Annealing method for improving bonding effect of multi-layer wafer and application

Abstract

The invention discloses an annealing method for improving the bonding effect of multiple layers of wafers and application. The method comprises the following steps: step 1, carrying out primary silicon-silicon bonding on a bonding surface of a first wafer and a first bonding surface of a second wafer; step 2, first high-temperature annealing is carried out on the bonded first wafer and second wafer, protective gas at least comprises nitrogen, oxygen is introduced before annealing and temperature rise, a silicon oxide layer with a certain thickness is formed on the second wafer, and then nitrogen is introduced; 3, obtaining a composite wafer through first high-temperature annealing, and carrying out second silicon-silicon bonding on the second bonding surface, on which the silicon oxide layer is formed, of the second wafer and the bonding surface of the third wafer; the thickness of the silicon oxide layer on the second bonding surface is 30-50 angstroms; and step 4, carrying out second high-temperature annealing on the bonded composite wafer and the third wafer. The silicon oxide layer can isolate nitrogen from the silicon surface, prevent generation of silicon nitride from affecting the appearance of the silicon surface, and do not affect the secondary bonding effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor manufacturing, and in particular relates to an annealing method for improving the bonding effect of multi-layer wafers and its application. Background Art

[0002] In the production of optoelectronic semiconductor devices, three-layer Si-Si bonding and post-bonding annealing processes are commonly used. The main process is as follows: first, two wafers are used for the first Si-Si bonding, and after the bonding is completed, the first annealing is performed. Then, the two wafers that have completed the first bonding and annealing are used to bond with a third wafer for the second Si-Si bonding, and after the bonding is completed, the second annealing is performed.

[0003] The annealing process mentioned above usually requires a high-temperature process of 1150°C or above. In a high-temperature environment, nitrogen as a protective gas will react with the silicon surface to produce an uneven silicon nitride film. This silicon nitride film will not only cause abnormal appearance of the silicon surface, but also the silicon nitride film produced on the wafer surface during the first annealing will affect the bonding effect of the second bonding. Conventional annealing processes generally use a high-temperature process of 1150°C in a pure nitrogen environment, and the annealing time often takes several hours. In a long-term high-temperature nitrogen atmosphere, silicon will react with nitrogen to produce a small amount of silicon nitride. After Si-Si bonding, a long-term high-temperature annealing at 1150°C must be performed, and the silicon nitride problem cannot be solved by reducing the temperature and process time. Summary of the Invention

[0004] In view of all or part of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide an annealing method and application for improving the bonding effect of multi-layer wafers, wherein the formed silicon oxide layer can isolate nitrogen from the silicon surface, prevent the generation of silicon nitride from affecting the appearance of the silicon surface, and will not affect the bonding effect of the subsequent second bonding.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides an annealing method for improving the bonding effect of multi-layer wafers, comprising the following steps:

[0007] Step 1: Performing a first silicon-silicon bonding between the bonding surface of the first wafer and the first bonding surface of the second wafer;

[0008] Step 2: performing a first high-temperature annealing on the bonded first and second wafers, wherein the protective gas during the first high-temperature annealing process comprises at least nitrogen, oxygen is introduced before the annealing temperature is raised, and a silicon oxide layer of a certain thickness is formed on the second wafer, and then the nitrogen is introduced;

[0009] Step 3: Obtaining a composite wafer through the first high-temperature annealing, performing a second silicon-silicon bonding between the second bonding surface of the composite wafer having the silicon oxide layer formed thereon and the bonding surface of the third wafer; during the second silicon-silicon bonding, the thickness of the silicon oxide layer on the second bonding surface is 30-50 angstroms;

[0010] Step 4: performing a second high-temperature annealing on the bonded composite wafer and the third wafer.

