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The emerging 3-dimensional integration relies on components such as through-silicon vias (TSVs) and micro-bumps for vertical stacking [1, 2]. In a typical micro-bump fabrication process, a thin Cu seed layer is coated with a thick layer of photoresist. After exposure and development, cylindrical openings are formed in the photoresist layer. Depending on the application, bumps with single or multiple metal layers, one of them typically being Cu, are deposited. The removal of photoresist is followed by wet etch of the Cu seed, revealing individual micro-bumps. While Cu in the ‘body’ of the micro-bump is deposited electrochemically, the Cu seed layer is typically fabricated using physical vapor deposition (PVD). As the plated Cu etches up to 30% faster than PVD Cu [3], the Cu part of the bumps may suffer from significant radial etch before the seed is removed completely. Figure 1 shows the post-etch SEM image of a micro-bump with Cu-Ni-Sn metal stack. For 4 µm micro-bumps plated on a 200 nm Cu seed, the contact area with the pad will be reduced by about 25 % after seed etch. In order to minimize the ‘under-etch’ of the micro-bumps, the thickness of the Cu seed layer should be reduced. However, the reduction of Cu seed layer thickness leads to the well-known ‘terminal effect’ on the wafer scale [4]. Figure 2 illustrates the cross-section of a plating cell. Due to the potential drop along the radial direction, the plating current density decreases from the wafer edge to the wafer center, resulting in the micro-bump height decreasing from the wafer edge to the wafer center. In this study a quick and cost-effective approach was developed to evaluate the micro-bump height uniformity on wafer level. We studied dependency of micro-bump height uniformity on Cu seed thickness using 300 mm wafers and demonstrated the impact of terminal effect on height variation along the radial direction. By using only the experimentally measured i-ηcurve for the kinetics, numerical simulation of the wafer-scale plating uniformity was significantly simplified. As the proposed methodology required no explicit knowledge of the plating bath composition and only involved coupon-scale experiments, it provided a fast and cost-effective way to evaluate the effectiveness of proprietary plating chemistries with respect to wafer-scale height uniformity of micro-bumps. The usefulness of the proposed approach was supported by the good match between simulation results and the corresponding experimental measurement. Although the simulation capability was only demonstrated for Cu, the same approach can be applied to other layers deposited for micro-bumps. The overall uniformity can be readily calculated through summation of all the layers involved. The simulation program can also be modified to address different profile control approaches such as additional current collectors or anodes with modified geometry. References: [1] Y. Civale, D. S. Tazcan, H. G. G. Philipsen, P. Jaenen, R. Agarwal, F. Duval, P. Soussan, Y. Travely, E. Beyne, Die stacking using 3D-wafer level packaging copper/polymer through-Si via technology and Cu/Sn interconnect bumping,” IEEE International 3D System Integration Conference, 2009. [2] R. Agarwal, W. Zhang, P. Limaye, R. Labie, B. Dimcic, A. Phommahaxay, P. Soussan, Cu/Sn microbumps interconnect for 3D TSV chip stacking, IEEE Electronic Components and Technology Conference (ECTC), 858 (2010). [3] Wet etch rate PVD Cu vs ECD Cu is internal imec data for a given specific process. [4] S. Armini, P.M. Vereecken, Impact of terminal effect on Cu plating: theory and experimental evidence, ECS Transactions 25 (2010) 185. Figure 1

In this paper, we evaluate PECVD carbon coating for TiN/Si substrate as a technologically relevant current collector for 3D thin film Lithium ion battery applications. Carbon coated TiN/silicon pillars (with aspect ratio of 30) served as substrate for electrolytic manganese dioxide (EMD) deposition from aqueous bath to be tested as positive electrode for Lithium ion batteries. Reversible insertion and extraction of Li+ is observed showing an improved cycle performance for carbon coated TiN substrates compared to Pt coated TiN substrates. The average lithium storage capacity for planar EMD films is about 50mAh/cm2 (at 10mV/s comparable with charging rate of 20 C). Based on a surface area gain of 28 for the 3D pillar structures, a capacity of 0.2mAh/cm2 is expected for conformal coated 200 nm film at these high rates.

Among all the Ru-based substrates, mixed-phase RuTa liners grown by physical vapor deposition have been explored as potential directly-platable diffusion barrier candidates. In order to understand the full-wafer copper direct plating process that occurs on these liners, the effect of the applied waveform, electrical contacts and suppressor chemistry have been investigated. In order to enhance copper nucleation and wafer scale edge-to-center copper front propagation rate, a liner surface cleaning protocol is developed. The copper front propagation across the 300 mm wafer is reported as a function of the RuTa film thickness. An optimized copper direct plating process on RuTa layers as thin as 4 nm is integrated in 90 and 30 nm half pitch single and dual damascene structures. Results in terms of compatibility of the direct plated copper with the following chemical mechanical planarization step complemented with physical and electrical characterization data are reported. The direct filling of ~25 nm sacrificial fin structures is also investigated.

