Formation method of semiconductor structure
By combining wet oxygen oxidation and dry oxygen oxidation processes to form the gate oxide layer of semiconductor devices, the leakage problem after device size is reduced is solved, and the device performance and reliability are improved.
Patent Information
- Application Number
- CN202510293023.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-12
- Publication Date
- 2025-05-30
AI Technical Summary
In existing semiconductor devices, as the device size shrinks, the thickness of the gate oxide layer becomes thinner, resulting in obvious leakage of the device, and the manufacturing process faces huge challenges.
The gate oxide layer is formed by a combination of wet oxygen oxidation process and dry oxygen oxidation process. In the wet oxygen oxidation process, the reaction gas is diluted by the first protective gas, which reduces the oxidation rate and improves density and thickness uniformity; the dry oxygen oxidation process optimizes interface performance and reduces interface defects.
It improves the density and interface performance of the gate oxide layer, reduces micropore defects, improves the TDDB performance of MOS devices and reduces the HCI effect.
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Figure CN120076390A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the field of semiconductor manufacturing technology, and particularly to a method for forming a semiconductor structure. Background Art
[0002] In semiconductor manufacturing technology, silicon dioxide, as a gate dielectric layer (i.e., gate oxide layer), has the advantages of simple preparation process, good bonding force with the silicon substrate, and few defects.
[0003] However, with the miniaturization of semiconductor device sizes, the thickness of the gate oxide layer gradually becomes thinner, and device leakage is obvious, which poses a great challenge to the manufacturing process of the gate oxide layer.
[0004] Therefore, the existing methods for forming a gate oxide layer need to be further improved. Summary of the Invention
[0005] The technical problem solved by the present invention is to provide a method for forming a semiconductor structure to improve the performance of the formed semiconductor structure.
[0006] To solve the above technical problem, the technical solution of the present invention provides a method for forming a semiconductor structure, including: providing a substrate; performing a first oxidation treatment on the substrate by using a wet oxidation process to form a first oxide layer on the surface of the substrate, the first oxide layer having a first thickness, and the step of performing the first oxidation treatment on the substrate by using the wet oxidation process includes an oxidation reaction stage, in which hydrogen and oxygen are used as reaction gases, and while introducing hydrogen and oxygen, a first protective gas is introduced to dilute the reaction gases; after the wet oxidation process, performing a second oxidation treatment on the substrate by using a dry oxidation process to form a second oxide layer between the surface of the substrate and the first oxide layer, and using the first oxide layer and the second oxide layer as the gate oxide layer, the second oxide layer having a second thickness, and the second thickness being less than the first thickness.
[0007] Optionally, in the oxidation reaction stage, oxygen has a first reaction gas flow rate, hydrogen has a second reaction gas flow rate, and the ratio range of the first reaction gas flow rate to the second reaction gas flow rate is from 5:1 to 1:1; the first protective gas has a first protective gas flow rate, and the ratio range of the first reaction gas flow rate to the first protective gas flow rate is from 1:8 to 1:20.
[0008] Optionally, the range of the first reaction gas flow rate is from 1 SLM to 15 SLM; the range of the second reaction gas flow rate is from 1 SLM to 15 SLM; the range of the first protective gas flow rate is from 5 SLM to 20 SLM.
[0009] Optionally, the substrate is placed in a reaction chamber; before the oxidation reaction stage, the first oxidation treatment of the substrate using the wet oxidation process further includes: a first stabilization stage, raising the temperature in the reaction chamber from a first process temperature to a second process temperature and maintaining the second process temperature for a first stabilization time; a second stabilization stage, raising the temperature in the reaction chamber from the second process temperature to a first reaction temperature and maintaining the first reaction temperature for a second stabilization time; a preparation stage, introducing oxygen with a third reaction gas flow rate and the protective gas with a second protective gas flow rate into the reaction chamber, where the third reaction gas flow rate is less than the first reaction gas flow rate and the second protective gas flow rate is less than the first protective gas flow rate.
[0010] Optionally, the process parameters of the wet oxidation process include: the first process temperature ranges from 500 °C to 700 °C; the second process temperature ranges from 600 °C to 850 °C; the first stabilization time ranges from 5 min to 20 min; the first reaction temperature ranges from 700 °C to 900 °C; the second stabilization time ranges from 5 min to 20 min; the third reaction gas flow rate ranges from 1 SLM to 10 SLM; the second protective gas flow rate ranges from 5 SLM to 20 SLM.
