Suspended grating coupler with variable period and gradually-changed tooth height

Abstract

The invention relates to a variable-period and gradually-changed tooth height suspended grating coupler, and belongs to the technical field of integrated optics and silicon-based photonics. Comprising a silicon substrate layer, a silicon dioxide buried oxide layer, a grating device layer and a silicon dioxide upper cladding layer which are arranged from bottom to top, the tooth height of the whole grating area continuously changes in the light propagation direction according to a preset change rule, and the whole tooth height is in a monotone increasing trend; the silicon dioxide buried oxide layer below the grating is selectively etched to form a suspended structure, and the refractive index difference between air and a grating material is utilized, so that the leakage loss of light energy to the substrate is effectively inhibited. Through multi-dimensional collaborative optimization of the tooth height, the period and the suspended structure, the problem that the bandwidth and the efficiency of a traditional grating coupler are difficult to consider at the same time is solved, the gradually-changed tooth height structure is prepared through the gray exposure and etching technology, and the device is compact in structure, compatible with the CMOS technology and high in reliability. The method has a wide application prospect in the fields of high-speed optical interconnection, sensing, large-scale photonic integrated chips and the like.

Description

Technical Field

[0001] This invention relates to a suspended grating coupler with variable period and gradually increasing tooth height, belonging to the fields of integrated optics and silicon-based photonics. Background Technology

[0002] In recent years, photonic integrated circuit (PIC) technology has demonstrated significant advantages in power consumption, bandwidth, and cost, and its irreplaceable advantages in information transmission and processing have led to its widespread application in optical communication, optical sensing, and quantum information processing. A key challenge in PIC is achieving efficient coupling between single-mode fiber and submicron waveguides. Grating couplers (GCs), with their flexible spatial positioning and simple manufacturing process, are core devices for achieving efficient coupling between silicon-based photonic chips and optical fibers. Traditional uniform-period grating couplers suffer from two main bottlenecks: first, mode-field mismatch. Because the gratings in traditional grating couplers are uniformly distributed, the coupled light field distribution exhibits an approximately exponential decay, while the light field in optical fibers has a near-Gaussian distribution, resulting in a mismatch and errors; second, substrate leakage, as some light energy is coupled downwards into the high-refractive-index silicon substrate, leading to losses. To improve mode-field mismatch, existing technologies have proposed variable-period grating (apodized grating) designs. However, while simple variable period and variable tooth height designs can improve bandwidth, they often fail to address substrate leakage issues, resulting in limited improvements in peak coupling efficiency and potentially introducing additional losses due to mode mismatch. To reduce substrate leakage, suspended structures are introduced.

[0003] In summary, there is an urgent need in the field for a grating coupler solution that can simultaneously achieve high coupling efficiency and wide bandwidth. This solution should have advantages such as simple fabrication process, flexible control, and strong compatibility with advanced complementary metal-oxide-semiconductor (CMOS) photonic platforms. Summary of the Invention

[0004] To address the problems and shortcomings of the existing technologies, this invention provides a suspended grating coupler based on variable period and variable tooth height design. By coordinating the design of grating tooth height variation, grating period variation, and suspended structure, the three elements form a coupling effect in controlling the light field distribution, phase matching, and suppressing substrate leakage, thereby achieving a simultaneous improvement in coupling efficiency and operating bandwidth without the need for additional complex structures.

[0005] The technical solution of the present invention is: a floating grating coupler with variable period and gradually varying tooth height, comprising: a substrate layer 1, a silicon dioxide buried oxide layer 2, a grating device layer 3 and a silicon dioxide upper cladding layer 4 arranged sequentially from bottom to top; The grating device layer 3 includes, from front end to rear end, a tightly connected single-mode waveguide region 31, a mode field conversion region 32, an apodization grating region 33, and a uniform grating region 34. The apodized grating region 33 and the uniform grating region 34 together constitute the grating region; The grating period of the apodization grating region 33 varies along the direction of light propagation, while the grating period of the uniform grating region 34 remains constant. The buried oxide layer 2 of silicon dioxide is selectively removed below the grating region to form a region 21 that suspends the grating region, thereby suppressing light energy leakage towards the substrate and improving coupling efficiency.

