A bandgap thin film structure device and a method of fabricating the same

Abstract

The application discloses a bandgap thin film structure device and a preparation method thereof. The bandgap thin film structure device comprises a substrate, a sacrifice layer formed on a first surface of the substrate, a first silicon nitride layer formed on the sacrifice layer and parallel to the substrate, and the first silicon nitride layer has a continuous and stepless plane. The first silicon nitride layer comprises a bandgap thin film area and a peripheral part surrounding the bandgap thin film area, the surface of the first silicon nitride layer has bandgap holes arranged periodically, each bandgap hole has a release hole around it, the bandgap holes and the release holes are distributed in the bandgap thin film area, the bandgap holes and the release holes are used for etching the sacrifice layer between the substrate and the bandgap thin film area, the central area of the bandgap thin film area has a defect area, and the defect area does not have the bandgap holes. A second silicon nitride layer is formed on a second surface of the substrate opposite to the first surface. A cavity in which the bandgap thin film area is suspended is formed between the bandgap thin film area of the first silicon nitride layer and the first surface, and the peripheral part is supported by the unetched part of the sacrifice layer.

Description

Technical Field

[0001] This application relates to the field of bandgap thin film technology, specifically to a bandgap thin film structure device and its fabrication method. Background Technology

[0002] In recent years, with the rapid development of quantum information science and nano-opto-electro-mechanical systems (NEMS / MEMS), high-quality factor (High-Q) bandgap thin films have gradually become a research hotspot in scientific research and application fields. Bandgap thin films typically refer to phononic or photonic bandgap films formed in specific two-dimensional or three-dimensional micro / nano structures by periodically controlling the geometric morphology or mechanical properties of the material. This effectively suppresses energy leakage and dissipation, and improves the system's coherence and signal retention time. These thin films are often applied in cutting-edge fields such as high-sensitivity sensors, quantum state storage devices, precision measurement platforms, and next-generation hybrid quantum interfaces.

[0003] High-Q thin films are particularly important in quantum technology. A higher quality factor results in lower energy loss and longer coherence time of quantum states. For example, in quantum networks, suspended high-Q bandgap films can serve as interfaces between photons and phonons, facilitating efficient quantum state transitions and storage. In precision measurements, high-Q films can significantly reduce thermal noise and environmental coupling, thereby achieving sub-quantum limit measurement sensitivity. Therefore, how to fabricate highly intact and consistent suspended bandgap films is one of the core technical challenges in this field.

[0004] Currently, commonly used thin film release techniques mainly rely on silicon-based substrates and potassium hydroxide (KOH) etching processes. While the traditional silicon-based + KOH etching process is widely used in some MEMS / NEMS fields, it still has significant limitations in the fabrication of high-Q bandgap thin films. To meet the demands for high integrity and high reliability in quantum information and cutting-edge device research, it is necessary to explore more efficient, rapid, and stable thin film release methods. Summary of the Invention

[0005] To address the aforementioned deficiencies in this field, this application aims to provide a bandgap thin-film structure device and its fabrication method.

[0006] According to one aspect of this application, a bandgap thin-film structure device is provided, comprising:

[0007] Substrate;

[0008] A sacrificial layer is formed on the first surface of the substrate.

[0009] A first silicon nitride layer is formed on the sacrificial layer and parallel to the substrate. The first silicon nitride layer has a continuous and stepless plane. The first silicon nitride layer includes a bandgap thin film region and an outer periphery surrounding the bandgap thin film region. The surface of the first silicon nitride layer has periodically arranged bandgap holes, each bandgap hole surrounded by a release hole. The bandgap holes and release holes are distributed in the bandgap thin film region and are used to etch the sacrificial layer located between the substrate and the bandgap thin film region. The central region of the bandgap thin film region has a defect region, which does not have bandgap holes.

[0010] A second silicon nitride layer is formed on the second surface of the substrate opposite to the first surface;

[0011] In this structure, a cavity is formed between the bandgap thin film region of the first silicon nitride layer and the first surface, which allows the bandgap thin film region to be suspended, and the outer periphery is supported by the unetched portion of the sacrificial layer.

[0012] According to some embodiments of this application, band gap holes distributed in the band gap film region form multiple band gap hole groups, each band gap hole group is hexagonal, and the band gap hole groups are periodically arranged into a honeycomb structure.

[0013] According to some embodiments of this application, each bandgap aperture group forms a basic unit, the basic unit has a rectangular shape, and each of the two opposite long sides of the basic unit passes through the center of two bandgap apertures, and each of the two opposite short sides of the basic unit has a complete bandgap aperture, and the centers of the bandgap apertures of the basic unit are arranged in a regular hexagon.

[0014] The release holes are arranged in a hexagonal close-packed pattern in the region surrounded by the gap holes of the basic unit.

[0015] According to some embodiments of this application, the short side of the basic unit has a length of a, and the radius of the gap hole is r, where r = 0.25a - 0.27a.

[0016] According to some embodiments of this application, the tangential distance between the edge of the gap hole and the edge of the nearest release hole is greater than 1.5 μm.

[0017] According to some embodiments of this application, the diameter of the release hole is 1-5 μm, and the distance between the edges of two adjacent release holes is 3-18 μm.