GaN nanopillar arrays are fabricated by inductively-coupled-plasma dry etching of a GaN epitaxial layer, using self-assembled Ni clusters as mask. Pillars of 0.4-1.6 μm height were prepared and were investigated photoelectrochemically. After the roughening, the surface area increases up to 6 times and the plateau photocurrent increases by 84% with respect to the planar GaN. The enlarged GaN/electrolyte interface promotes charge transfer and photocarrier separation in the pillar array. The defect-mediated carrier recombination becomes more important after the dry etching process. The impact of surface damage is studied by the photoelectrochemical and photoluminescence measurements.

For the application of durable micro-storage for autonomous systems and implants, 3D thin-film batteries are leading candidates. Lithium ion batteries have the highest energy density of all known systems and are thus the best choice also for these rechargeable micro-batteries. Since liquid electrolyte based batteries present safety issues and limitations in size and design, pure solid state devices are considered particularly for miniaturization. Planar thinfilm lithium and lithium ion batteries are at present commercially available but have limited capacity (2). The capacity can be increased by coating the thin-film stacks on micro-or nanostructured surfaces (3D thin-film batteries). The main advantage of thin-film (TF) batteries is that its power can be increased by scaling down the film thickness. The main technological challenges in the introduction 3D TF batteries are pinhole-free and chemically uniform conformal coating of the cathode, electrolyte and anode thin-film stack.

The effect of boric acid on the electrochemical deposition of nickel nanoparticles is presented in this paper. We show that lowering the concentration of H3BO3 in the electrolyte increases the particle density. At the same time, it was observed that the deposition of nickel stops at an earlier stage when a low concentration of boric acid was used. Both observations can be explained by the formation of an insoluble Ni(OH)2 which was counteracted by the addition of H3BO3. We were able to reduce the current density needed to obtain similar particle densities by almost an order of magnitude.

The effect of the molecular mass (Mm) of polyethylene glycol (PEG) and its derivatives on the copper island density (Np) on ruthenium-tantalum (RuTa) alloy was investigated by utilizing scanning electron microscopy (SEM) and galvanostatic methods. The experimental results show that the island density is affected by the strength of suppression which can be modified by using different suppressor molecules. Furthermore, we use galvanostatic measurements to evaluate the suppression strength and show that potential-time transients recorded during galvanostatic deposition of Cu can be used to estimate the island density.

The copper interconnect technology is constrained by the significant current distribution due to the terminal effect for resistive thin seeds. As a result, the current in the wafer center can become insufficient for cathodic protection of thin copper seeds leading to center seed corrosion. To improve the current distribution, the exchange current density can be lowered e.g. by lowering the copper concentration. Our approach is to investigate and develop acid ultra-low copper baths with feature fill capability. For this goal, the optimum additives concentrations were sought and fill studies in 30 nm trenches were performed. In addition, proposing a high acid chemistry that is also compatible with direct plating, would enable the in-situ copper seed formation and filling in one and the same process step for direct plating on non-copper seed.

Silicon nanowires are attractive for photovoltaic applications where they can be used along with bulk silicon in an all-Si tandem solar cell. The larger band gap, caused by the quantum confinement effect in narrow silicon nanowires (< 5 nm), provides a more efficient light absorption. Most common processes for nanowire synthesis are rather expensive and require high temperatures, high vacuum and hazardous precursors. A simple and cheap method is the silver-assisted electroless Si etching process. Silicon is selectively etched in HF based solutions with help of silver particles which are deposited beforehand or in-situ. Both the etch process and the deposition of silver particles were studied. The silver nanoparticles were deposited by electroless deposition (galvanic displacement) from HF and non-HF containing solutions. The effects of silver coverage, Si doping and illumination on the Si etching process were investigated. The experimental observations were used to get more insight into the mechanism.

The electrochemical nucleation and growth of copper on a ruthenium-tantalum (RuTa) alloy has been studied using galvanostatic methods. The experimental results show that the island density is highly dependent on the bath composition and deposition parameters. High island density was achieved with higher applied current, addition of suppressor and low copper concentration in the plating bath.