[0011] Nitrogen, as a protective gas, has become one of the preferred protective gases in semiconductor manufacturing and other industries due to its advantages such as good stability, low cost, pollution-free and easy operation. In traditional furnace annealing, nitrogen is usually introduced during the annealing process to create an oxygen-free environment. In this case, nitrogen should be introduced before heating. This solution adopts the annealing process after the first Si-Si bonding, introducing a small amount of oxygen in the stage before heating. Oxygen is introduced first to produce a sufficient silicon oxide layer at a lower temperature, and then nitrogen is introduced to prevent annealing in a nitrogen environment without forming a sufficient silicon oxide layer. The silicon oxide layer can isolate nitrogen from the silicon surface and prevent the formation of silicon nitride.

[0012] From the perspective of material stress, silicon nitride > single crystal silicon > silicon oxide; from the perspective of material hardness, silicon nitride > single crystal silicon > silicon oxide. It can be seen that silicon oxide has smaller stress and lower hardness, while silicon nitride has larger stress and higher hardness. The smaller the stress and hardness, the tighter the bond. In the second silicon-silicon bonding process, the presence of a silicon oxide layer on the second bonding surface of the second wafer can make the bond between the second wafer and the third wafer tighter, improving the bonding effect. Compared to adding a high-temperature thermal oxidation treatment process before annealing after the first silicon-silicon bonding, this solution chooses to oxidize with oxygen before heating in the first high-temperature annealing process. Through reasonable design, the silicon oxide layer is directly formed in the front stage of the annealing process (temperature stabilization stage), which can save one oxidation process step, while not affecting the subsequent process and reducing production costs.

[0013] A silicon oxide layer is generated on the surface of the wafer. If the silicon oxide layer is too thin, for example, less than 30 angstroms, it cannot prevent the production of silicon nitride. In the actual production process, the wafer produces a natural oxide layer (thickness of about 20-30 angstroms) due to exceeding Q-TIME (not entering the next process machine operation within the specified time). When there is a natural oxide layer, it cannot prevent the production of silicon nitride. Therefore, the solution requires that the thickness of the silicon oxide layer be no less than 30 angstroms. However, it should also be noted that although the thicker the silicon oxide layer, the better for the bonding effect, the thickness of the silicon oxide layer is required to be as small as possible (less than 50 angstroms) for the product structure. An excessively thick silicon oxide layer, for example, a thickness greater than 50 angstroms, may cause stress problems or interface defects during the second silicon-silicon bonding and annealing process, thereby affecting the reliability of the product structure.

[0014] In step 2, the oxygen introduction process lasts for 8-12 minutes at a flow rate of 6-9 L / min. After the wafer enters the process chamber, the stable phase (typically 10 minutes) precedes heating. This phase stabilizes the chamber temperature before heating, and oxidation during this phase does not increase the heat treatment time.

[0015] In step 2, the thickness of the silicon oxide layer formed on the second wafer is 65-75 angstroms. A small amount of oxygen is introduced before heating, and the oxygen flow rate and time are controlled to generate a silicon oxide film of about 70 angstroms. This is achieved by adjusting the stable phase time and oxygen flow rate before heating (the ventilation time and oxygen flow rate ratio is not unique, as long as a silicon oxide layer of about 70 angstroms is generated).

[0016] In step 2, after forming the silicon oxide layer and before annealing, furnace cleaning is carried out to remove part of the silicon oxide layer, and the removed part is a silicon oxide material with a thickness of 25-35 angstroms. In step 2, the thickness of the silicon oxide layer is controlled to about 70 angstroms in advance. On the one hand, considering the loss of furnace cleaning, the furnace cleaning process will cause the silicon oxide layer thickness on the wafer surface to reduce, so a certain, slightly larger initial thickness is set to compensate for subsequent losses. On the other hand, if the initial thickness of the silicon oxide layer is too small, the furnace tube process control is difficult. It is easier to directly control the thickness of the silicon oxide layer at about 70 angstroms than to directly control the thickness below 50 angstroms. Compared with 50 angstroms, the process cost increase is negligible at 70 angstroms, but process parameters including film thickness and uniformity are easier to control. If the initial thickness is set to exceed 70 angstroms, the thicker the initial thickness is set, the longer the furnace tube film (forming the silicon oxide layer) and the subsequent furnace cleaning cost will increase.

