Silicon-based silicon dioxide rectangular waveguide manufacturing method based on weak coupling Bragg grating

Abstract

The invention provides a method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weak coupling Bragg grating. The method comprises the following steps: growing a silicon dioxide layer on a silicon substrate as a waveguide lower cladding; sequentially growing a core region layer and a polycrystalline silicon mask layer on the waveguide lower cladding layer; transferring the grating pattern to a wafer, and manufacturing a Bragg grating on the core region layer by adopting dry etching to form a weak coupling grating; removing the polycrystalline silicon mask layer, and growing a grating covering layer on the Bragg grating; growing a polycrystalline silicon layer on the grating covering layer again to serve as a mask for waveguide etching; aligning the waveguide pattern with the Bragg grating through a photoetching overlay process; etching and forming the core region layer by using a dry etching method and taking the polycrystalline silicon layer as a mask to obtain a rectangular waveguide; and removing the residual polycrystalline silicon layer on the rectangular waveguide, and growing an upper cladding on the rectangular waveguide. By optimizing the technological process and the structural design, manufacturing of the silicon-based silicon dioxide rectangular waveguide Bragg grating which is low in loss, high in reflectivity and good in consistency is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical waveguide device manufacturing, and relates to a manufacturing process technology of a silicon-based silica waveguide and a Bragg grating, and particularly relates to a silicon-based silica rectangular waveguide manufacturing method based on a Bragg grating. BACKGROUND

[0002] In the fields of optical communication, optical fiber sensing, optical interconnection, etc., a silicon-based silica waveguide becomes a core basis of integrated optical devices due to good optical performance, high compatibility with CMOS technology, and the like. As a key structure in a waveguide device, a Bragg grating realizes functions such as reflection and filtering of an optical signal through periodic modulation of a waveguide refractive index, and the performance directly affects the stability and consistency of the device.

[0003] In the manufacturing of an optical waveguide device, as a key structure, the manufacturing process of a grating directly affects the performance of the device. A general technical solution in the prior art is to manufacture the grating together when the waveguide is manufactured, and the grating is usually located above the waveguide and etched to the lower cladding silicon oxide; or a fishbone grating structure is used. However, this kind of process has obvious defects: etching to the lower cladding silicon oxide will cause excessive refractive index modulation of the grating and the waveguide, easily causing enhanced light field disturbance and increased loss; and the fishbone grating is difficult to control in etching precision due to the complex structure, resulting in poor consistency of grating period and depth of different batches or even the same batch of devices, directly affecting the stability of optical signal reflection and filtering, and failing to meet the needs of high-precision integrated optical systems.

[0004] At present, in the traditional process of manufacturing a Bragg grating on a silicon-based silica waveguide, the following problems are often encountered: in order to obtain higher reflectivity, a strong coupling grating design is often used, but this will cause the uniformity of the grating to decrease; and if the refractive index difference is not properly controlled, the coupling coefficient will be too large, causing increased waveguide loss.

[0005] A general technical solution is to manufacture the grating together when the waveguide is manufactured, and the grating is usually located above the waveguide and etched to the lower cladding silicon oxide; or a fishbone grating structure is used. However, this kind of process has obvious defects: etching to the lower cladding silicon oxide will cause excessive refractive index modulation of the grating and the waveguide, easily causing enhanced light field disturbance and increased loss; and the fishbone grating is difficult to control in etching precision due to the complex structure, resulting in poor consistency of grating period and depth of different batches or even the same batch of devices, directly affecting the stability of optical signal reflection and filtering, and failing to meet the needs of high-precision integrated optical systems. In addition, if the etching depth is too deep or the mask selection is unreasonable, the grating structure will be deformed and the surface roughness will increase, ultimately causing increased device loss and reduced reflectivity, and failing to meet the needs of high-precision integrated optical systems.

