Lithium niobate crystals, single crystal thin films and their future layout in the optical chip industry

Lithium niobate crystals, single crystal thin films and their future layout in the optical chip industry

Abstract of the article

With the rapid development of application fields such as 5G/6G communication technology, big data and artificial intelligence, the demand for the new generation of photonic chips is increasing day by day. Lithium niobate crystals, with their excellent electro-optic, nonlinear optical and piezoelectric properties, have become the core material of photonic chips and are known as the "optical silicon" material of the photonic era. In recent years, breakthroughs have been made in the preparation of lithium niobate single crystal thin films and device processing technology, demonstrating advantages such as smaller size, higher integration, ultrafast electro-optic effect, wide bandwidth, and low power consumption. It has broad application prospects in high-speed electro-optic modulators, integrated optics, quantum optics and other fields. The article introduces the domestic and international research and development progress and relevant policies of the preparation technology of optical-grade lithium niobate crystals and single crystal films, as well as their latest applications in the fields of optical chips, integrated optical platforms, quantum optical devices, etc. The development trends and challenges of the lithium niobate crystal-thin-film - device industrial chain were analyzed, and suggestions were put forward for the future layout. At present, China is in a stage of catching up with the international advanced level in the fields of lithium niobate single crystal thin films and lithium niobate-based optoelectronic devices, but there is still a considerable gap in the industrialization of high-quality lithium niobate crystal materials. By optimizing the industrial layout and strengthening basic research and development, China is expected to form a complete lithium niobate industrial cluster from material preparation to device design, manufacturing and application.

ZMSH's LiNbO3 Wafers

Quick Overview of the Article

With the rapid development of fields such as 5G/6G communication technology, big data, artificial intelligence, optical communication, integrated photonics and quantum optics, the demand for the new generation of photonic chips and their basic crystal materials is becoming increasingly urgent. Lithium Niobate (LN) is a multifunctional crystal with properties such as piezoelectricity, ferroelectricity, pyroelectricity, electro-optics, acoutooptics, photoelasticity, and nonlinearity. It is currently one of the crystals with the best comprehensive performance in photonics. The role of lithium niobate in future optical devices is similar to that of silicon-based materials in electronic devices, and thus it is also known as the "optical silicon" material of the photonic age. Lithium Niobate Thin Film (LNOI) is a kind of thin film material based on lithium niobate crystals and has excellent photoelectric properties: ① High electro-optic coefficient. Lithium niobate single crystal thin films have excellent electro-optic effects and are suitable for high-speed optical modulators. ② Low optical loss. The thin-film structure reduces light propagation loss and is suitable for high-performance optoelectronic devices. ③ Wide transparent window. It has high transparency in the visible light and near-infrared bands. ④ Nonlinear optical characteristics. Support nonlinear optical effects such as Secondary Harmonic Generation (SHG). ⑤ Compatible with silicon-based integration. Integration with silicon-based optoelectronic devices can be achieved through bonding technology. In recent years, many research projects deployed at home and abroad have taken lithium niobate crystals and single crystal films as important development directions , especially in the fields of microwave photonic chips, optical waveguides, electro-optic modulators, nonlinear optics, and quantum devices.

Table 1 Important technological events lithium field

Lithium niobate thin films have become an important candidate material for the substrate of a new generation of multifunctional integrated photonic information processing chips. The market capacity of optical modulators based on lithium niobate crystal materials is predicted to be 36.7 billion US dollars in 2026. Compared with silicon photonic modulators and indium phosphide modulators, thin-film lithium niobate modulators have the advantages of high bandwidth, low insertion loss, low power consumption, high reliability and high extinction ratio. At the same time, they can also be miniaturized, which can meet the increasingly miniaturized requirements of coherent optical modules and data communication optical modules. China is independently controllable in crystal materials, crystal films, processing methods, devices and systems. At present, many domestic manufacturers have released 800 Gbps thin-film lithium niobate solution optical modules. Downstream customers have tested the corresponding products. In the future, the application advantages of 1.6T optical modules will be more obvious.

