Low-reflection attenuation thin film with wave band of 10.6 microns and preparation method of low-reflection attenuation thin film
By designing low-reflection attenuation thin films with alternating layers of dielectric and absorbing materials in precision optical systems, the problem of return loss caused by high reflectivity is solved, achieving extremely low reflectivity and adjustable transmittance, which is suitable for high-end optical systems.
Patent Information
- Application Number
- CN202511968366.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-24
- Publication Date
- 2026-02-27
AI Technical Summary
Existing absorptive neutral density filters suffer from severe return loss due to their high reflectivity in precision optical systems, which affects system performance.
A low-reflection attenuation thin film in the 10.6µm band is designed by alternately depositing multiple layers of dielectric materials with different refractive indices and thicknesses, along with absorbing materials, and combining the principle of optical interference cancellation to achieve extremely low reflectivity and adjustable transmittance.
It achieves extremely low reflectivity (no more than 0.11%) and adjustable transmittance (0.1%-100%), making it suitable for high-end optical systems such as high-power laser systems, fiber optic communication and precision measurement, and reducing return loss.
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Figure CN121578431A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a low-reflection attenuation thin film in the 10.6µm band and its preparation method, belonging to the field of low-reflection attenuation thin film technology. Background Technology
[0002] Attenuation films, also known as neutral density filters, differ from ordinary filters and are divided into absorptive and reflective types. Absorptive neutral density filters generally utilize thin film materials to absorb light of specific wavelengths. The amount of light passing through the filter depends on the type of thin film material, the film thickness, and the film system structure. A typical absorptive attenuation film can be understood as a "uniform, absorbing glass or material." Its working principle is as follows: when light is incident on the film, some energy is reflected at the front and back surfaces (Fresnel reflection), while the light entering the film is absorbed by the material itself, and the remaining light is transmitted. Such films generally have high back reflection: due to the mismatch between the refractive index of the film material and air, according to Fresnel's equations, unavoidable reflection occurs at the front and back surfaces. This reflected light (especially from the front surface) returns along the same path, forming an "echo," which is fatal to many precision optical systems (such as fiber optic communication, precision laser processing, and interferometry). They are typically used in non-critical applications where return loss is not sensitive and power is low, such as simple light intensity control and background light suppression for inexpensive sensors.
[0003] To suit precision optical systems, this application proposes a low-reflection attenuation thin film in the 10.6µm band and its fabrication method. This is a precision optical interference thin film, whose design is far more complex than that of ordinary absorptive films. It is formed by alternating deposition of multiple layers of dielectric materials with different refractive indices and thicknesses, along with absorbing materials. Its working principle mainly includes absorption attenuation and destructive interference. Absorption attenuation: The thin film contains absorbing material layers responsible for converting a portion of the light energy into heat energy, achieving the basic attenuation function. Destructive interference: By precisely designing the thickness and refractive index of the multilayer film, the light reflected from the outermost surface and the light reflected back from the interfaces inside the film layers have nearly equal amplitudes but opposite phases when emitted. When these light waves meet, they interfere destructively, thereby reducing the total reflected light intensity to an extremely low level. Summary of the Invention
[0004] This invention provides a low-reflection attenuation thin film in the 10.6µm band and its preparation method, which has extremely low reflectivity and adjustable transmittance, making it suitable for precision optical systems.
[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:
[0006] A low-reflection attenuation thin film in the 10.6µm band, when OD≥1 (transmittance≤10%), has the following film structure: SUB / aMbNcEdFeEfHgEhL / A;
[0007] When OD < 1 (permeability > 10%), its membrane structure is: SUB / aMbN(k1Hk2L)^ x gEhL / A;
[0008] Wherein, SUB represents zinc selenide substrate, A represents air, M represents zinc sulfide, N represents yttrium oxide, E represents silicon, F represents metallic chromium, H represents silicon monoxide, and L represents aluminum oxide; a, b, c, d, e, f, g, h, k1, k2 represent coefficients of the optical thickness of each layer at one-quarter of the reference wavelength, the magnitude of which is related to the reference wavelength λ. In this design, the reference wavelength λ is set to 550 nm; the superscript X represents the number of cycles of the absorption control layer (k1Hk2L).
[0009] Absorbing film materials that can be used in the 10.6µm band include alumina, silicon oxide, silicon monoxide, silicon, and metallic chromium. However, when using dielectric materials alone for absorbing film coating, the film thickness and stress cannot be effectively controlled in order to obtain low reflectivity.
[0010] This application achieves both optical and mechanical properties of a low-reflection attenuation film in the 10.6µm band by alternating and synergistic deposition of multiple layers of dielectric materials with different refractive indices and thicknesses, along with absorbing materials. The reflectivity can be as low as below 0.001%, making it widely applicable in high-end fields such as high-power laser systems, fiber optic communication, precision measurement, lidar, and spectral analysis. For example, it can be used directly at the laser output end or before optical isolators, locations that are extremely sensitive to reflected light.
