A deep cut infrared bandpass filter and a method of making the same

Abstract

The application relates to the technical field of optical films, and discloses a deep-cut infrared band-pass filter and a preparation method thereof. The base sheet is provided with a band-pass film system on one side, the band-pass film system is formed by alternately stacking high-refractive-index layers and low-refractive-index layers into multiple Fabry-Perot cavities; the high-refractive-index layers are first or second hydrogenated silicon layers, the hydrogen content of the second hydrogenated silicon layer is higher than that of the first hydrogenated silicon layer by more than 5 at.%; the spacer layers of the cavities are low-loss second hydrogenated silicon layers, the first hydrogenated silicon layers with absorption are all located in the reflection stack, and deep cutting is realized by using intrinsic absorption. In the preparation, the two kinds of hydrogenated silicon layers are sputtered by using different hydrogen and argon flow ratios, and the hydrogen barrier layer is plated and then subjected to step annealing. The single-side film system can realize that the optical density is not less than 6 in the range of 400 nm to 890 nm, the optical density is not less than 5 in the range of 990 nm to 1150 nm, the average transmittance in the passband is not less than 92%, the central wavelength shift is not more than 11 nm in the range of 0 to 30 degrees of incident angle, and the drift is not more than 1 nm after reflow soldering and wet heat testing.

Description

Technical Field

[0001] This invention relates to the field of optical thin film technology, and more specifically, to a deep cutoff infrared bandpass filter and its preparation method. Background Technology

[0002] Infrared bandpass filters are commonly required at the receiver end of optoelectronic systems such as infrared sensing, imaging, and recognition. These filters only allow a narrow wavelength band (typically the center wavelength λ0 = 850nm or 940nm) corresponding to the system's infrared light source to pass through, while deeply blocking other wavelengths, especially visible light and ambient light in adjacent infrared bands, to suppress noise and improve the system's signal-to-noise ratio. As the requirements for detection distance and accuracy under strong ambient light conditions, such as outdoor sunlight, continue to increase, the required cutoff depth has increased from optical density OD4 to OD5 or even OD6, hence the term deep-cutoff infrared bandpass filters.

[0003] Furthermore, such systems typically need to receive incident light within a certain angular range, requiring filters to accommodate incident angles of 20° to 40°. According to multilayer interference theory, the center wavelength λ of the passband at an incident angle θ... θ λ0 satisfies the condition of perpendicular incidence. θ =λ0(1-sin 2 θ / n e 2 ) 1 / 2 , where n e Let n be the equivalent refractive index of the film system. Therefore, increasing n... e This is the fundamental way to reduce angular deviation.

[0004] Existing technologies can be mainly categorized as follows:

[0005] The first type uses oxides such as titanium dioxide, niobium pentoxide, and tantalum pentoxide as high refractive index materials. The refractive index of these materials at 940 nm is only 2.1~2.4, and the equivalent refractive index of the film system is about 2.0~2.2. When the incident angle changes from 0° to 30°, the center wavelength shift reaches more than 30 nm. In order to accommodate this shift, the passband must be widened, and the ambient light noise entering the detector increases accordingly.

[0006] The second type uses silicon hydride as a high refractive index material. Silicon hydride has a refractive index greater than 3.5 in the 800nm~1800nm ​​wavelength range, and the equivalent refractive index of the film system can reach over 3.2, with a significantly reduced angular offset. Chinese patent application CN107841712A discloses a method for preparing high refractive index silicon hydride thin films, Chinese patent application CN110737040A discloses an infrared bandpass filter using silicon hydride as the high refractive index layer, and Chinese patent applications CN111736250A and CN113075758B also disclose narrowband infrared filters using silicon hydride as the high refractive index layer.

[0007] However, existing technologies using silicon hydride present an unresolved contradiction in achieving deep cutoff. The optical band gap of silicon hydride monotonically increases with its hydrogen content (expressed as atomic percentage at.%), meaning the percentage of hydrogen atoms in the film relative to the total number of atoms in the film, the same below), and the absorption edge shifts towards shorter wavelengths. Silicon hydride with a high hydrogen content (e.g., 18 at.%) has an absorption edge around 650 nm, with almost no absorption at 940 nm, which is beneficial for achieving high passband peak transmittance; however, it is also nearly transparent in the 750 nm–890 nm band, where interference reflection alone is insufficient to achieve OD6, making it the weakest band for deep cutoff. Conversely, if silicon hydride with a low hydrogen content (e.g., 6 at.%) is used, its absorption edge redshifts to approximately 780 nm, significantly improving the cutoff depth in the 750 nm–890 nm band, but simultaneously increasing the Urbach absorption tail at 940 nm, raising the extinction coefficient to 10. -3 This magnitude reduces the peak transmittance to below 87%. In other words, cutoff depth and peak transmittance are mutually exclusive and cannot be balanced.

[0008] To circumvent the aforementioned contradictions, Chinese patent application CN110737040A proposes a solution involving adding a cutoff film with at least 15 layers to the other surface of the substrate and introducing a medium refractive index layer on the bandpass film surface. While this solution can extend the OD4 cutoff range to the entire visible light band, it suffers from the following drawbacks: First, double-sided coating doubles the total number of film layers and coating processes, resulting in a long preparation cycle and low yield. Second, the residual stress of the two surface film systems is difficult to balance, which can easily cause warping in thin substrates with a thickness of less than 0.3 mm, leading to poor in-plane uniformity of the center wavelength. Third, the cutoff edge of the back cutoff film also moves towards shorter wavelengths with the incident angle, encroaching on the short-wavelength side of the passband at large incident angles and compressing the effective passband width. Fourth, this solution still has limited improvement in cutoff depth for the 800 nm to 890 nm band and is difficult to stably achieve OD6.

[0009] On the other hand, in silicon hydride films with high hydrogen content, the hydrogen binding energy in the form of Si—H2 and Si—H3 is low, making it easy for hydrogen to escape when heated. As a result, the film shrinks and the refractive index increases. Consequently, after existing silicon hydride filters undergo reflow soldering at 260°C or damp heat reliability testing at 85°C / 85% relative humidity, the passband center wavelength will drift by several nanometers, resulting in poor batch consistency.

