Suspended photonic crystal film and preparation method thereof

Abstract

The invention belongs to the technical field of nano photonic crystal micromachining, and particularly relates to a suspended photonic crystal film and a preparation method thereof. Sequentially depositing an aluminum oxide layer, a silicon dioxide layer and a silicon nitride layer on the silicon substrate, and then forming a patterned photonic crystal structure by etching the silicon nitride layer; part of the silicon substrate is removed through dry etching, the exposed aluminum oxide layer and the exposed silicon dioxide layer are sequentially removed through a wet etching or dry etching synergistic process, and the suspended photonic crystal film is obtained. Through the design of the composite sacrificial layer and the inverted etching strategy, the problems of structure collapse, etching residue and low alignment precision in the traditional process are solved, and the method is suitable for the fields of Moire photonics, integrated photonics and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of nano-photonic crystal micro-machining, and in particular relates to a suspended photonic crystal film and a preparation method thereof. Background Art

[0002] Moiré superlattices are formed through tiny twist angles / lattice mismatches of two-dimensional materials or artificial lattices, providing a new platform for basic physics and applied research: in the field of electronics, magic-angle graphene has overturned traditional condensed matter theory due to its flat-band induced superconductivity and correlated insulating states; in the field of photonics, twisted double-layer photonic crystals exhibit unique optical properties such as flat-band induced light field localization, slow light effect and nonlinear enhancement through similar mechanisms, opening up new dimensions for topological photonics, quantum light sources and optoelectronic integration.

[0003] Compared to two-dimensional materials, moiré photonic crystals have a higher degree of design freedom. That is, by regulating the material's refractive index, lattice constant, interlayer twist angle, and gap, precise control of the light field state density and band structure can be achieved. However, substrate contact can introduce photon leakage and parasitic absorption, severely restricting the efficiency of light field localization and measurement accuracy. Therefore, the preparation of suspended photonic crystal films has become a necessary condition for achieving efficient optical coupling and dynamic control. However, the substrate silicon is usually several hundred microns thick, and deep silicon etching with a faster etching speed is usually used. Its etching depth is difficult to control at the nanometer level, and it is easy to cause over-etching damage to the silicon nitride layer. Summary of the Invention

[0004] The purpose of the present invention is to provide a suspended photonic crystal film and a preparation method thereof. By using a composite sacrificial layer of aluminum oxide and silicon dioxide, the probability of damage to the silicon nitride film caused by over-etching can be effectively reduced. At the same time, the residual stress at the interface between the sacrificial layer and silicon nitride can be released in stages, effectively avoiding problems such as collapse and warping of the silicon nitride suspended film.

[0005] To achieve the above object, the present invention provides a method for preparing a suspended photonic crystal film, comprising the following steps:

[0006] S1, sequentially depositing an aluminum oxide layer, a silicon dioxide layer, and a silicon nitride layer on a silicon substrate, and then etching the silicon nitride layer to form a patterned photonic crystal structure;

[0007] S2. Remove part of the silicon substrate by dry etching, and sequentially remove the exposed aluminum oxide layer and silicon dioxide layer by wet etching or dry etching coordinated process to obtain a suspended photonic crystal film.

[0008] Furthermore, the thickness of the silicon substrate is 200-800 μm; the thickness of the aluminum oxide layer is 50-200 nm; the thickness of the silicon dioxide layer is 50-500 nm; and the thickness of the silicon nitride layer is 100-800 nm.

[0009] Furthermore, the silicon substrate is removed by deep reactive ion etching technology with an etching rate of 4-6 μm / min.

[0010] Furthermore, the wet etching includes: removing the aluminum oxide layer by phosphoric acid, and the etching time is 1-5 minutes.

[0011] Furthermore, the silicon dioxide layer is removed by using a buffered oxide etching solution, and the etching time is 1-10 minutes.

[0012] Furthermore, the dry collaborative process includes: removing the aluminum oxide layer by a plasma etching process, and removing the silicon dioxide layer by a plasma etching or reactive ion etching process.

[0013] Furthermore, the aluminum oxide layer is prepared by atomic layer deposition; the silicon dioxide layer is prepared by plasma enhanced chemical vapor deposition; and the silicon nitride layer is prepared by plasma enhanced chemical vapor deposition or low pressure chemical vapor deposition.

[0014] Furthermore, the deposition temperature of the aluminum oxide layer is 120-300°C, and the precursors are trimethylaluminum and water vapor; the deposition temperature of the silicon dioxide layer is 150-350°C; and the deposition temperature of the silicon nitride layer is 700-850°C.

