Impact of mask corner rounding on pitch 40 nm contact hole variability

Abstract

Contact holes are traditionally designed as squares or rectangles on the mask. The lithography tool acts as a low-pass filter of the spatial frequencies and is not able to image the corners. Therefore, the contact holes will appear as circles or ellipses on the wafer, even if they are square on the mask. Nevertheless, the presence of the designed corners imposes a limit to the center-to-center distance that can be obtained in certain contact hole configurations. Indeed, as the distance between contact holes is decreased, these corners are the first features to create violations in the Mask Rule Check (MRC). Rounded contact hole corners therefore give more design freedom.

The recent introduction of multi-beam mask writing makes writing curvilinear shapes on actual masks practical, including contact holes with corner rounding. While the benefits to design freedom are obvious, there is a need for studies identifying the impact of this additional corner rounding parameter on the imaging properties.

In this work, we investigate the imaging impact of corner rounding on pitch-40nm contact holes, exposed with an EUV scanner (ASML NXE:3400B at imec). The study aims to experimentally establish the difference in imaging quality introduced by the corner rounding. More specifically, we first characterize the pattern fidelity on the mask by means of mask CD-SEM images. Next, we assess the impact of corner rounding on imaging properties such as Local Critical Dimension Uniformity (LCDU), Mask Error Enhancement Factor (MEEF) and Exposure Latitude.