[0006] The application patent with the application number 202010198505.9 discloses a preparation method of a birefringent waveguide Bragg grating reflector, comprising: disposing a lower cladding layer on the surface of a substrate; disposing a waveguide core layer on the surface of the lower cladding layer away from the substrate; etching a plurality of grooves on the surface of the waveguide core layer away from the lower cladding layer to form a grating; the wave vector direction of the grating is parallel to the waveguide direction of the waveguide core layer; and disposing an upper cladding layer covering the grating on the surface of the waveguide core layer away from the substrate to form the birefringent waveguide Bragg grating reflector. The above application can change the shape symmetry of the waveguide core layer by etching the grating on the surface of the waveguide core layer, thereby improving the shape birefringence value of the birefringent waveguide Bragg grating reflector; meanwhile, the grating can make the waveguide core layer and the upper cladding layer and the lower cladding layer have strong anisotropic stress, thereby making the birefringent waveguide Bragg grating reflector have a high stress birefringence value, and making the birefringent waveguide Bragg grating reflector have a high birefringence value. However, after etching the grating, the waveguide needs to be etched, and if the grating is etched very deeply, it is difficult to make the upper surface of the grating flat when the waveguide is etched by using polycrystalline silicon or photoresist as a mask, which leads to a poor edge morphology of the waveguide. In addition, when the grating grows the upper cladding layer, since the upper cladding layer is extrinsic silicon dioxide, the grating morphology on the core area and the upper cladding layer form mutual solubility, and the grating morphology changes greatly. SUMMARY

[0007] In view of the technical problems of large loss, unstable reflectivity and poor consistency in the Bragg grating fabrication process on a silicon-based silica waveguide, the application provides a silicon-based silica rectangular waveguide fabrication method based on a weakly coupled Bragg grating, which realizes the fabrication of a silicon-based silica rectangular waveguide Bragg grating with low loss, high reflectivity and good consistency by optimizing the process flow and structure design.

[0008] To achieve the above-mentioned purpose, the technical scheme of the application is as follows: a silicon-based silica rectangular waveguide fabrication method based on a weakly coupled Bragg grating, comprising the following steps: Substrate pretreatment: growing a silicon dioxide layer on a silicon substrate by a thermal oxidation process as a waveguide lower cladding layer; Growth of core layer and mask layer: growing a core layer and a polycrystalline silicon mask layer on the waveguide lower cladding layer in sequence by a PECVD process; Bragg grating fabrication: transferring a grating pattern to a wafer, fabricating a Bragg grating on the core layer by dry etching to form a weakly coupled grating; Removal of mask layer and growth of grating covering layer: removing the polycrystalline silicon mask layer and growing a grating covering layer on the Bragg grating by PECVD; Waveguide etching mask preparation: growing a polycrystalline silicon layer on the grating covering layer again by PECVD as a waveguide etching mask; Waveguide and grating overlay: through the lithography overlay process, the waveguide pattern is aligned with the Bragg grating which has been made in horizontal and vertical directions, and the profile of the rectangular waveguide is defined; Waveguide etching: the core layer is etched into a rectangular waveguide by using dry etching with the polysilicon layer as a mask; Upper cladding growth: the remaining polysilicon layer on the rectangular waveguide is removed, and the upper cladding is grown on the etched rectangular waveguide by using PECVD, and the device is completed.

[0009] Preferably, the material of the core layer is doped silicon dioxide, and the refractive index is controlled by doping the concentration of germane, and the core layer and the waveguide lower cladding form a refractive index difference of 2%.

[0010] Preferably, the etching depth of the Bragg grating is 200-500nm.

[0011] Preferably, the period of the Bragg grating is designed according to the target reflection wavelength, and a uniform period structure is adopted.

[0012] Preferably, the grating cover layer is intrinsic silicon dioxide, and the thickness is 1μm, and the grating cover layer completely covers the grating structure of the Bragg grating.

