Adsorption and Thermal Decomposition of Alkanethiols on Cu(110)

Self-Formation of a Ru/ZnO Multifunctional Bilayer for the Next-Generation Interconnect Technology via Area-Selective Atomic Layer Deposition

This study suggests a Ru/ZnO bilayer grown using area-selective atomic layer deposition (AS-ALD) as a multifunctional layer for advanced Cu metallization. As a diffusion barrier and glue layer, ZnO is selectively grown on SiO2 , excluding Cu, where Ru, as a liner and seed layer, is grown on both surfaces. Dodecanethiol (DDT) is used as an inhibitor for the AS-ALD of ZnO using diethylzinc and H2 O at 120 °C. H2 plasma treatment removes the DDT adsorbed on Cu, forming inhibitor-free surfaces. The ALD-Ru film is then successfully deposited at 220 °C using tricarbonyl(trimethylenemethane)ruthenium and O2 . The Cu/bilayer/Si structural and electrical properties are investigated to determine the diffusion barrier performance of the bilayer film. Copper silicide is not formed without the conductivity degradation of the Cu/bilayer/Si structure, even after annealing at 700 °C. The effect of ZnO on the Ru/SiO2 structure interfacial adhesion energy is investigated using a double-cantilever-beam test and is found to increase with ZnO between Ru and SiO2 . Consequently, the Ru/ZnO bilayer can be a multifunctional layer for advanced Cu interconnects. Additionally, the formation of a bottomless barrier by eliminating ZnO on the via bottom, or Cu, is expected to decrease the via resistance for the ever-shrinking Cu lines.

Vapor-Phase Cleaning and Corrosion Inhibition of Copper Films by Ethanol and Heterocyclic Amines

Cleaning and passivation of metal surfaces are necessary steps for selective film deposition processes that are attractive for some microelectronic applications (e.g., fully aligned vias or self-aligned contacts). For copper, there is limited knowledge about the mechanisms of the copper oxide reduction process and subsequent passivation layer formation reactions. We have investigated the in situ cleaning (i.e., oxidation and reduction by vapor-phase species) and passivation of chemical-mechanical polishing (CMP)-prepared Cu films in an effort to derive the mechanisms associated with selectively tailoring the surface chemistry. By monitoring the interaction of vapor-phase ethanol with the surface species generated after ozone cleaning at 300 °C, we find that the optimum procedure to remove these species and avoid byproduct redeposition is to use atomic layer deposition (ALD)-like binary cycles of ethanol and N₂, with active pumping. We have further explored passivation of clean Cu using benzotriazole and 2,2'-bipyridine in an ALD environment. Both molecules chemisorb on clean Cu in an upright orientation, with respect to the metal surface at temperatures higher than the melting point of the organic inhibitors (100 ≤ T < 300 °C). Both molecules desorb without decomposition from clean Cu above 300 °C but not from Cu₂O. Previous studies related to the passivation of Cu surfaces using heterocyclic amines have focused on solution-based or ultrahigh vacuum applications of the passivation molecules onto single crystalline Cu samples. The present work explores more industrially relevant vapor-phase passivation of CMP-cleaned, electroplated Cu samples using ALD-like processing conditions and in situ vapor-phase cleaning.

Synthesis of Cu Nanoparticles: Stability and Conversion into Cu2S Nanoparticles by Decomposition of Alkanethiolate

A lean synthesis of copper nanoparticles (Cu NP) from CuCl₂ in dodecane via formation of Cu(I)-dodecanethiolate (Cu(I)-DDT) and their decomposition paths including spontaneous C-S bond cleavage of the alkanethiol on the surface of Cu NP is presented. The reduction of Cu(I)-DDT by the tert-butylamine-borane complex (TBAB) in dodecane under N₂ at elevated temperatures leads to the formation of thiol-protected Cu NP with narrow size distribution in the size range of 3-10 nm depending on the reaction conditions. The Cu NP in the presence of excess dodecanethiol reacts further to Cu₂S NP under decomposition of the ligand on the particle surface. The Cu₂S formation occurs after a short time at T > 175 °C or within ∼12 h at room temperature. If excess thiol is removed immediately after the synthesis, the resulting colloid shows irreversible aggregation within days or hours. Our results suggest that alkanethiols are not long-term stable on nanocopper surfaces and that the formation of copper(I) sulfide under the cleavage of the C-S bond occurs even at room temperature.

Adsorption of single 1,8-octanedithiol molecules on Cu(100)

STM experiments and calculations have allowed identifying the most favorable conformation of a single octanedithiol molecule on a copper surface.

