Superconformal Electrodeposition of Copper in 500–90 nm Features

T. P. Moffat, J. E. Bonevich, W. H. Huber, A. Stanishevsky, D. R. Kelly, G. R. Stafford, D. Josell
2000 Journal of the Electrochemical Society  
The successful implementation of copper electroplating technology in the metallization of electronic devices derives from the use of electrolyte additives to affect the local deposition rate, thereby allowing superconformal or "superfilling" of trenches and vias in the Damascene process. 1 The first description of this process was adopted from the theory of additive-induced leveling whereby the electrolyte additive(s) adsorb at the metal electrolyte interface, reducing the active area and
more » ... tive area and thereby inhibiting the metal reduction process. 1,2 Since the inhibitor concentration is dilute (10 Ϫ3 to 10 Ϫ6 mol/L) the adsorption process is typically considered to be diffusion limited while the copper deposition process proceeds under interfacial charge-transfer control. In order to maintain transport control of the inhibitor flux, the adsorbate must either be incorporated in the growing film or undergo reductive desorption during metal deposition. For the Damascene process, the trench or via geometry results in a lower flux of the inhibitor to the bottom of the features than to the external surface. Consequently, the metal deposition kinetics proceed more rapidly at the bottom of the trench which results in superconformal filling. This model also predicts that the slowest growth occurs at the edge demarcating the entrance of the trench or via where the inhibitor flux is expected to be greatest. 3 However, it was necessary to empirically modify the simple area-blockage leveling theory in order to capture the experimentally observed corner rounding and general shape change behavior. 1 The complexity of additiveelectrode interactions was recently highlighted by the observation of "overfill" where the originally concave surface profile becomes convex due to a sustained differential rate of metal deposition that cannot be rationalized by the transport-limited inhibition model outlined here. 4-6 Rather, this observation has focused attention on the competition between species that accelerate vs. suppress deposition in the multicomponent additive packages, thereby demonstrating the need for detailed information concerning the surface chemistry of copper in solutions containing additives. In addition to influencing the kinetics of metal reduction, electrolyte additives are also known to exert a dramatic effect on the metallurgical microstructure. Specifically, certain additives sharply inhibit epitaxial growth, leading to fine-grained deposits and incorporation of the additives and/or the corresponding breakdown products as impurities within the film. 7-9 In certain instances, the physical defects and light element impurities lead to room temperature recrystallization within a few hours of deposition. This relaxation is typically accompanied by a beneficial reduction in the electrical resistivity of the deposits. 7,10 One of the difficulties associated with studying superconformal electrodeposition has been the proprietary nature of the processing parameters employed throughout the industry. Nonetheless, a brief survey of the literature suggests that electrolyte formulations that have previously been used for plating throughholes in printed circuit boards may be an effective starting point for exploring submicrometer superconformal deposition. 11-15 The first complete report of the chemistry and processing conditions used to demonstrate "superfill" in submicrometer dimensions was recently published based on the polyether, thiol, chloride, and diethyl safranine azo dimethyl aniline (Janus Green B) chemistry 16 that stems from the patent literature. 17 The films were deposited galvanostatically under quiescent conditions. The most effective superfilling under these conditions required the use of all four additives that resulted in the filling of 90% of the trenches examined. In contrast, an electrolyte closely related to that used in this study reportedly filled only 50% of the trenches. In this paper superconformal deposition over a wide range of feature sizes and aspect ratios is demonstrated using a polyether (polyethylene glycol, PEG), thiol (3-mercapto-1-propanesulfonate, MPSA), and chloride (Cl) electrolyte. Complete filling was observed for films grown potentiostatically in a regime corresponding to mixed kinetic/diffusion control for copper deposition. For control purposes, deposition from an additive-free electrolyte was also investigated. Additional experiments with various combinations of Cl-PEG-MPSA as well as experiments with the sole addition of benzotriazole (BTAH) were performed in an attempt to assess the possibility of correlating simple i-E measurements of inhibition with superconformal deposition. BTAH, which is widely used as a corrosion inhibitor, 18 is also known to inhibit copper deposition and yield bright deposits, 19-21 thereby enabling the effect of surface roughness on feature filling to be explored. Furthermore, analysis of electrodeposits revealed that the incorporated BTAH molecule could be recovered in the native form, which suggests that this electrolyte might be an ideal agent for examining inhibition and additive incorporation effects. 19 These experiments were complemented by resistivity measurements of blanket films that provide a preliminary assessment of the extent of additive incorporation through its influence on metallurgical structure.
doi:10.1149/1.1394096 fatcat:d5o6uukvkjeoroh66joeuahezy