Image sensor transfer gate dielectric layer structure and method of making same

Abstract

The application provides an image sensor transfer gate dielectric layer structure and a manufacturing method thereof. The structure comprises a first gate dielectric layer, a second gate dielectric layer and a third gate dielectric layer which are stacked in sequence. The first gate dielectric layer is an oxide layer, the second gate dielectric layer is an oxide layer containing nitrogen elements, and the third gate dielectric layer is an oxide layer or a high dielectric constant material layer. The content of nitrogen elements in the second gate dielectric layer gradually increases from bottom to top. In the application, the first gate dielectric layer directly contacts the substrate surface to ensure the interface quality. The content of nitrogen elements in the second gate dielectric layer increases from bottom to top, forming a stress gradient, which helps to improve the quality of the gate dielectric layer, reduce the interface trap, lower the leakage current and the interface defect density. The third gate dielectric layer is used to provide overall insulation and improve reliability. The combination of the first gate dielectric layer, the second gate dielectric layer and the third gate dielectric layer not only ensures the quality and insulation performance of the gate dielectric layer structure, but also reduces the leakage current and the interface defect density, and improves the reliability.

Description

Technical Field

[0001] This application relates to the field of semiconductor integrated circuit technology, and in particular to an image sensor transmission gate dielectric layer structure and its fabrication method. Background Technology

[0002] In the transmission gate of the pixel region of a CMOS image sensor, the gate dielectric layer is the core medium controlling the channel carriers. As process nodes become smaller, traditional single-layer silicon dioxide as the gate dielectric layer faces many problems. For example, there is the problem of electric field concentration. PN junctions are usually planar or slightly curved. At the junction edge (channel edge or corner of shallow trench isolation structure), due to the large geometric curvature, abrupt change in dielectric constant, and high interface state density, the electric field strength is significantly enhanced. The strong electric field accelerates the collisional ionization of carriers, generating additional electron-hole pairs. The bending of the surface band intensifies the formation of surface leakage paths. Therefore, the local leakage current / dark current increases at the channel edge or corner of shallow trench isolation structure, which can easily cause the gate dielectric layer to break down, affecting the device performance and power consumption.

[0003] Another example is the requirement for high uniformity of gate dielectric layer thickness. If the gate dielectric layer is too thin, it is prone to breakdown, and if it is too thick, the coupling is poor. It is difficult to ensure both low defects and low leakage current at the same time with the thickness of a single gate dielectric layer. Summary of the Invention

[0004] This application provides an image sensor transmission gate dielectric layer structure and its fabrication method, which not only ensures the quality and insulation performance of the image sensor transmission gate dielectric layer structure, but also reduces leakage current and interface defect density, thereby improving reliability.

[0005] To address the aforementioned technical problems, according to a first aspect of this application, an image sensor transmission gate dielectric layer structure is provided, comprising a first gate dielectric layer, a second gate dielectric layer, and a third gate dielectric layer stacked sequentially, wherein the first gate dielectric layer is an oxide layer, the second gate dielectric layer is a nitrogen-containing oxide layer, and the third gate dielectric layer is an oxide layer or a high dielectric constant material layer; and the nitrogen content in the second gate dielectric layer gradually increases from near the first gate dielectric layer to far away from the first gate dielectric layer.

[0006] Optionally, the thickness of the third gate dielectric layer gradually increases from the center to the edge.

[0007] Optionally, the image sensor transmission gate dielectric layer structure includes a central region, a near-edge region, and an edge region. The thickness of the third gate dielectric layer in the edge region is greater than the thickness of the third gate dielectric layer in the near-edge region, and the thickness of the third gate dielectric layer in the near-edge region is greater than the thickness of the third gate dielectric layer in the central region.

[0008] Optionally, the central region occupies more than two-thirds of the entire area of ​​the image sensor transmission gate dielectric layer structure.

[0009] Optionally, the overall thickness of the image sensor transmission gate dielectric layer structure in the central region is 3.5nm~6.5nm, the overall thickness of the image sensor transmission gate dielectric layer structure in the near-edge region is 4.5nm~7.5nm, and the overall thickness of the image sensor transmission gate dielectric layer structure in the edge region is 5.5nm~8.5nm; wherein, the thickness of the first gate dielectric layer is 1nm~1.5nm and the thickness of the second gate dielectric layer is 1nm~2nm throughout the entire region; the thickness of the third gate dielectric layer in the central region is 1.5nm~3nm, the thickness of the third gate dielectric layer in the near-edge region is 2.5nm~4nm, and the thickness of the third gate dielectric layer in the edge region is 3.5nm~5nm.

[0010] Optionally, the oxide layer may be made of silicon dioxide, and the nitrogen-containing oxide layer may be made of silicon oxynitride.

