Atomic layer deposition of ultrathin Cu2O and subsequent reduction to Cu studied by in-situ X-ray photoelectron spectroscopy
Abstract: The growth of ultrathin (< 5 nm) Ru-doped Cu2O films deposited on SiO2 by atomic layer deposition (ALD) and Cu films by subsequent reduction of the Cu2O using HCOOH or CO is reported. Ru-doped Cu2O has been deposited by a mixture of 99 mol% of [(nBu3P)2Cu(acac)] as Cu precursor (1) and 1 mol% of [Ru(η5‑C7H11) (η5‑C5H4SiMe3)] as Ru precursor (2). The catalytic amount of Ru precursor was to support low temperature reduction of Cu2O to metallic Cu by formic acid (HCOOH) on arbitrary substrate. In-situ XPS investigations of the Cu2O ALD film indicated nearly 1 at-% of carbon contamination and a phosphorous contamination below the detection limit after sputter cleaning. Systematic investigations of the reduction of Ru-doped Cu2O to metallic Cu by HCOOH or CO as reducing agents are described. Following the ALD of 3.0 nm Cu2O, the ultrathin films are reduced between 100 °C and 160 °C. The use of HCOOH at 110 °C enabled the reduction of around 90% Cu2O. HCOOH is found to be very effective in the removal of oxygen from Ru‑doped Cu2O films with 2.5–4.7 nm thickness. In contrast, CO was effective for the removal of oxygen from the Cu2O films only below 3.0 nm at 145 °C. Root mean square surface roughness of 0.4±0.1 nm was observed from AFM investigations after the ALD of Cu2O, followed by the subsequent reduction of 3.0 nm Cu2O using either HCOOH at 110 °C or CO at 145 °C on SiO2. Furthermore, ex‑situ LEIS and AFM investigations confirmed that the Cu2O film after ALD and Cu films after subsequent reduction was continuous on the SiO2 substrate.
Surface chemistry of a Cu(I) beta-diketonate precursor and the atomic layer deposition of Cu2O on SiO2 studied by x-ray photoelectron spectroscopy
Abstract: The surface chemistry of the bis(tri-n-butylphosphane) copper(I) acetylacetonate, [(nBu3P)2Cu(acac)] (1) and the thermal atomic layer deposition (ALD) of Cu2O using this Cu precursor as reactant and wet oxygen as coreactant on SiO2 substrates are studied by in-situ x-ray photoelectron spectroscopy (XPS). The Cu precursor was evaporated and exposed to the substrates kept at temperatures between 22 °C and 300 °C. The measured phosphorus and carbon concentration on the substrates indicated that most of the [nBu3P] ligands were released either in the gas phase or during adsorption. No disproportionation was observed for the Cu precursor in the temperature range between 22 °C and 145 °C. However, disproportionation of the Cu precursor was observed at 200 °C, since C/Cu concentration ratio decreased and substantial amounts of metallic Cu were present on the substrate. The amount of metallic Cu increased, when the substrate was kept at 300 °C, indicating stronger disproportionation of the Cu precursor. Hence, the upper limit for the ALD of Cu2O from this precursor lies in the temperature range between 145 °C and 200 °C, as the precursor must not alter its chemical and physical state after chemisorption on the substrate. Five hundred ALD cycles with the probed Cu precursor and wet O2 as coreactant were carried out on SiO2 at 145 °C. After ALD, in-situ XPS analysis confirmed the presence of Cu2O on the substrate. Ex-situ spectroscopic ellipsometry indicated an average film thickness of 2.5 nm of Cu2O deposited with a growth per cycle of 0.05 Å/cycle. Scanning electron microscopy and atomic force microscopy (AFM) investigations depicted a homogeneous, fine, and granular morphology of the Cu2O ALD film on SiO2. AFM investigations suggest that the deposited Cu2O film is continuous on the SiO2 substrate.
In-situ XPS Investigation of ALD Cu2O and Cu Thin Films after Successive Reduction
Presented in the 14th International Conference in Atomic Layer Deposition, Kyoto, Japan, June 2014
Abstract: Atomic Layer Deposition (ALD) is emerging as a ubiquitous method for the deposition of conformal and homogeneous ultra-thin films on complex topographies and large substrates in microelectronics. Electrochemical deposition (ECD) is the first choice for the deposition of copper (Cu) into the trenches and vias of the interconnect system for ULSI circuits. The ECD of Cu necessitates an electrically conductive seed layer for filling the interconnect structures. ALD is now considered as a solution for conformal deposition of Cu seed layers on very high aspect ratio (AR) structures also for technology nodes below 20 nm, since physical vapor deposition is not applicable for structures with high AR. Cu seed layer deposition by the reduction of Cu2O, which has been deposited from the Cu(I) β-diketonate [(nBu3P)2Cu(acac)] (1) used as Cu precursor, has been successfully carried out on different substrates like Ta, TaN, SiO2, and Ru [1, 2]. It was found that the subsequent gas-phase reduction of the Cu2O films can be aided by introducing catalytic amounts of a Ru precursor into the Cu precursor, so that metallic copper films could potentially obtained also on non-catalytic substrates [3, 4]. In this work, in situ X-ray photoelectron spectroscopy (XPS) investigation of the surface chemistry during Cu2O ALD from the mixture of 99 mol% of 1 and 1 mol% of [Ru(η5 C5H4SiMe3)(η5-C7H11)] (2) as ruthenium precursor, and the reduction of Cu2O to metallic Cu by formic acid carried out on SiO2 substrate are demonstrated. Oxidation states of the Cu in the film are identified by comparing the Cu Auger parameter (α)  with literature data. α calculated after ALD equals 362.2 eV and after reduction equals 363.8 eV, comparable to the Cu2O and metallic Cu in thin-films  respectively. In addition, <10 % of Cu(I), Cu(II), and Cu(OH)2 species are identified from the Cu 2p3/2 and Cu L3VV Auger spectrum after reduction. Consequently, the ALD Cu2O is successfully reduced to metallic copper by in-situ thermal reduction using HCOOH.
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