| At present,the manufacturing methods of complex micro-nano size structures mainly include electron beam lithography,ultra-precision machining technology,femtosecond laser-induced two photo polymerization technology,and so on.However,these manufacturing methods have complex manufacturing procedures,long cycles,and high costs.In recent years,maskless lithography combined with Digital Micromirror Device(DMD)has been used to fabricate different optical microstructures,because DMD can be used as a virtual mask instead of a physical mask,which can be easily and quickly formed.The maskless lithography machine has become extremely important manufacturing equipment in the field of optical devices.However,the projection lens system is one of the core components of DMD maskless lithography,and its imaging quality directly affects the transfer of exposure patterns.A common method to improve the imaging quality of optical systems is to replace the surface of some conventional lenses with aspherical surfaces.Based on the traditional projection lens system,a diffractive optical element(DOE)and aspheric surface are designed to optimize the diffractive hybrid projection lens system and improve the imaging quality of the system.This paper studies and analyzes the design of the DMD maskless lithography projection objective,the illumination system of the digital lithography system and the design of the projection optical system at home and abroad.Combining the optical characteristics of the aspheric surface and the DOE,the imaging characteristics and aberration characteristics of the aspheric surface,in order to meet the requirement of lithography linewidth of 2.16?m,a refractive/diffractive hybrid projection lens system for DMD maskless lithography was designed.The third generation DMD chip was used as the spatial light modulator(SLM).Based on the traditional projection lens system,the combinations of single-layer DOE and aspherical surface were used to optimize lens system.The optimization simulation and image quality evaluation are carried out in the optical design software ZEMAX.The results show that when the DOE is very close to the front lens group in front of the hybrid system diaphragm,the performance is the best.The image-side numerical aperture NA of the projection optical system was increased from 0.33 to 0.45,the theoretical minimum line width was reduced from 4?m to 2.16?m.And its magnification is-0.2,the rear working distance is 29.07 mm,and the total lens length is 196.97 mm.Compared with the traditional projection lens system,the maximum curvature of field of this hybrid projection lens system was reduced from 1.5?m to 0.9?m,and the maximum relative distortion was reduced from2×10-4%to 1.5×10-5%.The maximum mean square radius of was reduced from 0.522?m to0.112?m.And due to the increase of the modulation transfer function(MTF)value,the projection objective can collect higher frequency light to improve the lithography resolution.In addition,this article takes the main wavelength of the lithography light source at 405 nm as an example.In order to maintain a high uniformity of illumination on the DMD,DOE was introduced as a homogenizing device into the illumination system to shape the Gaussian distributed beam into a uniform light intensity distribution.Under the numerical simulation of MATLAB software,this article uses angular spectrum diffraction theory and AA(Adaptive-Additive)algorithm to design and optimize the phase distribution of the two-dimensional diffractive optical beam shaping device.The final relative light intensity is generally maintained above 95%,which shows its good uniform light effect numerically. |
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