[0015] Furthermore, the preparation method comprises the following steps:

[0016] S1. Growing an aluminum oxide layer on a silicon substrate by atomic layer deposition, preparing a silicon dioxide layer by plasma enhanced chemical vapor deposition to form a composite sacrificial layer; growing a low stress silicon nitride layer on the composite sacrificial layer by chemical vapor deposition, and processing the silicon nitride layer by electron beam exposure combined with etching to form a photonic crystal structure;

[0017] S2. Fix the silicon nitride layer on the surface of a high-temperature resistant carrier with the silicon substrate facing upward to form an inverted structure, and remove the back silicon substrate through photolithography combined with deep reactive plasma etching process; selectively release the composite sacrificial layer through a wet etching or dry etching collaborative process to obtain a suspended silicon nitride photonic crystal film.

[0018] The present invention also provides a suspended photonic crystal film obtained by any of the above preparation methods.

[0019] Furthermore, the silicon nitride layer is a periodic hole array, the lattice type of which includes at least one of tetragonal, hexagonal, triangular, and cage mesh, the hole diameter is 50-500nm, and the lattice constant is 100-1000nm.

[0020] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0021] 1. The present invention provides a method for fabricating a suspended photonic crystal film. By hollowing out a silicon substrate and incorporating a collaborative aluminum oxide / silicon oxide composite sacrificial layer, this creates a suspended silicon nitride structure enclosed in an air environment. This design eliminates the substrate constraints of traditional designs, reduces photon leakage and parasitic losses, and significantly improves the quality factor of the photonic crystal. Furthermore, the mechanical freedom of the suspended film supports external force control—dynamic modulation of the lattice constant and symmetry through stretching or torsion—providing a reconfigurable vehicle for moiré photonic crystal band engineering, topological photon state manipulation, and on-chip quantum light source integration.

[0022] 2. The aluminum oxide / silicon oxide composite sacrificial layer design employed in this invention creates a dual etch stop during the deep silicon etch and release phases, reducing the probability of damage to the silicon nitride film caused by overetching. Furthermore, the composite sacrificial layer, in synergistic dry / wet processes, can phase-release residual stress at the sacrificial layer-silicon nitride interface, effectively preventing problems such as collapse and warping of the suspended silicon nitride film. The silicon dioxide layer further reduces the density of interface defect states, improving the optical transmission efficiency of the silicon nitride photonic crystal.

[0023] 3. The present invention solves the problems of structural collapse, etching residue and low alignment accuracy in traditional processes through a composite sacrificial layer design and an inverted etching strategy, and is suitable for fields such as moiré photonics and integrated photonics. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the preparation flow chart of suspended photonic crystal films.

[0025] Figure 2 This is an electron microscope image of a photonic crystal structure (after being suspended) prepared in the example.

[0026] Figure 3 This is an electron microscope image of another photonic crystal structure (after being suspended) prepared in the example. DETAILED DESCRIPTION

[0027] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0028] The present invention provides a method for preparing a suspended photonic crystal film, comprising the following steps:

[0029] S1. Growing an aluminum oxide layer on a silicon substrate by atomic layer deposition, preparing a silicon dioxide layer by plasma enhanced chemical vapor deposition to form a composite sacrificial layer; growing a low stress silicon nitride layer on the composite sacrificial layer by chemical vapor deposition, and processing the silicon nitride layer by electron beam exposure combined with etching to form a photonic crystal structure;

[0030] S2. Fix the silicon nitride layer on the surface of a high-temperature resistant carrier with the silicon substrate facing upward to form an inverted structure. Completely remove the back silicon substrate through photolithography combined with deep reactive plasma etching process; selectively release the composite sacrificial layer through a wet etching or dry etching collaborative process to obtain a suspended silicon nitride photonic crystal film.

[0031] Preferably, the aluminum oxide layer is prepared by ALD, has a thickness of 50-200 nm, a deposition temperature of 120-300° C., and the precursors are trimethylaluminum and water vapor.

[0032] Preferably, the silicon dioxide layer is prepared by PECVD and has a thickness of 50-500 nm.

[0033] Preferably, the silicon nitride layer is prepared by PECVD or LPCVD and has a thickness of 100-800 nm.

[0034] Preferably, the silicon nitride layer photonic crystal structure is prepared by electron beam exposure and plasma etching / reactive ion etching process, and its area is 0.01×0.01mm 2 Up to 2×2mm 2 .

[0035] Preferably, the silicon nitride layer photonic crystal is a periodic hole array, the lattice type of which includes at least one of tetragonal, hexagonal, triangular, and cage mesh, the hole diameter is 50-500 nm, and the lattice constant is 100-1000 nm.

