Design and manufacturing method of six-bandpass optical filter

Abstract

The invention introduces a design and manufacturing method of a six-bandpass optical filter, the optical filter comprises six bandpasses, a high-refractive-index film layer and a low-refractive-index film layer are alternately plated by adopting a magnetron sputtering coating process, and high isolation of the six bandpasses is realized through stress matching design. According to an optical film basic theory, simulation software is combined to carry out film layer design optimization, and the six-bandpass optical filter overcomes the defects of the optical filter in passband number, visible light region cut-off depth and film layer reliability, and has important practical significance and engineering value.

Description

Technical Field

[0001] The present invention relates to the field of optical thin film devices, and in particular to the design and manufacture of a multi-bandpass filter in the visible-near infrared band. Background Art

[0002] Multispectral imaging technology is developing toward more spectral channels, higher integration, smaller size, and lighter weight. Multichannel filters used for spectroscopic analysis are key optical components, necessitating the development of novel multi-bandpass filter fabrication technologies. Current multispectral imaging systems generally employ a combination of multiple single-bandpass filters or liquid crystal tunable filters (LCTFs) for spectroscopic analysis. The former results in a bulky system (e.g., six channels require six separate filters), while the latter suffers from slow response speeds and high costs. Integrated multi-bandpass filters can significantly reduce the number of optical components, but existing technologies struggle to achieve six or more highly isolated passbands in the visible-to-near-infrared range (400-1200nm).

[0003] Traditional multi-bandpass filters are typically fabricated using the Fabry-Perot (FP) principle, using a multiple-shell method. An FB cavity is a cavity separated by two parallel flat mirrors. When both mirrors have the same high reflectivity, the interferometer achieves high transmittance for a specific wavelength range. Fine-tuning the cavity thickness can alter the wavelength of transmission. In practice, the cavity consists of a dielectric film forming a spacer layer, with a multilayer coating on each side, acting as the two mirrors mentioned above. Simply adjusting the thickness of the spacer layer can control the wavelength of transmission. Due to the inherent limitations of Fabry-Perot (FP) filters, which lack a wide cutoff depth in the visible light range, these filters are not suitable for multi-channel filtering within the visible light band, significantly limiting their application. Furthermore, conventional coating techniques struggle to control the coating rate and thickness of numerous layers (over 200). Summary of the Invention

[0004] In view of the shortcomings of existing multi-bandpass filters in terms of the number of passbands, visible light cutoff depth, and film reliability, the present invention provides a design and preparation method for a six-bandpass filter, which specifically solves the following problems: 1. How to overcome the limitation of the traditional FP structure on the number of passbands (from ≤3 to ≥6) through asymmetric membrane design; 2. How to achieve compatibility between high transmittance (>98%) and deep cutoff (OD ≥ 4) across a wide wavelength band (400-1200nm); 3. How to reduce the risk of film collapse of ultra-multilayer films through Ta2O5 / SiO2 stress matching design.

[0005] The present invention provides a method for manufacturing a visible-near-infrared six-bandpass filter. The filter comprises a substrate, a front surface film, and a rear surface film. The method is characterized in that the front surface film and the rear surface film are respectively coated on the front and rear surfaces of the substrate using a magnetron sputtering coating process (technology). The front surface film and the rear surface film both comprise multilayer films, which are layered and coated using high and low refractive index materials. The coating raw materials include a substrate, a target material, a sputtering gas, a reaction gas, etc. The magnetron sputtering coating technology is implemented using a magnetron sputtering coating device, including a gas activation system (ICP), a rotating placement table, a reaction chamber, a target power supply, and other units. The substrate is also referred to as a substrate. The high refractive index generally refers to a refractive index greater than that of the substrate, and the low refractive index generally refers to a refractive index less than that of the substrate. When the incident angle of light is constant, in order to minimize the filter offset, the refractive index n of the high-refractive-index material should be as large as possible, and the refractive index n of the low-refractive-index material should be as small as possible. Tantalum (Ta) and its oxide (Ta2O5) (refractive index 2.2) are important high-refractive-index materials, especially playing a key role in ultra-multi-layer (>100 layers) multi-bandpass filters. Compared with silicon nitride, the high refractive index of tantalum oxide can reduce the number of layers required to achieve the same optical effect. At the same time, the tantalum oxide film material has less stress and is suitable for narrow-band filter design. Therefore, the present invention considers the use of tantalum oxide material; the low-refractive-index material is selected from silicon dioxide (SiO2) (refractive index 1.45) which is often used in film design; Ta2O5 films typically exhibit compressive stress, meaning the film tends to shrink toward the substrate. SiO2 films typically exhibit tensile stress, meaning the film tends to stretch outward. The present invention achieves dynamic stress equilibrium by alternating Ta2O5 and SiO2 film layers, with the net stress of each Ta2O5 / SiO2 bilayer pair approaching zero. This also reduces overall film stress, preventing excessive stress accumulation in a particular direction (e.g., perpendicular to the substrate), thereby inhibiting film warping or delamination. Finally, an annealing treatment (250–400°C) is performed to release local stress through heat treatment and promote lattice relaxation of Ta2O5 and SiO2. The compressive stress of Ta2O5 partially relaxes with increasing temperature, while the tensile stress of SiO2 is relieved by the reorganization of silicon-oxygen bonds.

