Thomas P. Moffat

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.

Superconformal Electrodeposition of Copper

A model of superconformal electrodeposition is presented based on a local growth velocity that is proportional to coverage of a catalytic species at the metal/electrolyte interface. The catalyst accumulates at the interface through reaction with the electrolyte. More importantly, if the concentration of the catalyst precursor in the electrolyte is dilute, then surface coverage within small features can change far more rapidly due to changing interface area. In such a case, the catalyst effectively floats on the interface during deposition, with changes in coverage coupled to alterations in arc-length of the moving surface. The local coverage therefore increases during conformal growth on a concave surface, resulting in a corresponding increase in the local deposition rate. The opposite is true for a convex surface. The model is supported by experiments and simulations of superconformal copper deposition in 350-100 nm wide features. The model also has significant implications for understanding the influence of adsorbates on the evolution of surface roughness during electrodeposition.

Electrodeposition of Copper in the SPS-PEG-Cl Additive System I. Kinetic Measurements: Influence of SPS

1to form a passivating film that inhibits the metal deposition rate by two orders of magnitude. Subsequent adsorption of short chain disulfide or thiol molecules with a sulfonate-end group~s! leads to the disruption and/or displacement of the passivating surface complex and acceleration of the metal deposition rate. The effect of submonolayer quantities of catalytic SPS is sustained even after extensive metal deposition, indicating that the catalyst largely remains segregated on the growth surface. Multicycle voltammetry reveals a significant potential dependence for SPS adsorption as well as its subsequent deactivation. Catalyst deactivation, or consumption, was examined by monitoring the quenching of the metal deposition rate occurring on SPS-derivatized electrodes in a SPS-free electrolyte. Catalyst consumption is a higher order process in terms of its coverage dependence and a maximum deactivation rate is observed near an overpotential of 20.1 V. Derivatization experiments are shown to be particularly effective in revealing the influence of molecular functionality in additive electroplating. Specifically, the charged sulfonate end group is shown to be central to effective catalysis. In the last three years, a curvature-enhanced accelerator coverage ~CEAC! mechanism has been shown to quantitatively describe superconformal film growth which is responsible for ‘‘bottom-up superfilling’’ of submicrometer features in damascene processing. 1-3 The mechanism has also been shown to apply to silver electrodeposition 4 as well as copper chemical vapor deposition. 5 A key characteristic of superfilling electrolytes, disclosed to date, is the competition between inhibitors and accelerators for electrode surface sites. According to the CEAC model, a thiol or disulfide accelerator, or catalyst, displaces an inhibiting halide-cuprouspolyether species from the interface and remains segregated at the surface during metal deposition. 1-3,6,7 A key consequence of these two stipulations is the possibility that local area change associated with metal deposition on a nonplanar surface may give rise to changes in the local catalyst coverage, ~e.g., increases on concave sections and decreases on convex segments! and thereby superconformal film growth. This process is particularly important for surface profiles with dimensions in the submicrometer regime and naturally provides an explanation for the beneficial effects induced by certain additives known as ‘‘brighteners.’’ 1,6 In this first of a series of papers, a more complete assessment of the electrochemical response of planar electrodes in copper superfilling electrolytes is presented. A typical electrolyte contains a dilute, i.e., micromolar, concentration of accelerator in the presence of an inhibitor concentration that is usually an order of magnitude greater. This configuration gives rise to hysteretic voltammetric curves, rising chronoamperometric transients, and decreasing chronopotentiometric traces, all of which reflect the competitive adsorption dynamics occurring between the two species. An underdeveloped aspect of this system is a quantitative description of the mass balance of the additives during plating. Of specific interest is the partitioning of the catalyst between segregation to the free surface vs. deactivation by either incorporation into the growing deposit or desorption into the electrolyte. Examination of the metal deposition kinetics on catalyst-derivatized electrodes in a catalyst-free electrolyte is shown to be particularly helpful in quantifying the deactivation process. These experiments also provide an avenue for exploring the impact of various additive functional groups on the metal deposition kinetics. Experimental

H2 evolution at Si-based metal–insulator–semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

Seedless Superfill: Copper Electrodeposition in Trenches with Ruthenium Barriers

Superfilling of fine trenches by direct copper electrodeposition onto a ruthenium ban ier is demonstrated. The ruthenium layer, as well as an adhesion promoting titanium or tantalum layer, was deposited by physical vapor deposition onto patterned silicon dioxide. Copper was deposited from an electrolyte previously shown to yield superconformal feature filling on copper seeded features. The single-step deposition process offers significant processing advantages over conventional damascene processing.

A Simple Equation for Predicting Superconformal Electrodeposition in Submicrometer Trenches

We present a single variable first-order differential equation for predicting the occurrence of superconformal electrodeposition. The equation presumes that the dependence of deposition rate on surface coverage of the accelerator is known (e.g., derived from voltammetry experiments) on planar electrodes A simplified growth geometry, based on the recently proposed mechanism of curvature enhanced accelerator coverage, is used to permit simplification of the trench-filling problem. The resulting solution is shown to reduce computational time from hours to seconds, while yielding reasonably accurate predictions of the parameter values required for trench filling.

