Applications of conventional optics in photolithography are generally constrained by the diffraction limit. This spatial-resolution barrier can be overcome in the near field through exploration of the optical properties of subwavelength features. A near-field scanning optical-microscope (NSOM) system uses a pulled optical-fiber tip to provide nanoscale spatial-imaging resolution through weak evanescent waves and sample interactions.1 Functions such as near-field photolithography2 and biological perturbation need significant light enhancement.
Various approaches for concentrating electromagnetic energy in the near field have been investigated, including use of tapered waveguides,3 air gaps,4metal wedges,5 and tapered conic rods.6,7 The scanning probe we have developed enhances the local optical-field intensity through a surface-plasmonic wave generated along its metal-coated silicon oxide tip (see Figure 1). This evanescent wave is locally confined at the metal/dielectric interface, and coupled into air media around the aperture with a significant electromagnetic-field enhancement.8
Figure 1. Schematic of our scanning probe with tip light enhancement. The surface-plasmonic wave is excited, propagates along the metal/dielectric interface, and is then coupled into the air media around the aperture. Al: Aluminum. Si: Silicon. SiO2: Silicon dioxide.
We have developed a batch fabrication process to manufacture such probe tips containing a subwavelength aperture (see Figure 2) through close synergy of probe design, nanofabrication, precision engineering, and device characterization. The resulting scanning probe is a silicon cantilever with a hollow pyramidal tip. The shape of the tip was constructed using an anisotropic wet-etching process. An aperture was milled using a focused ion beam on the tip. Compared to the standard tapered optical fibers used in NSOMs, our probe provides enhanced local-field intensity and high throughput by means of the carefully designed conic tip. The silicon material and probe structure also provide a higher thermal threshold and mechanical robustness.
Figure 2. Scanning-electron micrograph of our probe.
The scanning probe is fully integrated with a customized NSOM system, with built-in scanning control of the probe-sample distance using a tuning fork (see Figure 3). The sample stage can perform three-degrees-of-freedom linear scanning. The optical fiber, coupled with the laser source, is fed into the hollow pyramidal structure. The system is fully automated to adjust the relative lateral movement of the sample surface and probe tip at nanometer resolution. The tuning fork controls the vertical distance between the tip and the sample surface.
Figure 3. Schematic of our scanning probe integrated with a near-field scanning optical-microscope (NSOM) system. (inset) Pyramidal scanning-probe tip coupled with optical fiber using a laser source at (left) 632.8 and (right) 405nm. BS: Beam splitter. 3-DOF: Three degrees of freedom. PMT: Photomultiplier tube. OB: Optical bench.
We performed preliminary photolithography experiments on AZ5209E photoresist. A tapered optical fiber, connected to a 405nm laser source, was aligned with the aperture for light delivery to the tip. The photoresist was spun on a glass cover, which was then attached to the tuning fork. The probe was controlled for near-field sample scanning. A series of 15 point-by-point exposures were carried out stepwise at 6.5μm separation. The exposure time varied from 0 to 8min, with subsequent 60s development periods.
We evaluated lithography patterns using atomic-force-microscopy measurements (see Figure 4). Their depths and widths correlated linearly with increasing exposure time, with slopes of 760 and 3.5nm/s, respectively. We measured a lateral resolution of 200nm. Both the surface-plasmonic wave and the lower transmitted modes through the hollow tip contributed to the subwavelength features in the patterned photoresist. A finer resolution is expected with further reduced aperture size, where all modes are cut off except for the tightly localized evanescent wave.
Figure 4. Near-field photolithography results measured using an atomic-force microscope. FWHM: Full width at half maximum. R: Lateral resolution.
In summary, we have designed and fabricated plasmonic scanning probes that are fully compatible with NSOM systems to develop a multifunctional platform for near-field imaging and patterning. We are working on further down-scaled optical apertures and tip design, in the context of which the probe shows great promise for noninvasive in vivo imaging, sensing, and manipulating of delicate biological samples.
The probe fabrication work was carried out at the Microelectronics Research Center and Nano Science and Technology at the University of Texas at Austin. We gratefully acknowledge financial support from the National Science Foundation (grant 0826336). We also thank Micah Glaz for his help in making pulled-fiber probes for use in the NSOM for comparison experiments.
Yu-Yen Huang, Yuyan Wang, Kazunori Hoshino, Yujan Shrestha, David Giese, Xiaojing Zhang
Department of Biomedical Engineering
The University of Texas at Austin
Yu-Yen Huang is a PhD student.
Kazunori Hoshino is a postdoctoral researcher. His research interests are in microelectromechanical systems (MEMS) and optical MEMS, biologically inspired microvisual systems, nanoscale light-emitting devices, and nanophotonics.
Yujan Shrestha and David Giese are senior undergraduate students in the Zhang research group.
Xiaojing (John) Zhang is an assistant professor affiliated with both the Department of Biomedical Engineering and the Microelectronics Research Center. His research interests include integration of photonics with MEMS and microfluidic devices for in vivo imaging, and near-field microscopy and sensing, in particular for investigation of cellular processes that are critical to development and disease.