Research On Lateral Shearing Interferometry In Measurement Of Wavefront Aberration Of Lithography Lens

Projection lithography lens is the core part of the lithography machine.With the development of lithography technology,the imaging quality of projection lithography lens is getting higher and higher.System wavefront aberration is not only one of the objective standards to evaluate the imaging quality of the projection lens,but also the basis in refining and assembling projection lithography lens.Therefore the measurement of system wavefront aberration is one of the core technologies for the development of lithography.Foreign suppliers of projection lithography lens have developed dedicated equipment of system wavefront aberration measurement.However,such precision equipment is not commercially available for us due to the maintaining of technological dominance of the suppliers.Grating lateral shearing interferometry is the most promising technology for system wavefront aberration measurement,and the related research will promote the development of lithography industry in China.The design of the phase-restoration algorithms for suppressing the zeroth order effect in the grating lateral shearing interferometry is studied.Due to the unavoidable zeroth order effect error in the lateral shearing interferometer,the algorithm should be designed specially to suppress the zeroth order effect error effectively.A new method to design an odd-numbered phase shifting phase-restoration algorithm is proposed for the problem.According to the influence degree of the zeroth order effect and phase shifting error,we design two classes of phase-restoration algorithms.Class A algorithms consist of four phase-restoration algorithms,with different degrees of ability to suppress phase shifting error while maintaining the same ability to suppress the error of zeroth order effect.The theoretical results show that the restoration errors of the four algorithms are all proportional to the sine of double phase,and are respectively proportional to the second,the fourth,the sixth and the eighth power of the phase shifting error.With the same phase shifting error,the phase-restoration error gets smaller as more frames are used.Class B algorithm can more effectively suppress the error of zeroth order effect.The correctness of theoretical analysis is verified by simulation.With the shearing interferometer,the wavefront aberration of the lens system is obtained by 9-step phase shifting phase-restoration algorithm of Class A,and the experiment of repeatability precision is carried out.The results show that the repeatability precision is 0.1421 nm(RMS),which means that the measurement is stable in the grating lateral shearing interferometry.A calibration method of the zeroth order effect in the grating lateral shearing interferometry is proposed.The principle of calibration is analyzed theoretically,The relationship between the phase restoration error affected by the calibration error and the intensity modulation ratio is deduced.The results show that the phase-restoration error introduced by the calibration error of zeroth order effect is proportional to the phase shifting error and to the amplitude ratio.The theoretical analysis is verified by simulation.The simulation results show that the phase-restoration error is proportional to phase shift error and to the intensity modulation ratio,when the phase shifting error is less than 2°(0.035rad)and the intensity modulation ratio is less than 0.2.Compared with the 9-step phase shifting phase-restoration algorithm of Class A,the relative error of the RMS value of the phase-restoration error is less than 6%.With the shearing interferometer,the experiment is carried out to verify the calibration method of the zeroth order effect and compare with the 9-step phase shifting phase-restoration algorithm of Class A.For the same lens,the RMS value of the reconstructed phase by the two algorithms are close,and the error of the reconstructed phase is0.37 nm.By using the calibration method of the zeroth order effect,the precision of phase-restoration is similar to the 9-step phase shifting phase-restoration algorithm of Class A,with only 5-step phase-shift interferometry.