Research On The Thermal Aberration And Active Compensation In Deep Ultraviolet Lithographic Projection Objective

Deep ultraviolet lithography is the currently popular key equipment in the great scale integrated circuit manufacturing, and the lithography machine working at 193 nm with the resolution of 90 nm is being developed in our country. At the wavelength of 193 nm, all the materials have significant absorption. As the core-block of lithography machine, the projection lens absorbs laser power during the exposure process, which causes refractive index change, thermal deformation and thermal stress, and then thermal aberration is produced, which decreases the resolution and degrades the imaging quality. Resolution enhancement technologies like off-axis illumination are employed in the modern photolithography, and the projection lens is nonuniformly heated, the locally temperature rising causes non-rotational symmetrical and time–changing thermal aberration. To reach the expected resolution, the fast prediction and real-time compensation of this kind of aberration is important. Simulation research is the effective approach of thermal aberration prediction which offers the reliable feedforward information for the real-time compensation.Aiming at the dioptric lithographic projection objective with NA of 0.75, finite element analysis is used to get the temperature distribution under four different illumination conditions, and the mathematical model of temperature distribution is constructed, with which the thermal aberration is obtained in CODE V. Compared with the commercial software, it is more efficient with the temperature distribution function for the prediction of thermal aberration. According to the characteristic of thermal aberration, the author proposes to use active optics technology on the lens for the compensation. The compensation ability of active lens with optimized lens supporting condition is analyzed and experiment is carried out to verify the feasibility of the approach. The details are as follows:(1) The mathematical model of temperature distribution is constructed and the thermal aberration is obtained. Based on the temperature distribution in dipole illumination condition, the dissertation proposed the combined two-dimension Gauss function to fit the temperature field, and model is extended for the expression of temperature in conventional illumination, annular illumination and quadrupole illumination. The results indicate that the average temperature fitting error is about 10-3°C and RMS is about 10-4°C. The aberration calculated by CODE V with temperature distribution function match well with the one based on Sig Fit, however the time cost for simulation shows a decrease of three orders of magnitude, which indicates that the simulation efficiency is improved by three orders of magnitude. While compared with the contribution to system aberration by refractive index change, the impact of thermal deformation and thermal stress are so insignificant to be ignored.(2) To compensate the thermal astigmatism and tetrafoil aberration in dipole and quadrupole illumination, active optics is proposed to use. The analysis has been made to find how the wavefront exiting from each lens affects the system aberration, and then L20(lens 20) is chosen as the active lens for compensation; the principle for aberration compensation is presented and the solution of exit wavefront of L20 is attained. By studying on the factors which affect the aberration characteristic of a simplified active refractive plate, the optimized lens supporting method and actuator locations is proposed and been used on L20. The simulation results show that the compensation approach can compensate the astigmatism up to 1λ, while the higher order aberration is slightly increased.(3) By testing the wavefront with interferometer before and after the active force loads are applied on the active plate, experiment is carried out as a proof-of-principle. Astigmatism dominates in the tested wavefront with the same level of simulation results and it has a linear positive correlation with the active force. The results have proved the feasibility of the compensation approach and the reliability of simulation results based on finite element analysis.The achievements are meaningful and valuable for the study of temperature distribution, thermal aberration prediction and compensation in lithographic projection objective.