[0017] The pre-furnace cleaning process conditions are: 0.5% hydrofluoric acid treatment for 40-80 seconds at a flow rate of 800-1500 sccm; SC1 treatment for 3-7 minutes, followed by SC2 treatment for 3-7 minutes. Hydrofluoric acid (DHF) is used to remove approximately 30 angstroms of the silicon oxide layer, typically used to remove the native oxide layer. However, this invention removes a portion of the generated silicon oxide layer. SC1 (HCl) / SC2 (NH4OH, H2O2) is used to remove particles, which does not affect the final results of this solution.

[0018] In step 1, the process conditions for the first silicon-silicon bonding are: bonding in a pure nitrogen environment, a bonding temperature of 40-60° C., a bonding pressure of 8-12 KN, and a bonding time of 8-12 min.

[0019] In step 2, the process conditions of the first high-temperature annealing are: annealing temperature of 1100-1200°C, annealing time of 2 hours or more, heating rate of 8-12°C / min, protective gas of pure nitrogen, and nitrogen flow rate of 8-12L / min. The heating rate will also affect the formation of the silicon oxide layer. During the heating process, the parameters of the thin film silicon oxide layer, such as film thickness and uniformity, are usually difficult to control, especially when the annealing heating rate is fast, such as exceeding 10°C / min. Therefore, it is chosen to form this silicon oxide layer before heating, and after heating, due to the protection of nitrogen, no other silicon oxide layer will be generated. When the temperature in the furnace is low (usually below 800°C), the reaction rate between oxygen and the silicon surface is extremely low, and almost no obvious oxide layer is formed; when the temperature rises to the critical temperature for the reaction between silicon and oxygen (usually ≥800°C), the oxidation reaction begins to occur significantly; for example: 800°C-900°C (i.e., the stable temperature in the process chamber during the stable stage before heating): the oxidation rate is slow, and a thin layer of silicon oxide is generated (such as the initial interface oxide layer).

[0020] In step 3, the process conditions for the second silicon-silicon bonding are: bonding in a pure nitrogen environment, a bonding temperature of 40-60° C., a bonding pressure of 8-12 kN, and a bonding time of 8-12 min.

[0021] In step 4, the process conditions of the second high-temperature annealing are: annealing temperature is 1100-1200°C, annealing time is 2 hours or more, heating rate is 8-12°C / min, protective gas is pure nitrogen, and nitrogen flow rate is 8-12L / min.

[0022] The first wafer, the second wafer and the third wafer are silicon wafers.

[0023] The present invention also provides an annealing method for improving the bonding effect of multi-layer wafers for use in the preparation process of optoelectronic semiconductor devices. In the production process of optoelectronic semiconductor devices, three-layer wafer silicon-silicon bonding and post-bonding annealing processes (including two bondings and two anneals) are often used. If there is no need to perform multi-layer wafer silicon-silicon bonding and annealing, that is, only a single silicon-silicon bonding and annealing process, it is usually not necessary to consider the impact of the annealing process on the second bonding or other subsequent processes caused by the production of silicon nitride on the wafer surface. The annealing method for improving the bonding effect of multi-layer wafers provided by the present invention is particularly suitable for the preparation of optoelectronic semiconductor devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is a schematic flow chart of an annealing method for improving multi-layer wafer bonding provided by the present invention;

[0026] Figure 2 The surface appearance of the wafer observed under an optical microscope after the first high-temperature annealing without oxygen, i.e., the annealing process provided in Comparative Example 1;

[0027] Figure 3 The surface appearance of the wafer observed under an optical microscope after the first high-temperature annealing process provided in Example 1 is performed by oxygen permeation;

[0028] Figure 4 The bonding interface of the second bonding observed under C-SAM after the second high-temperature annealing without oxygen, i.e., the annealing process provided in Comparative Example 1;

[0029] Figure 5 The bonding interface of the second bonding observed under C-SAM after the second high-temperature annealing according to the oxygen-through annealing process provided in Example 1. DETAILED DESCRIPTION

[0030] The following is a clear and complete description of the technical solutions in the specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0031] It should be noted that, in order to describe the technical solution more specifically, the steps described in the following embodiments do not strictly correspond one-to-one with the steps described in the summary of the invention.

