ALD HfO₂ Films for Defining Microelectrodes for Electrochemical Sensing and Other Applications

Microelectrodes are used in a wide range of applications from analytical electrochemistry and biomolecular sensing to in vivo implants. While a variety of insulating materials have been used to define the microelectrode active area, most are not suitable for nanoscale electrodes (<1 μm²) due to the limited robustness of these films when the film thickness is on the order of the nanoelectrode dimension. In this study, we investigate atomic layer deposited hafnium dioxide (ALD HfO₂) as an insulating film to coat planar platinum microelectrodes, with the active areas being defined where the HfO₂ is etched. Thermally grown films with thicknesses between 10 and 60 nm were deposited by 100 to 550 ALD cycles and were initially characterized by measuring their standard electrical properties and imaging incipient texture development. Electrochemical measurements on the structures were made, including linear sweep voltammetry and electrochemical impedance spectroscopy, which identified the presence of pinholes in films deposited over the range of 100 to 350 cycles, resulting in leakage. These measurements also suggest a lower limit to the size of microelectrodes below which the electrochemical current detected is no longer dominated by that through the exposed active area. A bilayer insulator comprising ALD HfO₂ coated with parylene-C was investigated to minimize the pinhole leakage. Steady-state currents were measured for different electrode areas, qualitatively agreeing with the theory for areas down to ∼1 μm². For sub-square micrometer electrode areas, bilayer-insulated devices with parylene-C apertures that exposed the smallest microelectrode area showed measured currents that were consistent with extrapolations, indicating that it reduces leakage through HfO₂.