Ultraviolet response enhancing structure applied to CMOS (complementary metal oxide semiconductor) image sensor and manufacturing method of ultraviolet response enhancing structure

Abstract

The invention discloses an ultraviolet response enhancing structure applied to a CMOS (Complementary Metal Oxide Semiconductor) image sensor and a manufacturing method of the ultraviolet response enhancing structure, belonging to the technical field of ultraviolet photoelectric detection. The light emitting diode structurally comprises an anti-reflection film and a P-type silicon epitaxial layer, the anti-reflection film and the P-type silicon epitaxial layer are sequentially stacked in the direction of incident light, a clamping photodiode is formed at the end, away from the anti-reflection film, of the P-type silicon epitaxial layer, and a p + layer is doped at the end, close to the anti-reflection film, of the P-type silicon epitaxial layer; the anti-reflection film comprises a SiO2 layer, an Al2O3 layer and an HfO2 layer, and the SiO2 layer, the Al2O3 layer and the HfO2 layer are sequentially stacked in the direction of incident light; the N well region and the P-type heavily-doped clamping layer are sequentially stacked on the clamping photodiode in the direction of incident light. According to the structure, the defect and trap concentration of an interface of silicon and hafnium oxide are reduced, so that dark current is reduced; the narrower surface P-type doping can reduce the recombination area, thereby enhancing the collection of ultraviolet charges.

Description

Technical Field

[0001] The present invention relates to the field of ultraviolet photoelectric detection technology, and in particular to an ultraviolet response enhancement structure applied to a CMOS image sensor and a manufacturing method thereof. Background Art

[0002] The rise of new technologies such as artificial intelligence, machine vision, and the Internet of Things has led to increased requirements for image sensor performance to achieve better data collection and analysis. Ultraviolet band imaging has been used in a variety of application scenarios, such as space ionizing ray detection, personal radiation protection, hazardous substance detection, surface defect detection, and clinical diagnosis. Complementary metal oxide semiconductor (CMOS) image sensors show good photon detection sensitivity in the visible spectrum, but because silicon materials show a high extinction coefficient in the ultraviolet wavelength range, the propagation distance of ultraviolet light incident on the sensor surface is only a few tens of nanometers at most, and even the attenuation length of 285 nanometer wavelength light is only 4.2 nanometers. In addition, there are a large number of surface traps caused by dangling bonds and oxide charges on the surface of silicon materials, which have the ability to accelerate the recombination of photogenerated charges in this area, and form a positive electric field on the surface to generate a potential well, making it difficult for electrons to transfer to the interior. Therefore, such a short absorption range makes it very easy for photogenerated charges to recombine on the surface, resulting in a significant reduction in sensitivity and insufficient detection capability.

[0003] In response to the challenges of silicon-based CMOS image sensors in enhancing ultraviolet signals, the industry has adopted a variety of methods to improve their performance. A common method is to prepare organic or inorganic ultraviolet enhancement materials on the surface of the device. These materials can absorb ultraviolet light and emit visible light, thereby enhancing the sensor's response to ultraviolet light, but these materials are often difficult to integrate with existing CMOS manufacturing processes, resulting in increased manufacturing costs of devices and more complicated production processes. In the prior art, several effective enhancement methods have been developed based on semiconductor process methods. First, the back thinning technology can reduce the scattering and absorption of light inside the sensor, thereby improving the quantum efficiency of ultraviolet light. Secondly, surface ion implantation technology can change the electrical properties of the silicon surface and enhance its sensitivity to ultraviolet light. In addition, laser annealing technology can repair the lattice damage that may occur during ion implantation, while reducing surface defects and further improving the performance of the sensor. Finally, the application of anti-reflection film is also the key to improving the quantum efficiency of ultraviolet light. By depositing an anti-reflection film on the surface of a CMOS image sensor, the reflection loss of light can be reduced and the transmittance of light can be increased, thereby improving the sensor's ability to detect ultraviolet light. However, the above techniques cannot enhance the UV signal on the silicon surface and inside the silicon at the same time. Therefore, a UV response enhancement method applied to CMOS image sensors is needed to simultaneously enhance the transmittance of the UV signal on the silicon surface and the charge transfer inside the silicon to achieve high quantum efficiency. Summary of the invention

[0004] The object of the present invention is to propose an ultraviolet response enhancement structure applied to a CMOS image sensor and a manufacturing method thereof.

[0005] An ultraviolet response enhancement structure applied to a CMOS image sensor includes an antireflection film and a P-type silicon epitaxial layer, and the antireflection film and the P-type silicon epitaxial layer are stacked in sequence along the direction of incident light. A clamping photodiode is formed at one end of the P-type silicon epitaxial layer far from the antireflection film, and a p+ layer is doped at one end close to the antireflection film; the antireflection film includes a SiO2 layer, an Al2O3 layer, and a HfO2 layer, and the SiO2 layer, the Al2O3 layer, and the HfO2 layer are stacked in sequence along the direction of incident light; the clamping photodiode stacks an N-well region and a P-type heavily doped clamping layer in sequence along the direction of incident light.