[0011] When high transmittance is required, silicon monoxide (with high absorption at 10.6µm and an extinction coefficient of around 0.7) is chosen as the absorbing film material. When very low transmittance is required, metallic chromium (with an extinction coefficient of over 30 at 10.6µm) is the preferred material. By controlling the film thickness of chromium and silicon monoxide, the transmittance of the film can be effectively controlled. However, the reflectance of the film cannot be effectively controlled. Therefore, a combination of metallic chromium and dielectric film materials was ultimately selected for film system design and fabrication, resulting in a low-reflectance attenuation film with a specific transmittance.
[0012] Double-sided coatings can be applied to a zinc selenide substrate. The first side is a conventional low-reflection antireflection coating with the structure SUB / mBnC / A, where SUB represents the zinc selenide substrate, A represents air, B represents ytterbium fluoride, and C represents zinc selenide; m and n represent the coefficients of the optical thickness of each layer at one-quarter of the reference wavelength, and their values are related to the reference wavelength λ. In this design, the reference wavelength λ is set to 550 nm, m is 12.0751, and n is 4.3232. The physical thickness of the ytterbium fluoride layer is 1082.00 nm, and the physical thickness of the zinc selenide layer is 216.16 nm. The relationship between the physical thickness and one-quarter of the reference wavelength is d = k * λ / (4n), where d is the physical thickness, k is the coefficient of the optical thickness of each layer at one-quarter of the reference wavelength, λ is the reference wavelength, and n is the refractive index of each layer. Ytterbium fluoride refractive index: n=1.535@550nm, yttrium oxide refractive index: n=2.75@550nm; the second side is a low-reflection attenuation thin film in the 10.6um band of this application.
[0013] To balance the mechanical and optical properties of the obtained low-reflection attenuation film in the 10.6µm band, the values for a are: 0.30–0.60, b: 0.30–0.50, c: 0.80–1.00, d: 0.85–1.05, e: 5.51–7.51, f: 2.30–4.30, g: 8.94–10.94, and h: 7.26–9.26.
[0014] When OD < 1, the values of k1 are 0.7 to 3, the values of k2 are 2 to 8, and the values of X are 1 to 5.
[0015] As one specific preferred implementation, when OD≥1 (transmittance≤10%), aM is a zinc sulfide layer, bN is a yttrium oxide layer, cE is a first silicon layer, dF is a chromium metal layer, eE is a second silicon layer, fH is a first silicon monoxide layer, gE is a third silicon layer, and hL is a first aluminum oxide layer; wherein, the physical thickness of the zinc sulfide layer is 30-80nm, the physical thickness of the yttrium oxide layer is 30-80nm, the physical thickness of the first silicon layer is 30-70nm, the physical thickness of the chromium metal layer is 10-90nm, the physical thickness of the second silicon layer is 200-250nm, the physical thickness of the first silicon monoxide layer is 210-260nm, the physical thickness of the third silicon layer is 300-380nm, and the physical thickness of the first aluminum oxide layer is 700-800nm.
[0016] As one specific preferred implementation, when OD < 1 (transmittance > 10%), aM is a zinc sulfide layer, bN is a yttrium oxide layer, and (k1Hk2L)^ xAs the absorption control layer, k1H is the second silicon monoxide layer, k2L is the second aluminum oxide layer, gE is the third silicon layer, and hL is the first aluminum oxide layer; the physical thickness of the zinc sulfide layer is 30-80 nm, the physical thickness of the yttrium oxide layer is 30-80 nm, the physical thickness of the second silicon monoxide layer is 50-200 nm, the physical thickness of the second aluminum oxide layer is 200-700 nm, the physical thickness of the third silicon layer is 300-380 nm, and the physical thickness of the first aluminum oxide layer is 700-800 nm.
[0017] The single-sided reflectivity of the aforementioned low-reflection attenuation film in the 10.6µm band is no greater than 0.11%, and the double-sided transmittance is between 0.1% and 100%. However, in practice, a maximum of 90% is generally required.
[0018] The preparation method of the above-mentioned low-reflection attenuation film is as follows:
[0019] A Cathay 1150 coating machine was used, and the crystal control system employed a Membrane Technology MXC-3B controller. The thickness of the thin film was measured by utilizing the change in the oscillation frequency of the quartz crystal. A Cathay GTRF-HMI2000 radio frequency ion source was used. The vacuum chamber achieved the required vacuum level through a combination of a mechanical pump and a dual condenser pump system, and the vacuum level was measured using an ionization gauge.
