Experimental characterization of the receding meniscus under conditions associated with immersion lithography

摘要

Immersion lithography allows the semiconductor industry to create next-generation devices without requiring a large shift in infrastructure, making it an appealing extension to optical lithography. Improved resolution is enabled by placing an immersion fluid with a high refractive index between the final lens of the optical system and the resist-coated wafer. Several engineering challenges accompany the insertion of the immersion fluid in a production tool, one of the most important being the confinement of a relatively small amount of liquid to the under-lens region. The semiconductor industry demands high throughput, leading to relatively large wafer scan velocities and accelerations. These result in large viscous and inertial forces on the three-phase contact line between the liquid, air, and substrate. If the fluid dynamic forces exceed the resisting surface tension force then residual liquid is deposited onto the substrate. Liquid deposition is undesirable; as the droplets evaporate, they will deposit impurities on the substrate. In an immersion lithography tool, these impurities may result in defects. An experimental investigation was undertaken to study the static and dynamic contact angle under conditions that are consistent with immersion lithography. A semi-empirical model is described here to predict the velocity at which liquid loss occurs. This model is based on fluid physics and correlated to measurements of the dynamic and static contact angles. The model describes two regimes, an inertial and a capillary regime, characterized by two distinct liquid loss processes. The semi-empirical model provides the semiconductor industry with a useful predictive tool for reducing defects associated with film pulling.