Short-wave infrared transmission enhanced backside illumination CCD image sensor and manufacturing method thereof
By employing a back-illuminated CCD image sensor structure and a multi-layer antireflection coating design, the problem of low short-wave infrared transmittance in CCD image sensors is solved, enabling wide-spectrum detection and making it suitable for multi-band integrated device applications.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-01-09
- Publication Date
- 2026-04-07
AI Technical Summary
Existing CCD image sensors lack short-wave infrared transmittance design, making it difficult to achieve wide-spectrum detection from 400nm to 3000nm, which limits their application in multi-band integrated devices.
A back-illuminated CCD image sensor structure is adopted, and a high-resistivity single crystal material wafer is used as a carrier wafer through thinning and polishing. Multiple anti-reflection films are set on the back of the chip, and the thickness of the bonding material is optimized to enhance the transmittance of short-wave infrared photons.
It improves short-wave infrared transmittance to over 70%, meeting the needs of multi-band integrated devices and is suitable for space-constrained applications such as aerospace and portable devices.
Smart Images
Figure CN121815783A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of image sensor technology, and in particular relates to a short-wave infrared transmission enhanced back-illuminated CCD image sensor and its manufacturing method. Background Technology
[0002] CCD (Charge Coupled Device) image sensors are key components for applications ranging from industrial inspection and scientific imaging to smart mobility. Currently, CCD image sensors generally do not have requirements for short-wave infrared transmittance, nor are there structural optimization designs specifically for this. However, by improving short-wave infrared transmittance, a single CCD sensor can simultaneously cover the visible, near-infrared, and short-wave infrared bands, achieving broad-spectrum detection from 400nm to 3000nm, meeting the core requirements of multi-band integrated devices. Optimizing short-wave infrared transmittance also allows for multi-band integration on a single chip, significantly reducing system size, weight, and power consumption, making it particularly suitable for space-constrained applications such as aerospace and portable devices. Therefore, research on short-wave infrared transmittance optimization technology has significant practical implications for multi-band integrated devices. Summary of the Invention
[0003] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a short-wave infrared transmission enhanced back-illuminated CCD image sensor and its manufacturing method.
[0004] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A short-wave infrared transmission enhanced back-illuminated CCD image sensor includes an ortho-illuminated CCD chip and a carrier sheet. The ortho-illuminated CCD chip is integrated onto the carrier sheet by bonding material, and a contact metal electrode is disposed on the back side of the ortho-illuminated CCD chip. The carrier sheet is made of a high-resistivity single-crystal material sheet that has undergone thinning and polishing treatment.
[0005] Furthermore, the epitaxial layer thickness of the positive-illumination CCD chip is 10μm to 30μm.
[0006] Furthermore, the resistance of the carrier sheet is 3000 ohms / cm to 7000 ohms / cm; the thickness of the carrier sheet is 300μm to 500μm.
[0007] Furthermore, the thickness of the bonding layer formed by the bonding material is 3μm to 7μm.
[0008] Furthermore, the material of the back contact metal electrode is metal Al, and the thickness of the back contact metal electrode is 500nm to 700nm.
[0009] Furthermore, an infrared anti-reflection film is provided on the back side of the positive-illumination CCD chip.
[0010] Furthermore, the infrared antireflection film includes a first SiO2 layer, a first TiO2 layer, a second SiO2 layer, a second TiO2 layer, and a third SiO2 layer formed sequentially on the back side of the positive-illuminated CCD chip; the thickness of the first SiO2 layer is 20±2nm, the thickness of the first TiO2 layer is 80±5nm, the thickness of the second SiO2 layer is 20±2nm, the thickness of the second TiO2 layer is 25±3nm, and the thickness of the third SiO2 layer is 200±10nm.
