3:15 PM - EL08.01.04
Two-Step Approach for Conformal Chemical Vapor-Phase Deposition of Ultra-Thin Conductive Silver Films
Sabrina Wack1,Petru Lunca Popa1,Noureddine Adjeroud1,Christèle Vergne1,Renaud Leturcq1
Luxembourg Institute of Science & Technology1
In the fabrication of microelectronic devices, which include DRAM capacitors, transistors, and back-end-of line (BEOL) interconnects, copper (Cu) films are widely studied. However, in this field, silver (Ag) is of particular interest due to its low electrical resistivity and low residual stress compared to Cu. Moreover, a downscaling to less than 100 nm is possible without a considerable increase in resistivity.  All these arguments impose Ag as a potential replacement of Cu for contacts and interconnects. Besides microelectronics field, the outstanding optical properties of silver make it a good choice for several applications including optical coatings for plasmonics, windows or lenses, mirrors or sensors, while its chemical reactivity is used in antibacterial surfaces. More and more of these applications require a conformal growth of ultra-thin silver layers, which represents a challenge for most deposition methods. 
The deposition of highly uniform and conformal conductive ultra-thin films is also of great interest in the microelectronics industry where the miniaturization of semiconductor devices introduces complex three-dimensional structures with high aspect ratio. [3,4] Consequently, one of the main challenges is to be able to uniformly fill the metallic films into these structures. Conductive ultra-thin silver films are commonly deposited by line-of-sight methods (mainly sputtering) that do not allow conformal deposition on 3D structures with complex morphology such as in microelectronic devices, and have strong uniformity limitations for non-flat substrates such as curved glass. On the other hand, non-line-of-sight methods, such as chemical vapor deposition or atomic layer deposition, usually produce non-electrically-conductive films for low thickness, due to island growth mode. [4,5]
Our new approach relies on an original two-step plasma-enhanced chemical vapor-phase deposition , allowing us to reach the electrical performances of silver films obtained by physical approaches without the need of additional wetting layer. Indeed, we synthetized highly conductive and uniform Ag films with a critical thickness lower than 15 nm and a sheet resistance of 1.6 Ω/sq. for 40 nm thin film, corresponding to a resistivity of 6.4 µΩ.cm. The high reflectance (up to 94%) and low absorbance (3%) in the infrared region further demonstrate the optical quality of the films, despite a still large rms roughness of 8.9 nm and justify the relevance of the product for high-performance IRR coatings. Moreover, we successfully demonstrate the high conformality of the deposited film on complex lateral high aspect ratio structures (up to 100), with better coverage than the one reported up to now for atomic layer deposition of silver.
This new processing approach opens a very promising route for the use of ultra-thin silver films for electronic and optoelectronic applications, and could be extended to other metals deposited from metal-organic precursors, in particular copper and gold, for which the deposition using chemical vapor based methods is a very active field.
 Amusan et al., J. Vac. Sci. Technol. A 2016, 34, 01A126.
 Wack et al., ACS Appl. Mater. Interfaces 2020, 12, 36329.
 Cremers et al., Appl. Phys. Rev. 2019, 6, 021302.
 Hagen et al., Appl. Phys. Rev. 2019, 6, 041309.
 Wack et al., J. Phys. Chem. C 2019, 123, 27196.