A study of diffraction limitation and minimum run length for immersion lithography

As the semiconductor technology node shrinks to 28 nm and beyond, not only the difficulty of lithographic process becomes higher and higher, the design rule setup is becoming more challenging. As the critical dimension (CD) enters the range that it shrinks faster than the shrink of the coherence length, which is defined by both numerical aperture and degree of coherence in the illumination condition. This will increase the difficulty in dealing with optical proximity. The coherence length exhibits itself in the correlation between neighboring spatial points in the image plane. In this paper, we discuss the several cases that can be used to probe the correlation length that is commonly found in the active area layer, such as, a semi-dense narrow space sandwiched by two wider pads, within which, the run length between the pads and its relation to the coherence length is studied. Self-developed codes with finite-difference time-domain (FDTD) algorithm are used for the optical simulation. Resist acid diffusion is taking into account as a Gaussian term. We will present our results and a short discussion.