[0006] Further, the refractive indices of the SiO2 layer, the Al2O3 layer, and the HfO2 layer of the antireflection film increase in sequence along the direction of incident light.

[0007] Further, the thickness of the p+ layer is less than 5 nm.

[0008] A manufacturing method of an ultraviolet response enhancement structure applied to a CMOS image sensor includes the following steps:

[0009] Flip the CMOS image sensor so that the silicon-based part faces upward, and use wet etching to thin the back silicon material to a thickness of 2 - 3 μm;

[0010] Adopt an ion implantation process to perform high-doping boron implantation with an energy of 100 eV - 5 keV and a dose of 10^14 / cm 2 magnitude from the entire upper surface of the silicon substrate to form a p+ layer, and activate the impurities through ultraviolet laser to form a doping gradient and a potential gradient;

[0011] Deposit a HfO2 layer through atomic layer deposition process to form a fixed charge layer on the silicon-based part, and then deposit an Al2O3 layer and a SiO2 layer in sequence through chemical vapor deposition process.

[0012] Further, the thickness of the HfO2 layer is less than 20 nm, the thickness of the Al2O3 layer is less than 30 nm, and the thickness of the SiO2 layer is greater than 30 nm.

[0013] The beneficial effects of the present invention are as follows:

[0014] 1. The antireflection film uses inorganic oxide materials and can be integrated into the CMOS process. By appropriately selecting and optimizing the material thickness, an average transmittance higher than 60% can be achieved.

[0015] 2. P-type heavy doping is carried out on the surface, reducing the defect and trap concentration at the silicon / hafnium dioxide interface, thereby reducing the dark current; further, the narrow surface P-type doping can reduce the recombination region, thereby enhancing the collection of ultraviolet charges.

[0016] 3. After low-energy boron ion implantation on the surface, ultraviolet laser is used to activate the dopant, which is beneficial to precisely control the activation region and produce a steep impurity distribution curve. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of the ultraviolet response enhancement structure of a CMOS image sensor.

[0018] Figure 2 It is a schematic diagram of the antireflection film structure.

[0019] Figure 3 It is a schematic diagram of the doping potential of the P-type silicon epitaxial layer. Detailed Embodiments

[0020] The present invention proposes an ultraviolet response enhancement structure applied to a CMOS image sensor and its manufacturing method. The present invention will be further described below with reference to the drawings and specific embodiments.

[0021] Figure 1 It is a schematic diagram of the ultraviolet response enhancement structure of a CMOS image sensor, including an antireflection film and a P-type silicon epitaxial layer. Light enters from the back of the image sensor, and the light beam is focused by refracting or reflecting the light through a microlens, and then enters the surface of the antireflection layer. The antireflection film material includes inorganic oxides such as Al2O3, SiO2, and HfO2. The oxide material can be conveniently realized by CMOS process, and excellent antireflection performance can also be achieved by appropriately selecting and optimizing the material thickness, so that the average transmittance in the 250nm - 350nm ultraviolet band reaches more than 60%. The light transmitted through the antireflection film generates photogenerated charges through the photoelectric effect in the silicon material. The surface dangling bonds and fixed charges cause the surface potential to rise and the energy level to drop, making the charges move from the internal high energy level to the surface low energy level. Therefore, the doping concentration and energy are reasonably selected on the surface to form P-type doping to achieve surface passivation, and a steep doping distribution forms a potential to promote the movement of photogenerated charges to the N-well region of the clamped photodiode. The clamped photodiode is formed in the P-type silicon epitaxial layer, which includes a P-type heavily doped clamping layer and an N-well region. The P-type heavily doped clamping layer can promote the N-well region to reach complete depletion to achieve a high potential region, generating an electric field region from the clamped photodiode to the surface to promote the collection of photogenerated charges.

[0022] Figure 2Schematic diagram of an anti-reflection film structure. Three layers of materials, SiO2, Al2O3, and HfO2, are stacked along the incident light direction to form a refractive index gradient that increases sequentially from air to the three-layer materials of silicon-based SiO2, Al2O3, and HfO2, which can effectively prevent the increase in reflectivity caused by a sharp change in refractive index. Among them, the HfO2 material can form a fixed surface charge layer on the silicon-based surface to generate field-effect passivation, which is used to achieve excellent surface passivation within a specified bandwidth and extend the carrier lifetime. The HfO2 is prepared by atomic layer deposition technology to form a uniform high-quality thin film and reduce surface defects and recombination rate.

[0023] Figure 3 Schematic diagram of the doping potential of the P-type silicon epitaxial layer. A high-concentration surface p+ layer with a steep doping concentration distribution is formed on the silicon surface, and the thickness of the surface p+ layer is within 5 nm. In the silicon body without surface doping, due to the large thickness of the undepleted layer, a wide-thickness potential barrier region will be formed, making it difficult for photo-generated charges to be absorbed by the clamped photodiode. Through the steep gradient doping on the surface, a potential gradient from the surface to the clamped photodiode is formed in the figure, which is beneficial to the collection of photo-generated charges.

