Gery R. Stafford

Superconformal Electrodeposition of Copper in 500–90 nm Features

Superconformal electrodeposition of copper in 500 nm deep trenches ranging from 500 to 90 nm in width has been demonstrated using an acid cupric sulfate electrolyte containing chloride (Cl), polyethylene glycol (PEG), and 3‐mercapto‐l‐propanesulfonate (MPSA). In contrast, similar experiments using either an additive‐free electrolyte, or an electrolyte containing the binary combinations Cl‐PEG, Cl‐MPSA, or simply benzotriazole (BTAH), resulted in the formation of a continuous void within the center of the trench. Void formation in the latter electrolytes is shown to be reduced through the geometrical leveling effect associated with conformal deposition in trenches or vias with sloping sidewalls. The slanted sidewalls also counterbalance the influence of the differential cupric ion concentration that develops within the trenches. Examination of the i-E deposition characteristics of the electrolytes reveals a hysteretic response associated with the Cl‐PEG‐MPSA electrolyte that can be usefully employed to monitor and explore additive efficacy and consumption. Likewise, resistivity measurements performed on corresponding blanket films can be used to quantify the extent of additive incorporation and its influence on microstructural evolution. The films deposited from the Cl‐PEG‐MPSA electrolyte exhibit spontaneous recrystallization at room temperature that results in a 23% drop in resistivity within a few hours of deposition.

Electrochemistry of Titanium and the Electrodeposition of Al-Ti Alloys in the Lewis Acidic Aluminum Chloride–1-Ethyl-3-methylimidazolium Chloride Melt

The chemical and electrochemical behavior of titanium was examined in the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl 3 -EtMeImCl, molten salt at 353.2 K. Dissolved Ti(II), as TiCl 2 , was stable in the 66.7-33.3% mole fraction ( m/o composition of this melt. but slowly disproportionated in the 60.0-40.0 m/o melt. At low current densities, the anodic oxidation of Ti(0)did not lead to dissolved Ti (II). but to an insoluble passivating film of TiCl 3 . At high current densities or very positive potentials, Ti (0) was oxidized directly to Ti(IV); however, the electrogenerated Ti (IV) vaporized from the melt as TiCl 4 (g). As found by other researchers working in Lewis acidic AlCl 3 -NaCl, Ti(II) tended to form polymers as its concentration in the AlCl 3 - EtMeImCl melt was increased. The electrodeposition of Al-Ti alloys was investigated at Cu rotating disk and wire electrodes. Al-Ti alloys containing up to ∼19% atomic fraction (a/o) titanium could be electrodeposited from saturated solutions of Ti (II) in the 66.7-33.3 m/o melt at low current densities, but the titanium content of these alloys decreased as the reduction current density was increased. The pitting potentials of these electrodeposited Al-Ti alloys exhibited a positive shift with increasing titanium content comparable to that observed for alloys prepared by sputter deposition.

Electrodeposition of Aluminum from the Aluminum Chloride‐1‐Methyl‐3‐ethylimidazolium Chloride Room Temperature Molten Salt + Benzene

The constant current electrodeposition of bulk aluminum on copper substrates was investigated in the Lewis acidic [> 50 mole percent (m/o) AlCl 3 ] aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt (AlCl 3 -MeEtimeCl). Although aluminum can be electroplated from the neat molten salt, we have found that the quality of the electrodeposit is greatly enhanced by the addition of benzene as a cosolvent. Electrodeposits produced in such melts exhibited a grain size on the order of 5 to 15 μm. The lattice parameters of these electrodeposits were slightly smaller than the Joint Committee on Powder Diffraction Standards (JCPDS) value; this was attributed to the presence of vacancies in the aluminum lattice that were present at concentrations ranging from about 0.1 to 0.5 atomic percent (a/o). The copper substrate was found to have a slight (311) texture as determined by x-ray diffraction; however, all of the electrodeposits exhibited a preferred (220) crystallographic orientation with the intensity of the (311) reflection being equal to that of a randomly oriented sample. The (200) and (111) reflections were relatively weak. The relative intensity of the (220) reflection increased and those for the (311), (200), and (111) orientations decreased as the benzene concentration was increased. A Williamson-Hall treatment indicated that microstrain in the electrodeposits was essentially nonexistent.

