Low-stress high-reflectivity optical composite film and preparation method and application thereof

Abstract

The invention belongs to the technical field of optical components, and particularly relates to a low-stress high-reflectivity optical composite film and a preparation method and application thereof. The method comprises the following steps: S1, respectively weighing a SiO evaporation material and a Ge evaporation material, putting the materials into an evaporation table, and vacuumizing; and S2, sequentially depositing SiO of # imgabs 0 #, Ge of # imgabs 1 #, SiO of # imgabs 2 #, Ge of # imgabs 3 #, SiO of # imgabs 4 # and Ge of # imgabs 5 # on the silicon wafer to obtain the optical composite film with low stress and high reflectivity. The prepared optical composite film can be used as a high-reflection film of a tunable F-P resonant cavity, and narrower transmission spectrum peak bandwidth and higher transmissivity are achieved.

Description

Technical Field

[0001] The present invention belongs to the field of optical components, and in particular relates to a low-stress and high-reflectivity optical composite film, a preparation method thereof, and applications thereof. Background Art

[0002] A Fabry-Perot resonant cavity device, or FP cavity, is composed of two parallel plane mirrors. Light is reflected multiple times within the cavity to form an oscillating beam that interferes with the output. A tunable FP resonant cavity is one that effectively controls the central resonant wavelength by adjusting the cavity length, incident angle, and refractive index of the medium within the cavity, thereby achieving regulation of the FP cavity's transmission spectrum. Depending on the tuning method, FP cavities can be roughly divided into fiber-type FP cavities, micromechanical FP cavities, and electro-optical FP cavities. In an electrostatically driven micromechanical FP cavity, the transmission spectrum can be tuned by installing a fixed reflector on the bottom wafer and a movable reflector on the upper wafer, driven by electrostatic force.

[0003] The parallelism of the movable and fixed mirror structures in a tunable micromechanical FP cavity significantly impacts module performance. Therefore, the parallelism of the upper and lower plates should be maintained as closely as possible during process preparation. The optical thin film on the resonant cavity surface is typically a multilayer composite thin film material, and its stress significantly impacts wafer warpage and the morphology of the sensitive microelectromechanical system (MEMS) structure. Wafer warpage not only makes processing difficult, but deformation of the MEMS sensitive structure also directly impacts the performance of the filter module. Different stresses in different materials can cause stress deformation in the parallel plates, thereby affecting the performance of the filter module. Therefore, low-stress optical composite films can effectively ensure the stability of the filter module. Furthermore, the film material must not only have a relatively stable refractive index, but also meet basic requirements for spectral transparency, mechanical robustness, chemical stability, and resistance to high-energy radiation.

[0004] With the development of optoelectronic technology, the performance requirements for multifunctional integrated optical components are becoming increasingly stringent. Infrared optical thin films are required to provide anti-reflection, high-reflection, filtering, spectroscopic, and protective functions across different wavelengths. These functions can be selectively integrated into a single infrared film system based on specific application requirements, thereby simplifying the optoelectronic system structure and reducing costs. To achieve the high reflectivity performance of optical thin films in tunable resonant cavities, optical thin film materials with different refractive indices can be periodically stacked to form a distributed Bragg reflector ladder.

[0005] Existing multilayer optical composite film technologies are mostly composed of SiO and Si stacked together. Both materials exhibit negative stress after being prepared using the vapor deposition process. In addition, excessive film stress after multi-layer stacking can lead to film damage and increased wafer warping. The large wafer warping will cause difficulties in subsequent device processing. Summary of the Invention

[0006] In order to overcome the above-mentioned defects in the prior art, the present invention provides a low-stress, high-reflectivity optical composite film and its preparation method and application. The optical composite film of the present invention has the advantages of low stress, small wafer warpage, and high reflectivity.

[0007] To achieve one of the above purposes, the present invention adopts the following technical solutions:

[0008] A method for preparing a low-stress, high-reflectivity optical composite film comprises the following steps:

[0009] S1. Weigh the SiO evaporation material and Ge evaporation material separately and place them into the crucible of the evaporation table, and evacuate the crucible to avoid thermal reaction with air, thereby ensuring the purity of the material;

[0010] S2, sequential deposition on silicon wafer SiO, Ge, SiO, Ge, SiO, Ge, to obtain an optical composite film with low stress and high reflectivity.

[0011] Preferably, in step S2, a layer of Ge is deposited on the top layer. of SiO2.

