Copper Coverage Research Articles

Adsorption of oxygen on ultrathin Cu/Pt(111) films

Adsorption of oxygen on ultrathin Cu/Pt(111) films was studied using Auger electron spectroscopy, low-energy electron diffraction, and quadrupole mass spectroscopy. Comparing the oxygen-saturated copper films, the density of adsorbed oxygen is doubled as the copper coverage is reduced from 2 to 0.4 monolayers. In the submonolayer range, the more edge sites at the copper islands and larger Cu–Cu distances due to pseudomorphic growth provide a higher possibility for the adsorption of oxygen. A different adsorption rate of oxygen was resolved for the exposure of copper films to oxygen. As oxygen starts to incorporate into the film, the adsorption rate decreases. For a film prepared by alternate steps of copper deposition and oxygen adsorption, the oxygen distribution is not homogeneous from sputter profiling measurements. The amount of oxygen at both interfaces is higher than within the film. From the study of the annealing effects for Cu/Pt(111), the onset of diffusion for Cu adatoms into the Pt(111) substrate is slightly influenced by oxygen.

An X-ray absorption spectroscopy study of the structure and reversibility of copper adsorbed to montmorillonite clay

X-ray absorption spectroscopy (XAS) and adsorption-desorption measurements have been performed to assess the relationship between the structure and reversibility of copper complexes on montmorillonite clay. By varying the solution pH and background electrolyte concentration, the adsorption of copper on either the edge sites or permanent charge sites of montmorillonite was controlled. This allowed the structure and reversibility of copper complexes on each of these site types to be assessed independently of each other. XAS analysis of copper adsorbed on the permanent charge sites indicated outer-sphere surface complexes, with these complexes showing sorption reversibility. For copper complexes formed on the edge sites of montmorillonite, XAS data confirmed the presence of monomer and dimer copper surface complexes. Sorption irreversibility at edge sites was noted at copper coverages less than 20 μmoles/g clay at pH=4.2 and at coverages greater than 50 μmoles/g clay at pH=6.8. At pH=6.8, higher Cu-Cu coordination numbers indicated the copper sorption irreversibility may be due, in part, to the formation of dimer surface complexes. The coordination numbers at pH=4.2 indicated the irreversibility could be due to the formation of dimers or due to formation of surface complexes on high energy edge sites.

Underpotential Deposition on Alloys

Results from underdeposition experiments on alloy surfaces offer the exciting possibility of providing information on site-specific chemical interactions between an adsorbate and one of the constituents in the alloy. Eventually, this information may be useful in developing a quantitative understanding of how site-specific chemical interactions influence such phenomena as electrocatalysis or dealloying. Herein, we report the first such set of results. We examined the underpotential deposition of copper on polycrystalline alloys. Copper is known to underpotentially deposit on elemental Au electrodes, but not on Ag electrodes. We found that the charge associated with the adsorption process decreased with increasing Ag atomic fraction in the alloy. No measurable amount of copper adsorption was detected for The peak potential of the copper stripping waves shifted toward the metal/metal ion equilibrium electrode potential with increasing Ag atomic fraction in the alloy. A percolation cluster model was used to explain the underpotential deposition of copper in the system investigated. According to the model each Ag atom blocks three adsorption sites in the Au-rich composition range. In Ag-rich alloys an adsorption site must be formed by at least three Au atoms. A theoretical dependence for the copper coverage as a function of alloy composition was derived and compared to the experimental results. © 2001 The Electrochemical Society. All rights reserved.

Multiprobe End-Point Detection for Precision Control of the Copper CMP Process

ABSTRACTSentinelä is an optical multiprobe endpoint detection (EPD) system that has been developed by SpeedFam-IPEC (SFI) for the 200-mm and 300-mm Momentum orbital polishers. Sentinel EPD for copper CMP is based on monitoring the visible spectrum of light from a Xenon (Xe) flash lamp, which has been reflected by the wafer surface during polish. Large representative areas of the wafer are monitored from multiple locations in the pad continuously throughout the polish process. Spectra of reflected light from cleared and uncleared copper on a pattern wafer demonstrate a clear transition from copper to the underlying dielectric layer.With the Sentinel EPD system, the wafer is probed at optimized multiple locations constantly throughout the polish process by small-diameter light spots. The coverage of the monitoring is extensive and the radial coverage is substantial on a real-time basis. The EPD call is made based on an EPD signal, which displays the fractional copper coverage as a function of polish time. The detection of copper clearing on the SFI Momentum tools is very reliable and predictable, and not sensitive to process changes.