[0006] The front surface adopts a bandpass design and the rear surface adopts a multi-FP series structure, which can adjust the center wavelength position (CWL) and half-maximum width (FWHM); The actual coating structure can be obtained by performing simulation tests using coating simulation software. Figure 2 This is the transmittance spectrum obtained by simulation.

[0007] Furthermore, the film system structure after the front surface film is optimized includes: S1:Sub| 0.13L 0.17H 0.24L 0.52H 0.15L 0.31H 0.83L 0.03H 0.35L 1.14H1.14L 1.09H 1.13L 0.45H 0.06L 0.47H 1.13L 1.08H 1.14L 0.31H 0.10L 0.26H 0.08L0.15H 1.06L 0.32H 0.08L 0.52H 0.12L 0.04H 1.02L 1.08H 1.13L 0.36H 0.13L 0.37H1.14L 1.10H 1.22L 0.28H 0.24L 0.18H 1.09L 0.28H 0.18L 0.37H 1.19L 1.08H 1.07L0.09H 0.02L 0.97H 1.13L 0.57H 0.04L 0.47H 0.66L 0.05H 0.49L 0.44H 0.21L 0.34H0.61L 0.14H 0.49L 0.29H 0.41L 0.16H 0.84L 0.07H 0.71L 0.13H 0.57L 0.21H 0.48L0.21H 0.57L 0.22H 0.42L 0.32H 0.42L 0.21H 0.57L 0.27H 0.33L 0.36H 0.46L 0.16H0.56L 0.35H 0.23L 0.38H 0.56L 0.05H 0.61L 0.37H 0.07L 0.64H 0.60L The above structure is mainly obtained based on optical coating theory and special simulation analysis tools.

