EUVA’s challenges toward 0.1nm accuracy in EUV at-wavelength interferometry

Conclusions

We have been developing the metrological techniques to achieve 0.1 nm accuracy for evaluating the EUV lithographic optics. To select the most suitable methods, six different methods are compared. As a result, we have concluded that the PDI, the LDI and the CGLSI are the most promising candidates installing the EWMS for evaluating the EUV lithographic optics. To achieve the ultra-high accuracy, we have analysed various error factors and developed various calibration methods. In order to assess the accuracy of our interferometer, the asymmetrical systematic errors are evaluated. The evaluated asymmetric error is less than 0.09 nm rms, which is small enough for measuring the wavefront of the EUV lithographic optics. The interferometry can extend to the extremely short wavelength of the EUV region and the ultra-high accuracy is achieved.