Fabrication and testing of optics for EUV projection lithography

Extreme Ultraviolet Lithography (EUVL) is a leading candidate as a stepper technology for fabricating the next generation of microelectronic circuits. EUVL is an optical printing technique qualitatively similar to Deep UV Lithography (DUVL), except that 11-13nm wavelength light is used instead of 193-248nm. The feasibility of creating 0.1µm features has been well-established using small- field EUVL printing tools, and development efforts are currently underway to demonstrate that cost- effective production equipment can be engineered to perform full-width ring-field imaging consistent with high wafer throughput rates. Ensuring that an industrial supplier base will be available for key components and subsystems is crucial to the success of EUVL. In particular, the projection optics are the heart of the EUVL imaging system, yet they have figure and finish specifications that are beyond the state-of-the-art in optics manufacturing. Thus it is important to demonstrate that industry will be able to fabricate and certify these optics commensurate with EUVL requirements. The goal of this paper is to demonstrate that procuring EUVL projection optical substrates is feasible

Calibration of symmetric and non-symmetric errors for interferometry of ultra-precise imaging systems

The azimuthal Zernike coefficients for shells of Zernike functions with shell numbers n<N may be determined by making measurements at N equally spaced rotational positions. However, these measurements do not determine the coefficients of any of the purely radial Zernike functions. Label the circle that the azimuthal Zernikes are measured in as circle A. Suppose that the azimuthal Zernike coefficients for n<N are also measured in a smaller circle B which is inside circle A but offset so that it is tangent to circle A and so that it has the center of circle A just inside its circular boundary. The diameter of circle B is thus only slightly larger than half the diameter of circle A. From these two sets of measurements, all the Zernike coefficients may be determined for n<N. However, there are usually unknown small rigid body motions of the optic between measurements. Then all the Zernike coefficients for n<N except for piston, tilts, and focus may be determined. We describe the exact mathematical algorithm that does this and describe an interferometer which measures the complete wavefront from pinholes in pinhole aligners. These pinhole aligners are self-contained units which include a fiber optic, focusing optics, and a 'pinhole mirror'. These pinhole aligners can then be used in another interferometer so that its errors would then be known. Physically, the measurements in circles A and B are accomplished by rotating each pinhole aligner about an aligned axis, then about an oblique axis. Absolute measurement accuracies better than 0.2 nm were achieved

Construction and testing of wavefront reference sources for interferometry of ultra-precise imaging systems

We have built and calibrated a set of 532-nm wavelength wavefront reference sources that fill a numerical aperture of 0.3. Early data show that they have a measured departure from sphericity of less than 0.2 nm RMS (0.4 milliwaves) and a reproducibility of better than 0.05 nm rms. These devices are compact, portable, fiber-fed, and are intended as sources of measurement and reference waves in wavefront measuring interferometers used for metrology of EUVL optical elements and systems. Keys to wave front accuracy include fabrication of an 800-nm pinhole in a smooth reflecting surface as well as a calibration procedure capable of measuring axisymmetric and non-axisymmetric errors

First lithographic results from the extreme ultraviolet Engineering Test Stand

The extreme ultraviolet ͑EUV͒ Engineering Test Stand ͑ETS͒ is a step-and-scan lithography tool that operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUV imaging and acquire system learning for equipment manufacturers to develop commercial tools. The initial integration of the tool is being carried out using a developmental set of projection optics, while a second, higher-quality, projection optics is being assembled and characterized in a parallel effort. We present here the first lithographic results from the ETS, which include both static and scanned resist images of 100 nm dense and isolated features throughout the ring field of the projection optics. Accurate lithographic models have been developed and compared with the experimental results