[0013] Preferably, after the core layer and the polysilicon mask layer are grown, photoresist is coated, then ultraviolet exposure is performed by using the grating mask, and finally development is performed to obtain the grating pattern; the grating pattern on the grating mask is transferred to the wafer.

[0014] Preferably, the cross-sectional size of the rectangular waveguide is: width 3-4μm, height 3-4μm, which meets the single-mode transmission condition.

[0015] Preferably, after the waveguide etching, the Bragg grating is located on the upper surface of the rectangular waveguide, and forms an integrated structure with the rectangular waveguide.

[0016] Preferably, the upper cladding is boron-phosphorus-doped silicon dioxide, and by adjusting the proportion of boron-phosphorus doping, the refractive index of the upper cladding is adjusted to be consistent with the waveguide lower cladding, and the rectangular waveguide and the Bragg grating are completely covered.

[0017] Preferably, the thickness of the waveguide lower cladding is 1μm-15μm; the thickness of the polysilicon mask layer and the polysilicon layer ranges from 500-800nm. The polysilicon mask and the polysilicon layer are removed by using solution wet etching.

[0018] The waveguide pattern is aligned with the Bragg grating by using a lithography machine through an overlay mark.

[0019] Compared with the prior art, the beneficial effects of the present application are: Low-loss characteristics: The weakly coupled grating design (shallow etching depth) reduces the strong disturbance of the grating structure on the light field. At the same time, precise overlay avoids additional loss caused by the offset between the waveguide and the grating, effectively reducing device loss.

[0020] High reflectivity and stability: The uniform periodic grating structure combined with a 2% refractive index difference design can achieve a reflectivity of over 90%, and the reflection spectrum is steep.

[0021] Excellent consistency: Through the polysilicon mask step-by-step etching and overlay process, the grating size deviation is controlled within ±20nm, and the device reflectivity consistency deviation in mass production is ≤5%.

[0022] Strong process compatibility: All process steps (thermal oxidation, PECVD, photolithography, dry etching) are compatible with CMOS processes, enabling large-scale integrated manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 This is the implementation process of the present invention.

[0025] Figure 2 This is a structural diagram of the product obtained by the present invention. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0027] Example 1 like Figure 1 As shown, the present invention provides a method for fabricating a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating. Through step-by-step growth, multiple masking and overlay processes, the precise alignment and structural uniformity of the grating and waveguide are ensured. This method is stable and highly controllable, and can significantly improve device performance. The steps of the present invention include: Substrate pretreatment: a 10-15 μm-thick silicon dioxide layer is grown on a silicon substrate by a thermal oxidation process as a waveguide lower cladding. The silicon dioxide structure directly grown by thermal oxidation of the silicon substrate is more compact and better matches the silicon lattice.

[0028] Growth of core layer and mask layer: a core layer (silicon dioxide-based material with a refractive index difference of 2%) and a polysilicon mask layer are sequentially grown on the waveguide lower cladding by a PECVD (plasma-enhanced chemical vapor deposition) process.

[0029] The material of the core layer is doped silicon dioxide (e.g., germanium-doped silicon dioxide), which forms a refractive index difference with the thermally oxidized silicon dioxide to serve as a waveguide for light transmission. The refractive index is controlled by the concentration of germane doping, and a refractive index difference of 2% is formed with the waveguide lower cladding.

[0030] Bragg grating fabrication: after the growth of the core layer and the polysilicon mask layer, photoresist is coated, then a grating mask is used for photolithography by ultraviolet exposure, and finally development is performed to transfer the grating pattern on the grating mask to the wafer. A Bragg grating is fabricated on the core layer by dry etching, which can make the waveguide morphology perpendicularity > 89 degrees to ensure that the grating morphology meets the requirements. To achieve weak coupling characteristics, the etching depth of the grating is controlled to be several hundred nanometers to ensure that the coupling coefficient is within the weak coupling range.

