A method for smoothing the surface of a silicon optical waveguide in cooperation

Abstract

The application discloses a method for smoothing the surface of a silicon optical waveguide. The method mainly comprises the following steps: firstly, performing overall wet oxygen oxidation on an integrated electro-optical device and removing the oxidation layer, the oxidation rate of the step is high, and the process of removing the oxidation layer eliminates residual stress; secondly, performing hydrogen annealing on the optical path part of the integrated electro-optical device, the step has a good improvement on the side wall ripple of the optical waveguide device, and plays a role in reducing scattering loss; finally, performing excimer high-energy pulsed laser beam (XeCl light beam) smoothing treatment on the secondary defect part and the area with large side wall roughness, and realizing small-range fine processing. The synergistic effect of the above steps can improve the smoothness of the optical waveguide, realize high-precision and low-loss of the device, and the process steps are compatible with the CMOS process, and have low cost and feasibility.

Description

Technical Field

[0001] This invention belongs to the field of silicon optical waveguide devices in integrated optical circuits, and particularly relates to a method for smoothing the surface of silicon optical waveguides through synergistic effects and compatible with CMOS processes, thereby reducing device transmission loss. Background Technology

[0002] As the demand for computing power and data transmission continues to grow in the information age, the development of electronic chips is gradually pushing physical limits, leading to energy consumption crises and cost pressures. Silicon photonics chips, however, break through the limitations of Moore's Law, achieving massive information throughput without altering the binary architecture and computing system. They directly utilize CMOS facilities (including the entire value chain encompassing design tools, materials, equipment, and wafer fabs) to achieve direct, rapid commercialization and large-scale production of silicon photonics chips.

[0003] Optical waveguides are passive photonic devices and fundamental signal transmission channels in optical systems. Research on the fabrication of silicon waveguides and related photonic components using Chinese standard CMOS foundries is of profound significance. Key parameters of optical waveguides include propagation loss, bending loss, absorption loss, and scattering loss. Silicon is an indirect bandgap semiconductor material with low intrinsic absorption, resulting in low absorption loss. Furthermore, due to the large refractive index difference between the transmission layer and the buried oxide layer in silicon waveguides, the transmission layer has strong light confinement capabilities, leading to low radiation loss. Bending loss is determined by the radius of curvature of the silicon optical waveguide. Therefore, when considering reducing silicon optical waveguide transmission losses, the focus can be on minimizing scattering loss. Scattering loss has a root mean square error relationship with roughness, i.e.:

[0004]

[0005] Where k is the wavenumber in vacuum; σ is the root mean square difference in roughness; n is the effective refractive index of the waveguide; and d is half the width of the waveguide. Therefore, the surface roughness of silicon optical waveguide devices can be improved by modifying the manufacturing process, thereby reducing the scattering loss during device operation. In practice, the loss of most silicon waveguides is 3–30 dB / cm. As devices are miniaturized and waveguide sizes gradually decrease, the scattering loss is more significantly affected by surface roughness, and the requirements for surface smoothness become increasingly stringent. Summary of the Invention

[0006] The purpose of this invention is to address the problem of high transmission loss in silicon waveguide devices caused by the large surface roughness during the fabrication process of silicon waveguide materials. This invention provides a synergistic smoothing method for the silicon optical waveguide surface. Through the synergistic optimization of three process steps, the edge roughness of the silicon optical waveguide device is reduced, thereby lowering its transmission loss.

[0007] The objective of this invention is achieved through the following three technical steps:

[0008] Step (1): Perform overall wet oxygen oxidation on the silicon wafer of the integrated optoelectronic device, and then remove the oxide layer on the surface;

[0009] Step (2): The silicon waveguide region of the integrated optoelectronic device silicon wafer processed in step (1) is subjected to hydrogen annealing.

[0010] Step (3): Use an excimer high-energy pulsed laser beam to perform melt self-repair on the waveguide region of the silicon wafer where the roughness of the pattern sidewall is greater than the threshold a.

