Silicon-silicon oxide-lithium niobate high bandwidth electro-optic modulator and integration method

Abstract

Silicon-silicon oxide-lithium niobate high bandwidth electro-optical modulator and integrated method, the wafer is from bottom to top silicon substrate layer, silicon oxide isolation layer, lithium niobate film layer, silicon oxide buffer layer and silicon film layer; The electro-optical modulator comprises a multimode interferometer, a mode spot converter, an electro-optical phase shift arm, a traveling wave electrode and a thermoelectric electrode. The present application forms a conventional silicon waveguide, a thin silicon waveguide and a silicon oxide waveguide by etching multiple times, and forms a traveling wave electrode and a thermoelectric electrode by metal deposition. The silicon oxide layer acts as a buffer layer to alleviate the thermal mismatch and lattice mismatch between the silicon wafer and the lithium niobate wafer, improve the reliability and yield of the electro-optical modulator based on the silicon-silicon oxide-lithium niobate wafer, and provide a new degree of freedom for the design of the electro-optical modulator. The present application is compatible with CMOS process, and takes advantage of silicon, silicon oxide and lithium niobate, which is conducive to improving the compactness and reliability of integration.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic device integration, specifically a high-bandwidth electro-optic modulator for silicon-silicon oxide-lithium niobate wafers and its integration method. Technical Background

[0002] With the rapid development of optical communication, microwave photonics, optical quantum computing, and photonic intelligent systems, electro-optic modulators, as one of the core devices in these systems, have received widespread attention and research. Meanwhile, with the development of integration technology, the methods for implementing integrated modulators on-chip are becoming increasingly diverse. Silicon-based integration technology can realize large-scale, low-cost electro-optic modulators. Furthermore, with the development of thin-film lithium niobate fabrication technology, electro-optic modulators can achieve higher linearity and bandwidth. At the same time, silicon / silicon nitride-lithium niobate heterogeneous integrated electro-optic modulators have also demonstrated their unique advantages. For example, lithium niobate wafers can be bonded to silicon-based chips using wafer bonding technology to form silicon-lithium niobate waveguides. This avoids the etching of lithium niobate while leveraging the mature fabrication process and high-density integration advantages of silicon-based materials (see reference 1: Peter O. Weigel, et al., "Bonded thin film lithium niobate modulator on a siliconphotonics platform exceeding 100 GHz 3-dB electrical modulation bandwidth", Opt. Express 26. 23728-23739 (2018)). Alternatively, silicon nitride thin films can be deposited on lithium niobate wafers using thin film deposition technology to form silicon nitride-lithium niobate waveguides, which also avoids the etching of lithium niobate (see reference 2: Amirmahdi Honardoost, et al., "Towards subterahertz bandwidth ultracompact lithium niobate electrooptic modulators", Opt. Express 27. 6495-6501 (2019)). However, the former is not suitable for mass production due to wafer bonding, while the latter faces challenges in integration size due to the use of silicon nitride waveguides.

[0003] To address these challenges, researchers proposed a high-speed, low-voltage electro-optic modulator based on lithium niobate-silicon wafers. Bonding between the silicon and lithium niobate wafers is achieved in the front-end process, and light transmission is controlled by adjusting the etching depth of the silicon wafer, thus simultaneously achieving high-volume, high-integration, and excellent electro-optic modulation (see Reference 1: Zou Weiwen, Wang Jing, Xu Shaofu, Wang Xingjun, "High-speed, low-voltage electro-optic modulator based on lithium niobate-silicon wafers CN111175999A"). However, due to the thermal mismatch between silicon and lithium niobate materials, the annealing temperature during bonding cannot be too high, ultimately resulting in low bonding strength.

[0004] WO2021258616A1 discloses a polarization-independent optical switch. Although a silicon dioxide cladding is grown on the lithium niobate waveguide 2, the purpose is to cover the polarization-independent modulation waveguide 2 and the metal electrode 7 with the silicon dioxide cladding, so as to achieve the purpose of the polarization-independent modulation waveguide 6 not being in direct contact with the air. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and propose a high-bandwidth electro-optic modulator and integration method based on silicon-silicon oxide-lithium niobate wafers. It leverages the advantages of both silicon-based and lithium niobate materials, while a silicon oxide layer serves as a buffer between the silicon wafer layer and the lithium niobate wafer layer. This buffer layer avoids significant thermal and lattice mismatches between the silicon and lithium niobate wafer layers, ultimately resulting in stronger bonding between the wafers. Furthermore, etching the buffer layer can form a silicon oxide waveguide, and the size of the silicon oxide waveguide provides new degrees of freedom for waveguide design.