[0018] According to some embodiments of this application, the first silicon nitride layer is polygonal in shape; the side length of the polygon is 1.5-5mm.

[0019] According to some embodiments of this application, the thickness of the substrate is 200-700 μm;

[0020] The thickness of the sacrificial layer is 0.5-2.0 μm;

[0021] The thickness of the first silicon nitride layer is 20-300 nm; and

[0022] The thickness of the second silicon nitride layer is 20-200 nm.

[0023] According to some embodiments of this application, the substrate is a sapphire substrate or a silicon substrate.

[0024] According to some embodiments of this application, the sacrificial layer is formed of silicon oxide.

[0025] According to another aspect of this application, a method for fabricating the above-mentioned bandgap thin-film structure device is also provided, comprising:

[0026] A sacrificial layer is grown after polishing the first surface of the substrate.

[0027] Simultaneously, a first silicon nitride layer is grown on the surface of the sacrificial layer and a second silicon nitride layer is grown on the second surface of the substrate;

[0028] A first photoresist is spin-coated onto the surface of the first silicon nitride layer, and then exposed and developed to obtain a photoresist pattern.

[0029] Reactive ion etching is used to pattern the photoresist and remove residual first photoresist to form bandgap vias and release vias;

[0030] A second photoresist is spin-coated onto the surface of a first silicon nitride layer having bandgap and release holes, and the second photoresist is exposed and developed to form a photoresist protective layer on the surface of the first silicon nitride layer and on the sidewalls of the bandgap and release holes.

[0031] After the photoresist protective layer is formed, a portion of the sacrificial layer is etched through the bandgap vias and release vias using a wet etching solution to form a cavity, thereby forming the bandgap thin film region and the outer periphery surrounding the bandgap thin film region.

[0032] According to some embodiments of this application, the process for growing a first silicon nitride thin film and a second silicon nitride thin film includes: a SiH2Cl2 flow rate of 10 sccm-25 sccm, an NH3 flow rate of 30-90 sccm, a process pressure of 150 mTorr-250 mTorr, and a furnace tube temperature of 700-850°C.

[0033] According to some embodiments of this application, the method for fabricating millimeter-scale bandgap thin film structure devices further includes: removing the substrate from the wet etching solution to obtain a planar silicon nitride bandgap thin film layer, cleaning, removing adhesive, and drying;

[0034] The drying process employs critical point drying technology.

[0035] Compared with the prior art, this application has at least the following beneficial effects:

[0036] This application provides a millimeter-scale bandgap thin film structure device, including a substrate and a silicon nitride bandgap thin film layer and a silicon nitride layer respectively grown on both sides of the substrate; wherein, there is a cavity between the silicon nitride bandgap thin film layer and the substrate; in particular, the cavity between the silicon nitride bandgap thin film layer and the substrate in this application is supported by a sacrificial layer; the silicon nitride bandgap thin film layer is a planar structure with bandgap holes.

[0037] This application also provides a method for fabricating millimeter-scale bandgap thin film structure devices, which utilizes a design that combines a sacrificial layer structure with bandgap thin film release holes, and combines buffered oxide etchant (BOE) to perform wet etching on the sacrificial layer. The etching process is stable, no bubbles are generated, and the etching rate is significantly improved.

[0038] The method for fabricating millimeter-scale bandgap thin film structures in this application can also solve the problem that the traditional KOH etching method is not applicable to sapphire systems. The bandgap thin films prepared by the method of this application have advantages such as low dielectric loss and high mechanical strength, making them more suitable for quantum device applications and applicable to a wider range of quantum and micro / nano devices. Attached Figure Description

[0039] Figure 1 This is a process flow diagram for fabricating bandgap thin-film structure devices according to an example embodiment of this application.

[0040] Figure 2 This is a detailed fabrication process flow diagram of the bandgap thin-film structure device according to an example embodiment of this application.

[0041] Figure 3 This is a detailed view of the etching pattern of the bandgap thin film and the densely packed hexagonal release holes in an example embodiment of this application.

[0042] Figure 4 The image shows the front bandgap pattern (left) and the back etching window (right) of a traditional bandgap thin film.

[0043] Figure 5 This is a flowchart of the fabrication process for traditional bandgap thin-film structure devices.

[0044] Figure 6 This is a schematic diagram of the combination of the bandgap thin film structure (X) and the bottom electrode (Y) in an example embodiment of this application.

[0045] Figure 7 This is a schematic diagram illustrating the preparation of Comparative Example 5 of this application.

[0046] Figure 8 This is a schematic diagram illustrating the corrosion effect of an example embodiment and a comparative example of this application.

[0047] Figure 9 This is a characterization diagram of a bandgap thin film from an example embodiment of this application.

[0048] Figure 10 This is a schematic diagram of the basic unit of the first silicon nitride layer in an example embodiment of this application. Detailed Implementation

[0049] The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0050] It should be particularly noted that similar substitutions and modifications made to this application are obvious to those skilled in the art, and they are all considered to be included in this application. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this application to implement and apply the technology of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0051] Unless otherwise specified, this application is conducted under standard conditions or conditions recommended by the manufacturer. The raw materials or excipients used, as well as the reagents or instruments used, whose manufacturers are not specified, are all conventional products that can be obtained commercially.

