High-transmittance ultraviolet filter and implementation method

Abstract

The application discloses a high-transmittance ultraviolet filter and a realization method. The filter film system is designed by combining light control regular film layers and double-cavity Fabry-Perot structure irregular film layers, and light control and time control are combined in film thickness monitoring. Through sensitive analysis on the film system structure and accurate control on the sensitive interval film layer, the ultraviolet filter test spectrum and the design spectrum are consistent, and the high transmittance of the ultraviolet filter is realized. The ultraviolet filter film system structure of the application has strong applicability and good process implementation, and is suitable for development of ultraviolet waveband filters which cannot be directly optically monitored.

Description

TECHNICAL FIELD

[0001] The present invention relates to optical thin film technology, in particular to a design method and process implementation of an ultraviolet filter with high transmittance. BACKGROUND

[0002] The wavelength range of ultraviolet band is 10-400nm. Ultraviolet band plays an important role in modern science and technology and daily life. In the field of space exploration, since the resonance lines of neutral and ionized states of elements are more abundant in ultraviolet region than in visible region, the resonance lines are extremely sensitive to the study of physical state and chemical composition of celestial bodies, and the working wavelength band of ultraviolet telescope and spectrograph has been extended to 115-400nm (see reference [1]). In the field of communication and remote sensing, since the spectral band of 240-280nm is almost completely absorbed by the ozone layer during the process of passing through the earth's atmosphere, the communication and detection in this band have the advantages of strong anti-interference ability and high sensitivity. In the field of medical treatment and detection, different bands of ultraviolet light are applied to phototherapy, ultraviolet sterilization, ultraviolet fluorescence analysis, etc. In these ultraviolet spectral application fields, ultraviolet filters play an important role as important optical elements for intercepting ultraviolet spectral information.

[0003] The basic structure of a band-pass filter based on Fabry-Perot (F-P) structure can be represented as [reflective layer M / spacer layer C / reflective layer M] k If H and L represent high and low refractive index medium materials of 1 / 4 reference wavelength, the structure of M can be HL, LH, HLH, HLHL, HLHLHL, etc., the thickness of the spacer layer C is usually an even multiple of 1 / 4 reference wavelength, and k represents the number of half-wave repetitions or the number of filter cavities. For F-P structure band-pass filter, the higher the reflectivity of the reflective layer, the thicker the spacer layer (the higher the interference order), the smaller the bandwidth, and the better the monochromaticity of the filter transmittance. The more the number of half-wave repetitions, the better the rectangularity of the filter, and the depth of the out-of-band cutoff is enhanced.

[0004] There are two main difficulties in the preparation of ultraviolet band-pass filter: first, the dispersion and absorption of ultraviolet band materials are relatively serious, and there is often a certain difference between the design results and the actual plating results; second, the thickness error of the film layer is not easy to control, and the spectral waveform is easy to deteriorate, because the white light source cannot be directly optically monitored in the ultraviolet band.

[0005] [1] Fan Zhengxiu, Shao Jianda, Yi Kui, etc. "Optical Thin Film and Its Application"

[0006] [2] Tang Jinfa, Gu Peifu, Liu Xu, etc. "Modern Optical Thin Film Technology" SUMMARY

[0007] This invention targets bandpass filters in the 200-400nm ultraviolet band. On the one hand, considering the large ultraviolet absorption of the material, too many cavities in the filter would lead to an increase in film thickness and a decrease in transmittance. On the other hand, considering the waveform and out-of-band cutoff depth requirements of the filter, this invention proposes a design and fabrication method for a dual-cavity ultraviolet bandpass filter based on a FP structure.

[0008] The film structure of the dual-cavity ultraviolet bandpass filter based on the FP structure described in this invention is as follows: Figure 1 As shown, it consists of a light-controlled ordered film layer 1 and a dual-cavity FP structure irregular film layer 2. If H and L represent high and low refractive index dielectric materials at 1 / 4 of the reference wavelength λ0, the film structure of the light-controlled film layer 1 is HLHLHL…, and its center wavelength is twice the optical thickness of the intermediate wavelength in the ultraviolet filter, located in the visible / near-infrared band. It can be accurately monitored for the initial film thickness using a white light source.