[0006] The technical solution of the present invention is as follows:

[0007] A high-bandwidth electro-optic modulator of silicon-silicon oxide-lithium niobate is characterized in that the wafer structure, from top to bottom, consists of a silicon substrate layer, a silicon oxide isolation layer, a lithium niobate thin film layer, a silicon oxide buffer layer, and a silicon thin film layer.

[0008] The electro-optic modulator is a push-pull structure, and the waveguide structure along the light propagation direction consists of a first multimode interferometer, a first mode-spot converter, an electro-optic phase-shifting arm, a second mode-spot converter, and a second multimode interferometer.

[0009] The first multimode interferometer and the second multimode interferometer are disposed on the silicon thin film layer, and a metal thin film is deposited near one arm of the first multimode interferometer as a thermal electrode.

[0010] The first mode converter, the electro-optic phase shifter, and the second mode converter are disposed on the silicon oxide buffer layer and the silicon thin film layer, and a metal thin film is deposited on the silicon thin film layer near the electro-optic phase shifter as a traveling wave electrode.

[0011] A silicon thin film layer is used for light transmission; a lithium niobate thin film layer is used for light modulation. Light is split into two beams by a multimode interferometer. These two beams pass through a mode converter and enter the electro-optic modulation arm, then are coupled out by the multimode interferometer. A thermoelectric electrode, located near one arm of the multimode interferometer, serves as the bias electrode for the electro-optic modulator, applying a bias voltage to suppress bias drift caused by charge relaxation in the lithium niobate modulator. A traveling wave electrode, located near the electro-optic phase-shifting arm, applies a high-speed microwave signal. From top to bottom, this arrangement is ground-signal-ground, with the ground electrode on either side of the electro-optic phase-shifting arm and the signal electrode in the middle of the two arms. In the multimode interferometer, the energy of the light wave is primarily confined within the silicon waveguide; in the electro-optic phase-shifting arm, the energy is primarily confined within the lithium niobate thin film layer, with a small portion confined within the silicon waveguide and silicon oxide layer. The mode converter enables light transmission from the multimode interferometer to the electro-optic phase-shifting arm.

[0012] This invention proposes an integration method for a high-bandwidth electro-optic modulator based on a silicon-silicon oxide-lithium niobate wafer, mainly including silicon waveguide etching, silicon oxide buffer layer etching, silicon oxide cladding deposition, and metal thin film deposition. The silicon waveguide etching includes two etching processes, ultimately forming a conventional silicon waveguide and a thin silicon waveguide; the silicon oxide buffer layer etching forms a silicon oxide waveguide. The conventional silicon waveguide forms a multimode interferometer; the conventional silicon waveguide, silicon oxide waveguide, and thin silicon waveguide form a mode converter; the thin silicon waveguide, silicon oxide waveguide, and lithium niobate thin film layer form an electro-optic phase-shifting arm. The silicon oxide cladding is fabricated on the multimode interferometer using a deposition process; the hot electrode and traveling wave electrode are fabricated through metal deposition.

[0013] The advantages of this invention are:

[0014] 1. Using silicon-silicon oxide-lithium niobate wafers to fabricate electro-optic modulators can simultaneously leverage the advantages of high integration of silicon waveguides and the linear electro-optic modulation of lithium niobate.

[0015] 2. Using a silicon oxide wafer as a buffer layer between the silicon wafer and the lithium niobate wafer can prevent thermal mismatch between the silicon wafer and the lithium niobate wafer, thereby improving the bonding strength of the wafers. In addition, silicon oxide waveguides can be formed by etching the buffer layer, and the size of the silicon oxide waveguide provides new degrees of freedom for waveguide design.

[0016] 3. Electro-optic phase-shifting arms are formed by using thin silicon waveguides, silicon oxide waveguides, and lithium niobate layers to avoid etching contamination of lithium niobate.

[0017] 4. Using the thermoelectric electrode as the bias electrode to suppress bias drift caused by charge relaxation in the lithium niobate modulator.

[0018] 5. The mode converter formed by using the conventional silicon waveguide, silicon oxide waveguide and thin silicon waveguide described above has a large transmission efficiency.

