A method for manufacturing a linear gradient filter

Abstract

The application discloses a preparation method of a linearly graded filter. The method comprises the following steps: preparing a substrate; preparing a first distributed Bragg reflector on one side of the substrate; preparing a resonant cavity layer on the side of the first distributed Bragg reflector away from the substrate; preparing a photoresist on the side of the resonant cavity layer away from the substrate; controlling the exposure amount of the photoresist in a first direction according to preset thickness variation information of the resonant cavity in the first direction, wherein the first direction intersects the thickness direction of the resonant cavity; performing dry etching on the photoresist and the resonant cavity layer, so that the thickness variation of the resonant cavity in the first direction meets the preset thickness variation information; and preparing a second distributed Bragg reflector on the side of the resonant cavity layer away from the substrate. The linearly graded filter prepared by the preparation method can make the thickness variation of the resonant cavity layer in the linearly graded filter closer to linear variation.

Description

Technical Field

[0001] This invention relates to the field of optical element technology, and in particular to a method for preparing a linear graded filter. Background Technology

[0002] A linear variable filter (LVF) is the core spectroscopic element of a miniature near-infrared spectrometer. Its working principle involves fabricating one (or more) optical thin films on an optical substrate, such that the optical thickness of the film changes continuously and linearly along a certain direction, thereby achieving the effect of a linear change in the transmission center wavelength with position.

[0003] There are currently two main methods for LVF manufacturing: Option 1: Mask-based coating method. This method uses a mask with a specific geometry to block the substrate during coating, resulting in varying amounts of film material received at different locations, thus creating a thickness gradient. While relatively simple, this method has the following drawbacks: 1. Diffraction effects at the mask edges cause blurring in the transition region, making it difficult to achieve steep wavelength gradient changes; 2. Thickness distribution accuracy is greatly affected by the mask-substrate distance and the geometry of the coating source, making repeatability control difficult; 3. It is difficult to fabricate complex two-dimensional gradient patterns or multiple independent gradient regions; 4. Inaccurate wedge angle control easily introduces channel crosstalk.

[0004] Option 2: Uniform Deposition followed by Etching. This method involves first depositing a multilayer optical film of uniform thickness across the entire substrate surface, then locally removing portions of the film material using photolithography and etching techniques to achieve the desired thickness. While this approach offers advantages such as: 1. High-quality film (due to uniform deposition under optimal conditions); 2. Precise control of thickness distribution by the photolithographic pattern, unaffected by deposition geometry; 3. Ability to achieve arbitrarily complex two-dimensional patterns; 4. Suitability for mass production, allowing simultaneous processing of multiple substrates on the same wafer, it also faces the following challenges: Aspect Ratio Depth Dependence (ARDE). For densely etched periodic structures, the etching rate varies with feature size (i.e., RIE-lag), leading to inconsistent etching depths in different width regions, affecting the final film thickness uniformity and wavelength gradient linearity. Summary of the Invention

[0005] This invention provides a method for fabricating a linear graded filter to solve the problem that the thickness variation of the resonant cavity layer in a linear graded filter fabricated using existing methods deviates from linearity.

[0006] This invention provides a method for preparing a linear graded filter, comprising: Preparing a substrate; A first distributed Bragg reflector is fabricated on one side of the substrate; A resonant cavity layer is fabricated on the side of the first distributed Bragg reflector away from the substrate; Photoresist is prepared on the side of the resonant cavity layer away from the substrate; The exposure amount of the photoresist in the first direction is controlled according to the preset thickness change information of the resonant cavity in the first direction, thereby controlling the thickness of the photoresist after development in the first direction, wherein the first direction intersects with the thickness direction of the resonant cavity; Dry etching is performed on the photoresist and the resonant cavity layer so that the thickness variation of the resonant cavity layer in the first direction satisfies the preset thickness variation information; A second distributed Bragg reflector is fabricated on the side of the resonant cavity layer away from the substrate.

[0007] Optionally, controlling the exposure amount of the photoresist in the first direction based on preset thickness variation information of the resonant cavity in the first direction includes: The spatial exposure distribution information of the exposure head is determined based on the preset thickness change information of the resonant cavity in the first direction, wherein the exposure head includes a digital micromirror device or a spatial light modulator; The light beam output by the exposure head is controlled to be directed to the surface of the photoresist away from the substrate based on the spatial exposure distribution information.