[0006] Furthermore, the tooth height of multiple grating teeth within the grating region changes continuously along the light propagation direction according to a preset variation law, and exhibits a monotonically increasing trend overall.

[0007] Furthermore, the tooth height variations of the apodization grating region 33 and the uniform grating region 34 are continuously connected at the connection point to form an overall continuously varying tooth height distribution.

[0008] Furthermore, the lateral dimension of the region 21 that suspends the grating region covers the grating region, so that the entire grating region is in a suspended state.

[0009] Furthermore, the changes in grating tooth height, grating period, and the region 21 that suspends the grating area work together to simultaneously regulate the light field distribution and phase matching relationship.

[0010] This invention also provides a method for fabricating a suspended grating coupler with variable period and gradually varying tooth height, the method comprising: The tooth height of the multiple grating teeth in the grating area is a gradient tooth height, which is prepared by grayscale exposure and etching process; The silicon dioxide buried oxide layer 2 below the grating region is selectively etched to form a region 21 that suspends the grating region; The grating device layer 3 defines a pattern on the top thin film using grayscale exposure technology, and then forms the required grating structure through dry or wet etching processes; in the variable period and variable tooth height grating regions, the grating period gradually changes within the apodization grating region 33, and the tooth height gradually changes within the grating region, which is used to achieve broadband matching and efficient mode field conversion.

[0011] The working principle of this invention is as follows: When the grating coupler is in operation, the optical signal is first incident obliquely from above onto the coupler surface via a single-mode fiber. After diffraction through the grating structure of grating device layer 3, the light is coupled into the single-mode waveguide 31 inside the chip for transmission, or conversely, the light in the waveguide is efficiently coupled into the fiber. Its core working principle is based on the synergistic effect of a variable period structure, a gradually varying grating tooth height, and a suspended structure. The grating period is gradually distributed along the light propagation direction within the apodized grating region, allowing light of different wavelengths to satisfy the Bragg diffraction condition at different positions. This design expands the phase matching range of the grating through dispersion modulation, thereby significantly widening the operating bandwidth of the device to meet the needs of broadband optical communication systems. The height of the grating teeth gradually increases from the connection end of the mode conversion region 32 towards the rear end, forming a highly varied diffraction structure. This design utilizes the diffraction effect caused by variations in tooth height to achieve an adiabatic conversion of the optical field from a waveguide-bound mode to a radiation mode. This effectively compensates for the mode-field mismatch between the optical fiber and the chip waveguide, improves coupling efficiency, and enhances directivity and anti-reflection performance. By selectively etching the buried oxide layer of silicon dioxide beneath the grating modulation region, an air cavity suspension region is formed. The refractive index difference between the air and the grating material is significantly increased, resulting in stronger optical field confinement. This suppresses the leakage of light energy to the high-refractive-index silicon substrate, thereby reducing substrate loss and improving energy utilization.

[0012] It should be noted that the aforementioned changes in grating period, tooth height, and suspended structure are not independent of each other. Their parameters need to be designed in a coordinated manner; otherwise, it will lead to mismatch in optical field distribution or increased leakage loss, thus failing to achieve the expected improvement in coupling performance.

[0013] The beneficial effects of this invention are as follows: By designing the grating coupler with a variable period and gradually changing grating tooth height, and supplementing it with a suspended design, this invention achieves a comprehensive improvement in coupling efficiency, operating bandwidth, and process robustness. Specifically, the fully gradient tooth height and variable period design work together to optimize the coupling intensity distribution and phase matching conditions of the grating, achieving high coupling efficiency over a wide spectral range. The suspended structure further optimizes energy utilization by suppressing substrate leakage loss. The gradient structure can be realized using a grayscale exposure process, one advantage of which is reducing the stringent requirements on fabrication process precision. Furthermore, all structures are compatible with standard CMOS platforms, facilitating fabrication and enhancing device manufacturability and performance consistency. Ultimately, this invention provides a high-performance, easily packaged, and highly efficient grating coupling solution compatible with standard silicon photonics processes. It achieves comprehensive optimization of coupling efficiency and bandwidth while maintaining a compact device structure, providing key technical support for the development of high-speed optical communication and large-scale photonic integrated chips. Attached Figure Description

[0014] Figure 1This is a front cross-sectional view of the grating coupler of the present invention, which highlights the gradual change characteristics of the grating tooth height and the suspended structure. Figure 2 This is a three-dimensional structural schematic diagram of the grating coupler of the present invention.

