Paper TF+VT-WeM9
Atomic Density and Nano-Porosity Thresholds for Nanoscale Low-k Moisture Diffusion Barrier Materials

Wednesday, October 30, 2013, 10:40 am, Room 104 A

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) Cu and moisture diffusion barrier materials are one specific example. Low k diffusion barrier materials are desired to replace the relatively high-k SiN_x:H (k = 7.0) material currently utilized as both a moisture and Cu 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 intentional incorporation terminal organic (CH_x) groups during plasma enhanced chemical vapor deposition (PECVD) of SiO₂ and SiN_x:H network materials. However while lowering k, the introduction of nano-porosity can seriously compromise the performance of such materials as potential moisture and metal diffusion barriers. 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-SiN(C):H dielectric materials is controlled by the concentration of terminal Si-CH₃ bonding versus Si-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-SiN(C):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 materials become easily penetrable by solvents and metals when the pore diameter approaches the size of these species. The implications of these critical nano-porosity thresholds on the performance of low-k materials as diffusion barriers and potential limitations on thickness scaling in the presence of defects will also be discussed.