[0008] Furthermore, the film system structure of the rear surface film is designed in two ways, including: S2 Implementation Case 1: Sub| 0.18L 0.38H 0.14L 0.30H 1.58L 0.45H 0.99L 1.70H1.49L 1.22H 0.94L 1.61H 1.26L 2.78H 0.46L 1.81H 1.24L 0.99H 1.28L 1.03H 1.66L0.74H 1.22L 3.03H 1.05L 1.39H 0.79L 0.87H 0.31L 0.13H 0.81L 1.58H 1.33L 0.84H0.73L 1.97H 0.64L 0.64H 1.34L 1.20H 0.83L 1.12H 0.41L 1.59H 1.72L 0.92H 0.99L0.11H 0.57L 0.91H 0.79L 0.77H 0.79L 1.02H 1.30L 0.88H 0.65L 2.68H 0.61L 0.24H0.21L 0.65H 0.47L 0.81H 1.73L 0.95H 0.80L 0.80H 0.83L 1.59H 0.62L 0.33H 0.63L1.35H 0.95L 0.94H 0.87L 0.80H 0.77L 1.86H 0.59L 2.03H 1.42L 0.34H 1.13L 1.14H0.86L 1.24H 1.04L 0.44H 0.63L 2.63H 0.49L 1.55H 1.54L 0.30H 2.76L 1.64H 1.70L1.50H 1.93L 1.43H 0.98L S2 Implementation Case 2: Sub| 0.18L 0.35H 0.12L 1.83H 1.41L 1.44H 1.81L 0.52H0.23L 0.71H 0.30L 0.57H 0.35L 1.76H 1.31L 1.19H 0.64L 1.50H 0.50L 0.37H 0.45L0.65H 0.60L 0.31H 0.40L 2.89H 1.62L 0.77H 1.21L 0.99H 0.98L 1.36H 0.59L 0.23H0.65L 0.73H 0.40L 0.27H 0.62L 2.23H 0.12L 0.43H 1.54L 0.97H0.58L 1.71H 1.49L1.41H 0.79L 1.70H 1.31L 2.70H 0.45L 1.87H 1.18L 1.00H 1.28L 1.03H 1.66L 0.76H1.28L 2.97H 1.07L 1.35H 0.81L 0.86H 0.24L 0.14H 0.82L 1.60H 1.31L 0.95H 0.71L1.94H 0.68L 0.62H 1.31L 1.14H 1.09L 0.83H 0.53L 1.62H 1.76L 0.76H 1.21L 0.15H0.35L 0.96H 0.84L 0.77H 0.82L 0.80H 1.50L 0.75H 0.68L 2.70H 0.61L 0.57H 0.12L0.52H 0.53L 0.70H 1.92L 0.63H 0.77L 0.84H 1.06L 1.73H 0.38L 0.29H 0.81L 1.08H1.07L 0.88H 0.93L 0.80H 0.74L 1.94H 0.54L 2.00H 1.32L 0.37H 1.25L 1.14H 0.73L1.31H 1.11L 0.30H 0.77L 2.35H 0.93L 1.48H 1.49L 0.15 H 2.80L 1.68H 1.64L1.50H 1.93L 1.44H 1.00L The above structure is mainly obtained based on optical coating theory and special simulation analysis tools.

[0009] Since magnetron sputtering has a high film-forming rate, low substrate temperature, and good film adhesion, large-area coating can be achieved. Magnetron sputtering coating technology can change parameters such as the coating thickness, refractive index, and extinction coefficient of the thin film by controlling the electric field and gas flow, and is convenient for testing and debugging. Therefore, the present invention adopts magnetron sputtering technology to coat the film and prepare the filter.

[0010] Furthermore, the substrate includes glass or a transparent organic plate, and the thickness of the substrate is 0.1 to 4 mm.

[0011] Furthermore, the target material includes single crystal silicon and tantalum target, the sputtering gas includes argon, and the reaction gas includes oxygen; when coating SiO2 film, the reaction gas is oxygen and the sputtering gas is argon, and when coating Ta2O5 film, the reaction gas is also oxygen and the sputtering gas is argon.

[0012] Furthermore, considering the firmness of the film layer and the substrate, the film system uses SiO2 as the outermost layer; since the stress in SiO2 is compressive stress, it can enhance the crack resistance of the film layer, so SiO2 is preferably used as the outermost layer.

[0013] Furthermore, the advantages and benefits of the present invention lie in the fact that the method for manufacturing a visible-near-infrared six-bandpass filter designed in the present invention utilizes magnetron sputtering technology and tantalum materials. The resulting visible-near-infrared six-bandpass filter exhibits high passband transmittance, low cutoff band transmittance, minimal offset, minimal film stress and deformation, and is less susceptible to film collapse and other issues. The filter also boasts a long lifespan, excellent filtering performance, and stable overall filter performance. Furthermore, the present invention can also be used to design and manufacture filters with even higher bandpass numbers.

[0014] Furthermore, the traditional method for producing multi-bandpass filters involves coating different regions of a substrate with films that meet the requirements for different wavelength bands. This method requires the use of a nano-mask. Plating a single region requires pre-treatment, spin coating, pre-baking, exposure, and development to create a protective layer. The film structure for the protective region is then arranged, and a filter layer with a specific function is then plated within the protective region. The mask position must be constantly adjusted when coating subsequent layers, and these steps must be repeated, requiring additional time and cost. Furthermore, when the filter produced by this method transmits light within a specific wavelength band, only that specific region can be used, resulting in limited information. The innovation of the present invention lies in the simultaneous production of the six-bandpass filter through magnetron sputtering coating technology, which utilizes carefully designed film layers. When transmitting light within a specific wavelength band, the entire filter is usable, significantly improving information acquisition and significantly saving time and cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the measured transmittance spectrum of the six-bandpass filter manufactured in the embodiment of the present invention. Figure 2 This is the transmittance spectrum simulated during the design process of the present invention Figure 3 Flow chart for manufacturing the six-bandpass filter in the present invention DETAILED DESCRIPTION The present invention provides a method for manufacturing a visible-near-infrared six-bandpass filter. The method uses magnetron sputtering technology to prepare a tantalum oxide thin film. During the sputtering process, oxygen is introduced into the tantalum target. The refractive index and absorptivity of the material are adjusted by adjusting the oxygen flow rate. The oxygen flow rate is greater than or equal to 100 sccm (standard milliliters per minute) and less than or equal to 200 sccm.