[0052] The following is a detailed description of this application.

[0053] Current silicon-based substrate etching processes using potassium hydroxide (KOH) typically involve first depositing a functional thin film, such as silicon nitride (Si3N4), on the silicon substrate, and then removing part of the silicon substrate using wet etching to obtain a locally suspended thin film structure. However, this method has the following problems:

[0054] Excessive etching time: KOH has a limited etching rate for silicon and exhibits strong directional etching. To achieve complete release, an immersion process of 6-8 hours is often required to fully release a 200μm thick silicon substrate. This not only significantly prolongs the process cycle but also increases the risk of film damage during etching.

[0055] Bubble and Stress Issues: The chemical reaction process of KOH etching silicon generates bubbles. The retention of these bubbles within the release channels can cause non-uniform mechanical stress in localized areas of the thin film, leading to structural warping, deformation, and even direct film rupture. This problem is particularly severe for high-precision, high-consistency quantum devices.

[0056] Insufficient selectivity and compatibility: KOH is highly corrosive and may damage other functional or protective layers during prolonged processing. Furthermore, silicon substrates are not ideal for all devices. For example, sapphire substrates often perform better in applications requiring low dielectric loss or high mechanical stability, but traditional KOH etching methods are not suitable for sapphire systems.

[0057] Unstable process yield: Because the etching process depends on the concentration, temperature, and hydrodynamic conditions of the solution, the repeatability between experiments is poor, often leading to large fluctuations in release success rate and making it difficult to guarantee device yield. This problem directly restricts the scalability of the process, especially in large-scale fabrication or device array fabrication.

[0058] To address the aforementioned technical problems, this application provides the following technical solutions.

[0059] This application provides a method for fabricating the above-mentioned bandgap thin-film structure device, such as... Figure 1 As shown, it includes:

[0060] Step I: After polishing the first surface of the substrate 1, a sacrificial layer 2 is grown; at the same time, a first silicon nitride layer 3 is grown on the surface of the sacrificial layer 2 and a second silicon nitride layer 4 is grown on the second surface of the substrate.

[0061] Step II: Spin-coat the first photoresist 5 onto the surface of the first silicon nitride layer 3, expose and develop to obtain a photoresist pattern.

[0062] Step III: Reactive ion etching of the photoresist pattern and removal of residual first photoresist 5 to form bandgap via C and release via D.

[0063] Steps IV-V: Spin-coat the second photoresist 6 onto the surface of the first silicon nitride layer 3, on which the bandgap via C and the release via D are formed; expose and develop the second photoresist to form a photoresist protective layer on the surface of the first silicon nitride layer 3 and on the sidewalls of the bandgap via C and the release via D; after forming the photoresist protective layer, etch a portion of the sacrificial layer 2 through the bandgap via C and the release via D using a wet etching solution to form a cavity F, thereby forming the bandgap thin film region A and the outer periphery B surrounding the bandgap thin film region.

[0064] In some specific examples, the processes for growing the first silicon nitride film 3 and the second silicon nitride film 4 include: a SiH2Cl2 flow rate of 10 sccm-25 sccm, an NH3 flow rate of 30-90 sccm, a process pressure of 150 mTorr-250 mTorr, and a furnace tube temperature of 700-850℃.

[0065] In some specific examples, the fabrication method of millimeter-scale bandgap thin film structure devices further includes: removing the substrate with the planar silicon nitride bandgap thin film layer from the wet etching solution, cleaning, removing the adhesive, and drying; wherein the drying adopts critical point drying technology.

[0066] Critical point drying is a physical method primarily used for sample preparation in scanning electron microscopy (SEM). Its core objective is to completely eliminate the destructive effects of liquid surface tension when drying samples with complex and intricate microstructures (especially biological soft tissues and porous materials), thereby perfectly preserving the original three-dimensional morphology of the sample. The main steps include: sample fixation – dehydration (gradient ethanol / acetone) – displacement (pure intermediate solvent) – CO2 displacement in a critical point dryer – heating and pressurizing to a supercritical state – slow depressurization – sample removal.

[0067] According to another aspect of this application, this application also provides a bandgap thin film structure device prepared by the above-described preparation method, comprising: a substrate 1, a sacrificial layer 2, a first silicon nitride layer 3, and a second silicon nitride layer 4.

[0068] The sacrificial layer 2 is formed on the first surface of the substrate 1.

[0069] A first silicon nitride layer 3 is formed on the sacrificial layer 2 and parallel to the substrate 1. The first silicon nitride layer 3 has a continuous and stepless plane. The first silicon nitride layer 3 includes a bandgap thin film region A and an outer periphery B surrounding the bandgap thin film region. The surface of the first silicon nitride layer has bandgap holes C arranged in a periodic pattern. Each bandgap hole is surrounded by a release hole D. The bandgap holes C and the release holes D are distributed in the bandgap thin film region A. The bandgap holes and the release holes are used to etch the sacrificial layer 2 located between the substrate 1 and the bandgap thin film region A. The central region of the bandgap thin film region A has a defect region E. The defect region E does not have bandgap holes C.

