Results and Discussions The Fourier synthesis illumination device composed of MEMS and off-axis ellipsoidal mirrors can achieve various illumination patterns such as disk, dipole, quadrupole, and annular and the illumination area size (representing the partial coherent factor) and spacing of the illumination area can be adjusted. The tested illumination profile has no distortion and the illuminating intensity distribution is relatively uniform. When the MEMS scanning angle is ±1° and the magnification of the ellipsoidal condenser is 10 (i.e., with an object distance of 1 m), the maximum illumination diameter can reach >30 mm on the condenser, and the 4×NA on the ellipsoid focal surface can reach >0.6. Moreover, the illumination area on the surface to be detected located at the imaging focal point of the ellipsoidal mirror was tested, and all scanning rays were concentrated in the same area on the surface to be detected. Neither the scanning mode nor scanning angle influences the position of the overlapping area. Ellipsoidal mirrors with different magnifications can be used to adjust the size of the illumination area on the surface, and the actual magnification of the illumination area is basically consistent with the theoretical value.
Off-axis illumination is an important resolution enhancement technology in lithography, and it can effectively enhance both resolution and the focal depth of the lithography tool. Conventional off-axis illumination methods, such as those using pupil filters, have the disadvantage of serious energy loss. Moreover, realizing a few special illumination patterns is difficult using transmission elements represented by an axicon, and diffractive elements have the problem that a single diffractive element corresponds to only one illumination pattern. In the EUV spectral band, because optical materials intensely absorb EUV radiation, the transmission elements and transmission type diffraction optical elements cannot be used. In the present study, we investigate an illumination system based on Fourier synthesis technology. It has advantages of realizing any off-axis illumination patterns, increasing imaging numerical aperture (NA), high energy efficiency, and wide applicability in various spectral bands, especially for applications in the EUV spectrum. We hope that our research results will improve the understanding of Fourier synthesis technology and achieve an illumination technology that limits illumination divergence and can easily provide uniform illumination, making it useful in applications such as lithography projection exposure and mask defect detection.
We use a micro-electro-mechanical systems (MEMS) mirror combined with an off-axis ellipsoidal mirror to construct our Fourier synthesis illumination device. The surfaces of the MEMS and ellipsoidal mirrors are coated with a high reflectivity film for working wavelength. Based on the characteristic high-frequency two-dimensional rotation of the MEMS mirror, with the support of an optimization scan program, we set ray-scanning paths of the MEMS mirror in the x and y directions, achieving various illumination patterns such as disk, dipole, quadrupole, and annular, and adjust the partial coherent factor. The scanning ray is then collected and imaged by using the ellipsoidal mirror with two imaging focal points, whose surface to be detected (such as the mask) is located at the focal point. The Fourier synthesis illumination device provides uniform illumination with the required illumination pattern and illumination divergence to the surface to be detected. In this study, the imaging characteristics of two ellipsoidal mirrors with different magnifications of M=10 and M=2.5 are verified, and the simulation results are found to be basically consistent with the experimental test results.
The Fourier synthesis technique based on MEMS and off-axis ellipsoidal mirror is studied and an experimental confirmatory device is set up. The feasibility of Fourier synthesis technology is verified, and it can achieve various illumination patterns and illumination size by adjusting the pupil and partial coherent factor. The experiment demonstrates that Fourier synthetic illumination technology can meet the requirements of off-axis illumination and illumination divergence of imaging systems. Our research shows that Fourier synthesis technology is an illumination method that can be easily meet illumination requirements. There are only two main reflection elements needed to minimize energy loss, and their reflection characteristics are widely applicable over a wide spectral range.