[0020] To ensure better adhesion, the zinc selenide substrate is cleaned before coating: wipe with a degreasing cloth dipped in a mixture of anhydrous ethanol and ether in a volume ratio of 1:3 to remove surface residue.
[0021] To better ensure the film density, the initial vacuum level is 2.0–6.0 × 10⁻⁶. -4 Pa, baking temperature 100-180℃; wherein, the ion source parameters of the M and N layers are set as follows: BEAM voltage 1000±200V, BEAM current 1000±200mA, accelerating voltage 600±100V; the ion source parameters of the E, F, H and L layers are set as follows: BEAM voltage 400±50V, BEAM current 600±50mA, accelerating voltage 600±100V.
[0022] To further enhance film adhesion, the zinc selenide substrate was bombarded with an ion beam for 5 minutes before coating. The ion beam parameters were: BEAM voltage 200±500V, BEAM current 200±50mA, and accelerating voltage 500±100V. The purpose was to clean the substrate, increase the aggregation coefficient, and strengthen film adhesion. During film deposition, a radio frequency ion source was used to assist deposition and increase aggregation density. The ion source was purged with argon gas to maintain a vacuum of 7.5–9.5 × 10⁻⁶. -3Pa utilizes an ion source to improve the film structure and stress matching, effectively solving the stress problem between film layers, thereby improving the film density, making the film more robust and longer-lasting. Controlling the conditions during film preparation is crucial. The preparation conditions of each film layer not only affect the density and other properties of a single layer, but also the bonding force with adjacent layers and the overall optical properties of the film. Among these, the selection of the evaporation rate is particularly important. For this film system, zinc sulfide is evaporated using an electron gun at a rate controlled at 0.1-1 nm / s; yttrium oxide is evaporated using an electron gun at a rate controlled at 0.2-0.8 nm / s; silicon is evaporated using an electron gun at a rate controlled at 0.2-0.6 nm / s; metallic chromium is evaporated using an electron gun at a rate controlled at 0.01-0.1 nm / s; silicon monoxide is evaporated using a molybdenum boat with a resistance evaporation rate controlled at 0.1-0.8 nm / s; and alumina is evaporated using an electron gun at a rate controlled at 0.1-0.6 nm / s. Long-term practice has shown that, for the bonding layer of the first two layers of this film system, the aforementioned rate yields the best film density and adhesion to the substrate and adjacent layers.
[0023] Argon ion-assisted deposition is used in the above coating process to improve the density of the film layer, effectively solve the stress problem between film layers, improve the density of the film layer, make the film layer more robust, and extend its service life.
[0024] The above method utilizes ion beam-assisted deposition technology at low temperatures, employing chromium as a highly efficient absorption layer. Ion beam-assisted deposition reduces residual stress in the film layer and improves adhesion, resulting in an attenuating film with excellent optical properties and strong adhesion. This attenuating film exhibits a reflectivity of no more than 0.11% in the 10.6µm wavelength range, and its transmittance can be adjusted from 0.1% to 100% according to application requirements, with an error within ±1%. It also meets laser resistance requirements, satisfying the practical needs of optical devices.
[0025] Any techniques not mentioned in this invention are based on existing technologies.
[0026] The present invention relates to a low-reflection attenuation film in the 10.6µm band. Based on the ordinary absorption type, it greatly suppresses surface reflection by introducing a complex optical interference design, thereby achieving excellent performance with low return loss while maintaining attenuation function, and is suitable for high-end optical systems. Attached Figure Description
[0027] Figure 1 This is a schematic diagram of the membrane system structure in Example 1;
[0028] Figure 2 The diagram shows the reflection (bold) and transmission (unbold) curves for Example 1.
[0029] Figure 3 This is a schematic diagram of the membrane system structure in Example 2;
[0030] Figure 4 The reflection (bold) and transmittance (unbold) curves for Example 2 are designed;
[0031] Figure 5 This is a front profile diagram of OD1 plating in Example 1;
[0032] Figure 6 This is a diagram of the OD1 plating profile in Example 1;
[0033] Figure 7 This is a front profile diagram of the OD0.7 plating in Example 2;
[0034] Figure 8 This is a diagram of the OD0.7 plating finish in Example 2;
[0035] Figure 9 The graph shows the measured OD1 reflection (red line) and transmittance (blue line) curves in Example 1.
[0036] Figure 10 The graph shows the measured OD0.7 reflection (black line) and transmittance (blue line) curves in Example 2. Detailed Implementation
[0037] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.
[0038] Example 1
[0039] Implementation examples of OD≥1 (transmittance≤10%):
[0040] like Figure 1 As shown, the low-reflection attenuation film of OD1 has a film structure of 8 layers of optical thin film SUB / aMbNcEdFeEfHgEhL / A, where SUB represents zinc selenide substrate, A represents air, M represents zinc sulfide, N represents yttrium oxide, E represents silicon, F represents metallic chromium, H represents silicon monoxide, and L represents aluminum oxide; ah represents the coefficient of one-quarter reference wavelength optical thickness of each layer. In this design, the reference wavelength λ is set to 550nm.
