Verification of Adhesive Forces Reduction By Surface Modification Treatment in Finfet Patterns: A Molecular Dynamics Simulation Study
Authors: Seki Ryuichi , Tabe Hirofumi , Fujiwara Naozumi , Sato Masanobu , Takayanagi Masayoshi 
Published: December 2024

Since the pattern collapse was reported for the first time it has been getting more critical issue in wet cleaning process along with semiconductor scaling. The model of the pattern collapse is that structures such as fins deform due to capillary force during drying step, then they contact and stick each other due to surface tension. One technique to prevent pattern collapse involves lowering the surface tension by modifying the surface’s silanol groups with functional groups[1]. The expected mechanism is that the adhesive force becomes smaller than the elastic force of the pattern due to the reduced surface tension. However, the details, including the dynamics, are still unclear. In particular, how functional groups affect fin contact from a molecular perspective remains unknown. Clarifying these issues is crucial because it will lead to a better understanding of the pattern collapse, subsequently enhancing semiconductor fabrication efficiency. Molecular dynamics (MD) simulation is a useful tool to elucidate these phenomena, as it can obtain physical properties and forces of the system from statistical thermodynamics and visualize nanoscale dynamics[2].
In this study, we prepared models of fins modified with trimethyl silanol (TMS) and calculated the adhesive forces between them by using MD techniques. We utilized the MD solver Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and the visualizer Open Visualization Tool (OVITO) [3, 4].
The fin model utilized in these calculations is depicted in Figure 1. The size of the simulation box was 15.360×2.304×42.000 nm. Periodic boundary conditions were applied in the x and y directions, while specular boundary condition was applied in the z direction. This model primarily consisted of Si atoms. The height of fins was 30 nm, with the top 10 nm terminated by OH groups (912 in total) and the remaining terminated by H atoms. Here we introduced TMS substitutions for randomly selected OH groups. The numbers of substitutions were 0, 91, 228, and 365, resulting in 0%, 10%, 25%, and 40% TMS models, respectively. To reduce bias arising from the random placement of TMS groups, five TMS models were created for each level of TMS substitution. The first step in the calculation process involved placing water between the fins to simulate a wet condition and induce pattern collapse, followed by conducting a 2.0 ns MD calculation. As a result, in every model, the fins came into contact each other (Figure 1b). Subsequently, all water molecules were removed to simulate a dried condition, followed by a 2.0 ns MD calculation. In all models, under the dry condition, the fin contact established during the wet condition was kept (Figure 1c, d). The average adhesive forces between 1.0 ns and 2.0 ns of the MD trajectory after water removal were analyzed, revealing that the forces in the 10%, 25%, and 40% TMS models were less than 1/3 of those in the 0% TMS models. We interpret that, despite the persistence of pattern collapse in the dry condition, restoring the initially separated fins appears more feasible in TMS-modified surface models. The details will be reported in an oral presentation.
[1] T. Koide et al., "Effect of Surface Energy Reduction for Nano-Structure Stiction," ECS Transactions, vol. 69, no. 8, p. 131, 2015, doi: 10.1149/06908.0131ecst.
[2] R. Seki et al., "Insights into FinFET Structure Collapse: A Reactive Force Field-Based Molecular Dynamics Investigation," Solid State Phenomena, vol. 346, p. 123, 2023, doi: 10.4028/p-mUO0Oa.
[3] A. P. Thompson et al., "LAMMPS - a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales," Computer Physics Communications, vol. 271, p. 108171, 2022, doi: 10.1016/j.cpc.2021.108171.
[4] A. Stukowski, "Visualization and analysis of atomistic simulation data with OVITO–the Open Visualization Tool," Modelling and Simulation in Materials Science and Engineering, vol. 18, no. 1, p. 015012, 2010, doi: 10.1088/0965-0393/18/1/015012.
Figure 1: (a) The wet Fin model with inter-fin water molecules for MD simulations. The top 10 nm of the fins were terminated with OH groups, while the remaining were terminated with H atoms. Through the introduction of TMS substitutions for randomly selected OH groups, we prepared 10%, 25%, and 40% TMS models. (b) The snapshot of MD simulation after 2.0 ns from the initial configuration of (a). (c) The dry model prepared by removing water molecules from the configuration of (b). (d) The snapshot of MD simulation after 2.0 ns from the configuration of (c).

Figure 1