[0032] Example 1

[0033] An annealing method for improving the bonding effect of multi-layer wafers, referring to Figure 1 , including the following steps:

[0034] Step 1: Perform a first silicon-silicon bond between the bonding surface of the first wafer and the first bonding surface of the second wafer. The process conditions for this first silicon-silicon bond are: bonding in a pure nitrogen environment, a bonding temperature of 40°C, a bonding pressure of 12 kN, and a bonding time of 12 minutes. These process parameters can be adjusted according to actual needs. The first and second wafers are silicon wafers.

[0035] Step 2: Perform the first high-temperature annealing on the bonded first wafer and the second wafer. The protective gas during the first high-temperature annealing is pure nitrogen. Before the annealing temperature is raised, that is, during the temperature stabilization stage after the wafer enters the process chamber and before the temperature is raised, a small amount of oxygen is first introduced. The oxygen flow rate is controlled to be 8L / min, and the ventilation time is 10min (using the stage when the process chamber temperature is stable before heating, generally 10 minutes, but it can also be adjusted according to actual conditions). At this time, the process chamber temperature is the machine static temperature of 800°C. At a lower temperature, a silicon oxide layer with a thickness of about 70 angstroms, that is, the initial thickness of the silicon oxide layer, is formed on the second wafer, and then nitrogen is introduced. The purpose of introducing oxygen first and then nitrogen is to prevent annealing in a nitrogen environment without forming a sufficient silicon oxide layer.

[0036] After the silicon oxide layer is formed and before the annealing temperature is raised, a furnace cleaning is performed to remove a portion of the silicon oxide layer. The removed portion is approximately 30 angstroms thick, leaving a remaining silicon oxide layer of approximately 40 angstroms thick. The furnace cleaning process conditions are: 0.5% hydrofluoric acid treatment for 50 seconds, flow rate 800-1500 sccm; SC1 treatment is continued for 5 minutes, and then SC2 treatment is performed for 5 minutes. Hydrofluoric acid (DHF) is used to clean away approximately 30 angstroms of the silicon oxide layer and is typically used to remove the native oxide layer. However, in the present invention, a portion of the generated silicon oxide layer is removed. SC1 (HCL) / SC2 (NH4OH, H2O2) is used to remove particles. Pre-furnace cleaning is a step before each annealing. The general purpose of pre-furnace cleaning is to: 1. Remove particulate contaminants on the wafer caused by the environment and previous processes to prevent contamination of the furnace tube and wafer; 2. Remove the natural oxide layer produced by the wafer and oxygen in the air (the natural oxide layer is about 20-30 angstroms thick. In this embodiment, this cleaning is used to remove part of the silicon oxide in the silicon oxide layer formed by oxygen).

[0037] A silicon oxide layer is formed first, and then nitrogen is introduced before the temperature rise phase begins. The process conditions for the first high-temperature annealing are: annealing temperature of 1150°C, annealing time of 2 hours, a heating rate of 8°C / min, and pure nitrogen as the protective gas at a nitrogen flow rate of 8L / min. These process parameters can be adjusted according to actual needs.

[0038] Step 3: After the first high-temperature annealing, a composite wafer is obtained, and the second bonding surface with a silicon oxide layer formed on the second wafer in the composite wafer is bonded to the bonding surface of the third wafer for a second time. The third wafer is a silicon wafer. The process conditions for the second silicon-silicon bonding are: bonding in a pure nitrogen environment, a bonding temperature of 40°C, a bonding pressure of 12KN, and a bonding time of 12 minutes. These process parameters can be adjusted according to actual needs. During the second silicon-silicon bonding, the thickness of the silicon oxide layer on the second bonding surface is about 40 angstroms. Before bonding, the silicon oxide layer of about 40 angstroms can be flattened, but the bonding surface usually has a product structure, so attention should be paid to the risks brought by grinding.