1. Research Progress of Lithium Niobate Crystals and Single Crystal Films

The physicochemical properties of lithium niobate single crystals largely depend on [Li]/[Nb] and impurities. The Congruent Lithium Niobate (CLN) crystal with the same composition is deficient in lithium, so it contains a large number of Li vacancies (VLi) and inverse Nb (Nb) point defects. The [Li]/[Nb] ratio of Stoichiomentric Lithium Niobate (SLN) is close to 1∶1. Although it has excellent performance, its preparation is difficult and the production cost is high. Lithium niobate single crystals are classified into acoustic grade and optical grade. The relevant units mainly engaged in the growth of lithium niobate crystals are shown in Table 1. Among them, the company mainly engaged in the growth of optically grade lithium niobate is a Japanese enterprise. At present, the domestic production rate of optical-grade lithium niobate wafers is less than 5%, and they are highly dependent on imports. Yamashiro Ceramics Co., LTD. (referred to as Yamashiro Ceramics) has industrialized 8-inch lithium niobate crystals and wafers (Figure 1 (a)). In China, Tiantong Holdings Co., LTD. (referred to as Tiantong Co., LTD.) and China Electronics Technology Group Corporation Deqinghua Ying Electronics Co., LTD. (referred to as Deqinghua Ying) respectively produced 8-inch lithium niobate crystals and wafers in 2000 and 2019, but they have not yet achieved industrial mass production. In terms of stoichiometric ratio and optically grade lithium niobate, there is still a technological gap of about 20 years between Chinese lithium niobate crystal growth enterprises and Japanese enterprises. Therefore, there is an urgent need in China to make breakthroughs in the growth theory and process technology of high-quality optically grade lithium niobate crystals.

Fig. 1 Lithium niobate crystal and single-crystal thin film

The breakthroughs in lithium niobate photonic structures and photonic chips and devices worldwide are mainly attributed to the development and industrialization of lithium niobate thin film material technology. However, due to the high brittleness of lithium niobate single crystals, it is extremely difficult to prepare hundred-nanometer-scale films (100-2,000 nm) with low defects and high quality. Ion implantation and direct bonding techniques exfoliate bulk single crystals into nanoscale lithium niobate single crystal films, making large-scale lithium niobate photonic integration possible. At present, only a few companies in the world, including Jinan Jingzheng, French Soitec SA Company, and Japanese Kiko Co., LTD., have mastered the preparation technology for lithium niobate single crystal thin films. Jinan Jingzheng has adopted the core technologies of ion beam slicing and direct bonding, and has been the first in the world to achieve industrialization. It has formed a globally leading lithium niobate thin film brand (NanoLN), supporting over 90% of the basic research and development of lithium niobate thin film devices worldwide. In 2023, Jinan Jingzheng launched an 8-inch optical-grade lithium niobate film (Figure 1 (b)), and it is also the first enterprise in the industry to produce lithium niobate films from 8-inch X-axis lithium niobate crystals. The key indicators of Jinan Jingzheng series products, such as physical properties, thickness uniformity, defect suppression and elimination, are all at the international leading level. The situation of enterprises related to the preparation of lithium niobate crystals and single crystal films is shown in Table 2.

Table 2 Manufacturing companies of lithium niobate crystals and single-crystal thin films

2. Advanced applications of lithium niobate

Compared with traditional lithium niobate single crystal materials, thin-film lithium niobate has a smaller size, lower cost, higher integration, and can operate stably under a wider range of temperature and electric field conditions. These advantages make it have broad application prospects in fields such as 5G communication, quantum computing, optical fiber communication and sensors, especially demonstrating great potential in photoelectric modulation, optical signal processing and high-speed data transmission (Table 3).

Table 3 Main application fields of lithium niobate crystal and single-crystal thin film

2.1 High-speed Electro-optic Modulator

Lithium niobate modulators are widely used in ultra-high-speed trunk optical communication networks, submarine optical communication networks, metropolitan core networks and other fields due to their advantages such as high speed, low power consumption and high signal-to-noise ratio. Key technologies such as large-scale lithography technology, ultra-low loss waveguide processing technology and heterogeneous integration have promoted the development of thin-film lithium niobate modulators, enabling them to support applications of 800 Gbps and 1.6T high-speed optical modules. Compared with materials such as indium phosphide, silicon photonics and traditional lithium niobate, thin-film lithium niobate has outstanding features such as ultra-high bandwidth, low power consumption, low loss, small size and the ability to achieve large-scale production at the wafer level (Table 4), making it an ideal material for photoelectric modulators. The global thin-film lithium niobate modulator market is growing steadily. It is expected that the total global market value will reach 2 billion US dollars in 2029, with a compound annual growth rate of 41.0%.