[0008] Optionally, controlling the exposure amount of the photoresist in the first direction based on preset thickness variation information of the resonant cavity in the first direction includes: The micro-dot distribution information of the halftone mask is designed based on the preset thickness variation information of the resonant cavity in the first direction; The exposure head is controlled to output a uniform light beam, which, after passing through the halftone mask, is incident on the surface of the photoresist away from the substrate.

[0009] Optionally, controlling the exposure amount of the photoresist in the first direction based on preset thickness variation information of the resonant cavity in the first direction includes: Based on the preset thickness variation information of the resonant cavity in the first direction, different binary templates are used sequentially, wherein the different binary templates refer to the different opening areas of the binary templates; The photoresist is exposed multiple times to the surface away from the substrate, and a different binary template is used for each exposure.

[0010] Optionally, before dry etching the photoresist and the resonant cavity layer, the method further includes: The photoresist is then subjected to a post-baking process.

[0011] Optionally, dry etching is performed on the photoresist and the resonant cavity layer, including: The photoresist is then etched using an ion beam.

[0012] Optionally, the photoresist is subjected to ion beam etching, including: The photoresist and the resonant cavity layer are vertically etched using an ion beam. The resonant cavity layer is polished using the ion beam.

[0013] Optionally, after dry etching the photoresist and the resonant cavity layer, the method further includes: The surface of the resonant cavity layer is subjected to thermal annealing.

[0014] Optionally, the substrate may be cooled while the photoresist and the resonant cavity layer are being dry etched.

[0015] Optionally, after fabricating the substrate, the process may also include: The substrate is cleaned.

[0016] The technical solution of this invention is that by using the above-described preparation method to prepare a linear graded filter, the thickness change of the resonant cavity layer in the linear graded filter can be made closer to a linear change.

[0017] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating a method for preparing a linear gradient filter according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating a method for preparing a linear gradient filter according to an embodiment of the present invention. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0021] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or device that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices. The terms "upper," "lower," "left," "right," etc., indicate orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings and are only used to describe the relative positional relationships between components or constituent parts, and do not specifically limit the specific installation orientation of each component or constituent part.

[0022] Figure 1 A flowchart illustrating a method for fabricating a linear graded filter according to an embodiment of the present invention is provided, with reference to... Figure 1 The method for preparing the linear graded filter in this embodiment of the invention includes: S110, Prepare the substrate.

[0023] For example, the substrate material can be fused silica or single-crystal germanium (Ge); the substrate size can be a circle with a diameter of 25mm-30mm, or a square of 20mm×20mm to 30mm×30mm; the substrate thickness ranges from 1 to 3mm; the substrate surface accuracy can be λ / 4 @ 632.8nm; the substrate parallelism is less than 3 arcmin; the substrate surface quality can meet the 60-40 scratch / pit specification.

[0024] Optionally, after preparing the substrate, the process may also include cleaning the substrate.

[0025] For example, the substrate can be sequentially subjected to ultrasonic multi-step cleaning, nitrogen drying, and ultra-clean storage. This embodiment of the invention, by cleaning the substrate before fabricating the first distributed Bragg mirror, helps improve the uniformity of the film thickness in the subsequently fabricated first distributed Bragg mirror.

[0026] S120. A first distributed Bragg reflector is fabricated on one side of the substrate.

[0027] In this embodiment of the invention, the first distributed Bragg reflector is a multilayer reflective structure consisting of periodically alternating high-refractive-index dielectric films and low-refractive-index dielectric films. The high-refractive-index dielectric film can be made of tantalum pentoxide, and the low-refractive-index dielectric film can be made of silicon dioxide. The films can be formed using a sputtering method.

[0028] S130. A resonant cavity layer is prepared on the side of the first distributed Bragg mirror away from the substrate.

[0029] For example, the resonant cavity layer can be made of silicon dioxide material, and the thickness of the resonant cavity layer obtained in this step is uniform.

