Controlled Cracking-Induced Delamination for Polymer-Based Cleaning
ABSTRACT
The continuous miniaturization of microelectronic devices imposes increasingly strin gent requirements on surface cleanliness, as nanoparticles above 10 nm critically threaten device performance. Conventional cleaning methods often damage delicate nanostructures [1] or lack sufficient nanoscale efficiency. In this context, new cleaning methods such as polymer film coating and removal emerge as a promising gentle cleaning approach [2] [3].
This study focuses on the controlled delamination of polymethyl methacrylate (PMMA) thin films through Environmental Stress Cracking (ESC), a phenomenon where polymers crack under the combined influence of mechanical stress and chemical exposure. The susceptibility to ESC is strongly influenced by the chemical affinity between the polymer and the solvent, which can be quantitatively predicted using Hansen Solubility Parameters (HSP) [4]. These parameters enabled us to select polymethyl methacrylate and ethanol as the polymer-solvent pair for this study, optimizing the conditions to induce controlled cracking. The overall cleaning process is schematized in Figure 1, illustrating the key steps: coating of the polymethyl methacrylate film by spin-coating on a Si wafer previously contaminated with particles, baking, immersion in ethanol to induce environmental stress cracking, and finally spontaneous delamination and particle removal by rinsing.
The cracking behavior of polymethyl methacrylate (PMMA) films was systematically investigated by varying key film preparation parameters, namely the solvent used to dis solve PMMA, the PMMA concentration in the spin-coated solution and the baking conditions. Crack morphology and total crack length were used as quantitative indicators of cracking severity. The choice of solvent used to dissolve PMMA was found to have a significant impact on the resulting crack morphology. Films prepared using toluene, butanone, and chloroform exhibited markedly different crack patterns following immersion in ethanol, as shown in Figure 2. Previous research has demonstrated that the morphology of cracking is primarily controlled by the adhesion strength at the polymer-substrate interface [5]. Therefore, the observed variations in crack morphology are attributed to solvent-dependent modulation of interfacial adhesion, which governs crack initiation, spacing, and propagation. The PMMA concentration of the spin-coated solution strongly affects cracking behavior, with a saturation effect observed beyond a certain concentration range. This behavior may be attributed to variations in film thickness, which in our experiments ranged from 2 µm to 10 µm. We hypothesize that thicker films, obtained at higher PMMA concentrations, promote more extensive cracking. Baking further influences crack propagation: increasing the bake temperature or ex tending its duration intensifies the cracking phenomenon. This suggests that a higher thermal budget induces greater thermal stresses within the film, which are subsequently relaxed through crack formation and growth. Applying this process to silicon substrates contaminated with cerium oxide particles, we achieve a particle removal efficiency above 80% after PMMA film deposition, baking, and ethanol-induced delamination. These results demonstrate the potential of ESC-driven polymer peeling as an effective cleaning technique for microelectronics. This approach offers a sustainable alternative to aggressive chemical or mechanical cleaning, minimizing substrate damage and chemical waste. Future work will focus on mechanistic studies of adhesion and fracture.
BIOGRAPHY
Mona Moukrati is a second-year PhD student working on an STMicroelectronics research project conducted in collaboration with the Laboratoire Interdisciplinaire de Physique (LiPhy), France. Her research focuses on fracture-induced delamination of thin films as an alternative cleaning strategy for microelectronic wafers. She investigates the use of controlled cracking of polymer thin films to enable particle and contaminant removal from silicon substrates. Her work combines experimental studies, time-resolved imaging, and image analysis to characterize cracking dynamics and identify the key parameters governing cleaning efficiency. By linking thin film mechanics and fracture physics to industrial cleaning constraints, her research aims to contribute to the development of more efficient and less aggressive wafer cleaning processes.