[0010] Therefore, there is a need in the art for an infrared bandpass filter that can achieve deep cutoff while maintaining high peak transmittance and small angular offset on a film system only on one side of the substrate, and a method for its preparation; furthermore, it is also desirable for such a filter to have good thermal and hygrothermal stability. Summary of the Invention

[0011] To overcome the above-mentioned defects of the prior art, embodiments of the present invention provide a deep cutoff infrared bandpass filter and a method for preparing the same.

[0012] To achieve the above objectives, the present invention provides the following technical solution:

[0013] This invention provides a deep cutoff infrared bandpass filter, comprising a substrate and a bandpass film system located on one side of the substrate. The bandpass film system includes multiple Fabry-Perot cavities formed by alternating stacks of high-refractive-index layers and low-refractive-index layers. Each Fabry-Perot cavity includes two reflectors and a spacer layer located between the two reflectors. The high-refractive-index layer is a first silicon hydride layer or a second silicon hydride layer, wherein the hydrogen content of the second silicon hydride layer is 5 at.% higher than that of the first silicon hydride layer. Each spacer layer is a second silicon hydride layer, and the first silicon hydride layers are all located within the reflectors.

[0014] Preferably, the hydrogen content of the first silicon hydrogenation layer is 4 at.% to 8 at.%, and the extinction coefficient at 850 nm is 5 × 10⁻⁶. -3 ~5×10 -2 The hydrogen content of the second silicon hydride layer is 13 at.% to 22 at.%, and the extinction coefficient at 850 nm is no greater than 5 × 10⁻⁶. -4 .

[0015] Preferably, the high refractive index layer closest to the spacer layer in each reflector is a second silicon hydride layer, and the remaining high refractive index layers in the reflector are first silicon hydride layers; the total physical thickness of all first silicon hydride layers in the bandpass film system is 0.25 to 1.5 times the total physical thickness of all second silicon hydride layers.

[0016] Preferably, the outermost layer of the bandpass film system facing away from the substrate is a hydrogen barrier layer, which is a silicon nitride layer or an aluminum oxide layer with a thickness of 15nm~50nm; the low refractive index layer is a silicon dioxide layer.

[0017] Preferably, the bandpass film system further includes a long-wavelength cutoff stack located on the side of the plurality of Fabry-Perot cavities away from the substrate. The long-wavelength cutoff stack is formed by stacking a second silicon hydride layer and a low refractive index layer alternately with a quarter-wavelength reference wavelength of 1.15 to 1.28 times the center wavelength λ0 of the passband, and the number of layers is 7 to 13.

[0018] Preferably, the optical thickness of the spacer layer is 4, 6, or 8 times one-quarter of the passband center wavelength λ0, the number of Fabry-Perot cavities is 3 to 7, the number of bandpass film layers is 40 to 75, and an antireflection film is provided on the other side of the substrate.

[0019] Preferably, the passband center wavelength λ0 is 930nm~950nm, and the optical density of the filter at a 0° incident angle is not less than 6 in the 400nm~890nm band, not less than 5 in the 990nm~1150nm band, and the average transmittance in the 930nm~950nm band is not less than 92%; the shift of the passband center wavelength when the incident angle increases from 0° to 30° is not greater than 11nm.

[0020] This invention also provides a method for preparing a deep cutoff infrared bandpass filter, which includes the following steps:

[0021] S1. Clean and pre-treat the substrate;

[0022] S2. A silicon target reactive magnetron sputtering is used to deposit the first silicon hydride layer with a hydrogen-argon flow rate ratio of R1, and it is deposited alternately with a low refractive index layer. The hydrogen-argon flow rate ratio is the ratio of the hydrogen flow rate to the argon flow rate during sputtering.

[0023] S3. Deposit a second silicon hydride layer with a hydrogen-argon flow ratio of R2, and alternately deposit it with a low refractive index layer, R2-R1≥7%;

[0024] S4. Perform and repeat steps S2 and S3 according to the membrane layer sequence of the bandpass membrane system to obtain the bandpass membrane system;

[0025] S5, Deposit hydrogen barrier layer;

[0026] S6. Perform stepped annealing in a vacuum through at least two temperature plateaus.

[0027] Preferably, the magnetron sputtering uses a medium-frequency power supply with a power of 6kW~12kW and a frequency of 20kHz~60kHz; R1 is 2%~5%, R2 is 12%~22%; the sputtering working pressure is 0.2Pa~0.8Pa; the substrate temperature during deposition does not exceed 80℃; when switching between depositing the first silicon hydride layer and depositing the second silicon hydride layer, the hydrogen flow rate is continuously adjusted to the target value within 5s~40s.

[0028] Preferably, the stepped annealing process comprises: heating to 150℃~180℃ at a rate of 1℃ / min~5℃ / min and holding for 30min~90min, then heating to 220℃~260℃ at a rate of 1℃ / min~5℃ / min and holding for 60min~180min, and then cooling to room temperature at a rate not exceeding 2℃ / min; the furnace pressure during the stepped annealing process does not exceed 5×10⁻⁶. -3 Pa.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1. Deep cutoff is achieved on a single-sided film system on only one side of the substrate: the optical density in the 400nm~890nm band is not less than 6, and the optical density in the 990nm~1150nm band is not less than 5; among them, the minimum optical density in the 800nm~890nm band, which is the weakest of the existing silicon hydride film system, is increased from about 3.6 to more than 6.4.

[0031] 2. High peak transmittance in the passband: The average transmittance in the 930nm~950nm band is not less than 92%, and the measured transmittance reaches 93.6%, which is about 7.4 percentage points higher than the scheme that uses all low-hydrogen-content hydride silicon.

[0032] 3. Small angular offset: When the incident angle increases from 0° to 30°, the shift in the center wavelength of the passband is no more than 11nm, and when it increases to 40°, it is no more than 19nm.

[0033] 4. Narrow transition band: The width of the short-wavelength transition band from 90% transmittance to OD5 is no more than 6nm.

