Lithium niobate heterogeneous gain type modulator and integration method thereof

Abstract

According to the lithium niobate heterogeneous gain type modulator and the integration method thereof, heterogeneous integration of erbium-doped lithium niobate and other materials (such as silicon nitride or silicon) is achieved through the wafer-to-wafer bonding technology, and the modulator comprises a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer, a silicon oxide buffer layer, an erbium-doped lithium niobate layer, a silicon oxide wrapping layer and a metal electrode layer. In the non-wafer bonding area, optical signals are only transmitted in the waveguide layer; in a wafer bonding area, an erbium-doped lithium niobate layer is introduced, so that amplification of an optical signal can be realized; the device further comprises a multi-mode interferometer, an interlayer coupler, an electro-optical phase shifting arm, a traveling wave electrode and other structures, and beam splitting, coupling, electro-optical modulation and beam combination of light are achieved. The integration method comprises the steps of selecting a substrate wafer and a bonding wafer, patterning the waveguide layer, depositing, bonding the erbium-doped lithium niobate wafer and the like. The advantages of different materials are combined, the gain characteristic of erbium-doped lithium niobate is introduced, and brand new performance improvement is brought to the electro-optical modulator.

Description

Technical Field

[0001] The invention belongs to the technical field of photon heterogeneous integration, and in particular to a lithium niobate heterogeneous gain modulator and an integration method thereof. Technical Background

[0002] Microelectronics technology has enabled electronic devices to move from discrete to integrated, bringing decades of exponential improvements in cost, reliability, power consumption and volume, and is the key to the widespread application of electronic information technology. At present, electronic information technology is facing bottlenecks such as bandwidth and energy consumption, and Moore's Law is struggling. Photons, as another major information carrier, have the characteristics of high bandwidth, high speed, low power consumption and high parallelism. Electro-optic modulators are common and key core components in typical optoelectronic processing systems. They can realize the conversion of electrical signals to optical signals. They are widely used in current optical computing, photon analog-to-digital conversion, laser radar and other fields. There is no shortage of large-scale cascades of electro-optic modulators in optoelectronic processing systems. However, with the rapid development of fields such as artificial intelligence, the current explosive growth in data processing volume, and the changing application scenarios have put forward lower latency requirements for devices, so electro-optic modulators are required to have lower optical losses, larger integration scale, and higher computing speed.

[0003] At present, lithium niobate material has become the main integrated platform for electro-optical modulators due to its excellent electro-optical properties. For example, in 2018, Harvard University developed a Mach-Zehnder electro-optic modulator based on lithium niobate film, using traveling wave electrodes to modulate the lithium niobate electro-optic phase shift arm. Its electro-optic bandwidth exceeds 100 GHz and the device loss is 1 dB (Cheng W., et al., "Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages, Nature. 562, 101-104, 2018). In 2019, Sun Yat-sen University proposed an electro-optic modulator heterogeneously integrated with silicon and lithium niobate. The two materials are bonded by benzocyclobutene. Its electro-optic bandwidth exceeds 70 GHz and the device loss is 2.5 dB (Xinlun C., et al., "High-performance hybrid silicon and lithium niobate Mach-Zehnder modulators for 100 Gbit s-1 and beyond, Nature photonics. 13, 359-364, 2019). The electro-optic modulators reported so far can already achieve high-speed signal processing, but they still face many challenges. First, modulator loss is one of the key factors affecting system performance and integration scale. Reducing modulator loss and improving system performance is one of the current research focuses. Secondly, lithium niobate wafers are small in size and are not suitable for large-scale integration. How to combine lithium niobate with other materials (such as silicon) through heterogeneous integration technology to achieve larger-scale integration is another important direction of current research. In addition, how to further improve the computing speed of electro-optic modulators to meet the growing demand for data processing is also a problem that needs to be solved urgently. Summary of the invention

[0004] The purpose of the present invention is to solve the above-mentioned problems of the prior art. In view of the numerous challenges faced by large-scale integration, a lithium niobate heterogeneous gain modulator and its integration method are proposed. By doping lithium niobate with rare earth ions, it has the advantage of on-chip light amplification, thereby compensating for the loss problem in the link. At the same time, by utilizing the high integration of silicon and the compatibility of CMOS technology, the low loss of silicon nitride and the compatibility of CMOS technology, and the high-speed electro-optical modulation performance of lithium niobate, it aims to break through the limitations of traditional modulators in terms of loss and single material platform, and realize high-density integration of devices. The bonding processing scheme adopted also increases the freedom of integration, so that the scale of integration is no longer constrained by the size of lithium niobate wafers.