Crystal-bound vs surface-bound thiols on nanocrystals

The use of thiol ligands as a sulfur source for nanocrystal synthesis has recently come en vogue, as the products are often high quality. A comparative study was performed of dodecanethiol-capped Cu2S prepared with elemental sulfur and thiol sulfur reagents. XPS and TGA-MS provide evidence for differing binding modes of the capping thiols. Under conditions where the thiol acts only as a ligand, the capping thiols are "surface-bound" and bond to surface cations in low coordination number sites. In contrast, when thiols are used as a sulfur source, "crystal-bound" thiols result that sit in high coordination sites and are the terminal S layer of the crystal. A (1)H NMR study shows suppressed surface reactivity and ligand exchange with crystal-bound thiols, which could limit further application of the particles. To address the challenge and opportunity of nonlabile ligands, dodecyl-3-mercaptopropanoate, a molecule possessing both a thiol and an ester, was used as the sulfur source for the synthesis of Cu2S and CuInS2. A postsynthetic base hydrolysis cleaves the ester, leaving a carboxylate corona around the nanocrystals and rendering the particles water-soluble.

Synthesis and application of hybrid polymer composites based on silver nanoparticles as corrosion protection for line pipe steel

A facile method was developed to synthesize in high yield dispersed silver nanoparticles (AgNPs) with small particle sizes of less than 10 nm. Silver nitrate was reduced to silver nanoparticles by p-chloroaniline in the presence of polyoxyethylene maleate 4-nonyl-2-propylene-phenol (NMA) as a stabilizer. The produced AgNPs were used to prepare hybrid polymer based on N-isopropylacrylamide (NIPAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-methylenebisacrylamide (MBA) and potassium persulfate (KPS) using a semi-batch solution polymerization method. The prepared AgNPs and hybrid polymer were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM). The corrosion inhibition activity of the AgNPs and hybrid polymer towards steel corrosion in the presence of hydrochloric acid has been investigated by polarization and electrochemical impedance spectroscopy (EIS) methods. Polarization measurements indicate that the AgNPs and hybrid polymer acts as a mixed type-inhibitor and the inhibition efficiency increases with inhibitor concentration. The results of potentiodynamic polarization and EIS measurements clearly showed that the inhibition mechanism involves blocking of the steel surface by inhibitor molecules via adsorption.

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(111)

The adsorption and thermal stability of 2-octylthieno[3,4-b]thiophene (OTTP) on the Au(111) surfaces have been studied using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). UHV-STM studies revealed that the vapor-deposited OTTP on Au(111) generated disordered adlayers with monolayer thickness even at saturation coverage. XPS and TPD studies indicated that OTTP molecules on Au(111) are stable up to 450 K and further heating of the sample resulted in thermal decomposition to produce H(2) and H(2)S via C-S bond scission in the thieno-thiophene rings. Dehydrogenation continues to occur above 600 K and the molecules were ultimately transformed to carbon clusters at 900 K. Highly resolved air-STM images showed that OTTP adlayers on Au(111) prepared from solution are composed of a well-ordered and low-coverage phase where the molecules lie flat on the surface, which can be assigned as a (9×2√33)R5° structure. Finally, based on analysis of STM, TPD, and XPS results, we propose a thermal decomposition mechanism of OTTP on Au(111) as a function of annealing temperature.

The surface chemistry of dimethyl disulfide on copper

The surface chemistry of dimethyl disulfide (DMDS) is studied on a Cu(111) single crystal and a polished copper foil in ultrahigh vacuum as a basis for understanding its tribological chemistry using a combination of temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Low-energy electron diffraction reveals that the polished foil becomes ordered on heating in vacuo and displays identical surface chemistry to that found on the Cu(111) surface. Dimethyl disulfide reacts with the copper surface at 80 K to form thiolate species. Heating the surface to ∼230 K causes a small portion of the thiolate species to decompose to form methyl groups adsorbed on the surface. Further heating results in methane and C(2) hydrocarbon desorption at ∼426 K, due to a reaction of adsorbed methyl species, to completely remove carbon from the surface and to deposit atomic sulfur.

