Curvature Enhanced Accelerator Coverage Research Articles

Mechanism of Bismuth Stimulated Bottom-up Gold Feature Filling

The mechanism underlying -stimulated bottom-up Au filling and self-passivation in trenches and vias in slightly alkaline electrolytes is explored. The impacts of electrolyte components and and potential-dependent kinetic factors on the rate of Au electrodeposition are quantified experimentally. Derived parameters are applied within the surfactant conservation Curvature Enhanced Accelerator Coverage model to simulate the filling of high aspect ratio trenches. It is observed that Bi adsorption accelerates the Au deposition rate with a non-linear dependence occurring around a critical coverage. Further, the impact of electrolyte composition is such that gradients of and derived from reduction of during deposition accentuate deposition farther from the feature opening. These factors and surface area reduction result in Bi enrichment at the bottoms of filling features that localizes active deposition to feature bottoms. Ultimately, weakening of the concentration gradients and associated kinetics as bottom-up feature filling progresses reduces the Bi coverage on the growth front below the critical value and bottom-up growth terminates. Good agreement is observed with key experimental features including the incubation period of conformal deposition, transition to bottom-up growth, subsequent bottom-up filling and finally self-passivation as the growth front nears the field.

(Electrodeposition Division Research Award Address) Superconformal Deposition: Where We Are and Where We Might Go

Superconformal deposition processes for void-free filling of features have developed remarkably over the first two decades of this century. Superconformal copper deposition, aka “superfill”, enabled void-free copper filling of features with conductive surfaces having aspect ratios unimaginable when implemented in on-chip interconnects in 1997. The only known mechanism, already more than 40 years old, provided only more modest superconformal feature filling through a gradient of deposition rate that scaled with a gradient of suppressor concentration arising from incorporation in the depositing metal. A new mechanism, for the copper “superfill”, was determined in 2001: local coverage of a strongly bound but surfactant accelerator arose through area change that accompanied deposition on nonplanar surfaces. Copper “superfill” through this Curvature Enhanced Accelerator Coverage, or CEAC, mechanism accompanied and accounted for the decrease of feature size in on-chip interconnects over the next ten years. Along the way the CEAC mechanism was shown to be generic to the coinage metals of silver and gold. In all cases, feature filling was shown to be predicted a priori by CEAC-based models using only kinetics derived from studies on planar substrates.In the meantime, the microelectronics industry began to pursue wafer stacking that hinged upon void-free copper filling of much larger Through Silicon Vias, TSV, for which the CEAC mechanism was poorly suited. Demonstrations of bottom-up copper filling incompatible in both geometry and additive-chemistry with the CEAC mechanism required a new mechanistic understanding. In 2012 the old, linear model for suppressor containing electrolytes was replaced by a model reflecting suppressors whose deactivation was so non-linear in potential and concentration that suppression breakdown yielded increasing negative current with increasing positive potential during cyclic voltammetry. Models based on this S-shaped negative differential resistance (S-NDR) predicted the extreme bottom-up nature of the copper filling. Subsequent years brought demonstrations of generality with gold, nickel, cobalt, zinc and copper, albeit manifesting in two very different filling geometries: activation of only the bottom-up surface and activation of the bottom surface with a portion of the adjacent sidewalls. Filling of features having much larger dimensions as well as geometries such as through holes was also demonstrated. In all cases, the filling evolution was shown to be a-priori predictable by S-NDR based models using only kinetics derived from studies on planar substrates.Today we find ourselves with the next superconformal filling process requiring new mechanistic understanding. Gold filling exhibits the bottom-up geometry of suppressor-derived S-NDR filling but with a bismuth additive that accelerates as in CEAC filling. It is thereby explained by neither. Void-free, bottom-up filling has been achieved in trenches whose depths range from a few micrometers to a quarter of a millimeter. Trenches with aspect ratios as low as unity and as high as fifty-six (!) have been successfully bottom-up filled, and surface profiles indicate the limit has not been reached. Adsorption and deposition exhibit a unique nonlinear dependence on transport, and bottom-up filling halts automatically at a depth within patterned features defined by potential that, equivalently, yields a tell-tale potential signature upon filling under galvanodynamic control. Uniformity of filling is excellent across patterned substrates. The new process satisfies a need outside the area of microelectronics: gold filling of trenches in x-ray diffraction gratings. As in the past, understanding will enable superconformal electrodeposition to meet the evolving needs of a new technology. Figure 1