Cu interactions with α-Al 2O 3(0001): effects of surface hydroxyl groups versus dehydroxylation by Ar-ion sputtering

X-ray photoelectron spectroscopy studies and first principles calculations compare Cu adsorption on heavily hydroxylated α-Al 2O 3(0001) with dehydroxylated surfaces produced by Ar + sputtering followed by annealing in O 2. Annealing a cleaned sapphire sample with an O 2 partial pressure of ∼5×10 −6 Torr removes most contaminants, but leaves a surface with ∼0.4 ML (ML: monolayer) carbon and ∼0.4 ML OH. Subsequent light (6 min) Ar-ion sputtering at 1 keV reduces the carbon to undetectable levels but does not dehydroxylate the surface. Further sputtering at higher Ar-ion excitation energies (>2 keV) partially dehydroxylates the surface, whereas 5 keV Ar-ion sputtering creates oxygen vacancies in the surface region. Further annealing in O 2 repairs the oxygen vacancies in the top layers, but those beneath the surface remain. Deposition of Cu on the hydroxylated surface at 300 K results in a maximum copper(I) coverage of ∼0.35 ML, in agreement with theoretical predictions. Maximum copper(I) coverage at 300 K decreases with decreasing surface hydroxylation. Exposure of a partially dehydroxylated sapphire(0001) surface to either 2 Torr H 2O vapor or air results in recovery of surface hydroxylation, which in turn increases the maximum copper(I) coverage. These results demonstrate that the hydroxyl surface coverage critically affects the ability of Cu to ‘wet’ sapphire(0001) at 300 K. In addition, first principles density functional calculations show that while an ad-OH species stabilizes copper(I) at room temperature, any nearby in-surface OH does the opposite.

Behavior of Cu 0.6Al 0.4 films at the SiO 2 interface

The interaction of a sputter-deposited Cu 0.6Al 0.4 film with SiO 2 (Cu 0.6Al 0.4/SiO 2) is investigated using X-ray photoelectron spectroscopy (XPS) and ex-situ scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX) capability. The data indicate that aluminum segregates to the SiO 2 interface and becomes oxidized. The formation of the aluminum oxide at the interface results in stronger chemical interaction of copper with the substrate. For copper coverages less than ∼0.31 ML (based on a Cu/O atomic ratio), only Cu(I) formation is observed. At higher coverages, Cu(0) is observed. The change in slope is coincident with the change in copper oxidation state. The adhesion behavior of copper in the Cu 0.6Al 0.4/SiO 2 case is similar to that of copper metal sputter-deposited onto α-Al 2O 3(0001). These data are in contrast with the observed behavior of copper metal deposited onto SiO 2 (Cu/SiO 2). The data for Cu/SiO 2 show that copper does not wet SiO 2 and forms 3-D nuclei. Furthermore, post-annealing experiments performed on Cu 0.6Al 0.4/SiO 2 show that neither de-wetting nor diffusion of copper occurs for temperatures up to 800 K. The data indicate that aluminum alloyed with copper at the SiO 2 interface serves as an effective adhesion promoter and thermal diffusion barrier.

