Triple Complex Phase Computer-Generated Holograms for Aspherical Surface Testing in Extreme Ultraviolet Lithography
Objective Aspherical surface testing plays an important role in projection optics for extreme ultraviolet lithography(EUVL)systems and determines the resolution and overlay accuracy of the lithography system.The substrate transmission wavefront error and the pattern placement error are the main factors influencing the accuracy in the metrology of aspherical surface based on computer-generated hologram(CGH),which remains a great challenge to the traditional calibration methods.To this end,a new method for calibrating the wavefront error introduced by pattern placement error based on a triple complex phase is proposed and experimentally studied.Methods We propose a triple complex CGH that simultaneously emits three wavefronts for aspheric surface testing,and apply it to wavefront error calibration.By conducting six combined measurements,the manufacturing error and graphic position error of the CGH substrate can be calibrated,and they can also be eliminated in the measurement results.The first measurement is for interferometer TF and RF verification,and with the calibrating method,the surface form of TF and RF can be known.The second measurement is adopted for calibrating the first-order diffraction wavefront of CGH in the+X direction.The third measurement is employed for calibrating the first-order diffraction wavefront of CGH in the-X direction.The fourth measurement is for calibrating the first-order diffraction wavefront of CGH in the+Y direction.The fifth measurement is for calibrating the first-order diffraction wavefront of CGH in the-Y direction and the sixth measurement is to utilize TF and CGH for testing the aspheric surface.Results and Discussions Based on the measurement results of test 1-6,TF and RF surface form and tested aspheric surface form can be calculated.The peak-to-valley(PV)value of TF surface form is 74.2 nm and root mean square(RMS)error is 13.9 nm.The PV value of RF surface formform is 53.3 nm and RMS error is 6.6 nm.The PV value of surface form of the tested aspheric surface is 31.6 nm and RMS error is 4.88 nm.The same tested aspheric surface is also measured by refraction null compensation test,and the tested PV value is 38.6 nm and RMS error is 5.14 nm.The comparison between the two results indicates that the proposed method has subnanometer RMS accuracy.Conclusions Aspherical surface testing plays a vital role in projection optics for EUVL systems,and determines the resolution and overlay accuracy of the lithography system.The substrate transmission wavefront error and the pattern placement error are the main factors that influence the accuracy in the metrology of aspherical surface based on CGH,which remains a great challenge to traditional calibration methods.Therefore,we propose a triple complex CGH that simultaneously emits three wavefronts for aspheric surface testing and apply it to wavefront error calibration.By carrying out six combined measurements,the manufacturing error and graphic position error of the CGH substrate can be calibrated,and they can be eliminated in the measurement results.The PV value and RMS error are 31.6 nm and 4.88 nm for a tested aspheric surface respectively.Furthermore,the same aspheric surface is also measured by a refractive aspherical null testing system,with the corresponding PV value and RMS error being 38.6 nm and 5.1 nm respectively.This method intrinsically has the subnanometer accuracy necessary for EUVL aspherical optical components.
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