The acid copper sulfate solutions used for Damascene plating contain several additives which provide the differential kinetics inside cavities for void-free fill. Although the synergetic effects between these additives were studied quite thoroughly for the last 2 decades, the underlying mechanisms are still not fully understood. In this work we provide some further insight into the physicochemical mechanism of copper deposition inhibition by the suppressor polymer. The quantitative participation of each solution component in the formation of Cu(I)-Suppressor-Cl complex is examined. In addition, our experiments show that ionic strength has a profound effect on the adsorption/desorption mechanism.

CoFe alloys with 50%-70% Fe have the highest magnetic moment of 2.4 Tesla. There have been numerous research and development studies dedicated to achieving 2.4 Tesla soft magnetic films by electrodepositing CoFe and CoFeNi alloys. The high tensile stress of the electrodeposited high moment films is one of the key causes slowing down the implementation of CoFe alloys. There have been few reported studies on the structure and stress evolution during the deposition of CoFe film either by vacuum processes or solution processes. This work reports on the preliminary finding related to the tensile stress in electrodeposited CoFe films as a function of grain size, alloy composition, and other deposition parameters. The residual tensile stress was found to increase with descreasing grain size in the film, which was accompanied by the increase of iron content in the films. Pulse plating study demonstrated to have little effect on film stress. Solution temperature was found to be highly effective in reducing tensile stress in the film.

We have studied electrochemical deposition of copper on ruthenium-tantalum (Ru-Ta) alloy, tantalum (Ta), and cobalt (Co) substrates using cyclic voltammetry and galvanostatic methods. We show that a single-step direct-plating from acidic Cu bath approach is favorable on thin Ru-Ta films, while it presents a significant challenge for plating on resistive Ta and Co substrates.

Carbon nanotubes (CNTs) are considered a promising material for interconnects for future generation microchips. The integration of vertical CNT in a processing environment is evaluated in this work. Extrapolated performances of CNT-based interconnects are compared with existing technologies at different hierarchy levels including the limitations of present deposition methods for copper and tungsten. For practical implementation, CNT bundles were selectively grown into contact holes using physical vapor deposited and electrochemical deposited cobalt or nickel catalysts. A polishing step was used to control the CNT length after embedding the CNT into an oxide matrix. A CNT metal decoration method based on electrodeposition is presented, which can be used to assess the yield of electrically conductive CNT as well as to form top contacts for electrical characterization. Finally, the importance of having suitable and robust structures for evaluating the integration process is highlighted after the electrical characterization of CNT in a nanoprober station.

Nickel catalyst nanoparticles acting as nucleating seeds for carbon nanotube (CNT) growth were selectively deposited on the sidewalls of titanium nitride (TiN) electrodes by electrochemical deposition (ECD) from a Ni2+ electrolyte solution. Horizontal aligned CNTs were grown selectively from the sidewalls of these TiN electrodes forming a bridging CNT contact between the electrode gaps. Current-voltage measurements demonstrated this could be a promising technique towards nanoscale interconnections and nanoelectronic devices with resistance values for the bridging CNTs as low as 300Ω.

We have studied electrochemical nucleation and growth of copper on ruthenium and ruthenium-based substrates using galvanostatic methods. We show that potential-time transients recorded during galvanostatic deposition of Cu on Ru can be used to study the effects of additives, deposition parameters, and substrate pre-treatment on Cu island density, and help determine experimental parameters needed to facilitate high island densities and quick sweep of a Cu film front across the resistive substrates.

The formation of thin alumina films directly on Si is reported. Galvanostatic and potentiostatic anodization in two- and three- electrode cell configurations are compared. Control of the electrode potential with a 3-electrode setup provides faster pore formation and organization than with control of the cell potential in a 2-electrode configuration. Higher pore density and smaller pore sizes are obtained for potentiostatic control than for galvanostatic control. Internal consistency was found between equations describing the steady-state current-voltage characteristics and the pore density and size.

In this paper, we report on the electrochemical deposition of manganese and copper-manganese alloy on silicon from sulfate solutions with and without complexing agents. We show that the composition of copper-manganese films deposited on n-type (100) silicon can be controlled using different deposition parameters and bath compositions. Manganese was deposited on p-type (100) silicon only from the simplest solutions, containing no copper and complexing agents.

The electrochemical nucleation and growth of nickel on silicon from NiNO3 solution has been studied based on the Scharifker and Hills model for nucleation and diffusion limited growth of 3D hemispherical islands. The analysis of the experimental transients shows that a potentials between -1.2V and -1.3V the nucleation is progressive but becomes dominantly instantaneous at -1.4V and beyond. The deposited particle density for the different deposition potentials has been determined both by analysis of the current transients and by SEM analysis. The best result for particle size and size distribution was 25{plus minus}10nm and was obtained at a potential of -1.5V.