Interactions of copper with oxidized TaSiN

Tantalum rich TaSiN exposed to the ambient results in a homogenous surface composition of a silicon rich Ta x Si y O z mixture. The interaction of sputter deposited copper with oxidized TaSiN (O/TaSiN) is investigated using X-ray photoelectron spectroscopy (XPS). Copper is found to wet (grow conformally on) O/TaSiN at 300 K. For copper coverages of less 0.4 ML (based on the Cu to O atomic ratio), copper is present as Cu(I). At higher coverages, Cu(0) is observed. A change in slope of the copper coverage curve is coincident with the change in copper oxidation state. The data indicate that copper is initially deposited in a conformal ionic layer followed by Cu(0) formation in subsequent depositions. The data also show that although the O/TaSiN surface contains significant amounts of silicon and oxygen, the ability of copper to wet O/TaSiN is superior to that of SiO 2. Post-deposition annealing experiments performed indicate that although diffusion does not occur for temperatures less than 900 K, copper “de-wetting” occurs for temperatures above 500 K.

Energy and temperature dependent sticking coefficients of CO on ultrathin copper layers on Ru(001)

The chemisorption of CO on ultrathin copper layers on Ru(001) has been investigated using a supersonic molecular beam and temperature programmed desorption. We are comparing the adsorption kinetics of CO on clean Ru(001) with those on a pseudomorphic Cu monolayer and a fully relaxed Cu(111) multilayer (>40 layers) on Ru(001). For all three surfaces, a decrease of the initial sticking coefficient, S 0, with increasing beam energy was observed. For beam energies below 0.3 eV S 0 is highest on the Cu monolayer, even though the binding energy of CO is significantly lower than on Ru(001). For higher energies S 0 scales with the CO binding energy on the different substrates. The dependence of S 0 of a low energy CO beam (0.09 eV) on the copper coverage is consistent with the Stranski–Krastanov growth mode of Cu on Ru(001) that is known from STM measurements.

Surface Chemistry of CH3Br and Methyl Modified by Copper Deposition on Ru(001)

The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing work function change (Δφ) and temperature-programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydrogenation at the completion of a single copper layer and the significant difference in reactivity of the Cu(2 ML)/Ru(001) or Cu(111) surfaces are the focus of this study. A decrease in work function at the completion of 1 ML CH3Br of 2.15 ± 0.02 eV and 1.33 ± 0.05 eV was measured, respectively, for Ru(001) and Cu(2 ML)/Ru(001) held at 82 K. Methyl bromide does not dissociate upon adsorption on clean or the copper-covered surfaces, and it is bound with the bromine down. Copper modifies the reactivity of the Ru substrate, gradually decreasing the dissociated fraction of CH3Br from 0.55 of the initial one monolayer on clean Ru(001) to 0.06 on Cu(2 ML)/Ru(001), probably because of defects in the copper layer. The methyl fragment dehydrogenation rate slows as the copper coverage increases. At a narr...

Cluster variation study of the underpotential deposition of Cu on Au(111) in the presence of bisulfate

A cluster variation method is developed to study the phase transitions and the structures of phases which occur at the fluid-crystal interface during the underpotential deposition of a metal on an electrode in the presence of an anion, such as bisulfate. In addition to the possibility of first-order condensation phase transitions occurring during the deposition of a metal, the steric repulsion of adsorbed anions can also cause an order-disorder transition. Using clusters containing six adsorption sites, the method is applied to the underpotential deposition of copper on (111) gold in the presence of bisulfate. In order to fix a constant in the expression for the entropy, the effect of the hard-core exclusion of a pair of first-neighbor bisulfates, in addition to the effect of finite interactions, is calculated exactly in the limit of high temperature. The cluster method yields two coupled adsorption isotherms for copper and bisulfate in terms of their activities and coverages. The resulting isotherms show an order-disorder transition due to the hard-core exclusion of neighboring bisulfate ions, as well as two first-order phase transitions in the copper and bisulfate coverages which correspond to the two spikes in the experimental voltammogram. The cluster method also gives the local structure of the phases which occur as the voltage is changed.

