Effect Of Post Plasma Treatment On Reliability Of ECRCVD SioF Films

Abstract

As the dirnensions of ULSl devices continue to shrink, the RC delay of long interconnections will limit the device high speed performance. In advance logic devices, the stack of the interlayer dielectrics has increased to five or six layers. One of the most effective ways to reduce the parasitic capacitance is using low dielectric constant materials for the interlayer dielectrics. Resent works have indicated that the incorporation of fluorine atoms in the plasma deposited silicon oxide can reduce the effective dielectric constant below 3.5. However, the dielectric constant of SiOF films increases monotonically when exposed to atmosphere because of the high water absorptivity. In an effort to prevent water absorption, SiO, and/or SIN capping layers on the SiOF were examined. However, these methods turned out to be insufficient for reducing the wiring delay, because the structure of the capped SiOF was not effective in decreasing the total capacitance between metal lines. Therefore, our research goal is to develop a surface modification technique by in-situ plasma treatment of SiOF films with low dielectric constant and low water absorptivity which do not need any capping layer. Also, in sub-half micron region, it is necessary to reduce the resistivity of interconnection materials for implementation of new matertals. Copper is a leading candidate because of its lower electrical resistivity and higher resistance to electromigration than aluminum. However, the high diffusivity of Cu atoms into Si0,-base interlayer dielectrics causes device failure. Thus the purpose of this research is to study the effect of post plasma treatment on the reliability of SiOF films for interlayer dielectrics in the multilevel interconnections of ULSls. Figure 1 shows that the angie-resolved XPS spectra of Fls after about 50A etching with various plasma treatment RF bias power. In the cases of 50 W and 150 W, fluorine atoms almost disappear at the top surface. The larger the plasma treated RF bias power, the deeper the fluorine desorbed region. Therefore, as the plasma treated RF power increases, the chemical properties of the plasma treated SiOF films near the top surface tend to resemble those of thermal oxides because of the reduction in the Si-F bonding in the films. The changes in the film density measured by RBS analysis and the refractive index by ellipsometry of the plasma treated SiOF films as a function of RF bias power are shown in Fig. 2 . The film density and refractive index both increase with increasing the RF bias power. The density of the film with RF bias power of 0 W is 1.927 g/cd and that of RF bias power of 150 W is 2.1 13 g/ c” which are lower than that of the thermally grown silicon oxide by about 15% and 7%, respectively. The change in the refractive index can be an indication of variation in important film properties, including the amount of polarizable species, composition and film density. Therefore, this results implies that the chemical composition of the SiOF films at the surface changed from that of the as-deplosited and the film density of the surface increased due to the ion bombardment effect of the post plasma treatment. Figure 3 shows relative dielectric constant of the as-plasma treated SiOF films and the boiled SiOF films as, a function of RF bias power of the plasma treatment. The relative dielectric constant of the post plasma treated SiOF films was increased from 3.14 to about 3.43 with increasing the RF bias power. This result suggests that the increment of the relative dielectric constant is due to the change in the surface chemical composition by desorption of fluorine atoms and the densification of the film by oxygen ion bombardment. Moreover, as the RF bias power in the plasma treatment increases, the change in the relative dielectric constant of the plasma treated films by the boiling treatment was decreased in magnitude. .This result implies that the water absorption resistance increased with increasing the RF bias power because of the densification of the top surface and reduction in the number of Si-F bonds which tend to associate with OH bonds.