[0070] The second silicon nitride layer 4 is formed on the second surface of the substrate 1 opposite to the first surface.

[0071] A cavity F is formed between the bandgap thin film region A of the first silicon nitride layer and the first surface of the substrate 1, suspending the bandgap thin film region A. The outer periphery B is supported by the unetched portion of the sacrificial layer.

[0072] In some specific examples, the band gap holes C distributed in the band gap film region A form multiple band gap hole groups Z, each band gap hole group Z is hexagonal, and the band gap hole groups Z are periodically arranged into a honeycomb structure.

[0073] In some specific examples, each bandgap aperture group Z forms a basic unit U, which has a rectangular shape, and each of the two opposite long sides of the basic unit U passes through the center of two bandgap apertures C. Each of the two opposite short sides of the basic unit has a complete bandgap aperture C, and the centers of the bandgap apertures C of the basic unit are arranged in a regular hexagon. The release apertures D are arranged in a hexagonal close-packed arrangement in the region surrounded by the bandgap apertures C of the basic unit U.

[0074] In some specific examples, such as Figure 10 As shown, the short side of the basic unit U has a length of *a*, and the radius of the bandgap aperture C is *r*, where *r* = 0.25a - 0.27a. The internal tangent distance *d* between the edge of the bandgap aperture C and the edge of the nearest release aperture D is greater than 1.5 μm. The diameter of the release aperture is 1-5 μm, and the distance between the edges of two adjacent release apertures is 3-18 μm.

[0075] In some specific examples, the sacrificial layer 2 is formed of silicon oxide. The substrate 1 is a sapphire substrate or a silicon substrate.

[0076] In some specific examples, the first silicon nitride layer 3 is polygonal in shape; the side length of the polygon is 1.5-5 mm.

[0077] In some specific examples, the thickness of substrate 1 is 200-700 μm; the thickness of sacrificial layer 2 is 0.5-2.0 μm; the thickness of first silicon nitride layer 3 is 20-300 nm; and the thickness of second silicon nitride layer 4 is 20-200 nm.

[0078] In some specific examples, this application employs a hierarchical structure of "sapphire substrate—silicon oxide sacrificial layer—bandgap film," selectively removing the silicon oxide sacrificial layer via BOE wet etching to achieve rapid, low-defect release of the bandgap film. Specifically, replacing the Si substrate + KOH wet release path with a SiO2 sacrificial layer + BOE wet release avoids long-term corrosion and bubble problems from the source, significantly shortening the release time and improving yield.

[0079] In some instances, this application adopts the following approach:

[0080] The substrate is selected from single- or double-polished c-plane sapphire (low dielectric loss, mechanical stability). The sacrificial layer material is selected from SiO2 (thickness 0.5-2.0 μm). The bandgap film material can be selected from high-stress silicon nitride (stress 0.8–1.2 GPa) to achieve a higher mechanical quality factor. In addition to the bandgap pattern, additional release holes are required. The release holes are designed to minimize release time while ensuring yield. The parameters include: hole diameter, arrangement, hole density, and safe distance from the bandgap holes.

[0081] In some instances, the method for fabricating bandgap thin-film structure devices of this application (such as...) Figure 2 (As shown) includes:

[0082] Step I-II: Preparation of sacrificial layer: Take a sapphire substrate that is polished on one or both sides, select one polished side, and grow a 0.8–1.5 μm SiO2 sacrificial layer using plasma chemical vapor deposition (PECVD).

[0083] Optionally, the SiO2 growth process parameters are as follows: SiH4 (concentration of 5%, diluted with Ar gas) flow rate of 100 sccm, N2O flow rate of 460 sccm, process pressure of 80 Pa, RF power of 50 W, growth temperature of 350℃, and growth time determined according to different sacrificial layer thicknesses.

[0084] Step III: Preparation of the first and second silicon nitride layers: On a sapphire substrate with a pre-grown sacrificial layer, low-pressure chemical vapor deposition (LPCVD) is used to grow Si3N4 thin films with high pre-stress (0.8-1.2 GPa) on both sides, with a thickness ranging from 20-200 nm. The process parameters for Si3N4 film growth are as follows: SiH2Cl2 flow rate 10-25 sccm, NH3 flow rate 30-90 sccm, process pressure 150-250 mTorr, and furnace temperature 700-850℃. The growth time is determined according to the different film thicknesses.

[0085] The first silicon nitride layer has a polygonal structure, and its size can be adjusted by changing the lattice period number of the phononic crystal, typically ranging from 1.5 to 5 mm. The surface of the first silicon nitride layer has periodically arranged bandgap holes, each surrounded by a release hole. The bandgap holes and the release holes are used to etch a portion of the sacrificial layer. The central region of the first silicon nitride layer has a defect region, which does not have bandgap holes. Optionally, the diameter of the bandgap holes is between tens and hundreds of micrometers.

