Method for improving efficiency of electro-optical modulator and electro-optical modulator

Abstract

The invention discloses a method for improving the efficiency of an electro-optical modulator and the electro-optical modulator, and belongs to the technical field of integrated photoelectronics. The method comprises the following steps: firstly, preparing a silicon optical device on an SOI wafer, and bonding the silicon optical device on a lithium niobate wafer in an inverted manner to serve as a new substrate; removing the original silicon substrate and thinning the buried oxide layer; and finally, bonding a lithium niobate film on the thinned surface and preparing a modulation structure. By replacing a high-loss silicon substrate with a low-loss lithium niobate substrate, a symmetrical medium environment is constructed, and the problems of large radio frequency loss and mismatch of microwave and light wave refractive indexes in the prior art are effectively solved. According to the method, the microwave loss is obviously reduced, the speed matching is realized, the modulation efficiency and the bandwidth of the modulator are greatly improved, meanwhile, the thermal stress matching is improved, and the device yield is improved.

Description

Technical Field

[0001] This invention belongs to the field of integrated optoelectronics technology, specifically relating to a method for improving the efficiency of an electro-optic modulator and an electro-optic modulator, and more particularly to a method for improving the modulation efficiency and bandwidth of an electro-optic modulator based on heterogeneous integration technology. Background Technology

[0002] The rapid development of artificial intelligence (AI) technology is leading the Fourth Industrial Revolution, and co-packaged optoelectronic (CPO) technology has become a key path to overcome the bottleneck of network interconnection in AI computing centers. In optoelectronic interconnection systems, the electro-optic modulator, as a core device, undertakes the important task of converting electrical signals into optical signals. Its bandwidth, insertion loss, power consumption, and integration directly determine the overall performance of the optical communication system.

[0003] Lithium niobate-on-insulator (LNOI) films are ideal for realizing compact, low half-wave voltage, and high-bandwidth modulators due to their excellent electro-optic effects and large refractive index difference with surrounding materials. However, the strong chemical inertness of lithium niobate (LN) makes it difficult to perform fine micro- and nano-etching, which greatly increases the fabrication cost and process difficulty of all-lithium niobate modulators. In contrast, the silicon-on-insulator (SOI) platform has mature CMOS-compatible processes and can easily realize complex waveguide coupling, beam splitting / combining, detection, and thermal modulation functions, but silicon itself lacks second-order electro-optic effects.

[0004] To combine the advantages of both, existing technologies widely employ heterogeneous integration schemes that bond thin-film lithium niobate to etched SOI surfaces. This approach utilizes silicon-based photonics technology for passive and thermally modulated functions, achieving electro-optic modulation using only the lithium niobate layer. However, the buried oxide (BOX) layer of conventional SOI wafers is typically thin (e.g., 2-3 micrometers or even thinner), which means that the radio frequency (RF) signal applied to the electro-optic modulator electrodes cannot be effectively isolated, resulting in leakage into the underlying silicon substrate. Due to the high dielectric constant and semiconductor properties of the silicon substrate, this not only causes significant RF losses but also leads to a severe mismatch between the microwave refractive index and the optical wavegroup refractive index, thus limiting the electro-optic interaction length and high-frequency response efficiency of the modulator, becoming a bottleneck restricting the performance of high-end optical modules. Summary of the Invention

[0005] The purpose of this application is to address the problems of existing SOI-based heterogeneous integrated modulators, which cannot effectively isolate radio frequency signals from high-dielectric-constant silicon substrates due to the limitation of buried oxide layer thickness, resulting in microwave and optical wave velocity mismatch and excessive radio frequency loss, thus limiting the modulation efficiency and bandwidth of the devices. A "flip-chip substrate replacement" process is proposed, which involves flip-chip bonding a pre-prepared SOI functional layer onto a low-loss lithium niobate wafer, removing the original silicon substrate and thinning the oxide layer, and then integrating the lithium niobate modulation film to construct a heterogeneous integrated modulator structure supported entirely by a lithium niobate substrate.

