Carbon-doped oxide materials (SiCOH films) with ultralow dielectric constants have been prepared by plasma-enhanced chemical vapor deposition (PECVD) from mixtures of SiCOH precursors with organic materials. The films have been characterized by Rutherford backscattering and forward recoil elastic scattering analysis, Fourier transform infrared spectroscopy and index of refraction measurements, and measurement of step heights in the films. The electrical properties of the films have been measured on metal–insulator–silicon structures. By proper choice of the precursor and deposition conditions, the dielectric constants of the SiCOH films can be reduced to values below 2.1, demonstrating the extendibility of PECVD-prepared carbon-doped oxides as the interconnect dielectrics for future generation of very large scale integrated chips.
Dielectric films have been prepared by radio-frequency plasma-enhanced chemical vapor deposition from mixtures of tetramethylsilane with oxygen. The films have been characterized as-deposited and after annealing at 400 °C to determine the thermal stability of their properties. Rutherford backscattering and forward recoil elastic scattering have been used for determination of the composition of the films. Optical properties were characterized by Fourier transform infrared spectroscopy and measurements of the index of refraction and optical gap. The electrical properties were measured in a Si/insulator/metal configuration. It has been found that the index of refraction decreases and the optical gap and dielectric constant increase with increasing oxygen concentration in the gas feed. While the materials did not show a mass or composition loss after annealing, the annealing resulted in a reduction of the dielectric constant of the films. Dielectric constants as low as 3.1 have been obtained after annealing the film deposited from pure tetramethylsilane.
The low dielectric constant (k) of plasma enhanced chemical vapor deposited SiCOH films has been attributed to porosity in the films. We have shown previously that the dielectric constant of such materials can be extended from the typical k values of 2.7–2.9 to ultralow-k values of k=2.0. The reduction in the dielectric constants has been achieved by enhancing the porosity in the films through the addition of an organic material to the SiCOH precursor and annealing the films to remove the thermally less-stable organic fractions. In order to confirm the relation between dielectric constant and film porosity the latter has been evaluated for SiCOH films with k values from 2.8 to 2.05 using positron annihilation spectroscopy, positron annihilation lifetime spectroscopy, small angle x-ray scattering, specular x-ray reflectivity, and ellipsometric porosimetry measurements. It has been found that the SiCOH films with k=2.8 had no detectable porosity, however the porosity increased with decreasing dielectric constant reaching values of 28%–39% for k values of 2.05. The degree of porosity and the pore size determined by the dissimilar techniques agreed within reasonable limits, especially when one takes into account the small pore size in these films and the different assumptions used by the different techniques. The pore size increases with decreasing k, however the diameter remains below 5 nm for k=2.05, most of the pores being smaller than 2.5 nm.
Comparison of Fourier transform infrared (FTIR) absorption spectroscopy, high resolution 13C nuclear magnetic resonance spectroscopy, and hydrogen evaluation by forward recoil elastic scattering (FRES) measurements shows that FTIR cannot be used to determine the carbon hybridization or total hydrogen content of diamondlike carbon films (DLC). FTIR absorption reflects only the optically active hydrogen in the DLC and, using it together with FRES, it can provide an estimate of the unbound hydrogen contained in the films. The fraction of unbound hydrogen can reach values of up to ∼50%.
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