J.A. Kelber

Cu interactions with α-Al2O3(0001): effects of surface hydroxyl groups versus dehydroxylation by Ar-ion sputtering

XPS studies and first principles calculations compare Cu adsorption on heavily hydroxylated sapphire (0001) with a dehydroxylated surface produced by Ar{sup +} sputtering followed by annealing in O{sub 2}. Annealing a cleaned sapphire sample with an O{sub 2} partial pressure of {approximately}5 x 10{sup {minus}6} Torr removes most contaminants, but leaves a surface with {approximately}0.4ML carbon and {approximately}0.4ML 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, while 5 KeV Ar ion sputtering creates oxygen vacancies in the surface region. Further annealing in O{sub 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 Cu(I) coverage of {approximately}0.35 ML, in agreement with theoretical predictions.

Cu-metal interfacial interactions during metal organic chemical vapour deposition

Abstract The interactions of Cu(II) bishexafluoroacetylacetonate (Cu11(hfac)2) with polycrystalline Ta surfaces under ultrahigh vacuum conditions are reported here for temperatures between 115 and 1000 K. Cu11(hfac)2 adsorbed at 115 K is reduced to Cu(0) by 450 K without the use of an external reducing agent. The reduction occurs without disproportionation of Cu(I) intermediates. This behavior is unique to Ta and differs sharply from reported results for TiN or other metallic substrates. The Cu overlayer is stable on the Ta surface to 750 K. Between 750 and 950 K a decrease in Cu intensity is observed which becomes more pronounced above 950 K. This decrease in Cu intensity is due to diffusion of Cu into the Ta substate.

STM, LEED and AES studies on the oxidation of a CrN overlayer on Fe0.8Cr0.2(100)

Abstract A combination of Scanning Tunneling Microscopy (STM), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) was used to study the CrN surface phase grown on a Fe0.8Cr0.2(100) substrate. The co-segregation of Cr and N was achieved by annealing the Fe0.8Cr0.2(100) crystal to 833 K. LEED data of the annealed surface showed a sharp (1 × 1) pattern corresponding to the segregated CrN phase. STM images showed that the CrN overlayer is rough and inhomogeneous, exhibiting many deviations from ideal regularity. Steps were observed with terrace widths ranging from 5 to 65 nm. The step heights are in the range 0.2–3.5 nm. Most of the steps are oriented preferentially in the 〈100〉 direction. Oxidation of the surface results in disordering of the CrN layer (1 × 1) LEED pattern. STM measurements revealed oxide island formation at saturation coverage. Our Auger data suggest that a Cr oxide overlayer forms over the surface nitrogen. Auger spectra indicated that exposure to O2 results only in the oxidation of Cr while the underlying Fe is still metallic. Tunneling current-voltage measurements and Auger spectra at different O coverages can be correlated with the surface oxidation behavior of the CrN overlayer. Comparison of the CrN(1 × 1) layer with a N-depleted surface indicates that surface N retards oxidation at 300 K.

Seedless electrodeposition of Cu on unmodified tungsten

The electrodeposition of Cu onto barrier surfaces is of considerable importance in the development of Cu interconnect processes for deep submicrometer devices. Current processing usually involves electrodeposition from a sulfate bath onto a Cu seed layer, which is first deposited by plasma vapor deposition ~PVD! or metallorganic chemical vapor deposition ~MOCVD!. Conformal, uniform seed layer deposition becomes increasing problematic as device dimensions continue to shrink; 1 electrodeposition in the absence of a seed layer is desirable. Surface science studies carried out in ultrahigh vacuum ~UHV!, however, indicate that the ability of Cu adlayers to wet ~grow conformally on! a Ta or W barrier surface is severely degraded by the presence of even monolayer coverages of oxygen. 2-4 The electrodeposition of Cu onto reactive metal surfaces in aqueous environments therefore presents obvious difficulties. Under acidic conditions ~pH ;1 or lower!, and cathodic potentials, W metal is predicted 5 to be thermodynamically stable relative to its oxides. Exploratory studies of Cu electrodeposition on W in the absence of a Cu seed layer were therefore carried out under these conditions. Experimental Studies were carried out in a flat three-electrode cell fitted with a Luggin capillary and a platinum-rhodium counter electrode. All potentials reported here are referenced to Ag/AgCl. The cell was configured so that the area of the electrode accessible by the electrolyte was 1c m 2 . The solutions used for these studies consisted of 0.05 M CuSO4 in H2SO4 at pH 1.0. Solutions were purged with N2 for .1.5 h prior to each experiment. Polycrystalline W foils ~.99.95% pure! were used as working electrodes. Foils were rinsed in acetone, ethanol, and distilled water prior to use. Electrochemical measurements were carried out using a commercially available potentiostat ~EG&G 263A! and software. Scanning electron microscopy ~SEM! data were acquired using a JEOL 300S model with an energy dispersive analysis by X-ray ~EDAX! attachment for elemental analysis. X-ray photoelectron spectroscopy ~XPS! data were acquired with a hemispherical analyzer operated in constant pass energy mode ~50 eV! and unmonochromatized Mg Ka radiation.