Kinetic study of CO oxidation on copper modified Pt(111)

The kinetics of CO oxidation on Pt(111) modified with submonolayer copper adlayers were studied using mass spectrometry (MS) and photoemission electron microscopy (PEEM). The submonolayer 2D copper adlayers were formed on approximately half of the Pt(111) crystal, by evaporating copper through a mask. More precisely, the article deals with the modifications of the hysteresis in the reaction rate compared to the hysteresis occurring under certain reaction conditions on pure Pt(111) when the CO pressure is cyclically varied. When copper was evaporated onto the Pt crystal, the kinetic phase transition between the high and low rate regimes shifted to higher CO pressures and the reaction rate increased. In addition, the CO pressure range in which bistability was observed became wider. Upon increasing the copper coverage, the shift showed a maximum at a coverage of 0.3 ML until at 0.7 ML no clear first-order transition in the reaction rate could be observed any more. The results suggest that the active site responsible for the increased activity lies at the copper–platinum bimetallic border sites.

Electrochemical characterisation of a semiconductor–metal junction: n-type InP|Cu; influence of the structure of the metallic layer

Copper electrodeposition was performed on an n-type InP electrode at two different potentials: −0.55 and −0.90 V (MSE). For each deposition potential value, the compactness of the deposit is discussed, on the basis of SEM observations and line profile analysis of the surface of the coated electrode. A clear difference in the morphology of the layer appeared and a more homogeneous film was observed, when the deposition was performed in the higher overpotential conditions (at −0.9 V). Copper deposits obtained in an acidic solution were then reoxidized in an alkaline medium (10−2 M KOH), and two different voltammograms could be observed for a given copper amount depending on the initial deposition potential. This difference poses the problem of the nature of the path of the interfacial charge transfer on a semiconducting InP sample with homogeneous or heterogeneous copper coverage.

Study of copper underpotential deposition on Au(111) surfaces

First-principles total energy calculations are carried out to study the structure of copper underpotential deposition on Au(111) surfaces in sulfuric acid solutions. The norm-conserving method is used to construct the pseudopotentials of all the elements involved. The copper adlayer structure under various copper coverage is investigated. The results show that the proposed honeycomb structure with 2/3 monolayer copper coverage is unstable without the co-adsorption of sulfate. The co-adsorbed sulfate is found to bind to copper. The calculated structural parameters are in general agreement with those obtained from a recent X-ray experiment. In addition, the sulfate adsorption on clean Au(111) surface is studied. The results show that sulfate molecule binds much more weakly with clean Au(111) surfaces. Total energy calculations for bisulfate adsorption suggest that even though it is the dominant species in acidic electrolyte, the adsorbed bisulfate may dissociate thus leave sulfate adsorbed on the surface.

Adhesion improvement of epoxy resin/copper lead frame j oints by azole compounds

The adhesion strength of epoxy resin/copper joints is often very poor, due to the naturally formed copper oxide having a low mechanical strength. To improve the adhesion strength of epoxy resin/copper lead frame joints, copper lead frames were treated with azole compounds as adhesion promoters. The azole compounds used were benzotriazole (BTA), benzotriazole-5-carboxylic acid (CBTA), 8-azaadenine, imidazole, 2-methyl imidazole, urocanic acid, adenine, benzimidazole, and polybenzimidazole (PBI). The dependence of the adhesion strength of epoxy resin/azole-treated copper joints on the structure of the azole compound, the azole treatment time, and the azole treatment temperature was investigated. The surface coverage of azole-treated copper was examined by contact angle measurements, a surface defect test, optical microscopy, and scanning electron microscopy (SEM), and the locus of failure was studied by X-ray photoelectron spectroscopy (XPS). Triazole compounds such as CBTA and 8-azaadenine showed excellent adhesion strength; imidazole-based azole compounds did not improve the adhesion strength. However, the adhesion strength of CBTA- and 8-azaadenine-treated joints decreased with increasing treatment time, since thick porous Cu-azole complexes had a weaker mechanical strength when formed. The polymeric azole compound PBI showed the highest adhesion strength, 785 N/m, because of complete coverage of the copper surface. The thermal stability of azole compounds and epoxy resin/azole-treated copper joints was also investigated. CBTA and 8-azaadenine did not decompose up to 250°C, while PBI was stable up to 500°C in an air atmosphere.

