Surface irradiation and materials processing using polyatomic cluster ion beams

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We developed a polyatomic cluster ion beam system for materials processing, and polyatomic clusters of materials such as alcohol and water were produced by an adiabatic expansion phenomenon. In this article, cluster formation is discussed using thermodynamics and fluid dynamics. To investigate the interactions of polyatomic cluster ions with solid surfaces, various kinds of substrates such as Si(100), SiO2, mica, polymethyl methacrylate, and metals were irradiated by ethanol, methanol, and water cluster ion beams. To be specific, chemical reactions between radicals of polyatomic molecules and surface Si atoms were investigated, and low-irradiation damage as well as high-rate sputtering was carried out on the Si(100) surfaces. Furthermore, materials processing methods including high-rate sputtering, surface modification, and micropatterning were demonstrated with ethanol and water cluster ion beams.

I. INTRODUCTION

Ion-assisted materials processing has generated interest in surface treatments, such as deposition and etching,1–4 and has been responsible for advancements of functional materials. Ion beam technology is one example of ionassisted material processing that has recently attracted attention due to the controllability and variety of ion beams that can be used. For example, the energy, current, and size of the ion beams can be controlled over a wide range by applying electric and magnetic fields, and they are used in applications such as focused ion beams, ion rockets, accelerators, and fusion reactors.5–10 In addition, ion beams can transfer mass and material properties, which is not possible with other beams, such as electron or laser beams. By controlling the kinetic energies as well as by selecting the ion species, ion beams can be used for surface treatments such as implantation, sputtering, and deposition. It is very hard to transfer a high-current beam of low-energy ions, because the space charge effect is enhanced. Highly accelerated ion beams can be used to sputter the material surface; however, they are also responsible for irradiation damage to the surface. There is a trade-off between a high sputtering rate and a low damage irradiation, although material processing with damage-free sputtering is of course desirable. Chemical sputtering results in less irradiation damage than physical sputtering. Because physical sputtering is caused by a transfer of momentum from incident particles to surface a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.426 806

J. Mater. Res., Vol. 27, No. 5, Mar 14, 2012

atoms, it results in irradiation damage to the surface of materials. During chemical sputtering, ion bombardment facilitates a chemical reaction that produces weakly bound particles, which are easily desorbed into the gas phase.11 The release of the weakly bound particles is mostly thermally driven, and the irradiation damage caused by ion bombardment is less than that caused by physical sputtering. In a polyatomic molecule, various kinds of radicals suc