[0011] Preferably, in step (1), the wet oxidation process involves generating H2O gas in a closed reaction chamber at 950-1100℃ by mixing oxygen and hydrogen in a molecular ratio of 2:1. At this time, the oxygen and the generated H2O gas are pumped into the reaction chamber where the silicon wafer for integrated optoelectronic devices is placed, and the following reaction occurs:

[0012] Si + 2H₂O = SiO₂ + 2H₂

[0013] Preferably, the reaction time of the wet oxidation method in step (1) is 1.5-3h.

[0014] As a preferred option, the hydrogen annealing process in step (2) is specifically annealing at 950-1100℃ for 3-5 minutes in a hydrogen atmosphere, and more preferably annealing at 1050℃ and 40 torr pressure in a hydrogen atmosphere for 3 minutes.

[0015] Preferably, in step (3), the wavelength of the excimer laser is 308 nm and the pulse width is 20 ns.

[0016] Preferably, the excimer laser in step (3) is a rare gas halide, more preferably xenon chloride.

[0017] Preferably, in step (3), the laser beam emitted by the excimer laser is perpendicularly irradiated into the region of the waveguide region on the silicon wafer surface where the roughness of the sidewall of the pattern is greater than the threshold α, and the angle α between the waveguide region on the silicon wafer surface and the horizontal plane satisfies 0 < α < 90°. In order to pull the molten Si toward the waveguide ridge, a larger substrate tilt angle should be selected as much as possible.

[0018] As a preferred option, the threshold value a in step (3) is 10 nm.

[0019] In step (1), the present invention first employs a wet oxygen oxidation method. Oxidant molecules first enter the gas-solid phase interface through diffusion, gain greater energy, and then diffuse through the solid cladding to the solid-solid interface of the cladding-core layer and react with the silicon core layer. As the reaction time increases, the oxide layer thickens and the diffusion rate decreases.

[0020] As the oxidation process continues, the silicon at the peak position has a relatively large surface area, and more silicon is consumed during this oxidation process, gradually flattening the peak. Afterwards, removing the SiO2 capping layer will result in a lower surface roughness of the recovered silicon.

[0021] In step (2), a hydrogen annealing process is applied to the local optical path region. During the annealing process at 950-1100℃ in a hydrogen atmosphere, Si-H bonds are formed on the SOI (silicon on insulating substrate) surface under these high temperature and vacuum conditions. Furthermore, the atomic activity and migration rate of the silicon waveguide surface are enhanced at high temperatures. After the wet oxidation method and oxide layer removal in step (1), the presence of Si dangling bonds will facilitate the construction of Si-H bonds in step (2). Moreover, the smoothing of the Si material in step (1) will make the Si-H bonds more uniformly distributed, which will further facilitate their migration. In step (2), the formation of silicon-hydrogen bonds is an important factor in the changes in the morphology and structure of the silicon surface. Based on the principle of minimizing surface energy, hydrogen migrates from the high energy state at the sidewall peak to the low energy state, driving the silicon surface to become smoother.

[0022] In step (3), the hydrogen annealing process in step (2) can smooth the surface and eliminate lattice damage, but it also leaves secondary defects such as dislocation loops, stacking faults, and rod-shaped defects, as well as local stress. Therefore, it is necessary to further utilize a rare gas halide (e.g., xenon chloride 308nm) excimer high-energy pulsed laser beam as a heat source. After the laser passes through the transmission system, it generates a laser beam with uniform energy distribution and is projected onto the silicon material. The high-energy laser beam makes Si molten. Since molten Si has low viscosity and relatively high surface tension, the surface tension drives the liquid flow to minimize the surface energy, thereby obtaining a smooth surface. This achieves surface melting self-repair of secondary defects and areas with large side surface roughness, minimizing the instantaneous liquid surface free energy and thus achieving smoothing. This process is a small-scale fine processing.

