Daniel Wheeler

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

FiPy: Partial Differential Equations with Python

Many existing partial differential equation solver packages focus on the important, but arcane, task of numerically solving the linearized set of algebraic equations that result from discretizing a set of PDEs. Many researchers, however, need something higher level than that.

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.

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).

Accelerator Aging Effects During Copper Electrodeposition

Slow sweep rate voltammetric analysis of the Cu/Cu~II! deposition reaction is shown to be an effective tool for examining aging effects associated with thiol and disulfide additives that are widely employed as brighteners. Sulfonate-terminated short chain thiols are spontaneously oxidized by Cu~II! to form disulfide molecules with the conversion being complete within a few hours of electrolyte preparation. An additional aging effect occurs during electrolysis in conventional unseparated electrochemical cells. At the anode, the disulfide is reduced by Cu~I! forming thiolate complexes which subsequently affect the copper deposition reaction occurring at the cathode. The latter effect may be avoided by using a cation selective membrane to isolate the anode compartment. The application of electrodeposition in state of the art manufacturing of microelectronic devices together with advances in analytical methods has revitalized scientific investigations into the role of organic additives in electroplating. Of particular interest is the combined use of rate accelerating and inhibiting species for copper ‘‘superfilling,’’ or ‘‘bottom-up’’ filling, of submicrometer features in dual damascene processing. 1 Sulfonate-terminated alkanethiols or disulfides are representative of a class of accelerators which are usually present at micromolar concentrations in bright plating baths. 2,3 These species adsorb on the copper surface as either thiolates or disulfides and, when combined with a sulfonate end group, disrupt the inhibiting function of the polyether-halide-Cu ~I! layer. 3,4 The superconformal growth mode that arises from this competition is well described by the curvature enhanced accelerator coverage mechanism ~CEAC! whereby surface area decrease of an advancing surface of concave curvature results in enrichment of the more strongly bound surface species. 5-10 In the thiol/disulfide-polyetherhalide system, it is the accelerating thiol or disulfide species that are more strongly chemisorbed; they thus become concentrated during deposition on concave surfaces of trenches and vias leading to bottom-up filling. The overriding importance of this geometrical effect in superfilling was recently demonstrated by first derivitizing a patterned electrode with submonolayer quantities of thiolate or disulfide followed by electrodeposition of copper from an electrolyte containing only the polyether-halide inhibitor precursors as additives. 11 Feature filling proceeded in a manner analogous to that observed when the thiol or disulfide were present in the copper plating solution. This indicates that homogeneous thiol/disulfide chemistry has little to do with the superconformal feature filling process per se. Nevertheless, evidence of accelerator aging effects associated with homogeneous chemistry, beyond simple consumption, have been widely noted with an emphasis on the interactions occurring between copper, Cu~I! ,C u~II!, thiol/disulfide, oxygen, and related products. 12-15 From a practical perspective these reactions appear to significantly hamper process control. 12-15 A previous study of the instability of thiols and disulfides in copper plating indicates that Cu~II! slowly oxidizes thiols while Cu~I! stimulates the decomposition of disulfides. 12 The report was based on examination of the oxidation behavior of these compounds at glassy carbon electrodes in combination with colorimetric studies which were generally performed in electrolytes containing high accelerator concentrations ~up to 1 mmol/L!. 12 The high concentrations were used to provide a strong analytical signal although plating operations usually employ accelerator concentrations in the 5 mmol/L range. It was anticipated that the effect of concentration might be simply reflected in the kinetics of the stated decomposition reactions. From a technological perspective, aging effects are usually dealt with by using a ‘‘bleed and feed’’ scheme whereby new additives are continually added to the electrolyte while used electrolyte is drained to maintain the cell volume. In this paper, the sensitivity of the kinetics of the copper deposition reaction to accelerator chemistry will be exploited in order to examine accelerator aging under conditions directly relevant to the superfilling process. It is shown that these effects can be understood in terms of conversion between disulfides and thiols and vice-versa. Furthermore, it is demonstrated that significant improvements in process stability can be obtained by using a cation selective membrane to separate the anode and cathode compartments. Experimental Slow sweep cyclic voltammetry was used to examine the aging effects associated with thiol and disulfide-based accelerators. For all the experiments described herein, the base electrolyte was 0.24

Via Filling by Electrodeposition Superconformal Silver and Copper and Conformal Nickel

Superconformal deposition of silver in vias was studied. The observed experimental fill behavior is compared with predictions from a model based on the curvature-enhanced accelerator coverage mechanism of superconformal deposition. Superconformal copper deposition and conformal nickel deposition results are also modeled. The previously published model predicts via filling behavior using the dependence of deposition rate kinetics on the coverage of adsorbed catalyst. The requisite kinetic parameters are obtained from independent current-voltage and current-time transient studies conducted on planar substrates.

Superconformal Electrodeposition of Silver from a KAg ( CN ) 2 ­ KCN ­ KSeCN Electrolyte

Electrodeposition of silver from a KAg(CN) 2 -KCN electrolyte was investigated. The addition of potassium selenocyanate (KSeCN) results in a hysteretic current-voltage response, specular films, and superconformal growth in submicrometer vias. These observations are well described by the recently proposed curvature enhanced accelerator coverage model of film growth.