Two-wave pattern shift aberration monitor for centrally obscured optical systems

An aberration monitoring technique based on lateral shifts of two-wave interference patterns in centrally obscured optical systems is presented, and simulations are used to evaluate the performance of such a technique. The technique is being explored as a convenient means for monitoring the aberration level in the 0.3-NA Micro Exposure Tool (MET) optic over time. A binary mask was designed for observing phase differences across the MET optic on cut-lines at 0, 45, 90 and 135 degrees across the pupil. The mask consists of 5 line-and space patterns in a dark field that measure the side-to-side phase difference across the pupil at 7 equally spaced radial points extending from 35% to 95% of the pupil radius. For near on-axis illumination the blockage of the zero-order creates a two-wave, interferometric pattern at the wafer with half of the period expected under normal imaging conditions. The optical path difference between the two orders produces an image shift of one full period of the (frequency doubled) interference pattern per 360 degrees of side-to-side path difference. Shifts on the order of 5 to 20 nm are expected and are measured using a reference target of an array of 5 medium sized dots. Aerial image simulation is being utilized to predict the expected performance and to improve the initial design. The aberrations measured by interferometry are being used for this purpose. Also the quality of images at low partial coherence with the wavefront convergence present in the MET illumination is being studied. In addition to theory and simulation results, practical considerations in implementing this technique on actual lithography tools based upon MET-type optics are addressed, including pattern design, illumination characteristics, and data analysis

Spectral purity enhancement for the EUV Lithography Systems by suppressing UV reflection from multilayers

Plasma based radiation sources optimized to emit 13.5 nm Extreme UV radiation also produce a significant amount of light at longer wavelengths. This so called out-of-band (OoB) radiation is detrimental for the imaging capabilities of an EUV lithographic imaging system, particularly the ultraviolet (UV) parts of the light (λ=100-400 nm). To suppress these wavelengths while maintaining the high efficiency of the mirror for EUV light, several methods have been developed, including phase-shift gratings (PsG) and anti-reflection layers (SPE layer). Both methods have achieved a suppression factor of 10 - 30 around the target wavelength. To achieve a full band suppression effect with a minimum loss of EUV light, a new scheme based on surface pyramid structures was developed. An average suppression of more than 10 times was achieved with a relative EUV efficiency of 82.2% by using the Si pyramids structure (compared to a flat multilayer (ML)). Recently, we have successfully produced a pyramid structure consisting of multilayers which greatly improves the relative EUV efficiency to 94.2%