Multi-layer optical film structure for improving stress of DBR film layer and preparation method of multi-layer optical film structure

Abstract

The invention discloses a multilayer optical film structure for improving the stress of a DBR film layer and a preparation method of the multilayer optical film structure. The multi-layer optical film structure comprises a substrate and a multi-layer film stack arranged on the substrate, and the multi-layer film stack comprises a plurality of high refractive index layers and a plurality of low refractive index layers; the multilayer film stack further comprises at least one Al2O3 transition layer, and the at least one Al2O3 transition layer is arranged between the high refractive index layer and the low refractive index layer. The Al2O3 transition layer has a pressure stress characteristic which is opposite to the tensile stress characteristic of the DBR membrane stack, so that a stress buffer area can be formed on an interface to realize stress compensation; meanwhile, Al2O3 has a moderate thermal expansion coefficient between a high-refractive-index material and a low-refractive-index material, and plays a role in buffering thermal stress. By introducing the Al2O3 transition layer, the stress concentration between the film layers is effectively relieved, the risk of cracking and stripping of the film layers is remarkably reduced, and the mechanical stability and reliability of the device are improved.

Description

Technical Field

[0001] This invention belongs to the field of optical thin film technology, and specifically relates to a multilayer optical film structure for improving DBR film stress and its preparation method. Background Technology

[0002] In modern optoelectronic devices, such as light-emitting diode (LED) chips, lasers, and photodetectors, distributed Bragg reflectors (DBRs) play a crucial role. DBRs are typically constructed by alternating stacks of high-refractive-index (H) and low-refractive-index (L) materials, achieving high reflectivity for specific wavelengths of light through the coherent superposition of reflections from multiple interfaces.

[0003] In the preparation of DBRs, commonly used H / L material combinations include titanium dioxide (TiO2) and silicon dioxide (SiO2). However, such DBR membrane structures in the prior art have significant defects.

[0004] First, during film deposition (e.g., using electron beam evaporation (EBE)), due to film densification, cooling contraction, and ion bombardment effects, significant internal stress tends to accumulate in the film stack. This internal stress typically manifests as tensile stress. Second, the H-layer material (such as TiO2) has a coefficient of thermal expansion (TEC) of approximately 9.0 × 10⁻⁶. -6 / K) and L-layer materials (such as SiO2, whose TEC is approximately 0.5 × 10) -6 There is a significant mismatch in the coefficient of thermal expansion between the K and K regions. Furthermore, lattice mismatch between different materials can exacerbate stress at the interface.

[0005] The combined effect of the above factors leads to excessive internal stress in traditional DBR films (especially multilayer films with a relatively thick total thickness), resulting in greater brittleness of the film and making it prone to cracking and peeling during subsequent processes or device operation. This seriously affects the yield, mechanical stability and long-term reliability of the device.

[0006] Therefore, there is an urgent need in this field for a novel multilayer membrane structure to effectively reduce or compensate for the stress inside the DBR membrane layer and improve the stability of the membrane system. Summary of the Invention

[0007] In view of the technical problem in the background technology that the traditional DBR film layer is prone to cracking and peeling due to excessive internal stress (usually tensile stress) caused by factors such as material thermal expansion coefficient mismatch, lattice mismatch and deposition process, thereby affecting the mechanical stability and reliability of the device, the present invention aims to provide a multilayer optical film structure and its preparation method that can effectively alleviate film layer stress and improve structural stability.

[0008] To achieve the above objectives, the first aspect of the present invention provides a multilayer optical film structure, including a substrate and a multilayer film stack disposed on the substrate, wherein the multilayer film stack includes a plurality of high refractive index layers and a plurality of low refractive index layers;

[0009] The multilayer film stack is characterized in that it further includes at least one Al2O3 transition layer, wherein the at least one Al2O3 transition layer is disposed between the high refractive index layer and the low refractive index layer.

[0010] As a preferred embodiment, the at least one Al2O3 transition layer has compressive stress characteristics; the film stack composed of the plurality of high refractive index layers and the plurality of low refractive index layers has tensile stress characteristics; the compressive stress characteristics of the Al2O3 transition layer are used to compensate for the tensile stress characteristics of the film stack.

[0011] As a preferred embodiment, the thermal expansion coefficient of the Al2O3 transition layer is between that of the high refractive index layer and the low refractive index layer.

[0012] As a preferred embodiment, the high refractive index layer is made of TiO2, and the low refractive index layer is made of SiO2.

[0013] As a preferred embodiment, the physical thickness of the Al2O3 transition layer is 10 nm to 100 nm.

[0014] As a further preferred embodiment, the physical thickness of the Al2O3 transition layer is 30nm to 60nm.

[0015] As a preferred embodiment, the optical thickness of both the high-refractive-index layer and the low-refractive-index layer is λ / 4, where λ is the design wavelength.

[0016] As a preferred embodiment, the number of the at least one Al2O3 transition layer is two.