(Invited) Bottom-up Electrodeposition

Superconformal electrodeposition that enables the fabrication of high aspect ratio Cu interconnects is not unique to copper (or for that matter even to electrodeposition). I will detail two known mechanisms underlying such processes: Curvature Enhanced Accelerator Coverage (CEAC) and S-shaped Negative Differential Resistance (S-NDR). I will describe how voltammetric measurements on planar substrates can quantify kinetics for models based on both mechanisms, enabling accurate, a-priori prediction of filling from a given electrolyte in features of various size and aspect ratio. I will summarize the chemistries and associated mechanisms for superconformal Ag, Au, Co, and Ni deposition for fabrication of defect-free nanometer scale damascene to millimeter scale through silicon via interconnects, including model predictions. I will finish by detailing truly bottom-up Au filling of high aspect trenches.

(Invited) Superconformal Electrodeposition

State of the art manufacturing of semiconductor devices involves electrodeposition of copper for device wiring and chip stacking. The success of the electroplating process depends on electrolyte additives that affect the local deposition rate to yield void-free superconformal, or bottom–up filling of trenches and vias that comprise 3D interconnects. Two distinct processes associated with the prerefential filling of recessed surface features, namely, curvature enhanced accelerator coverage and critical phenomena related to supressor induced negative differential resistance. Quantitative chemical process models based on the above processes have been shown to be effective in describing the filling of high aspect ratio features that range from nanometers (on-chip wiring1) to micrometers (through-silicon-vias2) to millimeters (printed circuit boards) in scale. Nevertheless, much remains to be known about the molecular nature of the additives and the competitive, co-adsorption dynamics involved in both suppression and acceleration of metal deposition. Accordingly, a variety of surface science tools, such as scanning tunneling microscopy (STM) and surface-enhanced infrared absorption spectroscopy (SEIRAS) are being used to investigate additive function. The lecture will detail experiments that reveal the important role of adsorbate water interactions, i.e. hydrophilicity, in the design and operation of additives used for superconformal film growth. The talk will conclude with a discussion of the lesson learned and their application to the deposition of other metals.

Bottom-up Gold Filling of High Aspect Ratio Trenches :Toward a Mechanistic Understanding

Cu is the standard choice for the formation of multi-level 3-D interconnects allowing for microelectronic chip-stacking (1). This is predicated on the well-defined additive chemistries that drive the superconformal Cu filling of through silicon vias (TSVs) and trenches (2). To transition to Au filling of complex 3-D Si architectures for optoelectronic applications, analogous chemistries that enable preferential filling within recessed silicon features must be developed. To date the state of the art for void free Au filling has come from the use of a Pb additive in both KAu(CN)2 (3, 4) and Na3Au(SO3)2 (5) electrolytes. Both of these systems yield superconformal filling via a curvature enhanced accelerator coverage (CEAC) mechanism. However, due to the toxicity of CN- and Pb2+ containing electrolytes, alternatives must be explored. Recently, it has been demonstrated that the addition of Bi to alkaline Na3Au(SO3)2 electrolytes yields preferential bottom-up Au electrodeposition in Si trenches, with limited deposition on the free surface (6). The addition of Bi to the intrinsically suppressed Na3Au(SO3)2 electrolyte clearly accelerates deposition(6, 7). However, growth in this system does not propagate via the typical CEAC mechanism demonstrated in other accelerated systems (3). Instead the growth propagates via a mechanism more closely resembling that of S-shaped negative differential resistance (S-NDR) based on suppression breakdown, typical for suppressor based additive systems(8). This talk will explore the utilization of this Bi, Na3Au(SO3)2 electrolyte system for the bottom-up filling of high aspect ratio silicon trenches. The effect of; potential, additive concentration and electrolyte transport will be explored on the filling of features with aspect ratios ranging from 1.5 to 17. Additionally, the chemical and morphological nature of the accelerated bottoms of features, and of the passive free surface, will be compared via x-ray photoelectron spectroscopy, scanning electron microscopy and anodic stripping voltammetry to gain mechanistic insight into the role of Bi in deposition acceleration and feature localization. P. C. Andricacos, C. Uzoh, J. O. Dukovic, J. Horkans and H. Deligianni, IBM J. Res. Dev., 42, 567 (1998).T. P. Moffat, D. Wheeler, M. D. Edelstein and D. Josell, IBM J. Res. Dev., 49, 19 (2005).D. Josell, C. R. Beauchamp, D. R. Kelley, C. A. Witt and T. P. Moffat, Electrochem. Solid-State Lett., 8, C54 (2005).D. Josell, D. Wheeler and T. P. Moffat, J. Electrochem. Soc., 153, C11 (2006).D. Josell and T. P. Moffat, J. Electrochem. Soc., 160, D3009 (2013).D. Josell and T. P. Moffat, J. Electrochem. Soc., 166, D3022 (2019).I. D. E. McIntyre and W. F. Peck, J. Electrochem. Soc., 123, 1800 (1976).S.-K. Kim, J. E. Bonevich, D. Josell and T. P. Moffat, J. Electrochem. Soc., 154, D443 (2007).

