Effect Of Internal Surface Bondings On The Etching Of SiO2 Aerogel Film

Abstract

Etching behavior of novel Si02 aerogel film for applying to interlevel dielectric in multilevel interconnections of ultra large-scale integration (ULSI) has been investigated. Recently, low dielectric property has been a critical issue for developing ULSl devices, because the device performance has been limited by signal delay and cross talk in the multilevel interconnections. Si02 aerogel film by sol-gel method and supercritical drying shows very low dielectric constant about 2. This low dielectric property is originated from its porous nature and network structure of Si02 aerogel film consists of a number of SiOR (R = alkoxy1 group) and Si-OH bonds and adsorbed water as surface terminal species. In application to multilevel structure, an understanding of its etching behavior IS indispensable matter Especially, different properties from thermally grown Si02, I e., porous nature and internal surface chemical bondings, must be considered during the etching process. Si02 aerogel films coated on Si wafer were used in high density inductively coupled plasma (ICP) etching experiment. Scanning electron microscopy (SEM) observation showed that microstructure of Si02 aerogel films was maintained during the etching process, as shown in Fig. 1. Interconnected network of Si02 aerogel was not changed under the attack of reactive species, even though the porous nature of the film seemed to enable the etching to be occurred inside of the film. Some factors have to be considered to play a role in blocking the etching inside of the film. Fig. 2 shows the conceivable etching model of Si02 aerogel particle compared with thermally grown Si02. It can be adaptable for Si02 aerogel to be etched 3 dimensionally other than planar, i.e., 2 dimensional etching of thermally grown Si02. Although the etching species transport through porous media, interconnected structure of aerogel particle must be guarded by surface terminal hydrogen bondings. The surface compositions of partially etched samples are given in Fig. 3 using X-ray photoelectron spectroscopy (XPS) with take-off angle variation. In this variation, contribution of surface component to the observed peak intensity increases with the decrease of take-off angle. In thermally grown Si02, a steep change in the composition with angles was observed. But with Si02 aerogel film, composition was not varied much with angles so it could be said that almost uniform composition-distribution was applied to the partially etched Si02 aerogel film within information depth of photoelectrons. This means that C-F polymer residue is uniformly formed inside of Si02 aerogel film and the etching of Si02 aerogel can be delayed by internal surface chemicals. In this work, gas transport phenomenon and reaction using CHF3 or C2F6 gas have been studied to investigate the role of hydrogen on the etching of Si02 aerogel film. The evolutions in physical and chemical properties of Si02 aerogel film during the etching were also evaluated.