[0011] To address the aforementioned technical problems, according to a second aspect of this application, a method for fabricating an image sensor transmission gate dielectric layer structure is also provided, comprising the following steps:

[0012] A first gate dielectric layer is formed on a substrate, wherein the first gate dielectric layer is an oxide layer;

[0013] A second gate dielectric layer is formed on the first gate dielectric layer, wherein the second gate dielectric layer is a nitrogen-containing oxide layer, and the nitrogen content in the second gate dielectric layer gradually increases from near to far from the first gate dielectric layer; and,

[0014] A third gate dielectric layer is formed on the second gate dielectric layer, wherein the third gate dielectric layer is an oxide layer or a high dielectric constant material layer.

[0015] Optionally, the second gate dielectric layer is formed using a deposition process; during the formation of the second gate dielectric layer, nitrogen-containing gas is introduced into the chamber, and the content of nitrogen-containing gas gradually increases.

[0016] Optionally, after forming the third gate dielectric layer, the method further includes:

[0017] A first patterned photoresist layer is formed, which covers the third gate dielectric layer in the edge region and exposes the third gate dielectric layer in the near-edge region and the center region.

[0018] Using the first patterned photoresist layer as a mask, the third gate dielectric layer is etched such that the thickness of the third gate dielectric layer in the edge region is greater than the thickness of the third gate dielectric layer in the near-edge region and the center region.

[0019] Remove the first patterned photoresist layer;

[0020] A second patterned photoresist layer is formed, which covers the third gate dielectric layer in the edge region and the near-edge region, exposing the third gate dielectric layer in the center region;

[0021] Using the second patterned photoresist layer as a mask, the third gate dielectric layer is etched such that the thickness of the third gate dielectric layer in the near-edge region is greater than the thickness of the third gate dielectric layer in the center region; and...

[0022] Remove the second patterned photoresist layer.

[0023] Optionally, after forming the third gate dielectric layer, the process further includes annealing the first gate dielectric layer, the second gate dielectric layer, and the third gate dielectric layer.

[0024] The image sensor transmission gate dielectric layer structure and its fabrication method provided in this application include a first gate dielectric layer, a second gate dielectric layer, and a third gate dielectric layer stacked sequentially. The first gate dielectric layer is an oxide layer, the second gate dielectric layer is a nitrogen-containing oxide layer, and the third gate dielectric layer is an oxide layer or a high-dielectric-constant material layer. Furthermore, the nitrogen content in the second gate dielectric layer gradually increases from near to far from the first gate dielectric layer. The first gate dielectric layer is in direct contact with the substrate surface to ensure interface quality. The increasing nitrogen content in the second gate dielectric layer from bottom to top creates a stress gradient, which helps improve the quality of the gate dielectric layer, reduces interface traps, and thus effectively reduces leakage current and interface defect density. The third gate dielectric layer provides overall insulation and improves reliability. The combination of the first, second, and third gate dielectric layers ensures both the quality and insulation performance of the image sensor transmission gate dielectric layer structure, while also reducing leakage current and interface defect density, thereby improving reliability.

[0025] Furthermore, the thickness of the third gate dielectric layer gradually increases from its center to its edge, meaning the third gate dielectric layer has a structure that is thinner in the middle and thicker at the edges. This makes the entire image sensor transmission gate dielectric layer structure also thinner in the middle and thicker at the edges. The thinner central region improves capacitive coupling efficiency and charge transfer performance, while the thicker edge region alleviates electric field concentration, reducing the probability of gate dielectric layer breakdown and improving reliability. It also reduces dark current. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the image sensor transmission gate dielectric layer structure provided in an embodiment of this application.

[0027] Figure 2 This is a schematic diagram of the image sensor transmission gate dielectric layer structure provided in another embodiment of this application.

[0028] Figure 3 This is a schematic flowchart illustrating a method for fabricating an image sensor transmission gate dielectric layer structure according to an embodiment of this application.

[0029] Figure 4 This is a schematic diagram of the structure after the formation of the first patterned photoresist layer, provided in an embodiment of this application.

[0030] Figure 5 This is a schematic diagram of the structure after forming the second patterned photoresist layer according to an embodiment of this application.

[0031] Explanation of reference numerals in the attached figures:

[0032] 10-Substrate; 20-Image sensor transmission gate dielectric layer structure; 21-First gate dielectric layer; 22-Second gate dielectric layer; 23-Third gate dielectric layer; 31-First patterned photoresist layer; 32-Second patterned photoresist layer. Detailed Implementation

[0033] To make the objectives, advantages, and features of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and specific embodiments. It should be noted that the drawings are all in a very simplified form and are not drawn to scale, used only to facilitate and clarify the illustration of the embodiments of this application. Furthermore, the structures shown in the drawings are often part of the actual structures. In particular, different figures may emphasize different aspects and sometimes use different scales.

[0034] As used herein, the singular forms “a,” “an,” and “the” include plural objects unless otherwise expressly indicated. As used herein, the term “or” is generally used to include “and / or” unless otherwise expressly indicated. As used herein, the term “a number” is generally used to include “at least one” unless otherwise expressly indicated. As used herein, the term “at least two” is generally used to include “two or more” unless otherwise expressly indicated. Furthermore, the terms “first,” “second,” and “third” are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as “first,” “second,” or “third” may explicitly or implicitly include one or at least two of that feature, unless otherwise expressly indicated.