[0017] A second aspect of the present invention provides a method for preparing a multilayer optical film structure as described above, comprising the steps of sequentially depositing a plurality of high refractive index layers and a plurality of low refractive index layers on a substrate;

[0018] The method is characterized in that it further includes:

[0019] At least one Al2O3 transition layer is deposited between the high refractive index layer and the low refractive index layer.

[0020] As a preferred embodiment, the deposition step employs electron beam evaporation (EBE) technology, and the thickness control error of the film layer in the deposition step is within ±2%.

[0021] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0022] Stress compensation significantly reduces the total stress of the film layer: The Al2O3 transition layer introduced in this invention plays a crucial role in stress regulation. Studies have shown that Al2O3 films (especially under processes such as EBE) typically exhibit compressive stress characteristics due to their high densification and lattice compression effects caused by ion bombardment, while traditional DBR film stacks usually exhibit tensile stress characteristics. This invention introduces a compressive Al2O3 layer into a tensile DBR film stack, with the stress directions opposite, forming a "stress buffer zone" at the film interface to achieve stress compensation, thereby significantly reducing the overall net stress of the film system. This effectively reduces wafer warpage and breakage after coating, reduces the risk of film cracking in subsequent processes, and improves structural integrity.

[0023] Thermal expansion coefficient (TEC) buffering, relieving interfacial stress: Al2O3 materials have a moderate thermal expansion coefficient (approximately 7.2 × 10⁻⁶). -6 By inserting an Al2O3 transition layer with a TEC between the H and L layers, which have a large difference in TEC, the interfacial stress caused by the severe mismatch in thermal expansion coefficients and lattice mismatch between the two layers can be effectively buffered, thus smoothing the change in thermal stress gradient.

[0024] Improved mechanical and thermal stability: Through the dual effects of stress compensation and TEC buffering, the present invention effectively alleviates stress concentration between film layers, reduces defects such as film cracking and peeling, significantly improves the mechanical and thermal stability of the film system structure, and extends the service life of the device.

[0025] Maintaining excellent optical performance: Through reasonable design (e.g., using the MacLeod optimization algorithm), the introduced Al2O3 transition layer (even at a thickness of 60nm) has virtually no negative impact on the original optical properties of the DBR film system (such as center wavelength, reflectivity, and bandwidth), thus ensuring the optical performance of the device. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a multilayer optical film structure according to a specific embodiment of the present invention.

[0027] Figure 2 This is a schematic diagram illustrating the stress compensation principle of the Al2O3 transition layer of the present invention.

[0028] Figure 3 This is a comparison diagram showing the optical performance (reflection spectrum) of the embodiments of the present invention with that of the prior art. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0030] This invention provides a multilayer optical film structure disposed on a substrate (e.g., sapphire, silicon, glass, GaAs, or InP). The main body of the film structure is a DBR film stack, comprising multiple high refractive index layers (H layers) and multiple low refractive index layers (L layers) stacked alternately.

[0031] In a preferred embodiment, the high refractive index layer (H layer) is made of TiO2, and the low refractive index layer (L layer) is made of SiO2. To construct the DBR structure, the H layer and L layer are typically designed as a uniform stack, i.e., their optical thicknesses are both one-quarter (λ / 4) of the center wavelength λ, and they are stacked alternately with an optical thickness ratio of 1:1.

[0032] The core improvement of this invention lies in the addition of at least one Al2O3 transition layer in the multilayer film stack, that is, between the high refractive index layer and the low refractive index layer.

[0033] The Al2O3 transition layer exhibits good uniformity and density, playing a crucial role in stress regulation. On one hand, as described in the invention, the Al2O3 layer itself possesses compressive stress characteristics, which can be used to compensate for the inherent tensile stress of the TiO2 / SiO2 film stack. On the other hand, Al2O3 has a moderate coefficient of thermal expansion (approximately 7.2 × 10⁻⁶). -6 / K) is between TiO2 (approximately 9.0 × 10) -6 / K) and SiO2 (approximately 0.5 × 10 -6 The value between / K) acts as a thermal stress buffer, alleviating stress concentration at the H / L layer interface caused by TEC mismatch. As shown in Table 1, this value falls between that of typical high refractive index materials (such as TiO2, with a TEC of approximately 9.0 × 10⁻⁶). -6 / K) and low refractive index materials (SiO2, TEC approximately 0.5 × 10) -6 Between / K).

[0034] Table 1: Comparison of Thermal Expansion Coefficients (TEC) of Related Materials

[0035] Material Role Coefficient of thermal expansion ( / K) <![CDATA[SiO2]]> Low refractive index layer <![CDATA[About 0.5×10 -6 > <![CDATA[Al2O3]]> Transition layer of the present invention <![CDATA[Approximately 7.2×10 -6 > <![CDATA[TiO2]]> High refractive index layer <![CDATA[Approximately 9.0×10 -6 >

[0036] The physical thickness of the Al2O3 transition layer is preferably between 10 nm and 100 nm. More preferably, the thickness range is between 30 nm and 60 nm. This thickness range is sufficient to achieve effective stress buffering and compensation, while having a small impact on the overall optical performance. It is also easy to correct using subsequent film system optimization algorithms (such as the MacLeod optimization algorithm) to ensure that the film system still has excellent reflective performance over a wide angle range.

