Low dielectric constant fluorinated diamond-like carbon thin films by plasma enhanced chemical vapor deposition

Abstract

Fluorinated amorphous carbon (a-C:F) thin films are developed for a low dielectric constant interlayer dielectric material from hexafluorobenze (C₆F₆) or 1,1,1,2-tetrafluoroethane (FCH₂CF₄) as the source gas and argon as the diluent gas in an asymmetric capacitively coupled rf (radio frequency) plasma reactor and an inductively coupled plasma reactor. Effects of input rf power, fluorination, applied bias voltage and post annealing on the properties of a-C:F films are investigated. For depositing a-C:F films from highly diluted C₆F₆ (3%) and argon (97%) in the capacitively rf plasma reactor at 150 mTorr, the dielectric constant of the film increases from 2.0 to 2.8 as the rf power is increased from 10 W to 70 W, while the optical energy gap decreases from 2.6 eV to 1.9 eV and the transparency in a ultra-violet range is degraded. At input power of 100 W, the deposited film exhibits high residual stress of 40 MPa and easily peeled off by a Scotch tape test. This is due to high self-bias voltage (-230 V) developed at the substrate holder during deposition. When depositing amorphous carbon films from tetrafluoroethane (TFE) and methane in the capacitively coupled plasma reactor, the incorporation of fluorine in the film is increased with increasing TFE fraction in the feed gas mixture. The dielectric constant of the a-C:F film deposited from pure TFE is as low as 2.3, but the film exhibits poor thermal stability while a-C:H (diamond-like carbon) film deposited from pure methane has a dielectric constant of 3.8 and shows good thermal stability up to 400°C. As the TFE content in the feed is increased, the dielectric constant and the refractive index decrease while the transparency of the film is enhanced significantly. When depositing a-C:F films from C₆F₆ (4 sccm) and Ar (5 sccm) in the inductively coupled rf plasma reactor, the bias voltage (from a separate 100 KHz source) applied to the substrate holder affects the film properties significantly. As the negative bias voltage is increased from 0 V to 200 V, the dielectric constant of the film increases from 2.1 to 2.6, and the degree of graphitization in the film is enhanced. When no bias voltage is applied, interface traps are formed between the film and the silicon substrate at a significant level, whereas a high bias voltage of -200 V degrades the film quality-the film exhibits high water absorption and high leakage current. Post annealing in N₂ or H₂ enhances the electrical properties of a-C:F films. At annealing temperatures greater than 300°C, the films exhibit few defects and high degree of crosslinking. Annealing in H₂ environment is effective in decreasing the interface traps and the charge density in the film. Annealing in N₂ environment is effective in eliminating the water absorption by the film is prevented and in enhancing the adhesion of the film to glass substrates. To deposit low dielectric constant fluorinated amorphous carbon thin films, low rf powers should used in capacitively coupled rf plasma reactors or low bias voltages should be used in inductively coupled plasma reactors. Post annealing at 400°C in a H₂ environment is recommended for improving film quality.