[0035] Figure 1 This is a schematic diagram of the image sensor transmission gate dielectric layer structure provided in one embodiment of this application. Please refer to... Figure 1 As shown, the image sensor transmission gate dielectric layer structure 20 provided in this application embodiment includes a first gate dielectric layer 21, a second gate dielectric layer 22, and a third gate dielectric layer 23 stacked sequentially. The first gate dielectric layer 21 is an oxide layer, the second gate dielectric layer 22 is a nitrogen-containing oxide layer, and the third gate dielectric layer 23 is an oxide layer or a high dielectric constant material layer. Furthermore, the nitrogen content in the second gate dielectric layer 22 gradually increases from near the first gate dielectric layer 21 to far away from the first gate dielectric layer 21.

[0036] The image sensor transmission gate dielectric layer structure 20 is located on the substrate 10. The substrate 10 can be made of silicon, germanium, silicon-germanium, silicon carbon, indium arsenide, gallium arsenide, indium phosphide, or other III / V compound semiconductor materials. The substrate 10 can also be implanted with certain dopant particles to change the electrical parameters according to design requirements. In this embodiment, the substrate 10 is a silicon substrate.

[0037] The first gate dielectric layer 21 is located on the substrate 10. The first gate dielectric layer 21 is an oxide layer, which is in direct contact with the substrate 10 to ensure interface quality. In one embodiment, the thickness of the first gate dielectric layer 21 is 1 nm to 1.5 nm, for example, 1 nm, 1.2 nm, or 1.5 nm, but it is not limited to these. The material of the oxide layer includes, but is not limited to, silicon dioxide.

[0038] The second gate dielectric layer 22 is located on the first gate dielectric layer 21, and the second gate dielectric layer 22 is a nitrogen-containing oxide layer. The presence of nitrogen can suppress interface defects, and the incorporation of nitrogen can reduce the formation of interface defects through chemical passivation. Interface traps are usually caused by incomplete reactions during the oxidation process or lattice mismatch between silicon and oxide. The addition of nitrogen can improve the silicon-oxide connection at the interface, reduce undesirable interface reactions, and thus reduce the trap density. Nitrogen can form stable nitrogen-silicon bonds, reduce oxidation defects in the oxide layer, and at the same time, nitrogen can also act as a passivator in the oxide, reducing interface defects that trap electrons and holes, thereby suppressing leakage current and noise.

[0039] In this embodiment, the nitrogen content in the second gate dielectric layer 22 gradually increases from near the first gate dielectric layer 21 to far away from it. Specifically, the side of the second gate dielectric layer 22 closest to the first gate dielectric layer 21 is designated as the bottom, and the side of the second gate dielectric layer 22 closest to the third gate dielectric layer 23 is designated as the top. From the bottom to the top of the second gate dielectric layer 22, the nitrogen content gradually increases, creating a stress gradient. The incorporation of nitrogen into silicon oxide leads to lattice changes, typically increasing the compressive stress of the oxide layer. Nitrogen atoms replace some silicon atoms in the oxide (forming silicon-nitrogen bonds) and form different chemical bonds with oxygen atoms in the oxide, which generates different stresses in localized areas. As the nitrogen concentration increases, this stress gradually increases, thus forming a stress gradient within the oxide layer. That is, the nitrogen concentration is lower and the stress is lower near the surface of the first gate dielectric layer 21, while the nitrogen concentration is higher and the stress is greater near the surface of the third gate dielectric layer 23.

[0040] The introduction of nitrogen atoms alters the lattice structure of the oxide layer. At low nitrogen concentrations, the effect of nitrogen on the oxide layer is minimal, resulting in lower stress. However, as the nitrogen concentration gradually increases, the introduction of nitrogen causes lattice deformation in the oxide layer (e.g., silicon oxide), typically generating compressive stress, which concentrates in regions with higher nitrogen concentrations. The addition of nitrogen may lead to localized lattice distortion, making the oxide layer's lattice more compact than that of a pure silicon oxide layer, thereby generating internal stress. By finely adjusting the nitrogen concentration gradient, the stress distribution in the oxide layer (i.e., the second gate dielectric layer 22) can be controlled to ensure that the stress in different regions of the overall image sensor transmission gate dielectric layer structure 20 reaches an optimal state, thereby maximizing the suppression of interface defects and improving device stability.

[0041] In one embodiment, the thickness of the second gate dielectric layer 22 is 1 nm to 2 nm, for example, the thickness of the second gate dielectric layer 22 is 1 nm, 1.5 nm or 2 nm, but it is not limited to these. The material of the second gate dielectric layer 22 includes, but is not limited to, silicon oxynitride.