[0034] 5. Fewer total number of film layers and total physical thickness: There is no need to add a cut-off film on the other side of the substrate. The bandpass film system in Example 1 has only 53 layers and a total physical thickness of about 7.2 μm. The coating process and coating time are significantly shortened, and the yield is improved.

[0035] 6. Good thermal and damp heat stability: After three reflow soldering cycles at 260℃ for 10s, the center wavelength drift of the passband is no more than 1nm, and after a damp heat test at 85℃ / 85% relative humidity for 500h, the drift is no more than 0.6nm.

[0036] 7. Low membrane stress: The absolute value of the total membrane stress is no more than 50 MPa, the warpage of the substrate with a thickness of 0.21 mm is no more than 8 μm, and the in-plane uniformity of the center wavelength is good. Attached Figure Description

[0037] Figure 1 This is a cross-sectional view of the film structure of the filter of the present invention;

[0038] Figure 2 This is a diagram showing the membrane layer arrangement of the bandpass membrane system in Embodiment 1 of the present invention;

[0039] Figure 3 This is a flowchart of the filter preparation method of the present invention;

[0040] Figure 4 The transmittance curves of the filter of Embodiment 1 of the present invention at incident angles of 0° and 30°;

[0041] Figure 5 The optical density curves of the filters of Embodiment 1 and Comparative Example 1 at an incident angle of 0° are shown.

[0042] Figure 6 This is the temperature-time curve of stepped annealing in Embodiment 1 of the present invention.

[0043] In the figure: 1—substrate; 2—bandpass film system; 21—first silicon hydride layer; 22—second silicon hydride layer; 23—low refractive index layer; 24—spacer layer; 25—long-wavelength cutoff layer; 26—hydrogen blocking layer; 27—reflector layer; 28—Fabry-Perot cavity; 3—antireflection film system. Detailed Implementation

[0044] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0045] To ensure clarity of terminology, the terminology and testing methods used in this paper will be explained first.

[0046] (1) Hydrogen content: expressed as atomic percentage (at.%), which refers to the percentage of hydrogen atoms in the film layer relative to the total number of atoms in the film layer. It is determined by elastic recoil detection analysis, or by Fourier transform infrared spectroscopy based on the integrated intensity of the Si-H bond stretching vibration absorption peak and calibrated by the results of elastic recoil detection analysis.

[0047] (2) Refractive index n and extinction coefficient k: A single-layer film sample with a thickness of 500nm±50nm was deposited on a single-sided polished substrate under the same process conditions as the filter. Its transmission and reflection spectra were measured and obtained by inversion using the Tauc-Lorentz dispersion model. Unless otherwise specified, the value at 940nm is used.

[0048] (3) Optical band gap E g : Based on the absorption coefficient α according to (αhν) 1 / 2 Plotting hν and extrapolating to the horizontal axis yields the absorption edge, where the absorption coefficient is equal to 1 × 10⁻⁶. 4 cm -1 The wavelength at that location.

[0049] (4) Optical density OD: OD = -lgT, where T is the transmittance expressed as a decimal. The optical density of a certain band is not less than a certain value, which means that the optical density at each wavelength in that band is not less than that value.

[0050] (5) Passband center wavelength λ0: the arithmetic mean of the two wavelengths with transmittance of 50% on both sides of the passband.

[0051] (6) Reflector: A film layer group consisting of alternating high-refractive-index layers and low-refractive-index layers, with each layer having an optical thickness of 0.9 to 1.1 times one-quarter of λ0; In the long-wavelength cutoff stack, each layer uses a wavelength longer than λ0 as a reference wavelength, and its optical thickness exceeds the above range, so the long-wavelength cutoff stack does not belong to the reflector referred to in this article.

[0052] (7) Spacer layer: A high-refractive-index layer with an optical thickness that is an even multiple of one-quarter of λ0, located between two reflectors in a Fabry-Perot cavity. One reflector, one spacer layer and another reflector constitute a Fabry-Perot cavity; adjacent Fabry-Perot cavities are connected by a low-refractive-index coupling layer.

[0053] like Figure 1 and Figure 2 As shown, the deep cutoff infrared bandpass filter of the present invention includes a substrate 1, a bandpass film system 2 deposited on one side of the substrate 1, and an antireflection film system 3 deposited on the other side of the substrate 1. The bandpass film system 2 includes a plurality of Fabry-Perot cavities 28 formed by alternating stacking of a first silicon hydride layer 21, a second silicon hydride layer 22, and a low refractive index layer 23; each Fabry-Perot cavity 28 includes two reflectors 27 and a spacer layer 24 located between the two reflectors 27. On the side of the plurality of Fabry-Perot cavities facing away from the substrate 1, the bandpass film system 2 further includes a long-wavelength cutoff layer 25 and a hydrogen blocking layer 26 in sequence. Figure 1 The thickness of each layer is not drawn to scale; the actual thickness of each layer is shown in Table 2. Figure 2 Starting from the substrate 1 side, the layers of the bandpass membrane system 2 are shown in sequence according to their film layer numbers. The materials of each layer are distinguished by the diagram. Figure 1 and Figure 2 The arrows in the diagram all indicate the direction of the incident light.

[0054] The substrate 1 transmits light and provides support for the working wavelength of the filter. Its material can be silica-based optical glass (such as borosilicate glass like D263T or B270, or clear glass), colored glass (such as blue glass), quartz glass, sapphire, silicon, or plastic, with a preferred thickness of 0.15mm to 0.50mm. Chemical strengthening of the glass substrate can further improve its mechanical strength. When an additional visible light cutoff is required, colored glass that absorbs visible light can be used as the substrate 1.

[0055] The low-refractive-index layer 23 has a refractive index of less than 1.6 at 940 nm, and is preferably a silicon dioxide layer, but can also be an aluminum oxide layer, a magnesium fluoride layer, or a silicon oxynitride layer. In addition to participating in interference, the low-refractive-index layer 23 also separates the first silicon hydride layer 21 and the second silicon hydride layer 22 from each other, thus acting as a hydrogen diffusion barrier.