[0039] Step 4: Perform a second high-temperature anneal on the bonded composite wafer and the third wafer. The process conditions for this second high-temperature anneal are: annealing temperature of 1150°C, annealing time of 2 hours, a heating rate of 8°C / min, and pure nitrogen as the shielding gas at a nitrogen flow rate of 8 L / min. These process parameters can be adjusted based on actual needs.

[0040] In this embodiment, the first, second, and third wafers are silicon wafers. The three-layer wafer bonded structure obtained in this embodiment can be used in optical communication products. For such products, the three-layer wafers must be silicon wafers. In other embodiments, the wafer type can be adjusted according to actual needs, and more layers of wafers can be bonded. When multiple silicon-silicon bonding is required, this method can be used to improve the annealing process.

[0041] The present invention introduces a small amount of oxygen into the annealing process after the first silicon-silicon bonding step before heating. This creates a silicon oxide layer on the wafer surface before high-temperature annealing. The silicon oxide layer isolates nitrogen from the silicon surface, preventing the formation of silicon nitride. During the second silicon-silicon bonding process, the presence of the silicon oxide layer on the second bonding surface of the second wafer strengthens the bond between the second and third wafers, improving the bonding effect.

[0042] Comparative Example 1

[0043] The difference from Example 1 is that oxygen is not introduced in the pre-heating stage of the first high-temperature annealing. Compared with the annealing method for improving the bonding effect of multi-layer wafers provided in Example 1:

[0044] See also Figure 2 and Figure 3 , Figure 2 The surface appearance of the wafer observed under an optical microscope after the first high-temperature annealing in the annealing process without oxygen, i.e., the annealing process provided in Comparative Example 1, Figure 3 The appearance of the wafer surface observed under an optical microscope after the first high-temperature annealing process provided by the oxygen-pass annealing process in Example 1. Figure 2 The brown area in the figure is the uneven silicon nitride formed on the wafer surface; Figure 3 There is no silicon nitride on the surface of the wafer, and the wafer surface is smoother.

[0045] C-SAM (Confocal Scanning Acoustic Microscopy) is a confocal scanning acoustic microscopy technique used to analyze the internal structure of samples and detect potential defects such as cracks, voids, and delamination. In semiconductor manufacturing, particularly after wafer bonding or packaging processes, C-SAM can effectively assess the quality of the bond interface and detect the presence of voids or other defects that could impact device performance and reliability.

[0046] See also Figure 4 and Figure 5 , Figure 4 The second bonding interface observed under C-SAM after the second high-temperature annealing without oxygen, i.e., the annealing process provided in Comparative Example 1, Figure 5 The bonding interface of the second bonding observed under C-SAM after the second high temperature annealing for oxygen permeation, i.e. the annealing process provided in Example 1. It can be seen that Figure 4 and Figure 5 The white area in the middle represents the cavity produced after bonding. Fewer cavities indicate better bonding. Compared with the bonding interface of Comparative Example 1, Example 1 shows significantly fewer cavities and better bonding. The silicon oxide film on the second bonding surface of the second wafer obtained by the treatment of Example 1 also facilitates the subsequent second silicon-silicon bonding, making the bond tighter and reducing the generation of cavities.

[0047] In the present invention, in the annealing process after the first bonding, oxygen is passed through before heating, and by controlling the time of oxygen passing and the oxygen flow rate, an oxide layer of about 70 angstroms is generated. The silicon oxide layer prevents the generation of silicon nitride, and also ensures that the oxide layer before annealing after the second bonding is less than 50 angstroms, thereby improving the bonding effect without affecting the stability of the device. Compared with adding a separate oxidation process before annealing after the first bonding, although an oxide layer can also be generated, the oxidation process is a high-temperature process, resulting in an extra heat treatment process, which affects the subsequent processes and also increases the production cost. The oxidation process of this solution is selected in a stable stage after the wafer enters the process chamber and before heating. The original function of this stage is to stabilize the temperature of the process chamber before heating. Oxidation in this stage will not increase the heat treatment time. Adding an extra oxidation process (the oxidation process is also a high-temperature process, and annealing is also taking place during oxidation) will increase the heat treatment time.