Self-terminating growth of platinum by electrochemical deposition

Atomic Layers from Solution Growth of flat thin films is often plagued by the formation of mounds and pyramids. To avoid this problem, atomic-layer deposition (ALD) can be used whereby alternating self-termination reactions stop the layer growth. Electrochemical approaches to ALD use surface alloys to slow film growth, but often lead to film contamination. Yihua Liu et al. (p. 1327; see the Perspective by Switzer) show that for platinum films, controlling surface potential can lead to adsorbed hydrogen on the surface, which can terminate film growth at one layer, leaving platinum species in solution available for further reduction. Rapid changes in applied potential can oxidize the hydrogen, which allows efficient contamination-free growth of an additional atomic layer. Control over the presence of adsorbed hydrogen enables rapid sequential deposition of metal monolayers from aqueous solution. A self-terminating rapid electrodeposition process for controlled growth of platinum (Pt) monolayer films from a K2PtCl4-NaCl electrolyte has been developed that is tantamount to wet atomic layer deposition. Despite the deposition overpotential being in excess of 1 volt, Pt deposition was quenched at potentials just negative of proton reduction by an alteration of the double-layer structure induced by a saturated surface coverage of underpotential deposited H (Hupd). The surface was reactivated for further Pt deposition by stepping the potential to more positive values, where Hupd is oxidized and fresh sites for the adsorption of PtCl42– become available. Periodic pulsing of the potential enables sequential deposition of two-dimensional Pt layers to fabricate films of desired thickness, relevant to a range of advanced technologies.

Modeling Superconformal Electrodeposition Using the Level Set Method

Superconformal deposition enables the void-free filling of high aspect ratio features such as trenches or vias in the Damascene metallization process, Superconformal electrodeposition, also known as superfill, occurs when particular combinations of chemical additives are included in the electrolyte. The additives enable preferential metal deposition at the bottom surface which leads to bottom up filling before the sidewalls close off. Two crucial mechanisms by which the additives enable superfill to occur are (i) accelerator behavior increasing the copper deposition rate as a function of coverage and (ii) conservation of accelerator coverage with increasing/decreasing interface area. Thus, the adsorbed catalytic accelerator species floats upon the growing metal/ electrolyte interface. An effective modeling approach must accurately track the position of the interface as well as preserving surfactant coverage while the interface is advancing. This must be achieved in an Eulerian framework due to the necessity of modeling the diffusion of electrolyte species. To this end, the level set method is used to track the interface while a scalar variable approach governs the surfactant coverage. Modeling of additive accumulation and conservation on a deforming interface in conjunction with the level set method presents areas for novel numerical approaches. Several test cases are examined to validate the surface coverage model. Comparison of superfilling simulations with experimental results is also presented.

Curvature Enhanced Adsorbate Coverage Model for Electrodeposition

The influence of a catalyst deactivating leveling additive in electrodeposition is explored in the context of the previously developed curvature enhanced accelerator coverage model of superconformal film growth. Competitive adsorption between a rapidly adsorbed suppressor, rate accelerating catalyst, and catalyst-deactivating leveler is examined. Rate equations are formulated where the leveling agent is capable of deactivating the adsorbed catalyst by either direct adsorption from the electrolyte or by deactivation/displacement during surface area reduction that accompanies advancing concave surfaces. The influence of a prototypical cationic surfactant leveler on electrochemical kinetics and feature filling is examined for copper electrodeposition from an electrolyte containing polyethylene glycol-chloride-bis(3-sulfopropyl)disulfide (PEG-Cl-SPS).

Electrodeposition of Cu in the PEI-PEG-Cl-SPS Additive System Reduction of Overfill Bump Formation During Superfilling

The impact of branched polyethyleneimine (PEI) on Cu electrodeposition from an acidified cupric sulfate electtrolyte containing a combination of superfilling additives, specifically polyethylene glycol, bis(3-sulfopropyl)disulfide, and chloride (PEG-Cl-SPS), is examined. Electroanalytical measurements reveal that adsorption of cationic PEI leads to inhibition of the metal deposition reaction to an extent similar to that provided by PEG-Cl adsorption. However, unlike the PEG-Cl suppressor, PEI is shown to deactivate adsorbed SPS accelerator. As a result, addition of PEI quenches the hysteretic voltammetric response that is a signature of competitive adsorption in the PEG-Cl-SPS additive system. Trench-filling experiments in a PEI-PEG-Cl-SPS electrolyte demonstrate that the deactivating interaction between PEI adsorption and adsorbed SPS can be optimized to prevent overfill bump formation without substantial detrimental impact on bottom-up, void-free feature filling.