Hexagonal boron nitride thin film synthesis with a ns-pulsed MHCD: in-situ plasma diagnostics and post-growth film characterization

TL;DR

This study demonstrates the synthesis of hexagonal boron nitride thin films using a ns-pulsed micro hollow cathode discharge plasma, with comprehensive in-situ diagnostics and post-growth characterization revealing promising results and areas for optimization.

Contribution

It introduces a novel PECVD method employing a ns-pulsed MHCD with aluminum nitride dielectric, providing detailed plasma and film diagnostics to advance scalable h-BN film production.

Findings

Successful synthesis of h-BN confirmed by Raman spectroscopy.

Surface coverage of the film was nearly complete with localized delamination.

The process shows potential for scalable production with further optimization.

Abstract

Hexagonal boron nitride (h-BN) is deposited on Si <100> wafer ($\approx$20 cm2) via Plasma Enhanced Chemical Vapor Deposition (PECVD) using a ns-pulsed N2/Ar Micro Hollow Cathode Discharge (MHCD) as a microplasma source. For the first time, aluminum nitride (AIN) is employed as the dielectric material in the MHCD to mitigate film contamination by atomic oxygen, an issue previously observed with conventional Al 2O3 dielectrics. A comprehensive multi-diagnostic approach is followed to characterize the deposited h-BN, including Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron microscopy (XPS). In parallel, in-situ diagnostics such as optical emission spectroscopy (OES) and intensified CCD (ICCD) imaging are used to monitor plasma properties, including emission profiles, gas temperature and discharge morphology. Raman spectra reveal the E2g phonon mode of h-BN around 1366 cm_____, confirming successful synthesis. SEM imaging reveals an almost complete surface coverage by the film, with localized delamination. This is probably due to an uneven resistive heating of the Si wafer, rapid post-deposition cooling ($\sim$13 K/min) and ambient exposure. AFM analysis indicates an average thickness of about 33 nm after 90 minutes of deposition ($\sim$22 nm/h deposition rate). XPS measurements reveal an average B/N atomic ratio of $\sim$1.5 along the wafer diameter. Deviations from ideal film properties (e.g., stoichiometric unity, uniform morphology) are attributed to plasma-induced inhomogeneities (such as non-uniform species flux and temperature gradients) among other factors (e.g., 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 for scalable h-BN synthesis, with further optimization of the reactor design, plasma conditions, and gas chemistry required to grow ideal films.