Manual alignment is also possible, or for example, if there are no positioning marks, the substrates 10, 12 are not aligned at all.
[0048] In the fifth step e) of this method, the first and second substrates 10, 12 are pre-bonded by a fusion pre-bonding process, in particular a plasma-based low-temperature fusion pre-bonding process. This is achieved by bringing the cleaned and hydrated surfaces into (atomic) contact with each other.
[0049] Due to the (atomic) contact between the substrates 10, 12, some of the Si-OH silanol groups on the surfaces of the substrates 10, 12 begin to form hydrogen bonds. In an embodiment, the fusion pre-bonding is performed at room temperature. Of course, in order to obtain a higher pre-bonding strength, higher temperatures, for example up to 100 °C, especially up to 150 °C, can be applied during this processing step.
[0050] The result of the fifth step e) is a pre-bonded substrate stack 16 composed of the first substrate 10 and the second substrate 12. In this embodiment, it is still possible to peel both substrates 10, 12 from each other. However, the bonding strength of the pre-bonded substrate stack 16 is sufficient to handle the substrates in subsequent processing steps without losing the (relative) alignment and connection between the substrates 10, 12.
[0051] In the sixth step f), the alignment between the first substrate 10 and the second substrate 12 in the pre-bonded substrate stack 16 is observed, for example, by a camera system.
[0052] If as a result of the observation it is found that there is a misalignment between the first substrate 10 and the second substrate 12, in particular if the first substrate 10 and the second substrate 12 are relatively displaced by more than a predetermined threshold value, the alignment step and the pre-bonding step are repeated, in particular after peeling (debonding).
[0053] The substrates 10 and 12 are firmly bonded only when the alignment meets a predetermined specification.
[0054] In this embodiment, the first substrate 10 and the second substrate 12 are not bonded by a conventional high-temperature fusion or annealing step. FIG. 1 symbolizes this by the processing step g) with a strikethrough.
[0055] Instead, the pre-bonded first substrate 10 and second substrate 12 are firmly and / or permanently bonded in the seventh processing step h) by applying a voltage containing not only a DC component but also a pulse component between them. Alternatively, a voltage containing an AC component can also be used.
[0056] In this electrically driven bonding process step, the hydrogen bonds formed in the molten pre-bonding process step are converted into covalent bonds.
[0057] For example, the covalent bond can be a Si-O-Si covalent bond. The voltage is applied to the first substrate 10 and the second substrate 12 via two electrodes (not shown) sandwiching the pre-bonded substrate stack 16.
[0058] The voltage is applied in the substrate plane direction and defines the movement of substances, particularly ions or vacancies, perpendicular to the contact interface 14 between the first substrate 10 and the second substrate 12.
[0059] In an embodiment, the voltage between the first substrate 10 and the second substrate 12 is applied for less than 30 minutes, particularly less than 20 minutes, and more particularly less than 10 minutes. It has been found that such a short time is sufficient to achieve a strong and homogeneous bond across the contact interface 14.
[0060] Furthermore, less than 200°C, particularly 150°C, is applied to the substrates 10 and 12 during the bonding in the seventh processing step h). As the temperature increases, the movement of materials is promoted, resulting in improved bonding quality and reduced time required for bonding.
[0061] What is important is that the temperature applied in the seventh processing step h) is still significantly lower than that of the conventional fusion process in which a processing temperature of 200°C to 400°C or higher is generally applied over a long period of time, for example, 2 to 10 hours or more.
[0062] In other words, the high-temperature treatment or annealing step of the conventional fusion bonding method is replaced by an electrically driven bonding process. As a result, the processing time and the thermal stress on the substrates 10, 12 are generally reduced, and a firmly bonded substrate stack 18 (wafer stack) is obtained.
Claims
1. A method for bonding substrates (10, 12), comprising: providing a first substrate (10) and a second substrate (12); aligning the first substrate (10) and the second substrate (12); pre-bonding the first substrate (10) and the second substrate (12) by a fusion pre-bonding process; bonding the first substrate (10) and the second substrate (12) by applying a voltage therebetween, the voltage including a pulse component or an AC component; and the method having the above steps.
2. In the fusion pre-bonding process, Si-OH groups on the surfaces of the first substrate (10) and the second substrate (12) form hydrogen bonds at a contact interface (14) between the first substrate (10) and the second substrate (12). The method according to claim 1.
3. The pre-bonding step is carried out at a temperature lower than 200°C, particularly at room temperature. The method according to claim 1 or 2.
4. In the step of bonding the first substrate (10) and the second substrate (12), a voltage is applied to the first substrate (10) and the second substrate (12) so as to define the movement of substances, particularly ions or vacancies, perpendicular to a contact interface (14) between the first substrate (10) and the second substrate (12). The method according to any one of claims 1 to 3.
5. The step of bonding the first substrate (10) and the second substrate (12) is carried out at a temperature lower than 250°C, particularly at a temperature between 100°C and 200°C. The method according to any one of claims 1 to 4.
6. An additional step of cleaning and / or planarizing the first substrate (10) and / or the second substrate (12), and / or An additional step of treating the first substrate (10) and / or the second substrate (12) with reactive plasma, and / or An additional step of hydrating the first substrate (10) and / or the second substrate (12) before the pre-bonding step, and / or An additional step of observing the alignment of the first substrate and the second substrate (10, 12) after the pre-bonding step and before the bonding step, and if it is found as a result of the observation that there is a misalignment between the first substrate (10) and the second substrate (12), repeating the alignment step and the pre-bonding step. including at least one additional step of The method according to any one of claims 1 to 5. **Claim 7** In the step of bonding the first substrate (10) and the second substrate (12), a voltage is applied between the first substrate (10) and the second substrate (12) for a time of less than 20 minutes, particularly less than 10 minutes. The method according to any one of claims 1 to 6.