High-transmittance ultraviolet short-wave pass filter and preparation method thereof

Abstract

The application belongs to the technical field of optical film, and discloses a high-transmittance ultraviolet short-wave pass filter and a preparation method thereof. The filter comprises a substrate, a filter film on one side of the substrate, and an antireflection film on the other side of the substrate. The filter film is a light control regular film layer, the light control regular film layer comprises first refractive index layers and second refractive index layers arranged alternately, the refractive index of the first refractive index layers is greater than that of the second refractive index layers, and the total thickness of the light control regular film layer is 1-2 µm. The thickness of the antireflection film is 30%-85% of the thickness of the filter film. The transmittance of the ultraviolet short-wave pass filter is improved by optimizing the refractive index matching of the film layer, the structure design of the film system, and the adjustment of the deposition process parameters.

Description

Technical Field

[0001] This invention belongs to the field of optical thin film technology, specifically relating to a high-transmittance ultraviolet short-wave pass filter and its preparation method. Background Technology

[0002] As ultraviolet (UV) technology expands towards shorter wavelengths (such as deep UV lithography, extreme UV lithography (EUV), short-wavelength UV laser processing, and 200-280 nm biofluorescence excitation), optical systems are placing increasingly stringent demands on the "purity" of UV light sources. This requires not only precise output of specific short-wavelength UV light but also strict suppression of interference from long-wavelength stray light (such as visible or infrared light above 300 nm) on the detector. Short-pass filters for UV lasers are key optical functional devices specifically designed for UV laser systems. Their core function is to accurately transmit and reflect specific short-wavelength UV light emitted by different UV lasers (typically specific wavelengths within the 200-300 nm band, such as 248 nm, 266 nm, and 355 nm). These filters are core optical components in applications such as UV laser processing, UV precision measurement, UV spectroscopy analysis, and national defense security. Their performance directly determines the signal-to-noise ratio, processing accuracy, and reliability of UV laser systems.

[0003] The realization of ultraviolet short-pass filters relies on the precise design of multilayer dielectric film systems, and the intrinsic absorption limitations of materials, the surface roughness of optical components, and the complex film system design are currently the core challenges. In existing technologies, some filters achieve short-pass functionality through single or multiple metal films (such as aluminum and silver), but metal films suffer from high absorption in the ultraviolet region (e.g., the transmittance of aluminum drops sharply below 250 nm) and are prone to oxidation, making it difficult to meet the requirements of high transmittance and long-term stability. Another type of solution based on dielectric films can improve ultraviolet transmittance, but it is limited by the material system (e.g., the ultraviolet high refractive index material HfO2 has significant absorption in short-wave ultraviolet) and the level of process (e.g., insufficient oxidation leads to large film absorption), generally resulting in problems such as low passband transmittance (<80%), insufficient cutoff depth (OD<2), or poor short-wave cutoff edge steepness, which cannot meet the stringent requirements of high-end ultraviolet detection systems. Summary of the Invention

[0004] The purpose of this invention is to provide a high-transmittance ultraviolet short-pass filter and its preparation method. The filter is based on an ultraviolet low-absorption oxide material system (such as HfO2 / SiO2 or Al2O3 / SiO2) and is designed specifically for ultraviolet lasers of a specific wavelength (such as 266 nm). The transmittance of the ultraviolet short-pass filter is improved by optimizing the refractive index matching of the film layer, the film structure design, and adjusting the deposition process parameters.

[0005] The objective of this invention is achieved through the following technical solution: This invention provides a high-transmittance ultraviolet short-pass filter, comprising a substrate, a filter film located on one side of the substrate, and an antireflection film located on the other side of the substrate; the filter film is a light-regulating film layer, which includes alternating first and second refractive index layers, wherein the refractive index of the first refractive index layer is greater than that of the second refractive index layer, and the total thickness of the light-regulating film layer is 1-2 µm; the thickness of the antireflection film is 30%-85% of the thickness of the filter film.

[0006] In some embodiments, the substrate is at least one of fused silica and CaF2; the film material of the first refractive index layer is at least one of HfO2 or Al2O3; and the film material of the second refractive index layer is SiO2.

[0007] In some embodiments, the antireflective membrane material is the same as that of the light filter membrane.

[0008] In some embodiments, the substrate is a double-sided polished substrate, wherein the surface accuracy PV < λ / 10 and the surface roughness RMS < 0.2 nm.

