Advanced Atomic Layer Deposition: Ultrathin and Continuous Metal Thin Film Growth and Work Function Control Using the Discrete Feeding Method

Abstract

The ultrathin and continuous ruthenium (Ru) film was deposited through an improved atomic layer deposition (ALD) process with a discrete feeding method (DFM), called DF-ALD, employing a cut-in purge step during the precursor feeding. The excess precursor molecules can be physically adsorbed onto the chemisorbed precursors on the substrate during precursor feeding, which screens the reactive sites on the surface. Using DF-ALD, surface coverage of precursors was enhanced because the cut-in purge removes the physisorbed precursors securing the reactive sites beneath them; thus, nucleation density was greatly increased. Therefore, the grain size decreased, which changed the microstructure and increased oxygen impurity concentration. However, a more metallic Ru thin film was formed due to thermodynamic stability and improved physical density. Consequently, DF-ALD enables the deposition of the ultrathin (3 nm) and continuous Ru film with a low resistivity of ∼60 μΩ cm and a high effective work function of ∼4.8 eV.

CFD Simulation of the Dosing Behavior within the Atomic Layer Deposition Feeding System

“…It can be expected that a longer pulse time will result in more precursors being injected into the reaction chamber. However, more precursor per cycle does not necessarily correspond to a better quality of the deposited film as excess precursor can sometimes get physisorbed onto the chemisorbed precursors on the substrate changing the microstructure of the film . Additionally, due to the finite amount of the surface active groups on the substrate the deposition rate will decrease with time during each ALD cycle.…”

“…Therefore, the choice of pulse time should be decided by considering the overall process. The ALD process is sometimes optimized by employing novel operations of the ALD valve, such as the "super cycle" 48 (also known as the "discrete feeding method" 26 ), where the cut-in purge between short pulses can remove the physisorbed precursors enhancing the surface coverage of precursors. The "discrete feeding method" can increase the nucleation density at the initial stage of film deposition by improving the surface coverage of precursors and reduce the critical thickness (the minimum thickness for continuous film formation), which is more conducive to the preparation of ultra-thin films.…”

Advanced atomic layer deposition: metal oxide thin film growth using the discrete feeding method

“…Due to this, precursor molecules cannot cover the substrate surface efficiently, which means the initial growth behavior of a thin film, such as nucleation density, would be degraded, resulting in a deterioration of the quality of the thin film. 5–7…”

Enhancing chemisorption efficiency and thin-film characteristics via a discrete feeding method in high-k dielectric atomic layer deposition for preventing interfacial layer formation

“…The steric hindrance effect, a factor that interferes with the adsorption behavior of a precursor, is a phenomenon wherein precursors cannot reach the chemisorption sites because of the physical size of the ligand of the adjacent precursors. 9–11 In the ideal case, when the chemisorption sites are fully occupied, the exposed area of the TiN surface is an inherent property because the steric hindrance effect only governs the exposed area. In this regard, the area exposed by the steric hindrance effect depends on the precursor employed in the ALD process; in turn, this is inherent.…”

“…During this stage, some part of the physisorbed precursor screens the unoccupied chemisorption site, unintentionally, thereby reducing the precursor chemisorption density, which is called the “screening effect”. 11–14 Accordingly, the screening effect is one of the mechanisms that increases the exposed area in the early stage of the insulator ALD process. In contrast to the steric hindrance effect, the screening effect can be effectively eliminated by reducing the physisorbed precursor.…”