Mikihiko Ishii

Rigorous wavefront analysis of the visible-light point diffraction interferometer for EUVL

In visible-light point diffraction interferometer (PDI), in order to achieve measurement error <0.1 - 0.2 nm rms, wavefront irregularity from the pinhole must be supressed as 0.05 - 0.1 nm rms in designing. It is so difficult to execute such high accurate (10-4λ) simulation because the numerical electromagnetic simulation shows slow convergence in the visible-region. We discussed this problem by using 2D-model and found simulation conditions to obtain significant results. By using the simulator, several kind of systematic erros have been analyzed and optimized.

Development of an experimental EUV interferometer for benchmarking several EUV wavefront metrology schemes

An experimental extreme UV (EUV) interferometer (EEI) using an undulator light source was designed and constructed for the purpose of developing wavefront measurement technology with the exposure wavelength of the projection optics of EUV lithography systems. EEI has the capability of performing five different EUV wavefront metrology methods.

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

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.

Experimental comparison of absolute PDI and lateral shearing interferometer

We present the experimental results of EUVA Absolute Point Diffraction Interferometer (ABSPDI) and Lateral Shearing Interferometer (LSI) for at-wavelength characterization of the projection lens for use in extreme-ultraviolet lithography (EUVL). The attained repeatability of either type of the interferometers is within 0.04nmRMS. The experimental results have shown good consistency between the LSI and ABSPDI. The reasons for the residual differences have been analyzed and we believed it is mainly due to the CCD tilt effect in the experimental system. After the CCD tilt effect was removed, a better consistency below 0.33nm RMS has been achieved.

Double-grating lateral shearing interferometer for EUV optics at-wavelength measurement

A Calibration technology for double-grating lateral shearing interferometer1 (DLSI) and lateral shearing interferometer (LSI) is proposed in this paper. In this method, two measurements are used for calibration. One is the measurement by using the first- and zero-order diffraction beams of grating in the interferometer; the other one is the measurement by using the minus-first-order and zero-order diffraction beams. The phase distributions were calculated out from the two measurements. After shifted one phase distribution to superpose the other one, in the sum of the two phase distributions, the test wavefront is canceled. The system error caused by the grating diffraction and grating tilt can be calculated out from the sum of the superposed phase distributions. For calculating out the system errors, the sum of the two phase distributions is fitted to Zernike-Polynomials. From the coefficients of the Zernike-polynomials, the system error is calculated. This method is carried out to calibrate the system error of DLSI. We performed an experiment to verify the available of our calibration method.