Novel low-temperature fabrication process for integrated high-aspect ratio zinc oxide nanowire sensors

Abstract

The authors present a new low-temperature nanowirefabrication process that allows high-aspect ratio nanowires to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 μm) aluminum film deposited on a silicon substrate. This technique is in sharp contrast to the traditional free-standing thick film methods, and the use of an integrated thin aluminum film greatly enhances the utility of such methods. The authors have demonstrated the method by integrating ZnOnanowires onto the metal gate of a metal-oxide-semiconductor (MOS) transistor to form an integrated chemical field-effect transistor (ChemFET) sensor structure. The novel thin film AAO process uses a novel multistage aluminum anodization, alumina barrier layer removal, ZnOatomic layer deposition(ALD), and pH controlled wet release etching. This new process selectively forms the ZnOnanowires on the aluminum gate of the transistor while maintaining the remainder of the aluminum film intact for other integrated device components and interconnects. This self-assembled high-density AAO template was selectively formed in an ultrasmooth 2.5 μm thick aluminum layer deposited through e-beam evaporation without the electropolishing required in AAO template formation in traditional 100 μm thick free standing films. The resulting nanopore AAO template consists of nanopores of 90 nm in diameter and 1 μm in height at an aerial density of 1.3 × 1010 nanopores/cm2. This thin film AAO template was then filled with ZnO using ALD at 200 °C, forming polycrystalline ZnOnanowires inside the pores. The alumina template was then removed with a buffered NaOH solution, leaving free standing ZnOnanowires of 1 μm height and 90 nm diameter, offering an increase in 38× the surface area over a standard flat ZnO film for sensing applications. The aluminum film remains intact (unanodized) in nonselected regions of the device as well as underlying the ZnOnanowires, acting as the gate of the MOS transistor. The ZnOnanowires were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy to verify stoichiometry and crystal structure. Additionally, the response of a ZnOnanowire ChemFET was measured using ammonia as a target gas. I-V characterization and transient response to ammonia in the range of 25–200 ppm were examined. The ammonia response to the threshold limit value concentration of ammonia (25 ppm) shows a 56 mV shift in threshold voltage, an overall sensitivity of 14%, an 8 min response time, and a 27 min recovery period. The ZnOnanowirefabrication sequence that the authors present is accomplished at low-temperature (<200 °C) and can be accomplished selectively, making it readily amenable to integration with standard metal-oxide-semiconductor field-effect transistor processing as well as other microelectronic sensors such as surface acoustic wave devices. This new process has initially been demonstrated using ZnO, but is also adaptable to a variety of nanowire materials using appropriate deposition methods as well as selective nanowire release methods. This allows the potential to conveniently fabricate a variety of high-aspect ratio nanowire based microelectronic sensors for a range of applications.