SiO2 films deposited on Si by an ionized cluster beam

Abstract

SiO2 films of 0.1∼1 μm thickness were deposited on Si substrates (4 in. in diameter) by the reactive ionized cluster beam (RICB) developed from the ICB. Fine powder of SiO2 is vaporized to form a jet stream, which, on being ejected from a nozzle into a high vacuum chamber, is cooled and clustered by adiabatic expansion. The clusters thus obtained are ionized in an electron shower, accelerated, and made to bombard the Si substrate. The distribution of film thickness depends on the directivity of the cluster beam and the density distribution of ion current, the latter varying with the density distribution of the electron shower and the lens effect of an accelerating electrode. The optimum design concepts to obtain uniform films are discussed in detail, focusing on the configuration of a nozzle for the cluster beam and an electron extractor and on the location of an accelerating electrode. The physical and chemical properties of SiO2 films obtained varies with ion current, accelerating voltage, substrate temperature, and oxygen partial pressure. Above all, oxygen pressure plays a very important role in the growth of SiO2 films; with approximately 4×10−3 Pa oxygen partial pressure, the SiO2 films formed by the RICB have a refractive index of 1.46, an etching rate of 540 Å/min with a solution of HF/NH4F=1/15, and the infrared spectrum peaked at 1060 cm−1, which indicates that the RICB can form almost the same quality SiO2 films as thermal oxidation but at a very low temperature of below 300 °C. When the RICB technique is applied to deposit SiO2 films as interlevel insulators for multilayer metallization, uniform step coverages are formed at a low temperature, below 300 °C, which is considered to be possible because of the migration of ionized clusters on the surface of the substrate.