[0009] The film structure of the dual-cavity FP structure film layer 2 is [reflective layer M / spacer layer C / reflective layer M / coupling layer / reflective layer M / spacer layer C / reflective layer M / matching layer]. Here, the optical thickness of the filter at its center wavelength is half of the optical control reference wavelength λ0. The reflective layer M consists of high and low refractive index films with a thickness of 1 / 4 of the filter's center wavelength, i.e., 1 / 8 of the reference wavelength λ0, and can be 0.5H 0.5L, 0.5L 0.5H, 0.5H 0.5L 0.5H, etc. The spacer layer C is an even multiple of 1 / 4 of the filter's center wavelength optical thickness, and can be 0.5mH or 0.5mL (m is an even number). The coupling layer can be 0.5L or 0.5H. The matching layer is determined based on film system optimization.

[0010] The specific implementation steps of the high-transmittance ultraviolet filter proposed in this invention under ion beam sputtering process are as follows:

[0011] 1) Select the number of light-controlled regularized film layers based on the change of film deposition rate with film layers and time under ion beam sputtering coating process; determine the light control wavelength based on the center wavelength of the ultraviolet filter and the material dispersion; set the corresponding reflective layer structure and spacer layer thickness according to the bandwidth requirements of the filter.

[0012] 2) Using Optilayer optical thin film design software, perform relative sensitivity analysis on the film structure, such as... Figure 2 As shown, the two spacer layers in this film structure are identified as sensitive layers (18 and 34 layers, with an optical thickness of 2L). The accuracy of the optical thickness of the spacer layers determines the accuracy of the center wavelength of the dual-cavity FP filter; the consistency of the optical thicknesses of the two spacer layers determines the waveform quality of the dual-cavity FP filter. The accuracy of the optical film thickness of the two 2L spacer layers directly determines the center wavelength and spectral quality of the dual-cavity FP filter structure.

[0013] 3) Using FilmWizard optical thin film design software, the optical thicknesses of other film layers are kept constant. Meanwhile, thickness errors of -1%, 0%, and 1% are applied to the optical thicknesses of the 18th and 34th layers, respectively, resulting in optical thicknesses of 1.98L, 2L, and 2.02L for the 18th and 34th layers. Their transmittance spectra are as follows: Figure 3 As shown. It was found that if the two spacer layers change synchronously, the waveform of the filter remains basically unchanged within a 1% error, but the center wavelength shifts. When the optical thickness of the 34th layer was individually given thickness errors of -1%, -0.5%, 0%, 0.5%, and 1%, respectively (i.e., optical thicknesses of 1.98L, 1.99L, 2L, 2.01L, and 2.02L), its transmittance spectrum is as follows. Figure 4 As shown, the thickness variation of the 2L film layer has a significant impact on the spectral waveform of this bandpass filter. A 1% error significantly worsens the waveform, exhibiting a certain pattern. With a negative thickness error, the waveform symmetry deteriorates, with the top left being higher than the right, and the right half-wave lengthening overall. With a positive thickness error, the waveform symmetry deteriorates, with the top right being higher than the left, and the left half-wave lengthening overall. Based on this trend, the measured curve of the filter can be used to determine the error in the optical thickness of the 34th layer during actual deposition, and then adjustments can be made, along with some modifications to the film structure.

[0014] 4) The ultraviolet filter was deposited using dual-ion beam sputtering deposition technology according to the initial film structure: substrate / 1H 1L 1H 1L 1H1L 1H1L 1H 1L(0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L 0.5H)^3 0.5L(0.5H 0.5L)^30.5H 2L 0.5H(0.5L 0.5H)^3 0.2L / air (λ0@535nm).