[0011] Optionally, the oxidation reaction stage includes: introducing the reaction gas and the first protective gas into the reaction chamber; performing an ignition operation in the reaction chamber to form water vapor from the reaction gas, and then performing the first oxidation treatment on the substrate.
[0012] Optionally, after the oxidation reaction stage, the first oxidation treatment of the substrate using the wet oxidation process further includes: introducing oxygen into the reaction chamber to discharge the residual water vapor and hydrogen in the reaction chamber; after discharging the residual water vapor and hydrogen in the reaction chamber, introducing the second protective gas into the reaction chamber to discharge the residual oxygen in the reaction chamber; under the atmosphere of the second protective gas, lowering the temperature in the reaction chamber from the first reaction temperature to room temperature.
[0013] Optionally, the cooling rate for lowering the temperature in the reaction chamber from the first reaction temperature to room temperature ranges from 1 °C / min to 5 °C / min.
[0014] Optionally, the second protective gas includes nitrogen, argon or noble gas.
[0015] Optionally, the first protective gas includes nitrogen, argon or noble gas.
[0016] Optionally, the substrate is placed in a reaction chamber; the second oxidation treatment of the substrate by using a dry oxygen oxidation process to form a second oxide layer between the surface of the substrate and the first oxide layer includes: after the temperature in the reaction chamber rises to a second reaction temperature, introducing oxygen into the reaction chamber to perform the second oxidation treatment on the surface of the substrate.
[0017] Optionally, the range of the second reaction temperature is from 700 °C to 950 °C.
[0018] Optionally, the ratio range of the first thickness to the second thickness is from 2:1 to 5:1.
[0019] Optionally, the range of the first thickness is from 40 Å to 120 Å; the range of the second thickness is from 20 Å to 35 Å.
[0020] Compared with the prior art, the technical solution of the embodiment of the present invention has the following beneficial effects:
[0021] In the method for forming a semiconductor structure provided by the technical solution of the present invention, a gate oxide layer is formed by combining a wet oxygen oxidation process and a dry oxygen oxidation process. And in the wet oxygen oxidation process, through the dilution effect of the first protective gas on the reaction gas, the amount of the reaction gas reaching the surface of the substrate is reduced, thereby reducing the oxidation rate, which is beneficial to improving the denseness and thickness uniformity of the gate oxide layer. The dry oxygen oxidation process is beneficial to improving the performance of the interface between the gate oxide layer and the substrate, and is beneficial to reducing interface defects. In addition, by controlling the thicknesses of the second oxide layer and the first oxide layer so that the second thickness is less than the first thickness, while optimizing the above interface performance, the introduction of excessive micropore defects by the dry oxygen oxidation process can be avoided, and the performance of the gate oxide layer can be improved as a whole, which is beneficial to improving the "time-dependent dielectric breakdown" (TDDB) performance of the formed MOS device and reducing the hot carrier injection effect (HCI) of the MOS device. BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figure 1 and Figure 2 is a schematic flowchart of a method for forming a semiconductor structure according to an embodiment of the present invention;
[0023] Figures 3 to 5 is a schematic structural diagram of each step of a method for forming a semiconductor structure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS
[0024] It should be noted that the "surface" and "upper" in this specification are used to describe the relative positional relationship in space and do not limit whether there is direct contact.
[0025] As described in the background art, the performance of semiconductor structures formed by existing gate oxide layer processes urgently needs to be improved. Here, an existing gate oxide layer process will be described.
[0026] Existing gate oxide layers are usually formed on the surface of substrate silicon by thermal oxidation processes. Thermal oxidation processes include wet oxygen oxidation and dry oxygen oxidation. Among them, dry oxygen oxidation uses dry and pure oxygen as the oxidation atmosphere and directly reacts with silicon at high temperatures (generally around 1000 °C). Wet oxygen oxidation uses water to replace oxygen and reacts with silicon at high temperatures. The dry oxygen oxidation rate is slower. The oxide film grown by dry oxygen oxidation has a dense structure, uniform film thickness, and fewer defects. However, it has more structural micropores than the wet oxygen oxidation process. These structural micropores are prone to cause electric field breakdown. The wet oxygen oxidation rate is faster. The filling effect of the OH bonds in wet oxygen oxidation on the structural micropores reduces the occurrence of structural micropores and can improve the electric field breakdown characteristics. However, the loose structure of the oxide film grown by wet oxygen oxidation will introduce more defects. In short, the existing methods for forming gate oxide layers need to be further improved.