[0005] The technical solution of the present invention is as follows:

[0006] A lithium niobate heterogeneous gain modulator and its integration method, characterized in that: the device realizes heterogeneous integration by wafer-to-wafer bonding, the wafer is an erbium-doped lithium niobate wafer, and the wafer is a silicon nitride or silicon wafer. In the wafer bonding area, the waveguide cross section is, from bottom to top, a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer, a silicon oxide buffer layer, an erbium-doped lithium niobate layer, a silicon oxide cladding layer, and a metal electrode layer, and the waveguide layer material is silicon or silicon nitride. In the non-wafer bonding area, the waveguide cross section is, from bottom to top, a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer, and a silicon oxide buffer layer, and the waveguide layer material is silicon or silicon nitride.

[0007] The optical structure of the electro-optic modulator is from left to right: multi-mode interferometer, interlayer coupler, electro-optic phase shift arm, interlayer coupler, multi-mode interferometer. The electrical structure of the electro-optic modulator is a traveling wave electrode, which is composed of a signal electrode in the middle and ground electrodes on both sides. The electro-optic phase shift arm is located between the signal electrode and the ground electrode. The pump light and the signal light are combined through an off-chip wavelength division multiplexer and then input to the chip. The multi-mode interferometer is used for light splitting and combining; the interlayer coupler is used for light transmission between the waveguide layer and the erbium-doped lithium niobate layer to form an erbium-doped lithium niobate heterogeneous waveguide; the electro-optic phase shift arm is used to achieve high-speed electro-optical interaction, as well as optical-optical interaction between the pump light and the signal light; the traveling wave electrode is used to load high-speed radio frequency signals.

[0008] The multimode interferometer is located in the waveguide layer, the waveguide layer material is silicon or silicon nitride, and belongs to the non-bonding area, and the optical mode field is distributed in the waveguide layer; the interlayer coupler is composed of a waveguide layer, a silicon oxide buffer layer, and an erbium-doped lithium niobate layer, and belongs to the bonding area. The optical mode field is gradually distributed from the waveguide layer to the waveguide layer and the erbium-doped lithium niobate layer to form a hybrid waveguide; the electro-optic modulator is composed of a waveguide layer, a silicon oxide buffer layer, and an erbium-doped lithium niobate layer, and belongs to the bonding area. The optical mode field is distributed in the waveguide layer and the erbium-doped lithium niobate layer. The pump light realizes on-chip optical amplification of the signal light in this area, and the signal light and the radio frequency signal of the traveling wave electrode have electro-optical interaction; the traveling wave electrode is composed of a metal electrode layer, and belongs to the bonding area.

[0009] The above-mentioned lithium niobate heterogeneous gain modulator and its integration method, the method comprises the following steps:

[0010] 1) Select a substrate wafer and a bonding wafer, and use a commercial SOI wafer or a silicon nitride wafer as the substrate wafer. The SOI wafer comprises a silicon substrate layer, a silicon oxide isolation layer, and a silicon waveguide layer from bottom to top; the silicon nitride wafer comprises a silicon substrate layer, a silicon oxide isolation layer, and a silicon nitride waveguide layer from bottom to top; the bonding wafer is an erbium-doped lithium niobate wafer, which comprises a silicon substrate layer, a silicon oxide isolation layer, and an erbium-doped lithium niobate layer from bottom to top.

[0011] 2) patterning the waveguide layer of the substrate wafer, etching the entire waveguide layer, and preparing the multi-mode interferometer, interlayer coupler assembly, and electro-optic phase shift arm waveguide;

[0012] 3) depositing a silicon oxide buffer layer on the patterned wafer, and then performing chemical mechanical polishing to make the wafer surface smooth and flat, wherein the buffer layer has a thickness of 50 nm to 350 nm;

[0013] 4) Bonding the erbium-doped lithium niobate wafer to the patterned substrate wafer, and annealing for several hours after bonding to improve the bonding strength. After bonding, removing silicon and silicon oxide on the erbium-doped lithium niobate wafer by chemical mechanical polishing and tetramethylammonium hydroxide;

[0014] 5) depositing a silicon oxide cladding layer on the erbium-doped lithium niobate layer, and then depositing a metal as a metal electrode.

