A Nanofountain Probe with Sub-100 nm Molecular Writing Resolution

摘要

Controlled patterning of materials at the nanoscale is a preparatory but fundamental step in the development of nanostructures and nanodevices. Dip-pen nanolithography (DPN), a direct-write lithographic process, has achieved this objective and opened up many possibilities for nanoscale patterning.[1]-[7] In the DPN process, molecular inks coated on an atomic force microscope (AFM) tip are transferred to a substrate while the tip is held or laterally moved along a surface as programmed. Feature sizes less than 15 nm can be routinely patterned by molecular self-assembly of the ink on the substrate. However, the necessity of molecular-ink replenishment in the DPN technique leads to limited throughput and inevitable realignment during patterning processes, especially when complex or large-area patterns are to be generated. Hence, the concept of continuous feeding of molecular inks is a natural goal following the historical paradigm shift in the handwriting world: from quills to fountain pens. In this regard, several devices and technologies have been proposed. Micropipettes have been developed and referred to as fountain nanopens because of their capabilities to deliver liquid or gaseous materials,[8] proteins,[9] and photoresist[10] to surfaces. However, micropipettes cannot realize nanoscale patterning. The common problem of micropipette-based probes is that they suffer from ill-controlled aperture shapes and sizes, especially at the nanoscale. Moreover, the feature size of the patterns made by micropipettes is limited since it depends critically on the probe outer diameter, which typically leads to larger dimension of the patterns due to the formation of a liquid meniscus around the pipette tip during patterning. As a variation of this approach, a microfabricated aperture on an AFM tip was proposed, in which the aperture was constructed at the apex of a hollow pyramidal tip utilizing the back of the tip as a reservoir.[11] Like micropipette-based probes, the resolution of these AFM probes is determined by the outer diameter size. A while ago, we reported a novel microfluidic probe that was micromachined to have a microfluidic system integrated into AFM probes.[12]-[14] The concept was later adopted by Deladi et al. and a micromachined fountain pen for writing and etching was reported.[15] However, the size of patterned features obtained with such chips was about 1 m. To our knowledge, none of the reported devices actually reached the stage of true nanoscale molecular writing, that is, sub-100 nm patterning. It is noted that for micropipette-based probes and apertured AFM probes, the ink in the liquid state at the lumen of the aperture floods the substrate when the tip is brought into contact with the surface due to surface tension.