[0023] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0024] This invention provides a method that is compatible with CMOS technology and reduces the transmission loss of silicon optical waveguide devices. This method achieves the reduction of the edge roughness of silicon optical waveguide pattern lines from 10-15 nm to 2-4 nm through the synergistic optimization of a series of process steps compatible with CMOS, thereby smoothing the surface of the optical waveguide and realizing high precision and low loss of the optical waveguide device. Attached Figure Description

[0025] Figure 1 This is a technical roadmap of the present invention;

[0026] Figure 2Two common SOI waveguide structures are shown, where (a) is a strip waveguide structure and (b) is a ridge waveguide structure.

[0027] Figure 3 This is a schematic diagram of the diffusion motion of silicon atoms on the surface during the hydrogen annealing stage, which is the process by which the waveguide surface gradually becomes flat.

[0028] Figure 4 This is a schematic diagram of an excimer high-energy pulsed laser beam acting on the surface of a silicon waveguide, illustrating that the smoothing principle of the silicon waveguide is to use a high-energy laser pulse at a certain incident angle to melt the sidewalls. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0030] Figure 1 The main technical roadmap of this invention combines multiple methods to reduce silicon surface roughness. It employs a series of processes, including overall wet oxidation followed by oxide layer removal, and high-temperature (950-1100℃) hydrogen annealing to further reduce the surface roughness of the silicon material in the optical path. Finally, it uses a rare gas halide (xenon chloride 308nm) excimer high-energy pulsed laser beam to smooth the roughened areas. This technical approach can reduce the average sidewall roughness from 21nm to 5nm, thereby reducing waveguide transmission loss to 3dB cm⁻¹. -1 . Figure 2 These are two common SOI waveguide structures in silicon photonics chips, where (a) is a strip waveguide structure and (b) is a ridge waveguide structure. The waveguide structure mainly includes a SiO2 cladding structure and a Si core layer structure. Transmission loss primarily depends on the surface (sidewall) roughness of the core layer structure. Methods for reducing optical waveguide roughness are detailed below:

[0031] (1) The wet oxygen oxidation process generates H2O gas in a closed reaction chamber where oxygen and hydrogen are mixed in a certain proportion. At this time, the oxygen and the generated H2O gas are pumped into the reaction chamber where the silicon wafer (which integrates SOI structure silicon optical waveguide device) is placed and the following reaction occurs:

[0032] Si + 2H₂O = SiO₂ + 2H₂

[0033] During the hydrogen annealing stage, silicon atoms undergo diffusion motion on the surface, which is the process by which the waveguide surface gradually becomes flat. Figure 3As shown, this process gradually flattens the uneven, spiked structure on the silicon surface as oxidation progresses. Wet oxidation has a much higher oxidation rate than dry oxidation, thus shortening the oxidation time. In this invention, at 1050℃ and for 2.5 hours, a SiO2 layer of 500 nm thickness was grown in a planar manner. The oxide layer was then washed away with HF acid solution, ultimately resulting in a silicon waveguide with an average surface roughness of 32 nm and a transmission loss of 36 dB cm⁻¹. -1 Further steps are required to reduce transmission loss.

[0034] (2) A hydrogen annealing process is applied to the silicon waveguide region. The two-dimensional diffusion process on the silicon surface can be explained using the following Mullins model:

[0035]

[0036] Among them, v n γ is the diffusion rate; γ is the surface tension of the material; Ω is the molar volume; n s D is the number of silicon atoms per unit area. s denoted by , where s is the diffusion coefficient, s is the arc length, K is the curvature, and T is the temperature. According to the Mullins model, the atomic mobility on the silicon surface is determined by the material, the diffusion coefficient, and the curvature. As the temperature increases, the atomic movement on the silicon surface accelerates. The rough surface has higher surface energy and is more likely to react with hydrogen to form Si-H bonds. Under the influence of atomic migration and Si-H bonds, the surface morphology of the rough silicon waveguide gradually improves.

[0037] The main parameters of the annealing process include gas pressure, annealing time, and annealing temperature. This invention selects annealing at 1050℃ and a pressure of 40 torr in a hydrogen atmosphere for 3 minutes. By adjusting the temperature, gas pressure, and time, the activation energy of the Si surface is affected, effectively eliminating surface sidewall ripples during hydrogen annealing, resulting in an SOI silicon waveguide with a surface roughness of 7 nm.

