Research Of Wavefront Reference Source In Fiber Point Diffraction Interferometer

Extreme ultraviolet lithography (EUVL) is regarded as a main choice for nextgeneration lithography. The wavefront aberration of lithography objective, which isthe most important part of lithography machine, should be less than λ/14RMS (forEUVL, λ=13.5nm) or1nm RMS, in order to satisfy the need of lithographyresolution and critical control dimension. Point diffraction interferometer which canproduce nearly perfect spherical wavefront with pinhole or fiber is free from theprecision of reference element, so it can meet the measurement accuracy for EUVLobjective. Thus point diffraction interferometer has occupied a significant position inEUVL optical test.The method enhancing test accuracy of point diffraction interferometer has beenstudied in this paper. Then a new wavefront reference source (WRS) which gathersthe advantage of pinhole and fiber diffraction has been designed. Analysis in theoryindicates that in order to meet the need of EUVL objective wavefront test, thediffracted spherical wavefront of this new wavefront reference source should have aasymmetry error value of less than8.85E-5λ (λ=532nm) when NA is less than0.6.(1) First the problem of the design for WRS should be explored: because othershave done a detailed study on optical collimation system and pinhole alignmentsystem already, this paper emphasizes the study on polarization system. Thechoosing of polarization state for WRS has been analyzed in theory, then we find interference of circularly polarization light is better than other state of polarizationon both contrast of interference fringes and the diffracted spherical wave error.Above all, we should design a polarization control system for WRS. The control ofpolarization is achieved by fiber polarization controller. Based on analysis on thefiber polarization controller, we’ve proposed an algorithm to control the polarizationfast, and then a polarization control system is designed. Also we have done anexperiment on polarization control to prove the algorithm correct, the error ofpolarization control is less than3.41%, it means that we can realize the polarizationcontrol fast without introducing extra error to wavefront.(2) The three main element of WRS (optical collimation system、polarizationcontrol system and pinhole alignment system) have been taken into account together.In order to meet the requirement that the asymmetry deviation of diffracted sphericalwave must be less than8.85E-5λ (λ=532nm), the tolerance of the WRS should beanalyzed. Based on the work above, we have established this WRS. What is of agreat importance to enhance the test accuracy is to calibrate the WRS error. We haveanalyzed the calibration algorithm thoroughly, and then the simulation of calibrationis carried out using the original data getting by testing two mirrors EUVL wavefrontaberration. Thus the tolerance of some key device is got by simulating calibrationrepeatedly.(4) Point diffraction interferometer with this new WRS is used to test the opticalobjective wavefront aberration. First the error introduced by this WRS when testwavefront aberration is analyzed, then the method to eliminate this error isdiscovered. The other WRS is established, which will connect with the front opticalpart. Finally this two WRS are used to test the wavefront aberration.