[0042] The third gate dielectric layer 23 is located on the second gate dielectric layer 22, and the third gate dielectric layer 23 is an oxide layer or a high dielectric constant material layer. The third gate dielectric layer 23 is used to provide overall insulation and improve reliability. In one embodiment, the third gate dielectric layer 23 may also be a hybrid layer of an oxide layer and a high dielectric constant material layer. The oxide layer may include, but is not limited to, silicon dioxide, and the high dielectric constant material layer may include, but is not limited to, hafnium oxide (HfO2), titanium oxide (TiO), lanthanum oxide (LaO), tantalum oxide (Ta2O5), strontium titanium oxide (SrTiO3), hafnium silicon oxide (HfSiO), or zirconium oxide (ZrO2).

[0043] In one embodiment, the thickness of the third gate dielectric layer 23 is 1.5nm to 3nm, for example, the thickness of the third gate dielectric layer 23 is 1.5nm, 2nm, 2.5nm or 3nm, but it is not limited to these.

[0044] In one embodiment of this application, the first gate dielectric layer 21, the second gate dielectric layer 22, and the third gate dielectric layer 23 together constitute the image sensor transmission gate dielectric layer structure 20. The first gate dielectric layer 21 is in direct contact with the surface of the substrate 10 to ensure interface quality. The nitrogen content in the second gate dielectric layer 22 increases from bottom to top, forming a stress gradient, which helps to improve the quality of the gate dielectric layer, reduce interface traps, and thus effectively reduce leakage current and interface defect density. The third gate dielectric layer 23 is used to provide overall insulation and improve reliability. The combination of the first gate dielectric layer 21, the second gate dielectric layer 22, and the third gate dielectric layer 23 not only ensures the quality and insulation performance of the image sensor transmission gate dielectric layer structure 20, but also reduces leakage current and interface defect density, thereby improving reliability.

[0045] In another embodiment of this application, based on the previous embodiment, the thickness of the third gate dielectric layer 23 gradually increases from the center to the edge, that is, the third gate dielectric layer 23 has a structure that is thin in the middle and thick at the edges. This makes the entire image sensor transmission gate dielectric layer structure 20 also thin in the middle and thick at the edges. The thinner central region can improve capacitive coupling efficiency and charge transfer performance, while the thicker edge region can alleviate electric field concentration, reduce the breakdown probability of the gate dielectric layer, and improve reliability. At the same time, it can also reduce dark current.

[0046] Figure 2This is a schematic diagram of the image sensor transmission gate dielectric layer structure provided in another embodiment of this application. Please refer to... Figure 2 As shown, in this embodiment, the image sensor transmission gate dielectric layer structure 20 is divided into a central region I, a near-edge region II, and an edge region III. The edge region III surrounds the near-edge region II, and the near-edge region II surrounds the central region I. The thickness of the third gate dielectric layer 23 in the edge region III is greater than the thickness of the third gate dielectric layer 23 in the near-edge region II, and the thickness of the third gate dielectric layer 23 in the near-edge region II is greater than the thickness of the third gate dielectric layer 23 in the central region I, resulting in the third gate dielectric layer 23 having a structure that is thinner in the middle and thicker at the edges.

[0047] In one embodiment, the central region I occupies more than two-thirds of the entire area of ​​the image sensor transmission gate dielectric layer structure 20, that is, the thinner area of ​​the third gate dielectric layer 23 needs to occupy more than two-thirds of the entire area, so as to avoid affecting the performance of the image sensor transmission gate dielectric layer structure 20. Of course, it is not limited to this and can be determined according to actual needs.

[0048] In one embodiment, the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the central region I is 3.5 nm to 6.5 nm, the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the near-edge region II is 4.5 nm to 7.5 nm, and the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the edge region III is 5.5 nm to 8.5 nm. It should be noted that this embodiment provides thickness ranges for each region. Although these ranges overlap, the thickness of each region should satisfy the premise of being thinner in the middle and thicker at the edges. For example, when the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the central region I is 6.5 nm, the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the near-edge region II needs to be greater than 6.5 nm, for example, 7.5 nm, and the overall thickness of the image sensor transmission gate dielectric layer structure 20 in the edge region III needs to be greater than 7.5 nm, for example, 8.5 nm.

[0049] In one embodiment, the first gate dielectric layer 21 has a consistent thickness throughout the entire region (i.e., including the central region I, near-edge region II, and edge region III), for example, 1 nm to 1.5 nm. The second gate dielectric layer 22 also has a consistent thickness throughout the entire region, for example, 1 nm to 2 nm. The thickness of the third gate dielectric layer 23 in the central region I is 1.5 nm to 3 nm, the thickness of the third gate dielectric layer 23 in the near-edge region II is 2.5 nm to 4 nm, and the thickness of the third gate dielectric layer 23 in the edge region III is 3.5 nm to 5 nm. However, this is not the only possible embodiment.

[0050] It should be noted that the numerical range given in the embodiments of this application includes the endpoints on both sides. For example, the thickness of the first gate dielectric layer 21 is 1nm~1.5nm, which means that the thickness of the first gate dielectric layer 21 is greater than or equal to 1nm and less than or equal to 1.5nm.