[0011] A method for fabricating a short-wave infrared transmission enhanced back-illuminated CCD image sensor includes the following steps: S100. Take an epitaxial wafer and complete the fabrication of an orthophoto CCD chip on the epitaxial wafer; S200. Take a high-resistivity single-crystal material sheet and use it as a carrier sheet after thinning and polishing. S300: Prepare bonding material on the thinned and polished surface of the carrier sheet; S400: After flipping the carrier sheet, it is integrated onto the front side of the positive-illumination CCD chip through a bonding process, and a bonding layer is formed between the positive-illumination CCD chip and the carrier sheet using bonding material. S500, perform thinning and polishing treatment on the back side of the positive-image CCD chip; S600: Deposit a metal material on the back side of the positive-illuminated CCD chip, and etch the metal material using a photolithography process to form a contact metal electrode; S700, an infrared anti-reflection film is formed on the back side of the positively irradiated CCD chip.
[0012] Furthermore, in step S200, the resistance of the high-resistivity single-crystal material sheet is 3000 ohms / cm to 7000 ohms / cm; the thickness of the high-resistivity single-crystal material sheet after thinning and polishing is 300μm to 500μm. In step S300, the thickness of the bonding material prepared on the thinned and polished surface of the carrier sheet is 3 μm to 7 μm. In step S400, after the back side of the positive-illuminated CCD chip is thinned and polished, the epitaxial layer thickness of the positive-illuminated CCD chip is 10μm to 30μm. In step S500, the deposited metal material is metal Al, and the deposition thickness is 500 nm to 700 nm.
[0013] Furthermore, step S700 includes the following sub-steps: S710. A first SiO2 layer is formed on the back side of the positively irradiated CCD chip, and the thickness of the first SiO2 layer is 20±2nm. S720. A first TiO2 layer is fabricated on the first SiO2 layer, and the thickness of the first TiO2 layer is 80±5nm. S730. A second SiO2 layer is fabricated on the first TiO2 layer, and the thickness of the second SiO2 layer is 20±2nm. S740. A second TiO2 layer is fabricated on the second SiO2 layer, and the thickness of the second TiO2 layer is 25±3nm. S750. A third SiO2 layer is fabricated on the second TiO2 layer, wherein the thickness of the third SiO2 layer is 200±10nm.
[0014] This invention employs a back-illuminated CCD device structure, where light is directly incident from the back, bypassing the circuitry layer on the front of the CCD chip. A high-resistivity single-crystal material wafer is selected as the carrier wafer, and its thickness is precisely controlled through a thinning and polishing process, enhancing the absorption of short-wave infrared photons and reducing charge recombination. By optimizing the thickness of the bonding material and designing and depositing multilayer antireflection films for specific short-wave infrared bands, the transmittance of short-wave infrared photons can be increased, further improving the transmittance. This back-illuminated CCD image sensor structure has strong practical value and great application prospects in semiconductor inspection, material sorting, night vision, and spectral analysis, and can be applied to industrial inspection, autonomous driving, and other application scenarios. Attached Figure Description
[0015] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of an embodiment of the short-wave infrared transmission enhanced back-illuminated CCD image sensor of the present invention.
[0016] Figure 2 This is a schematic diagram of the structure of an infrared antireflective coating.
[0017] Figure 3 This is a flowchart of an embodiment of the method for manufacturing a short-wave infrared transmission enhanced back-illuminated CCD image sensor according to the present invention.
[0018] Figure 4 This is a schematic diagram of the structure of a CCD chip.
[0019] Figure 5 This is a schematic diagram of a high-resistivity single-crystal material wafer after thinning and polishing.
[0020] Figure 6 A schematic diagram of preparing bonding material on the thinned polished surface of the carrier sheet.
[0021] Figure 7 This is a schematic diagram of the structure after the carrier sheet and the positive-image CCD chip are bonded.
[0022] Figure 8 This is a schematic diagram of the structure after the back side of the positive-image CCD chip has been thinned and polished.
[0023] The diagrams in the instruction manual are labeled as follows: The CCD chip 100 is illuminated; the epitaxial layer 110 is covered; the carrier sheet 200 is covered; the bonding material 210 is covered; the infrared antireflection film 300 is covered; the first SiO2 layer 311 is covered; the first TiO2 layer 321 is covered; the second SiO2 layer 312 is covered; the second TiO2 layer 322 is covered; and the third SiO2 layer 313 is covered. Detailed Implementation
[0024] The following specific examples illustrate the implementation of the present invention. The illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.