[0048] Example 2

[0049] Also provided is an annealing method for improving the bonding effect of multi-layer wafers and its application in the preparation process of optoelectronic semiconductor devices. In the production process of optoelectronic semiconductor devices such as optical fiber semiconductor devices, three-layer wafer silicon-silicon bonding and post-bonding annealing processes are often used. If there is no need to perform multi-layer wafer silicon-silicon bonding and annealing, that is, there is only a single silicon-silicon bonding and annealing process, it is usually not necessary to consider the impact of silicon nitride generated on the wafer surface during the annealing process on the second bonding or other subsequent processes. The annealing method for improving the bonding effect of multi-layer wafers provided by the present invention is particularly suitable for the preparation of optoelectronic semiconductor devices.

[0050] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. An annealing method for improving the bonding effect of multi-layer wafers, characterized in that: The following steps are involved: Step 1: Performing a first silicon-silicon bonding between the bonding surface of the first wafer and the first bonding surface of the second wafer; Step 2: performing a first high-temperature annealing on the bonded first and second wafers, wherein the protective gas during the first high-temperature annealing process comprises at least nitrogen, oxygen is introduced before the annealing temperature is raised, and a silicon oxide layer of a certain thickness is formed on the second wafer, and then the nitrogen is introduced; Step 3: obtaining a composite wafer through the first high-temperature annealing, and performing a second silicon-silicon bonding between the second bonding surface of the composite wafer having the silicon oxide layer formed on the second wafer and the bonding surface of the third wafer; During the second silicon-silicon bonding, the thickness of the silicon oxide layer on the second bonding surface is 30-50 angstroms; Step 4: performing a second high-temperature annealing on the bonded composite wafer and the third wafer.

2. The annealing method for improving multi-layer wafer bonding according to claim 1, wherein: In step 2, the process of introducing oxygen: the ventilation time is 8-12 minutes, and the oxygen flow rate is 6-9 L / min.

3. The annealing method for improving multi-layer wafer bonding according to claim 2, wherein: In step 2, the thickness of the silicon oxide layer formed on the second wafer is 65-75 angstroms; after the silicon oxide layer is formed and before annealing and heating, furnace cleaning is performed to remove part of the silicon oxide layer, and the removed part is a silicon oxide material with a thickness of 25-35 angstroms.

4. The annealing method for improving multi-layer wafer bonding according to claim 3, wherein: The process conditions for the furnace cleaning are: 0.5% hydrofluoric acid treatment for 40-80 seconds, flow rate 800-1500 sccm; continuing SC1 treatment for 3-7 minutes, and then using SC2 treatment for 3-7 minutes.

5. The annealing method for improving multi-layer wafer bonding according to claim 1, wherein: In step 1, the process conditions for the first silicon-silicon bonding are: bonding in a pure nitrogen environment, a bonding temperature of 40-60° C., a bonding pressure of 8-12 KN, and a bonding time of 8-12 min.

6. The annealing method for improving multi-layer wafer bonding according to claim 1, wherein: In step 2, the process conditions of the first high-temperature annealing are: annealing temperature of 1100-1200°C, annealing time of 2 hours or more, heating rate of 8-12°C / min, protective gas of pure nitrogen, and nitrogen flow rate of 8-12L / min.

7. The annealing method for improving multi-layer wafer bonding according to claim 1, characterized in that: In step 3, the process conditions for the second silicon-silicon bonding are: bonding in a pure nitrogen environment, a bonding temperature of 40-60° C., a bonding pressure of 8-12 kN, and a bonding time of 8-12 min.

8. The annealing method for improving multi-layer wafer bonding according to claim 1, characterized in that: In step 4, the process conditions of the second high-temperature annealing are: annealing temperature is 1100-1200°C, annealing time is 2 hours or more, heating rate is 8-12°C / min, protective gas is pure nitrogen, and nitrogen flow rate is 8-12L / min.

9. The annealing method for improving multi-layer wafer bonding according to claim 1, wherein: The first wafer, the second wafer and the third wafer are silicon wafers.

10. Use of the annealing method for improving multi-layer wafer bonding according to any one of claims 1 to 9 in the preparation process of optoelectronic semiconductor devices.

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