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42461-G5
Novel Molecular Simulations for Understanding the Function of Additives in Electrodeposition
The work supported by this Type-G grant led to increased understanding of and control over deposition of metals on surfaces by means of organic additives and templates. Two distinct surface-modification problems were addressed.
Organic accelerator for copper electrodeposition
The first problem dealt with electrodeposition of copper metal into small concave features, such as on computer chips. Organic additives are used in the bath to deposit metal from the "bottom up" so that holes or defects in the deposit are avoided. However, little is understood about the molecular surface mechanisms by which the combination of organic additives performs its function. We investigated the problem with molecular dynamics simulations as well as select experiments. In particular we investigated the molecule 3-mercaptopropanesulfonic acid (MPSA), which is used as a deposition accelerator, in combination with chloride.
Our molecular simulations required us to obtain intermolecular interaction potentials for no less than 10 molecular, ionic, and metallic species present in the system, several of which had not been previously simulated. A significant amount of effort was therefore devoted to performing quantum chemical calculations and equation fitting in order to obtain the interaction parameters.
The molecular dynamics (MD) simulations of the deposition bath used a pioneering simulation technique known as electrode charge dynamics that allows the simulated metal surfaces to polarize as real electrodes would. The model was used to simulate adsorption on both atomically smooth (111) and atomically rough electrode surfaces. We investigated how the presence of sodium chloride affects the surface adsorption and surface action of MPSA as well as the charge distribution in the system. We found that NaCl addition decreases the adsorption strength of MPSA at a simulated copper surface and may weaken the interaction of MPSA with copper ions. Both mechanisms may be necessary to generate the observed deposition acceleration.
The associated deposition experiments yielded additional insight in showing how the two chemical endgroups on MPSA (a sulfonate group and a thiol group) have distinct roles in controlling the behavior of MPSA in the bath.
Nanografting of silanes onto surfaces as a step toward nanoelectronic circuitry
The second problem is to develop a new method for generating templates on surfaces for nanoscale electronic circuits to be used in future generations of computer chips. The organic templates change the surface chemistry to the effect of directing electroless deposition of copper onto selected regions of the otherwise insulating substrate.
Our approach is to change the local chemical composition on a surface by a means known as nanografting. In nanografting an atomic force microscope (AFM) tip is dragged over a functionalized surface. The interaction of the tip with the surface changes the chemical bonding and causes a species in solution, in this case a silane, to adsorb onto the surface following the path of the tip. By this means we achieve chemical contrast. For instance, the scribed lines can be made hydrophilic, while the background surface is hydrophobic. These steps permit further chemical work for a number of possible applications. In the case of this work, the next intended step is to deposit metals onto the functionalized lines by means of an electroless wet process.
We were successful in nanografting controlled lines of both perfluorinated silanes and an aminosilane (APDES) onto surface monolayers of both octadecyldimethylmonochlorosilane (C18DMS) and octyldimethylmonochlorosilane (C8DMS). Subsequent analysis showed that the modified areas had relatively small changes in surface-layer thicknesses (less than 0.2 nm) and yet showed substantial changes in surface friction (AFM lateral force). At the same time, we showed that the nanografting does not compromise the insulating silicon oxide substrate. Our success in chemically modifying an insulating surface through mechanical patterning is encouraging and is leading to further surface-modification steps, namely the plating of metal ions onto the lines.
Summary of accomplishments
The work funded by this grant has led to a number of advances concerning control of surface chemistry and deposition of metals and other species using organic molecules. The work has led to two peer-reviewed publications, with a third one submitted, all of which include student co-authors. One graduate student and two undergraduate students were supported. The ACS-PRF-supported work has led to additional funding from the National Science Foundation and to long-term collaboration with other researchers, including another recipient of an ACS-PRF Type-G grant.
