An approach to increase efficiency of DOE based pupil shaping technique for off-axis illumination in optical lithography.

Off axis illumination (OAI) is one of the key resolution enhancement technologies in projection lithography system. Recently, phase type diffractive optical elements (DOEs) are adopted by most of the lithography machine manufactures to realize OAI. In general, the efficiency of the OAI unit is the main consideration compared with uniformity. However, the main goal of the traditional constraint of iterative Fourier transformation algorithm (IFTA) is used to optimize the SNR, while diffraction efficiency is the secondary consideration. Therefore a constraint for IFTA is well designed to increase the efficiency of DOE in this paper. This constraint can guarantee the pole balance at the same time. The main idea of this constraint is to apply amplitude and phase freedoms in the signal window and to control the total energy of each pole. Based on the proposed constraint several DOEs are designed for high NA lithography illumination system. And remarkable improvement in the efficiency is observed compared with the traditional constraint. Furthermore, the efficiency and uniformity could be weighted by adjusting the parameter with extending this constraint to general case. To demonstrate the proposed constraints, experiments are carried out where a spatial light modulator is utilized to represent DOEs.