[0025] Because the photosensitive area of an orthophoto-based CCD image sensor contains multiple layers of metal wires, polysilicon gates, and other structures used for signal transmission and control, significant light loss occurs due to the shielding and reflection effects of the metal interconnect layers and polysilicon gates, parasitic absorption in non-photosensitive areas, and reflection and scattering at the interface between air and silicon dioxide. Furthermore, due to the inherent properties of silicon and the device structure, the photosensitive area of an orthophoto-based CCD image sensor is relatively shallow, allowing short-wavelength infrared photons to penetrate this shallow area and reach the device substrate.
[0026] Furthermore, the front-illuminated CCD image sensor, with its front-incident illumination structure and the intrinsic properties of silicon material, inevitably introduces inherent defects. These defects severely limit the device's applications in low-light environments, high-precision scientific imaging, broadband detection, and short-wave infrared fields. Therefore, optimizing the structure of the front-illuminated CCD image sensor to meet the short-wave infrared transmittance requirements of multi-band integrated devices is quite challenging. For this reason, this embodiment uses a back-illuminated CCD image sensor. However, the short-wave infrared transmittance of back-illuminated CCD image sensors is generally only 30%–50%, still insufficient for multi-band integrated devices. Therefore, structural optimization of the traditional back-illuminated CCD image sensor is necessary to improve its short-wave infrared transmittance.
[0027] Please see Figure 1 , Figure 1This is a schematic diagram of an embodiment of the short-wave infrared transmission enhanced back-illuminated CCD image sensor of the present invention. The short-wave infrared transmission enhanced back-illuminated CCD image sensor of this embodiment includes a front-illuminated CCD chip 100 and a carrier sheet 200. The epitaxial layer 110 of the front-illuminated CCD chip 100 is generally thinned and polished to reduce the reflection and absorption of short-wave infrared photons and increase the transmittance of short-wave infrared photons. The thickness of the epitaxial layer 110 is preferably 10 μm to 30 μm; for example, the thickness of the epitaxial layer 110 can be 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm.
[0028] The carrier sheet 200 is made of a high-resistivity single-crystal material wafer that has undergone thinning and polishing. A high-resistivity single-crystal material wafer refers to a single-crystal material wafer with high resistivity, used as the support structure for a back-illuminated CCD image sensor. The most common high-resistivity single-crystal material wafer is a high-resistivity single-crystal silicon wafer, including high-resistivity single-crystal silicon wafers, silicon carbide (SiC) single-crystal wafers, gallium nitride (GaN) single-crystal wafers, sapphire (Al2O3) single-crystal wafers, gallium arsenide (GaAs) single-crystal wafers, and other single-crystal material wafers. The carrier sheet 200 uses a high-resistivity single-crystal material wafer, and its thickness is precisely controlled through thinning and polishing, which can enhance the absorption capacity of short-wave infrared photons and reduce charge recombination. In this embodiment, the resistance of the carrier sheet 200 is preferably between 3000 ohms / cm and 7000 ohms / cm; for example, the resistance of the carrier sheet 200 can be 3000 ohms / cm, 4000 ohms / cm, 5000 ohms / cm, 6000 ohms / cm, or 7000 ohms / cm. The thickness of the carrier sheet 200 is preferably between 300 μm and 500 μm; for example, the thickness of the carrier sheet 200 can be 300 μm, 350 μm, 400 μm, 450 μm, or 500 μm.
[0029] The positive-illumination CCD chip 100 is integrated onto the carrier sheet 200 via bonding material 210. Common bonding materials 210 used in back-illuminated CCD image sensors include dielectric materials such as silicon dioxide (SiO2), silicon nitride (SiN), and silicon oxynitride (SiON). These materials are bonded to form a bonding layer using chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD) processes. Since the bonding material 210 absorbs short-wave infrared photons, thereby reducing the transmittance of short-wave infrared photons, this embodiment also limits the thickness of the bonding layer formed by the bonding material 210 to avoid a significant impact on the transmittance of short-wave infrared photons. The thickness of the bonding layer is preferably 3μm to 7μm. For example, the thickness of the bonding layer can be 3μm, 4μm, 5μm, 6μm, or 7μm.
