Acceleration Effect
Fabrication of microscale and nanoscale structures can be implemented by electrodeposition of copper onto conductive templates that contain recesses in form of the desired structure. The current distribution that results from deposition from a simple acid
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Acceleration Effect Dale P. Barkey
3.1 Introduction Fabrication of microscale and nanoscale structures can be implemented by electrodeposition of copper onto conductive templates that contain recesses in form of the desired structure. The current distribution that results from deposition from a simple acid copper solution is concentrated near the upper corners of recesses and results in incomplete filling, formation of voids, and a substantial overburden of nonfunctional material. In industrial practice, effective filling of features such as trenches and vias by copper electrodeposition relies on additive ensembles that promote bottom-up filling, uniform current macrodistribution and minimal overburden. The components of these additive systems include suppressors, accelerants, and levelers. Suppressors and levelers both inhibit electrodeposition. For our purposes, a suppressor is an inhibitor that can be deactivated by an accelerant. Suppressors include polyalkyl glycols (PAG) such as polyethylene glycol (PEG). They are weakly adsorbed on the surface in combination with chloride and are not consumed or chemically transformed on the metal surface. Levelers, such as Janus Green B, are inhibitors that are not deactivated by accelerants. They are strongly adsorbed and are consumed on the metal surface. Filling of microscale and nanoscale features is achieved through one of two broad categories of mechanism [1]. One of these is the diffusion-limited consumption of leveler. This mechanism has been understood for decades and is the basis for leveling on many different physical scales. Since it does not involve an accelerant, it will not be considered further. The other mechanism relies on the combination of suppressor and accelerant. In this latter type of mechanism, the accelerant is concentrated at the bottom of features and deactivates or excludes the suppressor there. Accelerants have been postulated to lift or displace adsorbed suppressors from the surface and restore the D. P. Barkey (&) Department of Chemical Engineering, College of Engineering and Physical Science, Kingsbury Hall, Room W305, Durham, NH 03824, USA 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_3, Springer Science+Business Media New York 2014
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higher rate of deposition observed on the suppressor-free surface. While the application considered here is the filling of trenches, vias, or other substrate features that act as templates for fabrication of microscale and nanoscale structures, the principle is the same as that which produces bright deposits in more conventional plating, and the accelerant is sometimes referred to as a brightener. While many suppressors and levelers can be found in the literature, few effective accelerants are known. In the open literature, most reports concern bis(3-sulfopropyl) disulfide (SPS) or its reduced monomer, 3-mercaptopropylsulfonate (MPS) (see,
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