Resolution enhancement with source-wavelength optimization according to illumination angle in optical lithography

Abstract. Background: To increase the resolution and depth of focus (DOF) of flat panel display (FPD) exposure systems, off-axis illumination (OAI) conditions are used extensively. OAI using narrowband wavelength illumination has been studied sufficiently. In contrast, new techniques that consider broadband wavelength illumination are needed because the effects of OAI differ between broadband and narrowband illumination. Aim: This paper presents a divided spectrum illumination (DSI), a new design concept that achieves both high resolution and a large DOF. Approach: The source wavelength is optimized according to the illumination angle. Results: Experimental imaging results for line and space patterns with a line width of 1.0 and a pitch of 2.0  μm demonstrate that the DSI design provides improved resolution. Exposure results also indicate that resist profiles using DSI are sufficiently sharp to retain pattern fidelity at the top of the resist. The DOF with DSI is also improved by 21% compared to that obtained with traditional OAI. Conclusions: DSI achieves both high resolution and a large DOF while maintaining high productivity.