Measuring adhesion forces between hydrophilic surfaces with atomic force microscopy using flat tips

Abstract

A semiconductor wafer is exposed to several processing steps when it is converted from a bare silicon wafer to one populated with millions of transistor circuits. Lithography is one of the most important and critical steps in semiconductor wafer manufacturing. It is directly responsible for shrinking feature sizes to increase transistor densities. However, with further down scaling of the feature size on wafers, the magnitude of adhesive forces becomes a prohibitive factor in further increasing handling speeds and throughput of wafers. This thesis aims at providing a better understanding of the factors that are of prime importance for the magnitude of the adhesion forces between two parallel hydrophilic surfaces as in the wafer stepper machines between the wafer and the wafer table. Atomic Force Microscopy was chosen as the key method in this study due to its ability of sensing the interaction forces between surfaces with force – distance spectroscopy. Different from most adhesion studies in literature flat topped, micron sized silicon tips were used. The individual variables, which could play a role on the magnitude of the adhesion forces, are systematically varied in experiments, while controlling the others in order to arrive at a complete picture of the parameters that affect the adhesion forces. These variables were the surface roughness (that varied with an anisotropic wet chemical etching method), humidity of the environment, the applied load on tip, the residence time of the tip on the substrate, the tip’s retraction speed, and the tip size.