Substrate Vibration Accelerates Film Dewetting: Insights from Molecular Dynamics Simulations

Significance 

The thermally-induced dewetting of thin films on nonwetting substrates has garnered significant attention in the field of nanofabrication. This phenomenon occurs when the film thickness is reduced to tens of nanometers or even at the nanoscale, resulting in spontaneous film rupture and assembly patterns. The study of film dewetting not only holds intrinsic scientific interest but also has substantial applications in the fabrication of micro-nanostructures, particularly particle arrays, over a wafer scale. Researchers previously have investigated how delicate nanostructures and surface acoustic waves (SAWs) can influence film dewetting, enabling more precise control over the size, shape, and arrangement of nanostructures. Despite these advancements, the effect of substrate motion on film dewetting has not been thoroughly studied, and this aspect offers valuable insights into the underlying physics and potential applications.

A new study conducted by Professor Lei Wang from QingDao University of Science and Technology, published in the peer-reviewed Journal of Thin Solid Films, explored the intriguing interplay between surface tension and Van der Waals interactions within a medium-film-substrate system, leading to the spinodal dewetting mechanism. This process involves the spontaneous rupture of a thin film, forming characteristic patterns of particles. While dewetting of metallic films has traditionally been achieved through thermally annealing in a furnace, recent advancements in nanosecond pulsed laser-induced dewetting have provided the ability to obtain various particle structures in real-time observation.

Professor Lei Wang used Molecular Dynamics (MD) simulations to investigate the effect of substrate vibration on the thermally-induced dewetting of a silver (Ag) film on a silicon dioxide (SiO2) substrate. The SiO2 substrate was modeled to be 1.5 nm thick, and the bottom was fixed. The Ag film’s thickness was varied at 0.2 nm, 0.4 nm, and 0.6 nm, with dimensions of 15 nm × 15 nm. Periodic boundary conditions were employed in three dimensions, and the simulations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The interactions between Ag-Ag, Si-O, Ag-Si, and Ag-O were described by the Tersoff potential and embedded-atom (EAM) potential.

The author first examined the thermally-induced dewetting of the Ag film with different thicknesses (0.2 nm, 0.4 nm, and 0.6 nm) on both immobile and vertically vibrating SiO2 substrates. Without substrate vibration, the dewetting process was relatively slow, taking more than 1 nanosecond (ns) for film rupture as the thickness exceeded 0.6 nm. However, under the influence of substrate vibration, the dewetting process was significantly accelerated. For example, a 0.4 nm thick Ag film on a vibrating substrate spontaneously ruptured and formed a single particle at around 200 picoseconds (ps), reducing the total energy of the system. This observation indicates that the substrate vibration effectively promotes the dewetting of the Ag film.

The dewetting time (τ) was defined as the time from the beginning of heating to the complete formation of discrete particle patterns. Without substrate vibration, τ increased dramatically for Ag films with a thickness greater than 0.6 nm, similar to the behavior observed during film dewetting induced by nanosecond pulsed lasers. However, with substrate vibration, τ showed a slower increase as the thickness increased, indicating that the substrate vibration effectively promotes the thermally-induced film dewetting.

When Professor Lei Wang conducted qualitative investigation of the dewetting promotion under substrate vibration, it revealed that the height of the 0.4 nm thick Ag film on an immobile SiO2 substrate increased only slightly due to thermally-induced lattice disordering. However, when the substrate was vibrating, the Ag film underwent more significant changes, resulting in spontaneous rupture and particle formation. This suggests that the substrate vibration destabilizes the film, enabling dewetting to occur.

The study also investigated how the vibration parameters, namely the vibration period (T) and amplitude (A), affected the dewetting of the Ag film. An appropriate vibration period in the range of 2-100 ps was found to promote dewetting, while a large vibration period, such as T = 200 ps, had no significant effect. Similarly, an amplitude of 0.1-0.4 Å promoted dewetting, while amplitudes ≥ 0.5 Å led to complete detachment of the Ag film from the SiO2 substrate.

The author employed MD simulations to investigate the effect of substrate vibration on the thermally-induced dewetting of a thin Ag film on a SiO2 substrate. The results revealed that substrate vibration significantly promoted dewetting, leading to reduced dewetting time. The appropriate vibration period and amplitude were found to accelerate the dewetting process, making picosecond pulsed lasers a possible heating source to trigger film dewetting. The study also highlighted the potential for further research into the coupling between substrate/interface vibration and thermally-induced film dewetting, opening up a promising avenue for advancing nanofabrication techniques.

In conclusion, understanding the interplay between film dewetting and substrate motion could offer valuable insights for the development of advanced nanofabrication methods and the creation of precise micro-nanostructures. The research presented by Professor Lei Wang marks an important step in this direction and serves as a foundation for future investigations in the field of thin film physics and nanotechnology.

About the author

Lei Wang is an associate professor of College of Mathematics and Physics, QingDao University of Science and Technology (QUST), China. He received his PhD degrees from the National Center for Nanoscience and Technology (NCNST), Chinese Academy of Sciences (CAS), China. His research interest focuses on the laser-matter interaction including laser nanofabrication and optical manipulation. He develops several paralleled laser nanofabrication strategies to achieve the high-density monolayer of sub-10 nm nanoparticles and nanoholes with potential values in electromagnetic field enhancement devices. He also investigates the light-driven manipulation of graphene-based materials including quantitative measurement, mechanism interpretation, pulsed-laser induced propulsion and macroscopical pulling, etc.

Reference

Lei Wang. The effect of substrate vibration on Ag nanoparticle formation on SiO2 via thermally-induced dewetting: A molecular dynamics study. Thin Solid Films 767 (2023) 139674

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