Superconformal CoED on patterned Si wafers
Until recently, the manufacture of microprocessor wiring has almost exclusively been based on superconformal Cu electrodeposition (CuED) processes on patterned Si wafers29. However, the critical dimension of current microchip structures has nowadays approached the electron mean free path (MFP) of copper, introducing new challenges to the continuous scaling of interconnects for the 7 nm technology node and beyond30-32. In this context, Co is thought to be less prone to resistance scaling effects and therefore more suitable for further downsizing device dimensions33-35. Figure 1a shows a micrograph of a 300 mm Si wafer containing hundreds of advanced processor dies whose components are interconnected by void-free electrodeposited Co and Cu wiring. The specimen displayed by Fig. 1b reveals the intricate architecture of individual dies before metal deposition that comprise micro and nanoscale circuitry. Note that the manufacture of the novel Co interconnects by wet methods poses new technological challenges36.
Figure 1c schematically summarizes the electrochemical components of the overall surface process occurring during superconformal Co electroplating of patterned Si wafers. Specifically, unlike CuED, CoED processes carried out from aqueous plating baths are unavoidably accompanied by the hydrogen evolution reaction (HER), thereby developing pH gradients adjacent to the surface of the patterned wafers as they proceed37-39. The reaction rate of these two components (CoED and HER) is expressed in Fig. 1c in terms of their respective position-dependent electric current densities jCo and jHER. The scheme illustrates the strong dependence of the effective Co deposition (jCo) and the parasitic HER (jHER) on the specific local topography of the substrate. Thus, bottom-up metal deposition and H2 evolution occur preferentially on the recessed patterned features and their inhibition takes place on the non-patterned/planar surface regions. This enables the so-called superconformal CoED on high aspect ratio patterned features that is schematically shown in the inset of Fig. 1c. These differential Co deposition and H2 evolution are achieved by the action of an inhibitor additive that is selectively adsorbed on the planar areas and upper side walls of the patterned trenches36, 40. Moreover, an electrochemically inactive but pH-sensitive compound is employed to probe the evolution of local pH gradients across the interface. The delayed activation of this compound is indeed achieved through its pH-guided aggregation.
pH-triggered aggregation of the pH-probe
Supplementary Fig. 1 presents beaker-scale TE experiments in which a green laser beam (532 ± 10 nm, 5 mW, beam diam. 2.5 mm) is passed through a series of inhibitor- and pH-probe-containing Co plating baths (50 mM CoSO4 in 0.5 M H3BO3) with different pH values ranging from 2 to 6. Control experiments carried out with plating solutions containing no pH-sensing compound are also shown. These results demonstrate that TE enhancement by aggregation phenomena occur when the solution pH surpasses the value of 4.5 only in the plating bath that contains the pH-sensing compound. This aggregation stems from deprotonation of the pH-probe compound at that critical pH 4.5 followed by complexation with the Co2+ ions present in the electrolyte, thus leading to local precipitation of cross-linked agglomerates. The presence of Co2+ species in those precipitates was revealed by elementary EDX analysis in combination with control beaker scale TE experiments (Supplementary Figs. 2 and 3).
As above mentioned, when conducting additive-assisted metallization of the patterned wafers both CoED and HER preferentially take place on the recessed areas. Under typically applied process conditions, as the Co deposition proceeds on the recessed areas, a local pH gradient appears and develops at the front of the rising metal growth owing to H+ consumption and mass transport limitations. Fig. 2a-b illustrates the corresponding dynamics of the interfacial double layer. In a further stage of the process the synchronized consumption of H+ ions on top of the trenches (with pH reaching the critical value) and completion of superconformal metal filling results in pH-probe precipitation in front of the filled patterned areas.