[0038] (3) A XeCl excimer laser with a wavelength of 308 nm and a pulse width of 20 ns was used to selectively melt the waveguide surface layer. The excimer high-energy pulsed laser beam was used to perform a melting self-repair process on areas with high surface roughness (e.g., approximately 10 nm higher than the smooth area). Figure 4As shown, the laser device is deliberately positioned so that the incident laser beam is perpendicularly irradiated onto the Si substrate, and placed at an angle. This configuration allows gravity to act on the Si along the direction of the incident laser beam. With sufficient energy density laser illumination, the Si melts, and the molten Si undergoes reforming, utilizing the surface tension of the liquid Si to flatten its surface, thereby reducing surface stress and thus surface roughness. The molten Si, like a normal liquid, will flatten along the direction of gravity with its surface normal aligned with the direction of gravity. Therefore, to pull the molten Si towards the waveguide ridge, a relatively large substrate tilt angle (e.g., 45°) should be chosen. This molten layer flows under the influence of surface tension and gravity, and then solidifies to form smooth sidewalls. This technique can achieve a local sidewall roughness reduction to 4 nm. Calculations show that the waveguide transmission loss caused by sidewall roughness in these waveguides is reduced to 3 dB cm⁻¹. -1 Because of the low viscosity of molten Si, this process can be completed within 100 ns, enabling rapid repair of defects.

Claims

1. A method for synergistic surface smoothing of silicon optical waveguides, characterized in that... The steps of this method are as follows: Step (1): Perform overall wet oxygen oxidation on the silicon wafer of integrated optoelectronic device for 1.5-3 h, and then remove the oxide layer on the surface. Due to the presence of Si dangling bonds, it will be more conducive to the construction of Si-H bonds. Furthermore, the overall smoothing of the Si material will make the Si-H bonds distributed evenly, which will be more conducive to their migration. Step (2): The silicon waveguide region of the integrated optoelectronic device silicon wafer processed in step (1) is subjected to hydrogen annealing. When the temperature rises, the atomic movement on the silicon surface accelerates, and the rough surface with high surface energy is more likely to react with hydrogen to form Si-H bonds. Under the action of atomic migration and Si-H bonds, the surface morphology of the rough silicon waveguide is gradually improved. The hydrogen annealing process is specifically annealing at 950-1100℃ for 3-5 min in a hydrogen atmosphere. Step (3): Use an excimer high-energy pulsed laser beam to perform melt self-repair on the region of the waveguide area on the silicon wafer surface where the roughness of the pattern sidewall is greater than the threshold a.

2. The method according to claim 1, characterized in that... In step (1), the wet oxidation process involves generating H2O gas in a closed reaction chamber at 950-1100℃ by mixing oxygen and hydrogen in a molecular ratio of 2:

1. At this time, the oxygen and the generated H2O gas are pumped into the reaction chamber where the silicon wafer for integrated optoelectronic devices is placed, and the following reaction occurs: 。 3. The method according to claim 1, characterized in that... In step (2), the hydrogen annealing process specifically involves annealing at 1050°C and a pressure of 40 torr in a hydrogen atmosphere for 3 minutes.

4. The method according to claim 1, characterized in that... In step (3), the wavelength of the excimer laser is 308 nm and the pulse width is 20 ns.

5. The method according to claim 4, characterized in that... In step (3), the excimer laser uses a rare gas halide.

6. The method according to claim 5, characterized in that... In step (3), the excimer laser uses xenon chloride.

7. The method according to claim 1, 4, 5, or 6, characterized in that... In step (3), the laser beam emitted by the excimer laser is perpendicularly irradiated into the region of the waveguide region on the silicon wafer surface where the roughness of the sidewall of the pattern is greater than the threshold a, and the angle α between the waveguide region on the silicon wafer surface and the horizontal plane satisfies 0 < α < 90°.

8. The method according to claim 1, characterized in that... In step (3), the threshold value a is 10nm.

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