[0006] To achieve the above-mentioned objectives, this application adopts the following technical solution: A method for improving the efficiency of an electro-optic modulator includes the following steps:

[0007] A silicon-on-insulator (SOI) wafer and a lithium niobate wafer are provided. The SOI wafer includes a silicon substrate layer, a buried oxide layer and a top silicon structure layer stacked sequentially. The top silicon structure layer of the SOI wafer is bonded to the surface of the lithium niobate wafer, making the lithium niobate wafer a supporting substrate. Remove the silicon substrate layer of the SOI wafer and thin the buried oxide layer to form an isolation dielectric layer of predetermined thickness. A lithium niobate film is bonded to the surface of the isolation dielectric layer; Optical waveguides and modulation electrodes are fabricated on lithium niobate thin films to form an electro-optic modulation structure.

[0008] Furthermore, before bonding the SOI wafer to the lithium niobate wafer, the process also includes: Silicon optical functional devices are prefabricated on the top silicon structure layer of an SOI wafer; Silicon photonic functional devices include at least one of optical couplers, beam splitters, beam combiners, photodetectors, or thermal modulators.

[0009] Furthermore, the specific steps of bonding one side of the top silicon structure layer of the SOI wafer to the surface of the lithium niobate wafer include: A first dielectric layer is deposited on the surface of an SOI wafer on which silicon photonics functional devices are fabricated; the SOI wafer is flipped so that its back side faces up, and the SOI wafer is bonded to the surface of a lithium niobate wafer through the first dielectric layer.

[0010] Furthermore, methods for removing the silicon substrate layer of an SOI wafer include one or a combination of mechanical polishing, chemical etching, or dry etching.

[0011] Furthermore, the specific steps for thinning the buried oxide layer are as follows: The buried oxide layer was thinned to below 200 nm using a chemical mechanical polishing process, so that the lithium niobate film and the underlying top silicon structure layer could meet the optical field coupling conditions.

[0012] Furthermore, after the step of bonding a lithium niobate film to the surface of the insulating dielectric layer, the method further includes: The bonded wafers are subjected to high-temperature annealing to enhance bonding strength and release stress.

[0013] Furthermore, the specific steps for fabricating an optical waveguide on a lithium niobate thin film include: A ridge waveguide structure is formed on a lithium niobate thin film by photolithography and etching processes. The ridge waveguide structure is located in the region above the top silicon structure layer.

[0014] Furthermore, after forming the electro-optic modulation structure, it also includes: A second dielectric layer is deposited on the electro-optic modulation structure as a protective layer; The protective layer, the isolation dielectric layer, and the lithium niobate wafer together constitute the low microwave loss environment of the electro-optic modulation structure.

[0015] Furthermore, the lithium niobate wafer and the lithium niobate film have matching lattice constants and coefficients of thermal expansion, and the lithium niobate wafer has low-loss tangent characteristics.

[0016] An electro-optic modulator, fabricated using the method described above, comprises: Lithium niobate substrate; Silicon photonic device layer located on a lithium niobate substrate; Thinned buried oxide layer located on silicon photonic device layer; A lithium niobate modulation layer is located on a thinned buried oxide layer, and the lithium niobate modulation layer includes a ridge waveguide and a modulation electrode; The silicon photonics device layer is encapsulated between the lithium niobate substrate and the thinned buried oxide layer.

[0017] Compared with the prior art, the present invention has the following significant advantages: 1. Achieving optimal refractive index matching and improving modulation bandwidth: In the structure formed by this invention, the upper and lower surfaces of the lithium niobate ridge waveguide are both wrapped with silicon dioxide layers of controllable thickness (the lower layer is the original buried oxide layer, and the upper layer is the protective layer). The thickness of the oxide-silicon layer below the lithium niobate ridge waveguide is greater than 10 micrometers. This symmetrical structural design effectively reduces the effective refractive index of microwaves, making it highly matched with the refractive index of optical wavegroups, thereby significantly improving the bandwidth and modulation efficiency of the modulator.

[0018] 2. Reduced microwave transmission loss and improved energy efficiency: Utilizing the extremely low loss tangent characteristic of lithium niobate material, this invention replaces the originally lossy silicon substrate with a lithium niobate wafer substrate. This fundamental material replacement completely eliminates parasitic absorption and loss of radio frequency signals in the substrate, resulting in lower loss of the modulation electrode when transmitting microwave signals, thereby achieving higher electro-optical conversion efficiency.