Colorimetry approach based on TE enhancement at electrified patterned Si dies
In the following we show how we exploit these pH-driven surface agglomeration phenomena to address visualization of local proton dynamics in real time at the microscopic level over extended electrode areas with high spatial resolution. Fig. 3a illustrates the operating principle of the developed operando TE-based colorimetry approach. The galvanostatic Co plating experiments (i.e., at constant j) were carried out on single patterned Si dies in an electrochemical glass cell with plating electrolytes (50 mM Co2SO4 in 0.5 M H3BO3) containing the inhibitor- and pH-sensing compound. The electrolytes with initial pH 2.536 were agitated at different frequencies by a magnetic stirrer at 100-300 rpm to mimic solution flow on the die surface similar to the solution flow applied in a real plating tool. A large surface area Pt wire acted as anode and the measurements were controlled by an Autolab 302N potensiostat/galvanostat Metrohm system. Two non-invasive optical probes consisting of two green laser beams operated at equal intensities, aligned in parallel and separated by 5 mm from each other were accurately positioned by a differential micrometer screw so that they passed just adjacent to the outermost sample surface during the Co electroplating experiments. The primary laser probe (L1) was passed over rows of patterned fields bearing features with varying feature pitch (40-120 nm) and 100 nm average trench depth while the secondary laser beam (L2) probed the surface of non-patterned mirror-like sample locations (Fig. 3b). A conventional camera outside the glass cell positioned in front of the sample surface as depicted in Fig. 3c was used to record the time-evolution of the TE enhancement and the associated pH dynamics.
Real-time visualization of pH dynamics and CoED process progression
The time evolution of surface pH and agglomeration events enabled by the proposed approach are exemplarily illustrated in Fig. 4. The micrograph presented in Fig. 4a shows the geometry and characteristic dimensions of the patterned domains that were monitored by the laser L1. Fig. 4b displays selected screenshots recorded by the camera at progressive stages during Co deposition. Additional experiments at 100 rpm applied solution stirring are provided by Supplementary Fig. 4 and the corresponding video. The labels on the upper left corner in each subpanel indicate the time elapsed (t) after the electrochemical process was initiated. The absence of TE enhancement on the die surface indicates that the surface pH remains lower than 4.5 during the first six seconds of electrodeposition at all sample locations. Then at t = 7 s light scattering events start becoming apparent only at specific sample locations adjacent to the probing laser L1 (only on patterned sample regions with feature pitch 40-50 nm that develop large local current densities). Conversely, no apparent scattering events other than background events can be observed at the interfacial sample regions probed by L2. This indicates that at this stage, precipitation of the pH-probe compound selectively takes place at those patterned sample locations because their local pH reaches the critical value due to HER as the front of the deposited Co approaches the outermost sample surface. Note that these agglomeration events remain physically confined to the parallel patterned domains that in the probed surface region are separated from each other by 50 mm spacings (see Supplementary Fig. 5) and that fields with larger feature pitch (85-120 nm) where the local current density is comparably lower have not yet reached the critical pH. This proves that our approach enables 3D mm-scale spatial resolution of the local pH-guided reactivity over laterally extended macroscopic samples and that the intensity of the associated TE enhancement directly correlates with the position dependent current density.
As the electrodeposition proceeds, within 8 s ≤ t ≤ 15 s, an increasing number of patterned fields gradually achieve successful superconformal filling and the concomitant change of pH and pH-probe precipitation occurring in the locations probed by L1 can be instantly and straightforwardly followed based on the observed scattering enhancement. At t = 16 s the increase in pH far beyond the critical value and the accumulation of precipitated Co2+–pH-probe complex become obvious from the increased TE enhancement at those locations that were firstly filled. At this moment light scattering becomes observable for the first time on the non-patterned sample surface probed by the L2 beam. The reason for this delay in local pH increase and accompanying pH-probe precipitation is the presence of the inhibitor layer adsorbed on the flat sample areas that slows down the kinetics of both the HER and CoED36. Continuation of the galvanostatic Co deposition eventually leads to the interfacial pH surpassing the marker threshold at all sample locations that results in the even Co deposition along the entire Si die. Specifically, at t ≥ 24 s the increased pH > 4.5 on top of the flat sample locations separating recessed domains leads to losses in spatial resolution. Finally, at t ≥ 35 s massive Co2+–pH-probe precipitation due to the progressive alkalinisation of the diffusion layer all over the sample sets in and both L1 and L2 probes reveal increasingly similar scattering phenomena. The higher intensity of the light scattering is, nonetheless, still observable on the patterned regions.
