Impact of attenuated phase-shifting mask for 157-nm lithography with high numerical aperture lens

Abstract

A phase-shifting mask (PSM) is one of the most effective resolution enhancement technologies to improve the resolution limit and process margins such as exposure latitude (EL) and depth of focus (DOF). The attenuated phase-shifting mask (Att-PSM) is the most practical PSM, because it has a simple structure and can be easily fabricated. However, it is very difficult to evaluate the impact of using Att-PSMs on the resolution limit and process margin, under the condition of both a shorter wavelength and higher numerical aperture (NA). The reason is that the resolution improvement of the Att-PSM is very small under the above condition. In this study, we investigate the impact of using the Att-PSM instead of a binary mask under the conditions of shorter wavelength (157-nm) and higher-NA (0.85-NA). We evaluated the resolution limit by both aerial image simulation and exposure experiment. The aerial image simulation confirmed that the resolution improvement in the line and space pattern that can be expected from an Att-PSM of 5% transmittance diminished by decreasing wavelength and increasing NA. In particular, when a wavelength of 157-nm and an NA of 0.85 are used, we obtained a 6% resolution improvement compared to the binary mask. In the exposure experiment, we obtained an 11% resolution improvement when using a TaSiOx-type Att-PSM of 5.7% transmittance. From these results, we found that the Att-PSM can be used to fabricate smaller size features even shorter wavelength of 157-nm and the higher NA of 0.85.