Superconformal Film Growth: From Smoothing Surfaces to Interconnect Technology

Published

April 20, 2023

Author(s)

Thomas P. Moffat, Trevor Braun, David Raciti, Daniel Josell

Abstract

CONSPECTUS: State-of-the-art manufacturing of electronics involves the electrodeposition of Cu to form 3-D circuitry of arbitrary complexity. This ranges from nanometers wide interconnects between individual transistors to increasingly large multilevel intermediate and global scale on-chip wiring. At still 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 these applications is void-free Cu filling of lithographically defined trenches and vias. Significantly, while line-of-sight physical vapor deposition processes are unable to accomplish this feat, the combination of surfactants and electrochemical or chemical vapor deposition enables preferential metal deposition within recessed surface features known as superconformal film growth. The same 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 Cu〖SO〗_4 electrolytes include a combination of halide, polyether suppressor, sulfonate-terminated disulfide and/or thiol accelerator and in many cases a N-bearing cationic leveler. A variety of competitive and co-adsorption 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 formation of a polyether suppressor layer. At the same time halide serves as co-surfactant supporting the adsorption of disulfide species on the surface while inhibiting sulfide formation and incorporation into the growing deposit. Further still, the dangling hydrophilic sulfonate end-group of the accelerator enables activated metal deposition by hindering assembly of the polyether suppressor. A common thread in superconformal feature filling is additive-derived positive feedback of the metal deposition reaction within recessed, or re-entrant, regions. For sub-micrometer features or, more generally, optically rough surfaces, area reduction that accompanies motion of concave surface segments results in enrichment of the most strongly bound adsorbates, which for the above suppressor-accelerator systems is the sulfonate-terminated disulfide accelerator species. The superfilling and smoothing process is quantitatively captured by the curvature enhanced adsorbate coverage (CEAC) 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 rise to a negative differential resistance and related non-linear effects on morphological evolution. Significantly, for certain suppressor-only electrolytes, remarkable bottom-up feature filling occurs where metal deposition disrupts the inhibiting adsorbates at the bottom of the TSV and/or overruns the ability of the suppressor to form due to kinetic and/or transport limitations. Because the electrical response to changes in interface chemistry is far more rapid than mass transport processes deposition on planar substrates proceeds by bifurcation into passive and active zones giving rise to Turing patterns while on patterned substrates active zone development is biased towards the most recessed regions. Looking to the future the distinction between packaging and on-chip metallization is blurring as the dimension of the former merge with that of early day on chip 3D metallization.

Citation

Accounts of Chemical Research

Pub Type

Journals

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Keywords

microelectronics, metallization, elecrodeposition, copper, superconformal growth, electrochemistry

Citation

Moffat, T. , Braun, T. , Raciti, D. and Josell, D. (2023), Superconformal Film Growth: From Smoothing Surfaces to Interconnect Technology, Accounts of Chemical Research, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=936061 (Accessed October 19, 2024)