[0037] The preparation method of the present invention can employ conventional deposition techniques in the field of optical thin films. In a preferred embodiment, electron beam evaporation (EBE) is used.

[0038] Specifically, the multilayer film structure can be deposited using an Optorun coating machine (such as the Optorun series). During the coating process, the deposition rate, vacuum level, and substrate temperature are precisely controlled to ensure a dense and uniform film layer. In particular, the complex film system design, including an H layer, an L layer, and an Al2O3 transition layer, is precisely achieved through high-precision film thickness control (e.g., controlling the thickness error within ±2%).

[0039] Example

[0040] This embodiment provides a specific multilayer optical film structure, the structural design of which can be found in [reference needed]. Figure 2 As shown.

[0041] This embodiment is based on an existing 49-layer (49L) DBR film system, which is composed of alternating stacks of TiO2 (H layer) and SiO2 (L layer), with the optical thickness of all H and L layers designed to be λ / 4.

[0042] The technical solution of the present invention is applied to the 49L membrane system, specifically: two Al2O3 transition layers are inserted into the membrane system.

[0043] like Figure 2 As shown, the first Al2O3 thin film layer is disposed between the 14th and 15th layers of the original film system; the second Al2O3 thin film layer is disposed between the 32nd and 33rd layers of the original film system.

[0044] In this embodiment, the physical thickness of the two inserted Al2O3 thin film layers is set to 60 nm.

[0045] The film stress measurement performance of this embodiment was evaluated using a 375μm thick Si wafer as a carrier.

[0046] Table 2 Comparison of Stress Data

[0047]

[0048] As shown in Table 2, compared with the conventional 49L membrane system without an Al2O3 transition layer, the membrane structure of this embodiment has significantly improved mechanical and thermal stability due to the relief of stress concentration.

[0049] Reference Figure 2 As shown, the arrow indicates the stress direction. The compressive stress characteristics of the introduced Al2O3 layer are opposite to the tensile stress characteristics of the TiO2 / SiO2 film stack, thus forming an effective stress compensation.

[0050] Reference Figure 3 As shown, through optical simulation and actual fabrication verification, compared with the original 49L film system, the reflectance spectrum curve of this embodiment (with two 60nm Al2O3 layers inserted) remains basically unchanged, proving that the present invention can greatly improve the film stress and stability without sacrificing the optical performance required by the device.

[0051] The multilayer optical film structure and its preparation method described in this invention can be widely used in various optoelectronic devices that have high requirements for reflectivity and structural reliability, such as the reflective layer of LED chips, the reflector of lasers (such as VCSELs), or the photonic crystal structure in photodetectors.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A multilayer optical film structure, comprising a substrate and a multilayer film stack disposed on the substrate, the multilayer film stack comprising a plurality of high refractive index layers and a plurality of low refractive index layers; Its features are, The multilayer film stack further includes at least one Al2O3 transition layer, which is disposed between the high refractive index layer and the low refractive index layer.

2. The multilayer optical film structure according to claim 1, characterized in that: The at least one Al2O3 transition layer has compressive stress characteristics; the film stack composed of the plurality of high refractive index layers and the plurality of low refractive index layers has tensile stress characteristics; the compressive stress characteristics of the Al2O3 transition layer are used to compensate for the tensile stress characteristics of the film stack.

3. The multilayer optical film structure according to claim 1 or 2, characterized in that: The thermal expansion coefficient of the Al2O3 transition layer is between that of the high refractive index layer and the low refractive index layer.

4. The multilayer optical film structure according to any one of claims 1 to 3, characterized in that: The high refractive index layer is made of TiO2, and the low refractive index layer is made of SiO2.

5. The multilayer optical film structure according to any one of claims 1 to 4, characterized in that: The physical thickness of the Al2O3 transition layer is 10nm to 100nm.

6. The multilayer optical film structure according to claim 5, characterized in that: The physical thickness of the Al2O3 transition layer is 30nm to 60nm.

7. The multilayer optical film structure according to any one of claims 1 to 6, characterized in that: The optical thickness of both the high-refractive-index layer and the low-refractive-index layer is λ / 4, where λ is the design wavelength.

8. The multilayer optical film structure according to any one of claims 1 to 7, characterized in that: The number of at least one Al2O3 transition layer is two.

9. A method for preparing a multilayer optical film structure as described in any one of claims 1 to 8, comprising the step of sequentially depositing a plurality of high refractive index layers and a plurality of low refractive index layers on a substrate; Its features are, The method further includes: At least one Al2O3 transition layer is deposited between the high refractive index layer and the low refractive index layer.

10. The method according to claim 9, characterized in that: The deposition step employs electron beam evaporation (EBE) technology, and the thickness control error of the film layer in the deposition step is within ±2%.

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