The wavefront measurements have been performed with the EUV Wavefront Metrology System (EWMS) for the first
time using a prototype projection optic as a test optic. The wavefronts of the test optic was measured at the five positions
in the exposure field with the Digital Talbot Interferometer (DTI). The RMS magnitude of the wavefront errors ranged
from 0.71 λ (9.58 nm) to 1.67 λ (22.75 nm). The results obtained with the DTI were compared to those with the Cross
Grating Lateral Shearing Interferometer (CGLSI). As a result of a repeatability assessment, it was found that the EWMS
can stably measure the wavefronts of the test optic. Additionally, unwrapping of the phase map was found to be related
to the precision of the measurement.
Comparisons between several at-wavelength metrological methods are reported. The comparisons are performed by measuring one test optic with several kinds of measurement methods from the viewpoints of accuracy, precision and practicality. According to our investigation, we found that the PDI, the LDI, and the CGLSI are the most suitable methods for evaluating optics for EUV lithography.
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.
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.
We are developing an at-wavelength interferometer for EUV lithography systems. The goal is the measurement of the wavefront aberration for a six-aspherical mirror projection optic. Among the six methods that EEI can measure, we selected CGLSI and PDI for comparison. PDI is a method well-known for its high accuracy, while CGLSI is a simple measurement method. Our comparison of PDI and CGLSI methods, verified the precision of the CGLSI method. The results show a difference between the methods of 0.33nm RMS for terms Z5-36. CGLSI measurement wavefronts agree well with PDI for terms Z5-36, and it is thought of as a promising method. Using FFT analysis, we estimated and then removed the impact of flare on the wavefront. As a result of having removed the influence of flare, the difference between CGLSI and PDI improved to only 0.26nm RMS in Zernike 5-36 terms. We executed PDI wavefront retrieval with FFT, which has not been used till now. By confirming that the difference between methods using FFT and Phase shift is 0.035nm RMS for terms Z5-36, we have proven that PDI wavefront analysis with FFT is possible.
We present the theoretical measurement accuracy analysis for at wavelength characterization of the projection lens to
be used in extreme-ultraviolet lithography (EUVL) and the first experimental result from the lateral shearing
interferometer (LSI) test system. LSI is one of the potential candidates for high Numerical Aperture (NA) optics testing
at the EUV region during alignment of the projection optics. To address the problem of multiple-beam interference, we
propose a general approach for derivation of a phase-shift algorithm that is able to eliminate the undesired 0th order
effect. The main error source effects including shear ratio estimation, hyperbolic calibration, charge coupled device
(CCD) size effect, and CCD tilt effect are characterized in detail. The total measurement accuracy of the LSI is
estimated to be within 7mλ rms (0.1 nm rms at 13.5 nm wavelength).
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.
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.
An extreme ultra-violet phase-shifting point diffraction interferometer (PS/PDI) was studied by using the NewSUBARU[1] undulator radiation. The beam line was equipped with a monochromator for PDI measurement. To improve the converging performance of the undulator radiation, a new beam line suitable for PDI was designed. From the examination of monochromaticity required for PDI, the 0th-order light of the monochromator was used in the experiment. The higher-order radiation of the undulator was eliminated by the reflection band of the Mo/Si multilayer mirrors. By means of improvements of the pre-alignment method and of the mask structure, a higher contrast than ever was achieved in the interference fringes.
We have been studying phase-shifting point diffraction interferometry (PSPDI) as a technique evaluating extreme-ultraviolet (EUV) lithographic optics at the working wavelengths. In the PSPDI, the wavefront error of the test optic affects the measurement itself. One of these effects is that flare of a spot focused onto a pinhole of a PSPDI mask is mixed with a test beam as an optical noise. To mitigate the flare effect, we changed the PSPDI mask design and replaced the convex mirror of a test optic. The other effect is reducing the contrast of the interference fringe. To reduce the misalignment of the test optic, we have improved the accuracy of the PSPDI using visible light. Since the residual wavefront error of the test optic is not small enough for at-wavelength PSPDI measurement, we obtained an at-wavelength wavefront using a rather large second pinhole. The obtained EUV wavefront qualitatively agreed with the visible one.
Extreme-ultraviolet phase-shifting point diffraction interferometer (PS/PDI) was studied by using the NewSUBARU undulator radiation. The wave-front error of a Schwarzchild test optics was measured. Since this is a common path PDI technique, optics pre-alignment is very important to receive enough power at the second pinhole. We carried out this pre-alignment by using the same common path PS/PDI system but by using a He-Ne laser. A temporal wave-front error attained by pre-alignment was 4.4 nm rms. We then studied band width requirement to carry out this PS/PDI in EUV. We found that the wavelength ((lambda) ) dependency of grating diffraction angle plays an important role in phase matching at the CCD camera location, although significant optical path difference exists at the edge of the fringe field. A 1 micrometers square double window experiment was carried out with (lambda) /(Delta) (lambda) is congruent to 30, and straight fringes were observed throughout the CCD field. A PDI experiment using larger pinholes compared with nominal sizes was also conducted, and various factors, which were posed onto the experimental results, were investigated.
The precise alignment of Extreme Ultra-Violet Lithography (EUVL) imaging system is necessary in order to achieve diffraction-limited performance. Interferometric testing at the exposure wavelength is needed to ensure proper alignment and to achieve an acceptable final wavefront. We have built a prototype at-wavelength interferometer at the NewSUBARU facility. This interferometer is a phase-shifting point diffraction interferometer (PS/PDI) testing specially constructed Schwarzschild optics. Preliminary experiments using visible light were performed in order to learn this PS/PDI. The Schwarzschild optics were aligned using visible wavefront measurements with the interferometer. The precision of the visible measurements was evaluated. Experiments using EUV radiation have been started.
This paper presents a two-color optical path-1ength-cdulated reversible fringe-counting interferometer system for iieasuring the change of the air refractive index. By measuring the changes of the optical path for infrared and visible light the interferoieter cancels the effect of the geoietrical distance change. This system is being tested over a 235 ii path in the tunnel at NRLM. 1.
The penetration depth of light into a processed surface is interferometrically ieasured by optically contacting it on a glass plate with a resolution of 3 urn. This principle is based on the ieasurement of the difference between the phase changes of light in reflections on the object and the glass plate. Also the depth is siiiultanecusly measured by an optical systeii with a phase-locked laser interferometer and an optical displacement sensor. 1 .
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