[0027] To solve the above problems, in a method for forming a semiconductor structure provided by the present invention, a gate oxide layer is formed by combining a wet oxygen oxidation process and a dry oxygen oxidation process. And in the wet oxygen oxidation process, through the dilution effect of the first protective gas on the reaction gas, the amount of the reaction gas reaching the surface of the substrate is reduced, thereby reducing the oxidation rate, which is beneficial to improving the denseness and thickness uniformity of the gate oxide layer. The dry oxygen oxidation process is beneficial to improving the performance of the interface between the gate oxide layer and the substrate and is beneficial to reducing interface defects. In addition, by controlling the thicknesses of the second oxide layer and the first oxide layer so that the second thickness is less than the first thickness, while optimizing the above interface performance, excessive micropore defects introduced by the dry oxygen oxidation process can be avoided, and the performance of the gate oxide layer can be improved as a whole, which is beneficial to improving the "Time dependent dielectric breakdown" (TDDB) performance of the formed MOS device and reducing the "Hot Carrier Injection" (HCI) effect of the MOS device.
[0028] To make the above objects, features, and beneficial effects of the present invention more obvious and understandable, the following will describe in detail the specific embodiments of the present invention with reference to the accompanying drawings.
[0029] Figure 1 and Figure 2 is a schematic flow chart of a method for forming a semiconductor structure according to an embodiment of the present invention.
[0030] Please refer to Figure 1 and Figure 2 , the method for forming the semiconductor structure includes the following processes:
[0031] Step S101, providing a substrate;
[0032] Step S102, performing a first oxidation treatment on the substrate by using a wet oxidation process to form a first oxide layer on the surface of the substrate. The first oxide layer has a first thickness. The first oxidation treatment of the substrate by using the wet oxidation process includes an oxidation reaction stage. In the oxidation reaction stage, hydrogen and oxygen are used as reaction gases, and a first protective gas is introduced while introducing hydrogen and oxygen to dilute the reaction gases;
[0033] Step S103, after the wet oxidation process, performing a second oxidation treatment on the substrate by using a dry oxidation process to form a second oxide layer between the surface of the substrate and the first oxide layer. Taking the first oxide layer and the second oxide layer as a gate oxide layer, the second oxide layer has a second thickness, and the second thickness is less than the first thickness.
[0034] In this embodiment, before the oxidation reaction stage (step S1024), the first oxidation treatment of the substrate by using the wet oxidation process further includes:
[0035] Step S1021, a first stabilization stage, raising the temperature in the reaction chamber from a first process temperature to a second process temperature and maintaining a first stabilization time at the second process temperature;
[0036] Step S1022, a second stabilization stage, raising the temperature in the reaction chamber from the second process temperature to a first reaction temperature and maintaining a second stabilization time at the first reaction temperature;
[0037] Step S1023, a preparation stage, introducing oxygen with a third reaction gas flow rate and the protective gas with a second protective gas flow rate into the reaction chamber. The third reaction gas flow rate is less than the first reaction gas flow rate, and the second protective gas flow rate is less than the first protective gas flow rate.
[0038] In this embodiment, after the oxidation reaction stage, the first oxidation treatment of the substrate by using the wet oxidation process further includes:
[0039] Step S1025, introducing oxygen into the reaction chamber to discharge the residual water vapor and hydrogen in the reaction chamber;
[0040] Step S1026, after discharging the residual water vapor and hydrogen in the reaction chamber, introduce a second protective gas into the reaction chamber to discharge the residual oxygen in the reaction chamber;
[0041] Step S1027, under the atmosphere of the second protective gas, lower the temperature in the reaction chamber from the first reaction temperature to room temperature.
[0042] The following will be described in detail with reference to the accompanying drawings.
[0043] Figures 3 to 5 It is a schematic structural diagram of each step of the method for forming a semiconductor structure according to an embodiment of the present invention.