[0015] Compared with the prior art, the present invention has the following outstanding advantages:

[0016] 1) The lithium niobate heterogeneous gain modulator and its integration method described in the present invention integrate the advantages of multiple material systems, including the advantages of high-speed electro-optic modulation and on-chip optical amplification of erbium-doped lithium niobate, the advantages of high compactness of silicon materials, and the advantages of low loss of silicon nitride, so that the device is lossless or even has gain, breaking through the limitation of optical loss on the integration scale and providing strong technical support for large-scale integration.

[0017] 2) The lithium niobate heterogeneous gain modulator and its integration method described in the present invention are integrated based on chip-to-wafer bonding, which only requires bonding in the modulation area and avoids the problem of current erbium-doped lithium niobate wafers limiting the integration scale due to their small size. It has the advantage of monolithic integration with other passive devices.

[0018] 3) The lithium niobate heterogeneous gain modulator and its integration method described in the present invention can directly utilize the rich silicon and silicon nitride device library, so the modulator can be directly integrated with silicon-based and silicon nitride photonic circuits, and has the potential for large-scale monolithic integration.

[0019] 4) The interlayer coupler described in the present invention enables the pump light and the signal light to be transmitted from the waveguide layer to the erbium-doped lithium niobate layer to interact with each other, thereby realizing efficient optical interconnection between the erbium-doped lithium niobate and the mature silicon photonic circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic cross-sectional view of the waveguide structure of the lithium niobate heterogeneous gain modulator of the present invention, wherein (a) is a cross-sectional view of the wafer in the non-bonding area, and (b) is a cross-sectional view of the wafer in the bonding area.

[0021] Figure 2It is a schematic diagram of the structure of the lithium niobate heterogeneous gain modulator of the present invention.

[0022] Figure 3 It is a cross-sectional view of the modulation region of Example 1 of the lithium niobate heterogeneous gain-type modulator of the present invention, wherein (a) is a cross-sectional view of the interlayer coupler, and (b) is a cross-sectional view of the electro-optic phase shift arm.

[0023] Figure 4 It is a cross-sectional view of the modulation region of Example 2 of the lithium niobate heterogeneous gain-type modulator of the present invention, wherein (a) is a cross-sectional view of the interlayer coupler, and (b) is a cross-sectional view of the electro-optic phase shift arm. DETAILED DESCRIPTION

[0024] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments, and detailed implementation methods and structures are given, but the protection scope of the present invention is not limited to the following embodiments.

[0025] Embodiment 1 provides a lithium niobate heterogeneous gain modulator and its integration method. This modulator realizes heterogeneous integration of erbium-doped lithium niobate and other materials (such as silicon nitride or silicon) through wafer-to-wafer bonding technology. It not only combines the advantages of different materials, but also introduces the gain characteristics of erbium-doped lithium niobate, bringing a new performance improvement to the electro-optic modulator.

[0026] See also Figure 1 , Figure 1 The schematic diagram of the cross-section of the waveguide structure of the lithium niobate heterogeneous gain modulator of the present invention is shown in FIG. Figure 1 (a), the optical signal is transmitted only in the waveguide layer without gain modulation. The waveguide structure includes:

[0027] The silicon substrate layer 1, as the base of the entire structure, provides stable mechanical support and electrical properties.

[0028] The silicon oxide isolation layer 2 is located on the silicon substrate layer 1 and plays the role of isolation and protection to prevent the lower layer material from having adverse effects on the upper waveguide layer 3.

[0029] The waveguide layer 3 is made of silicon or silicon nitride and is a transmission channel for optical signals;

[0030] The silicon oxide buffer layer 4, located on the waveguide layer 3, serves as a transition layer for subsequent layer deposition and helps to optimize the transmission characteristics of the optical signal.

