硅酸盐学报

铌酸锂晶体是一种具有压电效应、非线性光学效应、双折射效应等性能的多功能晶体，其物理化学性质稳定，易掺杂调控，易于制备高质量的光波导，在集成光学领域具有较大应用潜力。然而，大尺寸铌酸锂晶体生长由于其热导率低，晶体内结晶潜热的输运速度和结晶界面上结晶潜热的释放速度不匹配，导致热量在固液界面累积，易引发晶体偏心生长和界面回熔。针对6英寸晶体等径后期偏心生长的问题，本工作从提高坩埚相对感应线圈位置和减弱上保温2个角度优化热场并开展实验，结果表明适当提高坩埚相对感应线圈位置和减弱上保温能够增大等径后期熔体表面径向温度梯度，改善晶体偏心生长情况。此外，大尺寸晶体生长各阶段对热场需求之间的矛盾加剧。为此，还开发了动态热场提拉法晶体生长技术，在固定热场中引入后加热器，通过调节中频电源和后加热器的功率，实现了晶体生长过程中温度梯度的实时调节，从而兼容不同阶段晶体生长的需求，显著改善了6英寸铌酸锂晶体的偏心生长情况，使晶体的等径长度从70 mm增加至90 mm。

Introduction Lithium niobate crystal exhibits versatile properties including piezoelectric, nonlinear and birefringence. Its stable physicochemical characteristics, doping amenability, and suitability for high-quality optical waveguide fabrication make it promising for integrated optics. However, the growth of large-size lithium niobate crystals presents significant challenges, with several critical issues remaining unresolved. Firstly, its low thermal conductivity causes a heat accumulation at the crystal-melt interface due to mismatched latent heat transport and release rates. This leads to eccentric growth and interface remelting. Secondly, the large crucible required for large-size crystal growth results in a reduced radial temperature gradient across the melt surface and an oversized cold core, which complicates necking initiation and shouldering control, ultimately compromising crystallinity. Finally, fixed thermal field configurations fail to accommodate divergent thermal requirements across different growth stages as crystal size increases. In this study, the fixed thermal field configuration and methodologies for mitigating eccentric crystal growth were investigated. In addition, the impact of dynamic thermal fields on stage-specific thermal requirements during crystal growth was also analyzed. Methods This work conducted the Czochralski growth of 6 in CLN(congruent lithium niobate) crystals in a fixed thermal field. A preliminary 6 in thermal field was firstly designed according to the CLN crystal growth requirements, and then by root-cause analysis of eccentric growth and thermal field optimization. The crucible position relative to the induction coil was gradually elevated, while maintaining identical growth parameters. Subsequently, comparative experiments of fixed crucible position and constant process conditions with the diminished upper insulation assessed their impact on the eccentric growth. To resolve conflicting thermal requirements across growth stages, a dynamic thermal field Czochralski technique was developed via integrating an active afterheater. This enabled real-time regulation of the temperature gradient by adjusting power to the main heater and active afterheater. Comparative analysis of the constant-diameter lengths in grown crystals could demonstrate the efficacy of the dynamic thermal field technique. Results and discussion In a fixed thermal field, the causes of eccentric growth during the late constant-diameter growth stage are analyzed. During the actual crystal growth, the melt level progressively decreases as the constant-diameter length increases, gradually revealing an exposed crucible wall effect. This reduces the radial temperature gradient across the melt surface. Simultaneously, a radiative influence from the crucible wall appears on the crystal intensifies. When crystallization latent heat cannot be efficiently transported from the crystal, a heat accumulation occurs, triggering random protrusions at the solid–liquid interface that radiate heat directly upward, resulting in eccentric growth. Thermal field optimization is achieved via elevating the crucible position relative to the induction coil and reducing upper insulation. Consequently, the constant-diameter length increases from 15 mm to approximately 70 mm, effectively mitigating an eccentric growth. Conclusions This work designed a fixed thermal field for 6 in CLN crystal growth. The analysis revealed that the primary cause of eccentric growth in 6 in CLN crystal was an insufficient radial temperature gradient across the melt surface during the late constant-diameter growth stage. The thermal field optimization was achieved via elevating the crucible position relative to the induction coil and reducing upper insulation, extending the constant-diameter length from 15 mm to approximately 70 mm. The development of a dynamic thermal field crystal growth technique, enabled by the introduction of an active afterheater, demonstrates critical advantages over conventional fixed thermal fields, i.e., 1) Fixed thermal fields exhibit temperature gradients solely determined by structural design, precluding manual intervention during growth. Conversely, dynamic thermal fields facilitate different temperature gradient control in different stages through coordinated power modulation of the primary and secondary heaters, thereby altering system heat dissipation, 2) Large-size crystal growth requires substantial melt volumes with a high thermal mass, resulting in prolonged adjustment response time and significant thermal hysteresis. The dynamic approach achieves a superior diameter control via directly manipulating furnace heat dissipation by an active afterheater power. This rapidly modifies temperature gradients within the crystal bulk and near the solid–liquid interface, reducing hysteresis and enabling diameter precision, and 3) Unlike fixed configurations necessitating furnace shutdowns due to thermal design failures, dynamic systems resolve thermal incompatibilities control through power adjustments to the MF(Medium-Frequency) Power Supply and active afterheater. This eliminates costly interruptions, saving both time and cost. To address limitations in fixed thermal field optimization, a dynamic thermal field Czochralski technique was developed, which enabled real-time regulation of temperature gradients during crystal growth. This approach could accommodate divergent thermal requirements across different growth stages, significantly mitigating an eccentric growth in 6 in CLN crystal and further increasing the constant-diameter length from 70 mm to 90 mm, thus establishing a novel paradigm for large-sizer crystal growth.