[0044] Please refer to Figure 3 , perform step S101 to provide a substrate 200.
[0045] In this embodiment, the material of the substrate 200 is silicon.
[0046] In other embodiments, the material of the substrate includes silicon carbide, silicon germanium, a multi-element semiconductor material composed of group III-V elements, silicon on insulator (SOI) or germanium on insulator (GOI). Among them, the multi-element semiconductor material composed of group III-V elements includes InP, GaAs, GaP, InAs, InSb, InGaAs or InGaAsP.
[0047] Please refer to Figure 4 , perform step S102 to perform a first oxidation treatment on the substrate 200 by a wet oxidation process to form a first oxide layer 201 on the surface of the substrate 200. The first oxide layer 201 has a first thickness d1. The first oxidation treatment of the substrate 200 by the wet oxidation process includes an oxidation reaction stage. In the oxidation reaction stage, hydrogen and oxygen are used as reaction gases, and a first protective gas is introduced while introducing hydrogen and oxygen to dilute the reaction gases.
[0048] Here, the dilution effect of the first protective gas on the reaction gases reduces the amount of the reaction gases reaching the surface of the substrate 200, thereby reducing the oxidation rate and facilitating the improvement of the density and uniformity of the first oxide layer 201.
[0049] In this embodiment, the substrate 200 is placed in a reaction chamber.
[0050] Specifically, the oxidation reaction stage includes: introducing the reaction gases and the first protective gas into the reaction chamber; performing an ignition operation in the reaction chamber to form water vapor from the reaction gases, and then performing the first oxidation treatment on the substrate 200.
[0051] In the oxidation reaction stage, oxygen has a first reaction gas flow rate, and hydrogen has a second reaction gas flow rate. The ratio range of the first reaction gas flow rate to the second reaction gas flow rate is from 5:1 to 1:1. In this embodiment, the ratio of the first reaction gas flow rate to the second reaction gas flow rate is 1:1, that is, the flow rate ratio of oxygen to hydrogen is 1:1.
[0052] The first protective gas includes nitrogen, argon, or noble gas. In this embodiment, the first protective gas is nitrogen.
[0053] The first protective gas has a first protective gas flow rate. The ratio range of the first reaction gas flow rate to the first protective gas flow rate is from 1:8 to 1:20. In this embodiment, the ratio of the first reaction gas flow rate to the first protective gas flow rate is 1:16, that is, the flow rate ratio of oxygen to nitrogen is 1:16.
[0054] The range of the first reaction gas flow rate is from 1 SLM to 15 SLM; the range of the second reaction gas flow rate is from 1 SLM to 15 SLM. In this embodiment, both the first reaction gas flow rate and the second reaction gas flow rate are 1 SLM.
[0055] The range of the first protective gas flow rate is from 5 SLM to 20 SLM. In this embodiment, the first protective gas flow rate is 16 SLM.
[0056] The range of the first thickness d1 is from 40 Å to 120 Å. In this embodiment, the first thickness d1 is 68 Å.
[0057] In this embodiment, before the oxidation reaction stage (step S1024), steps S1021 to S1023 are also executed, including: a first stabilization stage, raising the temperature in the reaction chamber from a first process temperature to a second process temperature and maintaining a first stabilization time at the second process temperature; a second stabilization stage, raising the temperature in the reaction chamber from the second process temperature to a first reaction temperature and maintaining a second stabilization time at the first reaction temperature; a preparation stage, introducing oxygen with a third reaction gas flow rate and the protective gas with a second protective gas flow rate into the reaction chamber, where the third reaction gas flow rate is less than the first reaction gas flow rate and the second protective gas flow rate is less than the first protective gas flow rate.
[0058] Here, through the first stabilization stage and the second stabilization stage, stable temperature conditions are provided for the surface oxidation of the substrate 200; through the preparation stage, introducing oxygen and the second protective gas with smaller flow rates into the reaction chamber is beneficial to uniformly adsorb oxygen molecules onto the surface of the substrate 200 and improve the uniformity of oxidation.