[0031] In the wafer bonding area (such as Figure 1 (b) shows that, based on the non-bonded area, an additional

[0032] The erbium-doped lithium niobate layer 5 is the core part of the modulator. By doping erbium ions, the lithium niobate has gain characteristics and can amplify the optical signal.

[0033] The silicon oxide cladding layer 6 covers the erbium-doped lithium niobate layer 5 and plays a role of protection and isolation, and also helps to optimize the transmission of optical signals.

[0034] The metal electrode layer 7 is used to apply an electrical signal and modulate the optical signal through the electro-optical effect.

[0035] In the bonding area, the waveguide structure consists of a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer (silicon or silicon nitride), a silicon oxide buffer layer, and an erbium-doped lithium niobate layer. The optical signal is transmitted in the waveguide layer and enters the erbium-doped lithium niobate layer through an interlayer coupler to achieve gain modulation.

[0036] Figure 2 This is a schematic diagram of the structure of the lithium niobate heterogeneous gain modulator of the present invention. As shown in the figure, the optical structure is a multimode interferometer 1-1, an interlayer coupler 1-2, an electro-optical phase shift arm 1-3, an interlayer coupler 1-2 and a multimode interferometer 1-1 from left to right. This structure realizes the splitting, coupling, electro-optical modulation and beam combining of light. The electrical structure is a traveling wave electrode 1-4. The traveling wave electrode 1-4 is composed of a signal electrode in the middle and ground electrodes on both sides, and the electro-optical phase shift arm is located between the signal electrode and the ground electrode. This structure allows the loading of high-speed radio frequency signals to achieve high-speed electro-optical interaction.

[0037] The pump light and the signal light are combined by an off-chip wavelength division multiplexer and then input to the chip. After the multimode interferometer 1-1 combines the input light, the light is coupled from the waveguide layer 3 to the erbium-doped lithium niobate layer 5 through the interlayer coupler 1-2 to form an erbium-doped lithium niobate heterogeneous waveguide; the electro-optical phase shift arm 1-3 is used to realize high-speed electro-optical interaction, as well as optical interaction between the pump light and the signal light; the traveling wave electrode 1-4 is used to load high-speed radio frequency signals, interact with the optical signal in the electro-optical phase shift arm 1-3, and realize electro-optical modulation. The modulated and amplified signal light is split and output through the multimode interferometer 1-1.

[0038] The multimode interferometer 1 - 1 is used for light beam splitting and combining and is located in the waveguide layer 3 . The waveguide layer material is silicon or silicon nitride and belongs to the non-bonding area. The optical mode field is distributed in the waveguide layer 3 .

[0039] The interlayer coupler 1-2 is used for light transmission between the waveguide layer 3 and the erbium-doped lithium niobate layer 5 to form an erbium-doped lithium niobate heterogeneous waveguide. Figure 3 As shown in (a), it is composed of a waveguide layer 3, a silicon oxide buffer layer 4, and an erbium-doped lithium niobate layer 5, which belongs to the bonding area. Evanescent wave coupling is realized through a tapered gradient waveguide. The optical mode field is gradually distributed from the waveguide layer 3 to the waveguide layer 3 and the erbium-doped lithium niobate layer 5, thereby forming a hybrid waveguide.

[0040] The electro-optic phase shift arm 1-3 is located between the signal electrode and the ground electrode, and is used to realize high-speed electro-optical interaction and optical-optical interaction between the pump light and the signal light. The electro-optical phase shift arm 1-3 is composed of a waveguide layer 3, a silicon oxide buffer layer 4, and an erbium-doped lithium niobate layer 5, and belongs to the bonding area. The optical mode field is mainly distributed in the waveguide layer 3 and the erbium-doped lithium niobate layer 5. The pump light realizes on-chip optical amplification of the signal light in this area. At the same time, the signal light and the radio frequency signal of the traveling wave electrode 1-4 undergo electro-optical interaction.

[0041] The traveling wave electrodes 1-4 are composed of a signal electrode in the middle and ground electrodes on both sides. They are used to load high-speed RF signals and are composed of metal electrode layers, belonging to the bonding area. By loading RF signals onto the traveling wave electrodes, electro-optical interactions occur with the signal light in the electro-optical phase shift arm, thereby modulating the signal light.

