Thickness-dependent thermal reliability of low-dielectric constant polycrystalline PTFE submicron dielectric thin films

Abstract

According to the SIA National Technology Roadmap for Semiconductors, interlevel dielectrics (ILDs) with a relative dielectric constant less than two will be needed for future integrated circuit devices beyond 0.1μm generation. For possible low-dielectric constant (low-k) candidates with a relative dielectric constant less than two, polytetrafluoroethylene (PTFE) has the lowest dielectric constant among nonporous low-k materials, and thus is a strong future ILD candidate. As the feature size decreases, the ILD thickness is also expected to decrease. Thus needs exist for characterizing and understanding the possible thickness-induced thermal reliability of PTFE thin films for deep-submicron multilevel interconnection applications. The majority of low-dielectric constant candidates for ULSI ILD applications are amorphous polymers; techniques exist for characterizing the glass transition temperatures of amorphous polymers, which is the critical measure of their thermal stability. However, a simple but reliable method remained to be introduced for characterizing the thermal stability of submicron crystalline thin films such as PTFE. It is determined in the present work that the directly measured ellipsometric angles Δ and ψ can be used for detecting the solid↔liquid transition temperatures of on-wafer polycrystalline thin films. The novel approach is applied for investigating the solid↔liquid transitions of on-wafer PTFE thin films. The results show that the solid–liquid transitions depend on the film thickness as a result of film/surface, film/substrate interactions and the thickness-dependent crystal size. The results can be well described by modifying a previous model for size dependent solid–liquid transitions of nanocrystals.