[0030] S140. Photoresist is prepared on the side of the resonant cavity layer away from the substrate.

[0031] It should be noted that the photoresist in the embodiments of the present invention can be either positive or negative.

[0032] For example, a suitable thickness of adhesive can be applied to the side of the resonant cavity layer away from the substrate first. Then, to remove solvent and enhance adhesion, the adhesive is heat-treated (i.e., pre-baking) to complete the photoresist preparation. During the heat treatment, the heating temperature range is 90-110°C (depending on the type of photoresist), and the heating time ranges from 1 to 5 minutes.

[0033] S150. The exposure amount of the photoresist in the first direction is controlled according to the preset thickness change information of the resonant cavity in the first direction, thereby controlling the thickness of the developed photoresist in the first direction, wherein the first direction intersects with the thickness direction of the resonant cavity.

[0034] Understandably, the thickness of the photoresist after development varies depending on the amount of exposure it receives. Ultimately, a photoresist relief pattern with continuously varying thickness can be formed on the surface of the resonant cavity. For example, at one end (such as the short-wavelength end): the photoresist is almost completely removed (leaving only a residue of less than 100nm), at the other end (such as the long-wavelength end): the photoresist is close to its original thickness (almost unprotected by etching), and in the middle region: a smooth transition.

[0035] In one feasible implementation, controlling the exposure amount of photoresist in a first direction based on preset thickness variation information of the resonant cavity in the first direction includes: determining spatial exposure distribution information of the exposure head based on the preset thickness variation information of the resonant cavity in the first direction, wherein the exposure head includes a digital micromirror device or a spatial light modulator. The light beam output from the exposure head is then controlled to be directed to the surface of the photoresist away from the substrate based on the spatial exposure distribution information.

[0036] The final thickness of the photoresist at different locations can be precisely controlled by adjusting the exposure dose at each location. Taking positive photoresist as an example, areas with high exposure are fully photosensitive and will be completely removed after development; areas with low exposure are partially photosensitive and will leave a thin layer after development; areas with zero exposure are completely unphotosensitive and will retain their original thickness after development.

[0037] In another feasible implementation, the exposure amount of the photoresist in the first direction is controlled according to preset thickness variation information of the resonant cavity in the first direction, including: designing the micro-dot distribution information of the halftone mask according to the preset thickness variation information of the resonant cavity in the first direction; controlling the exposure head to output a uniform beam, which, after passing through the halftone mask, is incident on the surface of the photoresist away from the substrate.

[0038] For example, a halftone mask (HTM) is composed of a micro-dot pattern with different duty cycles. Areas with high local density of micro-dots have a large equivalent exposure and leave a thinner photoresist after development, while areas with sparse micro-dots have a small equivalent exposure and leave a thicker photoresist after development.

[0039] In another feasible implementation, controlling the exposure amount of photoresist in the first direction based on preset thickness variation information of the resonant cavity in the first direction includes: determining the sequential use of different binary templates based on the preset thickness variation information of the resonant cavity in the first direction, wherein different binary templates refer to binary templates with different aperture areas. Multiple exposures are performed on the surface of the photoresist away from the substrate, with a different binary template used for each exposure.

[0040] For example, N independent exposures are performed (each using a different binary mask). The mask pattern of each exposure gradually approximates the gradient outline of the target. After N superpositions, a continuous distribution is approximately obtained. The larger N is, the more accurate the approximation, but the more complex the process. In practical applications, N=3-5 can meet the requirements of most LVFs.

[0041] Optionally, before dry etching the photoresist, a post-baking process may be performed on the photoresist.

[0042] For example, heating the developed photoresist can further enhance its resistance to etching. The heating temperature range is 110-130℃, but it should not be too high to avoid deformation of the photoresist pattern.

[0043] S160. Dry etching is performed on the photoresist and the resonant cavity layer so that the thickness change of the resonant cavity layer in the first direction meets the preset thickness change information.

[0044] For example, the dry etching time is the same in different regions. The etching depth of the resonant cavity layer corresponding to the thinner photoresist is deeper, and the final thickness of the resonant cavity layer is thinner; while the etching depth of the resonant cavity layer corresponding to the thicker photoresist is shallower, and the final thickness of the resonant cavity layer is thicker. In this way, the resonant cavity layer with continuously varying thickness can be formed by using the photoresist relief pattern with continuously varying thickness.

