Katsumi Sugisaki

Lithographic performance of high-numerical-aperture (NA=0.3) EUV small-field exposure tool (HINA)

Three sets of projection optics (Sets 1, 2, and 3) were fabricated to the mark of a wave front error (WFE) of less than 1 nm. The RMS WFE is 7.5 nm for Set 1, 1.9 nm for Set 2, and at most 0.9 nm for Set 3. In addition, the RMS mid-spatial frequency roughness (MSFR), which affects flare, is 0.34 nm for Set 2 and 0.17 nm for Set 3. This paper discusses the current lithographic performance of HINA, especially the evaluation of flare and the replication of fine-pitch patterns. Several EUV masks were fabricated to evaluate the effects of flare and to replicate fine-pitch patterns. In the case of Set 2 optics, 90 nm lines and spaces were barely delineated using a bright-field mask due to the RMS MSFR of 0.34 nm, and replication of 70 nm lines and spaces were achieved using a dark-field mask. Since the RMS WFE and the RMS MSFR for Set 3 optics are half as much as that for Set 2 optics, the lithographic performance of HINA is markedly improved. 50 nm lines and spaces of non-chemically-amplified resist were delineated with the illumination condition of a partial coherence, σ, of 0.8 and 45 nm lines and spaces were delineated with the annular illumination condition of outer σ of 0.8 and inner σ of 0.5. In addition ultimate resolution of 30 nm lines and spaces of chemically-amplified resist was performed under the coherent illumination condition of σ of 0.0.

Angular spectrum calculations for arbitrary focal length with a scaled convolution

Nyquist sampling theorem in an image calculation with angular spectrum method restricts a propagation distance and a focal length of a lens. In order to avoid these restrictions, we studied suitable expressions for the image computations depending on their conditions. Additionally, a lateral scale in an observation plane can be magnified freely by using a scaled convolution in each expression.

EUV wavefront measurement of six-mirror optics using EWMS

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.

Comparison of EUV interferometry methods in EUVA project

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.

Recent Progress of EUV Wavefront Metrology in EUVA

The recent experimental results of EUV wavefront metrology in EUVA are reported. EUV Experimental Interferometer (EEI) was built at the NewSUBARU synchrotron facility of University of Hyogo to develop the most suitable wavefront measuring method for EUV projection optics. The result is to be reflected on EWMS (EUV Wavefront Metrology System) that measures wavefront aberrations of a six-aspherical mirror projection optics of NA0.25, of a mass-production EUV lithography tool. The experimental results of Point Diffraction Interferometer (PDI) and Lateral Shearing Interferometer (LSI) are shown and the error factors and the sensitivity of astigmatism measurements of these methods are discussed. Furthermore, for reducing these kinds of errors, another type of shearing interferometer called DTI (Digital Talbot interferometer) is newly introduced.

Shearing Interferometry for at Wavelength Wavefront Measurement of Extreme-Ultraviolet Lithography Projection Optics

We present a type of lateral shearing interferometer (LSI) for at-wavelength characterization of the projection lens for use in extreme-ultraviolet lithography (EUVL). LSI is one of the potential candidates for high Numerical Aperture (NA) optics testing at the EUV region. 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 estimate, hyperbolic calibration, and charge coupled device (CCD) size, etc. are characterized and the measurement accuracy of the LSI is estimated to be within 7 mλ rms (0.1 nm rms at 13.5 nm wavelength) for testing the wavefront of EUVL projection optics.

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.

Wave-front errors of reference spherical waves in high-numerical aperture point diffraction interferometers

In phase-shifting point diffraction interferometry (PS/PDI), a pinhole with a diameter of 34 nm is necessary to measure a wave-front aberration of extreme ultraviolet projection optics of numerical aperture (NA) 0.20. However, it is extremely difficult to process such a small pinhole, and light transmission through the pinhole becomes too low. Here, the diameter of a pinhole is optimized, together with the thickness of a Ta membrane, for a converging wave of NA 0.20 with no aberration so that the difference between a wave front produced by the pinhole and that of a spherical wave is minimized. On that condition, the optimum values are a diameter of 50 nm and a thickness of 200 nm. For these values, behaviors are examined in real cases, including focal point shifts and aberrations with incident light. Astigmatism in the aberrations has the most impact on a wave-front error, and a 0°–90° astigmatism (Z5) coefficient in the FRINGE Zernike polynomials of test optics is required to be less than 50 mλ to use th...

Wavefront measurement interferometry at the operational wavelength of extreme-ultraviolet lithography.

Two basic types of interferometer, a point diffraction interferometer (PDI) and a lateral shearing interferometer (LSI) suitable for operation in the extreme-ultraviolet (EUV) wavelength region, are described. To address the challenges of wavefront measurement with an accuracy of 0.1 nm rms, we present a calibration method for the PDI that places a mask with two large windows at the image plane of the illumination point light source and a general approach to deriving the phase-shift algorithm series that eliminates the undesired zeroth-order effect in the LSI. These approaches to improving the measurement accuracy were experimentally verified by the wavefront measurements of a Schwarzschild-type EUV projection lens.

EUV wavefront metrology system in EUVA

An Experimental extreme ultraviolet (EUV) interferometer (EEI) using an undulator as a light source was installed in New SUBARU synchrotron facility at Himeji Institute of Technology (HIT). The EEI can evaluate the five metrology methods reported before. (1) A purpose of the EEI is to determine the most suitable method for measuring the projection optics of EUV lithography systems for mass production tools.