Thermal Field Study for Czochralski Growth of 6-inch Lithium Niobate Crystals

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.

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.

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.

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.

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.

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.