[0015] The labels in the figure are as follows: 1-substrate layer, 2-silicon dioxide buried oxide layer, 21-region that suspends the grating area, 3-grating device layer, 31-single-mode waveguide region, 32-mode conversion region, 33-apodization grating region, 34-uniform grating region, 4-silicon dioxide cladding layer. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0017] Example 1: As Figures 1-2 As shown, a suspended grating coupler with variable period and gradually increasing tooth height includes: a substrate layer 1, a silicon dioxide buried oxide layer 2, a grating device layer 3, and a silicon dioxide upper cladding layer 4 arranged sequentially from bottom to top. The grating device layer 3 includes, from front end to rear end, a tightly connected single-mode waveguide region 31, a mode field conversion region 32, an apodization grating region 33, and a uniform grating region 34. The apodized grating region 33 and the uniform grating region 34 together constitute the grating region; The grating period of the apodization grating region 33 varies along the direction of light propagation, while the grating period of the uniform grating region 34 remains constant. The buried oxide layer 2 of silicon dioxide is selectively removed below the grating region to form a region 21 that suspends the grating region, thereby suppressing light energy leakage towards the substrate and improving coupling efficiency.

[0018] Furthermore, the tooth height of multiple grating teeth within the grating region continuously varies along the light propagation direction according to a preset variation law, and exhibits a monotonically increasing trend overall. The regions are closely connected and located on the same device layer. The gradient tooth height is fabricated using grayscale exposure and etching processes. The buried oxide layer of silicon dioxide beneath the grating region is selectively etched to form a suspended structure, thereby reducing the influence of the substrate refractive index and improving the light field confinement and coupling efficiency.

[0019] Furthermore, the tooth height variations of the apodization grating region 33 and the uniform grating region 34 are continuously connected at the connection point to form an overall continuously varying tooth height distribution.

[0020] Furthermore, the lateral dimension of the region 21 that suspends the grating region covers the grating region, so that the entire grating region is in a suspended state.

[0021] Furthermore, the changes in grating tooth height, grating period, and the region 21 that suspends the grating area work together to simultaneously regulate the light field distribution and phase matching relationship.

[0022] This invention also provides a method for fabricating a suspended grating coupler with variable period and gradually varying tooth height, the method comprising: The tooth height of the multiple grating teeth in the grating area is a gradient tooth height, which is prepared by grayscale exposure and etching process; The silicon dioxide buried oxide layer 2 below the grating region is selectively etched to form a region 21 that suspends the grating region; The grating device layer 3 defines a pattern on the top thin film using grayscale exposure technology, and then forms the required grating structure through dry or wet etching processes; in the variable period and variable tooth height grating regions, the grating period gradually changes within the apodization grating region 33, and the tooth height gradually changes within the grating region, which is used to achieve broadband matching and efficient mode field conversion.

[0023] The suspended region is formed by partially removing the buried oxide layer of silicon dioxide beneath the grating region using back-side or side-side etching techniques. This device can be fabricated using semiconductor processes such as CVD deposition, photolithography, reactive ion etching (RIE), wet etching, and bonding.

[0024] Furthermore, the grating coupler is fabricated on an SNOI (Silicon Nitride-on-Insulator) wafer, which comprises a silicon substrate, a silicon dioxide buried oxide layer, and a top silicon nitride thin film from bottom to top.

[0025] Furthermore, the suspended region is partially formed by selective wet etching (such as using hydrofluoric acid (HF) buffer solution) from the side of the chip to remove the silicon dioxide buried oxide layer below the grating area, thereby reducing the influence of the substrate refractive index and improving the light field confinement and coupling efficiency.

[0026] Furthermore, the grating device layer is formed on the top silicon nitride thin film using photolithography and dry or wet etching processes. Furthermore, the fabrication process of this device includes one or more of the following: CVD / PVD deposition, photolithography, reactive ion etching, wet etching, and bonding processes.

