Understanding Molecular Layer Deposition Growth Mechanisms in Polyurea via Picosecond Acoustics Analysis

Molecular layer deposition (MLD) is an increasingly important thin film synthesis technique in areas such as sensors, microelectronics, protective coatings, and catalysis. However, new analytical approaches are needed to advance fundamental understanding of deposition reaction mechanisms. This work introduces ultrafast laser-based pump–probe picosecond acoustics analysis to characterize thickness-dependent properties of MLD films. Polyurea films are deposited on hydroxylated SiO2 substrates using 1,4-phenylene diisocyanate and a diamine reactant, either ethylenediamine (PDIC/ED polymer) or 1,6-hexanediamine (PDIC/HD), and the expected polymer structure is confirmed by Fourier transform infrared spectroscopy. During the first ∼20 nm of deposition, spectroscopic ellipsometry shows constant refractive index but decreasing growth rate before reaching steady state. X-ray reflectivity also shows approximately constant density during initial growth. However, the measured picosecond acoustics signatures demonstrate a marked increase in sound speed initially, indicating a transition in the physical film structure. The observed trends are ascribed to a transition in the kinetics of active site production and termination with increasing thickness, leading to changes in polymer and oligomer connectivity within the film. These findings provide a basis for better understanding MLD processes and reaction mechanisms that determine deposited film properties.