Superconformal Film Growth: From Smoothing Surfaces to Interconnect Technology.

ConspectusElectronics manufacturing involves Cu electrodeposition to form 3D circuitry of arbitrary complexity. This ranges from nanometer-wide interconnects between individual transistors to increasingly large multilevel intermediate and global scale on-chip wiring. At larger scale, similar technology is used to form micrometer-sized high aspect ratio through-silicon vias (TSV) that facilitate chip stacking and multilevel printed circuit board (PCB) metallization. Common to all of these applications is void-free Cu filling of lithographically defined trenches and vias. While line-of-sight physical vapor deposition processes cannot accomplish this feat, the combination of surfactants and electrochemical or chemical vapor deposition enables preferential metal deposition within recessed surface features known as superfilling. The same superconformal film growth processes account for the long-reported but poorly understood smoothing and brightening action provided by certain electroplating additives. Prototypical surfactant additives for superconformal Cu deposition from acid-based CuSO4 electrolytes include a combination of halide, polyether suppressor, sulfonate-terminated disulfide, and/or thiol accelerator and possibly a N-bearing cationic leveler. Many competitive and coadsorption dynamics underlie functional operation of the additives. Upon immersion, Cu surfaces are rapidly covered by a saturated halide layer that makes the interface more hydrophobic, thereby supporting the formation of a polyether suppressor layer. Also, halide serves as a cosurfactant supporting the adsorption of amphiphilic molecular disulfide species on the surface while inhibiting copper sulfide formation and incorporation into the growing deposit. Furthermore, the dangling hydrophilic sulfonate end group of the accelerator enables activated metal deposition by hindering polyether suppressor assembly. A common thread in superconformal feature filling is additive-derived positive feedback of the metal deposition reaction within recessed or re-entrant regions. For submicrometer features or optically rough surfaces, area reduction that accompanies the motion of concave surface segments results in the most strongly bound adsorbates' enrichment, which for the suppressor-accelerator systems is the sulfonate-terminated disulfide accelerator species. The superfilling and smoothing process is quantitatively captured by the curvature-enhanced adsorbate coverage mechanism. For larger features, such as TSV, whose depths approach the thickness of the hydrodynamic boundary layer, significant compositional and electrical gradients couple with the metal deposition process to give a negative differential resistance and related nonlinear effects on morphological evolution. For certain suppressor-only electrolytes, remarkable bottom-up feature filling occurs where metal deposition disrupts inhibiting adsorbates at the bottom of the TSV or overruns the ability of the suppressor to form due to kinetic or transport limitations. Because the electrical response to changes in interface chemistry is more rapid than mass transport processes, deposition on planar substrates proceeds by bifurcation into passive and active zones, generating Turing patterns. On patterned substrates, active zone development is biased toward the most recessed regions. The distinction between packaging and on-chip metallization will be blurred as the dimensions of the former merge with those of early day on-chip 3D metallization.

Filament Growth and Related Instabilities during Adsorbate Suppressed Electrodeposition.

Anisotropic growth of a single filament on a microelectrode is demonstrated by galvanostatic electrodeposition in a bistable passive-active critical system. Specifically, a Cu filament is formed by disruption of a passivating polyether-halide bilayer triggered by metal deposition with positive feedback guiding highly localized deposition. For macroscale electrodes, complex passive-active Turing patterns develop, while for micrometer-sized electrodes, bifurcation is frustrated and a single active zone develops, which is reinforced by hemispherical transport. As deposition proceeds, hemispherical symmetry is broken with lateral propagation of a single filament while an increasing fraction of the applied current supports expansion of the passive sidewall area that eventually leads to termination of anisotropic growth. Different polyether suppressors alter the dynamic range between passive and active growth that determines the shape and extent of filament formation. The impact of electrode area, geometry, and applied current on morphological evolution was also briefly examined. The results highlight the utility of appropriately scaled microelectrodes in the study of growth instabilities during breakdown of additive suppressed layers in critical electrodeposition systems.

A High-Throughput Structural and Electrochemical Study of Metallic Glass Formation in Ni-Ti-Al.

On the basis of a set of machine learning predictions of glass formation in the Ni-Ti-Al system, we have undertaken a high-throughput experimental study of that system. We utilized rapid synthesis followed by high-throughput structural and electrochemical characterization. Using this dual-modality approach, we are able to better classify the amorphous portion of the library, which we found to be the portion with a full width at half maximum (fwhm) of >0.42 Å-1 for the first sharp X-ray diffraction peak. Proper phase labeling is important for future machine learning efforts. We demonstrate that the fwhm and corrosion resistance are correlated but that, while chemistry still plays a role in corrosion resistance, a large fwhm, attributed to a glassy phase, is necessary for the highest corrosion resistance.

Exploring the Kinetic and Thermodynamic Relationship of Charge Transfer Reactions Used in Localized Electrodeposition and Patterning in a Scanning Bipolar Cell.

Bipolar electrochemistry involves spatial separation of charge balanced reduction and oxidation reactions on an electrically floating electrode, a result of intricate coupling of the work piece with the ohmic drop in the electrochemical cell and to the thermodynamics and kinetics of the respective bipolar reactions. When paired with a rastering microjet electrode, in a scanning bipolar cell (SBC), local electrodeposition and patterning of metals beneath the microjet can be realized without direct electrical connections to the workpiece. Here, we expand on prior research detailing electrolyte design guidelines for electrodeposition and patterning with the SBC, focusing on the relationship between kinetics and thermodynamics of the respective bipolar reactions. The kinetic reversibility or irreversibility of the desired deposition reaction influences the range of possible effective bipolar counter reactions. For kinetically irreversible deposition systems (i.e., nickel), a wider thermodynamic window is available for selection of the counter reaction. For kinetically reversible systems (i.e., copper or silver) that can be easily etched, tight thermodynamic windows with a small downhill driving force for spontaneous reduction are required to prevent metal patterns from electrochemical dissolution. Furthermore, additives used for the bipolar counter reaction can influence not only the kinetics of deposition, but also the morphology and microstructure of the deposit. Cyclic voltammetry measurements help elucidate secondary parasitic reduction reactions occurring during bipolar nickel deposition and describe the thermodynamic relationship of both irreversible and reversible bipolar couples. Finally, finite element method simulations explore the influence of bipolar electrode area on current efficiency and connect experimental observations of pattern etching to thermodynamic and kinetic relationships.