N.P. Magtoto

Effect of surface impurities on the Cu/Ta interface

Abstract Auger electron spectroscopy and temperature programmed desorption studies under ultra-high vacuum conditions demonstrate that even sub-monolayer coverages of oxygen or carbide on polycrystalline Ta significantly degrade the strength of Cu/Ta chemical interactions, and affect the kinetics of Cu diffusion into bulk Ta. On clean Ta, monolayer coverages of Cu will de-wet only above 600 K. A partial monolayer of adsorbed oxygen (3 L O2 at 300 K) results in a lowering of the de-wetting temperature to 500 K, while saturation oxygen coverage (10 L O2, 300 K) results in de-wetting at 300 K. Carbide formation also lowers the de-wetting temperature to 300 K. Diffusion of Cu into the Ta substrate at 1100 K occurs only after a 5-min induction period at this temperature. This induction period increases to 10 min for partially oxidized Ta, 15 min for carbidic Ta and 20 min for fully oxidized Ta.

The Effects of an Iodine Surface Layer on Ru Reactivity in Air and during Cu Electrodeposition

X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, and cyclic voltammetry have been used to study the adsorption of iodine on the Ru(0001), air, and water exposure to both clean and iodine covered Ru(0001) surfaces and the stability of the iodine adlayer during Cu overpotential electrodeposition. A Ru(0001)-(√3 X √3)R30°-1 structure was observed after I 2 vapor exposure of the Ru(0001) surface at room temperature. The Ru(0001)-(√3 X √3)R30°-I structure was found to be stable toward ambient air and water exposure. The I ad-layer passivates the Ru(0001) surface against significant hydroxide, chemisorbed oxygen, or oxide formation during exposure to air. Immersion of I-Ru(0001) results in greater hydroxide and chemisorbed oxygen formation than air exposure. A saturation coverage of I on a Ru(poly) electrode similarly passivated the Ru surface against oxidation upon exposure to water vapor over an electrochemical cell in an ultrahigh vacuum electrochemistry transfer system. Studies with combined electrochemical and XPS techniques show that iodine surface adlayer remained on top of the surface after cycles of overpotential electrodeposition/dissolution of copper on Ru(0001). These results indicate the potential bifunctionality of the iodine layer to both passivate the Ru surface in the microelectronic processing and to act as a surfactant for copper electrodeposition.

Characterization of oxidized Ni3Al(1 1 0) and interaction of the oxide film with water vapor

Abstract X-ray photoelectron spectroscopy was used to characterize the oxidation of Ni3Al(1xa01xa00) and interaction of the oxide film at ambient temperature with increasing pressures of water vapor (10−6–5 Torr). The Ni3Al(1xa01xa00) substrate was oxidized in 10−6 Torr O2 at 800 K followed by annealing to 1100 K. The Ni(2p), Al(2p) and O(1s) spectra were monitored to investigate the changes in spectral features and binding energy due to oxidation. The data indicate that the oxide film, upon annealing in UHV, is composed of NiO, Al2O3 and an intermediate phase denoted here as “AlOx”. Upon exposure of the oxide film at ambient temperature to increasing water vapor pressure, a shift in both the O(1s) and Al(2p)oxide peak maxima to lower binding energies is observed. In contrast, exposure of Al2O3/Al(poly) to water vapor under the same conditions results in a high binding energy shoulder in the O(1s) spectra (indicating hydroxylation). Spectral decomposition provides further insight into the difference in reactivity between the two oxide films. The corresponding trends of the O(1s)/Ni0(2p3/2) and Al(2p)/Ni0(2p3/2) spectral intensity ratios suggest conformal changes of the oxide film on Ni3Al(1xa01xa00).

Dielectric breakdown of ultrathin aluminum oxide films induced by scanning tunneling microscopy

Dielectric breakdown of 7-A-thick Al2O3 (111) films grown on Ni3Al(111) under ultrahigh vacuum conditions is induced by increasing the bias voltage on the scanning tunneling microscopy tip under constant current feedback. Breakdown is marked by the precipitous retreat of the tip from the surface, and the formation of an elevated feature in the scanning tunneling microscopy image, typically greater than 5 nm high and ∼100 nm in diameter. Constant height measurements performed at tip/sample distances of 1 nm or less yield no tip/substrate physical interaction, indicating that such features do not result from mass transport. Consistent with this, current/voltage measurements within the affected regions indicate linear behavior, in contrast to a band gap of 1.5 eV observed at unaffected regions of the oxide surface. A threshold electric field value of 11±1 MV cm−1 is required to induce breakdown, in good agreement with extrapolated values from capacitance measurements on thicker oxides.

Copper wetting of a tantalum silicate surface: Implications for interconnect technology

X-ray photoelectron spectroscopy data show that sputter-deposited Cu (300 K) displays conformal growth on oxidized TaSi films (TaSiO6). The TaSiO6 films, 6 A thick, were formed by sputter deposition of Ta onto ultrathin SiO2 substrates at 300 K, followed by annealing to 600 K in 2 Torr O2. The photoelectron spectra of the films are characterized by a Si(2p) binding energy at 102.1 eV, indicative of silicate formation. Annealing the film to >900 K resulted in silicate decomposition and SiO2 formation. Cu(I) formation and conformal growth were not observed for the annealed films. The results are similar to those previously reported for oxidized TaSiN, and indicate that Si-modified Ta barriers should maintain Cu wettability under oxidizing conditions for Cu interconnect applications.