[0019] 6. Depositing a silicon oxide cladding layer near the electro-optic phase shifter can further reduce the absorption loss of light by the metal; depositing a silicon oxide cladding layer near a multimode interferometer or thermo-optic phase shifter can isolate the conventional silicon waveguide from the traveling wave electrode, allowing the traveling wave electrode to cross with the conventional silicon waveguide, thus reducing the absorption loss of the metal. Attached Figure Description

[0020] Figure 1 The diagram shows the structure of the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator of the present invention, where (a) is a three-dimensional view and (b) is a top view.

[0021] Figure 2 The diagram shows the structure of the mode converter in the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator of the present invention, where (a) is a side view and (b) is a top view.

[0022] Figure 3 The image shows a cross-sectional view of the waveguide in the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator of the present invention, wherein (a) is a cross-sectional view of the thermoelectrode and one arm of the multimode interferometer, and (b) is a cross-sectional view of the traveling wave electrode and the electro-optic phase-shifting arm, which is covered by air.

[0023] Figure 4 The cross-sectional view of the waveguide in the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator of another embodiment of the present invention is shown in the structural diagram of the electro-optic phase-shifting arm of another embodiment, wherein (a) is a side view of the mode converter and (b) is a cross-sectional view of the traveling wave electrode and the electro-optic phase-shifting arm. The mode converter and the electro-optic phase-shifting arm are covered by a silicon oxide cladding. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, providing detailed implementation methods and structures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0025] Figure 1The diagram illustrates the structure of the silicon-silicon oxide-lithium niobate electro-optic modulator of this invention. (a) is a three-dimensional view of the electro-optic modulator, which, from bottom to top, consists of a silicon substrate layer 1, a silicon oxide isolation layer 2, a lithium niobate thin film layer 3, a silicon oxide buffer layer 4, a silicon thin film layer 5, a silicon oxide cladding layer 6, a traveling wave electrode 7, and a thermoelectric electrode 8. (b) is a planar view of the electro-optic modulator, where the waveguide structure from left to right consists of a first multimode interferometer 5.1, a first mode-spot converter 5.2, an electro-optic phase-shifting arm 5.3, a second mode-spot converter 5.4, and a second multimode interferometer 5.5. The traveling wave electrode 7 is located near the electro-optic phase-shifting arm; the thermoelectric electrode 8 is located near one arm of the first multimode interferometer 5.1. After light passes through the first multimode interferometer 5.1, the phase of the light in the upper arm is controlled by the thermoelectric electrode 8. Subsequently, the light from both arms passes through the first mode-spot converter 5.2 and enters the electro-optic phase-shifting arm 5.3, where its phase is controlled by the traveling wave electrode 7. Finally, the light wave is transmitted to the second multimode interferometer 5.5 via the second mode converter 5.4, and then combined and output. The thermoelectrode 8 has a straight structure; the traveling wave electrode 7 consists of curved sections at both ends and a straight section in the middle, causing the microwave direction to change by ninety degrees to avoid the fiber optic port and the radio frequency port being in the same direction. The curved sections of the traveling wave electrode 7 intersect with the multimode interferometers 5.1 and 5.5, respectively. To reduce the light absorption loss of the traveling wave electrode 7, a silicon oxide cladding layer 6 is provided above the multimode interferometers 5.1 and 5.5.

[0026] Figure 2 The diagram shows the structure of the mode converter in the high-bandwidth electro-optic modulator of silicon-silicon oxide-lithium niobate of the present invention. (a) is a side view of the mode converter. From bottom to top, the thin film structure of the mode converter of the present invention consists of a silicon substrate layer 1, a silicon oxide isolation layer 2, a lithium niobate thin film layer 3, a silicon oxide waveguide 4.1, a conventional silicon waveguide 5.2.1, and a thin silicon waveguide 5.2.2. (b) is a top view of the mode converter. In this embodiment, the mode converter is composed of a silicon oxide waveguide 4.1, a conventional silicon waveguide 5.2.1, and a thin silicon waveguide 5.2.2. The conventional silicon waveguide 5.2.1 is formed by completely etching the silicon thin film layer 5, the thin silicon waveguide 5.2.2 is formed by partially etching the conventional silicon waveguide 5.2.1, and the silicon oxide waveguide 4.1 is formed by etching a silicon oxide buffer layer. The width of the conventional silicon waveguide 5.2.1 gradually decreases from left to right; the width of the thin silicon waveguide 5.2.2 can gradually decrease or remain unchanged; the width of the silicon oxide waveguide remains unchanged.