[0015] 5) Based on the waveform of the measured curve after deposition, the thickness error of the 34th layer is inverted, and the film structure is appropriately adjusted using a correction factor. Specifically: Substrate / 1H 1L 1H 1L 1H 1L 1H 1L1H 1L(0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L 0.5H)^3 0.5L(0.5H 0.5L)^3 0.5H 2xL0.5H(0.5L 0.5H)^3 0.2L / Air. If the waveform shows a higher left-lower top and a longer right half-wave, it indicates that the 2L layer is under-deposited, and x is a positive value; if the waveform shows a higher right-lower top and a longer left half-wave, it indicates that the 2L layer is over-deposited, and x is a negative value. Using the corrected film structure, the UV filter is finally prepared.

[0016] The beneficial effects of this invention are as follows:

[0017] 1. This invention provides a design and manufacturing method for achieving high transmittance of ultraviolet filters. By combining a light-controlled regular film layer and a dual-cavity Fabry-Perot structure irregular film layer, accurate control of the film thickness in the ultraviolet band is achieved under white light source.

[0018] 2. Through sensitivity analysis and inversion of the FP-based dual-cavity filter, and correction of the film layer, the consistency between the test spectrum and the design spectrum of the dual-cavity ultraviolet filter was achieved, and the transmittance of the ultraviolet filter with a center wavelength of 280nm reached 85%.

[0019] 3. This invention overcomes the disadvantages of difficult monitoring of film thickness in the ultraviolet band, poor waveform and low transmittance of ultraviolet filters, and can be widely applied to the preparation of ultraviolet filters with high transmittance in the 200-400nm band. Attached Figure Description

[0020] Figure 1 This is a structural diagram of an ultraviolet filter film system.

[0021] Figure 2 This is a schematic diagram illustrating the relative sensitivity analysis of ultraviolet filter film layers.

[0022] Figure 3 It is a transmittance curve given simultaneously for the 18th and 34th layers with -1% and 1% thickness errors.

[0023] Figure 4 It is a transmittance curve of a given UV filter layer 34 with a thickness error of -1% to 1%.

[0024] Figure 5 This is a comparison chart of the measured transmittance of the UV filter with a 2L layer on the 34th layer and the design result.

[0025] Figure 6 This is a comparison chart of the measured transmittance of the corrected 34th layer UV filter with the design result. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0027] Double-sided polished JGS-1 quartz was selected as the substrate, and hafnium oxide (HfO2) and silicon dioxide (SiO2) were used as high and low refractive index materials, respectively, with refractive indices of 2.08 and 1.49 (@280nm, the refractive index varies depending on the thin film deposition process).

[0028] Based on previous material experiments, it was found that the material deposition rate was relatively fast at the beginning under this process, and tended to stabilize after the 8th layer. Therefore, the initial photocontrol film thickness was set to 10 layers, i.e., the first 10 layers were of the HLHLHLHLHL structure. The center wavelength of this ultraviolet filter is 280nm. Considering the material dispersion, the photocontrol wavelength of the filter was set to 535nm. The bandwidth of the filter is 4.2nm, so the film system of the dual-cavity FP structure is: (0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L 0.5H)^3 0.5L(0.5H 0.5L)^30.5H 2L 0.5H(0.5L 0.5H)^3. Using thin film design optimization software, the film system structure combining the photocontrol ordered film layer and the dual-cavity FP irregular film layer is:

[0029] Substrate / 1H 1L 1H 1L 1H 1L 1H 1L 1H 1L(0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L0.5H)^3 0.5L(0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L 0.5H)^3 0.2L / Air (λ0@535nm)

[0030] Based on the waveform of the measured curve after deposition, the thickness error of the 34th layer was inverted, and the film structure was appropriately adjusted using a correction factor. During actual deposition, the transmittance test curve after the first deposition was as follows: Figure 5 As shown, the shape exhibits a top-left-high-right-low profile, with the right half-wave elongated overall, indicating that the 2L layer was insufficient. Therefore, the optical thickness of the 34th 2L layer was increased, ultimately correcting the film system to:

[0031] Substrate / 1H 1L 1H 1L 1H 1L 1H 1L 1H 1L(0.5H 0.5L)^3 0.5H 2L 0.5H(0.5L0.5H)^3 0.5L(0.5H 0.5L)^3 0.5H 2.02L 0.5H(0.5L 0.5H)^3 0.2L / Air (λ0@535nm)