[0024] The manufacturing method of the ultraviolet response enhancement structure applied to the CMOS image sensor in this embodiment includes the following steps:

[0025] Step 1: Flip the CMOS image sensor so that the silicon-based part faces upward, and use wet etching to thin the back silicon material to a thickness of 2 - 3 μm. This smaller thickness can reduce the path and time for surface photo-generated charges to transfer to the internal clamped photodiode, but it needs to be selectively adjusted according to the depth of the clamped photodiode applied to ensure that it does not harm visible light imaging.

[0026] Step 2: Use the ion implantation process to perform low-energy high-doping boron implantation with an energy of 100 eV - 5 keV and a dose of 10^14 / cm 2 order of magnitude from the upper surface, and the implantation range is the entire plane to form a high-concentration surface p+ layer with a steep doping concentration distribution. Since typical high-temperature annealing is incompatible with the image sensor structure, impurities are activated by ultraviolet laser. Its penetration depth is small, and it can activate doping ions with the least impurity diffusion after ion implantation to form a steep doping gradient and a potential gradient conducive to the transfer of photo-generated charges.

[0027] Step 3: Deposit HfO2 material with a thickness of less than 20 nm by atomic layer deposition to form a fixed charge layer on the silicon-based part to form field-effect passivation. Subsequently, deposit Al2O3 material with a thickness of less than 30 nm and SiO2 material with a thickness greater than 30 nm by chemical vapor deposition process to mutually compensate for the high reflectivity in the 250 nm - 350 nm region of the silicon material and achieve high transmission characteristics in the ultraviolet region and improve the quantum efficiency.

[0028] This ultraviolet enhancement method for a silicon-based CMOS image sensor with a three-layer inorganic oxide antireflection film combined with surface doping reduces the influence of the dark current of surface defects and enhances the transfer potential gradient from the surface to the clamped photodiode by using hafnium dioxide for surface passivation and P-type doping to reduce the trap concentration, thus solving the problem of difficult imaging of the CMOS image sensor in the 250nm - 350nm ultraviolet band.

[0029] In summary, the present invention adopts a dual quantum efficiency enhancement method of an antireflection film and silicon surface doping. By using the inorganic oxide antireflection film structure of semiconductor process, the high reflectivity of the silicon surface to ultraviolet light is reduced. Through P-type shallow junction doping of the silicon surface, the transfer of photo-generated charges from the surface to the inside of the clamped photodiode is promoted, further improving the photon utilization rate. At the same time, by making reasonable choices for the type and thickness of the antireflection film, and the dose and energy of surface doping in the design, the reflection loss of light on the surface of optical elements can be reduced or eliminated, and the charge separation and collection efficiency can be enhanced.

Claims

1. An ultraviolet response enhancement structure applied to a CMOS image sensor, characterized in that It includes an antireflection film and a P-type silicon epitaxial layer, and the antireflection film and the P-type silicon epitaxial layer are stacked in sequence along the direction of incident light. A clamping photodiode is formed at one end of the P-type silicon epitaxial layer away from the antireflection film, and a p+ layer is doped at one end close to the antireflection film; The antireflection film includes a SiO2 layer, an Al2O3 layer, and a HfO2 layer, and the SiO2 layer, the Al2O3 layer, and the HfO2 layer are stacked in sequence along the direction of incident light; The clamping photodiode sequentially stacks an N-well region and a P-type heavily doped clamping layer along the direction of incident light.

2. The ultraviolet response enhancement structure applied to the CMOS image sensor according to claim 1, wherein The refractive indices of the SiO2 layer, the Al2O3 layer, and the HfO2 layer of the antireflection film increase in sequence along the direction of incident light.

3. The ultraviolet response enhancement structure applied to the CMOS image sensor according to claim 2, wherein The thickness of the p+ layer is less than 5 nm.

4. A manufacturing method of the ultraviolet response enhancement structure applied to a CMOS image sensor according to any one of claims 1 to 3, characterized in that, It includes the following steps: Flip the CMOS image sensor so that the silicon-based part faces upward, and use wet etching to thin the back silicon material to a thickness of 2 - 3 μm; An ion implantation process is used to perform high-doping boron implantation with an energy of 100 eV - 5 keV and a dose of 10^14 / cm from the entire upper surface of the silicon substrate, and impurities are activated by ultraviolet laser to form a doping gradient and a potential gradient; 2 The p+ layer is formed, and the doping gradient and potential gradient are formed by activating impurities with ultraviolet laser; Deposit the HfO2 layer through atomic layer deposition process to form a fixed charge layer on the silicon-based part, and then deposit the Al2O3 layer and the SiO2 layer in sequence through chemical vapor deposition process.

5. The manufacturing method of the ultraviolet response enhancement structure applied to a CMOS image sensor according to claim 4, characterized in that, The thickness of the HfO2 layer is less than 20 nm.

6. The manufacturing method of the ultraviolet response enhancement structure applied to a CMOS image sensor according to claim 4, characterized in that, The thickness of the Al2O3 layer is less than 30 nm.

7. The manufacturing method of the ultraviolet response enhancement structure applied to a CMOS image sensor according to claim 4, characterized in that, The thickness of the SiO2 layer is greater than 30 nm.

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