Electrochemical Atomic Layer Deposition of Cobalt Enabled by the Surface-Limited Redox Replacement of Underpotentially Deposited Zinc

Abstract

Electrochemical atomic layer deposition (e-ALD) process for fabricating cobalt (Co) nano-films is reported. The e-ALD process employs a two-step approach in which underpotential deposition (UPD) is first used to form a sacrificial adlayer of zinc (Zn) on a ruthenium (Ru) substrate. The sacrificial Zn adlayer then undergoes spontaneous surface-limited redox replacement (SLRR) by nobler Co. This provides an atomic layer of Co on the substrate surface. The two-step process is repeated cyclically to build multilayers of Co. The unique feature of the e-ALD approach presented herein is that it utilizes Zn as the sacrificial adlayer instead of Pb or Cu used conventionally in the e-ALD sequence of noble metals. The use of sacrificial Zn uniquely renders its redox replacement by Co to be thermodynamically favorable, thereby enabling Co e-ALD. In the present report, we discuss the electrochemical characteristics of the UPD and SLRR process steps, the e-ALD deposition rate and the deposit surface roughness. The Co deposits formed via e-ALD do not exhibit roughness amplification during the first 10 cycles of e-ALD, which is indicative of atomic-scale layer-by-layer growth of Co on the underlying Ru substrate. High-performance integrated circuits utilize nano-scale, currentcarrying copper (Cu) interconnects. In accordance with the Moore's law, 1 miniaturized interconnects are required in modern devices to obtain superior device performance. The continued shrinkage in the size (cross-sectional area) of each interconnect leads to an increase in its electrical resistance. This detrimentally impacts the electrical performance of the integrated circuit.2 Furthermore, aggressive interconnect size scaling below the 10 nm node poses challenges to void-free interconnect fabrication. State-of-the-art interconnects are fabricated of Cu metal due to its low electrical resistivity and superior electromigration resistance.3 The interconnect signal delay (τ) is given by τ = RC, where R is the interconnect resistance and C is the capacitance of the surrounding inter-layer dielectric. The interconnect resistance R increases dramatically for Cu interconnect dimensions below 40 nm due to the substantial increase in the electrical resistivity of Cu at such narrow dimensions. The resistivity increase is typically attributed to electron scattering processes at grain boundaries and at interfaces within the Cu interconnect structure. Such scattering processes become dominant when the interconnect dimension approaches the electron mean free path (EMFP = 39 nm at room temperature) in Cu.4,5 As a consequence, the interconnect signal delay increases. To overcome this critical issue, an interconnect material which exhibits lower electrical resistivity at narrow (sub 10 nm) dimensions is required. Cobalt (Co) is considered as a promising alternative interconnect material to replace the conventionally used Cu.6,7 At narrow dimensions, i.e., below 10 nm, Co exhibits comparable or lower electrical resistivity compared to Cu, largely attributed to the lower ...

Electrocatalytic Rate Alignment Enhances Syngas Generation

“…Next, the Zn atoms were galvanically exchanged with the more noble metal cation (see Experimental Procedures). 23 While the aqueous environment likely induces restructuring of the deposited metal and some corrosion of the Zn, this approach enabled consistent deposition of Co, Ni, and Fe. Comparison of the relative X-ray absorption intensities suggests that similar amounts of these transition metals are present after exchange (Figure S5).…”

“…Postsynthetic stripping of Co reveals $1 mg/cm 2 after one deposition cycle, and that iterative deposition can be used to increase the loading of the secondary transition metal (Table S8). 23…”

Editors' Choice—Review—Cobalt Thin Films: Trends in Processing Technologies and Emerging Applications

“…This study examines how ligands may be cleanly eliminated and explores the possibility of certain side-reactions that yield impurities such as C, as well as potentially other undesired products such as CoO. 40 Hering et al (2016) Electrochemical ALD (e-ALD) As a replacement for Cu interconnects in emerging IC devices A two-step e-ALD process employs a Zn sacrificial layer that undergoes spontaneous surface-limited redox replacement by Co 26…”

“…36,51,54 ALD exhibits a number of attractive features. 26,[37][38][39][40][41] In addition to enabling excellent conformality in nanoscale device topographies and feature sizes, ALD tends to grow particle and pin-hole free films while also providing excellent management of film thickness down to a few atoms. Another emerging advantage of thermal Co ALD is its ability to enable or prevent area-specific or area-selective film growth, in what is commonly referred to as area-selective ALD.…”

Electroless Deposition of Pb Monolayer: A New Process and Application to Surface Selective Atomic Layer Deposition

“…Improved understanding of nucleation kinetics of thin films has led to discoveries of new deposition protocols and concepts in which the thin film growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. − Exploiting some of these results, new approaches were developed where the underpotentially deposited (UPD) monolayer is used to significantly improve the morphology of electrodeposited thin films. − The latest example, so-called, “deposition via surface limited redox replacement (SLRR) of UPD monolayer,” has gained a lot of applications for synthesis of monolayer catalysts and noble metal thin films with different functionalities. − The basic description of this deposition approach can be presented as combination of the potential controlled step (formation of UPD monolayer) and the electroless step representing the SLRR of UPD monolayer by more noble metal ions, i.e., galvanic displacement . The underlying phenomena controlling the morphology of the deposit and SLRR reaction kinetics have been studied and well understood. − However, recent works show that there is still a lot of room for improvements and further simplifications of this deposition approach. − These efforts point to the fact that better practicality, control, and cost effectiveness of the metal deposition via SLRR of UPD monolayer should enable expanded applications of this deposition approach into the areas of technology where traditional vacuum deposition methods are currently used.…”