Optimal 3D phase-shifting masks in partially coherent illumination.
Gradient-based phase-shifting mask (PSM) optimization methods have emerged as an important tool in computational lithography to solve for the inverse lithography problem under the thin-mask assumption, where the mask is considered a thin two-dimensional object. As the critical dimension printed on the wafer shrinks into the subwavelength regime, thick-mask effects become prevalent and thus these effects must be taken into account in PSM optimization methods. Thick-mask effects are particularly aggravated and pronounced in etching profiles with abrupt discontinuities and trench depths. PSM methods derived under the thin-mask assumption have inherent limitations and perform poorly in the subwavelength scenario. This paper focuses on developing three-dimensional PSM optimization methods that can overcome the thick-mask effects in lithography systems with partially coherent illumination. The boundary layer model is exploited to simplify and characterize the thick-mask effects, leading to a gradient-based PSM optimization method. Several illustrative simulations are presented.
Key Laboratory of Photoelectronic Imaging Technology and System of Ministry of Education of China, School of Optoelectronics, Beijing Institute of Technology, Beijing, China. maxu@bit.edu.cn
Xu
X
Ma
Gonzalo R
GR
Arce
Yanqiu
Y
Li