[0086] Optionally, the band gap holes of this application form multiple band gap hole groups, each band gap hole group is hexagonal, and the band gap hole groups are periodically arranged in a honeycomb structure; each of the band gap hole groups forms a basic unit, the basic unit has a rectangular shape, and each of the two opposite long sides of the basic unit passes through the center of two band gap holes, and each of the two opposite short sides of the basic unit has one band gap hole, and the centers of the band gap holes of the basic unit are arranged in a regular hexagon; the release holes are arranged in a hexagonal close-packed arrangement in the area surrounded by the band gap holes of the basic unit.

[0087] Further optionally, the side length of the short side of the basic unit is a, and the radius of the band gap hole is r, where r = 0.25a - 0.27a.

[0088] In addition to the bandgap orifice, the BOE release process employed in this invention requires additional release orifices to accelerate the release process and prevent excessive corrosion of the Si3N4 in the thin film region by BOE during release. The release orifices are arranged in a hexagonal close-packed configuration, with a diameter of 1-5 μm and a distance of 3-18 μm between the inner edges of two release orifices. Specifically, the inner tangent distance d between the edge of the bandgap orifice and the edge of the nearest release orifice must be greater than 1.5 μm (e.g., ...). Figure 3 This is to prevent fractures caused by stress concentration.

[0089] Step IV-V Pattern Preparation: S1813 photoresist is spin-coated onto the surface of the first silicon nitride layer, exposed, and developed to form the photoresist pattern of the designed bandgap film.

[0090] Optionally, the pre-spreading speed is 600 rpm for 6 seconds, the spin coating speed is 3000 rpm for 60 seconds, and the baking temperature is 115℃ for 120 seconds. The above pattern is then exposed using a laser direct-write system, followed by development with MIF319 developer for 60 seconds to form the photoresist pattern of the designed bandgap film.

[0091] Step VI: Etching the first silicon nitride layer: Reactive ion etching (RIE) is used. First, oxygen is introduced to remove residual photoresist in the developing area. Optionally, the O2 flow rate is 50 sccm, the pressure is 5 Pa, the RF power is 50 W, and the time is 20 seconds. Then, CHF3 and oxygen are used to etch the silicon nitride. Optionally, the CHF3 flow rate is 50 sccm, the O2 flow rate is 10 sccm, the pressure is 2 Pa, and the RF power is 100 W. For a 100 nm Si3N4 thin film, the etching time is 2 minutes and 40 seconds.

[0092] Step VII: Removal of Residual Photoresist: Perform ultrasonic cleaning sequentially with acetone and isopropanol, followed by drying with a nitrogen gun. Then, treat with O2 plasma to remove any remaining photoresist.

[0093] Step VIII: Spin-coating protective adhesive: Re-spin-coating S1813 photoresist onto the etched Si3N4 film surface as a BOE protective adhesive to prevent excessive BOE corrosion of the Si3N4 in the film area during the release process.

[0094] Optionally, the pre-spinning speed is 600 rpm for 6 seconds, the spin coating speed is 3000 rpm for 60 seconds, and the baking temperature is 115℃ for 120 seconds.

[0095] Step IX: Patterning of the Protective Resin: The diameter of the bandgap vias is reduced by 200-300 μm, and the diameter of the release vias is reduced by 200-800 nm. Then, a laser direct-write system is used for overlay etching, followed by development, so that the protective resin coverage area is slightly larger than the Si3N4 thin film area. This avoids exposure of the silicon nitride portion due to overlay errors and also protects the edge areas of the vias. The developed protective resin is then post-baked. O2 plasma is used to treat the areas not covered by photoresist after exposure and development to remove any possible residual photoresist.

[0096] Step X: Wet Release: Release is performed using buffered oxide etchant (BOE), which can be prepared with a volume ratio of HF:NH4F = 1:6 (commercially common specifications). The sample is immersed in the BOE at room temperature for a duration depending on the thickness of the sacrificial layer. This partially etches the silicon oxide sacrificial layer in the thin film region to create a cavity, resulting in a suspended Si3N4 thin film supported by the unetched portion of the sacrificial layer.

[0097] Step XI: Removing the Protective Gel: Remove the released film from the BOE and place it in a container filled with deionized water. Then remove the film, replace the deionized water, and re-place the film to terminate the reaction. Next, soak the film in NMP to thoroughly remove the protective gel. The soaking temperature is 80℃, and the time is approximately 1 hour. Special attention should be paid to the process of transferring from the aqueous phase to the NMP solution. To avoid convective impacts that could damage the film due to the large density difference between the two solvents, a solvent displacement buffer transition method can be used: by using a dropper or pipette, add NMP to the system in small amounts multiple times while simultaneously removing a portion of the aqueous phase. This allows the composition of the solution environment to gradually change, achieving a smooth transition from the aqueous phase to the NMP phase, thereby effectively protecting the integrity of the suspended film.

[0098] Furthermore, the preparation method of this application also includes critical point drying: to avoid film collapse caused by nitrogen gun drying, critical point drying technology is employed. The desizing film is transferred to an alcohol solution using the same solvent displacement buffer transition method described above, and critical point drying is achieved using a carbon dioxide critical dryer. The dryer replaces the existing solvent (alcohol) with liquid CO2, and by heating the CO2 to above its critical point (31°C, 7.38 MPa) under high pressure, a smooth liquid-to-gas transition is achieved, thereby avoiding structural collapse caused by capillary forces. This method can effectively ensure the flatness and integrity of large-size suspended films.

