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内容記述 |
Owing to the recent progress in laser technology, highly intense extreme ultraviolet<br /> (EUV) sources based on laser-produced plasma have been receiving strong interest for <br /> various applications, such as the processing technique of next generation semiconductor <br />based on EUV lithography, and spatially and temporally highly resolved imaging of <br />living cells.. Due to the variation of the applications, improvement of EUV sources to <br />suit each use has been intensively studied. Among these laser plasma EUV sources, an <br />x-ray laser is a characteristic source with short pulse duration of several pico-seconds, <br />narrow spectral width of less than 10<sup>-4</sup>for Δλ/λ, and possibility of high coherence. The <br />high spatial coherence is especially an important factor because the x-ray beam can be <br />focused to a spot diameter of the wavelength order if the focusing system has sufficient <br />precision. With pulse duration of several pico-seconds, the small focal-size realizes the <br />imaging of high-speed phenomenon in the micro domain. Furthermore, x-ray lasers are <br />expected as a source of interference measurements for evaluating EUV optical system <br />and surface accuracy of optics in the field of EUV lithography. For the present <br />measurement, synchrotron radiation is used by inserting a monochromator and pinholes <br />in the undulator line. This can also be done in a laboratory scale by using an x-ray laser <br />as EUV source. . <br /> At the Advanced Photon Research Center, the x-ray lasers generated with <br />transient gain collisional excitation (TCE) method have been studied and have achieved <br />the saturation amplification of a nickel-like silver x-ray laser at the wavelength of 13.9 <br />nm. The TCE scheme have several advantages, such as short pulse duration less than <br />ten pico-seconds, and a high gain generation with low excitation energy of several tens <br />of Joules. In particular, EUV with the wavelength of around 13 nm is useful for <br />lithography, imaging, and interference measurement because the multilayer mirror with <br />high reflectance is commercially prepared. However, the beam divergence of the x-ray <br />laser generated with TCE method is as large as 10 mrad and spatial coherence is not <br />sufficient. In this study, the beam divergence and spatial coherence of the TCE x-ray <br />laser was improved and applied to measurement of time resolved emission <br />spectroscopy. <br /> As an application of the x-ray laser, the scintillation properties of a zinc oxide , <br />(ZnO) single crystal are evaluated for EUV using a 13.9 nm x-ray laser. For <br />next-generation lithography applications, various efforts have been made not only for <br />demonstration of efficient EUV sources, but also for the development of functional <br />optical components in the EUV region. In particular, the development of efficient and <br />fast imaging scintillator devices with sufiicient size is a key element for lithographic <br />applications. in these aspects, hydrothermal method grown ZnO is a prominent <br />candidate. Currently, its growth characteristics have been greatly improved in the aspect <br />of crystalline quality and size of up to 3-inch-diameter. ZnO has been intensively <br />studied for the past ten years as a light-emitting diode material and as nano-structured <br />material to improve the optical properties including response time. The advantage of <br />bulk ZnO is the availability of large sized homogeneous crystal with a reasonable <br />fabrication cost. For the evaluation of hydrothermal method grown ZnO, a nickel-like <br />silver x-ray laser operating at 13.9 nm is the ideal light source; having large pulse <br />energy up to about micro-joules level and a sufiiciently short pulse duration down to <br />several picoseconds. <br /> The improvement of the nickel-like silver x-ray laser at the wavelength of 13.9 <br />nm is described in chapter 2. To decrease the beam divergence of the x-ray laser, we <br />have used the double target configuration. On this technique, a part of the x-ray laser <br />generated in the first medium is used as the seed x-ray laser, and it is injected into the <br />successive second medium, which is used as an amplifier with calm density gradient.<br />The gain medium plasmas are generated with a chirped pulse amplification Nd:glass <br />laser system with two beam lines. Each beam consisted of two pulses; a pre-pulse with a <br />pulse duration of 300 ps and a picosecond heating pulse with a temporal delay of 600 ps <br />from the pm-pulse. The pulse duration of the heating pulse for the first target is 4 ps <br />with the quasi-travelling wave arrangement and that for the second target is 12 ps <br />without the travelling wave. The total pumping energy is set to be 14 J for the first <br />target and 11 J for the second target The energy ratio of the pre-pulse and the heating <br />pulse is 1:8. The laser pulses are focused with a line shape of 6.5 mm × 20μm on flat. <br />silver targets with an irradiance of the heating pulse of ～10<sup>15</sup>W/cm<sup>2</sup>. The distance <br />between the two targets is 20 cm. The far field patterns of the x-ray laser are measured <br />for several seed timing by changing the optical delay of the pumping laser of the first<br />target. At the timing of -15 ps, only the seed x-ray laser from the first target is observed. <br />The beam divergence of the seed x-ray laser is approximately 6 mrad in full width at <br />half maximum (FWHM). The total energy of the seed x-ray laser is 270 nJ. For the <br />seed timing of 0 ps and +15 ps, the intense beams amplified in