[0016] Furthermore, the silicon dioxide film is prepared by magnetron sputtering technology, and oxygen is filled in the process of sputtering the silicon target. The refractive index and absorptivity of the material are adjusted by adjusting the oxygen flow rate, and the oxygen flow rate is greater than or equal to 60 sccm and less than or equal to 120 sccm.

[0017] Relatively speaking, SiO2 is a low-refractive-index material, and is relatively balanced in terms of refractive index and extinction coefficient, which can achieve better transmittance characteristics.

[0018] Furthermore, the infrared bandpass filter manufacturing method includes the steps shown in the accompanying drawings. Figure 3 , the following are the specific instructions; S1. Install the substrate and target. The target is selected according to the coating requirements of each layer. Filter coating generally uses single crystal silicon (Si) and tantalum (Ta) as the target; The installation involves placing the target and the substrate (substrate) to be coated on the concentric axis of the rotating platform of the magnetron sputtering coating equipment, with the target at the bottom and the substrate at the top. The basic principle of coating is to pass sputtering gas into the plasma-assisted deposition system (APS) for ionization. The ions bombard the target surface and then sputter onto the substrate surface, reacting with the reaction gas to form a thin film layer covering the substrate surface. The film thickness is monitored using a time control method. The installation also includes first installing the substrate to be plated with a fixture and placing it in a loading chamber, then using a robotic arm to move the installed substrate from the loading chamber to the reaction chamber, and adjusting the target surface and the base surface to an appropriate distance; the distance depends on factors such as gas pressure, vacuum, APS power, target power supply power, etc., and can be obtained through experiments. The empirical parameter is generally 40 to 50 cm; S2. Vacuum the reaction chamber. Turn on the mechanical pump and diffusion pump in turn to evacuate the reaction chamber so that the background vacuum of the reaction chamber is better than 4.0×10 -4 Pa; S3, substrate cleaning, including high-speed inert gas cleaning; The high-speed inert gas cleaning includes, before formal plating, increasing the speed of the rotating placement table to 50-80 rpm / min, turning on the inert gas and controlling its flow rate within the range of 100-150sccm, and turning on the APS and setting the power to 1000-1500W to bombard the surface of the plated substrate for cleaning for 0.5-3 minutes; ultrasonic cleaning can also be performed before the substrate is installed, that is, the material is placed in an ultrasonic cleaning machine for cleaning and then dried.

[0019] S4. Coat the optical thin film layer by layer. Coat the optical thin film layer by layer according to the film structure design of the front surface film and the back surface film. There is no order for coating the front and back surface films. Generally, the first layer of SiO2 film is coated first. The reaction gas is O2 and the sputtering gas is Ar. The target power supply should be turned off first, and the O2 gas is turned on in the APS. Turn on the target power supply and set the APS power between 1.0 and 3.0KW. Ionize Ar to bombard the target. The sputtered silicon atoms react chemically with oxygen to combine on the surface of the substrate to form a SiO2 film. When a Ta2O5 film is coated next to the SiO2 film, the reaction gas is O2 and the sputtering gas is Ar. Turn on the target power supply and set its power. The rate is between 4.0 and 6.0 kW, and the sputtering gas Ar and the reaction gas O2 are simultaneously introduced into the APS. The Ar gas is ionized by the APS, and high-speed argon ions bombard the target material, sputtering out target particles (generally tantalum atoms). The target particles and the reaction gas particles are affected by the electric field and the initial velocity, and a chemical vapor reaction occurs on the surface of the substrate and is deposited to form a Ta2O5 thin film layer. When the SiO2 film is deposited adjacent to the Ta2O5 film, the target power supply should also be turned off first, the O2 gas in the APS is turned on, and the target power supply is turned on and its power is set between 4.0 and 6.0 kW to deposit the SiO2 thin film layer. In this way, layer-by-layer plating is carried out according to the film system structure design.