[0009] In some embodiments, the filter film has a film structure of 0.5L1.25H(LH)^11 2L, where H is a high refractive index film material HfO2, L is a low refractive index film material SiO2, the center wavelength is 355 nm, and the total film thickness is 1.340 µm.

[0010] In some embodiments, the antireflective membrane has a W-type multilayer membrane structure, wherein at least the first layer of the W-type multilayer membrane structure is a regular membrane layer.

[0011] In some embodiments, the antireflective film has the following structure starting from the substrate: the first layer is a regular SiO2 layer of 5-20 L, where L is a low-refractive-index SiO2 material with a center wavelength of 266 nm; the second layer is an HfO2 layer with a thickness of 6.3 nm; the third layer is a SiO2 layer with a thickness of 103.1 nm; the fourth layer is an HfO2 layer with a thickness of 43.1 nm; and the fifth layer is a SiO2 layer with a thickness of 39.4 nm.

[0012] The present invention also provides a method for preparing the above-mentioned filter, comprising: preparing a filter film and an antireflection film on a substrate using ion beam assisted coating technology, wherein a hollow cathode Hall source is used as the ion source for the auxiliary coating.

[0013] In some embodiments, oxygen ions are also provided during the coating process to assist in the coating process.

[0014] In some embodiments, the specific parameters of the coating technology are as follows: the base vacuum degree before coating is <5.0 e-4Pa, the substrate instantaneous temperature is 250 degrees; the Hall ion source anode voltage is 160 V and the anode current is 2 A; the special oxygen ion activation source power is 500W and the anode voltage is 50V; the evaporation rate of HfO2 is 0.1-0.15 nm / s and the evaporation rate of SiO2 is 0.2-0.4 nm / s.

[0015] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) This invention improves the optical performance of ultraviolet short-wave pass filters by optimizing the film structure design and adjusting the deposition process parameters; it can obtain filters with high passband transmittance (T>95%@266nm), high cutoff (T<0.1%@355nm), and small surface distortion (PV<λ / 10); (2) The filter film of the present invention adopts a regular film layer design scheme to reduce random errors, improve the film thickness control accuracy, and simplify the coating process; the antireflection film of the present invention adopts a multi-layer + adjustable thickness film system design scheme. Compared with the traditional 2-layer V-shaped film system design scheme, it not only broadens the effective coating wavelength, but also adjusts the film thickness in real time to compensate for the surface distortion caused by the filter film, and does not affect the coating index. (3) The present invention uses a special oxygen ion activation source to ionize oxygen and provide oxygen ions for auxiliary coating, thereby further reducing ultraviolet absorption of the film layer and improving the transmittance of the filter. Attached Figure Description

[0016] Figure 1 This is a transmittance curve of the filter film in the filter of an embodiment of the present invention; Figure 2 This is a design reflectance curve of the antireflection film in the filter of this invention embodiment; Figure 3 This is a diagram showing the designed transmittance curve of the filter in an embodiment of the present invention; Figure 4 A comparison curve of transmittance between the short-pass filter designed for classic 266 nm transmission and 355 nm reflection and the filter of the embodiment; Figure 5 A comparison curve of the reflectance of the antireflection film (V-type) designed for conventional 266 nm and the antireflection film in the filter of the present invention; Figure 6 The design reflectivity curve of the antireflection coating is shown when the first layer of the antireflection coating is SiO2-5L (center wavelength 266 nm) and the total thickness of the coating is about 0.417 µm. Figure 7The design reflectivity curve of the antireflection coating is shown when the first layer of the antireflection coating is SiO2-20L (center wavelength 266 nm) and the total thickness of the coating is about 1.092 µm. Detailed Implementation

[0017] This invention provides a high-transmittance ultraviolet short-pass filter, comprising a substrate, a filter film located on one side of the substrate, and an anti-reflection film located on the other side of the substrate; wherein, the substrate is a double-sided polished substrate, the surface accuracy PV < λ / 10, the surface roughness RMS < 0.2 nm (UV level), and the substrate material is at least one of fused silica and CaF2; the filter film is a light-regulating film layer, comprising alternating first and second refractive index layers, the refractive index of the first refractive index layer being greater than the refractive index of the second refractive index layer, the film material of the first refractive index layer being at least one of HfO2 or Al2O3; the film material of the second refractive index layer being SiO2, and the total thickness of the light-regulating film layer being 1-2 nm. µm; the antireflective membrane material is the same as the filter membrane material, the antireflective membrane system structure is a W-type multilayer membrane structure, at least the first layer of the membrane system structure is a regular membrane layer, and the thickness of the antireflective membrane is 30%-85% of the thickness of the filter membrane.

