Direct Copper Plating
Direct plating is the term used in the damascene interconnect technology when copper (Cu) is plated on a substrate without a copper seed. The conductive copper seed is replaced by a nobler metal such as ruthenium (Ru) or metal compounds such as RuTa or Ru
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Direct Copper Plating Aleksandar Radisic and Philippe M. Vereecken
7.1 Introduction Direct plating is the term used in the damascene interconnect technology when copper (Cu) is plated on a substrate without a copper seed. The conductive copper seed is replaced by a nobler metal such as ruthenium (Ru) or metal compounds such as RuTa or RuTiN alloys in an effort to combine seed and barrier properties into one as the lining thin film material [1]. Copper electrodeposition on top of a foreign substrate by itself is of course not that unusual, copper plated on a platinum rotating disk electrode is quite standard in the lab, but there are many technological complications when bringing it to the wafer scale. In contrast to the conductive platinum disk in the lab, the thin barrier and seed lining layers are resistive which implies a significant potential drop from the wafer edge to the wafer center. The fact that the cost scaling trends demand for denser packing of transistors (i.e., smaller features with thinner lining) and larger wafer size (currently 300 and going to 450 mm soon), only worsens this so-called terminal effect. Of course, this is true for both thin copper seed plating and direct plating, and both will be discussed in Sect. 7.4. Copper electrodeposition on foreign substrates proceeds through an electrochemical nucleation and growth process. This has serious implications for the copper damascene plating process as a continuous copper thin film needs to be formed before the inlaid trench and blind hole features start to fill up with copper [2]. Hence, the coalescence thickness needs to be at least 1/3 of the effectively open feature size. This means that higher island densities will be required for each generation of feature scaling or node. This obviously sets limits to the extendibility of the direct plating process. The nucleation island density is a function of the A. Radisic P. M. Vereecken (&) IMEC, Kapeldreef 75, B-3001 Leuven, Belgium e-mail: [email protected]
K. Kondo et al. (eds.), Copper Electrodeposition for Nanofabrication of Electronics Devices, Nanostructure Science and Technology 171, DOI: 10.1007/978-1-4614-9176-7_7, Springer Science+Business Media New York 2014
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overpotential and substrate. The growth and thus shape of the islands is affected by the bath additives and by the condition of the substrate surface. Nucleation and growth phenomena will be extensively treated in Sect. 7.3. The real challenge of direct plating, however, lays in the combination of the terminal effect and the nucleation and growth of copper into a coalesced or closed film; i.e., to make it work all over the wafer. As the substrate potential and thus overpotential changes with wafer radius, also the nucleation island density is dependent on wafer radius. Fortunately, a more severe terminal effect and thus a sharper potential drop helps the direct plating process as the nucleation is confined within a small ring area on the wafer only. Indeed, the sheet resistance of the th
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