[0031] The period of the Bragg grating is designed according to the target reflection wavelength (the center wavelength of the Bragg grating is equal to twice the product of the Bragg grating period and the effective refractive index of the Bragg grating). A uniform period structure is adopted, and the etching depth is 200-500 nm to ensure weak coupling characteristics.

[0032] Removal of the mask layer and growth of the grating cover layer: after the Bragg grating layer is etched, the polysilicon mask is removed; then a grating cover layer is grown on the Bragg grating by PECVD. The layer is intrinsic silicon dioxide with a thickness of 1 μm, which is used to prevent the waveguide morphology from being greatly deformed due to high-temperature annealing after the subsequent growth of the upper cladding, and covers the grating structure of the Bragg grating.

[0033] Waveguide etching mask preparation: a polysilicon layer is grown again on the grating cover layer by PECVD to serve as a mask for subsequent waveguide etching. The thickness of the polysilicon layer is 500-800 nm.

[0034] Waveguide and grating alignment: by a photolithography alignment process, the waveguide pattern is aligned with the fabricated Bragg grating in the horizontal and vertical directions to define the profile of the rectangular waveguide (with a size of several microns). Since the grating width is wide, there is no requirement for alignment accuracy.

[0035] The cross-sectional size of the rectangular waveguide is: width 3-4 μm, height 3-4 μm, which meets the single-mode transmission condition.

[0036] Waveguide Etching: Using a dry etching process, the polysilicon layer is used as a mask to etch the waveguide structure into a rectangular waveguide. The Bragg grating is located on the upper surface of the rectangular waveguide, forming an integrated structure with the rectangular waveguide. At this time, the previously etched Bragg grating structure is completely etched away except for the rectangular waveguide.

[0037] Dry etching (ICP etching) is used for patterning to ensure that the edges of the mask are steep. The waveguide structure is the core layer.

[0038] Upper cladding growth: After etching the rectangular waveguide, the remaining polysilicon layer on the rectangular waveguide is removed, and PECVD is used to grow an upper cladding on the rectangular waveguide after etching. The upper cladding is silicon dioxide doped with boron and phosphorus. By adjusting the ratio of doping boron and phosphorus, the refractive index is adjusted to be consistent with the lower cladding of the waveguide, completely covering the rectangular waveguide and Bragg grating, completing the device fabrication.

[0039] Example 2 like Figure 1 As shown, the present invention provides a method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating, and the specific steps are as follows: Substrate pretreatment: A 6-inch silicon substrate is selected and a 15 μm thick SiO2 layer is grown on its surface through a thermal oxidation process to obtain the waveguide lower cladding.

[0040] Core and mask layer growth: PECVD is used to grow a 4μm thick Ge-doped SiO2 core layer on the waveguide lower cladding, with a refractive index difference of 2% with SiO2, followed by a 600nm thick polysilicon mask layer.

[0041] Bragg grating fabrication: Positive photoresist is coated on the polysilicon mask layer, and a grating pattern with a period of 1.605μm and a duty cycle of 50% is defined by UV lithography. The grating pattern is transferred to the core layer on the wafer using ICP dry etching. The grating is etched to a depth of 200nm, forming a weakly coupled grating.

[0042] Mask removal and capping: The polysilicon mask is removed using a solution wet etch. PECVD is then used to grow a 1μm-thick SiO2 capping layer on the etched grating structure on the wafer, completely covering the grating etch depth and the grating structure. This solution wet etch completely removes the polysilicon without affecting the thickness of the core layer material.

[0043] Waveguide etching mask preparation: PECVD is used to grow an 800nm ​​thick polysilicon layer on the grating cover layer as a waveguide etching mask.

[0044] Waveguide and grating overlay: waveguide pattern is aligned with grating by overlay mark using lithography machine, and 4 μm wide rectangular waveguide is defined.

[0045] Waveguide etching: core layer is etched to the surface of lower cladding layer by ICP etching with polysilicon layer as mask, and rectangular waveguide is formed, at this time Bragg grating is located on the upper surface of rectangular waveguide.