[0045] Optionally, after dry etching of the photoresist and resonant cavity layer, the process further includes thermal annealing of the surface of the resonant cavity layer.

[0046] To repair near-surface lattice damage caused by dry etching and restore the refractive index anomaly caused by etching (recovery rate greater than 95%), the surface of the resonant cavity layer will be thermally annealed in an atmospheric environment or oxygen atmosphere. The thermal annealing temperature range is 300-500℃ (lower than the glass transition temperature of the substrate and film layer), the thermal annealing time range is 30-60 minutes, and the thermal annealing heating and cooling rate meets the requirement of less than 5℃ / min (to prevent thermal stress).

[0047] Optionally, for components that cannot withstand high temperatures (such as those with glued parts), a low-temperature long-time annealing process at 150-200℃ (2-4 hours) can be used to improve minor damage.

[0048] Optionally, the substrate can be cooled while the photoresist and resonant cavity layer are being dry etched.

[0049] To suppress surface diffusion during the etching process, maintain sharp edges, and prevent photoresist from overheating, deforming, or leaking, embodiments of the present invention perform a cooling process on the substrate during dry etching, cooling it to a temperature below -20°C.

[0050] S170. A second distributed Bragg reflector is fabricated on the side of the resonant cavity layer away from the substrate.

[0051] In this embodiment of the invention, the second distributed Bragg reflector is a multilayer reflective structure consisting of periodically alternating low-refractive-index dielectric films and high-refractive-index dielectric films. The high-refractive-index dielectric film can be made of tantalum pentoxide, and the low-refractive-index dielectric film can be made of silicon dioxide. The films can be formed using a sputtering method.

[0052] The technical solution of this invention is that by using the above-described preparation method to prepare a linear graded filter, the thickness change of the resonant cavity layer in the linear graded filter can be made closer to a linear change.

[0053] Figure 2 This is a flowchart illustrating a method for preparing a linear graded filter according to an embodiment of the present invention. Figure 2 The illustrated embodiment provides a detailed description of how to perform dry etching on the photoresist and resonant cavity layer, see reference. Figure 2 The method for preparing the linear graded filter in this embodiment of the invention includes: S210, Prepare the substrate.

[0054] S220. A first distributed Bragg reflector is fabricated on one side of the substrate.

[0055] S230. A resonant cavity layer is prepared on the side of the first distributed Bragg mirror away from the substrate.

[0056] S240. Photoresist is prepared on the side of the resonant cavity layer away from the substrate.

[0057] S250. Control the exposure amount of photoresist in the first direction according to the preset thickness change information of the resonant cavity in the first direction, wherein the first direction intersects with the thickness direction of the resonant cavity.

[0058] S260. Ion beam etching is performed on the photoresist and resonant cavity layer to make the thickness change of the resonant cavity in the first direction meet the preset thickness change information.

[0059] For example, a directional high-energy ion beam (e.g., Ar) can be generated using a separate ion source. + Physically bombarding the photoresist surface to peel it off atom by atom, color printing ion beam etching has the following advantages: 1. Anisotropy (vertical etching, steep sidewalls); 2. Good material versatility (almost all inorganic films can be etched with Ar). + 3. No chemical reaction byproducts contamination; 4. Etching parameters (energy, beam current, incident angle) are flexibly adjustable.

[0060] Specifically, the parameters for ion beam etching can be as follows: 1. The ion beam energy range is 300-800eV. The basis for selecting this range is that if the energy is too low (less than 300eV), the etching rate will be too low and the production efficiency will be poor; if the energy is too high (greater than 800eV), the surface will be severely damaged and the roughness will deteriorate. The preferred ion beam energy is 500-600eV.

[0061] 2. The ion beam current density ranges from 0.5 to 2.0 mA / cm². 2It is proportional to the etching rate, and the uniformity of the ion beam current needs to be less than ±3%, covering a 30mm×30mm area.