[0041] The values of a are: 0.5212, b is: 0.3901, c is: 0.8912, d is: 0.9410, e is: 6.5116, f is: 3.2988, g is: 9.9433, and h is: 8.2553.
[0042] Starting from the zinc selenide substrate, the layers are sequentially: zinc sulfide layer, yttrium oxide layer, first silicon layer, metallic chromium layer, second silicon layer, silicon monoxide layer, third silicon layer, and aluminum oxide layer. The physical thicknesses of the zinc sulfide layer, yttrium oxide layer, first silicon layer, and metallic chromium layer are all 30 nm and 30 nm, respectively. The physical thicknesses of the chromium oxide layer, second silicon layer, and silicon monoxide layer are all 46.39 nm and 219.21 nm, respectively. The physical thicknesses of the silicon monoxide layer and third silicon layer are all 226.59 nm, respectively. The physical thicknesses of the third silicon layer and aluminum oxide layer are all 700 nm. The relationship between the physical thickness and the quarter-reference wavelength is d = k * λ0 / (4n), where d is the physical thickness, k is the coefficient of the quarter-reference wavelength optical thickness of each layer, λ0 is the reference wavelength, and n is the refractive index of each layer. Zinc sulfide: n=2.38877@550nm, Yttrium oxide: n=1.788@550nm, Silicon: n=4.089@550nm, Metallic chromium: n=2.7109@550nm, Silicon monoxide: 2.021@550nm, Aluminum oxide: 1.62158@550nm.
[0043] Example 2
[0044] Implementation examples of OD < 1 (transmittance > 10%):
[0045] like Figure 3 As shown, the low-reflection attenuation film with OD0.7 has a film structure of 12 layers of optical thin film SUB / aMbN(k1Hk2L)^ X gEhL / A, where SUB represents zinc selenide substrate, A represents air, M represents zinc sulfide, N represents yttrium oxide, E represents silicon, H represents silicon monoxide, and L represents aluminum oxide; a, b, g, h, k1, and k2 represent coefficients of the optical thickness of each layer at one-quarter of the reference wavelength. In this design, the reference wavelength λ is set to 550 nm.
[0046] The values of a are 0.5212, b is 0.3901, g is 3.3422, h is 2.9483, k1 is 1.3748, and k2 is 7.1752.
[0047] X represents the number of cycles of the (k1Hk2L) absorption control layer, which is 4, meaning (k1Hk2L) cycles 4 times, for a total of 8 layers.
[0048] Starting from the zinc selenide substrate, the layers are sequentially: zinc sulfide layer, yttrium oxide layer, first silicon monoxide layer, first aluminum oxide layer, second silicon monoxide layer, second aluminum oxide layer, third silicon monoxide layer, third aluminum oxide layer, fourth silicon monoxide layer, fourth aluminum oxide layer, silicon layer, and fifth aluminum oxide layer. The physical thickness of the zinc sulfide layer is 30 nm, the physical thickness of the yttrium oxide layer is 30 nm, the physical thickness of the first, second, third, and fourth silicon monoxide layers is 94.43 nm, the physical thickness of the first, second, third, and fourth aluminum oxide layers is 608.42 nm, the physical thickness of the silicon layer is 116.51 nm, and the physical thickness of the fifth aluminum oxide layer is 250 nm. The relationship between the physical thickness and one-quarter reference wavelength is d = k * λ0 / (4n), where d is the physical thickness, k is the coefficient of the one-quarter reference wavelength optical thickness of each layer, λ0 is the reference wavelength, and n is the refractive index of each layer. Zinc sulfide: n=2.38877@550nm, Yttrium oxide: n=1.788@550nm, Silicon monoxide: 2.021@550nm, Aluminum oxide: 1.62158@550nm, Silicon: n=4.089@550nm.
[0049] The preparation methods of the low-reflection attenuation films in the above examples are as follows:
[0050] A Cathay 1150 coating machine was used, and the crystal control system employed a Membrane Technology MXC-3B controller. The thickness of the thin film was measured by utilizing the change in the oscillation frequency of the quartz crystal. A Cathay GTRF-HMI2000 radio frequency ion source was used. The vacuum chamber achieved the required vacuum level through a combination of a mechanical pump and a dual condenser pump system, and the vacuum level was measured using an ionization gauge.
[0051] To ensure better adhesion, the zinc selenide substrate is cleaned before coating: wipe with a degreasing cloth dipped in a mixture of anhydrous ethanol and ether in a volume ratio of 1:3 to remove surface residue.
