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.
Precise measurements of the wavefront aberrations of projection optics with 0.1 nm RMS accuracy are indispensable to
develop the extreme ultraviolet (EUV) lithography. In order to study measurement methods, we built the Experimental
EUV Interferometer (EEI) that has built-in Schwarzschild-type optics as test optics and was supplied with EUV
radiation of 13.5 nm in wavelength from a synchrotron radiation facility as a source light. The EEI can evaluate several
methods of EUV interferometory replacing optical parts easily. Those methods are dividable into two categories,
namely point diffraction interferometer (PDI) and lateral shearing interferometer (LSI) and those were experimentally
compared. Finally, 0.045nm RMS of reproducibility was achieved with PDI method and the residual systematic error
after removing specified errors was reduced to 0.064nm RMS excluding axial symmetrical aberrations. In addition, one
of LSI-type methods also proved to have almost enough accuracy for the assembly of the projection optics.
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.
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 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.
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.
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.
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.
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).
Point diffraction interferometry (PDI) is a promising candidate of the wavefront metrology for EUV lithographic projection optics. However, the pinhole used in the PDI is easily filled up with carbon contamination induced by EUV irradiation. We have evaluated the filling rate of pinholes by measuring decreasing rates of intensity of EUV radiation that passed through the pinholes. As a result, we found the filling rates of the pinholes depend on their materials and blowing of the oxygen. The filling rate was the slowest when the pinhole made of Ni was used and oxygen was blown.
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.
We have developed a high numerical aperture (NA) small-field exposure system (HiNA) for EUV exposure process development. NA of projection optics of EUV exposure tools for 45-nm node lithography is expected to be around 0.25, which is higher than that previously expected (0.1). HiNA has compatible illumination system, which can be switched to partial coherent illumination and coherent illumination by changing some optical elements. Coherent illumination system was prepared for a high contrast imaging but the uniformity of intensity is less than that of partial coherent illumination. A reflected-type fly*fs-eye element was adopted for partial coherent illumination, which can provide uniformity of both coherency and intensity simultaneously. The coherency of the partial coherent illumination is 0.8. HiNA projection optics consists of two aspheric mirrors, with the NA and the imaging field of 0.3 and 0.3×0.5mm<sup>2</sup>, respectively. We fabricated two sets of projection-optics. Although the wavefront error of set-1 optics was 7nmRMS, that of set-2 optics was improved to 1.9nmRMS, which was measured with a point diffraction interferometer (PDI) using He-Ne laser. The wavefront error of the set-2 optics was improved by using a new mirror mount mechanism. The mount system consists of several board springs made of super invar in order to minimize the deformation of mirrors by mounting stress. The projection optics of the set-2 has a remote controlled mirror adjustment mechanism which has five degrees of freedom (X,Y,Z,X-Tilt and Y-Tilt). The position of the concave secondary mirror was adjusted precisely with measuring the wavefront error using PDI.
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 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 0<sup>th</sup>-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.
We have successfully developed a simple, laboratory-sized EUV reflectometer EUMOR (extreme Ultraviolet Monochromatic Reflectometer). A CO<SUB>2</SUB> gas-jet-target laser-plasma source was employed as the EUV source for EUMOR. EUMOR uses a single line emission at the wavelength of 12.98 nm from a CO<SUB>2</SUB> gas-jet-target laser-plasma source without a grating, therefore it can achieve simultaneous high spectral resolution and high throughput. The intensity of EUV emission from the CO<SUB>2 gas-jet target laser-plasma was quantitatively evaluated, and the EUV flux that irradiated the surface of a sample was estimated to be 5x10(superscript 5</SUB> photons/shot. Four Mo/Si multilayer mirrors which were deposited under the same conditions with different layer periods were measured by EUMOR. The parameters of these multilayer mirrors, which were obtained by parameter fitting to the measured angular distribution of the reflectivity, showed good agreement with each other, demonstrating the reliability of EUMOR data.