Self-Annealing Behavior of Electroplated Cu in Blind-Hole Structures

Blind-hole (BH) metallization via Cu electrodeposition is a critical technology for printed circuit boards (PCBs) with high-density interconnections (HDIs) because it provides excellent thermal and electrical conductivity and efficient space utilization. In this study, the self-annealing behavior of electroplated Cu was in-situ characterized via ohmmeter measurements, field-emission scanning electron microscopy (FE-SEM) in combination with electron backscatter diffraction (EBSD) analysis, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). A strong correlation among the electrical property transition, Cu crystallographic transition, and impurity distribution was established that supports the curvature enhanced accelerator coverage (CEAC) mechanism and advances our understanding of the Cu physical/chemical characteristics in BH structures.

Drop-in Electrodeposition Processes for Void-Free, High Aspect Ratio Structures of Five Metals

Superconformal electrodeposition processes enable void- and seam-free metal filling of high aspect ratio features from tens of nanometers to hundreds of micrometers in size. There are two known mechanisms underlying such processes: Curvature Enhanced Accelerator Coverage (CEAC) and S-shaped Negative Differential Resistance (S-NDR). Voltammetric measurements on planar substrates can quantify kinetics in models based on both mechanisms, enabling accurate, a-priori prediction of filling from a particular electrolyte in a particular size and aspect ratio feature. I will summarize the chemistries and associated mechanisms for superconformal Cu, Ag, Au, Co, and Ni feature filling, including experiment and model predictions, for processes that are drop-in compatible with those presently used for fabrication of Cu damascene and through silicon via interconnects.

The Impact of Adsorbates on Metal Deposition through the Curvature Enhanced Accelerator Coverage Mechanism

Superconformal electrodeposition utilizing deposition accelerating additives enabled the implementation of damascene interconnects in microelectronics using copper “superfill” in the late 1990’s. The underlying Curvature Enhanced Accelerator Coverage (CEAC) mechanism was defined in 2001. Since then, superconformal deposition of copper, silver and gold in fine features as well as the formation of optically smooth, soft deposits has been successfully predicted by CEAC-based models that capture the interplay of deposition-rate accelerating adsorbate coverage and metal deposition through area change on nonplanar surfaces. Void-free filling of high aspect ratio, submicrometer trenches and vias with silver and gold has been quantitatively predicted and demonstrated using additives selected from the literature on catalysis of metal deposition by additive adsorption. These additives also provide, through the same CEAC mechanism, a means of obtaining optically smooth deposits without relying on deposit-hardening grain refinement through additive incorporation. I will summarize results to date, including what we believe we understand as well as what we don’t.

The Impact of Adsorbates on Metal Deposition through the Curvature Enhanced Accelerator Coverage Mechanism and Beyond

Superconformal electrodeposition utilizing additives that adsorb on the deposit surface and either enhance or suppress metal deposition enabled the implementation of Cu damascene interconnects in microelectronics. The filling process is a consequence of the Curvature Enhanced Accelerator Coverage (CEAC) mechanism, which captures the interplay between adsorbate coverage and the metal deposition rate during area change that necessarily accompanies growth on nonplanar surfaces. CEAC-based models have successfully predicted superconformal deposition using a variety of different chemistries that yield void-free filling of fine features, as well as optically smooth, surfaces of metals such as Cu, Ag and Au. Herein a brief review of advances in the superconformal Au deposition will be detailed. Of particular interest are additives whose behavior is non-monotonic with adsorbate coverage that present new challenges for understanding as well as interesting opportunities for application.