[0019] 3. Improved thermal stress matching and increased device yield: This invention innovatively uses lithium niobate wafers as the final substrate, ensuring that the substrate material is completely identical to the upper thin-film lithium niobate functional layer. This homogeneous material stacking greatly reduces lattice mismatch and differences in thermal expansion coefficients, effectively avoiding thin-film cracking or wafer warping caused by excessive thermal stress in subsequent high-temperature annealing or bonding processes, thus significantly improving the yield and reliability of device fabrication. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the method of an embodiment of this application; Figure 2 This is a schematic diagram of the stacked structure according to an embodiment of this application; Figure 3 This is the preparation process of the method in the embodiment of this application. Figure 1 ; Figure 4 This is the preparation process of the method in the embodiment of this application. Figure 2 ; Figure 5 This is the preparation process of the method in the embodiment of this application. Figure 3 . Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0022] Those skilled in the art should understand that, in the disclosure of this application, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limitations on this application.

[0023] Example 1 This embodiment provides a method for improving the efficiency of electro-optic modulators. The core of this method lies in integrating silicon photonic passive devices and lithium niobate modulation structures onto a low-loss lithium niobate substrate using heterogeneous integration technology. For example... Figure 1-5 As shown, the method includes the following specific steps: Step S1: Provide SOI wafers and lithium niobate wafers, and pre-fabricate silicon photonic devices.

[0024] First, prepare a silicon-on-insulator (SOI) wafer. The SOI wafer consists of a silicon substrate, a buried oxide layer (SiO2), and a top silicon structure layer from bottom to top. The silicon substrate is 500-800 μm thick, the buried oxide layer is 500-3000 nm thick, and the top silicon layer is 100-500 nm thick.

[0025] On the top silicon structure layer, silicon photonic functional devices are pre-fabricated using standard CMOS-compatible processes. This application does not limit the scope of the silicon photonic functional devices to the actual optical path design requirements, and these devices may include: Silicon waveguides and couplers: used for optical input / output and optical path transmission, where couplers include gratings ( Figure 3 The toothed structure on the left is used for vertical coupling of optical fibers. Germanium photodetector (Ge PD): Figure 3 The purple portion is prepared by epitaxial growth of germanium material on silicon and is used for optical signal monitoring; Thermal modulator: Figure 3 The red part, such as the titanium nitride (TiN) heater, or titanium nitride thermal tuner, is used for low-speed, wide-range phase adjustment and temperature control; Metal interconnect layer: used to connect the electrodes of the above active devices, that is, the metal leads connecting the Ge PD and TiN heater to form an electrical path; SiN (green block): Silicon nitride, used for end-face coupling.

[0026] In this way, all photonic functional units (coupling, beam splitting / combining, detection, and thermal modulation) except for electro-optic modulation are pre-fabricated on the top silicon layer using mature CMOS technology.

[0027] Simultaneously, a lithium niobate (LN) wafer is prepared. In this embodiment, a high-resistivity or optical-grade lithium niobate wafer is preferably used, and its crystal orientation (such as Z-cut or X-cut) is selected according to the design requirements of the modulator.

[0028] Step S2: Wafer flip bonding.

[0029] Before bonding, a planarizing dielectric layer (such as SiO2) is usually deposited on the surface of the SOI wafer on which silicon photonics functional devices are fabricated, and chemical mechanical polishing (CMP) is performed to ensure surface flatness.

[0030] Subsequently, the SOI wafer is flipped so that its "top silicon structure layer" faces down, and bonded to the surface of the lithium niobate wafer. The bonding process can employ hydrophilic direct bonding or plasma-activated bonding. After bonding, the original SOI structure is transferred to the lithium niobate wafer, which then becomes the new supporting substrate.

[0031] Step S3: Remove the silicon substrate and thin the buried oxide layer.

[0032] First, the silicon substrate of the original SOI wafer is removed. Specific removal methods include mechanical polishing to remove most of the silicon substrate, followed by chemical etching (such as using TMAH or KOH solution) or dry etching to completely remove the remaining silicon until the buried oxide layer of the original SOI is exposed.