[0012] Preferably, the evaporation rate of SiO evaporation material is The evaporation rate of Ge evaporation material is

[0013] Preferably, the evaporation rate of SiO2 is

[0014] To achieve the second of the above objectives, the present invention provides a low-stress, high-reflectivity optical composite film, which is composed of SiO layers and Ge layers alternately deposited on a silicon wafer, and each of the SiO layers and the Ge layers is deposited in three layers.

[0015] Preferably, a SiO2 layer is deposited on the uppermost Ge layer to protect the optical composite film from being corroded by subsequent processes.

[0016] To achieve the third of the above objectives, the present invention provides an application of a low-stress and high-reflectivity optical composite film, wherein the composite optical film is applied as a high-reflection film in an FP resonant cavity.

[0017] Preferably, the application of a low-stress and high-reflectivity optical composite film comprises the following specific steps:

[0018] S1, growing on the upper and lower surfaces of the fixed mirror by hot oxygen Oxidize silicon to obtain a thermal oxide film layer; then use PECVD (plasma enhanced chemical vapor deposition) to deposit on the lower surface of the fixed mirror Silicon nitride, followed by dry etching of the thermal oxide film Then etched by BOE (buffered oxide etchant) Thermally oxidize the film layer and use sulfuric acid to remove the glue and build the resonant cavity pattern;

[0019] S2. In the resonant cavity pattern area, a 1 μm fixed mirror is etched by tetramethylammonium hydroxide (TMAH) to construct the resonant cavity structure. After photolithography, the thermal oxide film layer is dry-etched in the limit point and ground hole area. Then BOE corrosion Thermal oxide film layer, followed by sulfuric acid stripping, followed by photolithography patterning of Al sacrificial layer area, and then deposition by sputtering aluminum;

[0020] S3, sputtering Al sacrificial layer after photolithography After that, the resonant cavity structure is developed by photolithography, and then SiO / Ge / SiO / Ge / SiO / Ge composite optical films are alternately evaporated, and an additional layer of SiO2 is evaporated to protect the high-reflection film, and then the remaining Al sacrificial layer is etched;

[0021] S4, deposition after photolithography The metal lead is stripped and then a silicon oxide passivation layer is grown

[0022] S5. Deposition by sputtering Aluminum, then the metal aluminum bonding limit point is patterned by photolithography, and the metal aluminum limit point is wet-etched except for the metal aluminum limit point.

[0023] S6. After photolithography, dry etching is used to remove the passivation layer between adjacent fixed mirror structures. Reuse wet etching Complete etching of the scribe line;

[0024] S7. After photolithography, the Ti metal bonding surface and the Au metal bonding surface are sputtered in sequence, and then patterned by photolithography. The Ti metal bonding surface and the Au metal bonding surface not covered by the photoresist are removed by wet etching to complete the preparation of the tunable FP cavity fixed mirror.

[0025] Preferably, in step S4, after photolithography, deposition is performed in sequence Ti metal lead, Pt metal lead, Au metal leads, Ti metal lead.

[0026] Preferably, in step S7, hydrofluoric acid is used to wet-etch the Ti metal bonding surface, and thiourea is used to wet-etch the Au metal bonding surface.

[0027] The advantages of the present invention are:

[0028] (1) The optical composite film prepared by the present invention can be used as a high-reflection film for a tunable FP resonant cavity to achieve a narrower transmission spectrum peak bandwidth and a higher transmittance. The SiO / Ge / SiO / Ge / SiO / Ge composite optical film is compatible with CMOS technology and MEMS bulk silicon technology. The compressive stress SiO film and the tensile stress Ge film reduce the wafer warpage and achieve low stress requirements. At the same time, the alternating structure of the low-refractive-index SiO film and the high-refractive-index Ge film can also form a distributed Bragg reflector to achieve high reflectivity requirements.

[0029] (2) In the optical composite film prepared by the present invention, the SiO film exhibits compressive stress and the Ge film exhibits tensile stress. Alternating the deposition of SiO and Ge films can achieve low stress requirements. In addition, SiO has a low refractive index and is in a non-absorption region in the 200nm to 4μm band. It has good chemical stability and strong mechanical properties. SiO is inexpensive and is compatible with complementary metal oxide semiconductor (CMOS) processes and microelectromechanical systems (MEMS) bulk silicon processes. Ge can be used as a high refractive index material that matches the low refractive index SiO film. The infrared light transmittance of ultrapure germanium films in the 1μm to 7μm band is as high as 90% to 95%. By alternately depositing SiO / Ge / SiO / Ge / SiO / Ge, a low-stress, high-reflectivity optical composite film structure for use in a tunable FP resonant cavity can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a graphical schematic diagram of the resonant cavity of the present invention.