[0027] Furthermore, using an SNOI chip and based on semiconductor CMOS fabrication technology, a floating grating coupler with variable period and gradually increasing tooth height proposed in this invention can be realized. The main integration process flow is as follows: Step 1: Select an SNOI wafer, which from bottom to top includes a substrate layer 1, a silicon dioxide buried oxide layer 2, and a top silicon nitride thin film (later used to form the grating device layer 3). First, the wafer undergoes pretreatment: conventional wet cleaning processes are used to remove surface impurities, followed by rinsing with deionized water, then using an SPM solution to remove organic and metallic impurities, and finally, it is thoroughly cleaned with deionized water and dried to obtain a clean initial structure. Step 2: First, spin-coat photoresist and expose and develop it using a standard mask to precisely define the pattern of the grating area on the wafer surface. Then, dry etching is performed using the photoresist as a mask, etching away only a portion of the top layer of silicon nitride within the patterned area, thus initially forming the etched area of ​​grating device layer 3. The etching depth is determined according to the design. Step 3: Fill the recessed areas etched in Step 2 with silicon dioxide using chemical vapor deposition. Then, planarize the area using chemical mechanical polishing (CMP). This process must be precisely stopped on the surface of grating device layer 3 to restore the silicon dioxide-filled grating area to a flat surface with the surrounding silicon nitride area, preparing for subsequent grayscale photolithography. Step 4: Recoat the photoresist and expose it using a grayscale mask to form a photoresist mask layer with a continuously distributed thickness on the grating area. Then, transfer this thickness distribution pattern to the underlying silicon nitride layer by dry etching, thereby forming an apodized grating region 33 with a continuously gradient tooth height and a uniform grating region 34 within the grating device layer 3; Step 5: Perform photolithography again to cover the device areas to be preserved (including the single-mode waveguide region 31, mode conversion region 32, apodization grating region 33, and uniform grating region 34). Then, perform dry etching to completely etch the exposed silicon nitride area down to the underlying silicon dioxide buried oxide layer 2. This step ultimately forms the overall structure of the grating device layer 3 composed of silicon nitride, which also defines the specific functional areas of the single-mode waveguide region 31, mode conversion region 32, apodization grating region 33, and uniform grating region 34. Step 6: Deposit the upper cladding and define the wet etching windows. A silicon dioxide upper cladding layer is deposited on the device surface using plasma-enhanced chemical vapor deposition. Then, windows are defined on both sides of the silicon dioxide upper cladding layer 4 and the grating device layer 3 by dry etching, etching down to the silicon substrate surface to form the lateral channels for subsequent wet etching. Step 7: Retaining the photoresist as a protective layer, perform isotropic wet etching using a buffered hydrofluoric acid solution. The etching solution laterally removes the silicon dioxide buried oxide layer 2 material directly beneath the apodized grating region 33 and the uniform grating region 34 in the grating device layer 3 through the lateral opening made in Step 6, thereby forming a region 21 that suspends the grating region. This process requires precise time control to avoid over-etching that could affect the stability of adjacent structures; Step 8: Perform chemical mechanical planarization on the wafer with the deposited silicon dioxide cladding layer 4 to achieve global planarization. Then perform final cleaning, drying, and wafer dicing to obtain the final coupler chip.

[0028] Example 2: This example describes a floating grating coupler with variable period and gradually increasing tooth height, which is the same as the previous example, except that: The structure comprises: a substrate layer 1 with a thickness of 725 μm; a silicon dioxide buried oxide layer 2 with a thickness of 2 μm; a region 21 with a thickness of 2 μm and a length greater than the grating etching length of 2 μm, which suspends the grating region; a silicon nitride grating device layer 3 with a thickness of 300 nm, wherein the single-mode waveguide region 31 has a width of 1 μm and a length of 10 μm; an apodization grating region 33 with a period that linearly increases from 570 nm to 630 nm and a tooth height that gradually increases from 100 nm, for a total of 25 teeth; and a uniform grating region 34 with a constant period of 630 nm and a tooth height that continuously increases to 300 nm, for a total of 15 grating teeth. During the fabrication of this structure, a grayscale mask is used for exposure to form a photoresist mask with a continuously varying thickness in the grating region. Then, a waveguide structure with a continuously varying height is formed in this region by dry etching, thereby achieving a smooth transition of tooth height from 100 nm to 300 nm; and a silicon dioxide cladding layer 4 with a thickness of 3.0 μm. The region 21 that suspends the grating region is located below the apodized grating region 33 and the uniform grating region 34, and is formed by lateral wet etching of the buried oxide layer of silicon dioxide.

