Optical films and fluidic slits with one nanometer structure and function

K-T Liao, J. Schumacher, H.J. Lezec, S.M. Stavis
NIST,
United States

Keywords: focused ion beam milling, nanofluidics, nanoparticles, separation, characterization, structural color

Summary:

The relationship between structure and function is fundamental to nanotechnologies of all dimensionalities, including surfaces, films, and slits with nanoscale vertical dimensions and microscale lateral dimensions. Such nanostructures implement engineered functions with diverse applications in biomimetics, optics, magnetics, mechanics, and fluidics. In this presentation, I will describe our progress towards fulfilling Feynman’s vision of using a focused ion beam to fabricate such nanostructures with atomic resolution as the foundation of functional nanotechnologies. We first investigate the response of bulk silicon, as a reference material, to a focused beam of gallium ions. We then investigate the milling responses of submicrometer films of silicon dioxide and silicon nitride as functional materials. Our nanofabrication process can resolve vertical features across the nanoscale, with lateral features extending across the microscale and into the macroscale. Our results establish new limits of dimensional control in this triad of important hard materials. Moreover, our nanofabrication process is highly efficient, enabling rapid prototyping of complex patterns for practical applications. To demonstrate the utility of our new limits of dimensional control, we fabricate devices in silicon dioxide with numerous critical dimensions of approximately 1 nm that implement optical and fluidic functions. Submicrometer films, milled in silicon dioxide, show structural colors under illumination with white light. Colorimetric analysis of optical micrographs can inform of subnanometer variation in film thickness with high throughput in a nanomanufacturing process. Such measurements can obviate the need for atomic force microscopy, which we otherwise use to quantitatively characterize our complex surfaces. Nanofluidic slits arrayed in a staircase structure, milled in silicon dioxide and sealed with an adhesion layer of silicon resin, separate nanoparticles by size exclusion. In this size separation process, the nanofluidic staircase simultaneously functions as a separation matrix and as a reference material, allowing characterization of single nanoparticles by localizing them and mapping their channel positions of size exclusion to the excluded channel depth. In this way, we measure the size distribution of a reference sample of fluorescent nanoparticles with uncertainty of approximately 1 nm. Analysis of nanoparticle intensity confirms the expected volumetric scaling of dye loading, providing an orthogonal validation of the size separation and characterization mechanism. In conclusion, we apply our new limits of dimensional control to solve an important problem in nanoparticle characterization. More generally, our results open the door to a variety of novel structure–function relationships at the nanometer scale in complex surfaces, films, and slits.