[0052] To better ensure the film density, the initial vacuum level is 4.0 × 10⁻⁶. -4 Pa, baking temperature 120℃; among which...
[0053] The ion source parameters for the M and N layers are set as follows: BEAM voltage 1000V, BEAM current 1000mA, and accelerating voltage 600V.
[0054] The ion source parameters for the E, F, H, and L layers are set as follows: BEAM voltage 400V, BEAM current 600mA, and accelerating voltage 600V.
[0055] To further enhance film adhesion, the zinc selenide substrate was bombarded with an ion beam for 5 minutes before coating. The ion beam parameters were: BEAM voltage 200V, BEAM current 200mA, and accelerating voltage 500V. The purpose was to clean the substrate, increase the aggregation coefficient, and strengthen film adhesion. During film deposition, a radio frequency ion source was used to assist deposition and increase aggregation density. The ion source was purged with argon gas to maintain a vacuum of 8.5 × 10⁻⁶. -3 Pa utilizes an ion source to improve the film structure and stress matching, effectively solving the stress problem between film layers, thereby improving the film density, making the film more robust and longer-lasting. Controlling the conditions during film preparation is crucial. The preparation conditions of each film layer not only affect the density and other properties of a single layer, but also the adhesion to adjacent layers and the overall optical properties of the film. Among these, the selection of the evaporation rate is particularly important. In this film system, zinc sulfide was evaporated using an electron gun at a rate controlled at 0.5 nm / s; yttrium oxide was evaporated using an electron gun at a rate controlled at 0.3 nm / s; silicon was evaporated using an electron gun at a rate controlled at 0.3 nm / s; metallic chromium was evaporated using an electron gun at a rate controlled at 0.05 nm / s; silicon monoxide was evaporated using a molybdenum boat with a resistance evaporation rate controlled at 0.3 nm / s; and alumina was evaporated using an electron gun at a rate controlled at 0.5 nm / s. Long-term practice has shown that, for the adhesive layer of the first two layers of this membrane system, the aforementioned rate yields the best membrane density and bonding with the substrate and adjacent membrane layers.
[0056] One side of each zinc selenide substrate is a low-reflection attenuation film as described in each example, and the other side is a conventional low-reflection antireflection film. The structure is SUB / mBnC / A, where SUB represents the zinc selenide substrate, A represents air, B represents ytterbium fluoride, and C represents zinc selenide; m and n represent the coefficients of the optical thickness of each layer at one-quarter of the reference wavelength, and their values are related to the reference wavelength λ. In this design, the reference wavelength λ is set to 550 nm, m is 12.0751, and n is 4.3232. The physical thickness of the ytterbium fluoride layer is 1082.00 nm, and the physical thickness of the zinc selenide layer is 216.16 nm. The relationship between the physical thickness and one-quarter of the reference wavelength is d = k * λ / (4n), where d is the physical thickness, k is the coefficient of the optical thickness of each layer at one-quarter of the reference wavelength, λ is the reference wavelength, and n is the refractive index of each layer. Ytterbium fluoride refractive index: n=1.535@550nm, yttrium oxide refractive index: n=2.75@550nm. Ytterbium fluoride was evaporated using an electron gun at a rate controlled at 0.6nm / s, and zinc selenide was evaporated using an electron gun at a rate controlled at 1.0nm / s. Other conditions were the same as those for the preparation of low-reflection attenuation films.
[0057] Stress analysis
[0058] Optical thin film stress refers to the internal stress generated within the thin film and existing between the thin film and the substrate during and after the film's fabrication process. This stress is an inherent characteristic of thin films, and if it is not effectively understood and controlled, it can lead to a series of serious problems.
[0059] I. Types and Causes of Stress
[0060] Thin film stress is mainly divided into two categories:
[0061] 1. Intrinsic stress
[0062] Also known as intrinsic stress, it is determined by the structure and growth mechanism of the thin film material itself during thin film growth. Its causes are complex and mainly include:
[0063] Atomic / ion mosaic effect: When evaporated or sputtered particles reach the substrate, they may not have enough kinetic energy to migrate to the lowest energy position. Instead, they are "frozen" in non-equilibrium positions, forming lattice defects and compressive stress.
[0064] Lattice mismatch: The lattice constant of the thin film material does not match that of the substrate, resulting in mismatched dislocations at the interface, which leads to stress.
[0065] Microstructure evolution: In the early stages of thin film growth, the island-like structures undergo coalescence, and the connections between islands lead to atomic rearrangement, which usually generates tensile stress.
[0066] Phase transition process: During or after deposition, the thin film material changes from an amorphous state to a crystalline state, or from one crystalline phase to another. The volume change will generate stress.