[0032] The final spectral curve of the filter is as follows Figure 6 As shown, the transmittance of the single-sided coated quartz substrate 280-4.2nm UV dual-cavity bandpass filter reaches 82%. If the back side is anti-reflective, the transmittance can reach over 85%. It should be noted that the actual center wavelength of the filter is slightly lower than the design wavelength. This is because the light control wavelength is maintained at 535nm during the two coating processes, and the thickness of the first 10 light control layers is less than the actual coating thickness, resulting in a slightly shorter overall coating thickness.

[0033] The process of fabricating ultraviolet filters by ion beam sputtering is as follows:

[0034] The optical substrate is ultrasonically cleaned and dried, then placed on a fixture tray in a vacuum chamber. A mechanical pump and a cryogenic pump are used to achieve a vacuum chamber pressure of 2.0 x 10⁻⁶. -3 1) The substrate temperature is below 140°C by rotating the fixture disk at a speed of 300 rad / min and holding for 120 min. 2) The substrate is pre-cleaned using an auxiliary RF ion source for 10 min. 4) The deposition of each film layer is completed by combining light control and time control, with sputtering deposition rates of 0.17 nm / s and 0.26 nm / s for HfO2 and SiO2, respectively. 5) After the film deposition is completed, the substrate temperature is lowered to below 50°C, the vacuum chamber is vented, the substrate is removed, and the ultraviolet filter is completed.

[0035] The specific process parameters of the VEECO ion beam sputtering vacuum coating equipment are shown in Table 1 below.

[0036] Table 1. Specific process parameters for ion beam sputtering vacuum coating equipment

[0037] Preparation parameters SiO2 target [CAT] HfO2 target Primary ion source beam pressure (v) 1250 1250 Primary ion beam source current (mA) 600 600 Ion source argon flow rate (sccm) 3 3 Ion source oxygen flow rate (sccm) 15 15 Vacuum oxygen flow rate (scm) 5 5 Substrate temperature 140 140

Claims

1. A method for achieving a high-transmittance ultraviolet filter, the high-transmittance ultraviolet filter comprising a substrate and a bandpass film system located on one side of the substrate, characterized in that: The aforementioned film system combines a light-controlled ordered film layer and a dual-cavity Fabry-Perot structure irregular film layer, and the film system structure is as follows: Substrate / (1H 1L) m (0.5H 0.5L) n 0.5HA 0.5H (0.5L 0.5H) n 0.5L (0.5H 0.5L) n 0.5HB 0.5H (0.5L 0.5H) n 0.2L / air In the formula: H represents an HfO2 film with an optical thickness of λ0 / 4, L represents a SiO2 film with an optical thickness of λ0 / 4, the numbers before H and L are the thickness ratio coefficients of the film, the optical thicknesses of H and L are in the visible light band, the optical thicknesses of 0.5H and 0.5L are in the ultraviolet band, λ0 is the center wavelength, which is 200-400nm, m is the number of regular film periods, n is the number of reflective film periods; A and B are two spacer films, spacer film A has an optical thickness of 2L, spacer film B has an optical thickness of 2(1+x)L, and x is the film correction factor; The film layers are deposited by ion beam sputtering. In the film system structure, the HL... regular film layers are directly optically monitored using a white light source, and the irregular film layers are monitored by a time monitoring method. The steps of the method for controlling the thickness difference between spacer layer A and spacer layer B are as follows: (1) Given x varying from -1%, -0.5%, 0%, 0.5% and 1%, fit the spectrum of the membrane structure; (2) Based on the test results of the actual prepared ultraviolet filter, the thickness difference between the spacer layer A and the spacer layer B is inverted, and the value of the correction factor x is adjusted to control the physical thickness difference between the spacer layer A and the spacer layer B within 0.5%.

2. The method for achieving a high transmittance ultraviolet filter according to claim 1, characterized in that: The physical thickness difference between the spacer layer A and the spacer layer B is less than 0.5%.

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