Optimized low-frequency PECVD diamond-like carbon hard masks for high-selectivity thin-film lithium niobate etching

Abstract

Low-frequency plasma-enhanced chemical vapor deposition (LF PECVD) was developed for growing diamond-like carbon (DLC) films intended as hard masks for photonic nanofabrication. DLC offers high hardness and physical etch selectivity, enabling effective etching of such ferroelectric materials as lithium niobate (LNO), lithium tantalate, and barium titanate that possess high intrinsic electro-optic coefficient. We systematically studied the influence of deposition power, chamber pressure, and substrate temperature on DLC film properties, finding that LF growth produces denser films with higher ion beam etch selectivity compared to RF deposition. A linear correlation was observed between refractive index, density, stress, and etch selectivity, enabling quality assessment by ellipsometry. Optimized PECVD deposition conditions yielded DLC films with 2.8 etch selectivity over LNO, deposition rates near 10 nm/min, and thickness uniformity of about 2% across 4-in. wafers. High compressive stress, up to 1.5 GPa, limited DLC adhesion on non-silicon substrates, which was overcome by introducing a thin, low-temperature SiNx adhesion layer between DLC and LNO. Raman spectroscopy confirmed that etch selectivity is enhanced by sp2-rich clustering in a-C:H films.