[0042] The above-mentioned lithium niobate heterogeneous gain modulator integration method comprises the following steps:

[0043] 1) Select substrate wafer and bonding wafer: Use commercial SOI wafer or silicon nitride wafer as substrate wafer, where SOI wafer is composed of silicon substrate layer, silicon oxide isolation layer and silicon waveguide layer, and silicon nitride wafer is composed of silicon substrate layer, silicon oxide isolation layer and silicon nitride waveguide layer. The selection of these two wafers depends on the application requirements, and the silicon waveguide layer or silicon nitride waveguide layer will serve as the transmission channel of the optical signal. The bonding wafer is an erbium-doped lithium niobate wafer, which includes a silicon substrate layer, a silicon oxide isolation layer and an erbium-doped lithium niobate layer from bottom to top, where the erbium-doped lithium niobate layer will be used to realize the electro-optical effect and optical amplification function.

[0044] 2) Patterning the waveguide layer of the substrate wafer, etching the waveguide layer, and preparing a multimode interferometer 1-1, an interlayer coupler 1-2 component, and an electro-optical phase shift arm waveguide 1-3. The multimode interferometer 1-1 is composed of a wide waveguide with one input port and two output ports; the interlayer coupler component 1-2 is a tapered waveguide with a gradually changing width; and the electro-optical phase shift arm waveguide 1-3 is a narrow waveguide that can couple the optical mode field into the erbium-doped lithium niobate.

[0045] 3) depositing a silicon oxide buffer layer on the patterned wafer, and making the wafer surface smooth and flat by chemical mechanical polishing;

[0046] 4) Bonding the erbium-doped lithium niobate wafer to the patterned substrate wafer to form a heterostructure. Removing silicon and silicon oxide on the erbium-doped lithium niobate wafer by chemical mechanical polishing and tetramethylammonium hydroxide solution, so that the erbium-doped lithium niobate layer is exposed to air;

[0047] 5) A silicon oxide cladding layer is deposited on the erbium-doped lithium niobate layer, and then a metal layer is deposited on the silicon oxide cladding layer to form a traveling wave electrode structure.

[0048] Example 2

[0049] The multimode interferometer 1-1 described in Example 2 of the lithium niobate heterogeneous gain modulator and its integration method of the present invention is located in the waveguide layer 3, the waveguide layer material is silicon or silicon nitride, belongs to the non-bonding area, and the optical mode field is distributed in the waveguide layer 3; the cross-sectional view of the interlayer coupler 1-2 is as shown in Figure 4 As shown in (a), it is composed of a waveguide layer 3, a silicon oxide buffer layer 4, and an erbium-doped lithium niobate layer 5, which belongs to the bonding area. The evanescent wave coupling is realized by a tapered gradient waveguide. The optical mode field is gradually distributed from the waveguide layer to the waveguide layer 3 and the erbium-doped lithium niobate layer 5 to form a hybrid waveguide; the cross-sectional view of the electro-optic modulator is shown in Figure 4 As shown in (b), the electro-optic phase shift arm 1-3 is composed of an erbium-doped lithium niobate layer 5, which belongs to the bonding area. The optical mode field is distributed in the erbium-doped lithium niobate layer 5. The pump light realizes on-chip optical amplification of the signal light in this area, and the signal light and the radio frequency signal of the traveling wave electrode 1-4 interact electro-optically; the traveling wave electrode is composed of a metal electrode layer, which belongs to the bonding area. The difference between Example 2 and Example 1 is that the erbium-doped lithium niobate layer 5 is etched in Example 2.