[0062] 3. The working pressure range is 1×10 -4 Up to 5×10 -4 Pa, a lower pressure has the effect of a longer mean free path and better beam collimation, but attention needs to be paid to balancing the pumping speed and gas supply.

[0063] As a feasible implementation method, ion beam etching of photoresist and resonant cavity layer includes: vertical etching of photoresist and resonant cavity layer by ion beam; and polishing of resonant cavity layer by ion beam.

[0064] When the angle between the ion beam and the substrate normal is 0° (i.e., perpendicular incidence): the ion beam etching rate is the highest and the anisotropy is the best, but there may be a micro-trenching effect. Therefore, when the photoresist and resonant cavity layer are etched perpendicularly by the ion beam, the etching rate can be reduced by controlling the angle between the ion beam and the substrate normal to meet 30-60° (e.g., 30°) to improve the smoothness of the resonant cavity surface (especially the trench side).

[0065] S270. A second distributed Bragg mirror is fabricated on the side of the resonant cavity layer away from the substrate.

[0066] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method of making a linearly graded filter, characterized by, include: Preparing a substrate; A first distributed Bragg reflector is fabricated on one side of the substrate; A resonant cavity layer is fabricated on the side of the first distributed Bragg reflector away from the substrate; Photoresist is prepared on the side of the resonant cavity layer away from the substrate; The exposure amount of the photoresist in the first direction is controlled according to the preset thickness change information of the resonant cavity in the first direction, thereby controlling the thickness of the photoresist after development in the first direction, wherein the first direction intersects with the thickness direction of the resonant cavity; Dry etching is performed on the photoresist and the resonant cavity layer so that the thickness variation of the resonant cavity layer in the first direction satisfies the preset thickness variation information; A second distributed Bragg reflector is fabricated on the side of the resonant cavity layer away from the substrate.

2. The preparation method according to claim 1, characterized in that, Controlling the exposure amount of the photoresist in the first direction based on the preset thickness change information of the resonant cavity in the first direction includes: The spatial exposure distribution information of the exposure head is determined based on the preset thickness change information of the resonant cavity in the first direction, wherein the exposure head includes a digital micromirror device or a spatial light modulator; The light beam output by the exposure head is controlled to be directed to the surface of the photoresist away from the substrate based on the spatial exposure distribution information.

3. The preparation method according to claim 1, characterized in that, Controlling the exposure amount of the photoresist in the first direction based on the preset thickness change information of the resonant cavity in the first direction includes: The micro-dot distribution information of the halftone mask is designed based on the preset thickness variation information of the resonant cavity in the first direction; The exposure head is controlled to output a uniform light beam, which, after passing through the halftone mask, is incident on the surface of the photoresist away from the substrate.

4. The preparation method according to claim 1, characterized in that, Controlling the exposure amount of the photoresist in the first direction based on the preset thickness change information of the resonant cavity in the first direction includes: Based on the preset thickness variation information of the resonant cavity in the first direction, different binary templates are used sequentially, wherein the different binary templates refer to the different opening areas of the binary templates; The photoresist is exposed multiple times to the surface away from the substrate, and a different binary template is used for each exposure.

5. The preparation method according to claim 1, characterized in that, Before dry etching of the photoresist and the resonant cavity layer, the process further includes: The photoresist is then subjected to a post-baking process.

6. The preparation method according to claim 1, characterized in that, Dry etching of the photoresist and the resonant cavity layer includes: The photoresist and the resonant cavity layer are etched by ion beam etching.

7. The preparation method according to claim 6, characterized in that, Ion beam etching of the photoresist and the resonant cavity layer includes: The photoresist and the resonant cavity layer are vertically etched using an ion beam. The resonant cavity layer is polished using the ion beam.

8. The preparation method according to claim 1, characterized in that, After dry etching of the photoresist and the resonant cavity layer, the process further includes: The surface of the resonant cavity layer is subjected to thermal annealing.

9. The preparation method according to claim 8, characterized in that, While the photoresist and the resonant cavity layer are being dry-etched, the substrate is being cooled.

10. The preparation method according to claim 1, characterized in that, After the substrate is fabricated, the following steps are also included: The substrate is cleaned.

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