[0059] The process parameters of the wet oxidation process include: the first process temperature ranges from 500°C to 700°C; the second process temperature ranges from 600°C to 850°C; the first stabilization time ranges from 5 min to 20 min; the first reaction temperature ranges from 700°C to 900°C; the second stabilization time ranges from 5 min to 20 min; the third reaction gas flow rate ranges from 1 SLM to 10 SLM; the second protective gas flow rate ranges from 5 SLM to 20 SLM. In this embodiment, the first process temperature is 600°C, the second process temperature is 750°C, the first stabilization time is 5 min, the first reaction temperature is 800°C, and the second stabilization time is 10 min.
[0060] In this embodiment, after the oxidation reaction stage (step S1024), steps S1025 to S1027 are further performed, including: introducing oxygen into the reaction chamber to discharge the residual water vapor and hydrogen in the reaction chamber; after discharging the residual water vapor and hydrogen in the reaction chamber, introducing a second protective gas into the reaction chamber to discharge the residual oxygen in the reaction chamber; under the atmosphere of the second protective gas, reducing the temperature in the reaction chamber from the first reaction temperature to room temperature.
[0061] In this embodiment, the cooling rate for reducing the temperature in the reaction chamber from the first reaction temperature to room temperature ranges from 1°C / min to 5°C / min. The purpose of selecting the cooling rate is to reduce the stress problem introduced by too high a cooling rate.
[0062] The second protective gas includes nitrogen, argon, or noble gas. In this embodiment, the second protective gas is nitrogen.
[0063] Please refer to Figure 5 , after the wet oxidation process, a second oxidation treatment is performed on the substrate 200 using a dry oxidation process, and a second oxide layer 202 is formed between the surface of the substrate 200 and the first oxide layer 201. Using the first oxide layer 201 and the second oxide layer 202 as the gate oxide layer, the second oxide layer 202 has a second thickness, and the second thickness is less than the first thickness.
[0064] So far, the gate oxide layer is formed by combining the wet oxidation process and the dry oxidation process. In the wet oxidation process, through the dilution effect of the first protective gas on the reaction gas, the amount of the reaction gas reaching the surface of the substrate 200 is reduced, thereby reducing the oxidation rate, which is beneficial to improving the compactness and thickness uniformity of the gate oxide layer. The dry oxidation process is beneficial to improving the performance of the interface between the gate oxide layer and the substrate 200 and reducing interface defects. In addition, by controlling the thicknesses of the second oxide layer 202 and the first oxide layer 201 such that the second thickness is less than the first thickness, while optimizing the above interface performance, excessive micropore defects introduced by the dry oxidation process can be avoided, and the performance of the gate oxide layer can be improved as a whole, which is beneficial to improving the "time-dependent dielectric breakdown" (TDDB) performance of the formed MOS device and reducing the hot carrier injection (HCI) effect of the MOS device.
[0065] In this embodiment, the substrate 200 is subjected to a second oxidation treatment using the dry oxidation process to form a second oxide layer 202 between the surface of the substrate 200 and the first oxide layer 201, which includes: after the temperature in the reaction chamber rises to the second reaction temperature, oxygen is introduced into the reaction chamber to perform the second oxidation treatment on the surface of the substrate 200.
[0066] The range of the second reaction temperature is from 700 °C to 950 °C. In this embodiment, the second reaction temperature is 850 °C.
[0067] The ratio range of the first thickness d1 to the second thickness d2 is from 2:1 to 5:1. The reason for selecting the above thickness range is to take into account the performance of the oxide layer formed by the wet oxidation process and the dry oxidation process.
[0068] The range of the second thickness d2 is from 20 Å to 35 Å. In this embodiment, the second thickness d2 is 26 Å.
[0069] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be determined by the scope defined by the claims.
Claims
1. A method for forming a semiconductor structure, characterized in that: include: providing a substrate; Performing a first oxidation treatment on the substrate by using a wet oxygen oxidation process to form a first oxide layer on the surface of the substrate, wherein the first oxide layer has a first thickness, wherein the first oxidation treatment on the substrate by using the wet oxygen oxidation process includes an oxidation reaction stage, wherein hydrogen and oxygen are used as reaction gases in the oxidation reaction stage, and a first protective gas is introduced while the hydrogen and oxygen are introduced to dilute the reaction gas; After the wet oxygen oxidation process, a dry oxygen oxidation process is used to perform a second oxidation treatment on the substrate to form a second oxide layer between the substrate surface and the first oxide layer, the first oxide layer and the second oxide layer are used as gate oxide layers, and the second oxide layer has a second thickness, which is less than the first thickness.
