A novel process for electrochemical atomic layer deposition (e-ALD) of copper is presented. In this process, a sacrificial monolayer of zinc (Zn) is formed via underpotential deposition (UPD) on a copper (Cu) or ruthenium (Ru) substrate. The sacrificial Zn monolayer then undergoes surface-limited redox replacement (SLRR) by nobler Cu. This provides a monolayer of Cu on the substrate surface. UPD-SLRR cycles are repeated to build multi-layers of Cu with controlled thickness while minimizing surface roughness. The proposed Cu e-ALD process is attractive from the point of view of scalability and commercial viability for the following reasons: (i) it eliminates the use of lead-containing chemistries used in previous formulations of Cu e-ALD; (ii) it utilizes a single alkaline (pH = 11.2) electrolyte, which minimizes parasitic reactions such as hydrogen co-evolution and eliminates the need for frequent electrolyte switching as needed in previous e-ALD processes. In this publication, we report cyclic voltammetry, electrochemical quartz crystal microgravimetry, and anodic stripping coulometry studies to gain insights into the process efficiency and deposit morphology characteristics of the Cu e-ALD process.

The experimentally measured resistivity of Co(0001) and Ru(0001) single crystal thin films, grown on c-plane sapphire substrates, as a function of thickness is modeled using the semiclassical model of Fuchs-Sondheimer. The model fits show that the resistivity of Ru would cross below that for Co at a thickness of approximately 20 nm. For Ru films with thicknesses above 20 nm, transmission electron microscopy evidences threading and misfit dislocations, stacking faults and deformation twins. Exposure of Co films to ambient air, and the deposition of oxide layers of SiO2, MgO, Al2O3 and Cr2O3 on Ru degrade the surface specularity of the metallic layer. However, for the Ru films, annealing in a reducing ambient restores the surface specularity. Epitaxial electrochemical deposition of Co on epitaxially-deposited Ru layers is used as an example to demonstrate the feasibility of generating epitaxial interconnects for back-end of line structures. An electron transport model based on a tight-binding (TB) approach is described, with Ru interconnects used an example. The model allows conductivity to be computed for structures comprising large ensembles of atoms (10 5 -10 6 ), scales linearly with system size and can also incorporate defects.

The 60 nm-thick Ru(0001) layer was deposited epitaxially onto Al 2 O 3 (0001) by ultrahigh vacuum (UHV) sputter deposition at 500°C followed by a step anneal (ex situ) to 950°C. The Co layer was electrodeposited at room temperature from an acidic electrolyte (pH = 3.8) containing dilute Co metal ions. Previously unreported, evidence for the underpotential deposition (UPD) of Co on Ru(0001) is presented and was shown to affect the nucleation of Co during constant potential electrodeposition. This result demonstrates a strong interaction between Co and the Ru substrate necessary for epitaxial growth. Cross-sectional transmission electron imaging and diffraction confirmed the formation of an epitaxial layer of Co(0001) on Ru(0001) to practical thicknesses for interconnect gap-fill. This finding suggests the plausibility of electrodeposited, single crystal interconnects in future integrated circuit chips.

Co electrodeposition was performed onto single crystal Ru(0001) and polycrystalline Ru films to study the influence of such seed layers on the growth of epitaxial Co(0001). The effect of misfit strain on the electrodeposited Co(0001) films was studied using 60 and 10 nm-thick Ru(0001) seed layers, where the misfit strains of the Co layer on the two Ru(0001) seed layers are 7.9% and 9.6%, respectively. Despite a large misfit strain of 7.9%, the planar growth of Co(0001) was achieved up to a thickness of 42 nm before a transition to island growth was observed. Epitaxial Co films electrodeposited onto 10 nm Ru(0001) showed increased roughness when compared with Co electrodeposited onto the 60 nm seed layer. Co electrodeposition onto polycrystalline Ru resulted in a rough, polycrystalline film with faceted growth. Electrochemical experiments and simulations were used to study the influence of [Co2+] and solution pH on the throughput of the electrodeposition process. By increasing [Co2+] from 1 to 20 mM, the deposition rate of Co(0001) increased from 0.23 nm min−1 to 0.88 nm min−1 at an applied current density of −80 μA cm−2.

The electrodeposition of Cu onto epitaxial Ru(0001) seed layers was investigated from a sulfuric acid-based solution containing dilute copper(II) sulfate and chloride ions. Using galvanostatic deposition at −350 μA/cm2, Cu was deposited epitaxially onto a 30 nm-thick Ru(0001) seed layer, despite a compressive misfit strain between −6.9% and −8.3%, depending on the extent of strain relaxation of the Ru layer. However, rather than depositing as a single crystal, Cu grew as a bicrystal having a common out-of-plane orientation of Cu(111) and two equivalent in-plane orientations. The Cu grain size was large, on the order of micrometers, and the grain boundaries were identified as incoherent ∑3{211} twin boundaries. The Cu initially grew as isolated islands, coalescing into a contiguous film at thicknesses around 50 nm. The Cu film was rough, and thickness and coverage varied over the electrodeposited region. After the initial island growth, Cu void fraction and film roughness both decreased with thickness as the deposit transitioned into a planar film with nanometric islands growing on the film surface. However, at thicknesses exceeding 200 nm, anisotropic growth of large, faceted Cu islands on the planar Cu film again increased the surface roughness. The epitaxially deposited Cu bicrystal showed an improvement in resistivity when compared with polycrystalline Cu similarly electrodeposited onto a polycrystalline Ru seed.