Zhuo Wang, Tzu-Yu Wu, Mark Hamm, Alexander Altshuler, Anderson Mach, Donald Gilbody, Bin Wu, Santosh Ganesan, James Chung, Mitsuhiro Ikuta, Jacob Brauer, Seiji Takeuchi, Tokuyuki Honda
As one of the smallest endoscopes that have been demonstrated, the spectrally encoded endoscope (SEE) shows potential
for the use in minimally invasive surgeries. While the original SEE is designed for side-view applications, the forwardview
(FV) scope is more desired by physicians for many clinical applications because it provides a more natural
navigation. Several FV SEEs have been designed in the past, which involve either multiple optical elements or one
optical element with multiple optically active surfaces. Here we report a complete FV SEE which comprises a rotating
illumination probe within a drive cable, a sheath and a window to cover the optics, a customized spectrometer, hardware
controllers for both motor control and synchronization, and a software suite to capture, process and store images and
videos. In this solution, the optical axis is straight and the dispersion element, i.e. the grating, is designed such that the
slightly focused light after the focusing element will be dispersed by the grating, covering forward view angles with high
diffraction efficiencies. As such, the illumination probe is fabricated with a diameter of only 275 μm. The twodimensional
video-rate image acquisition is realized by rotating the illumination optics at 30 Hz. In one finished design,
the scope diameter including the window assembly is 1.2 mm.
For lithography of 45-nm half-pitch and beyond, the resist blur due to photoacid diffusion is a significant issue. On the other hand, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. We study the influence of the resist blur on resolution in hypernumerical aperture ArF immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist performance for some of the latest commercial resists and estimated their acid diffusion lengths as 8 nm to 9 nm in sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, which is directly related to the resist resolution and is controllable by photoacid generator (PAG) anion size, polymer resin size, and post-exposure bake (PEB) temperature. We confirmed that there is a trade-off between resist resolution and sensitivity. Our results indicate that the resist blur is still a concern in order to extend lithography for 45 nm and beyond; however, it will not likely be a showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve ultimate resolution in hyper-NA immersion lithography.
The resist blur due to photoacid diffusion is a significant issue for 45-nm half-pitch node and beyond.
Furthermore, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. In
this paper, we study the influence of the resist blur on resolution and sensitivity in hyper-numerical aperture ArF
immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist
performance for some of the latest commercial resists, and estimated their acid diffusion lengths as 8 to 9 nm in
sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, that is, the resist
resolution was controllable by PAG anion size, polymer resin size, and PEB temperature. We also found that there
was the trade-off between resist resolution and sensitivity. Our results indicated that the resist blur is still a concern
in order to extend ArF lithography for 45-nm half-pitch node and beyond, however, it will not likely be a
showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve
ultimate resolution in hyper-NA immersion lithography.
We have built a visible light point-diffraction interferometer with the purpose to characterize EUVL projection optics. The interferometer operates at the wavelength of 532 nm and utilizes two identical pinhole wavefront reference sources for generation of both signal and reference wavefronts. In the simple configuration of our interferometer, the main source of system error is the pinhole reference wavefronts. It is important that the reference wavefronts are calibrated and the calibration is stable. The calibration using our refractive test optic is reproducible to better than 0.1 nm RMS. The interferometer measured the wavefront of our refractive test optic with the repeatability of 0.1nm RMS. This paper will discuss the error sources and removal of the errors with experimental results.
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.
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.
Precise in-situ transmission testing is necessary for evaluation of materials to be used in lithography systems, which use light source at the wavelength of 157 nm. Fluorine (F2) excimer lasers have pulse-to-pulse energy variation (< 9 %, 3 sigma, from manufacturer's specification), and pulse energy monitoring is required for precise evaluation. Due to the uncontrolled fluctuations of the polarization and high pulse energy of the laser, it is difficult to monitor the laser pulse energy and to achieve precision measurement. We have built a precise in-situ transmittance measurement system, employing polarization insensitive beam splitters for precision pulse energy monitoring. The beam splitter consists of three parallel plates. This scheme eliminates the effects of polarization fluctuation and decreases the energy to the detector. We have obtained 0.1 % (3 sigma) stability in our transmission measurement using this assembly.
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.
KEYWORDS: Adhesives, Oxygen, Metals, Quartz, Lithography, Systems modeling, Transmittance, Information operations, Lenses, Scanning electron microscopy
Purging and reduction of out-gassing are very important issues that need to be treated in order to realize F2 laser lithography system. Several methods of purging are tried and out-gases from metals, O-rings, lubricants, and an adhesive are analyzed. Metal surfaces mainly release oxygen and water independent of surface roughness, Ni plating, or elements. Other substances are not detected by API-MS or GC-MS. Since O-rings are indispensable to make gas-tight structures, several kinds of O-rings made of fluoro-compounds are tested. Black fluoro-rubber o-ring, O-ring F, is recommended from the view of organic out-gassing but Teflon-based fluoro-elastomer, O-ring A, is a good candidate in terms of the water out-gassing. Greases emit a large amount of out-gases even when the samples are not irradiated by 157 nm laser. As an adhesive, Adhesive A is recommended because of the fact that it does not release as much organic and inorganic compounds which may absorb 157 nm laser light. Finally preliminary demonstration using a model exposure system is performed to obtain purging time for several cases.
Purging and contamination are two important issues that need to be treated in order to realize F2 laser lithography system. For the purpose of developing gas-purging and chemically clean technologies, we designed and constructed an experimental set-up. It is used for the study of purging and out-gassing evaluation in order to obtain useful data for development of exposure system. Preliminary experiments showed that purging condition has a strong effect on the residual oxygen and water concentration in the final gas-replaced atmosphere. And we have found that the amount of out-gas depends on the surface finish method of the material used through analyses of impurity gas examination with or without laser irradiation.
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