Simulation Research On Thermal Aberrations Of DUV Lithography Projection Objective

Following the decreasing trends of the dimensions of integrated circuits(IC), the lithography technology asks for increasing resolution which indicates CD(critical dimension) of projection lens and that claims the reduction of system aberrations. Aberrations of a DUV(deep ultraviolet) lithography projection lens come from two sides: the assembly residual aberrations and thermal aberrations caused by energy absorption while the latter is more remarkable. For the truth that the thermal aberrations occurs during working hours and change with exposure time, the traditional ways for aberrations compensation is useless and the only method is real-time compensation. To study the thermal aberrations, the thesis has modeled the DUV projection lens for thermal analysis; the solution has been done to get thermal aberrations and based on which a compensation scheme is proposed. The contents and innovations are as follows: 1. Models for thermal analysis are established; On the basis of the available project lens system and its mechanical supporting structure, the model for thermal aberrations analysis is established which consists of module for light intensity distribution calculation, FEA(finite element analysis) module for thermal-structure calculation and module for system aberration analysis, and during the full solving process the influence of illuminating conditions and printing conditions are both taken into consideration. 2. Thermal aberrations under different illumination conditions are analyzed; The traditional exposure way for lithography projection lens is Top-Hat style, while diverse off-axis exposure conditions of dipole, quadrupole and circular patterns are common employed to get higher resolution and larger focal depth, and all the four illuminating ways are set respectively to get the different thermal aberrations distributions. 3. Fast thermal aberrations calculating model is established; The heat conduction for single lens is derived theoretically to get the linear relation of lens temperature rising and surface strain towards the intensity distribution, based on which an algorithm for fast calculation of lens temperature and strain fields is proposed using a polynomial decomposition method on the intensity distribution; then the linear transforming relation between the temperature field in the middle surface of lens L20 and the system thermal aberrations is obtained using sensitivity matrix; by combining the two linear relations, the fast calculation algorithm is founded. 4. Compensation scheme is proposed and simulation experiment is accomplished; The compensation scheme is proposed according to the results of thermal aberrations analysis. The project utilizes CO2 laser, scanning mirror and reflector operating on the pupil lens to match the compensation targets which takes the advantage of the high absorption property of SiO2 towards CO2 laser. The simulation experiment shows that the compensation scheme is effective for kinds of aberrations, and the dynamic range meets the demand of thermal aberrations compensation which is about 1.5λ, though the precision and the algorithm for scanning mirror expects to be improved.