[0033] Next, the exposed buried oxide layer is thinned. This is one of the key steps in this embodiment. The thickness of the buried oxide layer is precisely controlled using a chemical mechanical polishing (CMP) process, reducing it to below 200 nm.

[0034] Technical principle explanation: The buried oxide layer is thinned to below 200 nm to meet the evanescent wave coupling conditions between the subsequent "thin-film lithium niobate" and the "underlying silicon waveguide". If the layer is too thick, the optical field will not be able to transfer effectively between the silicon waveguide and the lithium niobate waveguide.

[0035] Step S4: Bonding thin film lithium niobate and high-temperature annealing.

[0036] A thin film of lithium niobate (LN) with a submicron thickness (e.g., 300nm-900nm) is used. Figure 4 The lithium niobate film is directly bonded to the surface of the buried oxide layer after thinning in step S3. The bonding of the lithium niobate film to the buried oxide layer surface can be achieved using either a thin-film transfer method or a "smart cut" method. After bonding, the wafer undergoes high-temperature annealing.

[0037] Benefits: Since the supporting substrate of the entire device is a lithium niobate wafer and the top bonded functional layer is also a lithium niobate thin film, both have the same coefficient of thermal expansion (CTE) and lattice constant.

[0038] Therefore, this structure can withstand higher annealing temperatures (e.g., above 300°C or even higher) without generating severe thermal stress that could lead to wafer breakage or film peeling, as is common in traditional silicon-based heterojunction. High-temperature annealing significantly enhances bond strength and repairs lattice damage that may occur during the peeling or transfer of lithium niobate films, thereby reducing optical loss.

[0039] Step S5: Fabricate a lithium niobate ridge waveguide.

[0040] Photoresist is spin-coated onto the surface of the bonded lithium niobate film, and waveguide patterns are defined by electron beam lithography or ultraviolet lithography.

[0041] Subsequently, an inductively coupled plasma (ICP) etching process or an argon ion beam etching process is used to etch a ridge waveguide on the lithium niobate film. The position of this ridge waveguide needs to be precisely aligned with the underlying silicon photonics device (such as the silicon waveguide coupling port) to ensure that the optical signal can be transmitted efficiently between the silicon layer and the lithium niobate layer.

[0042] Step S6: Prepare the electrode and protective layer.

[0043] Modulation electrodes (such as gold Au electrodes) are fabricated on both sides of a lithium niobate waveguide to apply radio frequency microwave signals to generate an electro-optic effect.

[0044] Finally, a relatively thick layer (e.g., more than 1 micrometer) of silicon dioxide (SiO2) is deposited on the entire device surface as an upper cladding / protective layer.

[0045] Example 2 The electro-optic modulator fabricated using the above method has the following stacked structure: Figure 2 and Figure 5 As shown in the last step, from bottom to top, it includes: Lithium niobate substrate: used as a mechanical support to reduce microwave loss; Silicon photonics device layer: This layer contains silicon waveguides, detectors, and hot electrodes encased in an insulating dielectric. Originally the top layer of SOI, it is now located at the bottom using a flip-chip process. Ultrathin isolation layer: derived from the original buried oxide layer with a thickness of less than 200 nm, used to achieve optical field coupling in the vertical direction; Lithium niobate modulation layer: contains etched ridge waveguides in which optical signals undergo electro-optic modulation; Upper cladding and electrodes: A silicon dioxide (SiO2) layer covering the modulation layer, containing metal electrodes.

[0046] Analysis of the technical effects of this embodiment: Extremely low RF loss: In traditional structures, the RF field penetrates to the lossy silicon substrate. However, in this embodiment, the substrate below the electrodes is replaced with a low-loss lithium niobate material with a positive tangent, resulting in almost no substrate loss for the RF signal and significantly improving high-frequency response characteristics.

[0047] Perfect speed matching: The lithium niobate ridge waveguide is symmetrically wrapped with two thick layers of SiO2 (original buried oxide layer + new protective layer). This structure greatly reduces the effective refractive index of microwaves, making it easier to match the refractive index of optical wavegroups, thereby significantly improving the modulation bandwidth.