Mechanism of Copper Deposition on Silicon from Dilute Hydrofluoric Acid Solution

Metal deposition on silicon from HF‐based solutions is initiated by electrochemical reduction of metal ions, a process which is driven by the difference between the electron quasi Fermi energy in the silicon, EFn, and the redox energy level of the ions in solution, . Mechanisms for metal ion reduction are elucidated by aligning the silicon bands with the redox levels of ions in solution. For copper, the reduction reaction occurs by capture of conduction band electrons, a process which requires nucleation of nanometer‐sized precipitates on the silicon surface. As the concentration of electrons at the surface, ns, is increased (e.g., by n‐type doping, illumination, or cathodic biasing), the nucleation rate of precipitates, N, increases steeply. N is further dependent on the activity of the metal ion in solution and the amount of surface charge at the Si/HF interface. The growth rate, v, of copper nuclei is determined by the surface concentration of holes, ps, and is increased by illumination and p‐type doping. The predicted effects on N and v of illumination level, doping level, and substrate bias were verified by immersion tests on Czochralski wafers in 1:100 HF. Total reflection x‐ray fluorescence was used to measure copper coverage; atomic force microscopy and surface‐sensitive minority carrier lifetime measurements were used to determine the areal density of copper precipitates. Application of these findings to reduce copper deposition from HF in industrial wafer cleaning practice are discussed.

Electrodeposition of copper on plasma and thermally oxidized titanium substrates

The effect of plasma and thermal oxidation of titanium cathode blanks on copper electrodeposition was studied. Polarization, potential and current step experiments were performed to evaluate electrochemical changes caused by the oxidation treatments. Scanning electron microscopy, Auger electron spectroscopy and X-ray diffraction were used to characterize the titanium substrates and the deposited copper films. Particular emphasis was given to the initial stages of copper nucleation and growth. The results showed that copper electrocrystallization was strongly influenced by the condition of the oxidized titanium surface. High density, uniform copper coverage was found on titanium substrates oxidized at 400 and 500 °C for 3 h under plasma or low oxygen pressure (80 m Torr) environments. The copper nucleation density and uniformity decreased considerably on titanium surfaces oxidized at 100 and 200 °C. Attempts were made to identify changes in the mechanism of copper nucleation on the oxidized titanium substrates using models developed by Thirsk and Harrison ( A Guide to the Study of Kinetics, Academic, New York, 1972 , p. 115).

Irradiation Effects During XPS Studies of Cu(II) Activation of Zinc Sulphide

The surfaces of zinc sulphide particles after copper(II) activation at pH 9 have been studied by x-ray photoelectron spectroscopy. The influence of x-ray irradiation time on the forms and intensities of the photoelectron signals from zinc sulphide particles activated at various copper coverages was investigated. High binding energy components and satellites of the Cu 2p signals, characteristic of copper(II) hydroxide overlayers, reduced in intensity with increased x-ray exposure. An associated reduction in the concentration of surface oxygen was also observed. Differential charging analysis and curve fitting reveal the dehydration of Cu(OH) 2 to CuO, followed by photoreduction of CuO to Cu 2 O. No evidence was found for x-ray irradiation enhancement of continued Cu activation of or diffusion into ZnS.

Surface diffusion of H and CO on Cu/Ru(001): evidence for long-range trapping by copper islands