2. The method for forming a semiconductor structure according to claim 1, wherein: In the oxidation reaction stage, oxygen has a first reaction gas flow rate, hydrogen has a second reaction gas flow rate, and the ratio of the first reaction gas flow rate to the second reaction gas flow rate ranges from 5:1 to 1:1; the first protective gas has a first protective gas flow rate, and the ratio of the first reaction gas flow rate to the first protective gas flow rate ranges from 1:8 to 1:
20.
3. The method for forming a semiconductor structure according to claim 2, wherein: The flow rate range of the first reaction gas is 1SLM to 15SLM; the flow rate range of the second reaction gas is 1SLM to 15SLM; and the flow rate range of the first protective gas is 5SLM to 20SLM.
4. The method for forming a semiconductor structure according to claim 1, wherein: The substrate is placed in a reaction chamber; before the oxidation reaction stage, the first oxidation treatment of the substrate using a wet oxygen oxidation process also includes: a first stabilization stage, raising the temperature in the reaction chamber from a first process temperature to a second process temperature, and maintaining a first stabilization time at the second process temperature; a second stabilization stage, raising the temperature in the reaction chamber from the second process temperature to a first reaction temperature, and maintaining a second stabilization time at the first reaction temperature; a preparatory stage, introducing oxygen with a third reaction gas flow rate and the protective gas with a second protective gas flow rate into the reaction chamber, the third reaction gas flow rate is less than the first reaction gas flow rate, and the second protective gas flow rate is less than the first protective gas flow rate.
5. The method for forming a semiconductor structure according to claim 4, wherein: The process parameters of the wet oxygen oxidation process include: the first process temperature range is 500°C to 700°C; the second process temperature range is 600°C to 850°C; the first stabilization time range is 5min to 20min; the first reaction temperature range is 700°C to 900°C; the second stabilization time range is 5min to 20min; the third reaction gas flow range is 1SLM to 10SLM; the second protective gas flow range is 5SLM to 20SLM.
6. The method for forming a semiconductor structure according to claim 4, wherein: The oxidation reaction stage includes: introducing the reaction gas and the first protective gas into the reaction chamber; performing an ignition operation in the reaction chamber to make the reaction gas form water vapor, and then performing the first oxidation treatment on the substrate.
7. The method for forming a semiconductor structure according to claim 6, wherein: After the oxidation reaction stage, the first oxidation treatment of the substrate using the wet oxygen oxidation process also includes: introducing oxygen into the reaction chamber to discharge residual water vapor and hydrogen in the reaction chamber; after discharging the residual water vapor and hydrogen in the reaction chamber, introducing a second protective gas into the reaction chamber to discharge residual oxygen in the reaction chamber; under the atmosphere of the second protective gas, reducing the temperature in the reaction chamber from the first reaction temperature to room temperature.
8. The method for forming a semiconductor structure according to claim 7, wherein: The temperature reduction rate for reducing the temperature in the reaction chamber from the first reaction temperature to room temperature is in a range of 1°C / min to 5°C / min.
9. The method for forming a semiconductor structure according to claim 7, wherein: The second protective gas includes nitrogen, argon or a rare gas.
10. The method for forming a semiconductor structure according to claim 1, wherein: The first protective gas includes nitrogen, argon or a rare gas.
11. The method for forming a semiconductor structure according to claim 1, wherein: The substrate is placed in the reaction chamber; The method of performing a second oxidation treatment on the substrate using a dry oxygen oxidation process to form a second oxide layer between the substrate surface and the first oxide layer includes: after the temperature in the reaction chamber rises to a second reaction temperature, introducing oxygen into the reaction chamber to perform the second oxidation treatment on the substrate surface.
12. The method for forming a semiconductor structure according to claim 11, wherein: The second reaction temperature ranges from 700°C to 950°C.
13. The method for forming a semiconductor structure according to claim 1, wherein: The ratio of the first thickness to the second thickness ranges from 2:1 to 5:
1.
14. The method for forming a semiconductor structure according to claim 1, wherein: The first thickness ranges from 40Å to 120Å; the second thickness ranges from 20Å to 35Å.