Scanning tunneling microscopy atomic resolution images of sulfur overlayers on Fe(111)

We report the first atomic resolution scanning tunneling microscopy (STM) images of S overlayers on the Fe(111) surface. S overlayers were obtained by annealing the Fe(111) crystal to elevated temperatures to induce the segregation of S from the bulk. STM images of the (1×1)-S structure are consistent with the proposed model of one “geometric” monolayer of S atoms occupying on-top three-fold hollow sites of the Fe(111) surface. The STM data also revealed the presence of nanoscopic triangular pits on the (1×1)-S surface. These pits are only one atom deep. Increased segregation of S results in the formation of a (2√3 ×1)R30° structure and an increase in the size and depth of the triangular pits. This new structure corresponds to S coverage corresponding to more than one “geometric” monolayer of S based on one geometric monolayer coverage for the (1×1)-S structure. STM images obtained within large pits reveal a periodic “staircase” topography consisting of terraces with (111) orientation. These terraces are ...

STM-induced void formation at the Al2O3/Ni3Al(1 1 1) interface

Under ultrahigh vacuum conditions at 300 K, the applied electric field and/or resulting current from an STM tip creates nanoscale voids at the interface between an epitaxial, 7.0 A thick Al2O3 film and a Ni3Al(1 1 1) substrate. This phenomenon is independent of tip polarity. Constant current (1 nA) images obtained at +0.1 V bias and +2.0 V bias voltage (sample positive) reveal that voids are within the metal at the interface and, when small, are capped by the oxide film. Void size increases with time of exposure. The rate of void growth increases with applied bias/field and tunneling current, and increases significantly for field strengths >5 MV/cm, well below the dielectric breakdown threshold of 12±1 MV/cm. Slower rates of void growth are, however, observed at lower applied field strengths. Continued growth of voids, to ∼30 A deep and ∼500 A wide, leads to the eventual failure of the oxide overlayer. Density functional theory calculations suggest a reduction–oxidation mechanism: interfacial metal atoms are oxidized via transport into the oxide, while oxide surface Al cations are reduced to admetal species which rapidly diffuse away. This is found to be exothermic in model calculations, regardless of the details of the oxide film structure; thus, the barriers to void formation are kinetic rather than thermodynamic. We discuss our results in terms of mechanisms for the localized pitting corrosion of aluminum, as our results suggest nanovoid formation requires just electric field and current, which are ubiquitous in environmental conditions.

Copper metallization of hydroxyl-modified amorphous Si:C:H films

X-ray photoelectron spectroscopy (XPS) was used to examine the initial stages of copper deposited by Physical vapor deposition (PVD), or sputter deposition, interacting with amorphous silicon:carbon:hydrogen (a-Si:C:H) films and hydroxyl modified amorphous silicon:carbon:hydrogen (a-Si:C:H/OH) films under Ultra-high vacuum (UHV) conditions. Amorphous-Si:C:H films were formed by condensing vinyltrimethylsilane (VTMS) on a titanium substrate (temperature ≤90 K) followed by electron beam bombardment (500 eV), and annealing to 300 K in UHV. Amorphous Si:C:H/OH films were formed by condensing H2O on the condensed VTMS multilayer (≤90 K) followed by electron beam bombardment (500 eV) and annealing to 300 K in UHV. The stoichiometry of the unmodified and modified a-Si:C:H films was determined by XPS to be C4.5:Si and C4.3:O0.44:Si, respectively. XPS measurements of PVD Cu on the modified film at 300 K indicate initial conformal growth with Cu(I) and Cu(0) formation at the Cu/Si:C:H/OH film interface. At higher Cu coverages, only Cu(0) was observed. In contrast, 3-dimensional island formation (Volmer–Weber growth) of Cu(0) was observed on the unmodified film. Annealing both the modified and unmodified films up to 800 K in UHV produced no significant change in the Cu(3p)/Cu(2p3/2) intensity ratio, indicating negligible Cu diffusion through the film into the titanium substrate below 800 K.

S-Induced Destabilization of Aluminum Oxide at the Fe(poly)–S–Al2O3 Interface

Auger measurements reveal that, under UHV conditions, interfacial sulfurinduces the destabilization of an aluminum oxide overlayer at theFe–Al2O3 interface at temperatures above400 K. One monolayer deposition of Al onto Fe/S results in the insertion ofAl at the Fe–S interface. Exposure of Fe–Al–S to oxygenat 300 K gives rise to the complete oxidation of the aluminum adlayer asevidenced by the disappearance of the Al0 Auger signal and thestoichiometric formation of the aluminum oxide. When the resultingFe–S–Al2O3 is annealed progressively tohigher temperatures between 400 and 900 K, analysis of the Auger spectrashows a dramatic decline in the Al/O Auger intensity ratio. This declineis accompanied by the appearance of a small signal due to Al0,which maintains a constant intensity as the total Al signal (due mainly toAl3+) decreases. The appearance of the Al0 Augersignal accompanied by the attenuation of the Al3+ signalsignifies the chemical conversion of Al3+ into Al0,probably followed by diffusion of Al into the bulk. The possibility ofalumina dewetting and island formation, however, cannot be ruled out onthe basis of the present data. In the absence of interfacial sulfur, the alumina–Fe interface is stable to 900 K.