In a 193-nm immersion scaner, the water is confined to the gap between the front lens and the wafer, forming a water meniscus. This water meniscus stays with the exposure head as the wafer moves at peak speeds of ~500 mm/s (Fig. 4.1). The movement of the wafer applies a frictional force to the bottom of the water meniscus. This force changes the shape of the water meniscus, forming an advancing side and a receding side. To maintain a clean process, the water meniscus must keep its shape and move easily on the wafer surface without leaking water and leaving water droplets behind. Therefore, a hydrophobic resist surface is essential to the success of immersion lithography.
Both static and dynamic contact angles on resist surfaces have been investigated and the research on these angles has been beneficial to resist designers, scanner designers, and process engineers. This chapter covers the following topics: definitions of static and dynamic contact angles, dynamics of the water meniscus, experimental results from the model immersion head, water leakage mechanisms, contact angle measurement methods, and process-induced contact angle changes.
4.1 Definition of Static and Dynamic Contact Angles
The contact angle (CA) of water on the resist surface is a very important parameter in 193i lithography and is defined as the angle at which a water and air interface meets the resist surface.
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