[0099] The technical solution of this application will be further described below with reference to specific embodiments.

[0100] Example 1 Sapphire substrate

[0101] A 500μm double-sided polished sapphire substrate was used. On the first surface of the substrate, a SiO2 sacrificial layer with a thickness of 1.5μm was grown by plasma chemical vapor deposition. The SiO2 growth process parameters were: SiH4 flow rate of 100sccm, N2O flow rate of 460sccm, process pressure of 80 Pa, RF power of 50W, and growth temperature of 350℃.

[0102] A 100nm first silicon nitride layer is grown on the surface of the sacrificial layer, and a second silicon nitride layer is grown on the second surface of the substrate. The process parameters for growing the Si3N4 layer are: SiH2Cl2 flow rate of 10-25 sccm, NH3 flow rate of 30-90 sccm, process pressure of 150-250 mTorr, and furnace tube temperature of 700-850℃.

[0103] S1813 photoresist was spin-coated onto the surface of the first Si3N4 layer. The pre-coating speed was 600 rpm for 6 seconds, the spin-coating speed was 3000 rpm for 60 seconds, and the baking temperature was 115℃ for 120 seconds. Then... Figure 3 The pattern shown was exposed using a laser direct writing system, and then developed using MIF319 developer for 60 seconds to form the photoresist pattern of the designed bandgap film.

[0104] Reactive ion etching was used, with oxygen introduced to remove residual photoresist in the developing area. The parameters were: O2 flow rate 50 sccm, pressure 5 Pa, RF power 50 W, and etching time 20 seconds. Then, silicon nitride was etched with CHF3 and oxygen to prepare bandgap vias (80 μm diameter) and release vias (the release vias were arranged in a hexagonal close-packed configuration, with a diameter of 5 μm, a distance of 3 μm between the inner edges of two release vias, and an inner tangent distance d of 1.6 μm between the edge of the bandgap via and the edge of the nearest release via). The etching parameters were: CHF3 flow rate 50 sccm, O2 flow rate 10 sccm, pressure 2 Pa, RF power 100 W, and etching time 2 minutes 40 seconds to remove residual photoresist.

[0105] S1813 protective adhesive was re-spin-coated onto the etched Si3N4 thin film surface. The pre-coating speed was 600 rpm for 6 seconds, the spin coating speed was 3000 rpm for 60 seconds, the baking temperature was 115℃ for 120 seconds, and the protective adhesive was patterned. After development, the protective adhesive was post-baked at 100℃ for 90 seconds to remove residual adhesive.

[0106] The sample was placed in 10 mL of buffer oxide etching solution HF:NH4F=1:6 at room temperature and immersed for 20-30 min to obtain a suspended Si3N4 film supported by a sacrificial layer. The protective adhesive was removed and critical point drying was performed.

[0107] Example 2

[0108] The substrate is silicon, and the fabrication process is the same as in Example 1, except that the substrate is used.

[0109] Example 3

[0110] The fabrication process for the bandgap thin film structure device is basically the same as that in Example 1, except that the aperture of the release hole is 1 μm, the distance between the inner tangent edges of the two release holes is 9 μm, and the inner tangent distance d between the edge of the bandgap hole and the edge of the nearest release hole is 1.5 μm.

[0111] Example 4

[0112] The fabrication process for the bandgap thin film structure device is basically the same as that in Example 1, except that the aperture of the release hole is 4 μm, the distance between the inner tangent edges of the two release holes is 18 μm, and the inner tangent distance d between the edge of the bandgap hole and the edge of the nearest release hole is 2 μm.

[0113] Comparative Example 1: Traditional processes for fabricating bandgap thin-film structure devices, such as... Figure 5 As shown.

[0114] Step I: A silicon nitride thin film 222 is grown on both sides of a silicon substrate 111.

[0115] Step II: Uniformly spin-coat a layer of photoresist onto the surface of the silicon nitride thin film, and then apply the designed bandgap pattern (e.g., ...) Figure 4 (As shown) The sample is transferred to the photoresist layer through a photomask and exposed using ultraviolet light. After exposure, the sample is placed in a developing solution to develop, resulting in the desired patterned photoresist mask on the surface of the silicon nitride film.

[0116] Step III: In a reactive ion etching (RIE) apparatus, a silicon nitride thin film is plasma etched using a gas system such as CHF3 / O2 or CF4 / O2. A corresponding bandgap aperture structure 333 is formed on the front side of the silicon nitride thin film, and a KOH etching window 444 is formed on the back side.

[0117] Step IV: After etching, the residual photoresist is completely removed using an organic solvent NMP and O2 plasma stripping process to avoid adverse effects on subsequent release and device performance. At this point, only the required bandgap structure remains on the surface of the silicon nitride film.

[0118] Step V: Place the patterned silicon nitride thin film sample in a potassium hydroxide solution for 6-8 hours. The potassium hydroxide etches from the window area on the reverse side of the pattern. By utilizing the anisotropic etching effect of KOH on the silicon substrate, specific areas of the substrate are gradually removed, thereby forming a suspended thin film structure.