the second medium was <br />observed. When the timing is adjusted to be 0 ps, an intense x-ray laser beam with little <br />divergence of 0.5 mrad in direction parallel to the target surface is observed outside the <br />shadow of the second plasma. The beam is spread in the direction perpendicular for 2.5 <br />mrad. This means that the seed x-ray is amplified in the high gain region, where the <br />refraction is dominant Delaying the pumping time of the seed x-ray from the timing of <br />0 ps, the spreading x-ray laser becomes weaker, and an intense and narrow spot appears <br />inside the shadow of the plasma At the timing of -15 ps, only the narrow spot is <br />observed, which can be fit well with a Gaussian profile. The beam divergence is <br />obtained to be a much-reduced value of 0.20 mrad (FWHM) for the directions <br />perpendicular and parallel to the target surface The diffraction limited beam divergence <br />for a coherent Gaussian beam with a source size of 50 μm is analytically calculated to<br />be 0.11 mrad. The observed divergence is only 1.8 items greater than this value. This <br />implies that the obtained x-ray beam is quite close to the diffraction-limited condition. <br />The gain coefficient of the amplifier plasma at the timing of +15ps is estimated to be <br />7.9 cm<sup>-1</sup> with exponential fitting to the data. This value is smaller than the gain <br />coefficient of the single target, which is observed to be 35 cm<sup>-1</sup> in a previous study. This <br />small gain coefficient and small beam divergence suggest that the gain region of this<br />highly directed beam is located in a low density area, where the influence of refraction <br />is negligible. <br /> In order to characterize the spatial coherence of the x-ray laser beam, we carried <br />out a Young's double slit experiment. A pair of slits with width Of 16μm-and separation <br />of 150-350 μm is placed 2.3 m away from the second target where the beam diameter is <br />460 μm. Even the 350 μm separated double slit, the interference pattern with high <br />contrast was observed for both parallel and perpendicular direction Assuming the <br />Gaussian shell model for the x-ray laser source, the spatial coherence length can be <br />determined by the visibility of the fringe patterns. For both the parallel and <br />perpendicular direction, the spatial coherent length is estimated to be about 600μm, <br />which is longer than the beam diameter at the position of the double slit. It means that <br />this narrow beam is spatially fully coherent <br /> The evaluation of the scintillation properties of ZnO single crystal for EUV<br />lithography is described in chapter 3. The x-ray laser operated at 13.9 nm was employed <br />as the excitation source. The lasing scheme is the 4p4d transition of the nickel-like <br />silver ion pumped with TCE method. The typical energy of the x-ray laser <br />emission was 0.5 μJ and the duration was 7 ps. This value is sufficiently short for this <br />experiment The single( crystal ZnO sample is grown by hydrothermal ｍethod combined<br />with a platinum inner container. High-purity and transparent ZnO single crystal with a <br />large size of 50×50×15mm3 was sliced with a (0001) surface orientation. The x-ray <br />laser was focused on the sample using a molybdenum/silicon multilayer spherical<br />mirror suitable for 13.9 nm. In spite of the huge photon energy compared with the <br />bandgap, no visible co1or center was observed after about 50 shots irradiation. To <br />eliminate continuous emission from the plasma., a 0.2 μm-thick zirconium foil was<br />placed before the EUV mirror. The fluorescence spectrum and the fluorescence lifetime <br />of the ZnO sample were measured using the 25 cm-focal-length spectrograph coupled <br />with a streak camera with the temporal resolution of 100 ps in the fastest scanning range. <br />The trigger pulse of the streak camera was provided by a pulse generator, which also <br />served as the master clock of the x-ray laser. For comparison, the scintillation properties <br />were also evaluated using the 351 nm third harmonics from the 10S3 nm chirped <br />pumping source for the x-ray laser. The ZnO was excited at energy slightly above the <br />bandgap. The pulse duration of the 351 nm excitation is measured to be 110ps. <br /> One shot of x-ray laser was enough to obtain an image of the time-resolved <br />fluorescence spectrum. To reduce the noise level, three frames were integrated to obtain <br />a clear streak image. The time profile at the peak of the spectrum can be expressed by a <br />double exponential decay with time constants of 1 ns and 3 ns. The two decay constants <br />have been measured in several works for UV excited ZnO single crystals. The fast <br />decay is the lifetime of free exciton and the slower decay is assigned to be trapped <br />carriers. The corresponding fluorescence spectrum and the time profile of UV excitation <br />is also measured. In both the excitation conditions, a prominent fluorescence peak of the <br />ZnO exciton transition was observed at around 380 nm. This wavelength is still <br />convenient for high resolution imaging devices, since even BK7 glass is transparent at <br />this wavelength. Moreover, the two decay lifetimes observed in both cases were almost <br />similar regardless of the huge difference in the excitation photon energy. The <br />fluorescence lifetime is sufficiently short for the characterization of the laser plasma <br />EUV source with nanoseconds duration for lithographic applications. Furthermore, a<br />large-sized and homogeneous material is potentially attractive for EUV imaging<br />applications including lithography.<br /> |
要旨・審査要旨 (445.2 kB)