[0027] Figure 3The images show cross-sectional views of the waveguides in the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator of the present invention. (a) shows a cross-sectional view of the thermoelectrode and one arm of the first multimode interferometer, from bottom to top: silicon substrate 1, silicon oxide isolation layer 2, lithium niobate thin film layer 3, silicon oxide buffer layer 4, conventional silicon waveguide 5.1.1, silicon oxide cladding layer 6, and thermoelectrode 8. The first multimode interferometer 5.1 and the second multimode interferometer 5.5 are composed of conventional silicon waveguide 5.1.1, which is formed entirely by etching through the silicon thin film layer 5. (b) shows a cross-sectional view of the traveling wave electrode and the electro-optic phase-shifting arm, from bottom to top: silicon substrate 1, silicon oxide isolation layer 2, lithium niobate thin film layer 3, silicon oxide buffer layer 4, silicon oxide waveguide 4.1, thin silicon waveguide 5.3.1, and traveling wave electrode 7. The electro-optic phase-shifting arm 5.3 consists of a silicon oxide waveguide 4.1 and a thin silicon waveguide 5.3.1. The silicon oxide waveguide 4.1 is formed by etching a silicon oxide buffer layer, and the thin silicon waveguide 5.3.1 is formed by etching a silicon thin film layer 5. A silicon oxide buffer layer 4 with a thickness of 0 nm to 500 nm is located beneath the traveling wave electrode 7. The thin silicon waveguide 5.3.1 is located beneath the silicon oxide waveguide 4.1 with a thickness of 0 nm to 500 nm.

[0028] Figure 4 The diagram shows the waveguide structure of another embodiment of the silicon-silicon oxide-lithium erbium niobate electro-optic modulator of the present invention. (a) is a side view of the mode converter. In this embodiment, the mode converter, from bottom to top, has a thin film structure consisting of a silicon substrate layer 1, a silicon oxide isolation layer 2, a lithium niobate thin film layer 3, a silicon oxide waveguide 4.1, a conventional silicon waveguide 5.2.1, and a thin silicon waveguide 5.2.2. The first mode converter 5.2 is covered by a silicon oxide cladding layer 6. (b) is a cross-sectional view of the electro-optic phase-shifting arm. In this embodiment, the electro-optic phase-shifting arm, from bottom to top, consists of a silicon substrate layer 1, a silicon oxide isolation layer 2, a lithium niobate thin film layer 3, a silicon oxide buffer layer 4, a silicon oxide waveguide 4.1, a traveling wave electrode 6, and a thin silicon waveguide 5.3.1. The traveling wave electrode 7 is under a silicon oxide buffer layer 4 with a thickness of 0 nm to 500 nm. The thin silicon waveguide 5.3.1 is under a silicon oxide waveguide 4.1 with a thickness of 0 nm to 500 nm. The thin silicon waveguide 5.3.1 is covered by a silicon oxide cladding layer 6.

[0029] The following steps can be used for device fabrication:

[0030] Step 1: Select a silicon-silicon oxide-lithium niobate wafer (the wafer is a square piece removed from the wafer after dicing). The thin film structure from bottom to top is: silicon substrate, silicon oxide isolation layer, lithium niobate layer, silicon oxide buffer layer, and silicon layer. The wafer is overlaid using photolithography and dry etching techniques. The silicon layer is fully etched and partially etched, respectively, to obtain multimode interferometers 5.1 and 5.5.

[0031] Step 2: The wafer described in Step 2 is etched using photolithography and dry etching techniques. The silicon oxide buffer layer is etched to obtain the mode converter 5.2, mode converter 5.4, and electro-optic phase shifter arm 5.3.

[0032] Step 3: Using thin film deposition technology, a silicon oxide cladding layer is deposited on the wafer described in Step 3. The deposition areas are the multimode interferometer 5.1 and multimode interferometer 5.5 regions. According to another embodiment, the deposition areas are the multimode interferometer 5.1 and 5.5 regions, the mode converter 5.2 and 5.4 regions, and the electro-optic phase shifter 5.3 region.

[0033] Step 4: Deposit metal as a thermal electrode 8 on the wafer described in Step 4 using thin film deposition technology.

[0034] Step 5: Deposit metal as traveling wave electrode 7 on the wafer described in Step 5 using thin film deposition technology.

