Deposition of Cubic-SiC Thin Films on Si (111) using the Molecular Ion Beam Technique
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the difference in thermal expansion of Si and SiC would cause lattice defects. Moreover, in each method, Si and C atoms are separately provided, so it seems hard to form SiC with stoichiometric composition and a Si-C bonding network. In this study, using organosilicon ions of methylsilicenium (SiCH 3') that have a Si-C bond in the molecular ion structure, SiC thin films were formed by molecular ion beam deposition. In the case of a high-energy ion beam, the Si-C bond in molecular ions would be decomposed by collision on the substrate. However, using a low-energy ion beam in the energy range of 10-100 eV, ions with a Si-C bond are deposited on the substrate without decomposition. Therefore SiC thin films may be formed stoichiometrically. Furthermore, molecular ions have a dipole moment because of the difference in electronegativity between Si and C; this dipole moment helps in the bonding. EXPERIMENT In order to prepare a clean surface of Si (I 11) substrate for deposition, the substrate was heated at 1000 'C for 10 minutes in a deposition chamber that was kept at ultra high vacuum (106-10' Pa) by an oil-free turbo molecular pump (1500 l/s). We confirmed the Si (11 )-(7x7) superstructure of the Si (I 11) surface using reflection high-energy electron diffraction (RHEED) before deposition. The deposition of SiC thin films was performed using a low-energy mass-analyzed ion beam deposition system. A diagram of the apparatus is shown in Fig. 1. By surface ionization of dimethylsilane SiH 2(CH 3)2 on a heated tungsten filament, SiCH3' was produced. The SiCH 3' ions were extracted at 25 keV from the ion source. 165 Mat. Res. Soc. Symp. Proc. Vol. 585 ©2000 Materials Research Society
Deposition chamber
[on source
Figure 1. The schematic diagram of the [ow-energy ion beam deposition apparatus.
A 90 0 mass-selecting electromagnet is used to select only the SiCH3+ions. This mass-selector was designed for a resolution of m/e=40. In the ion beam transported at 25 keV, high-energy neutrals are generated through the collisions of ions with the residual gas. To eject these high-energy neutrals, the ion beam is deflected with electrostatic deflection plates. Finally, the ion beam is decelerated to 10-100 eV by deceleration electrodes, and the ions are deposited on the Si (111) substrate at 750-1000 'C [5]. The pressure during the deposition was 1.8xl0"6-l.3xl0.5 Pa. The current of the SiCH 3+ ion beam on the substrate was measured by a picoampere-meter to be 0.30 tiVcm". The spot size of the ion beam was 10.0 mm diameter. RESULTS AND DISCUSSIONS As a result of molecular orbital calculations, it is known that there are four isomeric SiCH 3' ions. As the charge on the silicon becomes more localized, the structure becomes more stable regardless of its valence, so that the most stable molecular structure is the one in which the silicon atom has a positive charge and the carbon atom unites the three hydrogen atoms [6]. The energy distribution of SiCH3' ions decelerated to 100 eV was measured by a PPM421 Plasma Process Monitor (Balze
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