[0051] In this embodiment, the image sensor transmission gate dielectric layer structure 20 includes a first gate dielectric layer 21, a second gate dielectric layer 22, and a third gate dielectric layer 23 stacked sequentially. The first gate dielectric layer 21 is an oxide layer, the second gate dielectric layer 22 is a nitrogen-containing oxide layer, and the third gate dielectric layer 23 is an oxide layer or a high-dielectric-constant material layer. The nitrogen content in the second gate dielectric layer 22 gradually increases from near to far from the first gate dielectric layer 21. The first gate dielectric layer 21 is in direct contact with the surface of the substrate 10 to ensure interface quality. The increasing nitrogen content in the second gate dielectric layer 22 from bottom to top creates a stress gradient, which helps improve the quality of the gate dielectric layer, reduces interface traps, and effectively reduces leakage current and interface defect density. The third gate dielectric layer 23 provides overall insulation and improves reliability. The combination of the first gate dielectric layer 21, the second gate dielectric layer 22, and the third gate dielectric layer 23 ensures both the quality and insulation performance of the image sensor transmission gate dielectric layer structure 20, while also reducing leakage current and interface defect density and improving reliability.

[0052] Furthermore, the thickness of the third gate dielectric layer 23 gradually increases from its center to its edge, meaning the third gate dielectric layer 23 has a structure that is thinner in the middle and thicker at the edges. This makes the entire image sensor transmission gate dielectric layer structure 20 also thinner in the middle and thicker at the edges. The thinner central region can improve capacitive coupling efficiency and charge transfer performance, while the thicker edge region can alleviate electric field concentration, reduce the breakdown probability of the gate dielectric layer, and improve reliability. It can also reduce dark current.

[0053] Accordingly, the present invention also provides a method for fabricating an image sensor transmission gate dielectric layer structure, for fabricating the image sensor transmission gate dielectric layer structure as described above. Figure 3 This is a schematic flowchart illustrating a method for fabricating an image sensor transmission gate dielectric layer structure according to an embodiment of this application. Please refer to it. Figure 3 As shown, the method for fabricating the image sensor transmission gate dielectric layer structure provided in this application includes the following steps:

[0054] S1: A first gate dielectric layer is formed on the substrate, wherein the first gate dielectric layer is an oxide layer;

[0055] S2: A second gate dielectric layer is formed on the first gate dielectric layer, wherein the second gate dielectric layer is a nitrogen-containing oxide layer, and the nitrogen content in the second gate dielectric layer gradually increases from near the first gate dielectric layer to far away from the first gate dielectric layer; and,

[0056] S3: A third gate dielectric layer is formed on the second gate dielectric layer, wherein the third gate dielectric layer is an oxide layer or a high dielectric constant material layer.

[0057] In step S1, please refer to Figure 1 As shown, a first gate dielectric layer 21 is formed on the substrate 10, wherein the first gate dielectric layer 21 is an oxide layer.

[0058] In one embodiment, the first gate dielectric layer 21 is formed using an oxidation process, such as a dry oxidation process. Exemplarily, the first gate dielectric layer 21 with a thickness of 1 nm to 1.5 nm is formed in a dry oxygen environment at 850°C to 900°C. The first gate dielectric layer 21 is in direct contact with the substrate 10 to ensure interface quality. The material of the first gate dielectric layer 21 includes, but is not limited to, silicon dioxide. The first gate dielectric layer 21 formed by thermal oxidation has an extremely low interface state density, thereby ensuring high carrier mobility and device stability.

[0059] In step S2, please continue to refer to Figure 1 As shown, a second gate dielectric layer 22 is formed on the first gate dielectric layer 21, wherein the second gate dielectric layer 22 is a nitrogen-containing oxide layer, and the nitrogen content in the second gate dielectric layer 22 gradually increases from near the first gate dielectric layer 21 to far away from the first gate dielectric layer 21.

[0060] In one embodiment, a deposition process is used to form the second gate dielectric layer 22 on the first gate dielectric layer 21. During the formation of the second gate dielectric layer 22, a nitrogen-containing gas is introduced into the chamber, and the content of the nitrogen-containing gas gradually increases, thereby causing the nitrogen content in the formed second gate dielectric layer 22 to gradually increase.

[0061] For example, the second gate dielectric layer 22 can be formed using any suitable process, such as chemical vapor deposition, physical vapor deposition, or atomic layer deposition. For instance, chemical vapor deposition can be used to form a second gate dielectric layer 22 with a thickness of 1 nm to 2 nm on the first gate dielectric layer 21 using reactive gases such as silane (SiH4), nitrous oxide (N2O), and ammonia (NH3). During the formation of the second gate dielectric layer 22, the flow rate of nitrous oxide can be gradually increased to increase the nitrogen content in the formed second gate dielectric layer 22.

[0062] The nitrogen content in the second gate dielectric layer 22 gradually increases from bottom to top, which is used to form a stress gradient and suppress interface traps, thereby effectively reducing leakage current and interface defect density.