To demonstrate that the observed pH dynamics and resulting agglomeration originate from surface-confined electrochemically-driven transformations we carried out the following control investigations. First, CoED experiments performed under identical plating conditions as discussed above but placing the laser system 50 mm away from the sample surface exhibited much weaker scattering enhancement. Further, a second series of reference experiments carried out with laser irradiation adjacent to the sample surface but without pH-probe in the plating electrolyte showed no TE enhancement at all surface metallization stages. Finally, experiments with all plating components but conducted at zero total transferred charge (in the absence of electrochemical processes) equally showed no TE enhancement. The corresponding supplementary Figs. 6-8 are included in the Supplementary Information file.
Confirmation by numerical simulations
The scenario described before can also be confirmed by finite element-based numerical simulations if we assume variables (Co2+ concentration, pH, applied current density j) that exactly match the applied experimental conditions, and we use physical parameter values (charge transfer and reaction rate coefficients) in the simulations that were described elsewhere for Co electrodeposition. In the simulation routine (details are explained in the Supplementary Information file), it is enough to handle two simultaneously occurring electrode processes (CoED and the HER) and to account for mass transfer processes supplying reactants. To deal with surface heterogeneities, we assumed that both electrode reactions have a 100-fold decreased activity over the (additive-blocked) planar surfaces, while the reactions run unhindered at trenched areas. Fig. 5 presents the time evolution of pH profiles on the surface of a simulated Si die segment that comprises three patterned fields (black recessed rectangles). Selected screenshots of the proton dynamics at times matching those displayed in Fig. 4b are shown and those of the corresponding Co2+ concentration profiles can be found in supplementary Fig. 9. Both pH and Co2+ concentration profiles show that protons and Co2+ ions are more rapidly consumed on the surface of the patterned fields than on flat sample areas in the course of the plating process. Note in Fig. 5 that as the electrolysis proceeds, the resulting accelerated alkalinisation beyond the critical pH value at t = 9 s initially remains confined on top of the recessed fields and that for the following 3 s there is no overlap of individually developing mushroom-like pH profiles. This matches the evolution of the pH-guided precipitation phenomena revealed by the L1 probe during the first 16 s in Fig. 4b. Despite the slight temporal mismatch between the experimental and the simulated data, their qualitative agreement with regards to pH dynamics and pH-guided agglomeration is remarkable. At more advanced process stages (t > 12 s in Fig. 5), attainment and subsequent surpassing of the critical pH value not only on recessed areas but also on top of the adjacent flat sample locations leads to the loss in lateral spatial resolution of the interfacial pH concomitant with the expansion of the diffusion layer. This correlates with the experimental Tyndall effect observations at t ≥ 24 s in Fig. 4b.
Overall, the presented operando TE-based approach constitutes a significant progress in analytical strategies aiming at investigating the dynamics of the interfacial double layer because it provides instantaneous extended pH read-out on surfaces with distinct local reactivities and unique spatiotemporal resolution only limited by the acquisition capabilities of the employed camera (here 30 three-dimensional reactivity maps per second). Besides straightforwardly enabling comparative reaction kinetics intrinsic to complex electrode architectures, the approach is also amenable to quantification and exhibits outstanding sensitivity28. Finally, its applicability can be universally extended to any other nanoaggregation/decomplexation process of interest (e.g., electrocoagulation, mineral removal and separation, electrosynthesis, among others) occurring at light-addressable interfaces provided the probed colloids are smaller than the wavelength of visible light. This work provides therefore useful means to improve the understanding of such interfacial processes and control over them.