Superconformal Film Growth: Challenges and Opportunities

State of the art manufacturing of semiconductor devices involves electrodeposition of copper for device wiring and more recently for through-silicon-vias (TSVs). The process depends on the use of electrolyte additives that affect the local deposition rate thereby resulting in superconformal, or bottom–up “superfilling” of trenches and vias. In the case of Damascene processing of submicron features the deposition process is explained by the recently developed curvature enhanced accelerator coverage (CEAC) mechanism. The model stipulates that 1.) the growth velocity is proportional to the local accelerator, or catalyst, surface coverage and 2.) the catalyst remains segregated at the metal/electrolyte interface during copper deposition. For growth on non-planar geometries this leads to enrichment of the catalyst on advancing concave surfaces and dilution on advancing convex sections; thereby giving rise to bottom-up superfilling of recessed surface features such as trenches and vias. In the case of larger features other avenues for feature filling become available that will be outlined in some detail. Despite the success of these superconformal growth processes much remains to be known concerning the detailed molecular nature of the competitive co-adsorption processes involved in both the suppression and acceleration of the metal deposition reaction. The application of in situ surface science tools such as in situ STM and SEIRAS towards a better understanding of the additive interaction will also be detailed. Finally, the talk will conclude with an overview of the current understanding of additive adsorption, consumption and/or deactivation dynamics relevant to superfilling and microstructural evolution in both Damascene and TSV Cu processing.

Superconformal Film Growth: Insights Provided By Seiras

State of the art manufacturing of semiconductor devices involves electrodeposition of copper for device wiring and more recently for through-silicon-vias (TSVs). The process depends on the use of electrolyte additives that affect the local deposition rate thereby resulting in superconformal, or bottom–up “superfilling” of trenches and vias. In the case of Damascene processing of submicron features the deposition process is explained by the recently developed curvature enhanced accelerator coverage (CEAC) mechanism. The model stipulates that 1.) the growth velocity is proportional to the local accelerator, or catalyst, surface coverage and 2.) the catalyst remains segregated at the metal/electrolyte interface during copper deposition. For growth on non-planar geometries this leads to enrichment of the catalyst on advancing concave surfaces and dilution on advancing convex sections; thereby giving rise to bottom-up superfilling of recessed surface features such as trenches and vias. In the case of larger features other avenues for feature filling become available that will be outlined in some detail. Despite the success of these superconformal growth processes much remains to be known concerning the detailed molecular nature of the competitive co-adsorption processes involved in both the suppression and acceleration of the metal deposition reaction. The application of in situ surface science tools such as in situ STM and SEIRAS towards a better understanding of the additive interactions will also be detailed. The interaction between adsorbed anions, polyether suppressors, sulfonate terminated thiol/disulfide accelerator additves and water at the growing interface reveal the critical role of hydrophobic/hydrophilic interactions in controlling the growth velocity during metal deposition. The talk will conclude with an overview of the current understanding of additive adsorption, consumption and/or deactivation dynamics relevant to superfilling and microstructural evolution in both Damascene and TSV Cu processing.

Numerical Simulation and Experimental Verification of Additive Distribution in Through-Silicon Via during Copper Filling Process

Numerical model was built based on a set of PDEs and the curvature-enhanced accelerator coverage (CEAC) mechanism. Arbitrary Lagrange-Eulerian (ALE) method was used to track the moving cathode surface for describing via filling process. The filling profile changed from conformal model to bottom-up model due to the CEAC mechanism. Contour lines were colorized to reflect the local surface additive distribution. Additive surface concentration distribution in through-silicon vias during the copper deposition process was investigated according to the texture coefficient Mhkl of deposits using the micro-area X-ray technique. Surface concentration of accelerator in the via bottom was much larger than that in the top region due to the additive concentration gradient in via and the reduction of bottom surface during via filling process. The V-shaped filling profile demonstrated the rationality of the achieved accelerator surface concentration distribution. Evolution of a submonolayer quantity of a surface-confined accelerator mainly caused the super-conformal filling of TSVs. These provide a novel method to investigate the additive surface concentration distribution in the micrometer scale through-silicon via. Simulation results were compared with the experimental data; the changing tendency of surface concentration of accelerator along the via direction in simulation result is in accordance with the experimental result qualitatively (decrease from via bottom to via top region, and there is a slight increase in the top region).

