Research On Optical System Aberration Balance With Active Free-form Surface

Optical projective objective is the key part of commercial lithography equipment to produce integrated circuit. The objective lens is more sensitive to aberration as higher resolution requirement. Meanwhilemultiple polar or self-definition illumination patternused in multiple exposures technology are introduced into the lithography system, so that thermal aberration in objective is real-time changing and has a complex distribution. It’s becoming a difficulty in advanced lithography system manufacture that to eliminate thermal aberration. Nowadays a normal objective would take a cataditroptric or reflective structure, and wavefront correction device has been placed in the objective as a reflective mirror.So the method of active correcting aberration can be used in lithography system.However, the lithography objective demands aberration of all the fields less than1 nanometer. By contrast, conventional wavefront correction devices in active&adaptive optics with a deformation accuracy of tens nanometers and a capability for correcting single field wavefront cannot match the requirements of lithography objective. This thesis focus on the two points: actualization of high accuracy wavefront correction device and process of all fields’ aberration balancing which active optics haven’t researched on yet.Those works in thesis were carried out by theory analysis, numerical simulation and some experiments in addition. The main content of this research included,(1)Improving accuracy of analysis:proceeding an error analysis of numerical simulation,the accuracy of finite element method and Zernike fitting algorithm in optomechanicalanalysis had been raised;(2) Study of optical mode deforming realization: Mechanical conditions of thin plate mirror achieving the lower 36 terms Zernike modes deformation was calculated respectively by elastic theory and simulation, and a mode static energy was drawn to illuminate the different performance of modes;(3) Formulating a scheme of active free-form surface in engineering implementation:Based on focus-load actuator and moment actuator which were able to actualize, three error sources from this scheme were summarized such as mirror positioning accuracy, actuators coupling and mode error;(4) An experimental active mirror was building to study bonding of mirror and actuators, mirrorpositioning and actuators coupling;(5) Structure design of active free-form surface:Following the demands for different Zernike mode deformation, a rim-actuated structure and a back-actuated structure were designed, besides a mix-actuated structure was proposed for decreasing number of actuators and it had an advantage of spherical deformation;(6) Research on process of aberration balancing in objective:Based on a deep Ultraviolet lithography objective model, an active mirror was designed with rectangle aperture. For mirror deformation expression and orthogonal aberration function for rectangle filed optical system aberration expression, a rectangle orthogonal polynomials was derived. A process of objective aberration balancing was completed with aberration PV=15nm decreasing to PV=3.1nm.The main results of this thesis are following:1, a Zernike fitting algorithm with weights calculated based on data distribution automatically was proposed, which could eliminate the error caused by nonuniform data distribution.2, a new scheme of rim-actuate free-form surface was proposed and designed,which had a remarkable accuracy on some Zernike mode deformation with a few actuators used.3, Orthogonal polynomials applied in rectangle zone were derived which solved the problem that precise expression of rectangle aperture mirror map.