Gold Superfill in Sub-Micrometer Trenches D. Josell, a,z C. R. Beauchamp, a D. R. Kelley, a C. A. Witt, b and T. P. Moffat a, * a National Institute of Standards and Technology, Metallurgy Division, Gaithersburg, Maryland20899, USA b Cookson Electronics, Enthone, Orange, Connecticut 06516, USA Bottom-up feature fill, also called ‘‘superfill,’’ of gold is demonstrated for the first time. Gold superfill is achieved in trenches as narrow as 60 nm, particularly relevant for interconnects on GaAs and GaN technologies. This was accomplished through the use of submonolayer coverage of a preadsorbed, deposition-rate accelerating surfactant followed by Au electrodeposition. The experi- mental results indicate the potential of this technologically relevant process and also support the generality of the CEAC mecha- nism for feature superfill and surface stabilization. © 2005 The Electrochemical Society. DOI: 10.1149/1.1854777 All rights reserved. Manuscript submitted September 27, 2004; revised manuscript received November 8, 2004. Available electronically January 24, 2005. Superconformal, bottom-up feature filling during electrodeposi- tion, called ‘‘superfill,’’ 1 is now the preferred means for fabricating the copper metallizations in silicon based integrated circuits because it yields defect-free filling of trenches and vias where a conformal filling process would yield either seams or voids. For semiconductor technologies based on GaAs 2 and GaN, 3 gold is the contact material and metallization of choice. Higher-density integration of III-V tech- nology might require a damascene process analogous to that used for Cu metallizations in silicon technology. As with Cu, supercon- formal metal deposition will be required. Recently, superconformal deposition utilizing a leveling mechanism was demonstrated for Au metallizations. 4 Here we demonstrate a different process, that yields bottom-up feature filling analogous to that implemented for Cu met- allizations, which is known as ‘‘superfill.’’ This work uses under- standing of the mechanism behind Cu superfill to determine and demonstrate such a process. A variety of mechanisms have been proposed to underlie the superfill process during electrochemical deposition ECD of copper. 1,5-8 The curvature enhanced accelerator coverage CEAC mechanism 7,8 in particular predicts that an electrolyte-additive sys- tem will yield superfill if two requirements are met: acceleration of the deposition rate in the presence of adsorbed additive and the ability of the adsorbate to segregate to the metal surface during deposition. Under these circumstances the CEAC predicts that the decreasing area of the metal surface at the bottoms of the filling feature will lead to locally increasing coverage of the adsorbed ac- celerator, which will naturally lead to increased local deposition rate and bottom-up superfill. The CEAC mechanism has been able to explain superfill during ECD of copper, 7-11 ECD of silver 12-16 and chemical vapor deposition CVD of copper. 17,18 In these analyses, the CEAC mechanism has been successfully used to quantitatively predict superfill in both trenches and vias, using a variety of deposition rate accelerating additives accelerators, and for accelerators either codeposited with the metal or preadsorbed on the patterned surface to be filled. Sig- nificantly, the CEAC-based models use only kinetics obtained from independent studies on planar substrates; the CEAC formalism has no parameters to fit feature filling experiments. Additionally, while quantitative comparisons to experiments have not been published, the CEAC mechanism has been shown to explain how the presence of accelerators in the electrolyte 19 or preadsorbed on the deposit surface 20 stabilizes surfaces, consistent with experimental observa- tion, and has also been proposed to explain new Ag superfill results. 21 This work assumes complete generality of the CEAC mechanism for superfill to focus on Au, a metal of industrial relevance due to its use in contact metallizations for semiconductor technologies such as those based on GaAs and GaN. Consistent with predictions of the CEAC mechanism, Au superfill in fine trenches is demonstrated during electrodeposition from a cyanide electrolyte in the presence of either preadsorbed Pb or Tl. System Selection The CEAC mechanism predicts feature superfill can be obtained from an additive-electrolyte system if two requirements are met: acceleration of the metal deposition rate occurs in the presence of adsorbed additive accelerator and the adsorbed accelerator has the ability to segregate to the metal surface with no, or minimal, con- sumption during the metal deposition process surfactant. For ECD of metals in particular, if the accelerator is contained in the electro- lyte used for the metal deposition, the acceleration associated with its time-dependent adsorption manifests in rising current-time tran- sients or analogous depolarization shifting of deposition to lower overpotentials during current-voltage sweeps. Such behavior is ob- served during Ag ECD on planar substrates in a cyanide electrolyte containing dilute arsenic 22 and copper ECD in a cyanide electrolyte containing dilute Se 23 with experiments seemingly demonstrating Cu superfill 24 20 years prior to its industrial introduction 1  in ad- dition to the cited CEAC-based superfill studies. For Au deposition, increased deposition rates have been detailed for adsorption of thallium in cyanide, 25-27 slightly acidic cyanoaurate, 28 and hypo- phosphate buffered cyanide 29 electrolytes, adsorption of antimony in a sulfate electrolyte, 30 and adsorption of lead, bismuth and mercury in a hypophosphate buffered cyanide electrolyte. 29 According to the CEAC mechanism, all should be capable of yielding superfill during Au electrodeposition in fine features. For this study of Au superfill, Pb and Tl were used as the accel- erators. As noted above, data on Pb enhancement of Au deposition rates is for a hypophosphate buffered cyanide electrolyte. However, because both Pb and Tl were found here to function as accelerators in the cyanide electrolyte published in the studies of Tl adsorbate, that electrolyte was used for all experiments. For the work detailed herein, the accelerating species was preadsorbed on the substrates prior to metal deposition rather than incorporated in the electrolyte used for the metal deposition. Such a process has been previously demonstrated to yield superfill during ECD of Cu 31 and Ag 14 as well as CVD of Cu. 17 There was no sparging to remove oxygen from the electrolyte. Experimental Substrates.—The patterned wafers were fabricated by Interna- tional Sematech c and possessed neither barrier nor seed layers. Elec- tron beam evaporation was used to deposit a tri-seed layer contain- ing, in sequence from the dielectric, a titanium layer, a palladium layer, and a gold layer. The deposition process was identical to that used previously to deposit Ti/Pd/Ag seeds with the substitution of Au for the Ag layer for study of size effects in the electrical prop- * Electrochemical Society Active Member. z E-mail: daniel.josell@nist.gov c Corporate and product names are provided only for completeness of description; their inclusion does not imply NIST endorsement. Electrochemical and Solid-State Letters, 8 3 C54-C57 2005 1099-0062/2005/83/C54/4/$7.00 © The Electrochemical Society, Inc. C54