Claims

1. A lithium niobate heterogeneous gain modulator, characterized in that: Through the wafer-to-wafer bonding technology, heterogeneous integration of erbium-doped lithium niobate wafers and silicon nitride wafers or silicon wafers is achieved, wherein in the wafer bonding area, the waveguide structure is composed of a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer, a silicon oxide buffer layer, an erbium-doped lithium niobate layer, a silicon oxide cladding layer and a metal electrode layer from bottom to top, and in the non-wafer bonding area, the waveguide structure is composed of a silicon substrate layer, a silicon oxide isolation layer, a waveguide layer and a silicon oxide buffer layer from bottom to top, and the waveguide layer material is silicon or silicon nitride; The pump light and the signal light are combined by an off-chip wavelength division multiplexer and then inputted onto the chip. After the multimode interferometer combines the input light, the light is coupled from the waveguide layer to the erbium-doped lithium niobate layer via an interlayer coupler to form an erbium-doped lithium niobate heterogeneous waveguide. High-speed electro-optical interaction and optical-optical interaction between the pump light and the signal light are realized by an electro-optical phase shift arm. High-speed radio frequency signals are loaded by traveling wave electrodes and interact with the optical signals in the electro-optical phase shift arm to realize electro-optical modulation. The multi-mode interferometer is used for light beam splitting and combining, is located in the waveguide layer, and is non-bonded; The interlayer coupler is used for light transmission between the waveguide layer and the erbium-doped lithium niobate layer to form an erbium-doped lithium niobate heterogeneous waveguide, and is located in the wafer bonding area; The electro-optic phase shift arm is used to realize high-speed electro-optical interaction and optical-optical interaction of pump light and signal light, and is located in the wafer bonding area; The traveling wave electrode is used to load high-speed radio frequency signals, interact electro-optically with the signal light in the electro-optic phase shift arm, and modulate the signal light. It is located in the metal electrode layer.

2. The lithium niobate heterogeneous gain modulator according to claim 1, characterized in that: The interlayer coupler is composed of the waveguide layer, the silicon oxide buffer layer and the erbium-doped lithium niobate layer, and realizes evanescent wave coupling through a tapered gradient waveguide. The optical mode field is gradually distributed from the waveguide layer to the waveguide layer and the erbium-doped lithium niobate layer to form a hybrid waveguide.

3. The lithium niobate heterogeneous gain modulator according to claim 1, characterized in that: The electro-optic phase shift arm is composed of the waveguide layer, the silicon oxide buffer layer and the erbium-doped lithium niobate layer, or is composed of the erbium-doped lithium niobate layer. The optical mode field is distributed in the waveguide layer and the erbium-doped lithium niobate layer. The pump light realizes on-chip optical amplification of the signal light in this area, and the signal light and the radio frequency signal of the traveling wave electrode undergo electro-optical interaction.

4. The lithium niobate heterogeneous gain modulator according to claim 1, characterized in that: The traveling wave electrode is composed of a signal electrode in the middle and ground electrodes on both sides of the signal electrode, and the electro-optical phase shift arm is located between the signal electrode and the ground electrode.

5. A method for integrating a lithium niobate heterogeneous gain modulator, characterized in that: The steps include: 1) Selecting a substrate wafer and a bonding wafer, wherein the substrate wafer is an SOI wafer or a silicon nitride wafer, comprising a silicon substrate layer, a silicon oxide isolation layer and a waveguide layer, and the bonding wafer is an erbium-doped lithium niobate wafer, comprising a silicon substrate layer, a silicon oxide isolation layer and an erbium-doped lithium niobate layer; 2) patterning and etching the substrate wafer waveguide layer to prepare a multimode interferometer, an interlayer coupler assembly, and an electro-optic phase shift arm waveguide; 3) depositing a silicon oxide buffer layer on the patterned wafer; 4) bonding the erbium-doped lithium niobate wafer to the patterned substrate wafer to form a heterostructure; 5) removing excess material on the erbium-doped lithium niobate wafer to expose the erbium-doped lithium niobate layer; 6) depositing a silicon oxide cladding layer on the erbium-doped lithium niobate layer, and then depositing a metal layer on the silicon oxide cladding layer to form a traveling wave electrode structure.

6. The method for integrating a lithium niobate heterogeneous gain modulator according to claim 5, characterized in that: The erbium-doped lithium niobate layer is x-cut and has a thickness of 300nm to 600nm.

7. The integration method of the lithium niobate heterogeneous gain modulator according to claim 5, characterized in that: The thickness of the silicon oxide buffer layer is 50nm-350nm.

8. The method for integrating a lithium niobate heterogeneous gain modulator according to claim 5, characterized in that: The thickness of the metal electrode layer is 600nm-900nm, and the material is gold, aluminum or copper.

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