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PhD Defence Mohammad Bazr Afshan Fadafan

roughness and adhesion effects on pre-sliding friction: modellilng and experiments

Mohammad Bazr Afshan Fadafan is a PhD student in the department of Mechanics of Solids, Surfaces & Systems (MS3). His supervisor is dr.ir. M.B. de Rooij from the Faculty of Engineering Technology (ET).

Controlling friction and adhesion at a wafer-waferstage interface is in direct relation with high precision and stability of the positioning mechanisms of a lithography machine. Understanding these two phenomena is the first and key step in controlling them. This thesis aims at developing a BEM (Boundary Element Method) model for an adhesive frictional contact of a rough interface, representing the wafer-waferstage interface, along with experiments to verify the validity of the model. The developed model consists of two main blocks which are interacting with one another: adhesion and pre-sliding friction.

Adhesion is considered to be dominated by the van der Waals forces (in vacuum conditions) and the capillary force (in ambient conditions). In the first step, a previously developed algorithm for the non-adhesive normal contact of rough surfaces is extended to include the adhesion effect due to van der Waals forces. This BEM model is further extended, in the second step, to account for the capillary force due to a humid environment and thin water films adsorbed on the contacting surfaces. In the developed model, the effects of various parameters, such as work of adhesion, roughness properties, and relative humidity are investigated. To verify the accuracy of the model, a series of pull-off force measurements, both in vacuum and ambient conditions, is conducted using an Atomic Force Microscope (AFM) at the contact of a cantilever with an SiO2 colloidal probe and a silicon wafer. The experimental results are then compared with the model predictions for the measured forces.

In the second block, a BEM model is developed for the pre-sliding behavior of a rough interface formed by two contacting surfaces. The adhesive terms are then embedded in this model. The influences of different parameters, such as work of adhesion and roughness parameters, on the friction hysteresis loops, pre-sliding displacement, and static friction force are studied. To validate the model, friction measurements are carried out in an in-house setup, named VAFT (Vacuum Adhesion-Friction Tester), for the contact of a polymeric ball against a silicon wafer under various normal loads.

The developed BEM model, as the output of this thesis, can be used as a tool to design textures on the waferstage in order to achieve a desirable level of friction and adhesion aiming at a higher level of precision, stability, and durability during the lithography process.