Sub-nanometer interferometry and precision turning for large optical fabrication

At Lawrence Livermore National Laboratory (LLNL), we have the unique combination of precision turning and metrology capabilities critical to the fabrication of large optical elements. We have developed a self-referenced interferometer to measure errors in aspheric optics to sub- nanometer accuracy over 200-millimeter apertures, a dynamic range of 5{approximately}10. We have utilized diamond turning to figure optics for X-ray to IR wavelengths and, with fast-tool-servo technology, can move optical segments from off-axis to on-axis. With part capacities to 2.3-meters diameter and the metrology described above, segments of very large, ultra-lightweight mirrors can potentially be figured to final requirements. precision of diamond-turning will carryover although the surface finish may be degraded. Finally, the most critical component of a fabrication process is the metrology that enables an accurate part. Well characterized machines are very repeatable and part accuracy must come from proper metrology. A self- referencing interferometer has been developed that can measure accurately to sub-nanometer values. As with traditional interferometers, measurements are fast and post- processed data provides useful feedback to the user. The simplicity of the device allows it to be used on large optics and systems

Sub-nanometer interferometry for aspheric mirror fabrication

Aspheric mirrors for extreme ultraviolet lithography (EUVL) at a wavelength of 13nm require surface figure accuracy approaching 0.10 nm rms. A new type of interferometry, based on the fundamental process of diffraction, is described that has the intrinsically ability to achieve this accuracy on aspherical surfaces. However, care must be taken in the design and implementation of the optical system that images the aspheric mirror onto the CCD camera. Non-common paths of the measurement and reference wavefronts within the optical system, as well as distortion of the image of aspheric mirror on the CCD, must be addressed in order to realize sub-nanometer accuracy. The phase shifting diffraction interferometer and the mitigation of potential imaging errors are described for measuring the surface figure on aspheric mirrors

X-ray characterization of a four-bounce projection system

A four-bounce two-element projection system designed to achieve 0.14 {mu}m resolution over a 1.2 {times} 5 mm{sup 2} ring field has been fabricated. The radiation transport properties of both the individual multilayer-coated optics and the assembled system has been measured. The individual mirror measurements demonstrated that the coatings were within 0.03 nm of d-spacing specifications; however, the mirrors exhibited significant scatter which reduced reflectance below the design specification of 60%. The peak radiation transport efficiency of the assembled projector was 7% at 13.2 nm. To the best of the authors` knowledge, this represented the first measurement of the radiation transport efficiency of a multi-element optical system for EUV lithography. Experiments performed at LLNL`s front-end test bed facility were consistent with the measured transport efficiency

Coating strategy for enhancing illumination uniformity in a lithographic condenser

A three-element Koehler condenser system has been fabricated, characterized, and integrated into an EUV lithographic system. The multilayer coatings deposited on the optics were designed to provide optimal radiation transport efficiency and illumination uniformity. Extensive EUV characterization measurements performed on the individual optics and follow-on system measurements indicated that the condenser was operating close to design goals. Multilayer d-spacings were within 0.05 nm of specifications, and reflectances were approximately 60%. Illumination uniformity was better than {plus_minus}10%. The broadband transport efficiency was 11%

Fabrication and testing of optics for EUV projection lithography

Extreme Ultraviolet Lithography (EUVL) is a leading candidate as a stepper technology for fabricating the next generation of microelectronic circuits. EUVL is an optical printing technique qualitatively similar to Deep UV Lithography (DUVL), except that 11-13nm wavelength light is used instead of 193-248nm. The feasibility of creating 0.1µm features has been well-established using small- field EUVL printing tools, and development efforts are currently underway to demonstrate that cost- effective production equipment can be engineered to perform full-width ring-field imaging consistent with high wafer throughput rates. Ensuring that an industrial supplier base will be available for key components and subsystems is crucial to the success of EUVL. In particular, the projection optics are the heart of the EUVL imaging system, yet they have figure and finish specifications that are beyond the state-of-the-art in optics manufacturing. Thus it is important to demonstrate that industry will be able to fabricate and certify these optics commensurate with EUVL requirements. The goal of this paper is to demonstrate that procuring EUVL projection optical substrates is feasible