Surface reaction of sulfur-containing amino acids on Cu(110)

Adsorption behaviors of sulfur-containing amino acids, cysteine, methionine, and cystine molecules on Cu(110) surface were studied by core level photoelectron spectroscopy using synchrotron radiation. We found the following through the systematic comparisons of core level peaks such as S 2p, N 1s, and O 1s from different amino acids. At low coverage regimes, all the molecules form two distinct thiolate species, and their S 2p binding energy difference was about 0.9 eV. The relative populations of the two thiolates were different for different molecules and their coverage, which is due to the different bond strength of the sulfur-containing functional groups. At high coverage regimes, only cysteine molecules form zwitterionic state, which is related to the molecular ordering on Cu(110) surface.

Mechanochemistry and thermochemistry are different: stress-induced strengthening of chemical bonds

Most chemical reactions require activation which is conventionally supplied by heat. In stark contrast, mechanical activation by applied external forces opens intriguing novel possibilities. Here, the first direct comparison of mechanical versus thermal activation of bond breaking is provided. Studying both thiolate-copper interfaces and junctions provides evidence for vastly different reaction pathways and product classes. This is understood in terms of directional mechanical manipulation of coordination numbers and system fluctuations in the process of mechanical activation.

Thermal decomposition of HSCH2CH2OH on Cu(111): identification and adsorption geometry of surface intermediates

X-ray photoelectron spectroscopy has been employed to study the surface intermediates from the thermal decomposition of HSCH2CH2OH on Cu(111) at elevated temperatures. On the basis of the changes of the core-level binding energies of C, O, and S as a function of temperature, it is found that HSCH2CH2OH decomposes sequentially to form -SCH2CH2OH and -SCH2CH2O-. Theoretical calculations based on density functional theory for an unreconstructed one-layer copper surface suggest that -SCH2CH2OH is preferentially bonded at a 3-fold hollow site, with an adsorption energy lower than the cases at bridging and atop sites by 15.6 and 47.5 kcal x mol(-1), respectively. Other structural characteristics for the energy-optimized geometry includes the tilted C-S bond (14.1 degrees with respect to the surface normal), the C-C bond titled toward a bridging site, and the C-O bond pointed toward the surface. In the case of -SCH2CH2O- on Cu(111), the calculations suggest that the most probable geometry of the adsorbate has its S and O bonded at hollow and bridging sites, respectively. With respect to the surface normal, the angles of the S-C and O-C are 27.9 and 34.0 degrees.

Structure and Bonding of Alkanethiols on Cu(111) and Cu(100)

The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.

Electronic origin of disorder and diffusion at a molecule-metal interface: self-assembled monolayers of CH3S on Cu111

The relationship between structure, interfacial electrostatics, bonding, and dynamics of organic molecules on metals is studied using a self-assembled monolayer of methylthiolate, CH3S, on Cu(111). The flat adsorption energy landscape of CH(3)S/Cu(111) results from metal-to-molecule charge redistribution which allows for a high mobility of CH3S. This contributes a nonuniform diffuse background to Bragg scattering, which needs to be considered in diffraction analyses. Ramifications on the interpretation of experimental data and the potential impact on the design of metal-organic interfaces are discussed.

Selective attachment of 1,4-benzenedimethanethiol on the copper mediated Si(111)-(7 x 7) surface through S-Cu linkage

The well-defined and patterned copper clusters formed on the Si(111)-(7 x 7) surface have been employed as a template for selective binding of 1,4-benzenedimethanethiol (HS-CH2-C6H4-CH2-SH, 1,4-BDMT), to form ordered molecular nanostructures. Scanning tunneling microscopic (STM) studies showed that each 1,4-BDMT molecule preferentially binds to two neighboring copper atoms within one copper cluster through the S-Cu interaction with its molecular plane parallel to the surface, whereas some 1,4-BDMT bond to individually adsorbed copper atoms, resulting in an upright configuration. Large-scale two-dimensional molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on the Si(111)-(7 x 7) surface.

Adsorption and Thermal Reactions of Alkanethiols on Pt(111): Dependence on the Length of the Alkyl Chain

The adsorption and thermal decomposition of alkanethiols (R-SH, where R = CH3, C2H5, and C4H9) on Pt(111) were studied with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. Dissociation of sulfhydryl hydrogen (RS-H) of alkanethiol results in the formation of alkanethiolate; the extent of dissociation at an adsorption temperature of 110 K depends on the length of the alkyl chain. At small exposure, all chemisorbed CH3SH, C2H5SH, and C4H9SH decompose to desorb hydrogen below 370 K and yield carbon and sulfur on the surface. Desorption of products containing carbon is observed only at large exposure. In thermal decomposition, alkanethiolate is proposed to undergo a stepwise dehydrogenation: R'-CH2S --> R'-CHS --> R'-CS, R' = H, CH3, and C3H7. Further decomposition of the R'-CS intermediate results in desorption of H2 at 400-500 K and leaves carbon and sulfur on the surface. On the basis of TPD and XPS data, we conclude that the density of adsorption of alkanethiol decreases with increasing length of the alkyl chain. C4H9SH is proposed to adsorb mainly with a configuration in which its alkyl group interacts with the surface; this interaction diminishes the density of adsorption of alkanethiols but facilitates dehydrogenation of the alkyl group.

