Ultra-thin metal layers by plasma-assisted spatial atomic layer deposition at atmospheric pressure

• Applicants: Professorin Dr. Anjana Devi; Professor Dr. Thomas Riedl
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 417279094

Ultra-thin metal films provide a unique combination of electrical, optical and plasmonic properties, which impact a wide range of applications, among them semitransparent electrodes for thin-film optoelectronic devices. Unfortunately, the deposition of thin metal films on a wide range of substrates results in nanometer sized island-like patterns. Thus, below the percolation threshold, the layer has a low conductivity. The conductivity increases by orders of magnitude once the percolation threshold is reached. Typically, the threshold for percolation of metals like Ag or Au which are thermally evaporated or sputtered is on the order of 10-15 nm. To improve wetting of the metal layer several approaches such as surface functionalization, the use of alloys, and slight oxidation of the metal during deposition have been considered.The vast amount of studies on ultra-thin metal layers employed either sputter deposition or thermal evaporation. On the contrary, the growth of coinage metals Ag or Cu by atomic layer deposition (ALD) is still in its infancy. Advantageously, ALD allows for a precise control of film thickness, homogeneous growth over large areas and a conformal deposition on micro- and nanostructures with high aspect ratios. Today, ALD is a well-established technique to grow dielectric layers, especially metal-oxides for a number of applications. Plasma-assisted ALD (PA-ALD) enables the growth of non-oxide materials, such as nitrides and metals. To overcome the limits of conventional vacuum based ALD, spatial ALD at atmospheric pressure has been introduced, which is even suitable for roll-to-roll processing. The central aim of this project is to explore spatial PA-ALD at atmospheric pressure for the deposition of ultra-thin metal films. We intend to gather a fundamental understanding of the nucleation and growth of these metal layers depending on the substrate, the processing parameters of the spatial APP-ALD (temperature, plasma and precursor dose, etc.) and the type of metal precursor. The ultimate goal is the APP-ALD of thin metal layers with a high conductivity and high optical transmittance. Therefore, low temperature (< 120°C) spatial APP-ALD of is targeted which requires the following research tasks:• To develop novel Ag and Cu precursors with high vapor pressure and high reactivity towards reducing plasma at low temperatures, with sufficient thermal stability. • To study the initial stages of nucleation and film formation. • To identify the limits of transmittance and conductivity of the APP-ALD metal layers. • To explore the possibility of surface functionalization, e.g. by organic functional groups or by non-oxide inorganic films. • Consider atmospheric pressure MLD to include trace amounts of organic molecules in the APP-ALD growth process of the metal to suppress clustering while at the same time retain high electrical conductivity.

• DFG Programme: Research Grants