[0020] S5, annealing. Ta2O5 and SiO2 prepared by conventional process have large internal stress. Therefore, after the preparation of the front and rear surface films of the filter is completed, the filter is annealed to prevent the film from collapsing due to excessive internal stress.

[0021] Furthermore, the annealing temperature is greater than or equal to 250° C. and less than or equal to 400° C.

[0022] Through the above process, six-bandpass filters can be produced at one time, greatly saving time and cost.

[0023] The following table shows some of the spectral data in the example, and the corresponding measured spectrum is shown in the attached figure. Figure 1 .

[0024] Bandpass sequence 1 2 3 4 5 6 CWL_T50% / nm 410 460 510 553 650 812 BW_50% / nm 23.2 17.2 22.5 18.0 34.2 58.5 BW_90% / nm 18.3 11.8 17.8 12.4 29.3 52.4

Claims

1. A design and manufacturing method of a six-bandpass filter, characterized in that: The filter comprises six passbands, and adopts a magnetron sputtering coating process to alternately coat high-refractive index and low-refractive index film layers, and achieves high isolation of the six passbands through a stress matching design.

2. The method for designing and manufacturing a six-bandpass filter according to claim 1, characterized in that: The filter comprises a substrate, a front surface film and a rear surface film, wherein the front surface film and the rear surface film are respectively plated on the front and rear surfaces of the substrate by a magnetron sputtering coating process, wherein the front surface film and the rear surface film both comprise multilayer films; The high refractive index material includes tantalum oxide (Ta2O5) material, and the low refractive index material includes silicon oxide (SiO2) material; The front surface adopts a bandpass design and the rear surface is a multi-FP series structure, which can adjust the center wavelength position (CWL) and half-maximum width (FWHM).

3. The method for designing and manufacturing a six-bandpass filter according to claim 1, wherein: The optimized film system structure of the front surface film includes: S1: Sub| 0.13L 0.17H 0.24L 0.52H 0.15L 0.31H 0.83L0.03H 0.35L 1.14H 1.14L 1.09H 1.13L 0.45H 0.06L 0.47H 1.13L 1.08H 1.14L 0.31H0.10L 0.26H 0.08L 0.15H 1.06L 0.32H 0.08L 0.52H 0.12L 0.04H 1.02L 1.08H 1.13L0.36H 0.13L 0.37H 1.14L 1.10H 1.22L 0.28H 0.24L 0.18H 1.09L 0.28H 0.18L 0.37H1.19L 1.08H 1.07L 0.09H 0.02L 0.97H 1.13L 0.57H 0.04L 0.47H 0.66L 0.05H 0.49L0.44H 0.21L 0.34H 0.61L 0.14H 0.49L 0.29H 0.41L 0.16H 0.84L 0.07H 0.71L 0.13H0.57L 0.21H 0.48L 0.21H 0.57L 0.22H 0.42L 0.32H 0.42L 0.21H 0.57L 0.27H 0.33L0.36H 0.46L 0.16H 0.56L 0.35H 0.23L 0.38H 0.56L 0.05H 0.61L 0.37H 0.07L 0.64H0.60L.

