Copper has become a focus of research activities over the last two decades due to its use as interconnect material in microchip design. Presently the circuit layout of these microchips is realized by lithography, the resolution of which is dependent on the wavelength of the applied light. To continue the ongoing process of miniaturization, cationic organic molecules have been studied as additives in the copper damascene process by an increasing number of research groups.
In the present investigation the influence of various halides on structure and reactivity of a Diphenylviologen (DPV) layer adsorbed on a Cu(100) surface have been studied by in-situ electrochemical STM and high-resolution XPS, conducted at the synchrotron source BESSY2.
N,N'-diphenyl-4,4'-bipyridinium molecules (Diphenylviologen, DPV) spontaneously adsorb on a halide-modified Cu(100)-surface, forming a striped pattern at potentials around 0mV vs. RHE, which has been characterized by in-situ Scanning Tunneling Microscopy. Cyclic Voltammetry indicates that the building block of this striped phase is the radical cation of the viologen. On the bromide- and chloride-modified surface, no other redox-state could be prepared, whereas the CV shows an anodic peak at 120mV vs. RHE for the iodide-modified surface, indicating an oxidation of the radical cation to the dication. These result have been confirmed by ex-situ high resolution XPS after transfer into UHV in a process allowing conservation of the surface redox-states upon emersion. A careful analysis of the N1s and C1s core level shifts then allows the determination of the redox-state of the adsorbed viologens.
The striped phase is assembled from planar DPV radical cations in a face to face configuration, while the main molecular axis is parallel to the copper surface. The step edges are decorated by exceptionally long rows of molecules, stabilising the edges against copper dissolution. As a result the onset of the copper corrosion has been found to be displaced by about 100mV to more positive potential, depending on the halide. Furthermore, the preferential orientation of the molecules with respect to the step edge leads to a rather anisotropic corrosion process. |