Table 4　Performance comparison of substrate materials for optical modules

Internationally, The research team from Harvard University successfully developed Complementary Metal Oxide Semiconductor with a bandwidth of 100 GHz in 2018. CMOS) compatible integrated Mach-Zehnder Interferometer (MZI) electro-optic modulator, while Fujitsu Optical Devices Co., Ltd. launched the world's first commercial 200 GBaud thin-film lithium niobate modulator in 2021. Domestic progress has also been remarkable. In 2019, a research team from Sun Yat-sen University achieved a hybrid integrated electro-optic modulator of silicon and lithium niobate. Ningbo Yuanxin Optoelectronic Technology Co., Ltd. released the domestically produced thin-film lithium niobate strength modulator product in 2021. In 2022, Sun Yat-sen University collaborated with Huawei to develop the world's first polarization-multiplexed coherent optical modulator chip based on lithium niobate thin films. The lithium niobate thin-film coherent modulator chip of Niobo Optoelectronics supports 100 km optical fiber transmission of 260 GBaud DP-QPSK (Gigabaud Dual Polarization Quadrature Phase Shift Keying) signals. In 2023, Zhuhai Guangku Technology Co., LTD. (referred to as Guangku Technology) showcased a thin-film lithium niobate strength modulator product featuring ultra-high bandwidth and small volume. Chengdu Xinyisheng Communication Technology Co., LTD. (referred to as Xinyisheng) has applied this technology to 800 Gbps optical modules, with a power consumption of only 11.2W. Thin-film lithium niobate shows great potential in related applications of long-distance transmission, metropolitan area networks and data center interconnection networks, as well as in four-level Pulse Amplitude Modulation (Pulse Amplitude Modulation 4, PAM-4) applications of data centers and artificial intelligence clusters. Such as the 130 GBaud coherent drive modulator and 800 Gbps PAM-4 product of Guangkuo Technology, as well as the PAM-4 transceiver jointly launched by HyperLight Corporation of the United States, Newesun and Arista Networks Corporation of the United States. These products fully demonstrate the significant advantages of thin-film lithium niobate technology in enhancing bandwidth and reducing power consumption. At present, China is in a stage of running neck and neck with the international advanced level in this field.

2.2 Lithium niobate Integrated Optical Platform

On the lithium niobate integrated optical platform, the application from frequency comb to frequency converter and modulator has been realized, while integrating the laser on the lithium niobate chip is a major challenge. In 2022, a research team from Harvard University, in collaboration with HyperLight and Freedom Photonics, achieved a chip-level femtosecond pulse source and the world's first lithium niobate chip fully integrated high-power laser on a lithium niobate integrated optical platform (Figure 2 (a)). This type of lithium niobate on-chip laser integrates high-performance, Plug-and-play lasers, which can significantly reduce the cost, complexity and power consumption of future communication systems. At the same time, it can be integrated into larger optical systems and can be widely applied in fields such as sensing, atomic clocks, lidar, quantum information, and data telecommunications. Further development of integrated lasers that simultaneously possess narrow linewidth, high stability, and high-speed frequency modulation performance is also an important demand in the industry. In 2023, researchers from the Swiss Federal Institute of Technology and IBM achieved low-loss, narrow linewidth, high modulation rate and stable laser output on a lithium niobate-silicon nitride heterointegrated optical platform. The repetition rate is approximately 10 GHz, the optical pulse is 4.8 ps at 1,065 nm, the energy exceeds 2.6 pJ, and the peak power exceeds 0.5 W.

3. Development Trends and Challenges

With the development of artificial intelligence and large models, the future growth points of lithium niobate will mainly focus on the high-end optical chip field (Table 5), specifically including breakthroughs in core optical chip technologies such as high-speed optical modulators, lasers and detectors; Promote the application of lithium niobate thin films in optical chips and enhance the performance of the devices; Strengthen the research and development of lithium niobate thin film preparation technology to achieve large-scale production of high-quality thin films; Promote the integration of lithium niobate films with silicon-based optoelectronic devices to reduce costs.

Table 5 Outlook of lithium niobate photonics and its applications

Optical lithium niobate is mainly applied in fields such as optical communication, fiber optic gyroscopes, ultrafast lasers, and cable television. The direction that may enter mature application the fastest might be optical communication. In the field of optical communication, the market size of lithium niobate modulator chips and devices is approximately 10 billion yuan. Many high-quality optical-grade lithium niobate substrates in China need to be imported from Japan. As Japan intensifies its restrictions on China's semiconductor sector, lithium niobate substrates may appear on the restricted list. As high-speed coherent optical transmission technology continues to expand from long-distance/trunk lines to regional/data center and other fields, the demand for digital optical modulators used in high-speed coherent optical communication will continue to grow. The global shipment of high-speed coherent optical modulators is expected to reach 2 million ports in 2024. Correspondingly, the demand for lithium niobate substrates will also increase significantly.