[0031] Figure 2 Schematic diagram of the resonant cavity etching and aluminum sacrificial layer deposition of the present invention.

[0032] Figure 3 Schematic diagram of the deposition of optical composite thin film structure in the resonant cavity of the present invention.

[0033] Figure 4 Schematic diagram of the construction of the metal lead Ti / Pt / Au / Ti, passivation layer and metal aluminum bonding limit point of the present invention.

[0034] Figure 5 This is a schematic diagram of the scribe line etching and metal bonding surface construction of the present invention.

[0035] Figure 6 The resistive evaporation deposited SiO thin film results are characterized by the white light interferometer of the present invention.

[0036] Figure 7 This is the SEM characterization image of the composite optical film SiO2 / Ge / SiO / Ge / SiO / Ge / SiO of the present invention.

[0037] Figure 8 This is the composite optical film result characterized by the white light interferometer of the present invention.

[0038] Figure 9 This is a transmittance test chart of the composite optical film characterized by Fourier spectrometer in the present invention.

[0039] Figure 10 This is a wafer image of the low-stress, high-reflectivity composite film of the present invention. DETAILED DESCRIPTION

[0040] Example 1

[0041] S1, on the upper and lower surfaces of the fixed mirror (silicon wafer) through thermal oxidation growth Silicon oxide is then deposited on the lower surface of the fixed mirror using PECVD Silicon nitride. The resonant cavity area is patterned using photolithography, and the thermal oxide silicon in the resonant cavity area is dry-etched. Then use BOE corrosion Then use sulfuric acid to remove the glue, such as Figure 1 shown.

[0042] S2, use TMAH to etch the fixed mirror in the resonant cavity area 1μm, use photolithography to pattern the limit point and ground hole area in the resonant cavity, and use dry etching to thermally oxidize silicon in the limit point / ground hole area. Then use BOE corrosion Then, sulfuric acid is used to remove the glue. Then, the Al sacrificial layer area is patterned by photolithography and then deposited by sputtering. Aluminum, such as Figure 2 shown.

[0043] S3, after removing the photoresist in step S2, pattern the resonant cavity area by photolithography, alternately deposit SiO / Ge / SiO / Ge / SiO / Ge by evaporation to obtain a composite optical thin film structure, remove the photoresist, and then etch the aluminum sacrificial layer, such as Figure 3 shown.

[0044] S4, patterning the metal lead area using photolithography, and then depositing Ti metal lead, Pt metal lead, Au metal leads, Ti metal lead, after removing the photoresist, grow a silicon oxide passivation layer Deposition by sputtering Aluminum, then the metal aluminum bonding limit point is patterned by photolithography, and the metal aluminum limit point is wet-etched except for the metal aluminum limit point. like Figure 4 shown.

[0045] S5: After removing the photoresist in step S4, pattern the scribe lines using photolithography, and then use dry etching to remove the silicon oxide passivation layer between adjacent fixed mirror structures. Then use wet etching of silicon oxide The etching of the scribe lines is completed.

[0046] S6. After photolithography, the Ti metal bonding surface and the Au metal bonding surface are sputtered in sequence, and then patterned by photolithography, and the Ti metal bonding surface and the Au metal bonding surface not covered by the photoresist are removed by wet etching.

[0047] S7, then use hydrofluoric acid to corrode the Ti metal bonding surface, and use thiourea to corrode the Au metal bonding surface to remove the Ti metal bonding surface and the Au metal bonding surface covered by the photoresist, and complete the preparation of the tunable FP cavity fixed mirror, as shown in FIG. Figure 5 shown.

[0048] Comparative Example 1

[0049] The SiO film in Example 1 was deposited by electron beam evaporation.

[0050] Comparative Example 2

[0051] The SiO film in Example 1 was deposited by resistive evaporation.

[0052] The SiO films deposited in Examples 1 and 2 were characterized using a white light interferometer. The results showed that the roughness of the SiO film prepared in Example 1 was approximately The roughness is large. This is because the SiO material is relatively active, and the energy during electron beam evaporation is high, which causes the SiO material to splash out of the plating pot before it sublimates, resulting in many particles and large roughness on the film surface. The surface of the SiO film prepared in Comparative Example 2 is clean and particle-free, with a roughness of like Figure 6 Therefore, the effect of resistive evaporation in SiO thin film evaporation is better than that of electron beam evaporation.