[0018] The present invention also provides a method for preparing the above-mentioned filter, comprising: preparing a filter film and an antireflection film on a substrate using ion beam assisted coating technology, wherein a hollow cathode Hall source is used as the ion source for auxiliary coating, and oxygen ions are also provided for auxiliary coating during the coating process; the coating process parameters for the filter film and the antireflection film are as follows: the base vacuum degree before coating is <5.0 e-4Pa, the substrate instantaneous temperature is 250 degrees; the Hall ion source anode voltage is 160 V and the anode current is 2 A; the special oxygen ion activation source power is 500 W and the anode voltage is 50 V; the evaporation rate of HfO2 is 0.1-0.15 nm / s and the evaporation rate of SiO2 is 0.2-0.4 nm / s.

[0019] It should be noted that the filter film is designed as a regular film layer to reduce random errors and improve the accuracy of film thickness control; in order to balance the effects of film layer accumulation and film layer stress, the total film thickness is controlled at 1-2 µm.

[0020] Since optical filters can cause surface distortion, this distortion can usually be corrected and compensated by matching the stress (film thickness or process scheme) of the back film layer. A conventional 266 nm antireflection film (V-type) has a thickness of <0.1 µm. Considering the impact of film thickness on absorption accumulation and stress, this invention adopts a W-type multilayer adjustable thickness film system design instead of the traditional two-layer V-type film system design (V-type has a "V" shaped reflectance spectrum, and W-type has a "W" shaped reflectance spectrum). The first layer is set as a regular film layer ML scheme, where M is an adjustable coefficient and L is a low-refractive-index film material (SiO2). The thickness of the antireflection film can be adjusted by adjusting the value of M. This invention preferably uses M between 5 and 20. The final thickness of the antireflection film can be controlled between 30% and 85% of the filter film thickness without affecting the coating performance.

[0021] During the coating process, the present invention preferably uses a special ion source to ionize oxygen, providing oxygen ions to assist in coating and further reduce ultraviolet absorption of the film layer.

[0022] The present invention will be further described in detail with reference to specific embodiments. The following embodiments can enable those skilled in the art to have a more comprehensive understanding of the present invention, but do not limit the present invention in any way.

[0023] Example: This embodiment provides a high-transmittance ultraviolet short-wave pass filter and its preparation method: 1) Using UV-grade fused silica as the substrate material, a high-precision double-sided polished substrate is processed using an ultra-smooth process: surface accuracy PV < λ / 10, surface roughness RMS < 0.2nm; 2) HfO2 is used as the high refractive index film material, while SiO2 is used as the low refractive index film material; and ion beam assisted coating technology is used to prepare filter films and antireflection films; 3) The specific optimized film system design for the filter film with T>96%@266nm and T<0.1%@355nm is: 0.5L1.25H(LH)^112L, center wavelength 355nm, H is HfO2, L is SiO2, and the total film thickness is 1.340 µm; optical performance indicators are as follows: Figure 1 As shown; 4) The antireflection coating with R < 0.1% @ 266 nm has the following film system: first layer SiO2-8L (center wavelength 266 nm, film thickness 360 nm), second layer HfO2-6.3 nm, third layer SiO2-103.1 nm, fourth layer HfO2-43.1 nm, fifth layer SiO2-39.4 nm, with a total film thickness of approximately 0.552 µm (41% of the filter film thickness); optical performance indicators are as follows... Figure 2 As shown; 5) The coating process parameters for both the filter film and the antireflection film are as follows: the base vacuum degree before coating is <5.0e-4Pa, the substrate instantaneous temperature is 250 degrees; the Hall ion source anode voltage is 160V and the anode current is 2A; the special oxygen ion activation source power is 500W and the anode voltage is 50V; the evaporation rate of HfO2 is 0.1nm / s and the evaporation rate of SiO2 is 0.3nm / s.

[0024] 6) The final high-precision ultraviolet laser short-pass filter: T>95%@266nm, T<0.1%@355nm; surface accuracy PV<λ / 10; surface roughness RMS<0.3nm; optical performance indicators are as follows: Figure 3 As shown.