[0056] The refractive indices of both the first silicon hydride layer 21 and the second silicon hydride layer 22 at 940 nm are greater than 3. Table 1 shows the hydrogen content, optical band gap, absorption edge, refractive index, and extinction coefficient of the obtained silicon hydride films under the condition that all process parameters are exactly the same except for the hydrogen-argon flow ratio R=Q(H2) / Q(Ar) on the same medium-frequency magnetron sputtering equipment.

[0057] Table 1. Effect of hydrogen-argon flow ratio R on the properties of silicon hydrogenation films.

[0058] As shown in Table 1, with increasing hydrogen content, the optical band gap increases, the absorption edge blue shifts, and both the refractive index and extinction coefficient decrease simultaneously. Accordingly, this invention uses silicon hydride with R = 2%~5% and a hydrogen content of 4 at.%~8 at.% as the first silicon hydride layer 21, which has an extinction coefficient of 5 × 10⁻⁶ at 850 nm. -3 ~5×10 -2 This is sufficient to provide effective absorption attenuation in the 750nm~890nm wavelength range; hydride with an R=12%~22% and a hydrogen content of 13at.%~22at.% is used as the second hydride layer 22, and its extinction coefficient at 850nm is no greater than 5×10. -4 The extinction coefficient at 940 nm is no greater than 1×10⁻⁶. -4When used as a spacer layer, it causes almost no passband loss. The difference in hydrogen content between the two should not be less than 5 at.%, to ensure that the absorption edges differ by at least about 60 nm, thus making the functional division between the two effective; the difference in hydrogen content is preferably not less than 8 at.%. Due to the upper limit of hydrogen content in the silicon hydride film (generally not exceeding about 25 at.%, when the hydrogen content is too high, the film structure becomes loose, and the mechanical strength and environmental stability deteriorate), this difference is generally not greater than 18 at.%. Table 1 shows the results of this equipment under a set of fixed process parameters; when the hydrogen content is the same, the extinction coefficient of silicon hydride also varies within a certain range with process parameters such as sputtering power, working gas pressure and ion-assisted energy, so the extinction coefficient of the first silicon hydride layer at 850 nm can be 5 × 10⁻⁶. -3 ~5×10 -2 Adjust and select within the range.

[0059] The first silicon hydride layer 21 is entirely located within the reflector 27, and is situated in the reflector 27 except for the high-refractive-index layer closest to the spacer layer 24; the spacer layer 24 and the high-refractive-index layer closest to the spacer layer 24 in each reflector 27 are both second silicon hydride layers 22. The total physical thickness of all the first silicon hydride layers 21 in the bandpass film system 2 is 0.25 to 1.5 times the total physical thickness of all the second silicon hydride layers 22 (including the second silicon hydride layers 22 located in the spacer layer 24 and the long-wavelength cutoff layer 25): if this ratio is too small, the absorption in the 750nm to 890nm band is insufficient, and the cutoff depth does not reach OD6; if this ratio is too large, the peak transmittance of the passband decreases.

[0060] The optical thickness of spacer layer 24 is 4, 6, or 8 times one-quarter of λ0, i.e., a high-order cavity of order 4, 6, or 8 is used. Increasing the cavity order can increase the equivalent refractive index n of the film system. e To reduce angular offset, but too high an order will increase phase mismatch between cavities and widen the transition band, so 4th, 6th, or 8th order is preferred. The number of Fabry-Perot cavities is 3 to 7, and the number of layers in bandpass membrane system 2 is 40 to 75.

[0061] The long-wavelength cutoff stack 25 is composed of a second silicon hydride layer 22 and a low-refractive-index layer 23 stacked alternately at quarter-wavelength intervals, with a reference wavelength of 1.15 to 1.28 times the center wavelength λ0 of the passband (e.g., when λ0 = 940 nm, the reference wavelength is 1080 nm to 1200 nm). The number of layers ranges from 7 to 13. This extends the high-reflection region on the long-wavelength side from the long-wavelength edge of the high-reflection region of the multiple Fabry-Perot cavities themselves towards the long-wavelength direction (e.g., when λ0 = 940 nm, it extends from approximately 1084 nm to above 1150 nm). Since this band is located on the long-wavelength side of the absorption edge of the two silicon hydrides, a second silicon hydride layer 22 with a small extinction coefficient is used here to avoid loss of passband transmittance.

[0062] The hydrogen barrier layer 26 is the outermost layer of the bandpass film system 2, facing away from the substrate 1, with a thickness of 15 nm to 50 nm. The material is silicon nitride, aluminum oxide, silicon oxynitride, or a combination thereof. A thickness less than 15 nm is insufficient for hydrogen blocking; a thickness greater than 50 nm will significantly perturb the spectral shape of the bandpass film system. The hydrogen barrier layer 26 also provides protection against abrasion and moisture.

[0063] Antireflective coating system 3 is deposited on the other side of substrate 1 to reduce back reflection and improve bandgap transmittance. It can be formed by alternating stacking of niobium pentoxide and silicon dioxide, with 3 to 9 layers.

[0064] The principle of the above-mentioned deep cutoff infrared bandpass filter and its preparation method is as follows.

[0065] Firstly, the optical band gap E of silicon hydride g With hydrogen content C H Monotonically increasing. When C H When E increases from approximately 3 at.% to approximately 24 at.% g As the voltage increases from approximately 1.50 eV to approximately 2.00 eV, the absorption edge blue-shifts from approximately 827 nm to approximately 620 nm. Therefore, the first hydride layer, with its low hydrogen content, exhibits considerable absorption in the 750 nm–890 nm range (the extinction coefficient at 850 nm is 5 × 10⁻⁶). -3 ~5×10 -2 The second hydrogenated silicon layer, with its high hydrogen content, exhibits almost no absorption at 940 nm (the extinction coefficient at 940 nm is no greater than 1×10). -4 This complementarity between the two forms is the material basis of this invention.

[0066] Secondly, in a multi-cavity bandpass film system, the center wavelength λ0 of the passband is in a resonant state, and the electromagnetic field energy is highly concentrated within each spacer layer. Therefore, the extinction coefficient of the spacer layer has a much greater impact on the peak transmittance of the passband than the film layer in the reflector. However, no resonance occurs in the cutoff band, especially in the 750nm~890nm band. The light attenuates layer by layer in the reflector, and the field distribution mainly falls on the high refractive index layer of the reflector. Therefore, the extinction coefficient of the high refractive index layer in the reflector contributes the most to the cutoff depth of this band.

