Fluorinated Amorphous Carbon as a Low-Dielectric-Constant Interlayer Dielectric

Abstract

Low-k organic polymers such as polytetrafluoroethylene (PTFE) are promising materials for use as interlayer dielectrics (ILD) because their dielectric constants are generally lower than those of inorganic materials. However poor adhesion with Si substrates, poor thermal stability, and production difficulties have hindered their use in microelectronics.On the other hand, plasma-enhanced chemical vapor deposition (PECVD) of polymer films (plasma polymerization) has many advantages that help to overcome these problems. Plasma-enhanced chemical vapor deposition uses a glow discharge to create activated species such as radicals and ions from the original monomer, and the polymer films are deposited through various gas-phase and surface reactions of these active species, including ablation of the deposited film. No water is generated during plasma polymerization, and the influence of a solvent can be ignored. Also a layered structure that promotes adhesion can be easily fabricated by changing the source compounds.Recently the use of fluorinated amorphous carbon thin films (a-C:F) as new low-dielectric-constant interlayer dielectrics has been proposed. These thin films have an amorphous C–C cross-linked structure (including sp3 and sp2 bonded carbon) and have the same C–F bonds found in PTFE. The strong C–F bonds decrease the dielectric constant, and the C–C crosslinked structure maintains the film's thermal stability. The a-C:F film can be deposited from fluorocarbon source materials using PECVD. Typically fluorocarbons such as CF4, C2F6, C4F8, and their hydrogen mixtures are used as source materials. First the a-C:F films for low-k ILD, with a dielectric constant of 2.1, were deposited from CH4 + CF4 mixtures by using parallel-plate PECVD.