Laser Ablation of Solid Ozone
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LASER ABLATION OF SOLID OZONE Hidehiko Nonaka, Tetsuya Nishiguchi,' Yoshiki Morikawa,' Masaharu Miyamoto,' and Shingo Ichimura' Materials Science Division and 2Frontier Technology Division Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, lbaraki, 305-8568, Japan. 'Meidensha Corp. 515 Kaminakamizo Higashimakado, Numazu, Shizuoka 410-8588, Japan. ABSTRACT Species ablated from solid ozone by a UV laser were investigated using a time-of-flight method through a quadrupole mass filter. The results show that UV-laser ablation of solid ozone can produce a pulsed ozone beam with a translational energy far above that of room temperature. Highconcentration ozone from an ozone jet generator is solidified on a sapphire substrate attached to a copper block which is cooled to 30 to 60 K on a cryocooler head and the solid ozone is irradiated by pulsed laser light from a KrF laser (248 nm). The ablated species were a mixture of ozone and molecular oxygen as well as atomic oxygen due to photodissociation of ozone. At a substrate temperature of 30 K, the total amount of ablated ozone increases as the laser fluence increases to 13 mJcm 2 . Beyond this fluence, enhanced decomposition of ozone occurs. Gaussian fitting of the time-of-flight signals of the ablated ozone reveals an average thermal energy exceeding 1,500 K. The velocity also increases when the laser fluence enters saturation at 2,300 K at 13 mJcm 2 .
INTRODUCTION Ozone, 03, has the second largest oxidation potential (+2.076 eV) next to fluorine (+2.866 eV). [1] It has been predominant as an oxidizing agent, for instance, in preparing oxide superconductor films by reactive evaporation of metals in an ozonic atmosphere of less than xl0-3 Pa. [2] In contrast, the pressure has to be at least 10-2 Pa in an oxygen atmosphere. [3] Furthermore, ozone is in the gas phase throughout the practical range of temperature and leaves only oxygen gas as a by-product of the oxidation reaction. Because of ozone's advantageous properties, we have been investigating the potential of ozone, especially for application to the semiconductor device process in which extremely high integration of the devices requires an ultrathin, high-quality gate oxide that may no longer be fabricable using conventional processes in atmospheric oxygen. Using the high-purity ozone jet generator we developed [4], we have so far shown that silicon dioxide (SiO 2 ) films can be grown with low-pressure ozone (8x 104 Pa) at a much lower temperature than with oxygen. [5] Furthermore, analysis of the
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oxide-silicon interface using medium-energy ion-scattering spectroscopy revealed that ozone-formed oxide is more homogenous than conventional oxide because silicon atoms near the interface have much smaller displacement in the ozone-formed oxide [6]. Thus, ozone is no doubt a promising agent in the device process. We are therefore trying to increase the advantages of ozone by producing a collimated hyperthermal ozone beam by laser ablation of solidified ozone. Laser ablation of solidified gas molecules is reported
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