Effects of Absorption of Water and Other Reactive Species on the Fracture Properties of Organosilicate Glass Thin Films

This paper investigates the effects of the absorption of water and several other reactive species on the adhesion energy between organosilicate glass (OSG) thin films and various capping layers. The adhesion energy of these interfaces was measured by four-point bending tests conducted at room temperature in an ambient environment. The amount of absorbed reactant in the organosilicate glass films was controlled by exposing the capped organosilicate films to aqueous solutions for various periods of time. Results show that the initial degradation of the critical adhesion energy of the interfaces between OSG and tantalum, tantalum nitride (TaNx), silicon nitride (SiNx), and silicon dioxide (SiO2) capping layers is very fast, but that the adhesion energy reaches a saturation value thereafter. The reduced adhesion energy can be fully recovered to the “dry” value by baking the samples. A quantitative model based on Henry’s law is used to predict the adhesion degradation rate as a function of moisture exposure time. This model leads to a new and simple method for measuring the diffusion coefficient of water in OSG coatings or their interfaces. Introduction Silica-based dielectrics are often doped with carbon atoms and other organic molecules in order to reduce the permittivity of back-end-of-line (BEOL) inter-layer dielectrics. The resulting organosilicate glass (OSG) has a dielectric constant value less than 3.0 with approximately one half the density of fused silica. The open network structure makes it easy for reactive molecules to diffuse and absorb into the OSG. Vlassak et al. [1-3] and Dauskardt et al. [4-6] have used the four-point-bend and double-cantilever beam techniques to demonstrate that the cohesive and adhesive fracture of OSG is strongly dependent on the chemical reactivity of the testing environment as measured by pH or relative humidity (% R.H.). However, there are very few, if any, studies to elucidate the effects of the absorption of water and other reactive species on the fracture behavior of OSG. The objective of this work is to quantitatively characterize this effect. The adhesion strength between OSG low-k films was measured as a function of exposure time to various aqueous solutions. The results demonstrate that the interfacial strength can be reduced by as much as 50% after a 2-week exposure. A diffusion model is used to extract the water diffusion coefficient in OSG from the time-dependent adhesion data. Experimental Procedure Organosilicate glass thin films (OSG) with a thickness of 400 nm were deposited on 200 mm diameter bare silicon wafers at 400°C by using the plasma-enhanced chemical vapor deposition (PECVD) techniques. The precursor gases used in the deposition were 1, 3, 5, 7-tetramethylcyclo-tetrasiloxane (TMCTS), carbon dioxide, and oxygen. The OSG density measured by using X-Ray Reflectivity (XRR) is approximately 1.4 g/cm; the relative dielectric constant of the OSG was 3.0. The precise chemical composition and mechanical properties of the OSG have been reported in previous studies by Vlassak and coworkers [1-3, 7]. Here, the effect of water