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

Abstract

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.

In situ analysis of growth rate evolution during molecular layer deposition of ultra-thin polyurea films using aliphatic and aromatic precursors

“…32,33 As growth proceeds, the rate of site consumption and diffusion/adsorption will become constant, resulting in a constant growth rate in a steady growth regime. 34 The transition from initial to steady growth will depend on deposition temperature, composition and reactivity of the growth surface, and precursors used. For example, longer, more flexible monomers are expected to undergo DRs more readily, as these flexible chains can bend more easily.…”

“…35 Polyurea, with its chemical and moisture resistant properties, 36 can be formed via MLD using diamine and diisocyanate precursors under a wide range of deposition temperatures (room temperature (RT) to 90 °C). Several options for diamine precursors include ethylenediamine (ED), 37 1,6-hexanediamine (HD), 34 1,4-diaminobutane, 38 and p-phenylenediamine. 39 However, the diisocyanate reactant is typically limited to p-phenylene diisocyanate (PDIC) for polyurea 20,35 and polyurethane 40 MLD.…”

Nonporous, Strong, Stretchable, and Transparent Electrospun Aromatic Polyurea Nanocomposites as Potential Anticorrosion Coating Films

“…Coatings are usually produced by spin casting [38][39][40], molecularlayer deposition (MLD) [41,42], vapor-deposition polymerization (VDP) [43,44], or spray coating techniques [45,46]. These methods adopt a layer-by-layer deposition approach, and each layer adheres to each other rapidly, hindering the crosslinking of polymer chains and resulting in cracks and low strength [47][48][49][50]. In order to prevent nucleation growth during MLD, self-assembled precursors are added; however, they also diffuse easily into the porous deposited films [49].…”

“…These methods adopt a layer-by-layer deposition approach, and each layer adheres to each other rapidly, hindering the crosslinking of polymer chains and resulting in cracks and low strength [47][48][49][50]. In order to prevent nucleation growth during MLD, self-assembled precursors are added; however, they also diffuse easily into the porous deposited films [49]. This process leads to additional film growth and affects the controllability of the MLD process [51].…”

Atomic Layer Deposition of Insulating AlF3/Polyimide Nanolaminate Films

“…While ALD is often considered being limited to inorganic coatings, molecular layer deposition (MLD) is a corresponding technique for vapor deposition of organic and hybrid films, which is also based on continuous self-limiting surface reactions. [28][29][30][31].…”