Claims

1. A silicon-silicon-oxide-lithium niobate high bandwidth electro-optic modulator characterized in that, The wafer structure, from bottom to top, consists of a silicon substrate layer (1), a silicon oxide isolation layer (2), a lithium niobate thin film layer (3), a silicon oxide buffer layer (4), and a silicon thin film layer (5). The electro-optic modulator is a push-pull structure, and the waveguide structure along the light propagation direction consists of a first multimode interferometer (5.1), a first mode-spot converter (5.2), an electro-optic phase-shifting arm (5.3), a second mode-spot converter (5.4), and a second multimode interferometer (5.5). The first multimode interferometer (5.1) and the second multimode interferometer (5.5) are formed by etching the silicon thin film layer (5), and a metal thin film is deposited near one arm of the first multimode interferometer (5.1) as a hot electrode (8). The first mode converter (5.2), the electro-optic phase shifter (5.3), and the second mode converter (5.4) are formed by etching the silicon oxide buffer layer (4) and the silicon thin film layer (5), and a metal thin film is deposited near the electro-optic phase shifter (5.3) as a traveling wave electrode (7). The electro-optic phase-shifting arm (5.3) includes a silicon oxide waveguide (4.1) formed by etching a silicon oxide buffer layer (4) and a thin silicon waveguide formed by etching a silicon thin film layer (5). The silicon oxide waveguide, the thin silicon waveguide and the lithium niobate thin film layer (3) together form the electro-optic phase-shifting arm. The thermoelectrode (8) is used to suppress the bias drift of the lithium niobate modulator due to charge relaxation. The traveling wave electrode (7) is a ground electrode-signal electrode-ground electrode, with the ground electrode on both sides of the electro-optic phase shifting arm and the signal electrode in the middle of the two electro-optic phase shifting arms.

2. The silicon-silicon oxide-lithium niobate high bandwidth electro-optic modulator of claim 1, wherein, The traveling wave electrode (7) is composed of curved portions at both ends and a straight portion in the middle. The curved portions of the traveling wave electrode (7) intersect with the first multimode interferometer (5.1) and the second multimode interferometer (5.5), respectively.

3. The silicon-silicon oxide-lithium niobate high bandwidth electro-optic modulator of claim 1 or 2, wherein, A silicon oxide cladding layer (6) is deposited on the first multimode interferometer (5.1) and the second multimode interferometer (5.5).

4. The silicon-silicon oxide-lithium niobate high bandwidth electro-optic modulator of claim 1 or 2, wherein, A silicon oxide cladding layer (6) is deposited on the regions of the first mode converter (5.2) and the second mode converter (5.4), and a silicon oxide cladding layer (6) is deposited on the region of the electro-optic phase shifter (5.3).

5. The silicon-silicon oxide-lithium niobate high bandwidth electro-optic modulator of claim 1 or 2, wherein, Silicon oxide wafers serve as a buffer layer to prevent thermal mismatch between silicon wafers and lithium niobate wafers, thereby improving bonding strength.

6. An integration method for a silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1. On a silicon-silicon oxide-lithium niobate wafer, the silicon thin film layer is etched in whole or in part to obtain a first multimode interferometer (5.1) and a second multimode interferometer (5.5). Step 2. Etch the silicon oxide buffer layer on the wafer to obtain the first mode converter (5.2), the second mode converter (5.4), and the electro-optic phase shifter (5.3). Step 3. Deposit a silicon oxide cladding layer in the regions of the first multimode interferometer (5.1) and the second multimode interferometer (5.5); Step 4. Deposit a thin metal film as a thermoelectric electrode (8) near one arm of the first multimode interferometer (5.1); Step 5. Deposit a thin metal film as a traveling wave electrode (7) in the vicinity of the electro-optic phase-shifting arm (5.3).

7. The integration method of the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator according to claim 6, characterized in that, The first multimode interferometer (5.1) and the second multimode interferometer (5.4) are composed of conventional silicon waveguides formed by etching the entire silicon thin film layer (5).

8. The integration method of the silicon-silicon oxide-lithium niobate high-bandwidth electro-optic modulator according to claim 6, characterized in that, The first mode converter (5.2) and the second mode converter (5.4) include a silicon oxide waveguide (4.1) formed by etching the silicon oxide buffer layer (4) once, a conventional silicon waveguide formed by etching the entire silicon thin film layer (5), and a thin silicon waveguide formed by etching a portion of the conventional silicon waveguide.

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