Ta metallization of Si–O–C substrate and Cu metallization of Ta/Si–O–C multilayer

Interfacial reactions of Ta with a Si–O–C low-dielectric constant (low-k) material and Cu/Ta/Si–O–C multilayers are investigated using x-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (TEM). Data indicate that Ta deposition on the low-k substrate results in the initial formation of Ta oxide and TaC. Subsequent deposition of Ta eventually results in the formation of a metallic Ta overlayer at 300 K. The thickness of the initial Ta oxide/TaC-containing layer varies with the Ta deposition rate. At a deposition rate of ∼1 Au200amin−1, no metallic Ta is observed, even after 32 min sputter deposition time. In contrast, a film of roughly the same thickness, obtained after 15 s deposition at a rate of ∼2 Au200as−1, is predominantly metallic Ta. Sputter deposition rates, derived from XPS data, are in agreement with film thicknesses derived from cross-sectional TEM data. Heating of Ta/low-k films in UHV results in no significant changes (as detected by XPS) up to 800 K. Cu deposit...

Cu wetting and interfacial stability on clean and nitrided tungsten surfaces

Abstract Cu growth/nucleation behavior and thermal stability on clean and nitrided tungsten foil have been characterized by Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) under controlled ultra high vacuum (UHV) conditions. At room temperature, Auger intensity ratio versus time plots demonstrate layer by layer Cu growth for the clean tungsten surface (W) and three-dimensional nucleation for the nitride overlayer (7.5–10xa0A, WNx/W). Auger intensity ratio versus temperature measurements for the Cu (1 monolayer)/W system indicate a stable interface up to 1000xa0K. For the Cu (1 ML)/WNx/W system, initial Cu diffusion into the nitride overlayer is observed at 550xa0K. Maximum diffusion of the Cu occurs at 750xa0K. The driving force for diffusion is due to an effective Cu–nitride repulsion and a more thermodynamically favorable Cu–W interaction. TDS measurements of the nitride overlayer demonstrate N2 decomposition from 800–1400xa0K. The addition of Cu (1 ML) to the nitride overlayer lowers the decomposition temperature range of 750–1350xa0K. The enhancement of N2 recombination is attributed to the perturbing effect of Cu on W–N bonding driven by a Cu–W surface alloy formation.

Copper interaction with a Ta silicate surface: implications for interconnect technology

Abstract The TaSiO 6 films, ∼6 A thick, were formed by sputter deposition of Ta onto ultrathin SiO 2 substrates at 300 K, followed by annealing to 600 K in 2 Torr O 2 . X-ray photoelectron spectroscopy (XPS) measurements of the films yielded a Si(2p) binding energy at 102.1 eV and Ta(4f 7/2 ) binding energy at 26.2 eV, indicative of Ta silicate formation. O(1s) spectra indicate that the film is substantially hydroxylated. Annealing the film to >900 K in ultra-high vacuum resulted in silicate decomposition to SiO 2 and Ta 2 O 5 . The Ta silicate film is stable in air at 300 K. XPS data show that sputter-deposited Cu (300 K) displays conformal growth on Ta silicate surface (TaSiO 6 ) but 3D growth on the annealed and decomposed silicate surface. Initial Cu/silicate interaction involves Cu charge donation to Ta surface sites, with Cu(I) formation and Ta reduction. The results are similar to those previously reported for air-exposed TaSiN, and indicate that Si-modified Ta barriers should maintain Cu wettability under oxidizing conditions for Cu interconnect applications.

Electrodeposition of adherent copper film on unmodified tungsten

Adherent Cu films were electrodeposited onto polycrystalline W foils from purged solutions of 0.05 M CuSO4 in H2SO4 supporting electrolyte and 0.025 M CuCO3∙Cu(OH)2 in 0.32 M H3BO3 and corresponding HBF4 supporting electrolyte, both at pH = 1. Films were deposited under constant potential conditions at voltages between -0.6 V and -0.2 V vs Ag/AgCl. All films produced by pulses of 10 s duration were visible to the eye, copper colored, and survived a crude test called the Scotch tape test, which stick the scotch tape on the sample, then peel off the tape and see if the copper film peels off or not. Characterization by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray photon spectroscopy (XPS) confirmed the presence of metallic Cu, with apparent dendritic growth. No sulfur impurity was observable by XPS or EDX. Kinetics measurements indicate that the Cu nucleation process in the sulfuric bath is slower than in the borate bath. In both baths, nucleation kinetics do not correspond to either instantaneous or progressive nucleation. Films deposited from 0.05 M CuSO4/H2SO4 solution at pH > 1 at -0.2 V exhibited poor adhesion and decreased Cu reduction current. In both borate and sulfate baths, small Cu nuclei are observable by SEM upon deposition at higher negative overpotentials, while only large nuclei (~ 1 micron or larger) are observed upon deposition at less negative potentials.

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.