Preparation of High-Quality Metallic Films By Hot-Wire Assisted Atomic Layer Deposition
Authors: Shimogaki Yukihiro , Shima Kohei , Yuan Guangjie , Momose Takeshi , Shimizu Hideharu 
Published: February 2020

The shrinkage of device dimension based on scaling rule is an effective method to realize high-performance and low-power consumption of ULSIs. However, it also increases complexity of ULSI device structure. For example, the half pitch of the Cu interconnects used in ULSIs are expected to be reduced to 15 nm or less with aspect ratio of 3 or higher. Three-dimensional FinFET are commonly selected for high-performance and low-power consumption devices. The memory capacitor in DRAM uses cylindrical shape with the diameter of 20 nm or less and the aspect ratio of 100. These structures require a novel deposition method to form metallic and dielectric thin films with excellent thickness uniformity in a high-aspect ratio features.

Atomic layer deposition (ALD) is a suitable method for these applications. ALD has a good advantage in that the process is driven by the chemical saturation of surfaces with the precursor. ALD process allows for conformal coatings of surfaces of nanoscale structure even with a high aspect ratio. Useful precursors for metal-ALD are, however, limited. Precursors have to meet the following requirements: first, they should have sufficient vapor pressure at moderate temperatures. The precursors should be stable unless they are exposed to a reactant. The precursors should not, however, contain oxygen or a halogen, to avoid these elements being included in the deposited metallic film. Therefore, it is preferable to choose oxygen-free and/or halogen-free precursors. These aspects reduce the options of precursors available for use in ALD.

Metallocenes have a long history in organometallic chemistry owing to their versatility. Some of the metallocenes have been used as catalysts or reducing agents in synthetic chemistry. Therefore, several metallocenes are reported and commercially available. ALD using metallocene is, however, generally quite difficult to achieve, despite metallocene having an appropriate vapor pressure. The carbon-metal bond in metallocenes is relatively more stable than that in other metalorganic compounds, making it difficult to form high quality metallic films.

Hot-wire-assisted ALD (HW-ALD) has been identified as a successful method to form high-quality metallic films using metallocene and NH3. NH2 radical formed by cracking NH3 using hot wire at temperature over 1300°C promotes the dissociation of the carbon-metal bond in metallocene during the reducing period. This is examined by elucidation of the gas-phase kinetics and estimation of the surface reactions by quantum chemical calculations. Co, Co(W)-alloy, Ni, and Ru films were successfully formed by HW-ALD. Ni films formed from (C5H5)2Ni contain only several percent of carbon and Co films had carbon impurity under detection limit of carbon in XPS measurement. We have examined the work function of Ru films obtained by HW-ALD using CV measurement of MIS structure to confirm that carbon is not so much contained in the deposited films. Co(W)-alloy films formed by HW-ALD was examined as the Cu-diffusion barrier layer using atom probe tomography (APT). We could confirm that Co(W)-alloy film can surpress the Cu diffusion effectively, if W content is over 20 at%.

In summary, high-quality metallic films with less impurities and uniform step coverage on high-aspect ratio feature were obtained by HW-ALD.