Lapping of Soft-Brittle Lithium Niobate Crystal with Fixed Abrasive Pad

Lithium niobate (LiNbO₃, LN) single crystal is widely used in optoelectronic fields due to its excellent performance. However, its low hardness, high brittleness, and strong anisotropy lead to low processing efficiency and poor surface quality. Hydrophilic fixed abrasive lapping technology was adopted for the thinning of LN wafers in this research. The effects of lapping pressure on the thinning process were investigated comprehensively in terms of the material removal rate (MRR), surface quality, and subsurface damage (SSD). The results show that lapping pressure exerted a significant influence on the machining performance. High pressure contributed to improving the MRR but aggravated surface roughness (Ra) and SSD. With low pressure, material removal was dominated by ductile removal machining, with fine scratches as the main damage form, which was favorable for obtaining low Ra and low SSD. The root mean square (RMS) of the acoustic emission (AE) signal rose with the increase in pressure, increasing slowly in the ductile removal regime and rising abnormally in the brittle removal regime. It was positively correlated with the MRR and SSD and can be used as an in situ monitoring indicator. After a comprehensive comparison of five groups of experiments, 7 kPa was determined to be the optimal lapping pressure, with the following corresponding parameters: wafer speed: 100 rpm; lapping table speed: 80 rpm; slurry flow rate: 100 mL/min; eccentricity: 60 mm; soft lapping pad; abrasive mass fraction: 50%; and lapping time: 5 min. Under these conditions, the Ra value was approximately 30 nm, the MRR exceeded 1 μm/min, and SSD was as low as 3.3 μm, realizing the synergistic optimization of high-efficiency and low-damage machining. It provides a favorable foundation for the subsequent processing of LN substrates, such as ultra-precision polishing, thin-film transfer, and bonding.