[0067] 2. Thermal stress
[0068] Because the thermal expansion coefficients of the thin film and the substrate are different, stress arises from the uneven shrinkage of the two materials when the deposition process cools from high temperature to room temperature. If the thin film expands and contracts more than the substrate, it shrinks more after cooling and is stretched by the substrate, generating tensile stress; if the substrate expands and contracts more than the thin film, it shrinks more after cooling and compresses the thin film, generating compressive stress.
[0069] Macroscopic manifestations of stress
[0070] Compressive stress: It causes the film to tend to "shrink" towards the substrate, resulting in the entire substrate-film system bending towards one side of the film, forming a convex bend (like a saddle).
[0071] Tensile stress: It causes the film to have a tendency to "stretch", resulting in the entire system bending towards the substrate side, forming a concave bend (like a bowl).
[0072] II. Effects and Consequences of Stress
[0073] Excessive or uneven stress on the thin film can have disastrous consequences:
[0074] 1. Film cracking or peeling: When the tensile stress exceeds the film's bond strength and tensile strength, the film will crack. When the compressive stress is too high, the film may wrinkle, blister, or peel off from the substrate.
[0075] 2. Substrate Deformation: Stress causes optical elements (especially thin substrates) to bend, altering their surface accuracy and introducing wavefront distortion, leading to a decrease in image quality of the optical system (such as defocus and astigmatism). This is fatal in precision optical systems (such as lithographic lenses and space telescopes).
[0076] 3. Deterioration of optical performance:
[0077] Stress birefringence: Stress can cause isotropic materials to become anisotropic, resulting in birefringence and changing the polarization state of light.
[0078] Spectral characteristic drift: Substrate bending and film stress can change the actual physical thickness and refractive index of the film, causing the center wavelength and transmittance / reflectance curves to deviate from the design values.
[0079] 4. Reduced laser damage threshold: Stress concentration areas are rich in microcracks and defects. Under high-power laser irradiation, these areas are more likely to absorb energy and become hot spots and the starting point of damage.
[0080] 5. Long-term reliability decline: Stress will be released or redistributed over time, leading to unstable thin film performance and affecting the lifespan and environmental adaptability of components (such as failure after temperature cycling).
[0081] III. Stress Measurement Methods
[0082] The most common and classic measurement method is the substrate curvature method.
[0083] Principle: The average thin film stress is calculated by measuring the change in the radius of curvature of the substrate before and after coating using the Stoney formula.
[0084] Stoney formula:
[0085] Young's modulus of elasticity; Poisson's ratio; : Substrate thickness; D: Film thickness; : Radius of the sphere after plating; : Radius of the spherical surface before plating; The change in sagittal height before and after coating can be calculated using the change in surface profile before and after coating. Symbol convention: Tensile stress R is "+", representing a low aperture; compressive stress R is "-", representing a high aperture.
[0086] Measurement techniques: Surface profilometer: Contact measurement, high precision; Laser interferometer: Non-contact, can measure surface shape changes and wavefront errors; Optical lever method: Measures curvature changes by the displacement of the laser beam reflected spot on the sample surface, simple and efficient, often used for online monitoring.
[0087] For the low-reflection attenuation film OD1 in Example 1, the stress condition was calculated by measuring the sample surface shape change using a Zygo interferometer: Zinc selenide substrate: Young's elastic modulus =67.2 GPa, Poisson's ratio =0.28, test piece thickness =3mm, film thickness =1.6um, substrate diameter D=25mm, the change in surface aperture of the substrate before and after coating (higher is positive, lower is negative) is: +0.795fr, corresponding to = -0.25nm, substituting into the Stoney formula, we can obtain the film stress coefficient σ (MPa) as: -282.0MPa.
[0088] For the low-reflection attenuation film with OD0.7 in Example 1, the stress was calculated by measuring the sample surface shape change using a Zygo interferometer: Zinc selenide substrate: Young's elastic modulus =67.2 GPa, Poisson's ratio =0.28, test piece thickness =3mm, film thickness =3.09um, substrate diameter D=25mm, the change in surface aperture of the substrate before and after coating (higher is positive, lower is negative) is: +0.863fr, corresponding to = -0.27nm, substituting into the Stoney formula, we can obtain the film stress coefficient σ (MPa) as: -153.5MPa.
[0089] Optical performance testing: The transmittance and reflectance of the thin film were tested using a Perkin Eimer Spectrum 100 infrared spectrophotometer. The measured reflectance of the OD1 sample at the absorption surface was 0.08%, and the transmittance on both sides was 10.00%. The reflectance of the OD0.7 sample at the absorption surface was 0.11%, and the transmittance on both sides was 19.45%. The obtained spectral curves meet the design requirements: reflectance at 10.6 μm <1%, and transmittance satisfying both OD1 and OD0.7. The measured reflectance and transmittance curves are shown below. Figure 9-10 As shown.