[0036] Preferably, the thickness of the high temperature resistant carrier is 200-1000 μm, and the chip is fixed to the surface of the carrier by physical adsorption or temporary bonding.

[0037] Preferably, the wet collaborative process includes: removing the aluminum oxide layer by a phosphoric acid solution with an etching time of 1-5 minutes, and removing the silicon dioxide layer by a BOE solution with an etching time of 1-10 minutes.

[0038] Preferably, the dry collaborative process includes: removing the aluminum oxide layer by a plasma etching process, and removing the silicon dioxide layer by a plasma etching / reactive ion etching process.

[0039] Example

[0040] The present invention discloses a method for preparing a suspended photonic crystal film. Figure 1 Shown, including:

[0041] Step 1: First, provide a 500μm thick single crystal silicon substrate and perform ultrasonic cleaning with acetone and isopropyl alcohol for 10 minutes and 5 minutes respectively to remove surface organic pollutants and particles, and then blow dry with nitrogen.

[0042] Step 2: Next, a 100nm aluminum oxide film is grown on the surface of the silicon substrate using ALD technology. The process parameters are: the precursors are trimethylaluminum and water vapor, and the deposition temperature is 300°C.

[0043] Step 3: A 300nm thick silicon dioxide layer is deposited on top of the aluminum oxide layer using PECVD at 300°C. This layer forms a composite sacrificial layer with the aluminum oxide layer to mitigate risks such as overetching, collapse, and warping of the suspended silicon nitride film that may occur during the following process. The silicon dioxide layer also further reduces the density of interface defect states, improving the light transmission efficiency of the silicon nitride photonic crystal.

[0044] Step 4: Then, a 200 nm silicon nitride film is grown on the composite sacrificial layer by LPCVD technology at a deposition temperature of 850°C.

[0045] Step 5: Next, a layer of photoresist (such as ARP 6200.13 or ZEP 520) is spin-coated on the silicon nitride film. Electron beam exposure and development are used to form the photonic crystal pattern. Four alignment marks are simultaneously defined at the four corners of the pattern for subsequent temporary bonding precision control.

[0046] Step 6: On this basis, the silicon nitride layer is etched by inductively coupled plasma (ICP). During the etching process, by finely controlling the etching parameters, the sidewall steepness of the silicon nitride film and the selectivity with the silicon oxide sacrificial layer can be optimized, thereby ensuring the precise preparation of the photonic crystal structure. In particular, the fine-tuning parameters are: gas: CHF3100sccm+O215sccm+Ar 50sccm, ICP power: 1200W, RF bias power: 150W, silicon nitride etching rate of approximately 110nm / min, and silicon nitride to silicon oxide selectivity of approximately 15:1.

[0047] Step 7: Then, prepare a high-temperature resistant carrier, which can be quartz, and use polyimide tape to temporarily bond the inverted chip to the quartz; spin-coat photoresist, which can be AX10XT, on the back of the silicon substrate, and finely control the spin-coating speed to make the photoresist thickness about 7μm; after photolithography exposure and development, a hollow window pattern is formed.

[0048] Step 8: After this, the silicon substrate is removed by deep reactive ion etching technology, with an etching rate of about 5μm / min. During the etching process, by precisely controlling the etching parameters (SF6200sccm+C4F8 100sccm, ICP power 1000W+RF bias power 150W), a high selectivity between silicon and photoresist or aluminum oxide is achieved. For example, the selectivity of AX10XT to silicon is about 1:100, while the selectivity of silicon to aluminum oxide is greater than 300:1. Under these parameters, the complete removal of the silicon substrate can be achieved and damage to the silicon nitride structure can be avoided.

[0049] Step 9: Next, the chip was immersed in 85% phosphoric acid solution (temperature 85° C.) for 5 minutes to remove the aluminum oxide layer; then, it was ultrasonically cleaned with deionized water, neutralized with ammonia water, and dried with nitrogen.

[0050] Step 10: The chip is then immersed in a BOE solution (buffered oxide etchant) for 6-8 minutes to remove the silicon dioxide layer. The chip is then ultrasonically cleaned with isopropyl alcohol and dried with nitrogen. The wet etching process has a high selectivity between the sacrificial layer and the silicon nitride layer, significantly reducing overetch damage to the silicon nitride layer.

[0051] Step 11: Finally, remove the polyimide tape, separate the quartz carrier, and then place the chip upright to obtain a suspended silicon nitride photonic crystal film. At this point, the entire process is completed. Figure 2 and 3 , which are electron microscope images of two silicon nitride photonic crystals with different photonic crystal structures prepared by the present invention. It can be seen that both have good regularity and do not suffer from problems such as collapse and warping.