[0067] Thirdly, this invention separates two types of hydride with different hydrogen contents according to their "function-position": the first hydride layer, which has strong absorption but residual absorption at λ0, is used only as a high-refractive-index layer in the reflector, specifically to bear the absorption attenuation in the cutoff band; the second hydride layer, with low loss, is used as each spacer layer to ensure that the peak transmittance in the passband is not lost; and furthermore, the high-refractive-index layer closest to the spacer layer in each reflector is also set as the second hydride layer, making it a buffer layer between the resonant field and the absorption layer, thereby minimizing the impact of the residual absorption of the first hydride layer at λ0 on the peak transmittance. Thus, the mutually restrictive indicators of deep cutoff and high peak transmittance are decoupled, and can be simultaneously achieved on only one side of the substrate film system, without the need to deposit a cutoff film on the other side of the substrate.

[0068] Fourth, in the above-mentioned film system, the first and second silicon hydride layers are always separated by a low-refractive-index layer. The low-refractive-index layer also acts as a hydrogen diffusion barrier, preventing the loss of hydrogen content difference between the two silicon hydrides due to hydrogen interdiffusion. The outermost hydrogen barrier layer of the bandpass film system further seals the channels for hydrogen to escape to the outside. Stepped annealing first releases hydrogen from Si-H2 and Si-H3 with low binding energy and pre-relaxes the film layer at a first temperature platform below the service temperature, and then shapes the film layer structure at a second temperature platform above the service temperature, so that the filter will no longer experience significant hydrogen escape and refractive index changes during subsequent reflow soldering and damp heat testing.

[0069] Example 1

[0070] The deep cutoff infrared bandpass filter in this embodiment has λ0 = 940 nm. The substrate 1 is a 0.21 mm thick D263T chemically strengthened glass, double-sided polished, with a refractive index of 1.51 at 940 nm. The first silicon hydride layer 21 is deposited with R1 = 5% and a hydrogen content of 7.8 at.%, where n = 3.71 and k = 1.7 × 10⁻⁶ at 940 nm. -3 At 850 nm, k = 1.1 × 10 -2 The second silicon hydride layer 22 is deposited with R²=17% and a hydrogen content of 18.3 at.%, with n=3.49 and k=3.0×10 at 940 nm. -5 At 850 nm, k = 1.1 × 10 -4 The difference in hydrogen content between the two is 10.5 at.%. The low refractive index layer 23 is a silicon dioxide layer with n=1.470 at 940 nm. The hydrogen barrier layer 26 is a silicon nitride layer with a thickness of 30 nm and n=2.02 at 940 nm.

[0071] Bandpass film system 2 consists of 53 layers, including 3 Fabry-Perot cavities, a 6th-order spacer layer 24 (optical thickness is one-quarter of λ0, which is 6 times its original value), 9 long-wavelength cutoff layers 25 (reference wavelength 1130 nm), and 1 hydrogen blocking layer 26. Using λ0 = 940 nm as the reference wavelength, its film system expression is as follows:

[0072] G|L·[(H1L) 2 H2L·6H2·LH2(LH1) 2 ]·L·[(H1L) 2 H2L·6H2·LH2(LH1) 2 ]·

[0073] L·[(H1L) 2 H2L·6H2·LH2(LH1) 2 ]·L·(H2′L′) 4 H2′·B|Air

[0074] In the formula, G represents substrate 1, Air represents air; H1 represents the first silicon hydride layer 21, H2 represents the second silicon hydride layer 22, L represents the low refractive index layer 23, and each symbol represents a quarter-wavelength optical thickness with a reference wavelength of 940nm; the apostrophes H2′ and L′ represent a quarter-wavelength optical thickness with a reference wavelength of 1130nm; B represents the hydrogen blocking layer 26. The actual values ​​of each layer thickness after optimization and fine-tuning by optical thin film design software are shown in Table 2, and the markings in the table are consistent with... Figure 1 , Figure 2 The corresponding reference numerals in the attached figures correspond to each other; in the table, n for layers 44-52 is the value at 1130nm, and n for the remaining layers is the value at 940nm. Figure 2 For easy identification, spacer layer 24 is shown separately, and its material is second silicon hydride layer 22.

[0075] Table 2. Membrane structure of bandpass membrane system 2

[0076] The total physical thickness of the bandpass film system 2 is 7183.5 nm. The first silicon hydride layer 21 consists of 12 layers with a total physical thickness of 735.7 nm; the second silicon hydride layer 22 (including the spacer layer 24 and the high refractive index layer in the long-wavelength cutoff stack 25) consists of 14 layers with a total physical thickness of 1994.9 nm, and the ratio of the two is 0.37; the low refractive index layer 23 consists of 26 layers with a total physical thickness of 4422.9 nm.

[0077] The antireflective coating system consists of 5 layers, as shown in Table 3. Its average reflectivity in the 900nm~980nm wavelength range is less than 0.4%.

[0078] Table 3. Membrane structure of antireflection membrane system 3

[0079] The preparation method in this embodiment is as follows: Figure 3 As shown, it includes the following steps:

[0080] S1. Substrate Cleaning and Pretreatment. Substrate 1 was sequentially ultrasonically cleaned in an alkaline cleaning agent (frequency 40kHz, power 600W, temperature 55℃, time 10min), rinsed with deionized water, dehydrated with isopropanol, and dried with nitrogen. It was then mounted on the water-cooled rotating rack of a medium-frequency magnetron sputtering coating machine; a vacuum of 5×10⁻⁶ was applied. -4 Below Pa, the surface of substrate 1 to be plated is bombarded for 60s with an argon ion source at an ion energy of 300eV and a beam current of 200mA.