[0030] The back side of the positive-illumination CCD chip 100 is provided with a contact metal electrode (not shown in the figure). In this embodiment, the material of the back contact metal electrode is metal Al, and the thickness of the back contact metal electrode is 500nm to 700nm. For example, the thickness of the back contact metal electrode is 500nm, 550nm, 600nm, 650nm, or 700nm.
[0031] The contact metal electrode is primarily used to form an ohmic contact with the photosensitive area of the CCD chip, ensuring the effective collection and transport of photogenerated charge carriers. The contact metal electrode typically forms a low-resistance contact with the semiconductor material through an alloying process, reducing contact resistance and improving charge transfer efficiency. As an electrical connection point, the contact metal electrode also serves to lead the photogenerated charge signal generated by the photosensitive area of the CCD chip to external circuitry. Electrical interconnection with the readout circuit is achieved through bonding wires or flip-chip bonding, enabling signal reading and processing. Furthermore, in back-illuminated CCD image sensors, since the epitaxial layer 110 of the CCD chip is thinned through a thinning process, the contact metal electrode can also provide mechanical support for the thinned CCD chip, preventing it from breaking during subsequent packaging and testing. It should be noted that the contact metal electrode also blocks some incident light; therefore, the electrode pattern design is optimized to minimize light obstruction while ensuring electrical performance. These are conventional techniques and are not relevant to the improvements in this embodiment, and will not be elaborated upon here.
[0032] To further increase the transmittance of short-wave infrared photons, an infrared anti-reflection film 300 can also be provided on the back side of the positive-illumination CCD chip 100. (See also...) Figure 2 In this embodiment, the infrared antireflection film 300 employs a multilayer antireflection film system, comprising five layers: a first SiO2 layer 311, a first TiO2 layer 321, a second SiO2 layer 312, a second TiO2 layer 322, and a third SiO2 layer 313, sequentially formed on the back side of the positively irradiated CCD chip 100. The thickness of the first SiO2 layer 311 is preferably 20±2 nm, the thickness of the first TiO2 layer 321 is preferably 80±5 nm, the thickness of the second SiO2 layer 312 is preferably 20±2 nm, the thickness of the second TiO2 layer 322 is preferably 25±3 nm, and the thickness of the third SiO2 layer 313 is preferably 200±10 nm. The infrared antireflection film 300, with this structure, can further increase the transmittance of short-wave infrared photons and improve the transmittance of short-wave infrared photons.
[0033] In this embodiment, to address the disadvantage of low short-wave infrared transmittance in front-illuminated CCD image sensors, a back-illuminated CCD device structure is adopted, where light is directly incident from the back, bypassing the circuit layer on the front of the CCD chip. A high-resistivity single-crystal material wafer is selected as the carrier wafer 200, and its thickness is precisely controlled through a thinning and polishing process, enhancing the absorption of short-wave infrared photons and reducing charge recombination. By optimizing the thickness of the bonding material 210 and designing and depositing a multilayer antireflection film system for specific short-wave infrared bands, the transmittance of short-wave infrared photons can be increased, further improving the transmittance. This embodiment, by employing the above-mentioned optimized structure, can increase the short-wave infrared transmittance of the back-illuminated CCD image sensor structure to over 70%, possessing strong practical value and great application prospects in fields such as semiconductor detection, material sorting, night vision, and spectral analysis.
[0034] Please see Figure 3 , Figure 3 This is a flowchart illustrating an embodiment of the method for fabricating a short-wave infrared transmission enhanced back-illuminated CCD image sensor according to the present invention. The method for fabricating this short-wave infrared transmission enhanced back-illuminated CCD image sensor in this embodiment includes the following steps: S100, please refer to Figure 4 Take an epitaxial wafer and fabricate the positive-image CCD chip 100 on the epitaxial wafer. Since the epitaxial wafer needs to be thinned, a thin epitaxial wafer is generally selected. The fabrication of the positive-image CCD chip 100 is a mature current process and will not be described in detail here.