Electrodeposition of Al-Mo Alloys from the Lewis Acidic Aluminum Chloride-1-ethyl-3-methylimidazolium Chloride Molten Salt

The electrochemistry of Zr(IV) and Zr(II) and the electrodeposition of Al-Zr alloys were examined in the Lewis acidic 66.7-33.3 mol % aluminum chloride-1-ethyl-3-methylimidazolium chloride molten salt at 353 K. The electrochemical reduction of Zr(lV) to Zr(II) is complicated by the precipitation of ZrCl 3 ; however, solutions of Zr(II) can be prepared by reducing Zr(IV) with Al wire. Al-Zr alloys can be electrodeposited from plating baths containing either Zr(IV) or Zr(II), but for a given concentration and current density, baths containing Zr(IV) lead to Al-Zr alloys with the higher Zr content. This result was traced to the diminutive concentration-dependent diffusion coefficient for Zr(II). It was possible to prepare Al-Zr alloys containing up to ∼17% atomic fraction (atom %) Zr. The structure of these deposits depended on the Zr content. Alloys containing less than 5 atom % Zr could be indexed to a disordered face-centered cubic structure similar to pure Al, whereas alloys containing ∼17 atom % Zr were completely amorphous (metallic glass). The chloride pitting potentials of alloys with more than 8 atom % Zr were approximately +0.3 V relative to pure Al.

Electrodeposition of Cobalt and Cobalt‐Aluminum Alloys from a Room Temperature Chloroaluminate Molten Salt

The electrodeposition of magnetic cobalt-aluminum alloys was investigated in the Lewis acidic aluminum chloride-l-methyl-3-ethylimidazolium chloride [60.0-40.0 mole percent molten salt containing electrogenerated Co(II) at 25°C. Rotating disk electrode voltammetry indicated that it is possible to produce alloy deposits containing up to 62 atomic (a/o) aluminum at potentials positive of that for the bulk deposition of aluminum. The onset of the underpotential-driven aluminum codeposition process occurred at around 0.40 V vs. the Al/Al(III) couple in a 5.00 mmol liter -1 Co(II) solution but decreased as the Co(II) concentration increased. The Co-Al alloy composition displayed an inverse dependence on the Co(II) concentration but tended to become independent of concentration as the potential was decreased to 0 V. A rotating ring-disk electrode voltammetry technique was developed to analyze the composition and structure of the Co-Al alloy deposits. This technique takes advantage of the fact that the mass-transport-limited reduction of cobalt(II) occurs at potentials considerably more positive than that at which aluminum codeposition occurs. Scanning electron microscopy and energy dispersive x-ray analysis of bulk electrodeposits revealed that deposit morphology depends strongly upon aluminum content/deposition potential ; deposits produced at 0.40 V from 50.0 mmol liter -1 Co(II) solutions consisted of 10 to 20 μm diam multifaceted nodules of pure hcp cobalt, whereas those obtained at 0.20 V were dense and fine grained, containing about 4 a/o Al. Deposits produced at 0 V had the visual appearance of a loosely adherent black powder. x-ray diffraction measurements revealed a lattice expansion and a decrease in grain size as the hcp cobalt was alloyed with increasing amounts of aluminum.

Superconformal Electrodeposition of Silver in Submicrometer Features

The generality of the curvature-enhanced accelerator coverage (CEAC) model of superconformal electrodeposition is demonstrated through application to superconformal filling of fine trenches during silver deposition from a selenium-catalyzed silver cyanide electrolyte. The CEAC mechanism involves (i) increase of local metal deposition rate with increasing coverage of a catalytic species adsorbed on the metal/electrolyte interface and (ii) significant change of local coverage of catalyst (and thus local deposition rate) in submicrometer features through the changing area of the metal/electrolyte interface. Electrochemical and X-ray photoelectron experiments with planar electrodes (substrates) are used to identify the catalyst and obtain all kinetic parameters required for the simulations of trench filling. In accord with the model, the electrolyte yields optically shiny, dense films, hysteretic current-voltage curves, and rising current-time transients. Experimental silver deposition in trenches from 350 down to 200 nm wide are presented and compared with simulations based on the CEAC mechanism. All kinetics for the modeling of trench filling come from the studies on planar substrates. The results support the CEAC mechanism as a quantitative formalism for exploring morphological evolution during film growth.