[0063] In step S3, please continue to refer to Figure 1 As shown, a third gate dielectric layer 23 is formed on the second gate dielectric layer 22, wherein the third gate dielectric layer 23 is an oxide layer or a high dielectric constant material layer.

[0064] In one embodiment, the third gate dielectric layer 23 can be formed using any suitable process, such as chemical vapor deposition, physical vapor deposition, or atomic layer deposition. Exemplarily, the third gate dielectric layer 23 is formed on the second gate dielectric layer 22 using atomic layer deposition. The third gate dielectric layer 23 can be an oxide layer, such as silicon dioxide, or a high-dielectric-constant material layer, such as hafnium oxide, titanium oxide, lanthanum oxide, tantalum oxide, strontium titanium oxide, hafnium silicon oxide, or zirconium oxide. The third gate dielectric layer 23 serves to provide overall insulation for the image sensor transmission gate dielectric layer structure 20 and improve reliability.

[0065] In one embodiment, please refer to Figure 1 As shown, the thickness of the third gate dielectric layer 23 is the same in different regions, for example, the thickness of the third gate dielectric layer 23 is 1.5nm~3nm. The overall thickness of the image sensor transmission gate dielectric layer structure 20 is 3.5nm~6.5nm.

[0066] In this embodiment, the first gate dielectric layer 21 is in direct contact with the surface of the substrate 10 to ensure interface quality. The nitrogen content in the second gate dielectric layer 22 increases from bottom to top, forming a stress gradient, which helps to improve the quality of the gate dielectric layer and reduce interface traps, thereby effectively reducing leakage current and interface defect density. The third gate dielectric layer 23 is used to provide overall insulation and improve reliability. The combination of the first gate dielectric layer 21, the second gate dielectric layer 22 and the third gate dielectric layer 23 not only ensures the quality and insulation performance of the image sensor transmission gate dielectric layer structure 20, but also reduces leakage current and interface defect density, and improves reliability.

[0067] In another embodiment, after forming the third gate dielectric layer 23, the method further includes etching a portion of the third gate dielectric layer 23, such that the thickness of the third gate dielectric layer 23 gradually increases from its center to its edge, i.e., the third gate dielectric layer 23 has a structure that is thinner in the middle and thicker at the edges. It is understood that if etching of the third gate dielectric layer 23 is required, the thickness of the third gate dielectric layer 23 formed in step S3 should be appropriately increased to avoid the thickness of the central region of the third gate dielectric layer 23 being too small after subsequent etching. For example, the thickness of the third gate dielectric layer 23 before etching is 3.5 nm to 5 nm, and the overall thickness of the image sensor transmission gate dielectric layer structure 20 is 5.5 nm to 8.5 nm.

[0068] In this embodiment, the thickness of the third gate dielectric layer 23 gradually increases from its center to its edge, meaning the third gate dielectric layer 23 has a structure that is thinner in the middle and thicker at the edges. This makes the entire image sensor transmission gate dielectric layer structure 20 also thinner in the middle and thicker at the edges. The thinner central region improves capacitive coupling efficiency and charge transfer performance, while the thicker edge region alleviates electric field concentration, reducing the probability of gate dielectric layer breakdown and improving reliability. It also reduces dark current.

[0069] Please refer to Figure 2 As shown, in this embodiment, the image sensor transmission gate dielectric layer structure 20 is divided into a central region I, a near-edge region II, and an edge region III. The edge region III surrounds the near-edge region II, and the near-edge region II surrounds the central region I.

[0070] For example, a method for etching a portion of the third gate dielectric layer 23 includes the following steps. Figure 4 This is a schematic diagram of the structure after forming the first patterned photoresist layer according to an embodiment of this application. First, please refer to... Figure 4 As shown, a first patterned photoresist layer 31 is formed, which covers the third gate dielectric layer 23 in edge region III and exposes the third gate dielectric layer 23 in near-edge region II and center region I. Exemplarily, the first photoresist layer is first formed, and then exposed and developed to form the first patterned photoresist layer 31.

[0071] Then, using the first patterned photoresist layer 31 as a mask, the third gate dielectric layer 23 is etched such that the thickness of the third gate dielectric layer 23 in the edge region III is greater than the thickness of the third gate dielectric layer 23 in the near-edge region II and the center region I. Specifically, the third gate dielectric layer 23 in the near-edge region II and the center region I is etched. For example, after etching, the thickness of the third gate dielectric layer 23 in the near-edge region II and the center region I is 2.5 nm to 4 nm.

[0072] In one embodiment, atomic layer etching (ALE) is used to etch the third gate dielectric layer 23, but it is not limited to this, and any suitable process known to those skilled in the art can be used to etch the third gate dielectric layer 23.

[0073] Next, the first patterned photoresist layer 31 is removed.

[0074] Figure 5 This is a schematic diagram of the structure after forming the second patterned photoresist layer according to an embodiment of this application. Please refer to... Figure 5 As shown, a second patterned photoresist layer 32 is then formed, which covers the third gate dielectric layer 23 of the edge region III and the near-edge region II, exposing the third gate dielectric layer 23 of the center region I. Exemplarily, the second photoresist layer is first formed, and then exposed and developed to form the second patterned photoresist layer 32.