Surface reactions of 1-propanethiol on GaAs(100)

The adsorption and decomposition pathways of 1-propanethiol on a Ga-rich GaAs(100) surface have been investigated using the techniques of temperature programmed desorption, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). 1-Propanethiol adsorbs dissociatively on a clean GaAs(100) surface to form propanethiolate and hydrogen. Further reactions of these species to form new products compete with the recombinative desorption of molecular propanethiol. The C-S bond scission in the propanethiolate results in the formation of propyl species and elemental sulfur. The generation of propene via beta-hydride elimination then follows. In addition, propane and hydrogen form via reductive elimination processes. A recombinative high-temperature propanethiol desorption state is also observed. XPS and TOF-SIMS analyses confirm the presence of sulfur on the GaAs(100) surface following thermal decomposition. This paper discusses the mechanisms by which these products form on the GaAs(100) surface.

Halogen-substituted thiophenol molecules on Cu(111)

Para-halosubstituted thiophenols (X-TPs, where X is Br, Cl, or F) form ordered islands and monolayers on Cu(111) at temperatures as low as 81 K. At incomplete coverages, all X-TPs adsorb with the dehydrogenated thiol group attached to the substrate and the substituted ring inclined toward the surface, as verified experimentally and theoretically. The structure of ordered islands has a pronounced dependence on the nature of the halogen substituent: while unsubstituted TP and pentafluoro-TP molecules do not self-assemble into extended ordered patterns at 81 K, X-TP molecules form a range of different structures which depend both on the size and electronegativity of the substituent, as well as on the coverage.

Dimethyl methylphosphonate decomposition on Cu surfaces: supported Cu nanoclusters and films on TiO2(110)

The thermal decomposition of dimethyl methylphosphonate (DMMP), which is a simulant molecule for organophosphorus nerve agents, has been investigated on Cu clusters as well as on Cu films deposited on a TiO(2)(110) surface. Scanning tunneling microscopy studies were conducted to characterize the cluster sizes and surface morphologies of the deposited Cu clusters and films. Temperature-programmed desorption experiments demonstrated that the surface chemistry of DMMP is not sensitive to the size of the Cu clusters over the range studied in this work. DMMP reaction on an annealed 40 monolayer Cu film resulted in the desorption of H(2), methane, methyl, formaldehyde, methanol, and molecular DMMP, and reaction on the small (4.4 +/- 0.9 nm diameter, 1.8 +/- 0.6 nm height) and large (10.7 +/- 1.9 nm diameter, 4.8 +/- 1.0 nm height) Cu clusters generated similar products. Formaldehyde and methane production is believed to occur via a methoxy intermediate on the Cu surface. These products are favored on the higher coverage Cu films that completely cover the TiO(2) surface since competing reaction pathways on TiO(2) are suppressed. X-ray photoelectron spectroscopy studies showed that DMMP begins to decompose on the Cu clusters upon adsorption at room temperature and that atomic carbon, atomic phosphorus, and PO(x) remain on the surface after DMMP decomposition.

Low-temperature mobility and structure formation of a prochiral aromatic thiol (2,5-dichlorothiophenol) on Cu(111)

We present a low-temperature scanning tunneling microscopy study of increasing coverages of 2,5-dichlorothiophenol, an asymmetrically halo-substituted aromatic thiol, on Cu(111). At low coverage, deprotonation of the thiol occurs spontaneously upon adsorption at 80 K. Albeit the low deposition temperature, we find the formation of adsorbate islands at low coverage, which coalesce into a well-ordered film of horizontally adsorbed molecules at increasing coverage. This behavior indicates (i) significant mobility of the thiols on Cu(111) even at low temperatures and (ii) attractive adsorbate-adsorbate interactions. At higher coverages intermolecular interactions prevent long-range diffusion of adsorbates and thermal activation of the S-H bond becomes necessary. A close analysis of the molecular films reveals chiral recognition between neighboring molecules, which leads to the formation of enantiopure areas on the surface. Upright orientation of individual molecules starts at the boundaries between such phases and can be induced by scanning tunneling microscopy.