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18 August 2014Experiment analysis of freeform testing based on absolute testing method
Requirements for the measurement resolution in the sub-nanometer range have become quite common which includes
not only the repeatability or reproducibility but also the absolute measurement accuracy. The freeform lens for wavefront
compensating contains some medium spatial frequency terms. The wavefront error of lithographic object lens is very
small. One method to reduce the wavefront error of lithographic object lens is to use the freeform lens. The freeform lens
for compensation needs more accuracy than the object lens. We can also use freeform lens of sphere or aspheric for
compensation. The testing accuracy of sphere and aspheric lens are hard to achieve 1nm. The sphere and aspheric will
contain the power term and are hard to find the cat-eye. The ion beam figure system (IBF) is the best polishing machine
for nanometer manufacture which will polish the PV of 2um for several weeks even months. Usually we use the PV
200nm lens for compensation. So the freeform for compensation looks like a flat. In this paper we will show the testing
experiment of the freeform and the testing problem. The freeform surface is created by 66 Zernike polynomials which
are based on the flat lens. The freeform flat is polished by the ion figuring machine of NTG. The environment such as
temperature, vibration, humidity is controlled well. The Zygo's interferometer Verifire Ashpere with absolute testing
method is used to test the freeform. Position Accuracy is a problem in optical testing and manufacture. The high
accuracy testing can’t be determined by one method, we need the different method to compare the result especially these
method will contain some defects. The defects of the recently absolute testing method are discussed.
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Xin Jia, Fuchao Xu, Tingwen Xing, "Experiment analysis of freeform testing based on absolute testing method," Proc. SPIE 9203, Interferometry XVII: Techniques and Analysis, 920315 (18 August 2014); https://doi.org/10.1117/12.2060496