Boron nitride thin film synthesis with a ns-pulsed MHCD: <i>In situ</i> plasma diagnostics and post-growth film characterization

Abstract

Boron nitride (BN) is deposited on a Si 〈100〉 wafer (≈20 cm2) via plasma enhanced chemical vapor deposition (PECVD) using a ns-pulsed N2/Ar micro hollow cathode discharge (MHCD) as a microplasma source. Aluminum nitride (AIN) is employed as the dielectric material in the MHCD instead of more conventional Al2O3 to exclude a potential source of film contamination by atomic oxygen. For the first time, in situ diagnostics such as optical emission spectroscopy and intensified CCD imaging are used to monitor plasma properties during BN synthesis, including species emission profiles, gas temperature, and discharge morphology. Furthermore, a comprehensive ex situ multi-diagnostics approach is followed to characterize the films, including Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray photoelectron microscopy (XPS), and transmission electron microscopy coupled to energy dispersive x-ray spectroscopy (EDX). Raman spectra reveal the E2g phonon mode of BN around 1366 cm−1 (representative of hexagonal, rhombohedral, or turbostratic BN polytypes), confirming successful synthesis. Further analysis using selected area electron diffraction reveals diffraction rings, characteristic of BN films with a turbostratic structure. SEM imaging reveals an almost complete surface coverage by the film, with localized delamination though. This is probably due to an uneven resistive heating of the Si wafer, rapid post-deposition cooling (∼13 K/min), and ambient exposure. AFM analysis indicates an average thickness of about 33 nm after 90 min of deposition (∼22 nm/h deposition rate). XPS measurements reveal an average B/N atomic ratio of ∼1.5 along the wafer diameter. EDX analyses confirm the dominant presence of B- and N-atoms in the film, in fair agreement with XPS. Deviations from ideal film properties (e.g., stoichiometric unity and uniform morphology) are attributed to plasma-induced inhomogeneities (such as non-uniform species flux and temperature gradients) among other factors (e.g., oxygen impurities in the chamber and ambient exposure post-deposition), which affect nitrogen and boron incorporation and localized film properties. Despite these challenges, the MHCD-driven PECVD process demonstrates strong potential toward h-BN synthesis, with further optimization of the reactor design, plasma conditions, and gas chemistry required to grow ideal films.