[0035] S200, please refer to Figure 5 A high-resistivity single-crystal material wafer is taken and thinned and polished to serve as the carrier wafer 200. The resistance of the high-resistivity single-crystal material wafer is preferably 3000 ohms / cm to 7000 ohms / cm; the thickness of the high-resistivity single-crystal material wafer after thinning and polishing is preferably 300μm to 500μm. Using a high-resistivity single-crystal material wafer and precisely controlling the thickness of the carrier wafer 200 through thinning and polishing can enhance the absorption capacity of short-wave infrared photons and reduce charge recombination.
[0036] S300, please refer to Figure 6 A bonding material 210 is prepared on the thinned and polished surface of the carrier sheet 200. Since the bonding material 210 absorbs short-wave infrared photons, thereby reducing the transmittance of short-wave infrared photons, in order to avoid the bonding material 210 having a significant impact on the transmittance of short-wave infrared photons, this embodiment also limits the thickness of the bonding material 210. The thickness of the bonding material 210 prepared on the thinned and polished surface of the carrier sheet 200 is preferably 3μm to 7μm.
[0037] S400, please refer to Figure 7The carrier sheet 200 is flipped and integrated onto the front side of the positive-illumination CCD chip 100 using a bonding process. A bonding layer is formed between the positive-illumination CCD chip 100 and the carrier sheet 200 using bonding material 210. By limiting the thickness of the bonding material 210 in the previous step, the absorption of short-wave infrared photons by the bonding layer can be minimized, thereby improving the transmittance of short-wave infrared photons.
[0038] S500, please refer to Figure 8 The back surface of the positive-illumination CCD chip 100 is thinned and polished. This thinning and polishing process reduces the reflection and absorption of short-wave infrared photons by the epitaxial layer 110, thereby increasing the transmittance of short-wave infrared photons. In this embodiment, after the thinning and polishing process, the thickness of the epitaxial layer 110 is preferably 10 μm to 30 μm.
[0039] S600: A metal material (not shown in the figure) is deposited on the back side of the positive-illuminated CCD chip 100, and the metal material is etched using a photolithography process to form contact metal electrodes. In this embodiment, the deposited metal material is Al, and the deposition thickness is 500nm to 700nm. The above processes are all mature existing technologies and will not be described in detail here.
[0040] S700, please continue reading Figure 1 and Figure 2 An infrared antireflection film 300 is fabricated on the back side of the positively irradiated CCD chip 100. By designing and depositing a multilayer antireflection film system for a specific short-wave infrared band, the transmittance of short-wave infrared photons can be increased, thereby further improving the transmittance of short-wave infrared photons. This step may include the following sub-steps: S710. A first SiO2 layer 311 is formed on the back side of the positively irradiated CCD chip 100, and the thickness of the first SiO2 layer 311 is 20±2nm.
[0041] S720. A first TiO2 layer 321 is fabricated on the first SiO2 layer 311, and the thickness of the first TiO2 layer 321 is 80±5nm.
[0042] S730. A second SiO2 layer 312 is fabricated on the first TiO2 layer 321, and the thickness of the second SiO2 layer 312 is 20±2nm.
[0043] S740. A second TiO2 layer 322 is fabricated on the second SiO2 layer 312, and the thickness of the second TiO2 layer 322 is 25±3nm.
[0044] S750. A third SiO2 layer 313 is fabricated on the second TiO2 layer 322, wherein the thickness of the third SiO2 layer 313 is 200±10nm.
[0045] The back-illuminated CCD image sensor structure fabricated using the method of this embodiment improves the short-wave infrared transmittance of the device by selecting a back-illuminated CCD image sensor structure and optimizing the thickness of the carrier sheet 200, resistivity parameters, bonding material 210 thickness, and antireflection film parameters. This increases the short-wave infrared transmittance of the back-illuminated CCD image sensor structure to over 70%, meeting the short-wave infrared transmittance requirements of multi-band integrated devices. This lays the foundation for integrated short-wave infrared detectors and is suitable for applications such as industrial inspection and autonomous driving.
[0046] The above embodiments merely illustrate preferred implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention should be determined by the appended claims.
