Simulation study of decoherence and light intensity uniformization for extreme ultraviolet of uniform light pipe

Abstract

Free Electron Laser (FEL) is a high-quality laser source with wavelengths ranging from short-wave X-rays to long-wave infrared ray. Extreme Ultra-violet (EUV) radiation at λ=13.5nm emitted by FEL can be used in integrated circuit manufacturing, such as EUV lithography exposure, mask defect inspection.However, the high spatial coherence characteristics and similar Gauss intensity profile distribution of FEL source has a negative effect on imaging, and cannot meet the requirements of imaging applications in EUV lithography. In this work, a newly light pipe for decoherence and intensity uniformity at EUV spectral range is designed through the simulation calculations. New light pipe consists of a two pairs of tilted elements are symmetrically distributed in the y-z plane and x-z plane, respectively.In this way, the beam transmission divergence in two dimensions can be widened at the same time, and the disturbance of the ray transmission track and spatial phase distribution is increased, so as to achieve the uniformization of light intensity and the reduction of spatial coherence. The simulation results show,for an EUV gaussian beam at the λ=13.5nm, with diameter of 200μm and a divergence of 20mrad, compared with the conventional light pipe structure, the newly designed light pipe has more significant decoherence and illumination field uniformity. When the inner diameter is 1mm, the total length is not less than 600mm, and the tilt angle is 10mrad of the newly light pipe, the basically uniform illumination field can be obtained, the coherence is completely disordered,and non-uniformity of light intensity distribution at the illumination field is reduced to 0.97 from 28.38 achieved by conventional cylindrical light pipe.At the same time, the light power transmission efficiency is about 37.6% and the maximum transmission efficiency is about 44.58%. With the increase of the number of reflections, the uniformity can be further improved. When the inner diameter and tilted angle of the light pipe are unchanged, the length of the light pipe is increased to 1m, the non-uniformity of intensity distribution at the illumination field is further reduced to 0.90, and the light power transmission efficiency is about 22.35%. The results show that the newly designed light pipe structure can meet the application requirements of decoherence and improving the uniformity of illumination field at EUV wavelength range, and it has great application prospects in EUV lithography and other imaging applications.