[0119] Comparative Example 2

[0120] The fabrication process for bandgap thin film structure devices is basically the same as that in Example 1, except that there are no release holes between the bandgap holes.

[0121] The proportional release time is long, such as Figure 8 As shown in the figure, the left figure shows the case without release holes: after 1 hour, the release was still not complete, and the original structure of the silicon nitride film has been destroyed by BOE. The circled part is the original arrangement of the bandgap holes, but due to the long-term corrosion of BOE, the SiN (polygonal part) has been corroded.

[0122] The image on the right shows the case with release holes (densely arranged small holes are release holes, and hexagonally arranged holes are band gap holes). It only takes half an hour to completely release the silicon nitride (light-colored part) and the silicon nitride is well preserved.

[0123] Comparative Example 3

[0124] The fabrication process for the bandgap thin film structure device is basically the same as that in Example 1, except that the diameter of the release hole is 8 μm and the distance between the inner edges of the two release holes is 20 μm.

[0125] The sacrificial layer release time is relatively long in this proportion.

[0126] Comparative Example 4

[0127] The fabrication process for the bandgap thin film structure device is basically the same as that in Example 1, except that the aperture of the release hole is 0.1 μm and the distance between the inner edges of the two release holes is 1 μm.

[0128] The sacrificial layer release time is relatively long in this proportion.

[0129] Comparative Example 5

[0130] The fabrication process for conventional MEMS devices mainly includes the following steps:

[0131] A SiO2 sacrificial layer 22 is grown on the surface of a double-polished sapphire substrate 11. The sacrificial layer 22 is etched to obtain a sacrificial layer pattern. A Si3N4 thin film 33 is grown on both sides of the sapphire substrate with the sacrificial layer pattern. A release window 44 is formed by photolithography / etching. The sacrificial layer 22 is removed by BOE etching through the release window to form a cavity.

[0132] like Figure 7 As shown, the MEMS device fabricated by this process has a step.

[0133] Comparative Example 6

[0134] The substrate was prepared using a conventional process as described in Comparative Example 1, with a sapphire substrate.

[0135] A suspended silicon nitride film is prepared by directly growing a silicon substrate on a silicon substrate and creating etching windows in the silicon nitride film through processes such as spin coating, exposure / development, and etching, so that the silicon surface in the etching window area is completely exposed. The sample is then immersed in a potassium hydroxide solution.

[0136] However, since the substrate is sapphire, potassium hydroxide does not react with it, and this traditional process cannot be adapted to sapphire substrates.

[0137] As can be seen from the above embodiments and comparative examples, this application solves the problems of long time consumption and low yield in the traditional KOH etching method for silicon substrates by introducing a silicon oxide sacrificial layer and BOE rapid release process, and provides a brand-new solution for the high-quality preparation of high Q value bandgap films.

[0138] Compared with Comparative Example 5, the bandgap film of this application is a continuous and stepless planar structure, which is stable and has a high device yield.

[0139] Compared to Comparative Example 6, this application uses double-polished or single-polished c-plane sapphire as the substrate. Sapphire has advantages such as low dielectric loss and high mechanical stability, avoiding many problems encountered by silicon substrates during the etching process. Furthermore, the use of a sapphire substrate allows the laser to penetrate directly, and the dissipation caused by the sapphire substrate is less than that of conventional substrates, improving device performance. This method also makes it directly compatible with the process requirements of quantum devices and high-performance micro / nano structures (such as microwave resonant cavity circuit chips), providing a solid foundation for the large-size fabrication of high-Q thin films.

[0140] The silicon dioxide sacrificial layer in this application features controllable thickness and excellent etching selectivity, ensuring uniform etching rate and clear edges during subsequent release processes. This prevents unnecessary erosion of the functional thin film, significantly improving release controllability and overall device success rate. At room temperature, a 1μm thick silicon dioxide sacrificial layer can be completely etched in approximately 30 minutes, resulting in a fully suspended bandgap film. Furthermore, a sacrificial layer remains in certain areas of the final device to support the film. Compared to traditional KOH etching of silicon substrates, which takes 6-8 hours and involves bubble formation, the method in this application significantly shortens the process time, provides a smooth release process with almost no bubble generation, exhibits an extremely low film collapse rate, and achieves a release success rate approaching 100%. This step is the core of this invention, enabling rapid, stable, and high-yield fabrication.

[0141] The oscillator thin film prepared in this application can be bonded to a superconducting electrode chip for efficient integration (e.g., Figure 6As shown in the diagram, the entire structure is then placed into a Fabry-Perot optical cavity for optical coupling. Compared to oscillators obtained through conventional processes, the latter require opening holes in the silicon substrate on the superconducting electrode side to allow light to enter due to the opaque substrate, resulting in a complex process that is prone to introducing additional defects. In contrast, the high-stress silicon nitride oscillator film obtained by the method of this invention has excellent light transmittance, allowing light to be directly incident and coupled from one side of the oscillator. This eliminates the need for substrate perforation, significantly simplifying the process and improving the overall mechanical stability and optical performance of the structure.