Claims
1. A short-wave infrared transmission enhanced back-illuminated CCD image sensor, comprising a front-illuminated CCD chip and a carrier sheet, wherein the front-illuminated CCD chip is integrated onto the carrier sheet by bonding material, and a contact metal electrode is disposed on the back side of the front-illuminated CCD chip; characterized in that: The carrier sheet is made of a high-resistivity single-crystal material that has undergone thinning and polishing.
2. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 1, characterized in that: The epitaxial layer thickness of the positive-illuminated CCD chip is 10μm to 30μm.
3. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 1, characterized in that: The resistance of the carrier sheet is 3000 ohms / cm to 7000 ohms / cm; the thickness of the carrier sheet is 300μm to 500μm.
4. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 1, characterized in that: The thickness of the bonding layer formed by the bonding material is 3μm to 7μm.
5. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 1, characterized in that: The material of the back contact metal electrode is metal Al, and the thickness of the back contact metal electrode is 500nm to 700nm.
6. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in any one of claims 1 to 5, characterized in that: An infrared anti-reflection film is provided on the back side of the positive-illuminated CCD chip.
7. The short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 6, characterized in that: The infrared antireflection film comprises a first SiO2 layer, a first TiO2 layer, a second SiO2 layer, a second TiO2 layer, and a third SiO2 layer formed sequentially on the back side of a positively irradiated CCD chip; the thickness of the first SiO2 layer is 20±2nm, the thickness of the first TiO2 layer is 80±5nm, the thickness of the second SiO2 layer is 20±2nm, the thickness of the second TiO2 layer is 25±3nm, and the thickness of the third SiO2 layer is 200±10nm.
8. A method for fabricating a short-wave infrared transmission enhanced back-illuminated CCD image sensor, characterized in that, Includes the following steps: S100. Take an epitaxial wafer and complete the fabrication of an orthophoto CCD chip on the epitaxial wafer; S200. Take a high-resistivity single-crystal material sheet and use it as a carrier sheet after thinning and polishing. S300: Prepare bonding material on the thinned and polished surface of the carrier sheet; S400: After flipping the carrier sheet, it is integrated onto the front side of the positive-illumination CCD chip through a bonding process, and a bonding layer is formed between the positive-illumination CCD chip and the carrier sheet using bonding material. S500, perform thinning and polishing treatment on the back side of the positive-image CCD chip; S600: Deposit a metal material on the back side of the positive-illuminated CCD chip, and etch the metal material using a photolithography process to form a contact metal electrode; S700, an infrared anti-reflection film is formed on the back side of the positively irradiated CCD chip.
9. The method for manufacturing a short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 8, characterized in that: In step S200, the resistance of the high-resistivity single-crystal material sheet is 3000 ohms / cm to 7000 ohms / cm; the thickness of the high-resistivity single-crystal material sheet after thinning and polishing is 300μm to 500μm. In step S300, the thickness of the bonding material prepared on the thinned and polished surface of the carrier sheet is 3 μm to 7 μm. In step S400, after the back side of the positive-illuminated CCD chip is thinned and polished, the epitaxial layer thickness of the positive-illuminated CCD chip is 10μm to 30μm. In step S500, the deposited metal material is metal Al, and the deposition thickness is 500 nm to 700 nm.
10. The method for manufacturing a short-wave infrared transmission enhanced back-illuminated CCD image sensor as described in claim 8, characterized in that, The S700 step includes the following sub-steps: S710. A first SiO2 layer is formed on the back side of the positively irradiated CCD chip, and the thickness of the first SiO2 layer is 20±2nm. S720. A first TiO2 layer is fabricated on the first SiO2 layer, and the thickness of the first TiO2 layer is 80±5nm. S730. A second SiO2 layer is fabricated on the first TiO2 layer, and the thickness of the second SiO2 layer is 20±2nm. S740. A second TiO2 layer is fabricated on the second SiO2 layer, and the thickness of the second TiO2 layer is 25±3nm. S750. A third SiO2 layer is fabricated on the second TiO2 layer, wherein the thickness of the third SiO2 layer is 200±10nm.