[0081] S2. Deposition of the first silicon hydride layer and the low refractive index layer. Mid-frequency magnetron sputtering was performed using a pair of twin silicon targets. The mid-frequency power supply was 7.2 kW, the frequency was 40 kHz, the working pressure was 0.4 Pa, the argon flow rate was 120 sccm (standard state milliliters per minute, the same below), and the hydrogen flow rate was 6.0 sccm, i.e., R1 = 5%, to deposit the first silicon hydride layer 21. During the deposition of the low refractive index layer 23, the hydrogen gas was turned off, and oxygen was introduced at 90 sccm. A silicon dioxide layer was obtained by reactive sputtering from the silicon target at a deposition rate of 0.30 nm / s. Throughout the deposition process, the temperature of substrate 1 did not exceed 80℃.

[0082] S3. Deposit the second silicon hydride layer. The hydrogen flow rate is continuously and linearly adjusted from 6.0 sccm to 20.4 sccm within 20 seconds, i.e., R2 = 17%. Simultaneously, the intermediate frequency power supply is synchronously increased from 7.2 kW to 8 kW. Other parameters remain the same as in S2. The deposition rate is 0.35 nm / s, resulting in the deposition of the second silicon hydride layer 22. When switching back from S3 to S2, the same continuous adjustment is performed in the reverse direction. Continuous adjustment of the hydrogen flow rate and synchronous micro-adjustment of the power supply are used to suppress abrupt changes in the deposition rate and film stoichiometry at the moment of switching, avoiding anomalies in refractive index and extinction coefficient at the film interface.

[0083] S4. Perform S2 and S3 in the order of the film layers of bandpass film system 2, and repeat them sequentially to deposit three Fabry-Perot cavities and a long-wavelength cutoff stack 25. The step numbers are only used to distinguish different deposition processes. The order and number of repetitions of steps S2 and S3 are determined by the order of the film layers of bandpass film system 2. For example, in this embodiment, the first layer is a silicon dioxide layer and the second layer is a first silicon hydride layer 21. The film thickness is controlled by a combination of quartz crystal monitoring and broadband optical monitoring: the film layer with an optical thickness of one-quarter wavelength is monitored by broadband optical monitoring, while each layer in the spacer layer 24 and the long-wavelength cutoff stack 25 is monitored by quartz crystal monitoring to avoid the accumulation of errors from a single monitoring method.

[0084] S5. Deposition of hydrogen barrier layer. In the same chamber, oxygen is shut off, and nitrogen gas is introduced at 60 sccm and hydrogen gas at 4 sccm. A silicon nitride layer with a thickness of 30 nm, i.e., hydrogen barrier layer 26, is obtained by reactive sputtering of silicon target.

[0085] S6. Stepped annealing and antireflection coating. The substrate 1 coated with the bandpass coating system 2 is placed in a vacuum annealing furnace, and a vacuum is drawn until the furnace pressure does not exceed 5 × 10⁻⁶. -3 Pa was heated to 165℃ at a rate of 3℃ / min and held for 60 min, then heated to 240℃ at a rate of 3℃ / min and held for 60 min, and finally cooled to room temperature at a rate of 2℃ / min. The temperature-time curve is shown below. Figure 6 As shown; then the substrate 1 is flipped over and an antireflection coating system 3 is deposited; finally, after cutting, appearance inspection and full spectral inspection, the deep cutoff infrared bandpass filter of this embodiment is obtained.

[0086] In step S2, an auxiliary ion source is simultaneously activated during the deposition of the silicon dioxide layer, and the ion energy is adjustable between 80 eV and 220 eV. As the ion energy increases, the residual stress of the silicon dioxide layer changes from tensile stress to compressive stress, which can be continuously adjusted within the range of -120 MPa to +60 MPa. In this embodiment, an ion energy of 150 eV is used to compensate for the residual stress of the silicon dioxide layer with that of the silicon hydride layer, resulting in a total film stress of 28 MPa and a warpage of 6 μm for the 0.21 mm thick substrate.

[0087] Figure 4 The transmittance curves of the filter in this embodiment at incident angles of 0° and 30° are shown. Figure 5 The solid line in the figure represents the optical density curve of the filter in this embodiment at an incident angle of 0°. Test results: λ0 = 940.2nm, the average transmittance in the 930nm~950nm band is 93.6%; the minimum optical density in the 400nm~890nm band is 6.4, with the minimum optical density in the 800nm~890nm band being 6.4; the minimum optical density in the 990nm~1150nm band is 5.3; the center wavelength shifts by 10.4nm when the incident angle increases from 0° to 30°, and by 17.6nm when it increases to 40°; the short-wavelength transition band width from 90% transmittance to OD5 is 5.2nm. After three reflow soldering cycles at 260°C for 10s, the center wavelength shifts by 0.6nm; after a damp heat test at 85°C / 85% relative humidity for 500h, the center wavelength shifts by 0.4nm.

[0088] Example 2

[0089] In this embodiment, the filter λ0 = 850 nm. The substrate 1 is a 0.30 mm thick chemically strengthened blue glass. The first silicon hydride layer 21 is deposited with R1 = 3%, and the hydrogen content is 5.2 at.%, with n = 3.79 and k = 3.2 × 10⁻⁶ at 850 nm.-2 The second silicon hydride layer 22 was deposited with R2=15% and a hydrogen content of 16.4 at.%, with n=3.56 and k=1.9×10 at 850nm. -4 The difference in hydrogen content between the two is 11.2 at.%. The bandpass film system 2 consists of 59 layers, including four Fabry-Perot cavities. The spacer layer 24 is of the 6th order, the long-wavelength cutoff layer 25 has 9 layers with a reference wavelength of 1030 nm, and the hydrogen blocking layer 26 is a 25 nm thick silicon nitride layer. The arrangement of the film layers is the same as in Example 1, i.e., the high refractive index layer closest to the spacer layer 24 in each reflector layer 27 is the second silicon hydride layer 22, and the remaining high refractive index layers are the first silicon hydride layers 21. The preparation method is the same as in Example 1, except that the two temperature plateaus for stepped annealing are 170°C for 45 min and 235°C for 90 min, respectively.