ZMSH's LiNbO3 Crystal

The biggest bottleneck in the mass production of optical lithium niobate materials is the consistency of optical quality, including the consistency of the composition, defects and microstructure of the crystal material itself, as well as the precision of the wafers processed by the Chemical Mechanical Polishing (CMP) process. Compared with foreign countries, the main problem lies in the insufficient research on deeper scientific and technological issues of crystal growth. The growth of high-quality optical-grade LN urgently requires in-depth research to understand its multi-scale physicochemical mechanisms. For instance, cluster structures in high-temperature melts, solid-liquid interface structures, interfacial ion transport, as well as dynamic defect structures and formation mechanisms during the growth process, and simulation of the real crystal growth process, etc. How to break through the preparation theory and technology of large-sized crystal materials? Ranking first among the 10 frontier scientific questions released by the China Association for Science and Technology in 2021 indicates that the fundamental scientific issues in the preparation of large-sized crystal materials have become the key factor restricting the rapid development of this industry.

The technical challenges of lithium niobate electro-optical devices mainly lie in thin-film formation, etching and CMP processes, with problems such as high surface roughness of ridge-shaped waveguides and low processing yield. Optical applications have high requirements for wafer and device processing, and high-precision equipment is basically monopolized by foreign equipment. The defect changes brought about by the thin-film formation of lithium niobate single crystals and their influence on the structure-performance relationship, such as the DC drift problem of lithium niobate thin films in integrated optical platforms.

4. Suggestions

（1）Strengthen strategic planning and policy guidance, establish an innovation ecosystem highland, and achieve cluster effects. Lithium niobate single crystal thin films have broad application prospects in optoelectronic chips, photonic chips, integrated photonic devices and other fields. The government has established strategic planning and policy guidance, built an ecosystem and industrial cluster area with "lithium niobate Valley" as the core, encouraged the cultivation of start-up companies, and promoted the rapid development and expansion of the lithium niobate industry.

(2) Strengthen cooperation among material, device and system enterprises and research institutes to form a collaborative innovation ecosystem. Universities and research institutions provide theoretical research and technical support, while enterprises are responsible for transforming research results into practical products and promoting the industrial application of lithium niobate technology. Relevant enterprises form cooperative alliances to jointly solve technical problems and share resources and markets. For instance, in the production of lithium niobate materials, the manufacturing of devices and application development, enterprises can enhance efficiency, reduce costs and strengthen market competitiveness through cooperation.

ZMSH's Lithium Niobate Single Crystal

(3) Strengthen the "first principles" and explore disruptive technological paths. From the perspective of "first principles", we should closely grasp the original technology and fundamental scientific issues to achieve the research and development of core technologies from lithium niobate crystals, films to devices, and explore a disruptive technological path. For instance, explore the application of lithium niobate in quantum technologies, such as quantum computing and quantum communication.

(4) Interdisciplinary cooperation and technological integration to cultivate compound talents. The research and development of lithium niobate crystals, films and devices requires knowledge and technology from multiple disciplines such as physics, chemistry, materials science, electronic engineering, software and artificial intelligence, and needs more compound talents. Therefore, the government's talent introduction policies (such as settlement subsidies and housing preferences) are needed to attract more high-end talents at home and abroad. The job market promotes the mobility of talents and the innovation of enterprises.

5. Conclusion

China is in a stage of keeping pace with the international advanced level in lithium niobate single crystal films and advanced devices, but there are still some problems in high-quality crystal growth, device industry, and advanced applications. For instance, to further enhance the uniformity and optical performance of lithium niobate single crystal films and achieve devices with higher quality factors and lower losses, it is still necessary to further break through the processing technology and material preparation techniques, and develop more precise numerical simulation and optimization methods. In the future, it is necessary to promote the large-scale integration of lithium niobate thin-film optoelectronic devices, reduce costs, and further expand the application of lithium niobate in emerging fields such as integrated optics, quantum computing, and biosensing. China has a complete layout in the optoelectronic industry chain and is expected to form a lithium niobate industrial cluster with international competitiveness.

ZMSH specializes in the supply and precision processing of lithium niobate (LiNbO₃) crystal substrates, while also providing customized services for semiconductor materials including silicon carbide (SiC) and sapphire (Al₂O₃), meeting advanced requirements in optoelectronics, 5G, and power electronics applications. Leveraging cutting-edge manufacturing processes and stringent quality control, we offer comprehensive support from R&D to mass production for global clients, driving innovation in the semiconductor industry.

ZMSH's 12inch SiC wafer and 12inch sapphire wafer:

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