Pitting Corrosion of Electrodeposited Aluminum‐Manganese Alloys

The corrosion behavior of a series of electrodeposited Al-Mn alloys has been investigated. Three different microstructures were examined. A supersaturated face-centered cubic (fcc) structure was produced with up to 5 atomic percent (a/o) Mn. Deposits containing 5--22 a/o Mn were composed of a glassy phase in combination with a supersaturated (1 a/o Mn) fcc phase, while alloys with 22--26 a/o Mn were single-phase metallic glasses. The pitting characteristics of these materials, in aqueous chloride solutions, were controlled by the alloy microstructure. Relative to aluminum, a 250 to 400 mV increase in the pitting potential was observed for the single-phase Al-Mn alloys. In contrast, the dual-phase material exhibited pitting potentials closer to that of aluminum due to selective pitting of the (1 a/o Mn) fcc phase. The enhanced pitting resistance of single-phase Al-Mn alloys is remarkable in view of the reactive nature of elemental manganese.

The microstructure of electrodeposited titanium-aluminum alloys

The microstructure of electrodeposited titanium-aluminide alloys containing 3.6 to 24.1 at. pct Ti was studied by transmission electron microscopy. The surface morphology of the deposits showed that they contained nodular and faceted grains which tended to be less faceted at higher Ti contents. Extensive 111 twinning was observed in all deposits, and growth striations parallel to were observed in the low Ti deposits. The growth of nodules was linked to the presence of these twins; it was hypothesized that the twin boundaries act as easy atomic attachment points and, therefore, enhance the growth rate. The presence of twins and striations was used to pro-pose a growth mechanism. The 5.3, 15.8, and 24.1 at. pct Ti deposits were single-phase grains of the Ll2 crystal structure, as opposed to the expected equilibrium two-phase mixture of face-centered cubic (fcc) Al (saturated with Ti) and D022 Al3Ti. Calculated electron diffraction in-tensity data were used to demonstrate that the decrease in intensity of the superlattice reflections in the substoichiometric deposits is due to a reduction in the difference in atomic scattering factors between the two lattice site types.

In situ measurements of stress evolution for nanotwin formation during pulse electrodeposition of copper

In situ stress measurements were performed during high frequency pulse electrodeposition of nanotwinned Cu thin films. Periodic stress changes during pulse-on and pulse-off periods were observed. The stress profile showed an abrupt increase in tensile stress to about 400 MPa during the pulse-on period and a stress relaxation during the pulse-off period. First-principles calculations predict that a complete relaxation of the tensile stress allows the formation of nanotwins separated by 28 nm or more. This is in good agreement with the results obtained from microstructural analysis of the Cu films fabricated during in situ stress measurements.

Electrodeposition of Silver-Aluminum Alloys from a Room-Temperature Chloroaluminate Molten Salt

The electrodeposition of silver-aluminum alloys was investigated at platinum and tungsten electrodes in the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl 3 -EtMelmCl) molten salt containing electrogenerated silver(I) at 25°C. Sampled-current voltammetry indicated that it is possible to electrodeposit Ag-Al alloys at potentials positive of that where the bulk deposition of aluminum is normally observed (∼0 V). The aluminum content of these alloys varied with the applied potential and displayed an inverse dependence on the silver(I) concentration, indicating that the alloy formation process was kinetically limited. Experiments conducted in melts with different compositions revealed that at a fixed potential and silver(I) concentration. the atomic fraction of aluminum in the alloy is virtually independent of the melt composition. At high concentrations, silver(I) adsorbs on platinum and tungsten, but this process is inhibited by the addition of benzene. The adsorption process does not appear to affect the codeposition of aluminum with silver. X-ray diffraction analysis of bulk Ag-Al alloy electrodeposits prepared in melt containing benzene as a cosolvent indicated the presence of both face-centered cubic Ag and hexagonal close-packed δ-Ag 2 Al.