[0090] Example 3

[0091] In this embodiment, the filter λ0 = 940 nm. The substrate 1 is sapphire with a thickness of 0.30 mm. The first silicon hydride layer 21 is deposited with R1 = 2% and a hydrogen content of 4.1 at.%; the second silicon hydride layer 22 is deposited with R2 = 21% and a hydrogen content of 21.0 at.%; the difference in hydrogen content between the two is 16.9 at.%. The bandpass film system 2 consists of 67 layers, including 5 Fabry-Perot cavities, an 8-layer spacer layer 24, an 11-layer long-wavelength cutoff stack 25 with a reference wavelength of 1160 nm, and a 20 nm thick alumina layer as the hydrogen blocking layer 26. The arrangement of the film layers is the same as in Example 1. The preparation method is the same as in Example 1, except that the two temperature plateaus of the stepped annealing are 155 °C for 90 min and 255 °C for 120 min, respectively.

[0092] Comparative Example 1

[0093] Except for replacing all the first hydrogenated silicon layers 21 in the bandpass film system 2 with the second hydrogenated silicon layers 22 (i.e., all high refractive index layers are deposited at R=17%, the hydrogen content is 18.3 at.%, and the thickness of each layer is re-optimized to maintain the same passband position and bandwidth), the rest is the same as in Example 1. Figure 5 The dashed line in the figure represents the optical density curve of the filter in Comparative Example 1 at an incident angle of 0°.

[0094] Comparative Example 2

[0095] Except for replacing all the second silicon hydrogenation layers 22 in the bandpass film system 2 with the first silicon hydrogenation layer 21 (i.e. all high refractive index layers are deposited at R=5%, the hydrogen content is 7.8 at.%, and the thickness of each layer is re-optimized), the rest is the same as in Example 1.

[0096] Comparative Example 3

[0097] Except for omitting the hydrogen barrier layer 26 in step S5 and the stepped annealing in step S6 (directly flipping over to deposit the antireflection film 3 after depositing the bandpass film system 2), the rest is the same as in Example 1.

[0098] Comparative Example 4

[0099] Except for swapping the positions of the first silicon hydride layer 21 and the second silicon hydride layer 22 (i.e., each spacer layer 24 and the high refractive index layer closest to the spacer layer 24 in each reflector 27 adopts the first silicon hydride layer 21, and the remaining high refractive index layers in the reflector 27 adopt the second silicon hydride layer 22, and the thickness of each layer is re-optimized), the rest is the same as in Example 1.

[0100] The performance test results of the above embodiments and comparative examples are listed in Table 4. In the table, the statistical bands for the average transmittance of the passband, the minimum optical density of the short-wavelength cutoff band, and the minimum optical density of the long-wavelength cutoff band are: 840nm~860nm, 380nm~800nm, and 900nm~1100nm for Example 2, and 930nm~950nm, 400nm~890nm, and 990nm~1150nm for the other examples.

[0101] Table 4 Performance test results of the examples and comparative examples

[0102] The following conclusions can be drawn from Table 4:

[0103] (1) All the high refractive index layers in Comparative Example 1 are second hydride silicon layers 22 with high hydrogen content. Although its average transmittance in the passband is slightly higher (94.1%), the minimum optical density in the short-wavelength cutoff band is only 4.2, and this minimum value appears in the 800nm~890nm band (the minimum optical density is 3.6, see Figure 5 (The dashed line) indicates that simply relying on hydride with a high hydrogen content cannot achieve deep cutoff in this band.

[0104] (2) All the high refractive index layers in Comparative Example 2 are first hydride layers 21 with low hydrogen content. The minimum optical density of its short-wave cutoff band is 6.8, but the average transmittance of the passband drops to 86.2%, which is 7.4 percentage points lower than that of Example 1. This shows that although hydride with low hydrogen content can achieve deep cutoff, it comes at the cost of sacrificing peak transmittance.

[0105] (3) Comparative Example 4 used the same two types of silicon hydride as the present invention, but interchanged their positions. Its average transmittance in the passband was reduced to 88.0%, which is close to that of Comparative Example 2. This shows that the key to achieving the effect of "both deep cutoff and high peak transmittance" does not lie in the simultaneous use of two types of silicon hydride with different hydrogen contents, but in the positional rules defined by the present invention, namely, the spacer layer must be the second silicon hydride layer with high hydrogen content, and the first silicon hydride layer with low hydrogen content must be located entirely within the reflector.

[0106] (4) Comparative Example 3 omitted the hydrogen barrier layer 26 and stepped annealing. Its spectral performance was similar to that of Example 1, but the center wavelength drifted by 4.8 nm after reflow soldering and by 3.6 nm after damp heat test. The total stress of the film system and the warping of the substrate increased significantly, indicating that the hydrogen barrier layer and stepped annealing are key measures to significantly improve thermal and damp heat stability.

[0107] (5) Examples 1 to 3 all achieved the expected indicators under the conditions of 3 to 5 cavities, 6th and 8th order spacer layers, different substrates and different hydrogen barrier layer materials, indicating that the technical solutions of the present invention can be implemented within the above-defined range.

[0108] Those skilled in the art should understand that the present invention is not limited to the above embodiments. Modifications can be made without departing from the concept of the present invention, and all such modifications fall within the protection scope of the present invention.

[0109] (1) The passband center wavelength λ0 can be any wavelength in the range of 780nm~1650nm. The reference wavelengths of each layer can be scaled up proportionally. At this time, the hydrogen content of the first and second silicon hydrogenation layers can be adjusted according to the relative position of their absorption edges and λ0.

[0110] (2) The first silicon hydrogenation layer and the second silicon hydrogenation layer can each be composed of two or more silicon hydrogenation sublayers with different hydrogen contents. As long as all the sublayers with lower hydrogen contents are located inside the reflector and each spacer layer is composed of sublayers with higher hydrogen contents, it belongs to the technical solution of the present invention.

[0111] (3) A small amount of nitrogen, oxygen or carbon can be added to the reaction gas of silicon hydride to form silicon nitride hydrogen, silicon oxide hydrogen or silicon carbide hydrogen, as long as its hydrogen content meets the above-mentioned hydrogen content relationship between the first silicon hydride layer and the second silicon hydride layer.