[0048] High process yield: By utilizing the homogeneous material system of "LN substrate + LN thin film", the most challenging thermal mismatch problem in heterogeneous integration is solved.

[0049] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

[0050] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0051] Although this document uses a significant amount of technical terminology, the possibility of using other terms is not excluded. These terms are used merely to facilitate the description and explanation of the nature of this application; interpreting them as any additional limitation would be contrary to the spirit of this application.

[0052] This application is not limited to the above-described preferred embodiments. Anyone can derive other products in various forms under the guidance of this application. However, regardless of any changes made to their shape or structure, any technical solution that is the same as or similar to that of this application falls within the protection scope of this application.

Claims

1. A method for improving the efficiency of an electro-optic modulator, characterized in that, Includes the following steps: A silicon-on-insulator (SOI) wafer and a lithium niobate wafer are provided, wherein the SOI wafer comprises a silicon substrate layer, a buried oxide layer and a top silicon structure layer stacked sequentially. One side of the top silicon structure layer of the SOI wafer is bonded to the surface of the lithium niobate wafer, so that the lithium niobate wafer becomes a supporting substrate. Remove the silicon substrate layer of the SOI wafer and thin the buried oxide layer to form an isolation dielectric layer of predetermined thickness. A lithium niobate film is bonded to the surface of the insulating medium layer; An optical waveguide and modulation electrode are fabricated on the lithium niobate thin film to form an electro-optic modulation structure.

2. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, Prior to bonding the SOI wafer to the lithium niobate wafer, the method further includes: Silicon optical functional devices are prefabricated on the top silicon structure layer of the SOI wafer; The silicon photonics functional device includes at least one of an optical coupler, a beam splitter, a beam combiner, a photodetector, or a thermal modulator.

3. The method for improving the efficiency of an electro-optic modulator according to claim 2, characterized in that, The specific steps of bonding one side of the top silicon structure layer of the SOI wafer to the surface of the lithium niobate wafer include: A first dielectric layer is deposited on the surface of an SOI wafer on which silicon photonics functional devices are fabricated; the SOI wafer is flipped so that its back side faces up, and the SOI wafer is bonded to the surface of the lithium niobate wafer through the first dielectric layer.

4. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, Methods for removing the silicon substrate layer of the SOI wafer include one or a combination of mechanical polishing, chemical etching, or dry etching.

5. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, The specific steps for thinning the buried oxide layer are as follows: The buried oxide layer is thinned to below 200 nm using a chemical mechanical polishing process, so that the lithium niobate film and the underlying top silicon structure layer can meet the optical field coupling conditions.

6. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, Following the step of bonding a lithium niobate film to the surface of the insulating dielectric layer, the method further includes: The bonded wafers are subjected to high-temperature annealing to enhance bonding strength and release stress.

7. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, The specific steps for fabricating an optical waveguide on the lithium niobate thin film include: A ridge waveguide structure is formed on the lithium niobate film by photolithography and etching processes. The ridge waveguide structure is located in the region above the top silicon structure layer.

8. The method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, After forming the electro-optic modulation structure, the following is also included: A second dielectric layer is deposited on the electro-optic modulation structure as a protective layer; The protective layer, the isolation dielectric layer, and the lithium niobate wafer together constitute the low microwave loss environment of the electro-optic modulation structure.

9. A method for improving the efficiency of an electro-optic modulator according to claim 1, characterized in that, The lithium niobate wafer and the lithium niobate film have matching lattice constants and coefficients of thermal expansion, and the lithium niobate wafer has low-loss tangent characteristics.

10. An electro-optic modulator, characterized in that, The electro-optic modulator, prepared by the method according to any one of claims 1 to 9, comprises: Lithium niobate substrate; Silicon photonic device layer located on the lithium niobate substrate; The thinned buried oxide layer is located on the silicon photonics device layer; A lithium niobate modulation layer is located on the thinned buried oxide layer, the lithium niobate modulation layer comprising a ridge waveguide and a modulation electrode; The silicon photonic device layer is sandwiched between the lithium niobate substrate and the thinned buried oxide layer.

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