Masataka Shiratsuchi

Development of a captured image simulator for 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. It is not well understood whether we can get sufficient sensitivity with conventional optical setups and wavelength with the assistance of some kind of resolution enhancement techniques (RET), or we should move toward inspection using revolutionary new optics or shorter inspection wavelength. To study precisely the sensitivity of inspection optics for common types of defects, we have made a captured image simulator based on the RCWA (Rigorous Coupled Wave Analysis) method with which we can take into account the effect of the three-dimensional structure of a mask. We tried to calculate captured images for some mask structures at two different wavelengths, namely 199 nm and 257 nm. We made certain that no significant differences were observed for larger scale defects, but that a considerable difference of image contrast was observed for small scale defects around 50 nm in size. Thus we confirmed that this simulator is effective for evaluating and designing optical systems of mask inspectors, in order to achieve a high sensitivity for the detection of small defects in Alt-PSMs.

Development of a captured image simulator for the differential interference contrast microscopes aiming to design 199 nm mask inspection tools

Recently, technologies of ArF laser exposure tools and alternating phase shifting masks (Alt-PSM) are expected to be used in actual production. To utilize such newly developed technologies, it is inevitable to develop a mask inspection technology to check them properly. But it is currently difficult to check them precisely because sufficient image contrast is hard to obtain with any conventional mask inspection tools. Among many observation methods, the differential interference contrast (DIC) is one of a few methods that can be used to observe a differentiated phase shift of transmitted light of an object with high resolution. To study precisely the performance of this optical configuration, we built a new captured image simulator in which Wollaston prisms were modeled as a kind of phase modulation plates. We built this simulator as an extension of the captured image simulator we reported formerly), which is based on Rigorous Coupled- Wave Analysis (RCWA) to calculate diffractions; this enables us to properly treat effects of polarization, high NA, and 3-dimensional mask structures. We applied this simulator to see sensitivities of DIC against bumps and divots with various sizes. We found that the image contrast for small phase defects 20 to 50 nm in sizes is much higher in DIC microscopes than in conventional optical setup with coherence factor less than 1. We also found the dependence of captured images on polarizations and optical axis directions. We expect our simulator to be a useful tool for studying, designing, and developing mask inspection tools.