[0090] To ensure the reliability of the optical components, the attenuation film samples in Examples 1-2 were subjected to the following performance tests in accordance with the requirements of GJB2485-95 General Specification for Optical Films:
[0091] (1) Immersion test: The samples obtained in each case were completely immersed in distilled water or deionized water. After 48 hours, the membrane layer showed no abnormalities.
[0092] (2) Thermal shock test: The samples obtained in each case were placed in an oven and baked at 300 degrees for 1 hour. The surface of the samples obtained in each case was intact and undamaged. The surface shape, mechanical properties and optical properties of the samples before and after baking were consistent with those before baking (the rate of change was within the allowable test error range).
[0093] (3) Adhesion test: 3M tape with a width of 1cm was used to stick firmly to the surface of the film. After the tape was quickly pulled up from the edge of the part in a vertical direction towards the surface, the film obtained in each case did not fall off or get damaged. The above process was repeated 10 times, and the film still did not fall off or get damaged.
[0094] In summary, the absorptive attenuation film made of chromium and dielectric film material can achieve very low reflectivity. At the same time, the film layer is robust and durable on the ZNSE substrate. The reflectivity in the 10.6µm band is below 0.11%, and the transmittance can be controlled by adjusting the thickness of the silicon monoxide and chromium film layers to meet the practical application requirements and satisfy various technical requirements for laser applications.
[0095] Comparative Example 1
[0096] In Example 1, the chromium layer was replaced with an alumina layer, and all other aspects were the same as in Example 1. Since the absorption coefficient of alumina is much smaller than that of chromium, in order to attenuate the transmittance of the 10.6µm band to the level of OD1, the thickness of the alumina film would be increased significantly. The stress of the film layer would be uncontrollable, and the film layer would develop cracks in a short time after the product was covered.
[0097] Comparative Example 2
[0098] In Example 2, the alumina layer was replaced with a chromium layer, and all other aspects were the same as in Example 2. Since the absorption coefficient of chromium is much greater than that of alumina, in order to control the transmittance in the 10.6µm band so that it does not attenuate too much to the point of OD0.7, the chromium film thickness is very thin, around a few nanometers. At this time, the chromium film layer has an "island structure" and has not yet formed a uniform film layer. At this time, the optical performance and physical properties of the chromium film layer will deviate from the values of the bulk material, which is a very big defect for the product.
[0099] Comparative Example 3
[0100] In Example 1, the chromium layer was replaced with a silicon oxide layer, and the rest was the same as in Example 1. Since the absorption coefficient of silicon oxide is relatively small, in order to attenuate the transmittance of the 10.6 μm band to the level of OD1, the thickness of the silicon oxide film in the film system design would be particularly large, which is beyond the scope of thin films and is basically impossible to achieve.
[0101] Comparative Example 4
[0102] In Example 2, the alumina layer is replaced with a silicon oxide layer, and the rest is the same as in Example 2. Since the absorption coefficient of silicon oxide is relatively small, in order to attenuate the transmittance of the 10.6um band to the level of OD0.7, the thickness of the silicon oxide film in the film system design will be particularly large, reaching about 5um; the film stress will be uncontrollable, and the film layer will generate film cracks in a short time after the product is covered.
[0103] Comparative Example 5
[0104] In Example 1, the silicon monoxide layer was replaced with a silicon oxide layer, and the rest were the same as in Example 1. Since silicon oxide has a relatively small absorption coefficient and a relatively low refractive index, this would result in a significant increase in the thickness of the aluminum oxide film. At the same time, the film system's reflection bandwidth would become narrower, which would be detrimental to the stress optimization and reflectivity control of the film.
[0105] Comparative Example 6
[0106] In Example 2, the silicon monoxide layer is replaced with a silicon oxide layer, and the rest is the same as in Example 2. Since the absorption coefficient of silicon oxide is relatively small, in order to reduce the transmittance of the 10.6um band to the level of OD0.7, the thickness of the alumina film in the film system design will be particularly large, reaching about 3um; the film stress will be uncontrollable, and the film layer will develop film cracks in a short time after the product is covered.
[0107] Comparative Example 7
[0108] In Example 1, the silicon monoxide layer was replaced with a silicon layer, and the rest were the same as in Example 1. Since the silicon monoxide absorption layer was reduced, the silicon film thickness increased, and the reflection bandwidth of the film system decreased. This resulted in a larger total stress in the film layer, which was also not conducive to controlling the reflectivity of the film layer.
[0109] Comparative Example 8
[0110] In Example 2, the silicon monoxide layer was replaced with a silicon layer, and the rest was the same as in Example 2. Since the absorption coefficient of silicon is relatively small, in order to reduce the transmittance of the 10.6 μm band to the level of OD0.7, the silicon film thickness in the film system design would be particularly large, which is beyond the scope of thin films and is basically impossible to achieve.