[0142] This type of transparent high-Q oscillator film can also be widely used in cutting-edge research and device development fields such as optomechanical coupling experiments, quantum state preparation and detection, weak force detection, and low-noise sensing, and has good versatility and promotion value.

[0143] The release integrity and smoothness of the film were characterized using 3D laser microscopy and SEM, such as... Figure 9 As shown.

[0144] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of this application. It should be noted that, for those skilled in the art, several improvements and modifications can be made to this application without departing from the principles of this application, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A bandgap thin-film structure device, characterized in that, include: Substrate; A sacrificial layer is formed on the first surface of the substrate; A first silicon nitride layer is formed on the sacrificial layer and parallel to the substrate. The first silicon nitride layer has a continuous and stepless plane. The first silicon nitride layer includes a bandgap thin film region and an outer periphery surrounding the bandgap thin film region. The surface of the first silicon nitride layer has periodically arranged bandgap holes, each of which is surrounded by a release hole. The bandgap holes and the release holes are distributed within the bandgap thin film region. The diameter of the release holes is smaller than that of the bandgap holes. The bandgap holes and the release holes are used to etch the sacrificial layer located between the substrate and the bandgap thin film region. The central region of the bandgap thin film region has a defect region, which does not have the bandgap holes. A second silicon nitride layer is formed on the second surface of the substrate opposite to the first surface; Wherein, a cavity is formed between the bandgap thin film region of the first silicon nitride layer and the first surface, suspending the bandgap thin film region, and the outer periphery is supported by the unetched portion of the sacrificial layer.

2. The bandgap thin-film structure device according to claim 1, characterized in that, The band gap holes distributed in the band gap film region form multiple band gap hole groups, each of the band gap hole groups is hexagonal, and the band gap hole groups are periodically arranged into a honeycomb structure.

3. The bandgap thin-film structure device according to claim 2, characterized in that, Each of the bandgap aperture groups forms a basic unit, the basic unit having a rectangular shape, and each of the two opposite long sides of the basic unit passes through the center of the two bandgap apertures, and each of the two opposite short sides of the basic unit has a complete bandgap aperture, the centers of the bandgap apertures of the basic unit being arranged in a regular hexagon. The release holes are arranged in a hexagonal close-packed pattern in the region surrounded by the gapped holes of the basic unit.

4. The bandgap thin-film structure device according to claim 3, characterized in that, The short side of the basic unit has a length of a, and the radius of the gap hole is r, where r = 0.25a - 0.27a.

5. The bandgap thin-film structure device according to claim 4, characterized in that, The tangential distance between the edge of the gap hole and the edge of the nearest release hole is greater than 1.5 μm.

6. The bandgap thin-film structure device according to claim 5, characterized in that, The diameter of the release hole is 1-5 μm, and the distance between the edges of two adjacent release holes is 3-18 μm.

7. The bandgap thin-film structure device according to any one of claims 1-6, characterized in that, The bandgap thin film region is polygonal, and the side length of the polygon is 1-5 mm.

8. The bandgap thin-film structure device according to any one of claims 1-6, characterized in that, The thickness of the substrate is 200-700 μm; The thickness of the sacrificial layer is 0.5-2.0 μm; The thickness of the first silicon nitride layer is 20-300 nm; and The thickness of the second silicon nitride layer is 20-300 nm.

9. The bandgap thin-film structure device according to claim 8, characterized in that, The substrate is a sapphire substrate or a silicon substrate; The sacrificial layer is formed of silicon oxide.

10. A method for fabricating a bandgap thin-film structure device according to any one of claims 1-9, characterized in that, include: A sacrificial layer is grown after polishing the first surface of the substrate. Simultaneously, a first silicon nitride layer is grown on the surface of the sacrificial layer and a second silicon nitride layer is grown on the second surface of the substrate; A first photoresist is spin-coated onto the surface of the first silicon nitride layer, and then exposed and developed to obtain a photoresist pattern. Reactive ion etching is used to etch the photoresist pattern and remove any remaining first photoresist to form bandgap vias and release vias; A second photoresist is spin-coated onto the surface of the first silicon nitride layer on which the bandgap aperture and the release aperture are formed, and the second photoresist is exposed and developed to form a photoresist protective layer on the surface of the first silicon nitride layer and on the sidewalls of the bandgap aperture and the release aperture. After the photoresist protective layer is formed, the sacrificial layer between the substrate and the bandgap thin film region is etched through the bandgap via and the release via using a wet etching solution to create the cavity, thereby forming the bandgap thin film region and the outer periphery surrounding the bandgap thin film region.

11. The method for fabricating a bandgap thin-film structure device according to claim 10, characterized in that, The process parameters for growing the first silicon nitride layer and the second silicon nitride layer include: SiH2Cl2 flow rate of 10 sccm-25 sccm, NH3 flow rate of 30-90 sccm, process pressure of 150 mTorr-250 mTorr, and furnace tube temperature of 700-850℃.

12. The method for fabricating a bandgap thin-film structure device according to claim 10, characterized in that, Also includes: The bandgap thin film structure device is removed from the wet etching solution and then cleaned, de-adhesive removed, and dried. The drying process employs critical point drying technology.

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