Abstract The surface diffusion of H and Co on Cu/Ru(001) was studied using laser-induced thermal desorption (LITD) techniques. The surface mobilities of H and CO on the Ru(001) surface were both dramatically attenuated by submonolayer copper coverages. At 300 K and Θ H =0.25 ML, the H diffusion coefficient decreased by a factor of ∼30 from D H =8.7 × 10 −7 cm 2 /s on clean Ru(001) to D H =3.0 × 10 −8 cm 2 /s at Θ Cu = 0.09 ML. At fixed copper coverages of Θ Cu = 0.10 ML and Θ Cu = 0.25 ML, the hydrogen diffusion coefficient was negligible at low hydrogen coverages and then increased at higher hydrogen coverages. The copper coverages also increased the hydrogen diffusion activation barrier from E H = 2.8 ± 0.2 kcal mol on clean Ru(001) to E H = 9.8 ± 1.0 kcal mol at Θ Cu = 0.15 ML . The dependence of hydrogen mobility on hydrogen coverage and the increase in diffusion activation barrier at higher copper coverages suggest that hydrogen is trapped by copper on Ru(001). The copper coverage on Ru(001) also reduced the CO diffusion coefficient. At 300 K and Θ CO = 0.41 ML, the CO diffusion coefficient decreased by a factor of 2400 as the copper coverage increased from Θ Cu = 0.00 ML to 0.10 ML. Recent scanning tunneling microscope (STM) studies have revealed that copper forms islands on Ru(001). Monte Carlo simulations of these surface diffusion results are consistent with a trapping interaction between H and the copper islands. The Monte Carlo results also argue for a very long-ranged perturbation of ∼200 A induced by the copper islands. These experimental and theoretical results for dramatic and long-ranged trapping indicate that copper seriously perturbs the surface electronic properties of Ru(001).

XPS, AES and LEED studies of Cu deposited on Cr 2O 3(0001) surfaces

The interaction between ultrathin layers of Cu and Cr 2O 3(0001) surfaces was studied by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy and low-energy electron diffraction (LEED). The XPS results indicate that copper deposited on the Cr 2O 3(0001) surfaces is initially in a Cu(I) state, and an agglomeration of copper on the Cr 2O 3(001)-1 × 1 surface was seen with increasing copper coverage. A Cu(111)R30° Cr 2O 3 (0001)-1 × 1 epitaxial relationship was observed by LEED.

Anion effects and the mechanism of Cu UPD on Pt(111): X-ray and electrochemical studies

We propose a mechanism for the underpotential deposition of Cu on Pt(111) in 0.1M H 2SO 4 in the presence and absence of halides. The mechanism is based on recent results from electrochemical and in situ surface EXAFS and X-ray standing wave (XSW) studies of Cu UPD on Pt on 0.1M H 2SO 4 and in the presence and absence of halides with emphasis on Cl −, Br − and I −. EXAFS data were obtained in the presence and absence of Cl − at a potential of +0.1 V corresponding to a coverage of approximately 0.75 ML. In the absence of chloride, the data were consistent with the presence of an incompletely discharged copper adlayer. The copper—copper bond distance was found to be 2.85Å. In the presence of chloride the X-ray data suggest the presence of a fully discharged copper layer and the Cu Cu bond distance, 2.59Å, approached the bulk copper value (2.56Å). In the presence of chloride, no oxygen is present as a backscatterer in the plane of the Cu Cu adlayer. However, oxygen (from either bisulfate or water) is present as a backscatterer in the absence of chloride giving rise to a copper—oxygen bond distance of 2.16Å. It appears that the chloride acts as a protective overlayer precluding oxygen (from either solvent or electrolyte) adsorption. Qualitatively similar results were obtained for Cu UPD on an iodine treated Pt surface. In addition, XSW data for this last system suggest the presence of electrochemically inactive (solvated) copper ions in the vicinity of but not in contact with the platinum surface which we describe as representing a “pre-adsorbed” state. From electrochemical studies at low (10 μM) copper concentrations, we find that the underpotential deposition of submonolayer amounts of copper induces an enhanced adsorption of chloride and bromide on Pt(111) that is reflected in exceedingly sharp voltammetric peaks that are transient in nature. The adsorbed anions are believed to be in contact with the platinum surface and in the vicinity of the electrodeposited copper. We ascribe this enhanced adsorption to the formation of a CuX adlayer on the surface of the electrode. These exceedingly sharp peaks increase in amplitude for copper coverages of up to half a monolayer and subsequently decrease so that upon completion of the copper monolayer they are absent. Based on these findings and on previous investigations, we propose a mechanism for the underpotential deposition of copper on Pt(111) which involves a “pre-adsorbed” state, the transient formation of a CuAn (An −x =anion) adlayer which in turn gives rise to a copper monolayer which itself is covered by an anion-adlayer.