[0025] A classic short-pass filter design for 266 nm transmission and 355 nm reflection: 0.5LH(LH)^12 0.5L, where H is HfO2 and L is SiO2, with a center wavelength of 355 nm. Optical performance specifications are compared with the example shown below. Figure 4 As shown (where the thin solid line represents the optical performance curve of an embodiment of the invention), the 266 nm transmittance of the filter system obtained by the present invention is increased from 93.5% to 96.6%. The conventional 266 nm antireflective coating (V-type) design consists of a first layer of 12.9 nm HfO2 and a second layer of 57.6 nm SiO2. The optical performance indicators are compared with those of the embodiments as follows: Figure 5 As shown (the thin solid line is the optical performance curve of the antireflection film in the embodiment of the present invention), the antireflection film in the embodiment of the present invention has a wider effective coating wavelength. The wider effective band has a greater tolerance. The V-shaped film has only about 18 nm of qualified band, while the W-shaped film has nearly 40 nm of qualified area available.

[0026] Furthermore, tests revealed that when the first layer of the antireflective coating was SiO2-5L (center wavelength 266 nm), the total coating thickness was approximately 0.417 µm (31% of the filter thickness), and the optical performance indicators were as follows: Figure 6 As shown, it can be seen that R < 0.1% @ 266 nm is still satisfied; when the first layer of the antireflection film is SiO2-20L (center wavelength 266 nm), the total film thickness is approximately 1.092 µm (82% of the filter thickness), and the optical performance indicators are as follows. Figure 7 As shown, it can be seen that R<0.1%@266 nm is still satisfied.

[0027] If oxygen assistance is used in the coating process instead of oxygen ions as in the embodiments of this invention, the transmittance of the 266 nm filter film is T>90%, which is lower than the transmittance of the filter film in this invention (T>95%).

[0028] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A high-transmittance ultraviolet short-wavelength filter, characterized in that, The filter includes a substrate, a filter film located on one side of the substrate, and an antireflection film located on the other side of the substrate; the filter film is a light-regulating film layer, which includes alternating first and second refractive index layers, wherein the refractive index of the first refractive index layer is greater than the refractive index of the second refractive index layer; the thickness of the antireflection film is 30%-85% of the thickness of the filter film. The filter film has a film structure of 0.5L1.25H(LH)^11 2L, where H is a high refractive index film material HfO2 and L is a low refractive index film material SiO2. The center wavelength of the filter film is 355 nm and the total thickness of the filter film is 1.340 µm. The antireflective membrane has a W-type multilayer membrane structure, in which at least the first layer is a regular membrane layer.

2. The filter according to claim 1, characterized in that, The substrate is at least one of fused silica and CaF2; the film material of the first refractive index layer is at least one of HfO2 or Al2O3; and the film material of the second refractive index layer is SiO2.

3. The filter according to claim 1, characterized in that, The antireflective membrane uses the same material as the filter membrane.

4. The filter according to claim 1, characterized in that, The substrate is a double-sided polished substrate with a surface accuracy PV < λ / 10 and a surface roughness RMS < 0.2 nm.

5. The filter according to claim 1, characterized in that, The antireflective coating has the following structure starting from the substrate: the first layer is a regular SiO2 layer of 5-20 L, where L is a low-refractive-index SiO2 material with a center wavelength of 266 nm; the second layer is an HfO2 layer with a thickness of 6.3 nm; the third layer is a SiO2 layer with a thickness of 103.1 nm; the fourth layer is an HfO2 layer with a thickness of 43.1 nm; and the fifth layer is a SiO2 layer with a thickness of 39.4 nm.

6. A method for preparing a filter as described in any one of claims 1-5, characterized in that, include: Filter films and antireflective films are prepared on a substrate using ion beam assisted coating technology, wherein a hollow cathode Hall source is used as the ion source for assisted coating.

7. The preparation method according to claim 6, characterized in that, Oxygen ions are also provided during the coating process to assist in the coating process.

8. The preparation method according to claim 6, characterized in that, The specific parameters of the coating technology are as follows: the base vacuum degree before coating is <5.0 e-4Pa, the substrate instantaneous temperature is 250 degrees; the Hall ion source anode voltage is 160 V and the anode current is 2 A; the special oxygen ion activation source power is 500 W and the anode voltage is 50 V; the evaporation rate of HfO2 is 0.1-0.15 nm / s and the evaporation rate of SiO2 is 0.2-0.4 nm / s.

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