[0112] (4) The low refractive index layer can be silicon dioxide, aluminum oxide, magnesium fluoride or silicon oxynitride; the hydrogen barrier layer can be silicon nitride, aluminum oxide, silicon oxynitride or a combination thereof; the antireflective coating system can be formed by alternating stacking of one of niobium pentoxide, titanium dioxide, tantalum pentoxide and silicon dioxide.

[0113] (5) In addition to medium-frequency magnetron sputtering, the deposition method can also be DC magnetron sputtering, radio frequency magnetron sputtering, ion beam sputtering or plasma-enhanced chemical vapor deposition; hydrogen gas can also be partially or completely introduced into the chamber through an auxiliary ion source.

[0114] (6) An adhesion layer or a blocking layer may be provided between the substrate and the bandpass film system; the bandpass film system may be deposited on the light-incident surface of the substrate or on the light-exit surface of the substrate; the long-wavelength cutoff stack may also be provided between multiple Fabry-Perot cavities and the substrate.

[0115] (7) Step annealing can be performed after all film layers have been deposited, or after some film layers have been deposited and after all film layers have been deposited respectively; the number of temperature plateaus in step annealing can be more than two.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A deep-cut-off infrared bandpass filter, comprising a substrate and a bandpass film system located on one side of the substrate, the bandpass film system comprising a plurality of Fabry-Perot cavities formed by alternating stacks of high-refractive-index layers and low-refractive-index layers, each of the Fabry-Perot cavities comprising two reflectors and a spacer layer located between the two reflectors, characterized in that, The high refractive index layer is a first silicon hydride layer or a second silicon hydride layer, wherein the hydrogen content of the second silicon hydride layer is more than 5 at.% higher than that of the first silicon hydride layer; each of the spacer layers is a second silicon hydride layer, and the first silicon hydride layers are all located within the reflector.

2. The deep cutoff infrared bandpass filter according to claim 1, characterized in that, The hydrogen content of the first silicon hydride layer is 4 at.% to 8 at.%, and the extinction coefficient at 850 nm is 5 × 10⁻⁶. -3 ~5×10 -2 The hydrogen content of the second silicon hydride layer is 13 at.% to 22 at.%, and the extinction coefficient at 850 nm is no greater than 5 × 10⁻⁶. -4 .

3. The deep cutoff infrared bandpass filter according to claim 1 or 2, characterized in that, The high-refractive-index layer closest to the spacer layer in each of the reflectors is the second silicon hydride layer, and the remaining high-refractive-index layers in the reflectors are the first silicon hydride layers; the total physical thickness of all the first silicon hydride layers in the bandpass film system is 0.25 to 1.5 times the total physical thickness of all the second silicon hydride layers.

4. The deep cutoff infrared bandpass filter according to claim 3, characterized in that, The outermost layer of the bandpass film system facing away from the substrate is a hydrogen barrier layer, which is a silicon nitride layer or an aluminum oxide layer with a thickness of 15nm~50nm; the low refractive index layer is a silicon dioxide layer.

5. The deep cutoff infrared bandpass filter according to claim 1 or 4, characterized in that, The bandpass film system also includes a long-wavelength cutoff stack, which is located on the side of the plurality of Fabry-Perot cavities away from the substrate. The long-wavelength cutoff stack is formed by stacking a second silicon hydride layer and a low refractive index layer alternately with a quarter wavelength reference wavelength of 1.15 to 1.28 times the center wavelength λ0 of the passband, and the number of layers is 7 to 13.

6. The deep cutoff infrared bandpass filter according to claim 5, characterized in that, The optical thickness of the spacer layer is 4, 6, or 8 times one-quarter of the center wavelength λ0 of the passband. The number of Fabry-Perot cavities is 3 to 7. The number of layers of the bandpass film system is 40 to 75. An antireflection film system is provided on the other side of the substrate.

7. The deep cutoff infrared bandpass filter according to claim 6, characterized in that, The passband center wavelength λ0 is 930nm~950nm, and the optical density of the filter at an incident angle of 0° is not less than 6 in the 400nm~890nm band, not less than 5 in the 990nm~1150nm band, and the average transmittance in the 930nm~950nm band is not less than 92%; the shift of the passband center wavelength when the incident angle increases from 0° to 30° is not greater than 11nm.

8. A method for preparing a deep cutoff infrared bandpass filter, used to prepare a deep cutoff infrared bandpass filter as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Clean and pre-treat the substrate; S2. A silicon target reactive magnetron sputtering is used to deposit a first silicon hydride layer with a hydrogen-argon flow rate ratio of R1, and the layer is deposited alternately with a low refractive index layer. The hydrogen-argon flow rate ratio is the ratio of the hydrogen flow rate to the argon flow rate during sputtering. S3. Deposit a second silicon hydride layer with a hydrogen-argon flow ratio of R2, and alternately deposit it with a low refractive index layer, R2-R1≥7%; S4. Perform and repeat steps S2 and S3 according to the membrane layer sequence of the bandpass membrane system to obtain the bandpass membrane system; S5, Deposit hydrogen barrier layer; S6. Perform stepped annealing in a vacuum through at least two temperature plateaus.

9. The preparation method according to claim 8, characterized in that, The magnetron sputtering uses a medium-frequency power supply with a power of 6kW~12kW and a frequency of 20kHz~60kHz; R1 is 2%~5%, R2 is 12%~22%; and the sputtering working pressure is 0.2Pa~0.8Pa. sediment During the process, the substrate temperature does not exceed 80℃; when switching between depositing the first hydrogenated silicon layer and depositing the second hydrogenated silicon layer, the hydrogen flow rate is continuously adjusted to the target value within 5s to 40s.

10. The preparation method according to claim 8 or 9, characterized in that, The stepped annealing process involves heating to 150℃~180℃ at a rate of 1℃ / min~5℃ / min and holding for 30min~90min, then heating to 220℃~260℃ at a rate of 1℃ / min~5℃ / min and holding for 60min~180min, followed by cooling to room temperature at a rate not exceeding 2℃ / min; the furnace pressure during the stepped annealing process does not exceed 5×10⁻⁶. -3 Pa.

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