Quasi-Ballistic Thermal Transport in Vertically Aligned h‑BN

Hexagonal boron nitride (h-BN) has attracted significant interest in the semiconductor industry due to its wide bandgap (5.95 eV), high anisotropic thermal conductivity, and low dielectric constant. However, the formation of crystalline h-BN conventionally requires high processing temperatures, which are incompatible with back-end-of-line (BEOL) integration and advanced packaging processes that must remain below 400 °C to avoid degradation of front-end devices. In this work, trichloroborazine (TCB) was systematically investigated using ammonia as the nitrogen co-reactant under both plasma-enhanced atomic layer deposition (PEALD) and atomic layer annealing (ALA) conditions. In the ALA process, low-energy inert ions are shown to enable in situ crystallization at temperatures well below those required for conventional MOCVD. As a result, highly ordered, vertically aligned polycrystalline h-BN films were deposited within a BEOL-compatible temperature window of 300-400 °C. The as-deposited ALA h-BN films exhibited clear signatures of cross-plane quasi-ballistic phonon transport over a thickness range of 20-50 nm, characterized by a thickness-independent effective thermal resistance of ∼6-9 m² K GW^-1. This invariance with film thickness indicates that phonon mean free paths exceed the film thickness, confirming quasi-ballistic heat transport enabled by the high crystallinity and preferential vertical alignment of the h-BN domains.