Effect of pH and Ion Concentration on Wetting of Nanoholes and Water Structuring
Authors: Cho Hwiwon , Vereecke Guy , Kenis Karine , Kim Tae-Gon , Park Jin-Goo , Wostyn Kurt , Sanchez Efrain Altamirano 
Published: July 2022

Like supervias in the back end of line, and deep contact holes with an aspect ratio higher than sixty in 3D-NAND memory, higher aspect ratio patterning is one of the important issue in advanced semiconductor manufacturing. As with the difficulty of patterning, the technique of cleaning the nanostructure so that it does not affect the next process is also considered important. In semiconductor cleaning, chemicals such as HF and NH4OH that include reactant ions are used for cleaning the semiconductor surface. Ions in bulk aqueous solutions were shown either as structure making or breaking effects in Marcus’s study (Chem. Rev., 2009). The classification of ions as structure making or breaking was confirmed using 1 M solutions in nanotrenches (Vereecke, Micro. Eng., 2021). This shows there is an ion effect, and it can change properties between solution and material surface. The addition of structure breaking ions in HF solutions was used to decrease the etch rate of oxide between fins (Vereecke, Micro. Eng., 2022). Going further from Marcus’s study, we focused on pH and ion concentration in the addition of structure making ions. In the nanostructure, it was considered necessary to study because the surface properties may vary depending on the degree of structuring and the influence of the zeta potential depending on the concentration and pH of the ions.
In this work, we used an in-situ ATR-FTIR (Attenuated Total Reflection – Fourier Transformed Infra-Red) spectroscopy technique (Nicolet iS50 AEM, Thermo Fisher Scientific, USA) to characterize the wetting of nanostructures embedded in a silica matrix by UPW (Ultra-Pure Water) and electrolyte solutions, and a streaming zeta potential analyzer (SURPASS3, Anton Paar, Austria) to characterize the surface potential of flat surfaces of the same material. Wetting in the nanostructures was characterized by an analysis of the ratio of the OH stretching peak to OH bending peak (Vrancken, Langmuir, 2016). Also, dissolution of CO2 in the wetted nanostructures was monitored to compare the solubility and diffusivity in the nano-confined solutions with that in bulk solutions. In this experiment, we used dense arrays of silicon nanoholes in a PEALD SiO2 matrix (depth of about 300 nm, diameter of about 20 nm, and pitch 90 nm) that were fabricated on Si wafers using arrays of nanostructures, as described in Vereecke (2021). Crystal drying, wetting with a solution, and applying CO2 were performed in this order. HI, HBr, HCl were used for chemicals and pH 1, 2, 3, 4 was used for pH.
Monitoring of the OH stretching to bending ratio showed little improvement in wetting as a function of pH between 1 to 4 as compared to UPW. Also, little difference was observed when changing the acid from HCl to HBr and HI, with anions of higher structure breaking properties according to Marcus (Chem. Rev., 2009). A higher CO2 solubility and a lower CO2 diffusivity were measured in the nanoconfined solutions as compared to bulk UPW, indicative of water structuring. A higher CO2 solubility at pHs 2-3 as opposed to pH 1 and 4 may originate from the proximity to the isoelectric point. Results will be complemented with tests performed at the isoelectric point and pH 0, with 1 M solutions where structure breaking properties of the used anions are expected to be stronger and wetting might be improved.