Claims
1. A low-reflection attenuation thin film in the 10.6µm band, characterized in that: When OD≥1, its membrane structure is: SUB / aMbNcEdFeEfHgEhL / A; When OD < 1, its membrane structure is: SUB / aMbN(k1Hk2L)^ x gEhL / A; Wherein, SUB represents zinc selenide substrate, A represents air, M represents zinc sulfide, N represents yttrium oxide, E represents silicon, F represents metallic chromium, H represents silicon monoxide, and L represents aluminum oxide; a, b, c, d, e, f, g, h, k1, k2 represent coefficients of the optical thickness of each layer at one-quarter of the reference wavelength, with a reference wavelength λ of 550 nm; the superscript X represents the number of cycles of the (k1Hk2L) absorption control layer.
2. The low-reflection attenuation thin film in the 10.6µm band as described in claim 1, characterized in that: The values for a are: 0.30–0.60, b is: 0.30–0.50, c is: 0.80–1.00, d is: 0.85–1.05, e is: 5.51–7.51, f is: 2.30–4.30, g is: 8.94–10.94, and h is: 7.26–9.
26.
3. The low-reflection attenuation thin film in the 10.6µm band as described in claim 1 or 2, characterized in that: The values of k1 are 0.7 to 3, the values of k2 are 2 to 8, and the values of X are 1 to 5.
4. The low-reflection attenuation thin film in the 10.6µm band as described in claim 1 or 2, characterized in that: When OD≥1, aM is a zinc sulfide layer, bN is a yttrium oxide layer, cE is a first silicon layer, dF is a chromium layer, eE is a second silicon layer, fH is a first silicon monoxide layer, gE is a third silicon layer, and hL is a first aluminum oxide layer; wherein, the physical thickness of the zinc sulfide layer is 30-80nm, the physical thickness of the yttrium oxide layer is 30-80nm, the physical thickness of the first silicon layer is 30-70nm, the physical thickness of the chromium layer is 10-90nm, the physical thickness of the second silicon layer is 200-250nm, the physical thickness of the first silicon monoxide layer is 210-260nm, the physical thickness of the third silicon layer is 300-380nm, and the physical thickness of the first aluminum oxide layer is 700-800nm.
5. The low-reflection attenuation thin film in the 10.6µm band as described in claim 1 or 2, characterized in that: When OD < 1, aM is a zinc sulfide layer, bN is a yttrium oxide layer, (k1Hk2L)^ x As the absorption control layer, k1H is the second silicon monoxide layer, k2L is the second aluminum oxide layer, gE is the third silicon layer, and hL is the first aluminum oxide layer; the physical thickness of the zinc sulfide layer is 30-80 nm, the physical thickness of the yttrium oxide layer is 30-80 nm, the physical thickness of the second silicon monoxide layer is 50-200 nm, the physical thickness of the second aluminum oxide layer is 200-700 nm, the physical thickness of the third silicon layer is 300-380 nm, and the physical thickness of the first aluminum oxide layer is 700-800 nm.
6. The low-reflection attenuation thin film in the 10.6µm band as described in claim 1 or 2, characterized in that: The single-sided reflectivity of the low-reflection attenuation film in the 10.6µm band is no more than 0.11%, and the double-sided transmittance is between 0.1% and 90.0%.
7. The method for preparing the 10.6µm low-reflection attenuation thin film according to any one of claims 1-6, characterized in that: Argon ion-assisted deposition is used in the coating process; zinc sulfide is evaporated using an electron gun at a rate controlled at 0.1-1 nm / s, yttrium oxide is evaporated using an electron gun at a rate controlled at 0.2-0.8 nm / s, silicon is evaporated using an electron gun at a rate controlled at 0.2-0.6 nm / s, metallic chromium is evaporated using an electron gun at a rate controlled at 0.01-0.1 nm / s, silicon monoxide is thermally evaporated using a molybdenum boat with a resistance to evaporation at a rate controlled at 0.1-0.8 nm / s, and alumina is evaporated using an electron gun at a rate controlled at 0.1-0.6 nm / s.
8. The preparation method according to claim 7, characterized in that: The initial vacuum degree for coating is 2.0–6.0 × 10⁻⁶. -4 Pa, baking temperature 100-180℃.
9. The preparation method according to claim 7 or 8, characterized in that: The ion source parameters for the M and N layers are set as follows: BEAM voltage 1000±200V, BEAM current 1000±200mA, and accelerating voltage 600±100V. The ion source parameters for the E, F, H, and L layers are set as follows: BEAM voltage 400±50V, BEAM current 600±50mA, and accelerating voltage 600±100V.