Simulation and experimental evaluation of double-exposure techniques

The process windows and capabilities of double exposure techniques with binary and attenuated masks are explored using simulation and experiment, including the effects of resist properties, illumination conditions and overlay error. Here it is shown that by using a low partial coherence factor (sigma) for the two exposures, the total window is considerably improved over that obtained using higher partial coherence illumination. We call this process ORAMEX, which stands for Ordinary Resist And Multiple EXposure. It was found that the process window for nested lines and spaces using ORAMEX is considerably better than that for conventional illumination. This is shown for aerial images and for aerial images plus a resist model with contrast and diffusion length similar to that of state of the art Deep UV resists. In fact, the total process windows found for ORAMEX show good process latitudes for both dense and isolated features, with ORAMEX usually enhancing dose latitude more than single exposure off axis illumination does. Overlay errors are found not to affect the process window for individual features. However, they do affect the common window for every other line (in positive resist) but not spaces. It was also found that using attenuated masks instead of binary masks further improves the process window and resolution of ORAMEX. Experimental results agree with simulation and show a process window for 150 nm lines and spaces with over 0.4 micrometer depth of focus and 15% dose latitude in 0.6 micrometer of resist using ORAMEX and chrome on glass masks. Using attenuated masks and ORAMEX a similar process window (0.4 micrometer DOF and 16% dose latitude) was obtained for 125 nm lines and spaces. Both results were obtained on a 0.6 NA Deep UV stepper using commercial positive resist.