[0075] Subsequently, using the second patterned photoresist layer 32 as a mask, the third gate dielectric layer 23 is etched, such that the thickness of the third gate dielectric layer 23 in the near-edge region II is greater than the thickness of the third gate dielectric layer 23 in the central region I. That is, the third gate dielectric layer 23 in the central region I is etched again. For example, after etching, the thickness of the third gate dielectric layer 23 in the central region I is 1.5 nm to 3 nm.

[0076] Next, the second patterned photoresist layer 32 is removed to form a shape as shown in the image. Figure 2 The structure shown.

[0077] In this embodiment, the thickness of the third gate dielectric layer 23 in the edge region III is greater than the thickness of the third gate dielectric layer 23 in the near edge region II, and the thickness of the third gate dielectric layer 23 in the near edge region II is greater than the thickness of the third gate dielectric layer 23 in the center region I, so that the third gate dielectric layer 23 has a structure that is thin in the middle and thick at the edges.

[0078] In this embodiment, only photolithography and etching processes are needed to form the third gate dielectric layer 23 with varying thickness, which is compatible with existing processes.

[0079] In one embodiment, after the formation of the third gate dielectric layer 23 (i.e., without etching the third gate dielectric layer 23), or after the removal of the second patterned photoresist layer 32 (i.e., after etching the third gate dielectric layer 23), the first gate dielectric layer 21, the second gate dielectric layer 22, and the third gate dielectric layer 23 can be annealed, that is, the formed image sensor transmission gate dielectric layer structure 20 can be annealed to passivate the interface states. For example, the image sensor transmission gate dielectric layer structure 20 can be annealed at a temperature of 300°C to 350°C in a hydrogen atmosphere or a hydrogen-nitrogen mixed atmosphere for 20 to 30 minutes to passivate the interface states.

[0080] It should be noted that in this embodiment, the image sensor transmission gate dielectric layer structure 20 is divided into three regions, requiring two photolithography and etching processes to form the third gate dielectric layer 23 with three different thicknesses. In other embodiments, the image sensor transmission gate dielectric layer structure 20 can also be divided into two regions, requiring one photolithography and etching process to form the third gate dielectric layer 23 with two different thicknesses. Alternatively, to achieve multiple slow changes in the thickness of the third gate dielectric layer 23, the image sensor transmission gate dielectric layer structure 20 can be divided into four or more regions, which would require three or more photolithography and etching processes, increasing the number of process steps and costs. An appropriate number of regions can be selected based on actual needs.

[0081] In the fabrication method of the image sensor transmission gate dielectric layer structure 20 provided in this embodiment, a first gate dielectric layer 21 is first formed on a substrate 10, wherein the first gate dielectric layer 21 is an oxide layer. Next, a second gate dielectric layer 22 is formed on the first gate dielectric layer 21, wherein the second gate dielectric layer 22 is a nitrogen-containing oxide layer, and the nitrogen content in the second gate dielectric layer 22 gradually increases from near to far from the first gate dielectric layer 21. Then, a third gate dielectric layer 23 is formed on the second gate dielectric layer 22, wherein the third gate dielectric layer 23 is an oxide layer or a high-dielectric-constant material layer. The image sensor transmission gate dielectric layer structure 20 thus formed ensures both the quality and insulation performance of the gate dielectric layer, reduces leakage current and interface defect density, and improves reliability. Furthermore, when the third gate dielectric layer 23 has a structure that is thinner in the middle and thicker at the edges, it can also improve capacitive coupling efficiency, improve charge transfer performance, enhance reliability, and reduce dark current.

[0082] In summary, the image sensor transmission gate dielectric layer structure and its fabrication method provided in this application include a first gate dielectric layer, a second gate dielectric layer, and a third gate dielectric layer stacked sequentially. The first gate dielectric layer is an oxide layer, the second gate dielectric layer is a nitrogen-containing oxide layer, and the third gate dielectric layer is an oxide layer or a high-dielectric-constant material layer. Furthermore, the nitrogen content in the second gate dielectric layer gradually increases from near to far from the first gate dielectric layer. The first gate dielectric layer is in direct contact with the substrate surface to ensure interface quality. The increasing nitrogen content in the second gate dielectric layer from bottom to top creates a stress gradient, which helps improve the quality of the gate dielectric layer, reduces interface traps, and thus effectively reduces leakage current and interface defect density. The third gate dielectric layer provides overall insulation and improves reliability. The combination of the first, second, and third gate dielectric layers ensures both the quality and insulation performance of the image sensor transmission gate dielectric layer structure, reduces leakage current and interface defect density, and improves reliability.