[0029] Example 3: This example describes a floating grating coupler with variable period and gradually increasing tooth height, which is the same as the previous example, except that: like Figures 1 to 2 As shown, this floating grating coupler with variable period and gradually increasing tooth height differs from Embodiment 1 in the silicon nitride thickness and grating period, including: a substrate layer 1 with a thickness of 725 μm; a silicon dioxide buried oxide layer 2 with a thickness of 2 μm; a region 21 with a thickness of 2 μm and a length greater than the grating etching length of 2 μm, which suspends the grating region; a silicon nitride grating device layer 3 with a thickness of 300 nm, wherein the width of the single-mode waveguide region 31 is 1 μm and the length is 10 μm; and the period of the apodized grating region 33 increases linearly from 560 nm to 6. The grating region 34 has 20 teeth, with a tooth height gradually increasing from 120 nm. The uniform grating region 34 has a constant period of 620 nm, with a tooth height continuously increasing to 400 nm, totaling 12 grating teeth. During fabrication, a grayscale mask is used for exposure, forming a photoresist mask with continuously varying thickness in the grating region. Then, dry etching is used to form a waveguide structure with a continuously varying height in this region, achieving a smooth transition in tooth height from 120 nm to 400 nm. A 3 μm thick silicon dioxide cladding layer 4 is also present. The region 21, which suspends the grating region, is located below the apodized grating region 33 and the uniform grating region 34, and is formed by lateral wet etching of the buried oxide layer in silicon dioxide.

[0030] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A suspended grating coupler with variable period and gradually varying tooth height, characterized in that, include: The layers arranged from bottom to top are: substrate (1), buried oxide layer (2), grating device layer (3), and top cladding layer (4). The grating device layer (3) includes, from front end to back end, a tightly connected single-mode waveguide region (31), a mode field conversion region (32), an apodization grating region (33), and a uniform grating region (34). The apodized grating region (33) and the uniform grating region (34) together constitute the grating region; The grating period of the apodization grating region (33) varies along the direction of light propagation, while the grating period of the uniform grating region (34) is constant. The buried oxide layer (2) of silicon dioxide is selectively removed below the grating region to form a region (21) that suspends the grating region, thereby suppressing the leakage of light energy toward the substrate and improving coupling efficiency.

2. The floating grating coupler with variable period and gradually varying tooth height according to claim 1, characterized in that: The tooth height of multiple grating teeth within the grating area changes continuously along the light propagation direction according to a preset variation law, and exhibits a monotonically increasing trend overall.

3. The floating grating coupler with variable period and gradually increasing tooth height according to claim 1, characterized in that: The tooth height variations of the apodization grating region (33) and the uniform grating region (34) are continuously connected at the connection point to form an overall continuously varying tooth height distribution.

4. The floating grating coupler with variable period and gradually varying tooth height according to claim 1, characterized in that: The lateral dimension of the region (21) that suspends the grating region covers the grating region so that the entire grating region is in a suspended state.

5. The floating grating coupler with variable period and gradually varying tooth height according to claim 1, characterized in that: The changes in grating tooth height, grating period, and the region (21) that suspends the grating area work together to simultaneously regulate the light field distribution and phase matching relationship.

6. The method for fabricating a suspended grating coupler with variable period and gradually increasing tooth height as described in any one of claims 1-5, characterized in that, The preparation method includes: The tooth height of the multiple grating teeth in the grating area is a gradient tooth height, which is prepared by grayscale exposure and etching process; The buried oxide layer (2) of silicon dioxide below the grating region is selectively etched to form a region (21) that suspends the grating region. The grating device layer (3) defines a pattern on the top thin film using grayscale exposure technology, and then forms the required grating structure through dry or wet etching processes; in the variable period and variable tooth height grating regions, the grating period gradually changes in the apodization grating region (33), and the tooth height gradually changes in the grating region, which is used to achieve broadband matching and efficient mode field conversion.

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