Effect Of Material Inhomogeneity On Image Quality Of Optical Systems

In the common optical system design, refractive index of the optical materials are thought to be homogeneous. However, absolutely homogeneous optical material does not exist in fact. To some common optical systems, since the system itself has visible aberration, so the effect of refractive index inhomogeneity to the imaging quality of the system can be ignored. However, to some minimal aberration optical systems,such as high numerical aperture projection lithography lens, high resolution microscope, the various aberrations have been corrected well, the main factors that affect the image quality is the stress birefringence effect and inhomogeneity of the refractive index. The material inhomogeneity could make an optical system can not function as intended, or even can’t work correctly. Therefore, we must model and simulate the refractive index homogeneity of the material when we are doing optical system design,and then it will guide the further optimization and alignment.The existing methods for analyzing index homogeneity of the optical materials are some two-dimensional methods and the simulation accuracy is not enough.So, we propose a three-dimensional simulation and ray tracing method. Specifically, finished the following work:1, Finish the optical system design of an NA0.75projection lithography objective lens. The NA0.75projection lithography objective lens exposure for the realization of the90nm node replication. Based on the extensive collection and research internationally of high NA projection, designed a lithography objective lens with NA0.75, the working wavelength193nm, the MTF of the visual field reaching the diffraction limit, the wave aberration RMS<2.0nm, full field distortion<2.0nm.2, Presents a three-dimensional refractive index homogeneity analysis simulation and ray tracing method. In the traditional analysis methods, the inhomogeneity of optical materials is interfering as the aberration information and is added to the lens surface directly, ignoring the oblique incident light and the lens thickness, so the simulation accuracy is not high. In such cases, I proposed the three-dimensional simulation and ray tracing method,which works in ZEMAX through custom macro program, modeling the three-dimensional distribution of the refractive index homogeneity, setting the ray tracing step size, and controlling the entire ray tracing trace process, to improve the simulation accuracy. To compare the traditional two-dimensional analysis method and the three-dimensional simulation and analysis ray tracing method, a NA0.7optical system with small aberration is analyzed.The result showed that the simulation accuracy of three-dimensional simulation and ray tracing method is very close to the experiment result, and the simulation accuracy is significantly improved. The results show that simulation accuracy of the proposed three-dimensional simulation and ray tracing method is reliable.3, Using the three-dimensional simulation and ray tracing method, analyzed the effect of refractive index homogeneity to the NA0.75projection lithography objective lens. The result shows that the effect of optical refractive index homogeneity to the optical system ray aberration, MTF, PSF and distortion are very serious and must be further optimized, calibrated or compensated. The simulation result has great guiding significance to further optimization of design and optical alignment.This paper presents a three-dimensional simulation and ray tracing method for the analysis of the refractive index homogeneity of optical materials.It has very important significance and application value especially for some optical system with small aberrations.