[0046] Upper cladding growth: polysilicon layer is removed by wet etching, and 15 μm thick SiO2 upper cladding is grown by PECVD, which completely covers rectangular waveguide and Bragg grating, and the fabrication is completed.

[0047] Through the above process, the reflectivity of Bragg grating reaches 90%, and the consistency of batch production is good, which meets the demand of high-performance integrated optical devices.

[0048] The fabricated device is shown in FIG. 1, and the last upper cladding is not shown in the figure. Figure 2

[0049] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. A method for fabricating a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating, characterized in that: The steps are as follows: Substrate pretreatment: A silicon dioxide layer is grown on the silicon substrate by thermal oxidation process to serve as the waveguide lower cladding; Growth of the core layer and mask layer: The core layer and polysilicon mask layer are grown sequentially on the waveguide lower cladding using the PECVD process; Bragg grating fabrication: The grating pattern is transferred to the wafer, and a Bragg grating is fabricated on the core layer using dry etching to form a weakly coupled grating. Removing the mask layer and growing the grating cover layer: removing the polysilicon mask layer and growing the grating cover layer on the Bragg grating using PECVD; Waveguide etching mask preparation: PECVD is used to grow a polysilicon layer on the grating cover layer to serve as a mask for waveguide etching; Waveguide and grating overlay: The waveguide pattern is aligned horizontally and vertically with the fabricated Bragg grating through a photolithography overlay process to define the outline of the rectangular waveguide. Waveguide etching: Dry etching is used to etch the core layer using the polysilicon layer as a mask to obtain a rectangular waveguide; Upper cladding layer growth: Remove the remaining polysilicon layer on the rectangular waveguide, and use PECVD to grow the upper cladding layer on the etched rectangular waveguide to complete the device fabrication.

2. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 1, characterized in that: The material of the core layer is doped silicon dioxide, and the refractive index is controlled by adjusting the concentration of doped germane. A refractive index difference of 2% is formed between the core layer and the waveguide lower cladding.

3. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 1 or 2, characterized in that: The etching depth of the Bragg grating is 200-500 nm.

4. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 3, characterized in that: The period of the Bragg grating is designed according to the target reflection wavelength and adopts a uniform periodic structure.

5. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to any one of claims 1, 2, and 4, wherein: The grating covering layer is intrinsic silicon dioxide with a thickness of 1 μm. The grating covering layer completely covers the grating structure of the Bragg grating.

6. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 1, wherein: After the core layer and the polysilicon mask layer are grown, photoresist is coated, and then a grating mask is used for photolithography using ultraviolet exposure, and finally developed to obtain a grating pattern; the grating pattern on the grating mask is transferred to the wafer.

7. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 1, wherein: The cross-sectional dimensions of the rectangular waveguide are: width 3-4 μm, height 3-4 μm, meeting single-mode transmission conditions.

8. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 7, wherein: After the waveguide is etched, the Bragg grating is located on the upper surface of the rectangular waveguide and forms an integrated structure with the rectangular waveguide.

9. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to claim 1, characterized in that: The upper cladding is silicon dioxide doped with boron and phosphorus. By adjusting the ratio of doped boron and phosphorus, the refractive index of the upper cladding is adjusted to be consistent with that of the waveguide lower cladding, completely covering the rectangular waveguide and the Bragg grating.

10. The method for manufacturing a silicon-based silicon dioxide rectangular waveguide based on a weakly coupled Bragg grating according to any one of claims 1, 2, 4, 6 to 9, wherein: The thickness of the waveguide lower cladding is 1 μm to 15 μm; the thickness of the polysilicon mask layer and the polysilicon layer is in the range of 500-800 nm; removing the polysilicon mask and the polysilicon layer by using a solution wet etching method; The waveguide pattern is aligned with the Bragg grating by overlay marks using a photolithography machine.

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