Ion beam controlled to etch single silicon atom depth

Like sandblasting at the nanometer scale, focused beams of ions ablate hard materials to form intricate three-dimensional patterns. The beams can create tiny features in the lateral dimensions—length and width, but to create the next generation of nanometer-scale devices, the energetic ions must precisely control the features in the vertical dimension—depth. Now, researchers at the National Institute of Standards and Technology (NIST) have demonstrated that a standard ion-beam technique can be fine-tuned to make structures with depths controlled to within the diameter of a single silicon atom.

Above – nanofluidic staircase machined with subnanometer precision by a focused ion beam separates nanoparticles by size. The device is also a reference material to accurately measure nanoparticle size and compare it to optical brightness, which could aid in the quality control of consumer products. Credit: NIST

Taking advantage of that newly demonstrated precision, the NIST team used this standard machining technique to fabricate devices that allow precise measurement of the size of nanoparticles in a liquid. The nanofluidic devices, which have the potential for mass production, could become a new laboratory standard for determining nanoparticle size. Such measurements could expedite quality control in industrial applications of nanoparticles.

Although engineers have for years used ion beams to fix defects in integrated circuits and machine tiny parts in optical and mechanical systems, those applications did not require the depth control the team has now reported.

To realize the full potential of the process, the team explored several ways of using a focused beam of gallium ions to mill the surfaces of silicon, silicon nitride and silicon dioxide—materials that are common for the fabrication of nanoscale devices used in electronics, optics and mechanics. The researchers used an atomic force microscope, which features a sensitive probe to measure the depth of the topography formed by the ion beam. Careful measurements were important to testing the limits of the ion-beam technique. The facilities at NIST enabled the team to undertake both tasks—precision fabrication and precision measurement.

The team applied the new capability to improve the measurement of the size of nanoparticles. Using a gallium ion beam, the researchers machined staircase patterns in silicon dioxide and then enclosed them to control the flow of fluid at the nanoscale. In some devices, the researchers machined a staircase with a step size of 1.1 nanometers; they machined others with a step size of 0.6 nanometers—just a few atoms in depth.

Lab on a Chip – Subnanometer structure and function from ion beams through complex fluidics to fluorescent particles

The vertical dimensions of complex nanostructures determine the functions of diverse nanotechnologies. In this paper, we investigate the unknown limits of such structure–function relationships at subnanometer scales. We begin with a quantitative evaluation of measurement uncertainty from atomic force microscopy, which propagates through our investigation from ion beam fabrication to fluorescent particle characterization. We use a focused beam of gallium ions to subtractively pattern silicon surfaces, and silicon nitride and silicon dioxide films. Our study of material responses quantifies the atomic limits of forming complex topographies with subnanometer resolution of vertical features over a wide range of vertical and lateral dimensions. Our results demonstrate the underutilized capability of this standard system for rapid prototyping of subnanometer structures in hard materials. We directly apply this unprecedented dimensional control to fabricate nanofluidic devices for the analytical separation of colloidal nanoparticles by size exclusion. Optical microscopy of single nanoparticles within such reference materials establishes a subnanometer limit of the fluidic manipulation of particulate matter and enables critical-dimension particle tracking with subnanometer accuracy. After calibrating for optical interference within our multifunctional devices, which also enables device metrology and integrated spectroscopy, we reveal an unexpected relationship between nanoparticle size and emission intensity for common fluorescent probes. Emission intensity increases supervolumetrically with nanoparticle diameter and then decreases as nanoparticles with different diameters photobleach to similar values of terminal intensity. We propose a simple model to empirically interpret these surprising results. Our investigation enables new control and study of structure–function relationships at subnanometer scales.

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