Chemical Vapor-Phase Deposition of Nanostructured Silver Layers

Among the noble metals, silver (Ag) presents the lowest electrical resistivity at low thickness, the highest reflectance from visible to infrared spectra and the lowest thermal conductivity. Consequently, it is used in a large range of applications, either as nanoparticle (NP) films, e.g. in devices based on localized surface plasmon resonance, or as continuous thin films for highly reflective optical mirrors or as infrared reflecting coatings. Although copper (Cu) is widely used in the field of microelectronics, Ag is a good candidate as a potential replacement of Cu for interconnects in integrated circuits (ICs) thanks to its lower residual stress and the absence of considerable increase of resistivity when downscaling. Among the non-line-of-sight vacuum deposition method, atomic layer deposition (ALD) is known for its ability to produce an accurate and precise thickness control giving uniform and conformal film growth thanks to the sequential and self-limiting surface reactions of precursors. However, the ALD of silver is still weakly understood, and the uniform deposition over large scale is often not demonstrated. It is also very challenging to obtain an ultra-thin continuous Ag films due to the growth mechanism leading to islands films rather than continuous layer.
One part of this thesis work is dedicated to take advantage of the nanoparticle morphology and overcome the challenge of the good control of the morphology of Ag nanostructures on large-scale surfaces by an understanding of the growth mechanism. The uniformity of the deposition of silver NPs by plasma-enhanced ALD (PE-ALD) is quantified in terms of film morphology as well as chemical composition and crystalline structure over an 8-inch surface area. After a careful investigation of the PE-ALD parameters and their impact on the Ag deposition, we prove the self-saturated regime required for obtaining good control of the deposition and large-scale uniformity. An empirical model which explains the silver NPs growth mechanism correlated with the experimental results obtained is also proposed. This fine control of the Ag NP morphology opens the way for interesting applications requiring precise NP dimensions.
The thesis also reports a new approach relying on an original two-step plasma-enhanced chemical vapor-phase deposition as an alternative process regime which might be more promising for obtaining the challenging continuous and highly conductive ultra-thin films deposition. After reviewing the influence of the deposition conditions, we prove that the first step provides a uniform new morphology made of compact Ag NPs that is usually not achieved at low thickness with CVD or ALD. Based on the experimental results obtained, a proposed model for the growth mechanism of this peculiar Ag morphology is discussed and suggests that the chemical reaction occurs not only on the surface but also in the gas phase. After the second step of plasma post-treatment of silver made of compact nanoparticles, the sintering of the Ag NPs enhances the electrical conductivity of silver films by increasing the connectivity between particles due to the presence of higher density of energetic radicals. This is highlighted by a rigorous investigation of the influence of the post-processing conditions on the film morphology and its electrical properties. Therefore, we manage to reach the electrical performances of silver films obtained by physical approaches, i.e. by achieving a critical thickness as low as 15 nm and an electrical conductivity of 3.9×10^5 S/cm for a 40-nm thick Ag film. The described method can also be extended to other noble metals, in particular copper and gold, for which the deposition using chemical vapor-based methods is a very active field.
The two different processes developed in this work, i.e. standard PE-ALD and the novel two-step approach, are compared on complex substrates. The later path demonstrates a high film conformality on complex lateral high aspect ratio structures (LHAR with AR of 100), with better coverage than the one reported up to now for ALD of Ag. Moreover, the quasi-substrate-independency of silver films chemical-vapor phase deposited using both approaches confirms the weak influence of the underlayers known in the literature for Ag. On the other hand, we show that the study of the optical behavior of Ag films gives information about their morphology. Indeed, the presence of an absorbance visible peak is a signature of metallic NP morphology causing localized surface plasmon resonance. By following the film reflectivity spectrum of Ag NP films, a ‘continuous-like’ behavior understood by the Drude model is found at low wavelengths whereas higher wavelengths highlight a ‘particle-like’ behavior sticking to the oscillator model. This evolution of the optical properties is very similar for separated NPs using standard PE-ALD and film made of compact NPs obtained by the novel approach, except a stronger and broader absorbance peak typical for a film of aggregated NPs in the last case. On these compact NPs, a plasma post-processing gives strong increase of the infrared reflectance up to 97% and a strong decrease of near infrared transmittance as low as 3% for a 40-nm-thick film. Due to the wide ranges of applications of thin metal films, and the challenge to provide methods for conformal deposition, this work is interesting for the whole community of material scientists and could inspire many investigations.