Computational Lithography

Chapter 2

Optical Lithography Systems

2.1 Partially Coherent Imaging Systems

Most practical illumination sources in optical lithography systems have a nonzero line width and their radiation is more generally described as partially coherent [75]. Partially coherent illumination (PCI) is desired, since it can improve the theoretical resolution limit. PCI is thus introduced in practice through modified illumination sources having large coherent factors or through off-axis illumination. In partially coherent imaging, the mask is illuminated by light traveling in various directions. The source points giving rise to these incident rays are incoherent with one another, such that there is no interference that could lead to nonuniform light intensity impinging on the mask [92, 93]. Common partially coherent illumination modes include dipole, quadrupole, and annular illumination. Partially coherent imaging models are discussed in this section.

In partially coherent optical lithography systems, the Köhler illumination configuration is assumed, which is shown in Fig. 2.1. In the Köhler illumination configuration, the light source is considered to be located at the focal plane of the condenser and the object plane is located at the condenser exit pupil [4]. Each point source on the illumination emits a coherent, linearly polarized plane wave with a spatial frequency determined by the position of the point source related to the optical axis. Under the assumption of the Köhler illumination configuration, ...