[0083] Furthermore, the thickness of the third gate dielectric layer gradually increases from its center to its edge, meaning the third gate dielectric layer has a structure that is thinner in the middle and thicker at the edges. This makes the entire image sensor transmission gate dielectric layer structure also thinner in the middle and thicker at the edges. The thinner central region improves capacitive coupling efficiency and charge transfer performance, while the thicker edge region alleviates electric field concentration, reducing the probability of gate dielectric layer breakdown and improving reliability. It also reduces dark current.

[0084] The above description is merely a description of preferred embodiments of this application and is not intended to limit the scope of this application in any way. Any changes or modifications made by those skilled in the art based on the above disclosure shall fall within the protection scope of the claims.

Claims

1. A transmission gate dielectric layer structure for an image sensor, characterized in that, It includes a first gate dielectric layer, a second gate dielectric layer and a third gate dielectric layer stacked in sequence, wherein the first gate dielectric layer is an oxide layer, the second gate dielectric layer is a nitrogen-containing oxide layer, and the third gate dielectric layer is an oxide layer or a high dielectric constant material layer; and from the proximity to the first gate dielectric layer to the distance from the first gate dielectric layer, the nitrogen content in the second gate dielectric layer gradually increases, forming a stress gradient from bottom to top.

2. The image sensor transmission gate dielectric layer structure according to claim 1, characterized in that, The thickness of the third gate dielectric layer gradually increases from the center to the edge.

3. The image sensor transmission gate dielectric layer structure according to claim 2, characterized in that, The image sensor transmission gate dielectric layer structure includes a central region, a near-edge region, and an edge region. The thickness of the third gate dielectric layer in the edge region is greater than the thickness of the third gate dielectric layer in the near-edge region, and the thickness of the third gate dielectric layer in the near-edge region is greater than the thickness of the third gate dielectric layer in the central region.

4. The image sensor transmission gate dielectric layer structure according to claim 3, characterized in that, The central region occupies more than two-thirds of the entire area of ​​the image sensor transmission gate dielectric layer structure.

5. The image sensor transmission gate dielectric layer structure according to claim 3 or 4, characterized in that, The overall thickness of the image sensor transmission gate dielectric layer structure in the central region is 3.5nm~6.5nm, the overall thickness of the image sensor transmission gate dielectric layer structure in the near-edge region is 4.5nm~7.5nm, and the overall thickness of the image sensor transmission gate dielectric layer structure in the edge region is 5.5nm~8.5nm; wherein, the thickness of the first gate dielectric layer is 1nm~1.5nm and the thickness of the second gate dielectric layer is 1nm~2nm throughout the entire region; the thickness of the third gate dielectric layer in the central region is 1.5nm~3nm, the thickness of the third gate dielectric layer in the near-edge region is 2.5nm~4nm, and the thickness of the third gate dielectric layer in the edge region is 3.5nm~5nm.

6. The image sensor transmission gate dielectric layer structure according to claim 1, characterized in that, The oxide layer is made of silicon dioxide, and the nitrogen-containing oxide layer is made of silicon oxynitride.

7. A method for fabricating a transmission grating dielectric layer structure for an image sensor, characterized in that, Includes the following steps: A first gate dielectric layer is formed on a substrate, wherein the first gate dielectric layer is an oxide layer; A second gate dielectric layer is formed on the first gate dielectric layer, wherein the second gate dielectric layer is a nitrogen-containing oxide layer, and the nitrogen content in the second gate dielectric layer gradually increases from near to far from the first gate dielectric layer, forming a stress gradient from bottom to top; and, A third gate dielectric layer is formed on the second gate dielectric layer, wherein the third gate dielectric layer is an oxide layer or a high dielectric constant material layer.

8. The method for fabricating the image sensor transmission gate dielectric layer structure according to claim 7, characterized in that, The second gate dielectric layer is formed using a deposition process; during the formation of the second gate dielectric layer, nitrogen-containing gas is introduced into the chamber, and the content of nitrogen-containing gas gradually increases.

9. The method for fabricating the image sensor transmission gate dielectric layer structure according to claim 7, characterized in that, After forming the third gate dielectric layer, the method further includes: A first patterned photoresist layer is formed, which covers the third gate dielectric layer in the edge region and exposes the third gate dielectric layer in the near-edge region and the center region. Using the first patterned photoresist layer as a mask, the third gate dielectric layer is etched such that the thickness of the third gate dielectric layer in the edge region is greater than the thickness of the third gate dielectric layer in the near-edge region and the center region. Remove the first patterned photoresist layer; A second patterned photoresist layer is formed, which covers the third gate dielectric layer in the edge region and the near-edge region, exposing the third gate dielectric layer in the center region; Using the second patterned photoresist layer as a mask, the third gate dielectric layer is etched such that the thickness of the third gate dielectric layer in the near-edge region is greater than the thickness of the third gate dielectric layer in the center region; and... Remove the second patterned photoresist layer.

10. The method for fabricating the image sensor transmission gate dielectric layer structure according to claim 7, characterized in that, After forming the third gate dielectric layer, the process further includes annealing the first gate dielectric layer, the second gate dielectric layer, and the third gate dielectric layer.

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