Characterization of Batch-Microfabricated Scanning Electrochemical-Atomic Force Microscopy Probes

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Abstract
A procedure for the batch microfabrication of scanning electrochemical-atomic force microscopy (SECM-AFM) probes is described. The process yields sharp AFM tips, incorporating a triangular-shaped electrode (base width 1 μm, height 0.65 μm) at the apex. Microfabrication was typically carried out on 1/4 3-in. wafers, yielding 60 probes in each run. The measured spring constant of the probes was in the range 1−1.5 N m-1. To date, processing has been carried out twice successfully, with an estimated success rate for the fabrication process in excess of 80%, based on field emission-scanning electron microscopy imaging of all probes and current−voltage measurements on a random selection of ∼30 probes. Steady-state voltammetric measurements for the reduction of Ru(NH3)63+ in aqueous solution indicate that the electrode response is well-defined, reproducible, and quantitative, based on a comparison of the experimental diffusion-limited current with finite element simulations of the corresponding mass transport (diffusion) problem. Topographical imaging of a sputtered Au film with the SECM-AFM probes demonstrates lateral resolution comparable to that of conventional Si3N4 AFM probes. Combined electrochemical−topographical imaging studies have been carried out on two model substrates: a 10-μm-diameter disk ultramicroelectrode (UME) and an array of 1-μm-diameter UMEs, spaced 12.5 μm apart (center to center). In both cases, an SECM-AFM probe was first employed to image the topography of the substrates. The tip was then moved back a defined distance from the surface and use to detect Ru(NH3)62+ produced at the substrate, biased at a potential to reduce Ru(NH3)63+, present in bulk solution, at a diffusion-controlled rate (substrate generation−tip collection mode). These studies establish the success of the batch process for the mass microfabrication of SECM-AFM tips.

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