EUV lithography is a successor to DUV/VUV lithography, and is the final photon base lithography technology. The concept of EUV scanners for 50nm node and below is considered by clarifying the similarities and differences between EUV scanners and DUV scanners. Illumination optics, projection optics, wafer alignment sensors and wafer focus sensors are examined. And the throughput model, overlay budget and focus budget are introduced. The concrete design of illumination optics and the requirements for sources are described. Numerical aperture, magnification and field size are discussed. EUV scanners for 50nm node and below are realized.
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.
In order to evaluate the performance of multilayer optics, we have successfully developed a simple, laboratory-sized reflectometer that can be operated readily on a routine basis. This reflectometer makes use of a single line emission at the wavelength of 12.98 nm from a CO<SUB>2</SUB> gas-jet laser-plasma x-ray source that can be readily operated on a routine basis. Our reflectometer achieved repeatability of less than +/- 0.8% in reflectivity measurements. The peak reflectivity of a sample determined by calculation based on multilayer mirror parameters obtained from our reflectometer was within +/- 1.3% of that obtained by an SR-based reflectometer. These results confirm that our reflectometer performs well enough to evaluate multilayer optics.
A three-aspherical-mirror system for Extreme Ultraviolet Lithography has been developed. The aspherical mirrors were fabricated using the computer controlled optical surfacing (CCOS) process and a phase shift interferometer. The mirrors have a figure error of 0.58 nm and surface roughness of 0.3 nm. In order to obtain a high efficiency mirror, M1 and M2 were coated with a graded d-spacing Mo/Si multilayer and mirror M3 was coated with a uniform d-spacing Mo/Si multilayer. The peak reflectivity is 65% at the wavelength of 13.5 nm. The wavelength matching of each mirror spans 0.45 nm. The mirrors were aligned with a Fizeau-type phase shift interferometer, and a final wavefront error of less than 3 nm was achieved. Exposure experiments carried out at new Subaru synchrotron facility revealed that this system is capable of replicating a 56 nm pattern in a 10 mm X 1 mm exposure field.
We have assembled and aligned projection optics for extreme ultraviolet (EUV) lithography. The projection optics consists of three aspherical mirrors. First, the positions of the mirrors were coarsely adjusted using the side and back surface of the mirrors. Next, the mirrors were finely aligned to minimize the wavefront errors which were measured by an interferometer. The adjustable axes were selected according to the results of the analysis of the allowable error range. The compensation values for each adjustable axis were calculated by commercially available ray-tracing software. After the alignment procedure, the wavefront error of 3 nm RMS was achieved.
Extreme ultraviolet lithography (EUVL) is one of the candidates to fabricate a sub-0.1 micrometer-pattern. We have developed an Engineering Test Stand (ETS-0) which consists of three aspherical mirrors imaging optics for EUVL. This optics meets the specification of sub-0.1 micrometer generation. The key technology of EUVL is a development of reduction optics. The requirements of both figure error and surface roughness are less than 0.3 nm, and the wave-front error (WFE) of optical system has to be reached to be less than (lambda) /14 rms, where (lambda) is the exposure wavelength. Therefore, the high-precision fabrication and alignment techniques for the optics are required. We have developed the alignment procedure of three-aspherical-mirror optics to minimize the WFE, by the Fizeau-type interferometer using He-Ne laser ((lambda) equals 632.8 nm) and by the ray trace program (CODE-V). Namely, we have found the effective mirror-adjustment-axis to realize the high-precision alignment. The effective axis is decided by the priority for the adjustment axis. The priority is lead by two methods. One method is decided by the contribution to the WFE reduction that was calculated by CODE-V. The other method is decided by the correlation between the amount of decentration (shift for X-axis or Y-axis direction), despacing (shift for Z-axis direction), tilt of each mirror and the F.Z.- coefficients. The mirror is adjusted in the order of the priority of mirror axis. As a result, the WFE of 3 nm RMS has been achieved by using this alignment procedure in three- aspherical-mirror optics.
Soft X-ray microscopy, especially using the wavelength region from 2.4nxn to 4.4nxn, is expected in biology. Because the microstructure of live and thick biological specimens containing water can be observed by x-ray absorption difference of protein and water with low damage. The final goal of this work is to produce an imaging x-ray microscope for biological studies.