4. The method for designing and manufacturing a six-bandpass filter according to claim 1, wherein: The film system structure after the rear surface film optimization includes: S2 design 1: Sub| 0.18L 0.38H 0.14L 0.30H 1.58L 0.45H0.99L 1.70H 1.49L 1.22H 0.94L 1.61H 1.26L 2.78H 0.46L 1.81H 1.24L 0.99H 1.28L1.03H 1.66L 0.74H 1.22L 3.03H 1.05L 1.39H 0.79L 0.87H 0.31L 0.13H 0.81L 1.58H1.33L 0.84H 0.73L 1.97H 0.64L 0.64H 1.34L 1.20H 0.83L 1.12H 0.41L 1.59H 1.72L0.92H 0.99L 0.11H 0.57L 0.91H 0.79L 0.77H 0.79L 1.02H 1.30L 0.88H 0.65L 2.68H0.61L 0.24H 0.21L 0.65H 0.47L 0.81H 1.73L 0.95H 0.80L 0.80H 0.83L 1.59H 0.62L0.33H 0.63L 1.35H 0.95L 0.94H 0.87L 0.80H 0.77L 1.86H 0.59L 2.03H 1.42L 0.34H1.13L 1.14H 0.86L 1.24H 1.04L 0.44H 0.63L 2.63H 0.49L 1.55H 1.54L 0.30H 2.76L1.64H 1.70L 1.50H 1.93L 1.43H 0.98L; S2 design 2:Sub| 0.18L 0.35H 0.12L 1.83H 1.41L 1.44H 1.81L 0.52H 0.23L 0.71H0.30L 0.57H 0.35L 1.76H 1.31L 1.19H 0.64L 1.50H 0.50L 0.37H 0.45L 0.65H 0.60L0.31H 0.40L 2.89H 1.62L 0.77H 1.21L 0.99H 0.98L 1.36H 0.59L 0.23H 0.65L 0.73H0.40L 0.27H 0.62L 2.23H 0.12L 0.43H 1.54L 0.97H0.58L 1.71H 1.49L 1.41H 0.79L1.70H 1.31L 2.70H 0.45L 1.87H 1.18L 1.00H 1.28L 1.03H 1.66L 0.76H 1.28L 2.97H1.07L 1.35H 0.81L 0.86H 0.24L 0.14H 0.82L 1.60H 1.31L 0.95H 0.71L 1.94H 0.68L0.62H 1.31L 1.14H 1.09L 0.83H 0.53L 1.62H 1.76L 0.76H 1.21L 0.15H 0.35L 0.96H0.84L 0.77H 0.82L 0.80H 1.50L 0.75H 0.68L 2.70H 0.61L 0.57H 0.12L 0.52H 0.53L0.70H 1.92L 0.63H 0.77L 0.84H 1.06L 1.73H 0.38L 0.29H 0.81L 1.08H 1.07L 0.88H0.93L 0.80H 0.74L 1.94H 0.54L 2.00H 1.32L 0.37H 1.25L 1.14H 0.73L 1.31H 1.11L0.30H 0.77L 2.35H 0.93L 1.48H 1.49L 0.15H 2.80L 1.68H 1.64L 1.50H 1.93L1.44H 1.00L.

5. The method for designing and manufacturing a six-bandpass filter according to claim 2, wherein: The substrate comprises glass or a transparent organic plate, and the thickness of the substrate is 0.1 to 4 mm; the target material comprises single crystal silicon and tantalum target material, the sputtering gas comprises argon, and the reaction gas comprises oxygen; when depositing SiO2 film, the reaction gas is oxygen and the sputtering gas is argon; when depositing Ta2O5 film, the reaction gas is also oxygen and the sputtering gas is argon.

6. The method for designing and manufacturing a six-bandpass filter according to claim 2, wherein: The multilayer film uses SiO2 as the outermost layer. Since the stress in SiO2 is compressive stress, it can enhance the crack resistance of the film layer, so SiO2 is used as the outermost layer.

7. The method for designing and manufacturing a six-bandpass filter according to claim 2, wherein: The tantalum oxide thin film is prepared using magnetron sputtering technology, and oxygen is introduced during the sputtering process of the tantalum target. The refractive index and absorptivity of the material are adjusted by adjusting the oxygen flow rate. The oxygen flow rate is greater than or equal to 100 sccm (standard milliliters per minute) and less than or equal to 200 sccm.

8. The method for designing and manufacturing a six-bandpass filter according to claim 2, wherein: The silicon dioxide film is prepared by magnetron sputtering technology. Oxygen is filled in the process of sputtering the silicon target. The refractive index and absorptivity of the material are adjusted by adjusting the oxygen flow rate. The oxygen flow rate is greater than or equal to 60 sccm and less than or equal to 120 sccm.

9. The method for designing and manufacturing a six-bandpass filter according to claim 2, wherein: The magnetron sputtering coating process includes: S1 installation of substrate and target; S2 vacuuming of reaction chamber; S3 cleaning of substrate; S4 coating of optical thin film layer by layer; S5 annealing. After the preparation of the front and rear surface films of the filter is completed, the filter is annealed.

10. The method for designing and manufacturing a six-bandpass filter according to claim 9, wherein: The annealing temperature is greater than or equal to 250° C. and less than or equal to 400° C.

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