Hrtem and Eels Studies of Reacted Materials From Caf 2 by Electron Beam Irradiation
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ABSTRACT The change of calcium fluoride (CaF 2) by electron beam irradiation has been investigated in TEMs operated at 200 kV. By irradiation fluorine is desorbed rapidly from CaF 2 crystal. Although plasmon peaks in EELS spectra suggest the formation of Ca metal, no evidence for the existence of Ca metal is found in electron diffraction pattern or HRTEM images. This is due to perfect topotactic formation of Ca metal from CaF 2 with a similar crystal structure and closely similar lattice parameters. Irradiation also forms amorphous material near the edge of Ca/CaF 2 and randomly oriented CaO crystallites grow from the amorphous material. This amorphous material is regarded as hydroxide formed through the reaction of Ca and water, considering the residual gas composition and the formation rate of the material. INTRODUCTION CaF 2 has been investigated as a potential insulating layer on Si or GaAs substrates for future semiconductor technology[1-3]. This material is also expected to be used as an inorganic resist for high-resolution electron beam lithography [4, 5], although CaF 2 is not the only candidate material since other inorganic halides can be used [6, 7]. Such inorganic halides are prone to lose halogen ions by intense electron radiation, which is known as electron-stimulated desorption (ESD) [8]. Some electron beam lithography experiments with inorganic halides utilize this phenomenon, changing halides to metals and forming holes by the displacement of the metals [4, 6]. On the other hand, Mankiewich et. al. [5] formed narrow lines of CaO in CaF 2 by electron irradiation and successive oxidation of Ca metal. Water was used as a developer to remove CaO because CaO dissolves easily in water compared to CaF2 . To understand the atomistic mechanism of these processes using CaF2 , we have investigated the structural change of CaF 2 by electron beam irradiation in TEMs using high-resolution (HR) TEM and other techniques.
183 Mat. Res. Soc. Symp. Proc. Vol. 504 ©1998 Materials Research Society
EXPERIMENTAL The specimen investigated was natural CaF 2 (fluorite.). No impurity was detected using energy dispersive X-ray (EDX) analysis. Synthetic MgF 2 with 99.99% (Rare Metallic Co., Ltd., Japan) was also used to investigate the difference of irradiation effects between these two materials (see the next section). They were crushed and dispersed on holy carbon films using ethanol for TEM experiments. Two TEMs were used for the experiments. For HRTEM and EDX analyses, a JEOL JEM-2010 (nominal Cs value is 0.5 mm) with a LaB6 filament and an ultra-thin-window (UTW) EDX detector, was used, operating at 200 kV. Electron energy loss spectroscopy (EELS) analyses were performed at 200 kV using another TEM, a Hitachi HF-2000 with a cold field emission source and a Gatan 666 PEELS system. RESULTS AND DISCUSSION Fig. 1 shows the change of EDX spectra as a function of irradiation time. The experimental condition is described in the figure caption. The X-ray intensity of fluorine decreases rapidly with irradiation whereas that of
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