Control of Nano-Porosity In Plasma Deposited Low-k Diffusion Barrier and Inter-Layer Dielectrics for Nano-Electronic Applications

As the semiconductor industry strives to keep pace with Moore’s Law, new materials with extreme properties are increasingly being introduced and tighter control of these material properties is being demanded. Low dielectric constant (i.e. low-k) materials are one specific example. Lower k materials are desired to replace SiO₂ (k=4.0) as the interlayer dielectric (ILD) and SiN_x:H (k = 7.0) as the Cu capping diffusion barrier layer in order to reduce resistance-capacitance (RC) delays in nano-electronic Cu interconnect structures. Typical methods for producing low-k materials consist of introducing controlled levels of nano-porosity via carbon doping during plasma enhanced chemical vapor deposition (PECVD) of SiO₂ and SiN_x:H matrix materials. While lowering k, the introduction of nano-porosity can seriously compromise the performance of these layers in their respective applications. In this presentation, we will demonstrate that critical thresholds in nano-porosity exist for the diffusion of water and solvents through low-k materials. Specifically, we utilize Fourier Transform Infra-Red (FTIR) spectroscopy, to show that the concentration and size of nano-pores formed in low-k a-SiC(N):H dielectric materials is controlled by the concentration of terminal Si-CH₃ bonding versus Si-C/N network bonding. We further combine moisture / solvent diffusivity measurement with x-ray reflectivity (XRR) and positron annihilation lifetime spectroscopy (PALS) to demonstrate that low-k a-SiC(N):H dielectrics become poor moisture diffusion barriers at mass densities < 2.0 g/cm³ and when the pore size approaches that for the molecular diameter of water. Similarly, we show that low-k a-SiOC:H ILDs become easily penetrable by solvents and susceptible to downstream processing damage when the pore diameter approaches the size of the solvent and pores become interconnected. The implications of these critical nano-porosity thresholds on continued scaling of low-k diffusion barrier and ILD materials will be discussed as well as methods for overcoming these limitations.