[0052] This embodiment hollows out the silicon substrate and combines it with an aluminum oxide / silicon oxide composite sacrificial layer to create a suspended silicon nitride structure enclosed in an air environment. This design removes the substrate constraints of traditional designs, reduces photon leakage and parasitic losses, and significantly improves the quality factor of the photonic crystal. Furthermore, the mechanical freedom of the suspended membrane supports external force control—dynamic modulation of the lattice constant and symmetry through stretching or torsion—providing a reconfigurable platform for moiré photonic crystal band engineering, topological photon state manipulation, and on-chip quantum light source integration.

[0053] The aluminum oxide / silicon oxide composite sacrificial layer design adopted in this embodiment forms a double etching stop during the deep silicon etching and release stages, reducing the probability of damage to the silicon nitride film caused by over-etching; at the same time, the composite sacrificial layer can release the residual stress at the interface between the sacrificial layer and silicon nitride in stages under the coordination of dry / wet processes, effectively avoiding problems such as collapse and warping of the silicon nitride suspended film.

[0054] It will be easily understood by those skilled in the art that the above description is merely 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 in the scope of protection of the present invention.

Claims

1. A method for preparing a suspended photonic crystal film, characterized in that: The following steps are involved: S1, sequentially depositing an aluminum oxide layer, a silicon dioxide layer, and a silicon nitride layer on a silicon substrate, and then etching the silicon nitride layer to form a patterned photonic crystal structure; S2. Part of the silicon substrate is removed by etching, and the exposed aluminum oxide layer and silicon dioxide layer are removed in sequence by wet etching or dry etching to obtain a suspended photonic crystal film.

2. The method for preparing a suspended photonic crystal film according to claim 1, wherein: The thickness of the silicon substrate is 200-800 μm; the thickness of the aluminum oxide layer is 50-200 nm; the thickness of the silicon dioxide layer is 50-500 nm; and the thickness of the silicon nitride layer is 100-800 nm.

3. The method for preparing a suspended photonic crystal film according to claim 1, wherein: The silicon substrate is removed by deep reactive ion etching technology at an etching rate of 4-6 μm / min.

4. The method for preparing a suspended photonic crystal film according to claim 1, wherein: The wet etching comprises: removing the aluminum oxide layer by phosphoric acid, the etching time being 1-5 minutes; And / or, the silicon dioxide layer is removed by using a buffered oxide etchant, with the etching time being 1-10 minutes.

5. The method for preparing a suspended photonic crystal film according to claim 1, wherein: The dry etching includes: removing the aluminum oxide layer by a plasma etching process, and removing the silicon dioxide layer by a plasma etching or reactive ion etching process.

6. The method for preparing a suspended photonic crystal film according to claim 1, wherein: The aluminum oxide layer is prepared by atomic layer deposition; the silicon dioxide layer is prepared by plasma enhanced chemical vapor deposition; and the silicon nitride layer is prepared by plasma enhanced chemical vapor deposition or low pressure chemical vapor deposition.

7. The method for preparing a suspended photonic crystal film according to claim 6, wherein: The deposition temperature of the aluminum oxide layer is 120-300°C, and the precursors are trimethylaluminum and water vapor; The deposition temperature of the silicon dioxide layer is 150-350°C; The deposition temperature of the silicon nitride layer is 700-850°C.

8. The method for preparing a suspended photonic crystal film according to any one of claims 1 to 7, characterized in that: The preparation method specifically comprises the following steps: S1. Growing an aluminum oxide layer on a silicon substrate by atomic layer deposition, preparing a silicon dioxide layer by plasma enhanced chemical vapor deposition to form a composite sacrificial layer; growing a silicon nitride layer on the composite sacrificial layer by chemical vapor deposition, and processing the silicon nitride layer by electron beam exposure combined with etching to form a photonic crystal structure; S2. Fix the silicon nitride layer on the carrier surface with the silicon substrate facing upward to form an inverted structure, and remove the back silicon substrate by photolithography combined with deep reactive plasma etching process; selectively release the composite sacrificial layer by wet etching or dry etching to obtain a suspended silicon nitride photonic crystal film.

9. A suspended photonic crystal film obtained by the preparation method according to any one of claims 1 to 8.

10. The suspended photonic crystal film according to claim 9, characterized in that: The silicon nitride layer is a periodic hole array, the lattice type of which includes at least one of tetragonal, hexagonal, triangular, and cage mesh, the hole diameter is 50-500nm, and the lattice constant is 100-1000nm.

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