[0053] A layer of SiO2 was deposited on the Ge layer of the top layer of the composite optical film prepared in Example 1. After evaporation, the thickness of each film layer was characterized using SEM. The specific thickness and evaporation rate of each layer of the composite optical film are shown in Table 1 and Figure 7 As shown:

[0054] Table 1

[0055]

[0056] The roughness and refractive index of the composite optical film prepared in Example 1 were characterized using a white light interferometer and a Fourier spectrometer. The results showed that the roughness of the composite optical film was And in the mid-infrared band (3μm ~ 5μm) the refractive index is less than 5%, achieving a high reflectivity performance of more than 95%, such as Figure 8 and Figure 9 .

[0057] The BOW value and stress of the composite optical thin film layer were tested using a stress tester. The results are shown in Table 2 below:

[0058] Table 2

[0059]

[0060]

[0061] As shown in Table 2, due to the stress offset between the SiO film and the Ge film, the overall stress of the composite optical film is less than -230 MPa, and the BOW value is less than 10 μm, achieving a low stress effect.

[0062] The above are only preferred embodiments of the present invention and are 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 low-stress and high-reflectivity optical composite film, characterized in that: The steps include: S1. Weigh SiO evaporation material and Ge evaporation material separately and place them into the evaporation table, and evacuate the table; S2, sequential deposition on silicon wafer SiO, Ge, SiO, Ge, SiO, Ge, to obtain an optical composite film with low stress and high reflectivity.

2. The method for preparing a low-stress, high-reflectivity optical composite film according to claim 1, wherein: In step S2, a layer of Ge is deposited on the top layer. of SiO2.

3. The method for preparing a low-stress, high-reflectivity optical composite film according to claim 1, wherein: The evaporation rate of the SiO evaporation material is The evaporation rate of the Ge evaporation material is 4. The method for preparing a low-stress, high-reflectivity optical composite film according to claim 2, wherein: The evaporation rate of SiO2 is 5. A low-stress, high-reflectivity optical composite film prepared by the method according to any one of claims 1 to 4, characterized in that: The optical composite film consists of SiO layers and Ge layers alternately deposited on a silicon wafer, wherein the SiO layers and the Ge layers are both deposited in three layers.

6. The low-stress, high-reflectivity optical composite film according to claim 5, characterized in that: A SiO2 layer is also deposited on the uppermost Ge layer.

7. Use of the low-stress, high-reflectivity optical composite film according to claim 5 or 6, characterized in that: The composite optical film is used as a high-reflection film in a FP resonant cavity.

8. The use of the low-stress and high-reflectivity optical composite film according to claim 7, characterized in that: The specific steps are as follows: S1, growing on the upper and lower surfaces of the fixed mirror by hot oxygen Oxidize silicon to obtain a thermal oxide film layer; then deposit it on the lower surface of the fixed mirror Silicon nitride, followed by dry etching of the thermal oxide film Re-corrosion Thermally oxidize the film layer and use sulfuric acid to remove the glue and build the resonant cavity pattern; S2, etch a 1μm fixed mirror in the resonant cavity pattern area to build the resonant cavity structure, and dry-etch the hot oxide film layer in the limit point and ground hole area after photolithography. Re-corrosion Thermal oxide film layer, followed by sulfuric acid stripping, followed by photolithography patterning of Al sacrificial layer area, and then deposition by sputtering aluminum; S3, sputtering Al sacrificial layer after photolithography After that, the resonant cavity structure is developed by photolithography, and then the SiO / Ge / SiO / Ge / SiO / Ge composite optical film is alternately evaporated, and an additional layer of SiO2 is evaporated, and then the remaining Al sacrificial layer is etched; S4, deposition after photolithography The metal lead is stripped and then a silicon oxide passivation layer is grown S5. Deposition by sputtering Aluminum, then the metal aluminum bonding limit point is patterned by photolithography, and the metal aluminum limit point is wet-etched except for the metal aluminum limit point. S6. After photolithography, dry etching is used to remove the passivation layer between adjacent fixed mirror structures. Reuse wet etching Complete etching of the scribe line; S7. After photolithography, the Ti metal bonding surface and the Au metal bonding surface are sputtered in sequence, and then patterned by photolithography. The Ti metal bonding surface and the Au metal bonding surface not covered by the photoresist are removed by wet etching to complete the preparation of the tunable FP cavity fixed mirror.

9. The use of the low-stress, high-reflectivity optical composite film according to claim 8, characterized in that: In step S4, after photolithography, deposition is carried out in sequence Ti metal lead, Pt metal lead, Au metal leads, Ti metal lead.

10. The use of the low-stress and high-reflectivity optical composite film according to claim 8, characterized in that: In step S7, hydrofluoric acid is used to wet-etch the Ti metal bonding surface, and thiourea is used to wet-etch the Au metal bonding surface.

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