Bottom-up Copper Deposition in Damascene Features Using Alkali Electrolytes

I will discuss deposition processes in patterned features such that filling starts towards the feature bottoms with deposition then proceeding upward. In particular I will present results for alkaline electrolytes that exhibit critical behavior in the presence of a single deposition-rate suppressing additive and, for deposition under conditions that take advantage of the critical behavior, exhibit localized filling within patterned features and bottom-up filling of those features. I will detail bottom-up filling during copper electrodeposition in submicrometer features. I will point out the electrochemical and feature filling characteristics shared with previously observed bottom-up filling with both copper and gold in TSVs by a mechanism based on the resistively stabilized negative differential resistance evident in the voltammetry for the electrolytes. I will also point out the differences from superfilling of fine features using the curvature enhanced accelerator coverage mechanism.

Superfilling When Adsorbed Accelerators Are Mobile

Bottom-up superconformal feature filling during electrodeposition, called “superfilling,” that is used for industrial processing of damascene copper interconnects has also been demonstrated during electrodeposition of silver and gold. The curvature enhanced accelerator coverage (CEAC) mechanism has been proposed to underlie all three processes and has been used to quantitatively predict observed filling of patterned features. The key feature of the CEAC mechanism is redistribution of adsorbed additives through changes of local surface area as dictated by mass conservation and the relative strengths of adsorption. Previous studies of CEAC-mediated superfilling have neglected adsorbate diffusion along the surface that might arise during deposition due to the CEAC-induced gradients in surface coverage. This paper extends the CEAC model to include such diffusion, applying the resulting formulation to understand differences in the geometries of experimental superfilling systems.

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).

Gold Superfill in Submicrometer Trenches: Experiment and Prediction

Bottom-up deposition of gold in fine trenches, also called “superfill,” was recently demonstrated using a submonolayer coverage of preadsorbed, deposition-rate-accelerating lead followed by gold electrodeposition. The present study has used experiments on planar substrates to quantify the effect of Pb adsorption on the Au deposition rate and the rate at which the adsorbed Pb was consumed during the Au deposition process. The values obtained have been incorporated into the curvature enhanced accelerator coverage mechanism of superfill where they were used to quantitatively predict the nonconformal, bottom-up deposition observed during filling of the patterned features. The results indicate the potential of the process for damascene interconnect fabrication in GaAs, GaN, and related technologies.

Geometry Effect on Impurity Incorporation and Grain Growth in Narrow Copper Lines

Knowledge of the geometry effect on impurity incorporation and grain growth in narrow lines is important for reducing copper line resistivity. In this paper, we investigate impurity incorporation in narrow lines with time-of-flight secondary ion mass spectroscopy. We also study the influence of linewidth, trench depth, pattern density, and overburden on copper grain growth in reduced dimensions. The concentration of chlorine and carbon is found to increase with decreasing linewidth, while the concentration of sulfur is close to the detection limit. This effect contributes to copper superfilling and is consistent with the curvature-enhanced accelerator coverage model. Copper self-anneal slows down as linewidth decreases, although an opposite trend is observed for lines below a width of about . Impurity incorporation and geometric constraint retard copper grain growth in narrow lines, resulting in an inverse relationship between copper line resistivity and geometry. However, copper overburden, which has a much larger grain size, can enhance grain growth in narrow lines depending on the line geometry. Reducing impurities and balancing trench depth and copper overburden can be used to reduce the resistivity of narrow copper lines.

Superconformal film growth: Mechanism and quantification

Superconformal electrodeposition of copper is explained by the recently developed curvature-enhanced-accelerator coverage (CEAC) model, which is based on the assumptions that 1) the local growth velocity is proportional to the surface coverage of the accelerator, or catalyst, and 2) the catalyst remains segregated at the metal/electrolyte interface during copper deposition. For growth on nonplanar geometries, this leads to enrichment of the catalyst on advancing concave surfaces and dilution on advancing convex sections, thereby giving rise to bottom-up superfilling of submicrometer trenches and vias. In this paper the robustness of the CEAC model is demonstrated by characterizing the kinetics of catalyst accumulation and consumption in a series of electroanalytical experiments on planar electrodes. The model is then used to successfully predict interface shape evolution during feature filling in a variety of experiments, without using adjustable parameters.

Impurity Incorporation during Copper Electrodeposition in the Curvature-Enhanced Accelerator Coverage Regime

Impurity Incorporation during Copper Electrodeposition in the Curvature-Enhanced Accelerator Coverage Regime

Superfilling and the Curvature Enhanced Accelerator Coverage Mechanism

State-of-the-art manufacturing of semiconductor devices involves the electrodeposition of copper for on-chip wiring of integrated circuits.